Layered double hydroxide polymer nanocomposites 9780081019047, 0081019041

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Layered Double Hydroxide Polymer Nanocomposites

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Woodhead Publishing Series in Composites Science and Engineering

Layered Double Hydroxide Polymer Nanocomposites Edited by

Sabu Thomas International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

Saju Daniel International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India St. Xavier’s College Vaikom, Kottayam, Kerala, India

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2020 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-08-102261-0 (print) ISBN: 978-0-08-101904-7 (online) For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Gwen Jones Editorial Project Manager: Andrea Gallego Ortiz Production Project Manager: Debasish Ghosh Cover Designer: Greg Harris Typeset by MPS Limited, Chennai, India

Contents

List of contributors Preface Acknowledgement 1.

Layered double hydroxides: fundamentals to applications Saju Daniel and Sabu Thomas 1.1 Introduction 1.2 Layered double hydroxides 1.2.1 Structure of layered double hydroxides 1.2.2 Differences between Ordinary Clays and Layered Double Hydroxides 1.2.3 Synthesis Routes of Layered Double Hydroxides 1.3 Organic Modification of Layered Double Hydroxides 1.3.1 One step Co-precipitation 1.3.2 Anion exchange reaction 1.3.3 Memory Effect or Regeneration Method 1.3.4 The delamination/restacking method 1.4 Characterization of layered double hydroxides and modified layered double hydroxides 1.5 Potential applications of layered double hydroxides, organically modified layered double hydroxides and layered double hydroxide nanocomposites 1.5.1 Flame retardant applications 1.5.2 Catalysis 1.5.3 Water splitting 1.5.4 Environmental remediation 1.5.5 Electrode for super capacitor 1.5.6 Biomedical applications 1.6 Introduction to Layered Double Hydroxide Polymer Nanocomposites 1.7 Different Fabrication Techniques of Layered Double Hydroxide polymer Nanocomposites 1.7.1 Melt Compounding 1.7.2 Solution Blending 1.7.3 In
Situ Methods 1.7.4 Layer By Layer Assembly 1.7.5 Two roll mill mixing

xvii xxi xxv 1 1 2 2 4 4 7 11 12 12 13 14

21 23 25 26 27 30 32 35 35 35 37 38 39 41

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1.7.6 Sonication 1.7.7 High energy ball milling 1.8 Applications of Layered Double Hydroxide Polymer Nanocomposites 1.8.1 Flame Retardant Application 1.8.2 Biomedical Application 1.8.3 Gas sensing Applications 1.8.4 Energy Applications 1.8.5 Food Packaging Applications 1.8.6 Water Purification 1.8.7 Gas Barrier Materials 1.8.8 Agricultural Applications 1.8.9 Anti Corrosion Materials 1.9 LDH based polymer hybrid nanocomposites 1.10 Conclusion and perspectives 1.11 Abbreviations References 2.

3.

FTIR characterization of layered double hydroxides and modified layered double hydroxides Meisam Shabanian, Mohsen Hajibeygi and Ahmad Raeisi 2.1 Introduction 2.2 Fourier transform infrared spectra of layered double hydroxides 2.2.1 Fourier transform infrared characteristic absorption bands of layered double hydroxides with different anions 2.2.2 Fourier transform infrared characteristic absorption bands of layered double hydroxides with different metals 2.2.3 FTIR spectra of layered double hydroxides containing three metals 2.3 FTIR spectra of organo-modified layered double hydroxides 2.4 Conclusion References Fabrication technologies of layered double hydroxide polymer nanocomposites Shadpour Mallakpour and Farbod Tabesh 3.1 Introduction 3.1.1 Layered double hydroxides 3.1.2 Modification of layered double hydroxides 3.2 Preparation of polymer/layered double hydroxide nanocomposites 3.2.1 Introduction 3.3 Preparation of Natural polymer/layered double hydroxide nanocomposites 3.3.1 Preparation of carboxymethylcellulose/layered double hydroxide nanocomposites

42 42 43 43 44 49 50 51 52 53 54 55 56 63 66 68

77 77 79 79 83 85 87 95 96

103 103 103 104 106 106 115 115

Contents

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3.3.2

3.4

3.5

Preparation of pectin/layered double hydroxide nanocomposites 3.3.3 Preparation of chitosan/layered double hydroxide nanocomposites 3.3.4 Preparation of natural rubber/layered double hydroxide nanocomposites 3.3.5 Other natural polymer/layered double hydroxide nanocomposites Preparation of synthetic polymer/layered double hydroxide nanocomposites 3.4.1 Preparation of polyimide/layered double hydroxide nanocomposites 3.4.2 Preparation of poly(methyl methacrylate)/layered double hydroxide nanocomposites 3.4.3 Preparation of polyvinyl acetate/layered double hydroxide nanocomposites 3.4.4 Preparation of P(MMA-co-BA)/layered double hydroxide nanocomposites 3.4.5 Preparation of wood flour/polypropylene/layered double hydroxide nanocomposites 3.4.6 Preparation of poly(amide-imide)/layered double hydroxide nanocomposites 3.4.7 Preparation of low-density polyethylene/layered double hydroxide nanocomposites 3.4.8 Preparation of polyvinyl alcohol/layered double hydroxide nanocomposites 3.4.9 Preparation of polyester/layered double hydroxide nanocomposites 3.4.10 Preparation of polyvinyl chloride/layered double hydroxide nanocomposites 3.4.11 Preparation of polypropylene-ethylene vinyl acetate/layered double hydroxide nanocomposites 3.4.12 Preparation of silicone rubber/layered double hydroxide nanocomposites 3.4.13 Preparation of epoxy resin/MoS2/layered double hydroxide nanocomposites 3.4.14 Preparation of polyurethane/nitrile butadiene rubber Blend/layered double hydroxide nanocomposites 3.4.15 Preparation of polyethyleneimine/poly(sodium 4-styrene sulfonate) hybrid/layered double hydroxide nanocomposites 3.4.16 Preparation of isotactic polypropylene/layered double hydroxide nanocomposites Conclusions and future perspectives

116 117 118 120 120 122 123 124 124 127 127 131 134 136 137 139 140 141 143

144 145 148

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Contents

Acknowledgments References 4.

5.

6.

Microscopic characterization techniques for layered double hydroxide polymer nanocomposites Shadpour Mallakpour and Shima Rashidimoghadam 4.1 Introduction 4.2 Microscopic characterization techniques for PNCs 4.2.1 Optical microscope 4.2.2 Scanning electron microscope 4.2.3 Transmission electron microscope 4.2.4 Field ion microscope 4.2.5 Scanning probe microscope 4.2.6 Scanning tunneling microscope 4.2.7 Atomic force microscope 4.2.8 X-Ray diffraction topography 4.3 Microscopic characterization of polymer/LDH NCs 4.3.1 Microscopic characterization of elastomer/LDH NCs 4.3.2 Microscopic characterization of thermoplastic polymer/LDH NCs 4.3.3 Microscopic characterization of thermosetting polymer/LDH NCs 4.3.4 Microscopic characterization of polymer blend/LDH NCs 4.4 Conclusions Acknowledgments References Further reading X-ray diffraction analysis of layered double hydroxide polymer nanocomposites Rodrigo Botan and Sabrina de Bona Sartor 5.1 Introduction 5.2 X-ray diffraction analysis 5.3 X-ray diffraction analysis of layered double hydroxides and Modified Layered Double Hydroxides 5.4 X-ray diffraction analysis of layered double hydroxide polymer nanocomposites 5.5 Conclusion References Spectroscopic characterization techniques for layered double hydroxide polymer nanocomposites Shadpour Mallakpour and Faezeh Azimi 6.1 Introduction

152 152

157 157 158 159 159 160 162 162 163 164 165 165 166 170 183 192 197 197 197 203

205 205 206 210 216 225 226

231 231

Contents

6.2

Spectroscopy of polymer nanocomposites 6.2.1 Fourier transform infrared spectroscopy 6.2.2 Raman spectroscopy 6.2.3 Energy-dispersive X-ray spectroscopy 6.2.4 Fluorescence spectroscopy 6.2.5 Dielectric spectroscopy 6.2.6 Nuclear magnetic resonance spectroscopy 6.2.7 UV
vis spectrophotometry 6.2.8 X-ray photoelectron spectroscopy 6.3 Spectroscopic characterization of layered double hydroxide polymer nanocomposites 6.3.1 Fourier transform infrared spectroscopy of layered double hydroxide polymer nanocomposites 6.3.2 Raman spectroscopy of layered double hydroxide polymer nanocomposites 6.3.3 Energy-dispersive X-ray spectroscopy of layered double hydroxide polymer nanocomposites 6.3.4 Fluorescence spectroscopy of layered double hydroxide polymer nanocomposites 6.3.5 Dielectric spectroscopy of layered double hydroxide polymer nanocomposites 6.3.6 Nuclear magnetic resonance spectroscopy of layered double hydroxide polymer nanocomposites 6.3.7 UV
vis spectroscopy of layered double hydroxide polymer nanocomposites 6.3.8 X-ray photoelectron spectroscopy of layered double hydroxide polymer nanocomposites 6.4 Spectroscopic characterization for the aging process 6.5 Conclusions Acknowledgments References Further reading

7.

Melt rheological properties of layered double hydroxide polymer nanocomposites Appukuttan Saritha and Kuruvilla Joseph 7.1 Introduction 7.1.1 The importance of rheological studies of polymer nanocomposites 7.1.2 Rheology of polymer layered double hydroxide nanocomposites 7.2 Rheology of thermoplastic polymer layered double hydroxide nanocomposites 7.3 Rheology of thermosetting polymer layered double hydroxide nanocomposites

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281 281 281 282 284 297

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Contents

7.4 Modeling of rheological properties 7.5 Conclusions and future scope References Further reading 8.

9.

Thermal properties and flame-retardant characteristics of layered double hydroxide polymer nanocomposites Yanshan Gao, Lei Qiu, Dermot O’Hare and Qiang Wang 8.1 Introduction 8.2 The techniques for determining thermal stability properties and flame retardancy performance 8.2.1 The techniques for determining thermal stability properties of polymers 8.2.2 The techniques for determining the flame-retardant performance of polymers 8.3 LDH-based thermal stabilizer materials and their applications 8.3.1 Thermal stabilizer introduction 8.3.2 Thermal stability properties of LDH-based nanocomposites 8.3.3 The mechanism of thermostability using LDHs 8.4 LDH-based flame-retardant materials and their applications 8.4.1 Flame retardant introduction 8.4.2 Flame-retardant performance of LDH-based nanocomposites 8.4.3 Posttreatment of LDHs as flame retardants 8.4.4 The mechanism of flame retardancy using LDH 8.5 Conclusions and future development References Mechancial and dynamical mechanical properties of layered double hydroxide-filled elastomer and elastomeric blend nanocomposites Suneel Kumar Srivastava 9.1 Introduction 9.2 Preparative methods of LDH-elastomer and LDH-elastomeric blend nanocomposites 9.3 Different types of layered double hydroxide fillers used in the fabrication of elastomer and elastomeric blend nanocomposites 9.4 Morphology of elastomer-LDH and elastomeric blend-LDH nanocomposites 9.4.1 Morphology of elastomeric-LDH nanocomposites 9.4.2 Morphology of elastomeric blend-LDH nanocomposites 9.5 Mechanical properties of elastomer-LDH and elastomeric blend-LDH nanocomposites 9.5.1 Mechanical properties of elastomer-LDH nanocomposites 9.5.2 Mechanical properties of elastomeric blend-LDH nanocomposites

301 305 305 310

311 311 313 313 314 317 317 318 322 323 323 324 337 338 340 340

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Contents

Dynamical mechanical properties of LDH-filled elastomer and elastomeric blend nanocomposites 9.6.1 Dynamical mechanical properties of elastomer-LDH nanocomposites 9.6.2 Dynamical mechanical properties of elastomer blend-LDH nanocomposites 9.7 Conclusion References

xi

9.6

10. Layered double hydroxide nanocomposites based on carbon nanoforms Gonzalo Abella´n, Jose A. Carrasco and Eugenio Coronado 10.1 A general introduction to LDH-carbon nanoform nanocomposites 10.2 Graphene and graphene oxide/LDH nanocomposites 10.2.1 Synthesis 10.3 Carbon nanotubes/LDH nanocomposites 10.3.1 Synthesis 10.4 Other CNF/LDH nanocomposites 10.4.1 Fullerene/LDH nanocomposites 10.4.2 Carbon quantum dot/LDH nanocomposites 10.4.3 Carbon spheres/LDH nanocomposites 10.4.4 Carbon (nano)fibers/LDH nanocomposites 10.4.5 Graphene/single-walled CNT/LDH nanocomposites 10.5 Applications of CNF/LDH nanocomposites 10.5.1 Energy storage and conversion 10.5.2 Catalysis 10.5.3 Miscellanea 10.6 Conclusions 10.7 Perspectives Acknowledgments References Further reading 11. Recent advances in layered double hydroxide/polymer latex nanocomposites: from assembly to in situ formation V. Prevot and E. Bourgeat-Lami 11.1 Introduction 11.2 Use of latex technology for the production of LDH-based composite materials and macroporous structures 11.2.1 Assembly of preformed LDH and latex particles 11.2.2 LDH-based nanocomposites by in situ emulsion and suspension polymerizations 11.2.3 Latex-templating approaches 11.3 Properties of LDH-based nanocomposites and LDH macroporous structures

388 388 398 402 403

411 411 415 417 425 427 431 431 431 431 432 436 436 436 443 445 448 449 450 450 460

461 461 464 464 468 475 481

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11.3.1 LDH-based nanocomposites 11.3.2 LDH-based macroporous structures 11.4 Concluding remarks and general trends References 12. Fabrication, assembly, and optoelectric properties of layered double hydroxide/conjugated polymer nanocomposites Yaping Huang, Harrone Muhammad Sohail and Jun Lu 12.1 Fabrication and assembly of LDHs/conjugated polymer nanocomposites 12.1.1 Introduction 12.1.2 Fabrication and assembly of LDH/CP nanocomposites 12.2 Optical and optoelectric properties of LDH/CP nanocomposites 12.2.1 Optical properties 12.2.2 Optoelectric properties 12.3 Conclusions and outlook References 13. Polymer layered double hydroxide hybrid nanocomposites Shadpour Mallakpour and Elham Khadem 13.1 Introduction 13.2 Modification of LDHs with organic compounds 13.3 Layered double hydroxide/Carbonaceous nanofiller hybrids 13.4 Synthesis of LDH/Carbonaceous nanofiller hybrids 13.5 Applications of LDH/Carbonaceous nanofiller hybrids 13.5.1 Removal of pollution 13.5.2 Supercapacitor 13.5.3 Catalyst 13.6 Polymer/LDH/Carbonaceous nanofiller hybrid nanocomposites 13.6.1 Polymer/LDH/CNT hybrid nanocomposites 13.6.2 Polymer/LDH/graphene hybrid nanocomposites 13.6.3 Polymer/LDH/Other nanofiller hybrids 13.7 Conclusions Acknowledgments References 14. Electrical and electronic applications of layered double-hydroxide polymer nanocomposites Shadpour Mallakpour and Forough Motirasoul 14.1 Introduction 14.2 Supercapacitors 14.2.1 Application of Layered Double Hydroxide Nanocomposites in Supercapacitors 14.2.2 Application of Layered Double Hydroxide/Polymer Nanocomposites in Supercapacitors

481 485 489 489

497

497 497 499 515 515 522 525 526 531 531 532 535 536 538 538 543 546 548 550 554 556 558 559 559

565 565 568 568 571

Contents

14.3

Batteries 14.3.1 Application of Layered Double Hydroxide Nanocomposites in Batteries 14.3.2 Application of Layered Double Hydroxide/Polymer Nanocomposites in Batteries 14.4 Fuel Cells 14.4.1 Application of Layered Double Hydroxide/Polymer Nanocomposites in Fuel Cells 14.5 Other Electrical and Electronic Applications of Layered Double Hydroxide/Polymer Nanocomposites 14.6 Conclusions Acknowledgments References 15. Applications of layered double hydroxide biopolymer nanocomposites Shadpour Mallakpour and Leila khodadadzadeh 15.1 Introduction 15.2 Biopolymer/layered double hydroxide nanocomposites 15.2.1 Polysaccharide/layered double hydroxide nanocomposites 15.2.2 Protein/layered double hydroxide nanocomposites 15.2.3 PHA/layered double hydroxide nanocomposites 15.2.4 PLA/layered double hydroxide nanocomposites 15.2.5 PVA/layered double hydroxide nanocomposites 15.3 Conclusions Acknowledgments References 16. Layered double hydroxide based nanocomposites for biomedical applications Raji Vijayamma, Nandakumar Kalarikkal and Sabu Thomas 16.1 Introduction 16.1.1 Layered double hydroxide nanocomposites 16.1.2 Layered double hydroxide nanocomposites in the medical field 16.2 Biomedical applications of layered double hydroxide nanocomposites 16.2.1 Layered double hydroxide nanocomposites in drug-delivery applications 16.2.2 Layered double hydroxide nanocomposites in gene-delivery applications 16.2.3 Bioimaging applications 16.2.4 Biosensor 16.2.5 Layered double hydroxide nanocomposites for tissue engineering applications

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Contents

16.3

Layered double hydroxide polymer nanocomposites for biomedical applications 16.3.1 Alginate
layered double hydroxide nanocomposites 16.3.2 Chitosan
layered double hydroxide nanocomposites 16.3.3 Other polymer
layered double hydroxide nanocomposites 16.4 Summary References Further reading

17. Layered double hydroxide nanocomposites for agricultural applications Luı´z Paulo Figueredo Benı´cio, Frederico Garcia Pinto and Jairo Tronto 17.1 Introduction 17.2 The history and evolution of chemical use in agriculture 17.3 Principal agricultural problems to resolve with new technologies 17.4 Layered double hydroxide applications in agriculture 17.4.1 Layered double hydroxide matrices of slow-release fertilizers 17.4.2 Layered double hydroxides for storage and gradual herbicide release 17.4.3 Layered double hydroxides for storage and slow release of plant growth regulators 17.4.4 Use of layered double hydroxides for pesticide removal 17.5 Final considerations List of abbreviations References Further reading 18. Layered double hydroxide polymer nanocomposites for food-packaging applications Giuliana Gorrasi and Andrea Sorrentino 18.1 Introduction 18.1.1 Characterization and analytical techniques of polymer nanocomposites for food-packaging applications 18.2 Layered double hydroxides as hosts of active molecules for potential in food-packaging applications 18.3 Polymeric nanocomposites based on layered double hydroxide-active molecules 18.3.1 Nanocomposites from oil-derived polymers 18.3.2 Nanocomposites of bioplastics from fossil-based resources 18.3.3 Nanocomposites of bioplastics from renewable sources 18.4 Regulation issues 18.5 Conclusions and future perspectives

695 695 698 709 710 710 714

715 715 716 718 719 720 726 727 730 733 734 734 741

743 743 747 748 754 756 760 764 766 767

Contents

Acknowledgment References 19. Layered double hydroxide polymer nanocomposites for water purification Shadpour Mallakpour and Vajiheh Behranvand 19.1 Introduction 19.2 Pollutant elimination from water: why layered double hydroxides? 19.2.1 Structural properties of layered double hydroxides 19.2.2 Layered double hydroxide modification 19.2.3 Water pollutants 19.2.4 Mechanisms of adsorption 19.3 Pollutant elimination by polymer/layered double hydroxide nanocomposites 19.3.1 Importance of using polymer/layered double hydroxide nanocomposites in water purification 19.3.2 Polymer/layered double hydroxide-based adsorbents 19.4 Conclusions Acknowledgments References

xv

768 768

781 781 782 782 782 783 783 790 790 791 798 798 799

20. Layered double hydroxide polymer nanocomposites for catalysis Shadpour Mallakpour and Hashem Tabebordbar 20.1 Introduction 20.2 Applications of layered double hydroxides in catalysis 20.3 Polymer/layered double hydroxide nanocomposites 20.3.1 Preparation of polymer/layered double hydroxide nanocomposites 20.4 Applications of polymer/layered double hydroxide nanocomposites in catalysis 20.5 Conclusions Acknowledgments References

805

Index

835

805 807 811 812 815 829 829 829

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List of contributors

Gonzalo Abella´n Institute of Molecular Science (ICMol), University of Valencia, Valencia, Spain; Department of Chemistry and Pharmacy and Joint Institute of Advanced Materials and Processes (ZMP), University Erlangen-Nu¨rnberg, Fu¨rth, Germany Faezeh Azimi Chemistry Group, Pardis College, Technology, Isfahan, Islamic Republic of Iran

Isfahan

University

of

Vajiheh Behranvand Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Luı´z Paulo Figueredo Benı´cio Soil Department, Federal University of Vic¸osa, Brazil Rodrigo Botan Unifacvest University, Lages, SC, Brazil E. Bourgeat-Lami University of Lyon, Universite´ Claude Bernard Lyon 1, CPE Lyon, CNRS, UMR 5265, Chemistry, Catalysis, Polymers and Processes (C2P2), Villeurbanne, France Jose A. Carrasco Institute of Molecular Science (ICMol), University of Valencia, Valencia, Spain Eugenio Coronado Institute of Molecular Science (ICMol), University of Valencia, Valencia, Spain Saju Daniel International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India; St. Xavier’s College Vaikom, Kottayam, Kerala, India Sabrina de Bona Sartor Unifacvest University, Lages, SC, Brazil Yanshan Gao College of Environmental Science and Engineering, Beijing Forestry University, Beijing, P.R. China

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List of contributors

Giuliana Gorrasi Department of Industrial Engineering, University of Salerno-via Giovanni Paolo II 132, Fisciano, Italy Mohsen Hajibeygi Faculty of Chemistry, Kharazmi University, Tehran, Iran Yaping Huang Beijing University of Chemical Technology, Beijing, P.R. China Kuruvilla Joseph Department of Chemistry, Indian Institute of Space Science and Technology, Valiamala, Thiruvananthapuram, Kerala, India Nandakumar Kalarikkal International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India; School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, India Elham Khadem Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Leila Khodadadzadeh Chemistry Group, Pardis College, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Jun Lu Beijing University of Chemical Technology, Beijing, P.R. China Shadpour Mallakpour Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran; Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran; Chemistry Group, Pardis College, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Forough Motirasoul Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Dermot O’Hare University of Oxford, Oxford, United Kingdom Frederico Garcia Pinto Institute of Exact and Technological Sciences, Federal University of Vic¸osa - Rio Paranaı´ba Campus, Brazil V. Prevot CNRS, ICCF - Institut de Chimie de Clermont-Ferrand, Universite´ Clermont Auvergne, Clermont-Ferrand, France Lei Qiu College of Environmental Science and Engineering, Beijing Forestry University, Beijing, P.R. China

List of contributors

xix

Ahmad Raeisi Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Arak University, Arak, Iran Shima Rashidimoghadam Department of Chemistry, Organic Polymer Chemistry Research Laboratory, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Appukuttan Saritha Department of Chemistry, School of Arts and Sciences, Amrita Vishwa Vidyapeetham, Amritapuri, Kollam, Kerala, India Meisam Shabanian Faculty of Chemistry and Petrochemical Engineering, Standard Research Institute (SRI), Karaj, Iran Harrone Muhammad Sohail Beijing University of Chemical Technology, Beijing, P.R. China Andrea Sorrentino Institute for Polymers, Composites and Biomaterials (IPCB), National Research Council (CNR), Lecco, Italy Suneel Kumar Srivastava Department of Chemistry, Indian Institute of Technology, Khragpur, India Hashem Tabebordbar Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Farbod Tabesh Chemistry Group, Pardis College, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Sabu Thomas International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India; School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India Jairo Tronto Institute of Exact and Technological Sciences, Federal University of Vic¸osa - Rio Paranaı´ba Campus, Brazil Raji Vijayamma International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India Qiang Wang College of Environmental Science and Engineering, Beijing Forestry University, Beijing, P.R. China

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Preface

Recently the researchers have turned their attention toward layered double hydroxide (LDH) polymer nanocomposites because of the distinctive properties of LDHs such as wide range of chemical compositions, structural homogeneity, unique and excellent anion exchanging ability, easy synthesis, high bound water content, memory effect, nontoxicity and biocompatibility, and their wide range of applications. This book really showcases the researches of many famous scientists who have been working in the field of LDHs. The aspire of introducing this compendium is to replenish a solid understanding of the recent innovative advances in the fabrication, characterization, and applications of polymer LDH nanocomposites in various fields such as biomedical, aerospace, electrical and electronics, automotive industry, agricultural, water treatment, and catalysis. This book consists of 20 chapters and all the chapters cover various relevant topics and state of the art, new challenges, and opportunities in each field. Chapter 1 is actually a voyage from LDHs to LDH polymer nanocomposites so that one can acquire the knowledge of different strategies required to convert LDHs to exfoliated LDH polymer nanocomposites. Chapter 2 provides Fourier transform infrared spectroscopic characterization of various LDHs and organically modified LDHs with the help of which success of organic modification can be easily identified. Chapter 3 illustrates recent advances in the fabrication technologies of LDH polymer nanocomposites with suitable examples and it helps to find the novel and green procedure for the fabrication of LDH
based nanocomposites of each type of polymer. Chapters 4
6 highlight advanced morphological characterization techniques such as X-ray diffraction analysis, wide range of spectroscopic and microscopic characterization techniques for LDH, organically modified LDHs, and LDH polymer nanocomposites. In chapter 4 basal spacing of various organically modified LDHs are provided so that suitable organic modifiers required for the preparation of exfoliated polymer nanocomposites can be easily detected and the diffraction pattern of the polymer nanocomposite helps to predict the morphology of the composite—intercalated or exfoliated. Chapters 5 and 6 focus on spectroscopic characterization techniques such as nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, ultraviolet visible spectroscopy, Fourier transform infrared spectroscopy, dielectric spectroscopy, fluorescence spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and microscopic characterization techniques such as optical microscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy to help in characterizing the

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Preface

nanoscale dispersions, phase segregation, and interface/interphases of polymer nanocomposites. Chapters 7
9 discuss specific properties of LDH polymer nanocomposites such as melt rheological properties, thermal and flame retardancy, and mechanical and dynamic mechanical properties. Chapter 7 reviews the rheology of nanocomposites of thermoplastics and thermosetting polymers filled with LDH and modified LDH with special emphasis on the modeling of rheological properties to understand the LDH particle dispersion and its influence on the melt flow behavior of the nanocomposites. Chapter 8 summarizes the thermal stability and flame retardancy of LDH polymer nanocomposites, their characterization techniques such as thermogravimetric analysis, cone calorimetry, limiting oxygen index and UL94 investigations, and the synergistic effect of LDH with other thermal stabilizers and fire retardants. Chapter 9 reviews recent advances on the mechanical and dynamical mechanical properties of LDH filled elastomer and elastomeric blend nanocomposites. Chapters 10
13 discuss fabrication and applications of carbon nanoform/LDH nanocomposites, LDH/polymer latexes nanocomposites, LDH/conjugated polymer nanocomposites, and LDH
based polymer hybrid nanocomposites. Chapter 10 deals with the fabrication of carbon nanoform/LDH nanocomposites and their applications in super capacitor, water splitting, catalysis, drug delivery, and environment protection. In Chapter 11 three main routes for the fabrication of LDH/polymer latexes nanocomposites such as electrostatic assembly, in situ polymerization, and latex-templating and their flame retardant applications are reviewed. Chapter 12 introduces various LDH/conjugated polymer nanocomposites fabricated by layerby-layer assembly through electrostatic interaction, hydrogen bonds, van der Waals force of attraction, and miscellaneous interactions for optoelectronic applications. Chapter 13 focuses on the state of the art in the preparation of polymer/LDH/carbonaceous nanomaterial hybrids and the influence of hybrid fillers on the properties such as thermal, mechanical, permeability, and drug delivery properties of polymers. Chapters 14
20 discuss wide range applications of LDH polymer nanocomposites, such as electrical and electronic applications such as super capacitors, batteries, and fuel cells, biomedical applications, such as controlled drug delivery, gene delivery, tissue engineering, and photodynamic therapy, and agricultural applications, such as controlled fertilizer, herbicide and plant growth regulators release and pesticide removal, food packaging, water purification, and catalysis. The main focus of Chapter 15 is on the fabrication of LDH
based polymer nanocomposites containing biopolymer matrix for the environmental protection and their important applications in the fields of water treatment, drug delivery, tissue engineering, packaging, and catalysis. This book is really an one stop reference book emphasizing recent advances in the technologies for the fabrication and characterization of LDHs, organically modified LDHs, LDH nanocomposites, LDH
based polymer nanocomposites, and LDH-based polymer hybrid nanocomposites and their wide range of applications thereby covering almost all the points of LDH chemistry. Therefore this book will

Preface

xxiii

lend a hand to academics, researchers, scientists, engineers, industrialists, and students in the field of polymer nanocomposites especially polymer LDH nanocomposites to discern solutions to their unreciprocated posers. Sabu Thomas and Saju Daniel

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Acknowledgement

We express our sincere thanks to all the authors who have taken so much effort and spent their valuable time for contributing chapters to this book. We are very much grateful to the peer reviewers for the valuable guidance. We would like to appreciate and acknowledge all the Elsevier team members of this project for their continuous and unforgettable support throughout the editing of this book.

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Layered double hydroxides: fundamentals to applications

1

Saju Daniel1,2 and Sabu Thomas1,3 1 International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India, 2St. Xavier’s College Vaikom, Kottayam, Kerala, India, 3School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

1.1

Introduction

In recent years, Layered double hydroxides have achieved a lot of attention both from academia and industries due to their easy and ecofriendly synthesis, low cost, non toxicity, structural and compositional tunability, high chemical and thermal stability, high biocompatibility and broad spectrum applications. By exploiting the three outstanding properties of layered double hydroxides, compositional flexibility in cations and anions, excellent anion exchange ability, memory effect, it is very easy to tailor layered double hydroxides into functional hybrid materials and nanocomposites for vast field applications. LDHs can be synthesized by simple methods which enable control over structure, composition and shape by properly varying the conditions. Recent developments in the synthesis methods have offered various morphologies such as microspheres, fibrous structures, nano sized belt, LDH films on substrates etc leads to tremendous range of possibilities for the fabrication of smart high-performance multifunctional materials. So the fabrication of layered double hydroxide based functional hybrid materials and nanocomposites have become one of the most fascinating topics of today’s research. One way of producing the LDH hybrid is the intercalation of smart inorganic or organic functional materials into the nanospace in the intergallery space by anion exchange to form inorganic/ inorganic or inorganic/organic hybrid system for controlled drug delivery, pesticide delivery, gene delivery, biosensor applications, bioimaging etc. Second way is to mix LDH with smart nanomaterials like graphene, carbon nanotube etc to form nanocomposites for energy storage and conversion like electrode for super capacitors, batteries, fuel cell, solar cell, photo catalysts and electro catalysts for water splitting, environmental remediation such as water treatment, removal of toxic gases from atmosphere. Third way is to introduce organically modified LDH or exfoliated LDH sheets or LDH/other nanofillers hybrid into the polymer matrix to develop polymer nanocomposites or LDH based polymer hybrid nanocomposites for energy, food packaging, agricultural, biomedical, flame retardant, gas barrier, anticorrosion, waste water treatment etc. Thin films and core-shell hierarchical hybrid nanostructures formed from LDH sheets and any other nanofillers or polymers attracted much Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00001-X © 2020 Elsevier Ltd. All rights reserved.

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Layered Double Hydroxide Polymer Nanocomposites

attention because of their high performance in photoluminescence, drug delivery, electrode for super capacitors etc. The aim of this chapter is to make aware of the readers the importance of layered double hydroxides, layered double hydroxide nanocomposites and layered double hydroxide based polymer nanocomposites so as to motivate them to develop hierarchical hybrid nanostructures for the benefit of the society. For this purpose, from fundamentals to applications of layered double hydroxides, LDH hybrids and polymer nanocomposites are reviewed.

1.2

Layered double hydroxides

Layered double hydroxides are versatile and emergent class of two dimensional inorganic layered nanomaterials, natural or synthetic anionic clay minerals, of which the general formula is [M211-x Mx31 (OH) 2] x1 (An2) x/n. yH2O where MII is a divalent ion, MIII is a trivalent ion, An2 is an anion and charge density of LDH layers, x 5 MIII/MII 1 MIII whose value lies between 0.2 and 0.33 for pure LDH phase (Taviot-Gue´ho et al., 2018; Basu et al., 2014). This formula gives rise to a generic layer sequence [AcBZAcB]n for layered double hydroxides in which A and B represents layers of hydroxide anions, c represents layers of metal cations and Z represents layers of other anions such as carbonate ion, chloride ion, nitrate ion etc and neutral molecules like water molecules (https://en.wikipedia.org/wiki/ Layered_double_hydroxides). Hydrotalcite is one of the naturally occurring LDH clays and the parent member of the family layered double hydroxides with the chemical formula Mg6 Al2 (OH) 16CO3.4H2O and its name attributable to high water content (hydro) and talc like appearance (Selvam et al., 2014; Maheskumar et al., 2014; Mishra et al., 2018). Its existence was first declared by Hochstetter in 1842 and synthesized 100 years later by Feitknecht (Basu et al., 2014; Grosu et al., 2018). It is most common and its structure and properties were studied extensively and is considered as the representative of LDHs. So LDHs are also known as hydrotalcite-like compounds (Evans and Duan, 2006).

1.2.1 Structure of layered double hydroxides The structure of layered double hydroxides can be easily reviewed by analogizing its structure with that of brucite which has the formula Mg (OH) 2. Brucite has hexagonal close packing of hydroxide ions in which alternate octahedral sites are occupied by Mg21 ions so that hydroxide layers are neutral. The neutral hydroxide layers are stacked one upon the other and are held together by Vanderwaal’s force of attraction which results in a basal spacing of about 0.48 nm. Schematic representation of brucite structure is shown in Fig. 1.1 (Arizaga et al., 2007). It can be imagined that the substitution of some divalent ions in brucite structure by some trivalent ions isomorphously results in the formation of a positively charged mixed metal hydroxide layers [M1-x II M x III (OH) 2] x1 and the intercalation of anions in the inter layer regions counterbalance the residual positive charge on the

Layered double hydroxides: fundamentals to applications

3

Figure 1.1 Schematic representation of the brucite structure. (a) Side and (b) top view of the layer. Source: Adapted from Arizaga, G.G.C., Satyanarayana, K.G. and Wypych, F., 2007. Layered hydroxide salts: synthesis, properties and potential applications. Solid State Ionics, 178 (15
18), pp. 1143
1162. with kind permission of elsevier

Figure 1.2 Schematic representation of the structure of a generic LDH. (a) Side and (b) top view of the layer. Source: Adapted from Arizaga, G.G.C., Satyanarayana, K.G. and Wypych, F., 2007. Layered hydroxide salts: synthesis, properties and potential applications. Solid State Ionics, 178 (15
18), pp. 1143
1162. with kind permission of elsevier

metal hydroxide layers resembles layered double hydroxide structures. Water molecules in the interlayer region bind to the metal hydroxide layers and anions via extensive hydrogen bonding and help to stabilize the crystal structure of layered double hydroxides. Due to the intercalation of water molecules and anions in the inter lamellar region, the basal spacing has been increased from 0.48 nm in brucite to about 0.77 nm in hydrotalcite (Basu et al., 2014). Schematic representation of structure of layered double hydroxide is shown in Fig. 1.2 (Arizaga et al., 2007).

4

Layered Double Hydroxide Polymer Nanocomposites

1.2.1.1 Metal cations in the layers The metal cations both divalent and trivalent ions in the layers of LDHs are mainly from third and fourth periods of the periodic table. The divalent metal ions that are found commonly in the layers are Mg21, Ni21, Zn21, Co21, Fe21, Mn21, Cu21, Ti21, Cd21, Ca21 etc and the common trivalent metal ions that are found in the layers are Al31, Cr31, Fe31, Mn31, Ga31, V31, In31, Y31, La31, Ru31 etc.

1.2.1.2 Interlamellar anions The generally found interlamellar anions in LDHs are halides e.g. fluoride, chloride etc oxoanions such as carbonate, nitrate, sulphate, bromate etc, oxo and polyoxometallates like chromate, dichromate, (Mo7O24) 62, (V10O28) 62etc, anionic complexes such as ferro and ferricyanide, (PbCl4) 22 etc and organic anions like carboxylates, phosphonates, alkyl sulphates etc.

1.2.2 Differences between Ordinary Clays and Layered Double Hydroxides Even though LDHs have layered crystalline structures with substitutable ions in the interlamellar region similar to layered silicates such as montmorillonite, their chemical and structural features such as composition, geometries, and layer thickness are not identical. As already mentioned LDHs are anionic clays because of the presence of anions in the interlamellar gallery of positively charged layers whereas reversed structure can be observed in the case of layered silicates so the name cationic clays. Each crystal layer in LDHs consists of single octahedral metal hydroxide sheet whereas in layered silicates two or more sheets of metal oxides in the sandwiched structure are observed. In montmorillonite, single crystal layer is a sandwiched system of one octahedral sheet containing Fe, Al, and Mg between two silica tetrahedral sheets. Hence crystal layer thickness and rigidity of LDHs are lower than that of layered silicates (Basu et al., 2014; Das et al., 2008). Schematic diagram showing structural and chemical difference between LDH and MMT is given in Fig. 1.3 (Das et al., 2008).

1.2.3 Synthesis Routes of Layered Double Hydroxides A number of techniques can be applied for the synthesis of LDHs and the selection of the method highly depends on the type of cations in the hydroxide layers, the intercalated anions and the desired physicochemical properties such as phase purity, crystallinity, porosity, morphology, and electronic and optical characteristics of the final materials. The direct methods used for the synthesis of LDHs are co precipitation, urea hydrolysis, salt- oxide method, sol-gel synthesis, electrochemical synthesis, and in-situ film growth and the indirect method used for the synthesis of LDHs are anion exchange, reconstruction by memory effect, and delamination followed by restacking (Richetta et al., 2017).

Layered double hydroxides: fundamentals to applications

5

Figure 1.3 Schematic diagram showing structural and chemical difference between LDH and MMT. Source: Adapted from Das, A., Costa, F.R., Wagenknecht, U. and Heinrich, G., 2008. Nanocomposites based on chloroprene rubber: effect of chemical nature and organic modification of nanoclay on the vulcanizate properties. European Polymer Journal, 44 (11), pp. 3456
3465 with kind permission of elsevier

1.2.3.1 Co-Precipitation It is the most common and useful method for the preparation of large amount of layered double hydroxides and this method is also known as salt-base method. This method involves the slow addition of mixed aqueous solution of salts of both divalent and trivalent ions in proper proportion into water taken in a reactor and the co-precipitation of both the metal ions from the aqueous solution by the simultaneous addition of dilute solutions of sodium hydroxide and/or sodium bicarbonate, sodium carbonate, or ammonium hydroxide solution at controlled temperature and under vigorous shaking. Generally the pH of the reaction medium is kept constant in the range of 7
10, on the basis of nature of metal ions. Finally the suspension is subjected to hydrothermal treatment to obtain well crystallized sample (Basu et al., 2014; Richetta et al., 2017; Radulescu et al., 2008; Yan et al., 2016; Rives et al., 2013).

1.2.3.2 Urea Hydrolysis In this sophisticated method, an aqueous solution of the selected metal ions that required to be present in the LDH and urea in the stoichiometric molar ratio is heated in the temperature range from 100 C to reflux temperature for two to three days. At the end of the process, urea hydrolyses to ammonium carbonate that leads to the precipitation of LDH with carbonate ion as interlayer anion. This homogeneous precipitation method via urea hydrolysis offers LDH with high crystalline and narrow particle size distribution (Basu et al., 2014; Radulescu et al., 2008).

6

Layered Double Hydroxide Polymer Nanocomposites

1.2.3.3 Hydrothermal Crystallization In this method, an aqueous suspension of two oxides, one of trivalent metal ion, M2O3 and another of divalent metal ion, M2O are taken in a pressurized vessel and subjected to hydrothermal treatment at elevated temperature for a few days. During this process the precursor amorphous hydrated M2O3 crystallizes in the presence of reactive basic oxide M2O which acts as the crystallizing agent (Basu et al., 2014; Radulescu et al., 2008; Xu and Lu, 2005).

1.2.3.4 Sol-Gel Method In this method the sol- gel transition occurs during the strong acid hydrolysis of required metal precursors such as metal based alkoxides or acetyl acetonides in ethanolwater system into metal hydroxides using HCl or HNO3. The heating of the mixture to reflux with stirring should be continued until gel formation (Richetta et al., 2017).

1.2.3.5 Salt-oxide Method This is actually a solid-liquid reaction in which aqueous solution of the chloride salt of trivalent ion in excess is treated with an aqueous suspension of the metal (II) oxide (Richetta et al., 2017).

1.2.3.6 Electrochemical deposition In this method nitrate ions are reduced electrically to hydroxide ions on the working electrode due to which local pH value increases that induces the precipitation of LDH films. This is a good method for depositing LDH films of any desired thickness, morphology and film density on metal substrate with good adhesion (Richetta et al., 2017).

1.2.3.7 In-situ film growth In this method the substrate is immersed in water solution of other metal salt and a base is added to control the pH. Here the substrate shows dual role as a source of one of the reacting metal ions and act as the surface for film deposition (Richetta et al., 2017).

1.2.3.8 Anion exchange It is an indirect method widely used for preparing LDH with any desired anions in the inter lamellar region. In actual practice, an aqueous suspension of the LDH precursors or of the pre-synthesized LDH is stirred in the presence of a large excess of the salt of the anion to be intercalated. The anionic exchange occurring in the solution can be expressed as 

   M21 2 M31 2 A 1 B ! M21 2 M31 2 B 1 A

where A and B represent different anions

Layered double hydroxides: fundamentals to applications

7

The exchange of the anion depends on the electrostatic forces between positively charged LDH layers and the exchanging anions. The main criterion for anionic exchange is that the force of attraction between the host ion and the LDH sheet must be lower than that of guest ion and the sheet. The affinity between various anions and the positively charged LDH sheets is in the order NO32 , Br2 , Cl2 , F2 , OH2 , MoO422 , SO422 , CrO422 , HPO422 , CO322. This method can be used for the preparation of layered double hydroxides with any inorganic anions or organic anions. The entire process of anion-exchange must be carried out in an inert atmosphere (Basu et al., 2014; Selvam et al., 2014; Mishra et al., 2018; Richetta et al., 2017; Bullo Saifullah, 2015).

1.2.3.9 Reconstruction / rehydration method The noticeable property of LDHs is the regaining of original structure back after subjecting it to calcination between 400
500 C followed by rehydration. When LDH is heated between 400
500 C, it changes to mixed metal oxides. It is immersed in water or any other solution of anions to be intercalated. The overall process of rehydration must be carried out in an inert nitrogen atmosphere. This method can also be used for the synthesis of LDH with any inorganic or organic anions (Basu et al., 2014; Mishra et al., 2018; Richetta et al., 2017; Bullo Saifullah, 2015). Some examples for the synthesis of layered double hydroxides by various methods are given in Table 1.1.

1.3

Organic Modification of Layered Double Hydroxides

The prime objective of organic modification of layered double hydroxides is to enhance the interlayer spacing of LDH materials (Fig. 1.4) so as to make it easily accessible for the intercalation of large hydrophobic polymer chains. Some important organic modifiers used for the modifications of LDHs are given in Table 1.2 The fine tuning of the inter lamellar galleries of layered double hydroxides with suitable organic anions offer exfoliated layered double hydroxide polymer nanocomposites with multifaceted applications such as biomedical, energy, food packaging, flame retardant, gas barrier, agricultural, water purification etc. This is also an efficient way to tailor the nano space in the intergallery region with functional materials like pesticides, drugs, nucleic acids, enzymes etc for vast applications such as controlled pesticides release, drug delivery, gene delivery, biosensors etc. Organic modification can be carried out successfully by following any one of the important strategies such as anion exchange reaction, reconstruction, direct synthesis and restacking (Leroux and TaviotGue´ho, 2005). Sometimes bilayers, instead of mono layers of organic anions are formed in the inter gallery space as a result of which basal spacing becomes double that is a boon for producing polymer nanocomposites. This happens when the process is carried out in the presence of excess of organic anions for a long time with

Table 1.1 Some examples for the synthesis of layered double hydroxides by various methods (Yan et al., 2016; Prince et al., 2009; Wu et al., 2014; Xie et al., 2008; Baskaran et al., 2014; Yarger et al., 2008; Raynal et al., 2014; Liu et al., 2006a; Liu et al., 2006b). Method of Synthesis

Precursors

Reaction conditions

Type of LDH formed

References

Urea hydrolysis

Ni (NO3) 2  6H2O Fe (NO3) 3  9H2O Ni (NO3) 2  6H2O Fe (NO3) 3  9H2O Co (NO3) 2  6H2O, Al (NO3) 3  9H2O

NO32 /urea molar ratio of 0.25; hydrothermal treatment at 110 Cfor 24 h NO32/urea molar ratio of 3.0; hydrothermal treatment at 110 C for 24 h solution1of Co21, Al31salt; solution 2 of (NH4) 2CO3 and NH4OH; solution 2 was added dropwise to solution1 with constant stirring at 40 C for 1 h, with pH 8.5; washed, filtered, and dried at 80 C overnight NaOH solution was continuously dropped into the mixed salt solution until pH 7.8; stirred for 30 min; hydrothermally treated at 110 C for 3 h; filtered, washed, and then dried at 80 C for 12 h Solution 1of NaOH and Na2CO3, and solution2 of mixed salt were continuously dropped. into a beaker with constant pH 9.5; aged at 100 C for 13 h; filtered, washed, and dried at 100 C for 24 h HNO3 as hydrolysis acid, temperature of 0 C, ethanol as solvent, dried at 70 C for 24 h HNO3 as hydrolysis acid, temperature of 0 C, ethanol as solvent, dried at 70 Cfor 24 h HNO3 as hydrolysis acid, temperature of 0 C, ethanol as solvent, dried at 70 C for 24 h

NiFeCO3LDH

Wu et al. (2014)

NiFe
NO3LDH CoAl
CO3LDH

Wu et al. (2014)

MgCoAl
NO3 LDH

Xie et al. (2008)

MgAl
CO3 LDH

Zhao et al. (2014)

Mg Al LDH

Prince et al. (2009)

Ni Al LDH

Prince et al. (2009)

NiCoAl LDHs

Prince et al. (2009)

Co precipitation

Mg (NO3) 2  6H2O, Co (NO3) 2  6H2O, Al (NO3) 3  9H2O Mg (NO3) 2  6H2O, Al (NO3) 3  9H2O

Sol-Gel method

Mg (OEt) 2, Al (OsBu) 3 Ni (OAc) 2, Al (Os-Bu) 3 Co (OAc) 2, Ni (OAc) 2, Al (Os-Bu) 3

Baskaran et al. (2014)

Electro deposition

Zn (NO3) 2  6H2O Al (NO3) 3  9H2O

Working electrodes preparation by e-beam evaporation ˚ titanium and 600 A ˚ of platinum, followed by of 200 A ˚ gold on a cleaned glass substrate. After each 2000 A electro deposition wash the film with de ionized water and dry with gentle stream of N2 gas. Ag/AgCl electrode-reference electrode Pt wire counter electrode

Zn-Al NO3LDH

Yarger et al. (2008)

ZnSO4  7H2O CoSO4  7H2O

Zn21: Co21 5 2: 1 in molar ratioH2O2: Co21 55 5 1: 2 in molar ratio. A cleaned Ni foil (1 cm 3 1 cm in square) -working electrode, Ag/Ag Cl electrodereference electrode Pt wire counter electrode Al substrate dipped in Zn21 solution

ZnCo-LDH films

Li et al. (2014)

Zn Al LDH

Liu, Jinping, et al (2006a) Liu, Zhaoping, et al (2006b)

In-situ growth Anion exchange

CoCl2.6H2O AlCl3.6H2O

Co (NO3) 2.6H2O Al (NO3) 3. 9H2O

Co-Al-CO3 LDH was synthesized by urea method by mixing and refluxing 10, 5, and 35 mM solutions of CoCl2.6H2O, AlCl3.6H2O, and urea in 1 dm3 of deionized water for 2 days in the presence nitrogen with magnetic stirring. Treat Co-Al-CO3 LDH with salt-acid mixed solution (NaCl-HCl) in an inert atmosphere of nitrogen at ambient temperature. Disperse NaCl-HCl treated LDH sample into 500 cm3 of an aqueous solution containing 0.1 M sodium nitrate and sodium per chlorate respectively Solution 1 Co (NO3) 2.6H2O (1 M) and Al (NO3) 3. 9H2O (0.33 M) in 2.6 L deionized H2O. Solution 2 (NH4) 2CO3 (0.67 M) and NH4OH (3.27 M) in 2.225 L deionized H2O. Add solution 2 to solution1 with constant stirring at 40  C for 1 h. Silicate solutions of appropriate concentrations were added slowly to the HT gel at room temperature and stirred for 48 h. Exchange of CO322 by silicate anion

Co-Al-Cl LDH Co-Al-NO3LDH Co-Al-ClO4LDH

CoAl-HT-Si

Baskaran et al. (2014)

10

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.4 Schematic representation of ion-exchange technique. Source: Adapted from Mishra, G., Dash, B. and Pandey, S., 2018. Layered double hydroxides: A brief review from fundamentals to application as evolving biomaterials. Applied Clay Science, 153, pp. 172-186 with kind permission of Elsevier

Table 1.2 Some important organic modifiers used for the modifications of LDHs (Basu et al., 2014). Organic Modifiers

Structure

Symbol

Sodium dodecyl benzenesulfonate

SDBS

Sodium dodecyl sulfate

SDS

Stearic Acid

SA

Oleic Acid

OA

O OH

Lauric acid

LA

Bis (2-ethylhexyl) hydrogen phosphoric acid

BEHP

Layered double hydroxides: fundamentals to applications

11

Figure 1.5 Schematic showing the arrangement of MAPK anions in the OLDH interlayer region. (b) TEM images and size distribution of the OLDH. Source: Adapted from Xie, J., Wang, Z., Zhao, Q., Yang, Y., Xu, J., Waterhouse, G.I., Zhang, K., Li, S., Jin, P. and Jin, G., 2018. Scale-up fabrication of biodegradable poly (butylene adipate-co-terephthalate) /organophilic
clay nanocomposite films for potential packaging applications. ACS Omega, 3 (1), pp. 1187-1196 with kind permission of ACS

vigorous shaking. Xie et al. (2018) intercalated mitogen activated protein kinase (MAPK) into the interlamellar region of Zn Al LDH by solvent free high energy ball milling and the organically modified ZnAl-MAPK OLDH is incorporated into PBAT matrix to form nanocomposite for food packaging applications. By carefully analyzing the basal spacing for OLDH obtained from XRD analysis (4.07 nm), the alkyl chain length of mitogen activated protein kinase (MAPK) (1.86 nm) and the thickness of metal hydroxide layer (0.47 nm) in the LDH sheets, it can be obviously concluded that bilayer of MAPK anions are formed in the interlayer space (Fig. 1.5).

1.3.1 One step Co-precipitation In early days, organically modified layered double hydroxides are prepared via two step procedure. The first step is the preparation of LDH containing CO322, NO32 and Cl2 and the second step is the exchange of these anions by organic anions. Now- adays, in most of the reported works, researchers merge these two steps into one and called it as one step co- precipitation. One step co precipitation involves the slow addition of a solution of target anion into a solution containing divalent and trivalent ion. It is noted that thermal treatment is essential to improve the crystallinity and the addition of alkali maintains the pH at the required level to achieve the co precipitation of the two metallic salts (Mishra et al., 2018). Wang et al. (2015) extracted bio-based modifier (cardanol-BS) from renewable resource cardanol through the ring-opening of 1, 4-butane sultone (BS). Cardanol-BS modified layered double hydroxide (m-LDH) was prepared by one-step co- precipitation method and fabricated EP/ m-LDH nanocomposite for flame retardant application (Scheme 1.1).

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Layered Double Hydroxide Polymer Nanocomposites

Scheme 1.1 Diagrammatic Illustration of the Synthetic Route of Cardanol-BS Modified LDH. Source: Adapted from Wang, X., Kalali, E.N. and Wang, D.Y., 2015. Renewable cardanol-based surfactant modified layered double hydroxide as a flame retardant for epoxy resin. ACS Sustainable Chemistry & Engineering, 3 (12), pp. 3281-3290. with kind permission of ACS

1.3.2 Anion exchange reaction The procedure for the anion exchange reaction implicates the dispersal of pristine LDH into the aqueous solution of the anionic surfactant that has to be introduced into the inter lamellar galleries so as to replace the existing anion to produce the organically modified LDHs, with constant stirring at room temperature for several hours (Basu et al., 2014). Anion exchange plays a major role in the widespread applications of layered double hydroxides and their composites as this method is the principal step for introducing functional anions in the interlamellar region, pillaring and delamination.

1.3.3 Memory Effect or Regeneration Method It is the widely accepted method for the modification of layered double hydroxides in which heating of LDH in a Muffle furnace at 450 C for 3 hours leads to the formation of amorphous mixed metal oxide. The mixed oxide is dispersed into water containing anionic surfactant which is to be introduced into the inter lamellar galleries of LDH and kept for 24 hours with constant magnetic stirring. During this stirring carbonate anions already present in the inter lamellar galleries are replaced by the organic anions and the material memorize its original structure so the name

Layered double hydroxides: fundamentals to applications

13

3 hours @ 450°C Calcination in a muffle furnace

Unmodified LDH

Centrifuging and drying @ 60°C to a constant weight Modified LDH as solid residue

Addition of mixed oxide into a specific volume of a surfactant solution (0.1–0.2 M)

Mixed oxide

Stirring for 24 hours at ambient temperature Dispersion into aq. solution of the desired surfactant

Figure 1.6 A simplified schematic showing the synthesis route to obtain modified LDH from unmodified LDH. Source: Adapted from Basu, D., Das, A., Sto¨ckelhuber, K.W., Wagenknecht, U. and Heinrich, G., 2014. Advances in layered double hydroxide (LDH) -based elastomer composites. Progress in Polymer Science, 39 (3), pp. 594-626 with kind permission of elsevier

Figure 1.7 Mg/Al LDH loaded with 5-fluorouracil (5-FU) via memory effect. Source: Adapted from Peng, F., Wang, D., Cao, H. and Liu, X., 2018. Loading 5-Fluorouracil into calcined Mg/Al layered double hydroxide on AZ31 via memory effect. Materials Letters, 213, pp. 383-386 with kind permission of elsevier

memory effect (Basu et al., 2014) (Fig. 1.6). Peng et al. (2018a) successfully intercalated anticancer drug 5-Fluorouracil in the intergallery space of Mg/Al LDH via memory effect for drug delivery applications. Mg/Al LDH prepared in-situ by a hydrothermal process and the original layered structure is recovered by adding calcined LDH into a solution containing anticancer drug molecules 5-Fluorouracil (Fig. 1.7).

1.3.4 The delamination/restacking method In this method, the complete separation of LDHs into single sheets by using suitable solvent so that a stable colloidal suspension is obtained. The aqueous

14

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.8 Schematic illustration of the preparation process for metallo- porphyrin intercalated LDH nanocomposites via the exfoliation/ restacking route.

solution of organic anion is added to this colloidal solution in an inert atmosphere of nitrogen. Then the colloidal solution is subjected to drying, well-ordered LDHs intercalated with desired anions are obtained (Richetta et al., 2017). Ma et al. (2014) intercalated metalloporphyrin into the intergallery space of Ni-Al LDH and Mg-Al LDH by delamination /restacking method. The prepared LDH-CO3 is converted into LDH-NO3 by direct salt-acid method. It is delaminated by shaking with formamide and aqueous solution of metalloporphyrin is added to this colloidal solution in the presence of an inert atmosphere of nitrogen (Fig. 1.8) (Ma et al., 2014).

1.4

Characterization of layered double hydroxides and modified layered double hydroxides

The success of synthesis of LDHs and modification of LDHs can be evaluated by the characterization techniques such as SEM, TEM, AFM, XRD, FTIR, EDS, Raman spectroscopy and X-ray photoelectron spectroscopy and thermo gravimetric analysis. The sharp reflections corresponding to the (00n) planes in the powder X-ray diffraction patterns confirm the formation of LDH crystals and shifting of the typical and characteristic Bragg reflections in the diffraction pattern confirm the successful organic modification of the layered double hydroxides. The basal spacing of LDH and modified LDH can be calculated from the 2θ value corresponding to the first-order basal plane reflection by using Bragg’s equation. The shifting of the first-order basal plane reflection towards higher d-values or lower

Layered double hydroxides: fundamentals to applications

15

2θ values indicates the successful intercalation of organic anions into the inter lamellar gallery of layered double hydroxides. XRD analysis goes into the in-depth study of the crystal by providing the unit cell parameters, miller indices crystallite size or particle size (calculated with the help of Scherrer equation) etc. SEM and TEM give valuable information regarding the size and shape of the LDHs and modified LDHs. The chemical compositions of the LDHs and modified LDHs are determined by energy-dispersive X-ray spectra (EDS). The thickness of LDHs and modified LDHs can be detected from the AFM height images and the corresponding height profiles. The FTIR spectrum confirms the formation of LDH by providing peaks corresponding to interlayer anion, interlayer water molecules, O-H of metal hydroxide layer, M-O lattice etc (Nagendra et al., 2017a; Nagendra et al., 2015; Nagendra et al., 2017b). The increase in basal spacing obtained from XRD and the characteristic peak corresponding to the organic modifier from the FTIR spectrum and visualization of increase in basal spacing by taking the image by high resolution TEM etc indicates the successful modification of layered double hydroxides with the organic modifiers. Some of the characterization techniques are illustrated here with suitable examples, which are taken from some previously reported research work, for the better understanding of this topic. Nagendra et al (2017b) synthesized Co 2 Al, Zn 2 Al LDH and Co 2 Zn 2 Al LDH by co precipitation method. The size and shape of these LDHs were characterized by SEM and TEM analysis and visualized as circular platelets, hexagonal platelets with rounded edges, and hexagonal platelets with sharp edges morphologies respectively for Co 2 Al LDH, Zn 2 Al LDH, and Co 2 Zn 2 Al LDH (Fig. 1.9). This difference in shape is attributed to the difference in nucleation and growth mechanism of LDH in the presence of different metal salts. The crystallinity was confirmed by XRD and the prepared LDH crystals are found to be highly pure as there were no peaks other than typical LDH (Fig. 1.10a). Because of water molecules in the interlamellar region and hydroxyl groups in the LDH layers, a dominant oxygen peak was observed in EDS of different LDH. (Fig. 1.9) The FTIR bands at 1356 and1382 cm21 indicated the presence of carbonate and nitrate respectively, the bands at 3440 cm21 (ν (O-H) ) and 1632 cm21 (δ (H2O) ) indicated the presence of water molecules in the inter lamellar region (Fig. 1.10b). Xu et al. (2013b) synthesized LDH-CO3 by urea method and converted it to LDH-NO3 by an acid
salt direct exchange method. The reason for the conversion is that it is difficult to substitute interlayer CO322 anions by organic anions directly, because of high electrostatic attraction between CO322 and LDH platelets. After the first conversion, LDH NO3 was again converted to LDH-DS and LDH -DBS by anion exchange reaction. From the XRD data, it is clear that the basal space of LDH-NO3, LDH-DS, and LDH-DBS are 0.88 nm, 2.78 and 2.96 nm respectively which are found to be well agreement with the theoretical values (Fig. 1.11 and Fig. 1.12). The characteristic reflection peak of LDH-CO3 corresponding to 2θ 5 11.8 is absent in XRD graphs of all the other LDHs mentioned above which indicates complete conversion of CO322. XRD data of unmodified and some organically modified LDHs are given in Table 1.3. In addition to that, the characteristic absorption band of the carbonate at around 1352 cm21 was not observed in the

Figure 1.9 SEM, TEM, and EDS analysis of the as-synthesized LDH: (a) Co 2 Al LDH, (b) Zn 2 Al LDH, and (c) Co 2 Zn 2 Al LDH. Source: Adapted from Nagendra, B., Rosely, C.S., Leuteritz, A., Reuter, U. and Gowd, E.B., 2017. Polypropylene/layered double hydroxide nanocomposites: Influence of LDH intralayer metal constituents on the properties of polypropylene. ACS Omega, 2 (1), pp. 20-31 with kind permission of ACS

Figure 1.10 (a) Powder XRD patterns and (b) FTIR spectra of as-prepared Co 2 Al LDH, Zn 2 Al LDH, and Co 2 Zn 2 Al LDH. Source: Nagendra, B., Rosely, C.S., Leuteritz, A., Reuter, U. and Gowd, E.B., 2017. Polypropylene/layered double hydroxide nanocomposites: Influence of LDH intralayer metal constituents on the properties of polypropylene. ACS Omega, 2 (1), pp. 20-31 with kind permission of ACS

Layered double hydroxides: fundamentals to applications

Figure 1.11 Schematic diagrams showing the theoretical calculation results of the anion alignments in the interlayer of LDH-DS (a) and LDH-DBS (b). Source: Adapted from Xu, K., Chen, G. and Shen, J., 2013. Exfoliation and dispersion of micrometer-sized LDH particles in poly (ethylene terephthalate) and their nanocomposite thermal stability. Applied Clay Science, 75, pp. 114-119 with kind permission of Elsevier

Figure 1.12 XRD patterns of the LDH-NO3, LDH-DS and LDH-DBS samples. Source: Adapted from Xu, K., Chen, G. and Shen, J., 2013. Exfoliation and dispersion of micrometer-sized LDH particles in poly (ethylene terephthalate) and their nanocomposite thermal stability. Applied Clay Science, 75, pp. 114-119. with kind permission of elsevier

17

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Layered Double Hydroxide Polymer Nanocomposites

Table 1.3 XRD data of unmodified and some organically modified LDHs (Basu et al., 2014; Xu et al., 2013b; Costa et al., 2008; Liu et al., 2008; Manzi-Nshuti et al., 2009). Type of LDH

Basal spacing (nm)

LDH NO3 LDH-DS LDH- DBS LDH-laurate LDH-oleate LDH-stearate LDH-BEHP (Bis (2-ethylhexyl) hydrogen 3phosphate-modified LDH)

0.89 2.78 2.96 2.45 3.4 3.37 1.52

FTIR spectrum of LDH-NO3, LDH-DS, and LDH-DBS. At the same time, some new peaks were observed at 1384 cm21, 2957, 2920 and 2845 cm21. The first peak arises due to the stretching mode of NO32 and the three other peaks arises as a result of asymmetric and symmetric stretching vibrations of CH3/CH2 group of long alkyl chains of the DS and DBS anions. Some additional bands were observed at 1220 cm21, 1200 and 1042 cm21, 1133 and 1011 cm21, first band results from the DS anions, a couple of bands (second and third) characteristic of the DBS absorption and the next pair of bands (fourth and fifth) characteristic of the C-H aromatic in-plane bendings of the LDH-DBS. In addition to all these bands, a broad absorption band appeared between 3700 and 3000 cm21 can be assigned to the O-H group stretching (Fig. 1.13). FTIR bands of some organically modified LDHs are given in Table 1.4. The information obtained from thermo gravimetric analysis of various types of LDHs reported in literature is that thermal stability of layered double hydroxides is only up to 200 C and above that temperature it decomposes to amorphous mixed metal oxides. Yang et al. (2002) conducted several in situ techniques such as DRIFTS, TG/DTA, MS, and HTXRD to detect the thermal evolution of the structure of an Mg
Al
CO3 layered double hydroxide (LDH) under an inert atmosphere (Fig. 1.14). The diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) helped to determine the changes in the functional groups. TG/DTA investigate the changes in mass and energy changes (Fig. 1.15), MS identifies the products liberated during each stage of heating and HTXRD monitors the structure evolution. A model has been proposed on the basis of the study to describe the structural evolution of the Mg
Al
CO3 LDH. In this model, the conversion of layered double hydroxides into mixed metal oxides is divided into four stages. In the first stage, between 70
190 C, removal of interlayer water takes place and a change from Phase I with a basal spacing ranging from 7.5 to 7.3 A to Phase II with basal spacing of 6.6 A which indicates that no change in the LDH tactoid structure. In stage 2, between 190
280 C, the OH2 group bonded to Al31 disappears, in the third stage, between190
280 C, the OH 2 group linked to Mg21disappears and in the final stage, between 405
580 C, loss of CO322 takes

Layered double hydroxides: fundamentals to applications

19

Figure 1.13 FTIR spectra of the LDH-NO3, LDH-DS and LDH-DBS samples. Source: Adapted from Xu, K., Chen, G. and Shen, J., 2013. Exfoliation and dispersion of micrometer-sized LDH particles in poly (ethylene terephthalate) and their nanocomposite thermal stability. Applied Clay Science, 75, pp. 114-119. with kind permission of elsevier

place. At this temperature range, the material completely changes to an amorphous meta stable mixed solid oxide solution (Selvam et al., 2014; Yang et al., 2002). It is to be noted that thermal behaviour of modified LDH is different from that of unmodified LDH because of the presence of organic anions in the interlamellar region. The thermal behaviour of modified layered double hydroxides was greatly influenced by the nature of the anionic surfactant which can be easily understood from the Fig. 1.16. Costa et al. (2008) prepared Mg-Al-LDH by urea hydrolysis and modified it with DS, DBS, laurate and BEHP. In the Fig. 1.16 the loss of interlayer water below 225 C in all the modified form LDH-DS, LDH-DBS and LDH-laurate except LDH-BEHP are in a similar manner as that of unmodified LDH. The difference in LDH-BEHP is due to the less amount of water in the interlayer region due to the branching of alkyl chain. The shifting of first decomposition stage to lower temperatures in the modified samples can be attributed to the decrease in force of attraction of interlayer water to the sheet due to the increase in basal spacing during modification. There is marked variation in the second decomposition temperature in all the modified forms. In the case of SDBS-LDH, two step decomposition was observed between 240
600 C and the two peaks are found to be greater than that of unmodified LDH. It is due to the decomposition of SDBS in this region and the decomposition of aromatic ring and alkyl chain in the absence of oxygen delay

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Layered Double Hydroxide Polymer Nanocomposites

Table 1.4 FTIR bands of some organically modified LDHs (Basu et al., 2014; Xu et al., 2013b; Costa et al., 2008; Liu et al., 2008; Manzi-Nshuti et al., 2009). Modified LDH

Band Region

Types of vibrations

LDH
SDS

2850
2965 1229 1065 630 671, 820, 1379 and 1468 426 2850
2965 1186 1038 615 1602, 1496, 1409 and 1450 674, 833, 1379 and 1467 426 2850
2965 1563 1412 680, 870, 1378 and 146 425 3500 3012 2800-3000 1400-1600 3500

ν -CH2ν S 5 O symmetric ν S 5 O asymmetric ν C-S Different vibration modes of CO322 M-O lattice vibration ν -CH2ν S 5 O symmetric ν S 5 O asymmetric ν C-S ν C-H aromatic in plane bending

LDH
DBS

LDH-laurate

LDH-oleate

LDH- stearate

LDH-BEHP (Bis (2-ethylhexyl) hydrogen phosphate-modified LDH)

2800-3000 1542 2850
2965 1037 and 1136 1220 671,880,1380 and 1465 443

Different vibration modes of CO322 M-O lattice vibration ν -CH2ν COO- (asymmetric) ν COO- (symmetric) Different vibration modes of CO322 M-O lattice vibration ν-OH layer hydroxide νCH attached to double bond ν-CH2-and CH3ν-COO2symmetric and asymmetric ν-OH layer hydroxide and interlayer water ν-CH2- and CH3ν COO- (asymmetric) ν -CH2ν P-O-C symmetric ν P 5 O anti symmetric Different vibration modes of CO322 M-O lattice vibration

the decomposition of the host material. A greater loss below 250 C was observed only for LDH-SDS because of the decomposition of SDS between 210250 C. In the case of LDH-laurate and LDH
BEHP large weight loss occurs between 250 C-350 C and is due to the decomposition of the interlayer surfactant anions.

Layered double hydroxides: fundamentals to applications

21

Figure 1.14 The thermal evolution of Mg
Al
CO3 LDH as a function of temperature. Source: Adapted fromYang, W., Kim, Y., Liu, P.K., Sahimi, M. and Tsotsis, T.T., 2002. A study by in situ techniques of the thermal evolution of the structure of a Mg
Al
CO3 layered double hydroxide. Chemical Engineering Science, 57 (15), pp. 2945-2953 with kind permission of elsevier

1.5

Potential applications of layered double hydroxides, organically modified layered double hydroxides and layered double hydroxide nanocomposites

Layered double hydroxides can be regarded as promising layered nano materials due to the remarkable properties such as uniqueness in structure, distribution of two or three different types of metal cations in the LDH layer, hydroxyl groups in the surface, simple synthesis methods, non toxicity, flexible tunability in both anions and cations, excellent anion exchangeability, memory effect, appreciable chemical and thermal stability, high power to deliver the intercalated anion in a sustained manner, biocompatibility, electrochemical activity, high surface to volume ratio, high adsorbing power, endothermic decomposition etc. Because of these improvising properties, layered double hydroxides itself, LDH intercalated with functional materials and LDH based nanocomposites with superior nano materials especially carbon nano materials are good precursors for world wide applications to meet the requirements of the society. Some important applications of layered double hydroxides and hybrid layered double hydroxides-intercalated and nanocomposites such as flame retardancy, catalysis, energy storage and conversion such as electrode for super capacitor,

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Layered Double Hydroxide Polymer Nanocomposites

Figure 1.15 In situTG/DTA of Mg
Al
CO3 LDH as a function of temperature. Source: Adapted fromYang, W., Kim, Y., Liu, P.K., Sahimi, M. and Tsotsis, T.T., 2002. A study by in situ techniques of the thermal evolution of the structure of a Mg
Al
CO3 layered double hydroxide. Chemical Engineering Science, 57 (15), pp. 2945-2953 with kind permission of elsevier

Figure 1.16 TGA plots of LDH and its modified forms. Source: Adapted from Costa, F.R., Leuteritz, A., Wagenknecht, U., Jehnichen, D., Haeussler, L. and Heinrich, G., 2008. Intercalation of Mg
Al layered double hydroxide by anionic surfactants: preparation and characterization. Applied Clay Science, 38 (3-4), pp. 153-164. with kind permission of elsevier.

Layered double hydroxides: fundamentals to applications

23

splitting of water etc, environment remediation such as waste water treatment and preventing air pollution, controlled drug delivery and pesticide release and all these applications are illustrated here with most relevant example for each application.

1.5.1 Flame retardant applications Layered double hydroxides are promising green flame retardant materials and can retard the growth of flame through three distinct means. (1) It performs as heat sink due to the endothermic decomposition (2) It decomposes to form mixed metal oxides which act as an insulating film on the surface (3) It releases bound water and carbon dioxide thereby diluting the flammable gases (Radulescu et al., 2008). The flame retardancy of LDHs can be enhanced by intercalating suitable anions such as borate, phosphate etc into the inter lamellar region of LDH so that a single substance such as LDH containing zinc cations and borate anion can achieve the synergistic effect of three commercial flame retardants magnesium hydroxide, aluminium hydroxide and zinc borate. The main advantage of usage of LDH over the commercial flame retardants is the requirement of only very low concentration, non toxicity and high efficiency (Gao et al., 2014b). Guo et al. (2017) applied a coating of Mg 2 Al layered double-hydroxide (LDH) on wood surface for flame retardant applications by a two step synthetic method and the formation of coating on the surface of wood was confirmed by SEM analysis (Fig. 1.17).

Figure 1.17 Schematic diagram and SEM images show the formation of the Mg 2 Al LDH coating via one-step co precipitation (a, e) and two-step process with (b, f) 8, (c, g) 10, and (d, h) 12 h hydrothermal treatment, respectively. Source: Adapted from Guo, B., Liu, Y., Zhang, Q., Wang, F., Wang, Q., Liu, Y., Li, J. and Yu, H., 2017. Efficient flame-retardant and smoke-suppression properties of Mg
Al-layered double-hydroxide nanostructures on wood substrate. ACS applied materials & interfaces, 9 (27), pp. 23039-23047 with kind permission of ACS.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 1.18 CONE combustion parameters of untreated wood and the Mg 2 Al LDH-coated wood: (a) Total smoke production, (b) smoke production ratio, (c) heat release rate, (d) specific extinction area, (e) effective heat of combustion, and (f) total heat release. Source: Adapted from Guo, B., Liu, Y., Zhang, Q., Wang, F., Wang, Q., Liu, Y., Li, J. and Yu, H., 2017. Efficient flame-retardant and smoke-suppression properties of Mg
Al-layered double-hydroxide nanostructures on wood substrate. ACS applied materials & interfaces, 9 (27), pp. 23039-23047 with kind permission of ACS

The limiting oxygen index enhanced from18.9% to 39.1% and heat release index and smoke emission decreased by 40% and 58% respectively with respect to untreated wood (Fig. 1.18). The enhancement in flame retardancy can be attributed to the endothermic decomposition of LDH into mixed metal oxide which resulted in char formation. The hydrophilicity of LDH is changed to hydrophobic by surface modification by fluoro alkyl silane which is evidenced from contact angle measurement and the value is 152 . The increment in storage modulus by32% indicates that there is also increment in mechanical strength.

Layered double hydroxides: fundamentals to applications

25

1.5.2 Catalysis LDHs both as directly prepared and after thermal treatment and/or reduction have been widely used as stable and recyclable heterogeneous catalysts or catalyst supports for a variety of reactions benefitting the flexible tunability and uniform distribution of metal cations in the LDH layers and the facile exchangeability of intercalated anions. Layered double hydroxides are promising heterogeneous solid base catalysts for a number of organic transformations due to the abundance of hydroxyl groups. On heating between 450-500 C, LDHs have been changed to mixed metal oxides having a number of Lewis base sites and enhanced surface area which results in high catalytical efficiency. The rehydration of mixed metal oxide in the absence of CO2 can reproduce LDH with OH intercalated anion and this activated LDH possesses abundant Bronsted type basic sites. The number and the strength of OH sites can be controlled by varying the conditions of calcination. It is reported that the activated MgAl
LDH nano crystallites supported on carbon nanofibers (CNFs) is a very good catalyst for the trans esterification of glycerol with diethyl carbonate to form glycerol carbonate. This improvement in catalytical activity was attributed to the small size of the MgAl
LDH crystallites and number of accessible active OH sites in catalysts due to the accessible pores of the CNFs. The catalytic efficiency of binary LDH can be enhanced by incorporating third cation to form a ternary LDH. Polyoxometalate-intercalated catalysts can be widely used as heterogeneous catalysts for the acid-catalyzed esterification, oxidation of alcohols, alkenes etc (Fan et al., 2014). Wu et al. (2014) synthesized NiFe
NO3-LDH by urea hydrolysis for applying it as a catalyst for the one-pot synthesis of benzoin ethyl ether from benzaldehyde and ethanol. It was observed that the maximum percentage of conversion of benzaldehyde is 51.5% with a selectivity of 100% for benzoin ethyl ether. The catalytical activity of LDH in the above reaction is mainly attributed to the porous structure and L acid site of LDH. Baskaran et al. (2014) synthesized cobaltaluminium hydrotalcite (CoAl-HT-Si) with silicate ion as intercalated anion and proved that this LDH is a promising catalyst for variety of alcohol oxidation. The layer by layer assembly of LDH with oppositely charged species through electrostatic forces or/and hydrogen bonding will result in the formation of nanocomposites with excellent catalytic performances. Because of high specific surface areas, high aspect ratio, mechanical strength, thermal stability and electronic conductivity, carbon based nano materials such as carbon nanotube, graphene, carbon fibres etc are found to be excellent LDH catalyst supports in various heterogeneous reactions by enhancing their dispersion, heat, and mass transfer during the reaction, and provides mechanical strength for the whole composites. NiAl
LDH/CNTs nanocomposite modified electrode shows more electro catalytical activity for glucose electro oxidation than NiAl
LDH modified electrode or CNTs modified electrode. This can be attributed to the contribution of CNT in transporting more charge between Ni centers and electrode and promoting the diffusion of the reactants by providing a porous network like structure. A big challenge in this field is to improve simultaneously the activity, selectivity and stability of these LDH-based materials for catalytical purpose by finding new strategies to tailor the electronic structure of the catalysts and supports.

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Layered Double Hydroxide Polymer Nanocomposites

1.5.3 Water splitting Layered double hydroxide nanocomposites can act as excellent photo catalysts and electro catalyst for water splitting process there by providing hydrogen fuel for energy production which is essential for solving the energy crisis in the coming years as its production via these methods is low cost, high efficiency, environmentally benign. Because of the unique properties of layered double hydroxide such as cation-exchange ability, anion-exchange ability, adsorption capacity etc, LDHs offer enhancement in light absorption, charge separation, electron transfer, electrode reaction kinetics and durability. For the last two decades, there has been rapid progress in the design of LDH based nanocomposites for water splitting purpose in which the above phenomena were increased by properly tuning the structure and size of LDH and selecting novel materials such as graphdiyne (Shi et al., 2018), graphene oxide (Ma et al., 2015), graphitic carbon nitrides (Bhowmik et al., 2018), carbon quantum dot (Tang et al., 2014), etc which are capable of promoting the photo and electro catalytic ability of layered double hydroxides. LDH materials containing transition metal cations, especially Ni and Co have been widely applied in electro catalytic reactions. The intercalation of simple metal oxy-anions and POMs in LDH hosts can lead to photo catalytic capability. Yan et al. (2016) have published a review article on recent advances in the synthesis of layered double hydroxide based materials and their applications in hydrogen and oxygen evolution. Shi et al. (2018) synthesized GDY@NiFe LDH composite by coupling reaction followed by electro deposition method (Fig. 1.19). The enhancement in the OER activity of the composite can be attributed to the increase in electrical conductivity as well as surface active areas due to the synergistic effect of LDH and GDY. The high electrical conductivity of GDY due to unique electronic structure helped the

Figure 1.19 (a) Schematic illustration for the formation of GDY@NiFe architectures. SEM images of (b) pure Cu foil, (c) GDY, (d) NiFe LDH, and (e) the GDY@NiFe composite. Source: Adapted from Shi, G., Yu, C., Fan, Z., Li, J. and Yuan, M., 2018. Graphdiynesupported NiFe layered double hydroxide nanosheets as functional electrocatalysts for oxygen evolution. ACS applied materials & interfaces, 11 (3), pp. 2662-2669. with kind permission of ACS

Layered double hydroxides: fundamentals to applications

27

rapid flow of electrons and LDH provide abundant active sites for oxygen evolution (Fig. 1.20). GDY@NiFe LDH composite shows good catalytic activity in an alkaline electrolyte for a small over potential of 260 mV to acquire the current density of 10 mA cm22 Scheme 1.2.

1.5.4 Environmental remediation Layered double hydroxides are capable of adsorbing toxic metals and ions and organic dyes from waste water and toxic gases liberated into the atmosphere from automobiles and industries owing to its large specific area, porosity, high anion exchangeability and non toxicity and are promising candidates for environment remediation technologies (Mishra et al., 2018). Mahjoubi et al. (2017) synthesized four layered double hydroxides with different anions Zn-Al-SO4, Zn-Al-Cl, Zn-AlNO3 and Zn-Al-CO3 by co precipitation method and dye adsorption experiments were conducted at various conditions. The precursors showed exceptional Langmuir maximum adsorption capacities of 2758, 2455, 2270 and 1684 mg/g for Zn-Al-SO4, Zn-Al-Cl, Zn-Al-NO3 and Zn-Al-CO3, respectively indicating that these materials are potential broad-spectrum adsorbent for dye removal from wastewater. Li et al. (2016) synthesized MgAl-CO322LDH by an ethanol-water mediated solvothermal method for adsorbing congo red dye from waste water and determined that maximum adsorption efficiency of the LDH and calcined LDH are 129.9 and 143.27 mg g21, respectively. The adsorption mechanism of LDH is anion exchange where as that of calcined LDH is reconstruction. LDH hybrids obtained by coupling it with different anions like humate, EDTA, glutamate, tartrate, MoS422, polysulfide, carbon-based materials like CNT, graphene, iron ferrite nanoparticles and with some other compounds like TiO2, MnO2 and silica are found to be more powerful adsorber of toxic pollutants than LDH. This is due to the enhancement in surface area, anion exchange power, stability, chelating and binding sites, excellent selectivity for different metal ions, low toxicity and easy way of separation and reusability when it is integrated with magnetic particles. Koilraj, P et al (2018) fabricated Mg Al-LDH/graphene oxide (GO), 2D/2D multifunctional nanocomposite by dispersion-coagulation method for the adsorption of Sr21 and SeO422 from waste water. The SeO422 removal happened due to the anion exchange ability of LDH and it exchanges NO32 with SeO422. The Sr21 removal takes place as a result of its coordination with
COO- or
CO- group in GO produced by the ring opening of epoxides. (Scheme 1.3) The observed sorption efficiency of SeO422 on the Mg Al-LDH/GO (5%) composite was found to be 0.835 mmol/g (65.9 mg/g) and that of Sr21 on the Mg Al-LDH/GO (5%) composite was 213.35 mg/g (2.435 mmol/g) of GO and the values remained constant for pH range 4-10 (Fig. 1.20). The Anionic/LDH obtained by incorporating anions such as humate, EDTA, tartrate etc into the interlamellar region of LDH by anion exchange method have cumulative characteristics of both the LDH and the anion agents, which consequently increased the adsorptive power towards toxic pollutants (Zubair et al., 2017). Garcia-Gallastegui et al. (2012) designed and fabricated Mg-Al-LDH /GO hybrid by the self mode layer by layer assembly of positively charged LDH sheet and

Figure 1.20 Sorption isotherms of (A) Sr21 cation and (B) SeO422 anion on MgAl-LDH/GO (5%) and MgAl-LDH/GO (20%) composites in the single electrolytic solution. Source: Adapted from Koilraj, P., Kamura, Y. and Sasaki, K., 2018. Cosorption Characteristics of SeO42
and Sr2 1 Radioactive Surrogates Using 2D/2D Graphene Oxide-Layered Double Hydroxide Nanocomposites. ACS Sustainable Chemistry & Engineering, 6 (11), pp. 13854-13866 with kind permission of ACS

Layered double hydroxides: fundamentals to applications

29

Scheme 1.2 Schematic Diagram of the Proposed Process of OER on the GDY@NiFe Sample. Source: Adapted from Shi, G., Yu, C., Fan, Z., Li, J. and Yuan, M., 2018. Graphdiynesupported NiFe layered double hydroxide nanosheets as functional electrocatalysts for oxygen evolution. ACS applied materials & interfaces, 11 (3), pp. 2662-2669. with kind permission of ACS

Scheme 1.3 Mechanism of Sr21 sorption onto the alkoxide moiety present in the MgAl-LDH/GO composite and their nano aggregate formation. Source: Adapted from Koilraj, P., Kamura, Y. and Sasaki, K., 2018. Cosorption Characteristics of SeO42
and Sr2 1 Radioactive Surrogates Using 2D/2D Graphene Oxide-Layered Double Hydroxide Nanocomposites. ACS Sustainable Chemistry & Engineering, 6 (11), pp. 13854-13866 with kind permission of ACS

negatively charged graphene oxide (Scheme 1.4). Here graphene supports layered double hydroxide for adsorbing carbon dioxide thereby increasing the efficiency. The enhancement in adsorption capacity and multicycle stability can be attributed

30

Layered Double Hydroxide Polymer Nanocomposites

Scheme 1.4 Schematic Representation of the LDH and GO at Different Mass Ratios Highlighting the Degree of Surface Coverage Expected. Source: Adapted from Garcia-Gallastegui, A., Iruretagoyena, D., Gouvea, V., Mokhtar, M., Asiri, A.M., Basahel, S.N., Al-Thabaiti, S.A., Alyoubi, A.O., Chadwick, D. and Shaffer, M.S., 2012. Graphene oxide as support for layered double hydroxides: enhancing the CO2 adsorption capacity. Chemistry of Materials, 24 (23), pp. 4531-4539 with kind permission of ACS

to the increased particle distribution. The adsorption capacity of LDH has been increased by more than 60 % for only 7 wt% GO concentration (Fig. 1.21).

1.5.5 Electrode for super capacitor Layered double hydroxides are superior material for making pseudo-capacitor electrodes because of the presence of electrochemically active surfaces, environment friendly nature and low cost. On the basis of Faradic electrochemical reaction, LDHs containing transition metals show larger specific capacity in aqueous alkaline electrolytes than LDH with other metal ions. LDHs have some limitations for acting as super capacitor electrode due to low conductivity and strong stacking tendency. In order to avoid this, it is coupled with nano materials with high conductivity and specific area like graphene, multiwalled carbon nanotube etc and the nanocomposite thus produced have large redox activity. Lin, Yan, et al (2013) engineered three-dimensional activated reduced graphene oxide nanocup/nickel aluminum layered double hydroxides composite (3D-ARGON/NiAl-LDH) by the hydrothermal synthesis via in situ growth of ultrathin NiAl-LDH nanoflakes on the 3D-ARGON in an ethanol medium. The procedure for the preparation of 3D-ARGON/NiAl-LDH nanocomposite is shown in Fig. 1.22. The 3D nanocomposite thus produced have a macropore on the rim of a cup and large mesoporous structure on the wall of a cup which help the electron

Layered double hydroxides: fundamentals to applications

31

Figure 1.21 Average CO2 sorption capacities per mass of total adsorbent of pure LDH and carbon hybrids at 573 K and P (CO2) 5 0.2 bar, based on measured GO content, shown with standard errors. Source: Adapted from Garcia-Gallastegui, A., Iruretagoyena, D., Gouvea, V., Mokhtar, M., Asiri, A.M., Basahel, S.N., Al-Thabaiti, S.A., Alyoubi, A.O., Chadwick, D. and Shaffer, M.S., 2012. Graphene oxide as support for layered double hydroxides: enhancing the CO2 adsorption capacity. Chemistry of Materials, 24 (23), pp. 4531-4539 with kind permission of ACS

Figure 1.22 Procedure for the preparation of 3D-ARGON/NiAl-LDH. Source: Adapted from Lin, Y., Ruiyi, L., Zaijun, L., Junkang, L., Yinjun, F., Guangli, W. and Zhiguo, G., 2013. Three-dimensional activated reduced graphene oxide nanocup/nickel aluminum layered double hydroxides composite with super high electrochemical and capacitance performances. Electrochimica Acta, 95, pp. 146-154. with the kind permission of elsevier

transfer and mass transport during the faradaic redox reaction taking place in the supercapacitor. The maximum specific capacitance reported was 2712.7 F g21 at the current density of 1 A g21, which is more than 7 times that of pure NiAl-LDH, 3 times that of common reduced graphene oxide/NiAl-LDH and 1.8-fold that of twodimensional activated reduced graphene oxide/NiAl-LDH (Fig. 1.23). Some examples of LDH nanocomposites for energy applications are given in Table 1.5.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 1.23 Specific capacitances of the 3D-ARGON/NiAl-LDH (a) and pure NiAl-LDH (b) capacitor cells in different discharge current density. Source: Adapted from Lin, Y., Ruiyi, L., Zaijun, L., Junkang, L., Yinjun, F., Guangli, W. and Zhiguo, G., 2013. Three-dimensional activated reduced graphene oxide nanocup/nickel aluminum layered double hydroxides composite with super high electrochemical and capacitance performances. Electrochimica Acta, 95, pp. 146-154. with kind permission of elsevier

1.5.6 Biomedical applications LDHs have got a lot of attention from researchers in biomedical field for the innovative applications such as drug delivery, gene delivery, bio sensing and bio imaging area because of its non toxicity and in vivo and in vitro bio compatibilities. The great capabilities of LDHs in exchanging anions with drugs, nucleic acids (DNA, RNA), enzymes etc and the efficiency of layered double hydroxides in releasing the drug to the target location in a controlled and sustained manner at a particular pH give them an important position in the drug delivery applications. From the literature survey it is clear that many drugs such as non steroidal inflammatory, anti diabetic, cardiovascular, antibiotics, anti cancer, antioxidant, antimicrobial etc can be intercalated into the interlamellar region either by anion exchange reaction or by memory effect or by one step co precipitation. At the time of drug release, the drug delivery system breaks slowly releasing the drug at the required site. The slow releasing of drug from the interlayer sheet of LDH with respect to time is also a big factor in suppressing the toxicity of the drug. A dissolution test must be conducted to detect the drug release ability of the pillared LDH materials in a simulated intestinal fluid (buffer at pH 7.8). Researchers have been attempting to deliver RNA and

Layered double hydroxides: fundamentals to applications

33

Table 1.5 Some examples of LDH nanocomposites for energy applications. LDH nanocomposite

Electrode for super capacitor /battery

Specific capacitance/ Voltage

Reference

NiMn-LDH/CNT nanocomposite CoAl-LDH/MnO2

Electrode for super capacitor Electrode for super capacitor Electrode for super capacitor

2960 F g 21 at 1.5 A g 21 1088 F g 21 at 1A g 21 781.5 F/g at 5 mV. s21

(Zhao et al., 2014)

Electrode for super capacitor

1740 mF cm22 at 1 mA cm22

(Wan et al., 2015)

Electrode for super capacitor

1133.3 mF cm22 at 1 mA cm22

(Sekhar et al., 2017)

Electrode for super capacitor Electrode for super capacitor Cathode active material for Ni- metalhydride secondary battery

2130 F g21 at 2 A g21 2682 F g 21 at 3 A g 21 147 mV after 300 cycles 157 mV after 869 cycles

(Le et al., 2019)

Graphene Nanosheet/ Ni21/Al31 Layered Double-Hydroxide NiCo2S4 nanotube@ NiMn layered double hydroxide arrays/three dimensional graphene sponge Conductive silver nano wires-fenced carbon cloth fibers-supported layered double hydroxide nanosheets NiCo-LDH/rGO composites Ni Co-LDH@Ni Ni-AlLDH/C composites

(Diao et al., 2014) (Gao et al., 2011)

(Chen et al., 2014) (Be´le´ke´ et al., 2014)

DNA to mammalian cells in vivo by incorporating them with LDH alone or LDH and drug with the purpose of treating diseases (Mishra et al., 2018; Bullo Saifullah, 2015; Kuthati et al., 2015; Nakayama et al., 2010). Li, et al. (2014) designed CDsiRNA-5-FU/LDH nano complexes and delivered into the cancer cells and proved that this nano drug design is efficient in suppressing cancer cell growth. Schematic representation of immobilization of DNA in the inter gallery space of LDH and cellular uptake of DNA@LDH system followed by its action on the cell are demonstrated in Figs. 1.24 and 1.25 respectively (Mishra et al., 2018). Li et al. (2011) used layered double hydroxides as DNA vaccine delivery vector for enhancing antimelanoma immune response. The enzyme immobilized LDHs have been widely accepted as an ideal material for the construction of biosensor due to the high thermal stability, biocompatibility and the ability of LDH to protect the immobilized enzymes. The enzyme urease

34

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.24 Schematic image of DNA intercalated LDH. Source: Adapted from Mishra, G., Dash, B. and Pandey, S., 2018. Layered double hydroxides: A brief review from fundamentals to application as evolving biomaterials. Applied Clay Science, 153, pp. 172-186. with kind permission of elsevier

Figure 1.25 Schematic image of cellular uptake of DNA intercalated LDH nano hybrid and its action on cells. Source: Adapted from Mishra, G., Dash, B. and Pandey, S., 2018. Layered double hydroxides: A brief review from fundamentals to application as evolving biomaterials. Applied Clay Science, 153, pp. 172-186. with kind permission of elsevier

Layered double hydroxides: fundamentals to applications

35

was immobilized into the inter lamellar region of Zn/Al LDH and used the enzyme/ LDH system for the construction of urea biosensor. The rate at which urease dissociates urea is converted into an electric pulse from which the information regarding the amount of urea surrounding it can be obtained. Some examples for the applications of layered double hydroxides, layered double hydroxide nanocomposites and organically modified layered double hydroxides are given in Table 1.6.

1.6

Introduction to Layered Double Hydroxide Polymer Nanocomposites

It is very important to mention that layered double hydroxide polymer nanocomposites have a vital role in the material chemistry field because of their wide range of applications such as flame retardancy, thermal stability, water purification, catalysis, drug delivery, photo luminescence, agricultural applications, food packaging applications, energy applications etc. The high efficiency of layered double hydroxide polymer nanocomposite in these application fields can be achieved mainly by increasing the extent of exfoliation of LDH nanosheets in the polymer matrices. The remarkable exfoliation of LDH nano sheets in the polymer matrices is a big challenge due to the high interlayer interaction which arises from the high charge density of the layers. In order to solve this problem, there has been a prompt advancement in the fabrication of LDH
polymer nanocomposites to meet the promising applications in the recent years and so many new strategies have been developed and are categorized into three modes of preparation. First one is the intercalation of monomers into LDH or organically modified LDH followed by polymerization, which help to exfoliate and distribute LDH nanosheets uniformly throughout the polymer matrix and the second one is the direct intercalation of polymer into organically modified LDH and the intercalation of this large molecule leads to exfoliation of LDH nanosheets and the third one is to delaminate the sheets by using suitable solvents followed by mixing with the polymer (Fig. 1.26) (Wang and O’Hare, 2012). The following are the general techniques used for the fabrication of layered double hydroxide polymer nanocomposites through any one of the three modes of preparation which are mentioned above.

1.7

Different Fabrication Techniques of Layered Double Hydroxide polymer Nanocomposites

1.7.1 Melt Compounding Melt Compounding is the mixing of polymer and organically modified LDH or preexfoliated LDH by applying high local shear stresses in a melt mixer at high temperature, which should be above the softening point of the polymer (Radulescu et al., 2008; Ardanuy and Velasco, 2011; Unalan et al., 2014; Xiong, 2018).

Table 1.6 Some examples for the applications of layered double hydroxides, layered double hydroxide nanocomposites and organically modified layered double hydroxides. LDH/OLDH

Synthesis method

Mg-Fe/LDH and Ni-Fe/LDH

co-precipitation method Hydrothermal method

Mg-Al LDH Cu-Al layered double hydroxides

Hydrotalcite (Mg-Al) and Hydrotalcite-like compounds (MgFe, Zn-Al, and Zn-Fe) Mg
Al layered double hydroxide LDH (Cl) ) and LDH (CO3)

co-precipitation method

[NiFe]- (NO32) -LDH

pulsed laser ablation in liquids

anthraquinone-2-sulfonic acid sodium salt monohydrate (AQS) Ni-Fe LDH Homogeneous precipitation.

Graphene oxide, GOmodified Hummers method. Reduction of the Graphene Oxide Nanosheet. co-precipitation method

Mg
Al
CO322LDHs

Intercalating anion/other nanomaterial

5-Fluorouracil Memory effect Cu-Al/carbon fiber- LDH (CuAl/CF-LDH) Deoxyribonucleic acid (DNA) anion- exchange Intercalation of DNA into LDH ion-exchange and reconstruction BF4 2, Cl2, ClO4 2, CO322 C2O422, F2, I2, PO432 SO422 Anion exchange Anion exchange with sodium dodecyl sulfate exfoliation in water/ethanol LDH/rGO nanocomposite

GO from natural graphite powders by Hummer’s method LDH-assembled GO hydrogelsself-assembly approach

Applications

References

Arsenic Removal

(Nakahira et al., 2007) (Peng et al., 2018a) (Peng et al., 2018b)

corrosion resistance, Anti cancer agent catalysts for the degradation of ammonia and adsorption of azo dye Nonviral Gene Delivery Vehicles

(Balcomb et al., 2015)

DNA carrier

(Nakayama et al., 2010)

water oxidation catalysis

(Hunter et al., 2016)

oxygen evolution reaction (OER) efficiency

(Ma et al., 2015)

water purification

(Fang and Chen, 2014)

Layered double hydroxides: fundamentals to applications

37

Figure 1.26 Schematic illustration of the polymer nanocomposite preparation based on isotactic polypropylene and Mg 2 Al LDH layered double hydroxide. Source: Adapted from Nagendra, B., Mohan, K. and Gowd, E.B., 2015. Polypropylene/ layered double hydroxide (LDH) nanocomposites: influence of LDH particle size on the crystallization behavior of polypropylene. ACS applied materials & interfaces, 7 (23), pp. 12399-12410 with kind permission of ACS

The shearing helps the delamination of LDH tactoids and high residence time allow the polymer chain to intercalate into the interlayer gallery space to obtain exfoliated polymer nanocomposites. From the literature survey, it has been found that this approach was successfully applied for the fabrication of polypropylene (Ardanuy and Velasco, 2011; Purohit et al., 2011; Purohit et al., 2014), poly (L-lactide) (Tang et al., 2016), poly (methyl methacrylate) (Nyambo et al., 2008), ethyl vinyl acetate (Wang et al., 2011), poly ethylene (Costa et al., 2011) Carboxylated nitrile rubber (Laskowska et al., 2014) layered double hydroxide nanocomposites.

1.7.2 Solution Blending In this method polymer LDH nanocomposites are prepared by dispersing organically modified LDH or exfoliated LDH into a polymer solution by heating at high temperature or mechanical stirring or ultra sonic assisted stirring followed by the evaporation of the solvent (Radulescu et al., 2008; Unalan et al., 2014; Xiong, 2018). Usually a homogeneous dispersion is produced by mixing the polymer which is dissolved in a suitable solvent and nanoparticles in the same or different solvent. The advantage of this method is that it is easy to prepare exfoliated polymer nanocomposites by delaminating LDHs using suitable solvent followed by the addition of polymer solution into it so that the interaction between the polymer and individual LDH nanosheet is high (Fig. 1.26). This method has been utilized for the successful fabrication of poly propylene (Nagendra et al., 2017a; Nagendra et al., 2015; Yang et al., 2015), poly acryl amide (Fu et al., 2010), silicone rubber (Pradhan and Srivastava, 2014), poly styrene (Suresh et al., 2016; Edenharter et al., 2016), poly amide (Shabanian et al., 2014), high density polyethylene (Gao et al., 2014a) layered double hydroxide nanocomposites.

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Layered Double Hydroxide Polymer Nanocomposites

1.7.3 In-Situ Methods 1.7.3.1 In-Situ Polymerization The preparation of monomer intercalated LDH followed by polymerization with the addition of initiators and/or excitation by heat or radiation is the basic principle of this method (Radulescu et al., 2008; Unalan et al., 2014; Xiong, 2018). (Fig. 1.27) Polyamide 6 (Peng et al., 2009), poly aniline (Hu et al., 2014), poly styrene (Nyambo et al., 2008), polyurethane (Guo et al., 2011; Kotal and Srivastava, 2011), polyimide (Dinari et al., 2015), PBMA (Kovanda et al., 2010) layered double hydroxide nanocomposites have been fabricated successfully by this method.

1.7.3.2 In-Situ LDH Synthesis In this method, synthesis of LDH from two constituent metal salts is carried out by co-precipitation in the presence of a polymer solution so that polymer chains can easily intercalate into the interlamellar galleries of LDHs (Radulescu et al., 2008; Xiong, 2018). Li et al. (2018) prepared beads of chitosan /layered double hydroxide nanocomposite for the adsorption of selenium oxoanions via in situ LDH synthesis. The precursors required for LDH synthesis were added into a chitosan solution and the gel thus formed was allowed to fall into sodium hydroxide solution, both of them co precipitated to form nanocomposite beads. The bead formations have taken place effectively only when the precursor concentration becomes 60%.

Figure 1.27 Schema of LDH/PBMA nanocomposite formation during in situ polymerization. Source:Adapted from Kovanda, F., Jindova´, E., Lang, K., Kuba´t, P. and Sedla´kova´, Z., 2010. Preparation of layered double hydroxides intercalated with organic anions and their application in LDH/poly (butyl methacrylate) nanocomposites. Applied Clay Science, 48 (1-2), pp. 260-270 with kind permission of elsevier

Layered double hydroxides: fundamentals to applications

39

1.7.3.3 Double in-situ method This method includes both in situ LDH synthesis as well as in situ polymerization. In this method, precursors for the preparation of LDH and monomers and initiators for the preparation of polymers are mixed together. The LDH is formed by the precipitation of metal ions and the monomer or polymer intercalate into the interlayer gallery of the LDH and the initiator continuously helps the conversion of monomers into polymers to form exfoliated layered double hydroxide polymer nanocomposites (Xiong, 2018).

1.7.4 Layer By Layer Assembly The general strategy involved in this method is the layer by layer assembly of the positively charged nanosheets produced by the delamination of LDHs alternatively with anionic polymers via electrostatic attraction. Yan, Dong peng, et al fabricated a multilayer (APPP/LDH) n thin film by alternate LBL assembly of sulfonated poly (p-phenylene) anionic derivate, APPP and exfoliated Mg-Al-LDH. The fluorescence properties of this UTF was found to be greater than APPP due to the suppression of π-π stacking of polymer chain with the introduction of exfoliated LDH nano sheets (Yan et al., 2009). In addition to this poly anions, neutral polymer which contains NH2 or -OH group can form hydrogen bond as driving force for LBL assembly and some other neutral conjugated polymer molecules can form Vander Waal’s forces as driving force for LBL assembly by utilizing delocalized π electrons on the conjugated polymer. The growth of the film can be monitored by uv-vis absorption spectroscopy Many research works reported on the development of LDH/polymer multilayer films by LBL assembly demonstrated that these nanocomposites are new platform materials for optical, electrical and magnetic applications. Xu et al. (2013a) developed (ZnAlLDH/PANI) n multilayer films by LBL technique in which UTF has been prepared by dipping the substrate into ZnAl-LDH colloidal solution first then into PANI solution and the procedure was repeated n times. The washing with pure water and drying in nitrogen was carried out after each deposition (Scheme 1.5).

Scheme 1.5 LBL assembly process for (ZnAl-LDH/PANI) n films. Source: Adapted from Xu, D.M., Guan, M.Y., Xu, Q.H. and Guo, Y., 2013. Multilayer films of layered double hydroxide/polyaniline and their ammonia sensing behavior. Journal of hazardous materials, 262, pp. 64-70. With kind permission of elsevier

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Layered Double Hydroxide Polymer Nanocomposites

The preparation of core-shell assembly of hybrid nanofillers followed by the incorporation of it into suitable polymer system leads to the formation of LDH/ polymer hybrid nanocomposites with wide range of novel applications such as flame retardancy, (Zhang et al., 2017) drug delivery, electrode for super capacitors etc. The core- shell structured materials can also be prepared from LDH and polymer by using layer by layer method which can also be used for wide range of promising applications.

1.7.4.1 Preparation of LDH Nanosheets The LDH nanosheets have very important role in the fabrication of multifunctional multilayer (LDH/polymer) n thin films and core-shell structured materials by LBL assembly. LDH nanosheets can be produced by dispersing either unmodified LDH or organically modified layered double hydroxides in suitable solvents such as butanol, formamide, a mixture of dimethyl formamide and ethanol, xylene (Nagendra et al., 2017b), water (Zhang et al., 2017), etc with the help of mechanical shaking, stirring, ultra sonication etc (Wang and O’Hare, 2012; Mao et al., 2017). Liu et al. (2006b) delaminated Co-Al-NO3LDH by agitating the dispersion in formamide with the help of mechanical shaker (Fig. 1.28). AFM analysis is an effective characterization technique for predicting the delamination of LDH into single sheets by providing the thickness of LDH and that of delaminated sheet from the AFM height images and the corresponding height profiles of the layered double hydroxide before and after exfoliation. Nagendra et al (2017b) synthesized Co 2 Zn 2 Al LDH by co precipitation and delaminated the LDH sheets by shaking it with xylene solvent and measured the thickness of the synthesized LDH and delaminated sheets by conducting AFM analysis. From the AFM height images and the corresponding height profiles, it is proved that the thickness of LDH before exfoliation is around a few hundreds of

Figure 1.28 Schematic illustration of the possible delamination mechanism for LDHs in formamide. Source::Adapted from Liu, Z., Ma, R., Osada, M., Iyi, N., Ebina, Y., Takada, K. and Sasaki, T., 2006. Synthesis, anion exchange, and delamination of Co 2 Al layered double hydroxide: assembly of the exfoliated nanosheet/polyanion composite films and magneto-optical studies. Journal of the American Chemical Society, 128 (14), pp. 4872-4880 with the kind permission of ACS

Layered double hydroxides: fundamentals to applications

41

Figure 1.29 AFM images and height profiles of (a) as-synthesized and (b) exfoliated singlelayer Co 2 Zn 2 Al LDH nanosheets. Source: Adapted from Nagendra, B., Rosely, C.S., Leuteritz, A., Reuter, U. and Gowd, E.B., 2017. Polypropylene/layered double hydroxide nanocomposites: Influence of LDH intralayer metal constituents on the properties of polypropylene. ACS Omega, 2 (1), pp. 20-31 with kind permission of ACS

nanometers whereas the thickness of exfoliated nanosheets is less than 1.0 nm which is in good agreement with the reported values of single sheet of LDH (Fig. 1.29).

1.7.5 Two roll mill mixing This method has been widely applied for the compounding of rubber and the first step in this method is the mastication of rubber for about 2 minutes before adding all the ingrediants. The ingrediants are added into the rubber in the sequential order, activators, accelerators, nanofillers and cross linking agents. The friction ratio between the two rolls must be kept constant throughout the cycle. The mixing is completed within 15 minutes and kept for maturation for about 24 hours. The compounded rubber has been cured in a compression molding press at 150  C for pre determined cure time which is detected with the help of moving die rheometer. Das et al. (2011) fabricated zinc oxide free rubber/St-LDH nanocomposites by two roll mill mixing and proved that St-LDH is a good candidate for the vulcanization of rubber as activator and it substitute the role of zinc oxide which make the method

42

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.30 Schematic presentation for the preparation procedure of LDH (LDH-St) and rubber/LDH-St composites.

more ecofriendly. Schematic presentation for the preparation procedure of LDH (LDH-St) and rubber/LDH-St composites is shown in Fig. 1.30. Recently, sonication and high energy ball milling techniques have been applied for the preparation of polymer nanocomposites.

1.7.6 Sonication In this method, the formation, growth, and collapse of bubbles in a liquid called acoustic cavitation produced by high intensity ultrasound waves leads to intense shockwaves that promote collisions between the particles, reduction in the size of LDH tactoids and, ultimately, the exfoliation of the layered filler (Unalan et al., 2014).

1.7.7 High energy ball milling Now- a-days, the fabrication of polymer nanocomposites has been carried out by implementing the high energy ball milling as this method requires no solvent and heating. It is mixing of components in the solid form at room temperature which makes the fabrication of composite more effective, convenient and green. It is a high-shear mixing technique in which the shear among balls of different diameters is capable of separating the sheets by overcoming the van der Waals force of attraction between the layers. The clay dispersion in the polymer is enhanced by the transfer of energy between the balls and polymer/clay mixture (Unalan et al., 2014).

Layered double hydroxides: fundamentals to applications

1.8

43

Applications of Layered Double Hydroxide Polymer Nanocomposites

1.8.1 Flame Retardant Application Most of the commercially available flame retardants release toxic gases during combustion and introduction of some of them into the polymer matrix deteriorates the mechanical properties of polymers (Gao et al., 2014b). For the reason that, in the past two decades, polymer layered double hydroxide nanocomposites have achieved great attention due to the excellent and environment friendly flame retardant action of the green nanofiller, LDH. It retards the propagation of flame by acting as heat sink due to the endothermic decomposition of metal hydroxides into metal oxides which forms an insulating char on the surface that impedes the combustion process by downgrading the oxygen supply to the matrix. It liberates carbon dioxide and bound water during combustion thereby diluting the flammable gases (Basu et al., 2014; Matusinovic and Wilkie, 2012; Costa et al., 2007; Elbasuney, 2015). The factors which influence the flame retardancy of polymer layered double hydroxide nanocomposites are nature of cations and interlamellar anions of LDH, size and shape of LDH particles, type of polymer used and extent of exfoliation of LDH sheets into the polymer matrix (Gao et al., 2014b; Matusinovic and Wilkie, 2012). Matusinovic and Wilkie (2012) published a review article of flame retardancy and morphology of polymer layered double hydroxide nanocomposites which are mainly focused on the correlation between dispersion of layered double hydroxides in the polymer matrix and reduction in PHRR. Gao et al. (2014a,b) well studied about flame retardant polymer layered double hydroxide nanocomposites and published a review article on this topic in which PHRR reduction values of polymer nanocomposites with different types of polymers and LDHs are itemized. The various polymers used for this purpose are polypropylene, polystyrene, polyethylene, ethylene vinyl acetate, polyvinyl chloride, acrylonitrile
butadiene
styrene, unsaturated polyesters, epoxy, poly (lactic acid) ethylene
propylene
diene terpolymer etc. The generally used organic modifiers of LDH for flame retardant application are lauryl alcohol phosphoric acid ester potassium, benzoic acid, benzene sulfonic,4-amino benzoic, benzene phosphonic, and N- (2- (5,5-dimethyl-1,3,2dioxaphosphinyl-2-ylamino) -hexyl) acetamide-2-propylacid, N- (2- (5,5-dimethyl1,3,2-dioxaphosphinyl-2ylamino) -N-hexyl) formamide-2-propenyl acid, dodecyl sulfate, stearate anion,2-aminotoluene-5-sulfonic acid, dodecyl benzene sulfonate, 2-methyl-2-propene-1-sulfonate, 2-ethylhexyl sulfate, bis (2-ethylhexyl) phosphate etc. The flame retardant additives commonly added to LDH for synergestic effect are magnesium hydroxide, ammonium polyphosphate, micro-encapsulated red phosphorus, expandable graphite, intumescent flame retardant, triphenol phosphate, resorcinol diphosphate, decabromophenyl oxide/antimony oxide, silica, melamine, zinc borate etc. The efficient flame retardant LDH/Polymer nanocomposite can be fabricated by the proper selection of the polymer, the anions required for the modification of the LDH, cations of the LDH and flame retardant additives (Gao et al., 2014b; Matusinovic and Wilkie, 2012).

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Layered Double Hydroxide Polymer Nanocomposites

Figure 1.31 Schematic illustration of the flame retardant mechanism of cardanol-BS modified LDH in EP composites. Source: Adapted from Wang, X., Kalali, E.N. and Wang, D.Y., 2015. Renewable cardanolbased surfactant modified layered double hydroxide as a flame retardant for epoxy resin. ACS Sustainable Chemistry & Engineering, 3 (12), pp. 3281-3290 with kind permission of ACS

Wang et al. (2015) fabricated EP/ Cardanol-BS modified layered double hydroxide nanocomposite by a combined effect of three roll mill and ultra sonication. Schematic illustration of the flame retardant mechanism of cardanol-BS modified LDH in EP composites is shown in Fig. 1.31. The LOI value of EP/m-LDH (6%) nanocomposite was found to be 29.2% with UL-94 V0 rating. The peak heat release rate, total heat release and total smoke production values of EP/m-LDH-6% were reduced by 62%, 19%, and 45%, respectively, compared to that of pure EP (Fig. 1.32). The flame retardant action depends on the dispersion of nanofillers in the polymer matrix which can improve the quality of char residue. The compact and continuous char residue restrict the escape of flammable gases liberating from the interior as well as acting as an insulating shield for the rest of the material.

1.8.2 Biomedical Application Layered double hydroxide polymer nanocomposites have vital role in biomedical sectors specifically in tissue engineering, drug delivery, gene therapy, photo dynamic therapy and stem cell therapy. In tissue-engineering the important role of fibrous scaffolds is to give biological and structural support to cell adhesion, proliferation, and differentiation there by helping the regeneration of tissues and organs. Fibrous scaffolds apt for such purpose can be prepared by introducing appropriate nanoparticles into a suitable polymer matrix. Shafiei et al. (2016) successfully prepared poly (ε-caprolactone) (PCL) / layered double hydroxide nanocomposite by electro spinning fabrication technique and observed that the addition of LDH in

Layered double hydroxides: fundamentals to applications

45

Figure 1.32 (a) Heat release rate and (b) total heat release versus time curves of EP and its flame retardant composites. Source: Adapted from Wang, X., Kalali, E.N. and Wang, D.Y., 2015. Renewable cardanolbased surfactant modified layered double hydroxide as a flame retardant for epoxy resin. ACS Sustainable Chemistry & Engineering, 3 (12), pp. 3281-3290 with kind permission of ACS

PCL-LDH scaffold influenced the cell adhesion and proliferation remarkably and increased the adipogenic differentiation of mouse adipose derived stem cells (mADSCs) significantly. Lee et al. (2017) fabricated layered double hydroxide poly peptide thermo gel nanocomposite system for chondrogenic differentiation of stem cells which is necessary for cartilage repair. (Scheme 1.6) This study led to the conclusion that LDH/thermogel system can overcome the disadvantages of hydrogel system and future perspective of this study is that it can act as an injectable system in stem cell therapy. An immune fluorescence study proved that there is an increased chondrogenic differentiation in the nanocomposite systems. The immunofluorescence

46

Layered Double Hydroxide Polymer Nanocomposites

Scheme 1.6 Schematic Presentation of the Research Stem cells (green circles with a blue core), KGN (red dots), and RGD-coated LDHs (yellow hexagons) are incorporated in a hydrogel during the thermal-energy-driven gelation of the polymer aqueous solution. Stem cells adhere to the LDH surface by RGD, and KGN is continuously released in the system and induces chondrogenic differentiation of TMSCs. Source: Adapted from Lee, S.S., Choi, G.E., Lee, H.J., Kim, Y., Choy, J.H. and Jeong, B., 2017. Layered double hydroxide and polypeptide thermogel nanocomposite system for chondrogenic differentiation of stem cells. ACS applied materials & interfaces, 9 (49), pp. 42668-42675. with kind permission of ACS

images of the proteins secreted by the cells, COL II and SOX 9 are given in Fig. 1.33. Kapusetti et al. (2013) prepared bone cement / layered double hydroxide nanocomposite for the application of joint arthroplasty. It has been proved that this nanocomposite achieved superiority in properties such as thermal stability, fatigue resistance behaviour over pure bone cement due to strong interaction between the polymer and LDH. The high biocompatibility and bioactivity of these nanocomposites has been verified by MTT assay and in vivo studies on rabbits clearly revealed that it is better healing agent than pure bone cement. Photodynamic therapy (PDT) is a treatment in which a drug called photo sensitizer (drug) is administered and the tumor is illuminated with a suitable light in order to activate the drug. Wei et al. (2015) developed LDH
indocyanine green (ICG)
chitosan nanocomposites for the application in photodynamic therapy. In this study the photo instability and biodegradation of ICG has been protected by intercalating it in the interlayer gallery of amine modified LDH then coated with chitosan, a natural polymer. The coating of polymer on the LDH-ICG nanocomposites was obtained by the cross linking formed between amine groups on the surface of LDH as well as that of chitosan using glutaraldehyde, the cross- linking agent.

Layered double hydroxides: fundamentals to applications

47

Figure 1.33 Immunofluorescence images of COL II (a) and SOX 9 (b). (c) GAG content. The images were taken for each system 21 days after 3D culture of TMSCs. Source: Adapted from Lee, S.S., Choi, G.E., Lee, H.J., Kim, Y., Choy, J.H. and Jeong, B., 2017. Layered double hydroxide and polypeptide thermogel nanocomposite system for chondrogenic differentiation of stem cells. ACS applied materials & interfaces, 9 (49), pp. 42668-42675. with kind permission of ACS

ICG has low toxicity and high absorbing power in the wavelength region 800
805 nm which is transparent to the tissues and exposing the dye to NIR light enhances the penetration depths in tissues. Lv et al. (2015) fabricated DAS- (PAA/PAH) 10-LDH magnetic assembly by LBL technique for targeted drug delivery (Fig. 1.34). They have sufficient saturated magnetic strength and are sensitive towards external magnetic field which is a criterion for targeting drug delivery. It was observed that the drug loading and release ability of the multilayer assembly can be controlled by treating with suitable acidic solution and verified by loading and release of a simulated drug, methylene blue (Fig. 1.35). Hu, H., et al. (2013) fabricated P(DMAEMA)-grafted LDH nano hybrids for advanced gene delivery application. The introduction of ATRP initiation sites containing disulfide bonds onto LDH surfaces by using three step method followed by atom transfer radical polymerization (ATRP) of 2- (dimethylamino) -ethyl methacrylate (DMAEMA) develop novel gene delivery vectors (Scheme 1.7). The resultant LDH-PDs showed better ability to condense plasmid DNA (pDNA) and much higher efficiency to delivery genes in different cell lines including COS7and HepG2 cell lines (Fig. 1.36).

48

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.34 Illustration for the fabrication of stabilized ZnAl-LDH via LbL process. Source: Adapted from Lv, F., Xu, L., Zhang, Y. and Meng, Z., 2015. Layered double hydroxide assemblies with controllable drug loading capacity and release behavior as well as stabilized layer-by-layer polymer multilayers. ACS applied materials & interfaces, 7 (34), pp. 19104-19111 with kind permission of ACS.

Figure 1.35 Release behavior of (a) (PAA/PAH) 10-LDH, (b) DAS- (PAA/PAH) 10-LDH, (c) B-DAS- (PAA/PAH) 10-LDH and (d) A-DAS- (PAA/PAH) 10
LDH toward MB. Source: Adapted from Lv, F., Xu, L., Zhang, Y. and Meng, Z., 2015. Layered double hydroxide assemblies with controllable drug loading capacity and release behavior as well as stabilized layer-by-layer polymer multilayers. ACS applied materials & interfaces, 7 (34), pp. 19104-19111 with kind permission of ACS.

Layered double hydroxides: fundamentals to applications

49

Scheme 1.7 Schematic Diagram Illustrating the Preparation Processes of P(DMAEMA) -Graft-LDH Hybrids via ATRP. Source: Adapted from Hu, H., Xiu, K.M., Xu, S.L., Yang, W.T. and Xu, F.J., 2013. Functionalized layered double hydroxide nanoparticles conjugated with disulfide-linked polycation brushes for advanced gene delivery. Bioconjugate chemistry, 24 (6), pp. 968-978. with kind permission of ACS

1.8.3 Gas sensing Applications Xu et al. (2013a) developed (ZnAl-LDH/PANI) n multilayer films by LBL technique for sensing ammonia gas. In this work it is proved that this UTF is an efficient sensor for ammonia gas. The change in resistance associated with the exposure of thin film to ammonia gas measures the ammonia sensing behaviour of UTF. The response of (ZnAl-LDH/PANI) n multilayer films towards ammonia is reversible at room temperature and it increases with increase in concentration of ammonia. When ammonia

50

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.36 Representative images of EGFP expression mediated by LDH (at the optimal weight ratio of 10), P(DMAEMA) (at the optimal N/P ratios of 15), and LDH-PD2 (at the N/P ratios of 15) in HepG2 cells. Source: Adapted from Hu, H., Xiu, K.M., Xu, S.L., Yang, W.T. and Xu, F.J., 2013. Functionalized layered double hydroxide nanoparticles conjugated with disulfide-linked polycation brushes for advanced gene delivery. Bioconjugate chemistry, 24 (6), pp. 968-978. with kind permission of ACS

comes in contact with poly aniline protonation
deprotonation of polyaniline will modulate the resistance change. It is mentioned that the introduction of LDH may enhance the voids for reaction with ammonia which facilitated the gas sensing response property of aniline.

1.8.4 Energy Applications Layered double hydroxide polymer nanocomposites can be used as proton exchange membrane in polymer electrolyte membrane fuel cells (PEMFCs) which are the most promising fuel cells due to their modularity and wide spectrum applications. Sulfonated polysulfone (SPSU), sulfonated poly (ether ketone) (SPEK), sulfonated poly (ether ether ketone) (SPEEK) are the polymers that are generally used for this application. Herrero, M., et al (2014) fabricated sulfonated polysulfone /layered

Layered double hydroxides: fundamentals to applications

51

double hydroxide nanocomposite membrane by solvent casting technique using the solvent dimethylacetamide for fuel cell application due to the easy availability, low cost and high thermal stability of polysulfone. In addition to these, polysulfone can be easily sulfonated very easily with a variety of sulfonating agents providing wonderful proton conducting membranes. The introduction of LDH enhances the mechanical properties, diminish the fuel and water permeability and keep humidity inside the membrane because of their hygroscopic properties and also affect the electrical and transport properties. Layered double hydroxide/conductive polymer (core/shell) nano platelet array can be applied as an electrode for high performance super capacitors as the LDH nanoplatelet core provide high energy-storage capability through a fast and reversible redox reaction where as the highly conducting polymer shell facilitates transport of electron during the charge
discharge process. Han, Jingbin, et al. (2013) developed LDH@PEDOT NPA electrode for supercapacitor with excellent electrochemical behavior-high specific capacitance, remarkable cycling performance and high specific energy and power. The largely enhanced pseudocapacitor behavior of the LDH@PEDOT NPA electrode can be correlated to the combined effect of LDH nanolatelet core and highly conductive PEDOT shell, the LDH nanoplatelet core offers abundant energy- storage capacity, whereas the highly conductive PEDOT shell and porous structural design smooth the progress of the electron/mass transport in the redox reaction. Dye-sensitized solar cells (DSSC) are promising clean energy device and have achieved a great deal of research attention because of low cost and easy processability and satisfactory power conversion efficiency. Layered Double Hydroxides are efficacious additive in polymer gelled electrolyte for the application in dye-sensitized solar cells. Ho et al. (2014) prepared LDH/poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) gelled electrolyte for DSSCs and have made the observation that there is hike in open circuit voltage. This is owing to the positive shifts in redox potential with increase in the amount of LDH which may be due to the high ion exchange efficiency of the LDH.

1.8.5 Food Packaging Applications Xie et al. (2018) fabricated excellent biodegradable food packing thin film of poly (butylene adipate-co-terephthalate) (PBAT) /OLDH nanocomposite by melt blending and blowing method (Scheme 1.8). A PBAT/OLDH film (1 wt % OLDH, O is alkyl phosphonate, C12H25PO422) showed excellent mechanical, thermal, optical, and water vapor barrier properties than pure PBAT film with a reduction of 37% in haze and 41.9% enhancement in nominal tensile strain at break. The water vapor barrier properties has been enhanced due to the hindrance to the flow of water molecules due to the uniform distribution of LDH sheets and their by increasing the path of water molecules. The food packaging experiment proved that the packaging efficiency of the PBAT/OLDH nanocomposite film containing 1% OLDH is greater than that of pure PBAT film and commercially available polyethene packing material (Fig. 1.37). Tammaro et al. (2014) fabricated Poly (ethylene terephthalate)

52

Layered Double Hydroxide Polymer Nanocomposites

Scheme 1.8 Schematic Illustration for the Manufacture of the Biodegradable PBAT/OLDH Nanocomposite Films. Source:Adapted from Xie, J., Wang, Z., Zhao, Q., Yang, Y., Xu, J., Waterhouse, G.I., Zhang, K., Li, S., Jin, P. and Jin, G., 2018. Scale-up fabrication of biodegradable poly (butylene adipate-co-terephthalate) /organophilic
clay nanocomposite films for potential packaging applications. ACS Omega, 3 (1), pp. 1187-1196 with kind permission of ACS

(PET) nanocomposites by using seven different modified layered double hydroxides with the help of high energy ball milling. LDH helped the composite to prevent the unwanted oxidation of food by providing lower oxygen diffusion and permeability coefficients to the composites due to its high exfoliation into the polymer matrix. So these composites are promising candidates for food packaging applications.

1.8.6 Water Purification Mohamed et al. (2018) prepared polypyrrole nanofiber /Zn-Fe layered double hydroxide nanocomposite and applied it for removal of safranin dye from waste water because of their excellent adsorption power and photo catalytic properties (Fig. 1.38). It was observed that 5mg/L of safranin dye was completely removed by utilizing 0.05 gm of the composite after 120 minutes illuminating time in alkaline or neutral medium (Fig. 1.39). The enhancement in the photo catalytic removal of dye is attributed to the large specific surface area and reduction in the band gap due to composite formation. The mechanisms involved in the photo catalytic removal

Layered double hydroxides: fundamentals to applications

53

Figure 1.37 Photographs of banana samples with following treatment: (a) exposed to air, (b) packaged with pure PBAT film, and (c) packaged with OLDH-1 film. Source:Adapted from Xie, J., Wang, Z., Zhao, Q., Yang, Y., Xu, J., Waterhouse, G.I., Zhang, K., Li, S., Jin, P. and Jin, G., 2018. Scale-up fabrication of biodegradable poly (butylene adipate-co-terephthalate) /organophilic
clay nanocomposite films for potential packaging applications. ACS Omega, 3 (1), pp. 1187-1196 with kind permission of ACS

Figure 1.38 Photo catalytic degradation mechanism of safranin dye using PpyNF/Zn-Fe LDH nanocomposite. Source: Adapted from Mohamed, F., Abukhadra, M.R. and Shaban, M., 2018. Removal of safranin dye from water using polypyrrole nanofiber/Zn-Fe layered double hydroxide nanocomposite (Ppy NF/Zn-Fe LDH) of enhanced adsorption and photocatalytic properties. Science of the Total Environment, 640, pp. 352-363 wit kind permission of elsevier

are the adsorption of dye, charge transfer and production of oxidizing radicals due to the absorption of photon (Fig. 1.38).

1.8.7 Gas Barrier Materials The two dimensional layered double hydroxide nano sheets in the polymer nanocomposites are capable of hindering the gas penetration through the composite by providing an extensive diffusion path for the gas molecules due to low free space owing to the large aspect ratio of LDH and high interaction between the filler and

54

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.39 Effect of illumination time in degradation of several concentrations of safranin dye, (B) effect of catalyst dose in the removal of the dye with different time intervals, (C) effect of solution pH on the removal of safranin, and (D) stability of the catalyst for several runs of dye removal. Source: Adapted from Mohamed, F., Abukhadra, M.R. and Shaban, M., 2018. Removal of safranin dye from water using polypyrrole nanofiber/Zn-Fe layered double hydroxide nanocomposite (Ppy NF/Zn-Fe LDH) of enhanced adsorption and photocatalytic properties. Science of the Total Environment, 640, pp. 352-363 with kind permission of elsevier

polymer. Hence LDH/polymer nanocomposites are very good gas barrier material in tire and device packing industry. Wang et al. (2017) fabricated (U-mLDH/NBR) n multilayer films by LBL assembly by repeating the spin coating of U-mLDH suspension and NBR solution alternatively onto the substrate n times (Scheme 1.9). There was reduction in the oxygen transmission rate of the (U-mLDH/NBR) 30 nanocomposite by 92.2% with respect to NBR film and the value is found to be 0.626 cm3 m22 day21 atm21 (Fig. 1.40). The relative permeability of the nanocomposite is found to be lower than other rubber composites reported in literature and possesses high thermal stability, satisfactory mechanical strength so that it can act as a good gas barrier material.

1.8.8 Agricultural Applications Low density polyethylene/organically modified LDH nanocomposites are widely used as agricultural plastic films in greenhouses due to its infrared absorbing

Layered double hydroxides: fundamentals to applications

55

Scheme 1.9 Schematic illustration for the assembly of (U-mLDH/NBR) n film on PET substrate by layer-by-layer assembly technique. Source: Adapted from Wang, L., Dou, Y., Wang, J., Han, J., Liu, L. and Wei, M., 2017. Layer-by-layer assembly of layered double hydroxide/rubber multilayer films with excellent gas barrier property. Composites Part A: Applied Science and Manufacturing, 102, pp. 314321 with kind permission of elsevier

ability. Wang et al.(2010) fabricated low density polyethylene (LDPE) / N, N-bis (phosphonomethyl) glycine modified LDH nanocomposite by master batch method. The IR absorption power of MgAlGLYPLDH/LDPE in both the broader 1428400cm21and narrower 1428-714 cm21ranges, and principally in the 1111-909 cm21range are higher than that of MgAl-CO3-LDH/LDPE film and it has selective and appropriate IR absorption to be applied in agricultural films.

1.8.9 Anti Corrosion Materials Hu et al. (2014) fabricated PANI/AD-LDHs nanocomposites through the grafting of PANI chemically onto LDH. The first step is the intercalation of anticorrosive decavanadate anion into Zn
Al
NO3 LDH through anion exchange reaction. After the intercalation, decavanadate- intercalated LDH (D-LDH) was treated with APTS in order to graft PANI onto the D-LDH (Scheme 1.10). The corrosion protection power of coated PANI/AD-LDHs nanocomposites on steel was studied by measuring the OCP versus time of exposure, Tafel and EIS after exposing the coating to 3.5 wt.% NaCl solution. The deviations in the OCP values, corrosion current and EIS spectra with time (Fig. 1.41) indicating that the coating of PANI/AD-LDH possesses higher corrosion resistant property than that of D-LDH and PANI. This can be attributed to the increase in barrier to diffusion, redox properties of PANI as well as the power of D-LDH to release inhibiting anions.

56

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.40 (A) O2TR values for pristine PET, (U-LDH/NBR) n and (U-mLDH/NBR) n films (n 5 10, 20 and 30); (B) O2TR values for (U-mLDH/NBR) n films as a function of relative humidity (0%, 30% and 60%); (C) O2TR values of PET substrates coated with (N-mLDH/NBR) n multilayer films as a function of bilayer number n; (D) O2TR values for pure NBR, (N-mLDH/NBR) 15 and (U-mLDH/NBR) 30 films with nearly the same thickness (1 mm), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Source: Adapted from Wang, L., Dou, Y., Wang, J., Han, J., Liu, L. and Wei, M., 2017. Layer-by-layer assembly of layered double hydroxide/rubber multilayer films with excellent gas barrier property. Composites Part A: Applied Science and Manufacturing, 102, pp. 314-321 with kind permission of elsevier

1.9

LDH based polymer hybrid nanocomposites

The introduction of hybrid nanoparticles system which consists of LDH and any other functionalized nanoparticles with high magnetic property, electrically conductivity, mechanical strength, flame retardancy etc, into any polymer matrix, there will be synergistic effect of properties of all the fillers of the hybrid system resulting in the fabrication of high performance polymer hybrid nanocomposites. The compatibility between the fillers and the polymer can be achieved by properly selecting the fillers in the hybrid system and the polymer. Kalali et al. (2016) fabricated Fe3O4@Ph-CDBS-LDH / epoxy hybrid nanocomposite by modifying LDH with green flame retardants followed by decoration of modified LDH with Fe3O4 to produce Fe3O4@Ph-CDBS-LDH hybrid and finally introduced this hybrid system into epoxy resin (Scheme 1.11). Schematic Illustration of the flame-retardant

Scheme 1.10 Schematic illustration of the preparation of AD-LDH/PANI nanocomposites. Source: Adapted from Hu, J., Gan, M., Ma, L., Li, Z., Yan, J. and Zhang, J., 2014. Synthesis and anticorrosive properties of polymer
clay nanocomposites via chemical grafting of polyaniline onto Zn-Al layered double hydroxides. Surface and Coatings Technology, 240, pp. 55-62. with kind permission of elsevier.

58

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.41 Electrochemical impedance diagrams obtained after different exposure times to 3.5% NaCl solution for the steel covered by AD-LDH/PANI coating. Source: Adapted from Hu, J., Gan, M., Ma, L., Li, Z., Yan, J. and Zhang, J., 2014. Synthesis and anticorrosive properties of polymer
clay nanocomposites via chemical grafting of polyaniline onto Zn-Al layered double hydroxides. Surface and Coatings Technology, 240, pp. 55-62. with kind permission of elsevier.

Scheme 1.11 Preparation Process of a Fe3O4@Ph-CDBS-LDH Hybrid. Source: Adapted from Kalali, E.N., Wang, X. and Wang, D.Y., 2016. Synthesis of a Fe3O4 nanosphere@ Mg
Al layered-double-hydroxide hybrid and application in the fabrication of multifunctional epoxy nanocomposites. Industrial & Engineering Chemistry Research, 55 (23), pp. 6634-6642. with kind permission of ACS

Layered double hydroxides: fundamentals to applications

59

Scheme 1.12 Schematic Illustration of the Flame-Retardant Mechanism of (a) LDH/EP and (b) Fe3O4@Ph-CDBS-LDH/EP with EDX Mapping. Source: Adapted from Kalali, E.N., Wang, X. and Wang, D.Y., 2016. Synthesis of a Fe3O4 nanosphere@ Mg
Al layered-double-hydroxide hybrid and application in the fabrication of multifunctional epoxy nanocomposites. Industrial & Engineering Chemistry Research, 55 (23), pp. 6634-6642. with kind permission of ACS

mechanism of LDH/EP and Fe3O4@Ph-CDBS-LDH/EP with EDX Mapping are shown in Scheme 1.12. The introduction of 8 wt % this hybrid system into the EP matrix increased LOI value by 26.8 % and reduced the total smoke production and the peak heat release rate of the EP composite by 34% and 55% respectively with respect to EP, and achieved the UL-94V0 rating in the vertical burn test (Fig. 1.42). In addition to flame retardancy, this composite achieved high mechanical strength and thermal stability. Jiang et al. (2014) fabricated EP/ silica@Co 2 Al layered double hydroxide spheres hybrid nanocomposites for flame retardant applications. In this study mesoporous silica@Co 2 Al layered double hydroxide (m-SiO2@Co 2 Al LDH) spheres was prepared by LBL assembly (Scheme 1.13) and its formation confirmed by TEM and uv- visible spectroscopic analysis (Fig. 1.43). The presence of m-SiO2@Co 2 Al LDH in EP enhanced the flame retardant property by decreasing the PHRR by 39.3% and TSR by 23.8% (Fig. 1.44). This can be attributed to the synergestic effect of both the fillers that is LDH catalyzed formation of char and labyrinth effect of m-SiO2. Pradhan and Srivastava (2014) prepared three LDH/MWCNT hybrids using three different types of LDH by dry grinding and the resultant LDH/MWCNT hybrids

60

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.42 HRR versus time curves of epoxy and LDH-based epoxy composites obtained from cone calorimetry tests. Source: Adapted from Kalali, E.N., Wang, X. and Wang, D.Y., 2016. Synthesis of a Fe3O4 nanosphere@ Mg
Al layered-double-hydroxide hybrid and application in the fabrication of multifunctional epoxy nanocomposites. Industrial & Engineering Chemistry Research, 55 (23), pp. 6634-6642. with kind permission of ACS

Scheme 1.13 Synthetic Route of m-SiO2@Co 2 Al LDH. Source;Adapted from Jiang, S.D., Bai, Z.M., Tang, G., Song, L., Stec, A.A., Hull, T.R., Hu, Y. and Hu, W.Z., 2014. Synthesis of mesoporous silica@ Co
Al layered double hydroxide spheres: layer-by-layer method and their effects on the flame retardancy of epoxy resins. ACS applied materials & interfaces, 6 (16), pp. 14076-14086. with kind permission of ACS.

were incorporated into silicon rubber by solution intercalation method. The tensile strength of SR/ Mg
Al-LDH/MWCNT, SR/Co
Al-LDH/MWCNT and SR/Li
AlLDH/MWCNT hybrid nanocomposites for 1% hybrid filler system were enhanced by 134%, 125% and 100% respectively with respect to neat SR. The solvent resistance property and cross link density were also found to be highest for SR/Mg
AlLDH/MWCNT. The maximum synergistic effect was observed for Mg
Al-LDH/

Layered double hydroxides: fundamentals to applications

61

Figure 1.43 TEM images, digital photos (A, B, C) and UV 2 vis absorption spectra (D) of m-SiO2, m-SiO2@Co 2 Al LDH (10 layers) and m-SiO2@Co 2 Al LDH (20 layers). Source;Adapted from Jiang, S.D., Bai, Z.M., Tang, G., Song, L., Stec, A.A., Hull, T.R., Hu, Y. and Hu, W.Z., 2014. Synthesis of mesoporous silica@ Co
Al layered double hydroxide spheres: layer-by-layer method and their effects on the flame retardancy of epoxy resins. ACS applied materials & interfaces, 6 (16), pp. 14076-14086. with kind permission of ACS.

Figure 1.44 HRR curves of EP and its nanocomposites. Source;Adapted from Jiang, S.D., Bai, Z.M., Tang, G., Song, L., Stec, A.A., Hull, T.R., Hu, Y. and Hu, W.Z., 2014. Synthesis of mesoporous silica@ Co
Al layered double hydroxide spheres: layer-by-layer method and their effects on the flame retardancy of epoxy resins. ACS applied materials & interfaces, 6 (16), pp. 14076-14086. with kind permission of ACS.

MWCNT hybrid system owing to the highest surface area which led to maximum interaction between the SR matrix and Mg
Al-LDH/MWCNT. Huang et al. (2010) fabricated polyamide-6/CNT/LDH hybrid nanocomposite by preparing CNT/LDH hybrid filler from negatively charged CNT produced by the oxidation of CNT with nitric acid and nitrate intercalated Co-Al LDH produced by

62

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.45 Schematic description of assembling exfoliated LDH/CNT hybrids: (a) CNT formamide suspension, (b) LDH formamide suspension, and (c) mixture of CNT and LDH suspensions. Source: Adapted from Huang, S., Peng, H., Tjiu, W.W., Yang, Z., Zhu, H., Tang, T. and Liu, T., 2010. Assembling exfoliated layered double hydroxide (LDH) nanosheet/carbon nanotube (CNT) hybrids via electrostatic force and fabricating nylon nanocomposites. The Journal of Physical Chemistry B, 114 (50), pp. 16766-16772 with kind permission of ACS

the anion exchange of carbonate intercalated LDH and the hybrid filler was incorporated into polyamide-6 by converting ε-caprolactam into polyamide-6 in the presence of hybrid filler and 6-aminocaproic acid. The positively charged LDH nanosheet adsorb negatively charged CNTs on their surface through electrostatic attraction and this introduced a new path for producing exfoliated LDH nanosheet/ CNT hybrids. (Fig. 1.45). The synergistic effect of both the nano fillers enhanced the mechanical strength of the PA-6 nanocomposites (Fig. 1.46) due to uniform distribution of nanofillers and the strong interaction between the fillers and the polymer. This hybrid nanofillers preparation is necessary to design polymer hybrid nanocomposites for energy storage and generation, sensing applications and for polymer reinforcement. Chen et al. (2010) developed electrically conductive poly (vinyl alcohol) /graphene /layered double hydroxide hybrid film by layer-by-layer self-assembly. The various steps involved are dipping the substrate sequentially into exfoliated LDH suspension, aqueous PVA solution, exfoliated GO suspension and poly vinyl alcohol. The procedure also includes washing with water and drying in nitrogen after every dipping process. The dipping operations were repeated n times to develop multilayer films of (LDH/PVA/GO/PVA) n (Scheme 1.14). Here hydrogen bonding act as the driving force for LBL assembly which is formed through the hydroxyl group of PVA, carboxylate and epoxide group on the surface of GO and hydroxyl group of LDH nanosheet. Different fabrication routes to polymer nanocomposites and LDH based polymer hybrid nanocomposites and their applications are shown in Table 1.7.

Layered double hydroxides: fundamentals to applications

63

Figure 1.46 Typical stress-strain curves of neat PA6 and its nanocomposites. Source: Adapted from Huang, S., Peng, H., Tjiu, W.W., Yang, Z., Zhu, H., Tang, T. and Liu, T., 2010. Assembling exfoliated layered double hydroxide (LDH) nanosheet/carbon nanotube (CNT) hybrids via electrostatic force and fabricating nylon nanocomposites. The Journal of Physical Chemistry B, 114 (50), pp. 16766-16772 with kind permission of ACS

Scheme 1.14 Schematic of Layer-by-Layer Assembly Procedure. Source: Adapted from Chen, D., Wang, X., Liu, T., Wang, X. and Li, J., 2010. Electrically conductive poly (vinyl alcohol) hybrid films containing graphene and layered double hydroxide fabricated via layer-by-layer self-assembly. ACS Applied Materials & Interfaces, 2 (7), pp. 2005-2011.

1.10

Conclusion and perspectives

This chapter is really an overview of current research on hybrid LDHs- intercalated, surface immobilized systems, LDH nanocomposites, LDH based polymer nanocomposites and LDH based hybrid polymer nanocomposites which are rapidly growing in the field of material chemistry. LDHs are good nano reservoirs for controlled

Table 1.7 Different fabrication routes to polymer nanocomposites and their applications. Type of LDH

Method of synthesis of LDH

Zn2Al 2 X LDH (X 5 CO322, Cl2, NO32, and SO422) Mg 2 Al LDH

Co precipitation

Exfoliation/organicmodification/ nanocomposites of LDH

Coprecipitation

Delamination in xylene

Zn-Fe LDH

Coprecipitation

Modified Zn-Al LDH

One step Coprecipitation

Mixing of pyrrole and Zn-Fe LDH. Addition of oxidant to pyrrole /Zn-Fe LDH mixture Salicylate, Parahydroxybenzoate, aleuriticcarboxylate, citrate, glycolate serine carboxylate, 2.2-bishydroxymethyl-propionate antioxidants

Sodiumdodecyl benzenesulfonateZnAl-LDH Mg/Al
Cl-LDH

One step Coprecipitation

NiFe, NiAl, and NiCr LDH-SDS

Coprecipitation

Coprecipitation

Flame-retardant Cyclophosphazene Compound

LDH based polymer nanocomposites, Method of fabrication, Application

Reference

HDPE/LDH Nanocomposites Solvent Mixing Method Flame Retardant Polypropylene/LDH nanocomposites Solution blending LDH improves the thermal stability, nucleation ability, and crystallization rate of iPP PpyNF/Zn-FeLDH nanocomposite Interfacial polymerization Water purification Highenergy ballmilling method Poly (ethylene terephtalate) (PET) Food packaging applications

(Gao et al., 2014a)

Polypropylene/DBS LDH nanocomposite melt blending Dielectric PCL-LDH fibrous scaffolds Electrospinning adipogenic differentiation of mesenchymal stem cells. PLA/HPCP/LDH-SDS composites thermal stability and flame retardancy

(Nagendra et al., 2015)

(Mohamed et al., 2018) (Tammaro et al., 2014) (Purohit et al., 2011) (Shafiei et al., 2016) (Shan et al., 2012)

Mg/Al-CO3-LDH

Mg 2 Al 2 CO3 LDH

Urea hydrolysis

Graphene Oxide (GO) - modified Hummers method surface modifications of LDH and GO using 3,4dihydroxybenzophenone 3,4dihydroxybenzophenone (DBP) and 1-dodecylamine (DDA), respectively

PS/LDH/GO hybrid nanocomposite solution blending from tetrahydrofuran utilizing a threeroll mill Flame retardant

(Edenharter et al., 2016)

Layered Double Hydroxide/Chitosan Nanocomposite Beads Sorbents for Selenium Oxoanions In situ LDH synthesis and Direct Mixing

(Li et al., 2018)

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Layered Double Hydroxide Polymer Nanocomposites

drug delivery, gene delivery, pesticide release etc due to their excellent anion exchange efficiency and intercalation properties. Biomolecule immobilization play a great role in the development of LDH chemistry as it contributes several innovative advanced materials for biotechnological applications such as biosensing, bioimaging etc. LDH based nanocomposites with carbon nanotubes; graphene, carbon nanofibers, graphydine etc. have remarkable importance in water splitting, environmental remediation like water and air purification, energy storage like super capacitor electrodes applications etc due to the reason that these hybrids are inherited by the combined properties of both the parental materials. For the usage of super capacitor electrode, LDHs containing transition metals are essential to achieve good results because of their variable valencies which are responsible for high redox activity. The hybrid LDHs are outstanding materials for the production of stimuli responsive systems on the basis of change in some properties by stimulating with heat, pH, light, pressure, magnetic field, and voltage for promising applications such as chemical switches, memory devices, or molecular sensor. The study in this field is found to be less so application of hybrid LDHs in this field has to be improved by properly selecting the candidates that having excellent stimuli response property. The exfoliation of LDH sheets into the polymer matrices by dispersing small amount of organically modified LDHs enhance properties like flame retardancy, mechanical strength, thermal stability etc. It is really a big challenge even though new strategies like pre-exfoliation have already been developed to overcome this problem which is not ecofriendly and economically viable because of the usage of highly expensive and toxic solvents. By properly designing the layered double hydroxides nanocomposites and its uniform distribution into the polymer system through suitable fabrication methods like melt compounding, solution blending etc enhance the dielectric, electric properties etc. thereby extending the applications to electrical, electronic, biomedical engineering fields. Because of these advantages, the fabrication of LDH-based polymer nanocomposites is a rapidly growing area in the research field. New technologies like layer by layer assembly has been extensively utilized for the fabrication of polymer/LDH thin films, core-shell structure with photoluminescence, magnetic, electrical and electronic properties which can be applied in light emitting diodes, targeting drug delivery, super capacitors etc, which is one of the reason for LDH based research more fascinating.

1.11 LDH XRD FTIR SEM TEM AFM MMT

Abbreviations Layered double hydroxide X-ray diffraction Fourier Transform Infrared Spectroscopy Scanning electron microscopy Transmission electron microscopy Atomic force microscopy Montmorillonite

Layered double hydroxides: fundamentals to applications

MAPK OLDH PBAT EP BS EDS DS DBS DRIFTS MS TG/DTA HTXRD BEPH SDS SDBS CNFs CNTs HT GO EDTA 3D-ARGON CD 5-FU PBMA LBL APPP PANI UTF St-LDH PHRR LOI PCL MADSCs PDT ICG KGN TMSCs COL II SOX RGD MTT NIR PAH PAA DAS ARPT P(DMAEMA) PEMFCs SPSU

Mitogen activated protein kinase Organically modified layered double hydroxide Poly (butylene adipate-co- terephthalate) Epoxy Resin 1, 4-butane sultone Energy-dispersive X-ray spectra Dodecyl sulfate anion Dodecyl benzene sulponate anion Diffuse reflectance infrared Fourier transform spectroscopy Mass Spectroscopy Thermogravimetry / Differential Thermal Analysis High temperature X-ray diffraction Bis (2-ethylhexyl) hydrogen phosphate Sodium dodecyl sulfate Sodium dodecyl benzene sulfonate Carbon nanofibers Carbon Nanotubes Hydrotalcite Graphene oxide Ethylene diamine tetra acetate ion Three-dimensional activated reduced graphene oxide nano cup Cell death 5-fluorouracil Poly butyl methacrylate Layer by layer Poly (p-phenylene) anionic derivate Poly aniline Ultra thin film Stearic acid modified LDH Peak of heat release rate Limiting oxygen index Poly (ε-caprolactone) Mouse adipose derived stem cells Photodynamic therapy Indocyanine green Kartogenin Tonsil-derived mesenchymal stem cells Type II A1 collagen Transcription factor Arginyl glycyl aspartic acid (3- (4, 5-dimethylthiazol-2-yl) 22, 5-diphenyltetrazolium bromide) Near infrared Poly- (allylamine hydrochloride) Poly (acrylic acid) 4, 4’diazostilbene-2, 2’-disulfonic acid disodium salt Atom transfer radical polymerization Poly ( (2-dimethylamino) ethyl methacrylate) Polymer electrolyte membrane fuel cells Sulfonated polysulfone

67

68

SPEK SPEEK PEDOT APTS NPA NBR LDPE GLYP D-LDH AD LDH EIS OCP Ph-CDBS EDX HRR TSR SR MWCNT CNT PA-6 PVA GO rGO PpyNF CF PET HPCP PLA PS

Layered Double Hydroxide Polymer Nanocomposites

Sulfonated poly (ether ketone) Sulfonated poly (ether ether ketone) Poly (3, 4-ethylenedioxythiophene) γ-aminopropyltriethoxysilane Nanoplatelet array Nitrile butadiene rubber Low density polyethylene N, N-bis (phosphonomethyl) glycine Decavanadate- intercalated LDH Decavanadate- intercalated and γ-aminopropyltriethoxysilane (APTS) -modified Zn
Al layered double hydroxide Electrochemical impedance spectroscopy Open circuit potential Phytic acid (Ph) and (hydroxypropyl) - sulfobutyl-β-cyclodextrin sodium Energy-dispersive X-ray spectroscopy Heat release rate Total heat release Silicone rubber Multiwalled carbon nanotube Carbon nanotube Polyamide-6 Poly vinyl alcohol Graphene Oxide Reduced Graphene Oxide Polypyrrole nanofiber Carbon fiber Poly (ethylene terephtalate) Hexaphenoxycyclotriphosphazene Poly (lactic acid) Poly styrene

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1162. Balcomb, B., Singh, M., Singh, S., 2015. Synthesis and characterization of layered double hydroxides and their potential as nonviral gene delivery vehicles. ChemistryOpen 4 (2), 137
145. Baskaran, T., Kumaravel, R., Christopher, J., Sakthivel, A., 2014. Silicate anion intercalated cobalt-aluminium hydrotalcite (CoAl-HT-Si): a potential catalyst for alcohol oxidation. Rsc Advances 4 (22), 11188
11196. Basu, D., Das, A., Sto¨ckelhuber, K.W., Wagenknecht, U., Heinrich, G., 2014. Advances in layered double hydroxide (LDH) -based elastomer composites. Progress in Polymer Science 39 (3), 594
626.

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292.

FTIR characterization of layered double hydroxides and modified layered double hydroxides

2

Meisam Shabanian1, Mohsen Hajibeygi2 and Ahmad Raeisi3 1 Faculty of Chemistry and Petrochemical Engineering, Standard Research Institute (SRI), Karaj, Iran, 2Faculty of Chemistry, Kharazmi University, Tehran, Iran, 3Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Arak University, Arak, Iran

2.1

Introduction

Infrared (IR) spectroscopy especially Fourier transform infrared (FTIR) spectroscopy is a technique that has been used over the years in both academia and industry for the structural and compositional analysis of organic, organometallic, polymeric, and inorganic materials, in addition to quality control of raw materials and commercial products. FTIR spectroscopy is a useful tool for functional group identification and quantification. Certain functional groups of an organic or inorganic structure can be identified easily using the FTIR technique. Also, FTIR spectroscopy can be used to confirm a pure compound or to detect the presence of specific impurities. The term “infrared” generally refers to any electromagnetic radiation falling in the region from 0.7 to 1000 μm. However, the region between 2.5 and 25 μm (4000
400 cm21) is the most attractive for chemical analysis. The relationship of the infrared region to other electromagnetic radiations is represented in Fig. 2.1. The “mid-IR” region includes the frequencies corresponding to the fundamental vibrations of virtually all of the functional groups and different bonds of metals in organic and inorganic compounds (Rives, 2001). The absorption bands in FTIR spectra are typically narrow and distinguished, making it possible to identify and monitor an absorption band related to the specific structural feature that is to be modified with a reaction. When a sample is exposed to an infrared beam, various wavelengths of radiation corresponding to the energies of the possible vibrational transitions in the molecule or crystal will be absorbed by the bonds of the sample. The remaining signals are recorded as an absorption band in the spectrum. In this process, those frequencies related to the infrared beam that match the natural vibrational frequencies of the bonds in the molecule are absorbed, and the energy absorbed serves to increase the amplitude of the vibrational motions related to the bonds. Note that a molecule can absorb only selected energies (frequencies) of infrared radiation, and not all bonds in a molecule are capable of absorbing infrared energy, even if the frequency of the Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00002-1 © 2020 Elsevier Ltd. All rights reserved.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 2.1 Electromagnetic spectrum and relationship of vibrational infrared to other radiations.

radiation exactly matches that of the bond motion (Pavia et al., 2008). Only the dipole bonds are capable of absorbing infrared radiation. Some molecules such as H2 or Cl2 are symmetric bonds and cannot absorb infrared radiation (Pavia et al., 2008). An electrical dipole must be present in an asymmetric bond in a molecule that is changing at the same frequency as the incoming energy of radiation to be transferred. Then the changing electrical dipole from the dipole bond can couple with the sinusoidally changing electromagnetic field of the incoming radiation. Thus a nonpolar bond (symmetric bond) that has identical or nearly identical groups on each end will not absorb in the infrared (Pavia et al., 2008). Near-infrared (NIR) spectroscopy is also known as “proton” infrared spectroscopy, as it covers the spectral region in which all the overtone and combination bands of vibrations involving hydrogen appear. The NIR spectral region has been defined by Kaye (1954, 1955) to extend from 700 to 3500 nm (14,285
2860 cm21). The only fundamental vibrations in the NIR region between 4000 and 10,000 cm21 are those associated with hydrogen atoms existing in hydroxyl groups or water in the case of minerals and inorganic compounds like layered double hydroxides (LDH). Whittet et al. (1997) reported average band positions for hydroxyl group and water in the NIR region around 4200 cm21 due to M
OH motions, 5200 cm21 as the H2O combination mode (bending 1 stretching), and around 7100 cm21 as the first OH stretching overtone. It is clear that the main structure can be obtained by mid-IR but NIR spectroscopy could be a suitable technique to study many compounds such as LDH, which contain both water and OH groups in their structure, to obtain more information about the local environments involved. By using the FTIR method, the structures and relative quantities of modifier molecules in the LDH surfaces can be analyzed. However, in some cases the low concentrations and aqueous environment for synthesized LDH can complicate the interpretation of measurement results. The absorbance of trace impurities or background noise can influence the FTIR absorbance result at low

FTIR characterization of layered double hydroxides and modified layered double hydroxides

79

concentrations of LDH. Since water strongly absorbs infrared light, the removal of water, as well as contamination, in the LDH and the nanocomposite films is necessary.

2.2

Fourier transform infrared spectra of layered double hydroxides

LDH or hydrotalcite-like compounds belong to the anionic clay family. The structures of these materials are made on the layers with a brucite-like structure carrying a net positive charge that is balanced by the anions intercalated between the positively charged layers. Positive charge on the electrostatically neutral brucite was created through the substitution of octahedral M21 by M31 cations (Qu et al., 2016a,b; Takehira, 2017; Chubar et al., 2017). One of the ways to identify the structures of LDH and intercalated anions between LDH layers is the FTIR technique. Mumpton et al. (1965) represented for the first time the FTIR spectrum of hydrotalcite like Mg
Al LDH. Also, Ross and Kodama exhibited the characteristic absorption bands of Mg
Al LDH (Ross and Kodama, 1967). In all FTIR spectra of prepared LDH, a broad absorption band was observed around 3480 cm21 with a shoulder band around 3000 cm21, which was related to OH stretching vibration. In general, the structure of LDH can be identified by different characteristic absorption bands in a typical FTIR spectrum. These bands were assigned to four series, as listed here. (1) The OH stretching vibration related to water molecules in the interlayer LDH and metal hydroxide layers, which usually appeared around 3300
3600 cm21. (2) The absorption band around 1620 cm21 related to the OH bending vibration. (3) The characteristic absorption bands in the region of 400
1100 cm21 can be attributed to the metal
oxygen and oxygen
metal
oxygen bands (Shabanian et al., 2014, 2016a,b). (4) The characteristic absorption related to anions in the interlayer LDH usually appeared in the range of 800
1700 cm21.

2.2.1 Fourier transform infrared characteristic absorption bands of layered double hydroxides with different anions 2.2.1.1 Mg
Al LDH
CO322 The OH stretching frequency of the Mg
Al LDH appeared in a broad absorption band in the range of 3300
3500 cm21. The absorption band at about 1600
1650 cm21 was attributed to the bending motion of interlayer water. Generally, the absorption bands related to carbonate anion for asymmetric and symmetric stretching vibration appeared at about 1450 and 880 cm21, respectively. Three different absorption bands at 590, 637, and 667 cm21 were attributed to the M
O and O
M
O (M: Mg or Al) stretching vibration. In the Mg
Al LDH

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Layered Double Hydroxide Polymer Nanocomposites

spectrum a medium absorption band appears at 450 cm21 due to the Al
O bond related to the [AlO6]32 structure (Valcheva-Traykova et al., 1993). Also some weak absorption bands around 3000 cm21 can be related to the OH hydrogen bonded stretching of water molecules intercalated in the LDH layer (Acharya et al., 2007). The main region in FTIR spectrum of Mg
Al LDH
CO322 as a typical LDH is illustrated in Fig. 2.2. In Fig. 2.2, for O
H stretching vibration a broad absorption band as well as a recognizable shoulder can be seen around 3300
3600 cm21. This

Figure 2.2 The approximate region of absorption bands of Mg
Al LDH
CO322.

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81

strong band is broad due to the overlapping of two and/or three possible absorption vibrations of the interlayer water. Due to a combination of lattice or vibrational modes, the brucite-type hydroxides can appear in this area as a strong and broad absorption band (Duan and Evans, 2006). Also due to O
H and CO322 interaction in the interlayer of LDH, the broad absorption band of hydroxyl group can appear as a broad shoulder band. In some FTIR spectra of Mg
Al LDH, a weak band appeared around 3040
3060 cm21, which could be related to water molecules that were solved in the microporosity area of the LDH structure (Chˆatelet et al., 1996). The O
H bond in LDH is shorter than in brucite, and its effect can shift the IR absorption band of the main O
H stretching vibration around 3550
3570 cm21 for brucite to around 3470 cm21 for Mg
Al LDH
CO322 (Kagunya et al., 1998).

2.2.1.2 Mg
Al LDH
NO32 Mg
Al LDH with nitrate anions has been reported many times (Shabanian et al., 2016a,b; Lennerova´ et al., 2015; Xu and Zeng, 2001; Zhao et al., 2012; Zhang et al., 2014; Wang et al., 2011; Nyambo et al., 2008). The characteristic absorption band around 1380 cm21 observed in the FTIR spectrum of all Mg
Al LDH
NO32 samples was attributed to the NO32 group. This intensive sharp absorption band was related to v3 vibrational mode with D3h symmetry in NO32 structure. In many FTIR spectra of Mg
Al LDH
NO32 a sharp and strong characteristic band appeared around 450 cm21, which was related to the metal
oxygen bond in the brucite-like lattice. The FTIR spectrum of Mg
Al LDH
NO32 prepared by the coprecipitation method from aluminum and magnesium nitrate is presented in Fig. 2.3. The characteristic absorption band centered at 3441 cm21 was attributed to the O
H stretching of the  metal
hydroxide layer and interlayer water molecules. The bending vibration of the water interlayer was reflected at 1621 cm21. Also, the NO32 stretching vibration appeared at 1383 cm21. The appeared absorption bands in the range of 580
870 cm21 were related to Al
O and Mg
O stretching modes. The shoulder bands at 2917 and 2856 cm21 were attributed to H2O
NO3 bridging vibration (Hajibeygi et al., 2017).

2.2.1.3 Mg
Al LDH
SO422 The Mg
Al LDH containing SO422 in its interlayer was prepared and the characteristic absorption band related to SO422 was reported. The characteristic absorption band related to sulfate ions in the interlayer LDH appeared at 1000
1300 cm21 (Acharya et al., 2007; Fahami and Beall, 2016). Also, OH as a weak absorption band appeared as two shoulder bands at 2920 and 2852 cm21 due to OH hydrogen bond stretching vibration of intercalated water molecules. In addition, the bands in the range of 500
1000 cm21 are related to M
O, O
M
O, and M
O
M lattice vibrations (M 5 Mg and Al) (Acharya et al., 2007; Obadiah et al., 2012).

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Figure 2.3 FTIR spectrum of Mg
Al LDH
NO32.

2.2.1.4 Mg
Al LDH
PO432 and Mg
Al LDH
HPO422 The Mg
Al LDH with phosphate in its interlayer as well as its corresponding FTIR absorption bands was also reported (Shimamura et al., 2012). The absorption bands related to HPO422 appeared at 1085, 995, and 860 cm21 which can be related to antisymmetric stretching of P
O, symmetric stretching of P
O, and antisymmetric stretching of P
OH, respectively (Dartiguelongue et al., 2016). Three absorption bands related to HPO422 are transformed into a single broad absorption band at 1056 cm21. Also, a shoulder absorption band was obvious at 870 cm21 close to the antisymmetric stretching of P
OH. The characteristic absorption band related to phosphate in the interlayer of Zn
Al LDH was reported by other authors at 1056 cm21 (Costantino et al., 1997; He et al., 2010; Cheng et al., 2010).

2.2.1.5 Mg
Al LDH
Cl2 The preparation of Mg
Al LDH
Cl2 was reported via a coprecipitation method (Yue et al., 2017). The broad bands at 3466 and 1636 cm21 are associated with the stretching and bending vibrations of the 2 OH group of LDH layers and interlayer water molecules. The sharp band observed at 1372 cm21 is due to antisymmetric stretching of the CO322 ion, which may be introduced into the interlayer of Mg
Al LDH by absorption of CO2 during the preparation procedure (Li et al., 2009, 2014). The bands observed below 1000 cm21 (400
850 cm21) correspond to the characteristic lattice vibrations of Mg
O and Al2O3. Chloride did not have a significant and clear absorption band in the FTIR spectrum of LDH.

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83

Figure 2.4 The approximate region of absorption bands of typical interlayer anions.

By comparison of FTIR of LDH with different anions, the approximate region of absorption bands for some common anions are obtained and represented in Fig. 2.4. The absorption band related to the anions such as carbonate, nitrate, phosphate, and sulfate can be found in two regions (Fig. 2.4). Due to the presence of O
H bonds in the hydrogen phosphate anion, its absorption bands appeared in four or more regions.

2.2.2 Fourier transform infrared characteristic absorption bands of layered double hydroxides with different metals The LDH metals could be replaced with different metals (Hajibeygi et al., 2015; Rastin et al., 2017). The FTIR spectra in the range of 400
2000 cm21 related to some LDH with different M21 and Al31 are illustrated in Fig. 2.5. The Ca
Al and Cu
Al LDH were prepared from nitrate salts of pristine metals and for preparation of Ni
Al LDH, nickel chloride and aluminum nitrate solution salts were used. By comparison of these three spectra, it is clear that with changing metals in the LDH structure, their absorption bands were also changed. Some absorption bands in all three spectra are held in common such as the absorption band around 1375
1450 cm21, which is related to the presence of the nitrate ion and the CO322 group because of possible adsorption of CO2 during aging processes (Qu et al., 2016a,b). Also, a broad and weak band was observed at 1644 cm21 in all FTIR spectra related to the bending vibration of water (Zhong et al., 2017). The main differences between these three FTIR spectra are observed in the range of 400
900 cm21 related to typical stretching vibrations of metal oxides and metal hydroxide as well as O
M
O bonds in the LDH structure. In the FTIR spectrum of Ca
Al LDH, three shoulder and individual broad absorption bands appeared around 525, 765
870, and 1023 cm21. All of these bands were related to the typical stretching vibrations of M
O and M
OH (M 5 Ca and Al) in the LDH (Plank et al., 2006; Chen et al., 2015; Perioli et al., 2006).

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Layered Double Hydroxide Polymer Nanocomposites

In FTIR spectrum of Cu
Al the mentioned three absorption bands related to metal oxides and metal hydroxide can be seen, although the bands around 780 cm21 appeared as a broad band. The absorption bands which appeared in the range of 400
1040 cm21 were due to the pulsation of metal
oxygen and oxygen
metal
oxygen as well as metal hydroxide bonds in the brucite type (Chakraborty et al., 2015). Also, the absorption band at 1384 cm21 can be related to the nitrate ion vibration bands (Fig. 2.5) (Sahu and Pugazhenthi, 2011; Chakraborty et al., 2014). In the FTIR spectrum of Ni
Al LDH, a clear difference can be seen at 429 cm21. It can be related to presence of Ni
O bonds in the LDH structure (Chakraborty et al., 2014). Other absorption bands appeared as a broad and shoulder band in the region 500
1050 cm21 attributed to metal
oxide (Ni and Al) in the LDH structure that is typical of this kind of layered solids (Fig. 2.5) (Chakraborty et al., 2014). The absorption band of NO32 was observed as broad and sharp in Ni
Al as compared to Cu
Al and Ca
Al LDH. The bending vibration of interlayer H2O molecules (H\OH) appeared at 1637 cm21 confirming the presence of water molecules as bending modes. Also, other absorption bands in the region of 500
1000 cm21 as broad overlapped bands can be related to metal
oxide vibration modes (Costa et al., 2008). The FTIR spectra of Cu
Fe, Ni
Fe, and Ca
Fe LDH are presented in Fig. 2.6. All of the metals were used as their nitrate salts except for Ni which was used as a chloride salt. The absorption bands related to water bending vibration and nitrate anions are obvious around 1623 and 1355 cm21, respectively (Li et al., 2004).

Figure 2.5 FTIR spectra of Ca, Cu, and Ni
Al LDH.

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85

Figure 2.6 FTIR spectra of Cu
Fe, Fe
Ca, and Ni
Fe LDH.

In FTIR spectra of Cu
Fe and Ni
Fe LDH, the absorption band around 500 cm21 appeared in both spectra and can be related to M
O and M
OH lattice mode vibration (Nejati and Rezvani, 2012; Nejati et al., 2013; Zhang et al., 2010; Li et al., 2010). Due to the presence of chloride and nitrate anions, the absorption band around 1480 cm21 related to carbonate was decayed (Iwasaki et al., 2012). In FTIR spectrum of Ca
Fe, the absorption bands at 467, 588, and 853 cm21 were related to stretching vibration of M
O (M: Ca or Fe) and Ca
Fe
O in the LDH structure (Fig. 2.6) (Ferraro, 2012; Frost et al., 2009; Wu et al., 2012). The absorption band at 588 cm21 can be related to the Fe
O bond (Shabanian et al., 2015, 2016a,b).

2.2.3 FTIR spectra of layered double hydroxides containing three metals In recent years the preparation of ternary metal LDH has been investigated due to their unique application in electronic, magnetic, and optical areas. Ternary metal LDHs in general have better properties as compared to two-metal LDHs. These materials have better crystallinity and a well-defined hexagonal shape (Ma et al., 2010; Han et al., 2008, 2009; Yang et al., 2013). Some ternary LDH derivatives have been prepared by reaction of different M21 cations with M31 cations with different concentrations. FTIR characterizations of these ternary LDH derivatives are reviewed in the following sections.

86

Layered Double Hydroxide Polymer Nanocomposites

It should be mentioned that all infrared spectra of LDH typically showed similar absorption bands, especially at high wavenumber regions such as the O
H stretching mode of the basal layer, the interlayer water, and stretching vibration of the anion (e.g., CO322). Moving to the wavenumber region below 1000 cm21, which showed the information on the absorption bands of the lattice, dual LDH materials showed bands of HO
M
OH and M
O, but in the ternary system usually the absorption bands shifted to higher wavenumbers with increasing concentration of the third ion. It should be noted that these shifts depended on the nature of the third ion. For example a broad absorption band of Cu substitution was found to be comparable to Co substitution. The possible explanation may be due to a Jahn
Teller distortion in Cu21 octahedral compounds that causes c-axis elongation (Fahel et al., 2016). The absorption bands of third cations are usually observed at lower wavenumbers as compared to the corresponding free hydroxide anions. The absorption bands related to different metals in LDHs may appear at different frequencies in the FTIR spectrum. Some ternary LDHs were prepared by different metals and their FTIR spectra were reported (Pe´rez et al., 2012). The characteristic absorption bands related to metal
oxide and O
M
O (M: Zn, Al, and Cr) appeared at around 428, 553, 608, 780, and 938 cm21. Also, for LDH containing Cd, Al, and Cr, the absorption bands appeared at around 419, 489, and 531 cm21. The preparation of ternary metal LDH containing Co, Ni, and Fe with carbonate anion has been reported (Ehlsissen et al., 1993). The FTIR of the above-mentioned LDH had some characteristic absorption bands that confirmed its structure. The OH related to water molecules in the interlayer of LDH appeared at around 3503 cm21 as a broad band due to hydrogen bonding (Ehlsissen et al., 1993). The antisymmetry vibration of carbonate anions in the interlayer was observed at around 1366 cm21 (Hernandez-Moreno et al., 1985). Also, some absorption bands appeared at 1385 and 1525 cm21, which can be attributed to the vibration mode of carbonate. The absorption band in the region 480
800 cm21 was related to metal
oxide vibration and the band at 646 cm21 was related to symmetric bending of carbonate that overlapped with absorption of metal
oxide and shifted to low frequency (Zhang et al., 2008). The FTIR spectrum of Mg
Zn
Al LDH that was prepared by the coprecipitation method has been reported (Eshaq and ElMetwally, 2016). The broad band in the range of 3445
3500 cm21 is ascribed to the O
H stretching vibration of the water molecule and metal hydroxide in the brucite-like layers (Yang et al., 2002). The band at 1640 cm21 is attributed to the O
H bending mode in the water molecule. The weak shoulder band appeared at approximately 3000 cm21 owing to the OH stretching mode of interlayer water molecules hydrogen bonded to interlayer anions. The characteristic bands in the low-frequency region (400
1000 cm21) are related to the metal
oxygen stretching vibrations (Zn
O, Mg
O, and Al
O). In addition, a strong band at 1370 cm21 indicates the presence of CO322 anions in the interlayer region. The FTIR spectra of Ni
Mg
Al LDH and Co
Mg
Al LDH have also been considered. The bands at 3478 and 1663 cm21 were ascribed to the stretching vibrations of the OH group of LDH layers and bending vibration of water molecules in

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87

the interlayer (Parida et al., 2012). The sharp band appeared at 1388 cm21 due to antisymmetric stretching of the CO322 ion and the broad bands between 500 and 800 cm21 are attributed to the characteristic M
O vibrations (Bharali et al., 2015).

2.3

FTIR spectra of organo-modified layered double hydroxides

The use of LDH as a nanofiller to improve thermal, mechanical, and flameretardancy properties of a polymer matrix is one of the recent applications being investigated in both academia and industry (Costa et al., 2007; Manzi-Nshuti et al., 2008; Raeisi et al., 2017). However, the modification of LDH is necessary for preparation of uniform polymer LDH nanocomposites. Different methods have been used in the modification of LDH such as anion exchange (Choy et al., 2000) and regeneration in situ synthesis (Desigaux et al., 2006), etc. These methods have some drawbacks and so can be replaced by a onestep synthesized method (Wang et al., 2009). Accordingly, a one-step method as the correct approach to synthesize organo-modified LDH was reported from solution of metal salts and the anionic surfactant in a reactor (Wang et al., 2009). Many organic compounds that include an anionic segment in their structures have been used for the modification of LDH. In FTIR spectra of organo-modified LDH the presence of characteristic absorption bands related to functional groups of organic modifier can be helpful in investigating their structures. For the FTIR spectra of the organo-modified LDH two types of bands would be expected, one corresponding to the intercalated anionic modifier and the other corresponding to the host LDH material. The approximate region of absorption bands of some functional groups that can be presented in organo-modifiers are represented in Fig. 2.7. It is

Figure 2.7 The approximate region of absorption bands in organo LDH modifiers.

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Layered Double Hydroxide Polymer Nanocomposites

noticeable that the FTIR absorption bands can slightly shift due to different factors such as hydrogen bonding, intramolecular interaction, and chemical structure. Characteristic P
O
C stretching vibration bands are usually observed about 1140 cm21 (symmetric) and 1037 cm-1 (antisymmetric). The P 5 O stretching vibrations are indicated by a strong band about 1225 cm21. The antisymmetric and symmetric stretching vibrations of COO2 (carboxylate anion) have usually been observed about 1570 and 1440 cm21, respectively. The corresponding FTIR bands of organic modifiers are expected to shift toward a lower wavenumber in comparison to their free-state absorption bands, as more energy is required for executing such vibrations due to the presence of restriction between the layers. The bands in the range 1000
1800 cm21 are mostly due to the functionalities of the modifier and also due to interlayer water molecules. The appearance of characteristic bands for CO32
(γ) means that some CO32
still exists in the interlayer region of the modified LDH. This is perhaps caused by partial free movement of the CO32
ions due to enlargement of the interlayer region after organic modification. Most of the modified LDH materials have exhibited strong absorption bands in the range 2850
3100 cm21, corresponding to the
CH
stretching vibration arising from the hydrocarbon tail present in each modifier. In FTIR spectra of the modified LDH, the presence of interlayer water is not clear. In this regard, the only difference observed is the disappearance of a weak band (in the form of a shoulder) in the region 3000
3100 cm21, which originates from the interaction between O
H groups and CO32
ions. One of the common organic compounds used as an organo-modifier of LDH is sodium dodecylbenzene sulfonate (SDBS). This organic sodium salt can change with the anions in the interlayer of LDH via an ion exchange reaction (Wang and O’Hare, 2012). The organo-modified LDH can be prepared by a one-step reaction from metal salt in the presence of organic modifier salt in sodium hydroxide solution with pH 5 10 (Wang et al., 2009). The structure of SDBS is represented in Fig. 2.8. In LDH
SDBS, the characteristic OQSQO stretching vibration bands have appeared about 1040 cm21 (antisymmetric) and 1070 cm21 (symmetric), whereas the corresponding bands in some modifiers appear at 1230 and 1186 cm21, respectively. The C
S stretching vibration band is also observed in the range of 610
630 cm21. SDBS additionally has shown multiple bands corresponding to the aromatic ring C
C vibrations in the range 1450
1610 cm21. In general, the presence of absorption bands of SDBS in FTIR spectrum of the modified LDH indicates a good intercalation anionic structure in the interlayers of

Figure 2.8 Molecular structure of SDBS.

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89

LDH. The important bonds of SDBS are aromatic C
H and aliphatic C
H, double bond carbon
carbon groups, which showed the bands around 3020
3100, 2930
2990, and 1550
1650 cm21, respectively. The bands related to SO32 appeared at 1037 cm21 as well as 1182 cm21 (symmetric and antisymmetric). The mentioned region absorption bands for SDBS in the FTIR spectrum of a typical LDH
SDBS are illustrated in Fig. 2.9. The FTIR spectrum of a typical SDBS-modified Zn
Al LDH is illustrated in Fig. 2.10. The characteristic absorption bands can be observed at 2858, 2927, and 2962 cm21 which are related to the antisymmetric and symmetric CH3 and CH2 group vibration modes resulting from the long alkyl chains of the SDBS anion. The absorption bands at 3063, 1600, and 1131 cm21 are attributed to the aromatic CH

Figure 2.9 The approximate region of absorption bands of SDBS in a typical LDH
SDBS.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 2.10 FTIR spectrum of SDBS-modified Zn
Al LDH.

stretching, double bond aromatic carbon
carbon and C
H aromatic in-plane bending of SDBS, respectively. The two strong absorption bands at 1036 and 1172 cm21 are attributed to symmetric and antisymmetric stretching vibration of SO32 bands in the SDBS structure. Also, the absorption band around 1398 cm21 is related to NO32 bands which are maintained from metal salt in the preparation processes of LDH (Xu et al., 2013; Pavel et al., 2012; Hajibeygi et al., 2015). Other absorption bands in FTIR are related to the LDH structure. One of the other common organic modifiers for LDH is sodium dodecyl sulfate (SDS). The FTIR spectrum of SDS is similar to that of SDBS with only slightly modification. In SDS molecular structure there is no aromatic ring, therefore there are no absorption bands related to aromatic C
H and double bond carbon
carbon of the aromatic ring (Xu et al., 2013). The structure of SDS is presented in Fig. 2.11. The aspartic acid-modified Li
Al LDH was prepared and used for preparation of poly(ethylene terephthalate) LDH nanocomposites (Bunekar et al., 2016). In this work the organo-modified LDH was prepared using a two-step reaction. At first neat Li
Al LDH was synthesized from LiNO3  3H2O and Al(NO3)3  9H2O, and then organo-modified LDH was prepared via an ion exchange reaction with aspartic acid solution salt. The structure of aspartic acid is presented in Fig. 2.12. The FTIR spectra of LDH and aspartic acid-modified LDH were reported. The aspartic acid-modified Li
Al LDH spectrum exhibited some bands, included an absorption band related to the neat LDH as well as absorption bands related to functional groups of aspartic acid as modifier. A broad absorption band around

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91

Figure 2.11 Molecular structure of SDS.

Figure 2.12 Molecular structure of aspartic acid.

Figure 2.13 Molecular structure of lauric acid.

3200
3500 cm21 appeared which can be ascribed to the hydrogen-bonded hydroxyl groups from both the hydroxide layers and interlayer water molecules. The antisymmetric vibration bands of NO32 or CO322 appeared at 1354 cm21. The absorption bands at 530 and 740 cm21 can be attributed to the metal
oxide stretching modes. Characteristic absorption bands related to alkyl C
H stretching vibration were observed in the region 2800
3000 cm21. The antisymmetric and symmetric stretching modes of the carboxylate group appeared at 1527 and 1404 cm21, respectively. The lauric acid-modified Mg
Al LDH was prepared by Katiyar et al. (2010). In this research work, laurate-modified LDH was prepared and used in the preparation of nanocomposite based on polylactic acid. The structure of lauric acid is illustrated in Fig. 2.13. The FTIR spectra of LDH, organo-modified LDH, and lauric acid are considered. In the FTIR spectrum of the neat LDH, the broad absorption band in the range of 3200
3700 cm21 related to OH stretching vibration as well as a shoulder band at 3000 cm21 indicating that the hydrogen bonding between water molecules and carbonate ions was observed (Komarewsky et al., 1953). The absorption bandrelated to bending vibration of water molecules appeared at 1638 cm21. The characteristic absorption bands attributed to the carbonate ion in the interlayer also appeared. Two absorption bands of the remaining nitrate ions were also observed at 1384 and 830 cm21 (Hansen et al., 1994) and the absorption band of lattice vibration of metal
oxide was observed at 411 cm21. In the FTIR spectrum of laurate-modified LDH some new absorption bands appeared and some absorption bands disappeared. The disappearance of the

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Layered Double Hydroxide Polymer Nanocomposites

absorption band related to carbonate ion at 1373 cm21 and the appearance of absorption bands related to carboxylate groups indicated the lauric acid salt presence in the interlayer of LDH. The associated absorption bands of the carbonyl group stretching vibration of carboxylate appeared at 1561 and 1411 cm21 related to antisymmetric and symmetric stretching vibration of laurate COO2 in the anionic form (Borja and Dutta, 1992; Venkataraman and Vasudevan, 2000). Also, the absorption band for stretching vibrations related to antisymmetric and symmetric stretching of CH2 groups that appeared in the range of 2953
2858 cm21, while the CH2 bending vibration and CH2 rocking vibration appeared as single bands at 1467 and 721 cm21, respectively. Lauryl alcohol phosphoric acid ester potassium also was used as an organic modifier of LDH (Xie et al., 2016). The FTIR spectrum of the mentioned organomodified LDH revealed absorption bands at 438 and 3434 cm21 due to the presence of metal
oxide bonds and water molecules in the interlayer of LDH. Also, the absorption band at 2952 cm21 was related to symmetric and antisymmetric vibration of aliphatic C
H groups in the organic modifier structure. Manzi-Nshuti et al. (2009) reported the preparation of oleate-modified Zn
Al LDH. The oleate-modified LDH was prepared by coprecipitation method (Wang et al., 2005). The molecular structure of oleic acid is presented in Fig. 2.14. The FTIR spectrum of oleate-modified Zn
Al LDH revealed the absorption bands related to LDH as well as absorption bands related to oleate carboxylate salt as an organic modifier (Xu et al., 2004; Hibino, 2004). The symmetric and antisymmetric mode attributed to C
H aliphatic groups appeared around 2800
3000 cm21. Two strong bands appeared around 1400
1600 cm21, which was attributed to symmetric and antisymmetric carboxylate bands in oleate salt. An absorption band that appeared at 3006 cm21 as a weak band is related to C
H attached to a carbon
carbon double bond (Simons, 1978). The molecular structure of taurine (2-aminoethanesulfonic acid) is presented in Fig. 2.15. This organic compound was used in the preparation of organo-modified Mg
Al LDH (Lennerova´ et al., 2015).

O OH

Figure 2.14 Molecular structure of oleic acid. O

HO S O

Figure 2.15 Molecular structure of taurine.

NH2

FTIR characterization of layered double hydroxides and modified layered double hydroxides

93

In this work the FTIR spectra of neat LDH, modified LDH, as well as taurine as modifier, were compared. In the FTIR spectrum of modified LDH, the vibrational absorption bands related to taurine can be found. Three characteristic absorption bands appeared at 1241, 1184, and 1046 cm21, which were related to the SO32 group in the modified LDH structure. The stretching vibration of C
N appeared at 1046 cm21. Also, the absorption bands at 1512, 1305, and 1114 cm21 were attributed to different vibrations of the NH2 group. In some cases the bands attributed to 2 NH31 that were observed in the pure powders decreased significantly or vanished in the corresponding modified LDH. This can be attributed to the interaction between the N atom and the metal ions in the layers of LDH. The stretching vibration of CH2 groups in the taurine structure appeared at 1305, 963, 894, and 741 cm21 (Ohno et al., 1992). A sharp absorption band was observed in the FTIR spectrum of the neat LDH at 1383 cm21, which was related to nitrate anions in the interlayer of LDH. This absorption band appeared as a weak band in the FTIR spectrum of modified LDH. It can be related to a trace of nitrate anions remaining in the rehydrated LDH. Two organic compounds included 2-naphthalene sulfonate and 2,6-naphthalene disulfonate, which contain a naphthalene ring with one and two sulfonate groups being used for modification of LDH (Kameda et al., 2006). The structures of two aromatic modifiers are presented in Fig. 2.16. The FTIR spectrum of modified LDH indicated the naphthalene containing organic modifier anions in the interlayer region of LDH. In FTIR spectra of 2naphthalene sulfonate- and 2,6-naphthalene disulfonate-modified LDH, the absorption bands related to the organic modifier appeared as well as absorption bands related to the LDH structure. The absorption bands related to metal
oxide bonds appeared in the region of 500
1000 cm21. Also, the OH hydroxyl group in water molecules appeared as a broad band centered at 3500 cm21. The aromatic CH stretching vibration related to the naphthalene ring appeared around 3000
3100 cm21 and the stretching vibrations of the SO32 group have been observed at around 1120
1150 cm21 (Kameda et al., 2008). For the preparation of chiral organo-modified LDH, N,N0 -(pyromellitoyl)-bis-Lisoleucine diacid was used for modification (Mallakpour and Dinari, 2013). The molecular structure of N,N0 -(pyromellitoyl)-bis-L-isoleucine diacid as an organic modifier is shown in Fig. 2.17. The FTIR spectrum of organo-modified LDH showed two types of absorption bands: one related to the anionic organic modifier intercalated between LDH layers and the other attributed to the LDH structure.

SO3H

SO3H

HO3S

Figure 2.16 Molecular structures of naphthalene derivative modifiers.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 2.17 Molecular structure of a chiral organic modifier.

Figure 2.18 Molecular structure of diacid-diimide as an organic modifier of LDH.

An absorption broad and shoulder band was observed in modified LDH in the range 1600
1640 cm21, indicating the presence of H2O molecules as a bending vibration appeared in this region (Kloprogge et al., 2004). The characteristic absorption bands at 2930
3100 cm21 related to the aliphatic and aromatic C
H modes in the dicarboxylate salt have also been observed. FTIR spectra of Mg
Al LDH and diacid-diimide-modified Mg
Al LDH were reported in a research work (Hajibeygi et al., 2017). A diacid-diimide organic compound containing imide heterocyclic ring and aliphatic long chains was synthesized and used for modification of LDH. The molecular structure of diacid-diimide is shown in Fig. 2.18. The neat Mg
Al LDH was prepared via the coprecipitation method from magnesium and aluminum nitrate. The modified LDH was prepared by an ion exchange reaction between the neat LDH and carboxylate dianion salt of organic modifier. The FTIR spectra of neat LDH (A) and modified LDH (B) are shown in Fig. 2.19. In the FTIR spectrum of neat LDH (LDH
NO32), a broad and strong absorption band related to OH stretching vibration due to the presence of interlayer water molecules and metal hydroxide layers appeared at 3466 cm21. The bending vibration of the interlayer water molecules was reflected as a broad band centered at 1618 cm21. Also, the characteristic absorption band was observed at 1378 cm21 related to nitrate interlayer anions stretching vibrations. The Al
O and/or Mg
O as well as M
O
M (M: Al and/or Mg) stretching modes appeared as broad and shoulder absorption bands around 590
890 cm21. The FTIR spectrum of modified LDH revealed the absorption bands related to neat LDH as well as absorption bands attributed to an organic modifier. The shoulder absorption bands, which appeared around 2850
2930 cm21, were related to stretching vibration of CH aliphatic groups in the organic modifier structure. Two clear absorption bands at 1774 and 1711 cm21 were related to antisymmetric and symmetric stretching vibration of carbonyl in an imide heterocyclic ring in the

FTIR characterization of layered double hydroxides and modified layered double hydroxides

95

Figure 2.19 FTIR spectra of (A) neat LDH and (B) modified LDH.

organic modifier. The absorption band at 1383 cm21 was related to C
N vibration mode of the imide group. It is clear that to understand FTIR spectra of organo-modified LDH, comparison of FTIR spectra of neat LDH, neat modifier, and organo-modified LDH would be a promising method. However, some shift would be expected in the FTIR bands of organo-modified LDH as compared to the neat structures. These can be attributed to the new van der Waals forces, including repulsion and attraction between the atoms.

2.4

Conclusion

The infrared spectroscopic method is an excellent technique to study the structure of LDH and modified LDH. The infrared absorption bands identify molecular components and structures. This technique measures the absorption of infrared radiation by the sample material versus the wavelength. The presence of different polar bonds, LDH sheet structures, and anions in the interlayers (inorganic and organic compounds) can easily be detected using FTIR spectroscopy. To achieve good results it is necessary to compare FTIR spectra of the neat LDH, modifier, and modified LDH. The main functional groups and sharp absorption bands can be helpful in investigating the structures. All LDH showed some similar bands in FTIR spectra, such as a broad absorption band in the range 3300
3500 cm21

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Layered Double Hydroxide Polymer Nanocomposites

attributed to O
H stretching mode of the basal layer and the interlayer water, the band at 1620
1650 cm21, assigned to the bending mode of the interlayer water, the anion bands, and the band related to M
O and M
OH between 400 and 800 cm21. For modified LDH two types of bonds are expected, one corresponding to the intercalated anionic modifier and the other corresponding to the host LDH material. Both the anionic modifier and LDH bands showed some shift in the FTIR bands of organo-modified LDH which can be attributed to the formation of new van der Waals forces in organo-modified LDH.

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Fabrication technologies of layered double hydroxide polymer nanocomposites

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Shadpour Mallakpour1,2,3 and Farbod Tabesh3 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 3Chemistry Group, Pardis College, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

3.1

Introduction

3.1.1 Layered double hydroxides Layered double hydroxides (LDHs) are one of the layered material types with metal oxide (positively charged)/hydroxide sheets with accommodated water molecules and exchangeable anions in the interlayer space. These materials are also known as hydrotalcite-like anionic clay, due to their resemblance to the natural structure of clays (e.g., montmorillonite), which   have been infixed with cations and water x1 p2 31 M21 M ð OH Þ ½A :mH OÞ and have a typical thickness of 0.5 nm 2 2 12x x x=p (Brnardi´c et al., 2013; Velasco et al., 2012; Yu et al., 2015, 2016). Octahedral M (OH)6 units form layers of LDH, where the metal (M) is coordinated by six hydroxyl groups (OH), therefore forming M(OH)2 brucite-like sheets. As shown in Fig. 3.1 (Richetta et al., 2016), divalent and trivalent metal cations and hydroxyl anions will occupy centers of the octahedral units and vertices, respectively (Velasco et al., 2012). Thus, various compositions can be prepared from the most prevalent Mg-Al LDHs to rare CaAl-LDHs. Also, monovalent cations (such as LiAl-LDHs) can be replaced by divalent cations and another type of LDH can be prepared where the trivalent cation is altered (most commonly to Fe31) (Brnardi´c et al., 2013). This possibility to prepare a wide range of LDHs created superiority over natural clay and the impact of the cation type on the specifications of LDHs and polymer/ LDH nanocomposites (NCs) is the aim of the research. Two general methods exist for the preparation of LDH nanosheets, which have been schematically illustrated in Fig. 3.2, top-down (delamination) and bottom-up (controlled nucleation), of which top-down is the most common (Wang and O’Hare, 2012).

Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00003-3 © 2020 Elsevier Ltd. All rights reserved.

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Figure 3.1 Schematic view of the general structure of (Zn, Al) LDH, with Cl2 and NO32 anions intercalated in the brucite-like structure. Other possible chemical species eventually presented in the interlamellar space are shown. The basal spacing, d, is also indicated. Source: Adapted from Richetta, M., Digiamberardino, L., Mattoccia, A., Medaglia, P., Montanari, R., Pizzoferrato, R., et al., 2016. Surface spectroscopy and structural analysis of nanostructured multifunctional (Zn, Al) layered double hydroxides. Surf. Interf. Anal. 48, 514
518. With kind permission of John Wiley and Sons.

Figure 3.2 Schematic synthesis of top-down and bottom-up methods for LDH single layers. Source: Adapted from Wang, Q., O’Hare, D., 2012. Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets. Chem. Rev. 112, 4124
4155. With kind permission of American Chemical Society.

The top-down method entails the modification of the interlayer environment of LDHs and, subsequently, the selection of a suitable solvent system, while, in bottom-up synthesis, an aqueous coprecipitation system is required (Omwoma et al., 2014).

3.1.2 Modification of layered double hydroxides Neighboring sheets in the LDH structure forcefully imbibe each other by hydrogen bonding and electrostatic attraction. The basal spacing (usually under 1 nm)

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between LDH layers prohibits polymer or monomer penetrating into the layers. Another barrier to homogeneous dispersal of LDH layers in the polymeric matrix is incompatibility of the LDH layers with polymer molecules which have hydrophilic and hydrophobic natures, respectively. The modification of LDHs to boost the interlayer spacing (Mallakpour and Khadem, 2017b) and diminish the hydrophilicity of the surface of the layer is an approach to overcome the barrier (Brnardi´c et al., 2013). There are four general methods to insert anions into the LDHs: direct synthesis, anionic exchange, reconstitution (rehydration) (Velasco et al., 2012), and mechanochemical approaches (Qu et al., 2016)

3.1.2.1 Direct synthesis The direct synthesis is conducted by coprecipitation and is the most common pathway to synthesize LDHs with inorganic anions. In this method, the desired anion is added to the solution of metal salts (usually chloride or nitrate salts) that will form the layers. To prevent coprecipitation of other phases such as oxide contamination from the metals, control of the pH is necessary. The tendency of the organic anions to hydroxide layers avoids the incorporation of metal salts anions used in the synthesis. The ability to control the charge density of the layers (M21 to M31 ratio) and producing high pure LDHs are the main benefits of this method.

3.1.2.2 Anionic exchange The anionic exchange route is very convenient and includes the dispersion of LDH precursor in a solution containing an excess of anions (e.g., PO432 can be incorporated) (Alibakhshi et al., 2016). Before picking out the precursor, it is crucial to be sure about the affinity of the anions with the LDH structure. Therefore, an enormous difficulty is carbonate anions, owing to the high tendency of the layers for small divalent anions. On the other side, among the most common monovalent anions presented in LDHs, hydroxide anions face the difficulty of the interchange.

3.1.2.3 Reconstitution (rehydration) In the reconstitution of LDHs, after being converted to oxides through calcination at temperatures between 500 C and 800 C, they can rehydrate to the original form in the presence of anions and water, which is called memory effect. A wide range of LDHs such as carbonates, organic-like naphthalene carboxylates, or carboxylates can be prepared with inorganic anions by this method. Despite the mentioned methods which are widely used, many issues should be considered, for example, treatment of aqueous waste, complex process, high energy expenditure, etc.— mechanochemical methods can effectively transcend these difficulties.

3.1.2.4 Mechanochemical approaches The mechanochemical methods are divided into two types: (1) a mechanohydrothermal process, in which at first precursors are ground and then hydrothermal

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treatment is done with a proper material (Zhang and Hou, 2018), and (2) direct mechanochemical synthesis, which has single-step grinding (including dry and wet grinding) and two-step grinding (dry and wet grinding) methods. Table 3.1 provides a comparison between these two methods. Irrespective of the method, the main obstacle in the preparation of high-purity hybrids is their affinity to carbonates, in which specific conditions should be conducted to avoid the presence of this anion. Thus, the alternative is the use of water-free carbonates (bidistilled water is normally used) and an inert atmosphere (nitrogen or other inert gases). Inorganic anions such as carbonates, chlorides, nitrates, and sulfates are the most common interlayer anions. The most used LDH is hydrotalcite, where magnesium and aluminum are the constituents, and carbonate, are the
main interlayer anions. Their typical composition formula is 31 22 Mg21 12x Alx ðOHÞ2 ðCO3x=n  mH2 OÞ (0.2 # x # 0.33) (Velasco et al., 2012).

3.2

Preparation of polymer/layered double hydroxide nanocomposites

3.2.1 Introduction Pure polymers suffer from some weaknesses such as poor mechanical strength, thermal stability, gas permeability, fire retardancy, adsorption capacity, etc. Therefore, some fillers such as SiO2 nanoparticles (NPs), CaCO3 NPs, carbon nanotubes, clay, graphene, cellulose, carbon dots, biochar, metal oxides, etc. can be used to cover those weaknesses. Due to the new mechanical, thermal, and optical properties, which seldom exist in the virgin macromolecule, the polymer/LDH NCs have grabbed significant attention in the material chemistry field. LDH would improve these limitations of polymers, for example, layer structure of LDHs can reduce the gas permeation through the polymeric matrix, existence of metal in the LDH Table 3.1 Advantages and disadvantages of the mechanochemical approaches (Qu et al., 2016) Approaches

Advantages

Disadvantages

Mechanohydrothermal process

Regular hexagonal shape particles Highly dispersed high crystallinity at low pressure Shorter time reaction The regular hexagonal shape of particles Salts, oxides, and hydroxides all possible raw materials Ease of operation Particle agglomeration

Solvent involvement Raw materials limited to soluble salts Energy waste Solvent involvement

Direct mechanochemical syntheses

Limited to Fe-based LDH Low crystallinity

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structure increases the char yield, thermal stability, and fire retardancy; the large surface area of this material can drastically increase the adsorption capacity of the polymer. The level of distribution (intercalation or exfoliation) of LDH layers into the polymeric matrix will determine the performance of the NCs, and NCs with exfoliated LDHs have favorable properties in comparison with NCs with intercalated LDHs, due to further dispersal into the polymeric matrix. On the other hand, the exfoliation of the LDHs into the polymer becomes more unfavorable due to powerful electrostatic interactions of the interlayer, which is the result of the great charge density (B300 mequiv/100 g) of LDH layers. As mentioned earlier, giant polymer chains cannot penetrate into the basal spacing of pristine LDH unless this distance is significantly increased. The three routes used for the fabrication of layered double hydroxide polymer nanocomposites shown in Fig 3.3 are: (1) intercalation of the monomers in the interlayer region of LDH or modified LDH and in situ polymerization resulting in intercalation, exfoliation, or both morphologies to the polymer nanocomposites; (2) direct intercalation of polymers into the interlayer region of LDH or modified LDH resulting in intercalated or exfoliated or both morphologies to the nanocomposite. The several methods coming under this category are: melt mixing, solvent blending, two-roll mill mixing, melt-spinning, freeze-drying, and melt extrusion; (3) preexfoliation of modified LDH with suitable solvents and restacking of the exfoliated layers over the polymer either by using polymers directly or monomer followed by in situ polymerization, which results in exfoliation, intercalation, or both morphologies to the nanocomposite. The several methods under this category are layer-by-layer (LbL) assembly, solvent blending, spin-spray LbL, and mechanical agitation. Intercalation of the monomers (in situ polymerization), in which the monomers are dissolved in LDH suspension and intercalated into LDH interlayer spaces. Then, the polymerization (radical, cationic, anionic, condensation, ring opening, etc.) will occur using a suitable initiator (depending on the type of polymerization), accelerator, and crosslinker (if needed) followed by exfoliation of LDH nanosheets within the polymer matrices. In some cases, an emulsifier agent is needed for better dispersion of hydrophobic substances (Fig. 3.4). Direct intercalation of expanded macromolecule chains, in fact, the polymer instead of monomer will be intercalated and lead to exfoliation of LDH nanosheets. In this method, modification of LDH with organic anions to increase the gallery spaces is required. This method contains several sub-categories, as described below. In melt mixing, the materials are melted in a mixer at desired conditions (specific temperature, pressure, roller speed, and time) and then mixed to produce a polymer/LDH NC. Predrying should be done to prevent moisture interference (Kong et al., 2018b). This method leads to less dispersion of LDH in the polymeric matrix. A wide range of polymer/LDH NCs can be produced with intercalated or partially exfoliated structures. The intercalation of polymer into the interlayer space can determine the degree of intercalation or exfoliation of the obtained polymer/ LDH NCs. One of the most important advantages of this method is independency on the solvent. Another advantage of this method is compatibility with common plastic processing technologies (Fig. 3.5).

Figure 3.3 Pathway of NC preparation by (A) monomer exchange and in situ polymerization, (B) direct polymer exchange, and (C) restacking of the exfoliated layers over the polymer. Source: Adapted from Wang, Q., O’Hare, D., 2012. Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets. Chem. Rev. 112, 4124
4155. With kind permission of the American Chemical Society.

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Figure 3.4 In situ polymerization. Source: Adapted from Velasco, J.I., Ardanuy, M., Antunes, M., 2012. Layered double hydroxides (LDHs) as functional fillers in polymer nanocomposites. In: Gao, F. (Ed.), Advances in Polymer Nanocomposites. Woodhead Publishing Cambridge, United Kingdom, pp. 91
130. With kind permission of Elsevier.

Figure 3.5 Melt mixing. Source: Adapted from Velasco, J.I., Ardanuy, M., Antunes, M., 2012. Layered double hydroxides (LDHs) as functional fillers in polymer nanocomposites. In: Gao, F. (Ed.), Advances in Polymer Nanocomposites. Woodhead Publishing Cambridge, United Kingdom, pp. 91
130. With kind permission of Elsevier.

Solvent blending requires dissolution of polymer and dispersion of LDH into a proper solvent. If the solvent is not water (like xylene), predrying for removal of moisture is essential (Suresh et al., 2018). The solvent is evaporated from the polymer/LDH NCs through several pathways, such as casting of the NCs to prepare NC films. In this method, the polymer can penetrate into LDH layers easier, and hence, this method causes better dispersion of LDH due to freedom of mobility of LDH in the solvent. Using organic solvents (in some cases) makes this method quite expensive (Fig. 3.6). Two-roll mill mixing is mainly used to fabricate rubbers. In this method, firstly, the polymer is added to a mixer, after a specific time, the LDH is included in this mixer. The mixing procedure breaks down the powder LDH particles, subsequently, diffusion of the particles and exfoliation of the layers occur. Depending on the conditions and procedure, after all these processes, the mixture is kneaded on to a

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Layered Double Hydroxide Polymer Nanocomposites

Figure 3.6 Solution blending. Source: Adapted from Velasco, J.I., Ardanuy, M., Antunes, M., 2012. Layered double hydroxides (LDHs) as functional fillers in polymer nanocomposites. In: Gao, F. (Ed.), Advances in Polymer Nanocomposites. Woodhead Publishing Cambridge, United Kingdom, pp. 91
130. With kind permission of Elsevier.

two-roll mill and the required additives, like accelerators and curing agents, are added (Kong et al., 2018a). Melt-spinning is suitable for fabrication of synthetic fibers (Aranishi and Nishio, 2017). This method entails mixing of the polymer and LDH powder in an extruder (sometimes a compatibilizer is needed to improve the miscibility) and melting of a polymer along with LDH in a cylinder and the mixed materials are forwarded through a die at a specific flow rate (Kutlu et al., 2013) (Fig. 3.7). Freeze-drying is suitable for preparing polymer/LDH aerogels. Thus, first, a suspension or gel of polymer/LDH using water-soluble/dispersive polymer and LDH is prepared, then, this suspension or gel is frozen at low temperatures (using carbon dioxide/solvent bath, liquid nitrogen, etc.), and finally, the ice is sublimed with a high vacuum to gain aerogels (Chen and Schiraldi, 2018). During the freezing part, the microstructure of the aerogels is formed. The lower temperature in the freezing section leads to a larger number of ice nuclei, smaller crystals, and a denser structure. Also, the lower viscosity of the nanocomposite leads to better growth of the ice in a large lamellar ice layer, leading to obtaining a good NC aerogel (Fig. 3.8). Melt extrusion, in this route dry polymer and LDH (and in some cases a compatibilizer) or any ingredient [like pharmaceuticals (Patil et al., 2016)] are placed in an extruder with specific conditions including the desired temperature, rotation speed, time, and shear force (Bunekar et al., 2018). This method is a solvent-free method, therefore, dispersion of LDHs into the polymeric matrix occurs with the aid of heating, but the dispersion state in this manner is not as good as with the solvent blending method (Fig. 3.9). Preexfoliation is followed by mixing with the macromolecule. In this method, first, the LDH in a colloidal solution is laminated into nanolayers using a proper solvent or devices such as ultrasonics, then mixing with the polymer will happen (Nagendra et al., 2015; Wang and O’Hare, 2012). This method is divided into its subgroups as described below.

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Figure 3.7 Illustration of a typical melt-spinning process. Source: Adapted from Aranishi, Y., Nishio, Y., 2017. Cellulosic fiber produced by melt spinning. In: Blends and Graft Copolymers of Cellulosics. Springer, pp. 109
125. With kind permission of Springer.

Figure 3.8 The formation of microstructures of polymer/clay aerogel composites. Source: Adapted from Chen, H.-B., Schiraldi, D.A., 2018. Flammability of polymer/clay aerogel composites: an overview. Polym. Rev. 1
24. With kind permission of Taylor and Francis.

Layer-by-layer (LbL) assembly involves a core or sheet (polymer or other substrate) covered with a layer of a negatively charged polymer [like poly(ethylene terephthalate) (Wang et al., 2017), poly(sodium 4-styrene sulfonate) (PSS) or other polyelectrolytes (Li et al., 2005)] and then the LDH sheets are deposited on the core or sheet by dipping the substrate in the LDH suspension. This process can be repeated several times. The growth of the film can be controlled by, for example,

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Layered Double Hydroxide Polymer Nanocomposites

Matrix API Feeder Feed rate output Feed rate input Screw speed input

Barrel temperatures output

Material Spectrum pressure

Motor

Die Motor

Torque Screw speed output

Barrel temperatures input

Material temperature

Figure 3.9 Schematic of a typical extruder system. Source: Adapted from Patil, H., Tiwari, R.V., Repka, M.A., 2016. Hot-melt extrusion: from theory to application in pharmaceutical formulation. AAPS PharmSciTech. 17, 20
42. With kind permission of Springer.

Figure 3.10 Schematic illustration for the assembly of (U-mLDH/NBR)n film on PET substrate by the layer-by-layer assembly technique. Source: Adapted from Wang, L., Dou, Y., Wang, J., Han, J., Liu, L., Wei, M., 2017. Layerby-layer assembly of layered double hydroxide/rubber multilayer films with excellent gas barrier property. Comp. Part A: Appl. Sci. Manuf. 102, 314
321. With kind permission of Elsevier.

microgravimetry, which changes in frequency, showing an increase in the mass of layers (Katagiri et al., 2018) (Fig. 3.10). In solvent blending, a polymer or copolymer is dissolved in a suitable solvent (whether water or other solvents) along with dispersion of the desired amount of the

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modified LDH (preexfoliated) and then the mixing occurs in one pot with a better dispersion state of LDH in a polymeric matrix. The obtained mixture in most cases is cast to evaporate the solvent from the NC (Chakraborty et al., 2016). In this method, the polymer can penetrate into LDH layers not only due to the existence of the solvent which increases the mobility but also owing to the presence of a modifier in the LDH layers, which increases the gallery space of the LDH (Fig. 3.11). In the spin-spray LbL method, polymer or polyelectrolytes are dissolved in water as a solvent and the modified LDH is dispersed in water. The mixture is sprayed in a rotating substrate by some nozzles (depending on the number of materials) which are fed by pumps and nitrogen gas. Control of the nozzles, spraying time, and drying time are monitored by a microcontrolled circuit. The spraying and drying time can vary based on the materials. Also, the rinse stage can be run between each deposition (Larocca et al., 2018) (Fig. 3.12). In the mechanical agitation method, polymeric matrix and desired modified LDH are blended through mechanical agitation at a specific temperature to obtain a homogeneous suspension. Then the mixture can be moved into the preheated mold to gain a specific shape (Zhu et al., 2018). By this method, modified LDH is dispersed in the polymers, an advantage of this method is that solvent may not be used (in the case of resins) (Fig. 3.13). This chapter has presented researches about the preparation of polymer/LDH NCs based on natural and synthetic polymers and their characterization and applications.

Figure 3.11 Schematic diagram for the preparation of PMMA/ONi-Al LDH nanocomposites. Source: Adapted from Chakraborty, S., Kumar, M., Suresh, K., Pugazhenthi, G., 2016. Investigation of structural, rheological and thermal properties of PMMA/ONi-Al LDH nanocomposites synthesized via solvent blending method: effect of LDH loading. Chin. J. Polym. Sci. 34, 739
754. With kind permission of Springer.

Figure 3.12 Schematic of spin-spray layer-by-layer apparatus. For the sake of clarity, the gas and liquid connections are shown for only one nozzle. Source: Adapted from Larocca, N.M., Bernardes Filho, R., Pessan, L.A., 2018. Influence of layer-by-layer deposition techniques and incorporation of layered double hydroxides (LDH) on the morphology and gas barrier properties of polyelectrolytes multilayer thin films. Surf. Coat. Technol. 349, 1
12. With kind permission of Elsevier.

Figure 3.13 Reaction scheme of MCLDH microcapsules. Source: Adapted from Zhu, P., Gu, Z., Hong, S., Lian, H., 2018. Preparation and characterization of microencapsulated LDHs with melamine-formaldehyde resin and its flame retardant application in epoxy resin. Polym. Adv. Technol. With kind permission of John Wiley and Sons.

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3.3

115

Preparation of Natural polymer/layered double hydroxide nanocomposites

Natural polymers, like polysaccharides, are good candidates for NC matrices due to their unique properties such as biocompatibility, biodegradability, nonhazardous nature, etc. Even these biosafe macromolecules can be used as stabilizers for LDH for better dispersion, for example, utilization of carboxymethylcellulose (CMC) as a compatibilizer of ZnAl-LDH in potato starch (Wu et al., 2011).

3.3.1 Preparation of carboxymethylcellulose/layered double hydroxide nanocomposites In a study, CMC was used for a matrix for bio-NC
containing MgAl-LDH (Yadollahi et al., 2014). In this study, the LDH was obtained by a coprecipitation method and CMC/LDH NCs were successfully made through the solvent blending method using bidistilled water as a solvent. Possible interactions in the NCs are shown in Fig. 3.14. In this work, the effect of the amount of LDH on the polymer and subsequently on the NC properties has been studied.

Figure 3.14 Schematic illustration of the possible interaction of LDH particles with CMC chains. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym. 108, 83
90. With kind permission of Elsevier.

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As the content of the LDH increased, the water vapor permeability (WVP) decreased. The WVP of NCs was decreased to 37%, with the maximum amount of the LDH (8 wt.% with respect to the polymer weight). Also, at the low amount of the LDH exfoliation was observed while with increasing the content of the LDH intercalation of the LDH were observed. These observations were obtained using transmission electron microscopy (TEM) images (Fig. 3.15). As mentioned earlier, the type of cation can affect the property of the resulting LDH and subsequently the NCs. In a study, MgAl-LDH and NiAl-LDH were used to prepare CMC/LDH NCs (Yadollahi and Namazi, 2013). As can be seen in the thermogravimetry analysis (TGA) curves (Fig. 3.16), the thermal stability of pure CMC was increased using LDHs. Also, the NC-containing MgAl-LDH is more thermally stable than NiAl-LDH and it shows that the LDH with Mg is more stable than Ni.

3.3.2 Preparation of pectin/layered double hydroxide nanocomposites In addition to WVP, the antimicrobial activity of polymer/LDH is one of the aims of using LDHs and biopolymers. For example, pectin/LDH NCs were prepared using LDH containing antimicrobial active molecules such as benzoate (Bz), 2,4-dichlorobenzoate (DCBz), para-hydroxybenzoate (p-OHBz), and ortho-hydroxybenzoate (o-OHBz) (Gorrasi et al., 2012). The solvent blending was selected to prepare the

Figure 3.15 TEM images of the CMC
LDH NC films with 3 wt.% LDH (A and B) and 8 wt.% LDH (C and D) at low and high magnifications, respectively. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym. 108, 83
90. With kind permission of Elsevier.

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Figure 3.16 TGA curves obtained for (a) Na
CMC, (b) Ni-Al
CMC
LDH, and (c) MgAl
CMC
LDH. Source: Adapted from Yadollahi, M., Namazi, H., 2013. Synthesis and characterization of carboxymethyl cellulose/layered double hydroxide nanocomposites. J. Nanopart. Res. 15, 1563. With kind permission of Springer.

pectin/LDH NCs in distilled water. The complete delamination of LDHs into apple pectin was confirmed using wide-angle mode X-ray diffraction patterns (WAX) and absence of any other diffraction peak (Fig. 3.17). The antimicrobial activity test took 12 months and, after that, no mold formation for NCs with LDH containing antimicrobial active molecules was observed, meanwhile, in the case of pure pectin, mold formation occurred after 2 weeks (Fig. 3.18). This result proved that such NCs have promise for use in the packaging industry.

3.3.3 Preparation of chitosan/layered double hydroxide nanocomposites As discussed earlier, one of the methods used to prepare polymer/LDH is preexfoliation of the LDH. Beside the anions, some drugs can be used for this. A drugdelivery system has been made up of chitosan/LDH biohybrid beads coated with pectin through the LbL method (Ribeiro et al., 2014). Mg2Al-LDH was synthesized through the coprecipitation method and a nonsteroidal antiinflammatory drug [5-aminosalicylic acid (5ASA)] was used to modify the LDH. Fig. 3.19 shows a schematic of this system. The bio-NCs were prepared by adding LDH-5ASA into chitosan solution (from crab shells) and then dropping this mixture into the NaOH solution. The beads were coated with pectin (from citrus fruits). This system was prepared to achieve three goals: (1) pectin on the surface of the beads resists low pH (in the stomach); (2) the mucoadhesive property would be presented by the

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Figure 3.17 X-ray diffraction patterns of (A): (a) LDH-Bz, (b) LDH-DCBz, (c) LDH-oOHBz, (d) LDH-p-OH-Bz; and (B): (a) pectin, (b) pectin/LDH-Bz, (c) pectin/LDH-DCBz, (d) pectin/LDH-o-OHBz, (e) pectin/LDH-p-OHBz. Source: Adapted from Gorrasi, G., Bugatti, V., Vittoria, V., 2012. Pectins filled with LDHantimicrobial molecules: preparation, characterization and physical properties. Carbohyd. Polym. 89, 132
137. With kind permission of Elsevier.

chitosan; and (3) controlling the kinetics of drug release would be achieved by incorporating the drug into the LDH. The in vitro results proved that this system is more efficient than only biopolymers or the immobilization of drug without LDH host.

3.3.4 Preparation of natural rubber/layered double hydroxide nanocomposites Also, natural rubbers can be reinforced with LDHs. For example, a mixture of metal LDHs (Zn/Mg-Al LDH) was prepared via an in situ pathway in the presence

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Figure 3.18 Pictures from a cast film of pectin and nanocomposites with nanohybrids after storage for 12 months at ambient temperature. Source: Adapted from Gorrasi, G., Bugatti, V., Vittoria, V., 2012. Pectins filled with LDHantimicrobial molecules: preparation, characterization and physical properties. Carbohyd. Polym. 89, 132
137. With kind permission of Elsevier.

of stearic acid (as an organic modifier) (Das et al., 2011). At first, stearic acidmodified LDH with zinc then accelerators and sulfur were added. Then, the nanocomposite was prepared using two-roll milling for 15 min. The obtained NCs were characterized using different analyses such as TEM, wide-angle X-ray scattering (WAXS), etc. Fig. 3.20 shows the WAXS and TEM image of the obtained NR/LDH NCs. The WAXS patterns of NR/LDH NCs reveal weak first basal reflection peaks in which the interlayer space obtained was 4 nm. This finding proves that the LDH in the rubber maintained its oriented structure. In addition, the moiety of intercalated/ partially exfoliated LDH reinforces the rubber matrix. Also, the TEM image of NR/ LDH NCs shows the exfoliated LDH in the rubber matrix. It is expected that the resultant NR/LDH NCs would be useful in heavy equipment industries, such as for gaskets, seals, etc.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 3.19 Pectin-coated chitosan bead incorporating 5-aminosalicylic acid (5ASA) intercalated in Mg2Al-LDH as a new drug-delivery system. Source: Adapted from Ribeiro, L.N., Alcˆantara, A.C., Darder, M., Aranda, P., Arau´joMoreira, F.M., Ruiz-Hitzky, E., 2014. Pectin-coated chitosan
LDH bionanocomposite beads as potential systems for colon-targeted drug delivery. Intern. J. Pharm. 463, 1
9. With kind permission of Elsevier.

3.3.5 Other natural polymer/layered double hydroxide nanocomposites Polymer/LDHs NCs can be used as a biocatalyst for C
C bond formation. For this, various polysaccharide/LDH aldolase biohybrid beads were prepared (Mahdi et al., 2015). In this work CMC, poly-galacturonic acid, sodium alginate, curdlan, oxidized cellulose, and carrageenan as biopolymers, fructose-6-phosphate aldolase (FSAwt) [purified from Escherichia coli (E. coli)] as an enzyme, and Mg2Al-LDH were used for the preparation of these biocatalysts. The FSAwt@Mg2Al-LDH and the beads were synthesized through coprecipitation and solvent-blending methods (in water), respectively. Oxidized cellulose, alginate, and carrageenan were the most compatible with the enzyme and its activity. As shown in Fig. 3.21 the aldol reaction of hydroxyacetone and formaldehyde was done to prove the catalytic efficiency of the biocatalytic and a yield of 80% was obtained after 3 h.

3.4

Preparation of synthetic polymer/layered double hydroxide nanocomposites

Some disadvantages of natural polymers encourage us to use synthetic polymers rather than natural polymers, such as designing polymers for specific uses,

Fabrication technologies of layered double hydroxide polymer nanocomposites

Mixed metal hydroxides layer Anionic layer

4.2 nm 3.1 nm

10

4

4.5-5.5 nm

Intensity (a.u.)

4.3 nm

LDH (pure) LDH-St (modified) NBR/LDH-St NR/LDH-St XNBR/LDH-St

121

Interlayer space

WAXS

103

0.9 nm

102

101

100 nm 2

4

6 8 2θ (deg)

10

12

14

Figure 3.20 WAXS patterns and TEM image of rubber/LDH nanocomposites. Source: Adapted from Das, A., Wang, D.-Y., Leuteritz, A., Subramaniam, K., Greenwell, H. C., Wagenknecht, U., et al., 2011. Preparation of zinc oxide free, transparent rubber nanocomposites using a layered double hydroxide filler. J. Mater. Chem. 21, 7194
7200. With kind permission of the Royal Society of Chemistry.

Figure 3.21 FSAwt@Mg2Al-LDH/carr beads mediated synthesis of L-1-deoxyerythrulose. Source: Adapted from Mahdi, R., Gue´rard-He´laine, C., Laroche, C., Michaud, P., Pre´vot, V., Forano, C., et al., 2015. Polysaccharide-layered double hydroxide
aldolase biohybrid beads for biocatalysed CC bond formation. J. Mol. Catal. B: Enzym. 122, 204
211. With kind permission of Elsevier.

withmore durability, more strength, more stability, lower processing cost, and so on. Thus, they can be used in many fields such as biomedical implants and devices, drug delivery, water treatment, wound dressing, etc.; on the other hand, modification of functional groups of synthetic polymers is easier than natural polymers. To overcome some weaknesses of synthetic polymers, blending two or more polymers or chemically linking (copolymers) makes an intermediate favorable property (Bhatia, 2016). In this section, the fabrication of important synthetic polymer/LDH using various methods is discussed.

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3.4.1 Preparation of polyimide/layered double hydroxide nanocomposites An organosoluble polyimide (PI) containing ZnCr-LDH has been synthesized (Dinari and Rajabi, 2016). The LDH was synthesized through the coprecipitation pathway, where Zn(NO3)2.6H2O and Cr(NO3)3.9H2O (Zn:Cr in the molar ratio of 2.0:1.0) were used. The NCs based on PI were prepared by in situ polymerization of 5-methyl-N,N-bis(4-nitrophenyl)naphthalene-1-amine with pyromellitic dianhydride (PMDA) using a different amount of LDHs. The resulting poly(amic acid) (PAA) as the precursor was further thermally imidized to the desired PI. Several techniques were applied to characterize the NCs. The morphology of LDH and PI/ LDH was studied via field emission scanning electron microscopy (FE-SEM) and the plate-like morphology and hexagonal crystallite for the LDH structure were given (Fig. 3.22). The incorporation of LDH into PI made it self-extinguishing, with the limiting oxygen index (LOI) for NCs in the range of 45.5
47.1%, which is good for these materials. It can be explained that the heat is distributed among layers. On the other hand, there are numerous
OH groups on the layers and they

Figure 3.22 FE-SEM images of NCs with (A and B) 2% and (C and D) 4% of LDH. Source: Adapted from Dinari, M., Rajabi, A.R., 2016. Structural, thermal and mechanical properties of polymer nanocomposites based on organosoluble polyimide with naphthyl pendent group and layered double hydroxide. High Perform. Polym., 1
9. doi:10.1177/ 0954008316665678. With kind permission of SAGE Journals.

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are H-bonded with polymer chains. Therefore, a large amount of energy should be consumed to break them. Thus, the heat transfer to the polymer will be postponed.

3.4.2 Preparation of poly(methyl methacrylate)/layered double hydroxide nanocomposites In another project (Tsai et al., 2016), LiAl- and MgAl-LDHs were organo-modified with two anionic modifiers, cocoamphodipropionate (K2) and sodium dodecyl sulfate (SDS). The multiorgano-modified LDHs were used as fillers in poly(methyl methacrylate) (PMMA) by in situ polymerization of methyl methacrylate (MMA) in the toluene and the properties of the cast NCs, for example, optical clarity and antiscratch, were investigated. From the wide-angle X-ray diffraction (WXRD) pattern of MgAl-LDH and MgAl-LDH-K2 1 SDS (Fig. 3.23), increasing in the basal space (d003) provides the evidence that modifiers were intercalated into the galleries of LDH. This change in basal spacing also occurred for the LiAl-LDH and LiAl-LDH-K2 1 SDS, as can be seen in Fig. 3.24. After adding LDHs into PMMA, the gas permeability was reduced from 0.9429 for pure PMMA to 0.1599 PMMA/LiAl-LDH-K2 1 SDS-3%. Also, the transparency of NCs was measured by ultraviolet-visible (UV-vis) spectroscopy and results indicated that LDHs did not affect the transparency of PMMA and show that LDHs were dispersed well in the matrix. Therefore, these three NCs are applicable as a glass substitute in many fields.

Relative intensity

(003)

(006) (b) (003) (006) (009)

0

20

40 2-Theta

(110) (113)

60

(a) 80

Figure 3.23 The WXRD diffraction patterns of (a) pristine MgAl-LDH and (b) MgAl-LDHK2 1 SDS. Source: Adapted from Tsai, T.Y., Bunekar, N., Liang, S.W., 2016. Effect of Multiorganomodified LiAl- or MgAl-layered double hydroxide on the PMMA nanocomposites. Adv. Polym. Technol. With kind permission of John Wiley and Sons.

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Relative intensity

(002)

(004)

(006) (b) (002)

(004)

(101) 0

20

(006) (200) (115) 40 2-Theta

(303) (10,10) (a) 60

80

Figure 3.24 The WXRD diffraction patterns of (a) pristine LiAl-LDH and (b) LiAl-LDHK2 1 SDS. Source: Adapted from Tsai, T.Y., Bunekar, N., Liang, S.W., 2016. Effect of multiorganomodified LiAl-or MgAl-layered double hydroxide on the PMMA nanocomposites. Adv. Polym. Technol. With kind permission of John Wiley and Sons.

3.4.3 Preparation of polyvinyl acetate/layered double hydroxide nanocomposites Based on the literature, the delaminated or modified LDHs have better dispersion into the polymeric matrix. Another example for this claim is the work by Chen et al. (2015), where the MgAl-LDHs were delaminated in a microemulsion composed of octane, N-lauroyl-glutamate (LGA), and butanol as a solvent, surfactant, and cosurfactant, respectively. These delaminated LDHs, with different percentages, were dispersed in the vinyl acetate and the chain-growth polymerization occurred using benzoyl peroxide to give polyvinyl acetate (PVAc). As a comparison, virgin LDH-PVAc was prepared. The TEM micrographs of the samples proved that delaminated LDH has a better dispersion state than that of the LDH-PVAc. The dispersed delaminated sheets in the polymer have a fiber-like shape at the nanoscale size (Fig. 3.25). The obtained NCs containing LDH are more thermally stable than that of the polymer without LDH.

3.4.4 Preparation of P(MMA-co-BA)/layered double hydroxide nanocomposites As shown in Fig. 3.26, the NC consists of MgAl-LDH as nanofiller and a copolymer of MMA and butyl acrylate (BA) as a matrix. (P(MMA-co-BA)/LDH) has

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Figure 3.25 TEM micrographs of different LDHs in PVAc. (A) Delaminated LDHs (prepared in the microemulsion composed of octane, LGA, butanol, and water and the ratio of octane to LGA is 10); (B) NO3-LDHs. Source: Adapted from Chen, J., He, M., Wang, Y., Hu, W., Lv, L., 2015. Nanoscale dispersion of delaminated sheets of layered double hydroxides in polyvinyl acetate. Micro Nano Lett. 10, 465
468. With kind permission of IET Digital Library.

Figure 3.26 Schematic illustration of the LDH/P(MMA-co-BA) nanocomposite film formation. Source: Adapted from Veschambres, C., Halma, M., Bourgeat-Lami, E., Chazeau, L., Dalmas, F., Prevot, V., 2016. Layered double hydroxides: efficient fillers for waterborne nanocomposite films. Appl. Clay Sci. 130, 55
61. With kind permission of Elsevier.

been prepared and the interaction between LDH layers and polymer chains has been reported as being electrostatic (Veschambres et al., 2016). This nanocomposite is suitable to be used in the coating industries. A coprecipitation approach was used to prepare the LDH and the latex was made through surfactant-free radical polymerization of MMA/BA (1:1) initiated by potassium persulfate (KPS) and the combination was transmitted into a Teflon cast. As shown in Fig. 3.27, the average particle size was obtained as 77 nm by dynamic light scattering (DLS), which fits well within the range of 30
140 nm from TEM images. TEM images of latex/LDH5 and latex/LDH10 samples are illustrated in Fig. 3.28. In the low-magnification image, the creation of a connected LDH lattice can be explicitly seen for either of the LDH polymer NCs. Since the network mesh

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Figure 3.27 DLS measurement (A) and TEM image (B) of MgAl-LDH. Source: Adapted from Veschambres, C., Halma, M., Bourgeat-Lami, E., Chazeau, L., Dalmas, F., Prevot, V., 2016. Layered double hydroxides: efficient fillers for waterborne nanocomposite films. Appl. Clay Sci. 130, 55
61. With kind permission of Elsevier.

Figure 3.28 Low- and high-magnification TEM images of microsections from latex/LDH5 and latex/LDH10 samples. Source: Adapted from Veschambres, C., Halma, M., Bourgeat-Lami, E., Chazeau, L., Dalmas, F., Prevot, V., 2016. Layered double hydroxides: efficient fillers for waterborne nanocomposite films. Appl. Clay Sci. 130, 55
61. With kind permission of Elsevier.

size is in the range of the latex bead diameter (ca. 700 nm), the latex particles handle the morphology of this network. In the TEM images at higher magnification, since the LDH content was increased, there was more LDH layer stacking in the network wall, and, as expected, the thickness of the network wall was related to the LDH content.

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3.4.5 Preparation of wood flour/polypropylene/layered double hydroxide nanocomposites The NC based on Mg3Al-LDH and wood flour/polypropylene (WF-PP) has been reported (Peng et al., 2017). The pristine LDH, poplar WF, and PP were mixed using a high-speed blender and, after that, the mixture was melt-blended in an extruder. Of course, the main idea in this work was to study the effect of modified LDH on the NCs; therefore, the modification of Mg3Al-LDH with 5-sulfosalicylic acid (SA) anions was done via an ion exchange reaction and the resulting NCs were examined against the WF-PP/LDH NCs. The effect of the LDH on the photostability of WF-PP has been investigated through accelerated ultraviolet (UV) weathering. The UV resistance mechanism of SA-LDH is shown in Fig. 3.29. Addition of LDH into WF/PP brought some features, such as less color change, better thermal stability, fewer surface cracks, and better mechanical properties than WF/PP and, also, it reduced the photo-oxidation of WF/PP. Another important effect of modified LDH in WF/PP is the shield effect of the modified LDH against the UV light through the layers, as well as the presence of an interlayer anion (SA), which helps in chemical absorbance of UV light.

3.4.6 Preparation of poly(amide-imide)/layered double hydroxide nanocomposites In one study, N-tetrabromophthaloyl-glutamic was used for the modification of LDH and, subsequently, reinforcing of a poly(amide-imide) (PAI) by Mallakpour et al. (2016). The modification was done through the coprecipitation method and the solvent-blending route was used to prepare the NCs. The schematic mechanism of the modification and preparation of NCs are shown in Figs. 3.30 and 3.31, respectively.

Figure 3.29 UV resistance mechanism of LDH and LDH-SA. Source: Adapted from Peng, Y., Wang, W., Cao, J., Huang, Y., 2017. Synthesis of 5-sulfosalicylic acid-intercalated layered double hydroxide and its effects on wood flour/ polypropylene composites during accelerated UV weathering. J. Appl. Polym. Sci. 134. With kind permission of John Wiley and Sons.

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Figure 3.30 Preparation of the M-LDH. Source: Adapted from Mallakpour, S., Dinari, M., Talebi, M., 2016. Novel nanocomposites obtained by dispersion of LDH modified with N-tetrabromophthaloyl-glutamic in poly (amide-imide) having N-trimellitylimido-l-leucine and 4, 40 -diaminodiphenylether units. Polym. Comp. 37, 1323
1329. With kind permission of John Wiley and Sons.

Figure 3.31 Preparation of the PAI/M-LDH NCs. Source: Adapted from Mallakpour, S., Dinari, M., Talebi, M., 2016. Novel nanocomposites obtained by dispersion of LDH modified with N-tetrabromophthaloyl-glutamic in poly (amide-imide) having N-trimellitylimido-l-leucine and 4, 40 -diaminodiphenylether units. Polym. Comp. 37, 1323
1329. With kind permission of John Wiley and Sons.

Fourier transform infrared spectroscopy (FT-IR), TGA, FE-SEM, TEM, and XRD were applied to study the properties of the samples. Thus, FE-SEM images were employed to observe the morphology of the neat LDH, modified-LDH (MLDH) nanolayers, and PAI/M-LDH NCs. The FE-SEM photographs exhibited the plate-like morphology for neat LDH layers, which were stacked on top of each other (Fig. 3.32A,B). Evidence of exfoliation is provided by the existence of single layers. In the case of M-LDH, FE-SEM results exhibited that the M-LDH layers were exfoliated and lost their ordered stacking structure (Fig. 3.32C,D). FE-SEM images of PAI/LDH NCs show the good and uniform dispersion of MLDH into PAI, although by increasing the amount of M-LDH to 8 wt.%, some agglomeration is revealed (Fig. 3.33). The existence of an amino acid in the polymer NC made it a biodegradable NC and suitable for industrial applications. In

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Figure 3.32 FE-SEM images of LDH-CO322 (A, B) and M-LDH (C, D). Source: Adapted from Mallakpour, S., Dinari, M., Talebi, M., 2016. Novel nanocomposites obtained by dispersion of LDH modified with N-tetrabromophthaloyl-glutamic in poly (amide-imide) having N-trimellitylimido-l-leucine and 4, 40 -diaminodiphenylether units. Polym. Comp. 37, 1323
1329. With kind permission of John Wiley and Sons.

addition, PAI/LDH NCs due to the presence of LDH as an acid scavenger and PAI with its chemical resistance is useful in the coating industry. A chiral NC based on PAI/organo modified LDH has been synthesized (Mallakpour et al., 2014). In this project, coprecipitation was carried out for the preparation of the MgAl-LDH under ultrasonic irradiation, then the LDH was modified with a diacid. A schematic illustration of the procedure is shown in Figs. 3.34 and 3.35. For the synthesis of PAI, a polycondensation reaction of N,N0 -(pyromellitoyl)-bis0 L-phenylalanine diacid and 4,4 -diaminodiphenyl sulfone was carried out and the chirality of the polymer was confirmed by specific rotation {[α]25D 5
134 degrees (at a concentration of 0.5 g/dL in DMF at 25 C)}. A solvent-blending method was applied to prepare NCs in ethanol. TEM, FT-IR, TGA, XRD, and FE-SEM were used to characterize the NCs. Fig. 3.36 shows FE-SEM images of the LDH and modified LDH. Based on these images, the nature of LDH particles roughly included plate-like shapes with side-long dimensions and thicknesses ranging over a few nanometers were retained. As commonly observed for typical organic anionic intercalated LDH compounds, the formation of plate-like NPs with regular shape has been observed from the images of M-LDH. Over a few hundred and several tens of nanometers have been calculated for the diameters and thicknesses of NPs, respectively. As shown in Fig. 3.37, a carboxylated CNT was used to prepare a hybrid of flower-like MgAl-LDH/CNT for reinforcing of chiral PAI (Mallakpour and Dinari, 2015). N,N0 -(pyromellitoyl)-bis-L-phenylalanine as diacid and 2-(3,5-diaminophenyl)-benzimidazole as diamine were utilized as monomers in a step-growth

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Figure 3.33 FE-SEM images of PAI (A, B), PAI/NCs 2 wt.% (C, D), PAI/NCs 4 wt% (E, F) and PAI/NCs 8 wt% (G, H). Source: Adapted from Mallakpour, S., Dinari, M., Talebi, M., 2016. Novel nanocomposites obtained by dispersion of LDH modified with N-tetrabromophthaloyl-glutamic in poly (amide-imide) having N-trimellitylimido-l-leucine and 4, 40 -diaminodiphenylether units. Polym. Comp. 37, 1323
1329. With kind permission of John Wiley and Sons.

polymerization in the presence of tetra-n-butylammonium bromide (TBAB). Preparation of NCs was carried out using absolute ethanol as solvent and ultrasonic irradiation as a dispersant. At first, the PAI was dispersed in ethanol and sonicated at room temperature. Then a different amount of LDH (2, 4, and 8 wt.%) was added to the PAI suspension and sonicated. After removal of ethanol and drying in a vacuum oven, the PAI/LDH NCs obtained. The hybrid can be easily observed via TEM images. Synthesized LDHs show smooth, well-formed hexagonal forms and overlapping crystals under ultrasonic irradiation (Fig. 3.38A,B). Fig. 3.38C,D clearly shows the stiff connection between CNTs and LDH platelets. Random dispersion of the individual CNTs inside the microspheres and on the surface of LDH platelets can be clearly seen in the image. Accumulation of LDHs has been prevented by good interaction between CNTs and LDHs, which is indicated by the results. Also, in the TEM images of PAI/ LDH
CNT NC2%, LDH
CNT is well seen in the PAI matrix as well as good dispersion state of the LDH
CNT (Fig. 3.39). These NCs have potential to be used as water pollutant adsorbents, for example, for heavy metals, dyes, etc.

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Figure 3.34 Intercalation of diacid in the interlayer of LDH. Source: Adapted from Mallakpour, S., Dinari, M., Hatami, M., 2014. Modification of Mg/Allayered double hydroxide with L-aspartic acid containing dicarboxylic acid and its application in the enhancement of the thermal stability of chiral poly (amide-imide). RSC Adv. 4, 42114
42121. With kind permission of the Royal Society of Chemistry.

Figure 3.35 Synthesis of NCs of PAI and M-LDH. Source: Adapted from Mallakpour, S., Dinari, M., Hatami, M., 2014. Modification of Mg/Allayered double hydroxide with L-aspartic acid containing dicarboxylic acid and its application in the enhancement of the thermal stability of chiral poly (amide-imide). RSC Adv. 4, 42114
42121. With kind permission of the Royal Society of Chemistry.

3.4.7 Preparation of low-density polyethylene/layered double hydroxide nanocomposites The NC composed of low-density polyethylene (LDPE)/organo-modified LDH has been synthesized (Gorrasi and Bugatti, 2016). Aleurate, citrate, glycolate, parahydroxybenzoate, salicylate, and serine carboxylate were the organic anions intercalated into the ZnAl-LDH. Coprecipitation and ball-milling procedures were used to prepare the LDH and LDPE/LDH NCs, respectively. In this method, milling of modified LDH and LDPE powder was done in a centrifugal ball mill using five

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Figure 3.36 FE-SEM images of (A and B) pristine LDH and (C and D) modified LDH. Source: Adapted from Mallakpour, S., Dinari, M., Hatami, M., 2014. Modification of Mg/Allayered double hydroxide with L-aspartic acid containing dicarboxylic acid and its application in the enhancement of the thermal stability of chiral poly (amide-imide). RSC Adv. 4, 42114
42121. With kind permission of the Royal Society of Chemistry.

Figure 3.37 Preparation of LDH
CNT hybrids. Source: Adapted from Mallakpour, S., Dinari, M., 2015. Hybrids of Mg
Al-layered double hydroxide and multiwalled carbon nanotube as a reinforcing filler in the l-phenylalaninebased polymer nanocomposites. J. Therm. Anal. Calorim. 119, 1905
1912. With kind permission of Springer.

steel balls, at a speed of 580 rpm, for 60 min, and at room temperature. Then, the mixture was molded at 130 C and quenched in an ice-water bath. The XRD patterns of the samples are given in Fig. 3.40. Peaks at 2θ 5 21.6 degrees and 2θ 5 23.8 degrees show the typical orthorhombic cell which proved the maintenance of LDPE in the nanocomposites. Also, the basal peak of modified LDHs has been observed at the same 2θ. The basal peak of salicylate and para-hydroxybenzoate-modified LDHs occurred at 2θ 5 5.2 degrees, related to the higher basal distance, which gives evidence for successful intercalation

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Figure 3.38 TEM micrographs of (A and B) the LDH and (C and D) LDH
CNT. Source: Adapted from Mallakpour, S., Dinari, M., 2015. Hybrids of Mg
Al-layered double hydroxide and multiwalled carbon nanotube as a reinforcing filler in the l-phenylalaninebased polymer nanocomposites. J. Therm. Anal. Calorim. 119, 1905
1912. With kind permission of Springer.

Figure 3.39 TEM micrographs of the PAI/LDH
CNT NC2% at different magnifications (a
d). Source: Adapted from Mallakpour, S., Dinari, M., 2015. Hybrids of Mg
Al-layered double hydroxide and multiwalled carbon nanotube as a reinforcing filler in the l-phenylalaninebased polymer nanocomposites. J. Therm. Anal. Calorim. 119, 1905
1912. With kind permission of Springer.

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Figure 3.40 XRD diagrams of the nanocomposite LDPE/LDHs modified with the six organic anions: (a) aleuritic carboxylate; (b) citrate; (c) glycolate; (d) para-hydroxybenzoate; (e) salicylate; (f) serine carboxylate. Source: Adapted from Gorrasi, G., Bugatti, V., 2016. Mechanical dispersion of layered double hydroxides hosting active molecules in polyethylene: analysis of structure and physical properties. Appl. Clay Sci. 132
133, 2
6. With kind permission of Elsevier.

between LDPE and LDHs. Oxygen barrier properties of pure LDPE and its NCs have been investigated at atmospheric pressure and 25 C. The results indicated that organo-modified LDPE/LDH NCs are a good candidate for food packaging. Also, an overall migration test was conducted in acetic acid 3% (at nonswelling conditions) and ethanol 10% (at more swelling conditions) for LDPE and LDPE/LDH-salicylate NCs. The overall migrations were below 10 mg/dm2 as standard by the European Union legislation for food contact plastics.

3.4.8 Preparation of polyvinyl alcohol/layered double hydroxide nanocomposites A bio-NC (BNC) consisting of poly(vinyl alcohol) (PVA) and phenylalaninemodified LDH (M-LDH) has been prepared via the solvent-blending method in water using sonication as a green energy source (Mallakpour and Dinari, 2014). N,N0 -(pyromellitoyl)-bis-L-phenylalanine diacid was used as a modifier to modify MgAl-LDH through direct synthesis. In addition FT-IR, TGA, XRD, TEM, and FE-SEM observations were applied for the further validation of the exfoliation behavior and to conceive the structure of the M-LDH nanolayers and PVA/M-LDH BNCs. The FE-SEM images of neat LDH, M-LDH, and PVA/M-LDH BNC containing 8 wt.% of M-LDH are displayed in Fig. 3.41. It has been illustrated by the FE-SEM image of neat LDH that the nature

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Figure 3.41 FE-SEM photographs of LDH (A
C), M-LDH (D
F), and PVA hybrid with 8 wt.% M-LDH (G
I). Source: Adapted from Mallakpour, S., Dinari, M., 2014. Novel bionanocomposites of poly (vinyl alcohol) and modified chiral layered double hydroxides: Synthesis, properties and a morphological study. Prog. Org. Coat. 77, 583
589. With kind permission of Elsevier.

of LDH particles roughly consists of a plate-like form, stacked on the top of each other with side-long dimensions ranging from a few micrometers and thickness of over a few hundred nanometers. The morphological features of the M-LDH were similar to that of the unmodified LDH, however it appeared floppier than the unmodified LDH. The FE-SEM images of PVA/MLDH BNC 8% showed that the morphology was changed. This morphological variation could be attributed to the reconstruction of the PVA matrix in the presence of the M-LDH, producing a packed lattice. It shows the uniform dispersion of particles in the polymeric matrix. The NCs of PVA/MLDH are a good candidate foruse in drug-delivery systems. Also, PVA was used as a matrix of a chiral LDH containing N-trimellitylimidoL-isoleucine and MgAl-LDH (Mallakpour and Dinari, 2016). The modification of LDH was conducted via direct synthesis assisted by ultrasonication. Through solvent blending, the NCs were prepared by mixing PVA and chiral LDH in distilled water and ultrasonic irradiation was used to make a well-dispersed LDH suspension. XRD, FT-IR, TGA, FE-SEM, and TEM were effective techniques to study the

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Figure 3.42 TEM micrographs of (A, B) LDH-CO322, (C, D) CLDH, and (E, F) NC4%. Source: Adapted from Mallakpour, S., Dinari, M., 2016. Bionanocomposite materials from layered double hydroxide/N-trimellitylimido-l-isoleucine hybrid and poly (vinyl alcohol) Structural and morphological study. J. Thermoplast. Comp. Mater. 29, 623
637. With kind permission of SAGE Journals.

properties of NCs. TEM photographs display smooth, overlapping crystals, wellformed, and in hexagonal form. The homogeneous contrast, uniform thickness, and reflection of the LDH ultrathin nature have been observed in the TEM images (Fig. 3.42). In the case of modified LDH, a hexagonal shape with rounded corners for the platelets and in the case of NCs of PVA and 4 wt.% of chiral diacid intercalated LDH (CLDH) a coexistence of LDH layers in the intercalated and partially exfoliated states have been observed.

3.4.9 Preparation of polyester/layered double hydroxide nanocomposites The manufacturing of fire-retardant LDH has been reported (Cai et al., 2016). Modification of MgAl-LDH (N-LDH) with spirocyclic pentaerythritol bisphosphorate

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Figure 3.43 Preparation of SPDP-LDH. Source: Adapted from Cai, J., Heng, H.-M., Hu, X.-P., Xu, Q.-K., Miao, F., 2016. A facile method for the preparation of novel fire-retardant layered double hydroxide and its application as nanofiller in UP. Polym. Degrad. Stabil. 126, 47
57. With kind permission of Elsevier.

diphosphoryl sodium (SPDP) anion was carried out by in situ synthesis (Fig. 3.43). An unsaturated polyester resin (UP) was applied as a matrix for NC preparation. At first, LDH was dispersed in the UP (with 1, 3, 5, and 10 wt.% of the UP weight) for 10 min at room temperature. Afterward, the mixture was moved to a mold and cured at room temperature for 1 h and in the next step 80 C curing for 3 h was done. The AFM was also used to study the particle size distribution and particle size of LDH and SPDP-LDH. Three-dimensional AFM images of LDH and SPDP-LDH are shown in Fig. 3.44. The lateral dimensions are in the range of 200
300 and 100
200 nm for the LDH and SPDP-LDH, respectively. Sizes of 40 and 30 nm were obtained for the height profiles of the images of LDH and SPDP-LDH, respectively. According to the data, to reduce the aggregation of LDH particles, the flame-retardant modification is very beneficial. According to the authors, the stacked layers of N-LDH are still stacked (Fig. 3.45A) and after modification with SPDP, some partial exfoliation in the polymeric matrix is observed (Fig. 3.45B). The fire retardancy of NCs was studied and the results proved the fire retardancy of NCs, and this property increased along with an increasing amount of LDHs. (LOI value of 21.74 for 10 wt.% of LDH in UP. The PHRR value for UP obtained was 736 kW/m2, while in the case of UP/modified LDH with 10 wt.% this value was 412 kW/m2.) This could be explained by the lower initial decomposition temperature of interlayer space and catalytic degradation action of the metal ion in LDH, causing fire retardancy of NCs.

3.4.10 Preparation of polyvinyl chloride/layered double hydroxide nanocomposites A reconstruction method was used for exfoliation of MgAl-LDH with laurylether phosphate through a urea hydrolysis method (Huang and Wang, 2009). The modified LDH was inserted into a polyvinyl chloride (PVC) matrix through solvent

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Figure 3.44 AFM images of (A) N-LDH and (B) SPDP-LDH. Source: Adapted from Cai, J., Heng, H.-M., Hu, X.-P., Xu, Q.-K., Miao, F., 2016. A facile method for the preparation of novel fire-retardant layered double hydroxide and its application as nanofiller in UP. Polym. Degrad. Stabil. 126, 47
57. With kind permission of Elsevier.

Figure 3.45 TEM micrographs of (A) 5 wt.% N-LDH/UP and (B) 5 wt.% SPDP-LDH/UP nanocomposites. Source: Adapted from Cai, J., Heng, H.-M., Hu, X.-P., Xu, Q.-K., Miao, F., 2016. A facile method for the preparation of novel fire-retardant layered double hydroxide and its application as nanofiller in UP. Polym. Degrad. Stabil. 126, 47
57. With kind permission of Elsevier.

blending in the tetrahydrofuran. The morphology and diameter of particles are disk-like, with sizes of 30
60 nm, respectively, obtained by atomic force microscopy (AFM). The low-magnification TEM image showed the LDHs well dispersed throughout the polymer. Higher magnification displayed that the LDH nanolayers are exfoliated in the PVC matrix, but intercalated tactoids were also present (Fig. 3.46). Also, the presence of modified LDH increased the activation energy of degradation of PVC for 10
26 kJ/mol, according to the Flynn
Wall
Ozawa method, as well as improving the stability of polymer chains against dehydrochlorination.

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Figure 3.46 TEM micrographs of PVC/LDH nanocomposites with 20% loading of LDH at different magnifications (a
c). Source: Adapted from Huang, N., Wang, J., 2009. A new route to prepare nanocomposites based on polyvinyl chloride and MgAl layered double hydroxide intercalated with lauryl ether phosphate. Express Polym. Lett. 3, 595
604. With kind permission of Budapest University of Technology and Economics, Dept. of Polymer Engineering.

The application of LDH in PVC led to stabilization of the chlorine atom on the PVC chain, therefore, this nanocomposite could be useful in the coating, medical fields etc.

3.4.11 Preparation of polypropylene-ethylene vinyl acetate/ layered double hydroxide nanocomposites LDHs can affect the storage modulus of polymers or blends. A blend copolymer consisting of polypropylene (PP) and ethylene vinyl acetate (EVA) has been prepared using organo-modified Mg2Al-LDH by the melt-mixing method (Rafiee et al., 2016). In this research, sodium dodecyl benzene sulfonate (SDBS) was used as a modifier agent of the LDH. Modified LDH was synthesized using a proper precursor in one step. Dynamic mechanical thermal analysis (DMTA) revealed that the presence of LDH into PP matrix (without EVA) increased storage modulus (SM), while the addition of EVA into the PP matrix resulted in a decrease in the SM. These observations can be explained as an effect of the stiffness of LDH layers increasing, and of the free volume of EVA and high entropy of this which annihilate the crystalline region of PP, decreasing of SM. The sample containing 95 wt.% of PP and 5 wt.% of LDH has the most SM at room temperature. As LDH can increase the SM, it also can increase the loss modulus (LM) due to the decrease in crystalline phase, as well as

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the addition of EVA with a rubbery-like nature. Therefore, the LDH and EVA have the same effect on SM, while they oppose each other in the case of LM. Fig. 3.47 shows a TEM image of PP-EVA/MgAl-LDH NCs.

3.4.12 Preparation of silicone rubber/layered double hydroxide nanocomposites The hydrophilic LDH has been changed into an organophilic LDH by modification of MgAl-LDH with dodecyl sulfate (DS) by Pradhan et al. (2011). The coprecipitation and ion exchange pathways were proper methods to prepare LDH and modified LDH, respectively. This organophilic LDH was used to improve the properties of crosslinked silicone rubber (SR), at first, modified LDH was dispersed in CCl4 at 80 C and lasted for 6 h, then, the suspension was added to the solution of SR (in CCl4) and was then crosslinked with V430. Figs. 3.48 and 3.49 show the schematics of this process. A larger smooth area for neat SR has been observed from SEM micrographs of tensile fractured surfaces of SR (Fig. 3.50A). Also, an identical setback was designated for the matrix with no weaker region for crack starting.

Figure 3.47 TEM images of P75E25L5 samples with different magnifications (a
d). Source: Adapted from Rafiee, F., Otadi, M., Goodarzi, V., Khonakdar, H.A., Jafari, S.H., Mardani, E., et al., 2016. Thermal and dynamic mechanical properties of PP/EVA nanocomposites containing organo-modified layered double hydroxides. Compos. Part B: Eng. 103, 122
130. With kind permission of Elsevier.

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+ 0.48

SO4– 2.77nm

4

SO –

SO –

4

SO4–

SO4–

0.78

SO4–

SO4– SO4– SO4– SO4– SO4– SO –

4

SO4–

SO4– SO4–

Dodecyl sulfate ion

LDH

SO4–

DS-LDH

Figure 3.48 Schematic diagram of pure LDH and DS
LDH. Source: Adapted from Pradhan, B., Srivastava, S.K., Ananthakrishnan, R., Saxena, A., 2011. Preparation and characterization of exfoliated layered double hydroxide/silicone rubber nanocomposites. J. Appl. Polym. Sci. 119, 343
351. With kind permission of John Wiley and Sons. SO4–

SO4–

CH3 H2C 2.77nm

SO4– SO4–

SO4– SO4–

+

HC

DS-LDH

SO4–

Si

CH

CH2

CH3 CH3 Silicone Rubber + H H Si

SO4–

CH3

Si

Si

Si

H Crosslinker SR/DS-LDH nanocomposite

Figure 3.49 Scheme of the intercalation process between DS
LDH and the SR matrix. Source: Adapted from Pradhan, B., Srivastava, S.K., Ananthakrishnan, R., Saxena, A., 2011. Preparation and characterization of exfoliated layered double hydroxide/silicone rubber nanocomposites. J. Appl. Polym. Sci. 119, 343
351. With kind permission of John Wiley and Sons.

Nevertheless, many microvoids (designated by the arrows in Fig. 3.50B) around the dispersed nanoparticles roughly fractured the surface when 5 wt.% DS
LDH was added to SR. This can be described as particle clusters trapping a significant quantity of polymeric matrix and subsequently, during distortion, some of the energy will be absorbed and mechanical properties of the polymer/LDH NCs will be maximized. It has been observed that at the organic
inorganic interface, there are secondary cracks and break stages before unity with the spreading out early of cracks based on the results. As well as many other organic polymer NCs, the superior mechanical properties of the NC would be described by the rough surface of the SR polymer NCs.

3.4.13 Preparation of epoxy resin/MoS2/layered double hydroxide nanocomposites LDHs can help to reduce the fire retardancy of epoxy resin, as epoxy resins are highly flammable and toxic smoke is produced by their combustion. Hence, in one

142

Layered Double Hydroxide Polymer Nanocomposites

Figure 3.50 SEM images of the tensile fracture surface morphology of (A) neat SR and (B) the SR/DS
LDH (5 wt.%) nanocomposite. Source: Adapted from Pradhan, B., Srivastava, S.K., Ananthakrishnan, R., Saxena, A., 2011. Preparation and characterization of exfoliated layered double hydroxide/silicone rubber nanocomposites. J. Appl. Polym. Sci. 119, 343
351. With kind permission of John Wiley and Sons.

Coprecipitation

Sonicaton

Fe3+ OH–

Ni2+ Co2+

LDH suspension Electrostatic attraction Self-assembly

n-BuLi

Hydrolysis Sonicaton

Solvothermal

MoS2-LDH hybrids

Bulk MoS2 Li+

Exfoliated MoS2 nanosheets

Figure 3.51 Illustration for the preparation of MoS2-LDH nanohybrids by the self-assembly method. Source: Adapted from Zhou, K., Gao, R., Qian, X., 2017. Self-assembly of exfoliated molybdenum disulfide (MoS 2) nanosheets and layered double hydroxide (LDH): towards reducing fire hazards of epoxy. J. Hazardous Mater. 338, 343
355. With kind permission of Elsevier.

study (Zhou et al., 2017), a hybrid of MoS2 and LDHs was used as a fire-retardant agent in the epoxy resin (EP). As cobalt, iron, and nickel are capable of increasing char yield of polymers, NiFe- and CoFe-LDH were chosen for this aim, and prepared through a coprecipitation method. The MoS2 and LDH stack together by electrostatic force, as MoS2 is negatively charged and LDH has a positive charge. Fig. 3.51 shows the exfoliation of MoS2, synthesis of LDHs, and preparation of the hybrids. The nanocomposite was synthesized through a solution-blending method. In this manner, the hybrid of MoS2-LDH was dispersed and sonicated into acetone (as a solvent). Then, the melted EP (at 95 C) was poured into the MoS2-LDH suspension

Fabrication technologies of layered double hydroxide polymer nanocomposites

Pyrolysis products

EP/LDH/MoS2 composites

LDH/MoS2 hybrids MoS2 nanosheets LDH

Small organic molecules

Metal oxides

143

Char

Water molecules Flammable gaseous products

Figure 3.52 Schematic illustration of the flame-retardant mode of action of EP nanocomposites with LDH/MoS2 hybrids. Source: Adapted from Zhou, K., Gao, R., Qian, X., 2017. Self-assembly of exfoliated molybdenum disulfide (MoS 2) nanosheets and layered double hydroxide (LDH): towards reducing fire hazards of epoxy. J. Hazardous Mater. 338, 343
355. With kind permission of Elsevier.

and the mixture was stirred with a mechanical stirrer for 6 h. The NC was then cured at 100 C and 150 C. The proposed mechanism of fire retardancy is as follows: 1. A maze or “tortuous path” was created by MoS2 to prevent the permeation of O2 and heat. 2. The degradation of LDHs generates a resistant oxide which protects the polymer from combustion and absorbs produced gases. 3. The release of volatile products is delayed by the catalytic carbonization effect of the LDH/MoS2 and it advances stable carbonaceous char as well as stopping the propagation of flammable gas, and oxygen and heat into the flame zone and polymer, respectively.

Also, water molecules from degradation of LDH play a role as cooling agents. In addition, the released heat is consumed for the degradation of LDH. These factors reduce the fire hazard risks of EP-LDH/MoS2. This mechanism is shown in Fig. 3.52. TEM images of different LDHs are presented in Fig. 3.53

3.4.14 Preparation of polyurethane/nitrile butadiene rubber Blend/layered double hydroxide nanocomposites In one study, MgAl-LDH was synthesized through a coprecipitation method and modification was done with SDS with a reconstruction (rehydration) method (Kotal

144

Layered Double Hydroxide Polymer Nanocomposites

Figure 3.53 TEM images of exfoliated MoS2 (A), CoFe-LDH (B), CoFe-LDH/MoS2 (C), NiFe-LDH (D), and NiFe-LDH/MoS2 hybrids (E, F). Source: Adapted from Zhou, K., Gao, R., Qian, X., 2017. Self-assembly of exfoliated molybdenum disulfide (MoS 2) nanosheets and layered double hydroxide (LDH): towards reducing fire hazards of epoxy. J. Hazardous Mater. 338, 343
355. With kind permission of Elsevier.

et al., 2010). These LDHs were used as filler in a blend of polyurethane (PU) and nitrile butadiene rubber (NBR) with amounts of 1, 3, 5, and 8 wt.% through the solution-blending method in tetrahydrofuran (THF). The LDH was added to the predissolved solution of PU/NBR (1:1) in THF. These NCs were crosslinked by sulfur. As the TEM images (Fig. 3.54) show, the LDH was well dispersed in NC 1 wt.% (Fig. 3.54A), which indicated the exfoliation of LDH and with increasing the amount of LDH (3 wt.%, Fig. 3.54B) both intercalation and exfoliation are seen. When increasing the amount of LDH continues to 8 wt.%, aggregation occurs (Fig. 3.54C). These NCs can be used in many applications such as tubing pipes, gaskets, protective covers, co-extrusion automotive gaskets, ballpen grips, automotive grips, etc.

3.4.15 Preparation of polyethyleneimine/poly(sodium 4-styrene sulfonate) hybrid/layered double hydroxide nanocomposites Polyethyleneimine (PEI) and poly(sodium styrene 4-sulfonate) (PSS) were used as a supporting polymer for LbL deposition of MgAl-LDH (Li et al., 2005). A coprecipitation method was utilized to prepare the LDH and then, formamide was used for exfoliation of the synthesized LDH. The procedure for preparation of the PSS/LDH NCs is as described here. First, a Si wafer and a quartz glass slide were cleaned with the aid of methanol/ HCl and subsequently H2SO4. This substrate was soaked first with PEI and then with PSS, followed by washing with water. This substrate was immersed in the

Fabrication technologies of layered double hydroxide polymer nanocomposites

145

Figure 3.54 TEM images of (A) PU/NBR/DS
LDH (1 wt.%) nanocomposite, (B) PU/NBR/ DS
LDH (3 wt.%) nanocomposite, and (C) PU/NBR/DS
LDH (8 wt.%). Source: Adapted from Kotal, M., Srivastava, S.K., Bhowmick, A.K., 2010. Thermoplastic polyurethane and nitrile butadiene rubber blends with layered double hydroxide nanocomposites by solution blending. Polym. Intern. 59, 2
10. With kind permission of John Wiley and Sons.

LDH suspension and then washed with water. This cycle was done several times to prepare a (PSS/LDH)n film. Fig. 3.55 shows an SEM image of LDH after ion exchange treatment and a TEM image of an LDH nanosheet. Uniform hexagonal crystals with a large lateral dimension can be found in the SEM image. Also, the TEM image clearly shows the ultrathin crystallite LDHs and implies a single LDH sheet. An AFM image of the PSS/LDH (Fig. 3.56) shows that the surface is tiled with LDHs with a lateral size of hundreds of nm to several microns. Although some overlaps can be seen, monolayer regions were predominant.

3.4.16 Preparation of isotactic polypropylene/layered double hydroxide nanocomposites The effect of LDH crystal size on the properties of isotactic polypropylene (iPP) has been investigated (Nagendra et al., 2015). In this study, two different

146

Layered Double Hydroxide Polymer Nanocomposites

Figure 3.55 SEM image of LDH crystals after ion exchange treatment (left). Some etched pits in the crystals may have formed during the ion exchange process using the acidic solution. TEM image of the LDH nanosheet (right). Source: Adapted from Li, L., Ma, R., Ebina, Y., Iyi, N., Sasaki, T., 2005. Positively charged nanosheets derived via total delamination of layered double hydroxides. Chem. Mater. 17, 4386
4391. With kind permission of the American Chemical Society.

Figure 3.56 AFM image of the first LDH nanosheet layer on an Si wafer precoated with PEI and PSS; nanosheet concentration, 0.5 g/dm. Source: Adapted from Li, L., Ma, R., Ebina, Y., Iyi, N., Sasaki, T., 2005. Positively charged nanosheets derived via total delamination of layered double hydroxides. Chem. Mater. 17, 4386
4391. With kind permission of the American Chemical Society.

suspensions of MgAl-LDH were prepared using gel preparation of LDH and sonication of the LDH. The MgAl-LDH was synthesized through the coprecipitation method. LDH gel was obtained by dispersion and washing of LDH in acetone followed by dispersion in xylene. On the other hand, MgAl-LDH was dispersed in xylene using an ultrasonic bath. The NCs were prepared via solvent blending in xylene and 2.5% and 5% for LDH gel and 1%, 2.5%, 5%, and 10% for sonicated LDH. Fig. 3.57 shows a schematic illustration of iPP/LDH NCs. XRD patterns of the synthesized LDH, LDH gel, and sonicated LDH show that sonication did not affect the crystallinity of the prepared LDH, while the LDH gel has been changed (Fig. 3.58).

Fabrication technologies of layered double hydroxide polymer nanocomposites

147

Figure 3.57 Schematic illustration of the polymer nanocomposite preparation based on isotactic polypropylene and MgAl-LDH layered double hydroxide. Source: Adapted from Nagendra, B., Mohan, K., Gowd, E.B., 2015. Polypropylene/layered double hydroxide (LDH) nanocomposites: influence of LDH particle size on the crystallization behavior of polypropylene. ACS Appl. Mater. Interf. 7, 12399
12410. With kind permission of the American Chemical Society.

Figure 3.58 Powder X-ray diffraction patterns of (a) as-prepared MgAl-LDH, (b) sonicated LDH, and (c) MgAl-LDH gel. Source: Adapted from Nagendra, B., Mohan, K., Gowd, E.B., 2015. Polypropylene/layered double hydroxide (LDH) nanocomposites: influence of LDH particle size on the crystallization behavior of polypropylene. ACS Appl. Mater. Interf. 7, 12399
12410. With kind permission of the American Chemical Society.

The particle sizes for LDH gel and sonicated LDH were reported as 3
4 microns and 50
20 nm, respectively. A TEM image of LDH gel shows delaminated LDH platelets into single-layer nanosheets, in contrast, SEM (Fig. 3.59) and TEM (Fig. 3.60) images of sonicated LDH show broken LDH layers. The pale

148

Layered Double Hydroxide Polymer Nanocomposites

Figure 3.59 Scanning electron microscope images of (A, B) MgAl-LDH dispersed in water and (C) sonicated LDH. Source: Adapted from Nagendra, B., Mohan, K., Gowd, E.B., 2015. Polypropylene/layered double hydroxide (LDH) nanocomposites: influence of LDH particle size on the crystallization behavior of polypropylene. ACS Appl. Mater. Interf. 7, 12399
12410. With kind permission of the American Chemical Society.

contrast of both gel and sonicated LDH shows their single-layer nature in comparison with the bold contrast of the prepared LDH. As can be inferred from the XRD patterns of the iPP/LDH NCs, no changes to the crystallinity of the obtained NCs were made. Therefore, the crystal size of LDHs had no effect on the iPP, but smaller crystal size of LDH (sonicated LDH) caused better nucleation ability of the iPP due to the higher surface area of the sonicated LDH compared with LDH gel. Furthermore, Table 3.2 provides a summary of the preparation of important polymer/LDH NCs and their preparation methods. Also, several valuable books and reviews based on polymer/LDH NCs have been published and the properties of these NCs have been studied in detail (Basu et al., 2014; Jlassi et al., 2017; Mallakpour and Hatami, 2017a).

3.5

Conclusions and future perspectives

LDHs are a new promising material, with exceptional features which can improve the weaknesses of neat polymers and will enhance properties of polymers such as mechanical properties, thermal stability, reduced gas permeability, flame retardancy, electrical properties, water vapor permeability, viscoelasticity, the activation energy of degradation, oxygen barrier, etc. These materials can be modified with an appropriate modifier to raise the basal distance, which results in ease of NC preparation and better dispersion and subsequently shows the best performance. Direct synthesis, anionic exchange, reconstitution (rehydration), and mechanochemistry are the approaches to modify LDHs. Even antimicrobial modifiers can make LDHs antimicrobial, thus it can be used as a drug carrier. LDHs are capable of being modified with a wide range of inorganic or organic anions, and even CNTs can be used for this. Based on the literature, of the three general methods, the most commonly used method for the preparation of polymer/LDH NCs is preexfoliation of the

Fabrication technologies of layered double hydroxide polymer nanocomposites

149

Figure 3.60 TEM images of (A, B) as-prepared MgAl-LDH and corresponding EDS spectrum, (C, D) delaminated MgAl-LDH and corresponding EDS spectrum (inset) selected area electron diffraction (SAED) pattern of delaminated MgAl-LDH, (E, F) sonicated LDH and corresponding EDS spectrum. Source: Adapted from Nagendra, B., Mohan, K., Gowd, E.B., 2015. Polypropylene/layered double hydroxide (LDH) nanocomposites: influence of LDH particle size on the crystallization behavior of polypropylene. ACS Appl. Mater. Interf. 7, 12399
12410. With kind permission of the American Chemical Society.

LDH. This is due to the good dispersion of LDHs into polymeric matrices, consequently giving better results in the obtained NCs. These NCs can be easily characterized using different techniques including FT-IR, FE-SEM, TEM, XRD, TGA, DSC, WAX, etc. As has been reviewed, polymer/LDH NCs are widely used, thus, it is expected that novel methods for the preparation of these NCs will be discovered in the future. LDHs can be combined with other nanoscale materials, such as

Table 3.2 A summary of preparation methods of polymer/LDH NCs and LDHs used Polymer/LDH NCs

LDH

LDH preparation method

CMC/LDH NCs

MgAl-LDH

Coprecipitation

Pectin/LDH NCs

LDH-NO3

Urea method

Chitosan/LDH NCs Natural rubber/LDH NCs CMC, polygalacturonic acid, sodium alginate, curdlan, oxidized cellulose, and carrageenan/LDH NCs PI/LDH NCs

Mg2Al-LDH Zn/Mg-Al LDH

Coprecipitation Coprecipitation

Mg2Al-LDH

Coprecipitation

ZnCr-LDH

Coprecipitation

PMMA/LDH NCs

LiAl- and MgAl-LDHs MgAl-LDH

Coprecipitation

MgAl-LDH

Coprecipitation

Octane, LGA, and butanol


Mg3Al-LDH

Coprecipitation

5-Sulfosalicylic acid

PVAc/LDH NCs P(MMA-co-BA)/ LDH NCs WF-PP/LDH NCs




Modifier or surfactant
Benzoate, 2,4dichlorobenzoate, para-hydroxybenzoate, and orthohydroxybenzoate 5-Aminosalicylic acid Stearic acid Fructose-6-phosphate aldolase


K2 and SDS

Modification method


Preparation method

References

Solvent blending

Ion exchange

Solvent blending

Yadollahi et al. (2014) Gorrasi et al. (2012)

Direct synthesis Direct synthesis

LbL assembly Two-roll mill mixing

Ribeiro et al. (2014) Das et al. (2011)

Direct synthesis

Solvent blending

Mahdi et al. (2015)

In situ polymerization In situ polymerization

Dinari and Rajabi (2016) Tsai et al. (2016)




In situ polymerization

Chen et al. (2015)




In situ polymerization

Veschambres et al. (2016) Peng et al. (2017)


Ion exchange

Ion exchange

Melt blending

PAI/LDH NCs

Mg2Al-LDH

Coprecipitation

Direct synthesis

Solvent blending

Coprecipitation

N-tetrabromophthaloylglutamic L -aspartic-based diacid

PAI/LDH NCs

MgAl-LDH

PAI/LDH NCs

Direct synthesis

Solvent blending

MgAl-LDH

Coprecipitation

CNTCOOH

Direct synthesis

In situ polymerization

LDPE/LDH NCs

ZnAl-LDH

Coprecipitation

Direct synthesis

Ball milling

PVA/LDH NCs

Mg2Al-LDH

Coprecipitation

Direct synthesis

Solvent blending

PVA/LDH NCs

MgAl-LDH

Coprecipitation

Direct synthesis

Solvent blending

Polyester/LDH NCs PVC/LDH NCs

MgAl-LDH MgAl-LDH

Coprecipitation Urea method

Aleurate, citrate, glycolate, parahydroxybenzoate, salicylate, and serine carboxylate N,N0 -(pyromellitoyl)-bisL-phenylalanine diacid N-trimellitylimido-Lisoleucine SPDP Laurylether phosphate

Ion exchange Ion exchange

Mechanical agitation Solvent blending

PP-EVA/LDH NCs SR/LDH NCs EP/MoS2/LDH NCs

Mg2Al-LDH MgAl-LDH NiFe- and CoFe-LDH MgAl-LDH

Coprecipitation Coprecipitation Coprecipitation

SDBS DS MoS2

Direct synthesis Ion exchange Direct synthesis

Melt mixing Solvent blending Solvent blending

Mallakpour and Dinari (2014) Mallakpour and Dinari (2016) Cai et al. (2016) Huang and Wang (2009) Rafiee et al. (2016) Pradhan et al. (2011) Zhou et al. (2017)

Coprecipitation

SDS

Reconstruction

Solvent blending

Kotal et al. (2010)

MgAl-LDH

Coprecipitation

formaldehyde

Direct synthesis

LbL assembly

Li et al. (2005)

MgAl-LDH

Coprecipitation

xylene

Direct synthesis

Solvent blending

Nagendra et al. (2015)

PU/NBR Blend/LDH NCs PEI/PSS hybrid/LDH NCs iPP/LDH NCs

Mallakpour et al. (2016) Mallakpour et al. (2014) Mallakpour and Dinari (2015) Gorrasi and Bugatti (2016)

152

Layered Double Hydroxide Polymer Nanocomposites

CNTs, metal oxides, graphene, etc. to create new properties or to improve and optimize some of their specific features. Currently, it is essential to mention that, in order to save our planet from pollutants, almost all scientists including physicians, pharmacists, materials scientists, chemists, physicists, and engineers in various fields, in academia as well as in industries, must use environmentally friendly methods and materials to develop technologies which will be compatible with the long-term future of the Earth (air, soil, and water). In this regard, using LDHs with biosafe materials such as amino acids, carbohydrates, vitamins, proteins, DNA, and other vital molecules from plants which are plentiful and have an endless natural supply, and which are biocompatible and biodegradable polymers, would be excellent choices to manufacture environmentally benign polymer/LDH NCs and utilize them for a wide variety of future technologies.

Acknowledgments The authors acknowledge the Research Affairs Division, Isfahan University of Technology (IUT), Isfahan, Iran, for partial financial support. Further financial support from National Elite Foundation (NEF), Tehran, Iran, Iran Nanotechnology Initiative Council (INIC), Tehran, Iran, and Center of Excellence in Sensors and Green Chemistry Research (IUT) is gratefully acknowledged.

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130. Ribeiro, L.N., Alcˆantara, A.C., Darder, M., Aranda, P., Arau´jo-Moreira, F.M., Ruiz-Hitzky, E., 2014. Pectin-coated chitosan
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518. Suresh, K., Kumar, R.V., Boro, R., Kumar, M., Pugazhenthi, G., 2018. Rheological behavior of polystyrene (PS)/Co-Al layered double hydroxide (LDH) blend solution obtained

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Microscopic characterization techniques for layered double hydroxide polymer nanocomposites

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Shadpour Mallakpour1,2 and Shima Rashidimoghadam1 1 Department of Chemistry, Organic Polymer Chemistry Research Laboratory, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Material Institute, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

4.1

Introduction

Layered double hydroxides (LDHs) are one type of anionic clay material, also recognized as hydrotalcite like compounds. The chemical composition of LDHs can be explained by the general formula [M1
x21 Mx31 (OH)2]x1(Am2)x/m  nH2O, while M21 is a divalent metal cation (e.g., Mg21, Fe21, Co21, Cu21, Ni21, or Zn21), M31 is a trivalent metal cation (e.g., Al31, Cr31, Ga31, In31, Mn31, or Fe31) and 2 Am2 is an anion with charge m, for example, Cl2, CO22 3 , NO3 , etc., which is placed in the hydrated interlayer galleries and can be relatively and easily replaced. The value of x is equal to the molar ratio M31/(M21 1 M31) and is normally between 0.2 and 0.33 (Fan et al., 2014; Wang and O’Hare, 2012; Elbasuney, 2015). LDHs can be prepared in the laboratory by a variety of synthetic methods, for example, coprecipitation, urea hydrolysis, hydrothermal crystallization, and ion exchange methods (Costa et al., 2008). The functionalization of LDH is an unavoidable process in the fabrication of polymer nanocomposites (PNCs) and is carried out by exchanging the interlayer anions with anionic surfactants such as fatty acid salts (Focke et al., 2010), sulfonates (Wang et al., 2005), and phosphates (Zhang et al., 2015) and so on. Nowadays, PNCs have attracted attention due to the considerable enhancements in material properties, for example, mechanical strength, gas, and solvent barrier, toughness, flame retardancy, etc., in comparison with traditional polymer composites. As a result of the small size of the filler particles, their uniform distribution in the polymer matrix and consequently good interaction between the polymer and nanofiller was achieved. These nano-scaled fillers have a specific surface area which leads to larger matrix/filler interface and so more mutual interactions. Thus a significant improvement in composite properties at very low filler volume fractions was achieved (Mittal, 2010; Mallakpour and Rashidimoghadam, 2017a). Various Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00004-5 © 2020 Elsevier Ltd. All rights reserved.

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nano-fillers, such as LDHs (Becker et al., 2011; Nogueira et al., 2011; Mallakpour and Dinari, 2014a, 2016a), carbon nanotubes (CNTs) (Mallakpour and Rashidimoghadam, 2017b, 2018), metal oxides (Mallakpour and Adnany Sadaty, 2016; Mallakpour and Motirasoul, 2016), clay (Mallakpour and Khani, 2015; Mallakpour and Shahangi, 2013), graphene (Behzadi and Mirzaei, 2016; Huang et al., 2016), carbon black (Zhang et al., 2018; Liu et al., 2018), and graphite (Sa´nchez-Sa´nchez et al., 2018; Yuan et al., 2018) were employed for the preparation of PNCs. Among them, LDHs have been arousing tremendous consideration and research interest in the scientific community owing to their wide range of available chemical compositions, ability of being modified by different types of organic anionic surfactants, and endothermic decomposition liberating water vapor and metal oxide residue (Mallakpour and Hatami, 2017; Mallakpour and Khadem, 2017; Reny Costa et al., 2007; Cao et al., 2016). Thus the preparation of polymer/ LDH NCs has attracted considerable technological and scientific interest (Nogueira et al., 2016) and several techniques for the synthesis of polymer/LDH NCs, such as in situ polymerization, meltmixing, and solution blending and various microscopic characterizations for giving evidence to intercalated or exfoliated morphologies of LDH in different PNCs have been reported.

4.2

Microscopic characterization techniques for PNCs

Characterization of the NCs is essential to obtain valuable information about these materials such as quality of distribution of filler in the polymer matrix, influence of filler surface modification on filler dispersion and composite properties, interactions of the filler modification with the polymer chains, changes in the process parameters on the resulting morphology and properties, and study a wide spectrum of properties to determine the application potential of the NCs (Mittal, 2012). Over the years, various techniques have been developed to explore the structure of PNCs (Bhattacharya et al., 2008), including scattering techniques (Cipelletti et al., 2016), microscopic techniques (Leng, 2010; Adhikari and Michler, 2009), spectroscopic techniques (Gurses, 2016; Ponnamma et al., 2016), chromatography (Bhattacharya et al., 2008), melt state rheometry, solid-state analysis, calorimetry, and others. Microscopic characterization techniques investigate and map the surface and subsurface structure of a material using photons, electrons, ions, or physical cantilever probes. Microscopy is a technique that, combined with other scientific techniques and chemical processes, allows the determination of both the composition and the structure of a material. Different microscopic techniques, such as optical microscopes, scanning electron microscopes (SEMs), transmission electron microscopes (TEMs), field ion microscopes (FIMs), scanning tunneling microscopes (STMs), scanning probe microscopes (SPMs), atomic force microscopes (AFMs), and X-ray diffraction topography (XRT) play a vital role in the characterization of PNC morphology on different length scales (Leng, 2010; Adhikari and Michler, 2009).

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4.2.1 Optical microscope The optical microscope, often referred to as a light microscope, is a type of microscope which uses visible light and a system of lenses to magnify images of small samples or to magnify the fine details of a larger object based on the principles of transmission, absorption, diffraction, and refraction of light waves, in order to examine minute specimens that cannot be seen by the naked eye. The optical microscope is the oldest design of microscope in which all of its parts work together. In the microscope, objects are enlarged or magnified with a convex lens that bends light rays by refraction. Diverging rays from points within the object (object points) are made to converge behind the convex lens and cross over each other to form image points (i.e., a focused image). The distance of the object from the lens divided by the distance of the focused image from the lens determines the magnification. The visibility of the magnified object depends on contrast and resolution. In general, the contrast or differences in light intensity between an object and its background or surroundings render the object distinct. An optical microscope has some advantages: cheap to purchase and operate, small and portable, the natural color of the specimen can be observed, living as well as dead material can be viewed, preparation is relatively quick and simple, requiring little expertise, and it is unaffected by magnetic fields. However, there are some drawbacks that provide limitations to its use in practice: magnifies objects up to 1500 3 , preparation may distort the specimen, the depth of the field is restricted, and it has a resolving power for biological specimens of around 1 nm. The advantages outweigh the disadvantages, however, in some circumstances an electron microscope maybe a better choice (Keller and Goldman, 1989).

4.2.2 Scanning electron microscope A SEM is a type of electron microscope that uses a focused beam of high-energy electrons (which are produced by an electron gun) to produce a variety of signals at the surface of solid specimens. The signals that derive from electron
sample interactions reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure and orientation of materials making up the sample. In most applications, data are collected over a selected area of the surface of the sample, and a two-dimensional image is produced that displays spatial variations in these properties. Areas ranging from approximately 1 cm to 5 μm in width can be imaged in a scanning mode using conventional SEM techniques (magnification ranging from 20 3 to approximately 30,000 3 , spatial resolution of 50
100 nm). The SEM is also capable of performing analyses of selected point locations on the sample; this approach is especially useful in qualitatively or semiquantitatively determining chemical compositions, crystalline structure, and crystal orientations. The design and function of the SEM is very similar to the electron probe microanalyzer and considerable overlap in capabilities exists between the two instruments. Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by

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electron
sample interactions when the incident electrons are decelerated in the solid sample. These signals include secondary electrons (that produce SEM images), backscattered electrons, diffracted backscattered electrons (that are used to determine crystal structures and orientations of minerals), photons (characteristic Xrays that are used for elemental analysis and continuum X-rays), visible light (cathodoluminescence
CL), and heat. Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples (i.e., for rapid-phase discrimination). X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete orbitals (shells) of atoms in the sample. As the excited electrons return to lower energy states, they yield X-rays that are of a fixed wavelength (i.e., related to the difference in energy levels of electrons in different shells for a given element). Thus, characteristic X-rays are produced for each element in a mineral that is “excited” by the electron beam. SEM analysis is considered to be “nondestructive”; that is, X-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyze the same materials repeatedly. The advantages of SEM include its wide array of applications, the detailed three-dimensional and topographical imaging and the versatile information garnered from different detectors. SEM is also easy to operate with the proper training and advances in computer technology and associated software make operation user-friendly. The technological advances in modern SEMs allow for the generation of data in digital form. Although all samples must be prepared before being placed in the vacuum chamber, most SEM samples require minimal preparation. The disadvantages of SEM start with the size and cost. SEMs are expensive, large, and must be housed in an area free of any possible electric, magnetic, or vibration interference. Maintenance involves keeping a steady voltage, currents to electromagnetic coils and circulation of cool water. Special training is required to operate SEM as well as prepare samples. The preparation of samples can result in artifacts. The negative impact can be minimized with knowledgeable experienced researchers being able to identify artifacts from actual data as well as preparation skill. There is no absolute way to eliminate or identify all potential artifacts. In addition, SEMs are limited to solid, inorganic samples small enough to fit inside the vacuum chamber that can handle moderate vacuum pressure. Finally, SEMs carry a small risk of radiation exposure associated with the electrons that scatter from beneath the sample surface. The sample chamber is designed to prevent any electrical and magnetic interference, which should eliminate the chance of radiation escaping from the chamber. Even though the risk is minimal, SEM operators and researchers are advised to observe safety precautions (de Assumpc¸a˜o and Ferri, 2017; Stokes, 2008).

4.2.3 Transmission electron microscope Transmission electron microscopy is a method in which a beam of electrons is transmitted through a very thin specimen and the interactions between the

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electrons and the atoms can be used to observe features such as structure and morphology. This technology can give information about the structure, crystallization, morphology, and stress of a substance, whereas SEM can only provide information about the morphology of a specimen. However, TEM requires very thin specimens that are semitransparent to electrons, which can mean sample preparation takes longer. The TEM operates on the same basic principles as the light microscope but uses electrons instead of light. Because the wavelength of electrons is much smaller than that of light, the optimal resolution attainable for TEM images is many orders of magnitude better than that from a light microscope. In TEM, the source of illumination is a beam of electrons of very short wavelength, emitted from a tungsten filament at the top of a cylindrical column of about 2 m height. The whole optical system of the microscope is enclosed in vacuum. Air must be evacuated from the column to create a vacuum so that the collision of electrons with air molecules and hence the scattering of electrons are avoided. Along the column, at specific intervals magnetic coils are placed. Just as the light is focused by the glass lenses in a light microscope, these magnetic coils in the electron microscope focus the electron beam. The magnetic coils placed at specific intervals in the column acts as an electromagnetic condenser lens system. The specimen stained with an electron-dense material and is placed in the vacuum. The electron beam passes through the specimen and scattered by the internal structures. The heated filament emits electrons which are then accelerated by a voltage in the anode. A higher anode voltage will give the electrons a higher speed. Thus the electrons will have a smaller de Broglie wavelength according to the equation, λ 5 h/mv. The resolving power of a microscope is directly related to the wavelength of the irradiation, which is used to form an image. The faster the electrons travel, the shorter their wavelength. As the wavelength is reduced, the resolution is increased. Therefore, the resolution of the microscope is increased if the accelerating voltage of the electron beam is increased. This technique involves a highvoltage beam of electrons emitted by a cathode and formed by magnetic lenses. The beam of electrons that has been partially transmitted through the very thin specimen carries information about the structure of the specimen. The spatial variation in this information (the “image”) is then magnified by a series of magnetic lenses until it is recorded by hitting a fluorescent screen, photographic plate, or light-sensitive sensor like a CCD (charge-coupled device) camera. The image detected by the CCD may be displayed in real time on a monitor or computer. The advantage of this technique is that it magnifies specimens to a much higher degree than an optical microscope. Magnification of 10,000 times or more is possible, which allows scientists to see extremely small structures. For biologists, the interior workings of cells, such as mitochondria and organelles, are clearly visible. However, this technique has some limitations. TEM requires that specimens be put inside a vacuum chamber. Because of this requirement, the microscope cannot be used to observe living specimens, such as protozoa. Some delicate samples may also be damaged by the electron beam and must first be stained or coated with a chemical to protect them. This treatment sometimes destroys the specimen, however (Tang and Yang, 2017; Brydson, 2011).

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4.2.4 Field ion microscope The FIM is the first imaging tool to directly observe individual atoms. FIM relies on the electric-field-induced ionization of inert gas atoms in the vicinity of a charged surface. When a very sharp metallic needle is subjected to a high voltage of a few kilovolts, an intense electric field is generated at the surface. This electric field is generated by the positive charges present at the surface. Indeed, the application of the high voltage induces the free electrons to be, on average, displaced inwards by a small amount to screen the electric field, leaving partly charged atoms at the very surface. For a nonflat surface, protruding atoms are subjected to a greater charge. Since the electric field at the surface is directly proportional to the charge density, it is higher around these local protrusions. In the case of an atomically smooth curved surface, these protrusions correspond to the edges of atomic terraces. By imaging the distribution of the field intensity at the surface, the FIM provides an atomically resolved image of the surface itself. This technique has been applied mainly to the study of metals and semiconductors, but a few biological images have been obtained (Lefebvre-Ulrikson et al., 2016; Gault, 2012).

4.2.5 Scanning probe microscope The SPM is a family of tools used to make images of nanoscale surfaces and structures, including atoms. The SPM has a probe tip mounted on the end of a cantilever. The tip can be as sharp as a single atom. It can be moved precisely and accurately back and forth across the surface, even atom by atom. When the tip is near the sample surface, the cantilever is deflected by a force. SPMs can measure deflections caused by many kinds of forces, including mechanical contact, electrostatic forces, magnetic forces, chemical bonding, van der Waals forces, and capillary forces. The distance of the deflection is measured by a laser that is reflected off the top of the cantilever and into an array of photodiodes (similar to the devices used in digital cameras). SPMs can detect differences in height that are a fraction of a nanometer, about the diameter of a single atom. The tip is moved across the sample many times. This is why these are called “scanning” microscopes. A computer combines the data to create an image. SPM work differently than optical microscopes because the operator does not have a direct view of the surface but an image that represents the structure of the surface. This method is widely utilized for exploration of the nanoscale structure of materials, as well as their electronic and mechanical properties with its related spectroscopic modes of operation. SPM has many advantages. It provides researchers with a larger variety of specimen observation environments using the same microscope and specimen, reducing the time required to prepare and study specimens. Specialized probes, improvements and modifications to scanning probe instruments continue to provide faster, more efficient, and revealing specimen images with minor effort and modification. Unfortunately, one of the downsides of SPM is that images are produced in black and white or grayscale which can in some circumstances exaggerate a specimen’s actual shape or size. Computers are used to compensate for these exaggerations and

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produce real-time color images that provide researchers with real-time information, including interactions within cellular structures, harmonic responses, and magnetic energy (Paul and Gru¨tter, 2015).

4.2.6 Scanning tunneling microscope The STM is an electron microscope with a resolution sufficient to resolve single atoms. The sharp tip in the STM is similar to that in the SEM, but the differences in the two instruments are profound. In the SEM, electrons are extracted from the tip with a series of positively charged plates placed a few centimeters downstream from the tip. The electrons at the apex of the tip are confined to the region within the metal by a potential barrier. The attractive force from the positive charge on the plates is sufficient to permit the electrons to overcome the barrier and enter the vacuum as free particles. The apertures in the downstream plates form an electron lens that converts the diverging beam from the tip into a beam converging to a focus on the surface of the sample. In the STM, the plates that form the lens in the SEM are removed, and the tip is positioned close to the sample. The electrons move through the barrier in a way that is similar to the motion of electrons in a metal. In metals, electrons appear to be freely moving particles, but this is illusory. In reality, the electrons move from atom to atom by tunneling through the potential barrier between two atomic sites. In a typical case, with the atoms spaced five angstroms apart, there is a finite probability that the electron will penetrate the barrier and move to the adjacent atom. The electrons are in motion around the nucleus, and they approach the barrier with a frequency of 1017 per second. For each approach to the barrier, the probability of tunneling is 1024, and the electrons cross the barrier at the rate of 1013 per second. This high rate of transfer means that the motion is essentially continuous and tunneling can be ignored in metals. Tunneling cannot be ignored in the STM; indeed, it is all-important. When the tip is moved close to the sample, the spacing between the tip and the surface is reduced to a value comparable to the spacing between neighboring atoms in the lattice. In this circumstance, the tunneling electron can move either to the adjacent atoms in the lattice or to the atom on the tip of the probe. The tunneling current to the tip measures the density of electrons at the surface of the sample, and this information is displayed in the image. STMs are helpful because they can give researchers a three-dimensional profile of the surface, which allows researchers to analyze a multitude of characteristics, including roughness, surface defects, and determining things about the molecule size and conformation. It is capable of capturing much more detail than other microscopes. This helps researchers better understand the subject of their research on a molecular level. STMs are also versatile. They can be used for ultra-high vacuum, air, water and other liquids, and gases. They will activate in temperatures as low as zero Kelvin up to a few hundred degrees Celsius. STM works faster than AFM. AFM maximum sample size is 150 3 150 μm. On the other hand, STM generates mm size length and width. Lastly, resolution of STM is much better than AFM. There are very few disadvantages to using an STM. STMs can however be difficult to use effectively. There is a very specific technique that requires a lot of

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skill and precision. STMs require very stable and smooth surfaces, excellent vibration control, and sharp tips. STMs use highly specialized equipment that is fragile and expensive. Although, STM analyzes only conductive materials, AFM is used for conductive and insulator materials. STM requires a vacuum atmosphere but AFM can work even in liquid. For that reason AFM can be used for biological materials (Kalinin and Gruverman, 2007; Chen, 2007).

4.2.7 Atomic force microscope An AFM is a very-high-resolution kind of SPM which provides images of atoms on or in surfaces. An AFM uses a cantilever with a very sharp tip to scan over a sample surface. As the tip approaches the surface, the close-range, attractive force between the surface and the tip causes the cantilever to deflect towards the surface. However, as the cantilever is brought even closer to the surface, such that the tip makes contact with it, an increasingly repulsive force takes over and causes the cantilever to deflect away from the surface. A laser beam is used to detect cantilever deflections towards or away from the surface. By reflecting an incident beam off the flat top of the cantilever, any cantilever deflection will cause slight changes in the direction of the reflected beam. A position-sensitive photo diode (PSPD) can be used to track these changes. Thus, if an AFM tip passes over a raised surface feature, the resulting cantilever deflection (and the subsequent change in direction of the reflected beam) is recorded by the PSPD. An AFM images the topography of a sample surface by scanning the cantilever over a region of interest. The raised and lowered features on the sample surface influence the deflection of the cantilever, which is monitored by the PSPD. By using a feedback loop to control the height of the tip above the surface (thus maintaining constant laser position) the AFM can generate an accurate topographic map of the surface features. AFM has many advantages. It is a powerful tool that is invaluable if you want to measure incredibly small samples with a great degree of accuracy. Unlike rival technologies it does not require either a vacuum or the sample to undergo treatment that might damage it. At the limits of operation however, researchers have demonstrated atomic resolution in high vacuum and even liquid environments. One of the major drawbacks of AFM is the single-scan image size, which is of the order of 150 3 150 μm, compared with millimeters for an SEM. Another disadvantage is the relatively slow scan time, which can lead to thermal drift on the sample. As the technology matures, researchers are relying on there being progress instrumentally, requiring improved signal-to-noise ratio, decreased thermal drift, and better detection and control of tip-sample forces, including the use of sharp probes. Novel solutions are steadily improving these performance issues. To further your understanding, you are welcome to follow this straight forward visual tutorial available online (Gautier et al., 2015; Johnson et al., 2009, https://www.researchgate.net/publication/259889212_Basic_Principles_of_Atomic_Force_Microscopy https://www.researchgate.net/publication/271709095_Atomic_force_microscopybased_force_measurements_on_animal_cells_and_tissueshttps://www.nanoscience.com/ techniques/atomic-force-microscopyhttps://www.weizmann.ac.il/Chemical_Research_

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Support/surflab/peter/afmworkshttps://warwick.ac.uk/fac/sci/physics/current/postgraduate/regs/mpagswarwick/ex5/techniques/structural/afm).

4.2.8 X-Ray diffraction topography XRT is an imaging technique based on Bragg diffraction. The basic working principle of XRT is as follows: an incident, spatially extended beam (mostly of X-rays or neutrons) impinges on a sample. The beam may be either monochromatic, that is, consist a single wavelength of X-rays or neutrons, or polychromatic, that is, be composed of a mixture of wavelengths (“white beam” topography). Furthermore, the incident beam may be either parallel, consisting only of “rays” propagating all along nearly the same direction, or divergent/convergent, containing several greatly different directions of propagation. When the beam hits the crystalline sample, Bragg diffraction occurs, that is, the incident wave is reflected by the atoms on certain lattice planes of the sample, on condition that it hits those planes at the right Bragg angle. Diffraction from the sample can take place either in reflection geometry (Bragg case), with the beam entering and leaving through the same surface, or in transmission geometry (Laue case). Diffraction gives rise to a diffracted beam, which will leave the sample and propagate along a direction differing from the incident direction by the scattering angle. The cross-section of the diffracted beam may or may not be identical to that of the incident beam. In the case of strongly asymmetric reflections, the beam size (in the diffraction plane) is considerably expanded or compressed, with expansion occurring if the incidence angle is much smaller than the exit angle, and vice versa. Independently of this beam expansion, the relationship of sample size to image size is given by the exit angle alone: the apparent lateral size of sample features parallel to the exit surface is downscaled in the image by the projection effect of the exit angle. A homogeneous sample (with a regular crystal lattice) would yield a homogeneous intensity distribution in the topograph (a “flat” image). This method is capable of providing information on the nature and distribution of structural defects such as dislocations, inclusions/precipitates, stacking faults, growth sector boundaries, twins, and low-angle grain boundaries in single-crystal materials (Raghothamachar et al., 2010).

4.3

Microscopic characterization of polymer/LDH NCs

LDHs have been extensively employed as nanofillers in PNCs as a result of their highly tunable properties. Thus several researchers have focused their attention on the preparation and characterization of polymer/LDH NCs (Kutlu et al., 2014; Donato et al., 2012; Wang et al., 2011; Hu et al., 2012; Lv et al., 2009; Peng et al., 2009; Kuila et al., 2008; Hajibeygi et al., 2015; Gorrasi et al., 2012; Wu et al., 2011; Chung and Lai, 2010; Matusinovic et al., 2013). This chapter has been written to present an updated overview of the recent advances in microscopic characterization techniques of various polymer/LDH NCs reported in the years 2013
17. The emphasis is placed on the recent advances in SEM, TEM, and AFM characterization techniques of these NCs.

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4.3.1 Microscopic characterization of elastomer/LDH NCs Laskowska et al. studied the effects of LDHs with different structures, specific surface areas, and Mg/Al ratios on the curing behavior, crosslink density, mechanodynamical properties, transparency, thermo-optical properties, barrier properties, and morphology of carboxylated acrylonitrile
butadiene rubber (XNBR) composites. As a result of the polar surface of LDHs, good compatibility with XNBR, which have polar nitrile and carboxyl groups, is expected. SEM images of XNBR containing 10 phr (parts per hundred rubber) of the Mg-ion rich LDH [which is characterized by the lowest specific surface area (13 m2/g) but has the highest platelet aspect ratio, with layer dimensions of approximately .500 nm in width and 10
50 nm in thickness] at various magnifications showed that the composite exhibited good dispersion of intercalated/exfoliated layers with a thickness of approximately 30 nm (Fig. 4.1). The mechanical, parallel orientation of anisotropic filler layers in the rubber matrix under the conditions of elevated temperature and high shearing force (internal mixer) is advantageous and produces elastomer composites with improved mechanical and gas barrier properties (Laskowska et al., 2014). Two organic and inorganic phosphorus-containing anions (HPO22 and 4 HDEHP2) were successfully intercalated into the Mg/Al LDH following the rehydration process, in basic media by Go´mez-Ferna´ndez et al. Afterward, unmodified LDH and different amounts (1, 3, and 5 pphp) of modified LDH were added into a flexible polyurethane foam (PUF) matrix having a castor-oil-based polyether polyol. SEM analysis showed that the hexagonal shape was not lost in modified LDH, but the surface appeared to be more irregular, especially in the case of LDH-HPO4, which also featured an aggregation of particles due to the high surface energy of the modified clay. SEM micrographs of LDH-HDEHP showed particles with softer edges than LDH-HPO4, which presented flake-like morphology probably due to the

Figure 4.1 SEM images of XNBR containing 10 phr of LDH70 (LDH70 represents MgO/ Al2O3 5 70:30) at various magnifications. Source: Adapted from Laskowska, A., Zaborski, M., Boiteux, G., Gain, O., Marzec, A., Maniukiewicz, W., 2014. Effects of unmodified layered double hydroxides MgAl-LDHs with various structures on the properties of filled carboxylated acrylonitrile
butadiene rubber XNBR. Eur. Polym. J. 60, 172
185. With kind permission of Elsevier.

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Figure 4.2 (A) SEM images of (a) LDH-CO3, (b) LDH-HPO4, and (c) LDH-HDEHP. Magnification 3 10,000 (left) and 3 100,000 (right). (B) SEM micrographs with different magnifications ( 3 25 on the left, 3 100 on the right) of the polyurethane foam nanocomposites, PUF-REF (a), PUF-LCO33 (b), PUF-LHPO43 (c), and PUF-LHDEHP3 (d). [Control sample was denoted as PUF-REF and NC foams were named PUF-LCO31, PUF-LCO33, and PUFLCO35 (with 1, 3, and 5 pphp of LDH-CO3); PUF-LHPO41, PUF-LHPO43, and PUF-LHPO45 (with 1, 3, and 5 pphp of LDH-HPO4) and finally PUFLHDEHP1, PUF-LHDEHP3, and PUF LHDEHP5 (with 1, 3, and 5 pphp of LDH-HDEHP)] (pphp 5 parts per hundred parts polyol). Source: Adapted from Go´mez-Ferna´ndez, S., Ugarte, L., Pen˜a, C., Zubitur, M., Angeles Corcuera, M., Eceiza, A., 2016. Flexible polyurethane foam nanocomposites with modified layered double hydroxides. Appl. Clay Sci. 123, 109
20. With kind permission of Elsevier.

higher size of the organic anion (Fig. 4.2a). The SEM images of the prepared NC foam surfaces showed that foam cells have a polyhedral shape, with most of them consisting of open cells. NC foams demonstrated an uneven cell size distribution but cell structure did not appear collapsed or damaged, probably due to an increase in the reactive mixture viscosity along with the hindering caused by the presence of LDH during the growth of the bubbles, thus favoring the formation of a heterogeneous structure. Additionally, some cells collapsed and thus, bigger cells were observed in PUF-LHPO43 (Fig. 4.2b) (Go´mez-Ferna´ndez et al., 2016). Xu et al. prepared MgAl-LDH-loaded graphene hybrid (RGO-LDH) through the coprecipitation technique. Then, the heptamolybdate (Mo7 O62 24 )-modified RGOLDH hybrid (RGO-LDH/ Mo) was synthesized via the ion exchange technique. They showed that the flame retardancy and smoke suppression properties of polyurethane elastomer (PUE) were improved by incorporation of RGO-LDH/Mo into the PUE. The structure and morphology of GO, RGO, RGO-LDH, and RGOLDH/ Mo were observed by TEM. Fig. 4.3A demonstrates that GO has a very thin twodimensional sheet structure. Compared with GO, the folded layer region of RGO is obviously increased and some areas are restacking in Fig. 4.3B, which is because

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Figure 4.3 TEM images of (A) GO, (B) RGO (C) RGO-LDH (D) and RGO-LDH/Mo. Source: Adapted from Xu, W., Zhang, B., Xu, B., Li, A., 2016. The flame retardancy and smoke suppression effect of heptamolybdate modified reduced graphene oxide/layered double hydroxide hybrids on polyurethane elastomer. Compos. Part A: Appl. Sci. Manuf. 91, 30
40. With kind permission of Elsevier.

graphene is easy to re-aggregate. From Fig. 4.3C, it can be seen that a lot of nanolamella-loaded graphene sheets are attributed to the unique LDH hexagonal structure, indicating that LDH has been successfully loaded on the surface of the graphene layer. After the Mo7 O62 24 was intercalated into the RGO-LDH interlayer, as shown in Fig. 4.3D, the contours of the MgAl-LDH sheets were not obvious, because, after modification, the lattice structure was damaged, resulting in the crystal’s incompleteness, which was consistent with the X-ray diffraction (XRD) results. The TEM images of PUE5 (PUE (98 wt%) 1 RGO-LDH (2 wt%)) and PUE9 (PUE (98 wt%) 1 RGO-LDH/Mo (2 wt%)) demonstrate that the RGO-LDH and RGOLDH/Mo have no obvious agglomeration and their basic size ranges from 200 to

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Figure 4.4 TEM images of PUE5 (A) and PUE9 (B) composites. Source: Adapted from Xu, W., Zhang, B., Xu, B., Li, A., 2016. The flame retardancy and smoke suppression effect of heptaheptamolybdate modified reduced graphene oxide/layered double hydroxide hybrids on polyurethane elastomer. Compos. Part A: Appl. Sci. Manuf. 91, 30
40. With kind permission of Elsevier.

400 nm. These results indicate that both the RGO-LDH and RGO-LDH/Mo are dispersed well in PUE (Fig. 4.4) (Xu et al., 2016). He et al. reported the preparation of LDH-NO3 and sodium p-styrenesulfonate hydrate (SSS) modified LDH complex (LDH-SSS) and the synthesis of two NBR/ LDH composites using sulfur as the curing agent. The aging behavior of NBR/LDH composites was studied. After being heated at 90 C for 96 h for aging, the morphology of NBR/LDH composites were evaluated as shown in Fig. 4.5. It was found that the morphology of LDH and NBR material falls into two different dispersed states in the two composites. Compared with the NBR/LDH-NO3 composites before aging (Fig. 4.5A), the smaller-sized and more uniformly dispersed LDH-SSS particles can be observed on the fractured surface of NBR/LDH-SSS (Fig. 4.5B). In addition, the microvoids appear around dispersed LDH-NO3 particles (Fig. 4.5A); this can barely be observed in the NBR/LDH-SSS composites before aging (Fig. 4.5B). This shows that there is a weaker LDH/polymer interfacial interaction between LDH-NO3 and NBR than that between LDH-SSS and NBR, which makes the local failure of the interface easier for the NBR/LDH-NO3 composite. Localized damage at the polymer/ LDH interface obviously induces interfacial microvoids and is very similar to profuse crazing. Therefore, such isolated microvoids around dispersed LDH are formed as a result of the energy-absorbing process during tensile deformation. This leads to higher tensile strength and elongation at failure for NBR/LDH-NO3 before aging, compared to the unfilled NBR/LDH and NBR/LDH-SSS. During the thermal-oxidative aging of NBR composites, however, large and dense microvoids around dispersed LDH-NO3 (Fig. 4.5C) are visible, while fewer and much smaller microvoids are around dispersed LDH-SSS (Fig. 4.5D). In addition, the obvious NBR/LDH interfacial phase separation

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Figure 4.5 SEM images of fractured surface of (A) NBR/LDH-NO3 before aging, (B) NBR/ LDH-SSS before aging, (C) NBR/LDH-NO3 after aging, and (D) NBR/LDH-SSS after aging. Source: Adapted from He, X., Li, T., Shi, Z., Wang, X., Xue, F., Wu, Z., et al., 2016. Thermal-oxidative aging behavior of nitrile-butadiene rubber/functional LDHs composites. Polym. Degrad. Stabil. 133, 219
226. With kind permission of Elsevier.

can be observed in Fig. 4.5C, but it is difficult to identify this in Fig. 4.5D. These all show that the LDH-SSS have stronger interfacial interactions and better thermaloxidative aging properties than LDH-NO3 in the aspect of modified NBR (He et al., 2016).

4.3.2 Microscopic characterization of thermoplastic polymer/LDH NCs Kutlu et al. utilized a homogeneous precipitation technique for preparation of LDHs with a 2:1 magnesium/aluminum ratio. The fabricated LDHs were then organically modified with camphorsulfonic acid (CSA) and ciprofloxacin. The CSA-modified LDH (LDH1) and ciprofloxacin-modified LDH (LDH2) were melt-compounded with

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high-density polyethylene (PE) and the obtained NCs were further melt-spun using a piston-type spinning device. The interlayer distance of LDH was increased after the modification with CSA and ciprofloxacin because of the intercalation of selected anionic molecules into LDH galleries. The TEM images of the NCs show that LDH1 produced much better distribution in the matrix in comparison with LDH2 (Fig. 4.6) and modification with ciprofloxacin was not fully successful. The layered structure of LDH was retained after melt-mixing in both cases and intercalation and exfoliation of the LDH layers were also partially observed (Kutlu et al., 2014). Xue et al. evaluated the Zn2Al-NO3 LDHs which were prepared by a coprecipitation method, for the adsorption of anionic dye acid red 97 (AC97). They showed

Figure 4.6 TEM images of (A) LDH1/PE nanocomposites in low magnitude, (B) LDH1/PE nanocomposites high magnitude, (C) LDH2/PE nanocomposites in low magnitude, and (D) LDH2/PE nanocomposites in high magnitude. Source: Adapted from Kutlu, B., Meinl, J., Leuteritz, A., Bru¨nig, H., Wießner, S., Heinrich, G., 2014. Up-scaling of melt-spun LDH/HDPE nanocomposites. Macromol. Mater. Eng. 299, 825
833. With kind permission of Elsevier.

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the possibility of resourcing the LDH adsorbent sludge by synthesizing polypropylene (PP)/Zn2Al-AC97 LDH NCs by incorporation of different concentrations (0.2, 0.4, 1, 2, 4, 10 wt%) of the obtained Zn2Al-AC97 LDH into PP using a newly developed solvent-mixing method. The good distribution of filler in the PP matrix was observed when its loading was low (0.2
1 wt%) (Fig. 4.7). However, with the enhancement of an LDH amount from 2 to 10 wt%, the smoothness of the NC surface declines and more LDH nanoparticles can be clearly seen. The rapid precipitation of the polymer composite in hexane leads to the formation of spherical particles in all NCs, as shown in Fig. 4.7 (Xue et al., 2014). Shabanian et al. showed that the incorporation of organo-modified LDH into long aliphatic chain polyamide resulted in a considerable increase in the thermal stability, char yields, and flame retardancy of the NCs as compared to pure polyamide. The NCs were synthesized using a solution intercalation method under ambient conditions in N,N-dimethylacetamide as a solvent. The polyamide was synthesized using direct polycondensation reaction from an oleic acid-based monomer and

Figure 4.7 SEM images of (A) 0.2 wt%, (B) 2 wt%, (C) 4 wt%, and (D) 10 wt% PP/Zn2AlAC97 nanocomposites. Source: Adapted from Xue, T., Gao, Y., Zhang, Z., Umar, A., Yan, X., Zhang, X., et al., 2014. Adsorption of acid red from dye wastewater by Zn2Al-NO3 LDHs and the resource of adsorbent sludge as nanofiller for polypropylene. With kind permission of Elsevier.

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Figure 4.8 TEM micrographs of PA/LDH 3 and PA/LDH 6 (PA 5 polyamide, PA/LDH 3 5 NC containing 3 wt.% SDBS-LDH, and PA/LDH 6 5 NC containing 6 wt.% SDBSLDH). Source: Adapted from Shabanian, M., Basaki, N., Ali Khonakdar, H., Jafari, S.H., Hedayati, K., Wagenknecht, U., 2014. Novel nanocomposites consisting of a semi-crystalline polyamide and Mg
Al LDH: Morphology, thermal properties and flame retardancy. With kind permission of Elsevier.

Mg
Al LDH was modified with sodium dodecyl benzenesulfonate (SDBS) by a one-step method. TEM images (Fig. 4.8) demonstrate the random dispersion of LDH sheets in the polymer matrix even at the high amount of filler, indicating direct evidence of crystal layer exfoliation along with small agglomerations representing a partially delaminated NC. It seems that the dimensions of these agglomerations are too small to be detected by XRD method (Shabanian et al., 2014). The effect of Ni/Al LDH concentration on the rheological behavior of poly (methyl methacrylate) (PMMA)/Ni/Al LDH blends, which were prepared by solvent blending method, was studied by Chakraborty et al. Firstly, Ni/Al LDH was synthesized by a coprecipitation method at constant pH using their nitrate salts and then modified with sodium dodecyl sulfate (SDS). Afterward, the NCs were fabricated by the introduction of different amount of LDHs (3 and 5 wt.%) into PMMA using a solvent blending technique in the presence of methylene chloride as a solvent. FE-SEM images (Fig. 4.9) showed that pristine and modified LDH are principally in the form of flakes and large agglomerates and the layered LDH structure can be seen in both cases. Modified LDHs showed large agglomeration. The good distribution of the particles needed extensive stirring or ultrasonication techniques to break the LDH into smaller dimensions (Chakraborty et al., 2014). Du et al. employed an effective technique for excellent distribution of various amounts of MgAl-LDH (0
5.0 wt%) in poly(vinyl alcohol) (PVA) aqueous dispersion, which can considerably improve the mechanical properties and water resistance of the PVA films while maintaining high film transparency. MgAl-LDH was

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Figure 4.9 FESEM images of (A) pristine Ni/Al LDH, and (B) modified Ni/Al LDH. Source: Adapted from Chakraborty, S., Kumar, M., Suresh, K., and Pugazhenthi, G. (2014). Influence of organically modified NiAl layered double hydroxide (LDH) loading on the rheological properties of poly (methyl methacrylate) (PMMA)/LDH blend solution. Powder Technol. 256, 196
203. With kind permission of Elsevier.

prepared via coprecipitation followed by hydrothermal treatment. The results of SEM and TEM analysis (Fig. 4.10) demonstrated the large LDH aggregates in NCs. Some PVA chains partially intercalated into the structure in the TEM images. At LDH contents 5 1.0 wt%, the LDH nanoplatelets uniformly disperse and show partial intercalating and partial exfoliating structures. At LDH content 5 5.0 wt%, partial intercalating and partial exfoliating structures can also be observed, however, some large LDH aggregates appeared synchronously (Du et al., 2014). Mallakpour et al. examined the effect of novel modified LDHs on thermal and structural properties of PVA. They employed ion exchange reaction of LDH and N, N0 -(pyromellitoyl)-bis-L-phenylalaninediacid in distilled water for fabrication of organically modified chiral LDH (MLDH). The PVA-based NCs were prepared with different amounts of LDH (0, 2, 4, 6, and 8 wt%) by solution-intercalation technique via the ultrasound-assisted technique. The results of FE-SEM and TEM analysis showed that the LDH platelets were well-distributed within the PVA matrix and oriented along the PVA axis in a disorderly fashion (Fig. 4.11). The thermal properties of NCs were enhanced due to the uniform distribution of modified LDH in a polymeric matrix and the strong hydrogen bonding between O
H groups of PVA and the hydroxyl groups of LDH layers or carbonyl group as well as other polar groups of intercalated chiral dicarboxylated anion (Mallakpour and Dinari, 2014b). Polymer electrolyte fuel cells (PEFCs) have attracted much interest recently because of their high power density, low operating temperature, low pollution level, quiet operation, lower corrosion, simplification of stack design, and relatively quick start-up and shut-down (de lasHeras et al., 2017). Chen et al. fabricated a series of novel thermo-responsive NCs by free-radical copolymerization of

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Figure 4.10 SEM (A, B, C) and TEM (A0 , B0 , C0 , Av, Bv, Cv) images of PVA/LDH nanocomposites with different LDH contents. (A, A0 , Av) ɸLDH 5 0.5 wt%; (B, B0 , Bv) ɸLDH 5 1.0 wt%; (C, C0 , Cv) ɸLDH 5 5.0 wt% (ɸLDH 5 LDH contents). Source: Adapted from Du, M., Ye, W., Lv, W., Fu, H., Zheng, Q., 2014. Fabrication of highperformance poly(vinyl alcohol)/MgAl-layered double hydroxide nanocomposites. Eur. Polym J. 61, 300
308. With kind permission of Elsevier.

N-isopropylacrylamide (NIPAm) and the silylanized Mg/Al LDH (SiLDHs) with different mass ratios (PNIPAm-co-SiLDHs). With the aim of protecting the layered structure of LDH in the copolymerization reaction, the silanization process was carried out by the hydrolysis of a silane coupling agent (c-methacryloxypropyltrimethoxysilane, MPTS) on the surface of the wet LDH plates. They also 22 investigated the effect of interlayer anions of LDH (NO2 3 and CO3 ) on the surface characteristics and the sorption properties of the NCs. The results of SEM analysis showed that compared with the NO3
Mg/Al LDH, the MPTS-modified NO3
Mg/ Al LDH has a much smoother surface, as a consequence of the modification of MPTS (Fig. 4.12). PNIPAm-co-Si LDH exhibits the porous structure with an average diameter of about 8 μm. The large porous structure supplies a gallery for water to come in and out. The NO2 3 intercalated LDHs composite showed the stronger sorption capacity for Orange-II than that of CO22 3 because of the larger exposed external surface of the NO2 intercalated LDH NCs (Chen et al., 2015). 3 Nicotera et al. incorporated LDHs with different Mg21/Al31 metal ratios (2:1 2 2 and 3:1) and various interlayer anions (CO22 3 ; ClO4 ; NO3 ) in Nafion matrix by

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Figure 4.11 (A) FE-SEM photographs of LDH (a
c), MLDH (d
f) and PVA hybrid with 8 wt% MLDH (g
i). (B) TEM micrographs of MLDH (a and b) and PVA hybrids containing 8 wt% of MLDH (c and d). Source: Adapted from Mallakpour, S., Dinari, M., 2014b. Novel bionanocomposites of poly (vinyl alcohol) and modified chiral layered double hydroxides: Synthesis, properties and a morphological study. Prog. Org. Coat. 77, 583
589. With kind permission of Elsevier.

solution intercalation technique, in order for preparation of high proton-conducting Nafion-based NCs. The results of SEM analysis of the Naf-LDH-2/1(Mg21/Al31) 2 membranes with ClO2 4 and NO3 countervailing anions, obtained both in surface and in cross-section showed that these films are quite uniform, compact, smooth, and homogeneous on the whole volume without pores or agglomerates, indicating that there is a very good dispersion of the nanoplatelets in the polymeric matrix. Also, the surface, although having some texture with a little deposit of LDH sheets, is compact and homogeneous, and we can affirm that the layers maintain submicrometric dimensions (Fig. 4.13) (Nicotera et al., 2015). Mallakpour et al. synthesized novel Ni-Al LDH intercalated with a bio-active amino acid containing dicarboxylate via coprecipitation reaction of Ni(NO3)2. 6H2O, Al(NO3)3. 9H2O, and N,N0 -(pyromellitoyl)-bis-L-phenylalanine under ultrasonic irradiation. They incorporated different amounts of LDH (2, 4, and 8%) into poly(amide-imide) (PAI) matrix. The presence of amide, imide, and phenol groups in the backbone of the polymer matrix causes hydrogen interactions with functional groups of the modified LDH. FE-SEM analysis (Fig. 4.14) showed the uniform distribution of LDH into the polymer matrix. TEM images of diacid modified NiAl LDH and NC of PAI with 4% diacid modified LDH is shown in Fig. 4.15. TEM images of NC 4% demonstrate a coexistence of LDH layers in the intercalated and the partially exfoliated states. TEM micrograph shows two-dimensional objects which are oriented largely parallel to the grid surface and thin sheet-like objects

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Figure 4.12 SEM images of prepared samples (A) NO3
Mg/Al LDH, (B) SiLDHN, (C) P/ SN-2, (D) P/SC-2, and (E) EDS of P/SN-2 [SiLDHN 5 MPTS modified NO3
Mg/Al LDH, P/SN-2 5 PNIPAm-co-SiLDHN with mass ratios of 3:1 (3 5 NIPAm and 1 5 SiLDHN), P/ SC-2 5 PNIPAm-co-SiLDHC with mass ratios of 3:1 (3 5 NIPAm and 1 5 SiLDHC)]. Source: Adapted from Chen, H., Qian, G., Ruan, X., Frost, R.L., 2015. Abatement of aqueous anionic contaminants by thermo-responsive nanocomposites: (Poly(Nisopropylacrylamide))-co-silylanized Magnesium/Aluminun layered double hydroxides. J. Colloid. Interface. Sci. 448, 65
72. With kind permission of Elsevier.

with similar lateral dimensions. Both modified LDH and PAI/modified LDH NC4% present disc-like images, which are actually the LDH platelets lying flat on the substrate. In addition, the figure also shows that some platelets are overlapping on the edge (Mallakpour et al., 2015).

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Figure 4.13 SEM images of Nafion nanocomposite membranes loaded with 3 wt.% of LDHMg21/Al31 (2/1) and countervailing anions NO32 (A) and ClO42 (B). Source: Adapted from Nicotera, I., Angjeli, K., Coppola, L., Enotiadis, A., Pedicini, R., Carbone, A., et al., 2015. Composite polymer electrolyte membranes based on Mg
Al layered double hydroxide (LDH) platelets for H2/air-fed fuel cells. Solid State Ionics 276, 40
46. With kind permission of Elsevier.

Zhao et al. incorporated Zn
Al LDH into poly(vinylidene fluoride) (PVDF) membranes using a phase inversion method. Fig. 4.16 demonstrates the threedimensional top surface images of the pure and LDH-incorporated PVDF membranes obtained from AFM analysis. The results showed that incorporation of LDH resulted in a great change in the surface morphologies of the pristine membranes. The surface of LDH-embedded membrane appeared to be smoother with both average roughness (Ra) and square roughness (Rq) dropping remarkably. The deep ups and downs with a large area and small quantities were replaced with flatter ones with a small area and large quantities. The change can be attributed to: (1) the existence of well-distributed LDH causing the morphology change of membrane surface; and (2) the incorporation of LDH influenced the shape, number, and size of pores on the membrane surface, producing more uniform pores with smaller size, which was confirmed by the SEM images (Fig. 4.17) (Zhao et al., 2016a). Mallakpour et al. reported the synthesis of bionanocomposite (BNC) materials based on isoleucine containing PAI and modified MgAl-LDH via solution intercalation technique for the first time. They modified MgAl-LDH by an ion exchange reaction in a solution of N,N0 -(pyromellitoyl)-bis-L-isoleucine in distilled water. Afterward, novel optically active PAI/modified LDH BNCs containing 2%, 4%, and 8% of modified LDH were successfully prepared using modified LDH with chiral isoleucine containing dicarboxylate and PAI chains with the same group via a solution intercalation technique for the first time. The results of FE-SEM analysis

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Figure 4.14 FE-SEM photographs of (A, B) modified NiAl LDH, (C, D) PAI/modified LDH NC 2%, and (E, F) PAI/modified LDH NC 8%. Source: Adapted from Mallakpour, S., Khani, M. (2015). Composites of semiaromatic poly (amide-ester-imide) based on bioactive diacid and oragnomodifiednanoclay produced by solution intercalation method: thermal and morphological study. Polym. Plast. Technol. Eng. 54, 541
547. With kind permission of Springer.

showed that the modified LDH is more floppy in comparison with neat LDH, which consists of plate-like shapes stacked on top of each other with lateral dimensions ranging from a few micrometers and thickness of over a few hundred nanometers. Also, it was found that, in BNC4%, the LDH nanosheets have a better dispersion in comparison with BNC with a high content of LDH (8%) (Fig. 4.18). The results of

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Figure 4.15 TEM micrographs of (A, B) modified LDH and (C, D) PAI/modified LDH NC 4%. Source: Adapted from Mallakpour, S., Khani, M. (2015). Composites of semiaromatic poly (amide-ester-imide) based on bioactive diacid and oragnomodifiednanoclay produced by solution intercalation method: thermal and morphological study. Polym. Plast. Technol. Eng. 54, 541
547. With kind permission of Springer.

Figure 4.16 AFM images of M0 and M3 membranes (M0 5 pure PVDF membranes, M3 5 2 wt.% LDH incorporated PVDF membranes). Source: Adapted from Zhao, Y., Li, N., Xu, B., Dong, B., Xia, S., 2016a Preparation and characterization of a novel hydrophilic poly(vinylidene fluoride) filtration membrane incorporated with Zn
Al layered double hydroxides. J. Ind. Eng. Chem. 39, 37
47. With kind permission of Elsevier.

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TEM studies showed that, for modified LDH, the platelets have a hexagonal shape with rounded corners. There are no signs of aggregation visible in the micrographs. The TEM analysis of BNC4% demonstrated a coexistence of modified-LDH layers in the intercalated and partially exfoliated states (Fig. 4.19) (Mallakpour and Dinari, 2016b). Zhao et al. reported the preparation of Zn-Al LDH-modified polyamide (PA) nanofiltration (NF) membranes with LDH loads varying from 0 to 0.250% (w/v) using interfacial polymerization of 1,3-phenylene diamine (MPD) in the aqueous phase and 1,3,5-benzenetricarbonyltrichloride (TMC) in the organic phase. The membranes with various contents of LDH were defined as PA-0, PA-1, PA-2, and PA-3. The surface morphology of various PA membranes was studied by FE-SEM analysis (Fig. 4.20i). The membrane without LDH showed the characteristic “ridgeand-valley” morphology but PA/LDH membranes showed fewer ridges and valleys and more “leaf-like” folds. Fig. 4.20i also presents the cross-section morphologies of PA-0 and PA-2. The addition of LDH to the PA membrane offered a much more even, or smoother surface without the same degree of notable outcropping against the background. Moreover, the thickness of the PA layer decreased, and PA-2 had a denser surface compared to the pure membrane. This phenomenon can be attributed to the hydrophilicity of LDH. Fig. 4.20ii demonstrates the AFM images. The ridges have been flattened and the valleys also rose in PA-2 compared to PA-0, which is consistent with the thickness change of the PA layer in them. In addition, the single ridge in PA-2 has a larger planar area, corresponding to the leaf-like surfaces of modified membranes (Zhao et al., 2016b). Kredatusova et al. described the multifunctional influence of modified LDH on the progress of ring-opening polymerization of ε-caprolactone under microwave irradiation. The modification of LDH was carried out with phosphonium-based ionic liquids (IL) containing phosphinate, carboxylate, and phosphate anions for the ion-exchange reaction with Mg-Al LDH. Poly(ε-caprolactone) (PCL)/LDH NCs were prepared by in situ ring-opening polymerization of ε-caprolactone under microwave irradiation (MROP) in the presence of organically modified LDH. They found that MROP of CL performed in the presence of LDH does not require any additional catalysts or initiators. The results of TEM analysis showed that the NCs prepared by in situ MROP of CL in the presence of phosphinate-LDH and carboxylate-LDH have highly delaminated morphology with a uniform distribution of nanolayers. But, the NC containing phosphate-LDH demonstrated homogeneous dispersion of LDH stacks smaller than 500 nm in size. The stacks of intercalated LDH were also observed in the PCL/LDH composite when the nonmodified LDH was applied as the catalyst for MROP of CL. The NC prepared from nonmodified LDH contained smaller stacks (diameter 200
300 nm) compared to the PCL/phosphate-LDH composite (Fig. 4.21) (Kredatusova´ et al., 2016). Shabanian et al. prepared novel poly(methyl-ether-imide) (PMEI)/ LDH NCs using the solution intercalation method. The synthesis of new PMEI was carried out by preparation of a methyl-rich bisphenol as starting material to produce a new diamine with an ether linkage which was utilized to the fabrication of PMEI. The SDBS was used for modification of Mg
Al LDH in one step and PMEI NCs were

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prepared by solution intercalation technique with various quantities of SDBS modified Mg
Al LDH (4 and 8 mass %). TEM images NC 4% showed that LDH sheets were dispersed in the PMEI matrix, indicating direct evidence of crystal layer exfoliation along with small agglomerations representing a partially delaminated NC (Fig. 4.22) (Shabanian et al., 2016). Hu et al. fabricated the decavanadate-anion with anticorrosive activity intercalated LDH (D-LDH) via anion-exchange reaction. Afterward, the D-LDH was grafted with ethoxy groups of γ-aminopropyltriethoxysilane (APTS) (AD-LDH) and employed in the fabrication of polyaniline (PANI) NCs by in situ polymerization. The results of SEM analysis showed that adding APTS in the D-LDH affected the morphology of the obtained material and reduced the D-LDH aggregation. Also, it is observed that the partial exfoliation of grafted intercalated LDH in the PANI matrix is quite unconspicuous but when comparing with the SEM image of PANI, partially exfoliated AD-LDH can be seen from the SEM image of AD-LDH/PANI composites. Therefore, the AD-LDH is well-dispersed in the PANI matrix and demonstrates partially exfoliated structures (Fig. 4.23) (Hu et al., 2015).

4.3.3 Microscopic characterization of thermosetting polymer/ LDH NCs

L

Li et al. showed that introducing 4.5 wt% of LDHs into sisal fiber reinforced phenolic composites might be effective to improve flame retardancy. The unmodified Mg-Al LDH (NLDH) and SDBS intercalated Mg-Al LDH (SDBSLDH) was incorporated into normal phenolic (PF)/silicone-modified novolac type phenolic resins (SPF) by solvent mixing. Then, biobased silicone modifier (TDS) was used to improve the electrical resistance and water absorption behavior. The results demonstrated that the modified composites optimally show a 60% reduction in total heat release (20.2 MJ/m2) compared to the composites without LDH. The SEM results revealed that all SPF-based composites (SPF-SF-NLDH, SPF-SFSDBSLDH, and SPF-SF) exhibit fiber breakage, while little pull out and debonding is observed. It displays remarkable toughness fracture surface morphology (Fig. 4.24). The excellent interfacial interaction is beneficial for the energy transfer and dissipation, leading to the SPF composites’ high impact strength. In comparison, the PF-based composites (PF-SFNLDH, PF-SF-SDBSLDH, and PF-SF) exhibit a large amount of Figure 4.17 The top surface (A) of M0 (a) and M3 (b) membranes and cross-section (B) morphologies of M0, M1, M2, M3, and M4 (a
e) membranes (M0 5 pure PVDF membranes, M1 5 0.67 wt.% LDH-incorporated PVDF membranes, M2 5 1.33 wt.% LDHincorporated PVDF membranes, M3 5 2 wt.% LDH-incorporated PVDF membranes, and M4 5 2.67 wt.% LDH-incorporated PVDF membranes). Source: Adapted from Zhao, Y., Li, N., Xu, B., Dong, B., Xia, S., 2016a Preparation and characterization of a novel hydrophilic poly(vinylidene fluoride) filtration membrane incorporated with Zn
Al layered double hydroxides. J. Ind. Eng. Chem. 39, 37
47. With kind permission of Elsevier.

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Figure 4.18 FE-SEM photographs of neat LDH (A, B), modified LDH (C, D), BNC4% (E, F), and BNC8% (G, H). Source: Adapted from Mallakpour, S., Dinari, M., 2016b. Using Mg-Al-layered double hydroxide intercalated with chiral dicarboxylic acid for the reinforcement of isoleucine amino acid containing poly(amide-imide). Polym. Compos. 37, 3288
3295. With kind permission of John Wiley and Sons.

Figure 4.19 TEM micrographs of modified LDH (A and B) and BNC4% (C and D). Source: Adapted from Mallakpour, S., Dinari, M., 2016b. Using Mg-Al-layered double hydroxide intercalated with chiral dicarboxylic acid for the reinforcement of isoleucine amino acid containing poly(amide-imide). Polym. Compos. 37, 3288
3295. With kind permission of John Wiley and Sons.

Figure 4.20 (i) FE-SEM images of the surfaces (A) of: (a) PA-0, (b) PA-1, (c) PA-2, (d) PA-3 and the cross-sections (B) of: (a) PA-0, and (b) PA-2. (ii) AFM images of PA-0 and PA-2 membranes. Source: Adapted from Zhao, Y., Li, N., Xia, S., 2016b. Polyamide nanofiltration membranes modified with Zn
Al layered double hydroxides for natural organic matter removal. Compos. Sci. Technol. 132, 84
92. With kind permission of Elsevier.

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Figure 4.21 TEM micrographs of poly(ε-caprolactone) composites prepared under microwave irradiation using various nanofillers: (A) nonmodified LDH, (B) phosphinateLDH, (C) phosphate-LDH, and (D) carboxylate-LDH. Polymerization conditions: constant power: 30 W, polymerization time: 10 min, filler content: 6 wt%. Source: Adapted from Kredatusova´, J., Beneˇs, H., Livi, S., Pop-Georgievski, O., Ecorchard, P., Abbrent, S., et al., 2016. Influence of ionic liquid-modified LDH on microwave-assisted polymerization of ε-caprolactone. Polymer. (Guildf). 100, 86
94. With kind permission of Elsevier.

fiber pull-out and debonding, which implies poor interactions between matrix and fiber, further affecting the energy transfer and dissipation. These two group composites are different only in the matrix resin, SPF versus PF, leading to such huge differences in impact strength and fracture surface morphology. The siliconecontaining SPF promotes the interactions between matrix and sisal fibers to be stronger than the PF composites (Li et al., 2016). Wang et al. reported the modification of LDH with a biobased modifier (cardanolBS) through a one-step coprecipitation technique. Then, various amounts of

Figure 4.22 TEM image of PMEIN 4. Source: Adapted from Shabanian, M., Ardeshir, H., Haji-Ali, S., Moghanian, H., Hajibeygi, M., Faghihi, K., et al., 2016. Efficient poly(methyl-ether-imide)/LDH nanocomposite derived from a methyl rich bisphenol: from synthesis to properties. Appl. Clay Sci. 123, 285-291. With kind permission of Elsevier.

Figure 4.23 SEM images of (A) D-LDH, (B) AD-LDH, (C) PANI, (D) AD-LDH/PANI. Source: Adapted from Hu, J., Gan, M., Ma, L., Zhang, J., Xie, S., Xu, F., et al., 2015. Preparation and enhanced properties of polyaniline/grafted intercalated ZnAl-LDH nanocomposites. Appl. Surf. Sci. 328, 325
334. With kind permission of Elsevier.

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Figure 4.24 SEM images of (A) SPF-SF-NLDH, (B) SPF-SFSDBSLDH, (C) SPF-SF, (D) PF-SF-NLDH, (E) PF-SF-SDBSLDH, and (F) PF-SF. Source: Adapted from Li, C., Wan, J., Pan, Y.-T., Zhao, P.-C., Fan, H., Wang, D.-Y., 2016. Sustainable, biobased silicone with layered double hydroxide hybrid and their application in natural-fiber reinforced phenolic composites with enhanced performance. ACS Sustain. Chem. Eng. 4, 3113
3121. With kind permission of the American Chemical Society.

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Figure 4.25 TEM micrographs of EP/m-LDH-1% (A, E), EP/m-LDH-4% (B, F), EP/mLDH-6% (C, G), and EP/LDH-6% (D, H) at different magnifications. Source: Adapted from Wang, X., Kalali, E.N., Wang, D.-Y., 2015. Renewable cardanolbased surfactant modified layered double hydroxide as a flame retardant for epoxy resin. ACS Sustain. Chem. Eng. 3, 3281
3290. With kind permission of the American Chemical Society.

Cardanol-BS-modified LDH (m-LDH) were incorporated into epoxy resins (EPs) using a combined technique of three-roll mill and ultrasonication. Fig. 4.25 gives the TEM images of EP/m-LDH and EP/LDH composites at various magnifications. The information about the distribution state was obtained from low-magnification images, whereas the high-magnification ones can distinguish whether intercalation and/or exfoliation have been obtained when LDHs are incorporated into epoxy matrices. In the case of EP/m- LDH-1%, a low-magnification TEM image (Fig. 4.25A) reveals a typical feature for epoxy-based NCs, a simultaneous presence of intercalated LDH plates, and small tactoids and stacks. Under high magnification (Fig. 4.25E), it was found that a well-intercalated NC structure has been formed. Similar TEM features were observed for EP/m-LDH-4% (Fig. 4.25B,F) and EP/m-LDH-6% (Fig. 4.25C,G). All of the EP/m-LDH NCs appear to be well-intercalated structures, and the m-LDH nanoplatelets are randomly oriented in the epoxy matrix. In contrast, the lowmagnification image of EP/LDH-6% (Fig. 4.25D) demonstrates that the distribution of neat LDH in the epoxy matrix is poor. Under high magnification (Fig. 4.25H), the unmodified LDHs form large aggregates with thick stacking. These results demonstrate that the uniform distribution of LDH within the epoxy matrix was achieved by modification of LDH by cardanol BS (Wang et al., 2015). Wang et al. reported that the fire hazard characteristics of EP can be improved by incorporation of LDH-wrapped β-FeOOH hybrid material into EP matrix. The SEM images show that the distribution of β-FeOOH rods throughout the EP matrix perform much better than that of LDH in the matrix (Fig. 4.26A). As can be observed in Fig. 4.26B, LDH agglomerates obviously in the EP matrix because of

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Figure 4.26 (A) SEM images for EP0 (a), EP/LDH (b), EP/b-FeOOH (c), EP/LDH-bFeOOH (d) of the fractured surfaces cryogenically broken after immersion in liquid nitrogen. (B) TEM ultrathin observations of the EP/LDH composites (a), EP/b-FeOOH composites (b) and EP/LDH-b-FeOOH composites (c, d). Source: Adapted from Wang, W., Pan, H., Shi, Y., Pan, Y., Yang, W., Liew, K.M., et al., 2016. Fabrication of LDH nanosheets on β-FeOOH rods and applications for improving the fire safety of epoxy resin. Compos. Part A: Appl. Sci. Manuf. 80, 259
269. With kind permission of Elsevier.

the H-bonding force of the LDH. The distribution of β-FeOOH throughout the EP matrix performs relatively well, but the structure of the rods collapses obviously because of stirring, ultrasonication, and shearing during the fabrication process. It can be seen that β-FeOOH rods surrounded by LDH demonstrate uniform distribution throughout the EP matrix. The physical area increases significantly because of the combination of LDH nanosheets and β-FeOOH rods, which is beneficial to the improvement of flame retardancy (Wang et al., 2016). Zhou et al. incorporated the LDH/MoS2 hybrids which were synthesized via self-assembly of exfoliated MoS2 nanosheets and LDH via electrostatic force, into epoxy to reduce its fire hazard risk. The results of SEM analysis showed that the pure epoxy has a smooth fracture surface, indicating the typical brittle failure of a thermosetting polymer. The fractured surface of epoxy systems shows entirely different fractographic features. For example, the MoS2-filled epoxy system demonstrates a rough surface with obvious agglomeration structure. A few individual MoS2 nanosheets exposed from the matrix can be observed, indicating the relatively weak interfacial interactions between MoS2 nanosheets and epoxy matrix. For the epoxy systems filled with LDH/MoS2 hybrids, it was observed that the LDH/MoS2 hybrids are dispersed well and completely embedded in the epoxy matrix without agglomeration, which is probably ascribed to the inhibition effect of deposited LDH on the restacking of MoS2 nanosheets. The uniform dispersion of LDH/MoS2 hybrids in epoxy matrix leads to prominent improvement of thermal stability and fire safety (Fig. 4.27) (Zhou et al., 2017). Shafiei et al. prepared the uniform and bead-free fibers of PCL composited with various amounts of LDH (ranging from 0.1 to 10 wt%) via an electrospinning method. They showed that the incorporation of LDH into PCL scaffold increased

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Figure 4.27 SEM images of the fractured surfaces for neat EP (A), EP/2% MoS2 (B), EP/ 2% NiFe-LDH-MoS2 (C), and EP/2% CoFe-LDH-MoS2 (D) composites (EP 5 epoxy). Source: Adapted from Zhou, K., Gao, R., Qian, X., 2017. Self-assembly of exfoliated molybdenum disulfide (MoS2) nanosheets and layered double hydroxide (LDH): Towards reducing fire hazards of epoxy. J. Hazard. Mater. 338, 343
355. With kind permission of Elsevier.

the in vitro degradation rate when subjected to accelerated degradation solution. The SEM analysis of fibers showed that the neat PCL scaffold has smooth surface morphology with a uniform diameter. However, it should be noted that all fabricated fibers were almost bead-free and randomly oriented in directions (Fig. 4.28). In order to use the PCL-LDH scaffolds in tissue engineering applications, the effect of electrospun PCL-LDH on the proliferation of mouse adipose-derived stem cells (mADSCs) was studied using SEM analysis (Fig. 4.29). It was found that, in both the PCL and PCL-LDH NC scaffolds, cells were well spread and flattened onto the fibers in an irregular pattern and secreted extracellular matrix (ECM). The sheet-like cells exhibited filopodia-like structures (star-like structures) and were elongated along the fibers as seen in the SEM image. It was also observed that cells organized into groups making chains of cells and colony layers. The spindle-like shapes of cells indicated appropriate media for supporting the growth of the cell. Some cells migrated into the pores of the PCL-LDH scaffolds, spreading on different layers, while this phenomenon could not be seen in PCL scaffolds. This might be because of the larger pores and presence of inducing agents that facilitate cell attachments and spreading (Shafiei et al., 2016).

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Figure 4.28 SEM micrograph of electrospun scaffolds; (A) PCL, (B) PCL 1 0.1% LDH, (C) PCL 1 1% LDH, (D) PCL 1 10%LDH (scale bars represent 10 μm). Source: Adapted from Shafiei, S.S., Shavandi, M., Ahangari, G., Shokrolahi, F., 2016. Electrospun layered double hydroxide/poly (ε-caprolactone) nanocomposite scaffolds for adipogenic differentiation of adipose-derived mesenchymal stem cells. Appl. Clay Sci. 127, 52
63. With kind permission of Elsevier.

4.3.4 Microscopic characterization of polymer blend/LDH NCs Rafiee et al. reported the preparation of PP/ethylene vinyl acetate copolymer (EVA) NCs containing organo-modified LDH by a one-step melt mixing process in the presence of a maleated PP(PP-g-MA) as compatibilizer. The results of TEM analysis demonstrated that LDH nanoparticles in the exfoliated and/or intercalated states were mainly localized in the EVA phase. The average particle size of EVA domains was reduced in the presence of LDH and PP-g-MA (Fig. 4.30). It has been observed that,

Figure 4.29 SEM observation of mADSCs cultured on (A, B) PCL, (C, D) PCL 1 0.1% LDH, (E, F) PCL 1 1% LDH, (G, H) and PCL 1 10% LDH scaffolds. Scale bars represent 100 and 2 μm. Source: Adapted from Shafiei, S.S., Shavandi, M., Ahangari, G., Shokrolahi, F., 2016. Electrospun layered double hydroxide/poly (ε-caprolactone) nanocomposite scaffolds for adipogenic differentiation of adipose-derived mesenchymal stem cells. Appl. Clay Sci. 127, 52
63. With kind permission of Elsevier.

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Figure 4.30 TEM images of P75E25C5L5 samples with different magnifications. Source: Adapted from Rafiee, F., Otadi, M., Goodarzi, V., Khonakdar, H.A., Jafari, S.H., Mardani, E., et al., 2016. Thermal and dynamic mechanical properties of PP/EVA nanocomposites containing organo-modified layered double hydroxides.Compos. Part B: Eng. 103, 122
130. With kind permission of Elsevier.

EVA as a minor phase forms droplet-type morphology within the PP matrix. EVA has a dark color because its electron density is much higher than that of PP and black areas represent LDH layers. TEM micrographs with high magnification (Fig. 4.30A, B) showed that LDH layers are broadly localized in the EVA domains in a disordered manner based on their polarization properties. LDH is seen mainly in the intercalated state within EVA domains. As shown in high-magnification TEM micrographs of the same sample (Fig. 4.30C,D), LDH layers were attracted to the EVA phase and formation of intercalated nanostructures is evident. There also exist some aggregates or stacks in this sample. Fig. 4.30A,B revealed characteristic TEM images of P25E75C5L5 (PP 5 25 wt%, EVA 5 75 wt%, compatibilizer 5 5 wt%, LDH 5 5 wt%) sample at different magnifications. In this sample, EVA is a major phase which attracts LDH particles. Fig. 4.31A confirms that LDH layers were partially

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Figure 4.31 TEM images of P25E75C5L5 samples with different magnifications. Source: Adapted from Rafiee, F., Otadi, M., Goodarzi, V., Khonakdar, H.A., Jafari, S.H., Mardani, E., et al., 2016. Thermal and dynamic mechanical properties of PP/EVA nanocomposites containing organo-modified layered double hydroxides.Compos. Part B: Eng. 103, 122
130. With kind permission of Elsevier.

dispersed within the EVA phase. The TEM image of this sample in low magnification in Fig. 4.31B shows the presence of intercalated LDH structures implying that EVA chains could penetrate into galleries of LDH layers (Rafiee et al., 2016). Bercea et al. showed that the chitosan/PVA/Mg-Al LDH NCs are a good candidate for preparation of novel composite biomaterials having various functional properties of chitosan coupled with good mechanical properties of PVA, on one side, and highly tunable properties, increased permeability, and retention of LDH nanoparticles, on the other. The Mg-Al LDH was prepared via the coprecipitation technique and was incorporated into chitosan/PVA mixture. The influence of pH on rheological and structural properties of the prepared NCs was also investigated. SEM images for chitosan/PVA/LDH NCs at pH 5 3, pH $ 7, and pH 5 10, showed that the polymer/clay mixture is uniformly dispersed at pH 5 3, but the network structure is not formed (Fig. 4.32). It was found that the porous structure obtained for a neutral or weakly basic environment where the gelation is induced by the polymer
polymer interactions and by the clay presence. Fracture surface analysis demonstrated that the LDH is uniformly distributed into the chitosan/PVA matrix. An SEM image of LDH shows that the LDH sample is highly crystalline and its structure consists of interconnected hexagonal-shaped particles (Bercea et al., 2015). Pak et al. prepared poly-3-hydroxybutyrate/poly(butyleneadipate-co terephthalate)/ LDH (PHB/PBAT/LDH) NCs from a binary blend of PHB/PBAT and stearate-Zn3Al LDH via a solution-casting technique. The dispersion of stearate-Zn3Al LDH in the

Figure 4.32 SEM micrographs of (A) CS/PVA/LDH composite at pH 5 3; (B) CS/PVA/ LDH composite at pH $ 7, and (C) LDH at pH 5 10 (CS 5 chitosan). Source: Adapted from Bercea, M., Bibire, E.-L., Morariu, S., Teodorescu, M., Carja, G., 2015. pH influence on rheological and structural properties of chitosan/poly(vinyl alcohol)/ layered double hydroxide composites. Eur. Polym J. 70, 147
156. With kind permission of Elsevier.

Figure 4.33 TEM micrographs of PHB/PBAT/2.0 wt% stearate-Zn3Al LDH. Source: Adapted from Pak, Y.L., Bin Ahmad, M., Shameli, K., Yunus, W.M.Z.W., Ibrahim, N.A., Zainuddin, N., 2013. Mechanical and morphological properties of poly-3hydroxybutyrate/poly(butyleneadipate-co-terephthalate)/layered double hydroxide nanocomposites. J. Nanomater. 2013, 8. Open access journal.

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NCs was studied by TEM analysis. The results demonstrated that the intercalated layers that were uniformly dispersed can be observed in PHB/PBAT. It was found that the stack consists of several layers, which showed the formation of intercalated NCs. The TEM analysis demonstrated that the LDH layers are in an intercalated but not well-ordered structure form and are dominantly distributed in the PHB/PBAT matrix (Fig. 4.33) (Pak et al., 2013).

4.4

Conclusions

Characterization of the NCs is necessary to obtain valuable information about these materials such as quality of distribution of filler in the polymer matrix, influence of filler surface modification on filler dispersion and composite properties, interactions of the filler modification with the polymer chains, changes in the process parameters on the resulting morphology and properties, and study of a wide spectrum of properties to determine the potential applications of NCs. This chapter describes the recent study of polymer/LDH NCs using different microscopic techniques such as SEM, TEM, and AFM. The capabilities of each microscopy technique allow investigation of NCs from different aspects. SEM analysis provides information from electrons scattered on the surface of the sample, so it is beneficial for imaging thicker and bulkier samples. TEM imaging, in which a high-energy beam of electrons is shone through a very thin sample, and the interactions between the electrons and the atoms can be used to observe ultrafine patterns on nanostructures. AFM can provide a topographical and mechanical view of the surface, with an easier sample preparation protocol. All of these techniques would be suitable and are vitally important and essential to understanding the better formation of polymer/ LDH NCs for a wide variety of applications in different technologies. We are sure that in the near future the advances in the above methods will provide more information in this field and a greater insight into polymer/NC structures.

Acknowledgments The authors wish to express their gratitude to the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, I. R. Iran, for partial financial support. Further financial support from National Elite Foundation (NEF), Tehran, I. R. Iran, Iran Nanotechnology Initiative Council (INIC), Tehran, I. R. Iran, and Center of Excellence in Sensors and Green Chemistry Research (IUT), Isfahan, I. R. Iran is gratefully acknowledged.

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2107. Mallakpour, S., Dinari, M., 2014a. Structural Characterization and thermal properties of chiral poly(amide-imide)/modified MgAl layered double hydroxide nanocomposites prepared via solution intercalation. Polym. Plast. Technol. Eng. 53, 1047
1055. Mallakpour, S., Dinari, M., 2014b. Novel bionanocomposites of poly(vinyl alcohol) and modified chiral layered double hydroxides: synthesis, properties and a morphological study. Prog. Org. Coat. 77, 583
589. Mallakpour, S., Dinari, M., 2016a. Bionanocomposite materials from layered double hydroxide/N-trimellitylimido-l-isoleucine hybrid and poly(vinyl alcohol). J. Thermoplast. Compos. Mater. 29, 623
637. Mallakpour, S., Dinari, M., 2016b. Using Mg-Al-layered double hydroxide intercalated with chiral dicarboxylic acid for the reinforcement of isoleucine amino acid containing poly (amide-imide). Polym. Compos. 37, 3288
3295. Mallakpour, S., Hatami, M., 2017. Condensation polymer/layered double hydroxide NCs: preparation, characterization, and utilizations. Eur. Polym. J. 90, 273
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547. Mallakpour, S., Motirasoul, F., 2016. Covalent surface modification of [small alpha]-MnO2 nanorods with l-valine amino acid by solvothermal strategy, preparation of PVA/[small alpha]-MnO2-l-valine nanocomposite films and study of their morphology, thermal, mechanical, Pb(ii) and Cd(ii) adsorption properties. RSC Adv. 6, 62602
62611. Mallakpour, S., Rashidimoghadam, S., 2017a. Chapter 8 Recent developments in the synthesis of hybrid polymer/clay nanocomposites: properties and applications. In: Thakur, V. K., Thakur, M.K., Pappu, A. (Eds.), Hybrid Polymer Composite Materials Volume 4: Applications. Woodhead Publishing, Sawston, Cambridge, Elsevier, pp. 227
265. eBook ISBN: 9780081007860 (https://doi.org/10.1016/B978-0-08-100785-3.00008-5). Mallakpour, S., Rashidimoghadam, S., 2017b. Starch/MWCNT-vitamin C nanocomposites: electrical, thermal properties and their utilization for removal of methyl orange. Carbohyd. Polym. 169, 23
32. Mallakpour, S., Rashidimoghadam, S., 2018. Application of ultrasonic irradiation as a benign method for production of glycerol plasticized-starch/ascorbic acid functionalized MWCNTs nanocomposites: investigation of methylene blue adsorption and electrical properties. Ultrason. Sonochem. 40, 419
432. Mallakpour, S., Shahangi, V., 2013. Bio-modification of cloisite Na1 with chiral L-leucine and preparation of new poly(vinyl alcohol)/organo-nanoclaybionanocomposite films. Synth. React. Inorg. Metal-Organic Nano-Metal Chem. 43, 966
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Mallakpour, S., Dinari, M., Nabiyan, A., 2015. Production of NiAl-layered double hydroxide intercalated with bio-safe amino acid containing organic dianion and its utilization in formation of LDH/poly(amide-imide) nanocomposites. J. Polym. Res. 22, 11. Matusinovic, Z., Feng, J., Wilkie, C.A., 2013. The role of dispersion of LDH in fire retardancy: the effect of different divalent metals in benzoic acid modified LDH on dispersion and fire retardant properties of polystyrene
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196. Wang, D.-Y., Leuteritz, A., Kutlu, B., Auf der Landwehr, M., Jehnichen, D., Wagenknecht, U., et al., 2011. Preparation and investigation of the combustion behavior of polypropylene/ organomodified MgAl-LDH micro-nanocomposite. J. Alloy. Compd. 509, 3497
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3290. Wang, W., Pan, H., Shi, Y., Pan, Y., Yang, W., Liew, K.M., et al., 2016. Fabrication of LDH nanosheets on β-FeOOH rods and applications for improving the fire safety of epoxy resin. Compos. Part A: Appl. Sci. Manufact. 80, 259
269. Wu, D., Chang, P.R., Maa, X., 2011. Preparation and properties of layered double hydroxide
carboxymethylcellulose sodium/glycerol plasticized starch nanocomposites. Carbohyd. Polym. 86, 877
882. Xu, W., Zhang, B., Xu, B., Li, A., 2016. The flame retardancy and smoke suppression effect of heptaheptamolybdate modified reduced graphene oxide/layered double hydroxide hybrids on polyurethane elastomer. Compos. Part A: Appl. Sci. Manuf. 91, 30
40. Xue, T., Gao, Y., Zhang, Z., Umar, A., Yan, X., Zhang, X., et al., 2014. Adsorption of acid red from dye wastewater by Zn2Al-NO3 LDHs and the resource of adsorbent sludge as nanofiller for polypropylene. Yuan, K., Zhou, Y., Sun, W., Fang, X., Zhang, Z., 2018. A polymer-coated calcium chloride hexahydrate/expanded graphite composite phase change material with enhanced thermal reliability and good applicability. Compos. Sci.Technol. 156, 78
86. Zhang, S., Liu, X., Gu, X., Jiang, P., Sun, J., 2015. Flammability and thermal behavior of polypropylene composites containing dihydrogen phosphate anion-intercalated layered double hydroxides. Polym. Compos. 36, 2230
2237. Zhang, G., Xu, Z., Liu, P., Su, Y., Huang, T., Liu, R., et al., 2018. A facile in-situ polymerization strategy towards polyimide/carbon black composites as high performance lithium ion battery cathodes. Electrochim. Acta 260, 598
605. Zhao, Y., Li, N., Xu, B., Dong, B., Xia, S., 2016a. Preparation and characterization of a novel hydrophilic poly(vinylidene fluoride) filtration membrane incorporated with Zn
Al layered double hydroxides. J. Ind. Eng. Chem. 39, 37
47.

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Zhao, Y., Li, N., Xia, S., 2016b. Polyamide nanofiltration membranes modified with Zn
Al layered double hydroxides for natural organic matter removal. Compos. Sci. Technol. 132, 84
92. Zhou, K., Gao, R., Qian, X., 2017. Self-assembly of exfoliated molybdenum disulfide (MoS2) nanosheets and layered double hydroxide (LDH): towards reducing fire hazards of epoxy. J. Hazard. Mater. 338, 343
355.

Further reading Liau, C.P., Bin Ahmad, M., Shameli, K., Yunus, W.M.Z.W., Ibrahim, N.A., Zainuddin, N., et al., 2014. Preparation and characterization of polyhydroxybutyrate/polycaprolactone nanocomposites. Sci. World J. 2014, 9.

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X-ray diffraction analysis of layered double hydroxide polymer nanocomposites

5

Rodrigo Botan and Sabrina de Bona Sartor Unifacvest University, Lages, SC, Brazil

5.1

Introduction

The polymers are macromolecules consisting of repeating monomeric units. At present there is a wide variety of polymers with properties and applications in several fields of engineering. However, traditional polymers have limits of application that force their improvement or even the development of new materials that meet these needs. One of the alternatives for polymer materials improvement was the development of the polymer composites, which have as a definition: “Composite materials are a macroscopic combination of two or more distinct materials, having a recognizable interface between them” (Reinhart, 1987). In this combination, the constituents retain their identities and function together, which makes the properties of the composite better than that of each constituent individually. However, through the continuous development of the technology, polymer composites and neat polymers are no longer meeting some needs for improved properties, such as tensile strength, fracture toughness, high temperature resistance, and gas permeation resistance. In search of perfecting or developing new materials, research centers and industries around the world are creating and studying polymer nanocomposites in order to meet these needs. Nanocomposites are hybrid materials in which at least one of the components has nanometric dimensions. As in traditional composites, one of the components is the matrix, in which the particles of the second material (filler) are dispersed (Esteves et al., 2004; Wing Mai and Zhen, 2006). Among the many existing polymer nanocomposite types, those synthesized with layered double hydroxides (LDHs) have been highlighted, mainly because of the great versatility that these LDHs present in their synthesis and the properties of the new polymer nanocomposites. The properties that show improvement are mainly the mechanical properties, thermal properties, flammability properties, and reduced gas permeability (Wing Mai and Zhen, 2006; Wang and O’Hare, 2012).

Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00005-7 © 2020 Elsevier Ltd. All rights reserved.

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The synthesis and characterization of polymer nanocomposites is a recent and developing area, so new methods of synthesis and properties never before reported are constantly presented in the literature. Among the most common methods used to synthesize polymer nanocomposites, it is possible to identify four routes: Melt mixing, exfoliation/adsorption, in situ nanoparticle synthesis, and in situ polymerization (Esteves et al., 2004; Wypych and Satyanarayana, 2005; Khan and O’Hare, 2002). These polymer nanocomposites in all cases must be characterized using several characterization techniques, with the main objective of proving their effective synthesis, as well as identifying their morphologies and properties. One of the most important techniques used in the characterization of polymer nanocomposites is X-ray diffraction (XRD) analysis. Through this analysis it is possible to obtain valuable information about polymer nanocomposites, especially information on morphology. Nevertheless, this technique is also used to characterize the fillers used in polymer nanocomposites, LDH. Thus this chapter discusses this important technique of analysis, XRD, used to characterize LDHs polymer nanocomposites.

5.2

X-ray diffraction analysis

In 1895, Wilhelm Conrad Ro¨entgen, while working with cathode rays produced by Crookes tubes, discovered the X-ray. After publishing photographic observations of his wife’s hand, where the bones could be observed, X-rays were very quickly used to generate medical radiographies and technical applications. However, Ro¨entgen could not evaluate interference, reflection, or refraction effects (Guinebretie`re, 2007; Epp, 2016). During the period after the publication of Ro¨entgen’s discovery until the beginning of the First World War, many researchers studied X-rays and their applications, such as Thomson, Stokes, Rutherford, and others (Guinebretie`re, 2007). The properties of X-rays were gradually discovered: they propagated in a straight line, they were able to penetrate materials of different thicknesses, especially the less dense ones, they produced fluorescence in some materials and that they did not undergo refraction and also they were not reflected. Thus, through these studies, it was possible to expand the study of physical rays, such as cathode rays, X-rays, alpha rays, beta rays, and gamma rays. In April of 1912, Max von Laue, a German physicist, and Walter Friedrich, Arnold Sommerfeld’s assistant, along with Paul Knipping, one of Ro¨entgen’s students, irradiated a copper sulfate crystal with a polychromatic X-ray beam and observed on the photographic plate, for the first time, that X-rays can be diffracted by crystals. This experiment was the starting point of a new field of experimental physics, crystallography. Furthermore, Laue published several articles showing that the diffraction spots are distributed along conic curves and established the fact that X-rays are electromagnetic in nature. For this work, Max von Laue won the

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Nobel Prize in Physics in 1914 (Guinebretie`re, 2007; Glazer, 2013; “Max von Laue
Biographical,” 2017). Shortly after Laue’s studies, from 1912 to 1914, William Henry Bragg and his son William Lawrence Bragg, conducted some experiments using X-ray with NaCl and ZnS crystals. From the study of XRD in these crystals, W.L. Bragg deduced a formula that made it possible to calculate the position of atoms within a crystal through the form in which an X-ray beam is diffracted by the crystal lattice, and, in 1913, used XRD patterns to deduce the NaCl structure (Helliwell, 2013). From the knowledge of the NaCl structure (the first crystalline structure to be determined), W.L. Bragg derived an absolute X-ray wavelength and his father, W.H. Bragg, developed the X-ray spectrometer, allowing different types of crystals to be analyzed (Helliwell, 2013). The Nobel Prize in Physics 1915 was awarded jointly to Sir William Henry Bragg and William Lawrence Bragg “for their services in the analysis of crystal structure by means of X-rays” (“William Bragg
Biographical,” 2017). Currently, XRD methods are the most effective methods for determining the crystal structure of materials from different origins. Fundamentally, the phenomenon of diffraction is observed when a wave passes through an orifice or a slit, whose dimensions are the same size of magnitude. Due to its reduced wavelength, in the range of 0.01
10 nm, X-rays can only be diffracted by structures with atomic dimensions (Guinebretie`re, 2007; Leng, 2008). In crystalline structures, the dispersion centers (atoms or groups of atoms) are spaced periodically at fixed distances and are considered as three-dimensional diffraction networks for X-rays. When the radiation strikes the crystal structure, it is reflected by each of the planes of parallel atoms. Scattering occurs without modification of the wavelength (coherent scattering or Thomson scattering) or incoherently, that is, with a change in wavelength (incoherent scattering or Compton scattering). Due to the fact that it is coherent, the X-rays from Thomson scattering are responsible for the diffraction (Guinebretie`re, 2007). Diffraction theory, according to W.L. Bragg, explains that for certain directions and wavelengths, very pronounced peaks of intensity of scattered radiation were observed. Bragg assumed that the incident waves were reflected specularly by parallel planes of atoms of the crystal, and that the rays reflected from successive planes would produce constructive interference under certain conditions (Guinebretie`re, 2007). In a crystal lattice with the space between its planes, the optical path difference between the rays reflected by adjacent planes is given by 2dsinθ which should be equal to the integral multiple (n) of wavelength (λ) of X-ray used, for constructive interference to take place (Guinebretie`re, 2007; Leng, 2008). 2d:sinθ 5 n:λ

(5.1)

Eq. (5.1) is Bragg’s law, where θ is the complement of the angle of incidence and n is known as the order of the diffraction. Although the reflection in each plane is to speculate, only for certain values of θ will the reflections from all the parallel

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planes be added. For diffraction to be possible, the wavelength should be at most equal to twice the interplanar distance (λ # 2d). Bragg’s law also describes conditions for diffraction occurring, through the planes of a crystal. For graphical expression and interpretation, the Ewald sphere may be used, whose can predict diffraction pattern in a polycrystalline material, considered as an aggregate of the crystals with all possible orientations in threedimensional space (Guinebretie`re, 2007; Leng, 2008). Thus, X-ray scattering and Bragg diffraction provide information for X-ray diffractometry, a method based on the ability of crystals to diffract X-rays, making possible the study of the crystalline structures. Furthermore, morphological and structural information, like crystal atomic structure (positions and symmetry of the atoms in the unit cell), size and shape of the domain, identification of the crystalline phases, and quantitative determination of their weight fractions can be provided (Giannini et al., 2016). ˚ or 100 nm), the When the size of the crystals is less than 0.1 μm (1.000 A Scherrer equation or Debye
Scherrer equation (Eq. 5.2) is used to determine its size. This equation relates the size of submicrometer particles, or crystallites, in a solid to the broadening of a peak in a diffraction pattern (Burton et al., 2009; Uvarov and Popov, 2013). L 5 ðK:λÞ=ðβ:cosθÞ

(5.2)

where, L is the crystallite size, λ is the X-ray wavelength (nm), β is the width of the peak [full width at half maximum (FWHM) or integral breadth] after correcting for instrumental peak’s broadening (in radians), θ is the Bragg angle (in degrees), and K is the Scherrer constant. K is a constant related to crystallite shape, with typical value of about 0.9, and K values can be varied with the actual shape of the crystallite (Giannini et al., 2016; Burton et al., 2009). The use of the Scherrer equation allowed the development of the microstructural analysis of nano-scale particles and materials. Since the beginning of research with XRD in crystals, this technique has been developed and is widely used to characterize molecules of different origins, especially in engineering and materials science. XRD methods can be classified into two types: spectroscopic and photographic. The spectroscopic technique, known as X-ray powder diffractometry, or simply X-ray diffractometry, is the most widely used diffraction method. Photographic techniques are not widely used as diffractometry in modern laboratories (Guinebretie`re, 2007; Leng, 2008). The XRD instrument is called an X-ray diffractometer. There are a wide variety of types, configurations, and shapes of X-ray diffractometers, depending on the type of experiment being performed. Essential components of a typical X-ray diffractometer include: a source of X-rays, filters, mirrors and monochromators, and detectors (Guinebretie`re, 2007). X-ray sources are usually a sealed X-ray tube (Crookes tubes, Coolidge tubes, high-intensity tubes), where high-speed electrons collide with a metal target, or a synchrotron radiation. The main advantage of synchrotron radiation sources lies in

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the very high intensity of the X-ray beams they produce. After the X-rays source, a sequence of filters, mirrors, and associated monochromators are used, allowing efficient use of the divergent X-rays leaving the X-ray tube. Therefore, it can be possible to associate a single diffraction peak with each family of crystal planes and select one peak among all of those emitted by the tube. Detectors generate a pulse of current when they absorb an X-ray and the most commonly used types are single-photon detectors, linear (position-sensitive) detectors, area detectors, and Xray film (Guinebretie`re, 2007). A diffractometer presents, as a basic function, the ability to detect the XRD of samples and to relate the intensity of the diffraction with the diffraction angle (2θ) (Leng, 2008). Originally, in XRD analysis, powder samples were used. However, samples of crystalline aggregates and even liquid samples can be analyzed, since samples should contain a large number of tiny crystals (or grains) which randomly orient in three-dimensional space because standard XRD data are obtained from powder samples of perfectly random orientation (Guinebretie`re, 2007; Leng, 2008). Data acquisition in an X-ray diffratometer covers a range of diffraction intensity, from low to high values of 2θ. By continuously changing the incident angle of the X-ray beam, a spectrum of diffraction intensity versus the angle between the incident and the diffraction beam is recorded (Leng, 2008). Processing diffraction spectra, and the intensity peaks, where each peak represents diffraction from a certain crystallographic plane, it is possible to extract three essential values from each peak: the angular position, the integrated intensity, and the intensity distribution (Guinebretie`re, 2007). The diffraction pattern is unique for each crystalline compound, providing a “fingerprint” that allows the identification and characterization of the phases in polycrystalline or multiphasic materials by their diffraction patterns. The identification of the crystalline structure is performed by comparing the data from the obtained spectrum with a database containing over 60,000 diffraction spectra of known crystalline substances (Guinebretie`re, 2007; Leng, 2008). The principal advantages of the XRD technique for characterization of phases are simplicity, quickness, and accuracy of the results obtained, the possibility of quantitative analysis of composed or a mixture of materials or phases. W.L. Bragg’s papers, especially “The diffraction of short electromagnetic waves by a crystal,” published in 1912 and “The structure of some crystals as indicated by their diffraction of X-rays,” published in 1914, jointly with subsequent development of the X-ray spectrometer apparatus by W.H. Bragg, conducted a revolution in the scientific understanding of crystals and their atomic arrangements and was the beginning of the field of X-ray crystallography (Helliwell, 2013). Traditionally, XRD had been used in the field of materials science and engineering, particular qualitative and quantitative phase analyses, investigations of crystallographic textures, and residual stress measurements (Epp, 2016). Nowadays, XRD has been used by modern genetics, medicine, and biochemistry fields, and has been an important tool to establish the complete structures of crystals, from very simple NaCl to the most complex structures, such as proteins,

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viruses, and DNA. Since the first publications, around 20 or so Nobel Prizes in Physics and Chemistry have been awarded for research that has used the ideas described in W.L. Bragg and W.H. Bragg’s works.

5.3

X-ray diffraction analysis of layered double hydroxides and Modified Layered Double Hydroxides

Layered materials are a class of materials whose basic characteristics are structured or formed by crystals. These crystals form layers that stack two-dimensionally and hold together through weak bonds. Therefore, through this definition LDHs can be classified as layered materials (Arizaga et al., 2007). LDHs were discovered around 1842 in Sweden, the material discovered at that time is today known as hydrotalcite. After this discovery, a large number of minerals with similar structure to the new discovered material were reported and they were called by the mineralogists of the time “sjo¨grenite-hydrotalcite.” However, the exact formula of hydrotalcite, [Mg6Al2(OH)16] CO3.4H2O, was only reported in 1915 by Manasse (Wang and O’Hare, 2012; Manasse, 1915). The increase in the interest in LDH structure and synthesis occurred after the publication of a series of articles by Feitknecht, in which materials produced as “doppelschichtstrukturen” (double-layer structure) were identified (Feitknecht, 1938; Feitknecht and Gerber, 1942; Feitknecht, 1942). These materials have been described as a structure in which there is a layer formed by a metal hydroxide intercalated by another layer of a second metal hydroxide. However, this theory for the structure of these materials was refuted by Allmann (1968) and Taylor (1969), who after XRD analysis in a single crystal, proved that the same layer coexisted with the two cations (Khan and O’Hare, 2002). The development of studies and research on LDH since its discovery has made it possible for these materials at present to have a well-known chemical composition and structure. The chemical composition of these materials is represented by the following general formula: 

 M21 12x M31 x ðOHÞ2 ½Am- x=mUnH2 O

M21 represents a divalent cation, M31 represents a trivalent cation, Am
represents an m valent anion, and n represents the number of water molecules. A simple way to understand the structure of LDHs is to compare them with the structure of brucite, a mineral composed of magnesium hydroxide, which has the chemical formula Mg(OH)2. Structurally, the brucite presents magnesium cations located in the center of slightly distorted octahedra, which have hydroxyl anions at their vertices. These octahedra share edges forming a structure of plain and neutral layers, which are held together due to intramolecular forces (Wypych and Satyanarayana, 2005; Khan and O’Hare, 2002; Crepaldi and Valim, 1998; Marangoni, 2009).

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When, in this layer structure, the isomorphic substitution of bivalent cations by trivalent cations occurs, the layer has a positive residual charge, but with the structure similar to the layer of brucite. This new structure does not satisfy the electrostatic valency principle and for the system, or layer, to become electrically neutral, the presence of ions (anions) between these layers (interlayer) is required. These anions, along with the water molecules, will promote the stacking of the LDH layers through weak forces in unorganized interlayer domains (Wypych and Satyanarayana, 2005; Khan and O’Hare, 2002; Arizaga et al., 2007; Crepaldi and Valim, 1998; Marangoni, 2009; Tronto, 2006). The interlayer domain in the LDHs corresponds to the region between the adjacent inorganic layers. Studies show that in this region there are basically water molecules and anions. The amount of water molecules is determined by factors, such as the nature of the interlayer anion, water vapor pressure, and temperature (Khan and O’Hare, 2002; Brindley and Kikkawa, 1979; Brindley and Kikkawa, 1980). Another important structural parameter observed in LDHs is the stacking sequence of their structural unit. Studies have shown that there are two main forms of lattice, rhombohedral and hexagonal (Khan and O’Hare, 2002). Thus, through the options in the variation of chemical composition and structure a vast number of natural and synthetic LDHs have been produced and studied. This variation occurs mainly through the types and proportions of the metal cations used, as well as the type of interlayer anion used in LDHs. This vastness of existing natural and synthetic LDHs must be characterized by some characterization techniques. One of the most important characterization techniques for the structural study of LDHs is the XRD. In the first section of this chapter, the importance of XRD analysis was discussed and it was shown that one of the most important uses of XRD analysis is for the characterization of crystalline materials. As LDHs are crystalline materials, this technique is fundamental for understanding the structure of these materials, making it possible to identify information as atomic arrangements in the unit cell, unit cell parameters, and defects in the structure (Rives, 2001). Most LDHs that are removed from nature or synthesized are in the form of a fine powder. Thus the vast majority of XRD analyses occurs in the material in the form of powder or powder XRD method. In general, the methodology for XRD analysis in LDHs is presented in a simple way. The equipment that performs the XRD analysis, as already presented above, is a diffractometer. In this equipment a sample in the form of LDH powder is placed in the equipment. The parameters of the equipment are set according to what is desired and the analysis is started. Basically, the analysis consists of an X-ray beam emission in the sample, varying the angle of incidence. After the analysis, the equipment generates an XRD pattern. An example of an XRD pattern for an LDH is shown in Fig. 5.1. Thus, through a simple analysis of Fig. 5.1, it is possible to understand that the interpretation of the LDH structure is based on the positions and intensity of the peaks presented in the XRD pattern.

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Figure 5.1 XRD pattern for LDH formed by ZnAl cations and intercalated with chloride anion.

Interpreting the XRD patterns and consequently their peaks makes it necessary to understand what each peak means in the LDH structure. The peaks obtained in the XRD pattern for the structure of LDHs are called basal reflections. The basal reflections that present (00l) are directly related to the layered stacking, basal spacing (d) (Marangoni, 2009; Rives, 2001). The basal reflections (0kl), when present, are related to the organization of a layer relative to another layer of the LDHs. When basal reflections (hk0) are found, these refer to the organization of the atoms inside the LDH layers (Marangoni, 2009; Rives, 2001). Thus, through the identification and study of the basal reflections found in the XRD pattern for LDHs, it is possible, as already described, to characterize in detail all the structures of these materials. Fig. 5.1 show an XRD pattern for a synthesized LDH formed by Zn and Al cations (using the 2:1 ratio of Zn/Al) and chloride as the anion (ZnAl/Cl). The observed XRD pattern of this LDH is identified as an organized crystalline structure, it being possible to identify several basal reflections referring to all its crystalline structure. It is important to describe that when LDHs are used to synthesize polymer nanocomposites, these LDHs are mostly synthetic. This fact occurs mainly due to the ease of LDH synthesis, as well as the composition control and properties that these synthesized LDHs present. Therefore, in this chapter we focus on the presentation of XRD patterns for synthetic LDHs. However, natural LDHs can also be used in the synthesis of polymer nanocomposites. These natural LDHs are found in nature, and as already described earlier,

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it was the discovery of these natural LDHs that triggered the study of this materials class. Natural LDHs as found in nature present their XRD pattern, characteristics, and properties presented in mineral databases. Hydrotalcite, for example, has all its characterization presented in the American Mineralogist Crystal Structure Database Record (Downs and Hall-Wallace, 2003) or RRUFF Project (Lafuentes et al., 2015). In these databases it is possible to observe the XRD patterns for hydrotalcite originating from different regions of the world. When the LDH structures are studied with the main purpose of using them as nanofiller in polymer nanocomposites, the information of the basal spacing (d) of these materials is highlighted. The basal spacing is highlighted because LDHs present higher tunable charge density on layers, which causes a stronger interaction among the hydroxide layers and hinders the adsorption of monomers or polymers between these layers and consequently the synthesis of polymer nanocomposites with better morphologies and properties (Wypych and Satyanarayana, 2005; Botan et al., 2016). Thus, when LDHs are used as nanofillers for polymer nanocomposite synthesis, these LDHs are usually modified with ions that allow less interaction between the layers and consequently make it easier to adsorb monomers or polymers in LDHs. The basal spacing for LDHs is calculated using the angle obtained in the XRD pattern for the basal reflections relative to layered stacking and the Bragg’s diffraction law (Eq. 5.1). Generally, to obtain more precise basal spacing for LDHs, a mean of the basal spacings obtained from the different basal reflections of the XRD pattern is performed (basal spacing (d)
(d003 1 2d006. . . 1 nd00(3n))/n) (Xu et al., 2004; Xu and Braterman, 2010). The result of the basal spacing obtained for the LDH shown in Fig. 5.1 is ˚ . Through the basal spacing it is also possible to define the interlayer spacing 7.72 A ˚ ). Thus the value of the LDH, only removing the layer value from the LDH (B4.8 A ˚. of the interlayer spacing of ZnAl/Cl is 2.92 A This basal spacing result found for LDH, which has chloride as an anion, is a small basal spacing value and all the most common anions found in LDH, such as ˚ (Wang and fluoride, carbonate, nitrate, and sulfate, have values around 7
9 A O’Hare, 2012; Khan and O’Hare, 2002; Rives, 2001). The basal spacing of the ˚. hydrotalcite calculated by Downs et al. (1993) is 7.75 A These small anions with a small basal spacing do not allow a decrease in the interaction of LDH layers and consequently the effective synthesis of polymer nanocomposites. Thus, as previously mentioned, when it is intended to use LDHs as nanofiller for polymer nanocomposites, it is necessary to modify or synthesize these materials with different anions. The anions that have been most used and studied in LDH synthesis as nanofillers for polymer nanocomposites are sodium dodecyl sulfate (SDS), carboxylates, dodecyl benzenosulfonate (SDBS), bis(2-ethylhexyl) phosphate, and others (Rives, 2001; Botan et al., 2016).

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Figure 5.2 XRD pattern for LDH formed by ZnAl cations and intercalated with dodecyl sulfate anion.

When LDHs are synthesized with these anions a change in their structure and consequently the XRD pattern occurs. Fig. 5.2 shows an example of an XRD pattern for an LDH synthesized with ZnAl cations and with a SDS anion (ZnAl/SDS). In general, when XRD patterns for modified LDHs with different anions are analyzed, the basal reflections characteristic of the layered structure (layered stacking) of these materials can be observed. Fig. 5.2 shows the basal reflections of ZnAl/ SDS. However, the basal reflections regarding the organization of a layer relative to another layer and the organization of the atoms inside the LDH layers on the LDH structure are not easily identified. Thus, when XRD patterns are compared to LDHs with common anions, or unmodified, with modified LDHs, important information on the structure of these materials can be identified. In order to compare, identify, and explain the differences in XRD patterns for an unmodified LDH and a modified LDH, the examples presented in Figs. 5.1 and 5.2 are used. The first important information obtained from the comparison of XRD patterns is that although both present clear differences, the two XRD patterns present basal reflections characteristic of LDHs, showing that an effective modification occurred in one of the LDHs. The second important piece of information is that the modified LDH (ZnAl/ SDS) presents a lower crystallinity, or organization, when compared to unmodified LDH (ZnAl/Cl). The identification of all basal reflections for the modified LDH is not clear and, in general, the intensity of the basal reflections is smaller. The third important piece of information obtained is that the basal reflections of ZnAl/SDS when compared to ZnAl/Cl are shifted to values lower of 2θ ( ). Thus, in summary, through the information obtained by comparison of XRD patterns, it is possible to identify that modified LDH exhibits a lower crystallinity than

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unmodified LDH. This lower crystallinity is directly related to LDH modification, thus demonstrating that LDH was effectively modified with the anions used. The effective modification can also be identified by calculating the basal spacing of the modified LDH and also explains the displacement of the basal reflections to lower values of 2θ ( ). Calculating the basal spacing of the ZnAl/SDS in the same way as calculated for ˚ . The value of the interlayer spacing was 20.46 A ˚. ZnAl/Cl the result was 25.26 A Values much higher than those found for ZnAl/Cl, were found when the basal spac˚ . Table 5.1 presents the basal spacing for LDHs with different anions ing was 7.72 A reported in the literature. Fig. 5.3 shows more examples of XRD patterns for modified LDHs. In this figure an XRD pattern is presented for an LDH formed by the Mg and Al cations and modified (intercalated) with the SDS anion and another LDH formed by the

Table 5.1 Basal spacing (d) for LDHs with variation of the intercalated anions (Xu et al., 2004; Xu and Braterman, 2010; Nogueira et al., 2011; Gonc¸alves, 2012, 2015; Gonc¸alves et al., 2014; Meyn et al., 1990; Wang et al., 2009) Anions

Cations

MgAl CaFe ZnAl ZnCr CaAl

Phthalate ˚) (A

Terephthalate ˚) (A

SDS ˚) (A

SDBS ˚) (A

Laurate ˚) (A

Stearate ˚) (A



14.90 15.00 14.70

2 2 14.10 14.00 13.20

26.12 27.23 25.26 2 27.52

29.00 2 30.20 30.00 28.50

23.38 34.48 24.50 29.34 33.69

49.04 2 45.97 2 2

Figure 5.3 XRD patterns for modified LDHs with different anions.

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cations Zn and Cr, and modified with the anion laurate. Calculating the basal spac˚ for MgAl and ing (d) for these LDHs, the following values are obtained: 26.12 A ˚ for ZnCr, as shown in Table 5.1. 29.34 A This large increase in the value of the basal spacing for ZnAl/SDS, and other modified LDHs as presented, is due to the ion that was placed in the interlayer domain, which is due to its larger size expanding this region in the LDH. One can try to relate the exact size of the intercalated ion with the found basal spacing, however there is still no clear relationship between the values found, mainly because the intercalation of the ions in the interlayer domain can occur in different modes as reported in numerous scientific articles (Khan and O’Hare, 2002; Arizaga et al., 2007; Rives, 2001; Botan et al., 2016; Xu et al., 2004; Xu and Braterman, 2010; Kuehn and Porllmann, 2010; Nhlapo et al., 2008). When LDHs are used as nanofillers for polymer nanocomposite synthesis, the expansion of the interlayer spacing is important, because through this expansion the higher tunable charge density on layers, which causes a stronger interaction among the hydroxide layers is decreased, and consequently the probability of the monomer or polymer adsorbing in this space increases, also generating an increase in the probability of polymer nanocomposite synthesis with better characteristics and properties (Nogueira et al., 2011; Botan et al., 2012; Nyambo et al., 2008; Matusinovic et al., 2013). Thus the XRD analysis presents fundamental importance for the structural characterization of LDHs. When focusing the study of LDH for use as nanofiller in polymer nanocomposites this technique also has significant importance. Through this technique as presented it is possible to characterize the structure of natural or synthesized LDHs. It also allows identifying and confirming modifications that occur in LDHs with the main objective of making them more compatible with the polymer materials, besides identifying the structural characteristics of the modified LDH, helping to understand the possible characteristics and properties that the polymer nanocomposites can present.

5.4

X-ray diffraction analysis of layered double hydroxide polymer nanocomposites

Nanocomposites can be defined as the combination of a continuous phase (matrix) and other material (filler) that have at least one dimension in the nanometer range (Esteves et al., 2004; Wing Mai and Zhen, 2006). Nanocomposites are recently developed materials. One of the initial milestone versions for the study and development of polymer nanocomposites occurred with Fujiwara and Sakamoto (Unitcha Ltd, 1976), when these researchers produced a nanocomposite of polyamide 6 and clay (montimorilonite). Later, Toyota researchers, along with Fujiwara and Sakamoto, optimized this new material (Unitcha Ltd, 1976; Toyota Motor Co, 1988; McAdam et al., 2008). This new nanocomposite produced an extraordinary improvement in its mechanical, thermal, and physical

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properties when compared to neat polymer (Usuki et al., 1993). Since then, the new nanocomposites developed have presented significant gains, mainly in the mechanical properties, thermal properties, and flammability properties, when compared to neat polymers or traditional composites, besides using a small amount of nanofiller (Botan, 2014). Nanocomposites can be formed by two or more materials. These materials may in principle be identified as two types: the matrix or base material, with the main functions of keeping the reinforcements together, transmitting to them the applied stress and affording the final product shape, and the reinforcement or filler, which supports the request transmitted by the matrix. When a polymer material is used as a matrix, the nanocomposite can be called a polymer matrix nanocomposite or polymer nanocomposite (Botan, 2014). The use of inorganic reinforcements in polymer nanocomposites has been shown to be very promising, as may lead to nanocomposites with higher mechanical strength, higher thermal stability (Botan et al., 2012, 2016; Nogueira et al., 2011) or with better optical (Fogg et al., 1997), magnetic (Du et al., 1998) and/or electrical properties (Morais et al., 2003) than neat polymers or traditional composites. This improvement in polymer nanocomposite properties is primarily due to reinforcing nanoparticles, which have a high surface area when dispersed in the polymer matrix. This large surface area modifies the properties of the matrix or polymer, mainly due to the specific interactions that arise between the reinforcement and matrix. These interactions may influence the molecular dynamics of the polymer, resulting in significant changes in its physical properties, mainly in its thermal and/or mechanical behavior (Botan, 2014; Klabunde, 2001). The use of inorganic reinforcements in new polymer nanocomposites is very interesting, however various types of inorganic reinforcements that differ, for example, in morphological properties or in properties such as thermal resistance or chemical reactivity, may be used. Among the most common reinforcements used in composites and nanocomposites of polymer matrix with inorganic reinforcement are carbonates, aluminosilicates, and clays. Most of these works have focused on natural clay cation exchangers, but other natural and synthetic layered materials are gaining prominence (Botan, 2014). A layered material that is gaining prominence in the synthesis of polymer nanocomposites is LDHs. LDHs, besides allowing the synthesis of new polymer nanocomposites with better characteristics and properties, are considered materials of great versatility, easy production, and low cost. However, there is a fact of extreme importance that must be considered in polymer nanocomposite synthesis with LDH: The compatibility of the polymer and the LDH. This compatibility is necessary because the LDH (hydrophilic nature) has a low chemical affinity with polymers or monomers, in general, being predominantly hydrophobic. The compatibility of the LDH with the polymer matrix can be improved by chemical modification of the components (Botan, 2014). An outstanding type of LDH modification can be performed by the anion intercalation as discussed in Section 5.3.

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The production of polymer nanocomposites is a recent and developing area, so there is not yet an unambiguous classification for the different types of hybrid materials and respective synthesis methods. The synthesis of these materials, as well as their morphologies, are in constant development, thus enabling new methods and structures never previously reported to be developed (Esteves et al., 2004; Botan, 2014). Using the most common and generally described techniques for the synthesis of polymer nanocomposites, it is possible to identify four main routes. Melt mixing, exfoliation/adsorption, in situ nanoparticle synthesis, and in situ polymerization (Esteves et al., 2004; Wypych and Satyanarayana, 2005; Khan and O’Hare, 2002; Botan, 2014). A brief description of the most common methods, the synthesis of nanocomposites from the melt mixing, as the name itself describes, consists in mixing the materials, polymer and reinforcement, in the molten state of the polymer material. For crystalline polymers, this temperature should be above the melting temperature (Tm) for amorphous polymers, above its glass transition temperature (Tg). Generally this process occurs in single or twin screw extruders. This method, until now, has been the most used in the synthesis of nanocomposites in the industry, mainly for nanocomposites that are reinforced with layered materials, because these present greater flexibility when compared with three-dimensional structures (Esteves et al., 2004; Wypych and Satyanarayana, 2005; Botan, 2014). In exfoliation/adsorption, the reinforcement is exfoliated in a solvent, where the polymer to be used should also be soluble. This polymer is mixed with solvent and exfoliated reinforcement. In this mixture, the polymer tends to adsorb on the surface of the reinforcing layer and when the mixture undergoes an evaporation or precipitation process the nanocomposite is synthesized (Wypych and Satyanarayana, 2005; Botan, 2014). In the synthesis of nanoparticles in situ, the synthesis of nanoparticles involves chemical methods of controlled preparation of inorganic solids. The materials prepared according to this strategy generally have chemical bonds between the components, which results in more homogeneous and more consistent hybrids. There are two main methods for preparing nanocomposites by synthesis of nanoparticles in situ, they are: sol
gel reaction and synthesis in the presence of structuring materials (Esteves et al., 2004; Botan, 2014). The polymerization of the matrix in situ, occurs through the mixing of reinforcement already previously produced in a monomer. This monomer will adsorb the reinforcement structure and will subsequently be polymerized. The synthesis of nanocomposites by in situ matrix polymerization occurs according to the most widely used polymer synthesis routes, such as solution polymerization, suspension polymerization, emulsion polymerization, and bulk polymerization. This strategy makes it possible to obtain a better dispersion of the reinforcements used, resulting in more homogeneous nanocomposites and, consequently, better processing (Esteves et al., 2004; Wypych and Satyanarayana, 2005; Khan and O’Hare, 2002; Botan, 2014). Thus, when polymer nanocomposites are synthesized, a uniform distribution of the reinforcements or fillers in the polymer matrix and a good adhesion at the

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interface of the two components is always sought, so that a synergism between the matrix and reinforcement occurs. The distribution and the form of the reinforcement in the polymer matrix are the morphology or structure of the polymer nanocomposites. The morphology of polymer nanocomposites can be classified or divided into three large groups. These three groups present profound dependence on the synthesis and process conditions by which these materials were subjected. Remembering that the reinforcement or filler used in this work is LDH, a layered material as already described, the definitions of the polymer nanocomposites morphology will be based on the use of the layered materials as fillers. The first group or class is called a microcomposite. In this group the layered compound does not delaminate. Delamination is the separation of layers from the layered compound in the polymer matrix. Thus there are aggregates of several intact layers of the layered compound distributed in the matrix. This morphology occurs mainly due to low or lack of affinity (miscibility) between the matrix and filler (Botan, 2014; Paul and Robeson, 2008). In this case the polymer or monomer, depending on the route of synthesis, cannot adsorb in the interlayer space of the layered compound. This structure shows the lowest gain in properties when compared to the other two morphologies. This structure also appears as the threshold between a nanocomposite material and a composite material. The second group is those named as intercalated nanocomposite. In this situation, the polymer or monomer is able to adsorb between the layers of the layered compound. Even though there is polymer between the layers of the layered compound, it is able to maintain its layered structure, which is only expanded. This structure is still not considered ideal, but presents considerable gains in properties when compared to neat polymers, traditional composites, and a microcomposition structure (Wypych and Satyanarayana, 2005; Botan, 2014; Paul and Robeson, 2008). The third group is the exfoliated nanocomposites. In this structure complete delamination of the layered compound occurs, its layers will be distributed randomly within the polymer matrix. In this case, there are nanosized layer particles increasing the surface area of contact with the matrix, resulting in stronger and larger interactions, allowing a wide synergism between the matrix and nanofiller. This structure presents the best gains in properties when compared to the other two structures discussed (Wypych and Satyanarayana, 2005; Botan, 2014; Paul and Robeson, 2008). However, these three structures are known and reported mainly in books or reviews for a better understanding of the polymer nanocomposites, but when studies are analyzed and developed with these materials, in the literature it is very common that polymer nanocomposites present a mixture of these structures, such as nanocomposites with intercalated and exfoliated structures at the same time. Nevertheless it is more usual to find nanocomposites with a morphology mix than fully intercalated or exfoliated nanocomposites (Botan et al., 2016; Nogueira et al., 2011, 2012; Botan et al., 2012, 2015; Botan, 2014).

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Thus the study and identification of polymer nanocomposite morphology is of great importance for its characterization. The XRD technique is of great importance and is widely used for the study and identification of the polymer nanocomposite morphology. XRD analysis for polymer nanocomposites is usually performed via powder XRD method. Thus, if the nanocomposites are not produced in the form of powder, these materials generally must be crushed. However, there are some cases in which the crushing of nanocomposites becomes improbable and therefore other techniques besides the crushing of the nanocomposite and the use of it in powder can be used. For example, there are nanocomposites that exhibit high impact strength or glass transition temperature below room temperature, so in these cases it is necessary to perform cryo grinding. There are also nanocomposites with a preferred orientation of crystallites and these orientations are lost when the nanocomposite is crushed, so for these cases it is common to use small films or sheets for nanocomposite analysis. The methodology of XRD analysis for polymer nanocomposites is very similar to that for LDHs and is described in Section 5.3. It consists of placing the polymer nanocomposites powder in the diffractometer, setting the analysis parameters, and initiating the analysis. As a final result of the analysis, an XRD pattern will be generated for the analyzed polymer nanocomposites. The use of XRD analysis for neat polymers occurs mainly when one intends to study the crystalline structure of crystalline polymers. However, because the polymers do not exhibit a crystallinity as characteristic as the layered materials, their XRD patterns show differences. Fig. 5.4 shows the XRD patterns for different neat polymers, polyamide 6 (PA6), polymethyl methacrylate (PMMA), and polystyrene (PS). Comparing the XRD patterns of neat polymers with those of LDHs, presented in Section 5.3, the difference is clear. The polymers do not have the same crystalline

Figure 5.4 XRD patterns for polyamide 6, polymethyl methacrylate, and polystyrene.

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structure as LDHs, thus causing their XRD patterns to not show such intense and characteristic peaks. When comparing the neat polymers, it is possible to observe a differentiation, mainly between PA6 and the other two, PMMA and PS. This occurs because the polymers exhibit variation in their crystallinity. PA6 shows higher crystallinity when compared to PMMA and PS. Analyzing the XRD pattern for PA6 it is possible to identify two peaks in the   region of 21 and 24 degrees (2θ), which are attributed to α(200) and α(002/202) crystal planes, respectively (Peng et al., 2009). The XRD pattern of PMMA due to its small crystallinity does not have a wellcharacterized diffraction peak, but small deviations of linearity or halos. Thus, for this material its XRD pattern has two halos, one more prominent and centered around 15 degrees and another with less intensity and centered around 30 degrees (2θ) (Nogueira et al., 2012). The PS also has low crystallinity, so its XRD pattern has two small characteristic halos and is centered approximately in 10 and 20 degrees (2θ) (Botan et al., 2015). Thus, when XRD patterns of polymer nanocomposites reinforced with LDHs are analyzed, it is important to know the XRD patterns for each material individually, so that they help in the identification and study of the XRD patterns that will be obtained for the nanocomposites. The polymer nanocomposites are mostly composed of the polymer matrix, in general, the majority of studies do not present the use of more than 10% of LDH, so the polymer matrix corresponds to 90% of the whole nanocomposite. Therefore, when analyzing the XRD patterns of polymer nanocomposites they will present mainly the same global form of the neat polymer that was used as matrix. However, although XRD patterns of polymer nanocomposites follow those of neat polymers, it is possible to identify small differences that will suggest the possible morphologies that nanocomposites will present. The XRD patterns can be divided into three types or classes, which will be directly related to the three most common morphologies that polymer nanocomposites can present. In the first group the XRD patterns of the polymer nanocomposites present the general form of the neat polymers, however in these XRD patterns it is also possible to identify several peaks referring to the LDH basal reflections used as filler. The identification of the LDH basal reflections in the XRD patterns of polymer nanocomposites suggests that the layered structure of LDH remains intact within the matrix. Therefore, no type of adsorption of the polymer occurred on the interlayer spacing of LDH, showing that the affinity between the polymer and LDH was not high. Through the low affinity and presence of LDHs with their structure intact within the polymer matrix, this type of XRD pattern suggests the morphology of a polymer microcomposition, which also has a poor overall distribution of LDH in the polymer matrix. In the second group, the XRD patterns of the polymer nanocomposites, as well as the first group, presents a general form very similar to that of the neat polymer

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that is used as matrix, but unlike the first group in these XRD patterns, it is not possible to identify several peaks referring to the basal reflections of LDHs. In this group, small reflection peak formations are identified mainly in low values of angles (2θ) in the XRD patterns. These small peak formations indicate that there is still some sort of organization of the LDH structure in the polymer matrix, suggesting that despite this small organization the vast majority of the layered structure of LDH was destroyed, suffering from delamination. Therefore, as the polymer was able to adsorb in the LDH structure, delaminating and/or expanding the LDH structure, the suggested morphology for this group is the intercalated. In the XRD patterns of these polymer nanocomposites, if a basal reflection peak characteristic of LDH is identified, it is possible to calculate the basal spacing value and compare it with the LDH spacing value, thus identifying how much the polymer has been able to expand the structure of the LDH. Fig. 5.5 shows an example of an XRD pattern for a polymer nanocomposite reinforced with a modified LDH (composition of 2% LDH), which shows the formation of a small peak. This suggests that the polymer nanocomposite has an intercalated morphology. Fig. 5.5 shows the XRD pattern of the neat polymer used in the matrix of this nanocomposite, the PS. In the third group the XRD patterns of the polymer nanocomposites are very similar to those of the neat polymers used as a matrix. In this case, unlike the others, there is no basal reflection peak which refers to the layered structure of LDH. Therefore, with the XRD pattern that does not present any peak regarding the layered structure of LDH, it can be suggested that in this nanocomposite a complete

Figure 5.5 XRD patterns for neat polystyrene and polymer nanocomposites formed by polystyrene and modified layered double hydroxide with a possible intercalated morphology.

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Figure 5.6 XRD patterns for neat polystyrene and polymer nanocomposites formed by polystyrene and modified layered double hydroxide with a possible exfoliated morphology.

breakdown of the layered structure of LDH has occurred. The polymer adsorbed into the interlayer space of LDH and ruptured its structure, thus demonstrating a great affinity between the polymer and LDH. Thus this characteristic of the XRD pattern suggests the exfoliated morphology for the polymer nanocomposites. The exfoliated morphology represents the complete disruption of the LDH layered structure, originating layers that will be randomly distributed in the polymer matrix. This justifies the absence of peaks in the XRD pattern referring to the layered structure of LDHs (Botan et al., 2016). Fig. 5.6 shows an example of a polymer nanocomposite with a possible exfoliated morphology, formed by PS and modified LDH, in which 3% of modified LDH was used in the polymer nanocomposite composition. Further examples of XRD patterns for polymer nanocomposites are shown in Figs. 5.7 and 5.8. Fig. 5.7 shows an XRD pattern for a nanocomposite formed by PMMA and a modified LDH, this nanocomposite presents in its composition 2% of the modified LDH. In this figure the XRD pattern for neat PMMA is also presented. As can be seen in Fig. 5.7 and following the XRD pattern analyzes for polymer nanocomposites described above, it is possible to identify that the polymer nanocomposite pattern is quite similar to the neat PMMA pattern, as well as no peaks being found in the XRD pattern of the polymer nanocomposite regarding the modified LDH. Thus it is possible to suggest that this PMMA/LDH nanocomposite presents a possible exfoliated morphology. Fig. 5.8 shows the XRD patterns for neat PA6 and a polymer nanocomposite formed by PA6 and modified LDH, using a composition with 3% modified LDH. Analyzing the XRD patterns, it is possible to suggest that this polymer nanocomposite may have an exfoliated morphology.

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Figure 5.7 XRD patterns for neat PMMA and polymer nanocomposites formed by PMMA and modified LDH with a possible exfoliated morphology.

Figure 5.8 XRD patterns for neat PA6 and polymer nanocomposites formed by PA6 and modified LDH with a possible exfoliated morphology.

An important point to be identified for the analysis of the XRD patterns of polymer nanocomposites is their composition because, in general, polymer nanocomposites are synthesized with small amounts of LDHs. Polymer nanocomposites with amounts less than 1% LDH by weight may exhibit some peculiarities in their XRD patterns. Due to the small amount of reinforcement and the detection limit of XRD equipment, the XRD patterns for these nanocomposites may not show any peak relative to the LDH layered structure, however this absence of peaks may not be due to a

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breakdown of the LDH layered structure but due to the limit of detection of the equipment or an optimal global distribution of LDH in the polymer matrix (Nyambo et al., 2008; Matusinovic et al., 2013). Therefore, for polymer nanocomposites with low concentrations of LDH in their compositions, this alternative for interpretation of XRD patterns should always be considered. Thus the XRD analysis presents the fundamental importance for the characterization of polymer nanocomposite morphology. Through this analysis it is possible to identify indications of which morphology the polymer nanocomposite presents and consequently to relate its possible properties with its morphology. This technique also makes it possible to confirm the effective synthesis of polymer nanocomposites. However, for a complete analysis of the polymer nanocomposite morphology, it is desirable that besides the XRD analysis another technique of analysis be performed to confirm and complement the information obtained through XRD analysis. Generally the analysis used to complement XRD analysis for polymer nanocomposites is microscopy, specifically using a transmission electron microscope, which makes it possible to visualize the LDH structure in the polymer matrix. However, the transmission electron microscopy technique presents negative points, such as not very clear images, besides representing a small part of the studied material, generally a slice of average thickness of 100 nm. Other negative points in this technique are the considerable time spent in sample preparation and analysis, in addition to its high cost. Thus, characterization alternatives have been searched for to complement XRD analysis, a technique with this potential is X-ray microtomography (Pakzad et al., 2011; Awaja et al., 2011).

5.5

Conclusion

XRD is an analysis technique of great importance for the study of material structure. Since the discovery of this technique, it has been possible to develop a deep understanding of material structures. This technique is so important for the development of science and technology that about 20 Nobel Prizes in Chemistry and Physics are related to the ideas presented by the Bragg’s. In the characterization of LDH polymer nanocomposites the XRD analysis has fundamental importance. This importance is demonstrated, because through this technique it is possible to characterize the entire structure of LDHs, as well as the effective modification of these LDHs with the main objective of using them as nanofillers in polymer nanocomposite synthesis. XRD analysis also makes it possible to identify the polymer nanocomposite morphology. The identification of the polymer nanocomposite morphology is of great importance, because it makes it possible to understand all the behaviors and properties of these materials.

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In addition to all the structural and morphological characterization of all the materials involved in LDH polymer nanocomposite synthesis, and LDH polymer nanocomposites themselves, this analysis, when compared to other analysis techniques, is considered an easy-to-prepare, fast, and low-cost analysis. Therefore, XRD analysis is an indispensable characterization technique when it is intended to study, develop and/or understand LDH polymer nanocomposites.

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Spectroscopic characterization techniques for layered double hydroxide polymer nanocomposites

6

Shadpour Mallakpour1,2,3 and Faezeh Azimi 3 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 3Chemistry Group, Pardis College, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

6.1

Introduction

Layered compounds are an interesting class of material, consisting of a twodimensional sheet-like structure with strong bindings within the individual layer and weak van der Waals forces between the layers (Nejati et al., 2015; Nagendra et al., 2017; Naik et al., 2011). Layered double hydroxides (LDHs) are inorganic compounds composed of positively charged brucite-like layers in which some of the divalent metal cations are substituted via trivalent metal cations, producing positively charged layers (Veschambres et al., 2016; Tammaro et al., 2014; Mallakpour et al., 2015). The general formula of LDHs is [M211
x M31x (OH)2]x1 [(Am2)x/m. nH2O]x2, where, in this formula, M21 and M31 are divalent and trivalent cations, respectively, and An2 is usually an exchangeable anion. The quantity of M21, M31, and, An2 together with the value of x are adjustable, and thus the structure and behaviors of LDHs can be easily tailored (Fig. 6.1) (Wang et al., 2015a; Kang et al., 2013; Mallakpour et al., 2016). Lately, there has been a rapid evolution in methodology growth for the fabrication of polymer nanocomposites (NCs) comprising LDH, because of synergistic effects of this filler on mechanical, thermal, electrical, magnetic, and fire-retardant behaviors of various polymer/LDHs NCs (Purohit et al., 2014; Gaume et al., 2013; Mallakpour et al., 2014b). Polymer NCs are an original group of materials that can be created by incorporation of inorganic/organic nanofillers within the polymeric matrix. They exhibit unique physicochemical properties that cannot be obtained with individual components acting alone (Abdolmaleki et al., 2017; Hu et al., 2014). LDH-type materials present the potential for wide applications in the highperformance hydrogel, supercapacitor, ion exchanger, improvement in thermal Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00006-9 © 2020 Elsevier Ltd. All rights reserved.

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Figure 6.1 Schematic illustration of LDH structure and chemical components. Source: Adapted from Gu, Z., Atherton, J.J., Xu, Z.P., 2015. Hierarchical layered double hydroxide nanocomposites: structure, synthesis and applications. Chem. Commun., 51(15), 3024
3036, with kind permission of Elsevier.

resistance and flame retardancy of polymer composites, treatment of wastewater, photoluminescence, and preparation of organic
inorganic NCs because of their structural characteristics and diverse compositions (Lv et al., 2016; Mallakpour et al., 2014a; Basu et al., 2014). Due to its layered structure, large aspect ratio, diameter in the nanometer range, heat stability, and flame retardancy at low concentration, LDH is a potential candidate as a nanofiller for the fabrication of polymer/ layered material NCs. The efficiency of reinforced NCs with LDH is limited because of powerful interlayer electrostatic interactions between the sheets and important hydrophilic behaviors which are obtained from the high charge density of the LDH layers and the high anionic species and water molecules (Dinari and Mallakpour, 2015; Wang and O’Hare, 2012; Andronescu et al., 2014; Mallakpour and Dinari, 2015a). Chemical or physical modifications, based on the covalent/noncovalent bonding of functional groups on the LDH layers, are methods to facilitate the dispersion stability of LDHs. A wide diversity of anionic coupling agents, such as fatty acids, sulfonates, phosphates, and amino acids has been reported as modifiers for surface treatment of LDHs (Mallakpour et al., 2013; Kumar et al., 2012; Costa et al., 2005). The interaction between electromagnetic radiation and substances, as a function of wavelength, can be identified by spectroscopy. Displayed data in the spectrum are a plot of the response or variation of the interaction as a function of wavelength or frequency (Siddiqui et al., 2013; Njuguna et al., 2015). Here we report eight useful and practical spectroscopic techniques for the analysis and characterization of modified LDHs and polymer/LDHs NCs.

6.2

Spectroscopy of polymer nanocomposites

Due to the growing interest in NCs, molecular characterization of these materials is essential for understanding their properties and for the development of new

Spectroscopic characterization techniques for layered double hydroxide polymer nanocomposites

233

materials. Spectroscopic techniques that bring information at a molecular level are unavoidable when characterizing polymers, fillers, and composites. The results of different analyses, such as Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, energy-dispersive X-ray (EDX) spectroscopy, fluorescence spectroscopy, dielectric spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, ultraviolet
visible (UV
vis) spectroscopy, and X-ray photoelectron spectroscopy (XPS) illustrate the surface of fillers, the state of filler dispersion in the host matrix, the extent of interaction between the polymer and the filler particles, or the dynamics of polymer chains at the polymer
filler interface. The potential applications of all these techniques can be summarized to solid-state structural simulations, mechanical, optical, and electrical properties of engineering materials, and quantum-chemical calculations of the electronic structure (Silva et al., 2012; Bokobza, 2017).

6.2.1 Fourier transform infrared spectroscopy FTIR spectroscopy is a powerful optical spectroscopy based on vibration measurements of an excited molecule by IR radiation at a specific wavelength range, which identifies the vibration characteristics of chemical functional groups in a sample (Ricci et al., 2015; Reichenb¨acher and Popp, 2012). FTIR spectroscopy provides information on molecular structures, chemical environments, orientations, and conformations of polymer chains. FTIR spectroscopy is one of the best methods for identifying molecular structure, such as functional group and bond, qualitatively according to the characteristic frequencies. In addition, FTIR spectra can determine the component content of the sample quantificatively according to band intensity (Chang and Tanaka, 2002).

6.2.2 Raman spectroscopy In Raman spectroscopy, the inelastic scattering of light is used to analyze vibrational and rotational modes of molecules. The recent method of coherent anti-Stokes Raman scattering possesses high sensitivity and is used for in vivo spectroscopy and imaging (Bokobza, 2017). Raman spectroscopy has the advantage of saving time when compared with other conventional methods. It is used to identify the structure, interface interactions, and physical properties of nanofillers, their functionalization as well as orientation. In a polymer NC, the interaction between nanofillers and polymers is reflected by a peak shift or a change in the peak intensity or width in the Raman spectrum. In other word, the shifts of the Raman band reveal mechanical deformation of nanofillers, polymer
nanofiller interactions, phase transitions of the polymer, stress state, as well as the Young’s modulus of the nanofillers (Yang et al., 2009).

6.2.3 Energy-dispersive X-ray spectroscopy EDX is a powerful method which has been utilized for the elemental investigation or chemical specification of a particular specimen. As a type of spectroscopy, it

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relies on the investigation of a sample by interactions among electromagnetic radiation and substance. The EDX method identifies the X-ray spectrum emitted by a sample bombarded via an electron beam to characterize the chemical composition of micro- and nanomaterials and shows the special distribution of the components through X-ray mapping (Michael and Iniyan, 2015; Mishra et al., 2017).

6.2.4 Fluorescence spectroscopy Fluorescence is a two-step process that requires absorption of light at a specific wavelength (excitation) followed by emission of light, usually at a higher wavelength. The emission of light is termed fluorescence. Fluorescence spectroscopy measures the change in the energy of the photons when the sample is exposed to high-energy photons which results in the emission of lower energy photons via the sample. It is an established technology for confocal microscopy, fluorescence resonance energy transfer, and fluorescence lifetime imaging in biochemical and medical fields (Albrecht, 2008). Fluorescence spectroscopy has a wide reach among polymer chemists and nanotechnologists. Their measurements can provide a wide range of detailed information regarding the molecular processes, including the solvent combinations in NC, rotational diffusion of molecules, distances between the nanofillers, conformational changes, and interaction of nanofiller with the polymer matrix (Goesmann and Feldmann, 2010).

6.2.5 Dielectric spectroscopy The dynamic measurements of nanoscale properties are performed with dielectric spectroscopy, which concludes the electric strength, complex dielectric constant components, and dielectric loss factor of numerous electric field frequencies and at various temperatures. The dielectric features of NCs are highly dependent on the filler content at lower frequencies. Further, alternating current and direct current dielectric spectroscopies were employed to determine the phase separation process, interactions between the divided phase, the influence of filler permittivity, filler purity (and conductivity), volume fraction, surface treatment, and processing situations (Krishnamoorti et al., 1996; Fumagalli et al., 2009).

6.2.6 Nuclear magnetic resonance spectroscopy NMR is a large range of phenomena associated with the interaction of electromagnetic radiation with materials. NMR spectroscopy involves putting a compound into a magnetic field and measuring the absorption of radio waves through the 1H, 13C, 19 F, 31P, or other nuclei. This analysis allows the observation of the special quantum mechanical magnetic behaviors of the atomic nucleus, especially for the macromolecular composite systems (M¨antylahti, 2014; Kitayama and Hatada, 2013). NMR is an effective tool to study polymers, their end-groups, branching,

Spectroscopic characterization techniques for layered double hydroxide polymer nanocomposites

235

functionalization, and various properties. The use of NMR in characterizing the NC does not depend on the nature of the polymer or filler. However, in most of the composite cases, such as elastomers, thermoplastics, and biopolymers, NMR explains the polymer
filler interfacial interactions, nature of dispersion, interspatial distances of the filler particles, bonding, presence of voids, etc., in similar ways (Ponnamma et al., 2013; Postma et al., 2006).

6.2.7 UV
vis spectrophotometry UV
vis spectroscopy is a characterization method to evaluate the absorbance of samples at a certain wavelength. This technique is based on the principle of electronic transition in molecules or atoms, which is caused by absorption of light in the visible area of the electromagnetic spectrum (400
800 nm) under excitation of an electron from the ground state into a higher orbital. In UV
vis spectroscopy, there is a linear relationship between absorbance and absorber concentration, which causes this characterization technique to be especially attractive for making quantitative measurements. The UV ray can be separated into three parts by wavelength, as UV-A (320
400 nm), UV-B (280
320 nm), and UV-C (200
280 nm). UV-C and most of UV-B are absorbed by the ozone layer in the upper atmosphere, UV rays in sunlight reaching the ground are mostly UV-A (90%
99%), and a smaller amount of UV-B (1%
10%). The optical features of NCs based on the LDHs were investigated using UV
vis spectroscopy. This method was carried out to study the state of transition metals incorporated within the layered lattice and also changes the absorption features by intercalating the anions in the interlayer area of the LDHs (Permyakov, 2012; Yu and Xie, 2012; Peng et al., 2016). UV
vis spectroscopy is one of the most important characterization techniques to study the optical properties of polymer NCs. It helps to understand the interaction between the matrix and the nanofiller and analyzes the role of nanofillers in enhancing the property of the NCs. In addition, the UV
vis technique demonstrates the transparency, dispersion regions, refractive index, and optical band gap (Sharma et al., 2011).

6.2.8 X-ray photoelectron spectroscopy XPS is a surface analysis technique and is created by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the top 1
10 nm of the material being studied. This analysis probes a limited depth of the sample and provides both elemental and chemical state information of the elements that exist into a material (Haasch, 2014). XPS investigates surface modification of natural and synthetic polymers, carbon nanofillers, elemental chemical composition of a surface, and the bonding states of those elements, and is applied in the analysis of tuned catalysts, engineered polymer coatings, and nanoelectronic heterostructures (Yang et al., 2010).

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6.3

Layered Double Hydroxide Polymer Nanocomposites

Spectroscopic characterization of layered double hydroxide polymer nanocomposites

6.3.1 Fourier transform infrared spectroscopy of layered double hydroxide polymer nanocomposites FTIR investigates the existence of functional groups and probable interactions between inorganic lamellae and intercalated anion in LDH structures. The FTIR spectrum of LDH containing CO322 anions usually shows a broad band in the 3100
3650 cm21, which is related to the O
H stretching vibration of the metal hydroxide layer and interlayer water molecules. A characteristic shoulder at 3000
3100 cm21 is caused by the interaction between the anions (such as CO322) and H2O molecules, present in the interlayer region. The peaks located at the 1350
1380 cm21 could be related to CO322 antisymmetric stretching modes in the interlayer of the LDHs. A group of bands ranging from 400 to 680 cm21 is attributed to lattice vibration of metal hydroxide and metal oxides (Theiss et al., 2013b). Tsai et al. (2016) employed cocoamphodipropionate (K2) and sodium dodecyl sulfate (SDS), for surface modification of various types of LDHs (LiAl-LDH and MgAl-LDH) and the preparation of NCs based on poly(methyl methacrylate) (PMMA). The higher aspect ratio of LDH displays great efficiency in thermal, antiscratch, optical, and barrier gas in comparison with neat PMMA. The FTIR analysis of the multimodified LDHs and pure LDHs was performed to prove the creation of modified LDHs as shown in Figs. 6.2 and 6.3. The spectra of both pristine LDH samples displayed a broad absorption band at about 3500 cm21 owing to aOH stretching vibration of hydroxyl groups of LDH. Incorporation of K2 1 SDS in the LDH layers of treated LDH samples was demonstrated via the presence of new strong absorption peaks. A broad band is observed around 3500 cm21, which is ascribed to the aOH and aNH stretching vibrations of linked groups to LDHs. The characteristic peaks at 460 and 540 cm21 could be related to stretching vibration of metal oxides such as Mg-O and Al-O, respectively. The alkane chain (
CH2) and
NH bending of modified LDHs revealed the absorption peaks at 2920 and 1649
1550 cm21, respectively. On the other hand, the peak of CQO stretching vibration was shown at 1649 cm21, which overlapped and appeared in lower frequency due to H-bonding. Also, the vibration bands at 1032
1160 and 1200 cm21 were ascribed to the SO322 and C
O
C, respectively. In another research work, to enhance the distribution characteristics of LDH into the hosting polymer and change surface behaviors of LDH from hydrophilic to hydrophobic, surface modification of the LDH was done by Elbasuney (2015). This author introduced dodecanedioic acid (DDA) as an organic ligand and poly(ethylene-co-acrylic acid) as a polymeric surfactant to prepare polymer NCs with excellent flame retardancy and thermal properties. FTIR spectroscopy verified the successful attachment of the surfactants to the LDH layers. As shown in Figs. 6.4 and 6.5, due to the same crystalline structure of both LDH and organic treated LDHs, the region between 500 and 1500 cm21 (the fingerprint region) displayed intense and identical absorption of poly(ethylene-co-acrylic acid)-LDH and

Relative intensity (%T)

-SO4–2

(B) -C=O

-NH,-OH

-C-O-C -CH2

-CH2

(A)

CO2 Al-O-Al -CO3–

Al+3-O Mg+2-O

-OH 4000

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Wavelength (cm–1) Figure 6.2 FTIR spectra of (A) MgAl-LDH and (B) MgAl-LDH-K2 1 SDS. Source: Adapted from Tsai, T.Y., Bunekar, N., Liang, S.W., 2016. Effect of multiorganomodified LiAl-or MgAl-layered double hydroxide on the PMMA nanocomposites. Adv. Polym. Technol. doi:10.1002/adv.21639, with kind permission of Elsevier.

Relative intensity (%T)

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SO4–2 CH2 C=O C-O-C -NH,-OH

-CH2

(A) Al-O-Al CO3–

-OH 4000

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Wavelength (cm–1) Figure 6.3 FTIR spectra of (A) LiAl-LDH and (B) LiAl-LDH-K2 1 SDS. Source: Adapted from Tsai, T.Y., Bunekar, N., Liang, S.W., 2016. Effect of multiorganomodified LiAl-or MgAl-layered double hydroxide on the PMMA nanocomposites. Adv. Polym. Technol. doi:10.1002/adv.21639, with kind permission of Elsevier.

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Figure 6.4 FTIR spectra of poly(ethylene-co-acrylic acid)-LDH and uncoated LDH. Source: Adapted from Elbasuney, S., 2015. Surface engineering of layered double hydroxide (LDH) nanoparticles for polymer flame retardancy. Powder Technol., 277, 63
73, with kind permission of Elsevier.

Figure 6.5 FTIR spectra of DDA-LDH and uncoated LDH. Source: Adapted from Elbasuney, S., 2015. Surface engineering of layered double hydroxide (LDH) nanoparticles for polymer flame retardancy. Powder Technol., 277, 63
73, with kind permission of Elsevier.

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Figure 6.6 FTIR spectra of the neat PAI and different NC materials. Source: Adapted from Mallakpour, S., Dinari, M., 2015b. Hybrids of Mg
Al-layered double hydroxide and multiwalled carbon nanotube as a reinforcing filler in the l-phenylalaninebased polymer nanocomposites. J. Therm. Anal. Calorim., 119(3), 1905
1912, with kind permission of Elsevier.

DDA-LDH in comparison with uncoated LDH. It can be seen that there is a significant difference in the infrared spectra (from 4000 to 1500 cm21) between organic treated LDHs and uncoated LDH which was related to the functional group area. The differences in FTIR absorption are due to the CQO and O
H stretching vibrations of the linked carboxylic surfactants. Mallakpour and Dinari (2015b) utilized hybrid nanofiller based on carboxylated multiwalled carbon nanotubes (MWCNTs) and Mg-Al LDH as a modifying agent by a simple coprecipitation method via ultrasonic radiation. Then, various loading amounts of LDH-MWCNTs were intercalated into the poly(amide imide) (PAI) and the obtained samples were studied by diverse techniques. Compared with the pure PAI, the FTIR spectra of PAI NCs were changed by intercalation of the LDHMWCNTs into the PAI matrix. Due to the presence of the LDH-CNTs in the PAI matrix, a new broad absorption band around 400
800 cm21 was observed which confirmed the formation of PAI/LDH-MWCNT NCs (Fig. 6.6).

6.3.2 Raman spectroscopy of layered double hydroxide polymer nanocomposites The Raman spectrum for LDHs containing MgAl illustrates strong and broad bands at 400
480 cm21, which are attributed to oxygen bonds of brucite-like layer,

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Layered Double Hydroxide Polymer Nanocomposites

M
O
M as well as M
OH2-coordinated water. The vibrational mode of CO322 interacting with the hydroxyl groups of the brucite-like layer shows a weak band at 800
830 cm21. A faintly stronger band is displayed in the 2400 cm21, which is related to weak vibrational modes of adsorbed CO2 interacting weakly with the interlayer area (Balcomb et al., 2015). Nam et al. (2016) described a synthetic strategy to fabricate biomimetic multifunctional NCs in which a thin film of polydopamine (PDA) was deposited on the LDHs via interlayer polymerization. They used Raman spectroscopy to confirm the LDH intercalation. In this study, PDA was employed as an organic modifier and anion exchange facilitated via CoAl-LDH-NO3 (CAN) and MgAl-LDH-NO3 (MAN) as reactive precursors for creation of PDA/LDH NCs. The prepared NCs were denoted as CAN-PD1 and MAN-PD1 (the reactions of 0.2 mL of dopamine solutions with CAN and MAN, respectively) and CAN-PD2 and MAN-PD2 (the reactions of 0.1 mL of dopamine solutions with CAN and MAN, respectively). Raman spectra of the samples are illustrated in Fig. 6.7 which confirmed the intercalation of PDA into LDH. The spectra of NCs displayed a strong band at 1065 cm21, which is attributed to the symmetry vibration of the NO32. The band broadening for NO32 was seen, which corresponded to the presence of PDA, confirming enhanced disorder of LDH molecules via the intercalation. On the other

Figure 6.7 Raman spectra of PDA/LDH NCs: (A) CAN, (B) CAN-PD1, (C) CAN-PD2, (D) MAN, (E) MAN-PD1, and (F) MAN-PD2 (absorption band of X: silicon, ▼: Al-OH, K: PDA, ’: NO32). Source: Adapted from Nam, H.J., Park, E.B., Jung, D.Y., 2016. Bioinspired polydopaminelayered double hydroxide nanocomposites: controlled synthesis and multifunctional performance. RSC Adv., 6(30), 24952
24958, with kind permission of Elsevier.

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hand, the broad peaks at 1390 and 1590 cm21, which are observed in Raman spectra of NCs, are attributed to the vibrational modes of the carbon atoms arranged in graphitic-like domains and the aromatic CQC and C
N stretching mode of the basic indole structure. Furthermore, the phenolic C
O stretching and N
H bending created the small shoulder bands at 1215 and 1510 cm21. Shao et al. (2015) reported the design and production of a sophisticated nanoarray containing an LDH shell and a conducting polymer such as a polypyrrole (PPy) core, which was created through a two-step electrosynthesis technique. This product exhibited its great feature in high-efficiency flexible electrochemical capacitors. Raman spectra of PPy-LDH presented two peaks at 1559 and 1330 cm21, which are attributed to the π-conjugated structure and ring stretching mode of PPy backbone, respectively. Also, the new peak at 523 cm21 resulted from the stretching of OH
O bonds between CO322 and H2O in the interlayer of CoNi-LDH (Fig. 6.8). In order to remove U(VI) from aqueous solution, Yu et al. (2017) synthesized graphene oxide and NiAl-LDH (GO-LDH) NCs through a one-pot hydrothermal process. The obtained Raman data of GO-LDH NCs demonstrated two significant peaks in Fig. 6.9. The G band (located around 1580 cm21) is related to the vibration of sp2 banded carbon atoms in a two-dimensional hexagonal lattice. Another band at about 1350 cm21, called the D band, depicted the vibration of sp3 carbon atoms of disorder and defects. In addition, the 2D band (around 2700 cm21) is a broad and weak peak, which is attributed to disorder an account of an out-of-plane vibration mode. The intensity ratios of the D and G bands (ID/IG) helps to estimate structural defects and content of functional groups. The ID/IG ratio of GO-LDH NCs (1.008) shifted to a higher value than that of GO(0.926), which suggested a size reduction of the sp2 domains after LDH assembly.

Figure 6.8 Raman spectra of the LDH, pristine PPy and PPy-LDH. Source: Adapted from Shao, M., Li, Z., Zhang, R., Ning, F., Wei, M., Evans, D.G., et al., 2015. Hierarchical conducting polymer@ clay core
shell arrays for flexible all-solid-state supercapacitor devices. Small, 11(29), 3530
3538, with kind permission of Elsevier.

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Figure 6.9 Raman spectra of the GO-LDH. Source: Adapted from Yu, S., Wang, J., Song, S., Sun, K., Li, J., Wang, X., et al., 2017. One-pot synthesis of graphene oxide and Ni-Al layered double hydroxides nanocomposites for the efficient removal of U (VI) from wastewater. Sci. China Chem., 60(3), 415
422, with kind permission of Elsevier.

In other study, Huang et al. (2010) successfully fabricated exfoliated LDH nanosheet/CNT hybrids through electrostatic force. For this purpose, first, the positively charged LDHs (Co-Al-CO3 LDH) were prepared by precipitation via urea hydrolysis. Afterwards, the negatively charged CNTs (CNT-COONa) were prepared by nitric acid oxidation followed by treating with sodium hydroxide. Finally, the exfoliated LDH/CNT hybrids were fabricated through mixing positively charged LDHs and negatively charged CNTs. Raman spectra investigated the assembling process and nanostructure of the sample (Fig. 6.10). In the case of Co-Al-CO3 LDHs, a strong peak is observed at around 1059 cm21, which is attributed to the symmetry vibration peak of NO32. Raman data of pristine CNTs exhibited the D band (1300
1400 cm21) and G band (1500
1600 cm21), which are assigned to the disorder graphitic structure of nanotubes and tangential C
C stretching vibrations both longitudinally and transversally on the nanotube axis. In contrast, CNT-COONa displays the D band (defect/disorder-induced mode) at 1323 cm21 and G band (in plane stretching tangential mode) at 1573 cm21. Additionally, after nitric acid treatment, the ID/IG ratio for CNT-COONa (0.18) was higher than that of pristine CNTs. The enhancement of peak intensity may be attributed to an increase in the disorder in the nanotube structure during the oxidation process. The Raman spectrum of LDH/CNT hybrids showed a red shift in the G band and D band with an ID/IG ratio of 0.71. The strong interaction between positive LDH nanosheets and negative CNTs led to an increase in the energy necessary for vibrations to occur, which is reflected in the higher frequency of the Raman peak. The upshift in ID/IG

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Figure 6.10 Raman spectra of (A) Co-Al-NO3 LDH, (B) pristine CNTs, (C) CNT-COONa, and (D) LDH/CNT hybrids. Source: Adapted from Huang, S., Peng, H., Tjiu, W.W., Yang, Z., Zhu, H., Tang, T., et al., 2010. Assembling exfoliated layered double hydroxide (LDH) nanosheet/carbon nanotube (CNT) hybrids via electrostatic force and fabricating nylon nanocomposites. J. Phys. Chem. B, 114(50), 16766
16772, with kind permission of Elsevier.

ratio may be related to the strong interaction between the LDH sheets with local defects and the CNTs. Moreover, there is no NO32 band at 1059 cm21, thus suggesting that the negative CNTs replaced the NO32 anions to keep the charge balance of the LDH.

6.3.3 Energy-dispersive X-ray spectroscopy of layered double hydroxide polymer nanocomposites EDX analysis of the synthesized Fe-Ni-Co LDH indicated that the Fe/Ni/Co ratios for FeNi2Co2 LDH, Fe2-Ni5-Co3 LDH, Fe-Ni3-Co LDH, and Fe2-Ni7-Co LDH were 100:25:12, 100:29:10, 100:33:6, and 100:35:4, respectively. The Fe content in the Fe-Ni-Co LDHs is the highest, while the Co content is the lowest, which showed a notable difference between the material input ratio and real element content in the samples. The Fe content in all samples was nearly the same, and the Co content increased with an added amount of CoCl2, which was identified as 3%, 4%, 7%, and 9%, respectively (Gao et al., 2017). Nicotera et al. (2015) successfully synthesized hybrid membranes based on layer nanoadditives of the anionic clay family by solution intercalation. This class of nanosized materials includes Mg12/Al13 LDHs (at two metal ratios, 2:1 and 3:1) with diverse anions in the interlayer region (CO322, ClO42, NO32) which was incorporated into the Nafion matrix for the advancement of innovative hybrid NCs exploitable in the high-temperature polymer electrolyte membrane fuel cells. In order to measure the influence of Mg12/Al13-LDHs on the final membrane,

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Figure 6.11 EDX mapping of (A) filler-free Nafion membrane and (B) NC membrane loaded with 3 wt.% of LDH-ClO42. Source: Adapted from Nicotera, I., Angjeli, K., Coppola, L., Enotiadis, A., Pedicini, R., Carbone, A., et al., 2015. Composite polymer electrolyte membranes based on Mg
Al layered double hydroxide (LDH) platelets for H 2/air-fed fuel cells. Solid State Ionics, 276, 40
46, with kind permission of Elsevier.

fabricated hybrid membranes were characterized through different techniques. There is no overlapping of emissions between fluorine (contained in Nafion) and magnesium (contained in LDH) and gold employed for the coating, therefore these elements were selected for composite membranes. Fig. 6.11 corresponded to EDX maps of the pristine Nafion and Naf-LDH-ClO42 composite, a detailed analysis of these maps obviously displays the dispersion of fluorine (yellow spots) that can be attributed to polymer matrix and fluorine and magnesium (violet spots) corresponding to composite membrane, indicating that the Mg is well dispersed in the crosssection of membrane. In another work, Wang et al. (2014) prepared CuMoO4/Zn-Al LDH hybrids through an ion exchange and precipitation route. Subsequently, polypropylene (PP) composites with different loadings of CuMoO4/Zn-Al LDH hybrids as flameretardant nanofillers were prepared via a masterbatch-based melt. The presence of CuMoO4/LDH hybrids as flame-retardant nanofillers in the polymer matrix illustrates an essential role in improving the thermal and flame retardancy behaviors of the created NCs. For smaller particles, diffraction techniques can be performed using transmission electron microscopy (TEM) for identification, TEM-EDX as an advanced technique was used to characterize the synthesized CuMoO4/LDH hybrids, as shown in Fig. 6.12. Fig. 6.12A illustrates the image of LDHs with highly transparent thin layers which were well dispersed in alcohol without showing any noticeable aggregation. It can be evidently observed from Fig. 6.12B and C that nanocrystalline CuMoO4 was dispersed well on the LDH nanosheets. The presence of CuMoO4 and Zn, Al, Mo, and Cu elements is further displayed from the EDX of CuMoO4/LDH hybrids which these peaks confirmed the formation of CuMoO4/LDH hybrids (Fig. 6.12D). A variety of inorganic anions was intercalated into the Mg3Al-LDH (LDHs) as nanofiller for fabrication of NCs by Gao et al. (2013). In this contribution, it was concluded that properties of PP/LDH NCs significantly depend on the mentioned

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Figure 6.12 TEM observation of LDH (A), CuMoO4/Zn-Al LDH hybrids (B), (C) and the EDX analysis of CuMoO4/Zn-Al LDH hybrids (D). Source: Adapted from Wang, B., Zhou, K., Wang, B., Gui, Z., Hu, Y., 2014. Synthesis and characterization of CuMoO4/Zn
Al layered double hydroxide hybrids and their application as a reinforcement in polypropylene. Ind. Eng. Chem. Res., 53(31), 12355
12362, with kind permission of Elsevier.

inorganic anions. The elemental analysis of PP matrix and the LDH nanoparticles by EDX is exhibited in Fig. 6.13. The spectra of Mg and Al were detected in Fig. 6.13A, showing that it is Mg3Al-CO3 LDH, while only C can be seen in Fig. 6.13B. A scanning electron microscope (SEM) image and the elemental mapping Al and Mg of PP/Mg3Al-CO3 LDH 9 wt.% depicted a good dispersion of Mg3Al-CO3 LDH in the PP matrix as displayed in Fig. 6.14. In 2015, the impact of unmodified Zn-Al LDH mixed with ethylene propylene diene monomer (EPDM) rubber composites was investigated by Basu et al. (2016). In the present study, employing unmodified Zn-Al LDH, the thermal stability, mechanical properties, dynamic mechanical behaviors, flame retardancy, and rheological properties of EPDM composite were considerably enhanced. While any metal cations were not observed in the SEM/EDX analysis of the pure EPDM, the existence of Zn and Al atoms in the surface of EPDM composite with a content of 4 wt.% of Zn-Al LDH was confirmed by this analysis. Also, great incorporation of the LDH particles into EPDM rubber was depicted by EDX elemental mapping

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Figure 6.13 SEM-EDX analysis of PP/Mg3Al-CO3 LDH NCs. (A) EDX elemental analysis of the point representing LDH particles and (B) EDX elemental analysis of the point representing PP. Source: Adapted from Gao, Y., Wu, J., Zhang, Z., Jin, R., Zhang, X., Yan, X., et al. (2013). Synthesis of polypropylene/Mg 3 Al
X (X 5 CO322, NO32, Cl2, SO422) LDH nanocomposites using a solvent mixing method: thermal and melt rheological properties. J. Mater. Chem. A, 1(34), 9928
9934, with kind permission of Elsevier.

of Al, Mg, S, and Zn atoms (Fig. 6.15A). The morphological characterization of EPDM filled with different contents of Zn-Al LDH is shown in Fig. 6.15B. It can be seen that the Zn-Al LDH particles in the submicron size were well dispersed in the EPDM matrix and the enhancement of inorganic filler amount did not lead to significant agglomeration. Due to the importance of the fabrication technologies for tissue engineering scaffolds, poly(ε-caprolactone) (PCL) is a well-known biopolymer which is employed as a fibrous scaffold in biomedical applications. Shafiei et al. (2016) prepared PCL-LDH NC scaffolds with different amounts of Mg/Al-LDH using an electrospinning technique. In Fig. 6.16, the EDX spectra of PCL-LDH NCs with 10 wt.% of LDH scaffold displayed low contents of Mg and Al atoms. In addition, it was demonstrated that the basic elements of the PCL are carbon and oxygen peaks. The EDX elemental map revealed a homogeneous dispersion of Mg and Al atoms within the fibers.

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Figure 6.14 (A) SEM image, (B) Mg mapping, and (C) Al mapping of 9 wt.% PP/Mg3AlCO3 NCs. Source: Adapted from Gao, Y., Wu, J., Zhang, Z., Jin, R., Zhang, X., Yan, X., et al. (2013). Synthesis of polypropylene/Mg 3 Al
X (X 5 CO322, NO32, Cl2, SO422) LDH nanocomposites using a solvent mixing method: thermal and melt rheological properties. J. Mater. Chem. A, 1(34), 9928
9934, with kind permission of Elsevier.

6.3.4 Fluorescence spectroscopy of layered double hydroxide polymer nanocomposites The fluorescence spectra of a fluorescent anion (CPBA) and CPBA intercalated LDH (ZnAl-CPBA LDH) were investigated. The excitation spectrum and emission spectrum of CPBA both show a red shift. After intercalation, the emission spectrum of ZnAl-CPBA LDH remains unchanged with the maximum emission wavelength

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Figure 6.15 (A) SEM-EDX analysis of the pure EPDM and EPDM-4 LDH composite and the corresponding Al, Mg, S, and Zn-mappings, (B) SEM images of EPDM filled with different amounts of LDH. Source: Adapted from Basu, D., Das, A., Wang, D.Y., George, J.J., Sto¨ckelhuber, K.W., Boldt, R., et al., 2016. Fire-safe and environmentally friendly nanocomposites based on layered double hydroxides and ethylene propylene diene elastomer. RSC Adv., 6(31), 26425
26436, with kind permission of Elsevier.

located at 430 nm, while the excitation spectrum shows a slight blue shift from 364 to 352 nm, which can be attributed to intermolecular interactions between the excited guest and the LDH layers. The fluorescence lifetime of ZnAl-CPBA LDH is 4.83 ns, significantly longer than the value of 1.26 ns for pristine CPBA. The improvement of the fluorescence lifetime can be assigned to the high degree of organization of the CPBA moieties in the interlayer galleries of the LDH layers, which suppresses the thermal vibration and rotation of the CPBA anions (Wang et al., 2015b). Yan et al. (2010b) successfully fabricated (2-hydroxy benzo[a]carbazole3-carboxylate) (BCZC) intercalated LDH with different layer charge densities

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Figure 6.16 (A) FE-SEM image of PCL 1 10% LDH scaffolds. (B) The elemental distribution mapping of Mg (green dots) and Al (red dots) within the LDH containing fibers (C) EDX analysis of PCL 1 10% LDH scaffolds. Source: Adapted from Shafiei, S.S., Shavandi, M., Ahangari, G., Shokrolahi, F., 2016. Electrospun layered double hydroxide/poly (ε-caprolactone) nanocomposite scaffolds for adipogenic differentiation of adipose-derived mesenchymal stem cells. Appl. Clay Sci., 127, 52
63, with kind permission of Elsevier.

(LCD). They investigated the photoluminescence property of pristine BCZC aqueous solution, solid BCZC, BCZC in the LDH matrix with high (Mg/Al ratio 5 1.801 defined as sample A) and low (Mg/Al ratio 5 3.132 defined as sample B) LCD. Displaying data in the fluorescence emission spectra is related to pristine BCZC aqueous solution (10 μM), solid BCZC, sample A, and sample B (Fig. 6.17). The symmetrical emission peaks located at 499 nm with the full width at half maximum (FWHM) of 90 nm and at 517 nm with FWHM of 78 nm can be observed for the BCZC aqueous solution and its solid state, respectively. The effect of the π
π or dipole
dipole interaction of the conjugated BCZC molecule can create a red shift for the BCZC solid sample. Also, photoemission behavior of BCZC in the LDH matrix with high and low LCD is similar to that of the BCZC solid and aqueous solution state, respectively, indicating which luminescent features can be adjusted and controlled by regulating two LCDs (Mg/Al 5 1.801 and 3.132) of LDH.

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Figure 6.17 Normalized photoemission spectra for (A) pristine BCZC aqueous solution (10 μM), (B) solid BCZC, (C) BCZC/Mg
Al-LDH (Sample A), and (D) BCZC/Mg
AlLDH (Sample B). Source: Adapted from Yan, D., Lu, J., Ma, J., Wei, M., Evans, D.G., Duan, X., 2010b. Benzocarbazole anions intercalated layered double hydroxide and its tunable fluorescence. Phys. Chem. Chem. Phys., 12(45), 15085
15092, with kind permission of Elsevier.

Yan et al. (2009) synthesized a sulfonated poly(p-phenylene) anionic derivate (APPP) and various exfoliated MgAl-LDH monolayers which were alternately assembled into ordered ultrathin films (UTFs) by a layer-by-layer method. The (APPP/LDH)n UTF (n 5 3
30) showed well-defined blue fluorescence and longrange order. Fig. 6.18A shows sharp emission peaks at around 415 nm (2.99 eV) of (APPP/LDH)n UTF, which are increased with rising “n.” In order to prove the obtained result, thin films were irradiated by UV light (Fig. 6.18B). The images display uniform bright blue luminescence the intensity of which is enhanced with an increase in the bilayer number “n.” Moreover, no red or blue shift of fluorescence spectra was detected from the UTFs, which proves a uniform distribution of APPP throughout the whole assembly processing. Compared with APPP pristine solution, an obvious blue shift (approximately 5 nm) occurred in the fluorescence spectra of UTFs without any broadening. This shift may be attributed to this consequence which the rigid framework of the LDH restricts the vibrations of the polymer backbone owing to nonbonding interactions. Also, they (Yan et al., 2010a) reported preparation of UFTs based on alternative layer-by-layer assembly of the sulfonated phenylenevinylene polyanion derivate (APPV) and exfoliated MgAl-LDH monolayers. The fluorescence peak at 2.25 eV (547 nm) of the (APPV/LDH)n UTFs (n 5 4
32) displayed an orderly growth of the UFTs upon increasing the number of deposition cycles (Fig. 6.19A). In comparison with the APPV pristine solution, the as-prepared UTFs excitation shifted, which indicates a homogeneous dispersion of APPV during the assembly process. Fig. 6.19B and C display irradiated thin films with UV light which can be depicted by homogeneous increased yellow luminescence of films with increasing “n.” Furthermore, according to the fluorescence lifetime values of (APPV/LDH)n UTFs (0.66
0.81 ns) compared to the pristine APPV solution (0.60 ns), it was concluded

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Figure 6.18 (A) Fluorescence spectra of (APPP/LDH)n (n 5 3
30) UTF and (B) photographs under 365 nm UV irradiation at different values of n. Source: Adapted from Yang, K., Gu, M., Guo, Y., Pan, X., Mu, G., 2009. Effects of carbon nanotube functionalization on the mechanical and thermal properties of epoxy composites. Carbon. N. Y., 47(7), 1723
1737, with kind permission of Elsevier.

that the LDH monolayers provided a confined environment for isolation of polymer chains and thus reduced the interlayer π
π stacking interaction. Anionic bis(2-sulfonatostyryl)biphenyl (BSB) especially can be used as a fluorescent brightener in the chemical industry; therefore, Yan and his co-workers (Yan et al., 2011) employed positively charged LDH nanosheets to assemble with BSB to obtain supramolecular UFTs. As shown in Fig. 6.20A, the fluorescence spectra of (BSB/LDH)n UTFs (n 5 4
32) have a maximum emission peak which is located at 444 nm with no shift or broadening of the emission peak. The obtained data demonstrated that no obvious change in intermolecular interactions of the BSB happen during the assembly process. Moreover, the thin films under UV illumination exhibited visible blue luminescence with enhanced brightness upon increasing “n” (Fig. 6.20B and C).

6.3.5 Dielectric spectroscopy of layered double hydroxide polymer nanocomposites LDHs containing Al or Ga as the trivalent ions and Mg or Zn as bivalent ions were investigated by broad band dielectric spectroscopy in a wide temperature range. Besides conduction effects, a relaxation process was observed which was assigned

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Figure 6.19 (A) Fluorescence spectra of (APPV/LDH)n (n 5 4
32) UTFs, (B) and (C) photographs of UTFs with different bilayer numbers under daylight and UV light (365 nm). Source: Adapted from Yan, D., Lu, J., Ma, J., Wei, M., Wang, X., Evans, D.G., et al., 2010a. Anionic poly (p-phenylenevinylene)/layered double hydroxide ordered ultrathin films with multiple quantum well structure: a combined experimental and theoretical study. Langmuir, 26(10), 7007
7014, with kind permission of Elsevier.

Figure 6.20 (A) Fluorescence spectra of (BSB/LDH)n (n 5 4
32) UTFs, (B) and (C) are photographs of UTFs with different n under daylight and UV light (365 nm). Source: Adapted from Yan, D., Lu, J., Ma, J., Wei, M., Evans, D.G., Duan, X., 2011. Reversibly thermochromic, fluorescent ultrathin films with a supramolecular architecture. Angew. Chem., 123(3), 746
749, with kind permission of Elsevier.

to the reorientational fluctuations of water molecules adsorbed on the oxide surface or in the interlayer voids. A nonmonotonous temperature dependence of the relaxation rates of this relaxation process has been found. A quantitative description of this dependence was possible based on a model assuming two competing processes: rotational fluctuation of water molecules and formation of additional defects (Frunza et al., 2015).

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Purohit et al. (2012) employed modified LDH (ZnAl-LDH) with sodium dodecylbenzene sulfonate (SDBS) as a surfactant to prepare NCs based on polyethylene (PE). Here, the structure/property relationships of NCs based on PE and LDH are investigated mainly by dielectric spectroscopy, which is discussed in more detail below. In comparison with amorphous polymers, polymers with a high degree of crystallinity like PE have a dielectric behavior which follows a different nomenclature of relaxation processes. The glassy dynamic of amorphous polymers and polymers with a high degree of crystallinity are called α-relaxation and β-relaxation, respectively. Sometimes this causes confusion but because this nomenclature is well established, dynamic glass transition will be called β-relaxation here. The dielectric behavior of the PE and prepared NCs (PE2
PE16) with the different contents of ZnAl-LDH (2
16 wt.%) in the temperature domain at a fixed frequency of 103 Hz is displayed in Fig. 6.21. The dielectric response of pure PE is very weak because the asymmetry in the repeating unit of PE leads to no intrinsic dipole moment. The dielectric losses are weak due to the preference of impurities and defect in PE. Thus the dielectric probe technique was employed to investigate the molecular dynamics of polyolefins in detail. The isochronal spectra of neat PE illustrate numerous weak relaxation processes. A weak process of so-called α-relaxation is seen at high temperature, which is attributed to the crystalline lamella. Presumably, this process is due to a rotational translation of chain segments helped by a chain twisting. For NCs, in the temperature range around 275K, a process related to segmental oscillation in the disordered areas of PE is observed which is

Figure 6.21 Dielectric loss, ε00 , versus temperature, T, at a frequency of 1 kHz for PE (squares) and different NCs: PE2 (circles), PE4 (triangles), PE6 (inverted triangles), PE8 (rhombus), PE12 (stars) and PE16 (pentagons). Source: Adapted from Purohit, P.J., Wang, D.Y., Emmerling, F., Thu¨nemann, A.F., Heinrich, G., Scho¨nhals, A., 2012. Arrangement of layered double hydroxide in a polyethylene matrix studied by a combination of complementary methods. Polymer. (Guildf)., 53(11), 2245
2254, with kind permission of Elsevier.

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shifted to higher frequencies with increasing temperature. At higher temperature, a further process is also observed in which its position and intensity are dependent on the concentration. This leads to the conclusion which this process is attributed to the existence of nanofiller. At first sight, this peak is attributed to an interfacial polarization (Maxwell
Wagner
Sillars polarization) which is originated by blocking of charge carried at the inner dielectric boundary layers on a mesoscopic length scale. In the case of NCs, this process is associated with Maxwell
Wagner
Sillars polarization and the effect of water absorption by the ZnAl-LDH. As can be seen, in comparison with the pure PE, the intensity of β-relaxation increases with increasing concentration of LDH. The increase in the measured dielectric loss is attributed to the concentration of polar molecules in the system. The only polar component in the system which increases with the concentration of LDH is the bulky anionic surfactant SDBS. In the polymer segments the alkyl tails of the SDBS are desorbed from the LDH surface and mixed with PE segments. This means the polymer segments near to the layers will oscillate together with the CH2 groups of the alkyl chains of the surfactant. Consequently, a remarkable increase in dielectric loss is observed with increasing concentrations of LDH. Due to the strong charge of the SDBS, this molecule is adsorbed at the exfoliated LDH layers. Thus dielectric spectroscopy selectively probes the mobility of segments located in an interfacial region near to the LDH sheets because the dielectric loss of the PE is low and so the matrix of the NCs is dielectrically invisible. Fig. 6.22 gives the dielectric loss for NC PE12 versus frequency and temperature in a 3D representation. The β-relaxation has a similar behavior to the pure PE at a lower temperature and higher frequency. At higher temperature and lower frequency regions, the Maxwell
Wagner
Sillars polarization is detected, which is not a relaxation process. The Maxwell
Wagner
Sillars polarization is very weak and so it was difficult to investigate. 00 Fig. 6.23 shows the variation of the dielectric constant (ε Þ of the NC PE6 versus frequency at the temperature 227K. As can be seen, two additional relaxation processes were identified. These processes are assigned to the molecular mobility of polymer segments to the surface of the LDH nanofiller. Process I at low frequency is related to the PE segments in close proximity to the LDH layers. Process II occurs at high frequency and is assigned to the oscillation of PE segments at a distance farther from the LDH sheets. In another research work, Purohit et al. (2011) focused on the fabrication and determination of dielectric behavior of new PP NCs (PE2
PE16) containing various amounts of organically modified ZnAl-LDH with SDBS (2
16 wt.%). The 3D representation of pure PP at a particular frequency depended on the temperature variation presented in Fig. 6.24. The asymmetry in the repeating unit of PP causes a low dipole moment which leads to reduced dielectric response of pure PP. As can be seen, the dielectric spectrum of pure PP has the main relaxation process, which is called β-relaxation. This process is assigned to the dynamic glass transition related to segmental fluctuations and is shifted to higher frequencies as the temperature increases. Moreover, due to the localized fluctuations, a γ-relaxation can be observed at lower temperatures (higher frequencies) than β-relaxation.

Spectroscopic characterization techniques for layered double hydroxide polymer nanocomposites

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Figure 6.22 Dielectric behavior of the sample PE12 versus frequency and temperature in a 3D representation. Source: Adapted from Purohit, P.J., Wang, D.Y., Emmerling, F., Thu¨nemann, A.F., Heinrich, G., Scho¨nhals, A., 2012. Arrangement of layered double hydroxide in a polyethylene matrix studied by a combination of complementary methods. Polymer. (Guildf)., 53(11), 2245
2254, with kind permission of Elsevier.

Figure 6.23 Dielectric loss versus frequency for the sample PE6 at T 5 277K. The dasheddotted lines correspond to individual relaxation processes. Source: Adapted from Purohit, P.J., Wang, D.Y., Emmerling, F., Thu¨nemann, A.F., Heinrich, G., Scho¨nhals, A., 2012. Arrangement of layered double hydroxide in a polyethylene matrix studied by a combination of complementary methods. Polymer. (Guildf)., 53(11), 2245
2254, with kind permission of Elsevier.

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Layered Double Hydroxide Polymer Nanocomposites

–2.0 β-relaxation

–3.0

400 350 300 250 T( K) 200

150

–1

0

1

2

5

4

3

log ε''

–2.5

6

z)]

f(H

[ log

00

Figure 6.24 Dielectric loss ε of pure PP versus frequency and temperature in a 3D representation. Source: Adapted from Purohit, P.J., Huacuja-Sa´nchez, J.E., Wang, D.Y., Emmerling, F., Thu¨nemann, A., Heinrich, G., et al., 2011. Structure
property relationships of nanocomposites based on polypropylene and layered double hydroxides. Macromolecules, 44 (11), 4342
4354, with kind permission of Elsevier.

Fig. 6.25 exhibits the dielectric spectrum of the NC PP16 in a 3D representation. The observed β-relaxation is similar to pure PP with an increased intensity in comparison with pure PP. At higher temperature, a dielectrically active process is observed which is not a relaxation process and may appear to be due to the preparation on NCs. Fig. 6.26 displays the dielectric behavior of the NCs containing various concentrations of LDH versus temperature at a frequency of 1 kHz. Compared to pure PP, the intensity of the β-relaxation process is strongly increased with the content of LDH. Also, the increase in the measured dielectric loss with the concentration of LDH is due to the increased content of the quite polar SDBS surfactant molecules, which increases with increasing concentration of LDH. The polar surfactant molecules are fluctuating together with the weakly polar PP segments and monitor the molecular mobility of the latter ones. Therefore, an increasing dielectric loss is observed with increasing concentration of LDH. Compared to the pure PP, for NCs the position of the β-relaxation is shifted by 30 K to lower temperature. According to the obtained result, the molecular mobility in the interfacial region between the LDH layers and the matrix is higher than that in the bulk unfilled PP. Therefore the glass transition temperature in the interfacial region is decreased compared to pure PP. Fig. 6.27 displays the dielectric loss of the NC PP16 at T 5 273.2K versus frequency. At higher frequencies, a well-defined loss peak can be observed. A more careful inspection of this peak shows that it has a so-called low-frequency contribution which originates from a further relaxation process. The two observed processes for the NCs are assigned to different regions of the molecular mobility of PP

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Process related to charge transport and/or percolation of electric exitations?

β-relaxation 2

log ε''

1 0

–1 –2

log

–2 0 2 4

z)] [f(H

6 150

250 300 200 T (K)

350 400

Figure 6.25 Dielectric behavior of the sample PP16 versus frequency and temperature in a 3D representation. Source: Adapted from Purohit, P.J., Huacuja-Sa´nchez, J.E., Wang, D.Y., Emmerling, F., Thu¨nemann, A., Heinrich, G., et al., 2011. Structure
property relationships of nanocomposites based on polypropylene and layered double hydroxides. Macromolecules, 44 (11), 4342
4354, with kind permission of Elsevier.

00

Figure 6.26 Dielectric loss ε versus temperature T at a frequency of 1 kHz for PP (squares) and different NCs: PP2 (circles), PP4 (triangles), PP6 (inverted triangles), PP8 (rhombus), PP12 (stars), and PP16 (pentagons). Source: Adapted from Purohit, P.J., Huacuja-Sa´nchez, J.E., Wang, D.Y., Emmerling, F., Thu¨nemann, A., Heinrich, G., et al., 2011. Structure
property relationships of nanocomposites based on polypropylene and layered double hydroxides. Macromolecules, 44 (11), 4342
4354, with kind permission of Elsevier.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 6.27 Dielectric loss versus frequency for the sample PP16 at T 5 273.2K. The dashed-dotted lines correspond to individual relaxation processes. Source: Adapted from Purohit, P.J., Huacuja-Sa´nchez, J.E., Wang, D.Y., Emmerling, F., Thu¨nemann, A., Heinrich, G., et al., 2011. Structure
property relationships of nanocomposites based on polypropylene and layered double hydroxides. Macromolecules, 44 (11), 4342
4354, with kind permission of Elsevier.

segments depending on the distance from the surface of the LDH sheets. Process I appears at lower frequencies and is assigned to the PP segments in close proximity of the LDH layers. Their mobility is hindered by the strong adsorption of the polar head group of the surfactants at the LDH layers. Process II at higher frequencies is related to the fluctuations of the PP segments at a distance farther from the LDH sheets.

6.3.6 Nuclear magnetic resonance spectroscopy of layered double hydroxide polymer nanocomposites In 1H NMR the resonances attributed to lactate anion in the exfoliated MgAl-lactate LDH were observed at 1.32 ppm and 1.30 ppm (CH3), 4.09 ppm and 4.07 ppm (CH), and 3.33 ppm (C
OH), and in 13C NMR at 20.10 ppm (C3) and 68.55 ppm (C2), thus verifying the presence of lactate anion. In addition, in exfoliated MgAllactate LDH, the 1H NMR peak due to (C
OH) of the lactate anion is observed at a higher field compared to that of the pure lactate anion and indicates that the lactate anion is hydrogen bonded to the hydroxyl groups of the brucite-like layers (Indrasekara and Kottegoda, 2011). 11 B MAS NMR spectra of Mg/Al and Zn/Al type LDHs synthesized with interlayer triborate anions consisted of a singlet and a complex second-order quadrupolar broadened pattern. Peaks at 19.6 and 2.9 ppm were assigned to trigonal and tetrahedral boron, respectively. The relative intensity of the two boron peaks was between 2 and 3, which was consistent with intercalation of the triborate, which contains two trigonal and one tetrahedral boron atom (Theiss et al., 2013a).

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Tsai et al. (2014) employed MgAl-LDHs with different particle sizes, which were modified with a sulfanilic acid salt (SAS) via the hydrothermal method for the preparation of polyethylene terephthalate (PET) NCs. PET/MgAl LDH-SAS NCs were synthesized by intercalation of modified MgAl-LDH in the bis-hydroxy ethylene terephthalate (BHET) via an in situ polymerization route using MgAl LDH-SAS as a catalyst. The aim of this work was an examination of the effect of MgAl LDH-SAS on the thermomechanical, gas barrier behavior, and crystallization property of PET-based NCs. The chemical structures of the NCs were characterized by 1H NMR and 13C NMR, as shown in Fig. 6.28. These results confirmed the structure of PET and the presence of additional chemical bonds in the NCs which can be influenced by the adhesion between LDH nanofiller and PET molecules and

Figure 6.28 NMR spectra of PET NCs: (A) 1H NMR; (B) 13C NMR. Source: Adapted from Tsai, T.Y., Naveen, B., Shiu, W.C., Lu, S.W., 2014. An advanced preparation and characterization of the PET/MgAl-LDH nanocomposites. RSC Adv., 4(49), 25683
25691, with kind permission of Elsevier.

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Layered Double Hydroxide Polymer Nanocomposites

do not do harm to the PET structures. The structure of PET NCs was further confirmed by protons of the polymeric CH2CH2 and C6H4 groups which are appeared as singlet peaks at 4.82 and 8.16 ppm, respectively. The integration ratio of these chemical shifts was exactly 1:1. However, neat PET and its NCs displayed the same peaks without any broadening or chemical shifts and/or the appearance of new peaks in the spectra. Thus, it is expected that linkages between BHET and SAS in LDH layers can happen during the ester interchange reaction, however, 1H NMR spectroscopy could not definitely identify extra peaks corresponding to bonds between SAS and PET. It seems that due to the very low content of LDH-SAS (1.0 wt.%), additional bonds between SAS and PET did not appear in the spectra. Finally, due to the results obtained from 1H NMR, it was demonstrated that there is no new chemical bond during loading of LDH to the polymerization. Hsueh and Chen (2003) synthesized LDH-amino benzoate (LDH-AB) by incorporating organic anions into the LDH. Subsequently, LDH-AB/polyimide (PI) NCs with different loadings of LDH-AB were successfully fabricated through the incorporation of LDH-AB in the PI matrix. In other words, the surface grafting of amino benzoate on the MgAl-LDH caused positive effects on the distribution and compatibility between inorganic MgAl nanolayers and organic PI matrix. The exfoliation of the MgAl nanolayers in the PI matrix to form LDH-AB/PI NCs was investigated by magic angle spinning 27Al NMR (27Al MAS NMR) spectrum and is shown in Fig. 6.29. The 27Al MAS NMR spectrum of LDH-AB in the narrow range of 210 to 120 ppm was referenced to the resonance of octahedrally coordinated aluminum within the brucite-like layers of the LDHs. The 27Al MAS NMR spectrum consisted of a signal resonance at 110 ppm, corresponding to octahedral coordination of all the aluminum atoms in the LDH-AB. Therefore NMR analysis indicated which LDH-AB with brucite-like layers were successfully fabricated. In 2005, Chen and Qu (2005) reported a facile approach to prepare NCs based on poly(methyl acrylate) (PMA) by in situ polymerization of methyl acrylate (MA)

Figure 6.29 27Al MAS NMR of LDH-AB. Source: Adapted from Hsueh, H.B., Chen, C.Y., 2003. Preparation and properties of LDHs/ polyimide nanocomposites. Polymer. (Guildf)., 44(4), 1151
1161, with kind permission of Elsevier.

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Figure 6.30 1H NMR spectra of (A) pure PMA and (B) PMA/OZnAl-LDH NC. Source: Adapted from Chen, W., Qu, B., 2005. Enhanced thermal and mechanical properties of poly (methyl acrylate)/ZnAl layered double hydroxide nanocomposites formed by in situ polymerisation. Polym. Degrad. Stability, 90(1), 162
166, with kind permission of Elsevier.

in the presence of Zn3Al(OH)8(C12H25SO4) (called OZnAl-LDH). Finally, the chemical structure of the PMA/OZnAl-LDH NCs was investigated compared with pure PMA. Fig. 6.30 shows the 1H NMR spectra of pure PMA and PMA/OZnAlLDH NC. The spectrum of pure PMA (Fig. 6.30A) displays chemical shifts centered at 3.46, 2.65, and 2.17
1.58 ppm which are assigned to the methyl, methane, and methylene protons, respectively, of the MA unit. The presence of the endstanding methyl group reveals a peak at 0.86 ppm. In Fig. 6.30B, the chemical shift of the PMA sample extracted from the PMA/OZnAl-LDH NCs is approximately the same as pure PMA. However, the small shift of position peaks is probably caused by the very small amount of OZnAl-LDH dispersed in the extracted PMA sample or the system error of the equipment. Also, the disappearance of peaks caused by CQC in MA monomer indicates the absence of MA molecules. In other words, the monomers absorbed in galleries of OZnAl-LDH have been converted to macromolecules after in situ polymerization. In another research work, Wang et al. (2005) presented PMMA NCs reinforced with MgAl-LDH as filler by two-stage in situ polymerization, where LDH layers had been modified with 10-undecenoate (LDH-U) to be well dispersed in PMMA matrix. The 27Al and 13C MAS NMR determined the structural and compositional details of the LDH-U. The 27Al MAS NMR spectrum of LDH-U demonstrated a signal resonance at 110.8 ppm. This resonance peak reveals that the aluminum atoms have an octahedral coordination geometry in the LDH-U. Therefore, the successful synthesis of LDH-U with a brucite-like layer was confirmed. Fig. 6.31 displays the 13C MAN NMR spectrum of the LDH-U. The spectrum exhibited three principal resonance peaks at 140.4 and 116.0 ppm, which are related to the carbon atoms of the vinyl group, Cb and Ca, respectively. The resonance peak of the

262

Layered Double Hydroxide Polymer Nanocomposites

Figure 6.31 13C MAS NMR spectra of LDH-U. Source: Adapted from Wang, G.A., Wang, C.C., Chen, C.Y., 2005. The disorderly exfoliated LDHs/PMMA nanocomposite synthesized by in situ bulk polymerization. Polymer. (Guildf)., 46(14), 5065
5074, with kind permission of Elsevier.

carboxylic group appears at 185.0 ppm. Moreover, the presence of methylene carbons in the 10-undecenoate anion framework is observed in the region of 27
40 ppm. According to the literature, it is well established that the resonance peak at 170.6 ppm, corresponding to the interlayer charge balancing carbonate anion of the LDH, was not observed in the spectrum and disappeared. Restated, the interlayer region of the LDH sheets contains significant amounts of the 10-undecenoate anion. On the other hand, the resonance peak of Ck is slightly deshielded (shift downfield value) compared with sodium 10-undecenoate. The incorporation of the 10-undecenoate anion into the LDH layers, followed by strong electrostatic interaction between the carboxylic group and the inner surface of the LDH, causes a significant shift in this resonance peak.

6.3.7 UV
vis spectroscopy of layered double hydroxide polymer nanocomposites According to the literature, the broad absorption band of Mg/Fe LDH is due to the metal ligand charge transfer band of O2p !Fe31 and the M
M-charge-transfer spectrum of Mg21
O
Fe31 (Mohapatra et al., 2016). The Mn-containing LDHs with the lowest Mn amount displayed an intense band about 250 nm that can be attributed to the O22!Mn21 charge transfer transition. Mg/Al-NO3 LDHs and Zn/Al-NO3 LDH have absorption spectra and the maximum absorption wavelength results at 297 and 305 nm, respectively, which correspond with the existence of NO32 in the LDH interlayer (Peng et al., 2016). In the UV area of Mn-Ru LDH, the charge transfer O!M (MQMn or Ru) of the inorganic layers is superimposed upon the ligand transitions. The absorption band at about 272 and 367 nm is based

Spectroscopic characterization techniques for layered double hydroxide polymer nanocomposites

263

on the direct charge transfer transitions from O22 2p to Mn21 3d (Dhanasekaran et al., 2017). The UV absorption intensity of ZnTi-LDH was nearly 10 times greater than that of MgAl-LDH and ZnAl-LDH at between 280 and 400 nm, which corresponds with the existence of Ti in the layers. The data displayed that the UV absorption intensity increase along with the enhancement of the Ti41 amount, demonstrating that the Ti component facilitates the absorption of photons and the obtaining electron transition (Wang et al., 2017). Hajibeygi et al. (2017) studied the influence of diacid-diimide modified Mg-Al LDH (DLDH) and sodium dodecylbenzene sulfonate (SDBS) modified Mg-Al LDH (SLDH) loading on the physical interactions and properties of developed NCs based on the PAI. It is clear from these results that high van der Waals interactions and hydrogen bonding between the modified LDH with a suitable modifier and PAI matrix caused a considerable increase in the thermal behavior of the PAI chain. The relative intensity of UV
vis spectra of PAI/DLDH (PAIDN) and PAI/SLDH (PAISN) was reduced by raising the LDH value. Solid-state UV
vis absorption of pure PAI and PAIDN 2, 5, and 8 mass% are displayed in Fig. 6.32. The absorption spectrum of pure PAI showed n!π and π!π transitions at 295
312 nm. These maximum absorption bands are ascribed to the atoms, such as oxygen, nitrogen, and also naphthalene rings, in the PAI structure. For LDH nanolayers, the maximum absorption bands at 450 nm are attributed to the metal charge transfer, which is caused by the 2p orbitals of oxygen to the 3d orbitals of Mg21 and Al31 ions (O2p!Mg3d, O2p!Al3d). The comparison of UV
vis spectra of PAI and PAINC showed that the maximum absorption bands were approximately in the range of

Figure 6.32 UV
vis spectra of PAI and PAIDN in the solid state. Source: Adapted from Hajibeygi, M., Shabanian, M., Omidi-Ghallemohamadi, M., 2017. Development of new acid-imide modified Mg-Al/LDH reinforced semi-crystalline poly (amide-imide) containing naphthalene ring; study on thermal stability and optical properties. Appl. Clay Sci., 139, 9
19, with kind permission of Elsevier.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 6.33 UV
vis spectra of PAI and PAINC at solution state. Source: Adapted from Hajibeygi, M., Shabanian, M., Omidi-Ghallemohamadi, M., 2017. Development of new acid-imide modified Mg-Al/LDH reinforced semi-crystalline poly (amide-imide) containing naphthalene ring; study on thermal stability and optical properties. Appl. Clay Sci., 139, 9
19, with kind permission of Elsevier.

400
460 nm. This region resulted from the transition electrons in the LDH layers and indicated the homogeneous dispersion of LDH nanostructures in the PAI matrix. Fig. 6.33 showed the UV
vis absorption spectra of virgin PAI and PAINC in the DMF solution. According to the related figure, the maximum absorption band of PAI solution at around 297 nm, is similar to solid state. Also, there are no maximum absorption bands of the LDH nanostructure in the spectrum of the PAINC solution, which is due to high dilution of the PAINC solutions. The absorption intensities were decreased by enhancement of LDH content. As can be seen, the UV
vis spectra of PAISN 2 and 5 mass% presented the lowest intensities. The strong interactions between PAI and LDH nanolayers are an important factor in this reduction. In another study, Shi et al. (2015) employed two surfactants, graphite-like carbon nitride (g-C3N4) and borate-modified LDH (LDH-B), for the preparation of PPgrafted maleic anhydride (PP-g-MA)/g-C3N4 and PP-g-MA/LDH-B NCs with 4 wt.% loadings. Among the nanoadditives, two-dimensional nanomaterials such as g-C3N4 with a stacked 2D structure have potential for significant improvement properties of NCs, especially by increasing the UV absorption capacity. On the other hand, the presence of LDH-B as nanofiller into the PP-g-MA matrix leads to a decrease in the optical behavior of the NCs (Fig. 6.34). The solution casting method in the presence of LDH layers and carboxymethyl cellulose (CMC) was carried out by Yadollahi et al. (2014). CMC-LDH NC films were fabricated with weight percentages of LDH from 0 to 8 wt.%. To evaluate the influence of LDH on the transparency of NC films, visible-light transmittance of

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265

Figure 6.34 UV
vis spectra of LDH-B, g-C3N4, PP-g-MA and its NCs. Source: Adapted from Shi, Y., Gui, Z., Yu, B., Yuen, R.K., Wang, B., Hu, Y., 2015. Graphite-like carbon nitride and functionalized layered double hydroxide filled polypropylene-grafted maleic anhydride nanocomposites: comparison in flame retardancy, and thermal, mechanical and UV-shielding properties. Comp. Part B: Eng., 79, 277
284, with kind permission of Elsevier.

neat CMC film and CMC-LDH NC films as displayed in Fig. 6.35. For example, the optical transmission properties were measured under the visible-light region, NC films with LDH content lower than 8 wt.% illustrated a transmittance of 70%, while the optical behavior of NC 8 wt.% was notably decreased with increasing LDH layers. The reduction in transparency of NC films could be related to the defective distribution of LDH or the occurrence of partial aggregation. The results were completed by micrographs of pure CMC and NCs with 3 and 8 wt.% of LDH, which gives information about the distribution state. Figs. 6.36 and 6.37 show the TEM and SEM of the samples. These images clearly illustrate that CMC films without LDH exhibited a smooth and homogeneous surface. In the following, the dispersion state of NC films is indicated only by a small region of intercalated and agglomerated LDH layers in the NCs with 3 wt.% of LDH, but when the LDH amount was enhanced to 8 wt.%, the intercalation and aggregation of LDH layers increased slightly, leading to enhanced light scattering.

6.3.8 X-ray photoelectron spectroscopy of layered double hydroxide polymer nanocomposites XPS was employed in order to better characterize the chemical structure of the Zn-Al-PO432 LDH nanoparticles. The XPS full-survey spectra of Zn-Al-PO432 LDH show the peaks related to aluminum, carbon, and oxygen elements at binding

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Layered Double Hydroxide Polymer Nanocomposites

Figure 6.35 Visible-light transmittance of pure CMC film and CMC-LDH NC films. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym., 108, 83
90, with kind permission of Elsevier.

energies of 72.4, 285, and 531 eV, respectively. Two peaks at binding energies of 1023 and 1045 eV are detected for zinc. Further, a peak corresponding to phosphorus element (P2p) at a binding energy of 133.8 eV was characterized. The observation of this peak represents the existence of phosphorus in the LDH structure. The deconvoluted spectra of the oxygen for Zn-Al-PO432 LDH demonstrate the highresolution O1s spectra in the range of 526
538 eV. The O1s spectrum was deconvoluted in three peaks showing three different oxygen species. The peaks centered at 530.55, 531.77, and 532.59 eV were ascribed to hydroxyl oxygen (aOH), the nonbridging oxygen (P
O) or Zn
O, and the bridging oxygen (P
O
P), respectively. The original O1s spectrum was then deconvoluted according to the assignments. These reveal that phosphorus was intercalated in the galleries of Zn-Al LDH. Also, there is a weak peak around 400 eV attributed to N1s, indicating the presence of a small amount of nitrate in the phosphate-intercalated LDH. The XPS results confirm successful loading of phosphate anions in the LDH structure (Alibakhshi et al., 2016). Huang et al. (2012) produced NCs based on ethylene vinyl acetate copolymer (EVA) as a host matrix mixed with a phosphorus nitrogen-containing compound, N-(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)hexylic)-acetamide-2-propyl acid (PAHPA) a modifier and LDH layers as nanofiller and obtained EVA/PAHPALDH NCs. They then investigated the effect of grafted PAHPA on the distribution and flame retardancy of LDH and EVA NCs. The XPS spectra of LDH are referenced to the O1s at 529.3 eV, C1s at 282.1 eV, Al2p at 71.8 eV, and Mg2p at 47.4 eV. The appearance of P2s, P2p, and N1s in the spectrum of PAHPA-LDH is due to nitrogen and phosphorus groups in modified LDHs. XPS of LDH revealed traces of carbonate group in the LDH layers with a 2.24% content of carbon, while the carbon amount of the PAHPA-LDH sample was around 6.33%. All of these

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Figure 6.36 SEM images of (A) CMC film, (B) the CMC-LDH NC film with 3 wt.%, and (C) CMC-LDH NC film with 8 wt.% LDH. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym., 108, 83
90, with kind permission of Elsevier.

results displayed favorable intercalation of PAHPA into the LDH layers by ion exchange (Fig. 6.38). Hu et al. (2015) reported an approach to increase the anticorrosion and flame retardancy of polyaniline (PANI). In this work, first, OH groups of decavanadate
intercalated LDH (D-LDH) were linked to ethoxy groups of γ-aminopropyltriethoxysilane (APTS) via covalent bonding, and finally, employed

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Layered Double Hydroxide Polymer Nanocomposites

Figure 6.37 TEM images of the CMC-LDH NC films with 3 wt.% LDH (A and B) and 8 wt.% LDH (C and D) at low and high magnifications, respectively. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym., 108, 83
90, with kind permission of Elsevier.

Figure 6.38 XPS spectra of LDHs and PAHPA-LDHs. Source: Adapted from Huang, G., Fei, Z., Chen, X., Qiu, F., Wang, X., Gao, J., 2012. Functionalization of layered double hydroxides by intumescent flame retardant: preparation, characterization, and application in ethylene vinyl acetate copolymer. Appl. Surf. Sci., 258 (24), 10115
10122, with kind permission of Elsevier.

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Figure 6.39 SEM images of (A) D-LDH, (B) AD-LDH, (C) PANI, and (D) AD-LDH/PANI, and (E) XPS survey spectra of AD-LDH and AD-LDH/PANI. Source: Adapted from Hu, J., Gan, M., Ma, L., Zhang, J., Xie, S., Xu, F., et al., 2015. Preparation and enhanced properties of polyaniline/grafted intercalated ZnAl-LDH nanocomposites. Appl. Surf. Sci., 328, 325
334, with kind permission of Elsevier.

(known as AD-LDH) for the preparation of PANI-AD-LDH NCs. The SEM micrographs of D-LDH, AD-LDH, and AD-LDH/PANI NCs are presented in Fig. 6.39. The morphology of the D-LDH seems to be aggregates of the plate-like particles due to relatively tight structural coherence in small particles. The images of ADLDH were changed by surface modification. The grafting agent caused a positive effect on the dispersion and, finally, reduction of agglomeration of clay particles. The SEM images of AD-LDH/PANI NCs illustrated well the distribution of ADLDH in the PANI matrix with the exfoliated structures. The XPS data of AD-LDH and AD-LDH/PANI are shown and summarized in the related figure and Table 6.1, respectively. The strong peak at 102.0 eV is due to the Si2p of AD-LDH, which demonstrated the existence of the APTS on the D-LDH surface as a source of silicon. No XPS signals from the Zn and Al were observed by this analysis. This result was in accordance with the relatively low concentration of the LDH in the

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Table 6.1 The chemical composition of AD-LDH and AD-LDH/PANI determined from the analysis of the XPS data Sample

AD-LDH AD-LDH/PANI

C1s

O1s

N1s

Si2p

S2p

BE (eV)

BE (eV)

BE (eV)

BE (eV)

BE (eV)

At. (%)

At. (%)

At. (%)

At. (%)

At. (%)

284.1 70.61 284.6 73.45

531.6 26.05 531.3 14.63

399.1 2.41 399.6 9.06

102.0 0.93 102.2 0.27

165.1 2.59

AD-LDH/PANI NCs. The bonding energies related to the nitrogen 1s XPS spectra of AD-LDH and AD-LDH/PANI indicated the creation of a chemical interaction between AD-LDH and PANI matrix by a shift to the higher bonding energy. Gore et al. (2016) fabricated a new LDH/PAN NC via a plan of interweaving LDH with nanofibers based on polyacrylonitrile (PAN) film as an efficient adsorbent for Cr(VI) elimination from aqueous solution in an original column experimental plan. The characteristic peaks at 398.1 eV assigned to the CRN groups from the PAN chains are in agreement with the bonding energy value for the LDH/PAN NCs at 399.1 eV (Fig. 6.40A). The C1s spectrum of graphite carbon exhibited a peak at 284.7 eV (Fig. 6.40B). Also, the located peak at 532 eV (Fig. 6.40C) is attributed to the carbonyl oxygen atoms in amides, and also the oxygen atoms in hydroxyl groups. As such, XPS analysis of LDH/PAN NCs indicated the presence of whole elements from the LDH and PAN (Fig. 6.40D).

6.4

Spectroscopic characterization for the aging process

Since LDHs are an ultraviolet-light (UV)-resistant material, they were used to modify bitumen by Liu and his co-workers. The aging resistance of LDH-modified bitumen was investigated using a UV-aging oven. The viscosities of the base and modified bitumen before and after UV aging were indicated, and the addition of LDHs increased the viscosity of base bitumen. After UV aging, the viscosity of each bitumen also increased. However, the viscosity aging index (VAI) value for LDH-modified bitumen was lower than for the base bitumen. Moreover, the greater the content of LDHs, the lower the VAI value. LDH-modified bitumen at 5 wt.% had the lowest VAI value, and exhibited the best of UV aging resistance performance. Fig. 6.41A shows the penetration of base bitumen and LDH-modified bitumen before and after UV aging. As indicated, the addition of LDHs decreased the

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Figure 6.40 XPS spectra of PAN/LDH NC showing the presence of (A) nitrogen, (B) carbon, (C) oxygen, and (D) survey spectra of all elements. Source: Adapted from Gore, C.T., Omwoma, S., Chen, W., Song, Y.F., 2016. Interweaved LDH/PAN nanocomposite films: application in the design of effective hexavalent chromium adsorption technology. Chem. Eng. J. 284, 794
801, with kind permission of Elsevier.

Figure 6.41 (A) Penetration of bitumen before and after UV aging; (B) PRR of bitumen after UV aging. Source: Adapted from Liu, X., Wu, S., Liu, G., Li, L., 2015. Optical and UV-aging properties of LDH-modified bitumen. Materials, 8(7), 4022
4033, open access journal.

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penetration value of base bitumen because the LDHs made the bitumen more viscous. However, this value for the base bitumen became lower than that for modified bitumen after UV aging. As shown in Fig. 6.41B, the base bitumen had the lowest penetration retention rate (PRR) of 18.1%, indicating poor UV-aging resistance. After the addition of 5 wt.% LDHs, this value increased to 44.8%. This showed that the addition of LDHs can improve the UV-aging resistance of base bitumen. Fig. 6.42A shows the softening points of base bitumen and LDH-modified bitumen before and after UV aging. As indicated, the addition of LDHs increased the softening point value of base bitumen because the LDHs made the bitumen more viscous. However, this value for the base bitumen became higher than that for modified bitumen after UV aging. As shown in Fig. 6.42B, the base bitumen had the highest softening point increment (SPI) of 28 C, showing poor UV-aging resistance. After mixing with 5 wt.% of LDHs, this value decreased to 9.0 C. This indicated that the addition of LDHs can improve the UV-aging resistance of base bitumen. This corresponded well with the results of viscosity and penetration. Fig. 6.43 shows the FTIR spectra of the base and LDH-modified bitumens before and after UV aging. The UV-aging process can accelerate the oxidation of bitumens, and increase the peak area of carbonyl at 1700 cm21 and sulfoxide at 1030 cm21. After UV aging, the carbonyl index (IC5O) of base bitumen increased by 0.0184, and the sulfoxide index (IS5O) by 0.0370. However, the IC5O values of 3% and 5% LDH-modified bitumen only increased by 0.0163 and 0.0123, and the IS5O by 0.0290 and 0.0200. Therefore the addition of LDHs can inhibit the oxidation of bitumen during UV aging.

Figure 6.42 (A) Softening point of bitumen before and after UV aging; (B) SPI of bitumen after UV aging. Source: Adapted from Liu, X., Wu, S., Liu, G., Li, L., 2015. Optical and UV-aging properties of LDH-modified bitumen. Materials, 8(7), 4022
4033, open access journal.

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Figure 6.43 FTIR spectra of base and LDH-modified bitumen (A) before and (B) after UV-aging. Source: Adapted from Liu, X., Wu, S., Liu, G., Li, L., 2015. Optical and UV-aging properties of LDH-modified bitumen. Materials, 8(7), 4022
4033, open access journal.

6.5

Conclusions

In the recent progress, polymer/LDH NCs were characterized as one of the most promising investigations in chemistry and material science due to their twodimensional structures, enormous variety in physicochemical behaviors, and potential practical applications. Characterization of these NC materials is necessary to understand/analyze different facets of polymer NCs. This chapter introduced the various techniques of spectroscopy, which are available, for example, FTIR, Raman, XPS, UV
vis, EDX, Fluorescence, Dielectric and NMR for the investigation of polymer/LDH NCs.

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Information obtained by these spectroscopic techniques includes the following: The chemical structure of samples, the attachment of the surfactants to the nanoparticle surface, the intercalation of modifiers in the basal spacing of the LDHs, and the existence of an intermolecular interaction between nanofillers and polymer matrix were confirmed by FTIR analysis. Also, Raman spectroscopy has been mainly employed to study the structural accommodation of interlayer species and the hydrogen bond network in LDH, particularly for oxo-anions such as CO322, NO32, SO422, CrO422, and organic carboxylate anions. The amount and distribution of nanoparticles and the elemental composition of NCs were confirmed by spectrum and elemental mapping using an EDX spectrometer combined with an SEM system. To characterize the optical behavior of samples, UV
vis and fluorescence analysis are performed which display the peak wavelength. The spectral bandwidth of UV
vis gave information such as particle size, shape, the material composition, and the local environment of NCs and nanoparticles. The dielectric behaviors of NCs as a function of frequency are measured by dielectric spectroscopy. Finally, empirical formula, the elemental composition, and electronic state of the elements in samples are identified by XPS technique.

Acknowledgments We thankfully acknowledge the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, I. R. Iran for partial financial support. Further financial support from Center of Excellence in Sensors and Green Chemistry Research, IUT, is gratefully acknowledged. We also thank Mr. M. Hatami, Dr. V. Behranvand, Dr. F. Tabesh, and Dr. S. Rashidimoghadam from the Department of Chemistry, IUT, for their great help.

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Further reading Agu´, U.A., Oliva, M.I., Marchetti, S.G., Heredia, A.C., Casuscelli, S.G., Crivello, M.E., 2014. Synthesis and characterization of a mixture of CoFe2 O4 and MgFe2 O4 from

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layered double hydroxides: band gap energy and magnetic responses. J. Magn. Magn. Mater. 369, 249
259. Huang, N.H., Wang, J.Q., 2009. A new route to prepare nanocomposites based on polyvinyl chloride and MgAl layered double hydroxide intercalated with lauryl ether phosphate. Express Polym. Lett. 3, 595
604. Iyi, N., Matsumoto, T., Kaneko, Y., Kitamura, K., 2004. Deintercalation of carbonate ions from a hydrotalcite-like compound: enhanced decarbonation using acid 2 salt mixed solution. Chem. Mater. 16 (15), 2926
2932. Newman, S.P., Jones, W., 1998. Synthesis, characterization and applications of layered double hydroxides containing organic guests. New J. Chem. 22 (2), 105
115. Parida, K.M., Mohapatra, L., 2012. Carbonate intercalated Zn/Fe layered double hydroxide: a novel photocatalyst for the enhanced photo degradation of azo dyes. Chem. Eng. J. 179, 131
139. Zhao, Y., Liang, J., Li, F., Duan, X., 2004. Selectivity of crystal growth direction in layered double hydroxides. Tsinghua Sci. Technol. 9 (6), 667
671.

Melt rheological properties of layered double hydroxide polymer nanocomposites

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Appukuttan Saritha1 and Kuruvilla Joseph2 1 Department of Chemistry, School of Arts and Sciences, Amrita Vishwa Vidyapeetham, Amritapuri, Kollam, Kerala, India, 2Department of Chemistry, Indian Institute of Space Science and Technology, Valiamala, Thiruvananthapuram, Kerala, India

7.1

Introduction

7.1.1 The importance of rheological studies of polymer nanocomposites The most important milestone achievement in polymer technology has been the recent progress in polymer/layered nanocomposites. Polymer nanocomposites are promising candidates which unearth diverse applications in the vast arena of science and technology. Hence the information on polymer solutions is crucial for the polymer processing industry. Analysis of the rheology of polymeric melts encompasses the analysis of the mechanical behavior of the melt upon the exertion of mechanical stress or strain. The rheological behavior of polymers is of key significance for various polymer processing methods like extrusion, blow molding, injection molding, fiber spinning, and calendaring. Because of the growing economic importance for manufacturing of polymers, it is essential to consider the equipment and operational parameters for optimizing the design processes. The melt flow behaviors of composites encompassing particles are thoroughly dependent on how the particles aggregate and the interparticle, as well as the interaction of particles with the polymer. The behavior exhibited in this case can be significantly diverse from that of the unfilled melts (Barnes, 1997; Leonov, 1990). In reality, the analysis of rheological property is an imperative tool to explore the state of dispersion of the filler in polymeric systems and their response under external force. This can be considered equivalent to the techniques for morphological analysis like X Ray Diffraction (XRD), scanning electron microscope, transmission electron microscope, etc. One of the major advantages of this analysis is that it replicates the bulk properties of the matrix and also offers flow performance of the melt that are considered crucial in the melt processing of composites. Furthermore, it provides an awareness of the microstructure in the molten state. The flow behaviors are connected, generally qualitatively or at least semiquantitatively, with the dispersion of particles in the matrix. Conventionally rheological analysis is carried out by investigating the Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00007-0 © 2020 Elsevier Ltd. All rights reserved.

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Layered Double Hydroxide Polymer Nanocomposites

viscoelastic response of a material in the linear flow regime, which means under very low shear strain or strain rate. As is known, polymeric melts are viscoelastic and their reaction to shearing is mainly dependent on the ratio between the timescales of shear experiments and the distinctive relaxation time of microstructures prevailing in those systems. In a high-molecular-weight unfilled melt, the term microstructure relates to molecular entanglements, whereas in a particle-filled system it refers to the structural relationship between the particles in the filler. Primarily, such microstructures indicate those structural features which physically function as a barrier against the flow of the polymer chains under stress. The system displays preferably an elastic response with an elevated value of storage modulus when the experimental timescale is far below the characteristic relaxation time whereas, at a large experimental timescale (experienced at low frequencies or shear rates), the system shows a viscous response. However, sufficiently high shearing actions can change and even exterminate these microstructures, which leads to an absolutely diverse material response. It is highly essential to have investigations in both these regions since they highlight the diverse mechanism of the reaction of the material towards external stress. The melts of polymeric systems are pigeon-holed by an acute strain beneath which stress and applied strain bear a linear relationship and their ratio (known as the relaxation modulus) shows a constant value independent of strain. Thus, the rheological behavior of polymeric melts below this critical strain to be a linear viscoelastic one. Above the critical strain, the relaxation modulus declines with strain, owing to changes in the microstructures, while stress becomes a nonlinear function of strain resulting in a nonlinear viscoelastic material response (Macosko, 1993). Consequently, the principal undertaking prior to performing rigorous rheological study is to define the point of transition between the linear and nonlinear regime of viscoelasticity. In order to determine this critical strain or a range about it, the polymeric melt is subjected to dynamic oscillatory shear using sinusoidal strain at constant frequency and varying strain amplitude. This serves to analyze the flow behaviors of the polymer melts devoid of terminating the interparticle interactions or network that may arise amongst them. The reaction of the fluid exhibiting viscoelasticity to nonlinear shearing (also known as flow reversal experiment) is employed to elucidate the development of structure amid the particles (Walker et al., 1995; Solomon et al., 2001; Li et al., 2003). To perform such an analysis, polymer melt is initially acted upon by a steady shear trailed by a definite relaxation period (shearing is stopped) followed by repeating the process in the opposite direction. It has been witnessed that development of any network structure is revealed in the stress shoot up in the flow reversal step. The time required to accomplish the stressshoot-up peak and its extent are absolutely dependent on the period of relaxation, rate of shear, and even the nature of dispersion of the particle in the melt.

7.1.2 Rheology of polymer layered double hydroxide nanocomposites Enhanced rheological properties for a system might be due to both particle
particle interactions and a polymer-based network where these two phenomena

Melt rheological properties of layered double hydroxide polymer nanocomposites

283

simultaneously contribute towards the reinforcement of the composite. As a result of complex interfacial interactions (polymer and particles) the molecular dynamics relaxation processes get reduced, which in turn suppresses relaxation, thereby affecting the performance of the nanocomposites and the nano-dispersion. Also, particle fractals within the polymer can disseminate stress and influence the system dynamics strongly. Nevertheless, it seems that the role of particle
particle interactions in inducing solid-like behavior is the more prevailing effect comparative to constrained layers, particularly at elevated concentrations and most notably for particles with active surfaces. Though the polymer
particle interactions are powerful, at concentrations higher than the threshold concentration, it is the formation of a network between the filler particles that provides the pseudo-solid rheological behavior at low frequencies. The percolation threshold depends on an assortment of parameters together with the morphology of the nanoparticles, the state of dispersion, molecular weight distribution of the polymer, and processing methods. Consequently, various values have been reported for percolation thresholds in diverse systems (Sinha Ray and Okamoto, 2003; Hussain et al., 2006; Habibi et al., 2010). It is noteworthy that a particle network is not inevitably the outcome of direct physical contacts between the fillers. Therefore, the reported rheological percolation threshold can be slightly less than the electrical percolation threshold. From a rheological perspective, a direct result of integration of filler in molten polymers is a noteworthy alteration in the resulting viscosity. The existence (or not) of yield stress is an important issue in polymer microcomposites (Hornsby, 1999), and therefore plays a key role in nanocomposites as these have even higher surface area. Below this specific stress, materials show solid-like behavior (i.e., they deform elastically) and, at stresses higher than the yield stress, the material behaves like a liquid. Studies conducted reveal that, in an approach analogous to the dynamic modulus, yield phenomena can also be correlated with the degree of particle interaction or polymer
filler interaction. Additionally, the accurate mechanism for yield stress in viscosity is also complicated. For example, in the case of clay, the percolation threshold was reduced and the modulus and viscosity at yield increased with increasing dispersion quality; while for silica exactly the opposite trend was observed (Jancar et al., 2010). The complexity of the interactions points toward the fact that depending on the system considered, the mechanism for solid-like behavior changes accordingly. The concept of percolation threshold concentration is very important from a practical perspective as it forms a crucial part in an assortment of applications such as mechanical reinforcement, electrical conductivity, flame retardancy, and permeability. Since the relaxation patterns change very strongly at percolation, a variation can be observed in many viscoelastic properties. Conversely, the storage modulus at low concentrations shows characteristic terminal behavior and, above percolation, a plateau is observed which increases with concentration. The rheological properties of the composites can be described by simple power-law equations (Winter and Mours, 1997) around percolation. These two types of interactions are displayed to an immense extent in the case of nanocomposites where the filler size falls in the nano regime. Undoubtedly, decreasing filler size and increasing surface area increase the tendency of the particles to interconnect. In addition, increased surface area of nanofillers enhances the probability of interfacial chain interactions. Hence micromechanical

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models fail to predict the resulting properties and excitingly particles with smaller sizes generally create stronger nanocomposites. In this context, it is apparent that the rheological analysis of nanocomposites requires mammoth attention and hence is considered decisive in the study of nanocomposites. Because of the diversity of particle morphology, the best candidate to be addressed is the aspect ratio of the filler counterparts, since a specific morphology must be defined accordingly (Kagarise et al., 2008). Although the part played by nanoparticles is significant, the relative size of the particle to the polymer chains is also another critical parameter influencing the viscoelastic properties. The literature suggests that when the radius of gyration of the tracer polymer (Rg) exceeds the radius of the nanoparticle (Rp), the subsequent reinforcement is much higher (Mu and Winey, 2007; Nusser et al., 2010). When the length of a polymeric chain is large, it can easily interweave with the particle and thereby amplify the time of relaxation of the polymer chain comparative to a shorter chain for which no entanglement with the particle is possible (Du et al., 2004; Picu and Rakshit, 2007; Anderson and Zukoski, 2009, 2010). Interestingly, most of the earlier studies were focused on polymer nanocomposites based on montmorillonite type of layered silicate clays. Lately, LDHs have emerged as an effective nanofiller for the preparation of polymer nanocomposites due to their tuneable properties and higher chemical purity. Moreover, polymer nanocomposites containing exfoliated LDH possessed more exfoliated clay layers as compared to layered silicate-based polymer nanocomposites, because each layer of the LDHs is comprised of a single octahedral sheet of mixed metal hydroxide, while layered silicates are composed of multiple octahedral/tetrahedral sheets of metal oxide/hydroxide (Leary et al., 2002; Lv et al., 2012). LDHs, generally termed anionic clays, constitute a type of anionic layered material consisting of positive brucite-like layers with an interlamellar space encompassing anions and solvation molecules to comthe charge. They possess the molecular formula of pensate x1 II III m
m2 MII12x MIII ð OH Þ ð A Þ  nH O, wherein M , M , and A represent a diva2 2 x=m x lent (or monovalent) cation, a trivalent cation, and the interlayer anion, respectively (Manzi-Nshuti et al., 2008; Zhao et al., 2009). Owing to their layered assembly and extraordinary anion exchange capacity, they are employed in numerous applications, such as catalysis (Wang et al., 2010), CO2 adsorbents, thermal stabilizers, UV absorbers, hosts for nanoscale reactions, and so on (Wang et al., 2012a,b, 2013a,b,c).

7.2

Rheology of thermoplastic polymer layered double hydroxide nanocomposites

Many thermoplastic polymers have been selected as the matrix for the incorporation of LDH-based fillers and they are found to exhibit excellent properties which make them befitting for quite a large number of applications. Polystyrene (PS) nanocomposites containing modified Co-Al layered double hydroxide (LDH) were prepared through a solvent blending technique (Suresh et al., 2016). The complex viscosity and rheological moduli of nanocomposites were found to be higher than that of

Melt rheological properties of layered double hydroxide polymer nanocomposites

285

pristine PS when the frequency increased from 0.01 to 100 s21. The storage modulus also showed increasing trend with a hike in the amount of LDH. The increase of storage modulus at lower frequency is the characteristic of pseudo-solid-like behavior due to the formation of network percolating LDH lamellae as seen in Fig. 7.1A. The growth of filler content in the nanocomposites causes it to change from a liquid-like to a solid-like nature. This transition concentration is called the rheological percolation threshold. The presence of a rheological percolation threshold in the nanocomposite samples can be credited to the development of incessant network of LDH and the polymeric chain. The same phenomena have been reported for PE/ MgAl LDH nanocomposites (Costa et al., 2006a,b,c) and polymer/layered silicate nanocomposites (Lim and Park, 2001). The nanocomposites show ascending complex viscosity values (Fig. 7.1B) with increasing concentration of LDH in the lower-frequency region, which slowly reverses as the frequency increases. The primary cause for this trend is adhesion between the LDH and PS and the cohesive interactions in the LDH layers. This also explains that the addition of LDH influences more frictional interactions. A changeover from Newtonian character to a shear thinning nature is also observed with increasing frequency, which is clarified by the point that the polymer chains have less time to entangle and the direction of randomly dispersed nanofiller at higher frequency. Since the elastic nature of the nanocomposites increases with increasing LDH content, it is seen that the loss factor of the nanocomposites is inferior to pristine PS and reduces with increasing LDH concentration, as seen in Fig. 7.2. In comparison with the storage moduli of samples, it is apparent that the loss modulus as seen in Fig. 7.3 is always higher than the storage modulus at lower frequency, indicating the dominance of the viscous part. Polypropylene (PP)/organomodified LDH nanocomposites were synthesized and the rheological properties investigated by Lonkar et al. (2012). It was observed that there is a robust effect of LDH particles on the flow performance of the composite melt which caused an augmentation of the rheological properties of nanocomposites. Figs. 7.4
7.6 show the disparity in the viscoelastic reaction of neat PP and

1000

(B) PS PS 1 PS 3 PS 5 PS 7

Complex viscosity (Pa-s)

Storage modulus (Pa)

(A) 10,000

100

10

0.01

0.1 1 10 Angular frquency (1/s)

100

PS PS 1 PS 3 PS 5 PS 7

1000

0.01

0.1 1 10 Angular frquency (1/s)

100

Figure 7.1 (A) Storage modulus versus angular frequency; (B) complex viscosity versus angular frequency of pristine PS and its nanocomposites (Suresh et al., 2016).

10

Loss factor

PS PS 1 PS 3 PS 5 PS 7

1

0.01

0.1 1 10 Angular frequency (1/s)

100

Figure 7.2 Loss factor versus angular frequency of pristine PS and its nanocomposites (Suresh et al., 2016).

Loss modulus (Pa)

10,000

1000

PS PS 1 PS 3 PS 5 PS 7

100

10 0.01

0.1 1 10 Angular frquency (1/s)

100

Figure 7.3 Loss modulus versus angular frequency of pristine PS and its nanocomposites (Suresh et al., 2016).

Figure 7.4 Difference in the viscoelastic response of unfilled polypropylene and high LDHfilled nanocomposite in a dynamic oscillatory frequency sweep experiment (Lonkar et al., 2012).

Melt rheological properties of layered double hydroxide polymer nanocomposites

287

Storage modulus (Pa), G'

104

103 PP PPL 1 PPL 3 PPL 5 PPL 7 PPL 10

102

101

100 0.1

1 10 Frequency (rad/s), ω

100

Figure 7.5 Storage modulus (G0) versus frequency (x) plots for unfilled PP and PP/LDH nanocomposite melts (Lonkar et al., 2012).

Figure 7.6 Complex viscosity (g ) versus frequency (x) plots for unfilled PP and PP/LDH nanocomposite melts (Lonkar et al., 2012).

nanocomposites containing high loadings of filler in a dynamic oscillatory frequency sweep experiment. In the experimental range of frequency, the loss modulus of the virgin PP was found to be higher than its storage modulus, which confirms that a dominant factor affecting the flow behavior of the unfilled PP melt is the viscous component. However, on increasing the frequency like a typical thermoplastic melt, the storage modulus of PP rose more than the loss modulus and at a certain

288

Layered Double Hydroxide Polymer Nanocomposites

high frequency, G0 crosses Gv. In contrast, the situation in PP/LDH nanocomposites is such that the storage modulus constantly exceeds the loss modulus demonstrating overriding elastic character of the material in the frequency range where the experiment is conducted. In the current situation, a high concentration of LDH in both the nano- as well as the microregime generates a robust physical barrier against the movement of the polymer chains and demonstrates that the liquid-like melt was gradually transformed into solid-like. The addition of LDH filler caused an upward shift of the low-frequency G0 values. The complex viscosity of nanocomposites increased with LDH concentration as illustrated in Fig. 7.6, and was attributed to the superior compatibility between the polymer matrix and LDH on account of its nano dispersion. The occurrence of nanostructured LDH particles in the melt not only enhances the melt viscosity but also provokes shear thinning nature in the lowfrequency region. The complex viscosity versus frequency plots for the polyethylene/LDH nanocomposites as shown in Fig. 7.7 were carefully analyzed by Costa et al. (2006a,b,c) and the figure clearly illustrates the traditional viscoelastic performance characterized by evolution from Newtonian at low frequency to shear thinning behavior at high frequency. The melt illustrates a changeover to pseudo-solid-like state from the liquid state and the extent of this transition with growing filler loading was studied by an index called shear-thinning exponent, “n.” In the case of Newtonian liquids, n is equal to zero and the neat polyethylene matrix acts more or less in a similar way with small negative value of n, while the nanocomposites demonstrate noteworthy divergence from this behavior. When the LDH concentration is increased, the negative value of n increases progressively in par with the budding

Complex viscosity,|η*| (Pa.s)

105

Shear thinning behavior

PE PEPB PE1LDH PE2.5LDH PE5LDH PE10LDH PE15LDH

104

103

Materials –n in|η*|~ ω PE 0.13 PEPB 0.04 PE1LDH 0.19 PE2.5LDH 0.23 PE5LDH 0.41 PE10LDH 0.71 PE15LDH 0.82

0

Newtonian behavior

102 0.1

1

10

100

Frequency, ω (rad/s)

Figure 7.7 Complex viscosity versus frequency plots for polyethylene/LDH nanocomposites. (PEPB is the blend of polyethylene and PE-g-MAH corresponding to the nanocomposite containing 15 wt.% LDH.) The variation of shear thinning exponent “n” with LDH concentration is also depicted (Costa et al., 2006a,b,c).

Melt rheological properties of layered double hydroxide polymer nanocomposites

289

shear-thinning nature which is distinctive of nanocomposites, where anchoring of polymer chains onto the surface of the particle leads to a decrease in relaxation (Costa et al., 2006a,b,c). Chakraborty et al. (2016) evaluated the dependence of frequency of PMMA nanocomposites containing Ni-Al LDH on the storage and loss modulus as depicted in Fig. 7.8. It is seen that at 190 C PMMA chains are completely stress-free in a relaxed state and display atypical homopolymer-like terminal performance provided the frequency is low. With a hike in frequency, there arises a corresponding enhancement in storage with a boom in the shear rate. When the shear rate shoots up, it is noticeable that there arises a perturbation in the long-time relaxation for all the filler loadings and LDH does not have much effect. The complex viscosity of PMMA and its nanocomposites analyzed at 190 C as displayed in Fig. 7.9 depicts the terminal relaxation zone. The viscosity diminishes

Figure 7.8 (A) Storage modulus and (B) loss modulus of PMMA and its nanocomposites (Chakraborty et al., 2016).

Figure 7.9 (A) Complex viscosity and (B) damping factor of PMMA and its nanocomposites (Chakraborty et al., 2016).

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Layered Double Hydroxide Polymer Nanocomposites

at higher frequency as the chains begin to orient in the direction of flow and unscramble from each another. It is clear that the reduction in viscosity arises at high frequencies due to partial disruption of the three-dimensional complex patterns. Costa et al. (2005, 2006a,b,c) first reported the synthesis of intercalated LDPE nanocomposites using a melt intercalation using dodecylbenzene sulfonate
LDH as filler. An intricate feature was witnessed in the morphology of the nanocomposites, with the filler particles mostly positioned in the form of lean platelets and agglomerates in the dispersed state in the LDPE matrix. These materials portrayed a substantial variation of the linear viscoelastic response when compared with the matrix in the low-frequency region, which is attributed to the development of a threedimensional structure. The nature of the polyethylene matrix, that is, unmodified or MAH grafted, is of great concern with regard to its rheological behavior. To study the collapse and restoration processes associated with dispersed LDH phase in the LDPE systems, the nanocomposite melts were initially subjected to constant shearing until steady state is reached, and then subjected to reshearing after a short time. It is estimated that MAH-grafted polyethylene sticks to the surface of the filler with better strength than pure hydrocarbon chains. Study of the kinetics of the development of this type of structural changes under dormant conditions and their consequent breakdown could pave the way toward determining the nature of dispersion of the filler particles during processing. Fig. 7.10 illustrates the effects of particle
particle interaction and its kinetics due to nonlinear shearing of the LDH-based nanocomposites. Oleate-modified LDH was used as a filler in poly(butylene succinate) (PBS) nanocomposites by Zhou et al. (2010) and the flow constraint of PBS melt due to the occurrence of filler particles was effectively explained as shown in Fig. 7.11. Furthermore, the occurrence of a percolation threshold at an unusually low filler loading of 3
5% w/w was described by the survival of exfoliated platelets. When Mg2Al/CO3 was used as filler, the enhancement of the rheological properties was not as great as that for the organo-modified entity (Fig. 7.12), thus emphasizing the imperative role of the modifier in causing the shear-thinning behavior. It is noteworthy that the hanging groups fastened at the surface of the LDH platelets may generate some abrasion and the lamellar plates and the chains. The PBS chains occupy the lamellar gap of Mg2Al/oleate leading to a pseudo-exfoliation state at LDH loadings less than 5% w/w which possibly explains the occurrence of percolation threshold at lower loadings. Wang et al. (2012a,b) studied the effect of LDH nanosheets, [Mg2Al(OH)6] (DDS)-2H2O (Mg
Al
DDS) prepared using a one-step reverse microemulsion method in PP matrix and found that the reduced G0 and Gv as seen in similar phenomenon reports (Costa et al., 2008; Zhu et al., 2010) were attributed to the enhanced flexibility (relaxation) of restricted polymer chains at the interface of the PP
LDH layer. Such a relaxation behavior is characteristic of nanofillers, which well illuminates the nanosize of the LDH particles in the composites (Huwe et al., 1999; Baschnagel et al., 2000). The variation of moduli with frequency is depicted in Fig. 7.13. The traditional viscoelastic behavior (Zhu et al., 2011; Ugaz et al.,

Melt rheological properties of layered double hydroxide polymer nanocomposites

291

Figure 7.10 Response to nonlinear shearing of the LDH-based nanocomposites showing the effects of particle
particle interaction and its kinetics (Costa et al., 2006a,b,c).

1997) was customary in the nanocomposites till 8.0 wt% of LDH. The viscosity curve turned linear within the whole frequency range when the filler loading increases to 16.0 wt%. It is clear that the nanocomposites have entered into a state of rheological percolation from the evolution in the complex viscosity (Shenoy, 1999) as illustrated in Fig. 7.14. PET nanocomposites containing LDH were thoroughly analyzed (Lee et al., 2006) and the rheological properties were carefully examined after organically modifying the filler and incorporating it into the PET matrix. LDH having carbonate anion was organo-modified by various anionic surfactants such as dodecylsulfate (DS), dodecylbenzene sulfonate (DBS), and octylsulfate (OS) by a rehydration process. The Cole
Cole plots were analyzed and it was found that PET nanocomposites with 2.0 wt% of each of the modified fillers exhibit similar curves with a slope of about 1.27 indicating the heterogeneity. The viscosity curves in Fig. 7.15 display the information that although the viscosity of PET with all the modified LDH fillers demonstrates a shear thinning behavior from a low-frequency range,

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Layered Double Hydroxide Polymer Nanocomposites

Figure 7.11 Frequency dependence of: (A) the storage modulus G0(x), (B) the loss modulus G0 0(x), and (C) the damping coefficient tan d of PBS, PBS:LDH/CO3 and PBS:LDH/oleate nanocomposite derivatives with different LDH/oleate loadings(expressed in % w/w) (Zhou et al., 2010).

the nanocomposites with LDH modified with DBS and OS reveal incessant curves exhibiting shear thinning due to the collapse of the structural network and slip between PET matrix and filler. It is also interesting to see that dodecylsulfatemodified LDH nanocomposites preserve the viscosity akin to a homo PET owing to the augmented interactions between filler and the PET matrix at high frequencies. LDH pillared by dodecyl benzenesulfonate (DBS) and modified with PANI/ DBSA prepared by solution adsorption method was melt-mixed with maleic anhydride modified polyethylene (PE-g-MA) and the properties of these nanocomposites were analyzed thoroughly (Kutlu et al., 2013). LDH was homogeneously dispersed throughout the matrix and accomplished a shear thinning exponent (STE) value of 20.84. An increase in filler loading also enhances the STE in a limited range, denoting the development of a denser network. Supply of an additional 5% of inorganic filler content does not smooth the progress of dispersion any further, as shown in Fig. 7.16.

Figure 7.12 Frequency dependence of complex viscosity of PBS, PBS:LDH/CO3 and PBS: LDH/oleate nanocomposite derivatives with different LDH/oleate loadings (expressed in % w/w) (Zhou et al., 2010). (A)

Storage modulus G' (Pa)

105

104 0 wt% 0.5 wt% 1 wt% 1.5 wt% 2 wt% 4 wt% 8 wt% 16 wt%

103

102

10–1 (B)

100 101 Frequency (Hz)

102

Loss modulus G'' (Pa)

105

104 0 wt% 0.5 wt% 1 wt% 1.5 wt% 2 wt% 4 wt% 8 wt% 16 wt%

103

10–1

100 101 Frequency (Hz)

102

Figure 7.13 (A) Storage modulus (G0 ) and (B) loss modulus (Gv) as a function of frequency for PP and PP/LDH nanocomposites (Wang et al., 2012a,b).

Complex viscosity (Pa.s)

0 wt% 0.5 wt% 1 wt% 1.5 wt% 2 wt% 4 wt% 8 wt% 16 wt%

104

103

102 100 101 Frequency (Hz)

10–1

102

Figure 7.14 Variation in complex viscosity as a function of frequency for PP and PP/LDH nanocomposites (Wang et al., 2012a,b). (B) 103

(A)

G' (Pa)

10

3

102 Eta' (Pa–s)

104

PET PET/MGALCO3 PET/MGALDS PET/MGALDBS PET/MGALOS

102 101

101

10

slope:2

0

100 101

102

103 G'' (Pa)

104

10–1 10–2

PET PET/MGALCO3 PET/MGALDS PET/MGALDBS PET/MGALOS

10–1

100 101 Freq. (rad/s)

102

103

Figure 7.15 (A) Cole
Cole plots, and (B) melt viscosity for PET nanocomposites with LDH content of 2.0 wt% at 270 C (Lee et al., 2006).

1,000,000 d 100,000 η (Pa-s)

c 10,000

b

1000 a 100 0.1

1 10 ω (rad/s)

100

Figure 7.16 Flow curves of PE-g-MA (A) and organomodified LDH/PE-g-MA conductive nanocomposites having 2.5% (B), 5% (C), and 10% (D) inorganic content. (The shear thinning exponent values in the low-frequency range are 20.09, 20.84, 20.91, and 20.94, respectively.) (Kutlu et al., 2013).

Melt rheological properties of layered double hydroxide polymer nanocomposites

295

In another study, poly(ethylene oxide co-propylene oxide-co-ethylene oxide) (PEO-PPO-PEO), a nonionic block copolymer, was utilized in an aqueous suspension of LDH for coagulating NBR latex. For understanding the effect of surfactant on the deagglomeration of LDH particles in aqueous media, the rheological behavior of the suspensions was analyzed and the particle size distribution, sedimentation testing, etc. were carried out. Owing to sturdier filler
rubber interaction there was a slight increase in viscosity of the latex suspensions (Fig. 7.17) (Braga et al., 2014) The role of LDH as a plasticizer was streamlined as a function of the intercalation of the nanocomposite to check how the intercalated LDH platelets behave with polarity of the polymer matrix (Nambo et al., 2008). The fact that the rheological behavior of a polymer is powerfully subjective to its molecular weight and molecular weight distribution (MWD) is made clear from the Cole
Cole illustration as

(A)

0.007 Dispersion water water/LDH water/LDHs

η (Pa.s)

0.006 0.005 0.004 0.003 0.002 0

200

(B)

400 600 • –1 γ (s )

0.010

1000

Suspension Latex Latex/LDH Latex/LDHs

0.008

η (Pa.s)

800

0.006

0.004

0.002 0

200

400 600 • γ (s–1)

800

1000

Figure 7.17 Viscosity of LDH aqueous dispersions (A) and latex/LDH suspensions (B) (Braga et al., 2014).

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Layered Double Hydroxide Polymer Nanocomposites

shown in Fig. 7.18 (Colby et al., 1987; Mead, 1994; Leroux et al., 2014). The semicircular-shaped graph which is convex-shaped and corresponding to zero at x intercept, tallies to the Newtonian zero-shear viscosity (Fox and Flory, 1951; Friedman and Porter, 1975; Nichetti and Manas-Zloczower, 1998). It is interesting to notice that the intercalated structure of the composite led to a decline of the molecular weight. Another interesting study was conducted (Fu et al., 2010) in which nanoscale LDH-nitrate particles and sheets produced a strong effect on the rheological behavior of polyacrylamide solution followed by an interesting property, like formation of a gel at low filler loading. There was also a robust relationship between frequency and viscosity in the lower-frequency regime. This behavior can be credited with the formation of a network structure of the filler, which confines the motion of the polymer chains. Due to the spatially linked network structure there will be a lack of free rotation, and hence the relaxation of the structure is dramatically prevented. LDPE/LDH composites synthesized via melt compounding with an organomodified LDH and polyethylene-grafted maleic anhydride as the compatibilizer demonstrated an important fact that the length of the surfactant alkyl chain is an

(B)

1.2 103

18

1 10 η´´ (Pa.s)

2 102

40

200

3

12

1.5 102

10

8 102

100

η´´ (Pa.s)

(A)

8 14

6 102 0 4 102

200

600

16 PP 1000

20

16 14 10 12 8

18

PBS

1 102 0 5 101

100

200

2 102 0 0 0

2

2

2

2

3

0

3

5 101

2 10 4 10 6 10 8 10 1 10 1.2 10 η´ (Pa.s)

(C) 2 104

6000

2 102

18

4000

4

1.5 10 η´´ (Pa.s)

1 102 1.5 102 η´ (Pa.s)

PDMS 8

16 10 1214

10000

20000

2000 1 104 0 5 103 PDMS

0 0

3

5 10

4

1 10 1.5 104 η´ (Pa.s)

2 104

Figure 7.18 Cole
Cole ηv
η0 (ω) for LDH/Cn polymer composites: (A) PP, (B) PBS, and (C) PDMS; the values of n are indicated (Leroux et al., 2014).

Melt rheological properties of layered double hydroxide polymer nanocomposites

297

essential component in determining the dispersion as well as the rheological properties of the nanocomposites (Muksing et al., 2011). The strong dependence of interlayer ions on the rheology of LDH
EVA nanocomposites was studied in detail and the results demonstrate the variation in the
2 22 storage and loss moduli following the order of SO22 4 . NO3 . CO3 . Cl . Furthermore, both storage and loss modulus versus frequency curves for the nanocomposites with relatively high LDH particle loadings (17 wt%) approach the plateau at low frequencies which might contribute to the formation of an interconnected structure of the fillers or a strong particle polymer interaction. It is interesting to note that the consequence of the addition of LDH on the rheological behavior is relatively weak at high frequencies, demonstrating less sensitivity of LDH to affect the short-range dynamics of the EVA chains (Gao et al., 2016). A complete study was accomplished on PP/ethylene vinyl acetate copolymer (EVA)/organo-modified LDH composites to explore the consequence of LDH loading on small-amplitude oscillatory shear (SAOS) rheological properties and to correlate the properties with microstructure. The occurrence of LDH causes an increase in the storage modulus and complex viscosity, particularly in the terminal region. Additionally, the elasticity augmentation in EVA-rich hybrids was greater than the PP-rich counterpart, which can be well explained by the greater affinity of LDH to EVA than to PP. It was also elucidated from the study that the LDH platelets in the case of PP-rich samples are localized at the interface or within the EVA dispersed particles. On the other hand, in the case of EVA-rich samples, they are primarily confined inside the matrix (Aghjeh et al., 2017)

7.3

Rheology of thermosetting polymer layered double hydroxide nanocomposites

Thermosetting polymers constitute a significant class of materials that have accomplished extensive application in many areas. The prime benefit in the use of thermosetting polymers lies in the advantage of starting with a low-viscosity liquid before curing. This permits the creation of expendable adhesives and coatings with custom-made rheological properties. The characteristic features of thermosetting polymers is determined by their curing mechanism, which integrates the chemical reactions that govern both the chemorheology preceding gelation and subsequent development of an intricate crosslinked network upon complete cure. An organoclay of composition Zn2Al/LS was employed as an additive in poly(lactic acid) (PLA), poly(butylene) succinate (PBS), and poly(butylene adipate-co-terephthalate) (PBAT) (Hennous et al., 2013). An intercalated nanocomposite structure is obtained in the case of PLA and PBS. A robust upsurge of the complex viscosity |η | is observed for both PLA and PBS nanocomposites due to a chain extender behavior of the intercalated Zn2Al/LS platelets towards polymer chains, while an exactly opposite behavior is witnessed for PBAT owing to a plasticizing effect of the organoclay filler as perceived in Figs. 7.19 and 7.20.

298

Layered Double Hydroxide Polymer Nanocomposites 40

2500 Tan δ

30

|η∗| (Pa.s)

2000

20

b

10

a

0 0.1

1500

1

10 ω (rad/s)

100

1000 a 500

b 0.1

1

10

100

ω (rad/s)

Figure 7.19 Complex viscosity |η | versus ω for (A) PBAT and (B) PBAT-Zn2Al/LS. Inset shows the variation of tan δ versus ω (Hennous et al., 2013). 220 Tan δ

a

200

b

|η∗| (Pa.s)

180 0.1

160

1

ω (rad/s)

10

100

140 120

b

100

a 0.1

10

1

100

ω (rad/s)

Figure 7.20 Complex viscosity |η | versus ω for (A) PBS and (B) PBS
Zn2Al/LS. Inset shows the variation of tan δ versus ω (Hennous et al., 2013).

Dimethicones comprising of dispersed hydrotalcite-type materials previously organo-modified with amino acids were dispersed in silicone and the resulting nanocomposites were analyzed (Naime Filho et al., 2012). An exfoliated structure as revealed from XRD is confirmed by the drastic change in the rheological behavior from a quasi-Newtonian to an extensive developed gel-like structure for the nanocomposite derivatives as depicted in Figs. 7.21 and 7.22. The percolation threshold was noticeable at a filler loading of 5 wt%, signifying the occurrence of a fundamentally advanced interface between the added filler and the matrix. The upsurge of more than one order of magnitude in viscosity was clarified by the rather strong attrition between the fastened amino acid anions and the silicone chains. The

Melt rheological properties of layered double hydroxide polymer nanocomposites

299

(A) 1,000,000

|η∗| (Pa.s)

100,000

10,000 12.5 1000

100 0.1

0 1

ω (rad/s)

10

100

(B) 1,000,000

|η∗| (Pa.s)

100,000

10,000 12.5 1000

100 0.1

1

ω (rad/s)

10

100

0

Figure 7.21 Complex viscosity versus ω for (A) Mg2Al/Gly x: PE-100 and (B) Mg2Al/Ala x: PE-100. The percentage hybrid LDH filler loading x is indicated (Naime Filho et al., 2012).

fact that the shear-thinning parameter, as well as viscosity hike, for the nanocomposite can be attributed to the sturdy interactions between the particles and the polymer chains (Sochi, 2010), where the silicate layers function as a barrier to the flow of the melt (Zhao et al., 2005). The transformation from pseudo liquid to gel-like assembly has a close relationship with the degree of exfoliation (Wagener and Reisinger, 2003). The shear-thinning exponent and the relaxation parameter as depicted in Fig. 7.22 take the shape of a sigmoid curve for evolution which is characteristic of the presence of a percolation threshold occurring between 2.5 and 5 wt.% fillers. This is associated with a drastic increase of more than one order of

300

Layered Double Hydroxide Polymer Nanocomposites

1

–1.2 –1 G′(Ala) G′′(Ala) G′(Gly) G′′(Gly) η∗(Ala) η∗(Gly)

0.6 0.4

–0.8 –0.6 –0.4

n in |η∗|→ ωn ω→0

n in G′, G′′ → ωn ω→0

0.8

0.2 –0.2 0 0

2

4 6 8 LDH filler (wt.%)

10

12

0

Figure 7.22 Variation of the relaxation parameters from the terminal zone using G0 versus ω, Gv versus ω and h versus ω curve (Naime Filho et al., 2012).

magnitude G0 and |η | versus ω values with no significant improvement being observed for higher percentage loading. Such low loading (2.5
5% w/w) can be explained by the presence of exfoliated platelets only, and the exfoliation degree is found to be correlated to the shear thinning exponent. While the pure silicone is a viscous liquid with no capacity for elastic deformation and low plastic deformation capacity, the nanocomposites are quite elastic and capable of moderate plastic deformation. Zwitter ionic imidazolium-based ionic liquid (ZIL) constituted by imidazolium cation and sulfonate anion was used to modify both cationic and anionic clay minerals, represented by montmorillonite (Na1-Mt) and calcined hydrotalcite (CHT) (Soares et al., 2017). Then, the unmodified and modified clays were mixed with epoxy prepolymer based on diglycidyl ether of bisphenol A (DGEBA). The modified clays resulted in epoxy networked materials with higher intercalation degree, and good transparency. Fig. 7.23 illustrates the dependence of complex viscosity, η , with frequency for ER dispersions prepared with 2.5 phr of different clays. The addition of the Na1-Mt resulted in an increase of the viscosity, whereas the addition of CHT did not significantly affect the viscosity of the ER prepolymer, suggesting better dispersion of the former clay. In both systems, the presence of the functionalized clay minerals gave rise to higher viscous ER dispersions as compared with those prepared with Na1-Mt and CHT. This behavior suggests a better dispersion of these modified clays inside the ER prepolymer. Probably, the sulfonate anions and the imidazolium cations in the interlayer space of ZIL-Mt and ZILCHT, respectively, contribute to a better intercalation of the epoxy prepolymer because of the affinity between these groups and the polar groups (epoxy and hydroxyl groups) present in the epoxy prepolymer, thus generating a better matrix
filler interaction and higher interfacial area. Such interactions may provoke an increase in viscosity of the dispersion.

Melt rheological properties of layered double hydroxide polymer nanocomposites

301

(A) Complex viscosity (Pa.s)

c 18 16

b

14 a 12

1

10 100 Angular frequency (1/s)

(B)

Complex viscosity (Pa.s)

18

e

16

14

d a

12

1

10 100 Angular frequency (1/s)

Figure 7.23 Dependence of complex viscosity η of the epoxy (DGEBA), with frequency for the clay/ER dispersions containing 2.5 phr of clay minerals: (a) ER prepolymer, (b) Na1Mt and (c) ZIL-Mt; (B) (a) ER prepolymer, (d) CHT and (e) ZIL-CHT (Soares et al., 2017).

7.4

Modeling of rheological properties

The melt-state viscoelastic properties, for instance, shear viscosity and shear modulus, are recognized to amplify in the filled polymer melts up to two orders of magnitude compared to their values for the pure polymer matrix. The agglomerated filler particles can be shattered upon the application of a momentary shear flow. In the dormant state, when the flow is switched off reaggregation of particles occur again (Osman and Atallah, 2006). Such thixotropic effects occur due to the deprivation of the filler superstructure, such as a decrease or disappearance of the shear overshoot or decline of complex shear viscosity in the succeeding shear cycles. The layered crystalline materials like LDH, when exfoliated, organize into highly anisometric particles that can be oriented upon application of shear flow (Lin-Gibson et al.,

302

Layered Double Hydroxide Polymer Nanocomposites

2004). It has been reported that there is noteworthy arrangement of the silicate layers in block copolymer nanocomposites when shear rates are above 10 s21 (Ren and Krishnamoorti, 2003). Monte Carlo simulations have revealed that incorporation of solid spherical nanoparticles boosts the small-strain module in proportion to the particle loading and the peak enhancement was estimated to be three times compared to the unfilled melt (Sharaf and Mark, 2004) and, accordingly, the improvement is found to be much less than that calculated for the melt-state nanocomposites and the theory does not consider the agglomeration of filler particles. Attractive interactions between the surface of the nanoparticle and the polymer segments were taken into account in molecular dynamic simulations; nevertheless, the viscosity and dynamic shear modulus have been calculated to amplify less than one order of magnitude, even at high loadings, in comparison with the pure melt (Smith et al., 2002). It is obvious that contemporary computer simulations cannot deal with such an exceedingly sluggish process like the evolution of the microscopic filler structure under the application of external mechanical forces. Hence the need of the hour is to develop appropriate rheological models for these complex systems. Conventional rheological models were satisfactory only in the case of pure polymer melts and hence take for granted only slight modification of the polymer structure even in the nonlinear region. Of late, a structure network model based on a modified upper convected Jeffreys model has been used to portray the shear reversal trial in PP
organoclay nanocomposites (Lertwimolnun et al., 2007). This model makes use of a supposed structure parameter whose evolution is specified by the kinetic equation that correlates the structure breakdown with the energy dissipation due to the flow process (Yziquel et al., 1999). In another interesting work in which lowdensity polyethylene (PE) melt is filled with Mn
Al-based LDH, an approach based on the well-known Wagner model has been employed to address a thixotropic response in the nonlinear shear regime is probed (Costa et al., 2005, 2006a,b,c). For the prediction of the storage modulus, two equations were formulated by Einstein (Einstein, 1956): Gc 5 Gm 1 1 1:25Vf G c 5 G m 1 1 Vf







(7.1) (7.2)

Guth modified the Einstein equations as (Guth, 1945):   Gc 5 Gm 1 1 1:25Vf 1 14:1 Vf2

(7.3)

By the rule of mixtures it can be shown that (Nielsen and Landel, 1993): tan δc 5 Vf tan δf 1 Vm tan δm

(7.4)

For rigid fillers, the former term in the above equation can be ignored: tan δc 5 Vm tan δm

(7.5)

Melt rheological properties of layered double hydroxide polymer nanocomposites

303

It is presumed that the matrix along with the fillers contributes a stiffness equivalent to the minimum elastic modulus of the composite; and hence the equation becomes (Tung and Dynes, 1987): tan δc 5 Vf ðGm =Gc Þtan δm

(7.6)

where G is the storage modulus of the material. The graphs of experimental and theoretical storage moduli values at 200 C for PMMA and its nanocomposites (Kumar et al., 2016) are shown in Fig. 7.24. (A)

4000

Storage modulus (MPa)

3500

3000

2500

Experimental value Einstein Eq. 1 Einstein Eq. 2 Guth Eq. 3

2000

1500 Pure PMMA (B)

4.5 4.0

PMMA-5

PMMA-5-G

Experimental value Theoretical Eq.5 Theoretical Eq.6

Loss factor

3.5 3.0 2.5 2.0 1.5 1.0 Pure PMMA

PMMA-5

PMMA-5-G

Figure 7.24 Plots of experimental and theoretical storage modulus (A) and loss factor (B) of pure PMMA, PMMA-5, and PMMA-5-G nanocomposites at 200 C (Kumar et al., 2016).

304

Layered Double Hydroxide Polymer Nanocomposites

Normally, polymer-based solutions are inherently non-Newtonian in nature, that is, they exhibit shear thinning as the viscosity is not linearly related with shear rate. It is familiar that at elevated shear rates, the viscosity decreases due to the alignment of polymer molecules which in turn permits effortlessness of flow of molecules. Diverse mathematical correlations are used to explain the shear stress versus the shear rate of non-Newtonian fluids and they pave the way toward the identification of the flow characteristics and proficiency of a fluid to execute specific functions. Power law and Bingham plastic models are the two parameter models in this regard. The power law model assumes the form: τ 5 Kγ n

(7.7)

η 5 Kγ n21

(7.8)

where n 5 1 for a Newtonian fluid; n , 1 for shear thinning; and when n . 1 it is a shear thickening fluid. The Bingham plastic model is stated as: τ 5 τ 0 1 γU μp

(7.9)

where μp is plastic viscosity, and τ 0 is yield stress. There are numerous models that include three or more parameters. The third parameter helps to distinguish the fluid flow in the lower and upper Newtonian regions as well as in the power law region. The plastic viscosity term is replaced in the Herschel
Bulkley model by adding the power law expression. τ 5 τ 0 1 K γn

(7.10)

The Sisko model (Sisko, 1958) is used to articulate the flow behavior in the upper Newtonian region and the power law region. It is a three-parameter model, which relates the apparent viscosity with shear rate. η 5 ηN 1 Kγ n21

(7.11)

The Casson model (Casson, 1965) for non-Newtonian behavior is stated as: τ 5 τ o1=2 1 ηαγ1=2

(7.12)

where ηα is viscosity at infinite shear stress. The models used to express polymer solutions, food, activated sludge, and blood flow characteristics are the Cross and Carreau models. Each model consists of four parameters. The Cross model is given as (Cross, 1965):   η 5 ηN 1 ðηo 2 ηNÞ=ð1 1 ðC  γ Þ mÞ

(7.13)

Melt rheological properties of layered double hydroxide polymer nanocomposites

305

where, m is dimensionless exponent and C is time constant. ηo and ηN denote the initial viscosity at zero shear rate and final viscosity at infinite shear rate, respectively. Whereas, the Carreau model is stated as (Carreau et al., 1968):   η 5 ηN 1 ðηo 2 ηNÞ  ð1 1 ðλ  γ Þ 2Þ ðn 2 1Þ=2

(7.14)

where λ and n correspond to relaxation time and gauge of shear thinning properties, respectively.

7.5

Conclusions and future scope

The most imperative parameters governing the rheological properties were found to be the interactions between polymer and particles, which are related to the alteration of chain dynamics at the surface of the particle, and interactions between particles, which are related to the tendency of the particles to interconnect and create local fractals leading to a continuous network. Alternative parameters such as state of dispersion, particle size, and morphology were found to be significant as well, but their contribution can be explored by considering their effect on the aforementioned interactions. Though stupendous work has been carried out in the area of LDH-filled polymer composites, still a systematic study of the rheology of the different systems is lacking. Thermoplastic polymers are mostly the chosen candidates for the study of rheology and hence there is an on-going prospect for research to be unveiled in the rheological analysis of thermosetting polymers. Hence it is worthwhile considering the gaps in the literature for future research. Organically modified LDH is found to exhibit excellent intercalation/exfoliation in polymers and hence pave the way for the exhibition of outstanding properties. Hence there is still a lot of work to be carried out toward the exploration of various types of organic modification in LDH, which leads to novel network formations in both thermoplastic as well as thermosetting polymeric nanocomposites.

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952. Wang, Q., Tay, H.H., Ng, D.J.W., Chen, L., Liu, Y., Chang, J., et al., 2010. The effect of trivalent cations on the performance of Mg-M-CO3 layered double hydroxides for hightemperature CO2 capture. ChemSusChem 3, 965
973. Wang, Q., Zhang, X., Wang, C.J., Zhu, J., Guob, Z., O’Hare, D., 2012a. Polypropylene/layered double hydroxide nanocomposites. J. Mater. Chem. 22, 19113
19121. Wang, Q., Tay, H.H., Guo, Z., Chen, L., Liu, Y., Chang, J., et al., 2012b. Morphology and composition controllable synthesis of Mg-Al-CO3 hydrotalcites by tuning the synthesis pH and the CO2 capture capacity. Appl. Clay Sci. 55, 18
26. Wang, Q., Gao, Y., Luo, J., Zhong, Z., Borgna, A., Guo, Z., et al., 2013a. Synthesis of nanosized spherical Mg3Al
CO3 layered double hydroxide as a high-temperature CO2 adsorbent. RSC Adv. 3, 3414
3420. Wang, Q., Gao, Y., Zhang, Z., Duan, L., Umar, A., O’Hare, D., 2013b. Synthesis and characterization of high surface area flower-like Ca-containing layered double hydroxides Mg 3
x Ca x Al1
NO3. Sci. Adv. Mater. 5, 411
420. Wang, Q., Yu, J., Liu, J., Guo, Z., Umar, A., Sun, L., 2013c. Na1 and K1-exchanged zirconium phosphate (ZrP) as high-temperature CO2 adsorbents. Sci. Adv. Mater. 5, 469
474. Winter, H., Mours, M., 1997. Rheology of polymers near liquid-solid transitions neutron spin echo spectroscopy viscoelasticity rheology. Adv. Polym. Sci. 134, 165
234. Yziquel, F., Carreau, P.J., Moan, M., Tanguy, P.A., 1999. Rheological modeling of concentrated colloidal suspensions. J. Non-Newtonian Fluid Mech. 86, 133
155. Zhao, J., Morgan, A.B., Harris, J.D., 2005. Rheological characterization of poly styreneclay nanocomposites to compare the degree of exfoliation and dispersion. Polymer. (Guildf). 46, 8641
8660. Zhao, Y., Wei, M., Lu, J., Wang, Z., Duan, X., 2009. Biotemplated hierarchical nanostructure of layered double hydroxides with improved photocatalysis performance. ACS Nano. 3, 4009
4016. Zhou, Q., Verney, V., Commereuc, S., Chin, I.-J., 2010. Fabrice Leroux, Strong interfacial attrition developed by oleate/layered double hydroxide nanoplatelets dispersed into poly (butylene succinate). J. Colloid. Interface. Sci. 349, 127
133. Zhu, J., Wei, S., Ryu, J., Budhathoki, M., Liang, G., Guo, Z., 2010. In situ stabilized carbon nanofiber (CNF) reinforced epoxy nanocomposites. J. Mater. Chem. 20, 4937
4948. Zhu, J., Wei, S., Li, Y., Sun, L., Haldolaarachchige, N., Young, D.P., et al., 2011. Surfactantfree synthesized magnetic polypropylene nanocomposites: rheological, electrical, magnetic, and thermal properties. Macromolecules 44, 4382
4391.

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Further reading Costa, F.R., Wagenknecht, U., Heinrich, G., 2007. LDPE/MgeAl layered double hydroxideS nanocomposite: thermal and flammability properties. Polym. Degrad. Stab. 92, 1813
1823. Cui, Z., Chin, 2010. Synergistic effects of layered double hydroxide with phosphorusnitrogen intumescent flame retardant in PP/EPDM/IFR/LDH nanocomposites. J. Polym. Sci. 28, 563
569. Drzal, L.T., Rich, M.J., Lioyd, P.F., 1983. Adhesion of graphite fibers to epoxy matrices: I The role of fiber surface treatment. Adhesion 16, 133
140. Feng, Y., Li, D., Wang, Y., Evans, D.G., Duan, X., 2006. Synthesis and characterization of a UV absorbent-intercalated Zn-Al layered double hydroxide. Polym. Degrad. Stab. 91, 789
794. Illaik, A., Taviot-Gue´ho, C., Lavis, J., Commereuc, S., Verney, V., Leroux, F., 2008. Unusual Polystyrene nanocomposite structure using emulsifier-modified layered double hydroxide as nanofiller. Chem. Mater. 20, 4854
4860. Katiyar, V., Gerds, N., Koch, C.B., Risbo, J., Hansen, H.C.B., Plackett, D., 2011. Melt processing of poly(L-lactic acid) in the presence of organomodified anionic or cationic clays. J. Appl. Polym. Sci. 122, 112
125. Krishnamoorti, R., Ren, J., Silva, A.S., 2001. Shear response of layered silicate nanocomposites. J. Chem. Phys. 114, 4968
4973. Li, D., Tuo, Z., Evans, D.G., Duan, X., 2006. Preparation of 5-benzotriazolyl-4-hydroxy-3sec-butylbenzene sulfonate anion intercalated layered double hydroxide and its photostabilizing effect on polypropylene. J. Solid State Chem. 179, 3114
3120. Lin, Y., Li, D., Evans, D.G., Duan, X., 2005. Modulating effect of Mg
Al
CO3 layered double hydroxides on the thermal stability of PVC resin. Polym. Degrad. Stab. 88, 286
293. Ren, J., Silva, A.S., Krishnamoorti, R., 2000. Linear viscoelasticity of disordered polystyrene 2 polyisoprene block copolymer based layered-silicate nanocomposites. Macromolecules 33, 3739
3746. Xu, Z., Saha, S.K., Braterman, P.S., D’Souza, N., 2006. The effect of Zn, Al layered double hydroxide on thermal decomposition of poly(vinyl chloride). Polym. Degrad. Stab. 91, 3237
3244. Zhao, X., Zhang, F., Xu, S., Evans, D.G., Duan, X., 2010. From layered double hydroxides to ZnO-based mixed metal oxides by thermal decomposition: transformation mechanism and UV-blocking properties of the product. Chem. Mater. 22, 3933
3942.

Thermal properties and flameretardant characteristics of layered double hydroxide polymer nanocomposites

8

Yanshan Gao1, Lei Qiu1, Dermot O’Hare2 and Qiang Wang1 1 College of Environmental Science and Engineering, Beijing Forestry University, Beijing, P.R. China, 2University of Oxford, Oxford, United Kingdom

8.1

Introduction

The PlasticsaEurope statistics suggest that the consumption of polymer-based materials has been increasing rapidly in recent years (Fig. 8.1). Production increased from 1.5 million tons in 1950 to 311 million tons in 2014. This growth is around 9% a year on average. Polymer-based materials are now recognized as key components in many important industries such as construction, automotive, electronics, and aerospace, due to their remarkable combination of properties, low weights, cost-effectiveness, and ease of processing. Most polymers, however, suffer from thermal degradation and are highly flammable, which increases their fire hazards when used in practical applications, significantly reducing service life and severely restricting their uses in many areas (Morgan and Wilkie, 2007; Gao et al., 2014a,b; Hilado, 1998; Feng et al., 2012). Consequently, improving polymer flame retardancy is a major challenge for extending polymer use to most applications. In order to improve the thermal stability properties and flame-retardant performance of polymer resins, effective nano-sized flame-retardant fillers have been added to polymer matrices. Halogenated flame retardants have been in use since the 1930s. They are the most widely produced and used flame retardants due to the advantages of low cost, ease of processing, high flame retardancy, and miscibility (Morgan and Gilman, 2013). However, it has been found that some halogen-containing flame retardants are toxic and will produce smoke and brominated dioxins when burning, which is a great threat to both the environment and people. As the awareness of health care and environmental protection increase, the use of halogen-based additives is diminishing in Europe and the United States. In 2004, two formulations of halogen flame retardants were banned in Europe and North America, while a third was banned in 2008 (Liu and Zhu, 2014). Organic phosphorus compounds can be vapor phase or

Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00008-2 © 2020 Elsevier Ltd. All rights reserved.

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Figure 8.1 Polymer production in million tons from 2004 to 2014.

condensed phase flame retardants, meaning that they can be useful in low loading levels when combined with polymers that inherently char on their own. But the phosphates in polymers can cause plasticization, which leads to a decrease in the mechanical properties of the polymers. Also, they can generate more smoke and CO during fire conditions because they help inhibit polymer combustion (Morgan and Gilman, 2013). Intumescent flame retardants provide excellent fire protection but tend to be limited to lower-temperature materials and fire protection barriers due to their water absorption issues. Metal hydroxides such as Al(OH)3 and Mg(OH)2 are the largest commercially manufactured flame retardants and are perceived to be very environmentally friendly. They generate greatly lower smoke and reduce overall toxic gas emissions when burning. Further, these fillers are fairly inexpensive and can be easily coated with surfactants to make their use in polymer easier. The main disadvantage of Al(OH)3 and Mg(OH)2 is their high loading (50
70 wt%) and inherent poor compatibility with hydrocarbon-based polymers. In addition, the mineral fillers can delay ignition and slow initial flame growth, but cannot stop it completely if enough constant external heat is applied (Morgan and Gilman, 2013). Layered double hydroxides (LDHs) are a class of lamellar compounds made up of positively charged mixed metal hydroxide layers with an interlayer region containing charge-compensating anions and water molecules. They can be described by the general chemical formula [Mz11
xM31x(OH)2]q1(Xn
)q/n  yH2O, where Mz1 represents divalent cations such as Mg21, Zn21, Ca21, etc., while M31 is trivalent cations such as Fe31 or Al31, and Xn
is a charge-balancing interlayer anion. LDHs are emerging as a new generation of thermal stabilizer and flame-retardant materials due to their unique chemical composition and layered structure. They are potentially eco-friendly flame retardants for polymer applications. In addition, by properly intercalating certain anions, such as borate into LDHs, LDHs might combine the advantages of both magnesium hydroxide (MH), aluminum hydroxide (AH), and zinc borate (xZnO  yB2O3  zH2O; known in the trade as Firebrake) (Gao et al., 2014a,b). In this chapter, we discuss the techniques for evaluating the thermal stability properties and flame-retardant performances and summarize LDH-based thermal

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stabilizer materials and fire-retardant materials, their applications, and the corresponding mechanisms.

8.2

The techniques for determining thermal stability properties and flame retardancy performance

In order to measure the thermal stability and flame retardancy of polymer composites, different standards and indices need to be considered. To date, thermogravimetric analysis (TGA) is usually used to measure the thermal stability of polymers and four approaches are commonly used to evaluate the fire properties of polymer/LDH nanocomposites, they are: microscale combustion calorimeter (MCC), limiting oxygen index (LOI), cone calorimeter (CONE), and Underwriters Laboratories (UL-94).

8.2.1 The techniques for determining thermal stability properties of polymers TGA is a kind of thermal analysis technology (Doyle, 1961; Wu et al., 2002), it is used to test the thermal stability and composition of polymer materials (Jain et al., 2016). It can offer the relationship between the sample weight and the heating temperature under the control of predefined program. Meanwhile it can be used with other analytical methods in the actual analysis of materials for carrying out comprehensive thermal analysis results (Qiu et al., 2015; Byrn et al., 1995; Zhao et al., 1997; Jeske et al., 2012). Briefly, TGA analysis was carried out with an established heating rate or a corresponding air (or CO2, N2, etc.) flow rate. When the substance which is being measured begins to sublimate, vaporize, decompose gas, or loss the crystal water in the heating process, the weight of measured materials will change. Then the thermogravimetric curve of the sample will not remain straight and instead drop little by little. By analyzing the curves of a material, we can discover the temperatures at which the sample starts to change, including the lost weight and its corresponding percentage. For example, in the TGA curves of ethylene-vinyl acetate copolymer (EVA)/LDH composites, all samples mainly underwent three stages of decomposition (Fig. 8.2) (Wang et al., 2011a,b,c,d). The first stage corresponds to the loss of physically absorbed water and interlayer water in lower temperatures (below 225 C) with a decomposition maximum at about 180 C in the DTG curve. The second step at higher temperatures (in the range of 225
500 C) is associated with the dehydroxylation of the metal hydroxide layers and the degradation of interlayer carbonates (Benito et al., 2010). Finally, the third step occurred at over 500 C, and is attributed to the removal of residual carbonate anions in TGA. Through TGA analysis, changes to crystal properties can be studied, such as the physical phenomena of melting, evaporation, sublimation, adsorption, and other experimental samples. At the same time, it is helpful to study the chemical phenomena of the materials including dehydration, dissociation, oxidation, reduction, and so on.

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100

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501 493

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Figure 8.2 TGA and DTG curves of pure EVA and its composites with 20 wt% LDHs.

8.2.2 The techniques for determining the flame-retardant performance of polymers MCC, LOI, CONE, and UL-94 are usually used to detect the flame-retardant properties of polymer composites. MCC is also known as pyrolysis combustion flow calorimeter (PCFC), which is a convenient and fast method for laboratory evaluation of flame-retardant properties. It is based on a TGA-like degradation of the polymer in nitrogen, followed by combustion of the gases produced in air (Gao et al., 2014a,b). MCC can quickly and easily measure the key fire parameters of plastics, wood and textiles, and composites. For the MCC test, just a few milligrams (c. 5 mg) of sample is heated to the setting temperature with a heating rate which

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was set up before, a wealth of information on material combustibility and fire hazard can be obtained in minutes, including the heat release rate (HRR), heat release capacity (HRC), total heat release (THR), and temperature of degradation. The LOI of a material is defined as the minimum oxygen concentration (expressed in volume percentage) required for the flame burning of the material to be carried out in a mixture gas system of oxygen and nitrogen. It is measured by passing a mixture of oxygen and nitrogen over a burning sample, and reducing the oxygen level until a critical level is reached. It is widely used to evaluate the flameretardant properties of materials (Weil et al., 1992). LOI values for different plastics are determined by standardized tests, such as the ISO 4589 and ASTM D2863. When doing the LOI test, the samples should be pressed into 120 3 60 3 3 mm sheets according to ASTM D2863. The values obtained by the test should be the averages of several tests for each sample. For LOI results, the higher the value, the more difficult it is for combustion to occur. Generally, we consider that the materials can burn easily when the LOI values are ,22, flammable when the value is between 22 and 27, and it is incombustible when the value is .27. LOI is a method to evaluate the relative combustibility of polymer materials, but it cannot give any useful information about the burning behavior (Alongi et al., 2011). Compared to MCC and LOI tests, CONE can provide useful information about the combustion of polymers and it is the most effective method for the laboratory evaluation of the flame-retardant properties of polymers. Approximately 30 g of composite samples was compression molded into 10 3 10 cm square plaques of uniform thickness (B3 mm) before the tests were performed. A cone-shaped heater with incident flux of 35 kW/m2 was used, and the spark was continuous until the sample ignited. The results obtained from CONE can be used to evaluate material-specific properties, setting it apart from many of the established fire tests which are designed to monitor the fire response of a certain specimen (Schartel and Hull, 2007). The CONE test can gather data regarding the ignition time (tig), average mass loss rate (AMLR), combustion products, HRR, THR, and other parameters associated with burning properties. HRR is generally considered to be the most important parameter for evaluating the flame-retardant performance of polymer composites. For example, pure acrylonitrile-butadiene-styrene (ABS) was observed to burn out within 830 s after ignition, and a very sharp HRR peak appears at the range of 150
700 s, with a peak heat release rate (PHRR) value of 489 kW/m2. While the ABS/MgAl LDH and ABS/ZnMgAl LDH composites showed a PHRR value of 196 and 214 kW/m2 with 60% LDH loading, respectively. The addition of LDHs prolonged the combustion times (Fig. 8.3) (Xu et al., 2012). UL-94 is a plastics flammability standard released by Underwriters Laboratories of the United States. The standard determines the material’s tendency to either extinguish or spread the flame once the sample has been ignited. UL-94 test is carried out with two standards: the vertical burn test (UL-94V) and the horizontal burn test (UL-94 HB) (Gao et al., 2014a,b). UL-94V provides useful information aimed at the dripping behavior of polymer composites. The dripping of the burning melt determines the spread of flame through secondary flaming during real situations

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Layered Double Hydroxide Polymer Nanocomposites

Figure 8.3 Dynamic curves of HRR versus time for the pristine ABS and two LDH/ABS composites. Surface burn

Vertical burn

Horizontal burn 45º

Doesn’t ignite Under hotter flame UL 94 5VA UL 94 5VB

Self extinguishing UL 94 V-0 (Best) UL 94 V-1 (Good) UL 94 V-2 (Drips)

Slow burn rating Takes more than 3 min to burn 4 inches

Figure 8.4 UL-94 flammability ratings summary.

(Costa et al., 2007). This standard involves five parts including HB, V-2, V-1, V-0, and 5V, as shown in Fig. 8.4. The vertical burning test is measured on sheets 127 3 12.7 3 3.3 mm3 according to the standard UL-94 test ASTM D635. A good flame-retardant material should reach the UL-94 V-0 rating and have no dripping during the test. This is primarily an evaluation that is used to qualify a product. But the UL-94 test is very dependent on operators and which version of the standard is used, so different labs may obtain different results. Jiang et al. (2016) studied PP nanocomposites consisting of Zn2Al-DBS LDHs in combination with zirconium 2-(2-(2-aminoethylamino)ethylamino)ethylphosphonate (ZrP) compounds. Fig. 8.5 shows that the flame for pure PP was very vigorous and spread rapidly. With an increasing load of LDHs in combination with ZrP, the combustion speed of the PP composites slowed significantly, at the same time, in the burning process, the flame of the PP composites was weaker than pure PP. When the loading of LDHs and ZrP was 5 and 15 wt%, respectively, the PP composites can reach UL-94 V-0 rating. UL-94 is the most widely used standard for the flammability of plastic materials which is used to evaluate the ability of materials to be extinguished after being

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Figure 8.5 Images of vertical flame test for: (A) PP, (B) PP
LDH20, (C) PP
ZrP20, (D) PP
LDH10
ZrP10, (E) PP
LDH6.7
ZrP13.3, and (F) PP
LDH5
ZrP15 composites at different time points.

ignited. But the UL-94 test is very dependent on operators and which version of the standard is used, so different labs may obtain different results. All of the above measures can be used for determining the flame retardancy performance of polymers. Different methods provide different information regarding the burning behavior. A variety of studies have been conducted to show correlations between each of the flammability tests. Weil et al. (1992) reported that the LOI value might be leveled with UL-94 or CONE data to some degree in certain conditions, but it was hard to show close relations between them. Also, it does not mean that higher LOI gives better UL-94V ratings.

8.3

LDH-based thermal stabilizer materials and their applications

8.3.1 Thermal stabilizer introduction The thermal stabilizer is one of the important additives to materials, especially polymers which are sensitive to high temperature. Generally, ideal thermal stabilizers should have properties such as high thermostability, good compatibility with polymer materials, as well as low volatility and proper lubrication. In recent years LDHs have attracted considerable interest from both industry and academia due to their good thermostability (Lin et al., 2005; Xu et al., 2006).

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Layered Double Hydroxide Polymer Nanocomposites

It was found that this kind of novel heat stabilizer could bring out great environmental protection and economic benefits. For instance, when combining LDHs with organotin, a synergistic stabilization effect was obtained for rigid poly(vinyl chloride) (PVC) and the cost was reduced (Hua et al., 2001). More and more researches on LDHs as thermal stabilizers for polymer materials have been carried out aiming to make sure that the polymers can be used for various aspects.

8.3.2 Thermal stability properties of LDH-based nanocomposites 8.3.2.1 Effect of inorganic LDHs Due to the unique structure of LDHs, all the members can act as a thermal stabilizer. But different interlayer anions have different results. LDHs composed of Mg, Zn, and Al are preferred as inorganic fillers within the polymer matrix since the usage of these metals preserves the original color of polymers (Yang et al., 2015). Qiu et al. (2015) carried out a series of experiments on polypropylene (PP)/ Mg3Al-CO3 LDH nanocomposites systematically. The morphology-dependent performance of Mg3Al-CO3 LDHs (plate-like, spherical, and flower-like) as nanofillers within PP matrix has been studied. The results showed that the thermal stability of PP/LDH nanocomposites was significantly improved after incorporating Mg3AlCO3 LDHs with different kinds of morphologies. Specifically, the temperature at 50% weight loss (T0.5) of the PP/plate-like LDH nanocomposites (with a LDHs loading of 13.0 wt%) was increased by 61 C compared to that of pure PP. The results also obtained that the influence of LDH morphologies on thermal stability follows the order of plate-like . spherical . flower-like (Fig. 8.6). Gao et al. (2014a,b, 2016a,b) investigated the thermal stability properties of Zn2Al LDHs with different inorganic anions on high-density polyethylene (HDPE) and EVA. The results obviously suggest that different interlayer anions intercalated LDHs could lead to different performances on the same polymer. But the inorganic anions showed the same influence order for HDPE and EVA resin, which is: Zn2Al-Cl LDHs . Zn2Al-CO3 LDHs . Zn2Al-NO3 LDHs . Zn2Al-SO4 LDHs. Lin et al. (2006) investigated the thermal stabilization of PVC with Mg2Al-CO3 and Mg3Al2Zn-CO3 LDHs. The results showed that both LDHs improved the thermal stability of PVC resin, but compared with Mg2Al-CO3 LDHs, Mg3Zn2Al-CO3 LDHs enhanced the thermal stability of PVC in terms of both long-term stability and early coloring. In addition, Wang et al. (2017) added Ni cation into Mg3Al LDHs and studied the performance of Ni0.2Mg2.8Al-CO3 LDHs on EVA. The decomposition temperature of PP with 5 and 10 wt% Ni0.2Mg2.8Al LDHs composites is 34 C higher than pure PP. Moreover, inorganic LDHs had a good effect on enhancing the thermal stability by influencing the reaction process of different kinds of polymers including PVC (Zhao et al., 2008; Zhang et al., 2007), PP(Cui, 2010; Nyambo et al., 2008a,b), etc., either by changing the valence state of the metal cations, interlayer anions, or the species of the metal cations, and even the ratios between the different layers of the metals.

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(A) 100

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Figure 8.6 TGA analysis of: (A) PP/spherical LDH, (B) PP/plate-like LDH, (C) PP/flowerlike LDH nanocomposites, and (D) graph of T0.5 versus LDH loading.

8.3.2.2 Effect of organic LDHs The above researches have demonstrated that LDHs with inorganic interlayer anions can improve the thermal stability of polymers effectively. However, pristine LDHs with hydrophilic surface properties are not compatible with hydrophobic polymers such as PP or polystyrene (PS), which will affect the dispersity of LDHs in the polymer matrix (Yang et al., 2015). In view of this problem, organic anionmodified LDHs were reported to improve the dispersion of LDHs in the polymer matrix (Wang et al., 2012a,b,c,d; Manzi-Nshuti et al., 2009,b,c). Yang et al. (2015) modified LDHs with various anionic surfactants, such as lauric acid (LA), palmitic acid (PA), stearate (SA), lauryl phosphate (LP), or dodecyl sulfate (DS), then systematically discussed the thermal stability of PP nanocomposites containing the appropriate hydrophobically modified LDHs. The T0.5 of the PP/organo-LDH nanocomposites was significantly improved by 37
61 C, respectively, depending on the type and loading content of organo-LDHs compared to that of pure PP (Fig. 8.7). The surfactant-dependent (DS and stearic) performance of Mg3Al LDHs as nanofillers for PP matrix was evaluated by Qiu et al. (2018). The results showed that the thermal stability of the PP/LDH nanocomposites was greatly improved in terms of the value of T0.5, especially for the PP/stearic-LDH nanocomposites with a LDH loading of 20 wt%, the T0.5 was increased by 80 C compared to that of pure PP (Fig. 8.8). Wang et al. (2013a,b) investigated the thermal stability of PP/

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Figure 8.7 TG curves of PP and PP/organic LDH nanocomposites depending on: (A) the kind of organic LDHs and (B) the loading content of lauric acid (LA) LDHs.

tartrazine intercalated LDH nanocomposites, and the thermal stability of Mg3Altartrazine LDHloaded PP nanocomposites was significantly enhanced compared to pure PP. With only 0.4
0.8 wt% of LDHs, the T0.1 and T0.5 were increased by 26.2 C and 41.3 C, respectively. In addition, Zhang et al. (2014) synthesized 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid (BP) intercalated Mg2Al LDHs (Mg2Al-BP LDHs) and investigated its thermal stabilization for PVC. Congo Red tests showed that the addition of the Mg2Al-BP LDHs can improve the static thermal stability time of PVC (Fig. 8.9). The dynamic thermal stability behavior of PVC was also enhanced after the addition of Mg2Al-BP LDHs. They also found that the stabilization mechanism may be attributed to the ability of Mg2Al-BP LDHs not only to scatter the incident light but also to absorb the released HCl, which improved the resistance of PVC to both accelerated weathering and thermal degradation. Besides, many studies have proved that other organic LDHs also can improve the thermostability of nanocomposites. For example, Nyambo et al. (2009a,b) prepared poly(methyl methacrylate) (PMMA)/Mg2Al-palmitate (C16) nanocomposites and the T0.1 was increased for all nanocomposites by 15 C. The T0.5 was increased compared to the pure PMMA by 27
35 C.

8.3.2.3 Effect of LDHs with other synergistic thermal stabilizers Some reports have conducted a series of researches on the synergistic effects of LDHs with other additives such as ammonium polyphosphate (APP), MH, and carbon-based materials, etc., as well. LDHs were used as synergistic agents of APP in poly(vinyl alcohol) (PVA) matrix by Zhao et al. (2008). The results showed that

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Figure 8.8 TGA analysis of: (A) PP/DS
LDH, (B) PP/stearic
LDH nanocomposites, and (C) graph of T0.5 versus LDHs loading.

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Figure 8.9 Thermal stability time of PVC, PVC/Mg2Al-CO3 LDH, PVC/BP, and PVC/ Mg2Al-BP LDH composites measured by Congo Red test.

LDHs exhibit an obvious synergistic effect with APP. The Tinitalis (the initial decomposition temperature, temperature at 5% weight loss) further increased compared to LDHs alone, which may be due to the physical crosslinking effect among layered particles, APP molecules, and polymer chains. Meanwhile, all PVA/APP/LDH composites showed higher char residues than that of PVA/APP at 500 C, 600 C, or 700 C. Besides, Zhang et al. studied the synergistic effects of LDHs with hyperfine magnesium hydroxide (HFMH) in EVA (Zhang et al., 2007). TGA data demonstrated that the addition of LDHs can raise 5
18 C of EVA/HFMH/LDH nanocomposite samples with 5
15 phr (parts per hundred resins) LDHs compared with that of the EVA/HFMH sample when 50% weight loss is selected as a point of comparison. Gao et al. (2016a,b) found that for PP/Mg3Al LDH-oxidized carbon nanotube (OCNT) nanocomposites with 10 wt% LDHs and 0.5, 1, and 2 wt% OCNTs, T0.5 was increased by 41 C, 41 C, and 43 C, respectively. These increases are much higher than observed with PP/LDH nanocomposites without OCNTs. DTG analyses also clearly showed that compared to pure PP (340 C), the temperature of the maxima rose after adding LDHs, OCNTs, or a combination of LDHs and OCNTs. The temperatures of the maxima in DTG lie in the range of 350a390 C, suggesting that LDHs, together with OCNTs, act as a good thermal stabilizer for PP. Many research articles report the addition of LDHs together with other thermal stabilizers does produce a synergistic improvement in thermal stability of the polymeric host material. The physical explanation of this effect is not clear, perhaps the physicochemical properties of host material is changed by the formation of some intermediate products with high thermal stability. To determine the origin of these effects requires further analysis and research.

8.3.3 The mechanism of thermostability using LDHs Since LDHs could improve the thermostability of polymer materials, many experts have done a lot of work to explain the mechanism. During the heating process,

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LDHs can release H2O and CO2 effectively, which can delay the scission of polymer chains, making the polymer composites more stable. However, if the loading of LDHs is too high, the T0.5 value will decrease when compared with low LDH loading. The reason is that high LDH loading results in a lot of metal and metal oxide, which in turn accelerates the catalytic degradation of polymer on heating (Jiang et al., 2014). In addition, when LDHs were modified with organic species, the thermal stability and the overall thermal decomposition behavior changed (Costa et al., 2009). The mechanism of chloride-containing polymers, such as PVC, is a little different from polymers without chloride such as PP. The HCl releases from PVC matrix can be absorbed by LDHs, preventing the further self-catalytic reaction of PVC. The addition of LDHs led to an enhanced and excellent thermostability of polymer materials, not only due to their barrier functions, but also concern with the changes of activation energy of thermal degradation, which play an important role in hindering the movement of small molecules during the polymer degradation process.

8.4

LDH-based flame-retardant materials and their applications

8.4.1 Flame retardant introduction As the American Chemistry Council has described, flame retardants are a key component in reducing the devastating impacts of fires on people, property, and the environment. They are added to different materials (e.g., textiles, plastics) to prevent fires from starting, limit the spread of fire, and minimize fire damage. Some flame retardants work effectively on their own, others act as “synergists” to increase the fire protective benefits of other flame retardants. A variety of flame retardants is necessary because materials that need to be made fireresistant are very different in their physical nature and chemical composition, so they behave differently during combustion. The elements in flame retardants also react differently with fire. As a result, flame retardants have to be matched appropriately to each type of material. Flame retardants work to stop or delay fire, but, depending on their chemical makeup, they interact at different stages of the fire cycle. When flame retardants are present in the material, they can act in three key ways to stop the burning process. They may work to: (1) Disrupt the combustion stage of a fire cycle, including avoiding or delaying “flashover,” or the burst of flames. (2) Limit the process of decomposition by physically insulating the available fuel sources from the material source with a fire-resisting “char” layer. (3) Dilute the flammable gases and oxygen concentrations in the flame formation zone by emitting water, nitrogen, or other inert gases. Therefore, the use of flame retardants is essential to stopping or slowing the spread of fire and LDHs have been increasingly used as fire retardants.

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8.4.2 Flame-retardant performance of LDH-based nanocomposites 8.4.2.1 Effect of inorganic LDHs Due to the poor compatibility between inorganic LDHs and the polymer matrix, only a few inorganic anions intercalated in LDHs have been investigated as flame-retardant additives for polymers. Carbonate is the first and the most extensively investigated; carbonate-intercalated LDHs have been shown to be highly efficient in improving the thermal stability and flame-retardant performance of many polymers, such as EVA (Gao et al., 2016a,b; Shi et al., 2005a,b; Jiao et al., 2008; Shi et al., 2005a,b; Nyambo and Wilkie, 2009a,b), PVC (Xu et al., 2006; Zhang et al., 2004; Molefe et al., 2015), ABS (Xu et al., 2012; Zhang et al., 2004), HDPE (Gao et al., 2014a,b; Zhang et al., 2004), and so on. Zhang et al. (2004) studied the fire retardancy of MgAl-CO3 LDHs in various polymers. After PS, ABS, HDPE, and PVC were filled with the nano-LDHs with a loading of 60 wt%, their LOI values could be increased up to 28, 27, 26, and 33, respectively, and the polymers produced less smoke than the materials free of the nanoLDHs during burning. Shi et al. (2005a,b) incorporated 60 wt% MgAl-CO3 and ZnMgAl-CO3 LDHs into EVA-28, the LOI of EVA can be increased from 21 to 34 and 40, respectively. ZnO present in the mixed metal oxide formed by decomposition of ZnMgAl-CO3 LDHs can promote charring of the composite. As a result, incorporation of Zn21 into the layers of the LDHs was found to promote material charring and smoke suppression. In addition, MgAl-CO3 and ZnMgAlCO3 LDHs also showed a good flame-retardant performance to ABS resin. Both ABS/MgAl-LDH and ABS/ZnMgAl-LDH composites exhibit higher LOI, lower smoke density values, and a prolonged combustion time, compared to pristine ABS (Xu et al., 2012). Molefe et al. (2015) observed that MgAl-CO3 LDHs are a promising functional filler for plasticized PVC. Both the thermal stability and flame-retardant performance can be improved with 30 phr LDHs loading. In addition, a series of MgAlFe-CO3 LDHs have been added to EVA by Jiao et al. (2008). The results show that MgAlFe-CO3 LDHs are better than MgAl-CO3 LDHs in improving the flame retardation of EVA at the same additive loading level and reached the UL-94 V-0 rating when the LDH loading was 50 wt%. The addition of ZnAl-CO3 LDHs coated with oleate also can promote charring to retard the generation of flame for PVC (Xu et al., 2006). Gao et al. (2014a,b) synthesized HDPE/Zn2Al 2 X (X 5 CO322, NO32, Cl2, SO422) LDH nanocomposites with different loadings from 10 to 40 wt% using a modified solvent-mixing method. The influence on flame-retardant properties followed the order of SO422 . NO32 . CO322 . Cl2. When adding 40 wt% LDHs, the PHRR was reduced by 54%, 48%, 41%, and 24%, respectively (Fig. 8.10). In 2005, Shi et al. (2005a,b) reported the borate intercalated MgAl LDHs as a flame-retardant filler for EVA for the first time. MgAl-LDHs showed a good flame-retardant performance, the LOI of EVA was increased from 21 to 29 after adding 60 wt% LDHs. EVA/MgAl-borate composites with 60 wt% LDH loading also showed a significantly better smoke suppression, which was 45% less than

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Figure 8.10 MCC analysis of: (A) HDPE/Zn2Al 2 Cl, (B) HDPE/Zn2Al 2 NO3, (C) HDPE/ Zn2Al 2 CO3, and (D) HDPE/Zn2Al 2 SO4 LDH nanocomposites.

that of pure EVA. Later, Nyambo and Wilkie (2009a,b) investigated the fire resistance performance of ZnAl- and MgAl-borate LDHs in EVA. At 40% loading, the reduction in PHHR observed in EVA composites containing LDHs reached to 74% and 77%, respectively. In addition, Wang et al. (2013a,b) synthesized PP/Zn2Al-borate and PP/Mg3Al-borate LDH nanocomposites using a modified solvent-mixing method. The results show that PP/Zn2Al-borate LDH nanocomposites exhibited superior performance to the equivalent PP/Mg3Alborate LDH nanocomposites. By considering both the thermal improvement and the flame-retardant performance, 15 wt% of the highly dispersed Zn2Al-borate LDHs in PP was found to be the optimal loading. The 15 wt% Zn2Al-borate LDHs in pristine (unmodified) PP resulted in a 64% reduction of the PHRR (Fig. 8.11). It is believed that borate promotes the formation of a ceramic-like MgO or Al2O3-based coating that forms over the char, which forms on the surface of a polymer during combustion and subsequently forms a vitreous phase, which acts as a binder to reinforce this ceramic coating, preventing further combustion (Shi et al., 2005a,b). In addition to borate, phosphate-intercalated LDHs were also studied by Ye and Qu (2008). They compared the flame-retardant properties of MgAl-CO3 and MgAlPO4 LDHs in the EVA blends. The LOI values of EVA/MgAl-PO4 samples with different loading levels are 2% higher than those of the corresponding MgAl-CO3 LDHs samples. Meanwhile, both EVA/MgAl-CO3 and EVA/MgAl-PO4 LDH

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Figure 8.11 (A) TGA analysis of PP/Zn2Al-borate nanocomposites with various LDH loadings (0, 3, 6, 9, 15, and 30 wt%). (B) HRR of PP/Zn2Al-borate nanocomposites with various LDH loadings (0, 6, 15, and 30 wt%).

composites can pass the V-0 rating when the LDH loading reached 60 wt%. However, the composites with 55 wt% MgAl-PO4 LDHs can pass the V-1 rating while the composites with 55 wt% MgAl-CO3 LDHs cannot pass any rating in the UL-94 test. The flame-retardant mechanism of MgAl-PO4 LDHs can be ascribed to its catalysis degradation of the EVA resin, which promotes the formation of charred layers with the P-O-P and P-O-C complexes in the condensed phase and the compact charred layers formed from the EVA/MgAl-PO4 sample effectively protect the underlying polymer from burning. The SEM observation gives further evidence of this mechanism (Fig. 8.12). To sum up, LDHs intercalated with inorganic anions such as CO322, NO32, Cl2, SO422, borate, phosphate, etc. are potentially promising flame-retardant additives for polymers such as ABS, EVA, PP, and so on. But one problem with inorganic

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Figure 8.12 SEM images of charred residues of (A) EVA/MgAl-CO3 and (B) EVA/MgAlPO4 LDH composites with 60 wt% LDH loading.

LDHs is the high loading as nanofillers. In order to obtain a high-efficiency flame retardant with a low inorganic intercalated LDHs loading, more work needs to be done in the future.

8.4.2.2 Effect of organic LDHs Although inorganic anion-intercalated LDHs are potentially promising flameretardant nanofillers for some polymers, the hydrophilic surface of the inorganic LDHs is incompatible with hydrophobic polymers, which severely inhibits homogeneous dispersion of LDH layers within the polymer matrix (Manzi-Nshuti et al., 2009,b,c; Bao et al., 2008). Furthermore, the high charge density in the metal hydroxide layer leads to a strong electrostatic interaction between the hydroxide sheets, making separation of these sheets (exfoliation) very difficult. Therefore, it is important to modify LDHs with suitable organic anions to increase the gallery distance as well as reduce the hydrophilic character of the surface (Gao et al., 2014a, b). Till now, many organic anions have been intercalated into LDH interlayers including oleate, DS, SA, undecenoate, dodecyl benzene sulfonate (DBS), C16,N(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)-N-hexyl)formamide-2-propenyl acid (DPHPA), 2-carboxylethyl-phenyl-phosphinic acid (CEPPA), etc. Among the organic anions, oleate is most extensively studied. The long chain of oleate acts to compatibilize the LDHs with many polymers, such as PP, PE, EVA, PMMA, and poly(ethylene-co-butylacrylate) (PEBuA). Oleate exhibits an excellent combination of high thermal stability, good water solubility, and relatively low cost, and as a result it is usually preferred to other possible surfactants (ManziNshuti et al., 2009,b,c). It was found that ZnAl-oleate LDHs revealed a good flame-retardant performance for PE, which PHRR reduction was 58% with 10 wt% LDHs loading, followed by PMMA (28%) and PEBuA (2%) (Manzi-Nshuti et al., 2009,b,c). Wang et al. (2011a,b,c,d) investigated the EVA/ZnAl-oleate nanocomposites, the result showed that the PHRR reduction was 33% with 10 wt% LDH loading. In addition, MgAl-oleate LDHs showed a similar flame-retardant performance, in which PHRR reduction was 36%, with the same LDH loading in EVA resin. In addition, Manzi-Nshuti et al. (2009,b,c) synthesized PP nanocomposites with a

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series of oleate-intercalated ZnMg LDHs. It was found that Zn0.5Mg1.5Al-oleate LDHs showed the largest PHRR reduction, 38% with 4 wt% LDHs loading. For Zn2Al-oleate LDHs, the PHRR reductions are 25% and 5% with 2 and 4 wt% LDH loading. DS is an important anion applied to modify LDHs for flame-retardant applications. Ye and Wu (2012) investigated the flame-retardant properties of low-density polyethylene (LDPE)/LDH nanocomposites with DS-modified MgAl-LDHs. When the LDH loading was 5 phr, the PHRR values were reduced by 14.5% and 5%, respectively. In addition, Wang et al. (2012a,b,c,d) proved that DS-intercalated NiAl LDHs and EVA matrix had good compatibility and when the LDHs loading was 20 wt%, the PHRR was reduced by 74.9%. DBS is another important anion applied to modify LDHs as polymer additives. Costa et al. (2007) investigated the flammability properties of the nanocomposites based on LDPE and MgAl-DBS LDHs. The PHRR values were found to be reduced significantly with increasing LDH concentration. When the LDHs loading was 16.2% (PE-LDH6), the PHRR value of the nanocomposites was reduced by 68% (Fig. 8.13A). Tig, a parameter defined as the time at which the test samples catch fire, is also significantly increased with increasing LDH content. The pure LDPE has a Tig below 100 s and that increased to above 120 s with the addition of 16.2 wt % LDHs (Fig. 8.13B). Except oleate, DS, DBS anions, 1,4-butane sultone (BS) (Wang et al., 2015a,b), phenyl phosphate (PP) (Edenharter and Breu, 2015), C16 (Majoni, 2015; Nyambo et al., 2009a,b), 2-carboxy lethyl-phenyl-phosphinic acid (CEPPA) (Ding et al., 2015), [((1,1,3,3-tetramethyldisiloxane-1,3-diyl)bis(propane-3,1-diyl))bis(2-methoxy-4,1-phenylene)bis(phenylphosphonochloridate)(SIEPDP) (Li et al., 2015), N(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)-hexylacetamide-2-propyl acid (PAHPA) (Huang et al., 2012), N-(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)-N-hexylformamide-2-propenyl acid (DPHPA) (Huang et al., 2011), and so on, were also intercalated into the LDH interlayer as flame-retardant nanofillers. For example, with only 6% cardanol BS modified MgAl LDHs (m-LDHs), the epoxy resin (EP) composite reached an LOI of 29.2% and UL-94 V-0 rating. The PHRR, THR, and total smoke production (TSP) values of EP/m-LDH-6% were decreased by 62%, 19%, and 45%, respectively, compared to those of pure EP (Wang et al., 2015a,b). Edenharter and Breu (2015) found that with a filler content of 5 wt%, MgAl-PP LDHs could be shown to significantly improve the flame-retardant properties of PS as compared to MgAl-CO3 LDHs, the PHRR values were 47% and 22%, respectively (Fig. 8.14A). Moreover, the heat release of the PP-LDH nanocomposites was spread over a wider range of time resulting in a higher burnout time (from 550 to 695 s), which indicates a slower transfer of mass and heat during the combustion of the polymer. The longer burning time at lower HRR may be related to the formation of a thin layer of char and residues of metal oxides that insulate the polymer from heat radiation (Fig. 8.14B). The addition of 5% and 10% of MgAl-C16 LDHs to PS also resulted in a substantial reduction in PHRR (47% and 61%, respectively) of the polymer (Majoni, 2015). In addition, Kang et al. (2013) investigated the effect of dye structure (acid yellow 36 and acid red 88)

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intercalated MgAl LDHs (d-LDHs) on the flammability of PP-g-MA composite. Compared with MgAl-NO3 LDHs, d-LDHs can significantly decrease the PHRR and THR of the composite, when the LDH loading was 5 wt%, the PHRR reduced by 22% and 33% for acid red 88 and acid yellow 36 intercalated LDHs, respectively, while it reduced by 11% for NO3-LDHs. In conclusion, organic intercalated LDHs show significantly enhanced flame retardancy compared with pure polymer matrix. Compared with the inorganic LDHs, the organic modified LDHs have much better compatibility with polymers, and much lower LDH loading is required to obtain a similar flame-retardant performance.

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Figure 8.14 (A) Heat release rate for pristine polystyrene (blue) and nanocomposites filled with CO3-LDH (black) or PP-LDH (red) and (B) residue of the cone test for Polystyrene/PPLDH nanocomposite.

8.4.2.3 Effect of LDHs with other synergistic fire retardants A problem of inorganic LDH nanofillers for polymers is that a very high loading is required. Many other flame-retardant additives were combined with inorganic LDHs, such as red phosphorus (RP), APP, carbon nanotubes (CNTs), graphene oxide (GO), melamine (MA), mesoporous silica (m-SiO2), and TiO2, etc. APP is an important flame-retardant additive, and previous studies showed that using LDHs together with phosphorus-containing flame retardants can help to improve the dispersion of these additives within the polymer matrix (Nyambo et al., 2008a,b). Zhao et al. (2008) combined different cation (ZnAl-CO3, ZnFe-CO3, NiAl-CO3, and NiFe-CO3) LDHs as synergistic agents with APP to improve the flame retardancy of PVA matrix. When the content of the LDHs in PVA is 0.3 wt% and the content of APP is 14.7 wt%, the LOI of PVA increased from 20 to 31, 33, 34, and 34, respectively. Furthermore, the amount of residue increased in the order: PVA/APP , PVA/APP/ZnAl , PVA/APP/ZnFe , PVA/APP/NiFe , PVA/APP/ NiAl. Among the PVA/APP/LDH samples, PVA/APP/NiAl showed the best flameretardant performance, which may be attributed to the slightly different decomposition behavior around 450 C from other ternary composites. Furthermore, a study on the effect of MgAlZnFe-CO3 LDHs on the flame-retardant properties of APP and melamine (mass ratio 5:1) poly(butylene succinate) (PBS) composites was investigated by Liu et al. (2014). It was revealed that IFR-PBS composites exhibited both excellent flame retardancy and antidripping properties when the content of MgAlZnFe-CO3 LDHs and IFR was 1% and 19%, respectively, for a goal of UL-94 V-0 rate and a limiting oxygen index value of 35. The results showed that a suitable amount of MgAlZnFe-CO3 LDHs had a noticeable synergistic effect on IFR-PBS composites. A possible interaction between APP and LDH was also proposed byZhao et al. (2008). During burning, APP is first thermally decomposed to form poly(phosphoric acid), which may undergo a further dehydration in two traditional ways. The phosphate ester may react with the PVA chain or itself, which subsequently crosslinks with the formation of a three-dimensional network structure.

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After adding the LDHs as synergistic agents, the approach of char formation contributed more to flame-retardant PVA. In addition to the traditional dehydration methods, poly(phosphoric acid) may react with the LDH layers, releasing water molecules and producing bridges between APP chains. The formation of a small number of such bridges will induce a stabilization of APP and a decrease in the volatility of phosphorus, and thus more APP will be available for phosphorylation and char formation (Song et al., 2006; Chen et al., 2005). The crosslinks can increase the degree of polymerization of poly(phosphoric acid), which will increase the viscosity of the melt during pyrolysis and combustion, and therefore enhance the formation of compact and dense charred layer. CNTs are another kind of flame-retardant additive for polymers. It has been found that adding a small amount of CNTs to polymers can improve the flameretardant performance of the polymer composites significantly (Xie et al., 2016). Considering the fact that LDHs and CNTs possess different flame-retardant mechanisms, the potential synergistic effect between them in polymers was investigated by Gao et al. (2016a,b). They found that a system with 10 wt% of aqueous miscible organic (AMO)-LDH and 1 wt% OCNT showed a PHRR reduction of 40%, even greater than the PHRR reduction with PP/20 wt% AMO-LDH (31%) (Fig. 8.15A). The increased PHRR reduction after adding OCNTs is because the dense network of nonflammable OCNTs acts as a physical barrier to the diffusion of oxygen and also slows the escape of combustion products formed during decomposition, which can shield the polymer resin from external radiation and heat feedback from the flame (Kashiwagi et al., 2002; Song et al., 2008; Wang and Jiang, 2011a). They also found the degree of mixing between AMO-LDH and OCNT has a significant effect on the flame-retardant properties. Better mixing can lead to better flame-retardant performance (Fig. 8.15B). In addition, the incorporation of AMO-LDH-OCNT hybrids led to better thermal stability and mechanical properties. CNT and ZnAl-CO3 LDHs with good solubility in liquid media were also synthesized by Xie et al. (2016). It was established that CNT/ZnAl-CO3 LDHs could improve the thermal stability while reducing the PHRR and the total smoke release of polyurethane (PU) foams efficiently. Jiang et al. (2014) combined m-SiO2 and CoAl LDHs to improve their flame retardancy effectiveness in EP. The m-SiO2@CoAl LDHs were synthesized through a layer-by-layer assembly process. The strong Si and O signal across the sphere confirms the m-SiO2 core, while the Co and Al signals both detected in the surface region clearly suggest the adsorption of CoAl LDH particles (Fig. 8.16A). Incorporation of m-SiO2@CoAl LDHs into EP led to an increase of the char yield and a decrease of PHRR as well as THR values. Compared to pure EP, the addition of 2 wt% m-SiO2@CoAl LDHs brings about a 39.3% maximum decrease in PHRR (Fig. 8.16B), and a 36.2% maximum decrease in THR. In addition, the incorporation of m-SiO2@CoAl LDHs results in a significant improvement of the char yield (Fig. 8.16C). The results exhibit that the EP/m-SiO2@CoAl LDH nanocomposites present a good flame retardancy. They also proposed the mechanism for the improved fire-resistant property of EP/m-SiO2@CoAl LDH nanocomposites. During the combustion process, m-SiO2 with catalytic activity leads to the

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Figure 8.15 PHRR reduction of PP and the various nanocomposites prepared.

formation of pyrolysis products with lower carbon numbers, which can be easily catalyzed by carbonization in the presence of metal oxides. Meanwhile, CoAl LDHs can catalyze carbonization of degradation products. Moreover, m-SiO2 plays a role as a barrier that can absorb degraded products to extend the contact time with metal compound catalyst. Furthermore, the degraded products are dehydrogenated and catalytically converted into char by the combination of the mSiO2 labyrinth effect and the CoAl LDH catalysis effect. In addition, Jiao and Chen (2011) proved that TiO2 has a good flame-retardant synergistic effect with LDHs in the EVA/LDH/TiO2 blends. Only 2 phr TiO2 can make the EVA/LDH/TiO2 pass the UL-94 test. The PHRR values of the composite samples decreased with increasing loading of TiO2, and their burning was also prolonged to 600
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Figure 8.16 (A) Dark-field STEM image and elemental mapping of m-SiO2@Co 2 Al LDH, (B) HRR curves of EP and its nanocomposites, and (C) digital photos of the residues from EP and its nanocomposites.

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mainly due to the physical processes in the condensed phase (Kong et al., 2006; Tang and Lewin, 2008). The accumulated TiO2 consequently formed a charred layer by collaborating with LDHs, which acts as a heat insulation barrier. This charred layer prevented heat transfer and transportation of degraded products between the melting polymer and surface, thus reducing the HRR and related parameters. Except APP, CNT, SiO2, and TiO2, microcapsulated red phosphorus (MRP) and intumescent flame retardant (IFR) also show good synergy with inorganic LDHs in polymers. The synergistic effect in the flame retardation between MgAlCO3 LDHs and MRP in EVA has been found by Jiao et al. (2006). The LOI value, the mechanical properties, and thermal stability are improved for the composites. When the loading of LDH and MRP is 70 and 10 phr, respectively, the LOI value increased from 32 to 39. Liu et al. (2014) studied the synergistic effect of IFR consisting of APP and melamine, and MgAlZnFe-CO3 LDHs. It was revealed that IFR-PBS composites exhibited both excellent flame retardancy and antidripping properties when the content of MgAlZnFe-CO3 LDHs was 1% (the total loading of flame retardant was 20%), for a goal of UL-94 V-0 rate and a limiting oxygen index value of 35. Although organically modified LDHs have much better flame-retardant performance than inorganic LDHs, organic anions are inherently combustible, thus the loading cannot be too high or both the thermal stability and flame-retardant properties will decrease. In addition, LDHs or organic LDHs alone, even at high concentrations, are not sufficient to obtain a high LOI value or V-0 rating in UL-94 testing. Thus, many synergistic flame-retardant additives are also combined with organic LDHs in order to achieve a desired result. MH is an example of a toxic-free, smoke-suppressing, halogen-free flameretardant additive. In order to improve the flame-retardant performance of polymer, a series of DS-intercalated LDHs, such as MgAl, ZnAl, and MgFe have been added to EVA/MH composites (Ding et al., 2011). The results show that the distribution of inorganics in EVA/MH/LDH composites is more homogeneous than the distribution in sample EVA/MH, which means that the addition of LDH can improve the distribution of MH in EVA. Composites containing LDH show good flame retardancy, when adding 45 wt% MH and 5 wt% LDHs, the PHRR reduction reached 88%, 68%, and 85% for MgAl LDHs, ZnAl LDHs, and MgFe LDHs, respectively. Especially for EVA/MH/MgAl LDH composite, it displays a remarkable reduction in PHRR of almost 60% relative to that of EVA/MH composite without LDHs. APP is a high-molecular-weight phosphate-based chain, it serves as both an acid source and a blowing agent in intumescent formulations to promote char formation during polymer decomposition. Phosphoric acids produced during pyrolysis promote charring, while the evolved NH3 improves swelling, hence slowing or preventing heat and mass transfer to and from the pyrolysis zone (Nyambo et al., 2008a,b). MgAl undecenoic acid LDH and APP were added to neat PS individually or in combinations at weight fractions no greater than 10% by Nyambo et al. (2008a,b). PS composites containing 5% and 10% LDH show reductions in PHRR of 17% and

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27%, respectively. When APP is added to PS at the same weight fractions, lower PHRR reductions were observed, 11% and 22%, respectively. Even though LDHs alone is more effective in reducing the PHRR than APP, their combination produced a better result than simply an additive effect. The observed reduction in PHRR for PS/LDH/APP was significantly higher, and was 42% with 5 wt% LDH and 5 wt% APP. Furthermore, APP/MgAl-DS LDH were added to poly(butyl acrylate
vinyl acetate) (P(BA-VAc)) (Zhao et al., 2011). Pure P(BA-VAc) is a readily flammable polymeric material with an LOI value of 20.0%, and it cannot pass the UL-94 test. The LOI value of flame-retardant P(BA-VAc) can reach up to 30.7 and UL-94 V-0 rating, particularly when the contents of organic LDH and APP in P (BA-VAc) are 0.5 and 14.5 wt%, respectively. IFR is an efficient flame-retardant system for polymer matrix. It is halogenfree and has low toxicity. A typical IFR system is APP and pentaerythritol (APP/ PER). Ding et al. (2010) proved that the addition of LDH nanofillers into the PP composites can obtain good flame-retardant synergistic effects with APP/PER additives at appropriate LDH loadings. Only small amounts of LDH fillers (lower than 5 wt%) can evidently increase the LOI values. For example, 5 wt% loading of LDH can increase the LOI value up to 35 of PP/IFR/ZnAl-DS LDH composites from 19 of pure PP. This indicates that a small amount of LDH fillers can give a good synergistic effect on flame-retardant properties with APP/PER additives. Wang et al. (2012a,b,c,d) synthesized maleic anhydride grafted ethylenepropylene-diene terpolymer (mEPDM)/IFR/MgAl-DBS composite; the results showed that the introduction of a small amount of LDH in the flame-retardant mEPDM led to a significant decrease in HRR. The PHRR value reduced by 55.2% when the IFR and LDH loading is 38 and 2 phr, respectively. In addition, when adding 30 phr IFR and 2 phr DBS-modified MgAl LDH into EPDM, the LOI value increased to 27% from 17.5% (pure EPDM), and the composite reached a V-0 rating in the UL-94 test (Shen et al., 2013). Furthermore, PMMA/IFR/ MgAl-DS LDH composites were investigated by Huang et al. (2014) when incorporating 5 wt% LDH and 10 wt% IFR, the PHRR reduction was 37.9%, the LOI value increased to 26.1 from 17.4 of pure PMMA. Wang et al. (2015a,b) investigated the effect of DBS-modified binary MgAl- and ternary MgZnAl-LDHs on flammability of flame-retardant PP composites in combination with IFR additives. The synergism between either binary or ternary LDH and IFR occurred during the combustion. The reduction of PHRR value was 79.2% and 77.7% for PP/18 wt% IFR/2 wt% MgAl-DBS and PP/18 wt% IFR/2 wt% MgZnAl-DBS LDH composites, respectively (Fig. 8.17A). In contrast to the MgAl LDHs, the MgZnAl LDHs showed superior char-formation ability and smoke suppression due to the presence of the element zinc. When IFR is partly substituted by LDH, most LOI values are slightly higher than that with IFR alone, except PP/16 wt% IFR/4 wt% MgAl-DBS LDH composites. An optimum is observed at 2.0 wt% of MgZnAl LDHs and 18.0 wt% of IFR, exhibiting the highest LOI of 32.5% and a UL-94 V0 rating in vertical burning test (Fig. 8.17B). The combustion process for PP/IFR/LDH composites can be divided into four stages based on the HRR curves: (1) predegradation of PP; (2) main burning

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Figure 8.17 (A) Heat release rate versus time curves of PP and its flame-retardant composites and (B) the LOI values and UL-94 results of PP and its flame-retardant composites.

process; (3) char formation; and (4) oxidation of char residues (Fig. 8.17A). The incorporation of both binary and ternary LDH has a significant influence on the fourth stage, which enhances the thermal oxidative resistance of the char layer, answering for the disappearance of the second peak of HRR curves for PP/IFR/ LDH composites. Except MH, APP and IFR, zinc borate (Wang et al., 2011a,b,c,d), triphenol phosphate (TPP) (Manzi-Nshuti et al., 2009,b,c), melamine (Manzi-Nshuti et al., 2008), and ZrP (Jiang et al., 2016) were also combined with organic LDHs as a synergistic flame-retardant additive. For example, as an effective synergistic flameretardant, the addition of the LDH/ZrP composites resulted in a significant decrease in the HRR compared with pure PP. When the loading of LDHs and ZrP was 5% and 15% (a ratio of 1:3), respectively, the PHRR was reduced by 28.2%. The improved flame-retardant performance may be because LDH and ZrP decomposed

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to produce noncombustible gases, which diluted the combustible gases, and a compact char layer that acted as a fire barrier (Jiang et al., 2016). Also, Huang et al. (2014) studied the flame-retardant effect of IFR/RGO/LDH on PMMA, when filled with 10 wt% IFRs, 1 wt% RGO, and 5 wt% LDHs; it can achieve an LOI value of 28.2% and UL-94 V-1 grade. Compared with neat PMMA, the PHRR of PMMA/ IFRs/RGO/LDHs is reduced by about 45%. Previous studies show that both the physical and chemical interactions between the LDHs and the halogen-free flame-retardant (HFFR) materials are responsible for the observed synergy in fire performance (Nyambo et al., 2008a,b). The LDHs are thought to impact char formation in the polymer/HFFR system. This char is very effective, making the polymeric substance less prone to combustion (Wang et al., 2010).

8.4.3 Posttreatment of LDHs as flame retardants Because of the intrinsic hydrophilic nature of LDHs, they generally do not exhibit native compatibility with nonpolar polyolefin hosts (e.g., PP), which may affect the flame-retardant characteristics of the materials. Therefore, to enhance the compatibility of the LDHs with the polymer matrices is very important. Till now, there are the following two methods to improve the dispersibility of LDH particles in polymers.

8.4.3.1 Organic modification of LDHs In order to apply LDHs as an inorganic filler in polymer nanocomposites, hydrophobic modification of LDH is necessary, and can be achieved by intercalating anionic surfactants with hydrophobic aliphatic carbon chains, such as fatty acids and dodecyl sulfate. Hydrophobic polymer chains can easily access the interlayer of LDH when the hydrophobic interlayer is swelled, resulting in polymer nanocomposites with highly dispersed LDH. Or, the LDH materials are treated by organic modification with anionic surfactants before they are incorporated into the polymers. The anionic surfactants include fatty acid, fatty acid metal salt, silane coupling agent, or titanate coupling agent, etc. (Feng et al., 2012; Tao et al., 2009). For example, pristine Mg2Al LDH was modified with three different organic acids (laurate, palmitate, and stearate) to increase its hydrophobicity. TEM analysis showed most of the layers of LDH modified with laurate and palmitate were sufficiently separated from each other, and randomly dispersed in the PP matrix, indicating that most of the LDH layers were exfoliated in the PP matrix (Fig. 8.18A,B). But in PP/ stearate-LDH nanocomposites, a swelled LDH structure with several layers (3
6 layers) and sufficiently separated layers was observed, indicating that both the PP intercalated LDH layers and the exfoliated LDH layers existed concurrently (Fig. 8.18C) (Yang et al., 2015). Treatment of the LDHs with 1
10 wt% of anionic surfactant, for example, sodium stearate or sodium oleate results in improvement of dispersibility and fluidity.

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Figure 8.18 Transmission electron microscopy images of (A) 3 phr laurate-LDH/PP, (B) 3 phr palmitate-LDH/PP, and (C) 3 phr stearate-LDH/PP, respectively.

8.4.3.2 Aqueous miscible organic solvent treatment In order to improve the compatibility between LDH and polymer, one solution is to intercalate surfactant anions into the LDH interlayers. The other method to improve the dispersibility of LDH particles in polymer matrix is using the solvent-mixing method. Currently, most polymer/LDH nanocomposites are prepared using the melt-mixing method, by which the polymer and dried LDH powders are mixed in an extruder at an elevated temperature. However, one problem is that the LDH nanoparticles aggregate severely when being dried, which can result in poor dispersion of the LDH nanoparticles in the polymer matrix. Therefore, in order to make highly dispersed polymer/LDH nanocomposites, solvent mixing is preferred. Recently, Wang et al. (2012a,b,c,d) reported a new method that makes it possible to disperse inorganic anion-intercalated LDHs in nonpolar solvents (e.g., xylene). Polymer/LDH nanocomposites can then be prepared by the solvent-mixing method. Due to the intrinsic hydrophilic nature of LDHs, they generally cannot be dispersed in nonpolar solvents and so they do not exhibit native compatibility with nonpolar polyolefin hosts. After aqueous miscible organic solvent treatment, these solvent-treated LDH nanoparticles can now be dispersed in xylene to give a stable suspension (see Fig. 8.19A). UV-visible data also indicate that the LDH suspension in xylene is optically transparent and stable (Fig. 8.19B) (Wang et al., 2012a,b,c,d). By using this method, polypropylene/Mg3Al-LDH nanocomposites were synthesized successfully by dissolving PP into a clear dispersion of Mg3AlLDH in xylene. Further, ZnAl and MgAl LDHs with different inorganic anions (such as Cl2, NO32, CO322, and SO422) were dispersed in the HDPE, EVA, and PP polymer matrix using the solvent-mixing method. The thermal stability and flame-retardant performance of the nanocomposites were significantly enhanced due to the good dispersion of LDHs (Gao et al., 2014a,b).

8.4.4 The mechanism of flame retardancy using LDH The origin of the excellent flame retardancy and smoke suppression properties of LDHs is derived from their unique chemical composition and layered structure.

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Figure 8.19 The effect of solvent treatment on the Mg3Al-LDH nanoparticles: (A) unmodified LDH cannot be dispersed in xylene, after the washing treatment the LDHs nanoparticles can be dispersed in xylene, (B) transmission UV-visible spectra of solventmodified Mg3Al-LDH dispersions in xylene.

Although no mechanism has been proposed for flame-retardant LDH nanocomposites (Nyambo et al., 2009a,b; Zhu et al., 2001; Chen et al., 2007), it is generally believed that the flame-retardant mechanism of LDHs is different from that of silicate clay-based nanocomposites (Nyambo et al., 2008a,b). During thermal decomposition, the LDH gradually loses its interlayer water when heated in the range from 50 C to 220 C. At higher temperature, it further loses its hydroxyl groups in the host sheet and carbonate anions in the interlayer spacings to produce water and carbon dioxide, and then it is converted into mixed metal oxides. The water vapor and carbon dioxide released from the LDHs can dilute flammable gases and prevent contact of the materials with oxygen and eventually stop the combustion when there is not enough fuel to propagate the reaction, and promote the formation of an expanded carbonaceous coating or char. Char formation protects the bulk polymer from exposure to air, thus reducing the heat release during the combustion and suppressing smoke production (Zammarano et al., 2005; Chen et al., 2002). The mechanism of action of the LDHs as the flame retardant can be described as the endothermic decomposition reducing the fire intensity, the shielding due to char formation, and stabilization of char and the dilution effect (Feng et al., 2012). Consequently, the mass loss rate will be significantly reduced due to the combination of the above-mentioned three functions (Gao et al., 2014a,b).

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Conclusions and future development

In this chapter, we have summarized the current research into flame-retardant polymer/LDH nanocomposites. As flame retardant nanofillers, the role of LDHs is summarized as having four functions: (1) heat absorption (endotherm), (2) gaseous dilution, (3) char formation, and (4) dispersion. A proper loading of LDH could also improve the thermostability of polymers because the released H2O and CO2 can delay the scission of polymer chains, so making the polymer composites more stable. In addition, the synergistic effect between LDHs and other HFFR additives was discussed. As a synergistic additive, LDH can not only enhance the flameretardant properties of polymer/LDH nanocomposites, but also reduce the loading of HFFR agents in polymer matrix so as to improve the thermal stability and mechanical properties of polymer nanocomposites. The synergistic function of LDHs is considered to impact the char formation of polymer/HFFR systems. But the detailed mechanism between LDHs and HFFR needs to be explored in future work.

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Mechancial and dynamical mechanical properties of layered double hydroxide-filled elastomer and elastomeric blend nanocomposites

9

Suneel Kumar Srivastava Department of Chemistry, Indian Institute of Technology, Khragpur, India

9.1

Introduction

Elastomers, according to the general IUPAC definition, are polymers exhibiting rubber-like elasticity (Evans and Slade, 2006). Their general characteristics involve viscoelastic behavior, low modulus of elasticity, high failure strain, along with very weak intermolecular forces. Their good heat resistance, ease of deformation at ambient temperatures, and exceptional elongation and flexibility before breaking make elastomers excellent and relatively cheap materials for extensive applications in a variety of commercial as well as domestic products (Choy et al., 2007; Bhowmick, 2008; Acharya et al., 2007b; Kura et al., 2014; Gu et al., 2015; Daud et al., 2006; Wang and O’Hare, 2012; Basu et al., 2013; Zvonimir and Wilkie, 2012; Velosco and Antunes, 2012; Kotal et al., 2009). The incorporation of nanofillers into elastomers further significantly improves their heat resistance, photo and UV stability, flammability, barrier properties, thermal, mechanical, and dynamic mechanical properties, etc., depending on the polymer matrices, nature of nanofillers, and their dispersion in elastomer matrices (Bhowmick, 2008). Although, a variety of inorganic fillers have been used in the fabrication of elastomeric nanococomposites, layered double hydroxides (LDHs) remain an important class of filler (Acharya et al., 2007b). LDHs exhibit a wide range of available chemical compositions and sufficient interlayer spacing. The general formula of LDHs can be represented as: [M z112xM31x(OH)2]q1(Am2)q/m  nH2O. Usually z 5 2, with M12 5 Ca12, Mg12, Zn12, Ni12, Mn12, Co12, or Fe12 and with M31 5 Al31, Cr31, Mn31, Fe31, Ga31, Co31, or Ni31 so that q 5 x. In one case z 5 1, with M1 5 Li1 and M31 5 Al31, giving q 5 2x
1. The values of x have been reported to fall between the range 0.1
0.5, however pure phases only exist for 0.2
0.33, that is, M12/M13 ratios are in the range of 2
4 and Am2 5 interlayer anions present in the LDH. The order of preference of exchangeable anions in the interlayer is: NO32 , Br2 , Cl2 , F2 , OH2 , SO422 , CO322. LDHs have a brucite-like Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00009-4 © 2020 Elsevier Ltd. All rights reserved.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 9.1 Structure of typical layered double hydroxide (Acharya et al., 2007b). Source: Reproduced with permission from Elsevier.

structure, where divalent octahedrally coordinated M(II) ions are partially substituted by trivalent M(III) ions. As a result, the positively charged metal oxide/ hydroxide layers are neutralized by other charge-balancing anions. Fig. 9.1 shows a schematic representation of a typical layered double hydroxide (Acharya et al., 2007b). A number of synthetic techniques have been successfully employed for the preparation of LDH, such as ion exchange process, coprecipitation, and reconstruction method. In addition, sol
gel synthesis, fast nucleation process followed by a separate aging step at elevated temperatures, and hydrothermal methods are also reported. However, pristine LDH prepared in this manner is not suitable in the development of elastomeric nanocomposites due to the smaller interlayer gallery space (B0.76 nm) (Velosco and Antunes, 2012). Therefore, pristine LDH is modified with anionic surfactants, for example, sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS). This replaces the counter anion and water molecules in the interlayer space. In addition, aromatic carboxylate anion, aliphatic carboxylate anion, phosphonates, and polymeric anions, etc., are also used for the organomodification of LDHs. These LDH-filled polymers typically show enhanced properties compared to neat polymer matrix. As a result, elastomer/LDH and elastomeric blend/LDH nanocomposites find their multifaceted applications in different fields (Acharya et al., 2007b; Kura et al., 2010a, 2013, 2014; Gu et al., 2015; Daud et al., 2006; Wang and O’Hare, 2012; Basu et al., 2013; Zvonimir and Wilkie, 2012; Velosco and Antunes, 2012; Kotal et al., 2009, 2010a,b, 2011, 2013; Kotal and Srivastava, 2011a,b; Kotal, 2012; Go´mez-Ferna´ndez et al., 2016; Srivastava and Kotal, 2013; Guo et al., 2011; Xiong et al., 2015; Yan et al., 2013; Zhang et al., 2007, 2008, 2013, 2016; Xu et al., 2016a,b; Roy et al., 2016a,b,c; Roy, 2017; Yu et al., 2013, 2014a,b; Yao et al., 2002; Wang and Pinnavaia, 1998; Kumar and Das, 2010; Kuila et al., 2007, 2008a,b, 2009a,b,c; Kuila, 2009; Ye et al., 2008; Costache et al., 2007; Wang et al., 2011, 2012a,b, 2013; Jiao et al., 2006, 2010; Pradhan, 2013; Pradhan et al., 2008, 2011, 2012; Pradhan and Srivastava, 2014; Nhlapo et al., 2008; Borja and Dutta, 1992; Meyn et al., 1990; Kanoh et al., 1999; Itoh et al., 2003; Shieh et al., 2010; Fornes et al., 2004; Fornes

Mechancial and dynamical mechanical properties

349

and Paul, 2003; Kong et al., 2006; Acharya et al., 2007a, 2008; Wu et al., 2011, 2004; Kim et al., 2001; Acharya, 2008; Chao et al., 2013; Das et al., 2011a,b,c, 2012; Basu et al., 2016; Zhao et al., 2014; Kuila et al., 2012; Cui and Qu, 2010; Kato, 2011; Acharya and Srivastava, 2017; Raman et al., 2013; Srivastava and Kuila, 2013; Srivastava and Himadari, 2010; Xiao et al., 2013, 2014; Wei-Na and Da-chen, 2012; Feng and Su, 2011; Sadhu and Bhowmick, 2004; Bharadwaj et al., 2013; He et al., 2016; Braga et al., 2014; Laskowska et al., 2013; Feng et al., 2013; Long-Chao et al., 2011; Fabiula et al., 2011; Eshwaran et al., 2015; Laskowska et al., 2014a; Costa et al., 2010; Srivastava and Mittal, 2017; Basu et al., 2014; Abdullah et al., 2010; Bottazzo et al., 2013; Srivastava, 2014). In view of this, the main aim of this review is to provide an overview of the mechanical and dynamical properties of LDH-filled elastomer and elastomeric blend nanocomposites. The elastomers addressed in this review consist of polyurethane (PU) (Kotal et al., 2009, 2010a,b, 2011, 2013; Kotal and Srivastava, 2011a,b; Kotal, 2012; Go´mezFerna´ndez et al., 2016; Srivastava and Kotal, 2013a; Guo et al., 2011; Xiong et al., 2015; Yan et al., 2013; Zhang et al., 2013, 2016; Xu et al., 2016a,b; Roy et al., 2016a,c; Roy, 2017; Yu et al., 2013), ethylene vinyl acetate copolymer (EVA) (Yao et al., 2002; Wang and Pinnavaia, 1998; Kumar and Das, 2010; Kuila et al., 2007, 2008a,b, 2009a,b,c; Kuila, 2009; Ye et al., 2008; Zhang et al., 2007, 2008; Costache et al., 2007; Wang et al., 2011, 2012, 2013; Jiao et al., 2010), silicone rubber (SR) (Pradhan, 2013; Jiao et al., 2006; Pradhan et al., 2011, 2012; Pradhan and Srivastava, 2014; Kong et al., 2006), ethylene propylene diene rubber (EPDM) (Acharya et al., 2007a; Wu et al., 2004; Kim et al., 2001; Acharya, 2008; Pradhan et al., 2008; Chao et al., 2013; Das et al., 2011a; Basu et al., 2016; Zhao et al., 2014; Kuila et al., 2012; Wang et al., 2012; Cui and Qu, 2010), styrene butadiene rubber (SBR) (Acharya and Srivastava, 2017; Acharya et al., 2008; Yu et al., 2014a,b; Raman et al., 2013; Xiao et al., 2013; Wei-Na and Da-chen, 2012; Das et al., 2011b, 2012; Feng and Su, 2011; Wu et al., 2011; Sadhu and Bhowmick, 2004), acrylonitrile butadiene rubber (NBR) (He et al., 2016; Braga et al., 2014; Xiao et al., 2014; Laskowska et al., 2013; Feng et al., 2013; LongChao et al., 2011; Fabiula et al., 2011; Eshwaran et al., 2015; Das et al., 2011a,c; Laskowska et al., 2014a; Costa et al., 2010; Srivastava and Mittal, 2017; Basu et al., 2014), natural rubber (NR) (Abdullah et al., 2010; Bottazzo et al., 2013; Srivastava, 2014), etc.

9.2

Preparative methods of LDH-elastomer and LDH-elastomeric blend nanocomposites

LDH/elastomers and LDH/elastometic blend nanocomposites have been prepared by in situ polymerization, solution blending, and melt blending. Depending on the filler distribution, polymer/LDH nanocomposites can be defined as intercalated nanocomposite, exfoliated nanocomposite, and partially exfoliated nanocomposite, depending on whether the LDH layers are arranged regularly and stacked parallel to

350

Layered Double Hydroxide Polymer Nanocomposites

each other, randomly distributed, or stacked parallel to each other, in addition to some randomly dispersed layers throughout the polymer matrix. The preparation of nanocomposites based on the solution blending method is considered not ecofriendly and cost-effective due to the use of excess amounts of organic solvents. In contrast, melt mixing of polymer with nanofiller is the most effective way to obtain nanocomposites for commercial application. The nanofillers are added to the molten polymer under a shearing force to obtain homogeneously dispersed inorganic filler
elastomer nanocomposites. The method is totally environmentally friendly and does not require solvent. Rubber naocomposites have also been prepared using two-roll mill mixing by introducing ingredients and controlling the gap distance between the rolls, setting the desired temperature and speed.

9.3

Different types of layered double hydroxide fillers used in the fabrication of elastomer and elastomeric blend nanocomposites

The incorporation of layered double hydroxide into elastomers improves significantly their mechanical, thermal, dynamic mechanical, barrier properties, flame retardancy, etc. Table 9.1 summarizes detailed information on the different types of LDH fillers used in fabrication of PU (and TPU), EVA, EPDM, SR, SBR, NBR, XNBR, and NR nanocomposites.

9.4

Morphology of elastomer-LDH and elastomeric blend-LDH nanocomposites

Polymer nanocomposites exhibit enhanced physicochemical properties compared to bulk polymer or conventional composites. This is ascribed to the nanolevel dispersion of the inorganic filler in the polymer matrix. Therefore, it is mandatory to investigate the morphology of polymer nanocomposites by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

9.4.1 Morphology of elastomeric-LDH nanocomposites 9.4.1.1 PU-LDH nanocomposites 9.4.1.1.1 XRD Kotal et al. (2009) studied XRD of dodecyl sulfate intercalated Mg-Al-LDH (referred to as DS-LDH)-filled TPU nanocomposites. These investigations revealed the absence of any diffraction peaks corresponding to neat PU in the range of 2θ  2
10 degrees. XRD of TPU/DS-LDH (1, 3, and 5 wt.%) showed the absence of diffraction peaks of DS-LDH corresponding to 2θ  3.41 (d003) and 6.89 degrees (d006), though a small hump appeared at around 2θ 5 2.5 degrees in the case of

Table 9.1 Different LDH used as fillers in preparation of elastomer and elastomeric blend nanocomposites Elastomer

Filler

Preparation method

References

TPU TPU PU prepolymer (isocyanate terminated) PU prepolymer

Solution blending Solution intercalation In situ polymerization

Kotal et al., 2009 Kotal et al., 2011 Kotal and Srivastava, 2011b Kotal et al., 2011a

PU prepolymer PU prepolymer Waterborne PU WPU Prepolymer PU PU prepolymer TPU/NBR (1:1 wt%) TPU/NBR TPU:NBR PU

Dodecyl sulfate intercalated LDH (1, 3, 5 and 8 wt.%) Stearate-intercalated LDH (1, 3, 5, and 8 wt%) DS- and St-LDH-modified LDHs, isocyanate grafted DS- and St-LDH Dodecyl sulfate intercalated Mg-Al layered double hydroxide (1, 3, 5, 8 wt.%) Stearate intercalated-LDH Dodecyl sulfate-modified Co-Al-LDH NiAl-LDH/ZnO ZnAl-LDH/ZnO Reduced graphite oxide-LDH DS-intercalated MgAl-LDH DS-Mg
Al-LDH CNF-LDH LDH:MWCNT Heptamolybdate intercalated MgAl LDHs

EVA-18

DS-Mg-Al-LDH

EVA-28

DS-LDH

EVA-40

DS-LDH

EVA-45)/EPDM(1:1 wt. ratio) EVA-60

DS-Mg-Al-LDH DS-Mg-Al-LDH

In situ polymerization In situ polymerization In situ polymerization In situ polymerization In situ polymerization Blending method In situ polymerization Solution blending, Solution intercalation Solution intercalation Prepolymerization method Solution blending method Solution blending method Solution blending method Solution blending method Solution blending method

Kotal et al., 2013 Guo et al., 2011 Xiong et al., 2015 Zhang et al., 2013 Xu et al., 2016a Zhang et al., 2016 Roy et al., 2016b Roy et al., 2016b Roy et al., 2016c Xu et al., 2016b Kuila et al., 2008b Kuila et al., 2007 Kuila et al., 2009a Kuila et al., 2009b Kuila et al., 2009c (Continued)

Table 9.1 (Continued) Elastomer

Filler

Preparation method

References

EVA-28/LDPE

DS-Mg-Al-LDH

Kuila et al., 2008a

EVA-18

DS-Zn-Al-LDH

LDPE/EVA-18 EVA-28 EVA-19 EVA-14 EVA-28 EVA-14 SR SR SR

Hyperfine magnesium hydroxide (HFMH)/DS-MgAl- LDH HFMH/organomodified Mg-Al-LDH LDH-CO3 modified with 2-aminotoluene-5-sulfonic acid Al2O3 with LDH Stearate-modified Mg-Al-LDH with different Ni content Nano hydrotalcite St- Mg
Al-LDH DS-Mg-Al-LDH MWCNT-Li
Al-LDH, MWCNT-Mg
Al-LDH, MWCNTCo
Al-LDH DS-Mg-Al-LDH DS-Mg-Al-LDH Sodium-1-decanesulfonate-modified LDH DS- Mg-Al-LDH, undecylenic acid-Mg-Al-LDH,(UA-LDH), oleic acid-Mg-Al-LDH(LDH-OA) Zn-Mg-Al-LDH Zn-Al-LDH Mg
Al, Ni
Al, and Cu
Al LDHs Sodium dodecyl benzene sulfonate-Mg-Al-LDH Zn-Al-LDH

Solution blending method Melt and solution intercalation Melt compounding Melt intercalation Melt blending Melt blending Melt blending Melt blending Solution blending Solution blending Solution blending

EPDM EPDM EPDM, XNBR EPDM EPDM EPDM EPDM EPDM Solution styrene butadiene copolymer (SSB) NBR NBR Latex

Sodium p-styrene sulfonate modified Mg-Al LDH Mg-Al-LDH

Solution blending Solution blending Solution intercalation Melt mixing Two-roll mill mixing Two-roll mill mixing Twin-roll mill Two-roll mill mixing Open two-roll mill mixing Roll mixing mill Mixing method

Zhang et al., 2008 Ye et al., 2008 Zhang et al., 2007 Costache et al., 2007 Jiao et al., 2010 Wang et al., 2013 Jiao et al., 2006 Pradhan et al., 2012 Pradhan et al., 2011 Pradhan and Srivastava, 2014 Acharya et al., 2007a Acharya, 2008 Pradhan et al., 2008 Chao et al., 2013 Das et al., 2011a Basu et al., 2016 Zhao et al., 2014 Wang et al., 2012b Das et al., 2012 He et al., 2016 Braga et al., 2014

NBR NBR

Sodum lignosulfonate-modified Mg-Al-LDH Mg-Al-LDH

Melt compounding Roll mixing mill

NBR NBR XNBR

LDH modified by sodium styrene sulfonate and sodium dodecyl benzene sulfonate Unmodified LDH and stearate-modified LDH MgAl-LDH MgAl-LDHs

X-NBR

Mg-Al-LDH

Open two-roll mill mixing Two-roll mill mixing Brabender Measuring Mixer Two-roll mill mixing

X-NBR

Organically modified Mg-Al-LDH

X-NBR NR NR/BR

Zn-Al-LDH DS-Zn-Al-LDH LDH (modified and unmodified)

Two-step melt compounding method Two-roll mill mixing Mixing method Two-roll mill mixing

Xiao et al., 2014 Laskowska et al., 2013 Feng et al., 2013 Eshwaran et al., 2015 Laskowska et al., 2014a Laskowska et al., 2014b Costa et al., 2010

Basu et al., 2014 Abdullah et al., 2010 Bottazzo et al., 2013

354

Layered Double Hydroxide Polymer Nanocomposites

TPU/8 wt.% DS-LDH nanocomposite. All these observations clearly suggest that DS-LDH layers are partially or completely exfoliated at lower DS-LDH loadings, followed by intercalation at higher filler loadings. XRD patterns of TPU nanocomposites containing 1, 3, and 5 wt.% stearate intercalated Mg-Al-LDH (referred to as St-LDH) also exhibited similar behavior (Kotal et al., 2011). However, a comparatively broad peak appeared at 2θ  2.07 degrees in TPU/St-LDH (8 wt.%) corresponding to the interlayer spacing of 4.26 nm due to the intercalation of TPU chains within the interlayer spaces of St-LDH (Kotal et al., 2013). Kotal and Srivastava (2011b) also studied XRD patterns of PU/MDI- and PU/IPDI-grafted LDHs (pristine and organomodified) nanocomposites. These findings clearly demonstrated the intercalation of PU chains in the interlayer space of the MDI- or IPDIgrafted pristine LDHs. In contrast, (003) basal diffraction peak as well as the weak (110) of MDI-g-pristine LDH and IPDI-g-pristine LDH, were completely absent in 3 wt.% filled MDI- and IPDI-grafted DS and St-LDHs/PU nanocomposites. This suggested the formation of uniform dispersion and exfoliation of MDI- or IPDI-grafted DS-LDH or St-LDH layers in PU matrix. Kotal et al. (2010a) also made XRD studies on DS-LDH, neat PU and 1, 3, 5, and 8 wt.% DS-LDH-incorporated PU nanocomposites prepared by solution blending method. A diffraction peak (003) appeared at 2θ B 3.2 degrees corresponding to 2.75 nm basal spacing of DS-LDH layers. XRD of neat PU showed an absence of diffraction peak (003) in 2θ range, corresponding to the DS-LDH. This peak was also found to be absent in PU filled with 1 wt.% DS-LDH content due to the delamination or exfoliation of DS-LDH in the PU matrix (Xu et al., 2016a; Roy et al., 2016b) However, a slight hump appeared below 2θ B 3.2 degrees in the diffractograms of PU filled with 3, 5, and 8 wt.% DS-LDH, indicating partial exfoliation or intercalation of DS-LDH in PU. Guo et al. (2011) also studied XRD of neat PU and its nanocomposite consisting different loadings of CoAl-DS-LDH. The (003) basal peak of CoAl-DS-LDH is found to be absent in 0.5
2.0 wt.% filled PU indicating exfoliation of CoAl-DSLDH layers in PU matrix. XRD of PU/APS-DS-LDH nanocomposites showed absence of (003) basal diffraction peak of the nanocomposites (in the range from 1 to 3.0 wt.% LDH loading) due to the highly dispersed and/or exfoliated layers of APS-DS-LDH (Zhang et al., 2016). At 5.0 wt.% APS-DS-LDH loading in PU, (003) reflection slightly appears due to the aggregation of APS-DS-LDH. Kumar and Das (2010) observed delamination/complete exfoliation of organically modified Mg-Al-LDH layers in the polyurethane matrix.

9.4.1.1.2 TEM The morphology of 1, 5, and 8 wt.% filled DS-LDH and St-LDH nanocomposites of TPU was studied and corresponding TEM images are shown in Fig. 9.2 (Kotal et al., 2009) and Fig. 9.3 (Kotal et al., 2011), respectively. It is noted that a fraction of the DS-LDH layers is uniformly exfoliated in TPU/DS-LDH (1 wt.%) and TPU/ DS-LDH (5 wt.%) nanocomposites. Interestingly, in either case, intercalated tactoids of LDH crystallites, mostly in the form of thin platelets, are also present along with the exfoliated LDH layers, suggesting the formation of partially exfoliated

Mechancial and dynamical mechanical properties

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Figure 9.2 TEM images of (A) PU/DS-LDH (3 wt.%) nanocomposite and (B) PU/DS-LDH (8 wt.%) nanocomposite (Kotal et al., 2009). Source: Reproduced with permission from Wiley.

morphology. However, an intercalated structure consisting of parallel DS-LDH layers is developed at 8 wt.% filler loading in TPU. The TEM of TPU/St-LDH (1 wt.%) and TPU/St-LDH (5 wt.%) nanocomposites suggested exfoliation of StLDH layers throughout the TPU matrix involving both exfoliated layers and intercalated tactoids of LDH crystallites in the form of thin platelets, respectively. However, exfoliated layers/intercalated tactoids of LDH crystallites and the aggregation of St-LDH are observed at 8 wt.% loading throughout the TPU matrix. All these findings clearly suggested St-LDH to be more compatible than DS-LDH in the formation of more exfoliated layers at lower loading of St-LDH. TEM images of modified and unmodified LDH (3 wt.%)-filled polyurethane nanocomposites showed delamination of filler in the polyurethane matrix (Kumar and Das, 2010). TEM images of PU/MDI-g-pristine LDH, PU/IPDI-g-pristine LDH, PU/MDI-g-DSLDH, PU/IPDI-g-DS-LDH, PU/MDI-g-St-LDH, and PU/IPDI-g-St-LDH nanocomposites at 3 wt.% filler loading are displayed in Fig. 9.4 (Kotal and Srivastava, 2011b).

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Figure 9.3 TEM images at higher magnification of PU nanocomposites containing (A) 1 wt. % (B) 5 wt.%, and (C) 8 wt.% LDH
stearate (Kotal et al., 2011). Source: Reproduced with permission from Wiley.

Figure 9.4 TEM images of (A) PU/MDI-g-pristine LDH, (B) PU/IPDI-g-pristine LDH, (C) PU/MDI-g-DS-LDH, (D) PU/IPDI-g-DS-LDH, (E) PU/MDI-g-St-LDH, and (F) PU/IPDI-gSt-LDH nanocomposites showing the nature of dispersion of MDI- and IPDI-grafted pristine, DS, and St-LDH layers in PU matrix (Kotal and Srivastava, 2011b). Source: Reproduced with permission from Royal Society of Chemistry.

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It clearly indicated aggregation of MDI- and IPDI-grafted pristine LDH in PU (cf. Fig. 9.4A and B), while such aggregates are found to be absent in MDI- and IPDI-grafted DS-LDH/PU nanocomposites (Fig. 9.4C and D), respectively. However, some tactoids composed of several metal hydroxide sheets (indicated by the arrows) with thicknesses 10
30 nm are also present, along with exfoliated MDI-g-DS-LDH and IPDI-g-DS-LDH layers in PU matrix, resulting in the formation of partially exfoliated nanocomposites. Fig. 9.4 also provides an idea about the dispersion of MDI-g-St-LDH and IPDI-g-St-LDH in PU. MDI-g-St-LDH/PU exhibited relatively better exfoliation and homogeneous dispersion of LDH layers in the PU matrix compared to the IPDI-g-St-LDH. Morphology of PU/dodecyl sulfate-modified Mg-Al-LDH (DS-LDH) nanocomposites by in situ synthesis of PU from prepolymer and polyol TG was also studied by TEM analysis (Kotal et al., 2010a). The lower-magnification image showed inhomogeneous distribution of DS-LDH layers in PU matrix. However, the corresponding higher-magnification image confirmed the partial delamination of DS-LDH layers. This was ascribed to the polar interaction between DS-LDH layers and PU prepolymer/hydroxylfunctionalized polyol-TG (mixtures of glycerol and trimethylolpropane). TEM images of PU/DS-LDH (5 wt.%) nanocomposite suggested intercalation of DSLDH layers in PU matrix. However, agglomeration is observed at 8 wt.% loading of DS-LDH in PU matrix. Scheme 9.1 represents the formation of partially exfoliated and intercalated PU nanocomposites from pristine Mg
Al LDH and PU.

Scheme 9.1 Schematic diagram for the formation of partially exfoliated and intercalated PU nanocomposites from pristine Mg
Al LDH and PU (Kotal et al., 2010a). Reproduced with permission from American Scientific Publishers.

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TEM study of PU/APS-DS-LDH (3 wt.%) nanocomposites suggested the formation of partially exfoliated states for nanocomposites (Yu et al., 2013). However, stacking of APS-DS-LDH layers in the PU matrix was clearly visible in PU/APSDS-LDH (5 wt.%). Fractured surface morphology analysis of WPU/NiAl-LDHs/ ZnO composites showed homogeneous distribution of NiAl-LDH/ZnO (0.5, 1.0, 1.5 wt.%) (Xiong et al., 2015). Such uniform distribution of the filler in the WPU matrix plays an important role in improving the mechanical performance of the prepared composites. The increasing content of NiAl-LDHs/ZnO (2.0 wt.%) led to microphase separation due to closer contact of fillers in WPU in the composites. In another work, TEM image of LDH in PU nanocomposite containing 2 wt.% CoAlDS-LDH also confirmed exfoliation of CoAl-DS-LDH layers (Guo et al., 2011). TEM of waterborne hyperbranched polyurethane acrylate (WHPUA)/dodecyl sulfate-modified LDH nanocomposites based on hyperbranched aliphatic polyester boltorn and MgAl-LDH showed formation of intercalated and exfoliated structures (Wang and Pinnavaia, 1998).

9.4.1.2 EVA-LDH nanocomposites 9.4.1.2.1 XRD Srivastava and his group reported XRD studies of EVA-18/DS-LDH (Kuila et al., 2008b), EVA-28/DS-LDH (Kuila et al., 2007), EVA-40/DS-LDH (Kuila et al., 2009a), and EVA-60/DS-LDH (Kuila et al., 2009c). A sharp basal peak (003) appeared in XRD of DS-LDH (modified) and LDH (unmodified) corresponding to an interlayer distance of B2.76 and 0.78 nm, respectively. The observed increase of 1.98 nm in basal spacing of DS-LDH is due to the intercalation of dodecyl sulfate anion. XRD patterns of pure EVA-18, EVA-28, EVA-45, and EVA-60 exhibit no peak in the 2θ range of 2
10 degrees. But original (001) peak of DS-LDH is shifted to a lower angle in EVA-18/DS-LDH nanocomposites due to the intercalation of polymer chains within the interlayer spaces of DS-LDH. However, EVA containing 8 wt.% of DS-LDH clearly showed the presence of a peak at 2θ  2.5 degrees in all probability due to the aggregation of DS-LDH particles in the EVA18 polymer matrix. It is also noted that the original (003) peak of DS-LDH disappeared in 1 and 3 wt.% DS-LDH-loaded EVA-28, EVA-45, and EVA-60 composites. This indicates that the DS-LDH platelets are either randomly oriented or separated far enough to produce insignificant diffraction peaks. At higher DS-LDH loadings (5 and 8 wt.%), corresponding EVA-28, 45, and 60 nanocomposites showed the appearance of a broad diffraction peak (2θ  2.5 degrees), possibly due to the formation of partially exfoliated or intercalated structure. XRD data established formation of the exfoliated EVA/LDH nanocomposites by controlling the LDH loading of about 10% for the melt intercalation and 5% for the solution intercalation (Zhang et al., 2008). In another study, XRD of organic-modified LDH with hyperfine magnesium hydroxide (HFMH) in the halogen-free flame-retardant EVA (28% vinyl acetate content)/HFMH/LDH nanocomposites suggested exfoliation of LDH depending on its LDH loading in EVA (Zhang et al., 2007).

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9.4.1.2.2 TEM The dispersion of LDH layers in the EVA (vinyl acetate contents: 18, 28, 40, 45, 60) has been adequately investigated by TEM as displayed in Figs. 9.5
9.8 (Kuila et al., 2007, 2008b, 2009a,c; Kuila, 2009). TEM micrographs of EVA-18/DS-LDH (1 wt.%) composites revealed the very complex nature of their morphology. This indicated wide variation in the shapes/sizes of partially exfoliated and homogeneously dispersed LDH layers. The average thickness and lateral dimension of the DS-LDH layers in EVA-28/DS-LDH (1 wt.%) varied in the range of 6
8 and 40
60 nm, respectively. It is noted that the DS-LDH layers are homogeneously dispersed throughout the EVA matrix. EVA-45 loaded with 1 wt.% DS-LDH (thickness: about 1 nm; lateral size: 40
60 nm) showed random distribution of the stacked LDH layers and were consistent with XRD analysis. A completely delaminated morphology is observed in EVA-60 at 1 wt.% DS-LDH (thickness: 6
8 nm; lateral dimension: 30
40 nm). The morphology of EVA-18, 28, 45, and 60 loaded

Figure 9.5 TEM images of (A) EVA-18/DS-LDH (1 wt.%) and (B) EVA-45/DS-LDH (5 wt.%) nanocomposites (Kuila et al., 2008b). Source: Reproduced with permission from Wiley.

Figure 9.6 TEM images of EVA-28/DS-LDH (3 wt.%) nanocomposite (A) at low magnification, (B) at high magnification (Kuila et al., 2007). Source: Reproduced with permission from Wiley.

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Figure 9.7 TEM images of (A) EVA-40/DS-LDH (1 wt.%) nanocomposite (B) EVA-40/DSLDH (3 wt.%) nanocomposite, and (C) EVA-40/DS-LDH (5 wt.%) nanocomposite (Kuila et al., 2009a). Source: Reproduced with permission from Wiley Interscience.

with 8 wt.% DS-LDH shows DS-LDH nanolayers not only intercalated, but also aggregated in the EVA matrix. These findings also established that the tendency toward exfoliation increases with increasing vinyl acetate (VA) content in EVA/ LDH nanocomposites, a fact that has also been inferred based on XRD analysis (Kuila, 2009). It is also noted that the degree of exfoliation predominates at low filler (DS-LDH) loading. However, equilibrium shifts towards intercalation at higher filler loadings in EVA. Zhang et al. (2008) observed homogeneous nano-dispersed layers of ZnAl-LDH (5 wt.%) in EVA through TEM in solution as well as melt intercalation methods. In another work, the effects of Ni cations on Mg-Al LDH prepared by conventional hydrothermal treatment (CHT) and microwave hydrothermal treatment (MHT) were studied on the morphology of corresponding EVA nanocomposites through TEM (Wang et al., 2011). More agglomerations of MgAl-CHT and NiMgAl-CHT compared to those of MgAl-MHT and NiMgAl-MHT in the EVA matrix were noted in EVA/LDHs (20 wt.%). However, NiMgAl-MHT shows the presence of more homogeneous nano-dispersed layers in NiMgAl-MHT-EVA composite, indicating better compatibility of NiMgAl-MHT with EVA matrix.

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Figure 9.8 TEM images of (A) fully exfoliated EVA-60/DS-LDH (1 wt.%) nanocomposite, (B) partially exfoliated EVA-60/DS-LDH (3 wt.%) nanocomposite, (C) partially exfoliated EVA-60/DS-LDH (5 wt.%) nanocomposites, and (D) aggregated morphology of EVA-60/ DS-LDH (8 wt.%) nanocomposite (Kuila et al., 2009c). Source: Reproduced with permission from Wiley.

9.4.1.3 SR-LDH nanocomposites 9.4.1.3.1 XRD Pradhan et al. (2012) studied XRD patterns of St-LDH, neat SR, and its St-LDH-filled nanocomposites. The (001) diffraction peak (2θB1.7 degrees) in the St-LDH pattern corresponds to basal spacing almost eight times greater (5.3 nm) than that of pristine LDH (Nhlapo et al., 2008; Borja and Dutta, 1992). The expanded basal spacing of StLDH clearly indicated intercalation of stearate ions (C18) in the interlayers of LDH (Meyn et al., 1990; Kanoh et al., 1999; Itoh et al., 2003). As a result of such an anion exchange reaction, formation of a bilayer structure between the LDH layers takes place as illustrated in Scheme 9.2 (Pradhan et al., 2012). It is noted that the characteristic crystalline basal peak of St-LDH (001) is completely absent in the patterns of 1, 3, 5, and 8 wt.% filled LDH in SR. This clearly suggested exfoliation of St-LDH layers in the polymer matrix. XRD of SR/DS
Mg-Al-LDH (1, 3, 5, 8 wt.%)

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Scheme 9.2 Effect of chain length of carboxylate anion on the d-spacing of LDH layers (Pradhan et al., 2012). Reproduced with permission from Wiley.

Figure 9.9 TEM images of (A) SS3 nanocomposite and (B) SS8 nanocomposite (Pradhan et al., 2012). Source: Reproduced with permission from Wiley.

nanocomposites also indicated the formation of exfoliated hydroxide layers dispersed in the SR matrix on a nanometeric scale (Pradhan et al., 2011). Pradhan and Srivastava (2014) noted disappearance of the diffraction peaks corresponding to Li-AlLDH/SR, MWCNT/SR, and Li-Al-LDH/MWCNT hybrid fillers in the corresponding LDH/MWCNT/SR nanocomposites (Pradhan and Srivastava, 2014).

9.4.1.3.2 TEM Fig. 9.9 displays TEM images of 3% and 8% loaded St-LDH-filled SR nanocomposites (Pradhan et al., 2012). It is clearly evident that St-LDH layers (thickness:

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Figure 9.10 TEM images of (A) SR/DS-LDH (5 wt.%) and (B) SR/DS-LDH (8 wt.%) nanocomposites (Pradhan et al., 2011). Source: Reproduced with permission from Wiley.

1
2 nm; lateral size: 75
100 nm) are exfoliated and homogeneously dispersed in the 3 wt.% filled SR matrix. In contrast, SR nanocomposite with 8 wt.% filler showed aggregation, although the corresponding peak(s) are found to be absent in the XRD pattern. TEM images of SR/DS-LDH (5 wt.%) and SR/DS-LDH (8 wt.%) nanocomposites are displayed in Fig. 9.10 (Pradhan et al., 2011). It confirmed that DS-LDH layers are well dispersed in the SR matrix to form exfoliated nanocomposites. The thickness and length of the individual exfoliated DS-LDH layers were found to be B1.5
2.5 and 30
80 nm, respectively. This TEM image provided positive evidence for the nanoscale dispersion of DS-LDH layers in the SR matrix. However, 8 wt.% filler loaded SR showed aggregation to some extent, resulting in the partial exfoliation of the SR chains in the gallery of DS-LDH. Pradhan and Srivastava, 2014) studied morphology of 1 wt.% loaded Li-Al-LDH/MWCNTs, Mg-Al-LDH/MWCNTs, and Co-Al-LDH/MWCNTs (1%)/SR through TEM as displayed in Fig. 9.11. It is inferred that MWCNTs are mainly aggregated in the LiAl-LDH/MWCNT/SR composite as compact bundles. In contrast, in Co-Al-LDH/ MWCNT/SR composite, MWCNTs are individually distributed in SR matrix along with a few bundled nanotubes. Interestingly, MWCNTs are homogeneously distributed in the SR matrix in Mg-Al-LDH/MWCNT/SR composite.

9.4.1.4 EPDM/LDH nanocomposites 9.4.1.4.1 XRD XRD of EPDM/LDH composites with varying DS-LDH loading showed disappearance of the basal (00 l) diffraction peaks of DS-LDH, indicating the possible formation of exfoliated nanocomposites (Acharya et al., 2007a). Pradhan and others (2008) used wide-angle X-ray scattering (WAXS) to understand the extent of dispersion of a varying amount of LDH particles in EPDM. It was noted that the position of the reflections corresponding to LDH-C10 (1-decanesulfonate-modified Mg
Al LDH) remained almost unaltered in EPDM/LDH nanocomposites.

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Figure 9.11 TEM images of Li
Al-LDH/MWCNT (1%)/SR (A), Mg
Al-LDH/MWCNT (1%)/SR (B), and Co
Al-LDH/MWCNT (1%)/SR (C) (Pradhan and Srivastava, 2014). Source: Reproduced with permission from Elsevier.

9.4.1.4.2 TEM The higher-magnification TEM images of EPDM/LDH composite with 3 wt.% DSLDH revealed a partially exfoliated nanostructure (Acharya et al., 2007a). TEM micrographs of EPDM containing 7.5 phr of MgAl-LDH-C10 are shown in Fig. 9.12 (Pradhan et al., 2008). It clearly shows that filler particles are dispersed as exfoliated fragments (lateral dimension ,100 nm) as primary particles (lateral dimension of a few hundred nm to 1
2 μm) and as soft clusters of the primary particles (lateral dimensions over a few μm). The proportions of the last two forms are much higher than the exfoliated fragments. Furthermore, the higher-magnification image of EPDM/Mg-Al LDH revealed highly disordered geometry of the MgAlLDH particles. EPDM/LDH (2 phr)/flame retardant comprised of pentaerythritol, ammonium polyphosphate, and methyl cyanoacetate (38 phr) confirmed formation of an exfoliated structure (Wang et al., 2012b).

9.4.1.5 SBR/LDH nanocomposites 9.4.1.5.1 TEM The low- and high-magnification TEM images of SBR/LDH-lignin composites in Fig. 9.13 showed improved dispersion of LDH in the presence of lignin

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Figure 9.12 TEM micrographs of EPDM/LDH nanocomposites with LDH-C10 content of 5 phr (magnification bar 1 μm) (Pradhan et al., 2008). Source: Reproduced with permission from Elsevier.

(Xiao et al., 2013). TEM studies also showed a decrease in the size of LDH particles with increasing lignin loading in SBR. This is clear evidence of lignin playing an important role in the dispersion of LDH in SBR.

9.4.1.6 NBR-LDH and XNBR-LDH nanocomposites 9.4.1.6.1 XRD Feng and Su (2011) studied XRDs of NBR and NO3-LDH, sodium styrene sulfonate (SSS)-LDH, and SDBS-LDH-filled NBR composites. It is noted that the basal spacing (d003) of NO3-LDH (0.83 nm) remained more or less unaltered, even after mixing with NBR matrix. The basal spacing (d003) of SSS (sodium styrene sulfonate)-LDH in NBR/SSS-LDH composite increased to 2.69 nm compared to 1.87 nm in SSS-LDH due to partial intercalation of polymer chains. In contrast, basal spacing (d003) of sodium dodecyl benzene sulfonate (SDBS)-LDH decreased from 2.96 to 2.69 nm in NBR/SDBS-LDH composite. Long-Chao et al. (2011), based on XRD analysis, reported formation of the exfoliated or partly intercalated NBR/organomodifed LDH (OLDH)/organomodifed MMT (OMMT)/hyperfine magnesium hydroxide (HFMH) nanocomposites. X-ray diffraction analysis of XNBR/organically modified MgAl-LDH nanocomposites shows dispersion of LDH particles as primary particles, as exfoliated layers, and as soft clusters (Pradhan et al., 2008). Das et al. (2011a) observed formation of the ordered structure by introducing ZnMg-Al LDH in XNBR rubber matrix. WAXS analysis of vulcanized XNBR/LDH-C10 (C10H21SO3Na) and XNBR/LDH-C16

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Figure 9.13 TEM micrographs of LDH/SBR and lignin-LDH/SBR composites (high magnification): (A) 30LDH/SBR; (B) 30LDH-3lignin/SBR; (C) 30LDH-9lignin/SBR; (D) 30LDH15lignin/SBR. Xiao et al. (2013). Source: Reproduced with permission from Wiley.

(C16H33SO3Na) nanocomposite clearly indicated a different state of dispersions (Costa et al., 2010).

9.4.1.6.2 TEM According to Xiao et al. (2014), TEM study indicated improvement in the dispersion of LDH particles in the presence of sodium lignosulfonate (SLS) in NBR matrix. Long-Chao et al. (2011) observed homogeneous dispersion of organomodified LDH/organomodified MMT/HFMH in the NBR matrix. The morphology investigations through TEM of NBR-LDH-NO3 (L), NBR-SSS-LDH (SL), and NBR-SDBS-LDH (SDL) nanocomposites clearly revealed differences due to the different organic modifications of LDHs (Feng et al., 2013). LDH particles are mostly clusters in NBR/LDH nanocomposites. In the case of NBR/SDBS-LDH (SDL) nanocomposites, the proportion of clusters decreased and intercalated fragments predominated. The best dispersion was visualized in NBR/SSS-LDH (SL) nanocomposite with no clusters evident and LDH particle fragments homogeneously dispersed. The higher-magnification TEM image showed predominance

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of the extent of the exfoliated fragments with less or no cluster formation in XNBR/LDH (Das et al., 2011c). The appearance of some agglomerates in XNBR/ MgAl-LDH (30 phr) through SEM indicated not good dispersion of filler in polymer matrix (Laskowska et al., 2014a). TEM images of vulcanized XNBR/LDH-C10 showed the presence of a higher extent of “soft clusters” compared to vulcanized XNBR/LDH-C16 (Costa et al., 2010).

9.4.1.7 NR/LDH nanocomposites 9.4.1.7.1 XRD DS-modified Zn-Al-LDH-filled NR showed increased basal spacing corresponding to 1 wt.% (3.90 nm) and 5 wt.% (3.66 nm) compared to ZnAl-LDH-DS (interlayer spacing: 2.53 nm) (Abdullah et al., 2010). Bottazzo et al. (2013) also reported an XRD study of NR/BR (cis-1,4-polybutadiene)/unmodified LDH and NRBR/modified LDH composites.

9.4.1.7.2 TEM Abdullah et al. (2010) recorded a TEM micrograph for NR/ZnAl LDH-NO3 (7 phr) and compared it with NR/ZnAl LDH DS (7 phr). They noted exfoliation in addition to the intercalation of nanolayers of Zn Al-LDH-DS in NR/Zn Al-LDH-DS composite.

9.4.2 Morphology of elastomeric blend-LDH nanocomposites 9.4.2.1 PU blend-LDH nanocomposites 9.4.2.1.1 XRD Kotal et al. (2010b) studied 1, 3, 5, 8 wt.% DS-LDH-filled thermoplastic polyurethane/nitrile butadiene (TPU/NBR referred to as TN) rubber blends. The corresponding XRD of blend composites showed shifting of the characteristic basal reflection peak (003) of DS-LDH (2θ B 3.4 degrees ) to 2θ B 2.2, 2.4, 2.5, and 2.6 degrees, respectively. This was attributed to the intercalation of polymer chains within the Mg
Al layers of DS-LDH. Furthermore, XRD of PU/NBR/DS-LDH nanocomposites showed almost complete absence of the slight hump initially present in DS-LDH at 2θ B 6.79 degrees. WAXD patterns of TN blend and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt.% in SFCNT-MgAl-LDH (Roy et al., 2016c; Roy, 2017) and SFCNTZnAl-LDH (Roy et al., 2016a; Roy, 2017) hybrids have also been investigated. The appearance of a broad peak at 2θ B 20 degrees in the XRD pattern of TN matrix is attributed to the amorphous phase of TPU along with short-range regular ordered structure of the hard and soft segment and the characteristic amorphous state of NBR. The absence of (003) and (110) planes of SFCNT-LDH hybrid in blend nanocomposites suggested the possibility of partial exfoliation of hybrid filler in the TN matrix. An XRD study also showed that the position of the TN peak at 2θ B 20 degrees remains almost unaltered in SFCNT-LDH hybrid filled nanocomposites. The calculations of full width at half-maximum values of neat TN and its

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nanocomposites suggested more ordered structure in SFCNT-LDH hybrid filled TN nanocomposites compared to neat TN. The WAXD pattern were recorded for TN and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt.% of both SFCNFMgAl-LDH (Roy et al., 2016b; Roy, 2017) and SFCNF-ZnAl-LDH (Roy et al., 2016a; Roy, 2017) hybrids. It is noted that a broad peak appeared in the hybridfilled TN nanocomposites with its position more or less identical with respect to neat TN. However, the absence of (003), (006), and (002) reflections of SFCNFLDH hybrid in TN nanocomposites indicated the formation of partially exfoliated TN nanocomposites.

9.4.2.1.2 TEM TEM images of PU/NBR blend nanocomposites containing 1, 3, and 8 wt.% DSLDH are displayed in Fig. 9.14A
C (Kotal et al., 2010b). It is inevitable that the DS-LDH (1 wt.%) layers are partially dispersed throughout the polymer matrix, suggesting the formation of partially exfoliated nanocomposites. A TEM image of PU/NBR containing 3 wt.% DS-LDH loading depicted the presence of both

Figure 9.14 TEM images of (A) PU/NBR/DS-LDH (1 wt.%) nanocomposite, (B) PU/NBR/ DS-LDH (3 wt.%), nanocomposite and (C) PU/NBR/DS-LDH (8 wt.%) nanocomposite (Kotal et al., 2010b). Source: Reproduced with permission from Wiley.

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Figure 9.15 TEM images of TPU/NBR nanocomposites containing (a) 0.50 and (b) 1 wt.% SFCNT-LDH hybrid (Roy et al., 2016c). Source: Reproduced with permission from Wiley.

exfoliated layers and intercalated tactoids of LDH crystallites. However, 8 wt.% DS-LDH loaded in PU/NBR exhibited aggregation of DS-LDH layers in the blend matrix. Fig. 9.15 shows an HRTEM image of TPU/NBR nanocomposites containing 0.50 and 1 wt.% of the SFCNT-Mg-Al LDH (Roy et al., 2016c). The presence of an interconnected SFCNT-LDH network is clearly demonstrated throughout the TPU/NBR matrix. The fine dispersion of 0.50 wt.% SFCNT-LDH hybrid filler in the TPU/NBR matrix is also visible. However, aggregation of blend nanocomposite is observed in TPU/NBR/SFCNT-LDH (1.0 wt.%) nanocomposite. HRTEM images of TPU/NBR nanocomposites containing 0.50 and 1 wt.% of both the SFCNTZnAl-LDH and SFCNF-ZnAl-LDH hybrid fillers are also displayed in Fig. 9.16 (Roy et al., 2016a). It is evident that 0.50 wt.% SFCNT-ZnAl-LDH and SFCNFZnAl-LDH hybrids form an interconnected network uniformly spread throughout the TN matrix. In contrast, 1.0 wt.% hybrid filler loaded in TN is found to undergo aggregation. All these findings clearly suggest that enhanced properties could be achieved by incorporating 0.50 wt.% hybrid filler in TN matrix.

9.4.2.2 EVA blend-LDH and EPDM blend-LDH nanocomposites 9.4.2.2.1 XRD Kuila et al. (2009b) carried out an XRD study of EVA/EPDM/DS-LDH prepared by solution blending. The absence of a basal peak of 1 and 3 wt.% DS-LDH-filled in EVA/EPDM blend suggested the formation of a completely exfoliated or partially exfoliated structure. However, a very weak and broad peak appeared at 2θ , 3 degrees at higher DS-LDH filler loadings (5 an 8 wt %) in EVA/EPDM. This could be attributed to the formation of partially exfoliated nanocomposites. Kotal (2012; Kuila et al., 2008a) made an XRD study of EVA/LDPE/DS-LDH (EVA/LDPE ratio:30/70, 50/50, 70/30) consisting of 1, 3, 5, and 8 wt.% of DSLDH. The close observation of the XRD patterns of the composites suggests the maximum degree of exfoliation in the EVA/LDPE (50/50) blend. This is possibly

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Figure 9.16 HRTEM images of TN nanocomposites containing (a) 0.50, (b) 1 wt. of SFCNT-LDH hybrid, and (c) 0.50, (d) 1 wt.% of SFCNF-LDH hybrid (Roy et al., 2016a). Source: Reproduced with permission from Springer.

due to its better compatibility compared to 30:70 and 70:30 EVA/LDPE blends. EVA/LDPE/DS-LDH nanocomposites with EVA/LDPE ratios of 30:70 and 70:30 with 1 and 3 wt.% of DS-LDH content show no diffraction peaks. However, a small and smooth hump appeared at 2θ , 2.5 degrees in the composites consisting of 5 and 8 wt.% of DS-LDH in the EVA/LDPE blend. This was ascribed to the intercalation of polymer chains inside the interlayer gallery of DS-LDH. Therefore, it was concluded that exfoliation of DS-LDH layers occurs at lower filler loadings (1 and 3 wt.%) and polymer chains intercalated or partially exfoliated at high loadings (5 and 8 wt.%). These observations clearly suggested that the tendency toward intercalation increases with increasing DS-LDH content in the EVA/LDPE blend. However, the possibility of aggregation of DS-LDH layers at higher loadings cannot be ruled out. XRD of PP/EPDM/intumescent flame retardancy (IFR) of phosphorus nitrogen (NP) compound/organically modified Mg-Al LDH suggested formation of intercalated and/or exfoliated nanocomposites (Cui and Qu, 2010).

9.4.2.2.2 TEM A TEM image of EVA/EPDM/DS-LDH (3 wt%) showed that DS-LDH layers are dispersed homogeneously in a disordered fashion in the EPDM matrix

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Figure 9.17 TEM images of EVA/LDPE (50:50) nanocomposites with (A) 3 wt.% DS-LDH content at lower magnification, (B) at higher magnification, and (C) 8 wt.% DS-LDH content (Kuila et al., 2008a). Source: Reproduced with permission from Elsevier.

(Kuila et al., 2009b). The thickness and lateral size of the exfoliated LDH layers correspond to 4
6 and 30
50 nm, respectively. These observations are found to be in good agreement with XRD analysis demonstrating the formation of partially exfoliated EVA/EPDM/DS-LDH nanocomposites. Ye et al. (2008) observed LDH layers undergoing exfoliation in LDPE/EVA/HFMH/LDH. They also suggested these layers act as synergistic compatibilizer and dispersant to make the HFMH particles homogeneously dispersed in the LDPE matrix. Kuila et al. (2008a) also studied the morphology of EVA/LDPE (50:50)/DS-LDH composites consisting 3 and 8 wt.% of DS-LDH and corresponding typical TEM images are displayed in Fig. 9.17. The grayish-white areas represent the EVA/LDPE matrix and the black area represents the DS-LDH layers. The TEM image showed more or less homogeneous dispersion of DS-LDH layers throughout the polymer blend matrix. However, a higher-magnification image confirmed the presence of disorderly oriented LDH layers in blend. This provided ample evidence for the formation of delaminated/exfoliated nanocomposites. The thickness and lateral sizes of the exfoliated LDH layers in EVA/LDPE/DS-LDH (3 wt.%) and EVA/LDPE/DS-LDH

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Layered Double Hydroxide Polymer Nanocomposites

(8 wt.%) nanocomposites correspond to 8
10 and 30
40 nm, respectively. In addition, there also exist some aggregates or stacking of DS-LDH layers in these nanocomposites (as indicated by arrows). This may be explained similar to that observed on LDPE/Mg-Al layered double hydroxide nanocomposites as reported by Costa et al. (2007). According to this, more and more polymer chains enter the interlayer region with time forcing the delamination of the surface layers one by one from the surface of large organomodified LDH particles. TEM images of the EVA/LDPE/DS-LDH (8 wt.%) confirmed not only aggregated but also intercalated/ partially exfoliated DS-LDH layers present in the EVA/LDPE matrix. TEM of PP/ EPDM/IFR/LDH confirmed findings of XRD analysis on the formation of the intercalated and/or exfoliated nanocomposites (Cui and Qu, 2010).

9.5

Mechanical properties of elastomer-LDH and elastomeric blend-LDH nanocomposites

The conventional inorganic fillers have already been used extensively as conventional fillers in enhancing the mechanical properties of polymers. However, the introduction of inorganic nanofillers in polymer and polymer blends marks a significant improvement in their mechanical properties. Therefore, the search for novel materials acting as reinforcing fillers in polymer
matrix composites exhibiting superior mechanical properties compared to neat polymers remains a big challenge today. Considering this, enhancement in mechanical properties, such as tensile strength (TS), elongation at break (EB), Young’s modulus, toughness, ductility, etc. of important elastomer/LDH and elastomer blend/LDH nanocomposites is reviewed below.

9.5.1 Mechanical properties of elastomer-LDH nanocomposites 9.5.1.1 PU-LDH nanocomposites Figs. 9.18(A, B) show the variation of TS and EB of PU filled with varying concentrations of dodecyl sulfate (DS)-LDH (Kotal et al., 2009) and stearate (St)LDH (LDH:MgAl-LDH) (Kotal et al., 2011), respectively. It is noted that the TS and EB of all the TPU nanocomposites is in general always greater with respect to neat TPU. TS of 3 wt.% of DS-LDH and 1 wt.% of St-LDH loaded neat TPU results in maximum improvements by 67% and 45%, respectively. This is due to the strong interfacial interaction between polar hydroxyl groups of St-LDH (and DS-LDH) and polar urethane groups of TPU as displayed in Scheme 9.3. The EB of TPU/DS-LDH (3 wt%) and TPU/St-LDH (3 wt%) showed maximum improvement by 27% and 53%, respectively. This is in all probability due to the entanglement of the TPU chains, chain slippage, and platelet orientation of DS-LDH and St-LDH layers. However, relatively higher values of EB at all St-LDH loadings in TPU are possibly due to the plasticization effect of the long alkyl chain of St-LDH.

(A) 65 800

55

750

50 700

45 40

650 Elongation at break (%) Tensile strength (MPa)

35 30

0

2 4 6 DS-LDH content (wt %)

8

600

1000

60

950

56

900 52 850 48

800 750

44 Tensile strength

40

700

Elongation at break

Elongation at break (%)

Tensile strength (MPa)

(B)

Elongation at break (%)

Tensile strength (MPa)

60

650

36 0

2 4 6 LDH-stearate content (wt %)

8

600

Figure 9.18 (A) Variation of tensile strength (TS) and elongation at break (EB) of PU/DSnanocomposites with DS-LDH content (Kotal et al., 2009). (B) Variation of TS and EB of PU/LDH
stearate nanocomposites with LDH-stearate content (Kotal et al., 2011). Source: Reproduced with permission from Wiley.

Scheme 9.3 Formation of hydrogen bonding between LDH-stearate and PU chains (Kotal et al., 2011). Reproduced with permission from Wiley.

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Layered Double Hydroxide Polymer Nanocomposites

The improvements in mechanical properties have also been correlated on the basis of fracture surface behavior of TPU/DS-LDH and TPU/St-LDH nanocomposites using SEM. TPU/Mg-Al-DS-LDH nanocomposites showed maximum improvements in TS (407%) and EB (159%) at 3 and 8 wt.% DS-LDH loading, respectively (Kotal et al., 2010a). The increase in TS may be ascribed to the strong interfacial interaction between the hydroxyl group of DS-LDH and polar urethane groups of polyurethane resulting in good compatibility between the two components. In other words, the high surface area of LDH and formation of partially exfoliated structure enhances the interfacial interaction through bridge, loop, and tail linkages of the polymer chains with the DS-LDH layers via hydrogen bonding and polar
polar interactions (Scheme 9.4). The mechanical property investigations also showed an increase in toughness with increasing DS-LDH loading up to 3 wt.% and then decreases in TPU/Mg-Al-DS-LDH nanocomposites

Scheme 9.4 Proposed mechanism for interactions between DS-LDH and PU chains (Kotal et al., 2010a). Reproduced with permission from American Scientific Publishers.

Kotal and Srivastava (2011b) investigated the effect of interlamellar grafting of pristine and organomodified LDHs in the corresponding PU nanocomposites. The corresponding tensile strength, elongation at break, and modulus at different percentages of strain are presented in Table 9.2. It is observed that isocyanate-grafted organo-LDHs impart an improvement in tensile strength as well as elongation at break for PU nanocomposites. Table 9.2 also shows the maximum improvement in

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Table 9.2 Mechanical data of polyurethane and its nanocomposites with 3 wt.% MDI- and IPDI-grafted LDH, DS-LDH, and St-LDH Modulus (MPa)

PU PU/IPDI-g-LDH PU/MDI-g-LDH PU/IPDI-g-DSLDH PU/MDI-g-DSLDH PU/IPDI-g-StLDH PU/MDI-g-StLDH PU/DS-LDH PU/St-LDH

TS/MPa

EB (%)

100%

200%

300%

0.28 6 0.01 0.32 6 0.01 0.35 6 0.01 0.40 6 0.02

0.41 6 0.01 0.43 6 0.03 0.4 6 50.03 0.53 6 0.03

0.54 6 0.01 0.56 6 0.02 0.58 6 0.02 0.70 6 0.02

1.10 6 0.6 1.76 6 0.3 2.48 6 0.7 3.75 6 0.8

626 6 27 850 6 32 800 6 15 900 6 40

0.43 6 0.02

0.60 6 0.03

0.80 6 0.02

4.07 6 0.4

872 6 20

0.48 6 0.03

0.65 6 0.02

0.84 6 0.03

4.60 6 0.6

906 6 22

0.62 6 0.02

0.88 6 0.03

1.14 6 0.02

5.40 6 0.5

822 6 29

0.38 6 0.01 0.35 6 0.02

0.49 6 0.02 0.52 6 0.03

0.64 6 0.03 0.67 6 0.03

3.20 6 0.7 2.60 6 0.4

866 6 24 895 6 35

Source: Modified from Kotal, M., Srivastava, S.K., 2011b. Synergistic effect of organomodification and isocyanate grafting of layered double hydroxide in reinforcing properties of polyurethane nanocomposites. J. Mater. Chem. 21, 18540
18551, reproduced with permission from the Royal Society of Chemistry.

tensile strength achieved in PU/3 wt.% of MDI-grafted St-LDH (391%) and PU/ 3 wt.% of IPDI-grafted St-LDH (318%). Such improvements in the tensile strength may also be attributed to the homogeneous dispersion of IPDI-grafted St-LDH and MDI-grafted St-LDH layers in PU matrix. It is also noted that the tensile strength of MDI-g-DS-LDH/PU and IPDI-g-DS-LDH/PU nanocomposites is improved by 270% and 241%, respectively. These mechanical studies also established a greater improvement in tensile strength in PU/isocyanate-grafted organomodified LDHs than the individual grafted or organomodified LDH/PU nanocomposites. Table 9.2 also shows that MDI-grafted LDHs/PU nanocomposites in comparison with their corresponding IPDI-grafted LDHs/PU nanocomposites exhibit relatively lower elongation at break. However, MDI-g-St-LDH/PU nanocomposite showed maximum decrease in the percent of elongation (31.3%). This is possibly due to the fact that the PU chains in the nanocomposites are more restricted by the MDI-grafted StLDH layers owing to their increased stiffness as well as crosslink density. PU/ CoAl-DS-LDH nanocomposites always showed superior Young’s modulus and tensile strength compared to neat PU (Guo et al., 2011). The maximum enhancement in Young’s modulus and tensile strength is observed in PU/CoAl-DS-LDH (2.0 wt.%) corresponding to B149% and 89%, respectively. This is ascribed to the exfoliation of LDH nanolayers in the PU matrix and strong interfacial interactions between hydroxyl groups of CoAl-DS-LDH and polar group of PU chains. The improvement in elongation at break in PU/LDH (0.5 wt.%) nanocomposite may be ascribed to the synergistic effect of platelet orientation and chain slippage. However, reduced mechanical properties of PU/CoAl-DS-LDH (5.0 wt.%) could be due to the aggregation of CoAl-DS-LDH nanolayers. According to Yan et al. (2013),

376

Layered Double Hydroxide Polymer Nanocomposites

1.05

800 Tensile strength (MPa) Elongation at break (%)

Tensile strength (MPa)

600 0.95

500 400

0.90 300 0.85

Elongation at break (%)

700 1.00

200 100

0.80 0

1

2 3 LDH-content (wt%)

4

5

Figure 9.19 Variation of tensile strength (MPa) and elongation at break (%) of PU/APS-DSLDH nanocomposites with different contents of APS-DS-LDH (Zhang et al., 2016). Source: Reproduced with permission from Royal Society of Chemistry.

increasing DS-LDH content in WPU increased the hardness and tensile strength of WPU/DS-LDH nanocomposite. In another study, mechanical properties showed improvements in polyurethane/NiAl-LDH/ZnO composites (Xiong et al., 2015). Zhang et al. (2013), incorporated ZnAl-LDH/ZnO in WPU and observed profound effect on the mechanical strength of WPU/ZnAl-LDHs/ZnO nanocomposite. This was ascribed to the good dispersion of ZnAl-LDH/ZnO in the WPU matrix. Zhang et al. (2016) studied the mechanical properties of PU/LDH nanohybrids. Fig. 9.19 shows the variation of tensile strength and elongation at break (%) of PU/APS-DSLDH nanocomposites. It is noted that EB is maximally increased by 430% for PU nanocomposites by incorporating 3 wt.% APS-DS-LDH loading. This has been attributed to interfacial bonding between APS-DS-LDH and PU matrix. This makes the PU/APS-DS-LDH hybrid less susceptible to breaking during extension. Alternatively, this may also be due to the plasticization effect of the long alkyl-chain of the intercalated DS-LDH, resulting in a flexible matrix. TS of PU/LDH nanocomposites was also maximally enhanced at 3 wt.% filler loading. This is ascribed to a stronger interfacial interaction between the
OH and
NH2 groups of APS-LDH and the
NHCOO group of PU. They also observed a slight decrease in modulus of PU/APS-LDH nanocomposites compared to neat PU. It was suggested that long hydrocarbon segments of APS and DS play a key role in the improvements of the matrix flexibility over the interfacial interaction between LDH and PU. Kotal and Srivastava (2011b) reported that the modulus of PU/MDI-g-St-LDH increased significantly compared to MDI-g-DS-LDH/PU nanocomposites. This is probably due to the nano-reinforcement effect/development of shear zones in nanocomposites under stress and strain conditions or high aspect ratio of LDH (Kotal et al., 2009).

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9.5.1.2 EVA/LDH nanocomposites The mechanical properties of LDH nanocomposites of EVA, consisting of different amounts of vinyl acetate content, have been studied (Kuila et al., 2007, 2008b, 2009c). The corresponding variation in tensile strength and elongation at breakage of EVA with varying DS-LDH is displayed in Fig. 9.20. It is noted that the tensile strength of 1 wt.% DS-LDH-filled EVA-18 and EVA-28 are maximally enhanced compared to neat EVA-18 and EVA-28, respectively. Such an improvement in tensile strength is likely due to the strong interfacial interaction between the polar hydroxyl groups present in the DS-LDH and the polar acetate groups of EVA. However, further addition of DS-LDH in EVA caused a reduction in TS due to aggregation of filler. It is also noted that elongation at break first decreases with the addition of DS-LDH in EVA-18 and EVA-28 followed by an increase at higher filler level. Such an improvement in elongation at break may be due to the platelet orientation of LDH nanolayers. In contrast, tensile strength as well as elongation at break increased with increased DS-LDH in EVA-45/DS-LDH and EVA-60/DSLDH nanocomposites due to the strong interfacial interaction between the polymer chains and DS-LDH. The tensile strength as well as elongation at break increased up to 5 wt.% DS-LDH content in EVA-45/DS-LDH nanocomposites. Beyond 5 wt. % of DS-LDH loadings, TS and EB of EVA-45 showed a decreasing trend. The tensile strength of EVA-60/DS-LDH nanocomposites also showed an increasing trend up to 3 wt.% of DS-LDH content. At higher filler loading (5 and 8 wt.%), a gradual decrease in TS is observed, though in either case the TS values are higher compared to neat EVA. Such a reduction in TS is associated with the extended aggregation of DS-LDH layers and augmented with increasing DS-LDH content in EVA. It is also evident from Fig. 9.20 that the addition of DS-LDH in EVA-60 results in an increase in the elongation at break and is at a maximum for 8 wt.% DS-LDH in EVA-60. Such improvements in EB may be due to the extensive entanglement of the crosslinkable polymer chains and synergistic effect of platelet orientation and chain slippage. The improvements in mechanical properties of EVA/DS-LDH nanocomposites have been correlated in terms of fracture behavior of the nanocomposites using SEM. The fracture surface morphology of neat EVA-18, EVA-45, and their corresponding nanocomposites consisting of 5 wt.% DS-LDH are shown in Fig. 9.21 (Kuila et al., 2008b). The fractured surface image of neat EVA-18 clearly shows the presence of some cracks, while their nanocomposites with 5 wt.% DS-LDH content do not show any prominent cracks. This is possibly due to the formation of some shear zones which reduce the formation of cracks and ultimately toughen the nanocomposite materials. A completely different fracture surface morphology is observed in EVA-45/DS-LDH (5 wt.%) compared to neat EVA-45. Considering the tensile mechanical data, it seems that the rougher the fracture surface, the better the mechanical properties of the related nanocomposites. Such a surface morphology variation is possibly due to the deviation of tear. It is also inferred from the SEM images that DS-LDH is compatible with EVA-45 and possibly its platelet orientation is likely to account for the improved mechanical properties in the corresponding nanocomposites.

378

Layered Double Hydroxide Polymer Nanocomposites

1000

(A) 32

950 900

28

850

26

800

24

Tensile strength Elongation at break

22 20

0

2

4

6

8

750

Elongation at break (%)

Tensile strength (MPa)

30

700

LDH content

(B)

950

50

Tensile strength (MPa)

45

925

40 900 35 875

Elongation break (%)

Tensile strength Elongation at break

30

850

25 0

2

4

6

8

DS-LDH content (wt %)

Figure 9.20 (A) Variation of TS and EB (%) of EVA-18/DS-LDH nanocomposites (Kuila et al., 2008b). (B) TS and EB of EVA-28/DS-LDH nanocomposites (Kuila et al., 2007). (C) Variation of tensile strength and elongation at break with DS-LDH contents for EVA-40/DSLDH nanocomposites (Kuila et al., 2009a). (D) Variation of tensile strength and elongation at break with 0, 1, 3, 5, and 8 wt.% DS-LDH contents in EVA 60 matrix (Kuila et al., 2009c). Source: Reproduced with permission from Wiley.

According to Wang et al. (2012a), the tensile strength of EVA composites filled with 2 wt.% of rare earth-doped Ni-containing LDHs (S-Ni0.1MgAl-La, S-Ni0.1MgAlCe, S-Ni0.1MgAl-Nd) and S-Ni0.1MgAl (surface-modified Ni-containing LDHs) was significantly increased compared to pristine EVA. The elongation at break values increased only for the EVA/rare earth-doped Ni-containing LDHs (2 wt.%) with respect to pristine EVA. In other investigations, although LDHs reduced the elongation at break and the tensile strength of the composites, the addition of nickel

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379

(C)

1000

14

900 10

850 800

8 Tensile strength Elongation at break

Elongation at break (%)

Tensile strength (MPa)

950 12

750

6 0

2

4 6 LDH content (wt %)

8

700

(D)

6

2000 1800

5 1600 4

1400

1200

3

Elongation at break (%)

Tensile strength (MPa)

Tensile strength Elongation at break

1000 2 1

3 5 DS-LDH content (wt %)

8

Figure 9.20 (Continued).

ions enhanced the ductility and strength of the composites in comparison with S-Mg3.0Al/EVA (Wang et al., 2013). EVA-NMH (nano-magnesium hydroxide) exhibited better mechanical properties than EVA-NLDH (nanohydrotalcite) (Jiao et al., 2006).

9.5.1.3 SR/LDH nanocomposites Pradhan et al. (2012) studied the variation of TS and EB of SR nanocomposites with varying St-LDH concentration. It is noted that SR/St-LDH (3 wt%) nanocomposite attained maximum improvement of tensile strength (97 %) and elongation at break (43%) compared to neat SR. Such improvements in the mechanical properties

380

Layered Double Hydroxide Polymer Nanocomposites

Figure 9.21 Tensile fracture surface morphology of (a) EVA-18, (b) EVA-18/DS-LDH (5 wt.%), (c) EVA-45, (d) EVA-45/DS-LDH (5 wt.%) (Kuila et al., 2008b). Source: Reproduced with permission from Wiley.

are possibly due to the larger aspect ratio of the LDH layers and interaction between-OH functionality of St-LDH layers and Si
O polar groups of SR matrix, homogeneity of the SR nanocomposites, and easier exfoliation (Shieh et al., 2010; Fornes et al., 2004). The improvement of elongation at break of St-LDH/SR nanocomposite may be attributed to platelet orientation of St-LDH or chain slippage, or both, in the SR matrix. Young’s modulus of St
LDH (3 wt.%)/SR nanocomposite was found to be 55% higher than that for the pure SR. They also correlated improvements in mechanical properties in terms of fracture behavior of the St-LDH/SR nanocomposites. Fig. 9.22(A
C) represents the stress
strain plots of SR filled with different loadings (0, 0.5, 0.75, 1.0, and 1.5 wt.%) of Li-Al-LDH/MWCNT, Mg-Al-LDH/ MWCNT, and Co-Al-LDH/MWCNT, respectively (Pradhan and Srivastava, 2014). It is noted that tensile strength and elongation at break of the hybrid-filled/SR composites are always higher compared to neat SR, the maximum improvements are observed for 1 wt.% of Mg-Al-LDH/MWCNT, Li-Al-LDH/MWCNT, and Co-AlLDH/MWCNT-loaded SR. Such improvements in the mechanical properties of SR are ascribed to the synergistic effect of 1D MWCNTs and 2D LDH fillers.

9.5.1.4 EPDM/LDH nanocomposites Acharya et al. (2007a) achieved superior tensile strength of EPDM/LDH nanocomposites compared to that of neat EPDM. Such an enhancement in the tensile

Mechancial and dynamical mechanical properties

381

Stress (MPa)

(A)

0.4 a 0.2 0.0

0

50

Stress (MPa)

(B)

100 150 Strain (%)

200 d

e c b

0.6 0.4

a 0.2 0.0

0

50

(C)

100 150 Strain (%)

200

0.8 Stress (MPa)

d

e c b

0.6

d c

e

0.6

b

0.4

a

0.2 0.0

0

50

100 150 200 Strain (%)

Figure 9.22 (A) Stress
strain plots (a) neat SR, (b) Li-Al-LDH/MWCNT (0.5 wt.%)/SR, (c) Li-Al-LDH/MWCNT (0.75 wt.%)/SR, (d) Li-Al-LDH/MWCNT (1.0 wt.%)/SR, and (e) LiAl-LDH/MWCNT (1.5 wt %)/SR (Pradhan and Srivastava, 2014). (B) Stress
strain plots (a) neat SR, (b) Mg-Al-LDH/MWCNT (0.5 wt.%)/SR, (c) Mg-Al-LDH/MWCNT (0.75 wt. %)/SR, (d) Mg-Al-LDH/MWCNT (1.0 wt.%)/SR, and (e) Mg-Al-LDH/MWCNT (1.5 wt. %)/SR (Pradhan and Srivastava, 2014). (C) Stress
strain plots (a) neat SR, (b) Co-Al-LDH/ MWCNT (0.5 wt.%)/SR, (c) Co-Al-LDH/MWCNT (0.75 wt.%)/SR, (d) Co-Al-LDH/ MWCNT (1.0 wt.%)/SR, and (e) Co-Al-LDH/MWCNT (1.5 wt.%)/SR (Pradhan and Srivastava, 2014). Source: Reproduced with permission from Elsevier.

strength value with the addition of LDH may be ascribed to the strong interfacial interaction between EPDM and DS-LDH. Furthermore, partially exfoliated rigid DS-LDH layers efficiently transfer stress from the polymer and directly enhance the stiffness in the corresponding nanocomposites. Interestingly, EB also increases with the DS-LDH content in EPDM. Such an increase in EB could be attributed to the platelet orientation, chain slippage, or plasticization. However, TS and EB both increase slowly at higher DS-LDH loadings in EPDM, owing to the aggregation of

382

Layered Double Hydroxide Polymer Nanocomposites

the filler. It is also noted that the modulus increases with increasing LDH content in all EPDM nanocomposites. Such an increase in modulus could be attributed to the resistance exerted by the sterically hindered LDH surface itself, strong polymer
filler interaction, or due to the formation of microvoids around DS-LDH nanolayers. Pradhan et al. (2008) observed significant improvement in the mechanical properties of EPDM/MgAl-LDH nanocomposites. Organomodified (sodium dodecyl sulfate, undecylenic acid, oleic acid) layered double hydroxide-filled EPDM composites showed more improved tensile strength and elongation modulus (Chao et al., 2013). Das et al. (2011a) reported tensile strength of NBR vulcanizates cured by LDH to be comparable with the vulcanizates cured by conventional approach. Zhao et al. (2014) reported that 10 phr of Cu-Al-LDH-filled EPDM composite resulted in no obvious changes to tensile strength compared to that of pure EPDM matrix.

9.5.1.5 SBR/LDH nanocomposites The tensile strength and modulus of lignin-LDH/SBR nanocomposites increased from 4.0 to 10.9 MPa following the lignin content in the range 0
9 phr (Xiao et al., 2013). It was suggested that better dispersion of LDH particles in the presence of lignin in SBR accounts for such enhanced mechanical properties of lignin-LDH/ SBR nanocomposites. Das et al. (2012) reported that tensile strength, elongation at break, and 100% modulus of SBR increased with increasing filler loading up to 80 phr.

9.5.1.6 NBR/LDH and XNBR/LDH nanocomposites He et al. (2016) studied the stress
strain behavior of different NBR composites (unmodified NBR, NBR with 7 phr LDH-NO3, and NBR with 7 phr LDH-SSS) before and after aging. It was noted that NBR/LDH-NO3 shows a better tensile strength and elongation at failure before aging than unmodified NBR and NBR/ LDH-SSS, with a decrease in value with increasing LDH-NO3 content. Nevertheless, tensile strength and elongation at failure of NBR/LDH-SSS after aging are higher than those of NBR/LDH-NO3 and unmodified NBR. NBR/organomodified LDH (OLDH)/organomodified MMT (OMMT)/hyperfine magnesium hydroxide (HFMH) nanocomposites showed improvement in tensile strength and stress at 100% elongation (Long-Chao et al., 2011). Xiao et al. (2014) observed improved mechanical properties in LDH/NBR and sodium lignosulfonate (SLS)LDH/NBR composites compared to pristine NBR. In another work, NBR/sodium styrene sulfonate (SSS)-modified LDH, which exhibited two times higher tensile strength than cured pure rubber without significant loss of elongation, was obtained (Feng et al., 2013). Eshwaran et al. (2015) studied the stress
strain behavior of NBR filled with unmodified commercial LDH, modified commercial LDH, and modified synthetic LDH. It was noticed that stretched nanocomposites follow two different patterns. The first region (below 300% strain) of the plot corresponds to the broken local filler network, disturbance of the chain conformation, Mullins

Mechancial and dynamical mechanical properties

383

effect, and other minor effects. The second region (above 300% strain) of the plot corresponds to the unstrained undisturbed regions, respectively. Tensile strength, modulus at 200% elongation, and elongation at break of XNBR/LDH-C10 were found to be always superior compared to neat XNBR (Pradhan et al., 2008). The tensile strength of the XNBR also improved on loadings with MgAl-LDH (Laskowska et al., 2014a). This was ascribed to the existence of an ionic/polar interaction between MgAl-LDH and XNBR. Furthermore, elongation at break of the XNBR MgAl-LDH composites is remarkably reduced compared to the unfilled neat gum. This is in all probability due to the reduced free volume as a result of the interaction between the filler and the matrix, as well as the higher crosslink density. Costa et al. (2010) observed that sodium 1-decanesulfonate- and 1-hexadecanesulfonate-modified Mg-Al LDH acted as a very efficient reinforcing agent for XNBR matrix. Basu et al. (2014) investigated the mechanical properties of the XNBR/Zn-Al LDH composites. They observed much improved tensile strength at 40 phr filler loading compared to XNBR gum. The 100% modulus also increases significantly with increasing LDH content and reaches its highest value at 100 phr loading of LDH.

9.5.1.7 NR/LDH nanocomposites NR/Zn-Al LDH-DS (nanocomposites) exhibit higher tensile strength than NR/ZnAl LDH-NO3 microcomposites (Abdullah et al., 2010). NR attained a 21.57% increment in tensile strength at 7 phr loading of Zn-Al LDH-DS. This is ascribed to the intercalation of NR into the galleries of Zn-Al LDH-DS. The ultimate tensile strength decreased at higher loadings due to poor dispersion or agglomeration.

9.5.2 Mechanical properties of elastomeric blend-LDH nanocomposites 9.5.2.1 PU blend-LDH nanocomposites Kotal et al. (2010b) investigated the effect of DS-LDH filler on the mechanical properties of neat PU/NBR blend and the corresponding findings are displayed in Fig. 9.23. It is noted that the tensile strength and elongation at break of PU/NBR/ DS-LDH (1 wt%) nanocomposite compared to neat blend are enhanced by 156% and 21%, respectively. Beyond this, the PU/NBR/DS-LDH nanocomposites show a gradual decrease in tensile strength and elongation at break due to agglomeration of DS-LDH layers in the PU/NBR blend. However, the possibility of the formation of crack growths through filler agglomerates in the PU/NBR nanocomposites cannot be ruled out. The variations of tensile strength and elongation at break of SFCNF-Mg-AlLDH-filled TN blends are displayed in Fig. 9.24 (Roy et al., 2016b). The enhancement in mechanical properties clearly indicates the reinforcing effect of SFCNF-MgAl-LDH in the TN matrix. It is also noted that 0.50 wt.% SFCNFMgAl-LDH filler-loaded TN nanocomposite exhibits maximum improvement in

Layered Double Hydroxide Polymer Nanocomposites

Tensile strength (MPa) Elongation at break (%)

Tensile strength (MPa)

14

630 600

12

570

10

540 8 510 6

Elongation at break (%)

384

480 4 0

2 4 6 DS-LDH content (wt %)

8

450

Figure 9.23 Variation of tensile strength and elongation at break of PU/NBR/DS-LDH nanocomposites with DS-LDH content (Kotal et al., 2010b). Source: Reproduced with permission from Wiley.

Figure 9.24 Variation of the tensile strength and elongation at break of the TN nanocomposites versus the SFCNF-MgAl-LDH hybrid content (Roy et al., 2016b). Source: Reproduced with permission from Wiley.

tensile strength of 167% and EB of 1.51 times compared to neat TN. Such improvements in mechanical properties suggest that the SFCNF-MgAl-LDH hybrid acts as a better reinforcing filler in improving the mechanical properties. This could be attributed to the highest specific surface area of SFCNF-MgAl-LDH and its homogeneous dispersion leading to strong polymer
filler interfacial interaction. At higher loadings, tensile strength and EB gradually decreased due to the aggregation of the filler into the matrix. The mechanical properties of the TN blend in the presence of SFCNF-MgAl-LDH hybrid filler has been explained as proposed in Scheme 9.5.

Mechancial and dynamical mechanical properties

385

Scheme 9.5 TN blend without and with filler in order to explain the mechanical properties (Roy et al., 2016b). Reproduced with permission from Wiley.

The synergistic effect of SFCNF and LDHs has also been established on mechanical properties of TN nanocomposites. Roy et al. (2016a) studied the effect of SFCNT-ZnAl-LDH and SFCNF-ZnAl LDH loadings on mechanical properties of the TN nanocomposites and the findings are displayed in Fig. 9.25. It is noted that 0.50 wt.% SFCNT-ZnAl-LDH (and SFCNF-ZnAl-LDH) loaded TN nanocomposites achieved maximum improvement in tensile strength of 171% (126%) and EB of 1.8 times (1.50 times) compared to neat TN. Such improvements in mechanical properties are attributed to the optimum dispersion of SFCNT-LDH hybrid fillers leading to an enhanced interaction between matrix and nanofiller. Furthermore, tensile strength and elongation at break slightly decrease at higher filler loadings due to the tendency of the filler to agglomerate. Their findings also clearly confirmed the synergistic effect of SFCNT and LDH in reinforcing of TN nanocomposites. The variations of tensile strength and elongation at break of TPU/NBR/SFCNTMg-Al-LDH nanocomposites have also been studied and corresponding findings are displayed in Fig. 9.26 (Roy et al., 2016c). It is observed that 0.50 wt.% SFCNTLDH hybrid-loaded TPU/NBR exhibits about 171% and 1.8 times enhancement in tensile strength and elongation at break compared to pure TPU/NBR. The variation of stress–strain of 0.25 wt.% SFCNTs/TPU/NBR, 0.25 wt.% LDH/TPU/NBR, and 0.50 wt.% SFCNT-LDH/TPU/NBR established role of synergistic effect of individual fillers on the mechanical properties of hybrid-filled TPU/NBR nanocomposites. Alternatively, this could also be ascribed to the optimum dispersion of SFCNTLDH hybrid filler that causes an enhanced interaction between matrix and nanofiller.

386

Layered Double Hydroxide Polymer Nanocomposites

Figure 9.25 (A) Variation of tensile strength and elongation at break of TN nanocomposites with SFCNT-ZnAl-LDH hybrid content. (B) Variation of tensile strength and elongation at break of TN nanocomposites with SFCNF-ZnAl-LDH hybrid content (Roy et al., 2016a). Source: Reproduced with permission from Springer.

9.5.2.2 EVA Blend-LDH nanocomposites Kuila et al. (2009b) investigated variation of tensile strength and elongation at break with DS-LDH loading in an EVA/EPDM blend. It is noted that tensile strength as well as elongation at break increases up to 3 wt.% DS-LDH content in EVA/EPDM matrix. Such an increase in TS and EB suggests that the EVA/ EPDM blend is strengthened and toughened simultaneously by increasing the concentration of DS-LDH. Furthermore, the interfacial interaction between the hydroxyl functionality of DS-LDH and the polar acetate group of EVA provided

Mechancial and dynamical mechanical properties

387

600 14 12

500

10

450 400

8 350 Tensile strength Elongation at break

6

Elongation at break (%)

Tensile strength (MPa)

550

300 250

4 0.0

0.2

0.4

0.6

0.8

1.0

MWCNT-LDH hybrid content (wt%)

Figure 9.26 Variation of tensile strength and elongation at break of TPU/NBR nanocomposites with SFCNT-Mg-Al-LDH hybrid content (Modified) (Roy et al., 2016c). Source: Reproduced with permission from Wiley.

good adhesion between these two phases, promoting their compatibility and reducing the extent of phase separation. The increase in EB may be attributed to the plasticization or chain slippage of the EVA/EPDM blend within the platelet orientation of DS-LDH nanolayers (Roy et al., 2016c). At higher DS-LDH loadings (5 and 8 wt.%), TS and EB of EVA/EPDM tend to decrease due to the aggregation of filler. The effect of DS-LDH contents (1, 3, 5, and 8 wt.%) on the tensile properties (tensile strength and elongation at break) has been investigated for EVA/LDPE (30:70, 50:50, and 70:30) blends (Kuila et al., 2008a; Kuila, 2009). The degree of improvement in mechanical properties of the nanocomposites is found to be different for all the blend systems. This is mainly attributed to the varying degree of dispersion of DS-LDH layers in the EVA/LDPE blends. It is also noted that EVA/ LDPE (30:70) blend with 3 wt.% of DS-LDH loadings led to the maximum improvements in tensile strength (103 %) and elongation at break (92%) with respect to the neat blend. Fig. 9.27 show that nanocomposites with EVA/LDPE (50:50) and 3 wt.% DS-LDH exhibited improvement in TS is 55%, whereas the EB of the nanocomposites decreases gradually with increasing concentration of DSLDH in the blend (Kuila et al., 2008a). Kuila (2009) also observed higher tensile strength (70%) and elongation at break (30%) compared to the pure blend in EVA/ LDPE (70:30)/DS-LDH (5 wt.%) blend nanocomposites. This was ascribed to the fine dispersion of DS-LDH particles offering the whole surface of the LDH layers available for the interaction with the polar group of the EVA in the EVA/LDPE blend. However, EVA/LDPE (30:70, 50:50) blends showed a decrease in tensile strength and elongation at break beyond 3 wt.% DS-LDH loading due to the aggregation of filler.

28

800

24

700

20

600

16 500 12 400

T.S. (MPa) E.B. (%)

8 0

2

4

6

8

Elongation at break (%)

Layered Double Hydroxide Polymer Nanocomposites

Tensile strength (MPa)

388

300

DS-LDH content (wt%)

Figure 9.27 Variation of tensile strength (TS) and elongation at break (EB) with 1, 3, 5, and 8 wt.% DS-LDH contents in EVA/LDPE matrix. Kuila et al. (2008a). Source: Reproduced with permission from Elsevier.

9.6

Dynamical mechanical properties of LDH-filled elastomer and elastomeric blend nanocomposites

Dynamic mechanical results are generally given in terms of complex moduli or compliance. The complex moduli are defined by E 5 E/ 1 iE//, where, E is the complex shear modulus; E/, the real part of the modulus; E//, the imaginary part of the modulus; and i 5 O
1. E/and E// are also called the storage modulus and loss modulus respectively. The angle that reflects the time lag between the applied stress and strain is δ, and is defined by a ratio called the loss tangent or dissipation factor: tan δ 5 E///E/. DMA is also by far the most sensitive technique. The glass transition temperature (Tg) of polymers can also be measured by DMA. In view of this, elastic modulus, loss modulus, glass transition temperature of elastomer, and elastomeric blend nanocomposites are reviewed below.

9.6.1 Dynamical mechanical properties of elastomer-LDH nanocomposites 9.6.1.1 PU-LDH nanocomposites Kotal et al. (2011) studied temperature dependence of logarithm of storage modulus, loss modulus and tan δ of TPU, and its St-LDH-filled nanocomposites as displayed in Fig. 9.28 and the findings are summarized in Table 9.3. It is noted that the storage moduli increase with St-LDH contents in TPU over a wide range of temperature (
80 C to 180 C). Such improvements in storage moduli are possibly due to the greater surface area available due to exfoliated St-LDH layers in the TPU matrix. This is likely to lead to the strong interaction (hydrogen bonding)

Mechancial and dynamical mechanical properties

389

Figure 9.28 Temperature dependence of logarithm of storage modulus, loss modulus, and tan δ of TPU, and its St-LDH-filled nanocomposites (a) neat PU and its nanocomposites containing, (b) 1 wt.%, (c) 3 wt.%, (d) 5 wt.%, and (e) 8 wt.% LDH
stearate (Kotal et al., 2011) Source: Reproduced with permission from Wiley. Table 9.3 Glass transition temperature (Tg), tan δmax, storage modulus (E0 ), and loss modulus (Ev) of neat PU and its LDH
stearate nanocomposites (Kotal et al., 2011) Sample

Neat PU PU/LDH
stearate (1 wt%) PU/LDH
stearate (3 wt%) PU/LDH
stearate (5 wt%) PU/LDH
stearate (8 wt%)

Tg ( C) Tan δmax

2 46 2 44 2 41 2 40 2 31

0.49 0.45 0.44 0.43 0.41

log E0 (MPa)

log Ev (MPa)

225 C

25 C

225 C

25 C

7.12 7.61 7.73 7.82 8.27

6.68 7.15 7.47 7.52 8.0

6.41 6.71 6.97 7.02 8.0

5.44 5.96 6.16 6.22 6.51

Source: Reproduced with permission from Wiley.

between polar urethane groups of TPU and hydroxyl group of St-LDHs. As a result, the mobility of TPU chains is restricted, influencing the increase in stiffness with agradual introduction of St-LDH layers in the TPU matrix. It is also evident from Table 9.3 that the improvement in log E0 is relatively much higher in TPU/St-LDH

390

Layered Double Hydroxide Polymer Nanocomposites

nanocomposites, possibly due to the increased interaction of
OH and
NHCOO groups present in St-LDH and TPU, respectively. The Tg of the soft segment of TPU is shifted with increasing loading of St-LDH to the higher-temperature regions and is found to be maximum (15 C) at 8 wt.% in TPU nanocomposites. It is also evident from Table 9.3 that increasing the filler loading in TPU is accompanied by a gradual reduction in the tan δmax values. This is in all probability due to the restricted molecular chains of TPU as a result of the increasing interaction between TPU and filler. DMA studies of dodecyl sulfate intercalated LDH-filled TPU nanocomposites also exhibited more or less similar behavior (Kotal et al., 2009). Kotal and Srivastava (2011a) also reported DMA analysis of PU/DS-LDH nanocomposites. Fig. 9.29(A, B) and (C) show the temperature dependence of storage moduli (E0 ) and loss factor (tan δ) of the relevant nanocomposites respectively. E0 is found to be maximum (176%) at 3 wt.% loading and considerably increased over a wide temperature range. Furthermore, it is noted that Tg is maximum for 3 wt.% DSLDH loading in PU, where it increases from 213 C (Tg) for neat PU to 26 C due to the increased interaction between DS-LDH and the PU matrix, which results in the restricted mobility of PU chains leading to relaxation of PU/DS-LDH systems at higher temperature. Usually, the temperature range with tan δ . 0.3 is taken as a standard to evaluate damping materials (Kuila et al., 2008b). In light of this, Zhang et al. (2016) studied the damping properties of the PU/APS-DS-LDH nanocomposites at 1 Hz. The temperature range with tan δ . 0.3 of neat PU was about 44.6 C and the glass transition temperature and tan δ (max) were found to be
34 C and 0.95, respectively. They also noted that PU/APS-DS-LDH (3 wt.%) exhibits temperature ranges B52 C and tan δ reached a maximum value of 1.04. This clearly suggested an improvement in tan δ of PU/APS-DS-LDH (3 wt.%) compared with neat PU and accounted for its enhanced damping properties.

9.6.1.2 EVA-LDH nanocomposites The temperature variation of dynamic storage modulus (E/) of neat EVA-18, EVA28, EVA-45, EVA-60, and corresponding DS-LDH-filled nanocomposites are reported in Fig. 9.30 (Kuila, 2009). It is noted that the storage modulus of EVA-18 nanocomposites with 1 wt.% DS-LDH content is higher compared to neat EVA. However, the storage modulus decreases with a higher amount of DS-LDH content. The storage moduli of EVA-28/DS-LDH nanocomposites are enhanced at Tg and at higher temperatures (50 C
80 C). However, in the ambient temperature (10 C
30 C) region, the storage modulus of the nanocomposites decreases compared to neat EVA-28. The storage moduli of EVA-45/DS-LDH nanocomposites are significantly higher compared to neat EVA-45 and the storage modulus values increase gradually with increasing concentration of DS-LDH in EVA. In contrast, the storage modulus of EVA-60/DS-LDH (3 wt.%) nanocomposites is higher compared to the neat counterpart or EVA-60/DS-LDH (1 wt.%) and EVA-60/DS-LDH (5 wt.%) nanocomposites. These findings clearly showed that the storage modulus values of different nanocomposites are improved with respect to neat EVA. In all

Mechancial and dynamical mechanical properties

391

(A) 4.0 3.5 PU

log E′ (MPa)

3.0

1 wt% DS-LDH

2.5

3 wt% DS-LDH

2.0

5 wt% DS-LDH

1.5

8 wt% DS-LDH

1.0 0.5 0.0

–0.5 –1.0 –60 (B)

–40

–20

40

60

80

0.9 PU

0.8

1 wt% DS-LDH

0.7

tan δ

0 20 Temperature (ºC)

3 wt% DS-LDH

0.6

5 wt% DS-LDH

0.5

8 wt% DS-LDH

0.4 0.3 0.2 0.1 0.0 –60

–40

–20

0 20 Temperature (ºC)

40

60

80

Figure 9.29 Temperature dependence of (A) storage modulus and (B) tan δ for PU nanocomposites with DS-LDH content (Kotal and Srivastava, 2011a). Source: Reproduced with permission from Springer.

probability, incorporation of DS-LDH in the polymer matrix restricts the mobility of the polymer chains through chemical bonding between the polar acetate group of EVA and hydroxyl group of DS-LDH, enhancing thereby the stiffness of the nanocomposites. As a result, the storage modulus of the nanocomposites exceeds that of neat EVA. However, the degree of improvement of the storage modulus for

392

Layered Double Hydroxide Polymer Nanocomposites

(A)

(B) c

1E9

b a

d 1E8

log E⬘ (Pa)

log E⬘ (Pa)

1E9

e

1E8 a

c

b

d

1E7 1E7 –40

–20

0 20 40 Temperature (°C)

60

(C)

0 20 40 60 Temperature (°C)

80

d c log E⬘ (Pa)

log E⬘ (Pa)

–20

(D) 1E9

1E9 1E8 1E7

–40

b

1E7

c a

a

1E6

1E6 –40

1E8

–20 0 20 40 Temperature (°C)

60

80

–40

d

b –20

0 20 40 Temperature (°C)

60

80

Figure 9.30 Storage modulus vs. temperature curve of (A) EVA-18/DS-LDH, (B) EVA-28/ DS-LDH, (C) EVA-45/DS-LDH, and (D) EVA-60/DS-LDH nanocomposites with (a) 0, (b) 1, (c) 3, (d) 5, and (e) 8 wt.% DS-LDH contents (Kuila, 2009).

different EVA/DS-LDH systems is not equivalent. This is probably due to the different degree of dispersion of DS-LDH layers in the nanocomposites. Fig. 9.31 displays variation of tan δ versus temperature for EVA-18/DS-LDH, EVA-28/DS-LDH, EVA-45/DS-LDH, and EVA-60/DS-LDH nanocomposites with 0, 1, 3, 5, and 8 wt.% DS-LDH contents (Kuila, 2009). It is noted that the tan δ peak position of EVA-18 nanocomposites shifted toward the negative temperature region. This indicates that the Tg of the nanocomposites decreases due to the plasticization effect of DS-LDH layers in EVA. The low concentration of vinyl acetate (VA) in EVA-18 is not sufficient for chemical interaction between the polar acetate group of EVA and the hydroxyl group of DS-LDH. The intensity of tan δ peak of the nanocomposites of EVA-28 and EVA-45 with DS-LDH filler decreases with increasing concentration of DS-LDH in EVA. The change in Tg is observed in EVA-60/DS-LDH nanocomposites. Hoewver, the heights of tan δ peak are similar to those of EVA-28 and EVA-45. The intensity of the tan δ peak decreased with increasing LDH content due to the stiffness of these materials. It is anticipated that incorporation of DS-LDH nanolayers restricts the mobility of the polymer molecules by the strong chemical bonding resulting in the reinforcement of the polymer

Mechancial and dynamical mechanical properties

393

(A)

(B)

0.25

ac

0.35

b d

0.20

a

b c

0.30

e

tan δ

tan δ

0.25 0.15 0.10

0.20

d

0.15 0.10

0.05

0.05 0.00 –40

–20

0

20

40

(C) 0.8

a

0.7 0.6

–40

60

Temperature (°C)

1.8 1.6

c

1.4

0

20

40

60

40

60

Temperature (°C)

(D)

d

b

–20

a

b c d

1.2

tan δ

tan δ

0.5 0.4 0.3

1.0 0.8 0.6

0.2

0.4 0.2

0.1 0.0 –40

–20

0

20

Temperature (°C)

40

60

0.0 –40

–20

0

20

Temperature (°C)

Figure 9.31 Tan δ vs. temperature curve of (A) EVA-18/DS-LDH, (B) EVA-28/DS-LDH, (C) EVA-45/DS-LDH, and (D) EVA-60/DS-LDH nanocomposites with (a) 0, (b) 1, (c) 3, (d) 5, and (e) 8 wt.% DS-LDH contents (Kuila, 2009).

matrix. Therefore, DS-LDH layers exhibit higher stiffness compared to the pristine EVA and account for the reduction in the magnitude of the tan δ peak. The broadness of the tan δ peak is determined at half of its height, and is associated with the mobility of the polymer chain in the thermal relaxation process. For compatible polymer/inorganic nanocomposites, local polymer chains that are located close to the filler surface require higher temperature to induce the motion of the polymer chains. However, other polymer chains, far from filler surfaces, which are unaffected by the fillers, maintain the same properties as those of the pristine polymer. Such improvements in mechanical properties have been correlated in terms of fracture behavior of the nanocomposites using SEM. Zhang et al. (2007) studied the effects of LDH loadings on the Tg of EVA/ HFMH/LDH. It is observed that Tg values decrease with increasing LDH contents toward the Tg value of pure EVA (Tg:
12 C). Furthermore, it is noted that Tg values of EVA/HFMH (Tg 5 22 C) filled with 2, 5, 10, and 15 phr of organomodified LDH (OM-LDH) correspond to
3,
5,
7, and
10 C, respectively. It seems that the addition of OM-LDH enhanced the mobility of the polymeric chain

394

Layered Double Hydroxide Polymer Nanocomposites

4.5 4.0 3.5 c Log E⬘ (MPa)

3.0 2.5 2.0

b

e a

d

1.5 1.0 0.5 0.0 –0.5 –100

–50

0

50

Temperature (°C)

Figure 9.32 Storage modulus versus temperature of the St
LDH/SR nanocomposites with (a) 0, (b) 1, (c) 3, (d) 5, and (e) 8 wt.% St
LDH (Pradhan et al., 2012). Source: Reproduced with permission from Wiley.

segments compared to EVA (100 phr)/HFMH (100 phr). Temperature variation of storage modulus of EVA/HFMH (100:100 phr), EVA/HFMH/LDH (100:2:98 phr), EVA/HFMH/LDH (100:5:95 phr), and EVA/HFMH/LDH (100:15:85 phr) have also been made at a constant frequency of 5 Hz. Their findings demonstrated much larger storage modulus value of EVA (without LDH) than that of the pure EVA sample. However, the storage modulus of the EVA/HFMH/LDH samples decreased with increasing loading of OM-LDH. This is attributed to the OM-LDH helping the dispersion of the HFMH evenly in the EVA matrix.

9.6.1.3 SR/LDH nanocomposites Figs. 9.32 and 9.33 display variation of the storage modulus (E) and tan δ for neat SR and the St-LDH/SR nanocomposites, respectively (Pradhan et al., 2012). These findings indicated the storage modulus of the St-LDH/SR nanocomposites at
75 C are greater compared to SR. It is also noted that the modulus is maximally improved by 714% higher for the St-LDH (3 wt.%)/SR nanocomposite. Such an enhancement in storage modulus could be attributed to the higher aspect ratio of the dispersed LDH layers. Alternatively, possibility of the interaction between the polymer chains and LDH restricting the flexibility of SR chains as well as the increase the stiffness of the composite also cannot be ruled out. The reduction in the storage modulus at higher filler loading in the SR matrix is likely to be due to the aggregation of St-LDH in SR (Kong et al., 2006). The tan δ versus temperature curves indicate that the incorporation of St-LDH lowers Tg of the corresponding SR

Mechancial and dynamical mechanical properties

395

0.08 a

0.06 0.04 Tan delta

0.02

b c d e

0.00 –0.02 –0.04 –0.06 –0.08 –100

–50

0

50

Temperature (°C)

Figure 9.33 Temperature dependence of tan δ of the St
LDH/SR nanocomposites with (a) 0, (b) 1, (c) 3, (d) 5, and (e) 8 wt.% St
LDH (Pradhan et al., 2012). Source: Reproduced with permission from Wiley.

nanocomposites approximately by 3 C and could be attributed to the plasticization effect of the St-LDH particles in the SR matrix.

9.6.1.4 EPDM-LDH nanocomposites The variation of tan δ and storage modulus (E0 ) versus temperature plots of the EPDM-LDH composites are displayed in Fig. 9.34 (Basu et al., 2016). It is noted that Tg shifted towards higher temperatures (by a small margin) on the addition of LDH. At a 4 phr loading the ZnO-containing sample shows a positive shift of B4 C as compared with the 4 phr LDH sample. However, on increasing the loading of LDH from 4 to 100 phr indicated an increasing tendency of Tg. The peak heights of the tan δ versus temperature plot decreased at a higher loading of LDH in the EPDM. The storage modulus also significantly improved in the region after the Tg and showed a steady rubbery plateau at a higher loading of LDH with EPDM. Acharya (2008) studied the variation of dynamic storage modulus (E0 ) and damping (tan δ) of EPDM/DS-LDH nanocomposites as a function of temperature, and the corresponding data are displayed in Table 9.4. It can be seen that the storage moduli of EPDM filled with 2-wt.% layered double hydroxide increased abruptly below Tg, after which, it decreases with increasing DS-LDH contents. Such a decrease is possibly due to the presence of amorphous regions in the corresponding nanocomposites. However, the storage modului of EPDM/LDH nanocomposites increase regularly with an increase in filler content above Tg. Such an improvement in the storage modulus of EPDM/DS-LDH nanocomposites is possibly due to the presence of stiff LDH filler in addition to the combined effect of

396

Layered Double Hydroxide Polymer Nanocomposites

(A) 2.0

–42.4°C

EPDM-4 ZnO EPDM-4 LDH EPDM-10 LDH EPDM-40 LDH EPDM-100 LDH

–38.5°C –37.5°C

1.5

–37.2°C

1.0

tan δ

–36.8°C

0.5

0.0 –80

–60

0

–40 –20

20

40

60

Temperature (°C) (B) EPDM-ZnO EPDM-4 LDH EPDM-10 LDH EPDM-40 LDH EPDM-100 LDH

E⬘ (MPa)

103

102

101

–80

–60 –40

–20

0

20

40

60

Temperature (°C) 4

(C)

log E⬘ (MPa)

3

EPDM-100 LDH EPDM-40 LDH EPDM-10 LDH EPDM-4 ZnO EPDM-4 LDH ~Ea/R

2

5.18 kJ mol

–1

2.78 kJ mol–1

1

1.54 kJ mol–1 1.12 kJ mol–1 1.34 kJ mol

0 0.0028

0.0035

–1

0.0042

0.0049

1/T (K–1)

Figure 9.34 (A) Loss tangent (tan δ) and (B) storage modulus (E’) vs. temperature and (C) log E’ vs 1/T plots of the EPDM
LDH composites (Basu et al., 2016). Source: Reproduced with permission from Royal Society of Chemistry.

Mechancial and dynamical mechanical properties

397

Table 9.4 Dynamic mechanical data of neat EPDM and EPDM/DSLDH nanocomposites (Acharya, 2008) Sample

DS-LDH contents (wt.%)

Tg

E0 (Pa) 3 107 at Tg

E0 (Pa) 3 106 at 25 C

Tan δ

EL0 EL2 EL3 EL4

0 2 3 4


61.2
48.5
47.4
47.2

3.8 4.7 3.7 3.5

2.5 4.2 4.5 6.6

1.18 0.76 1.05 0.92

aspect ratio and degree of dispersion of LDH particles. It is also noteworthy to mention that the glass transition temperature, Tg (β-relaxation), increases by  13.8 C for 3-wt.% of LDH content and remains more or less the same, with higher wt.% of LDH loading. Furthermore, it is noted that the tan δ value decreased and the peak area broadened in the nanocomposites. According to Das et al. (2012), storage modulus decreases with temperature, and loss modulus reaches a maximum in SBR/Zn-Al-LDH nanocomposites. The peak height of tan δ is reduced with the increase in the filler loading.

9.6.1.5 NBR/LDH and XNBR/LDH nanocomposites It is well known that ZnO along with stearic acid during vulcanization condition reacts with sulfur and organic accelerators. Eshwaran et al. (2015) reported that ZnAl LDH can be used in place of ZnO and stearic acid for vulcanization as well as in reinforcement of the rubber matrix. Fig. 9.35 shows dynamical mechanical thermal analysis of NBR nanocomposites consisting of unmodified LDH (uLDHc), modified commercial LDH (mcLDHc), modified synthesized LDH (mLDHs), and ZnO equivalent are presented in Fig. 9.35 (Eshwaran et al., 2015). A nominal negative shift and noticeable reduction in tan δ peak for mLDHc is also observed. NBR/ mLDHc exhibited B4 C lower Tg values as compared to uLDHc and mLDHs composites. Furthermore, it is noted that there is hardly any change in storage modulus (E0 ) with considerable change. Therefore, it can be concluded that mLDHc-based nanocomposites show a better filler
polymer interaction than uLDHc and mLDHs composites due to the reduction in the tan δ peak height. The variation of amplitude sweep with the amount of incorporated filler showed that mLDHs possesses a higher filler
filler interaction compared to the other nanocomposites. Pradhan et al. (2008) recorded that the tan δ maximum value steadily decreases with increasing filler concentration in XNBR/MgAl-LDH. Laskowska et al. (2014a) studied variation of tan δ, storage modulus (E0 ), and loss modulus (Ev) with temperature of XNBR and its MgAl-LDH nanocomposites at 10 Hz. They noted that the storage modulus (Ev) increases with increasing MgAl-LDH concentration, and the value is found to be highest at 30 phr loading in XNBR. This is probably due to the strong interfacial interactions between MgAl-LDH and XNBR. It is also evident from tan δ versus temperature plot that Tg of neat XNBR (
9.9 C) is increased

398

Layered Double Hydroxide Polymer Nanocomposites

(A)

1.5 Unmodified commercial LDH (uLDHc) Modified commercial LDH (mLDHc) Modified synthesized LDH (mLDHs)

3

10

10

tan δ

E⬘ (MPa)

1.0 102

0.5

1

100 –60

–40

–20

0

20

40

0.0 60

Temperature (°C)

(B) 4.0

Unmodified commercial LDH (uLDHc) Modified commercial LDH (mLDHc) Modified synthesized LDH (mLDHs)

E⬘ (MPa)

3.8

3.6

3.4

3.2

3.0 0.1

1

10

Dynamic strain (%)

Figure 9.35 (A) Temperature sweep curves and (B) amplitude sweep curves for NBR 1846 with different LDH (Eshwaran et al., 2015).

to
6.8 C in MgAl-LDH reinforced samples. Furthermore, a plot of loss modulus Ev versus temperature showed that Tg of XNBR (
22.2 C) increased in XNBR/ MgAl-LDH (
18.0 C) nanocomposites.

9.6.2 Dynamical mechanical properties of elastomer blend-LDH nanocomposites 9.6.2.1 PU blend/LDH nanocomposites The temperature dependence of storage modulus (E0 ), loss modulus (Ev), and tan δ of neat TN and its SFCNF-LDH hybrid-filled TN nanocomposites has been investigated and the findings are displayed in Fig. 9.36 (Roy et al., 2016b). It is noted that 0.50 wt.% SFCNF-LDH hybrid-filled TN achieved maximum E0 values up to 276% (at
60 C) and 261% (at 25 C). Ev values also increased in TN/SFCNF-LDH nanocomposites compared to the neat sample. At
30 C, the Ev values of the 0.50 wt.% hybrid-filled TN nanocomposite is maximally improved (254%) compared to the pure TN. The enhanced E0 and Ev values in TN nanocomposites suggest a strong influence of the hybrid filler on the elastic properties and their interaction with the

Mechancial and dynamical mechanical properties

399

(B)

(A) Neat blend Blend/CNF-LDH (0.25 wt%) Blend/CNF-LDH (0.50 wt%) Blend/CNF-LDH (0.75 wt%) Blend/CNF-LDH (1.0 wt%)

1000

100

Neat blend Blend/CNF-LDH (0.25 wt%) Blend/CNF-LDH (0.50 wt%) Blend/CNF-LDH (0.75 wt%) Blend/CNF-LDH (1.0 wt%)

100

E⬘⬘ (MPa)

E⬘ (MPa)

1000

10

10

1

0.1

1 –80

–60

–40

–20

0

20

40

60

–80

–60

–40

Temperature (°C)

–20

0

20

40

60

Temperature (°C)

(C) Neat blend Blend/CNF-LDH (0.25 wt%) Blend/CNF-LDH (0.50 wt%) Blend/CNF-LDH (0.75 wt%) Blend/CNF-LDH (1.0 wt%)

0.8

Tan δ

0.6

0.4

0.2

0.0 –80

–60

–40

–20

0

20

40

60

Temperature (°C)

Figure 9.36 (A) Temperature dependence of E0 values of the neat TN and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt.% SFCNF-LDH hybrid. (B) E” values of the neat TN and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt.% SFCNF-LDH hybrid and (C) tan δ of the neat TN and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt.% SFCNF-LDH hybrid (Roy et al., 2016b). Source: Reproduced with permission from Wiley.

polymer chains. This is likely to increase the friction between the filler and polymer because of the combined effect of the dispersion of filler and the filler
polymer interaction. The 0.50 wt.% SFCNF-LDH hybrid-filled TN nanocomposites showed a decrease in tan δ height (0.75) compared to that in the neat TN (0.79). Such findings were attributed to the internal friction among the nanofiller
nanofiller, nanofiller
polymer matrix, and polymer matrix
matrix under some external stresses. TN nanocomposites incorporated with 0.50 wt.% of filler exhibited the maximum positive shift in Tg (B3 C) compared to the neat sample because of strong polymer
filler interactions. Tg also shifted to higher values in the case of the 0.75 wt.% SFCNF-LDH filler loading followed by a reduction in the 1 wt.% filler-loaded TN blends in all probability due to the aggregation of the hybrid filler. DMA findings of neat TN and its SFCNT-LDH and SFCNF-LDH hybrid-filled nanocomposites are recorded in Table 9.5 (Roy et al., 2016a). The storage modulus and loss modulus of hybrid-filled TN nanocomposites are always higher compared to neat TN. TN nanocomposites filled with 0.50 wt.% hybrid showed maximum

400

Layered Double Hydroxide Polymer Nanocomposites

Table 9.5 Storage modulus at different temperatures, glass transition temperature (Tg), and height of tan δ value of pure TN and its composites (Roy et al., 2016a) Sample

E (MPa) at 260 C

% E (MPa) Improvement at 25 C

% Tg ( C) Improvement from Ev

tan δ height

Pure TN TN/SFCNT-LDH (0.25 wt.%) TN/SFCNT-LDH (0.50 wt.%) TN/SFCNT-LDH (0.75 wt.%) TN/SFCNT-LDH (1.0 wt.%) TN/SFCNF-LDH (0.25 wt.%) TN/SFCNF-LDH (0.50 wt.%) TN/SFCNF-LDH (0.75 wt.%)

571 1920


236

2.70 7.78


188

2 36 2 33

0.79 0.75

2402

321

10.30

281

2 30

0.66

1950

241

9.92

267

2 34

0.74

1840

222

7.77

187

2 35

0.76

1702

198

5.50

104

2 36

0.76

2158

278

10.15

276

2 32

0.71

1960

243

9.13

238

2 34

0.72

Source: Reproduced with permission from Springer (original Table 1, Roy, S., Srivastava, S.K., Mittal, V., 2016. Facile noncovalent assembly of MWCNT-LDH and CNF-LDH as reinforcing hybrid fillers in thermoplastic polyurethane/nitrile butadiene rubber blends. J. Polym. Res. 23: 36/1
36/11.

improvements in E0 . This could be ascribed to the excellent dispersion of filler into the matrix causing a strong interaction of filler with the polymer chains and resulting in efficient load transfer by the hybrid filler to the polymer. The loss modulus (E) value (at 230 C) of pure TN (47.22 MPa) has been improved by 237, 339, 219, and 200%, with 0.25, 0.50, 0.75, and 1 wt.% SFCNT-LDH hybrid loading, respectively. The corresponding improvements in loss modulus of SFCNF-LDHloaded TN blends are found to be 165, 262, 209, and 202%. These findings suggested that hybrid fillers not only influence the elastic properties strongly but also increase the friction between filler and polymer. The variation of tan δ versus temperature showed higher Tg of all the hybrid-filled nanocomposites compared to neat TN. It is noted that all the TN nanocomposites exhibit a slight positive shifting in Tg compared to neat TN. Furthermore, the SFCNT-LDH hybrid shows a relative increment in Tg compared to the SFCNF-LDH hybrid in TN matrix, which signifies that the SFCNT-LDH hybrid exerts comparatively better restriction than the other hybrid filler. Roy et al. (2016c) studied dynamic mechanical analysis of SFCNT-LDH hybridfilled TPU/NBR blend and the findings are displayed in Fig. 9.37. It is noted that the modulus of the 0.50 wt.% hybrid-filled TPU/NBR blend is significantly enhanced in both the glassy region (by 243% at 260 C) and rubbery state (by 241% at 25 C) compared to pure TPU/NBR. Such an enhancement in storage modulus is attributed to the homogeneous dispersion of the SFCNT-LDH nanofiller within the matrix, along with a strong interaction between the SFCNT-LDH hybrid filler and polymer matrix. The loss modulus (at 230 C) of TPU/NBR

Mechancial and dynamical mechanical properties

(B)

2500 Neat blend Blend/MWCNT-LDH (0.25 wt%) Blend/MWCNT-LDH (0.50 wt%) Blend/MWCNT-LDH (0.75 wt%) Blend/MWCNT-LDH (1.0 wt%)

2000

1500

1000

200

100

500

50

0

0

–80

–60

–40

–20

0

20

40

60

Neat blend Blend/MWCNT-LDH (0.25 wt%) Blend/MWCNT-LDH (0.50 wt%) Blend/MWCNT-LDH (0.75 wt%) Blend/MWCNT-LDH (1.0 wt%)

150

Loss modulus

Storage modulus (MPa)

(A)

401

80

–80

–60

–40

Temperature (°C)

–20

0

20

40

60

80

Temperature (°C)

(C) Neat blend Blend/MWCNT-LDH (0.25 wt%) Blend/MWCNT-LDH (0.50 wt%) Blend/MWCNT-LDH (0.75 wt%) Blend/MWCNT-LDH (1.0 wt%)

0.8

Tan δ

0.6

0.4

0.2

0.0 –80

–60

–40

–20

0

20

40

60

80

Temperature (°C)

Figure 9.37 Temperature dependence of (A) storage modulus, (B) loss modulus, and (C) tan δ of neat TPU/NBR and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt.% SFCNTMgAl-LDH hybrid (Roy et al., 2016c). Source: Reproduced with permission from Wiley.

nanocomposites filled with 0.25, 0.50, 0.75, and 1 wt.% hybrid is improved by 99, 254, 224, and 193%, respectively, compared to pure TPU/NBR. The temperature variation of the dissipation factor (tan δ) of neat TPU/NBR and its SFCNT-LDH hybrid-filled nanocomposites show decreases in the height of the tan δ curve in neat TPU/NBR filled with SFCNT-LDH hybrid. The maximum decrease in tan δ height (0.62) compared to neat TPU/NBR (0.79) is recorded in 0.50 wt.% SFCNTLDH hybrid-loaded TPU/NBR nanocomposite. Furthermore, it is also noted that a slight increase in Tg is observed in TPU/NBR/SFCNT-LDH nanocomposites compared to the TPU/NBR blend.

9.6.2.2 EVA-EPDM/LDH nanocomposites Kuila et al. (2009b) investigated the variation of dynamic storage modulus (E0 ), loss modulus (Ev), and damping (tan δ) of neat EVA/EPDM/DS-LDH nanocomposites as a function of temperature, which are displayed in Fig. 9.38. It is inferred that the storage modulus of EVA/EPDM/DS-LDH nanocomposites exceeds that of the neat EVA/EPDM blend. The absolute values of Ev at higher loss peak are

402

Layered Double Hydroxide Polymer Nanocomposites (B)

(A)

8

10 8

log E⬘ (Pa)

log E⬘ (Pa)

10

d c b

7

10

7

10

d c

a b a

6

10 6

10

–40

–20

0

20

40

60

80

–60

Temperature (°C)

–40

–20

0

20

40

60

80

Temperature (°C)

(C) 0.6 0.5

b a d c

Tan δ

0.4 0.3 0.2 0.1 0.0 –40

–30

–20

–10

0

10

20

30

40

Temperature (°C)

Figure 9.38 Temperature dependence of (A) Storage modulus (E0 ). (B) Loss modulus and (C) loss tangent tan δ of (a) pure EVA/EPDM blend, and their nanocomposites with (b) 3, (c) 5, and (d) 8 wt.% DS-LDH content (Kuila et al., 2009b). Source: Reproduced with permission from Wiley Interscience.

greater than the neat EVA/EPDM blend. The loss modulus (Ev) for the nanocomposites is found to be higher over the experimental temperature region. In EVA/ EPDM blends, Tg’s broaden and remain separated but shifted toward each other. It is also noted that both Tg’s shows little or no change with the addition of DS-LDH in EVA/EPDM. This may be attributed to the lack of surrounding entanglement of the polymer chains. The temperature dependence of tan δ of EVA/EPDM/DS-LDH nanocomposites shows that the height of tan δ peak decreases with increasing DSLDH concentration. This may be ascribed to the presence of rigid DS-LDH nanolayers accounting for increasing stiffness in the nanocomposites. Furthermore, it is noted that tan δ peaks of the nanocomposites are extended to a wider temperature range compared to that of the neat EVA/EPDM blend.

9.7

Conclusion

Several methods are reported for the synthesis of EVA, SR, EPDM, SBR, NBR, NR, and their blend nanocomposites using different types of individual/modified

Mechancial and dynamical mechanical properties

403

LDH and hybrid fillers of LDH. Following this, these nanocomposites have been investigated for the establishment of nanostructure and dispersion in polymer matrix through XRD, TEM, and SEM studies. Furthermore, mechanical and dynamical properties of most of these elastomer and elastomeric blend nanocomposites could be further enhanced compared to their neat counterparts under static and dynamic applications. The low mechanical properties of silicone rubber have also been successfully enhanced in LDHs and LDH-based hybrid-filled elastomer nanocomposites. However, more work in the future should be focused on investigation into the mechanical and dynamical properties of nanocomposites of LDH-filled SBR, SR, NR, NBR and XNBR elastomers and elastomeric blends.

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465. Pradhan, S., Costa, F.R., Wagenknecht, U., Jehnichen, D., Bhowmick, A.K., Heinrich, G., 2008. Elastomer/LDH nanocomposites: synthesis and studies on nanoparticle dispersion, mechanical properties and interfacial adhesion. Eur. Polym. J44, 3122
3132. Raman, V.S., Rooj, S., Das, A., Sto¨ckelhuber, K.W., Simon, F., Nando, G.B., et al., 2013. Reinforcement of solution styrene butadiene rubber by silane functionalized halloysite nanotubes. J. Macromol. Sci. Part A: Pure Appl. Chem. 50, 1091
1106. Roy, S., 2017. 3D Hybrid filler reinforced thermoplastic polyurethane and thermoplastic polyurethane acrylonitrile butadiene blend rubber nanocomposites. Ph.D Thesis, Indian Institute of Technology, Kharagpur. Roy, S., Srivastava, S.K., Mittal, V., 2016a. Facile noncovalent assembly of MWCNT-LDH and CNF-LDH as reinforcing hybrid fillers in thermoplastic polyurethane/nitrile butadiene rubber blends. J. Polym. Res. 23, 36/1
36/11. Roy, S., Srivastava, S.K., Mittal, V., 2016b. Noncovalent assembly of carbon nanofiber
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43470/10. Roy, S., Srivastava, S.K., Pionteck, J., Mittal, V., 2016c. Assembly of layered double hydroxide on multi-walled carbon nanotubes as reinforcing hybrid nanofiller in thermoplastic polyurethane/nitrile butadiene rubber blends. Polym. Inter. 65, 93
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588. Srivastava, S.K., Kotal, M., 2013. Recent advances on preparation, properties and applications of polyurethane nanocomposites. In: Davim, J.P., Charitdis, C.A. (Eds.), Nanocomposites Series: Advanced Composites
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4155. Wang, L., Li, B., Yang, M., Chen, C., Liu, Y., 2011. Effect of Ni cations and microwave hydrothermal treatment on the related properties of layered double hydroxide-ethylene vinyl acetate copolymer composites. J. Colloid Interface Sci. 356, 519
525. Wang, L., Li, B., Zhao, X., Chen, C., Cao, J., 2012a. Effect of rare earth ions on the properties of composites composed of ethylene vinyl acetate copolymer and layered double hydroxides. PLoS One 7, e37781/1-E37781/11. Wang, L., Li, B., Hu, Z., Cao, J., 2013. Effect of nickel on the properties of composites compoed of layered double hydroxides and ethylene vinyl acetate copolymer. Appl. Clay Sci. 72, 138
146. Wang, D.-Y., Das, A., Leuteritz, A., Mahaling, R.N., Jehnichen, D., Wagenknecht, U., et al., 2012b. Structural characteristics and flammability of fire retarding EPDM/layered double hydroxide (LDH) nanocomposites. RSC Adv. 2, 3927
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Yan, Y., Guigang, Z., Xiaoya, L., 2013. Research on UV-curable waterborne polyurethane/ LDH nanocomposite. PCI 43, 22
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3701. Yu, W.-W., Fu, H.-K., Zhang, D.-Z., Du, M., Zheng, Q., 2013. Preparation and relaxation behavior of layered double hydroxide modified PU/PMMA nanocomposites. Acta Polym. Sin. 9, 1212
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Layered double hydroxide nanocomposites based on carbon nanoforms

10

Gonzalo Abella´n1,2, Jose A. Carrasco1 and Eugenio Coronado1 1 Institute of Molecular Science (ICMol), University of Valencia, Valencia, Spain, 2 Department of Chemistry and Pharmacy and Joint Institute of Advanced Materials and Processes (ZMP), University Erlangen-Nu¨rnberg, Fu¨rth, Germany

10.1

A general introduction to LDH-carbon nanoform nanocomposites

Layered double hydroxides (LDHs) are anionic clays built from the stacking of positively charged brucite-type inorganic layers interleaved with anions. They can be formulated as [Mz11
xMy1x(OH)2]x1[An
]x/n  mH2O (Mz1, z 5 1 or 2; My1, y 5 3 or 4; An
5 organic or inorganic anions). Synthetic LDHs are quite promising based on their low cost, chemical versatility, and anion-exchange capabilities, which allows for tuning of their composition and properties in a wide range (Rives, 2001; Duan and Evans, 2006). Thanks to their layered nature, they can also be exfoliated in polar organic solvents by intercalation of suitable anions that minimize the interaction between layers to produce stable colloids containing single and fewlayer LDHs, which possess ultrahigh surface areas of c.1000 cm2/g in theory (Chen et al., 2015). These positively charged nanosheets can then be used as inorganic building blocks in the design of hybrid nanocomposites when mixed with other organic or molecular materials. In this context, an interesting ingredient of these nanocomposites is provided by the so-called carbon nanoforms (CNFs). These carbon-based materials include a wide variety of sizes and shapes ranging from C60 to metallofullerenes, carbon fibers, nanotubes, nano-onions, nanohorns, nanobuds, spheres, or graphene, among others (Delgado et al., 2008). Owing to their lowdimensional nature and sp2 hybridization of carbon atoms, and because of the sheer multitude of their esthetically pleasing structures, 0D fullerenes, 1D carbon nanotubes (CNTs), and 2D graphene, exhibit excellent physical and chemical properties, and stand as one of the most appealing building blocks in the development of nanocomposites. For instance, graphene (Geim and Novoselov, 2007; Geim, 2009), a one-atom-thick layer of graphite, is a semimetal, gapless two-dimensional (2D) semiconductor with unprecedented properties including ultrahigh room-temperature carrier mobility, or specific surface area (2630 m2/g, in theory). Moreover, graphene is an elastic film, with a high Young’s modulus (c.1.0 GPa) (Lee et al., 2008), it is Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00010-0 © 2020 Elsevier Ltd. All rights reserved.

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transparent like (or better than) plastic (Geim and Novoselov, 2007), and possesses outstanding electrical (c.106 S/cm) (Chen et al., 2008) and thermal (3000
5000 W/ m/K) (Seol et al., 2010) conductivities, better than any metal. Additionally, it behaves as an impermeable membrane (Abraham et al., 2017), and can be considered as both a solid and a macromolecule with molecular weights of more than 106
107 g/mol (Eigler and Hirsch, 2014). Graphene also exhibits a rich chemical phenomenology that ranges from supramolecular to covalent functionalization, endowing them with improved processability as well as allowing to controllably engineer the bandgap structure, create novel architectures, and manipulate the interfacial characteristics of this monolayer material (Eigler and Hirsch, 2014; Hirsch et al., 2013; Paulus et al., 2013; Criado et al., 2015; Abella´n et al., 2017). When it comes to nanocomposite formation, the limited solubility of pristine graphene is one of the main drawbacks. Along this front, negatively charged graphene oxide (GO) is usually employed as a very effective alternative. GO is a single layer of graphite oxide. During the formation of graphite oxide, the graphene layers in graphite become intercalated by an acid to form a stage 1 intercalation compound, with all layers being intercalated (Eigler and Hirsch, 2014). Subsequent oxygenation of such stage 1 intercalation compounds occurs on both sides of the basal plane and in this way graphite oxide is formed. Delamination of single layers of graphite oxide leads to GO. The exact nature of the functional groups in GO strongly depends on the reaction conditions, such as preparation time and temperature, as well as on the work-up procedure. Typically, GO consists of about 45 mass % carbon. Although several structure models have been proposed, GO is a rather polydisperse material, whose exact structure is very difficult to precisely define. The mobility of charge carriers depends on the density of defects and, therefore, mobility values range between 0.1 and 1000 cm2/V  s, very limited values compared to that of pristine graphene, which usually exceeds 15,000 cm2/V  s. In any case, its excellent processability has allowed the development of the first examples of graphene/LDH hybrid nanocomposites. With respect to 1D CNTs—which can exist with various flavors (i.e., a broad variation of helicities, single-walled, multiwalled)—they possess a low bandgap of 0
1.9 eV, high electrical conductivity of 0.17
2 3 105 S/cm (Baughman, 2002), and high thermal conductivity of 3000
6600 W/m  K (Kim et al., 2001). In addition, their excellent mechanical properties with Young’s moduli of 0.27
1.25 GPa and a tensile strength of 11
63 GPa (Yu, 2000), makes them excellent candidates for functional nanocomposite reinforcement. In stark contrast, LDHs exhibit an insulating behavior, low-to-moderate specific surfaces areas ranging from 20 to 365 m2/g, but a rich redox chemical behavior (Chen et al., 2015). The incorporation of transition metals in synthetic LDHs endowed them with excellent redox properties, which is extremely interesting in energy conversion and storage. Moreover, they can accommodate different metals in a well-defined atomic arrangement within the layers, a property that has been extensively used in heterogeneous catalysis. But LDHs are not only interesting as active materials per se, but also as functional precursors of layered mixed metal oxides or spinels, of utmost importance in catalysis or magnetism (Abella´n et al., 2014a, 2015b).

Layered double hydroxide nanocomposites based on carbon nanoforms

413

Probably, the most interesting property of LDHs is their anion-exchange ability (Rives, 2001). LDHs are capable of accommodating a multitude of inorganic and organic anions, including complex macrocycles, polymers, or even DNA strands (Duan and Evans, 2006). This allows the synthesis of hybrid functional materials with the coexistence, and in some cases, even interaction of different properties (Abella´n et al., 2014c, 2015a, 2015b; Clemente-Leo´n et al., 2011). Additionally, LDHs submitted to calcination at moderate temperatures exhibit the so-called “memory effect,” enabling the reconstruction/recrystallization of the initial solids in the presence of anions. This effect has been extensively used for environmental remediation and controlled delivery of anions (Goh et al., 2008). The intrinsic limitations of LDHs, mainly due to their poor conductivity, can be overcome by their combination with highly conducting CNFs, in the same way as the rich chemical reactivity of LDH perfectly complements the limited redox behavior of CNFs. In this sense, the design and synthesis of hierarchical nanocomposites is one of the most interesting approaches for combining their distinguishing properties, creating multifunctional hybrid materials endowed with high mechanical strength and hierarchical porosities, as well as improved heat and mass transfer properties (Abella´n et al., 2010; Coronado et al., 2010). The combination of LDHs with different CNFs has attracted an increasing level of attention during recent years due to their outstanding properties in different fields ranging from polymer reinforcement and environmental remediation, to catalysis or energy storage and conversion. Table 10.1 summarizes some properties of CNFs and LDHs, and Fig. 10.1 depicts a schematic illustration of an LDH material combined with the most typical CNFs. Along this line, Table 10.2 depicts the most typical LDH combination of metals and the capability of these inorganic layers to be mixed with CNFs giving rise to nanocomposites. There are different synthetic approaches for the preparation of CNF/LDH hybrids (Zhao et al., 2012). We can divide them into three main families, graphene/ LDH; CNT/LDH; and other nanocomposites combining LDHs with CNFs like fullerenes, carbon spheres, or mesoporus carbons. Fig. 10.1 summarizes the most common synthetic approaches pursued in the literature to prepare these systems. Firstly, based on the strong electrostatic interactions between exfoliated positively charged LDH nanosheets (Li et al., 2005b) and negatively charged CNFs, the reassembly of nanocarbons and LDHs is one of the most extensively used approaches for the synthesis of LDH/carbon hybrids. Secondly, owing to the strong adsorption ability of negatively charged CNFs for metal cations, their surface is perfectly suited for the direct seed nucleation and growth of LDH crystallites under basic pHs by means of coprecipitation or hydrothermal methods (Abella´n et al., 2014a; Du et al., 2014; Okamoto et al., 2007). Finally, LDHs can act as either as a catalytic support for the in situ formation of CNFs [employing chemical vapor deposition (CVD) using a carbon source like ethylene], or even as multilayer nanoreactors, where carbon-rich molecules are intercalated within LDH; the calcination of these systems leads to the formation of hybrid nanocomposites (Abella´n et al., 2012a, 2014a; Wang et al., 2014; Xu et al., 2001).

Table 10.1 Properties of different carbon nanoform materials and LDHs Properties

Fullerenes (Shi et al., 1992; Haddon et al., 1995; Yu et al., 1992)

CNTs (Yu, 2000; Krishnan et al., 1998; Yu et al., 2000; Wilder et al., 1998; Ebbesen et al., 1996; Berber et al., 2000; Peigney et al., 2001)

Graphene (Geim and Novoselov, 2007; Lee et al., 2008; Chen et al., 2008; Seol et al., 2010)

Graphene oxide (Montes-Navajas et al., 2013; Compton and Nguyen, 2010; Dreyer et al., 2010; Liu et al., 2012a; Suk et al., 2010; Go´mezNavarro et al., 2007)

LDHs (Chen et al., 2015)

Youngs modulus (GPa) Tensile strength/a Bandgap (eV) Electrical conductivity (s/cm)

15.9

0.27
1.25

c.1.0

380
470





1.5
2.0 10214
6  1028

11
63 0
1.9 0.17
2  105

130 0 c.106

130 0.1
4 1  1023
2

Thermal conductivity (W/m  K) Specific surface area (m2/g) Chemical reactivity

0.4

3000
6600

3000
5000

c.5000



Insulating (B62 S/cm for calcined CoNi-LDH)





150
1315

2630

736.6

20
365

Active

Active

Active

Active (redox)

Active (redox)

Source: Adapted from Zhao, M.-Q., Zhang, Q., Huang, J.-Q., Wei, F. 2012. Adv. Funct. Mater. 22 (4), 675 and extended with new data for graphene oxide.

Layered double hydroxide nanocomposites based on carbon nanoforms

415

Figure 10.1 Schematic illustration with some of the most typical nanocomposites between LDH and CNFs. (A) Nanocomposites with graphene: (A1) in situ growth of LDH flakes parallel or (A2) vertical to the graphene layer, (A3) intercalation of graphene layers into the interlayer space of the LDH and (A4) graphene growth on the surface of the LDH crystallite. (B) Nanocomposites with carbon nanotubes: (B1) in situ growth of LDH flakes in the surface of CNTs, (B2) in situ growth of CNTs vertical, or (B3) parallel aligned to the LDH surface. (C) Nanocomposites with, from left to right, fullerene, carbon spheres, and carbon fibers: (C1) direct assembly between LDH flakes and fullerenes located into the interlayer space, (C2) in situ growth of LDH crystallites on the surface of carbon spheres, and (C3) in situ growth of carbon fibers vertically aligned to the surface of the LDH flakes.

In the first part of this review we will discuss in detail the synthetic routes pursued to prepare LDH/carbon nanocomposites focusing first on 2D graphene, then on 1D CNTs, and finally on other CNFs. In the second part we will give an overview of the main applications of these nanocomposites in areas like energy storage and conversion and catalysis

10.2

Graphene and graphene oxide/LDH nanocomposites

As previously mentioned, all the examples of graphene/LDH nanocomposites developed in the literature using the reassembly of 2D building blocks have employed GO instead of pristine graphene, owing to its good processability and negatively charged surface. The reduction of GO can be performed by thermal or chemical treatment with reducing reagents like hydrazine, leading to rGO. The exfoliation of LDH leads to positively charged nanosheets with a unilamellar thickness (,1 nm) and ultimate 2D anisotropy (aspect ratio .100). The ideal perfect assembly should consist of electrostatic face-to-face stacking of positively charged LDH nanosheets

416

Layered Double Hydroxide Polymer Nanocomposites

Table 10.2 Most representative LDH compositions and their capability for synthesizing the main CNFs are labeled as follows: G (graphene), T (carbon nanotubes), F (fullerene), QD (carbon quantum dots), CS (carbon spheres), and F (carbon fibers) M3+,M4+/M2+,M+

Fe2+

Co2+

Ni2+

Cu2+

Zn2+

Ca2+

Mg2+

Mn2+

Li+

G, T, Fe3+

G, T, F

QD, CS

Co3+

G, T, F

Ni3+

G

F

G, T,

Al3+

CS, F

G, T, QD,

G, T

G

Mn3+ Ga

G

CS

CS Cr3+

G, T, F,

T, F

G, T

3+

In3+ Ti4+

G

Synthesized catalytic LDHs

LDHs without catalytic activity for nanocarbon growth

Not synthesized

and negatively charged GO in an alternating sequence at a molecular scale. However, due to the large disparity of particle sizes and intrinsic polydisperse nature of GO, a perfect sandwich-like hybrid remains a big challenge (Gao et al., 2011; Li et al., 2010). The first self-assembled graphene/LDH nanocomposite ever described was developed in 2010 (Chen et al., 2010). In that work, Li et al. synthesized a film of graphene and CoAl-LDH. Both halves were previously exfoliated and delaminated and then reassembled into the final nanocomposite. Graphene/LDH nanocomposites with different compositions such as NiAl (Gao et al., 2011), NiCo (Chen et al., 2014), or CoAl (Wang et al., 2011) have been described and applied as feasible supercapacitors, giving rise to excellent performance in terms of energy storage behavior, higher specific capacitance, and longer life cycles than expected for the separate materials.

Layered double hydroxide nanocomposites based on carbon nanoforms

417

10.2.1 Synthesis As mentioned above, we can classify the synthetic methods in three main branches, namely, reassembly of LDHs and graphene, direct growth of LDH on graphene, and formation of graphene in LDH layers.

10.2.1.1 Reassembly of graphene and LDHs This synthetic methodology lies in the self-assembly of preexfoliated 2D building blocks in solution, leading to the final restacked material, or in a layer-by-layer assembly on selected substrates (Daud et al., 2016; Cao et al., 2016) (Fig. 10.2). The exfoliation-restacking synthesis is based on the delamination of LDH previously intercalated with labile anions such as chlorides, nitrates, or surfactants prior to the combination with GO through electrostatic interactions (Wimalasiri et al., 2014). The main problem associated with the delamination of LDHs is the great facility in which the exfoliated nanosheets tend to reassemble, due to the electrostatic interactions between the cationic hydroxide layers and the interlamellar anions (Wang and O’Hare, 2012). So far, no exhaustive studies have been carried out on the full optimization of the liquid-phase exfoliation (LPE) (Latorre-Sanchez et al., 2012; Abella´n et al., 2012b) of LDHs, in contrast with the exfoliation of other 2D materials like graphene, transition metal dichalcogenides, or simple metal

Figure 10.2 (A) Scheme with the formation of a CoAl-LDH and graphene oxide (GO) nanocomposite. (B) Digital photographs of an aqueous dispersion of CoAl-LDH (left), of GO (middle), and the GO/CoAl-LDH nanocomposite (right). Source: Reproduced from Wang, L., Wang, D., Dong, X.Y., Zhang, Z.J., Pei, X.F., Chen, X. J., et al., 2011. Chem. Commun., 47 (12), 3556 with permission from the Royal Society of Chemistry.

418

Layered Double Hydroxide Polymer Nanocomposites

hydroxides which have been fully addressed (Backes et al., 2017). To avoid the inherent problems LDHs have in terms of delamination, polar aprotic solvents like formamide (Huang et al., 2015) or bulky amphiphilic interlamellar anions such as surfactant molecules (specially dodecyl sulfate) (Ma et al., 2015) are commonly used in order to enable the delamination of cationic nanosheets to be thermodynamically favorable, preventing unwanted aggregation. At this point, it is worth remarking that the conventional aqueous synthetic routes for LDHs usually lead to carbonate-intercalated LDH that requires further anion-exchange processes to weaken the interlamellar interactions, and allow proper exfoliation in solvents such as formamide or dimethylformamide. When it comes to nonaqueous routes, Gardner et al. described a synthetic procedure to obtain nanometric alkoxideintercalated Al-containing LDHs using alcohols as solvents (Gardner et al., 2001; Gursky et al., 2006). Recently, this methodology has been successfully extended to synthesize pure phases like: NiMn-, CoFe-, NiCo-, and NiFe-LDH, which can be exfoliated in water due to the hydrolysis of the interlamellar alkoxide groups (Fig. 10.3). (Chen et al., 2014; Latorre-Sanchez et al., 2012; Abella´n et al., 2012b, 2014b; Carrasco et al., 2016b; Xu et al., 2013). These chemical compositions are of utmost importance in magnetism, Li-ion batteries or water splitting. Along this line, Latorre-Sanchez et al. reported the first synthesis in water of a GO/NiMn-LDH with superparamagnetic behavior and high capacity values when used as an anode in Li-ion batteries (Latorre-Sanchez et al., 2012). The negative nature of GO reveals it to be crucial for combining with the positive LDH layers in the final nanocomposite. Controlling the GO synthetic process can modify the amount of negative charges. On the other hand, by differing the atomic ratio it is possible to modulate the positive charge in LDH to a certain extent. Fig. 10.4 (extracted from Wimalasiri et al., 2014) exhibits the schematic process for mixing GO (reduced with hydrazine) with LDH in a unique material, as well as electronic microscopy images that further corroborate this assembly.

Figure 10.3 (A) Idealized illustration of the preparation and structure of the hybrid GO/ NiMn-LDH. The scheme illustrates the size of the sheets and the large particle surface of GO on which the smaller NiMn-LDH nanosheets are supported. (B) Digital photographs of the aqueous dispersions of GO (left), NiMn-LDH (middle), and the hybrid GO/NiMn-LDH formed immediately after mixing the previous components. Source: Reproduced from Latorre-Sanchez, M., Atienzar, P., Abella´n, G., Puche, M., Forne´s, V., Ribera, A., et al., 2012. Carbon, 50 (2), 518 with permission from Elsevier.

Layered double hydroxide nanocomposites based on carbon nanoforms

419

Figure 10.4 (A) Schematic procedure for the synthesis of a graphene/NiAl-LDH nanocomposite and (B) the corresponding digital photographs of the different steps. Scanning electron microscopy (SEM) images of (C) NiAl-LDH platelets, (D) exfoliated NiAl-LDH nanosheets, (E) graphene nanosheets, and (F) graphene/NiAl
LDH nanocomposite. Source: Reproduced from Wimalasiri, Y., Fan, R., Zhao, X.S., Zou, L., 2014. Electrochimica Acta 134, 127 with permission from Elsevier.

Recently Ma and co-workers have revisited the synthesis of rGO/LDH hybrids by means of a fine control of the graphene:LDH mass ratios and adjusting the charge density of GO/rGO nanosheets, improving the stacking of the layers, which results in superlattice structures of up to 20 layers in the best case. Exploiting this strategy, the preparation of rGO/CoAl, rGO/MgAl, rGO/NiFe, and rGO/NiMn-LDH hybrids has been successfully developed, exhibiting great potential in supercapacitors, water splitting, and membranes (Ma et al., 2014, 2015, 2016; Sun et al., 2016) (Fig. 10.5). On the other hand, the hydrogen-bonding layer-by-layer (LbL) technique has been successfully applied in the assembly between positive hydroxide and negative GO nanosheets. The main difference with the exfoliation and restacking approach

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Layered Double Hydroxide Polymer Nanocomposites

Figure 10.5 (A) XRD patterns of LDH nanosheets flocculated with GO (black trace) and rGO (red trace) nanosheets, respectively. Indices 001 are basal series of superlattice lamellar composites, whereas L100 and L110 are in-plane diffraction peaks from LDH nanosheets. (Inset) Schematic illustration of sandwiched LDH nanosheets and graphene. (B) TEM observations of the lamellar nanocomposites. (C) High-resolution image showing lamellar lattice fringes with different contrast. (D) Electron diffraction indexed to be in-plane diffraction rings of LDH (L100 and L110) and graphene (G100 and G110), respectively. Source: Reproduced from Ma, R., Liu, X., Liang, J., Bando, Y., Sasaki, T., 2014. Adv. Mater. 26 (24), 4173 with permission from Wiley.

relies on the usage of substrates and a polymeric solution to assist the layer-bylayer assembly. In the work of Liu et al. (Chen et al., 2010) the authors prepare the GO nanosheets by a slightly modification of the Hummers method, and the CoAlNO3 LDH is delaminated in formamide. For the LbL assembly, polyvinyl alcohol (PVA) is dissolved in deionized water in order to obtain an aqueous solution of PVA 1% in weight. After preparing the different components, cleaned quartz glass slides (using H2SO4/H2O2, which introduces some negative charges in the SiO2 surface) are submerged first in a suspension of LDH nanosheets, followed by dipping the substrates in the PVA solution, then in the exfoliated GO, and finally again in the PVA solution. Between each step, a thorough rinsing with deionized water is carried out. The complete cycle is repeated several times to obtain the final nanocomposite film. The reduction of GO to rGO has been performed by immersing the final film in hydrazine/N,N-dimethylformamide (0.5 mL 50% hydrazine/30 mL DMF) solution. (Fig. 10.6). Similar assemblies have been obtained between CoAlLDH exfoliated in formamide and aqueous GO suspensions using PDDA-coated ITO [where PDDA 5 poly(diallyldimethylammonium chloride) and ITO 5 indium tin oxide] and flexible PET substrates. In this case an iterative self-assembly process has been developed without using a polymer between the different building blocks. The reduction is obtained by heating at 200 C for 2 h under H2 atmosphere (Fig. 10.6). LBL is a promising technique as other driving forces than electrostatic interactions can be used for directing the self-assembly of LDH and CNFs, such as hydrophobic interactions or hydrogen bonding.

10.2.1.2 Direct growth of LDH on graphene Two main approaches have been used for the direct growth of LDH on graphene. On the one hand, the coprecipitation of LDH starting from the selected precursor

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Figure 10.6 (A) Schematic procedure of a layer-by-layer assembly of LDH and GO nanosheets mediated through PVA chains. (B) Scheme of LbL assembly for the synthesis of multilayer films of GO/CoAl-LDH nanocomposite. Source: (A) Reproduced with permission from Chen, D., Wang, X., Liu, T., Wang, X., Li, J., 2010. ACS Appl. Mater. Interfaces 2 (7), 2005. Copyright 2010 American Chemical Society. (B) Reproduced with permission from Dong, X., Wang, L., Wang, D., Li, C., Jin, J., 2012. Langmuir 28 (1), 293. Copyright 2012 American Chemical Society.

salts (M21/M31) in the presence of ultrasonicated GO under controlled pH conditions. On the other hand, the hydrothermal approach used stainless steel Teflonlined autoclaves. Reducing agents like urea, hydrazine, glucose or sodium sulfide are commonly used to adjust the pH and reduce the GO to rGO (Liu et al., 2006). In this approach, GO acts as a template for the precipitation of the LDH, due to the adsorption capacity of the cations added during the precipitation process, thus limiting the final LDH particle size. Heating at moderate temperatures triggers the hydrolysis of urea that progressively increases the pH up to a final value close to 8. This enables the in situ reduction of GO layers into rGO, whilst assisting the formation of the LDH phase. Fig. 10.7 shows the first report on the synthesis of LDHrGO hybrids (Li et al., 2010). By following this synthesis, graphene/LDH nanocomposites of different compositions have been described, from the conventional CoAl-LDH (Huang et al., 2012) and MgAl-LDH (Yuan et al., 2013) to more sophisticated ZnCr-LDH (Lan et al., 2014), NiFe-LDH (Youn et al., 2015), or even ternary LDHs like NiCoAl-LDH (He et al., 2015). The work of Liu et al. (Gao et al., 2011) depicts a modified synthetic

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(A)

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RGO-Ni-Fe LDH

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Ni2+

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RGO-Ni-Fe LDH Intensity

500 cps Exfoliated GO

d = 0.82 nm

1000 cps

GO 10

20

30 40 2 Theta/degrees

50

60

70

Figure 10.7 (A) Schematic representation of the synthetic procedure of the hybrid rGO/ NiFe-LDH hybrid. (B) XRPD patterns of the precursor GO, the delaminated form, and the nanocomposite. The precursor GO exhibits a basal space of 0.82 nm, implying a complete oxidation of graphite into GO. The exfoliated sheets display a broad peak in the range 20
40 degrees, related with scattering effects between the exfoliated sheets and water as solvent. Lastly, the pattern of the hybrid material matches with an NiFe-LDH phase and no peaks of graphite are observed, therefore the restacking of the as-reduced graphene sheets is prevented. Source: Reproduced from Li, H., Zhu, G., Liu, Z.-H., Yang, Z., Wang, Z., 2010. Carbon 48 (15), 4391 with permission from Elsevier.

procedure in the formation of a graphene/NiAl-LDH nanocomposite. In this case, graphite oxide is exfoliated by ultrasonication and glucose is used as a reducing agent in order to obtain a graphene nanosheet suspension prior to the in situ growth of the LDH, avoiding the use of hydrazine due to its high toxicity (Zhu et al., 2010). After that, the suspension of graphene nanosheets is introduced in an autoclave where Ni and Al salts are present, as well as urea as a pH-controlling agent (Huang et al., 2013). The final nanocomposite has been successfully prepared after 24 h at 95 C (Gao et al., 2011) (Fig. 10.8). The annealing temperature is another important parameter that has to be optimized in order to avoid metal reduction at the expense of graphene that can act as a sacrificial reducing agent being converted into CO and CO2 (Fig. 10.9) (Abella´n et al., 2012b). In general, the direct growth of LDH on GO leads to strong interactions at the interface (Wang and Dai, 2013). Indeed, XANES measurement have been employed by Dai and co-workers to study the nature of these interactions. They observe for a related NiFe-LDH/CNT hybrid the formation of M 2 O 2 C (MQNi, Fe) bonding via the carboxyl group, leading to large perturbations to the carbon atoms in the carbonyl groups. The same is expected in the case of GO (Gong et al., 2013).

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GO

GNS Glucose 95ºC 1 h GO nanosheet Adsorption

95ºC 24 h Ni/Al LDH nanosheets

Hydrothermal

Ni2+

Ni2+, Al3+ and urea

Graphene nanosheets

Al3+

Figure 10.8 Scheme with the formation of a graphene/LDH nanocomposite. Source: Reproduced with permission from Gao, Z., Wang, J., Li, Z., Yang, W., Wang, B., Hou, M., et al., 2011. Chem. Mater. 23 (15), 3509. Copyright 2011 American Chemical Society.

Figure 10.9 Idealized structure of the GO/NiMn-LDH hybrid. The remarkable difference in size between NiMn-LDH (c.100 nm) and GO (c.3 μm) is illustrated, as well as how the thermal treatment affects the segregation of Ni NP and Mn oxides. The right panel shows an FESEM image of an Ni NP of c.20 μm formed in the nanometallurgic process. Source: Reproduced from Abella´n, G., Latorre-Sa´nchez, M., Forne´s, V., Ribera, A., Garcı´a, H., 2012b. Chem. Commun. 48 (93), 11416 with permission from the Royal Society of Chemistry.

The main limitation of the direct nucleation and growth of the LDH phases on GO is the restricted accessibility of electrolyte to the active metal centers due to the formation of thicker flakes compared to the exfoliation and restacking approach. On the other hand, the strong interaction between both phases leads to a dramatic improvement in the electrical properties of LDHs.

10.2.1.3 Graphene formation in LDH layers Owing to its unusual anion intercalation property, noncatalytic systems like MgAlLDH can act as “nanoreactors” for the formation of CNFs within their interlamellar space. Indeed, Yang et al. (Sun et al., 2012) described a way to synthesize graphene

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Step I

Step II

Step III

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Calcination

(B)

Monolayer graphene

Calcined LDH-1

LDH-1

Etching LDH-2

Calcined LDH-2

Bi-trilayer graphene

(C) Graphene LDH-3 DSO

Calcined LDH-3

Multilayer graphene

MMA

Figure 10.10 Scheme with the formation of MMA-derived graphene nanosheets with a different number of layers in the interlamelar space of an LDH. (A) Monolayer, (B) bi- and trilayer, and (C) multilayer graphene nanosheets after acid treatment of LDH-1, 2, and 3, respectively. Source: Reproduced from Sun, J., Liu, H., Chen, X., Evans, D.G., Yang, W., Duan, X., 2012. Chem. Commun. 48 (65), 8126 with permission from the Royal Society of Chemistry.

nanosheets with a certain control over their thickness after the calcination of MgAlLDH intercalated with dodecyl sulfonate (DSO) and methyl methacrylate (MMA) anions [see Fig. 10.10 extracted from Sun et al. (2012)]. These small and slightly oxidized graphene flakes can be isolated and characterized after acid etching of the nanocomposite. On the other hand, by template-directed CVD it is possible to use MgAl-LDH as precursors for the synthesis of mesoporous MgAl-layered double oxide (LDO) nanosheets consisting of MgO and MgAl2O4. The mesoporus structure arises from the Kirkendall diffusion of Mg21 during the transformation of octahedral Mg
O into tetrahedral Mg
O via dehydration of the interfacial
OH groups. These layered oxides have been proven excellent for actively decomposing CH4 (or related carbon sources) and depositing single-layer graphene due to the exposed oxygen atoms on their surface. In contrast to monometallic hydroxides like Mg(OH)2— which after calcination lead to the formation of MgO layers—the use of MgAlLDH gives rise to an uninterrupted and uniform unstacked graphene layer with protuberances rather than an aggregate of graphene nanocages (Xie et al., 2012). This is mainly due to the presence of mesopores (Fig. 10.11) (Zhao et al., 2014c). Aside from catalytically “innocent” metals like Mg, Zn, or Al, LDHs can be excellent catalyst precursors. Indeed, a wide variety of CNFs have been synthesized using the metal oxides or the metal nanoparticles obtained after calcination and/or reduction of LDH having Fe, Ni, or Co atoms (Abella´n et al., 2012a; Evans and Duan, 2006). A thorough revision can be encountered in the work of Wei et al., indeed using this methodology several graphene-containing hierarchical systems

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Figure 10.11 Scheme for the synthesis of unstacked double-layer template graphene. A mesoporous oxide derived from the calcination of a LDH is used as a template, followed by its removal by acid etching. Source: Reproduced from Zhao, M.-Q., Zhang, Q., Huang, J.-Q., Tian, G.-L., Nie, J.-Q., Peng, H.-J., et al., 2014c. Nat. Commun. 5 with permission from Nature publishing group.

have been synthesized, including SWCNT/graphene hybrids (see below) (Zhao et al., 2012). Authors like Zhang et al. have used methane as a carbon source combined with different LDHs to obtain a mixture of graphene and CNT hybrids with LDHs (Zhao et al., 2013b, 2014b). Fig. 10.12 depicts part of the characterization of the nanocomposite graphene/SWCNT/CoAlMg-LDO extracted from Zhao et al. (2013b). Despite the aforementioned efforts, the directly controlled synthesis of highquality graphene within the interlamellar space of LDH remains an open challenge. In any case, the rich chemical versatility of LDH assures a promising future in the design of new graphene synthetic concepts.

10.3

Carbon nanotubes/LDH nanocomposites

As introduced in the previous section, the first works reported in the literature were related to the use of LDH phases as catalysts for the formation of CNF, and the subsequent elimination of the LDH phase by acid leaching. Indeed, since 1997 several works have explored the influence of composition, size, surface reactivity, and calcination temperatures in the CVD growth of CNF using LDH as catalytic precursors. As a matter of fact, Wei et al. reported the use of different LDHs as catalysts for the large-scale synthesis of high-quality single-walled CNTs by means of a fluidized-bed catalytic CVD procedure (Zhao et al., 2010a). Other examples where LDHs have been used as catalysts can be found elsewhere (Li et al., 2005a; Xiang et al., 2009; Cao et al., 2009; Zhao et al., 2007; Xue et al., 2010). In 2008 the first CNT/LDH nanocomposite was described by Zhang et al. (Su et al., 2008). In that work, a MWCNT/CoAl-LDH nanocomposite was synthesized

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Figure 10.12 (A) SEM image of CoMgAl-LDH platelets, (B) XRPD patterns for the CoMgAl-LDH and LDO, (C) SEM, (D) TEM, and (E, F) HRTEM images for the graphene/ single-walled CNT/CoMgAl-LDO nanocomposite. Source: Reproduced from Zhao, M.-Q., Zhang, Q., Huang, J.-Q., Tian, G.-L., Chen, T.-C., Qian, W.-Z., et al., 2013b. Carbon 54, 403 with permission from Elsevier.

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by means of homogeneous precipitation, and used as electrode material for supercapacitor purposes at high currents, leading to improved performances. Similarly, in 2010, a CNT/NiAl-LDH nanocomposite with controllable LDH mass ratio was described by Li and co-workers, showing LDH platelets with a lateral size of 10
15 nm (Wang et al., 2010; Du and Fang, 2010). At that time the important influence of the functionalization degree of CNT was pointed out, paving the way for future works (Du and Fang, 2010).

10.3.1 Synthesis The two main approaches for the synthesis of these nanocomposites are LDH formation directly on CNTs and the in situ growth of CNTs on the LDH structure. The reassembly of both halves is not usually chosen as a synthetic method but some examples can be found in the literature.

10.3.1.1 Reassembly of CNTs and LDHs Reassembly of CNTs and LDHs has been carried out by authors such as Qiao et al. (2013) and Wang et al. (2016). In both cases, the LDH material was delaminated in formamide, followed by the addition of an NaOH (Qiao et al., 2013) or aqueous solution (Wang et al., 2016) with functionalized CNTs. Electrostatic interactions between opposing charges (cationic sheets of LDH and anionic groups in the CNT) are the driving force for the self-assembly process. Fig. 10.13, extracted from Qiao et al. (2013), represents a schematic description of the procedure.

Figure 10.13 Scheme of the formation of multiwalled CNT/LDH nanosheet nanocomposites. Source: Reproduced from Qiao, Z., Gao, C., Sun, B., Ai, S., 2013. J. Inorg. Organomet. Polym. Mater. 23 (4), 871 with permission from Springer.

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10.3.1.2 LDH formation on CNTs LDH formation on CNTs is the most typical method of synthesis for CNT/LDH nanocomposites, where the LDH phase has been usually synthesized by means of a coprecipitation or solvothermal method. In 2013, Dai and co-workers described the synthesis of a NiFe-LDH/CNT hybrid with an excellent electrocatalytic activity for the oxygen evolution reaction (OER), which boosted the research on NiFe applications in water splitting (Gong et al., 2013). They used the nucleation and growth of NiFe-LDH on the surface of dispersed oxidized CNTs at moderate temperatures (85 C), and then the solvothermal growth at 160 C obtaining crystalline NiFe-LDH platelets strongly interacting with partially reduced CNTs. However, this synthetic approach leads to the formation of spinel impurities, a typical problem associated with the aqueous synthesis of non-Al-containing LDH, like NiFe or CoFe, for example (Abella´n et al., 2013a, 2013c). A similar work was developed by. Yang et al. (2013b), this time combining a ternary NiCoAl-LDH with activated multiwalled CNTs via a homogeneous urea precipitation method. The resulting nanocomposite exhibited high specific capacitances and good results in rate capability or cyclic stability. Zhao et al. (2014a) explored the formation of a nanocomposite between NiMn-LDH or CoMn-LDH and CNTs previously treated with HNO3. The formation of the hybrid material was achieved by mixing the corresponding metal salts and the functionalized CNTs by an in situ coprecipitation method [Fig. 10.14, extracted from the work of Zhao et al. (2014a) depicts a scheme of the procedure]. The resulting nanocomposite was tested as an electrode material in supercapacitors, giving rise to excellent cyclability and high specific capacitance.

Figure 10.14 Scheme with the formation of CNT/NiMn-LDH nanocomposite. Step (I): surface modification of the CNT by functional groups such as
OH
,
CO or
COO
. Step (II): grafting of the NiMn-LDH nanosheets to the CNT structure by an in situ growth method. Source: Reproduced from Zhao, J., Chen, J., Xu, S., Shao, M., Zhang, Q., Wei, F., et al., 2014a. Adv. Funct. Mater. 24 (20), 2938 with permission from Wiley.

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10.3.1.3 CNTs formation on LDHs This approach relies on using CVD procedures, which usually give rise to a mixture of carbon species like graphene, carbon spheres, or CNTs (Abella´n et al., 2014a; Hima et al., 2008; Pacuła et al., 2015). The work of Hima et al. (2008) clearly defines how the CVD conditions directly affect the final CNFs (see Fig. 10.15 extracted from that work). The authors stated that the different CNFs may arise from an overgrowth of the initially formed 1D tubular structures, which gain volume as long as the CVD time is increased. This fact also affects the wettability properties of the nanocomposite, leading to superhydrophobic films in the case of the longest growing times. The selective formation of a specific CNF is governed by several factors, being the size and surface density (NP/m) of the catalytic nanoparticles (e.g., Fe NPs) generated in situ, one of the most important parameters. The fine-tuning of the size by using FeMgAl-LDH intercalated with MoO22 4 , which act as a pinning center for the Fe NPs generated after calcination, can lead to the selective formation of SWCNTs and more complex helical structures (Zhao et al., 2010b). Another factor that was revealed as very important in the specific growth of CNTs was the final

Interwoven CSs

Nanometric Co particles

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Calcined CoAl-LDH Sillicon wafer Sillicon wafer

CoAl-LDH Platelet-like particles

Calcination 700ºC/2 h

Sillicon wafer

Figure 10.15 Schematic formation of different nanocomposites of carbon structures after using CoAl-LDH particles as catalysts. Interwoven carbon spheres, caterpillar-like carbon fibers, and carbon nanotubes are reported. Source: Reproduced from Hima, H.I., Xiang, X., Zhang, L., Li, F., 2008. J. Mater. Chem. 18 (11), 1245 with permission from the Royal Society of Chemistry.

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disposition of the 2D crystallites. In this sense, magnetic NiFe-LDHs are a special class of hydroxides that exhibit the highest catalytic metal concentration allowed by these layered entities. However, it was not until 2014 that their application in direct nanocarbon formation was first studied (Abella´n et al., 2014a). Interestingly, bulk NiFe-LDH led to a wide variety of CNFs such as few-layer graphene, carbon fibers, and MWCNTs, whereas ultrathin films c.150 nm thick with parallel orientation of the crystallites conduce to a hierarchical growth of bamboo-like CNTs with a thickness of c.250 nm at relatively low temperatures (550 C) (Abella´n et al., 2014a) (Fig. 10.16).

Figure 10.16 FESEM images of bamboo-like CNT growth over NiFe-LDH platelets after (A, C, E) a chemical vapor deposition (CVD) and (B, D, F) a catalytic CVD (in absence of H2) procedures. Source: Reproduced from Abella´n, G., Carrasco, J.A., Coronado, E., Prieto-Ruiz, J.P., PrimaGarcı´a, H., 2014a. Adv. Mater. Interfaces 1 (6), 1400184 with permission from Wiley.

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Other CNF/LDH nanocomposites

Despite the fact that graphene/LDH and CNT/LDH nanocomposites are the most studied families of hybrid materials consisting of LDHs and carbon species, nanocomposites based on LDHs and other CNFs like fullerene, carbon spheres, nanofibers, carbon dots, or carbon rings have been reported.

10.4.1 Fullerene/LDH nanocomposites This type of material has been described by authors like Liu et al. (2012b), Liu (2013), and Fortner et al. (2012). The first example of C60/LDH nanocomposite dates from 1996, and was synthesized by simply mixing powders of dodecyl sulfate-intercalated MgAl-LDH with C60 solutions in toluene or hexane. The hydrophobic character of the interlamellar space of the alkyl-functionalized LDHs allowed the diffusion of the C60 molecules within the layers, and the ulterior calcination of the system led to the decomposition of dodecyl molecules forming the final C60/LDH nanocomposite (Tseng et al., 1996). A few recent examples were reported using the anion-exchange capacity of the LDHs, for example, by preparing benzoic acid-intercalated LDHs from NO3-LDHs and then adding the fullerene in the interlamellar space by means of ultrasonic treatment (Liu, 2013). It is worth remarking that no thorough characterization studies have been reported for this family of hybrids, as its physical and chemical properties are still unknown. Figure 10.1C1 exhibits a schematic representation of the C60/LDH nanocomposite.

10.4.2 Carbon quantum dot/LDH nanocomposites These nanocomposites were firstly reported in 2014 by Kang et al. They decorated NiFe-LDH with 0D carbon quantum dots (CQDs) of c.5 nm in a three-step synthetic process consisting of mixing first the metal salts and CQD solutions at 85 C for 4 h. Then, they transferred the mixture to a solvothermal reaction at 120 C for 12 h, followed by a final solvothermal process at 160 C for 2 h. A similar approach was developed by both Liu et al. and Zhang et al. (Liu et al., 2016a; Wei et al., 2016) for the synthesis of hybrids with MgAl and NiAl-LDH, respectively. Fig. 10.17, extracted from Tang et al. (2014), exhibits some parts of the characterization of the CQD/NiFe-LDH nanocomposite. The hybrids were tested as electrocatalysts for water oxidation as well as supercapacitors, with good results due to the synergy between both parts of the material.

10.4.3 Carbon spheres/LDH nanocomposites These nanocomposites were synthesized by authors like Zhang (Gong et al., 2011) and He (Xu et al., 2014) with the aim of exploring their electrochemical applications since in this field carbon spheres have been demonstrated to be of interest (Chen et al., 2012; Wickramaratne et al., 2014). Regarding the synthetic

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Figure 10.17 (A) SEM, (B) TEM, and (C) HRTEM images of a carbon quantum dot/NiFeLDH nanocomposite. (D) XRPD spectra of the nanocomposite and the pure LDH phase. Source: Reproduced with permission from Tang, D., Liu, J., Wu, X., Liu, R., Han, X., Han, Y., et al., 2014. ACS Appl. Mater. Interfaces 6 (10), 7918. Copyright 2014 American Chemical Society.

methodology, the direct reassembly of the spheres and LDHs has been explored by means of ultrasonication procedures, preparing both building blocks separately and then mixing them at the same time (Gong et al., 2011), whereas the most typical approximation of synthesizing these hybrids is by the CVD process, using the LDH material as a catalyst and a carbon source like ethylene (Carrasco et al., 2016a) for the formation of the spheres at high temperatures (e.g., 900 C) (Jin et al., 2005). Fig. 10.18 depicts the schematic formation by direct assembly of a carbon sphere/ LDH nanocomposite (Gong et al., 2011).

10.4.4 Carbon (nano)fibers/LDH nanocomposites The main approach pursued to obtain this kind of nanocomposite is the in situ LDH formation on carbon fibers (Yu et al., 2016; Zhao et al., 2013a; He et al., 2014; Warsi et al., 2014). In a typical procedure, the LDH is synthesized by means of a hydrothermal method, adding the carbon fibers to the mixture. He et al. (2014) prepared a mixture of bohemite AlOOH/carbon nanofibers and then an Ni salt plus urea were added to the aqueous solution. After some time, the final solution was transferred to an autoclave and the reaction was set at 100 C for 48 h. The resulting

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Figure 10.18 Schematic procedure for the synthesis of a carbon sphere/LDH nanocomposite through a direct assembly approach. Source: Reproduced with permission from Gong, J., Liu, T., Wang, X., Hu, X., Zhang, L., 2011. Environ. Sci. Technol. 45 (14), 6181. Copyright 2011 American Chemical Society.

nanocomposite was collected after a centrifugation process. Authors like Zhao et al. (2013a) also used MnO2 nanowires to enhance the supercapacitive properties of the nanocomposite [see Fig. 10.19 extracted from Zhao et al. (2013a)]. The main driving force in the development of these hybrids was their applications related to electrochemistry, especially for supercapacitors in this case (He et al., 2014). In addition to the previously described LDH nanocomposites, there are also some punctual examples of hybrids with other CNFs. For example, carbon nanorings or nano-onions/LDH have been synthesized by the groups of Zeng (Xu et al., 2001), Coronado (Abella´n et al., 2012a) (Fig. 10.20), and Yang (Sun et al., 2013) using Fe, Co, or Fe-containing LDH as catalytic nanoreactors. Indeed the pioneering works of Zeng and co-workers revealed that the in situ generated low-valencestate cobalt oxides during the calcination of terephthalate anion (TA2
)-intercalated CoMgAl-LDH led to the formation of MWCNTs with a diameter of 10
35 nm and a length of 20
200 nm (Xu and Zeng, 2000). The intercalation of poly(vinyl sulfonate)-intercalated CoAl-LDH led to the formation of sulfate-doped CNF (Xu et al., 2001). It is worth mentioning that since 1995 several efforts have been developed for the synthesis of meso/microporous 3D carbons using LDH as containers (Putyera et al., 1995, 1996; Hibino, 1996; Leroux and Dubois, 2006; Leroux et al., 2006; Pre´vot et al., 2011). More recently, the thermal decomposition of an NiFeLDH intercalated with sebacate molecules, acting as a carbon source, enables the low-temperature formation of carbon nano-onions (CNOs) and multiwalled nanotubes (MWNTs) through a simple reaction (Abella´n et al., 2012a). This process benefits from the catalytic activity of the FeNi3 nanoparticles formed in situ, which facilitate the decomposition of the confined sebacate molecules to produce CNFs

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Figure 10.19 (A, B, C) SEM images of the MnO2/LDH/carbon fiber nanocomposites. The inset in (B) corresponds to the EDS results. (D) EDS mapping of a single nanostructure. (E) TEM image of an individual MnO2/LDH composite (the inset corresponds to the FFT pattern). (F) Raman spectra of MnO2, carbon fiber/LDH and the MnO2/LDH/carbon fibers nanocomposite. Source: Reproduced from Zhao, J., Lu, Z., Shao, M., Yan, D., Wei, M., Evans, D.G., et al., 2013a. RSC Adv. 3 (4), 1045 with permission from the Royal Society of Chemistry.

embedded into a graphitic shell via a bottom-up approach. Remarkably, this was the first example of low-temperature (400 C) formation of CNFs and metal alloy nanoparticles, instead of LDO using LDH hybrids as precursors (Abella´n et al., 2012a, 2013b; Carrasco et al., 2016a). By following similar approaches carbon nanorings have been reported by Sun et al. (2013).

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Figure 10.20 (A) Schematic illustration of the formation of FeNi3-carbon nanocomposites and their corresponding carbon nanoforms after the acid leaching procedure. (B) HRTEM images of some carbon nanoforms and the FeNi3 nanoparticles acting as catalysts. Source: Reproduced from Abella´n, G., Martı´-Gastaldo, C., Ribera, A., Coronado, E., 2015. Hybrid Materials Based on Magnetic Layered Double Hydroxides: A Molecular Perspective. Acc. Chem. Res. 48 (6), 1601. Copyright 2015 American Chemical Society.

Iwasaki et al. (2013) reported a nanocomposite with nanocoils (also known as helical CNTs) applying a CVD process using NiFe-LDH as a catalyst precursor and acetonitrile as the carbon and nitrogen source for doping the nanocoils. On the other hand, nanobelts/LDH were observed by Su et al. (2011). This time, the composite was prepared by the in situ synthesis of the nanobelts in the CoAl-LDH phase after submitting a delaminated CoAl-dodecyl sulfate LDH colloidal suspension to a solvothermal technique (120 C for 24 h in a stainless steel Teflon-lined autoclave). The final hybrid was collected after filtering the resulting solution. Finally, other carbon/LDH (activated carbon, carbon cloth, or carbon dots) nanocomposites were reported by several authors (Zhang et al., 2014; Shi et al., 2016; Malak-Polaczyk et al., 2010). For example, carbon dot/LDH hybrids were prepared by direct reassembly of LDH and the carbon dot solution, and the resulting hybrid was tested for catalysis due to its peroxidase-like activity (Guo et al., 2015), and also as adsorbents to methyl blue due to the electrostatic interactions and hydrogen bonding that are established between the dye and the composite (Zhang et al., 2014). Be´le´ke´ et al. (2013) synthesized a carbon/NiAl-LDH by liquid-phase deposition (LDH formation on carbon black), whereas Malak-Polaczyk et al. (2010) prepared a similar hybrid by means of a homogeneous coprecipitation method. In both cases the

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Figure 10.21 Synthesis of the mildly oxidized graphene/single-walled CNT/NiFe-LDH electrocatalysts with a dual-sized distribution of the LDH platelets. Source: Reproduced from Zhu, X., Tang, C., Wang, H.-F., Zhang, Q., Yang, C., Wei, F., 2015. J. Mater. Chem. A 3 (48), 24540 with permission from the Royal Society of Chemistry.

resulting composites were electrochemically tested. Carbon black/LDH nanocomposites have also been reported by authors such as Debdipta et al. in the synthesis of elastomer composites (Basu et al., 2014). Lastly, a carbon cloth/LDH was reported by the group of Mai (Shi et al., 2016) with a honeycomb-like structure by means of a hydrothermal approach, which increases the electrochemically active sites for redox reactions, giving rise to excellent behavior for energy storage applications (Liu et al., 2017).

10.4.5 Graphene/single-walled CNT/LDH nanocomposites These nanocomposites have also been reported in the work of Zhu et al. (2015). In that work, a novel mildly oxidized graphene/single-walled CNT/NiFe-LDH electrocatalyst was synthesized by incorporating LDH crystallites into a carbon network by means of an in situ growth and defect-anchored nucleation (Fig. 10.21). The synergy between the properties of the different parts of the nanocomposite gave rise to a superior OER performance in basic media, highlighting the fact that the oxidized carbonaceous matrix allowed the LDH phase to grow in a highly uniform and fully accessible arrangement, which enhances the utilization efficiency of the active sites.

10.5

Applications of CNF/LDH nanocomposites

CNF/LDH nanocomposites have been used for several applications, Table 10.3 reports a brief overview of the most representative systems, detailing their preparation and highlighting their applications. In the following, we briefly summarize the most important achievements obtained with these nanocomposites in the different fields of application.

10.5.1 Energy storage and conversion As previously seen, energy storage and conversion has become one of the most appealing applied topics for LDH/carbon nanocomposites. In fact, preparation of

Layered double hydroxide nanocomposites based on carbon nanoforms

437

Table 10.3 Overview of some of the carbon material/LDH nanocomposites reported in the literature, highlighting the precursor salts used for the synthesis of the LDH phase, the synthetic method for obtaining the nanocomposite, the reducing agent, if used, and the main application of the nanocomposite Nanocomposite

Graphene oxide/CoAl-LDH

Precursor salts Chlorides

Synthetic Method

Reducing Agent

Layer by layer assembly

Applications

Films with enhanced electrical conductivity

Ref.

51

Graphene/NiAl-LDH

Nitrates

Hydrothermal

Glucose

Supercapacitor

Reduced graphene oxide/NiCo-LDH

Nitrate and chloride

Hydrothermal

Heating at 150ºC

Supercapacitor

Graphene oxide/CoAl-LDH

Chlorides

Coprecipitation

Hydrazine

Supercapacitor

73

Graphene/MgAl-LDH

Nitrates

Hydrothermal

Cr (VI) removal

74

Acetate and nitrate

Hydrothermal

Graphene/MgAl-LDH

Nitrates

CVD

Graphene/NiFe-LDH

Nitrates

Hydrothermal

Flame retardant

142

Graphene oxide/MgAl-LDH

Nitrates

Coprecipitation

CO2 adsorbent

143

Graphene/CoAl-LDH

Chlorides

Coprecipitation

Supercapacitor

144

Graphene/CNT/CoMgAlLDH

Nitrates

CVD

Graphene/CNT/FeMgAl-LDH

Nitrates

CVD

87

CNT/ZnAl-LDH

Nitrates

Hydrothermal

97

CNT/MgAl-LDH

Nitrates

Self-assembly

CO2 adsorbent

98

CNT/NiFe-LDH

Acetate and nitrate

Hydrothermal

Water oxidation

81

CNT/NiMn-LDH

Nitrates

Hydrothermal

Supercapacitor

102

Carbon spheres/CoAl-LDH

Nitrates

CVD

Super-Hydrophobicity

103

Carbon spheres/MgAl-LDH

Nitrates

Self-assembly

Water treatment

113

Fullerene/MgAl-LDH

Nitrates

Self-assembly

Carbon quantum dot/NiFe -LDH

Acetate and nitrate

Hydrothermal

Water oxidation

112

Carbon fiber/CoAl-LDH

Nitrates

Hydrothermal

Supercapacitor

120

Carbon nanoring/CoAl-LDH

Nitrates

CVD

Li-storage

123

Carbon nanobelt/CoAl-LDH

Nitrates

Hydrothermal

Reduced graphene oxide/NiFe-LDH

Hydrazine

Water oxidation

49 52

76 82

Urea

Supercapacitor

86

108

133

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Layered Double Hydroxide Polymer Nanocomposites

efficient catalysts based on LDH nanosheets has been restricted by their tendency to aggregate, insulating nature, and poor stability, etc. Hybridization with different CNFs has been proposed as the most effective solution for boosting the energy-related applications of LDH/CNF nanocomposites. Hereafter, we will succinctly summarize the most prominent examples in this field. We can divide this general topic into three subthemes, namely batteries, supercapacitors, and water-splitting electrocatalysts.

10.5.1.1 Batteries We can define a battery as a device composed of different connected electrochemical cells, therefore providing the required voltage or capacity when they are connected in series or in parallel, respectively (Liu et al., 2016a). The electropositivity of Li (the highest value in the periodic table with 23.04 V versus a standard hydrogen electrode), as well as its lightness, make this element an ideal metal as an anode in a battery device. The main advantages of Li-ion batteries (LIBs) are its high energy density, long cycling life, high voltage, and relatively good environmental compatibility, especially if compared with Pb and Ni batteries (Li et al., 2009; Liu et al., 2010). Nevertheless, the rapid increase in energy demand in recent years has prompted the search for low-cost alternatives for energy storage and conversion (Lewis and Nocera, 2006; Gray, 2009). There is a demand for better batteries with longer life, higher power, and higher energy density. In this context, carbon and its related allotropes have proven to be excellent electrode materials. It has been extensively reported that the synergy between carbon materials with metallic species and derivatives to form nanocomposites can improve the activity and performance of Li-ion batteries (Li et al., 2009), due to the optimized diffusion processes of metal atoms in a carbonaceous matrix and the redox properties of metallic species. In this context, carbon/LDH nanocomposites have provided a good opportunity. The first example was reported in 2012 by Garcı´a and coworkers (Latorre-Sanchez et al., 2012) showing that a NiMn-LDH/GO can act as a precursor for the formation of mixed metal oxides covered and wrapped by rGO layers. They showed a maximum capacity of 1030 mAh/g during the first discharge when used as an anode in LIBs and these values were kept higher than 400 mAh/g after 10 cycles. Cheng et al. (2013) reported a carbon nanofiber/NiAl-LDH nanocomposite as an anode for Li-ion batteries with an enhanced electrochemical activity after the thermal transformation of the LDH in a mixture of metal oxides and metal nanoparticles. The hybrid, synthesized at 550 C, exhibited a cycle discharge of 374 mAh/g and a charge capacity of 330 mAh/g, higher than that observed for carbon-only materials (Fan et al., 2011). In addition, a reversible discharge capacity of 374 mAh/g at a charge
discharge current rate of 0.2C and 119 mAh/g at 5.0C was found. Fig. 10.22, extracted from Cheng et al. (2013), exhibits the Li-ion storage performance of the hybrid material. Moreover, CNTs/LDH nanocomposites have been tested as anode materials for Ni-Zn secondary batteries (Yang et al., 2013a; Yang and Yang, 2013), with an average discharge capacity of about 385 mAh/g and high cycle stability with a capacity maintenance of 95% after 200 cycles.

Layered double hydroxide nanocomposites based on carbon nanoforms

(A)

439

(B)

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Figure 10.22 Li-ion storage performance of the herringbone carbon nanofibers and nanotubes obtained using LDHs as catalysts. (A) CV profiles, (B) voltage profiles of the galvanostatic charge
discharge at a current rate of 0.2C, (C) discharge capacities at different current densities, and (D) cycling performance of the carbon nanofibers at 0.5C. Source: Reproduced from Cheng, X.-B., Tian, G.-L., Liu, X.-F., Nie, J.-Q., Zhao, M.-Q., Huang, J.-Q., et al., 2013. Carbon 62, 393 with permission from Elsevier.

Another type of promising energy storage device, which has emerged in the last few years, is high-power lithium
sulfur batteries. In this line, unstacked graphene layers separated by a large amount of mesosized protuberances synthesized using MgAl-LDH as templates exhibited an excellent performance even after 1000 cycles, with high reversible capacities of c.530 and 380 mAh/g, which were retained at 5.0C and 10.0C, respectively (Zhao et al., 2014c).

10.5.1.2 Supercapacitors Electrochemical supercapacitors, also known as ultracapacitors, have gained increasing interest in the scientific community due to their high-power density and long lifecycle, among other properties (Wang et al., 2012a). Supercapacitors can be divided into two main types, the electrical double-layer capacitors (EDLCs), where the capacitance comes from the charge accumulation, thus being dependent on the surface area of the electrode materials, which is accessible to the electrolyte ions, and the pseudo-capacitors, where reversible and fast faradic processes are generated due to the presence of electro-active species (Zhang and Zhao, 2009). Carbonaceous materials like CNTs, graphene, or activated carbon are often used as electrodes (mainly in EDLC) due to their good conductivity, flexibility, high surface area, and electrocatalytic active sites allowing the generation of redox reactions

440

Layered Double Hydroxide Polymer Nanocomposites

(Pandolfo and Hollenkamp, 2006). Combining CNF with LDH gives rise to very promising nanocomposites for creating high-performance supercapacitors working at basic pHs, due to their full use of the double-layer capacitance and high activity of the faradaic reactions (Zhao et al., 2012). Some of the first examples of CNF/LDH nanocomposites as supercapacitors were developed using CNTs. Thus, a CNT/LDH was described in 2008 by Su et al. (2008). It exhibits better values of specific capacitance than the pure LDH (CoAlLDH in that work)
342 F/g in the hybrid compared to 192 F/g in the pure material —with a small loss of its capacitance of 10% after 400 cycles. The enhancement of the electrochemical properties relies on the high surface area and good electronic conductivity provided by the CNTs. Zhao et al. (2014a) reported a hybrid of CNT/ NiMn-LDH reaching a high specific capacitance (measured by galvanostatic charge
discharge curves) of 2960 F/g at 1.5 A/g, which is 1.8 times higher than that observed for a simple CNT/Ni(OH)2 composite. Even at high specific current, such as 30 A/g, the 79.5% of the capacitance was retained. Moreover, as the stability of these hybrid materials in alkaline solutions is crucial for developing commercial alkaline batteries, cycling life tests were measured at 12 A/g, exhibiting a loss of c.3% after 2000 cycles. Fig. 10.23, extracted from Zhao et al. (2014a), displays the electrochemical performance of the CNT/NiMn-LDH nanocomposite.

(A)

(B)

80 20 mV/s

0.5

Ni2Mn1-LDH/CNT

Current (A/g)

Ni3Mn1-LDH/CNT

40

Ni5Mn1-LDH/CNT

0.3

Ni(OH)3/CNT

Ni7Mn1-LDH/CNT

0

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Ni5Mn1-LDH/CNT

0.1

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Ni(OH)3/CNT

(C)

0.1

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(D)

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Ni2Mn1-LDH/CNT

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Ni7Mn1-LDH/CNT Ni(OH)3/CNT

2500 2000 1500 1000 0

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400 600 Times

3000

800 1000 I = 12 A/g

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2000

Ni2Mn1-LDH/CNT Ni5Mn1-LDH/CNT

1500 Ni(OH)3/CNT

1000

Ni7Mn1-LDH/CNT

0

500

1000 1500 2000 2500 3000 Cycles

Figure 10.23 Electrochemical performance of the CNT/NiMn-LDH and the CNT/Ni(OH)2 nanocomposites. (A) CV curves, (B) galvanostatic discharge curves, (C) specific capacitance, and (D) cycle stability. Source: Reproduced from Zhao, J., Chen, J., Xu, S., Shao, M., Zhang, Q., Wei, F., et al., 2014a. Adv. Funct. Mater. 24 (20), 2938 with permission from Wiley.

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As far as the graphene is concerned, the first examples were developed using self-assembled exfoliated CoAl-LDH and NiAl-LDH, and GO nanosheets, as detailed in Section 10.2.1.1. These nanocomposites exhibited specific capacitances up to 1031 F/g at a specific current of 1 A/g. Additionally, CoAl and CoNi-LDH/ rGO hybrids have been reported showing capacities as high as c.650 F/g (Ma et al., 2014). Other nanocomposites with good results were observed by authors like Zhang et al., with a carbon quantum dot/LDH nanocomposite (Wei et al., 2016), or Yang et al. (2013c), who reported the synthesis of a CNT/NiAl-LDH/rGO nanocomposite with high capacitances (1869, 1796, and 1562 F/g at 1, 2.5, and 5 mA/ cm2, respectively). A complete revision of the various LDH/carbon nanocomposites acting as supercapacitors can be found in the work of Cao et al. (2016). The research interest in the field of CNF/LDH supercapacitors is constantly increasing. However, it is well known that a test fixture configuration—like a two-electrode cell using large amounts (. 10 mg) of active materials in order to obtain reliable measurements—is more closely related to the performance of commercially available packaged cells than other conventional lab-scale configurations (Be´guin and Fra˛ckowiak, 2013; Stoller and Ruoff, 2010). Along this front, several efforts have still to be made in order to achieve the commercial application of CNF/LDH hybrids into the market.

10.5.1.3 Water splitting Electrocatalytic water splitting (WS) drives the conversion of water into hydrogen fuels and arguably qualifies as the most promising source of limitless, clean, renewable energy. The OER (2H2O ! O2 1 4H1 1 4e2) is the key limiting process as it is energetically disfavored by the thermodynamic stability of water (OliverTolentino et al., 2014; Dau et al., 2010). It involves the transfer of four electrons per molecule with a complex mechanism affecting the overall energy conversion efficiency (Liang et al., 2013). Practical electrochemical H2 production is generally limited by the high overpotentials necessary, imposed by the slow kinetics of O2 evolution, and the poor stability of electrode materials under alkali conditions. To date, the dominant electrocatalysts for OER are based on oxides of precious metals like iridium (IrO2) or ruthenium (RuO2) oxides based on their efficiency and sustainability (Walter et al., 2010). Their high cost and scarcity nonetheless have hindered integration into commercial electrolyzers and prompted the search for inexpensive, more abundant alternatives featuring similar efficiencies—high current densities at low overpotentials—and chemical stabilities. LDHs have been postulated as very efficient bimetallic earth-abundant electrocatalysts. Indeed, nanosheets of NiFe-LDH display very high OER activity in alkali solutions and have been proposed as excellent electrodes for ultrafast Zn
air batteries (Gong et al., 2013; Li et al., 2013; Gray, 2009; Lu et al., 2010). However, their poor electrical conductivity and tendency of the layers to re-pack might compromise the stability of the electrode featuring poor sustainability. This can be overcome by hybridization with conducting CNF, dramatically improving its performance. The surface exposure and the direct interfacial contact of all transition metal

442

Layered Double Hydroxide Polymer Nanocomposites

atoms and CNFs are very favorable in expediting the redox reaction due to a fast proton-coupled electron transfer process, and significantly shorter diffusion distances of the reactants. CNF/LDH nanocomposites have been tested recently by some authors focusing on this application. Dai et al. reported a CNT/NiFe-LDH nanocomposite with higher OER activity and stability than Ir-based catalyst, which it also beats in economic cost (Gong et al., 2013). The key to that activity is the growth of LDH nanoplatelets on the surface of CNTs, therefore combining the OER activity of pristine NiFeLDH (Carrasco et al., 2016b) with the electron transport properties of CNTs. The OER activity of the hybrid was studied under alkaline solutions in a three-electrode cell, and the onset potential was 1.50 V versus the reversible hydrogen electrode (RHE) in 0.1 M KOH. The Ir/C catalyst exhibits a similar value, whereas its OER current density value is lower than that of the CNT/NiFe-LDH at c.1.52 V. In 1 M KOH, the OER onset potential of the nanocomposite was c.1.45 V versus RHE. The durability of the CNT/NiFe-LDH composite was also studied, resulting in good (B)

IrlC

GC

0

/CNT

10

CFP Potential (V vs RHE)

IrlC

Potential (V vs RHE)

Potential (V vs RHE)

(D) 1.56

1.56 1.54

NiFe-LDH/CNT

1.52 1.50

IrlC

1.48 NiFe-LDH/CNT

1.46

/CN

0

Potential (V vs RHE)

(C) 1.60 0.1 M KOH 1 M KOH

IrlC

1.30 1.35 1.40 1.45 1.50 1.55 1.60

1.30 1.35 1.40 1.45 1.50 1.55 1.60

1.58

DH

20

T

-LDH

30

NiF e-L

/CN

5

DH

10

NiF e-L

NiFe

15

T

-LDH

20

NiFe

/CNT

25

0.1 M KOH 1 M KOH

40

IrlC

0.1 M KOH 1 M KOH

IrlC

Current density (mA/cm2)

35 30

Current density (mA/cm2)

(A)

0.1 M KOH 1 M KOH

1.54

IrlC

NiFe-LDH/CNT

1.52 IrlC

1.50 NiFe-LDH/CNT

1.48

GC

1.44 0

200

400 600 Time (S)

800 1000

CFP 0

10

20 30 40 Time (min)

50

60

Figure 10.24 Electrochemical performance of the CNT/NiFe-LDH nanocomposite as a OER catalyst. (A) Ir-corrected polarization curves of the nanocomposite and Ir/C catalyst on GC electrode in basic media. (B) Ir-corrected polarization curves of the nanocomposite and Ir/C catalyst on carbon fiber paper. (C) Chronopotentiometry curves of the nanocomposite and Ir/ C catalyst on GC electrode at a current density of 2.5 mA/cm2. (D) Chronopotentiometry curves of the nanocomposite and Ir/C catalyst on CFP at a current density of 5 mA/cm2. Source: Reproduced with permission from Gong, M., Li, Y., Wang, H., Liang, Y., Wu, J.Z., Zhou, J., et al., 2013. J. Am. Chem. Soc. 135 (23), 8452. Copyright 2013 American Chemical Society.

Layered double hydroxide nanocomposites based on carbon nanoforms

443

values under alkaline solutions. At 2.5 and 5 mA/cm2 the catalyst showed a nearly constant potential in 0.1 M KOH, whereas in 1 M KOH there was a decrease in the working potential at a current density of 5 mA/cm2. Fig. 10.24, extracted from the work of Dai (Gong et al., 2013), displays the electrochemical performance of this material as an OER catalyst in comparison with the Ir/C catalyst. Another CNT/LDH nanocomposite based on CNT/CoMn-LDH was described by Liu et al. (2016b) It exhibited excellent OER performance with an overpotential of 355 mV at 10 mA/cm2 in 0.1 M KOH and good values of electrochemical durability, resulting in a performance similar to that of RuO2. Furthermore, Wei et al. also synthesized a nanocarbon/NiFe-LDH hybrid (graphene and single-walled CNT) (Zhu et al., 2015), whose OER performance exhibited a small onset overpotential of c.240 mV and a low overpotential of 350 mV at 10 mA/cm2 in 0.1 M KOH. Since 2013, a systematic effort has been devoted to the design and synthesis of high-performance CNF/NiFe-LDH hybrids. However, several improvements are still required as, for example, a precise determination of the catalytic activity (the onset potential and the Tafel slopes are difficult to determine unambiguously due to the overlap of the Ni21/Ni31 redox current and OER current), the precise control over the mass ratio between the components and, in operando insights into the underlying mechanism of catalytic behavior. Moreover, an unambiguous control over the interfacial interactions through controlled covalent chemistry remains almost unexplored. A thorough revision of all the NiFe-LDH-based catalysts can be found elsewhere (Gong and Dai, 2015; Jia et al., 2016; Tang et al., 2016).

10.5.2 Catalysis LDHs have been widely used in the field of catalysis as supports or as active materials in different forms and for different purposes. CNF/LDH nanocomposites have been used as photocatalysts, in water oxidation, photodegradation of some pollutants, and for the elimination of NOx and SOx contaminant emissions, to name a few (Fan et al., 2014; Xu et al., 2011; Choudary et al., 2002). Moreover, combined with CVD processes, LDHs can act as a catalyst for the synthesis of CNFs (using a carbon source that can be extrinsically added as a gas into the reactor, or be present as anions in the interlamellar space of the LDH) (Abella´n et al., 2012a, 2014a; Carrasco et al., 2016a). As previously shown, the fact that the LDH phase evolves into a combination of metallic oxides and nanoparticles enhances the catalytic properties of the calcined material (Abella´n et al., 2014a). CNF/LDH nanocomposites are also reported as feasible catalysts, where the carbonaceous matrix increases the catalytic activity of the LDH, favoring the dispersion, mass transfer, and heat processes (Zhao et al., 2012). Li et al. (2003) prepared multiwalled CNT/NiAl-LDH, which was calcined, giving rise to a mixed Ni/Al oxide supported by the CNTs. The resulting nanocomposite was set again in a furnace and reduced with H2 at high temperature, followed by the addition of propylene as a carbon source for the synthesis of new multiwalled CNTs, therefore allowing the formation of more CNTs in relatively high quantity that can be later easily isolated after removing the mixed oxide catalyst through acidic treatment.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 10.25 (A) FESEM image of carbon spheres synthesized after a CVD procedure using NiFe-LDH as catalyst. (B) Histogram displaying the average size of the spheres. (C, D) Focused ion beam (FIB) SEM images of the milled region and a cross-section of the spheres, respectively. (E) 3D representation before and after the FIB, illustrating the solid inner structure. (F) Bright field TEM image of an electron-transparent lamella prepared by FIBSEM, highlighting the porosity between the carbon spheres. Source: Reproduced from Carrasco, J.A., Prima-Garcia, H., Romero, J., Herna´ndez-Saz, J., Molina, S.I., Abella´n, G., et al., 2016a. J. Mater. Chem. C, 4 (3), 440 with permission from the Royal Society of Chemistry.

Carrasco et al. (2016a) reported the synthesis of carbon spheres using NiFeLDHs as catalytic precursors. Fig. 10.25 depicts an electronic microscopic characterization of carbon spheres synthesized by CVD using NiFe-LDHs as catalysts. Photocatalysis has also been reported for some hybrids (Huang et al., 2014b). Fan et al. prepared a graphene/ZnCr-LDH nanocomposite by dispersing the LDH nanoplatelets on exfoliated graphene sheets (Lan et al., 2014). The hybrid was

Layered double hydroxide nanocomposites based on carbon nanoforms

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Figure 10.26 Photodegradation of Rhodamine B over graphene/ZnCr-LDH nanocomposite; (A) alone, (B) in the presence of 1 mM benzoquinone, and (C) in the presence of 10 mM t-BuOH. Source: Reproduced with permission from Lan, M., Fan, G., Yang, L., Li, F., 2014. Ind. Eng. Chem. Res. 53 (33), 12943. Copyright 2014 American Chemical Society.

tested in the photodegradation of Rhodamine B under visible light irradiation. Comparing the composite with the pure LDH phase, it was observed that the graphene matrix improved the transport and separation of the photogenerated charge carriers, therefore leading to better photodegradation results [see Fig. 10.26, extracted from Lan et al. (2014)].

10.5.3 Miscellanea Despite the fact that electrochemistry and catalysis are the main applications studied for these nanocomposites, other fields, such as environmental remediation, materials science, or drug delivery have been explored for the application of LDH/CNF nanocomposites.

10.5.3.1 Environment protection Thanks to the inherent anion-exchange features of the LDHs (Liu et al., 2006), these materials have been widely used as removal agents for different anions, such as Cr (VI) (Hsu et al., 2007), phosphates (Seida and Nakano, 2002), or 2chlorophenol (Chuang et al., 2008). In this context, added to the fact that some CNFs such as CNT or graphene have shown good results in the adsorption of organic pollutants due to their porous structure (Zhao et al., 2011), CNF/LDH nanocomposites are perfectly suited for this application. Yang and co-workers prepared a CNT/mixed metal oxide nanocomposite derived from a LDH for the adsorption of Congo red dye (Yang et al., 2015). The LDH with terephthalic anions in its interlamellar space was pyrolyzed under an N2 atmosphere, giving rise to the

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Layered Double Hydroxide Polymer Nanocomposites

CNT/Mg(Al)O nanocomposite. Its adsorptive properties were tested regarding Congo red dye, giving rise to a peak of 1250 mg/g (pH 5 7, adsorbent dosage of 30 mg). This value is higher than that exhibited by other adsorbents like FeOOH hollow spheres (275 mg/g), activated carbon fibers (557 mg/g), or functionalized CNTs (882 mg/g) (Yang et al., 2015). The main factor responsible for these high values is the interaction between the functionalized CNT surface and the dye, combined with the electrostatic attraction between the positively charged metallic components and the negatively charged Congo red ions. Wang et al. and Liu et al. reported the synthesis of graphene/LDH nanocomposites for the efficient removal of As (V) (Wen et al., 2013) and methyl orange (Yang et al., 2013d). The first (graphene/MgAl-LDH hybrid) exhibited adsorption peaks of 183.11 mg/g, higher than those observed for other adsorbents such as rGO/ magnetite (5.83 mg/g), α-FeOOH (58 mg/g), or Zr (IV)-loaded orange waste gel (88.0 mg/g) (Wen et al., 2013). Fig. 10.27, extracted from Wen et al. (2013), (A) 48.8

(B) 0.7 0.6 0.5

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Figure 10.27 (A) Time profile for the adsorption of As (V) on GO/MgAl-LDH; (B) pseudosecond-order kinetic plots of t/qt versus t; (C) effect of the pH on the adsorption of the As (V); (D) zeta-potential of the nanocomposite as a function of pH. All experiments were tested in m/V 5 0.5 g/L and I 5 0.01 M NaNO3 polyethylene test tubes. Source: Reproduced with permission from Wen, T., Wu, X., Tan, X., Wang, X., Xu, A., 2013. ACS Appl. Mater. Interfaces 5 (8), 3304. Copyright 2013 American Chemical Society.

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depicts the adsorption behavior of this hybrid. The second, a graphene/NiAl-LDH composite transformed in graphene/NiAl mixed oxides after the calcination of the material, presented a maximum adsorption of 210.8 mg/g for methyl orange, in line with that observed for porous materials (Haque et al., 2010). To sum up, due to the improved adsorption capacities of graphene/LDH nanocomposites, they have been used in CO2 adsoprtion showing excellent results (Garcia-Gallastegui et al., 2012). As a matter of fact, Shaffer and co-workers showed that the adsorption capacity of MgAl-LDH can be improved by 62% just by hybridization with rGO (7 wt.%) (Garcia-Gallastegui et al., 2012).

10.5.3.2 Drug delivery It is widely reported in the literature that CNFs such as graphene or CNTs can be used as efficient drug-delivery vectors because of their large surface area and electronic properties. Drugs with aromatic groups can be bonded through π
π interactions to the surface of nanocarbons (Liu et al., 2009; Sun et al., 2008). LDHs can also be used in this context due to their anion-exchange capabilities. Different LDHs, such as MgAl-, ZnAl-, LiAl-, or FeAl-LDH, have been tested as host materials for drug delivery, conforming drug
LDH host
guest supramolecular interactions (Li et al., 2004). For these reasons, CNF/LDH nanocomposites are expected to be good candidates for acting as nanovectors in drug delivery. However, reports regarding this topic are very scarce. Wang et al. (2012b) prepared films of graphene/MgAl-LDH nanocomposite with oxide-benzylpenicilin as the interlamellar anion. They observed a first-order kinetic regarding the controlled release of the drug. Moreover, the effectiveness in the release is enhanced as long as the density of the composite films is increased.

10.5.3.3 Materials science Carbon/LDH nanocomposites have also been used in the field of polymers, mainly in the form of CNT/LDH or graphene/LDH hybrids, taking advantage of the combination of properties of the two counterparts (Costa et al., 2007; Coleman et al., 2006). Fang and co-workers synthesized a CNT/NiAl-LDH nanocomposite by direct formation of the LDH on CNTs (Du and Fang, 2010). The resulting hybrid was tested as a flame retardant for polypropylene. Attending to the heat-release curves of polypropylene, the nanocomposite material exhibited better results than those found for the separated building blocks, with a peak heat-release rate of the hybrid of c.540 kW/m2. This effect can be ascribed to the combination of the free radical trapping effect of the CNT network and the barrier effect. The layered structure of the CNTs with the LDH shields the polymers from external radiation and heat feedback, while acting as excellent thermal insulator layers (Kashiwagi et al., 2005). Huang et al. reported the use of a graphene/LDH nanocomposite as flame retardant for poly(methyl methacrylate) (Huang et al., 2014a). The results depicted an improvement in the thermal stability of the polymer, as well as a reduction in the heat-release rate, CO, and smoke production when compared with the individual

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Layered Double Hydroxide Polymer Nanocomposites

Figure 10.28 Heat-release rate curve for the PMMA alone and hybridized with different components. Source: Reproduced from Huang, Z., Wu, P., Gong, B., Fang, Y., Zhu, N., 2014b. J. Mater. Chem. A 2 (15), 5534 with permission from Elsevier.

parts. Furthermore, the production of volatile compounds, such as hydrocarbons or carbonyl compounds, was also diminished. The enhancement of the properties from the composite was ascribed to the combination of the barrier effect of the graphene and the catalytic carbonization of the LDH. Fig. 10.28, extracted from Huang et al. (2014a), exhibits the heat-release rate for the poly(methyl methacrylate) combined with the LDH, the rGO, and the composite.

10.6

Conclusions

CNF/LDH nanocomposites have been widely studied in recent years thanks to the combination of properties of both LDH and carbon nanomaterials. In some cases they can exhibit newly improved or unexpected properties as a consequence of their hierarchical structures. Overall, three main approaches are reported to prepare the nanocomposites: reassembly of nanocarbons and LDHs, direct formation of LDHs on nanocarbon materials, or the direct synthesis of nanocarbons on the LDH phase. The first is usually achieved by means of liquid-phase exfoliation and dispersion of the materials separately, followed by the self-assembly of both parts driven by electrostatic interactions. Secondly, the direct growth of LDHs on the surface of nanocarbons is carried out generally by means of a coprecipitation or hydrothermal method where the carbonaceous material is added to a reaction mixture with the selected metallic salts. Lastly, the in situ growth of nanocarbons on the inorganic

Layered double hydroxide nanocomposites based on carbon nanoforms

449

materials is usually accomplished by CVD procedures, where the carbon source can be present in the form of carbon-rich intercalated anions in the interlamellar space of the LDH, or added in a gaseous phase (like ethylene) into the furnace. As far as their properties are concerned, the CNF/LDH nanocomposites exhibit improved conductivity, mechanical properties, and redox reactivity; moreover, the hybridization confers hierarchical porosities and better dispersion to the final materials. These hybrids are of fundamental interest in several fields, but they can also be used in a plethora of practical applications including batteries, supercapacitors, water splitting, catalysts, flame retardants, drug delivery, or environment protection. In summary, these classes of nanocomposites are expected to be of major importance because of their wide chemical versatility, straightforward synthesis, low cost, and excellent behavior toward very demanding topics such as energy storage and conversion or catalysis.

10.7

Perspectives

CNF/LDH nanocomposites are very promising candidates for their application in emergent and very demanding areas such as Li-ion batteries, supercapacitors, or water splitting. Indeed, great results have been achieved, like in water oxidation reaction, surpassing the performance of benchmark materials based on precious metals like Ir or Ru oxides. Moreover, these nanocomposites represent one of the most promising alternatives so far reported for ultrafast nickel
iron or metal-air batteries. In addition, the large porous structure of the employed carbon materials, such as graphene or CNTs, combined with the catalytic properties of some LDHs could give rise to hybrids very suitable for heterogeneous catalysis in polymerization processes. Furthermore, although topics like drug delivery or environment protection have started to be explored, further advances are required in those fields for increasing the capacity of adsorbing and transporting anions in these CNF/LDH hybrids. However, despite the great advances, several synthetic challenges should still be faced, as the development of high-quality defect-free graphene hybrids, the control over the metal distribution within the LDH layers, the stability of some LDH phases, or the synthesis of LDH nanocomposites with novel CNFs with promising properties like nanodiamonds or nanohorns. Moreover, hybridization with heteroatom-doped CNF is also a topic of utmost importance for several electrochemical applications. In this sense, more precise chemical control for tuning the particle size, the morphology of the building blocks and the final nanocomposites, or their processability, would boost the development of well-established applications, as well as new ones such as magnetism, nanoelectronics, or sensing, to name a few. Finally, the controlled assembly of functionalized LDH and CNF to create tailormade hybrid architectures remains completely unexplored.

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Acknowledgments The authors acknowledge financial support from the European Commission (ERC Starting Grant 2D-PnictoChem 804110 to G.A., and ERC Proof of Concept Grant HyMAC, 713704 to E.C.), the Spanish MINECO—Ministerio de Economia, Industria y Competitividad (Unit of Excellence Maria de Maeztu MDM-2015-0538 Grant, MAT2017-89993-R co-financed by FEDER and Ramo´n y Cajal Fellowship to G.A.), and the Generalitat Valenciana (SEJI/2018/034 to G.A. and Prometeo Program). G.A. also acknowledges Deutsche Forschungsgemeinschaft (FLAG-ERA, AB694/2-1), and La Caixa Foundation (Junior Leader Fellowship) for their economic support. J.A.C. thanks the University of Valencia for a doctoral fellowship Atraccio´ de Talent.

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Further reading Mallakpour, S., Khadem, E., 2017. Opportunities and Challenges in the Use of Layered Double Hydroxide to Produce Hybrid Polymer Composites. In: Hybrid Polymer Composite Materials. Elsevier, pp. 235
261. Tarascon, J.-M., Armand, M., 2001. Nature 414 (6861), 359. Taviot-Gue´ho, C., Pre´vot, V., Forano, C., Renaudin, G., Mousty, C., Leroux, F., 2018. Tailoring Hybrid Layered Double Hydroxides for the Development of Innovative Applications. Adv. Funct. Mater. 28 (27), 1703868. Wang, X., Zhou, S., Xing, W., Yu, B., Feng, X., Song, L., et al., 2013. J. Mater. Chem. A 1 (13), 4383. Zhang, L., Zhang, X., Shen, L., Gao, B., Hao, L., Lu, X., et al., 2012. J. Power Sour. 199, 395.

Recent advances in layered double hydroxide/polymer latex nanocomposites: from assembly to in situ formation

11

V. Prevot1 and E. Bourgeat-Lami2 1 CNRS, ICCF - Institut de Chimie de Clermont-Ferrand, Universite´ Clermont Auvergne, Clermont-Ferrand, France, 2University of Lyon, Universite´ Claude Bernard Lyon 1, CPE Lyon, CNRS, UMR 5265, Chemistry, Catalysis, Polymers and Processes (C2P2), Villeurbanne, France

11.1

Introduction

In the past two decades, there has been considerable interest in the development of polymer nanocomposite systems for various applications (Felline et al., 2016; Kumar et al., 2017). Some of the reported benefits of nanocomposite formation are improved barrier properties, scratch resistance, hardness/toughness, and fire resistance. In the case of clay particles, one of the main interests is in their ability to exfoliate or intercalate in the presence of polymer chains, allowing enhanced performances at very low filler loadings. It is generally accepted that clay exfoliation (rather than intercalation) favors performance improvement, especially for mechanical reinforcement and barrier properties (Galimberti et al., 2013; Ray, 2014). The benefits of exfoliated clay layers result from the many interactions between the polymer and the nanoclay layers, strengthening and reinforcing the polymer system, and yet being sufficiently small that the particles have no effect on the visual properties of the coating. The improved barrier performance is generally thought to arise from an increase in the tortuous path distance for water to reach the substrate, and this effect can be, and is, achieved by using lamellar platelets. Compared with the well-established use of 2D fillers from the smectite group, such as montmorillonite (MT) (Lambert and Bergaya, 2013), preparing waterborne polymer/layered double hydroxide (LDH) nanocomposites has only recently attracted considerable attention due to the peculiar physical and chemical properties of LDHs (Leroux and Besse, 2001; Costa et al., 2008; Matusinovic and Wilkie, 2012; Rives et al., 2013; Gao et al., 2014; Basu et al., 2014; Kalali et al., 2015). Because of their anion exchange

Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00011-2 © 2020 Elsevier Ltd. All rights reserved.

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Layered Double Hydroxide Polymer Nanocomposites

properties, LDHs are also often referred to as “anionic clays.” LDHs display many interesting features, such as wide-ranging chemical compositions, variable layer charge densities, ion-exchange properties, reactive interlayer spaces, and hydroxylated surfaces. The structure of hydrotalcite—one of the most studied minerals of the LDH family—is related to that of brucite, Mg(OH)2, in which a portion of the Mg21 has been replaced by Al31. Carbonate anions (along with water) are intercalated between the layers to maintain electroneutrality. The hydrotalcite chemical formula is therefore given as Mg0.75Al0.25(OH)2(CO3)0.125 0.5H2O. Based on a combination of a large variety of divalent and trivalent metal cations, the general formula for the LDH family can be written as (MII1
xMIIIx(OH)2)(Am
x/m nH2O), where (MII1
xMIIIx(OH)2)x1 represents the layer, and (Am
x/m nH2O) represents the interlayer species. LDHs are not naturally abundant, however synthetic LDH phases can be easily prepared in a large quantity and organically modified (Rives, 2001; Duan and Evans, 2006; Costantino et al., 2013; Forano et al., 2013). LDH particle size can be tuned over a large range with lateral dimensions ranging from 30 nm (Xu et al., 2006a; Xu et al., 2006b; Gunawan and Xu, 2009) to more than 5 μm (Okamoto et al., 2007) and thicknesses from 1 to 20 nm. The high aspect ratios are of interest for enhancing mechanical properties and permeability in nanocomposites. Moreover, the anion exchange capacity can be tuned from 200 to 450 meq/ 100 g by varying the proportion of trivalent metal cations. Such high values induce strong electrostatic cohesion between the stacked layers and the interlayer species, which limits swelling in water but does not hamper exfoliation under specific conditions, especially during polymer nanocomposite preparation (Wang and O’Hare, 2012). As for other types of nanofillers, most studies have focused on the inclusion of LDHs into solvent-borne formulations and much less attention has been dedicated to waterborne processes. In most cases, it is the nature of the polymer matrix that determines the choice of synthesis method. Typically, in the presence of water-insoluble monomers and/or polymers, organically modified LDHs are incorporated into the nanocomposite by melt intercalation, organic solution intercalation, exfoliation/adsorption or in situ polymerization, while water-soluble polymeric species allow pristine LDHs to be directly intercalated in water. Emulsion and suspension polymerization are important alternatives for producing polymeric particles and nanocomposites in water from water-insoluble monomers (Bourgeat-Lami and Lansalot, 2010). The synthesis of polymer/LDH nanocomposites by emulsion and suspension polymerization has been recently addressed by Qiu and Qu (2011) in a comprehensive review covering the different synthetic pathways for preparing LDH/polymer nanocomposites, their properties, and potential applications. However, the use of postsynthetic assembly techniques and the synthesis of LDH macroporous structures using latex particles as colloidal templates were not covered in this review. This chapter intends to present the current state of the art in the synthesis of waterborne polymer LDH nanocomposites and macroporous LDH materials using latex technology. Special attention will be paid to the interactions involved at the nanoscale level between the polymeric species and the inorganic LDH matrices,

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(A) Assembly of LDH and latex particles Physical blending, LbL assembly

LDH particles

(B) In situ polymerization Monomer, initiator, stabilizer LDH particles

(C) Latex templating

Calcination

In situ LDH synthesis

Reconstruction

LDH precursors Latex

Calcination reconstruction

Scheme 11.1 Scheme illustrating the three main routes to LDH-based nanocomposites and macroporous materials by latex technology: (A) assembly of preformed LDH and latex particles, (B) in situ polymerization, and (C) latex-templating.

since these are crucial for determining the properties of the final nanocomposites. Three main strategies will be distinguished: assembly of preformed LDH and latex particles, in situ polymerization, and latex-templating (Scheme 11.1). In the first approach, the self-assembly process is promoted by electrostatic interactions between the LDH and latex particles. Interfacial interactions and particle sizes thus play a key role in determining the morphology of the resulting nanocomposites. The second strategy relies on the in situ formation of polymer latexes in the presence of LDH particles using emulsion or suspension polymerization. Conversely, the third strategy is a templating approach in which LDH synthesis is performed at the latex surface or confined in a polymeric colloid crystal, giving macroporous materials with enhanced diffusion pathways and reactivities. Finally, the last part is devoted to the main properties of the resulting LDH nanocomposites, particularly mechanical and flame-retardancy properties, and the applications in which LDH macroporous structures have played an important role.

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Layered Double Hydroxide Polymer Nanocomposites

Use of latex technology for the production of LDHbased composite materials and macroporous structures

The possibility to associate latex technology and LDH inorganic building blocks has been investigated only recently compared with other nanocomposite preparation methods. This is mainly due to the specific properties of LDHs, such as poor swelling in water and highly stacked layered structures, limiting their efficient use in waterborne processes. However, recent advances in the synthesis of LDH nanosheets with tunable particle dimensions, exfoliation properties, and improved colloidal stability have enabled the development of new approaches. Waterbone processes permit the design of both LDH structured nanocomposites and LDHbased porous materials. Regardless of the method used, controlling the LDH/polymer interfacial properties is key to achieving the desired morphology and performance of the hybrid materials. The three main routes depicted above are detailed in the following sections and, for each of them, some key aspects and associated challenges are highlighted.

11.2.1 Assembly of preformed LDH and latex particles 11.2.1.1 Layer-by-layer assembly Since Decher and co-workers (Decher, 1997) demonstrated in 1997 that uniform polymer films could be deposited onto mineral substrates by the sequential adsorption of polyanions and polycations using the so-called layer-by-layer (LbL) assembly technique, interest in polyelectrolyte assembly has increased considerably. Over the past 20 years, this technique has evolved into a versatile and scalable tool for colloidal and surface engineering. In brief, the LbL technique relies on the adsorption of a polyelectrolyte onto an oppositely charged surface (typically a solid substrate or a colloid), removing the excess polyions by rinsing, and repeating the procedure sequentially with oppositely charged polyelectrolyte. Charge overcompensation occurs after every adsorption step, which allows adsorption of the subsequent oppositely charged layers. The LbL approach was used for the preparation of thin films involving polyelectrolyte/LDH multilayers on 2D substrates (Li et al., 2005; Liu et al., 2006; Han et al., 2008; Guo et al., 2010), but there has been much less work done on the synthesis of LDH/latex composite materials by the LbL approach. Li et al. (2006) reported the formation of core
shell particles by alternate deposition of Mg-Al-LDH nanosheets and poly(sodium 4-styrene sulfonate) (PSSNa) (up to 20 layer pairs in total) onto polystyrene (PS) beads (Dp 5 1.3 μm) (Fig. 11.1A,B). Hollow capsules were subsequently obtained after removal of the PS core and the PSS polyanion by calcination at 500 C (Fig. 11.1C,D). A slow heating (, 1 C/min) was key to preserving the core
shell morphology. The crystalline LDH structure was destroyed upon heating but was recovered after 12 h exposure to humid air (Fig. 11.1E,F), as attested by high-resolution transmission

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Figure 11.1 Scanning and transmission electron microscopy images of core
shell particles (A, B) and LDH capsules (C, D) obtained by LbL assembly followed by calcination of the colloidal template and reconstruction of the LDH crystalline structure by exposition to humid air (E, F). HR-TEM (G) and XRD (H) were used to confirm the crystalline LDH structure. Source: Reproduced from Li, L., Ma, R., Iyi, N., Ebina, Y., Takada, K. & Sasaki, T. (2006) Hollow nanoshell of layered double hydroxide. Chem. Commun., 3125
3127. Copyright 2017 The Royal Society of Chemistry.

electron microscopy (HR-TEM) (Fig. 11.1G) and wide-angle X-ray diffraction (XRD) (Fig. 11.1H) analyses. Another example of LbL assembly was reported by Bujdoso´ et al. (2011). In this work, sulfonated PS latex particles with 55 and 560 nm average diameters were first prepared and doped with silver particles through the adsorption of silver salts and subsequent reduction by sodium borohydride. A multilayer film was then grown on a glass substrate by deposition of the negatively charged PS@Ag particles and Mg/ Al LDH platelets. Surface plasmon resonance and atomic force microscopy were used to monitor the growth process and determine the thickness of the LbL films. The films made from the large particles were heterogeneous and contained a high amount of free silver nanoparticles, whereas those obtained from the small particles were more regular. Optical measurements revealed that the thickness of the LDH/ PS560@Ag films was built up mainly from the silver nanoparticles. As expected, the films obtained from the smaller particles displayed lower roughness values. The resulting hybrid films are of potential interest as antibacterial surfaces.

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11.2.1.2 Physical blending The production of nanocomposites through physical blending of a polymer latex with inorganic particles or carbonaceous materials such as carbon nanotubes (Regev et al., 2004), few-layer graphene (Noe¨l et al., 2014), graphene oxide, or silica particles (Prasertsri et al., 2013) has been already reported by several authors and is a rather straightforward process. In the case of 2D objects, a segregated network is formed by confining the plate-like fillers in the interstices between the latex particles during the drying process (Bourgeat-Lami et al., 2015). To secure the final segregated network morphology, self-assembly strategies have been reported that aim at promoting interactions between the latex and LDH particles during film formation. As LDH possesses a high positive surface charge in a large range of pH values, electrostatic interaction is often used to secure the cellular morphology. For instance, Veschambres et al. (2016) reported LDH-based nanocomposite films obtained by casting a mixture of LDH platelets (30
140 nm in lateral size) and oppositely charged poly(methyl methacrylate-co-n-butyl acrylate) [P(MMA-co-BA] latex particles (700 nm in diameter). The self-assembly process was driven by mutual electrostatic interaction between the positively charged platelets and the latex beads leading to the formation of a 3D nanostructure as illustrated in Fig. 11.2. Several parameters can influence the self-assembly behavior and the microstructure of the hybrid films. The size ratio between the LDH lateral size and the latex bead diameter is clearly an important parameter, as it determines the morphology of the clay network and the mesh size. Maintaining the colloidal stability of these binary systems is also critical to forming homogeneous films. Indeed, aggregation of the latex
LDH composites may lead to the formation of irregularly shaped clusters and to rough surfaces during coating. The mechanism of interaction between polymer latex particles and oppositely charged LDH platelets and its effect on the colloidal stability of the binary mixture, was studied in detail by Pavlovic et al. (2017). The authors used electrophoretic mobility and time-resolved dynamic lightscattering measurements to estimate the surface charge density and the stability

Figure 11.2 Scheme illustrating electrostatic self-assembly of LDH platelets with oppositely charged P(MMA-co-BA) latex particles and the subsequent formation of a nanocomposite film with honeycomb-like 3D microstructure. Source: Reproduced from Veschambres, C., Halma, M., Bourgeat-Lami, E., Chazeau, L., Dalmas, F. Prevot, V., 2016. Layered double hydroxides: efficient fillers for waterborne nanocomposite films. Appl. Clay Sci., 130, 55
61 with permission from Elsevier.

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ratio of the binary suspensions. Fast aggregation occurred for intermediate clay concentrations due to the neutralization of the negative surface charges of the polymer particles while stable suspensions were obtained for high and low LDH doses (Fig. 11.3A). The electrophoretic mobility increased with increasing LDH concentration due to their adsorption on the oppositely charged latex surface. Charge neutralization occurred at the isoelectric point, where the overall charge of the particles was zero, while further increase of the LDH dose led to charge inversion (Fig. 11.3C). TEM observation of the resulting composite suspensions revealed that the polymer spheres were completely coated with the clay platelets once the inorganic concentration was sufficiently high (Fig. 11.3B).

Figure 11.3 Scheme illustrating the mechanism of charging and aggregation of LDH/latex binary suspensions obtained by mixing stable colloidal suspensions of oppositely charged MgAl-CO3 LDH platelets and P(MMA-co-BA) latex particles (40 mg/L) at pH 9 for different ionic strengths. (A) Schematic representation of the aggregation mechanism as a function of the LDH dose, (B) TEM image for the highest LDH dose (1000 mg/g) where the latex underwent charge inversion, (C) electrophoretic mobility, and (D) stability ratio values. Source: Reproduced from Pavlovic, M., Rouster, P., Bourgeat-Lami, E., Prevot, V., Szilagyi, I., 2017. Design of latex-layered double hydroxide composites by tuning the aggregation in suspensions. Soft Matter, 13, 842
851 with permission from The Royal Society of Chemistry.

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The obtained results, and in particular the influence of ionic strength on the stability ratios, indicated that the aggregation behavior of the suspensions could be satisfactorily described by the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory with the contribution of two opposing interparticle forces: van der Waals attraction and electrostatic double-layer repulsion. Interestingly, an additional attractive Coulomb interaction between positively charged LDH patches on the particle surface and negatively charged empty places on other approaching latex particles, was also identified. The question of dispersion stability was also addressed by Braga et al. (2014) during the preparation of nitrile rubber/LDH composites by latex blending and coagulation. Pluronic F-127 was used as a dispersing agent together with ultrasound in order to decrease the size of the clay agglomerates in water and promote the formation of stable suspensions. Steric repulsion between the nitrile rubber and LDH particles enabled homogeneous dispersion of the clay platelets in the nitrile rubber matrix. During the drying process, with the evaporation of water, the viscosity increased, fixing likewise the LDH plates in the system while heat-curing crosslinked the different nitrile rubber particles together. Although XRD did not show any effect of Pluronic F-127 on clay exfoliation, field emission gun scanning electron microscopy (FE-SEM) revealed that the LDH particles were more homogeneously dispersed in the matrix when the surfactant was used.

11.2.2 LDH-based nanocomposites by in situ emulsion and suspension polymerizations In situ polymerization consists of performing the polymerization reaction in the presence of the filler material. In the in situ intercalative polymerization method pioneered by Toyota (Zhao et al., 2011; Usuki et al., 1993a, 1993b), the layered silicate is swollen with a liquid monomer (or eventually a monomer solution), and the polymerization occurs between the intercalated clay layers. The synthesis of nanocomposites via in situ emulsion or suspension polymerization proceeds according to a fundamentally different mechanism due to the multiphasic nature of the polymerization medium. In order to facilitate understanding of the following discussion, the specific features of these two polymerization processes are briefly described below before giving detailed examples illustrating the potential of these techniques for the production of LDH-based nanocomposites.

11.2.2.1 Conventional emulsion polymerization Emulsion polymerization allows the synthesis of stable aqueous colloidal dispersions of polymer particles, known as latexes (Chern, 2006). In “conventional” emulsion polymerization, the polymer particles are formed by starting from an insoluble (or scarcely soluble) monomer emulsified by the aid of a surfactant above its critical micellar concentration. The monomer is originally distributed between coarse emulsion droplets, surfactant micelles, and the water phase, where a small proportion of monomer (depending on its solubility) is molecularly dissolved. The initiator

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is soluble in water and polymerization thus starts in the aqueous phase by the formation of radicals through the initiator thermolysis and the addition of the first monomer units. The formed oligoradicals are captured by the monomer-swollen micelles where the polymerization continues, leading to solid particles. The growth of particles proceeds by diffusion of monomer from the monomer droplets, through the aqueous phase, into the growing particles. Emulsion polymerization is very versatile and allows tuning of the particle morphology and composition (e.g., formation of core
shell particles and other equilibrium morphologies) by successive additions of different monomers. Compared to bulk or solution polymerization, emulsion polymerization offers significant advantages, such as better control of heat and viscosity of the medium, and the possibility of increasing the molecular weight of the polymer chains without affecting the rate of polymerization. Over the last 20 years, emulsion polymerization has proven highly suitable for the production of polymer/inorganic particles (Bourgeat-Lami and Lansalot, 2010). In particular, there has been a large body of experimental work on the incorporation of smectite clays (mainly Laponite and MT) into polymer latexes (Bourgeat-Lami et al., 2011). In contrast, the amount of scientific publications reporting the synthesis of LDH-based composite latexes by emulsion polymerization can be brought down to a few papers. One main reason is that, as mentioned earlier, LDHs have poor swelling properties in water. To circumvent this issue, Chen et al. (2004) reported an all in situ approach wherein LDH platelets are formed at the same time as the latex particles. The composite material was obtained by adding an aqueous solution of sodium hydroxide to an emulsion consisting of magnesium and aluminum metal salts, methyl methacrylate as a monomer, sodium dodecyl sulfate (SDS) as surfactant, and benzoyl peroxide (BPO) as a thermal initiator, and increasing the temperature to 80 C to start polymerization. The authors reported that in situ intercalation of dodecyl sulfate (DS) anions allowed the polymerization to proceed in the clay galleries, resulting in intercalated PMMA chains. Unfortunately, since the latex suspension was unstable, no information was given at that time on particle morphology. The main advantage of the in situ approach however, is that the LDH platelets are well dispersed in the polymer matrix, leading to transparent composites even for high clay loadings (up to 50 wt%). Ding and Qu (2005) used a very similar approach to synthesize exfoliated PS/ZnAl LDH. The SDS surfactant was replaced by N-lauroyl-glutamate (LG), while n-hexadecane was used to promote clay exfoliation (Fig. 11.4). XRD confirmed the successful formation of the LDH platelets. Again, as the latex was precipitated before characterization, there was no information about latex stability and morphology. In a subsequent paper from the same group (Ding and Qu, 2006), the nature of the surfactant and the presence of n-hexadecane were shown to be key to the preparation of a fully exfoliated morphology. The authors studied the thermal properties of the resulting materials and showed that the thermal decomposition temperature of the PS/LDH nanocomposites was comparable to that of the PS matrix for low clay contents but decreased with increasing clay contents. This was associated with the presence of LG, which decomposed in the range 180
320 C. To prevent the drawback of the presence of a surfactant, Qiu and Qu (2006) prepared PS/LDH composites via surfactant-free

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Figure 11.4 Scheme illustrating the possible mechanism of formation of exfoliated PS/LDH nanocomposites via all in situ LDH and latex synthesis by emulsion polymerization of styrene in the presence of n-hexadecane. Source: Reproduced from Ding, P., Qu, B., 2005. Synthesis and characterization of exfoliated polystyrene/ZnAl layered double hydroxide nanocomposite via emulsion polymerization. J. Colloid Interface Sci. 291, 13
18 with permission from Elsevier.

emulsion polymerization using potassium persulfate (KPS) as a thermal initiator. XRD was used to monitor the changes to the interlaying spacing throughout the course of the reaction, and revealed successful intercalation of the persulfate initiator anions in between the LDH layers. As the polymerization progressed, the intercalated S2O82
ions as well as the initiator molecules in solution formed oligomers which entered the clay galleries and further enlarged the interlayer space, leading to a fully exfoliated PS/LDH nanocomposite at the end of the reaction as confirmed by TEM and XRD analyses. The precise location of the clay (i.e., either inside or outside the latex particles), however, could not be determined from the TEM images. Quite a few authors have also used organically modified LDH. For instance, Zhao et al. (2011) reported the preparation of Mg-Al-LDH intercalated by dodecyl sulfate (DS) anions through the coprecipitation method, and their subsequent incorporation into poly(n-butyl acrylate-co-vinyl acetate) (P(BA-co-VAc)) latexes by emulsion polymerization using KPS as initiator and poly(vinyl alcohol) (PVA) as stabilizer. A preemulsion was firstly formed by dispersing part of the monomer suspension containing the DS-intercalated LDH platelets in a water solution of PVA, and polymerized by slow addition of the initiator solution. The rest of the monomer and clay was then introduced in the reactor and the polymerization was continued until complete monomer-to-polymer conversion. The final latex was recovered by filtration and cast into films. Unfortunately again no information was given on particle size and morphology, the emphasis being placed instead on the thermal and flame-retardant properties of the resulting materials (see Section 11.3.1.2). In the same vein, Kovanda et al. (2010) reported the co-intercalation of DS and various monomeric anions ((meth)acrylates, 4-vinyl benzoate and 2-acrylamido-2-methyl-1propanesulfonate) into MgAl-NO3 and ZnAl-NO3 LDHs, either by ion exchange or by direct coprecipitation. The intercalated LDHs were used for the preparation of poly(butyl methacrylate) (PBMA)/LDH nanocomposites through in situ emulsion polymerization. The overall procedure was very similar to that of Zhao et al.

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Figure 11.5 (A) Scheme of LDH/PBMA nanocomposite formation by in situ starve
feed emulsion polymerization of BMA using LDH with intercalated monomeric and DS anions, and (B) SAXS patterns of PBMA films containing ZnAl-MA LDHs prepared by emulsion polymerization or by solution polymerization in 1-methyl-2-pyrolidone (NMP) or in a mixture of dimethylformamide and formamide (DMF 1 FA). Source: Reproduced from Kovanda, F., Jindova´, E., Lang, K., Kuba´t, P., Sedla´kova´, Z., 2010. Preparation of layered double hydroxides intercalated with organic anions and their application in LDH/poly(butyl methacrylate) nanocomposites. Appl. Clay Sci., 48, 260
270 with permission from Elsevier.

The organoclay was first suspended in the monomer (BMA) and the mixture was preemulsified in water using SDS as surfactant. Part of the preemulsion (around 5 wt%) was polymerized at 85 C for 10 min using ammonium persulfate as an initiator and the remaining part was fed into the reactor together with the initiator solution at a constant rate, slow enough to ensure starved conditions. The polymerization was finally pursued for 30 min, leading to a latex suspension of around 30% solids content. Compared to DS-intercalated LDHs, the additional presence of a reactive monomer in the clay galleries promoted clay exfoliation by in situ formation of polymer chains as schematically represented in Fig. 11.5A. However, compared to solution polymerization, in situ emulsion polymerization resulted in a poorer dispersion state of the LDH platelets within the PBMA matrix as attested by the higher slope value in the small angle X-ray scattering (SAXS) experiments (Fig. 11.5B).

11.2.2.2 Suspension polymerization Suspension polymerization can be roughly described as a bulk polymerization in which the monomer containing dissolved organosoluble initiator molecules is suspended as droplets in the aqueous continuous phase. Droplets of the organic phase are formed and maintained in suspension by the use of: (1) vigorous agitation throughout the reaction and (2) hydrophilic macromolecular stabilizers dissolved in water (e.g., low molar mass polymers such as PVA, polyvinylpyrrolidone, or hydroxymethylcellulose). Each droplet thus acts as a small bulk polymerization reactor to

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which the normal kinetics apply. Polymer is produced in the form of beads whose average diameter is close to that of the initial monomer droplets (0.01
2 mm). The resulting polymer beads are easily isolated by filtration provided they are rigid and not tacky. Therefore, suspension polymerization is unsuitable for preparing polymers that have low glass transition temperatures, but is widely used for styrene, methyl methacrylate, or vinyl chloride monomers, for instance. To the best of our knowledge, there are only two articles reporting the synthesis of polymer/LDH nanocomposites by suspension polymerization (Ding and Qu, 2006; Bao et al., 2006). Ding and Qu (2006) used a very similar all in situ approach as that described above for emulsion polymerization, where the water-soluble initiator was simply replaced by an oil-soluble initiator, and actually obtained poorer results and, in particular, lower clay exfoliation. However, strictly speaking, the process cannot be considered as a true suspension polymerization as a surfactant was used to stabilize the resulting particles. Bao et al. (2006) reported the synthesis of poly(vinyl chloride) (PVC)/LDH composite resins by in situ suspension polymerization of vinyl chloride monomer (VCM) using diethylhexyl peroxydicarbonate as a radical initiator and PVA as a suspending agent. The LDH particles were first intercalated by DS anions and then dispersed in VCM followed by vigorous stirring at room temperature to promote the formation of a homogeneous clay dispersion. The polymerization was initiated by increasing the temperature to 57 C. The resulting composite products were recovered by filtration and dried at 60 C prior to characterization. The polymerization performed in the presence of DS-LDH led to smaller particles than pure PVC, suggesting that the LDH platelets contributed to the stabilization of the polymer beads. TEM revealed a partially exfoliated/partially intercalated morphology and a better dispersion state than composite materials obtained by direct melt-blending, showing the superiority of the in situ approach for the synthesis of PVC/LDH nanocomposites.

11.2.2.3 Reversible deactivation radical polymerization (RDRP) Reversible deactivation radical polymerization [formally known as controlled/living radical polymerization (CRLP)] refers to a family of polymerization techniques enabling the synthesis of well-defined macromolecular architectures. RDRP processes are based on the reversible deactivation of propagating radicals, either through a reversible termination mechanism [e.g., nitroxide-mediated polymerization (NMP) or atom transfer radical polymerization (ATRP)] or reversible chain transfer [reversible addition fragmentation chain transfer (RAFT)] reaction. The rapid equilibrium between active propagating radicals and dormant species ensures that all chains have an equal opportunity to propagate, which gives narrow molar mass distributions. RDRP not only allows the preparation of polymers with welldefined molar mass, dispersity, end group functionality, topology, and architecture (e.g., block, stars, combs, etc.), but also offers unprecedented opportunities in materials design, including the ability to prepare organic/inorganic nanocomposites and surface tethered (co)polymers. The growth of polymer chains from inorganic surfaces is a topic of considerable interest and the reader is referred to a number of

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recent comprehensive reviews for an in-depth description of the various synthetic strategies (Pyun and Matyjaszewski, 2001; Barbey et al., 2009; Roghani-Mamaqani et al., 2012; Francis et al., 2014; Hui et al., 2014). A wide variety of inorganic substrates have been investigated including cationic clays [such as MT or Laponite (Konn et al., 2007)] and LDHs. In particular, various authors have shown that it is possible to grow polymer chains directly from the surface of LDH particles using either ATRP (Qiu et al., 2005; Hu et al., 2013) or RAFT (Ding et al., 2007; Reddy et al., 2016) polymerization providing access to exfoliated polymer/LDH nanocomposites. However, those polymerizations were performed in bulk or in solution, and are therefore beyond the scope of this review. RDRP can also be conducted in water solution and an extensive range of water-soluble or water-insoluble monomers can nowadays be polymerized using aqueous controlled radical polymerization methods (Zetterlund et al., 2015). The transposition of RDRP techniques to dispersed polymerization systems (i.e., emulsion polymerization and other related heterophase processes) is however not straightforward and significant effort has been devoted in the last 10 years to implementing RDRP in aqueous dispersed media (Zetterlund et al., 2015). One important recent development in the field is polymerizationinduced self-assembly (PISA), which is a special case of emulsion polymerization based on the self-assembly of in situ generated block copolymers. Briefly, the PISA process involves chain extending a hydrophilic polymer precursor prepared via RDRP with hydrophobic monomer(s) to form amphiphilic chains which selfassemble into spherical nano-objects or more complex morphologies depending on reaction conditions. The notable advantages of this process are the absence of lowmolecular-weight surfactant in the suspension, the possibility to polymerize a wide range of monomers, and to form a variety of morphologies at high solids content without the aid of an organic co-solvent. A few years ago, the approach was extended to the encapsulation of inorganic particles (Cenacchi-Pereira et al., 2015; Zetterlund et al., 2015; Bourgeat-Lami et al., 2016), recognizing that most of the hydrophilic polymer precursors used in the PISA process are also capable of interacting with inorganic compounds. Although there have been a few examples involving NMP (Qiao et al., 2015) and ATRP (Loiko et al., 2016), most work in this field has used RAFT polymerization. In short, the method, coined RAFT-assisted encapsulating emulsion polymerization (REEP), consists of the adsorption of a preprepared living amphiphilic random copolymer (hereafter denoted macro-RAFT agent) on the surface of the inorganic particles, followed by emulsion polymerization of a hydrophobic monomer. Following this method, Cenacchi et al. (2017) demonstrated very recently the first example of LDH encapsulation by REEP. A random copolymer of acrylic acid (AA) and BA [P(AA17-stat-BA17)] was first synthesized by RAFT polymerization, and then electrostatically adsorbed on Mg2Al-LDH particles to provide both colloidal stability and reactivatable groups from which the subsequent emulsion polymerization could proceed, leading to LDH encapsulation. Both nitrate- and carbonate-intercalated LDHs were investigated, showing noticeable differences between the two precursors. The nitrate-intercalated LDH showed higher adsorption capacity than its carbonate counterpart because interlayer nitrate ions (in addition to those on the outer surface) were also displaced by the macroRAFT

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agent. The two macroRAFT agent-modified LDHs were then engaged in the emulsion polymerization of methyl acrylate (MA) and BA (MA/BA 80/20 wt/wt) using 2,20 -azobis(2-methylpropionamide)dihydrochloride (AIBA) or 2,20 -azobis(N,N0 dimethylene-isobutyramidine)dihydrochloride (ADIBA) as radical initiators. Regardless of the type of LDH and initiator, cryo-TEM showed successful encapsulation of the LDH nanoplatelets in the core of the latex particles (Fig. 11.6). The nitrate produced, however, near-spherical isotropic nanocomposite latexes with several LDH platelets per particle (Fig. 11.6A), whereas the carbonate led to monoencapsulation under the same conditions (Fig. 11.6C). This was attributed to different initial dispersion states of the LDH-modified precursors. Indeed, elongated core
shell particles containing a single encapsulated LDH platelet were also obtained when the macroRAFT-modified nitrate LDH was submitted to an ultrasonic treatment prior to REEP (Fig. 11.6B). All polymerizations displayed limited conversions, which was attributed to the poor hydrolytic stability of AIBA. The

Figure 11.6 Cryo-TEM images of the final P(MA-co-BA)/LDH nanocomposite particles obtained by REEP using: (A, B) Mg2Al-NO3 and (C, D) Mg2Al-CO3-LDH with [macroRAFT] 5 3 mmol/L, MA/BA (80/20 wt/wt), pH 5 8.0, T 5 70 C, and initiator 5 AIBA (A
C) or ADIBA (D). In (B), the initial macroRAFT/LDH suspension was sonicated for 5 min before polymerization. The arrows point to individually encapsulated LDH platelets. Source: Reproduced from Cenacchi, A., Pearson, S., Kostadinova, D., Leroux, F., D’agosto, F., Lansalot, M., et al., 2017. Nanocomposite latexes containing layered double hydroxides via RAFT-assisted encapsulating emulsion polymerization. Polym. Chem., 8, 1233
1243 with permission from The Royal Society of Chemistry.

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conversion effectively increased when AIBA was replaced by ADIBA—an initiator known to be less sensitive to hydrolysis—which also resulted in more spherical particles due to the higher amount of polymer formed around each clay platelet. The process was further extrapolated to film-forming copolymers by Pearson et al. (2018). Three different nanocomposite morphologies were obtained by varying the nature of the macroRAFT agent, with the LDH platelets sandwiched between two polymer nodes, encapsulated inside polymer particles or located at their surface. The mechanical properties of the resulting nanocomposite films were studied by Dalmas et al. (2018), and correlated with their microstructure.

11.2.3 Latex-templating approaches Inorganic particles associated with latex particles are not only interesting in polymer science to produce polymer nanocomposite materials but also to design nanostructured materials. In this latter case, the polymer beads are no longer the main component of interest, but are instead considered as a “sacrificial hard template,” allowing the precursor solutions or particles to either coat the template surface or fill the voids (Fig. 11.7). The polymer beads are subsequently removed either by calcination or dissolution to introduce the porosity and generate nanostructured macroporous materials. Uniform polystyrene beads are probably the most common colloidal particles used since their size can be easily tuned in a large range and they are commercially available. Monodisperse silica spheres have also been extensively used as sacrificial templates but are beyond the scope of this chapter. Such synthetic strategies have been investigated to produce hollow capsules (Ma and Qi, 2009) and three-dimensionally ordered macroporous structures (3-DOM) (Stein et al., 2013; Petkovich and Stein, 2013) of various inorganic components including oxides and metals. In this section, we summarize several recent examples which demonstrate the efficiency of the latex-templating approach to produce nanostructured LDHs. Hard templating with infiltration Colloidal crystal Infiltrated template

Precursor solution

Gelled precursor in the template

Heat or chemical treatment

Colloidal Sphere

Hard templating with coating Coated sphere

3DOM material

Gelled precursor on the sphere

Heat or chemical treatment

Template removal

Hollow sphere

Template removal

Precursor solution

Figure 11.7 Schemes detailing the hard templating process. (Left) An example of hard templating conducted using an infiltration process. (Right) An example of hard templating conducted via a coating process. Source: Reproduced from Petkovich, N.D., Stein, A., 2013. Controlling macro- and mesostructures with hierarchical porosity through combined hard and soft templating. Chem. Soc. Rev., 42, 3721
3739 with permission from The Royal Society of Chemistry.

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As previously described in Section 11.2.1, hollow capsules were prepared using the LbL technique at the surface of colloidal PS spheres (Li et al., 2006). Other synthetic methods have been described to produce LDH core
shell structures or hollow spheres using the latex-templating approach. Du et al. reported on the possibility to directly perform LDH nucleation and growth on the surface of sulfonated PSdivinylbenzene cation-exchange resin beads (565 6 50 μm) (Du et al., 2009). To promote LDH nucleation, the beads were in a first step converted to an Mg21/Al31 exchanged form and then added to a sodium hydroxide solution. The LDH growth was carried out by adding the metal salt solution, while the pH was kept constant at a value of 9. SEM images evidenced the formation of an LDH shell of 150 nm constructed from LDH platelets aligned edge-on in a monolayer (Fig. 11.8). At a longer reaction time, more LDH materials were deposited on the beads as a thick additional layer lying flat on the previous LDH shell. However, for this bead size, the supported LDH shell structure was not sufficiently stable to produce hollow LDH spheres by calcination, with the spheres collapsing upon heating. Similarly, negatively charged PS nanospheres (22
360 nm) were used as a hard template to produce MgAl@PS core
shell particles by simple coprecipitation in the presence of the polymer spheres (Kartsonakis et al., 2016). SEM images combined with EDS and XRD confirmed the LDH precipitation, which led to a cracked appearance at the sphere surface. Interestingly, by tuning the rate of NaOH addition from 0.14 to 4.35 mL/min during LDH coprecipitation in the presence of the PS spheres (0.6
1.6 μm) as the template, CoFe-LDH hollow spheres with two different hierarchical morphologies were prepared (Xu et al., 2011). Low addition rates resulted in a flower-like shell consisting of edge-on orientated LDH platelets, while high rates favored rapid nucleation to give raspberry-like morphologies with LDH nanoparticles oriented face-on to the PS bead surface (Fig. 11.9). After calcination the flower-like morphology appeared close to collapsing, showing that the edge-on orientation is not favorable to the hollow sphere formation. Comparatively, the raspberry-like spheres maintained their morphology upon heating and LDH conversion in oxides and mixed oxides. By direct coprecipitation in the presence of PS beads (350 nm) and a subsequent filtration step, macroporous MgAl LDH were obtained. The MgAl LDH coprecipitation was carried out into a PS colloidal solution by the addition of Mg and Al nitrate salts dissolved in a 1:1 water:ethanol solution and aqueous ammonia as precipitant (Woodford et al., 2012). Extended porous architectures were formed by a calcination/reconstruction process through agglomeration and fusion of nanocrystalline LDH platelets. A thin-walled honeycomb nanostructure was observed by SEM with macropores distributed throughout the materials in a nonordered manner. Macroporous CoFe2O4 spinel microspheres were also obtained by a simple process involving spray-drying of a suspension containing CoFeIIFeIII LDH nanoparticles (Zhao et al., 2002) (3.0 wt%) and sulfonated PS spheres (0.3 wt%) (Zhang et al., 2010). The spinel phase was obtained by calcination at 700 C of the CoFeIIFeIII

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Figure 11.8 SEM images of (A) Mg21/Al31-exchanged polymer resin beads; (B) polymer beads recovered following exposure to NaNO3
NaOH solution for 1 h; (C, D) polymer beads recovered from the LDH synthesis reaction after 18 h; (E) polymer beads recovered from the LDH synthesis reaction after 30 h; (F) polymer beads recovered from the LDH synthesis reaction after 60 h. Source: Reproduced from Du, Y., Hu, G., O’hare, D., 2009. Nucleation and growth of oriented layered hydroxides on polymer resin beads. J. Mater. Chem., 19, 1160
1165 with permission from The Royal Society of Chemistry.

LDH/PS microspheres. The spherical morphology was retained upon heating, leading to macroporous microspheres with a twofold higher total pore volume (2.49 g/ cm) than microspheres prepared without PS spheres.

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Figure 11.9 (A) Schematic illustration of the CoIIFeIIFeIII-LDH spherical shell formed on PS beads and the product obtained after its calcination in air. SEM images of (B) the flowerlike PS/CoIIFeIIFeIII-LDH spheres obtained by slow addition of NaOH, (C) the raspberry-like sPS/CoIIFeIIFeIII-LDH spheres obtained by rapid addition of NaOH, (D) the CoFe2O4 porous spheres obtained by calcination of (B); and (E) CoFe2O4 hollow spheres obtained by calcination of (C). A hole is marked with a black arrow in (E), and the inset of (E) shows an AFM image. Source: Reproduced with permission from Xu, S., Yang, Y., Xu, T., Kuang, Y., Dong, M., Zhang, F., et al., 2011. Engineered morphologies of layered double hydroxide nanoarchitectured shell microspheres and their calcined products. Chem. Eng. Sci., 66, 2157
2163. Copyright 2017 Elsevier.

Another latex-templating strategy relies on the use of a colloidal crystal to confine LDH nanoparticles and to subsequently produce 3-DOM materials or inverse opals. This colloidal crystal templating method known to introduce well-ordered and interconnected pores into a material was recently reviewed in depth by A. Stein (Stein et al., 2013; Petkovich and Stein, 2013). Geraud et al. were the first to

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demonstrate the possibility of confining LDH coprecipitation into the interstitial voids of a PS colloidal crystal template (Halma et al., 2009; Geraud et al., 2006a, 2006b, 2007, 2008). Typically, monodisperse negatively charged PS spheres were firstly assembled into close-packed face-centered cubic (fcc) arrays by centrifugation forming millimeter- to centimeter-thick materials. Secondly, the LDH synthesis was carried out by coprecipitation using successive soaking of the well-organized colloidal crystal in a metal salt solution and (after a drying step) in sodium hydroxide. It is noteworthy that precursor solutions should be prepared using a water/alcoholic mixture to ensure good wettability of the PS colloidal crystal and precursor infiltration. Finally, 3-DOM LDH replicas were obtained either by template dissolution in toluene or using calcination/reconstruction. As shown in SEM images (Fig. 11.10) the inorganic component formed an interconnected wall-type structure (wall thickness 60
110 nm), in which the windows interconnecting the pores are observed. Such interconnected macroporous networks promote more efficient diffusion through the macroporous material compared with materials obtained by stack-

Figure 11.10 (A) SEM images of windows between macropores of NiAl matrix and three tilting TEM images showing the (B) [111], (C) [110], and (D) [211] directions (fcc lattice) of the macroporous Ni-Al-LDH matrix. Source: Reproduced with permission from Geraud, E., Rafqah, S., Sarakha, M., Forano, C., Prevot, V., Leroux, F., 2008. Three dimensionally ordered macroporous layered double hydroxides: preparation by templated impregnation/coprecipitation and pattern stability upon calcination. Chem. Mater., 20, 1116
1125. Copyright 2017 American Chemical Society.

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ing of core
shell spheres. Tilting TEM images in Fig. 11.10 provided further evidence of the 3-DOM structure. The different views of the pore array are consistent with views along the [111], [011], and [211] directions for replicas resulting from the fcc array. Obviously, the wetting interactions between the precursor and the template surface favored the formation of a coating following the curved surface, giving relatively smooth walls. By modifying the diameter of the initial PS beads, the pore size was easily tuned from 280 to 505 nm. This synthetic process was applied to a wide range of LDH layer compositions (MgAl, NiAl, CoAl, ZnAl, ZnCr, MgFeAl, MgCoAl, etc.) and a large variety of anions was incorporated. Calcination in air at around 450 C and 800 C produced oxides and mixed oxide walls while maintaining the periodical structure. In parallel, a macroporous carbon replica was also successfully prepared by carbonization and demineralization from a 3-DOM vinyl benzene sulfonate-intercalated MgAl LDH (Prevot et al., 2011b). Alternatively, NiAl LDH macroporous thin films were also successfully prepared on 2D substrates [tin-doped indium oxide (ITO), fluorine doped tin oxide (FTO) or platinum electrode] through the use of the inverse opal method and LDH electrodeposition (Prevot et al., 2011a; Martin et al., 2016). In these cases, colloidal crystal was formed either by evaporation and capillary attractive forces or by electrophoresis at constant potential of 1.5 V for 30 s. Then, the PS bead (350
430 nm) coated slides were used as working electrodes for LDH electrosynthesis. LDH thin films can indeed be directly and easily electrosynthesized at an electrode surface (Scavetta et al., 2007) thanks to nitrate reduction, forming small, ill-crystallized LDH particles which homogeneously coat the electrode surface. Typically, the PScoated electrode was soaked in an Ni and Al salt solution (0.03 M) containing potassium nitrate (0.3 M) and NiAl LDH was cathodically deposited by applying a constant potential of 20.9 V. The removal of the PS array was done by extraction with solvent such as toluene or a mixture of tetrahydrofuran and acetone. The films were examined at different stages of deposition from 30 to 180 s. After 60 s of electrodeposition, a deposited layer 1200 nm thick was characterized. As illustrated by the SEM images (Fig. 11.11), the LDH deposition in the interstices between the PS spheres was clearly seen, leading to a continuous LDH network surrounding interconnected cavities after PS dissolution (Prevot et al., 2011a; Martin et al., 2016). Such electrochemically induced precipitation of LDH appears more efficient for producing 2D macroporous thin films than the previously described successive impregnation method, which produced a coating with numerous cracks, bare regions, and overcoating. Still using a colloidal crystal template array, an alternative strategy has been employed to infiltrate a solution of LDH nanoclusters acting as colloidal building blocks (Tokudome et al., 2016). The LDH nanoclusters were synthesized using a one-pot route involving acetylacetone and propylene oxide in a mixture of ethanol and water (in addition to the metal salts). To prepare macroporous thin films, the diluted suspension of LDH nanoclusters was simply spin-coated on the PS (100 nm) array at 8000 rpm for 60 s and the polymer was chemically dissolved by

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Figure 11.11 (A and B) FE-SEM images of NiAl 3-DOM electrode (20.9 V, 60 s) at different magnification, (C) the corresponding AFM image, and (D) PXRD patterns of (a) coprecipitated NiAl LDH and (b) electrosynthesized NiAl LDH. Source: Reproduced from Prevot, V., Forano, C., Khenifi, A., Ballarin, B., Scavetta, E., Mousty, C., 2011a. A templated electrosynthesis of macroporous NiAl layered double hydroxides thin films. Chem. Commun., 47, 1761
1763 with permission from The Royal Society of Chemistry.

chloroform. A well-defined macroporous thin film with a pore diameter of 80 nm was successfully obtained.

11.3

Properties of LDH-based nanocomposites and LDH macroporous structures

11.3.1 LDH-based nanocomposites Fabrication of composites through the latex route has been widely used to produce clay
polymer composites with enhanced mechanical properties for high durability coatings, and with enhanced gas barrier properties for packaging applications. LDH platelets are good candidates to fulfill similar property enhancements. The following section describes the functional properties of materials formed from LDH-based nanocomposite latexes. The influence of physical properties and morphology of the precursor hybrid latexes on the final properties will be emphasized.

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11.3.1.1 Mechanical properties Clays are well known as low-cost fillers for thermoplastics and thermosets, imparting high modulus and tensile strength. It is generally accepted that the mechanical reinforcement of polymer/clay nanocomposites is due to the high aspect ratio and high surface area of the inorganic filler, and therefore requires the platelets to be well dispersed in the polymer matrix (Ray, 2014). A good clay dispersion also ensures optical transparency of the final material, which is a key aspect for coating applications. In addition, the establishment of clay
clay contacts and strong filler
filler interfacial interactions are known to be important requirements for good mechanical properties (Chabert et al., 2004). Recently, Veschambres et al. (2016) studied the mechanical properties of LDH-based nanocomposite films obtained by casting a mixture of LDH platelets (30
140 nm in lateral size) and oppositely charged poly(methyl methacrylate-co-n-butyl acrylate) latex particles (700 nm in diameter). The films displayed a well-defined cellular microstructure, which induced a large mechanical reinforcement characteristics of a mechanical percolation behavior (Fig. 11.12A). As expected, the thickness of the network wall

Figure 11.12 (A) Effect of LDH volume fraction (based on polymer) on the mechanical properties of polymer/LDH nanocomposite films obtained by heterocoagulation of positively charged LDH layers and negatively charged latex particles and subsequent drying of the dispersions. (B) Low and high magnification TEM images of ultrathin sections of the nanocomposite films, and (C) FIB-SEM image of the same material highlighting the remarkable homogeneity of the clay network at a very large scale. Source: Reproduced from Veschambres, C., Halma, M., Bourgeat-Lami, E., Chazeau, L., Dalmas, F. Prevot, V., 2016. Layered double hydroxides: Efficient fillers for waterborne nanocomposite films. Appl. Clay Sci., 130, 55
61 with permission from Elsevier.

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increased with increasing LDH content (Fig. 11.12B), while focused ion beam (FIB)-SEM analysis suggested some alignment of the platelets in the film plane, likely due to mechanical forces exerted on the clay network during film formation. Similar 3D morphologies have already been reported in the literature for smectite clays (Negrete-Herrera et al., 2007; Ruggerone et al., 2009a, 2009b) but to our knowledge this is the first time that such a morphology has been described for LDHs. The effect of surfactant on the mechanical properties of butadiene acrylonitrile (NBR) latex/LDH composites obtained by coagulation, was studied by Braga et al. (2014). It was shown that the addition of a surfactant significantly improved LDH dispersion in the NBR matrix as confirmed indirectly by a lower Payne effect. The Payne effect is a particular feature of filled rubbers, characterized by a pronounced decrease in the storage modulus, with increasing strain amplitude reflecting agglomeration of the filler particles due to filler
filler interactions. The nanocomposites obtained in the presence of surfactant showed a substantially lower variation of the shear modulus with the shear strain (ΔG 5 5.59 kPa) than the system without surfactant (ΔG 5 14.29 kPa), suggesting a better dispersion of the LDH platelets. Bao et al. (2006) reported the mechanical properties of PVC/LDH-DS composites synthesized by suspension polymerization for increasing LDH contents. The Young’s modulus, tensile strength, and Charpy notched impact strength were all greater than those of pristine PVC and PVC/LDH-DS composites obtained by meltblending, and all leveled off with increasing LDH content (Fig. 11.13). The increased mechanical performances were attributed in this case to a better clay dispersion in the polymer matrix and stronger clay/polymer interfacial interactions.

11.3.1.2 Flame retardancy Zhao et al. (2011) investigated the flame-retardancy properties of P(BA-co-VAc) membranes containing ammonium polyphosphate (APP)—a halogen-free flame retardant—and organically modified LDH platelets (O-LDH). The films were obtained by casting an O-LDH/APP/P(BA-co-VAc) latex suspension obtained by in situ emulsion polymerization of BA and VAc in the presence of APP and LDH, and evaporating the water at room temperature for 3 days, followed by vacuum drying at 50 C. Thermal gravimetric analysis showed that the composite sample displayed a higher solid residue (curve e in Fig. 11.14A) than calculated from the LDH and APP contents (curve b, Fig. 11.14A). The thermal behavior was thus not just the sum of the thermal degradations of the individual components but an additional stabilization mechanism was at play. Indeed, FTIR and XPS analysis showed the formation, during pyrolysis and combustion, of polyphosphoric and aromatic structures. The authors argued that O-LDH could catalyze the formation of a three-dimensional network by reaction of APP

(A) 3.4 b

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Figure 11.13 Influence of LDH-DS weight fraction on: (A) Young’s modulus, (B) tensile strength, and (C) Charpy notched impact strength of PVC/LDH-DS composites. (a) Meltblending and (b) in situ suspension polymerization. Source: Reproduced from Bao, Y.-Z., Huang, Z.-M., Weng, Z.-X., 2006. Preparation and characterization of poly(vinyl chloride)/layered double hydroxides nanocomposite via in situ suspension polymerization. J. Appl. Polym. Sci., 102, 1471
1477. Copyright 2016 Wiley Periodicals, Inc.

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(B)

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Figure 11.14 (A) TGA curves of: (a) P(BA-co-VAc), (b) O-LDH/APP/P(BA-co-VAc) calculated, (c) APP, (d) O-LDH, and (e) O-LDH/APP/P(BA-co-VAc) experimental. (B) SEM images of the char samples obtained for (a, b) APP/P(BA-co-VAc) and (c, d) O-LDH/APP/P (BA-co-VAc) [(a, c) outer surfaces and (b, d) inner surfaces]. Source: Reproduced from Zhao, C., Peng, G., Liu, B., Jiang, Z., 2011. Synergistic effect of organically modified layered double hydroxide on thermal and flame-retardant properties of poly(butyl acrylate
vinyl acetate). J. Polym. Res., 18, 1971
1981 Copyright 2011 Springer.

degradation products with the polymer chains resulting in crosslinking and compact char formation. SEM further confirmed the homogeneous and dense structure of the inner and outer char (Fig. 11.14B). The intumescent char residues formed a protective layer on the outer surface of the composite material, preventing further combustion at high temperatures.

11.3.2 LDH-based macroporous structures Numerous potential applications of macroporous LDH materials can be considered, including mainly chemical applications that rely on the enhanced porosity, high surface area, and rapid molecular diffusion within the interconnected nanostructured materials. In the following, the specific properties and applications of hollow nanostructured and macroporous LDHs obtained using latextemplating approaches are summarized, focusing on applications such as adsorption, sensing, catalysis/photocatalysis, and the development of electrochemical devices.

11.3.2.1 Adsorption and extraction As porous hosts, LDH materials prepared using PS templating approaches are obvious candidates for adsorption and extraction processes. Chloride entrapment was investigated using core
shell PS/MgAl-NO3, evidencing that equilibrium was reached after 4 h. The highest uptake capacity equal to 510.85 mg/g was recorded at pH 8, corresponding to a removal percentage of 51.55%. Unfortunately, the role of the PS core was not elucidated in this study and the comparison with the hollow

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LDH spheres resulting from the PS core removal, not provided. Enhanced loading of adsorbates on nanostructured LDHs has also been demonstrated for anionic dye molecules, such as orange II (Geraud et al., 2007). Three-DOM MgAl-CO3 LDH displayed very high adsorption capacities toward orange II, especially the calcined LDH materials (4.34 mmol/g) compared to the standard MgAl-CO3 LDH phase (0.504 mmol/g). Such difference was attributed to a better accessibility to the LDH layer surface. While MgAl-CO3 phases mainly displayed a surface exchange chemisorption process, the calcined LDH phases gave rise to the highest concentrations of adsorbed O-II due to a partial intercalation of O-II molecules through the calcination/reconstruction phenomenon. The interest of 3-DOM LDH materials was also shown for solid-phase microextraction of phenolic and polycyclic aromatic hydrocarbon compounds (Abolghasemi and Yousefi, 2014). Due to their pore structure, large surface area, and homogeneous macroporous morphology, enhanced adsorption capacity was obtained and compared favorably to commercial materials such as polydimethylsiloxane, allowing successful preconcentration of phenolic compounds in water samples (Abolghasemi and Yousefi, 2014; Geraud et al., 2007).

11.3.2.2 Catalysis and photocatalysis The introduction of porosity increases the surface area and ensures a good mass transport through interconnected pores and from large interfaces. These properties contribute to the high catalytic activity of LDH-based materials. LDHs have been reported as heterogeneous base catalysts for the transesterification of triglycerides (He et al., 2013); however, their efficiency is hampered by restricted diffusion of bulky triglycerides within the crystallites (Cantrell et al., 2005; Xi and Davis, 2008; Kim et al., 2010). Woodford et al. studied the production of triacylglycerides (C3H5(OOR)3 R 5 C4, C8, and C12) or unsaturated glyceryl diesel (Woodford et al., 2012). They demonstrated that the macropores conferred significant benefits for transesterification, especially the triglycerides of long chain, due to improved diffusion and accessibility to the active sites. For instance, a 10-fold rate enhancement was observed for triolein in using macroporous LDHs compared to conventional LDHs with improved conversion and without loss of fatty acid methyl ester selectivity. In addition, macroporous LDHs with well-defined porosity have been used as a bidimensional support to intercalate or immobilize homogeneous catalytic species. First- and second-generation anionic iron (III) porphyrins were immobilized on macroporous LDH phases using both the calcination reconstruction phenomenon and the anionic exchange and investigated as catalysts for cyclooctene, cyclohexane, and hetpane oxidation (Halma et al., 2009). The results obtained showed that the efficiency and selectivity of the metalloporphyrins immobilized on/ in macroporous LDH changed. A good selectivity for the alcohol product was systematically observed for the macroporous heterogeneous phases. Decatungstate W10 O42 32 was also efficiently intercalated into 3-DOM Mg-Al-LDH to produce heterogeneous photocatalysts. The photocatalytic activity of W10 O42 32 immobilized

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on both macroporous and conventional LDH was tested with two pollutants: 2,4 dimethylphenol and 2-(1-naphthyl) acetamide. The presence of macropores drastically increased the accessibility of light and the efficiency of the photodegradation. For instance, 60% of 2-(1-naphthyl) acetamide were photodegradated (λ 5 365 nm) after 17 h at pH 6.6 for an optimal concentration of photocatalyst of 60 mg/L. Complete mineralization of the pesticide was observed and the photocatalyst could be reused over four cycles without loss of activity.

11.3.2.3 Electrochemical and magnetic properties Electrochemical devices made of 3-DOM LDH capitalize on the relatively large and accessible surfaces in the modified electrodes. Usually, the electrodes are based on thin LDH films electrodeposited on a conductive support (Scavetta et al., 2007; Martin et al., 2016; Prevot et al., 2011a). The presence of redox metal cations such as Ni or Co into the LDH layers confers electrochemical properties to LDH membranes. Compared to a bare flat LDH electrode (Mousty, 2004; Shao et al., 2015), a 3-DOM NiAl LDH-modified electrode was shown to exhibit greater permeability and was further used to develop an amperometric electrochemical biosensing device for the detection of glucose (Prevot et al., 2011a). The nanostructured electrode was used to immobilize glucose oxidase. An increase in sensitivity from 13 to 18 mA/M/cm2 was measured for the nanostructured LDH film compared to the compact film, which was ascribed to the higher porosity facilitating glucose diffusion. Electrosynthesized 3DOM LDH electrode was also used as an electrochromic coating (Martin et al., 2016). Transmittance changes were followed by in situ measurements of the UV-vis spectra during potential cycling, showing a change in relative transmittance four times larger than the change observed for the compact film (Fig. 11.15). It was underlined that the reversibility of the color change was greatly improved by heating the films to convert the LDH phase to the corresponding mixed oxides. Magnetic properties of spinel nanostructured microspheres obtained from CoFeIIFeIII- and Ni0.5Co0.5FeIIFeIII-LDH microsphere precursors were investigated. In using an appropriate MII/(FeII 1 FeIII) ratio in the LDH precursor phase, pure spinel MIIFe2O4 was synthesized by simple calcination at 900 C in air. Thanks to the great versatility of composition in the LDH layer, the magnetic properties of the resulting spinels could be tuned over a large range (Abellan et al., 2015). CoFeIIFeIII- and Ni0.5Co0.5FeIIFeIII-LDH raspberry-like hollow spheres retained their morphology upon heating, leading to Ni0.5Co0.5Fe2IIO4 microspheres displaying unmodified magnetic properties compared to spinels obtained from conventional LDH powder. These could be of interest as drug carriers or catalysts (Xu et al., 2011). Similarly, CoFe2O4 microspheres prepared from LDH microsphere precursors obtained by combining spray-drying and PS templating displayed a saturation magnetization of 52.8 emu/g and a coercive force of 205 Oe, which is comparable to the powdered LDH precursors (Zhang et al., 2010).

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(A) –0.2

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Figure 11.15 (A) Transmittance (red) and current (blue) versus potential curves obtained for an inversed opal structure NiAl-LDH coating in presence of 0.1 mM [Fe(CN)6]42 (20 mV/s in 0.1 M borate buffer at pH 8). (B) Transmittance versus time of an inversed opal Ni-AlLDH film in the absence (a) and presence (b) of 0.1 mM [Fe(CN)6]42. (c) Transmittance versus time for a noninversed film in the presence of [Fe(CN)6]42 (first 15 scans at 20 mV/s between 20.5 and 1.4 V versus Ag/AgCl in a 0.1 M borate buffer at pH 8). Source: Reproduced from Martin, J., Jack, M., Hakimian, A., Vaillancourt, N., Villemure, G., 2016. Electrodeposition of Ni-Al layered double hydroxide thin films having an inversed opal structure: application as electrochromic coatings. J. Electroanal. Chem., 780, 217
224 with permission from Elsevier.

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489

Concluding remarks and general trends

Over the last decade, the use of latex technology has become a powerful tool for the production of LDH-based composite materials and macroporous structures. As illustrated in this chapter, several approaches have been described, including the assembly of preformed LDH and latex particles, in situ emulsion, or suspension polymerization in the presence of LDH particles and the use of polymer latexes as sacrificial templates. Even if their synthesis is often challenging, the large panel of LDH synthetic pathways using soft chemistry conditions has recently allowed the successful preparation of stable LDH colloidal suspensions and well-defined LDH/ polymer composite particles. The control of the synthetic process and the increased level of understanding have enabled the design of original morphologies which were until recently difficult to achieve. These include LDH-armored latex particles, polymer-encapsulated LDH, LDH inverse opals, and LDH hollow spheres. Waterborne polymer/LDH nanocomposites obtained by latex technology display great promise for the development of coatings with improved mechanical and flame-retardant properties. In addition, nanostructured LDH materials prepared by the latex-templating approach possessed real advantages for various applications such as adsorption for environmental purposes and photo/electro-catalysis. On the basis of the recent progress observed in this field, we strongly believe that more is to come for the development of LDH/polymer nanocomposite latexes. Some of the major possibilities may be in the preparation of nanostructured multicomponent systems by employing new functional hybrid LDH particles and introducing additional particles to open up new applications for LDH-based materials.

References Abellan, G., Marti-Gastaldo, C., Ribera, A., Coronado, E., 2015. Hybrid materials based on magnetic layered double hydroxides: a molecular perspective. Acc. Chem. Res. 48, 1601
1611. Abolghasemi, M.M., Yousefi, V., 2014. Three dimensionally honeycomb layered double hydroxides framework as a novel fiber coating for headspace solid-phase microextraction of phenolic compounds. J. Chromatogr. A 1345, 9
16. Bao, Y.-Z., Huang, Z.-M., Weng, Z.-X., 2006. Preparation and characterization of poly(vinyl chloride)/layered double hydroxides nanocomposite via in situ suspension polymerization. J. Appl. Polym. Sci. 102, 1471
1477. Barbey, R., Lavanant, L., Paripovic, D., Schu¨wer, N., Sugnaux, C., Tugulu, S., et al., 2009. Polymer brushes via surface-initiated controlled radical polymerization: synthesis, characterization, properties, and applications. Chem. Rev. 109, 5437
5527. Basu, D., Das, A., Stockelhuber, K.W., Wagenknecht, U., Heinrich, G., 2014. Advances in layered double hydroxide (LDH)-based elastomer composites. Prog. Polym. Sci. 39, 594
626. Bourgeat-Lami, E., Lansalot, M., 2010. Organic/inorganic composite latexes: the marriage of emulsion polymerization and inorganic chemistry. Adv. Polym. Sci. 233, 53
123.

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Bourgeat-Lami, E., Sheibat-Othman, N., Dos santos, A.M., 2011. Chapter 13 polymer-clay nanocomposite particles and soap-free latexes stabilized by clay platelets: state of the art and recent advances. Polymer Nanocomposites by Emulsion and Suspension Polymerization. The Royal Society of Chemistry, Cambridge. Bourgeat-Lami, E., Faucheu, J., Noe¨l, A., 2015. Latex routes to graphene-based nanocomposites. Polym. Chem. 6, 5323
5357. Bourgeat-Lami, E., D’agosto, F., Lansalot, M., 2016. Synthesis of nanocapsules and polymer/inorganic nanoparticles through controlled radical polymerization at and near interfaces in heterogeneous media. In: Vana, P. (Ed.), Controlled Radical Polymerization at and from Solid Surfaces. Springer International Publishing, Cham. Braga, F.C.F., Furtado, C.R.G., Oliveira, M.G., 2014. Influence of the surfactant on the hydrotalcite dispersion in NBR/LDH Composites produced by coagulation. Macromol. Symp. 343, 70
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Fabrication, assembly, and optoelectric properties of layered double hydroxide/conjugated polymer nanocomposites

12

Yaping Huang, Harrone Muhammad Sohail and Jun Lu Beijing University of Chemical Technology, Beijing, P.R. China

12.1

Fabrication and assembly of LDHs/conjugated polymer nanocomposites

12.1.1 Introduction 12.1.1.1 Conjugated polymers The majority of polymers (Zhang et al., 2017a; Knauert et al., 2010; Kularatne et al., 2012), such as broadly used commodity materials, polypropylene, polyethylene, polystyrene, and poly(ethylene terephthalate), have similar electrical and optical properties, which possess no mobile charges and no photoabsorption in the UV spectral region, and they are electric insulators and colorless. In fact, there is one particular class of polymers with quite different properties; they can be electric semiconductors or conductors and interact with light, and are denominated as conjugated polymers (CPs), having been discovered in the 1970s and awarded the Nobel Prize in chemistry in 2000. Conjugated polymers are a class of artificial synthesized polymers containing the delocalized π electrons in the main chain with the sp or sp2 hybridization carbon atoms like polythiophene, polyaniline, polypyrrole, polyvinylene, polyacetylene, etc. (Facchetti, 2011; Tennyson et al., 2010; Gibson et al., 2012). These are polymer photoelectric materials with metallic or semiconductive electric properties, concomitant with the processability and mechanical properties of polymers. The energy and charge can be transferred along the main chain of a large delocalized π electron conjugated system for rapid conduction at the applied voltage and/or photoexcitation, and some conjugated polymers are insulators or wide band gap semiconductors in their eigenstates, only after doping or chemical modification being transformed into a doping semiconductor or even a conductor. Conjugated polymers have strong light-capturing ability and can be used to amplify fluorescence sensoring signals, which play an increasingly important role in disease diagnosis and biological monitoring. As a new generation of

Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00012-4 © 2020 Elsevier Ltd. All rights reserved.

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photoelectric materials, conjugated polymer was compatible for flexible optoelectric devices, such as electronic papers, foldable display and wearable devices, and was the model system for flexible electronics.

12.1.1.2 LDHs/conjugated polymer nanocomposites Conjugated polymers have demonstrated their distinctive properties in electronic conductivity and light-emitting properties in a variety of applications (Gutierrez et al., 2015; Wang et al., 2015; Zhong et al., 2016; Leclerc et al., 2016), the synthesis and fabrication process of conjugated polymer devices may be subject to inherent physical constraints such as insolubility and poor thermo/photostability of these conjugated polymers with rigid rod main chains. Although the conjugated polymers with improved solubility can be synthesized by a chemical synthetic method, these methods are complicated and high-cost procedures and affect their optical performance, and the modified conjugated polymers can only be dispersed in hazardous organic solvents for further processing. A variety of methods were attempted to try to solve the solubility problems of conjugated polymers in the application. Layer-by-layer (LbL) assembly, proposed by Iler (1966) in 1991 originally reported that the charged solid substrate was immersed into the colloidal solution containing the species with opposite charge deposited alternately, so as to obtain the ultrathin film (UTF) of colloidal species. This UTF preparation technology, based on the interaction between the oppositely charged species with an electrostatic interaction as the driving force, did not attracted much attention, until 1997, when Decher (1997) proposed the electrostatic alternating layers of assembly technology, and utilized this technology in the preparation of polyelectrolytes and organic smallmolecule UTFs, which aroused wide attention. The LbL technology for assembling and preparing UTFs is very facile, for instance, the polyanions and polycations are assembled on a positively charged substrate to form a UTF. The positively charged substrate is first immersed in the polyanionic solution, after standing for some time, the substrate is washed with water, the surface physically adsorbed polyanions are then removed and dried; and then the substrate is immersed in polycationic solution, left to stand for some time, then rinsed with water, thus completing one cycle of the polyanion/polycation assembly, with the above steps repeated in order to obtain multilayer (polyanion/polycation) UTF. In recent years, it has been found that LbL assembly has many advantages in the fabrication of functional thin films (Joseph et al., 2015; Yao et al., 2011; Yang et al., 2014), such as abundant film-forming species, facile operation and low cost, and the potential synergistic effect between assembled members. This method has been widely used in polymer thin film. The nanocomposite thin films based on LDHs have become a new emerging class of nanocomposites (Ma and Takayoshi, 2015b; Ma and Sasaki, 2015a), which have received extensive attention. In addition to polyelectrolytes, other materials such as organic small molecules, organic/inorganic nanoparticles, microcapsules, biomacromolecules, etc. can be assembled with LDHs into the UTFs by suitable driving forces. LDH nanosheets are ideal building blocks for the preparation of LDH-based films by LbL assembly techniques. Recently, researchers have

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focused on the preparation of LDH nanosheets by effective exfoliation in polar solvent-like formides (Naik et al., 2011; Gordijo et al., 2007; Naik and Vasudevan, 2011; Zhao et al., 2011; Wongariyakawee et al., 2012), which has laid a solid foundation for the construction of nanocomposite films by the LbL method. LDH crystallites have a high charge density in hydroxide layers, so their exfoliation is a challenge. Adachi-Pagano et al. (2000) first reported LDH exfoliation, the delamination behavior of dodecyl sulfate intercalated ZnAl-LDHs was investigated in different solvents. Hibino and Jones (2001) studied amino acid anion-intercalated MgAl-LDHs in formamide to achieve exfoliation. To avoid the use of toxic formamide, they further developed an environmentally friendly LDH exfoliation method, which used lactate-intercalated LDHs in water for several days, where the plate LDH nanosheet colloidal solution can be obtained (Hibino and Kobayashi, 2005). The above-mentioned several exfoliation methods are required to modify the interlayer microenvironment of LDHs with organic species, and there is no direct evidence that the obtained LDH nanosheets are monolayers, most of which are formed by several layers of LDH hydroxides. Sasaki (Ma et al., 2015) used wellcrystalline MgAl-NO2 3 LDHs without organic anion intercalation as precursors to obtain the exfoliation in formamide. The resulting LDH nanosheets had typical two-dimensional morphology, and the transverse dimension was at the micron level. In addition, the Sasaki group (Liang et al., 2010; Liu et al., 2007) have successfully achieved the exfoliation of transition metal LDHs (such as CoNi, CoAl, NiAl, etc.), and the resulting nanosheets provide novel functional composite films with a very good structure/functional unit for further application in optical, electrical, magnetic, catalytic, and adsorptic fields. Because LDHs are a typical anionic layered compound, the chemical composition of the hydroxide layers, the species, and amount of interlayer anion are controllable, and thus they show many unique physical and chemical properties (Hibino and Kobayashi, 2005). LDH nanosheets and conjugated polymer can be assembled by LbL method in an assembly process that is simple and convenient. The composition of conjugated polymer with LDH nanosheets by LbL assembly cannot only solve some application shortcomings of conjugated polymers in optoelectronic devices, but can also optimize the performance of optoelectronic devices to some extent. In this chapter, firstly, according to the interaction between the LDH nanosheets and the CP layer, we highlight the development of assembly technology for LDH/CP nanocomposites. Secondly, the luminescent and optoelectric properties and some applications based on LDH/CP nanocomposite UTFs are reviewed.

12.1.2 Fabrication and assembly of LDH/CP nanocomposites With the development of electrostatic assembly technology, scientists have a deep understanding of the mechanism of electrostatic assembly, the structure of UTF, and the interaction between the components (Joao and Joao, 2014). The LDH/CP nanocomposite UTF can be fabricated conveniently by the LbL assembly method (Wongariyakawee et al., 2012). The assembled species with positive-charged LDH nanosheets can be polyanions, small anions, neutral molecules, metal complexes,

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and even cations. The assembled guest species include the polymer, biomacromolecules (protein, DNA, and RNA), inorganic metal nanoparticles, and semiconductor quantum dots and inorganic nanosheets (e.g., graphene). Researchers employed characterization techniques like UV-visible spectroscopy, fluorescence spectra, X-ray diffraction patterns (XRD), circular dichroism (CD) spectra, etc. to confirm the success of the LbL assembly. The morphology and structure of the LDH/CP nanocomposite UTFs is shown to prove this assembly by microscopic techniques like transmission scanning electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). For example, the morphology and supramolecular structure of luminescent UTFs were studied by small-angle X-ray diffraction patterns (SAXRD) and SEM techniques. The SAXRD pattern can illustrate whether the prepared films were ordered and periodic perpendicular to the substrate. Additionally, the periodic structure of UTFs was also verified by SEM, AFM, and fluorescence microscopy technique. The SEM images can show the thickness of UTFs, thus, calculating the UTF average thickness of one bilayer which is congruent with the SAXRD results, and if the film thickness was increased approximately linearly with the bilayer numbers (n), it illustrates films were ordered and periodic perpendicular to the normal of the substrates and that the film growth was linear. Other test data like XPS, NMR, IR spectroscopy, etc. can be used as supporting information to confirm LbL assembly. Through different testing measurements, the ordered and periodic perpendicular UTFs can be confirmed and the fabrication and assembly of LDH/CP nanocomposites by LbL assembly is feasible. The scheme of LbL assembly, the morphology and supramolecular structure of UTFs are documented in various places as following.

12.1.2.1 Layer-by-layer assembly method based on electrostatic interaction Conventional LbL assembly was suitable for polyanion with polycations or LDH nanosheet assemblies. The formation of a stable multilayer composite UTF is mainly the entropy effect of polyanion substitution and the charge reversal process. 2 The polyanions replace the small inorganic anions (e.g., CO22 3 ; NO3 , etc.) adsorbed on the LDH surfaces to form a stable polyanion adsorption layer, that is, a polyanion substitution was a thermodynamic entropy-increasing spontaneous process. At the same time, excessive negative charge also causes the polyanion adsorption layer to be negatively charged, and the charge polarity on the surface is reversed, to facilitate the subsequent assembly of the positively charged LDH nanosheets and multilayer formation. It is the first time that Sasaki’s group (Li et al., 2005) prepared the (LDH nanosheets/PSS)n multilayers using the electrostatic LbL assembly method. In order to improve the bonding force between the film and the substrate, the treated substrate (e.g., Si wafer or quartz glass) was soaked in polyetherimide (PEI) solution for 20 min, and rinsed in water, then transferred into poly (p-styrenesulfonate) (PSS) solution for 20 min and then water for rinsing, so that the substrate

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preassembled a layer of polymer. The substrates were then alternately assembled in the LDH nanosheet colloidal solution and PSS solution to finally get the multilayer thin film. The (MgAl-LDHs/PSS)n multilayers fabricated by the LbL assembly technique can be confirmed by UV-Vis absorption spectroscopy, which shows the linear absorption augmentation for PSS with the assembly cycle. This implied that the (MgAl-LDHs/PSS)n UTF has almost the same amount of assembly per cycle. The XRD characterization indicates the stacking order of the UTFs with a period in the normal direction of UTFs. Coronado and Martı´-Gastaldo (2013) fabricated PSS/ NiAl-LDH UTFs. The magnetic (PSS/LDH)n UTFs were performed according to the LbL method based on the presence of electrostatic attractive interactions between the corresponding charged components. The magnetic data of magnetic (PSS/LDH)n UTFs confirmed the effective transfer of the magnetic properties of the bulk LDH to the self-assembled film that displays glassy-like ferromagnetic behavior. LDH nanosheets can not only be used as components of hybrid films but also serve as a separator for separating the intercalated anions. To some extent, this achieves a long-range ordered arrangement of polyanions. Yan et al. (2009) combined the conjugated polymer, sulfonated poly(p-phenylene) (APPP) with LDH nanosheets mounted on a quartz glass substrate surface using the electrostatic LbL assembly method (Fig. 12.1). The results show that LDH/APPP UTF has an inorganic
organic hybrid quantum well structure; and the rigid LDHs can effectively separate the APPP anions, thus avoiding the red/blue shift of the luminescence of

Figure 12.1 (A) The chemical formula of APPP; (B) the representation of one sheet of MgAl-LDH; and (C) LbL assembly process for (APPP/LDH)n UTF (Yan et al., 2009).

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Layered Double Hydroxide Polymer Nanocomposites

APPP caused by the π
π interaction between the polymer chains. For the (APPV/ LDH)n (n 5 3
30) UTFs, its UV-Vis absorption and fluorescence intensity were increased approximately linearly with the bilayer numbers (n) shown in Fig. 12.2, which can illustrate that the assembled films were ordered and periodically perpendicular (Yan et al., 2009). Based on the electrostatic interaction between the positively charged LDH nanosheets and polyanions, researchers have achieved a number of polyanions and LDH nanosheet LbL fabrication assemblies of LDH/CP nanocomposites such as (APPV/LDH)n UTFs (Fig. 12.2) (Yan et al., 2009), (SPT/ LDH)n UTFs (Yan et al., 2011b) (Fig. 12.3), etc. Based on the electrostatic LbL assembly method, the organic polyanion can be used as a carrier for small functional molecules due to their hydrophobic interaction, to achieve small anions, small cations, and even neutral small molecules assembled with LDH nanosheets to obtain the LDH/CP@small molecule nanocomposites. The assembly process of LDH nanosheets with polyanion@small molecules is shown in Fig. 12.4. As the LDH nanosheets are positively charged, the principle of charge balance determines that it can only be assembled with the anionic guests by electrostatic interaction, which greatly restricts the development of LDH layered functional materials. The realization of LDHs and cationic functional molecules is also a challenge. By using the coassembly method, polyanion can be used as the carrier to construct the cation@polyanion/LDH UTFs. More LDH/CP nanocomposites with various functional properties can be fabricated based on the electrostatic interaction between LDH and polyanions. The steps of the program were as follow: Firstly, the small cation functional guest can be adsorbed on the polyanion backbone, and the polyanion negative charge balanced partly, forming (small cation@polyanion) pairs.

(A)

1.2

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207 nm

415 nm 30 bilayers

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A 0.4 0.2 0.0

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480 560 λ (nm)

640 720

800

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480 λ (nm)

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Figure 12.2 Characterization of (APPV/LDH)n (n 5 3
30) UTFs: (A) UV-Vis absorption spectra (the inset shows the absorbance at 207, 344 nm vs n), (B) fluorescence spectra (Yan et al., 2009).

Fabrication, assembly, and optoelectric properties of layered double hydroxide/conjugated

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Figure 12.3 (A) Chemical formula of SPT; (B) representation of one monolayer of MgAllayered double hydroxide (MgAl-LDH) (dark pink: Al(OH)6 octahedra; green: Mg(OH)6 octahedra); (C) the assembly process of (SPT/LDH)n UTFs (Yan et al., 2011b).

Figure 12.4 The electrostatic assembly process of LDH nanosheets with small anionic/ cationic/small neutral molecule@polyanion pairs.

Then, the LDH nanosheets and the (small cation@polyanion) pairs are alternatively assembled to obtain LDH/CP UTFs. The relationship between small cation and polyanion can be compared to the parasitic relationship in an ecological system,

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Layered Double Hydroxide Polymer Nanocomposites

that is, the small cation can be effectively parasitic on the surface of the polyanion, and the complex coexists within the interlayer of LDH nanosheets. Lu and coworkers assembled small tris (8-hydroxyquinolate-5-sulfonate) aluminum (III) (AQS32) anion with exfoliated MgAl-LDH nanosheets into ordered ultrathin films by employing the LbL assembly method (Li et al., 2011). The small anion, AOS32 together with polyanions such as one of poly(acrylic acid) (PAA), poly(styrene-4-sulfonate) (PSS), and poly [5-methoxy-2-(3-sulfopropoxy)-1,4-phenylene vinylene] (PPV), co-assembled. alternatively with LDH nanosheets, respectively; and found immobilizing small anions into the interlayer of LDHs by the electrostatic force between the polyanions and LDH nanosheets. Liu’s group researching exfoliated LDHs and montmorillonite (MMT) nanosheets with opposite charges, found that they can be assembled to form an ordered composite film through an electrostatic interaction (Liu et al., 2014, 2015; Wang et al., 2014). With the electrostatic LbL assembly, the photoactive divalent cation bis(N-methylacridinium) (BNMA) and polyvinyl alcohol (PVA) were intercalated between the LDHs and the MMT layer. The assembly process of (MMT/ BNMA@PVA/LDHs/BNMA@PVA)n UTFs is shown in Fig. 12.5. The MMT/LDH thin film formed a kind of electronic microenvironment (EME), and it was found that the lifetime of the inserted fluorescent species can be enhanced by about 40 times (Yan et al., 2010). The exfoliated LDHs and MMT nanosheets with opposite charges can provide a homogeneous 2D microenvironment for chromospheres, and offer the inorganic rigid building blocks at the same

Figure 12.5 The assembly process of (MMT/BNMA@PVA/LDHs/BNMA@PVA)n UTFs: (A) a representation of MMT, pink: [AlO6] octahedron, green: [SiO4] tetrahedron, yellow: [MgO6] octahedron, (B) a nanosheet of MMT, (C) chemical formula of BNMA, (D) structure of BNMA, (E) chemical formula of PVA, (F) structure of PVA, (G) a representation of BNMA@PVA solution, (H) a representation of LDHs, pink: Al(OH)6 octahedra, green: Mg (OH)6 octahedra, (I) a nanosheet of LDHs, (J) the MTFs in one cycle (Liu et al., 2015).

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time. The scheme of BNMA with the interlayer between the MMT and LDHs monolayers is shown in Fig. 12.6. Yan et al. (2010) selected BNMA and polyvinyl sulfonate (PVS) as the model system, the BNMA@PVS pair in their mixed solution can be used as a whole to alternately assemble with exfoliated MgAl-LDH monolayers to obtain the (BNMA@PVS/LDHs)n UTFs. Furthermore, the multiple component luminescence adjustable UTFs can be fabricated, such as (APPP/LDH)n(BNMA@PVS/LDHs)m, (APPP/LDH)n(APPV/LDH)m, (BNMA@PVS/LDHs)n(APPV/LDH)m, and (BNMA @PVS/LDHs)m(APPV/LDH)q UTF with blue/green, blue/orange, red/blue, and red/green dual color luminescence, the assembly design is shown in Fig. 12.7 (Yan et al., 2011a). Block copolymer can form the micelle in water with hydrophobic core, which can contain the neutral organic small molecules with excellent optoelectric properties; these negative-charged block copolymer micelles can also be assembled with the positive charge of LDH nanosheets, to realize the small neutral molecules assembly with LDH. Li et al. (2012a) introduced the neutral bis (8-quinolinolato) zinc complex (Znq2) into block copolymer (poly(tert-butylacrylate-co-ethyl acrylate-co-methacrylate), PTBEM), and the negatively charged spherical micelles were alternately assembled with positively charged LDH nanosheets. The process for the fabrication of Znq2@PTBEM micelle and (Znq2@PTBEM/LDH)n UTFs is shown in Fig. 12.8. Li et al. (2015) introduced spiropyran (SP) into spherical micelles formed by PTBEM and alternately assembled with LDH nanosheets to form (SP@PTBEM/LDHs)n UTF with optical switching function. Qin et al. (2015) introduced the neutral dye molecule 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) into the block copolymer polystyrene- b-polyacrylic

Figure 12.6 The scheme of BNMA in an electronic microenvironment between the MMT and LDH monolayers. The thick green arrows indicate the EME’s direction. For the BNMA cations, the blue dotted lines show the repulsion of LDH nanosheets, and the red dashed lines show the attraction of MMT nanosheets, and between the LDHs and MMT nanosheets, there exist the electrostatic attractive interaction (dashed green arrows) (Wang et al., 2014).

Figure 12.7 (A) Representation of one monolayer of MgAl-layered double hydroxide (MgAl-LDH) (dark pink: Al(OH)6octahedra; green: Mg(OH)6 octahedra); the chemical formulae of: (B) APPP (blue luminescence), (C) BNMA@PVS (green luminescence), (D) APPV (orange luminescence), and (E) APT (red luminescence); (F) the typical procedure for assembling dual-color-emitting UTFs with blue/green, blue/ orange, red/blue, and red/green luminescence (Yan et al., 2011a).

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Figure 12.8 Schematic representation of the PTBEM, Znq2, and MgAl/LDH nanosheet and the process for fabrication of Znq2@PTBEM micelle and (Znq2@PTBEM/LDH)n UTF (Li et al., 2012a).

acid (PS-b-PAA). The anionic micelles were then assembled with LDH nanosheets alternately, shaped (PS-b-PAA@DCM/LDH)n UTFs that can be used to detect the common volatile organic compound (VOC) vapors.

12.1.2.2 Layer-by-layer assembly method based on hydrogen bond interactions In addition to employing the electrostatic interaction between LDH nanosheets and polyanions to fabricate the LDH/CP UTF system, other weak interactions, such as hydrogen bonds, van der Waals forces, hydrophobic interactions, and so on can also be used as a driving force for the assembly of LDH/CP UTFs. Besides electrostatic assembly, the LDH nanosheets and CP molecules can also be interacted by hydrogen bonding due to the abundant hydroxyl groups in the LDH layers and water molecules within the interlayers to prepare a composite thin film. Neutral conjugate polymer with
OH,
NH2 groups can be selected and assembled with LDH nanosheets to form a LDH/CP composite UTF; the assembly process diagram is shown in Fig. 12.9. Han et al. (2011) fabricated (PVA/LDH)n UTF with the LbL assembly method based on hydrogen bond LbL assembly; polyvinyl alcohol (PVA) is an electrically neutral polymer that cannot be assembled by electrostatic forces with LDHs, whereas both PVA and LDHs are rich in
OH group, suggesting a hydrogen bond interaction is possible between them, which has been investigated by infrared spectroscopy and molecular dynamics simulations. Huang et al. (2009) fabricated (PVA/MMT/PVA/LDH)n with LbL hydrogen bonding assembly. Based on the multilayer assembly driving force of hydrogen bonds between LDH and neutral polymer, the more neutral polymer can be used as a carrier to

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Figure 12.9 The assembly process diagram of LDH nanosheets and neutral polymer interaction based on hydrogen bonding interaction (Han et al., 2011).

realize the coassembly of the neutral small molecules with the LDH nanosheets. Chen et al. (2010) fabricated (PVA@GO/LDH)n UTFs with the LbL assembly method based on the hydrogen bonding interaction. Li et al. (2012b) assembled the MgAl-LDH nanosheets with a (polyvinyl carbazole (PVK)/perylene) pair on the basis of hydrogen bonding interaction to fabricate the (PVK@ perylene/LDHs)n UTFs on the quartz substrate. A schematic diagram of the LbL assembly procedure is shown in Fig. 12.10. The contained N atoms in PVK exerted the hydrogen-bonding interaction with the
OH group on the LDH layer. This method has some universality, and many molecules can be assembled based on the hydrogen-bonding interaction with LDH nanosheets to form various UTFs, and the hydrogen-bonding-based assembly cycle can be repeated up to 50 times, being comparable with electrostatics assembly. Furthermore, the neutral complex Ir (F2ppy)3 can be mixed with (PVK) to form the (Ir(F2ppy)3@PVK) pair, which can be alternately assembled with the LDH nanosheets to form the (Ir(F2ppy)3@PVK/ LDH)n UTFs for the detection of VOCs (Qin et al., 2014). This method utilizes the hydrogen-bonding interaction between the neutral PVK with carbazole groups and LDH nanosheets with numerous hydroxyl groups to realize the encapsulation of neutral (Ir(F2ppy)3) complex into the interlayers of LDH nanosheets. The assembly process is shown in Fig. 12.11. Qin et al. (2016) fabricated (Alq3@DCM@PVK/LDH)n and (Ir(F2ppy)3@DCM@PVK/LDH)n nanocomposite UTFs with a 2D cascade

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Figure 12.10 Schematic representation of the fabrication process of the (PVK@perylene/ LDH)n UTFs, the molecular structure of perylene and PVK (top): (A) the pretreated quartz substrate, (B) Mg-Al LDH nanosheets, and (C) perylene@PVK complex (Li et al., 2012b).

Figure 12.11 Coassembly of the (Ir(F2ppy)3@PVK/LDH)n UTFs (Qin et al., 2014).

FRET process. Using the PVK polymer as a carrier, neutral molecules, Alq3, Ir (F2ppy)3 and DCM were successfully encapsulated within LDH layers to investigate the 2D cascade FRET process, the assembly process was shown in Fig. 12.12. The morphology and supramolecular structure of the luminescent UTFs were studied for the PVK@DCM/LDHs (F1), PVK@Alq3@DCM/LDHs (F2), and PVK@Ir (F2ppy)3 @DCM/LDHs (F3) UTFs. As shown in Fig. 12.13, the SAXRD showed that the Bragg peak at 2θ 5 1.34 , 1.37 , and 1.32 , indicated the periodic spacings about 6.58, 6.44, and 6.68 nm, corresponding to F1, F2, and F3 films, respectively, that is the UTFs had an ordered periodic structure along the film, normally up to 25 bilayers. The top-view SEM images showed the UTFs were smooth and the side-view SEM images showed the thickness of F1, F2, and F3 UTFs were about 122, 119, and 124 nm (n 5 18), respectively, Thus, the UTF average

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Layered Double Hydroxide Polymer Nanocomposites

Figure 12.12 (A) The scheme of the one- and two-step cascade FRET process, (B) molecular structures of the donor and acceptor, and (C) schematic representation of the fabrication of thin films process (Qin et al., 2016).

Figure 12.13 (Left) Small-angle XRD patterns for the UTFs-25: (a) F1, (b) F2, and (c) F3 UTFs. (Right) The structural and morphological characterization of F1, F2, and F3 UTFs-18: (A) top-view, (B) side-view SEM images, (C) fluorescence microscopy image, and (D) AFM images (Qin et al., 2016).

thickness of one bilayer can be calculated to be 6.78, 6.62, and 6.89 nm, which is congruent with the SAXRD results. The luminescent UTFs show a homogeneous bright orange color under a fluorescence microscope (Fig. 12.13, right, C),

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demonstrating that the fluorophores are distributed uniformly throughout the UTF. The AFM images of the fabricated UTFs illustrate that the root-mean-square (rms) roughness was 8 nm, and the roughness of F3 UTFs increased gradually from 5.40 to 12.75 nm as the n increased from 9 to 18, indicating a relatively smooth surface of these UTFs.

12.1.2.3 Layer-by-layer assembly method based on van der Waals forces Zhang et al. (2016) have illustrated the assembly of neutral conjugated polymer and LDH nanosheets based on van der Waals forces. Poly[(9,9-dihexylfuoreny-2,7diyl)-co-(9-ethylcarbazole-2,7-diyl)] (PFH-Ec), poly(9,9-n-diylhexyl-2,7-fuorenealt-9-phenyl -3,6-carbazole) (PFPC), poly(3-hexylthiophene-2,5-diyl)(P3HT) poly (2,5-di(2’-ethyl- hexyl)phenylene-1,4-ethynylene) (PPE), poly(9,9-n-dihexylfluorene-2,7-diyl)(PHF), polyphenylene vinylene (PPV), and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) (Fig. 12.14) were assembled with LDH nanosheets, respectively, and the UV absorption and luminescence spectroscopy of these LDH/ CP UTFs are consistent with those of the corresponding neutral conjugated polymers, and the absorbance increases linearly with increasing number of assembly cycles. It was found that the PPE, PPV, PHF, and P3HT were neutral conjugated polymers without polar groups, such as
OH,
NH2, etc., and the electronegative heteroatoms such as O, F, and N. Therefore, it is speculated that the assembly driving force of these LDH/CP UTFs should be different from the electrostatic interaction and hydrogen bonding; there may be weak van der Waals force (such as polarization, deformation, dipole, etc.). It is possible that the interaction between the positively charged LDH nanosheets and the delocalized π electrons on the main chain of the conjugated polymer play an indispensible role in the LbL assembly for these CH3

H3C

H3C

CH2(CH2)4CH3 C6H13

C6H13

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N CH3

m

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PFH-Ec

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H3C

O R

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60

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PVK

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Figure 12.14 The molecular formulae of conjugated polymer assembly with LDH nanosheets through van der Waal’s forces (Zhang et al., 2016).

OCH3

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Layered Double Hydroxide Polymer Nanocomposites

LDH/CP UTFs, and the specific reasons and evidence need to be further studied. There are also other issues worthy of further study for this assembly method, such as: how to balance the positive charge of the LDH hydroxides, the interaction between inorganic anions (NO32) existing in the interlayer and the neutral guest molecule.

12.1.2.4 Layer-by-layer assembly based on miscellaneous interaction Except the above method for the fabrication of LDH/CP UTFs, other interactions, such as magnetic-field-assisted assembly, and miscellaneous interaction including electrostatic, hydrogen bonding, and van der Waals force can also be used as a driving force for preparing LDH/CP UTFs. Shao et al. (2011) constructed the (CoFeLDH/MnTPPS)n UTFs through magnetic-field-assisted assembly, as shown in Fig. 12.15. Nowadays, biotechnology and nanotechnology are the two main trends in the development of science and technology, and the combination of these two fields has been paid more and more attention. The study of the analysis and identification of proteins and other biological small molecules is also significant (Han et al., 2011; Ji et al., 2008). Bioinorganic nanocomposites as the material basis of bionanotechnology have become the research focus. At present, the development of new biological/inorganic nanocomposites is relatively less commonly reported. In recent years, composite materials based on biomolecules and LDHs has also been of interest, with the development of LDH-based composites (Wu and Schanze, 2014; Chen et al., 2013; Mary-Ann et al., 2008; Bellezza et al., 2009; Vial et al., 2008; Le´a et al., 2006; Hu et al., 2013). For biomolecules, it is difficult to combine them with LDH nanosheets by conventional assembly methods without sacrificing their biological structure and functions. Positively charged LDH nanosheets with negatively charged biomolecules can be combined by the LbL assembly technique with miscellaneous interaction to obtain biomolecule/LDH composite UTFs. For

Figure 12.15 Schematic representation for the MFA LBL assembly of the (CoFe-LDH/ MnTPPS)n UTFs (Shao et al., 2011).

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complex biological macromolecules, such as proteins and nucleic acids, we can use their unique groups, based on electrostatic, hydrogen bonding, and van der Waals force to assemble LDH nanosheets, to fabricate a new kind of bioinorganic composite UTF functional material. Nucleic acids are mainly located in the cell nucleus, playing the role of storage and transmission of genetic information in protein replication and synthesis. Nucleic acids are not only the basic genetic carrier, but also play an important role in protein biosynthesis, individual growth, gene expression, and a series of major life phenomena. Nucleic acids were composed of nitrogenous organic bases, pentose, and phosphoric acids, and are a typical polyanion within hydrogen-bonding groups. Therefore, nucleic acid can be immobilized on the LDH interlayer to form a nucleic acid/inorganic composite UTF based on a variety of forces. For example, Shi et al. (2014) utilized MgAl-LDH nanosheets with DNA layers to prepare (DNA/LDH)n UTF. The (DNA/LDH)n UTFs were immersed in porphyrin (TMPyP) with chiral chromosphere solution, (TMPyP@DNA/LDH)n was obtained through the LbL method. These (TMPyP@DNA/LDH)n UTFs under different external conditions, had a binding arrangement between TMPyP and intercalated DNA in LDHs, resulting in different induced CD signals, so that the film can be used as a chiral optical switch. The characterization of (DNA/LDH)n UTF by LbL assembly was studied as shown in Fig. 12.16 by morphological characterization techniques like SEM, XRD, and AFM. Compared with exfoliated LDH nanosheets, the (DNA/LDH)n UTFs display a narrow, symmetric, and strong Bragg diffraction reflection at 2θ 5 3.98 , whose intensity is enhanced linearly along with an increase in bilayer number. The SEM image shows the average repeating distance is B2.58 nm, which is approximately consistent with the thickness augment per deposited cycle (B2.65 nm). DNA fragments and genetic units control basic biological function, such as the telomere which is a special junction of the eukaryotic chromosome end, and the human telomeres are essentially repeating nontranscribed sequences (TTAGGG) and some binding proteins to form a special structure. The telomere is closely related to cell apoptosis, cell transformation, and immortalization, and is the mitotic clock of cell life. When cells divide, DNA is replicated once, telomeres shorten the point, so telomere length reflects the cell copy history and replication potential. The greater the number of cell divisions, the more the telomere wears, and the more life is shortened; severely shortened telomeres are signals of cell aging. Recently, our research group has carried out the detection of different lengths of telomere fragments, based on the driving force of electrostatic and hydrogen bonding interaction, the fluorescent dyes, and long-chain telomere simulation nucleic acid were blended and assembled with LDH nanosheets to form a complex (fluorescent dyes @ ssDNA/LDHs)n UTF, the telomere fragments of its complementary sequence were tested to obtain the fluorescence changes for different lengths of telomere in the physiological condition, which show the potential application value in monitoring the shortening of telomeres. From the chemistry point of view, the protein is a class of organic macromolecules with a basic unit of more than 20 kinds of amino acids. There is some residual

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Figure 12.16 (A) XRD patterns of the exfoliated LDH nanosheets and the (DNA/LDH)n UTFs (n 5 10, 15, and 20). (B) Top view of the SEM image for the (DNA/LDH)20 UTF (inset: cross-section of the SEM image). (C) AFM image and (D) high-magnification SEM image of the (DNA/LDH)20 UTF (Shi et al., 2014).

within this amino acid containing carboxyl, amino, hydroxyl, mercapto, and other polar groups, which can be interacted with LDH nanosheets to prepare UTFs. Therefore, more protein molecules can be intercalated into the LDH nanosheet to obtain the protein/LDH UTFs. Hemoglobin (HB) and horseradish oxygenase (HRP) and NiAl-LDHs nanosheets were assembled by the LbL method; and (LDH/HB/ LDH/HRP)n UTFs were obtained (Kong et al., 2010). Green fluorescent protein (GFP) is a bioluminescent composed of 238 amino acids, which exhibited bright green fluorescence under UV light excitation. The immobilization of GFP is of significant interest for applications in biosensing due to its exquisite biological functions. However, there are some challenges for immobilization and application due to its vulnerable and sophisticated 3D structures. To keeping the 3D structure and biofunction of GFP, the LDH nanosheets from the colloidal mill method were employed to realize the assembly in the aqueous solution (Zhang et al., 2017b). Based on the special structure of fibrous silk fibroin (SF), and the interactions between SF and the CdTd QDs, the (CdTe QDs@SF/LDH)n UTF was fabricated,

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which showed the enhanced luminescence of CdTe QDs due to the β-sheet SF, compared with their solution counterpart. It has been found that these novel biomolecule/LDH UTFs by LbL assembly based on miscellaneous interactions have great potential applications in biological detection and sensoring.

12.2

Optical and optoelectric properties of LDH/CP nanocomposites

12.2.1 Optical properties 12.2.1.1 Photostability of LDH/CP nanocomposites Due to the intrinsic shortcoming of conjugated polymers in the application, including aggregation quenching, poor solubility, short service life, relatively poor thermal or optical stability, and various conjugated polymers were designed and synthesized to overcome these drawbacks. In the LDH/CP nanocomposites, the UV-proof properties of the LDH hydroxide layers protect the intercalated conjugated polymer from photodegradation, and have good light-resistant performance. Yan et al. (2009) fabricated (APPP/LDH)n UTFs with blue luminescence by LbL assembly and found that (APPP/LDH)n UTFs have longer fluorescence lifetimes and a higher photostability for UV irradiation than the counterpart samples, APPP/polycation UTFs. The alternative assembly of APPP with LDH nanosheets results in ordered stacking to form LDH/CP nanocomposites with well-defined blue photoluminescence and prolonged fluorescence lifetimes, which confirms that the LDH monolayers improved the luminescence properties of APPP by avoiding the formation of π-π stacking structure. Moreover, the existence of LDH monolayer leads to higher UV photostability for the blue luminescence of APPP as shown in Fig. 12.17. The LDH monolayer is rigid and layered in comparison to flexible polyelectrolyte. Thus, alternative assembly of conjugated polymer with rigid LDH monolayers can result in new inorganic/organic hybrid UTFs, in which LDH monolayers can provide more constraints for reducing the π
π stacking and suppressing thermal vibrations of polymers, and the nonradiative relaxation of excited states within polymer backbone. In addition, the presence of the inorganic layer can improve the photostability of CP. Yan et al. (2011b) fabricated luminescent ordered SPT/LDH UTFs by the LbL assembly method with good luminescent performance. The sulfonated polythiophene (SPT) polymer with red luminescence showed rather poor optical properties owing to low-band gap accompanied by strong nonradiative relaxation and π
π stacking interactions in its solution. The luminescence properties of SPT/LDH polymer nanocomposites confirmed that LDH monolayers improved the luminescence properties of SPT by avoiding the formation of π
π stacking of polymer backbones. Moreover, the existence of LDH monolayer leads to higher UV photostability for the red luminescence of SPT. The UV-resistant capabilities of the (SPT/ LDH)32 UTF are shown in Fig. 12.18.

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Figure 12.17 Decay of normalized maximal PL intensity IPL with time, normalized against the initial PL value; λex 5 365 nm to probe the UV irradiation resistance ability of (APPP/ PDDA)27 (black data points) and (APPP/LDH)27 UTF (red) irradiated under 344 nm UV light. Insets: photographs under UV light of (a) (APPP/PDDA)27 and (b) (APPP/LDH)27 UTF after the UV resistance experiment was finished (Yan et al., 2009).

Figure 12.18 The decay of the normalized maximal luminescence intensity with irradiation time (360 nm UV light) demonstrating the different UV-resistant capabilities of the (SPT/ LDH)32 UTF and the SPT drop-cast film (Yan et al., 2011b).

Based on the electrostatic interaction between LDHs and guest molecules, researchers achieved a number of LDH/CP nanocomposites with enhanced luminescence properties. The LDH/CP nanocomposite UTFs have an inorganic
organic hybrid quantum well structure with the electric insulating LDH hydroxide layers as

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the energy barrier, to effectively separate the conjugated polymer within the 2D confinement environments, thus avoiding blue/red shift of luminescence based on π
π interaction, therefore LDH/CP nanocomposites have improved luminescence properties. With the coassembly method, the polyanions can be used as a support to achieve a small cation@polyanion and electrically neutral organic fluorescent guest molecules/LDH UTFs. These luminescent UTFs have better photostability as well as fluorescent intensity.

12.2.1.2 Luminescence properties of LDH/conjugated polymer nanocomposites and applications According to the luminescence principles and our research conclusion, the criteria for getting good luminescence properties from LDH/CP nanocomposites can be summarized as follows: (1) the interlayered polymer or small molecules were dispersed homogeneously without concentration-quenching phenomenon. (2) The reversal of interlayer polarity from hydrophilic to hydrophobic ones was necessary for the homogeneous distribution of some neutral polymers and small molecules to achieve the good luminescence properties. (3) The interlayered two-dimensional confined space between the LDH hydroxide layers suppresses the vibration of the assembled molecule, which is favorable for enhancing the fluorescence properties of the polymer and small molecules. The LDH/CP nanocomposite UTFs realized the immobilization of fluorescent molecules, which is favored for the device application of these fluorescent molecules in light-emitting diodes, solar cells, and biological and chemical sensors. These immobilized conjugated polymers sometimes exhibited improved photostability, enhanced luminescence performance, and some novel luminescence properties. Yan et al. (2011b) fabricated an (SPT/LDH)n UTF showing red luminescence and reversible pH photoresponse. This UTF showed the polarized luminescence due to the incorporation of a photoactive polymer within a 2D interlayer of LDHs. They constructed the (Ir (F2ppy)3@PVK/LDH)n UTFs, which exhibited the fast, sensitive, and reversible response to common VOCs (Qin et al., 2016). The reversible luminescence response of this nanocomposite UTF is as shown in Fig. 12.19. They also constructed (PS-PAA@DCM/LDH)n UTFs, which exhibited solvatochromism luminescence in different solvent vapors, and can be applied in sensing for solvent polarity (Fig. 12.20) (Qin et al., 2015). Han et al. (2010) reported the preparation of reversible photoresponsive UTFs using a photoactive azobenzene polymer, poly{1
4[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1, 2-ethanediyl sodium salt} (PAZO) and exfoliated LDH nanosheets. In (PAZO/LDH)n multilayer films, azobenzene chromophores exhibited reversible trans
cis photoisomerization. The isolation effect of LDH nanosheets imposes enough free volume for the photoisomerization of the azobenzene group in PAZO, accounting for its complete trans
cis isomerization as well as high reversibility and reproducibility. These LDH/CP nanocomposites incorporated a photoactive moiety, providing an attractive and feasible methodology for

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Layered Double Hydroxide Polymer Nanocomposites

Figure 12.19 Reversible luminescence response of (Ir(F2ppy)3@PVK/LDH)n UTFs for VOCs (Qin et al., 2014).

(A)

f

a: atmosphere b: toluene c: THF d: CHCl3 e: pyridine f: DMSO

(B) 50 40 30

Δλ

Normalized intensity

a

20 10

a 0 525

550

575 600 625 Wavelength (nm)

650

675

34

36

38

b

40 42 Er(30)

c

d 44

e 46

f 48

Figure 12.20 Reversible photoluminescence response of the (PS-PAA@DCM/LDH)n UTFs to VOC polarity (Qin et al., 2015).

creating light-sensitive materials and devices with potential photo read/write capabilities; these UTFs can be potentially applied in the fields of optical coatings, photosensors, and optical information storage. The (LDH/HB/LDH/HRP)n UTF (Kong et al., 2010) modified electrode has good electrocatalytic behavior for substrate catechol, and has good reproducibility and storage stability. GFP/LDHs UTFs realize the immobilization of GFP (Zhang et al., 2017b), which was highly responsive to pH and could be detected in some small biological molecules. (QDs@SF/LDH) UTFs have enhanced fluorescence, longer lifetime, and larger quantum yield than QD aqueous solution, and displayed a fluorescence response to immune globulin. By introducing biomolecules with different functions between the LDH nanosheets, the biomolecules can be effectively protected without losing their biological function, and these biological/inorganic nanocomposites will be widely used in biosensing, bioidentification, biochemical analysis, biomarkers, bio-imaging, drug release, and so on.

Fabrication, assembly, and optoelectric properties of layered double hydroxide/conjugated

519

12.2.1.3 Fluorescence resonance energy transfer (FRET) of LDH/CP nanocomposites Fluorescence resonance energy transfer (FRET) is an energy transfer phenomenon between two fluorescent species [energy donor/accepter (D/A)] that are close to each other, and when the emission spectrum of the energy donor overlaps with the absorption spectrum of the energy acceptor, and the spacing between the two molecules is less than 10 nm, a nonradioactive energy transfer named FRET occurs, in which the fluorescence intensity of the donor is much lower or quenched sometimes, while the fluorescence of the acceptor is greatly enhanced. Within the LDH/CP nanocomposite UTFs, an effective two-dimensional FRET process can be realized with the luminous efficiency of the UTF and the lifetime of the luminescence has been greatly improved. The neutral polymer-polyvinyl carbazole (PVK), neutral fluorescent small molecule 2 perylene, and LDH nanosheets were coassembled to obtain ((perylene@PVK)/LDH)n nanocomposite UTFs by the hydrogen bonding LbL assembly method. The effective 2D FRET process within the interlayers of LDHs was realized. This 2D FRET process can be interrupted by common VOC vapor, which can be used as a new VOC fluorescence sensor (Qin et al., 2014). The possible mechanism is shown in Fig. 12.21. Qin et al. (2014) coassembled the neutral polymer-polyvinyl carbazole (PVK), neutral phosphorescent small molecule 2 iridium metal complex (Ir (F2ppy)3), and LDH nanosheets to obtain (PVK@Ir (F2ppy)3)/LDH)n nanocomposite UTFs by the hydrogen bonding LbL assembly method. The study indicated that the PVK and Ir (F2ppy)3 molecules worked as energy donor and acceptor, respectively, and can realize effective 2D FRET process within the interlayers of LDHs, the luminous efficiency and lifetime of the phosphorescence of Ir (F2ppy)3 have been greatly improved, as shown in Fig. 12.22, as has the lifetime of the acceptor and donor in the UTF, as shown in Fig. 12.23. Due to the confinement effect of LDH, the spacing between PVK and Ir (F2ppy)3 was restricted to within less than 10 nm, which is beneficial to improve the FRET efficiency and the luminous efficiency of UTF. Compared to the traditional FRET system with a D-A pair, the multistep cascade FRET system has two or more D-A pairs and has more advantages: larger Stoke shift, stronger emission intensity, and lower detection limits. Three different FRET

Figure 12.21 A representation of the reversible luminescence response to the VOC stimulus based on the FRET process in (perylene@PVK/LDH)n UTFs (Li et al., 2012b).

520

Layered Double Hydroxide Polymer Nanocomposites

250 (PVK/LDH)25 UTFs 200

λex = 294 nm

(Ir(F2ppy)3@PVK/LDH)25 UTFs

Intensity (a.u.)

λex = 294 nm 150

λex = 434 nm

100

50

0 350

400

450 500 Wavelength (nm)

550

Figure 12.22 Fluorescence spectra of (PVK/LDH)25 film excited at 294 nm, and (Ir (F2ppy)3@PVK/LDH)25 film excited at 294 and 434 nm (Qin et al., 2014).

Figure 12.23 (A) The luminescence decay curves of PVK (at 407 nm) of (PVK/LDH)25 (a); PVK (at 375 nm) (b); and Ir (F2ppy)3(at 471 nm) of UTF-25 (c) excited at 294 nm; inset is the magnified decay curve in the range of 0
50 ns. (B) The luminescence decay curves of Ir (F2ppy)3: in toluene solution (at 473 nm) excited at 434 nm (a); in Ir (F2ppy)3@PVK (8 wt %) drop-casted films (at 471 nm) (b); in UTF-25 (at 471 nm), both excited at 294 nm (c) (Qin et al., 2014).

luminescent films were successfully prepared based on the hydrogen-bonding LbL assembly method, (PVK@DCM/LDHs)n, (PVK@Alq3@DCM/LDHs)n, and (PVK@Ir(F2ppy)3@DCM/LDHs) (denominated as F1, F2, and F3 UTF, respectively), all of which exhibited the UTF prominent luminescence performance owing to the 2D cascade FRET process (Qin et al., 2016). The UV absorption and fluorescence spectra showed that the multistep FRET process was successfully realized in the UTFs. Those LDH/CP nanocomposite UTFs obtained a significant enhancement

Fabrication, assembly, and optoelectric properties of layered double hydroxide/conjugated

521

of light emission and extended lifetime of DCM dye. The fluorescence spectra and luminescence decay curves of DCM (557 nm) in three different FRET systems are shown in Fig. 12.24. Moreover, the F3 nanocomposite UTFs show fast, sensitive, and selective fluorescence signal patterns toward four common volatile organic compounds (VOCs) based on interfering with the 2D cascade FRET process, implying its potential application in the VOC selective sensing field (Fig. 12.25).

Figure 12.24 (A) Fluorescence spectra and (B) luminescence decay curves of DCM (557 nm) in three different FRET systems UTFs-18: F1 (curve a), F2 (curve b), and F3 (curve c), excited by 300 nm. Inset: pictures of the corresponding films under UV irradiation (DCM doping concentration is 6%, 4%, and 9% for F1, F2, and F3UTFs, respectively) (Qin et al., 2016).

Figure 12.25 The luminescence spectra of F3 film in different VOC vapors (the table is the output fluorescence signal of the film exposed to VOC vapors, ε is the solvent dielectric constant) (A); photographs taken under 365 nm UV irradiation (B) and the reversible fluorescence response (C) toward nitrobenzene vapor for eight consecutive cycles (Qin et al., 2016).

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Layered Double Hydroxide Polymer Nanocomposites

12.2.2 Optoelectric properties 12.2.2.1 Photodetectors Photodetectors that convert incident light into electrical signals to probe the light (wavelength and intensity) are widely used in industrial and scientific fields such as optical communications, aeronautical engineering, and biological and environmental surveys. Photodetectors based on novel inorganic nanostructures have been widely investigated, and exhibit excellent optoelectronic performances. However, photodetectors based on inorganic nanostructures need some complicated preparation technology and expensive equipment, which greatly restricts their practical applications. Furthermore, most inorganic photodetector materials have fixed light absorption and a narrow spectral sensitivity, which would be suitable for only fixed or narrow-band light detection. LDH/CP nanocomposites based on organic molecules with various types, broadband absorption range and excellent flexibility, have become suitable candidates for new-generation photodetectors. Zheng et al. (2016) coassembled a CP electron donor, poly[N-9’-heptadecanyl2,7- carbazole-alt-5,5-(4’,7’-di-2-thienyl
2’,1’,3’-benzothiadiazole)](PCDTBT), and electron acceptor, poly(5-(2-ethyl-hexyloxy)-2-methoxycyano-terephthalylidene) (CN-PPV) with Mg2Al-LDH nanosheets by hydrogen bonding LbL assembly method to obtain a novel photodetector based on two-dimensional (2D) confined CP electron donor 2 acceptor coassembled (PCDTBT@CN-PPV/LDHs)n UTFs. The photodetection mechanism of this UTF is shown in Fig. 12.26. As a novel photodetector, The UTFs exhibit broad-range visible-light absorption, from 400 to 650 nm,

Figure 12.26 (A) Schematic illustration of the (CN-PPV@PCDTBT/LDHs)20 UTF photodetector; (B) energy-level alignment of PCDTBT and CNPPV within the Mg2Al-LDH nanosheets under light irradiation; and (C) the proposed 2D PCT mechanism scheme (Zheng et al., 2016).

Fabrication, assembly, and optoelectric properties of layered double hydroxide/conjugated

(B) 300

0.5 mW/cm2 2.0 mW/cm2

300 200

3.5 mW/cm2

100

5.0 mW/cm2

0

15 mW/cm2

Current (nA)

Current (nA)

(A)

250

Experiment data

200

Power law fit, I-P0.88

150 100

–100

30 mW/cm2

–200

75 mW/cm2

50

–300

150 mW/cm2

0

–3

–2

–1

523

0

1

Voltage (V)

2

3

1

10

100

Light intensity (mW/cm2)

Figure 12.27 (A) I 2 V relationships of the (PCDTBT@CN-PPV/LDHs)20 UTFs excited by light with different intensities, ranging from 0.5 to 150 mWcm22; (B) corresponding fitting power law curve of photocurrent and light intensity (Zheng et al., 2016).

resulting from complementary absorption of PCDTBT and CN-PPV polymers. The fluorescence emission of the UTFs is completely quenched, implying the occurrence of a photo-induced charge transfer (PCT) process. The coassembled UTFs have a high photocurrent and on/off switching ratio (300 nA/B120), in contrast to those of the PCDTBT/CN-PPV drop-casting thin film (5.4 nA/B1.6); a fast response; a short recovery time (lower than 0.1 s); and excellent wavelength and light-intensity dependence. The PCT mechanism can be attributed to the formation of a 2D bulk heterojunction of the two polymers within the interlayers of the LDH nanosheets. Furthermore, the flexible UTFs on polyethylene terephthalate substrates are also fabricated, exhibiting excellent folding strength and electrical stability. The photoresponsive intensity dependence of (PCDTBT@CN-PPV/ LDHs)n UTFs is shown in Fig. 12.27.

12.2.2.2 Photocatalysis There exist numerous organic molecules with various HOMO/LUMO electron levels, which could form an electron donor
acceptor (D-A) system when in contact. The photocharge transfer process will occur when the D-A system has suitable energy level alignments which endow this system with various electric, magnetic, and photocatalytic properties. Organic D-A charge-transfer complex salt showed excellent conductivity and magnetic properties. The D-A pairs based on organic polymers and small-molecule semiconductors have been widely used in organic feld-effect transistors (OFETs), organic photoconductors, and D-A type heterojunctions as organic solar cells. In these D-A systems, the photo-induced electron transfer (PET) process across a D-A interface was the key process, dominating the efficiency of photoelectric conversion. However, the D-A organic systems were rarely used as photocatalyts for PEC water-splitting, possibly due to the expensive price of D-A molecules and the photo-instability of most organic molecules.

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Layered Double Hydroxide Polymer Nanocomposites

Zheng et al. (2015) fabricated the DAS-DNS/LDHs composite system by cointercalating the 4,4-diaminostilbene-2,2-disulfonate (DAS) and 4,4-dinitro-stilbene2,2-disulfonate (DNS) anions into Zn2Al LDHs. The DAS-DNS/LDHs composite exhibited broad UV-visible light absorption and fluorescence quenching, which was a direct indication of the photo-induced electron transfer (PET) process between the intercalated DAS (donor) and DNS (acceptor). The cointercalated DAS/DNS anions were orderly aligned within the interlayers and the HOMO/LUMO energy levels of the intercalated DAS and DNS anions were affected to match and couple as the electron donor and acceptor, respectively. The DAS-DNS/LDH composite was fabricated as the photoanode and Pt as the cathode. Under UV-visible light illumination, the enhanced photo-generated current (4.67 mA/cm2 at 0.8 V vs. SCE) was generated in the external circuit, and the photoelectrochemical water split was realized. Furthermore, this photoelectrochemical water-splitting performance had excellent crystalline, electrochemical, and optical stability. Therefore, this novel inorganic/organic hybrid photoanode exhibited potential application prospects in photoelectrochemical water-splitting. The proposed 2D PET mechanism scheme and the energy-level alignment illustrated in Fig. 12.28 indicate that the photo-generated electron would transfer from DAS to DNS anions upon illumination. The 2D PET process was realized within the interlayers, which resulted in charge separation in the photoanode, and the free 0 V vs vacuum level

–1 e–

–2 –3

e e–

–2.68

h+

–4

1 – O +2H+ 2 2

–3.48 –4.10 +

–5

H2

H /H2 –5.33

–6 O /H O 2 2 –7

–5.50 –5.82

DAS

2H+

H2O

DNS

DAS(50%)-DNS/LDHs/ITO

Pt

DAS e

e

e DNS

DAS(50%)-DNS/LDHs

Figure 12.28 The proposed 2D PET mechanism scheme of photo-generated current of DASDNS/LDHs photoanode and the energy level alignment of DAS and DNS anions in Zn2AlLDH interlayers, E (H1/H2) 5 0
0.059 pH (vs. NHE), pH 5 6.8, E (O2/H2O) 5 1.23
0.059 pH (vs. NHE), pH 5 6.8.

Fabrication, assembly, and optoelectric properties of layered double hydroxide/conjugated

525

electrons reached the Pt cathode to reduce the proton to hydrogen, while O2 was evolved at the photoanode by H2O molecules oxidized by the hole of the photoexcited DAS, and completed the entire PEC water-splitting. It can be concluded that the 2D confinement effect within the LDH interlayers can provide a good photocharge transfer condition for the electron donor and acceptor, and effectively realize the generation and separation of photo-generated electrons and holes. This novel inorganic/organic composite photoanode exhibited potential application prospect in PEC water-splitting, and the flexibility of organic D/A pair selection and cointercalation design for this inorganic/organic hybrid composite paved a broad and promising way to develop a kind of new, low-cost, and simple layered PEC system for solar energy conversion and application.

12.3

Conclusions and outlook

LDHs as a typical anionic inorganic layered material and its hydroxide layers can be exfoliated to form LDH nanosheets, which provide a basis for the construction of composite multifunctional film materials with various species. Based on the electrostatic interaction, the assembly of functional polyanions and LDH nanosheets can be achieved, a coassembly of small anions/cationic and polyanionic blends with LDH nanosheets was developed to construct the more LDH/CP nanocomposite UTFs. As a kind of novel nanocomposite, LDH/CP thin-film materials exhibited good photostability, and luminescent or optoelectric properties. Various assembly methods were developed to fabricate these nanocomposite materials based on the exfoliation and assembly features of LDH compounds. Based on the traditional electrostatic LbL method, hydrogen bond, van der Waals interaction, and miscellaneous interactions were employed into the LbL assembly process to encapsulate the small organic anions, cations, or neutral molecules/polymer into the interlayers of LDH nanosheets, which extended the assembled guest members for this nanocomposite and many neutral conjugated polymers, protein, and DNA/RNA can be assembled into the interlayers by this modified LbL assembly method. This LDH/ CP nanocomposite showed novel luminescence properties like 2D FRET process and PET process, which can be successfully applied into VOC probing, photocatalysis, and photodetectors. Thus, this nanocomposite has realized the immobilization of molecules with good photofunctional performance in a solution state, and is compatible for the device-orientation application involving photofunctional molecules/polymers. Above all, it is in the ascendance for LDH/CP nanocomposite photofunctional materials, which exhibited an unexpected performance based on the synergistic effect arising from the host
guest interaction within this composite material. This chapter has summarized the current research status of the LDH/CP nanocomposites, from which it can be witnessed that the LDH nanosheets played a significant role in the implementation of photofunction of the guest molecules, like a 2D “molecular container” at the nanometer scale. It must be admitted that more open problems

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Layered Double Hydroxide Polymer Nanocomposites

still exist and are worth solving in this field, such as the detailed interlayer structure, the nonelectrostatic assembly mechanism, and the 2D confinement effect. This is not only an opportunity for developing novel composite materials, but also a challenge for inorganic chemists to explore its functional potential and application scope as far as they can.

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3076. Yan, D., Lu, J., Chen, L., 2010. A strategy to the ordered assembly of functional small cations with layered double hydroxides for luminescent UTFs. Chem. Commun. 46, 5912
5914.

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Yan, D., Lu, J., Wei, M., Qin, Y., Chen, L., Zhang, S., et al., 2011a. Heterogeneous transparent ultrathin films with tunable color luminescence based on the assembly of photoactive organic molecules and layered double hydroxides. Adv. Funct. Matter 21, 2497
2550. Yan, D., Lu, J., Wei, M., Qin, Y., Chen, L., Zhang, S., 2011b. Fabrication of an anionic polythiophene/layered double hydroxide ultrathin film showing red luminescence and reversible pH photoresponse. AIChE J. 57, 1926
1935. Yang, Y.B., Su, L.N., Wang, A.N., 2014. Research progress of layer-by-layer self-assembly technology (in Chinese). Chem. World 10, 636
640. Yao, H., Fang, H., Wang, X., 2011. Hierarchical assembly of micro-/nano-building blocks: bio-inspired rigid structural functional materials. Chem. Soc. Rev. 40, 3764
3785. Zhang, P., Li, H., Shi, J., Lu, J., 2016. Assembly of neutral conjugated polymers with layered double hydroxide nanosheets by the layer by-layer method. RSC Adv. 6, 94739
94747. Zhang, M., Hong, Y., Chen, W., Wang, C., 2017a. Polymers for DNA vaccine delivery. ACS Biomater. Sci. Eng. 3, 108
125. Zhang, P., Hu, Y., Ma, R., Li, L., Lu, J., 2017b. Enhanced green fluorescence protein/layered double hydroxide composite ultrathin films: bio-hybrid assembly and potential application as a fluorescent biosensor. J. Mater. Chem. B 5, 160
166. Zhao, Y., Yang, W., Xue, Y., Wang, X., Lin, T., 2011. Partial exfoliation of layered double hydroxides in DMSO: a route to transparent polymer nanocomposites. J. Mater. Chem. 21, 4869
4874. Zheng, S., Lu, J., Yan, D.P., Qin, Y.M., Li, H.L., Evans, D.G., et al., 2015. An inexpensive co-intercalated layered double hydroxide composite with electron donor-acceptor character for photoelectrochemical water splitting. Sci. Rep. 5, 12170. Zheng, S., Lu, J., Duan, X., 2016. Novel visible-light photodetector based on twodimensional confined electron donor 2 acceptor co-assembled layered double hydroxide ultrathin films. ACS Omega 1, 1239
1246. Zhong, W., Xiao, J., Sun, S., Jiang, X.F., 2016. Wide bandgap dithienobenzo dithiophenebased π-conjugated polymers consisting of fluorinated benzotriazole and benzothiadiazole for polymer solar cells. J. Mater. Chem. C 4 (21), 4719
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Polymer layered double hydroxide hybrid nanocomposites

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Shadpour Mallakpour1,2 and Elham Khadem1 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

13.1

Introduction

Polymer nanocomposites (NCs) are encouraging materials due to their multipurpose properties and huge amount of alterable composition/preparation attainable by fine tuning. Compared to nanofillers used for hybrid nanomaterials, layered double hydroxide (LDH), due to its nontoxicity, structural homogeneity, high bound water content, and high reactivity toward organic anionic species is appropriate for several specific requirements (Mallakpour et al., 2016a; Omwoma et al., 2014). The layered crystalline geometry of LDH comprises different intercalating anionic species which can interchange with much larger organic anionic molecules and make it favorable as a filler (Basu et al., 2014; Radulescu et al., 2007). On the other hand, hybrid nanostructures composited from LDH and carbonaceous nanomaterials (e.g., carbon nanotube (CNT) and graphene) have drawn a great deal of attention. It is the combination of the special properties of the parent materials, which provides unique properties to hybrid materials. The carbonaceous nanomaterials combined with LDH not only avoid the restacking and aggregation of LDH, but also develop the conductivity, catalytic activity, and thermal stability of the polymeric composites (Cao et al., 2016; Daud et al., 2016; De Marco et al., 2017). Polymer nanofiller hybrid composites developed using LDH/carbonaceous nanofiller hybrids have drawn great interest lately because of the unique properties obtained by the combination of the parent materials. Modification of LDH with carbonaceous nanomaterials and changing surface properties not only improves the compatibility between the filler and the polymer, but also creates a strong interaction and good adhesion between the components of composite (Burgos-Ma´rmol and Patti, 2017; Pavlidou and Papaspyrides, 2008; Peng et al., 2006). LDHs, due to their capacity to separate into distinct layers, and the prospect of changing their surface chemistry through ion exchange reactions with organic and inorganic anions, can have a significant role in the production of polymer NCs with high thermal stability (Choudalakis and Gotsis, 2009; Ivanov et al., 2001; Mallakpour et al., 2015).

Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00013-6 © 2020 Elsevier Ltd. All rights reserved.

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Numerous reviews and book chapters have already been devoted to many aspects of polymer/LDH-NCs and LDH/carbonaceous nanofiller hybrids, highlighting the synthesis, characterization, properties, and use of LDH-type minerals as nanoreinforcers in polymer matrices (Basu et al., 2014; Costa et al., 2007; Leroux and Besse, 2004; Mallakpour and Khadem, 2017b; Papaspyrides and Kiliaris, 2014; Peng et al., 2006). The object of this chapter is first to update the study field of the polymer/LDH/carbonaceous nanofiller hybrid NCs by supplying as much data as needed regarding the treated LDH materials and interaction of polymer with the modified LDH/carbonaceous nanofiller hybrids and characterization, in addition to this academic point of view, the potential applications of those systems are also described.

13.2

Modification of LDHs with organic compounds

The modification of LDHs is as an inescapable method in the preparation of polymer NCs, to expand the interlayer space of LDH materials and facilitate intercalation of a large hydrophobic polymeric chain. Since the hydroxide layers of LDHs have a positive charge, organic anionic surfactants used for treatment have at least one anionic group and a long hydrophobic tail to reduce the surface energy of the treated LDHs (Costa et al., 2007; Mallakpour and Dinari, 2015a). Techniques such as coprecipitation, ion exchange, and the regeneration method not only can be used for the fabrication of LDH crystals, but also can be applied for the treatment of LDH using organic surfactants. In the LDH compound, the anion exchange capacity is highly related to the (1) electrostatic interaction among the host cationic brucite layers and the exchangeable anions and (2) quantity of free energy consisting of the changes of hydration. The order of preference for the anion exchange potential and conformation of the interlayer anions into LDH are controlled based on the electrochemical sequence shown below: 2 2 2 22 22 22 22 22 NO2 3 , Br , Cl , F 2 , OH , SO4 , CrO4 , HAsO4 , HPO4 , CO3

In the anion exchange process in the interlayer area of LDHs, factors such as increasing charge and decreasing ionic radius, pH value .4, appropriate choice of solvents, high temperature, and chemical composition and nature of the LDH, are key agents in improving the ability to exchange (Elbasuney, 2015; Saha et al., 2016). By the anionic exchange method, γ-poly(glutamic acid) (γ-PGA) is an anionic macromolecule used for the treatment of MgAl-LDH (Chiang and Wu, 2011). Changes in the basal spacing of LDH were studied at diverse temperatures by means of in situ wide-angle X-ray diffraction and in situ Fourier transform infrared (FT-IR) spectroscopy. They showed that a noteworthy reduction in the interlayer ˚ was possible by increasing the temperature spacing of LDH from 15.0 to 11.5 A   from 30 C to 130 C (Fig. 13.1). This occurrence may be related to the dehydration

Polymer layered double hydroxide hybrid nanocomposites

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Figure 13.1 Conceptual illustration of the possible structure formation of LDH-PGA at 30 C and 130 C. Source: Adapted from Chiang, M.-F., Wu, T.-M., 2011. Intercalation of γ-PGA in Mg/Al layered double hydroxides: an in situ WAXD and FTIR investigation. Appl. Clay Sci. 51, 330
334, with kind permission of Elsevier.

reaction and elimination of interlayer water molecules without destruction of the layered structure. Mallakpour et al. (2016b) used N-trimellitylimido-L-amino acids for modification of LDHs by the coprecipitation method under fast and green ultrasonic irradiation. The results of the thermogravimetric analysis (TGA) showed that the first degradation temperatures changed to lower temperatures and the thermal stability of LDH was reduced by intercalation of an organic chain into the space of the LDH. This is due to the inorganic carbonates, which have higher thermal stability than the organic dicarboxylate anions between layers. In analogous work, they prepared the modified LDHs by N,N0 -(pyromellitoyl)-bis-L-amino acids diacid as supporting agents via anion exchange reaction (Mallakpour and Dinari, 2015a). Amino acids used in this processes were L-isoleucine, L-leucine, L-phenylalanine, L-alanine, Lmethionine, and S-valine. FT-IR, X-ray diffraction (XRD), TGA, and transmission electron microscopy (TEM) were used for the characterization of the modified LDHs. For example, the XRD result showed that LDH/CO322 has a basal spacing of about 0.75 nm, which, after organic treatment via anionic exchange, XRD shifted to a lower angle position and basal spacing enlarged to maximum 2.15 nm for LDH/N,N-(pyromellitoyl)-bis-L-phenylalanine (Mallakpour et al., 2013). They showed that the best distribution was obtained for LDHs modified with N-trimellitylimido-L-amino acids compared with the N,N0 -(pyromellitoyl)-bis-L-amino acids. The TEM images of the LDH/CO322 and the LDH/N-trimellitylimido-L-leucine are displayed in Fig. 13.2. As can be observed, LDH has a smooth surface with overlapped crystal in a hexagonal form, while the modified LDH indicates dispersed hexagonal sheets with rounded corners. Li et al. (2011) mixed sulfate terminated low-molecular-weight polyethylene glycol (PEG) derivatives with MgAl-LDH and incubated it in a water-bath shaker. After surface modification of LDHs and fabrication of PEG haired LDHs, scanning electron microscopy (SEM) and TEM images (Fig. 13.3) did not show any changes in particle size, while the shape of NPs changed from hexagonal into smooth pielike disks.

Figure 13.2 TEM micrographs of the CO322/LDH (A, B) and LDH-PL (C, D). Source: Adapted from Mallakpour, S., Dinari, M., Behranvand, V., 2013. Ultrasonic-assisted synthesis and characterization of layered double hydroxides intercalated with bioactive N, N0 -(pyromellitoyl)-bis-l-α-amino acids. RSC Adv. 3, 23303
23308, with kind permission of the Royal Society of Chemistry.

Figure 13.3 TEM (A
C) and SEM (D
F) images of sulfate PEG0.6 k/LDH (A, D), sulfate PEG2 k/LDH (B, E), and sulfate PEG5 k/LDH (C, F) composite particles after 72 h reaction. Scale bar: 100 nm; subscript represents the number average molecular weight. Source: Adapted from Li, D., Xu, X., Xu, J., Hou, W., 2011. Poly (ethylene glycol) haired layered double hydroxides as biocompatible nanovehicles: morphology and dispersity study. Colloids Surf. A: Physicochem. Eng. Aspects 384, 585
591, with kind permission of Elsevier.

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13.3

535

Layered double hydroxide/Carbonaceous nanofiller hybrids

The mineral hydrotalcite was discovered in Sweden around 1842 and for the first time the stoichiometry formula of [Mg6Al2(OH)16]CO3  4H2O was assessed by Manasse in 1915. The major structural feature of LDH was determined by crystal XRD in 1960. LDH is a mineral material with positively charged layers made up of 2D highly tunable brucite-like layered crystal structure with an interlayer expanse comprising charge-compensating anions and some water particles, as required for the stabilization of the crystal structure (Evans and Slade, 2006; Rives, 2001). On the other hand, partial substitution of M(III) cations for M(II) cations, positively charged layers create in LDHs are neutralized by interlayer multivalent anions. The basal spacing of LDHs enlarges from 0.48 nm to about 0.77 nm due to presence of anions and solvation molecules. LDHs can be obtained by both synthetic and natural bases. Hydrotalcite with the chemical formula [Mg6Al2(OH)16]CO3  4H2O is the most usually recognized naturally occurring LDH (Rives et al., 2014; Theiss et al., 2016; Tonelli et al., 2013). Several methods, such as coprecipitation, anionic exchange, reconstruction, hydrothermal, and microwave treatments have been proposed for the synthesis of LDH. A number of reviews and books have already been issued that focus on the synthesis and application of LDHs (Galva˜o et al., 2016; He et al., 2006; Qu et al., 2016; Theiss et al., 2016). Despite the great properties that have been attained, these materials often have a rather low electric conductivity and poor adhesion, rare energy density, and low-rate performance. Another challenge is agglomeration and shrinkage of LDHs. These agents may limit the performance of LDHs in field charging/discharging cycles and lead to mechanical degradation of electrodes (Pacuła et al., 2016; Wang et al., 2016). To overcome this problem and increase the performance of pseudocapacitive materials, a combination of LDH and carbonaceous nanomaterials, such as carbon nanofiber (Ma et al., 2016), activated carbon (Lv et al., 2018; Ochai-Ejeh et al., 2017), CNT (Heli et al., 2016), and graphene (Kiran et al., 2017) have been proposed as an ideal method to optimize the electrochemical performance. Schematics of LDH/CNT and LDH/graphene hybrids are displayed in Fig. 13.4. Carbonaceous nanomaterials can serve as hard templates and support the LDHs, which facilitated the accommodation of large-volume changes during the charge/discharge process. Also, they provide a stable cycling performance and improve the chemical stability and conductivity. The resulting LDH/ nanocarbon hybrids can easily form an electron pathway and a network of stress transfer between carbonaceous nanomaterials and LDHs, which is a significant aspect for advanced functional materials. Thus, they enable fast redox charge transfer processes and synergistic enhancement of energy storage performance (Wang et al., 2016; Zhao et al., 2012). Additionally, compared to unmodified LDH, LDH/carbonaceous nanofiller hybrids exhibit the catalyst and adsorption tendency of anionic pollutants owning to their improvement in surface area, better stability, high interlayer spacing, higher anion

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Layered Double Hydroxide Polymer Nanocomposites

Figure 13.4 Schematic illustration showing: (A) graphene 1 LDH: (a0 ) intercalation of graphene layers into the interlayer space of LDHs; (b0 ) in situ grown LDHs parallel to the graphene layer; (c0 ) in situ grown LDHs vertical to the graphene layer; (d0 ) graphene in situ grown on the surface of LDHs; and (B) CNT 1 LDH: (a) CNTs uniformly attached to the surface of LDH flakes; (b) intercalation of CNTs into the interlayer space of LDHs; (c) LDHs in situ grown on the surface of CNTs; (d) randomly entangled CNTs grown from LDHs; (e) aligned CNT arrays grown from LDHs; (f) CNT-array double helix grown from LDHs. Source: Adapted from Zhao, M.Q., Zhang, Q., Huang, J.Q., Wei, F., 2012. Hierarchical nanocomposites derived from nanocarbons and layered double hydroxides-properties, synthesis, and applications. Adv. Funct. Mater. 22, 675
694, with kind permission of John Wiley and Sons.

exchange tendency, outstanding selectivity for diverse toxic metals, growth in chelating and binding sites, and low toxicity. According to these superior characteristics, they can act as a catalyst and promising adsorbent for wastewater treatment (Daud et al., 2016). Wang et al. (2017) prepared ultrathin NiCo-LDH on carbon cloth (CC) by one-pot coprecipitation approach. The prepared composite was used as a binderfree biosensor for glucose detection with a sensitivity of about 5.12 μA/μM/cm2.

13.4

Synthesis of LDH/Carbonaceous nanofiller hybrids

The first action to assay the required properties of LDH/carbonaceous nanofiller hybrids and in extensive applications is their synthesis and preparation. Several methods have so far been discovered to achieve a synergistic mixture of LDH/carbonaceous nanofiller hybrids (Cao et al., 2016; Daud et al., 2016; Zubair et al., 2017), as described below. 1. Coprecipitation synthesis This technique is widely applied for the synthesis of LDH/carbonaceous nanofiller hybrids. During the reaction, carbonaceous nanostructures are sonicated in distilled water

Polymer layered double hydroxide hybrid nanocomposites

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and combined with an aqueous solution of cations salts (M21 and M31) under stirring at a controlled pH. Materials, such as glucose, hydrazine, urea, and sodium sulfide are used as reducing agents to adjust the pH and reduce the carbonaceous nanostructures (Bai et al., 2016; Qiao et al., 2015). Heli et al. (2016) used this method to synthesize CoAl-LDH/ multiwalled carbon nanotubes (MWCNTs) and exerted in the electroreduction, electrocatalytic oxidation, and determination of hydrogen peroxide. In other work, MgAl-LDH/ MWCNT was prepared and characterized by SEM/energy-dispersive spectroscopy (EDX), FT-IR, BET, XRD, and TGA (Long et al., 2016). The prepared nanohybride was used for the removal of Congo red with an adsorption capability of 595.8 mg/g. 2. Exfoliation-restacking synthesis In this method, delamination of LDH was performed in a formamide solution under ultrasonication. Then, second nanofiller (carbonaceous nanomaterials) self-assembly to LDH via electrostatic interaction leads to exfoliated LDHs. The negatively charged species or functional groups with negative charge on the surface nanofillers have a critical role for attaching to positively charged LDHs and successful preparation of the LDH/ nanofiller hybrid (Qiao et al., 2013; Wang et al., 2016). In a research work, MWCNT was modified with melamine and 4,40 -diphenylether dicarboxylic acid and then combined with ZnAl-LDH via electrostatic force (Fig. 13.5) (Qiao et al., 2013). The structure properties of ZnAl-LDH/MWCNT hybrids were characterized by XRD, TEM, FT-IR, and TGA. TGA results demonstrated that functionalized-MWCNTs/LDH nanosheet hybrids due to high char yield at 800oC can serve as a potential flame-retardant material. %

Figure 13.5 Schematic description of F-MWCNTs/exfoliated LDHs nanosheet hybrid composites synthesis. Source: Adapted from Qiao, Z., Gao, C., Sun, B., Ai, S., 2013. Synthesis and characterization of functionalized multi-walled carbon nanotubes/exfoliated layered double hydroxide nanosheets hybrids via electrostatic force. J. Inorg. Organomet. Polym. Mater. 23, 871
876, with kind permission of Springer.

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Layered Double Hydroxide Polymer Nanocomposites

3. Layer-by-layer assembly This method involves the electrostatic interaction between negatively and positively charged materials. In this process, from a polymeric solution such as PVA, poly(ethylene imine) (PEI), and polyaniline (PANI) and substrate supports are used to obtain a cationic surface and deposit negatively charged materials and positively charged LDHs (Wang et al., 2015). For example, Chen et al. prepared multilayer films of (LDH/PVA/graphene oxide/PVA)n by hydrogen-bonding-based layer-by-layer assembly (Chen et al., 2010). They were characterized by XRD, atomic force microscope (AFM), UV-Visible Spectroscopy and SEM. To´th et al. introduced layer-by-layer assembly as a suitable method for building a sandwich-like structure by stratifying delaminated LDH and MWCNT in the presence of a tenside (To´th et al., 2014). 4. One-pot hydrothermal synthesis This method also uses a urea-hydrolyzed method for the preparation of LDH/graphene (LDH/G) NCs and LDH/CNT NCs. In this technique, urea was mixed with a salt solution, and then was added to an ultrasonicated suspension of second nanofiller (G or CNT). The obtained mixture was transferred to a Teflon-lined stainless autoclave and heated at the required temperature for a long time to produce the reduced GO. The hydrothermal method is limited to some cationic LDHs. However, it produces a well-crystallized NC with the same morphology as coprecipitation (He et al., 2015; Peng et al., 2017a). Liu et al. fabricated NiFe-LDH@single-walled carbon nanotubes (SWCNTs) by a hydrothermal method (Fig. 13.6A) (Liu et al., 2018). The SEM (Fig. 13.6B and C) and TEM (Fig. 13.6D and E) images of NiFe-LDH@SWNT showed that the NiFe-LDHs are efficiently grown around the SWCNT units and lattice fringes of 0.25 nm attributed to the (012) lattice plane of NiFe-LDH. In this regard, the use of a microwave method in LDH/carbonaceous nanofiller hybrids synthesis over conventional hydrothermal process is gaining importance and is introduced as a reliable method to attain highly crystalline layered structures. Lonkar et al. prepared LDH/graphene nanohybrids by a facile and rapid microwave method (Lonkar et al., 2015). 5. In situ synthesis This method was done in two states, either by in situ growth of LDH on the second nanofiller (Chen et al., 2014) or by in situ growth of nanofiller on the LDH. During the first state, oxygen functional groups on the nanofiller surface attract divalent and trivalent cations in the solution by electrostatic attractions due to having negative charge. Thus, LDHs form on the carbonaceous nanostructures by the precipitation method. However, the second state is uncommon and few have been reported. This method involved formation of carbonaceous nanomaterials inside LDH corridors by carbonization of interlayer organic anions, and growth of carbonaceous nanostructures through chemical vapor deposition (Momodu et al., 2015). Wang et al. (2010) used ZnAl-LDH/CNT hybrids prepared by this method (Fig. 13.7) for photodegradation of methyl orange molecules under UV irradiation.

13.5

Applications of LDH/Carbonaceous nanofiller hybrids

13.5.1 Removal of pollution The inherent structural properties allow LDH/carbonaceous nanomaterial hybrids to become favorable sorbents to remove a range of anionic pollutants in aqueous

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Figure 13.6 (A) Schematic illustration of the synthesis process of NiFe-LDH@SWCNT. (B, C) SEM images of NiFe-LDH@SWCNT in different magnifications. (D) TEM image of NiFe-LDH. (E) HRTEM image of NiFe-LDH@SWCNT. Source: Adapted from Liu, H., Zhou, J., Wu, C., Wang, C., Zhang, Y., Liu, D., et al., 2018. Integrated flexible electrode for oxygen evolution reaction: layered double hydroxide coupled with single-walled carbon nanotubes film. ACS Sustainable Chem. Eng. 6 (3), 2911
2915 with kind permission of the American Chemical Society.

solutions. The presence of carbonaceous nanomaterials with a distinctive nature, such as numerous oxygen-containing functional groups, robust chemical inertness, small particle sizes, low cost, high surface area, low toxicity, easy functionalization, and suitable biocompatibility, LDH hybrids can fabricate tailored functional composite-based LDHs (Cao et al., 2016; Zhao et al., 2012). Gong et al. (2011) prepared adsorbents based on direct assembling of the performed anisotropic LDH nanocrystals (LDH-NCs) onto the surface of carbon nanospheres (labeled as LDHNCs@CNs) for removal of Cu21. In an initial Cu21 concentration of 10.0 mg/L, the maximum adsorption capacity of the nanoassembly toward Cu21 was calculated

Figure 13.7 Schematic illustration of the synthesis pathway for in situ growth of ZnAl-LDH on to the modified CNTs in the presence of Lcysteine. Source: Adapted from Wang, H., Xiang, X., Li, F., 2010. Hybrid ZnAl-LDH/CNTs nanocomposites: noncovalent assembly and enhanced photodegradation performance. AIChE J. 56, 768
778, with kind permission of John Wiley and Sons.

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to be only B19.93 mg/g. As seen in Fig. 13.8, LDH nanosheets were arbitrarily deposited onto the CNs’ surface. Also, the suspension of LDH hybrid in a CH3OH/ H2O solvent was stable for more than 2 months without undergoing aggregation (Fig. 13.8C). In other work, the maximum uptake capacity of Cd(II) on carbon quantum dots (CQDs)/ZnAl-LDH was estimated at about 12.60 mg/g at 20 min, owing to the accessible exterior sites on the CQD/ZnAl-LDH (Rahmanian et al., 2018). Also, the experimental data confirmed that the adsorption isotherms and adsorption kinetics of Cd(II) on adsorbent were well-fitted by the Freundlich isotherm model and pseudo-second-order kinetic model, respectively. The amount of CQD materials can have an effective role in enhancing the adsorption capacity of anions (Koilraj et al., 2017). The results of adsorption efficiency of Sr21 and SeO422 on MgAl-NO3-LDH/CQD hybrids showed that the process of adsorption occurs via coordination with the 2 COO2 group of CQD, whereas that of SeO422 is accomplished through ion exchange with NO32 in the interlayer galleries of LDH. Based on TEM images, the diameters of CQD and LDH platelets were estimated at about 3
5 nm and 100 nm, respectively. Also,

Figure 13.8 TEM images of (A) LDH-NCs, (B) CNs, and (D) the assembly of LDHNC@CN composites. (C) A translucent and stable suspension of LDH-NCs in methanol/ water solvent. Source: Adapted from Gong, J., Liu, T., Wang, X., Hu, X., Zhang, L., 2011. Efficient removal of heavy metal ions from aqueous systems with the assembly of anisotropic layered double hydroxide nanocrystals@ carbon nanosphere. Environ. Sci. Technol. 45, 6181
6187, with kind permission of the American Chemical Society.

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TEM image showed that the CQD was well dispersed on the layered LDH nanosheets (Fig. 13.9A,c). The elemental mapping of the prepared hybrid is displayed in Fig. 13.9B. The MgAl-LDH/CQD hybrid was also used to remove anionic organic dye (Zhang et al., 2014b). The maximum uptake capability of methyl blue (MB) was only 185 mg/g. The adsorption performance of the obtained hybrid fitted well with the Langmuir isotherm and the pseudo-second-order kinetic model. The main reasons for the adsorption of MB on the surface of the LDH/CQD hybrid are attributed to the cooperative contribution of H-bonding between MB and CQD as well as electrostatic interaction between MB and LDH. Wang et al. (2018) synthesized hydrangea-like carbon sphere (CS)@Ni
Al LDH by a one-step hydrothermal synthesis strategy (Fig. 13.10). They reported that the maximum sorption capacity of U (VI) on CS@LDH (0.6 mmol/g) was twice as high as that of U(VI) on Ni
Al LDH (0.3 mmol/g) and approximately 1.5 times higher than that of U(VI) on CS (0.4 mmol/g) at pH 5 5.0 and T 5 298K.

Figure 13.9 (A) TEM images of (a) CQD, (b) MgAl-NO3-LDH, and (c) MgAl-NO3-LDH/ CQD (20%). (B) TEM EDX elemental mapping on MgAl-NO3-LDH/CQD (20%). Source: Adapted from Koilraj, P., Kamura, Y., Sasaki, K., 2017. Carbon-dot-decorated layered double hydroxide nanocomposites as a multifunctional environmental material for Co-immobilization of SeO42

and Sr21 from aqueous solutions. ACS Sustainable Chem. Eng. 5, 9053
9064, with kind permission of the American Chemical Society.

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Figure 13.10 The synthetic procedure of hydrangea-like CS@LDH nanocomposites using CS and Ni
Al LDH as monomer molecules by a two-step hydrothermal synthetic strategy. Source: Adapted from Wang, X., Yu, S., Wu, Y., Pang, H., Yu, S., Chen, Z., et al., 2018. The synergistic elimination of uranium (VI) species from aqueous solution using bifunctional nanocomposite of carbon sphere and layered double hydroxide. Chem. Eng. J. 342, 321
330, with kind permission of Elsevier.

Chen et al. (2017) reported a facile route for synthesis of Ca/Al-LDH@CNT hybrids, which displayed high adsorption capacity (382.9 mg/g at 289.15K) of U (VI) from aqueous solution. Furthermore, Ca/Al-LDH@CNT showed an endothermic and spontaneous process during adsorption. In other work, Yu and his coworkers investigated the effect of graphene oxide/Ni
Al LDH on removal of U (VI) ions (Yu et al., 2017). Based on Langmuir isotherms, the maximum adsorption ability of GO@LDH (160 mg/g) was much higher than those of GO (92 mg/g) and LDH (69 mg/g). Also, thermodynamic studies showed a spontaneous and endothermic chemical process.

13.5.2 Supercapacitor Compared with other batteries, supercapacitor electrode materials have attracted great attention as energy-storage devices and power sources due to high power capability, fast energy delivery, and long durability. Carbonaceous nanomaterials have significant properties, such as good conductivity, low cost, high stability, abundant resources, and lightweight design. In spite of their excellent rate capability and long cycle life, carbonaceous nanomaterials have a small specific capacitance (theoretical capacitance is 280 F/g). LDHs show high electrochemical activity, and good pseudocapacitive performance. Also, their theoretical capacitance is greater than that for carbonaceous nanomaterials. However, the limited velocity of ion diffusion and electron transfer of LDHs lead to low conductivity and poor cycle life. Therefore, the combination of LDHs with carbonaceous nanomaterials may be a good method to take the advantage of pseudocapacitance and double layer capacitance (Huang et al., 2017; Malak-Polaczyk et al., 2010). Li and his colleagues synthesized Ni
Al LDHs/CNT hybrids by a solution method with a specific capacitance of 1500 F/g at 1 A/g, and 70.3% retention at 10 A/g in 2 M KOH solution (Li et al., 2015). To prepare Ni
Al LDHs/CNTs, at first, the CNTs are precoated by γ-Al2O3 by annealing treatment of AlOOH/CNT in

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Figure 13.11 SEM images of (A) acid-treated CNTs, (B) Al2O3/CNTs, (C) Ni
Al LDHs/ CNTs, and (D) TEM image of Ni
Al LDHs/CNTs. Source: Adapted from Li, M., Liu, F., Cheng, J., Ying, J., Zhang, X., 2015. Enhanced performance of nickel
aluminum layered double hydroxide nanosheets/carbon nanotubes composite for supercapacitor and asymmetric capacitor. J. Alloys Compd. 635, 225
232, with kind permission of Elsevier.

a tube furnace at 700 C. Then, Ni(NO3)2U6H2O and NH4NO3 solutions were added into the AlOOH/CNT solution. Finally, the mixture was heated and refluxed at 100 C in a microwave reactor and Ni
Al LDH/CNT hybrids separated by filtration. The Ni
Al LDH/CNT hybrids can act as a positive electrode to produce an asymmetric capacitor with specific capacitance of 115 F/g and a high energy density of 52 Wh/kg at 1 A/g. Fig. 13.11A and B shows elongated CNTs (diameter of 20 nm) and Al2O3 deposited on the CNT surface, respectively. Also, SEM and TEM of Ni
Al LDHs/CNTs (Fig. 13.11C and D) clearly confirmed that LDHs are closely coated on the CNT surfaces. Fluorinated graphene (FGN) is introduced as a promising potential material, which has broadly served in redox for fuel cells, catalyst in hydrogen storage, and energy storage devices. This may be due to the semi-ionic C
F bonds in FGHs which enhanced the electrical conductivity, facilitated the ion transportation, and provided reactive sites for Faradic reaction. Peng et al. fabricated CoAl-LDH/fluorinated graphene hybrids by a two-step hydrothermal method (Peng et al., 2017b). The resulting hybrids exhibited the maximum specific capacitance (1222 F/g at 1 A/g), the best rate capability, and the most stable capacitance retention at the optimal fluorination time. The FE-SEM and TEM images in Fig. 13.12B displayed that the thin sheets of CoAl-LDH were disorderly deposited on the corrugated and scrolled FGN sheets, and many pores created in the hybrid. In other work, pseudocapacitive performance of the NiCo-LDH decorated with nitrogen-doped graphene was investigated (Mahmood et al., 2015). The results

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Figure 13.12 (A) Schematic presentation of the preparation technique for the LFG composites. (B) Typical FE-SEM images of pLH (a) and LFG-12 (b), and TEM images of LFG-12 (c) and (d). Source: Adapted from Peng W., Li H. and Song S., Synthesis of fluorinated graphene/CoAllayered double hydroxide composites as electrode materials for supercapacitors, ACS Appl. Mater. Interfaces 9, 2017b, 5204
5212, with kind permission of the American Chemical Society.

showed excellent capacitance of 2925 F/g at 1 A/g, as well as long cyclic stability of 10,000 cycles with good capacity retention of 90% at 16 A/g. Also, the hybrid displays excellent energy and power densities of 52 Wh/kg and 3191 W/kg, respectively, at a discharge rate of 16 A/g. In other work, CoNi LDH nanoflakes were grown on carbon nitride-coated Ndoped graphene hollow spheres by a facile chemical bath deposition method. Ndoped graphene, due to the large surface area and hollow structure, can increase the loading of electroactive materials and accelerate electron and ion transport (Hao et al., 2017). The composite showed a specific capacitance of 1815 F/g at 1 A/g and

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excellent cycling stability of 82.1% after 4000 cycles. Wei et al. fabricated Ni
Al LDH/carbon composites via a facile in situ water
ethanol system with capacitance of 1064 F/g at a current of 2.5 A/g (Wei et al., 2012). In this work, colloidal carbonaceous spheres (CCSs) were prepared under aromatization and carbonization of glucose at 180oC in an autoclave pressure vessel. They showed that the concentra% tion of both glucose and ethanol had an important effect on the morphology and capacitive performance of LDHs/carbonaceous nanofiller hybrids. Based on the SEM and TEM images, an increase in the amount of ethanol to 80% led to larger LDH nanosheets of 20
100 nm in length (Fig. 13.13). Also, LDH nanosheets roughened the surface of CCS and provided a higher specific area to use in catalysts, adsorbents, and electrode materials.

13.5.3 Catalyst The tendency to aggregate and poor mechanical properties of LDH usually limit their catalytic activity. Several studies have shown that a combination of carbonaceous nanomaterials and LDHs can be an ideal approach for promoting the exceptional catalytic activity. In prepared hybrids, the high surface area of carbonaceous nanomaterials allows adequate exposure of LDH active sites, which would result in superior catalytic performance. On the other hand, the existence of carbonaceous nanomaterials and heteroatom-doped carbonaceous nanomaterials with high electrical conductivity increases the carrier mobility strongly and amends the electron
hole separation efficiency of LDHs. As for photocatalysts, enhancing electron
hole separation efficiency caused prolonging of the life-time of photogenerated charge carriers, which is of serious significance in improving the photocatalytic efficiency. Consequently, LDH/nanocarbon hybrids as catalyst can greatly improve the heat and mass transfers during a reaction and can easily expose more active sites to the reactant, which is helpful for the high rate conversion of the reactants (Daud et al., 2016; Zhao et al., 2012). Zhang et al. used a one-step facile technique for the preparation of carbon dots and dodecyl benzene sulfonate (DBS)-LDHs NC (Zhang et al., 2014a). A TEM micrograph of carbon dot-DBS-LDH-NCs showed that carbon dots were well distributed on the DBS-LDHs surface (Fig. 13.14A). The resulting hybrid can serve as an effective heterogeneous Fenton-like catalyst for the decomposition of acidified H2O2 to produce abundant hydroxyl radicals (Fig. 13.14B), accompanied with a noteworthy improvement in the chemiluminescence signals. Tang and coworkers prepared a CQD/NiFe-LDH nanoplate hybrid by a plain coprecipitation
solvothermal route to the oxygen evolution reaction (Tang et al., 2014). The resulting CQD/NiFe-LDH hybrid reveals inexpensive, earth abundant, and easily constructed catalyst with an over potential of B235 mV in 1 M KOH at a current density of 10 mA/cm2, which was comparable to those of the most active perovskite-based catalyst. Shan et al. found that the combination of LDH/CNT hybrid shows a positive effect on catalytic performance and uses as a basic support for selective oxidation of benzyl alcohol to benzaldehyde (Shan et al., 2015). The LDH-CNT with amphiphilicity as solid emulsifiers exhibited good capability for

Figure 13.13 SEM and TEM images of (A, B) C sphere, (C, D) NAC (U)
0, (E
G) NAC (U) 50 and TEM images of (G) NAC-50, (H) NAC80. [NAC (U)
x, where x corresponds to the content of ethanol and U correspond to the addition of urea.] Source: Adapted from Wei, J., Wang, J., Song, Y., Li, Z., Gao, Z., Mann, T., et al., 2012. Synthesis of self-assembled layered double hydroxides/ carbon composites by in situ solvothermal method and their application in capacitors. J. Solid State Chem. 196, 175
181, with kind permission of Elsevier.

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Figure 13.14 (A) TEM image of carbon dot-DBS-LDH nanocomposites. (B) Possible mechanism for carbon dot-DBS-LDH nanocatalyst for the decomposition of acidified H2O2 to produce abundant  OH radicals. Source: Adapted from Zhang, M., Yao, Q., Guan, W., Lu, C., Lin, J.-M., 2014a. Layered double hydroxide-supported carbon dots as an efficient heterogeneous Fenton-like catalyst for generation of hydroxyl radicals. J. Phys. Chem. C 118, 10441
10447, with kind permission of the American Chemical Society.

assembling and stabilizing at the water
oil interface. Ahmed et al. investigated the catalyst performance of graphene-oxide-supported CuAl and CoAl-LDH for carbon
carbon coupling (classic Ullmann homocoupling reaction) (Ahmed et al., 2017). Based on the obtained results, CuAl- and CoAl-LDHs exhibited excellent yields of 91% and 98%, respectively, at very short reaction times of 25 min. Also, the catalytic activity of the LDH/GO hybrid was up to twice as high as for the LDH, after five reuse cycles (Fig. 13.15). This phenomenon can be attributed to the presence of GO, which provided a lightweight, charge complementary, and twodimensional material which interacts effectively with the 2D LDHs.

13.6

Polymer/LDH/Carbonaceous nanofiller hybrid nanocomposites

In recent years, LDHs combined with carbonaceous nanomaterials have provided a new insight in research to explore the outstanding potential applications of LDHs. As presented above, LDHs are stimulating materials with marvelous characteristics. However, intrinsic limitations restrict their applications. For instance, the low electrical conductivity of LDHs has severely hampered their electrochemical performance, although they show high chemical reactivity. Moreover, nanostructures such as carbonaceous nanomaterials and LDH nanolayers come have difficulty from aggregation during their application process in polymeric nanocomposites. After combination, this problem can be effectively hindered. It should be mentioned that most of the properties of carbonaceous nanomaterials and LDHs are complementary. Therefore, combining them into polymeric NCs is an effective way to integrate their distinguishing properties; for example, carbonaceous nanomaterials like

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Figure 13.15 The catalytic properties of the investigated catalysts. Reaction conditions: iodobenzene (1) (2.00 mmol), catalyst (0.25 g), DMSO (4 mL); reaction temperature, 110oC. % Source: Adapted from Ahmed, N.S., Menzel, R., Wang, Y., Garcia-Gallastegui, A., Bawaked, S.M., Obaid, A.Y., et al., 2017. Graphene-oxide-supported CuAl and CoAl layered double hydroxides as enhanced catalysts for carbon-carbon coupling via Ullmann reaction. J. Solid State Chem. 246, 130
137, with kind permission of Elsevier.

graphene or MWCNTs can introduce good electrical conductivity and high mechanical strength, while LDHs can contribute good chemical reactivity (De Marco et al., 2017; Zhang et al., 2017a). Preparation of polymer/LDH/other nanofiller hybrid nanocomposites is the first stage in surveying their extensive usages. Up to now, three basic procedures have been reported for the synthesis and preparation of a synergistic combination of nanofiller and polymer. Melt intercalation is introduced as one of the most widespread methods in the preparation of NCs. In this process, polymer melt is combined with nanofiller hybrids by processes such as injection molding and extrusion. The compatibility of materials and mechanical parameters has a good influence on the gradual loosening and exfoliating of LDH layers. However, complete dispersion of nanofiller hybrids would be difficult to obtain using this method. On the other hand, polymerization of a monomer in the gallery between the nanofillers leads to separation and dispersion of the nanofillers. The third method is solution-induced intercalation. During this process, nanofiller hybrids are immersed and expanded into a polymer solution. Limitations of this method include expensive solvents and their being environmentally dangerous in many reactions (Kuthati et al., 2015; Mallakpour and Khadem, 2018; Zu¨mreoglu-Karan and Ay, 2012).

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Layered Double Hydroxide Polymer Nanocomposites

13.6.1 Polymer/LDH/CNT hybrid nanocomposites As pointed out previously, the presence of filler in polymer matrix can provide promising properties, characteristics, and performance in polymeric NCs. Various approaches have been designed for the fabrication of polymeric nanocomposites, such as in situ polymerization, sol
gel, solution casting, etc., which are described in detail in other chapters in this book. In the following, we illustrate some of these composites and their applications. Mallakpour and Dinari (2015b) prepared poly(amide-imide)/LDH-MWCNT NCs under ultrasonic irradiation. At first, LDH-MWCNTs were prepared by in situ growth of LDH on the acid-functionalized MWCNTs in an alkali condition under ultrasonication. Then, the obtained LDH-MWCNTs with various percentages were dispersed into PAI to improve their thermal stability. According to the results, T5 and T10 (temperature at which there is 5% and 10% mass loss) from 418oC and 451oC for pure PAI reached 508oC and 532oC for PAI/LDH-CNT NC 8% wt.%, % % % role in removal of dyes and hearespectively. These composites showed a favorable vy metal ions from wastewater. This can be related to the large specific surface area and electrostatic interaction of surface charges between adsorbents and pollutants. In their research work, Mallakpour and Behranvand (2017a) prepared recycled poly(ethylene terephthalate) (R-PET)/MWCNT/Mg
Al LDH-NCs under ultrasonic irradiations, then they used R-PET NCs with 4 wt.% MWNT/LDH for Cd(II) adsorption. The maximum adsorption capacity was estimated to be about 38.91 mg/g. As can be seen in Fig. 13.16, crystalline hexagonal platelets with a size of 50 nm were loaded on the MWCNTs and there was no isolation of LDH. Liu et al. (2014b) mixed a desired amount of organo-modified CoAl-LDHs, MWCNTs, and ε-caprolactam and then they were added to 6-aminocaproic acid for the preparation of nylon-6 (PA6)/LDH-CNT NCs. Techniques such as XRD, TEM, and SEM were used for their characterization. Based on mechanical tests, insertion of 1 wt.% LDHs and 0.5 wt.% CNTs into PA6 increased the tensile modulus, yield strength, as well as the hardness of the ternary composite by about 230%, 128%, and 110%, respectively, in comparison with the neat PA6. The SEM images of the fractured surfaces of PA6/2 wt.% CNT and PA6/2 wt.% CNT/1 wt.% LDH samples are shown in Fig. 13.17A and B. Based on these images, the CNTs appeared as bright spots in PA6/2 wt.% CNTs, while some areas were without CNTs and other areas with CNT aggregations were observed for PA6/2 wt.% CNT/1 wt.% LDH hybrid composite, as indicated by white arrows. In TEM micrographs (Fig. 13.17C and D), it can be seen that the binary PA6 NCs with homogeneous dispersion of 2 wt.% CNTs or 1 wt.% LDHs have successfully been prepared. For the ternary PA6/CNT/LDH-NCs, the LDH aggregations are surrounded by nanotubes and the CNTs are not beholden to the open areas that have little or no LDH sheets; this indicates that a great affinity is created between the two kinds of nanofiller. A simple method to prepare MWCNT/LDH is noncovalent assembly. In this reaction, sodium dodecyl sulfate was used as a linker between one carbonaceous nanomaterial (MWCNT and carbon nanofiber [CNF]) and Zn
Al LDH, henceforth designated as SFCNT-LDH and SFCNF-LDH (Roy et al., 2016). The synthesized

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Figure 13.16 (A) FE-SEM images of MWCNT/LDH were taken at two magnifications and (B) TEM images corresponding to (a) pure MWCNT, (b) pure LDH, and (c) MWCNT/LDH hybrids.

nanofiller hybrids were incorporated within thermoplastic polyurethane/nitrile butadiene rubber (1:1 w/w) blends (TN). High-resolution transmission electron microscopic (HRTEM) images evidenced that 0.50 wt.% of loaded SFCNT-LDH and SFCNF-LDH hybrids have uniform network without aggregation into the polymer matrix than the polymer composite with 1 wt.% filler (Fig. 13.18). Also, they illustrated that TN/SFCNT-LDH blend NCs have better mechanical performance (storage modulus and tensile strength 321% and 126%, respectively) in comparison to the TN/SFCNF-LDH blend NCs (storage modulus and tensile strength 278% and 122%, respectively). The thermal behavior of TN/SFCNF-LDH blend NC exhibited enhanced thermal stability (25 C) and crystallization temperature (36 C) compared to the neat TN blend which had values of 16 and 23 C for TN/SFCNF-LDH blend NC, respectively. The roles of Li
Al-LDH/MWCNT, Mg
Al-LDH/MWCNT, and Co
Al-LDH/ MWCNT hybrids (with various weight ratios of 6:1, 3:1, 2:1, 1:1, 1:2) as nanoreinforcement were investigated to improve the thermal and mechanical properties of silicone rubber (SR) (Pradhan and Srivastava, 2014). The results showed that the presence of 1 wt.% Mg
Al-LDH/MWCNT, Li
Al-LDH/MWCNT, and Co
AlLDH/MWCNT hybrids into SR NC significantly improved the tensile strength by

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Layered Double Hydroxide Polymer Nanocomposites

Figure 13.17 SEM images showing an overall morphology of failure surface for (A) binary PA6/2 wt.% CNT nanocomposite and (B) ternary PA6/2 wt.% CNT/1 wt% LDH nanocomposite. TEM images of PA6 nanocomposites with (C) 2 wt.% CNT, (D) 2 wt.% CNT/1 wt.% LDH, and (E) 1 wt.% LDH. Source: Adapted from Liu, T., Peng, H., Miao, Y.-E., Tjiu, W.W., Shen, L., Wei, C., 2014b. Synergistic effect of carbon nanotubes and layered double hydroxides on the mechanical reinforcement of nylon-6 nanocomposites. Chin. J. Polym. Sci. 32, 1276
1285, with kind permission of Springer.

134%, 100%, and 125%, respectively, compared to the neat SR. Also, among the prepared SR NCs, Mg
Al-LDH/MWCNT displayed greater thermal stability and swelling behavior due to nanoscale dispersion and strong interfacial interaction as well as high surface area (Fig. 13.19). Kong and coworkers (2017) synthesized organic NiFe-LDH/CNT nanofiller hybrids by coprecipitation with various weight ratios (10:1, 20:1, 40:1) and used 4% of them in the preparation of epoxy resin (ER) hybrid NCs. The results of flame-retardant and thermal properties showed that the ER/NiFe-LDH-CNTs hybrid NCs (specific ratio of 10:1) have a better performance than the pure ER. For instance, the pure EP had 12.5% residues at 700 C, whereas EP/NiFe-LDH-CNTs10 composites had 27.2% residues. In addition, compared with the pure EP, the PHRRs of EP/NiFe-LDH-CNTs-10 reduced by 53% (from 922 to 424 kW/m2). These phenomena are attributed to better uniform distribution, stronger interfacial interaction, outstanding charring performance of NiFe-LDH, and a synergistic effect between NiFe-LDH and CNTs. In a research work, the effect of flame retardancy of layered double hydroxide wrapped carbon nanotubes (LDH-w-CNTs) on polypropylene (PP) was investigated by Du and Fang (2010). The presence of LDH-w-CNTs into the polymer matrix caused a reduction in the peak heat release rate (PHRR) of PP and a better flame retardancy on PP with respect to LDH and CNTs. Based on reports, the incorporation of LDH (5 wt.%), CNTs (0.5 wt.%), LDH-w-CNTs, and LDH-w-acidifying CNT into PP provide PHRR of 538, 549, 490, and 495 kW/m2, respectively.

Figure 13.18 Fabrication of TN nanocomposites with hybrid fillers (left). HRTEM images of TN nanocomposites containing (A) 0.50, (B) 1 wt.% of SFCNT-LDH hybrid and (C) 0.50, (D) 1 wt.% of SFCNF-LDH hybrid. Source: Adapted from Roy, S., Srivastava, S.K., Mittal, V., 2016. Facile noncovalent assembly of MWCNT-LDH and CNF-LDH as reinforcing hybrid fillers in thermoplastic polyurethane/nitrile butadiene rubber blends. J. Polym. Res. 23, 36, with kind permission of Springer.

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Figure 13.19 TEM images of (A) Li
Al-LDH/MWCNT, (B) Mg
Al-LDH/MWCNT, and (C) Co
Al-LDH/MWCNT. (D) Digital images showing dispersion of MWCNT (1), Li
AlLDH/MWCNT (2), Co
Al-LDH/MWCNT (3), and Mg
Al-LDH/MWCNT (4) in THF at room temperature (photographs recorded after 1 day). Source: Adapted from Pradhan, B., Srivastava, S., 2014. Layered double hydroxide/ multiwalled carbon nanotube hybrids as reinforcing filler in silicone rubber. Compos. Part A: Appl. Sci. Manuf. 56, 290
299, with kind permission of Elsevier.

13.6.2 Polymer/LDH/graphene hybrid nanocomposites Other considerable composites in this context are polymer/LDH-G NCs. Liu et al. (2014a) applied graphene nanosheets (GNSs) and MgAl-LDH as fillers and investigated their flame retardancy for epoxy resin (ER). With respect to the obtained results, 2.5 wt.% LDH and 2.5 wt.% GNS could reduce the total heat release (THR) of ER composites from 33.4 to 24.6 kJ/m2, better than 5 wt.% GNS (27.8 kJ/m2) or 5 wt.% LDH (25.7 kJ/m2) (Fig. 13.20). In other work by Wang and coworkers (2013), an Ni
Fe LDH/G nanofiller hybrid was prepared by a one-pot in situ solvothermal method and then its effect on the flame retardancy of ER was studied. They demonstrated that the distribution of graphene was better after hybridization with LDH. By adding 2 wt.% of NiFeLDH/graphene NC into ER composite, the char residue of resin composite was enhanced, and its PHRR and THR were reduced by 58.0% and 61.0%, respectively. The enhancement in the fire-retardant properties of composite in the gas phase can be associated with the Ni
Fe LDH/GNS hybrid, which acts as a physical barrier and retards and decreases the release of combustible gas (e.g., hydrocarbons and

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Figure 13.20 (A) THR of ER, ER/GNS, ER/LDH, and ER/GNS/LDH composites with different filler content. (B) SEM images of the surface char residues of (a) ER, (b) ER/ GNS5, (c) ER/LDH5, and (d) ER/GNS2.5/LDH2.5 after being calcined. Source: Adapted from Liu, S., Yan, H., Fang, Z., Guo, Z., Wang, H., 2014a. Effect of graphene nanosheets and layered double hydroxides on the flame retardancy and thermal degradation of epoxy resin. RSC Adv. 4, 18652
18659, with kind permission of Elsevier.

aromatic compounds). However, in the condensed phase, the Ni
Fe LDH/GNS hybrid, by forming a compact and insulating char layer on the composite surface, could protect the inner polymer matrix from further burning. Hong and coworkers (2014) fabricated Ni
Al-LDH and graphene hybrid (RGOLDH) by coprecipitation route and considered its role on the reducing flammability of polymethyl methacrylate (PMMA). Based on flame retardancy, the addition of 2 wt.% RGO-LDH in polymer decreased the PHRR of the pure PMMA from 918 to 688 kW/m2 in a PMMA/RGO-LDH composite. In other work, Huang et al. (2014) showed the presence of 10 wt.% of intumescent flame retardants (IFRs), 1 wt.% of graphene, and 5 wt.% of LDHs, into PMMA can reduce PHRR values by about 45% compared with neat PMMA, while the mechanical properties of PMMA/IFR/ RGO/LDH NC revealed almost no deterioration. Xu et al. (2016) studied the flame retardancy and smoke suppression characteristics of polyurethane elastomer (PUE) before and after incorporation of MgAl-LDHloaded graphene hybrid. In order to do this, MgAl-LDH-loaded graphene composite was synthesized by coprecipitation, and then heptamolybdateion (Mo7O2462) was intercalated into the interlayer space of LDH over ion exchange. Finally, RGOLDH/Mo was mixed with the PUE through blending under ultrasonication. They demonstrated that the heat release rate and maximum smoke density of PUE/RGOLDH/Mo 2 wt.% decreased to 448 kW/m2 (58.6%) and 331 compared to composites without Mo7O2462 (PUE/RGO-LDH 2 wt.%) with values of 509 kW/m2 (52%) and 380, respectively, which is evidenced to improve the catalytic carbonization and smoke suppression effect. In the TEM images in Fig. 13.21A and B it can be seen that the RGO-LDH and RGO-LDH/Mo are distributed well in the PUE without agglomeration. The combination of graphene with LDH can facilitate electron collection and transport as well as promote high current densities in supercapacitors. However,

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Figure 13.21 TEM images of PUE/RGO-LDH (A) and PUE/RGO-LDH/Mo composites (B), RGO-LDH (C) and RGO-LDH/Mo (D). Source: Adapted from Xu, W., Zhang, B., Xu, B., Li, A., 2016. The flame retardancy and smoke suppression effect of heptaheptamolybdate modified reduced graphene oxide/layered double hydroxide hybrids on polyurethane elastomer. Compos. Part A: Appl. Sci. Manuf. 91, 30
40, with kind permission of Elsevier.

their aggregation can reduce the total energy density of supercapacitors. The presence of pseudocapacitive conducting polymer-coated carbon as an alternative substrate can overcome this problem. Accordingly, in 2017, a novel sandwich-like hybrid with ultrathin CoAl-LDH nanoplates was electrostatically coated on the surface polypyrrole/graphene substrate (denoted as CoAl-LDH/PG) via a hydrothermal route (Zhang et al., 2017b). The result showed that it has the ability to transport super high-energy density of 46.8 Wh/kg at 1.2 kW/kg and, after 10,000 cycles, capacitance is maintained at about 90.1% of its initial capacitance.

13.6.3 Polymer/LDH/Other nanofiller hybrids Zhou et al. (2016) introduced LDH/MoS2 hybrids as a reinforcing agent for improving the flame-retardant property of poly(vinyl alcohol) (PVA). Therefore, LDH layers with positive charge are assembled with the negatively exfoliated MoS2 nanosheets in aqueous solution by electrostatic attraction. Then PVA composites with various ratios of LDH/MoS2 (0.5, 1, 3 wt.%) were fabricated via solutionblending technique. These composites were characterized by XRD and TEM and

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showed that the flame-retardant efficiency of PVA increased after insertion of LDH/MoS2, so that in SEM images of PVA/LDH/MoS2 NC 3 wt.% observed a compact and dense layer, protecting the composite during combustion. In other work, LDH/MoS2 was introduced into epoxy to reduce its fire hazard (Zhou et al., 2017). Compared with epoxy/MoS2, the addition of CoFe-LDH/MoS2 and NiFeLDH/MoS2 into epoxy showed a more homogeneous dispersion and provided more excellent fire resistance to epoxy resin (EP) matrix, so that their PHRR values were further reduced from 1863 to 708 and 643 kW/m2, respectively. Also, THR values are decreased from 109 MJ/m2 (pure EP) to 72 MJ/m2 (EP/NiFe-LDH/MoS2 NC 2 wt.%), which is about a 34% reduction. In SEM images, pure EP shows a discontinuous char layer containing evident cracks and holes. However, for EP incorporated with MoS2, LDH, and specific for LDH/MoS2, the amount of holes and cracks on the surface reduces and it forms a compact and dense morphology (Fig. 13.22).

Figure 13.22 SEM images of the char residue for EP (A), EP/MoS2 (B), EP/CoFe-LDH (C), EP/CoFe-LDH-MoS2 (D), EP/NiFe-LDH (E), and EP/NiFe-LDH-MoS2. (F) Composites after cone test. Source: Adapted from Zhou, K., Gao, R., Qian, X., 2017. Self-assembly of exfoliated molybdenum disulfide (MoS2) nanosheets and layered double hydroxide (LDH): Towards reducing fire hazards of epoxy. J. Hazard. Mater. 338, 343
355, with kind permission of Elsevier.

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Layered Double Hydroxide Polymer Nanocomposites

Multifunctional Fe3O4@ phytic acid (Ph)-hydroxypropyl-sulfobutyl-betacyclodextrin sodium (CDBS)-LDH hybrid is other composite that showed good performance in improving the flame-retardant and thermal conductivity properties of EP (Kalali et al., 2016). For example, the PHRR and total smoke production of the EP composite containing 8 wt.% of Fe3O4@Ph-CDBS-LDH were decreased by 55% and 34%, respectively, in comparison to those of the pristine EP. Also, thermal conductivity increased from 0.220 6 0.002 in pure EP to 0.270 6 0.005 (W/mK) in Fe3O4@Ph-CDBS-LDH/EP, association of the Fe3O4 NPs and their synergy in transferring and dissipation of the heat have an effective role in this behavior. In other work, modified SiO2 (m-SiO2), Co 2 Al LDH, and synthetic mSiO2@Co 2 Al LDH spheres were individually inserted into EP to organize specimens for study of their flame-retardant performance (Jiang et al., 2014). They found that incorporation of 2 wt.% m-SiO2@Co 2 Al LDH into EP caused an increment in the char yield and a decrease in the derivative thermogravimetric peak value. Moreover, the PHRR, THR, effective heat of combustion (EHC), total smoke release (TSR), and maximum average heat rate emission (MAHRE) values for EP/ m-SiO2@Co 2 Al LDH were clearly decreased. These can be related to labyrinth effect of m-SiO2 and formation of graphitized carbon char catalyzed by Co 2 Al LDH, which played essential roles in the flame retardance improvement.

13.7

Conclusions

Since the 20th century up to now, the use of nanofillers with unique properties to improve polymer properties had been challenged. The use of LDHs, due to positive charge of the layers, relatively easy preparation, tuning of the crystal structures, and chemically active nature, has been proposed as an interesting nanofiller by modern researchers. Based on the investigations reported by various groups of researchers in this chapter, unmodified LDHs may not really yield any favorable improvement in polymer composites. Therefore, solutions such as modifying LDHs with organic compounds or combining them with other nanofillers such as carbonaceous nanomaterials (graphene, carbon dots, and CNTs) and other nanofillers have been designed to improve the performance of LDHs in polymer and decrease their aggregation. In this chapter, we intended to focus on the current state of the art in the preparation of polymer/LDH/any other nanofiller hybrid composite, and studied the influence of treated LDHs (such as LDH/MWCNTs and LDH/graphene) on thermal, mechanical, flame retardancy, adsorption capacity, capacitance properties etc. of polymers. In most of the results, composites reinforced with LDHs/carbonaceous nanofiller hybrids showed attractive performance, such as in thermal properties and conductivity, compared with pure polymer and polymer/LDH NC,. This is due to the appropriate compatibility, interaction, and dispersion between polymers and treated LDHs with carbonaceous nanomaterials. This area of work is very interesting and attractive and we are sure many scientists are working on it and that exciting results will be published soon. Currently we are also working on this fascinating subject.

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Acknowledgments The authors would like to thank the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, I. R. Iran, National Elite Foundation (NEF), Tehran, I. R. Iran, Iran Nanotechnology Initiative Council (INIC), Tehran, I. R. Iran, and Center of Excellence in Sensors and Green Chemistry Research (IUT), Isfahan, I. R. Iran for financial support.

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10.

Electrical and electronic applications of layered doublehydroxide polymer nanocomposites

14

Shadpour Mallakpour1,2 and Forough Motirasoul1 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

14.1

Introduction

In the past decades, electrochemical progressive energy-storage apparatuses (including capacitors, fuel cells, and batteries) and conversion devices have fueled intense interest owing to growth demands of authentic and biocompatible energy resources for applications such as electric hybrid and movable electronic systems (Shao et al., 2015b). It is favorable to create new materials for increasing highrate efficiency (e.g., secondary batteries) due to strong energy debilitation under high current density with charge
discharge cycles (Hu et al., 2015). As already noted, systems for conversion and electrochemical energy storage consist of electrochemical capacitors [also known as ultracapacitors or supercapacitors (SCs)], batteries, and fuel cells. Fig. 14.1 exhibits their fundamental practice mechanisms. Electrical energy in fuel cells and batteries could be generated by transformation of chemical energy through redox interactions in a cathode and anode. Meanwhile energy cannot be passed over through redox interactions in SCs. SCs include two electrodes and an electrolyte as a separator that connects the electrodes. Electrical energy forms at the electrolyte stores in electrical double layers when the electrodes are polarized by applying a voltage (Winter and Brodd, 2004). SCs find applications in hybrid-electric transit buses, electric braking system in passenger cars, portable electronic device, trains, cranes, elevators, etc., potential applications for batteries include car batteries, mobile telephones, laptops, and other movable electronic systems. Fuel cells have applications ranging from safe electrical sources for power production, repartition of power, and emergency power. They are used in cars, buses, forklifts, motorcycles, aircrafts, boats, submarines, portable power systems, etc.

Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00014-8 © 2020 Elsevier Ltd. All rights reserved.

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Figure 14.1 Representation of (A) a battery showing the requirements on electron and ion conduction, (B) a fuel cell showing the continuous supply of reactants (hydrogen at the anode and oxygen at the cathode) and redox reactions in the cell, and (C) an electrochemical capacitor (supercapacitor), illustrating the energy storage in the electric double layers at the electrode
electrolyte interfaces. Source: Adapted from Winter, M., Brodd, R.J., 2004. What are batteries, fuel cells, and supercapacitors? With kind permission of ACS.

One category of two-dimensional layered solids is layered double hydroxides (LDHs, or hydrotalcite-type clays) with interlayer damping anions and positively charged layers which have high stability at alkaline conditions owing to being constructed in the presence of NaOH (Mallakpour and Dinari, 2015b; Mallakpour et al., 2014b). They have attracted a great deal of attention as fillers in polymer nanocomposites (NCs). Mallakpour et al. (2016b) fabricated novel NCs based on LDH modified with N-tetrabromophthaloyl-glutamic as a reinforcing filler and poly (amide-imide) as a matrix with improved thermal properties. Hajibeygi et al. (2017) prepared a new Mg-Al LDH modified with diacid-diimide as a filler in poly(amideimide) NCs. They studied the thermal stability and optical property of these NCs. The results showed an improvement in thermal behavior and optical property of poly(amide-imide). Zhou et al. (2017a) synthesized LDH nanoplates with high crystallinity and uniform size and incorporated LDH nanoplates into poly(vinyl alcohol) (PVA) matrix as a reinforcing agent based on a solution casting technique. The LDH nanoplates exhibited good dispersion in PVA matrix due to forming strong interfacial interactions with PVA that led to a significant improvement in their

Electrical and electronic applications of layered double-hydroxide polymer nanocomposites

567

thermal stability, flame retardancy, and mechanical properties. In another work, Kalali et al. (2016) synthesized a hybrid of Fe3O4 nano-sphere@Mg-Al LDH and used it as a filler in fabricating epoxy NCs. The incorporation of Fe3O4 nanosphere@Mg-Al LDH hybrid into epoxy matrix not only improved the flameretardant and mechanical properties of the epoxy resin but also endowed epoxy resin with increased thermal conductivity. LDHs have received intense attention serving as electrode substances in energy storage devices by virtue of their adjustable composition, easiness of production and rectification, cost effectiveness, and anion exchange capacity, over recent years (Chen et al., 2016b). Furthermore, LDHs have other excellent features like high specific surface areas, swelling ability, water retention, and numerous electrochemical active sites. Owing to these properties, preparation of LDH NCs and the incorporation of LDH nanofillers in the polymer matrix create novel materials which are suitable for electrochemical energy storage and conversion tools. LDHs, owing to their layered structure intercalated with anions and water molecules, are interesting materials for use as hydroxide ion conductors. The chemical combination of LDHs is portrayed through the generic principle: 

M21 1
x M31 x :ðOHÞ2

x1

ðAn
Þx=n :yH2 O

in which M21 is bivalency metals ranging from Cu, Zn, Mg, Ni, Co, Fe, Mn, etc.; M31 is trivalent metals including Cr, Al, Ga, Ni, Co, Fe, Mn, etc., and An2 is an 2 2 22 interlayer anion with load n such as Cl2 ; F2 ; CO22 3 ; NO3 ; OH ; SO4 , etc. The 21 31 value of x is the stoichiometric coefficient of M and M within 0.2
0.4 (Mallakpour and Hatami, 2017a; Mallakpour and Dinari, 2015a; Mallakpour et al., 2015). Due to unique features such as great surface areas, being environmentally friendly, thermal and chemical stability, crystallinity, uptake property, and catalytic activity (Nejati and Asadpour-Zeynali, 2014; Mallakpour et al., 2016a), LDHs find ´ lvarez et al., 2017), flame many applications in various fields such as catalysis (A retardants (Kaul et al., 2017), electrodes for secondary batteries (Yang et al., 2014b), electrode materials for SCs (Liu et al., 2017a), anode materials for batteries (Zhang et al., 2015a), fuel cells (Kubo et al., 2013), drug-delivery materials (Harrison et al., 2017), adsorbents (Lei, et al., 2017), nanofillers (Mallakpour et al., 2014a, 2013), anion exchangers (Beyki et al., 2017), electrochemical sensors (Zhan et al., 2017), and biosensors (Wang et al., 2017b). Despite the described features of LDHs, the electron-transfer rate is limited by low conductivity of LDHs, resulting in weak efficiency at a great current density of charge
discharge. A logical architecture of new electrodes considering constructional and combinational factors is very necessary. Therefore, in order to enhance the electrochemical properties, LDH NCs and LDH/polymer NCs have been developed (Han et al., 2013; Yan and Yang, 2016). Polymer NCs are a novel class of material comprising two or multiphases consisting of the polymer as the matrix and nanofiller as the dispersed phase that displays excellent thermal, optical, mechanical, and electrical properties (Mallakpour and Motirasoul, 2016, 2017b).

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The aim of this chapter is to cover recent electrical and electronic applications of LDH/polymer NCs. Applications such as SCs, batteries, fuel cells, etc. will be focused on by the selection of recent works.

14.2

Supercapacitors

SCs, owing to quick their charge
discharge rate, greater power density compared to conventional capacitors, great energy density, lengthy cycling lifetime, and low upkeep expenses are one of the most attractive electrochemical energy storage systems. It is noteworthy that power density is known as power per unit volume and energy density is the amount of energy that can be stored per unit volume (Huggins, 2009). The essential factor in the efficiency of SCs is electrode materials. SCs can commonly be categorized into two types according to the mechanism of charge
discharge: electrical double-layer capacitors (EDLCs) that mostly are carbon substances and pseudocapacitors that range from conductive polymers, oxides of transition metals (such as NiO, RuO2, MnO2, Co3O4, and their compounds), and hydroxides. EDLCs and pseudocapacitance appertain on their surface area and are surface phenomena. Thus, poriferous structures with greater surface area and nanoscale as electrode materials are more suitable for capacitors that can enable the forwarding of electrons and truncate the forwarding interval. Pseudocapacitors compared to EDLCs can create excellent special capacitance by usage quick and returnable redox interactions, however owing to their weak mechanical resistance and cycle lifetime they have limited applications (Wei et al., 2016; Yang et al., 2013b).

14.2.1 Application of Layered Double Hydroxide Nanocomposites in Supercapacitors LDHs, because of their ability for flexible ion exchange and numerous electrochemical active sites, have represented an appealing application potential in SCs. Chen et al. (2014) prepared very poriferous composite films created from super-thin LDH nanosheets grown on 3D macroporous nickel foam using an easy hydrothermal codeposition technique and applied to an electrode as a pseudocapacitor. These electrodes showed great specific capacitance (2682 F/g at 3 A/g) and energy density (77.3 Wh/kg at 623 W/kg). Hierarchical nanostructure and composition can present advantages for improved pseudocapacitor performances. The preparation of the three-dimensional (3D) composite electrode material involving carbon-coated NiMn-LDH on nickel foam prepared via a simple two-step hydrothermal method. The resulting performance of the composite material showed great specific capacitance (1916 F/g), high cycling stability (91% retention after 5000 charge
discharge cycles), and superior rate capability (79.5% retention at 10 A/g). Furthermore, prepared composite was applied as a positive electrode for the asymmetric SC and exhibited great specific capacitance of 120.67 F/g at 0.5 A/g and good cycling stability with 84.2% capacitance retention after 5000 cycles at a current density of

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5 A/g (Chen et al., 2016a). To understand the insightful mechanics of the effects of the trivalent metal ions on the electrochemical performance of ternary-component LDH, the triplex LDHs (NiCoAl-LDH, NiCoAl0.5Fe0.5-LDH, and NiCoFe-LDH) were prepared and characterized and the influences of Al31 and Fe31 on the pseudocapacitor efficiency have been investigated. XPS and FE-SEM of these triplex LDHs were showed in Fig. 14.2. The capacitance retention rate was obtained as B75.27% for NiCoAl-LDH and 42.59% for NiCoAl0.5Fe0.5-LDH after 1000 galvanostatic charge
discharge cycles at 20 A/g. NiCoAl-LDH exhibited a maximum specific capacitance of 1153 F/g, much higher than that of NiCoAl0.5Fe0.5-LDH (606 F/g) and NiCoFe-LDH (396 F/g)at 6 A/g. The capacitance retention rate was found to be B92.97% for NiCoAl-LDH after 3000 cycles showing an excellent cycling stability, while the capacitance retention rates of NiCoAl0.5Fe0.5-LDH and NiCoFe-LDH were found to be 73.16% and 50.65%, respectively, after 3000 cycles. The results displayed that the incorporation of Al31 into a metal hydroxide layer can greatly ameliorate the electrochemical activity of the reversible reaction of Ni21 and Co21. Therefore Al31 has a positive effect on the stabilization of host layers of LDH, which leads to an improvement of the pseudocapacitive performance (Wang et al., 2017a). The partly reduced electron transition, negligible electrochemical stability, and weak inherent electrical conductivity of LDH restricts its application in the electrode material. The introduction of extremely conductive carbon substances ranging from active carbon, graphene, and carbon nanotubes (CNTs) into LDH is an efficient approach to untangle this issue. Zhao et al. (2013) fabricated a solid-state SC system with fine flexibility using the CoMn-LDH/carbon fibers as electrode, which exhibited a specific energy of 126.1 Wh/kg and a specific power of 65.6 kW/kg. It should be noted that the energy and power available per unit weight are called specific energy and specific power, respectively (Huggins, 2009). This work illustrated a facile and inexpensive assembly of the SC with streamlined, very flexible, and weightless architecture that can be applied in wearable and miniature systems of energy storage. Bia et al. (2016) used an easy one-stage homogeneous precipitation method to prepare CNTs/NiAl-LDH NC. This NC demonstrated an enhanced capacitive performance in terms of supreme specific capacitance (694 F/g at 1 A/g) and 87% capacity retention at the current density of 10 A/g. Moreover, this electrode showed a lengthy lifetime of the cycle with a specific capacitance retention of 92% under high current densities if 20 A/g after 3000 cycles. These results indicated that synthesized NC is a potential candidate as electrode substances for SC applications. In other work, Yang et al. (2013b) evaluated a Ni-Al LDH, CNT, and diminished graphene oxide sheets (GNS) ternate NC as an electrode substance through an easy single-stage solvothermal approach. LDH/CNT/GNS electrodes presented superior electrochemical performance, including the excellent specific capacitance of 1562 F/g at 5 mA/cm2, ultrahigh rate capability, good charge
discharge durability, and long cycling life, which could be used in energy-storage devices. This enhancement of the electrochemical efficiency in this electrode is due to three reasons: (1) added carbon substances into LDH, which create a synergic efficacy with GNS serving as a conductive substratum and CNTs in the LDH/CNT as

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Figure 14.2 (A) Ni 2p (a), Co 2p (b), O1s (c) XPS spectra of NiCoAl-LDH, NiCoAl0.5Fe0.5LDH, and NiCoFe-LDH; the schematic illustration of three major hydroxyl groups (Ni-OH, CoOH, and M-OH) within the metal hydroxide layer matrix (d). (B) Cycling performance of the NiCoAl-LDH, NiCoAl0.5Fe0.5-LDH electrodes after 1000 cycles at 20 A/g (a); FE-SEM images of NiCoAl-LDH (b) and NiCoAl0.5Fe0.5-LDH (c) after 1000 cycles. Source: Adapted from Wang, X., Lin, Y., Su, Y., Zhang, B., Li, C., Wang, H., et al., 2017a. Design and synthesis of ternary-component layered double hydroxides for high-performance supercapacitors: understanding the role of trivalent metal ions. Electrochim. Acta, 225, 263
271. With kind permission of Elsevier.

channel for electron transfer, which cause weak inner electric resistance for effective charge transition in electrode; (2) the internal connection of flower-like Ni
Al LDH platelets provides a mesoporous structure (Fig. 14.3), which is effective for

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Figure 14.3 SEM (A
C) and TEM (D
F) images of LDH/CNT/GNS composite at different magnifications. Source: Adapted from Yang, W., Gao, Z., Wang, J., Ma, J., Zhang, M., Liu, L., 2013b. Solvothermal one-step synthesis of Ni
Al layered double hydroxide/carbon nanotube/reduced graphene oxide sheet ternary nanocomposite with ultrahigh capacitance for supercapacitors. ACS Appl. Mater. Interfaces, 5(12), 5443
5454. With kind permission of ACS.

transfer of electrolyte and reduces the routes of ion diffusion within the active substances; and (3) the high surface area of this hybrid with open pores prevents aggregation and this leads to wide usage of these materials.

14.2.2 Application of Layered Double Hydroxide/Polymer Nanocomposites in Supercapacitors Conducting polymers because of their great electric conductance, effective redox energetic places, and cheapness have attracted high interest as electrode materials. Therefore, the architecture of new electrodes, including conducting polymer and LDHs, is very efficient for high-performance SCs (Shao et al., 2015b). Li et al. (2016) fabricated a sandwich-like composite by incorporation of graphene/polypyrrole (GPPY) with NiAl LDH nanowires (NiAl-NWs) through a facile hydrothermal procedure. Characterization of composite showed that NiAl-NWs were homogeneously dispersed on both sides of GPPY. In this composite, GPPY was used as a scaffold and introduced many nitrogen species that could create additional pseudocapacitance and also improved the conductivity of the composite. This composite showed excellent specific capacitance (845 F/g) with great rate performance (67% retained at 30 A/g), significant cyclic stability (92% maintained after 5000 cycles), and high energy density (40.1 Wh/kg). Han et al. (2013) employed a potentiostatic electrodeposition approach to develop high-performance flexible SCs based on a LDH@poly(3,4-ethylenedioxythiophene) (PEDOT) core/shell nanoplatelet array (NPA) electrode (Fig. 14.4A
C). Here, high energy-storage capacity is provided by

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Figure 14.4 (A) Schematic illustration of the fabrication of the LDH@PEDOT core
shell nanoplatelet array electrode. SEM images of (B) the CoAl-LDH and (C) CoAlLDH@PEDOT NPA obtained by the electrodeposition method. (Adapted from Han, J., Dou, Y., Zhao, J., Wei, M., Evans, D.G., Duan, X., 2013. Flexible CoAl LDH@ PEDOT core/ shell nanoplateletarray for high-performance energy storage. Small, 9(1), 98
106. With kind permission of Wiley.) (D) A schematic illustration of the fabrication of PPY@LDH core
shell NW arrays; SEM images of (E) PPY NWs and (F) PPY@CoNiLDH core
shell NW arrays on a foam nickel substrate. (Adapted from Shao, M., Li, Z., Zhang, R., Ning, F., Wei, M., Evans, D.G., et al., 2015a. Hierarchical conducting polymer@claycore
shell arrays for flexible all-solid-state supercapacitor devices. Small, 11(29), 3530
3538. With kind permission of Wiley.)

the LDH core via a quick and reversible redox reaction, while the electron transfer during the charge
discharge process is facilitated by the very conductive PEDOT shell. The LDH/PEDOT NPA electrode demonstrated superior electrochemical behavior, including significant cycling performance, superior specific capacitance, good rate capability, and great specific energy and power. However, polymer endures a significant bulk change in time with frequent intercalation and ion evacuation in charge
discharge cycling that greatly reduce its mechanical resistance. To unravel this issue, Shao et al. designed a polypyrrole (PPY)@LDH core
shell array

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through a two-step electrosynthesis approach (Fig. 14.4D
F). In this unified structure, the PPY NWs core supplies a conductor lattice to simplify the electron muster and rapid transfer, while the LDH shell enhances the structural stability through inhibition of the volumetric swelling/shrinking of PPY within a lengthy charge/discharge process and serves as a protection layer. In this work, asymmetric SC with flexible solid state was prepared by utilizing film of reduced graphene oxide (RGO) as a negative electrode, foam Ni-supported PPY@LDH as a positive electrode, and PVA/KOH as the solid electrolyte. Because of the synergetic efficacy between PPY and LDH, SC presented a great energy density and high cycling stability (Shao et al., 2015a). In a study, hierarchical Ni-Co LDH nanosheets were entrapped on the flexible conductive textile substrate based on polyethylene terephthalate by a facile twoelectrode system-based electrochemical deposition method. The Ni
Co LDH nanosheets were grown on conductive textiles with good adhesion by applying an external cathodic voltage of 21.2 V. The electrochemical performance of this structure was enhanced in an electrolyte solution of 1 M KOH. The Ni10Co5 LDH nanosheets/conductive textile electrode exhibited a great specific capacitance of 2105 F/g at a current density of 2 A/g as well as a high cyclic stability as a pseudocapacitive electrode that can be used for low-cost energy storage device applications (Nagaraju et al., 2016). Recently, Zhao et al. (2015) constructed a hybrid based on a CoNi-LDH monolayer and mixture of poly(3,4-ethylene dioxythiophene) and poly(styrenesulfonate) (PEDOT:PSS) molecular-size through an electrostatic self-assembly method in an intermittent layer superlattice for SC applications. By virtue of a uniform and strong interaction, CoNi-LDH/PEDOT:PSS hybrids have a concurrent increase in ion transfer and charge-carrier that show ameliorated capacitive features with great specific capacitance (960 F/g at 2 A/g) and superior rate capability (83.7% survival at 30 A/g). Here, the LDH monolayers provide a surrounded microambience insert of polymer mixture conductive network with favorable spatial embellishment that causes resonances and conformational alteration of PEDOT and excellent mobility of the charge-carrier. This design can create an efficient regulation of the bus voltage in an aircraft. Therefore, an aircraft SC system with an interdigital design was prepared based on this hybrid, which delivers a remarkably increased energy and power generation (energy density of 46.1 Wh/kg at 11.9 kW/kg). These features represent a molecular-size hybrid of conductive polymer and LDH that is appropriate for energy-storage systems like lithium (Li) batteries, photodetectors, and water-splitting cells in miniaturized electronics. One of the most serious problems in energy storage/conversion systems is the self-safety and controllability of power delivery of these devices which still remains a challenge and should be given specific attention. Owing to the rapid current and high power transport an intense thermal efficacy is generated that causes the system to not work swiftly, particularly at high temperatures. To overcome this problem, it is suggested that smart matters based on irritant-responsive polymers be applied. They have the ability of conformational and chemical alterations, induced by exterior irritants. Hence, Dou et al. (2013) developed a smart SC with temperatureoperated on
off ionic channels in order to adjust electrochemical functioning by

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Figure 14.5 Schematic illustration of the LDH@P (NIPAM-co-SPMA) film on a flexible Ni foil substrate serving as a smart SC with thermally responsive ion channels. Source: Adapted from Dou, Y., Pan, T., Zhou, A., Xu, S., Liu, X., Han, J., et al., 2013. Reversible thermally-responsive electrochemical energy storage based on smart LDH@P (NIPAM-co-SPMA) films. Chem. Commun., 49(76), 8462
8464. With kind permission of RSC.

putting a temperature-sensitive polymer P(N-isopropylacrylamide-co-2-acrylamido2-methyl propane sulfonic acid) [P(NIPAM-co-SPMA)] onto the surface of NiAlLDH, which was developed on a pliable Ni foil substrate by a cast-coating technique (Fig. 14.5). Because of the spatial structure alteration of this polymer with temperature changes, the dissolved polymer at 20 C liberates the available ionic channels for the usual charge
discharge action, while the receded polymer at 40 C prevents ionic transfer, therefore diminishing the power conduction. Therefore, this research resulted in an excellent technique for the preparation of SCs, actuators, and switchable sensors which are sensitive to temperature.

14.3

Batteries

An electrochemical cell consists of two solid electrodes, the anode and the cathode, that are kept apart by an electrolyte-permeable separator. The chemical reaction between the anode and the cathode has an electronic and an ionic component. The electrolyte conducts the ionic component, referred to as the working ion, inside the cell, but it forces the electronic component to traverse an external electronic circuit. Current collectors at the anode and the cathode deliver the electronic current of large-area electrodes to/from posts that connect to the external circuit. If the external circuit is disrupted (open circuit), the working ion moves inside the cell, but it is not charge-compensated by electrons, so the cathode becomes positively charged and the anode negatively charged until the electrochemical potentials in the two electrodes become equal. A battery is fabricated of one or several interconnected cells. The output current I and time Δt to depletion of the stored energy can be

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enhanced by connecting cells in parallel (Goodenough, 2012). Batteries, depending on their capability of being electrically recharged, can be classified into primary (nonrechargeable) or secondary (rechargeable) (David and Thomas, 2001). In recent years, with rapid improvements in electronic technology, the demands have increased for rechargeable batteries with greater energy densities and lengthier lifetimes. Polymer electrolytes have a great role in the development of rechargeable batteries, because of advantages such as high energy density, nonleakage electrolyte, good cyclability, mechanical strength, flexible geometry, and safety (Wang et al., 2009).

14.3.1 Application of Layered Double Hydroxide Nanocomposites in Batteries LDH compounds have been widely designed for batteries, owing to the nature of interlayer changeable anions, the property of compositional flexibility, and their capability for undergoing an internal redox interaction in a restricted potential confine in alkaline ambiance. Hu et al. (2015) reported a Ni-AlLDH/graphene superlattice NC. This NC demonstrated a lasting excellent rate of performance for the cathode of the alkaline battery. In this NC, LDH blocks are far from each other separated by a graphene layer; as a result, all LDH layers easily accomplish fast charge transport by passage of the neighbor graphene and are activated. In another study, glucose-intercalated Ni-Mn LDH was synthesized by an easy one-step hydrothermal approach, which expanded interlayer distances to enhance cycling stability and break the bottleneck of LDH in applications. The obtained results showed that glucose-intercalated Ni-Mn LDH delivered a high specific capacity of 1464 F/g at a current density of 0.5 A/g (852 F/g for pristine Ni-Mn LDH). This improvement in performance is related to the small structure, lower charge transfer resistance, and faster reversible redox reactions. With increasing interlayer distance and stability LDH, the cycling stability was significantly increased from 45% to 90% for over 1000 cycles. These results suggest that glucose-intercalated Ni-Mn LDH could be successfully used in battery-type electrodes with enhanced performance (Lv et al., 2016). New zinc batteries with neutral pH have attracted increasing attention compared to Li batteries owing to their low cost and eco-friendliness. Elements that could appear as passive, such as the LDH additive, could play a critical role in improving the efficiency of the reaction. In particular, in one study, the addition of LDH to a zinc negative electrode was observed to enhance the zinc electrodeposition efficiency from 85% to 98%. This improvement was related to the deletion of a potential drop during the Zn21 reduction stage, avoiding the formation of H2 at the beginning of the discharging stage (Gonza´lez et al., 2016). The Ni-Zn battery, due to its high specific energy (373 Wh/kg), is an interesting power source for large-scale energy storage systems that can be used for hybrid/electric vehicles and new portable devices. In addition, it has attracted great interest because the active materials and electrolytes in Ni-Zn batteries have nontoxicity and are abundant in nature and are also low cost, more environmentally friendly than many other battery

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Layered Double Hydroxide Polymer Nanocomposites

systems, and can be used within a wide temperature range. Yang et al. (2014a) prepared Ag/ZnAl-LDH NCs through an easy Ag mirror reaction and evaluated their electrochemical function as an anode for Ni-Zn secondary batteries. They investigated the efficacy of Ag-additive on the electrochemical performance of LDH nanosheets. The outcomes of charge
discharge experiments exhibited Ag-additivetreated cycle efficiency and discharge ability of LDH as an anode for Ni-Zn secondary batteries. Subsequently, cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) illustrated the Ag-additive in Ag/ZnAl-LDH NC, which reduced the charge transport resistance and improved the electrochemistry activity of the ZnAl-LDH anode. In other work, Yang’s team presented the 3D LDH NC as a new anode substance for Ni-Zn secondary batteries. They synthesized 3D LDH/CNT NC incorporating 2D LDH and 1D CNTs simultaneously, by a facile precipitation method via the electrostatic force between the functional group with a negative charge on functionalized CNTs and a positively charged layer of LDH (Fig. 14.6). This NC as anode matter for an Ni-Zn cell exhibited greater cycle fixity contrasted with common ZnO and Zn-Al-LDH and a high discharge capacity (390 mAh/g after 200 cycling tests). Meanwhile, this NC showed a greater discharge voltage, less charge voltage, and 95.6% mean usage rate of the anode (Yang et al., 2013a). Rechargeable metal
air batteries have received remarkable attention due to their high theoretical specific energy density capacities that exceed those of Li batteries. The energy density is determined by the type of metal used for the negative electrode because no oxygen needs to be stored in the air as the active material in the positive electrode. Tsuneishi et al. (2014) fabricated hydroxide ion-conductive

Figure 14.6 Diagram of the chemical route to the LDH/CNT composite. Source: Adapted from Yang, B., Yang, Z., Wang, R., Wang, T., 2013a. Layered double hydroxide/carbon nanotubes composite as a high performance anode material for Ni
Zn secondary batteries. Electrochim. Acta, 111, 581
587. With kind permission of Elsevier.

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electrolytes based on Mg-Al LDHs in the presence and absence of KOH. The LDH solid electrolyte with KOH exhibited relatively high ionic conductivity. They applied the KOH
Mg-Al LDH solid as an electrolyte and Fe3O4 particle-supported carbon as anode in an all-solid-state Fe
air battery. The discharge performance was improved with the addition of K2S into the composite anode. Therefore, KOHLDH could be an important hydroxide ion-conductive solid electrolyte for all-solidstate Fe
air batteries in the future.

14.3.2 Application of Layered Double Hydroxide/Polymer Nanocomposites in Batteries Lead-acid batteries are broadly applied in motor systems to provide the great current required by machine starter motors because of their ability to supply a great surge current, along with their low cost. However, they cause serious pollution to the environment. Li batteries in comparison with lead-acid and nickel-based batteries display benefits of long cycling life, great energy density, environmental compatibility, and high operating voltage (Shao et al., 2015b). Li batteries are light, with an operating voltage of B3.6 V, a specific energy ranging between 100 Wh/kg and 150 Wh/kg, and a deliverable capacity ranging from 700 to 2400 mA/h for a battery. They are widely used in portable electronic tools such as cell phones, notebook computers, and video camcorders. Li batteries are generally charged and discharged at 0.2 2 1C current rate, meaning that the full capacity of the cell is stored or utilized in 5 and 1 h, respectively. The usual operating temperature (T) range of the Li batteries is 15 C 2 60 C. At T , 15 C, the capacity becomes low, whereas at T . 60 C slow degradation of the electrode/electrolyte materials sets in over a period of time (Reddy et al., 2013; Roy and Srivastava, 2015). However, batteries with greater energy densities and a lengthier life cycle for the expansion of newfound electronic systems are essential. Polymer electrolytes are environmentally friendly, with various applications that show good ionic conductivities, superior mechanical properties, and abiding electrochemical specifications (Nicotera et al., 2015; Liao and Ye, 2004a). They perform a significant role in the progress of Li-polymer rechargeable batteries as they can result in flexible exfoliated structure with tailormade geometries. Solid-state poly (ethylene oxide) (PEO)/LDH NC electrolyte systems have high ionic conductivity and can be used for fabricating Li
polymer secondary batteries (Liao and Ye, 2003, 2004a,b). Batteries based on lithium
sulfur (Li
S) have been propounded as a good applicant for novel energy-storage devices. Li
S batteries have many benefits such as their inexpensive material, high safety, and high energy density. Therefore, they have recently been pursued as the most promising alternative to Li batteries in many applications from electric vehicles to stationary grid storage. However, these batteries are greatly hindered by certain problems, especially their low utilization of sulfur and fast capacity depletion due to the dissolution of the intermediate discharge product, polysulfide, and its diffusion across the separator to the anode side

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Layered Double Hydroxide Polymer Nanocomposites

(Zhou et al., 2017b). The basic issue for Li
S batteries is the prevention of polysulfide resolvent while maintaining a high sulfur usage. Therefore, Zhang et al. (2016) fabricated a novel type of twice-shelled nanocages with an internal shell of cobalt hydroxide [Co(OH)2] and external shell of LDH (CH@LDH) as a new sulfur host for Li
S batteries (Fig. 14.7). The cathode of this composite has multiple advantages. Specifically, the CH@LDH/S composite is loaded with 75 wt.% of sulfur and twice-shelled nanocages of CH@LDH prepare somewhat great functional surfaces for a chemical bond with polysulfides to prevent their resolvent. The CH@LDH/S composite was investigated as cathode matter for Li
S batteries and exhibited a remarkably improved electrochemical performance and was capable of retaining high cycling stability at both 0.1 and 0.5C above 100 cycles and delivered great rate capacities with an excellent sulfur loading of 3 mg/cm2. In other work, Zhou et al. (2017b) reported utilization of LDH sheets as a modification layer on the polypropylene separator, synergistically working as both a physical confinement barrier and a chemical trap that efficiently blocked the crossover of polysulfide and accordingly improved the life cycle of Li
S batteries. The results showed that with LDH sheets blocking polysulfide crossover, the performance degradation was obviously ameliorated, going from 0.29% per cycle for the bare separator down to

Figure 14.7 (A) Schematic illustration of the synthesis of the CH@LDH/S composite. (B) SEM and TEM images of (a, e) ZIF-67, (b, f) single-shelled ZIF-67@LDH, (c, g) doubleshelled CH@LDH nanocages, and (d, h) CH@LDH/S. Source: Adapted from Zhang, J., Hu, H., Li, Z., Lou, X.W.D., 2016. Double-shelled nanocages with cobalt hydroxide inner shell and layered double hydroxides outer shell as high-efficiency polysulfide mediator for lithium
sulfur batteries. Angew. Chem. Int. Ed., 55 (12), 3982
3986. With kind permission of Wiley.

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0.18% per cycle for the modified separator. The success of introducing LDH sheets with positive charges for modifying the separator in an Li
S battery will open a new window for future development in designing high-performance Li
S batteries. Among rechargeable batteries, although Li batteries have high energy densities, they are expensive and have some problems of safety owing to inflammability that puts limitations on their diverse energy storage applications (Yan and Yang, 2016; Huang et al., 2015). Zinc electrodes as anode substances for alkaline secondary batteries were proposed for the new generation of power sources used in electric vehicles in recent years. This is due to their supreme electrochemical performance, such as good open circuit voltage, great energy density, and being inexpensive and environmentally friendly. However, due to the defects of the zinc electrode, such as shape alteration, dendrite growth, inactive surface, and zinc self-discharge, the development of Ni
Zn secondary batteries has been restricted by their poor cycle lifetime. These defects are mainly produced by the high solubility of zinc active materials in alkaline electrolytes. Hence, various additives, such as Bi (III) (Zhang et al., 2015c), In (III) (Wang et al., 2014b), calcium zincates (Wang et al., 2014a), polyaniline (Huang et al., 2014a), and polypyrrole (Huang et al., 2014b) have been added to zinc electrodes in order to overcome these problems (Huang et al., 2015). Recently LDHs and their materials have been studied as a new type of electrode in Ni
Zn secondary batteries. Yan and Yang (2016) successfully synthesized Zn-AlLDH/PPY composites by the polymerization of pyrrole in the slurry of hydrotalcite under sonication and stirring with the aim of combining the merits of LDH and PPY to attain an excellent electrochemical performance. They evaluated the electrochemical performance of Zn-Al-LDH/PPY composites as an electrode for the Ni
Zn secondary battery. The results showed better reversibility and superior cycle stability, in contrast to neat LDH electrode. In addition, the results of the EIS test showed that PPY modification reduced the charge transport resistance of the electrode and ameliorated the conductivity of the anode, which greatly increased the electrochemical performance of the Zn-Al-LDH/PPY composites.

14.4

Fuel Cells

Nowadays, to overcome difficulties of energy deficit, environmental pollution, and increasing global temperatures owing to using fossil fuels, great effort has gone into fuel cells development as a suitable power source. Fuel cells are electrochemical systems with fuels like methanol, hydrogen, and natural gas. They have advantages including high efficiency, simplicity, and biocompatibility (Herrero et al., 2014; Kim et al., 2015). LDHs possess many intrinsic properties such as swelling ability, anion exchange capacity, water retention, anion consolidation, and high specific surface areas. Due to these properties, the introduction of LDH nanofillers in the polymer matrix creates composite membranes, which are appropriate for fuel cell applications. The introduction of LDHs into polymer matrix enhances tangling polymers that diminish leakage of fuel and also improve the mechanical resistance of polymers in high temperatures (Angjeli et al., 2015).

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Layered Double Hydroxide Polymer Nanocomposites

14.4.1 Application of Layered Double Hydroxide/Polymer Nanocomposites in Fuel Cells Fuel cells, specifically polymer electrolyte membrane fuel cells (PEMFCs), consist of a polymer membrane, which separates the anode from the cathode and helps in proton conductance from the anode to the cathode. PEMFCs are one of the most promising fuel cells because of their low emissions, great electrical performance, and facility to operate. Proton exchange membrane (PEM) in PEMFCs plays a key role. The PEM must have several characteristics such as high proton conductivity, thermal, mechanical, and chemical stability during operation, low electronic conductivity, low cost, good segregation between oxidant and fuel, and low fuel permeance (Herrero et al., 2014; Angjeli et al., 2015). Herrero et al. (2014) investigated sulfonated polysulfone (SPSU)/LDH NC membranes for use in PEMFCs. In this work, polysulfone (PSU) was sulfonated at room temperature with trimethylsilylchlorosulfonate in 1,2-dichloroethane and NC membranes were fabricated by combining various amounts of Zn, Al-NO3 LDH (0%, 1%, 2%, and 5%) with SPSU in N,N-dimethylacetamide (DMAc) through a solvent casting approach. The Zn, Al-NO3 LDH was obtained through the coprecipitation technique. EIS was applied to investigate the membrane’s electrical properties. The EIS evaluation was accomplished with the membranes in contact with HCl solutions at various concentrations (1023 # c # 1021). The results demonstrated that the electrical resistance of membrane depends on the degree of sulfonation and the content of the LDH additive. The water uptake mensuration was applied as a quantitative value of membrane performance in fuel cell uses. The performance of a membrane is affiliated to the conductivity of the proton and water content. The excellent conductivity of protons is achieved by the great level of water uptake. The incorporation of LDH affected the water uptake, electrical and transfer parameters of SPSU membranes, which exhibited high water uptake property. This work illustrates that membranes of LDH composites are potential materials for proton exchange because their membrane transfer is entirely like those of the best PEM systems. Nafion has been broadly applied as a fuel cell membrane due to its excellent proton conductance, durability, and chemically consistency. However, it has some disadvantages such as a conductivity decrease at high-temperature conditions ( . 80 C) and low humidity. In recent years, various methods have been used to ameliorate the efficiency of Nafion at high temperatures. For this purpose, Nicotera et al. (2015) developed electrolyte membranes based on Nafion and Mg
Al LDHs for H2/air-fed fuel cells. They prepared LDHs with various Mg21/Al31metal ratios 2 2 (2:1 and 3:1) and several anions (CO22 3 ; ClO4 ; and NO3 ) that introduced Nafion for the development of hybrid NCs’ utilization in high-temperature PEMFCs. NC membranes were prepared through solution intercalation by loading 3 wt.% of LDH and the dynamic behavior of water trapped in NC membranes, within a range temperature of 20 C
130 C, was investigated by NMR techniques (self-diffusion and relaxation times). The water absorption analysis of the NC membranes showed the significant treatment of water mobility and retention with attention to filler-free

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Nafion in the metal ratio of 2:1 and independent from anions (Fig. 14.8A). This can be attributed to hydroxyl groups present in LDHs and the greater anion exchange capacity of LDHs with a 2:1 metal ratio in comparison to those of 3:1 because major anions are intercalated in the interlayer space that adsorbs a greater water content. These composite membranes showed the capability to retain a specified content of “mobile water” for some hours above 100 C in fully intense situations such as a lack of exterior humidity. The influence of intercalated anions was more observed in impedance evaluations (Fig. 14.8B) The LDH 2 NO2 3 composite membrane exhibited the best proton conductance value (2.7 3 1022 S/cm at 50% of relative humidity and 100 C). Instead, lower conductivity was attributed to LDH-CO22 3 because of the inconstancy of CO22 3 in electrochemical ambiance. In other work, these composites with a metal ratio of 2:1 for LDH were prepared and used for high-temperature PEMFCs. The results showed that the water uptake values at 95 C were greater compared to Nafion membrane. Also, the results indicated that the proton conductivity at least up to 100 C in the LDH-NO3 and LDH-CO3

Figure 14.8 (A) Histogram of the maximum water uptake value reached from the hybrid membranes. The red line indicates the maximum absorption of the recast Nafion. (B) Proton conductivities measured at a low humidity level (50% RH) for three composite membranes. Source: Adapted from Nicotera, I., Angjeli, K., Coppola, L., Enotiadis, A., Pedicini, R., Carbone, A., et al., 2015. Composite polymer electrolyte membranes based on Mg
Al layered double hydroxide (LDH) platelets for H2/air-fed fuel cells. Solid State Ionics, 276, 40
46. With kind permission of Elsevier.

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Layered Double Hydroxide Polymer Nanocomposites

composites is superior compared to Nafion membrane, while LDH-ClO4 introduced the least proton conductance, notwithstanding greater water survival, which shows the adsorbed water is not accessible for proton transfer. Experiments accomplished in severe situations [1.5 abs. bar, 50%, and 100% (relative humidity) RH, at 80 C and 100 C] displayed the efficacy of the various interlayer anions in membranes and the LDH-NO3 sample highlighted maximum efficiency in intense situations. Thus, the incorporation of LDH improved the mechanical resistance of the matrix: 0.22 W/cm2 reached at 100 C and 50% RH (Angjeli et al., 2015). Another approach to overcome the demerits of Nafion is the addition of LDH to a sulfonated poly(ether ether ketone) (SPEEK). SPEEK has features such as being low-cost, nonperfluorinated, and having great thermal, mechanical, and chemically constancies. This polymer, due to possessing sulfonic groups in the backbone, is able to simplify increased proton conductivity and water retention features of the membrane. Thus, SPEEK could be a promising substitute for Nafion. Compared to Nafion, the addition of LDH to SPEEK increased proton conductance, permeability, the thermal stability of SPEEK, and decreased the diffusion coefficient. Kim et al. (2013) prepared SPEEK membranes by the introduction of the Mg
Al LDH (with different amounts 1, 3, and 5 wt.%) as nanofiller. The membrane comprising 3 wt. % LDH exhibited proton conductance of 229.9 mS/cm, while the proton conductivity of neat SPEEK obtained 147.6 mS/cm at 80 C and 100% RH. Kim’s group (Kim et al., 2015) developed their work in 2015 and synthesized SPEEK/LDH composite membranes by applying two kinds of LDH [Mg
Al based LDH (MA)- and Ni
Ti-based LDH (NT) with varying amounts (1, 3, and 5 wt.%)] and two different times for sulfonation of PEEK (4 and 6 h) (SP4 and SP6). To prove the LDH efficacy on the conductivity of membranes, they incorporated these LDHs in Nafion and investigated their proton conductances. The SPEEK/LDH composite membranes represented higher water uptake and thermal stability properties in comparison to the neat PEEK membrane. The obtained results from proton conductivity (Fig. 14.9) showed that by adding LDHs to SPEEK, the proton conductance of NC membranes was enhanced for LDHs in amounts up to 3 wt.%, but then reduced. This is because of their good dispersion up to 3 wt.% LDH amounts and the existence of water in LDHs (MA and NT). MA-based membranes inferred superior proton conductance in contrast to NT-based membranes with a proton conductance of 229.6 mS/cm at 80 C and 100% RH for MA-based membranes containing 3 wt.% LDH and sulfonation of PEEK for 4 h. However, NT-based membrane exhibited greater power density (560 mW/cm2) compared to the MA-based membrane that was 19% greater than Nafion. Therefore, these membranes can be used in PEMFC. Anion-exchanged membrane fuel cells (AEMFCs) are one of the green and effective energy conversion systems that are being considered for mobile power and portable electronics because of their advantages, such as a wider selection of fuels, quicker electrokinetics, and increased material stability. The key component in AEMFCs is the anion exchange membrane (AEM). AEM has features like high thermal and mechanical stability, good conductivity, and high water uptake. In recent years, researchers have conducted numerous studies on AEM materials. Most of these studies have been done based on homogeneous polymers, while the AEMs

Electrical and electronic applications of layered double-hydroxide polymer nanocomposites

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Figure 14.9 Proton conductivities of the SP4 membrane and its (A) MA and (B) NT composites. Source: Adapted from Kim, N.H., Mishra, A.K., Kim, D.Y., Lee, J.H., 2015. Synthesis of sulfonated poly (ether ether ketone)/layered double hydroxide nanocomposite membranes for fuel cell applications. Chem. Eng. J., 272, 119
127. With kind permission of Elsevier.

based on composite materials containing laminated nanosheets are scarce. LDH is a promising substance for a blend with AEM materials as a conductor due to it possessing properties such as the capacity for anion exchange, water uptake, and weak swelling. Herein, we discuss research works which have been accomplished on AEMFCs, based on LDH/polymer NC. Recently, AEM-directed ethanol fuel cells (AEM DEFCs) were utilized in alkaline AEMs as a solid electrolyte. DEFCs are more abundant and biocompatible than other possible alcohol fuels and have lower toxicity and greater energy density. The commercial AEMs consist of

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Layered Double Hydroxide Polymer Nanocomposites

hydrocarbon chains, and quadric ammonium functional groups such as commercial A301 membrane have decreased thermal stability and low ion conductivity that limit the performance of AEM DEFCs. To overcome this issue, Zang et al. (2012) synthesized cross-linked PVA/Mg-Al LDH (PVA/LDH) membranes. The obtained outcomes showed an effective interfacial bond between PVA and LDH, which causes an increase in ion conductance and diminished ethanol permeance of membranes. The cell performance of PVA/LDH with 20 wt.% LDH demonstrated power densities of 61.31 and 81.92 mW/cm2 at 60 C and 80 C, respectively, which were greater than commercial A301 membrane (31.97 and 37.11 mW/cm2 at 60 C and 80 C, respectively). They examined the discharge process at 80 C for more than 80 h for PVA/LDH with 20 wt.% LDH. The ion conductance of membrane showed a small reduction from 23.9 to 22.7 mS/cm. These results display that this membrane has good potential for AEM DEFCs applications at high temperature. Di Vona et al. (2017) designed composite anion-conducting membranes containing PSU with grafted quaternary ammonium groups as ionomer and LDH with composition Mg0.62Al0.38(OH)2(Cl)0.38. 0.6H2O as the inorganic filler. They used two kinds of amines for the graft to PSU: large and rigid 1,4-diazabicyclo[2.2.2]octane (DABCO) to obtain DABCO-PSU, and small and flexible trimethylamine (TMA) to obtain TMA-PSU. Fig. 14.10 presents a schematic of LDHs intercalated in the

Figure 14.10 Schematic representation of LDHs intercalated in the polymeric matrix of TMA-PSU. Source: Adapted from Di Vona, M.L., Casciola, M., Donnadio, A., Nocchetti, M., Pasquini, L., Narducci, R., et al., 2017. Anionic conducting composite membranes based on aromatic polymer and layered double hydroxides. Int. J. Hydrogen Energy, 42(5), 3197
3205. With kind permission of Elsevier.

Electrical and electronic applications of layered double-hydroxide polymer nanocomposites

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Table 14.1 Conductivity in Water of Anionic Membranes at 25 C. Before Measurements, the Membranes Were Washed Several Times in Water Sample

σ (mS/cm)a

σ (mS/cm)b

TMA-PSU DABCO-PSU TMA-PSU/LDH DABCO-PSU/LDH

2.2 5.3 3.7 2.2



4.8 3.4

Membranes treated at 25 C in 2 M KOH for 24 h. Membranes treated in 2 M KOH at 25 C for 24 h and at 60 C for 24 h. Source: Adapted from Di Vona, M.L., Casciola, M., Donnadio, A., Nocchetti, M., Pasquini, L., Narducci, R., et al., 2017. Anionic conducting composite membranes based on aromatic polymer and layered double hydroxides. Int. J. Hydrogen Energy, 42(5), 3197
3205. With kind permission of Elsevier. a

b

matrix of TMA-PSU. Composites with DABCO due to lower water uptake showed slightly less ion conductivity than those with TMA. In quite humidified conditions, the composites exhibited improvement of mechanical properties with a nearly threefold increase of Young’s modulus. The experimental data showed the conductivity of the composite membranes to be comparable with that of pristine ionomers, being in the range 2
4 mS/cm, despite their drastically reduced hydration (as described in Table 14.1). Moreover, the membrane performance was improved in alkaline conditions at 60 C without missing their properties and the ion conductivity under 95% RH reached 30 mS/cm. In other work, Mg
Al LDHs were synthesized and introduced into quaternized polysulfone (QPSF) membranes to measure the efficacy of LDHs on properties of NC AEMs. QPSF NC membrane with 5 wt.% LDH compared to QPSF membrane showed increased tensile strength, lower water uptake, and greater ion conductivity (2.36 3 1022 S/cm at 60 C). This high performance suggests that it is a potential candidate for application of AEM fuel cells (Liu et al., 2017b). Fan et al. (2014) synthesized a novel polyphosphazene with pendant imidazolium and added LDH to polyphosphazene-based ionomer with the purpose of treating the conductance of AEMs. They designed a new method for casting and applied an exterior AC electric field in a trans-plane direction. In order to investigate the efficacy of added LDH by this new method they prepared three membranes designated as M1, M2, and M3, as shown in Fig. 14.11A. TEM images (Fig. 14.11B) showed the amorphous LDH nanoplatelets that were not in contact with one another and uniformly diffused in M2 with only small chunks appearing. The LDH were oriented by applying an electric field, as seen in the M3 TEM photograph. Normal LDH nanoplatelets were perpendicular to the trans-plane path and in contact together, creating a 3D dendritic construction. The incorporation of LDH in AEMs considerably enhanced ion exchange capacity, mechanical properties, water uptake, and ionic conductivity of AEMs, however, dimensional constancy was decreased. M2 showed a higher ionic conductivity (11.2 mS/cm) in comparison with M1 (2.74 mS/cm) because of the LDH presence as an anionic conductor in AEM. M3 with oriented LDH nanoplatelets showed an ion conductance 39% greater than M2 at 30 C with saturated

586

Layered Double Hydroxide Polymer Nanocomposites

Figure 14.11 (a) Scheme of fabrication of membranes. (b) TEM images of (A) MgAl
LDH platelets and ultrathin sections of (B) M1, (C) M2, and (D) M3. The scratch in each membrane made by a glass cutter in the slicing process is perpendicular to the trans-plane direction. Source: Adapted from Fan, J., Zhu, H., Li, R., Chen, N., Han, K., 2014. Layered double hydroxide
polyphosphazene-based ionomer hybrid membranes with electric field-aligned domains for hydroxide transport. Journal of Materials Chemistry A, 2(22), 8376
8385. With kind permission of RSC.

humidification. Hybrid membranes exhibited higher durability than M1 in an alkali ambiance and were flexible and stable below 163 C. These results showed that ionomer hybrid AEMs based on LDH-polyphosphazene with electric field treatment indicated a higher potential for application of AEMFC. Thus it is anticipated that this technique will be extended for a new range of materials with many new AEM device applications.

Electrical and electronic applications of layered double-hydroxide polymer nanocomposites

587

Zebda et al. (2011) fabricated Zn2Cr
2,20 -azinobis-3-ethylbenzothiazoline-6-sulfonate (ABTS) and Zn2Al
Fe(CN)6 LDHs. These synthesized hybrids were used in glucose/O2 biofuel cell systems as cathodes and anodes for electrical connection of laccase (Lac) and glucose oxidase (GOx). LDH hybrid and enzymes [Lac/Zn2Cr ABTS and GOx/Zn2Al
Fe(CN)6] were directly ensnared by electropolymerized PPY on tubular poriferous carbon electrode. In this biofuel cell device, Lac/Zn2Cr ABTS caused the electro-enzymatic diminution of O2 as a cathodic electrode, whereas GOx/Zn2Al
Fe(CN)6 caused the electro-enzymatic oxidation of glucose as the anodic electrode. The experimental data demonstrated the highest current density and power density of 0.43 mA/cm2 and 45 Mw/cm2, respectively, at 0.2 V by a Nafion membrane.

14.5

Other Electrical and Electronic Applications of Layered Double Hydroxide/Polymer Nanocomposites

One of greatest challenges for fuel cells and metal
air batteries is the progress of efficient and inexpensive electrocatalysts for oxygen reduction reaction (ORR). Therefore, through growing in situ CoFe-LDHs at the surface of polydopamine spheres (PDAS), the new hybrids were fabricated and their ORR catalytic activity was investigated. CoFe-LDHs/PDAS hybrids displayed greater ORR catalytic activity with a 4e-route tendency in comparison with neat CoFe-LDHs and PDAS and even calcinated CoFe-LDHs/PDAS. Also, synthesized hybrids indicated higher stability than commercial Pt/C ORR catalysts. The proposed ORR mechanism of these hybrids was a double-site ORR mechanism. Most probably O2 was adsorbed and diminished to peroxide at the site of Co-Nx and Fe-Nx between PDAS and CoFeLDHs, and the peroxide was then probably further diminished to OH2 catalyzed by CoFe-LDHs (Zhang et al., 2015b). They are a great area of attention to study high-performance and inexpensive bifunctional electrocatalysts toward ORR and oxygen evolution reactions (OER) for fuel cells and water splitting systems. In their study, Jia et al. (2017) prepared ternary CoNiMn-LDH/PPY/RGO composite in one step involving formation of the LDH and polymerization of the pyrrole in alkaline medium. CoNiMn-LDH/PPy/ RGO composite exhibited superior bifunctional electrocatalytic activities with the lowest ΔE (EOER; 10 mA=cm2
EORR;
3 mA=cm2 ) value of 0.85 V among the nonprecious metal bifunctional electrocatalysts as reported in the literatures, and good electrochemical durability for the ORR and OER. Thus, this composite is one of the potential bifunctional electrocatalysts for fuel cells and water splitting systems. Dou et al. (2012) reported ultrathin film (UTF) preparation of LDH/poly(N-isopropylacrylamide) (pNIPAM) on indium tin oxide electrodes. These UTFs demonstrated changeable electrocatalytic performance in responding to thermo-stimulants (Fig. 14.12). In this work, a layered intermittent structure with homogeneous and orderly growth of (LDH/pNTPAM)n UTFs was confirmed by UV-vis absorption spectroscopy and X-ray diffraction. Also, the returnable shrinkage
expansion

588

Layered Double Hydroxide Polymer Nanocomposites

Figure 14.12 Schematic illustration of the (LDH/pNIPAM)n UTFs with a temperaturesensitive contraction
expansion property. Source: Adapted from Dou, Y., Han, J., Wang, T., Wei, M., Evans, D.G., Duan, X., 2012. Temperature-controlled electrochemical switch based on layered double hydroxide/poly (Nisopropylacrylamide) ultrathin films fabricated via layer-by-layer assembly. Langmuir, 28 (25), 9535
9542. With kind permission of ACS.

transfer by thermal stimulant of UTFs was confirmed by the morphological and structural studies. The UTFs demonstrated returnable thermo-triggered CV and EIS on
off response between 20 C and 40 C. The mechanism of this behavior attributed to shrinkage
expansion of pNIPAM with low-high electrochemical impedance. Moreover, greater on
off electrocatalytic demeanor of the UTFs than the glucose oxidation was observed, which resulted from the temperature-controlled transport rate. Hence, present research presented a simple strategy for the design and synthesis of UTFs for utilization in changeable electrochemical sensors, controlled electroanalysis, and information storage-readout systems. Lv et al. (2012) synthesized magnetic LDH based on Zn and Al with embedding Fe3O4 in its interlayer. They introduced it into polyimide (PI) matrix after functionalization with sodium dodecyl sulfate in order to fabricate ZnAl-LDH/PI films through in situ polymerization with magnetic properties which have large potential in electromagnetic shielding application (Fig. 14.13). With the aim of electromagnetic shielding, an exterior magnetic field was used to investigate adjustment of magnetic LDH after imidization at room temperature. The results exhibited, in the imidization procedure of polyamic acid, that LDH missed hydroxyl group in layers and sodium dodecyl sulfate decomposed slightly, resulting in weak contact between PI and oxidized LDH. The saturated magnetization of ZnAl-LDH was increased partly owing to crystallinity treatment in Fe3O4 with conservation of sodium dodecyl sulfate. Meanwhile magnetic features of ZnAl-LDH/PI were not influenced by the imidization process. Polymer gel electrolyte is another application of polymer/LDHs. The polymer gel electrolyte NC films consist of poly(ethylene glycol diacrylate) (PEGDA)/ LiCF3SO3/Mg
Al LDHs with different contents of LDH. The ionic conductivity of NC showed that enhanced molecular weight of PEGDA and NC with 4.5 wt.% of

Electrical and electronic applications of layered double-hydroxide polymer nanocomposites

589

Figure 14.13 (A) Schematic representation of the ZnAl-LDH/polyamic acid preparation and imidization. (B) The photos of magnetic ZnAl-LDH suspended in water (a) in the absence and (b) presence of an external magnet. Source: Adapted from Lv, F., Wu, Y., Zhang, Y., Shang, J., Chu, P.K., 2012. Structure and magnetic properties of soft organic ZnAl-LDH/polyimide electromagnetic shielding composites. J. Mater. Sci., 47(4), 2033
2039. With kind permission of Springer.

LDH with a Mg21/Al31 ratio of 2 exhibited an ion conductance of 0.0016 S/cm at 25 C (Cho et al., 2004). Bao et al. (2008) prepared poly(vinylidene fluoride) (PVDF)/dodecyl sulfate (DS) intercalated LDH NC gel electrolytes and investigated their proton conductivity The proton conductivity was greatly increased in the NC gel
electrolyte systems and for NC containing 7.4 wt.% LDH, the proton conductivity was enhanced by about two and a half times in contrast to neat PVDF gel electrolyte. Kutlu et al. (2013) synthesized polyaniline (PANI) as a conjugated polymer. At first, Mg
Al LDH was pillared by dodecylbenzenesulfonate modifier and then was further modified with PANI/dodecylbenzensulfonic acid by applying

590

Layered Double Hydroxide Polymer Nanocomposites

a solution adsorption approach. LDH was melt-mixed with maleic anhydridemodified polyethylene. All synthesized compounds such as PANI modified LDH, and NC after melt compounding exhibited high conductivity, which can be used in electrical devices. Leroux et al. (2012) reported the preparation of a poly

Figure 14.14 (A) Illustration of the geometric templating of LDH on the PVDF substrate. (B) FE-SEM images of PVDF-LDH substrate. (C) Water uptake comparison of PVDF and PVDF-LDH pore-filling membranes. (D, E) Ionic conductivity and durability profile of PVDF and PVDF-LDH membranes. Source: Adapted from Sailaja, G.S., Zhang, P., Anilkumar, G.M., Yamaguchi, T., 2015. Aniosotropically organized LDH on PVDF: a geometrically templated electrospun substrate for advanced anion conducting membranes. ACS Appl. Mater. Interfaces, 7(12), 6397
6401. With kind permission of ACS.

Electrical and electronic applications of layered double-hydroxide polymer nanocomposites

591

(vinylpyrrolidone) (PVP) exfoliated NC using self-generated Zn-Al LDH platelets modified by alginate (ALG) and evaluated the conductive, capacitive properties of the NC films along with their dielectric behavior. NC films demonstrated ionic conductivity more than four times greater than that of the LiClO4 salt system and six times that of PVP. On the other hand, the dielectric properties were intensely modified. Owing to these properties, PVP: LDH/ALG can be used in electrical devices. Sailaja et al. (2015) presented an electrospun PVDF substrate with hexagonal platelets of Mg-Al LDH (Fig. 14.14). Features such as the distinctive morphology restructure, interior pore geometry, and modulate dynamic wetting profile of a nonwoven PVDF substrate, transformed it into a substrate with high performance for solid-state alkaline fuel cell anion conducting membranes. PVDF-LDH substrate showed extraordinary high durability ( . 140 C) compared to common substrates, for example, polyethylene, ion exchange capacity, great anion conductivity even at lower RH (50%), treated mechanical properties, and limited swelling, without compromising any of the desirable properties of the PVDF electrospun substrate. PVDF-LDH with these significant properties is expected to provide inimitable functionality, such as a substrate with high durability and great thermostability for utilization in fuel cell membrane, a flexible substrate for SC use, stable substrates for filtration/adsorption, and for battery separators.

14.6

Conclusions

LDHs, owing to their adjustable compound and ability of anion exchange, are promising electrode materials, but they have low conductivity that restricts the electrontransfer rate and reduces the performance of the electrode. In order to solve this problem, LDH NCs and polymer/LDH NCs have been developed. Polymer/LDH NCs are important materials for the preparation of electrodes and exhibit a synergistic effect because of the polymer supply in the dispersion of LDH and satisfactory conductivity. Therefore, polymer NCs based on LDH demonstrate excellent performance in energy storage. Due to the electrochemical performance of LDH NCs and polymer/ LDH NCs, they show great applications in SCs, batteries, fuel cells, and other electrical and electronic devices. This subject, owing to its uses, will be a very interesting research area, which many scientists in academia and industry will be working on for the further discovery for future advanced technologies.

Acknowledgments The authors thank the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, I. R. Iran, for partial financial support. In addition, financial support from the National Elite Foundation (NEF), Tehran, I. R. Iran, Iran Nanotechnology Initiative Council (INIC), Tehran, I. R. Iran, and the Center of Excellence in Sensors and Green Chemistry Research (IUT) is gratefully appreciated.

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Layered Double Hydroxide Polymer Nanocomposites

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246. Liao, C.S., Ye, W.B., 2004a. Effect of surface states of layered double hydroxides on conductive and transport properties of nanocomposite polymer electrolytes. Mater. Chem. Phys. 88 (1), 84
89. Liao, C.S., Ye, W.B., 2004b. Structure and conductive properties of poly(ethylene oxide)/layered double hydroxide nanocomposite polymer electrolytes. Electrochim. Acta 49, 4993
4998. Liu, S., Shackery, I., Patil, U., Lee, S.C., Park, B., An, H.B., et al., 2017a. Hierarchical MnCo-layered double hydroxides@Ni(OH)2 core
shell heterostructures as advanced electrodes for supercapacitors. J. Mater. Chem. A 5, 1043
1049. Liu, W., Liang, N., Peng, P., Qu, R., Chen, D., Zhang, H., 2017b. Anion-exchange membranes derived from quaternizedpolysulfone and exfoliated layered double hydroxide for fuel cells. J. Solid State Chem. 246, 324
328. Lv, F., Wu, Y., Zhang, Y., Shang, J., Chu, P.K., 2012. Structure and magnetic properties of soft organic ZnAl-LDH/polyimide electromagnetic shielding composites. J. Mater. Sci. 47 (4), 2033
2039. Lv, L., Xu, K., Wang, C., Wan, H., Ruan, Y., Liu, J., et al., 2016. Intercalation of glucose in NiMn-layered double hydroxide nanosheets: an effective path way towards battery-type electrodes with enhanced performance. Electrochim. Acta 216, 35
43. Mallakpour, S., Dinari, M., 2015a. Effect of organically modified Ni
Al layered double hydroxide loading on the thermal and morphological properties of l-methionine containing poly (amide-imide) nanocomposites. RSC Adv. 5 (35), 28007
28013. Mallakpour, S., Dinari, M., 2015b. Intercalation of amino acid containing chiral dicarboxylic acid between Mg
Al layered double hydroxide. J. Therm. Anal. Calorim. 119 (2), 1123
1130. Mallakpour, S., Dinari, M., Behranvand, V., 2013. Ultrasonic-assisted synthesis and characterization of layered double hydroxides intercalated with bioactive N, N0 -(pyromellitoyl)-bis-l-α-amino acids. RSC Adv. 3 (45), 23303
23308. Mallakpour, S., Dinari, M., Behranvand, V., 2014a. Anionic clay intercalated by multiwalled carbon nanotubes as an efficient 3D nanofiller for the preparation of highperformance l-alanine amino acid containing poly (amide-imide) nanocomposites. J. Mater. Sci. 49 (20), 7004
7013. Mallakpour, S., Dinari, M., Hatami, M., 2014b. Modification of Mg/Al-layered double hydroxide with L-aspartic acid containing dicarboxylic acid and its application in the enhancement of the thermal stability of chiral poly (amide-imide). RSC Adv. 4 (79), 42114
42121. Mallakpour, S., Dinari, M., Hatami, M., 2015. Novel nanocomposites of poly (vinyl alcohol) and Mg
Al layered double hydroxide intercalated with diacid N-tetrabromophthaloylaspartic. J. Therm. Anal. Calorim. 120 (2), 1293
1302. Mallakpour, S., Motirasoul, F., 2016. Covalent surface modification of α-MnO2nanorods with l-valine amino acid by solvothermal strategy, preparation of PVA/α-MnO2-l-valine nanocomposite films and study of their morphology, thermal, mechanical, Pb (II) and Cd (II) adsorption properties. RSC Adv. 6 (67), 62602
62611. Mallakpour, S., Dinari, M., Behranvand, V., 2016a. Design of one-pot green protocol for the synthesis of novel modified LDHs with diacids based on amino acids: morphology and thermal examinations. J. Iranian Chem. Soc. 13 (9), 1635
1642.

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Mallakpour, S., Dinari, M., Talebi, M., 2016b. Novel nanocomposites obtained by dispersion of LDH modified with N-tetrabromophthaloyl-glutamic in poly (amide-imide) having N-trimellitylimido-l-leucine and 4, 40 -diaminodiphenylether units. Polym. Comp. 37 (5), 1323
1329. Mallakpour, S., Hatami, M., 2017a. Condensation polymer/layered double hydroxide NCs: preparation, characterization, and utilizations. Eur. Polym J. 90, 273
300. Mallakpour, S., Motirasoul, F., 2017b. Preparation of PVA/α-MnO2-KH550 nanocomposite films and study of their morphology, thermal, mechanical and Pb (II) adsorption properties. Prog. Org. Coat. 103, 135
142. Nagaraju, G., Raju, G.S.R., Ko, Y.H., Yu, J.S., 2016. Hierarchical Ni
Co layered double hydroxide nanosheets entrapped on conductive textile fibers: a cost-effective and flexible electrode for high-performance pseudocapacitors. Nanoscale 8 (2), 812
825. Nejati, K., Asadpour-Zeynali, K., 2014. Electrochemical synthesis of nickel
iron layered double hydroxide: application as a novel modified electrode in electrocatalytic reduction of metronidazole. Mater. Sci. Eng. C 35, 179
184. Nicotera, I., Angjeli, K., Coppola, L., Enotiadis, A., Pedicini, R., Carbone, A., et al., 2015. Composite polymer electrolyte membranes based on Mg
Al layered double hydroxide (LDH) platelets for H2/air-fed fuel cells. Solid State Ionics 276, 40
46. Reddy, M.V., Subba Rao, G.V., Chowdari, B.V.R., 2013. Metal oxides and oxysalts as anode materials for Li ion batteries. Chem. Rev. 113 (7), 5364
5457. Roy, P., Srivastava, S.K., 2015. Nanostructured anode materials for lithium ion batteries. J. Mater. Chem. A 3 (6), 2454
2484. Sailaja, G.S., Zhang, P., Anilkumar, G.M., Yamaguchi, T., 2015. Aniosotropically organized LDH on PVDF: a geometrically templated electrospun substrate for advanced anion conducting membranes. ACS Appl. Mater. Interfaces 7 (12), 6397
6401. Shao, M., Li, Z., Zhang, R., Ning, F., Wei, M., Evans, D.G., et al., 2015a. Hierarchical conducting polymer@claycore
shell arrays for flexible all-solid-state supercapacitor devices. Small. 11 (29), 3530
3538. Shao, M., Zhang, R., Li, Z., Wei, M., Evans, D.G., Duan, X., 2015b. Layered double hydroxides toward electrochemical energy storage and conversion: design, synthesis and applications. Chem. Commun. 51 (88), 15880
15893. Tsuneishi, T., Sakamoto, H., Hayashi, K., Kawamura, G., Muto, H., Matsuda, A., 2014. Preparation of hydroxide ion conductive KOH
layered double hydroxide electrolytes for an all-solid-state iron
air secondary battery. J. Asian Ceram. Soc. 2 (2), 165
168. Wang, L., Yang, W., Wang, J., Evans, D.G., 2009. New nanocomposite polymer electrolyte comprising nanosized ZnAl2O4 with a mesopore network and PEO-LiClO4. Solid State Ionics 180 (4), 392
397. Wang, R., Yang, Z., Yang, B., Fan, X., Wang, T., 2014a. A novel alcohol-thermal synthesis method of calcium zincates negative electrode materials for Ni
Zn secondary batteries. J. Power Sour. 246, 313
321. Wang, R., Yang, Z., Yang, B., Wang, T., Chu, Z., 2014b. Superior cycle stability and high rate capability of Zn
Al
In-hydrotalcite as negative electrode materials for Ni
Zn secondary batteries. J. Power Sour. 251, 344
350. Wang, X., Lin, Y., Su, Y., Zhang, B., Li, C., Wang, H., et al., 2017a. Design and synthesis of ternary-component layered double hydroxides for high-performance supercapacitors: understanding the role of trivalent metal ions. Electrochim. Acta 225, 263
271. Wang, X., Zheng, Y., Yuan, J., Shen, J., Hu, J., Wang, A.J., et al., 2017b. Three-dimensional NiColayered double hydroxide nanosheetsarray on carbon cloth, facile preparation and its application in highly sensitive enzymelessglucose detection. Electrochim. Acta 224, 628
635.

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Wei, Y., Zhang, X., Wu, X., Tang, D., Cai, K., Zhang, Q., 2016. Carbon quantum dots/ Ni
Al layered double hydroxide composite for high-performance supercapacitors. RSC Adv. 6 (45), 39317
39322. Winter, M., Brodd, R.J., 2004. What are batteries, fuel cells, and supercapacitors? Chem. Rev. 104 (10), 4245
4269. Yan, J., Yang, Z., 2016. Based on the performance of hydrotalcite as anode material for a Zn
Ni secondary cell, a modification: PPY coated Zn
Al
LDH was adopted. RSC Adv. 6 (88), 85117
85124. Yang, B., Yang, Z., Wang, R., Wang, T., 2013a. Layered double hydroxide/carbon nanotubes composite as a high performance anode material for Ni
Zn secondary batteries. Electrochim. Acta 111, 581
587. Yang, W., Gao, Z., Wang, J., Ma, J., Zhang, M., Liu, L., 2013b. Solvothermal one-step synthesis of Ni
Al layered double hydroxide/carbon nanotube/reduced graphene oxide sheet ternary nanocomposite with ultrahigh capacitance for supercapacitors. ACS Appl. Mater. Interfaces 5 (12), 5443
5454. Yang, B., Yang, Z., Peng, Z., Liao, Q., 2014a. Effect of silver additive on the electrochemical performance of ZnAl-layered double hydroxide as anode material for nickel-zinc rechargeable batteries. Electrochim. Acta 132, 83
90. Yang, B., Yang, Z., Wang, R., 2014b. Facile synthesis of novel two-dimensional silvercoated layered double hydroxide nanosheets as advanced anode material for Ni
Zn secondary batteries. J. Power Sour. 251, 14
19. Zebda, A., Tingry, S., Innocent, C., Cosnier, S., Forano, C., Mousty, C., 2011. Hybrid layered double hydroxides-polypyrrole composites for construction of glucose/O2 biofuel cell. Electrochim. Acta 56 (28), 10378
10384. Zeng, L., Zhao, T.S., Li, Y.S., 2012. Synthesis and characterization of crosslinked poly (vinyl alcohol)/layered double hydroxide composite polymer membranes for alkaline direct ethanol fuel cells. Int. J. Hydrogen. Energy 37 (23), 18425
18432. Zhan, T., Song, Y., Tan, Z., Hou, W., 2017. Electrochemical bisphenol a sensor based on exfoliated Ni2Al-layered double hydroxide nanosheets modified electrode. Sensors Actuators B: Chem. 238, 962
971. Zhang, S., Yao, F., Yang, L., Zhang, F., Xu, S., 2015a. Sulfur-doped mesoporous carbon from surfactant-intercalated layered double hydroxide precursor as high-performance anode nanomaterials for both Li-ion and Na-ion batteries. Carbon. N. Y. 93, 143
150. Zhang, X., Wang, Y., Dong, S., Li, M., 2015b. Dual-site polydopamine spheres/CoFe layered double hydroxides for electrocatalytic oxygen reduction reaction. Electrochim. Acta 170, 248
255. Zhang, Z., Yang, Z., Huang, J., Feng, Z., Xie, X., 2015c. Enhancement of electrochemical performance with Zn-Al-Bi layered hydrotalcites as anode material for Zn/Ni secondary battery. Electrochim. Acta 155, 61
68. Zhang, J., Hu, H., Li, Z., Lou, X.W.D., 2016. Double-shelled nanocages with cobalt hydroxide inner shell and layered double hydroxides outer shell as high-efficiency polysulfide mediator for lithium
sulfur batteries. Angew. Chem. Int. Ed. 55 (12), 3982
3986. Zhao, J., Chen, J., Xu, S., Shao, M., Yan, D., Wei, M., et al., 2013. CoMn-layered double hydroxide nanowalls supported on carbon fibers for high-performance flexible energy storage devices. J. Mater. Chem. A 1 (31), 8836
8843. Zhao, J., Xu, S., Tschulik, K., Compton, R.G., Wei, M., O’Hare, D., et al., 2015. Molecularscale hybridization of clay monolayers and conducting polymer for thin-film supercapacitors. Adv. Funct. Mater. 25 (18), 2745
2753.

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Zhou, K., Gui, Z., Hu, Y., 2017a. Facile synthesis of LDH nanoplates as reinforcing agents in PVA nanocomposites. Polym. Adv. Technol. 28 (3), 386
392. Zhou, Y., Hu, G., Zhang, W., Li, Q., Zhao, Z., Zhao, Y., et al., 2017b. Cationic twodimensional sheets for an ultralight electrostatic polysulfide trap toward highperformance lithium-Sulfur batteries. Energy Storage Mater. 9, 39
46.

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Applications of layered double hydroxide biopolymer nanocomposites

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Shadpour Mallakpour1,2,3 and Leila khodadadzadeh3 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 3Chemistry Group, Pardis College, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

15.1

Introduction

The term “nanotechnology” is widespread nowadays not only in scientific fields, but also in many public sectors. According to the definition by the Royal Society and the Royal Academy of Engineering, “nanotechnologies are the plan, characterization, production and application of structures, devices and systems by controlling the size at the nanometer (nm) scale” (UNESCO, 2006), despite the fact that the US National Nanotechnology Initiative has defined that nanotechnology research and technology development are at the atomic, molecular, or macromolecular levels, in the length scale of approximately 1
100 nm range. For the answer of “what is so special about 100 nm?,” it has been said that in this scale (below 100 nm), engineers consider the properties of materials that ordinary engineers ignore, which in particular contain “quantum mechanical effects” and “surface science effects” (Gasman, 2006). Nanotechnology has a great influence on the economy and society in the early 21st century (Lau et al., 2013) compared to information technology, cellular and molecular biology, and semiconductor technology. According to many books and reviews, nanotechnology has had a significant impact on the oil and gas industry (Bera and Belhaj, 2016; Fakoya and Shah, 2017; Negin et al., 2016), petroleum exploration (He et al., 2016), energy research and solar cells (Abdin et al., 2013; Hussein, 2015, 2016), water treatment applications (Kunduru et al., 2017; Olvera et al., 2017; Qu et al., 2013), food safety and food packaging (Dimitrijevic et al., 2015; Duncan, 2011), drug and gene delivery (McDonald et al., 2015; Wen et al., 2016; Wong et al., 2017), cancer therapy (Beik et al., 2016; Xie et al., 2016), tissue engineering (Kingsley et al., 2013; Walmsley et al., 2015), and many other important areas. Composite materials are defined as solid materials with multiple phases. The economic importance of these materials is observed everywhere. Generally, Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00015-X © 2020 Elsevier Ltd. All rights reserved.

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composite materials are engineered materials, made up of more than one component with significantly different physical or chemical properties. The phases not only have intimate contact (at an atomic or molecular level) with each other but also remain individual on a macroscopic level within the finished structure. Matrix and reinforcement (filler) are two main phases in composite materials. Matrix is usually a continual and softer phase that can be a polymer (weak stiffness), metal (medium stiffness but high ductility), or ceramic (strong stiffness but brittle), while the reinforcing or dispersed phase is a strong and stiff part embedded in the matrix which enhances the physical and mechanical properties of matrix. For example, wood is composed of fibrous cellulose as reinforcement in a matrix of lignin, while bone and teeth are made up of hard inorganic crystals (hydroxyapatite or osteones) in a matrix of collagen. There are many routes for the combination of the phases and the most common, especially in polymer and material science, are filling, blending, compounding, mixing, melting, and assembling. It is noteworthy that the significance of composite materials is not only for their bettered strengths but also for their other useful applications such as electrical, biological, thermal, etc. (Durand, 2008; Hull and Clyne, 1996; Youssef, 2013; Zafar et al., 2016) The advance in nanotechnology has given new intuitions into applications of popular materials due to surprising properties as a result of nanoscale technology. As an example, polymer nanocomposites (NCs) are a comparatively new type of composite material, and are regarded as a new alternative to conventional polymers. In polymer NCs, the matrix is an organic polymer and nanoscopic inorganic or organic fillers (with at least one dimension less than 100 nm) are dispersed in it. Nanofillers (NFs) have very high aspect ratio which leads to their effective dispersion in matrix. Such a uniform dispersion creates an ultra large interfacial area between two phases and so the final NCs have predominant properties compared to conventional microcomposites. Furthermore, due to the unique properties of nanosized fillers, reinforcing with only a low loading of these fillers (, 5 wt.%) not only endows NCs with a similar performance to that of conventional composites (containing 40
50 wt.% of common fillers) but also resulting in a material with lower weight (Mousa et al., 2016). In comparison with pure polymers or conventional composites, NCs have remarkably ameliorated properties which in particular contain catalytic, gas barrier, thermal, mechanical, flame retardancy, etc. This aspect of nanotechnology has potential in applications such as biomedical applications, water treatment, engineered plastics, rubbers, adhesives, and coatings (Thakur and Thakur, 2015). It is crucial to diminish the environmental effect of materials manufacturing by reducing the environmental impression at all steps of their life cycle. Using nonbiodegradable petro-based synthetic polymers has caused many environmental problems, which in particular include high annual consumption and limitations of landfill sites for their waste accumulation, the production of carbon dioxide and hazardous emissions during incineration, uneconomical waste recycling, etc. (Mousa et al., 2016). Therefore, developing bio-based materials is one of the outstanding subjects for many researchers. As a consequence, fabrication of NCs in which both the matrix and NF are based on renewable resources is the order of the

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day. Therefore, the main focus of this chapter is on polymer NCs containing biopolymer matrix. Biopolymers are polymers that are biodegradable (Grossman et al., 2013) and according to ASTM standard D-5488-94d and European norm EN 13432, “biodegradable” means “capable of undergoing decomposition into water, carbon dioxide, inorganic compounds, methane, and biomass.” The principal mechanism for degradation is the enzymatic action of microorganisms (Ave´rous and Pollet, 2012). Different classifications of various biodegradable polymers are possible. It can be based on the synthesis procedure, processing technique, chemical constitution, application, economic significance, etc. and since each of them supplies different information, it is not simple to choose the classification. In this chapter we choose a classification of biopolymers based on their “origin” which classifies them into three main groups (Smith, 2005): 1. Natural polymers (renewable resource polymers), macromolecules obtained from natural origins. These polymers are subdivided into six classes: a. Polysaccharides; b. Proteins; c. Lipids; d. Polyesters obtained from plants and microorganisms (polyhydroxyalcanoates); e. Polyesters synthesized from bio-derived monomers [polylactic acid (PLA)]; f. A final group of various polymers such as composites and natural rubbers. Note: subdivisions (a
c) are called agro-polymers. 2. Synthetic polymers (a nonrenewable resource), macromolecules synthesized from mineral origins (crude oil). These polymers are subdivided into four classes: a. Aliphatic polyesters [polyglycolic acid, polybutylene succinate (PBS), polycaprolactone (PCL)]; b. Aromatic polyesters or blends of aliphatic and aromatic kinds (polybutylene succinate terephthalate); c. Poly(vinyl alcohols) (PVA); d. Modified polyolefin (polyethylene or polypropylene containing sensitive sectors for temperature or light). 3. Blends of polymers from miscellaneous origins. In spite of the appealing properties of biopolymers, such as vast accessibility of raw materials and improved biodegradability, some vital properties like barrier, mechanical, and thermal properties are limited and not comparable with conventional polymers. Production of biopolymer NCs by incorporation of NFs into biopolymers is an outstanding solution for the described problems (Averous and Boquillon, 2004), which makes them a suitable choice for a wide range of applications such as food packaging, water remediation, electrochemical and electroanalytical, medical and tissue engineering, coatings, etc. (Aranda et al., 2006; Costa et al., 2013; DeGruson, 2014; Grossman et al., 2013; Jorfi et al., 2013; Mallakpour et al., 2014; Mousa et al., 2016; Okamoto and John, 2013; Pilla, 2011; Reddy et al., 2013; Rhim et al., 2013; Tan et al., 2010; Zhou et al., 2011). One of the most important groups of NFs is inorganic types. Indeed, inorganic nanomaterials (NMs) are considered as basic building blocks in nanotechnology. Inorganic NMs may be classified according to their geometric shape and have at

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least one dimension in the range of 1
100 nm. According to this, they can be spherical (e.g., metal and metal oxide nanoparticles [NPs]), fibril-like (e.g., carbon nanotubes and metal wires), and platelet-shape (e.g., natural smectite clays, layered double hydroxide [LDH], graphite, and graphene sheets), producing particulate, elongated, and layered NCs, respectively (Zafar et al., 2016) (Fig. 15.1). Other geometric shapes and morphologies that have been reported include inorganic nanowires, nanotubes, nanoboxes, nanocubes, nanospheres, and nanorods (Chiu and Lin, 2012). A broad range of inorganic NMs have been used as NF for reinforcement of polymeric matrix but LDHs, a family of lamellar inorganic materials, has been much investigated for this purpose. This is due to their many outstanding properties which in particular include nontoxic, low price, eco-friendly, large surface area, great thermal and mechanical properties, and tendency to interchange their interlayer anions with other anions such as larger organic and inorganic ones (Mallakpour and Hatami, 2017). LDHs are also recognized as anionic clays due to the existence of exchangeable anions in the interlayer area. The structure of LDH composes of brucite-like sheets and the thickness of each sheet is about 0.5 nm. The positive charge of LDH sheets is a result of incomplete substitution of divalent metal cations with trivalent ones which are finally balanced with anions located inside the interlayer area. In general, LDH has the formula of (M211
x M31x (OH)2)(An
x/n  mH2O), where M21 and M31 are divalent and trivalent metallic ions which inhabit the center of octahedral units in the hydroxide layers, and An
is an interlayer anion. The metal (M) cations located in the octahedral units of hydroxide layers are coordinated by six hydroxyl groups, hence forming M(OH)2 brucite˚ ) and usually like sheets. The radii of these metal ions are similar to Mg21 (0.65 A contain Ni21, Co21, Mg21, Cu21, Zn21, or Cd21 (divalent), and Al31, Mn31, Fe31, Ga31, or Cr31 (trivalent). Many of the common metal ions used to synthesize LDH

Figure 15.1 Nanomaterial classification, together with examples, according to which the physical dimension is in the 1
100 nm range. Source: Adapted from Chiu, C.-W., Lin, J.-J., 2012. Self-assembly behavior of polymerassisted clays. Prog. Polym. Sci., 37(3), 406
444, with kind permission of Elsevier.

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Figure 15.2 Part of binary LDHs reported in patents and literature. Source: Adapted from Qu, J., Zhang, Q., Li, X., He, X., Song, S., 2016. Mechanochemical approaches to synthesize layered double hydroxides: a review. Appl. Clay Sci., 119, 185
192, with kind permission of Elsevier.

which are reported in many patterns and literatures are illustrated in Fig. 15.2. On the other hand, the usual interlayer anions are inorganic one such as CO322, Cl2, NO32, and SO422. It is noteworthy that the most commonly applied LDH is hydrotalcite, containing Mg21 and Al31 as metal cations and CO322 as interlayer anion [general formula: (Mg211
x Al31x (OH)2)(CO322x/n  mH2O) (0.2 # x # 0.33)] (Gao, 2012). The most common synthetic routes which have been used for the preparation of LDHs are coprecipitation, reconstruction, ion exchange, and hydrothermal method (Mallakpour and Hatami, 2017). It is noteworthy that recently, mechanochemical methods have received more attention from researchers to resolve the problems that remain with conventional solution methods such as high energy consumption, treatment of aqueous waste, complex operation, etc. (Qu et al., 2016) LDH is a favorable material for a broad range of practical applications such as catalysis, adsorption, pharmaceutics, photochemistry, electrochemistry, and other fields. This is due to its high versatility, easily tailored properties and low cost, which make it possible to produce materials designed to fulfill specific requirements (Li and Duan, 2006). In order for successful preparation of polymer NCs reinforced with lamellar NMs and to obtain the high efficiency of these NMs, rupture of the initial structure of lamellar NMs (exfoliation) or intercalation of polymer chains between their

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Layered Double Hydroxide Polymer Nanocomposites

layers (intercalation), as well as homogeneous distribution of them in the polymer matrix and good compatibility between two phases, are vital factors. On the other hand, it is not always possible to obtain complete exfoliated morphology and a microcomposite morphology can be observed too, in which the lamellar NM remains in its initial structure while mixed with the polymer matrix (Gao, 2012; Youssef, 2013). Several methods have been used to fabricate polymer/LDH NCs in which the most commonly applied are in situ polymerization, melt-mixing, and solution blending (Gao, 2012). This chapter focuses on the preparation, characterization, and application of NCs from most of the biopolymers reinforced with LDH.

15.2

Biopolymer/layered double hydroxide nanocomposites

15.2.1 Polysaccharide/layered double hydroxide nanocomposites Polysaccharides are renewable resource biopolymers and are a class of carbohydrate polymers consisting of multiple monosaccharide units which are linked together by glycoside linkage (Zafar et al., 2016). Due to possessing unique features such as more stability and usually not being irreversibly denatured on heating, polysaccharides are different from other agro-polymers. The diverse structures of polysaccharides, including linear or branched forms, a wide range of molecular weights from low to high, diverse polydispersities, existing in the forms of both monofunctional (
OH) or multifunctional (
OH,
COOH,
NH2), as well as specific properties such as water-soluble or insoluble properties, high level of chirality, environmentally safe, low toxicity, and nonimmunogenic, have attracted a great deal of attention for utilizing them in the production of NMs (as an organic NF) and NCs (as a matrix) (Zheng et al., 2015).

15.2.1.1 Cellulose/layered double hydroxide nanocomposites Cellulose, the most abundant and sustainable structure in nature, is obtained from cotton and wood pulp. It is considered as a renewable source polymer and believed to be an outstanding replacement for petroleum-based compounds. It is a β-1,4linked linear polymer of glucose units and is insoluble in water, dilute acidic solutions, and dilute alkaline solutions at normal temperatures (Chen, 2014). It has attracted considerable attention due to its unique properties such as biodegradability, biocompatibility, and chemical stability (Zafar et al., 2016). There are some reports for the incorporation of various types of LDH into both cellulose and modified cellulose (e.g., cellulose acetate, carboxymethylcellulose, oxidized cellulose, etc.), which have shown potential for applications such as adsorption and separation from solution and wastewater (biosorption for diclofenac and bovine serum albumin [BSA]- pollutant adsorption for fluoride, boron, and

Applications of layered double hydroxide biopolymer nanocomposites

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selenium separation of rare earth elements) (Beyki et al., 2017; Iftekhar et al., 2017; Latorre et al., 2013; Mandal and Mayadevi, 2008; Yan and Yi, 2013; Zhang et al., 2014), drug delivery (Barkhordari and Yadollahi, 2016), sensors and analytical applications (Lai et al., 2016), catalyst (for CC bond formation) (Mahdi et al., 2015), as well as important features for the packaging industry such as mechanical properties (Yadollahi and Farhoudian, 2015; Yadollahi et al., 2014), water vapor (Yadollahi et al., 2014) and gas barrier (Dou et al., 2014), swelling behavior (Yadollahi and Farhoudian, 2015; Yadollahi and Namazi, 2013; Yadollahi et al., 2015), and fire resistance (Ton-That et al., 2015). One of the most common and important effects of the incorporation of NFs into polymer matrix is improving the mechanical properties of the polymer. Yadollahi et al. (2014) investigated the mechanical properties, as well as water vapor permeability, of carboxymethyl cellulose/LDH NC films with an LDH concentration up to 8 wt.%. They synthesized Mg
Al-LDH by the coprecipitation method. Fabrication of Mg
Al-LDH was confirmed by observation of diffraction peak at 2θ value of 10.26 degrees in XRD spectrum, corresponding to a (003) spacing of pristine LDH (Fig. 15.3A). Carboxymethyl cellulose/LDH NCs (1, 3, 5, and 8 wt.%) were prepared by a casting/evaporation method. XRD patterns (Fig. 15.3A) of carboxymethyl cellulose/LDH NCs showed characteristic peaks of carboxymethyl cellulose (weak peak at 2θ B 11 degrees and a strong peak at 2θ B 20 degrees). When the LDH loading was 5 and 8 wt.%, the morphology of films was exfoliated/ intercalated due to a new broad peak appearing at 2θ values of 5 and 3.5 degrees. On the other hand, when the amount of LDH was below 5 wt.%, no diffraction peaks were observed around about 2θ 5 2 2 10 degrees, demonstrating the fully exfoliated structure. TEM images (Fig. 15.4) of carboxymethyl cellulose/LDH NCs revealed that the LDH sheets were well distributed in the carboxymethyl cellulose at LDH loadings below 8 wt.% and, when it was increased to 8 wt.%, the LDH tended to aggregate. According to the morphological analysis, the prepared carboxymethyl cellulose/LDH NCs were composed of highly exfoliated LDH layers, including the intercalated regions as well as agglomerated regions. The researchers investigated the water vapor permeability of carboxymethyl cellulose after incorporation with LDH. It was observed that the presence of LDH could decrease the water vapor permeability of carboxymethyl cellulose/LDH NC films and a reduction in the values of these parameters was observed with an increase of the LDH amount. It was decreased by 13%
37% depending on the LDH concentration. This observation is attributed to the concept of tortuous paths. In the presence of ordered and dispersed impermeable LDH layers the permeating water molecules are forced to follow longer and more tortuous pathways to diffuse through the carboxymethyl cellulose/LDH NC. They also observed the influence of incorporation of LDH into the carboxymethyl cellulose matrix on the mechanical properties of the final NCs. By incorporation of 3 wt.% LDH into the matrix, both tensile strength and tensile modulus were increased compared to the pure carboxymethyl cellulose, attributed to the strong interaction between LDH plates and polymer chains. However, by increasing the LDH content to 5 and 8 wt.%, both were decreased, due to the aggregation of LDH in the higher content. The elongation at break of the films was

Layered Double Hydroxide Polymer Nanocomposites

(A) CMC-LDH 1%

CMC-LDH 3%

Tensile strength (MPa)

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(B)

10 5 0 0

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3

4

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9

CMC-LDH 8%

CMC

Mg-Al-LDH 0 5 1015 202530 354045 5055 60 65 70

2θ (deg)

Tensile module (MPa)

CMC-LDH 5%

Elongation at break

Intensity (cps)

1200 1000 800 600

(C)

400 200 0

0

1

2

3

4

5

6

7

18 16 14 12 10 8 6 4 2 0

8

9

(D)

0

1

2

3

4

5

6

7

8

9

LDH content (Wt%)

Figure 15.3 (A) X-ray diffraction patterns of CMC film, Mg
Al-LDH, and CMC
LDH NC films and effect of LDH content (wt.%) on (B) tensile strength (C) tensile modulus, and (D) elongation at break of CMC-based films. CMC, carboxymethyl cellulose. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym., 108, 83
90, with kind permission of Elsevier.

decreased by incorporation and increasing the amount of LDH, which led to increasing the brittleness of the films (Fig. 15.3B
D). One of the most important factors for packaging applications, especially in the fields of food and drug packaging, is possessing high barrier properties for degradative gases (e.g., O2) and water. Although there are many benefits to polysaccharides being a matrix in polymer NCs for packaging applications, their performance is often limited by high gas permeability. To solve this problem, incorporation of LDH and other inorganic NFs into the polysaccharides has been proved to be an effective way of improving the barrier behavior through the extensive diffusion path for permeating molecules. One example in this area is the fabrication of cellulose acetate/LDHNC transparent and flexible films with a tremendous oxygen barrier, as reported by Dou et al. in 2014 (Dou et al., 2014). The films were prepared

Applications of layered double hydroxide biopolymer nanocomposites

607

Figure 15.4 TEM images of the CMC
LDH NC film with 3 wt.% LDH (A and B) and 8 wt.% LDH (C and D) at low and high magnifications, respectively. CMC, carboxymethyl cellulose. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym., 108, 83
90, with kind permission of Elsevier.

by spin-coating technique of cellulose acetate and LDH followed by thermal annealing. Compared to other oxygen barrier compounds, two factors including highly oriented nanoplatelets in the polymer matrix and hydrogen bonding network could cause coextensive diffusion length and strong resistance and finally lead to suppressing the oxygen permeability. Good dispersion of LDHs in cellulose acetate was successfully confirmed by XRD, SEM, and EDX. The absence of any nonbasal reflections (h, l 6¼ 0) compared with pure LDH demonstrated a preferred orientation of LDH platelets with the ab plane parallel to the substrate, which not only confirmed the uniform distribution of LDHs but also account for this high level of transparency. Furthermore, the SEM images of films showed the uniform layered architecture consisting of densely packed LDH nanoplatelets with good corientation. According to the EDX, the Mg, Al, and C were uniformly dispersed throughout the 2D-organized film. It is noteworthy that the ratio between metallic ions (divalent and trivalent) in the LDH can affect the reinforcement behavior of LDH for cellulose matrix and the thermal stability of the final NC. As an example, Mekdad et al. (2014) studied the effect of Mg/Al ratio and the rates of reinforcement for cellulose/hydrotalcite NCs. Hydrotalcite belongs to the family of LDHs. The authors synthesized hydrotalcite with two different ratio of Mg/Al, 2 and 3 (HT2 and HT3). Cellulose was extracted from yucca leaves and the NCs were prepared with 2, 5 and 10 wt.% of each NF. According to the XRD spectrum, the average sizes of the synthesized hydrotalcite particles confirmed the nanometric size of NFs (18.71 nm for HT2 and 19.70 nm

608

Layered Double Hydroxide Polymer Nanocomposites

for HT3, calculated by Debye
Scherer equation). The XRD spectra of cellulose showed peaks at 2θ 5 16, 22, and 34 degrees, where the first represented amorphous cellulose while the latter two were attributed to the crystalline shape of the cellulose. In terms of NCs, an extra wide stripe overlapped with the stripe (006) of hydrotalcite (2θ 5 20 degrees) in the XRD pattern of both NCs, corresponded to the most intense peak of microcrystalline cellulose. Furthermore, the disappearance of the peak corresponding to the amorphous shape of cellulose (2θ 5 16 degrees), indicated that the cellulose was in an ordered form in the NCs. A low intense peak at 2θ 5 29 degrees was observed for the NC of hydrotalcite with the ratio of Mg/Al 5 2 containing 5 and 10 wt.% hydrotalcite, while this peak was absent for the NC of hydrotalcite with the ratio of Mg/Al 5 3 (5 and 10 wt.%). This peak can be attributed to the formation of another phase when the load rate increased for the NC of HT2. This can be related to the load of the layer of the hydrotalcite, which was weaker in the case of HT3 due to the influence of the ratio of Mg/Al on the synthesis of composite materials. In addition to XRD, TEM observations (Fig. 15.5) were used for investigation of the morphology of the obtained NCs. Hydrotalcite NPs were well dispersed in the matrix, and the size of the majority of the clay particles was smaller than 100 nm. In addition, in the case of NCs of HT2, 2 wt.%, the particles were homogeneously dispersed and had a size of approximately 40
60 nm (Fig. 15.5C). The influences of the rate of reinforcement showed that the optima was 2% for the hydrotalcite of ratio of Mg/Al 5 2 while it was 5% for hydrotalcite of ratio of Mg/Al 5 3. Moreover, the hydrotalcite of ratio of Mg/Al 5 2

Figure 15.5 TEM micrographs of (A) MCC, (B) HT3, (C) MCC-HT2.2%, and (D) MCCHT3.5% NCs. MMC, microcrystalline cellulose; HT2 and HT3, hydrotalcite with ratios of Mg/Al 5 2 and 3.

Applications of layered double hydroxide biopolymer nanocomposites

609

[comparison between NC of HT2 (5%) and NC of HT3 (5%)] gave a more thermally stable NC. Natural cellulose is hydrophilic and therefore it absorbs moisture which can be indicated by its low water contact angle (20
30 degrees). This is despite the fact that hydrophobic cellulose materials are always in high demand in applications such as water-proof packaging, thin films, paper, sorbents, sanitation, fabrics, etc. There are several reports of modifying cellulose (e.g., paper) surfaces while there is little research on hydrophobic materials derived from cellulose fibers. Interestingly, Sobhana et al. (2017) used LDH as first-of-its-kind material in order to meet the challenges in cellulose-based super-hydrophobic materials. They hydrophobized cellulose by environmentally benign stearic acid with the aid of inorganic linker/ interface/sandwich material, LDH, which had layers linked with hydrophobic stearic acid and hydrophilic cellulose simultaneously (Fig. 15.6). Indeed, there was no direct attachment between hydrophobizer and cellulose but LDH created this conjugation. Cellulose conjugated with LDH through its hydrophilic layers, while the negatively charged polar head of stearic acid was attracted toward the positively charged brucite layers of LDH through an electrostatic interaction.

Figure 15.6 Schematic illustrations for the preparation of the super-hydrophobic SA/LDH/ cellulose NC. SA, stearic acid. Source: Adapted from Sobhana, S.L., Zhang, X., Kesavan, L., Liias, P., Fardim, P., 2017. Layered double hydroxide interfaced stearic acid 2 cellulose fibres: a new class of superhydrophobic hybrid materials. Colloids Surfaces A: Physicochem. Eng. Aspects, with kind permission of Elsevier.

610

Layered Double Hydroxide Polymer Nanocomposites

Fig. 15.7 shows SEM images of pristine cellulose fibers, cellulose fibers with stearic acid, and stearic acid/LDH/cellulose hybrid fibers with different metal precursors and LDH concentrations. The pristine cellulose fibers showed a wellpacked structure due to the compactness of fibers (Fig. 15.7A), while stearic acid/ LDH/cellulose hybrid fibers were much looser than they should have been

Figure 15.7 SEM images of the fibers: (A) pristine cellulose fibers, (B) cellulose fibers with stearic acid (0.02 M); SA-LDH/cellulose hybrid fibers at different metal precursor and stearic acid concentrations under static conditions (C) 500 mM M21:M31/0.02 M SA, (D) 100 mM M21:M31/0.02 M SA, (E) 100 mM M21:M31/0.002 M SA, (F) 100 mM M21:M31/0.001 M SA, (G) 50 mM M21:M31/0.002 M SA, (H) SA/LDH/cellulose fibers prepared under shaking conditions. SA, stearic acid. Source: Adapted from Sobhana, S.L., Zhang, X., Kesavan, L., Liias, P., Fardim, P., 2017. Layered double hydroxide interfaced stearic acid 2 cellulose fibres: a new class of superhydrophobic hybrid materials. Colloids Surfaces A: Physicochem. Eng. Aspects, with kind permission of Elsevier.

Applications of layered double hydroxide biopolymer nanocomposites

611

(Fig. 15.7C). Furthermore, when the metal salts used for making the LDH were highly concentrated, the quantitative making of LDH particles was very high, which richly covered the fiber surface and consequently resulted in loosening the fiber to fiber interaction. In all these cases, the dimensions of the LDH platelets remained uniform. The utilizing of LDH as an interface/linker/sandwich material in these hybrids was caused by increasing the hydrophobicity of cellulose, even in its wet and maiden condition, as well as decreasing the processing time of about 5 days in making hydrophobic cellulose material. It was also made possible using even a minimal amount of stearic acid, which could lead to great hydrophobicity. The hybrid of capsulated LDH in carboxymethyl cellulose was used for cephalexin (as a drug model) oral delivery in gastrointestinal tract conditions (Barkhordari and Yadollahi, 2016). For this purpose, cephalexin was intercalated between LDH layers through the coprecipitation route and the as-obtained LDHcephalexin nanohybrid was capsulated in pH-sensitive carboxymethyl cellulose hydrogel beads. The results showed that carboxymethyl cellulose/LDH-cephalexin NC beads had a pH-sensitive swelling behavior, which increased when the pH was increased. Furthermore, an in vitro release study of cephalexin in conditions that simulate the passage through the gastrointestinal tract, showed better protection against drug release for carboxymethyl cellulose/LDH-cephalexin beads compared to LDH-cephalexin at the stomach pH and a controlled release in the intestinal tract conditions due to the pH-sensitive swelling behavior of carboxymethyl cellulose. It was observed that drug release was decreased at acidic pH, attributed to the shrinkage of carboxymethyl cellulose at acidic pH, so the drug was protected against digestion within the stomach. The schematic illustration of cephalexin release from carboxymethyl cellulose/LDH-cephalexin beads is depicted in Fig. 15.8. The shapes of carboxymethyl cellulose/LDH-cephalexin beads and their SEM images are shown in Fig. 15.9. The large size of the wet beads confirmed their ability for great swelling and water retention. On the other hand, the appearance of LDH-cephalexin nanohybrids on the surface of beads proved a good interaction between LDH-cephalexin and carboxymethyl cellulose, resulting to good dispersion of them in the matrix.

Figure 15.8 Cephalexin release from CMC/LDH-CPX NC bead. CMC, carboxymethyl cellulose; CPX, cephalexin. Source: Adapted from Barkhordari, S., Yadollahi, M., 2016. Carboxymethyl cellulose capsulated layered double hydroxides/drug nanohybrids for Cephalexin oral delivery. Appl. Clay Sci., 121, 77
85, with kind permission of Elsevier.

612

Layered Double Hydroxide Polymer Nanocomposites

Figure 15.9 Digitals photo of CMC/LDH-CPX NC bead at (A) wet and (B) dry state; SEM micrographs of CMC/LDH-CPX NC beads at (A) low magnification ( 3 10,000) and (B) high magnification ( 3 70,000). CMC, carboxymethyl cellulose; CPX, cephalexin. Source: Adapted from Barkhordari, S., Yadollahi, M., 2016. Carboxymethyl cellulose capsulated layered double hydroxides/drug nanohybrids for Cephalexin oral delivery. Appl. Clay Sci., 121, 77
85, with kind permission of Elsevier.

15.2.1.2 Starch/layered double hydroxide nanocomposites Starch is one of the most widely available and inexpensive biopolymers. It is arranged in individual particles called granules which are partly crystalline regions consisting mostly of two homopolymers of glucopyranose: amylose, a linear polymer with α-(1 ! 4)-linked glucose units, while amylopectin is a highly branched polymer consisting of α-(1 ! 4)-linked glucose units with branches made by α-(1 ! 6) linkages at the branch points (Bertolini, 2009). Starch needs to be plasticized to obtain a thermoplastic material. Different plasticizers can be used, including sorbitol, maltose, xylitol, glucose, ethylene-bis-formamide, and glycerol (Mikus et al., 2014). There are many reports on the preparation of starch NCs incorporated with inorganic NFs, such as clay (Perotti et al., 2017), carbon nanotubes (Cheng et al., 2013), metal oxides (Ma et al., 2016), etc. Also, starch has been successfully reinforced with LDH (Chung and Lai, 2010; Privas et al., 2013; Wu et al., 2011). Starch/LDH NCs can be prepared by various methods. For example, Chung and Lai (2010) synthesized LDH in acid-modified corn starch (AMS) dispersion in a direct manner. This technique involves a rapid LDH nuclei precipitation followed by a

Applications of layered double hydroxide biopolymer nanocomposites

613

Figure 15.10 (A, B) TEM images of starch/LDH NCs and (C
E) schematic representations of LDH dispersion in starch matrix. (A, C) NCS4 and (B, D) AMS4. NCS, native corn starch; AMS, acid-modified starch. Source: Adapted from Chung, Y.-L., Lai, H.-M., 2010. Preparation and properties of biodegradable starch-layered double hydroxide nanocomposites. Carbohyd. Polym., 80(2), 525
532, with kind permission of Elsevier.

hydrothermal treatment that simultaneously leaches starches from the granules and grows the LDH nuclei. They compared the influence of using AMS and native corn starch (NCS) as matrices on the mechanical properties, opacity, and moisture adsorption of the final NCs. TEM images (Fig. 15.10) of NCs revealed good distribution of LDH in AMS while aggregation was observed in terms of NCS. This is due to the lower viscosity of modified starch which not only facilitated the distribution of LDH in the matrix but also improved the modulus of NCs without sacrificing their transparency and moisture sensitivity. Investigation of the mechanical properties revealed that when the LDH was absent, the Young’s modulus and tensile strength of the NCS were larger compared to the AMS, while the elongation at break was reduced. This was due to the lower molecular weight of starch as well as increasing the linear short chains after acid hydrolysis which leads to generating a softer material. As can be seen in Table 15.1, the tensile strength and elongation at break of NCS-based NCs decreased by increasing the amount of LDH (NCS1 . NCS2 . NCS3 . NCS4). This was due to the phase separation of NCs with a high LDH content which might destroy the integrity of the NCS-based NC structure. For AMS-based NCs, the Young’s modulus increased by increasing the amount of LDH (AMS4 . AMS3 . AMS2 . AMS1) (Table 15.1). This was due

614

Layered Double Hydroxide Polymer Nanocomposites

Table 15.1 Mechanical properties of NCS
LDH and AMS
LDH NCs Sample

Young’s modulus (MPa)

NCS1 NCS2 NCS3 NCS4 AMS1 AMS2 AMS3 AMS4

2805 2519 2734 2841 2424 2701 2734 3316

6 305a 6 258b 6 267a 6 177a 6 1 68a 6 152b 6 259b 6 363c

Tensile strength (MPa) 35.73 31.13 30.43 27.51 31.12 33.89 32.51 31.85

6 6 6 6 6 6 6 6

3.92a 2.25b 2.74b 2.84c 2.48a 2.32a 3.50a 3.30a

Elongation at break (%) 2.77 3.14 2.58 1.80 3.55 3.52 3.53 1.81

6 6 6 6 6 6 6 6

0.73a 1.38a 0.95a 0.66b 0.82a 0.99a 1.10a 0.54b

The samples were conditioned at 53% RH before measurement NCS, native corn starch; AMS, acid-modified starch. a,b,c Values followed by the different letters are significantly different at p , 0.05 within NCS
LDH and AMS
LDH nanocomposites Source: Adapted from Chung, Y.-L., Lai, H.-M., 2010. Preparation and properties of biodegradable starch-layered double hydroxide nanocomposites. Carbohyd. Polym., 80(2), 525
532, with kind permission of Elsevier.

to increasing the stiffness as well as decreasing the phase separation of AMS matrix by incorporation of LDH. This is despite the fact that the tensile strength of AMSbased NCs was not enhanced by increasing the amount of LDH. This indicated that the incorporation of LDH, which increased the stiffness of the matrix at a low strain, did not influence its stiffness at a high strain. Dropping the elongation at break of AMS-based NCs at higher LDH amount (AMS4) was attributed to the formation of a higher amount of aggregated LDH. Investigation of the opacity of NCs revealed that the opacity of NCS and AMS samples without LDH was similar. By increasing the loading of LDH, the opacity of NCS-based NCs increased significantly. However, the AMS-based NCs were uniformly translucent between 0% (AMS1) and 10% (AMS4) of LDH loadings. This suggested that the dispersion and miscibility of the nanophases in AMS were much better than those in the NCS matrix. The moisture content of samples slowly increased with an increase in the equilibrium relative humidity up to 70%. Above that a steep rise in moisture content was observed. The moisture content of NCs did not significantly change when LDH was added to both the NCS and AMS systems. This revealed that the water adsorption of starch/LDH NCs during storage is dominated by the starch matrix because of the relatively strong hydrophilicity of starch molecules. Wu et al. (2011) modified the LDH with modified polysaccharide, carboxymethyl cellulose via encapsulation of LDH in polysaccharide for stabilization of LDH in water. The prepared LDH-carboxymethyl cellulose NFs (with 37.3 wt.% of carboxymethyl cellulose) were then incorporated into the matrix of glycerol-plasticized starch for the fabrication of NCs. As can be seen in the TEM image (Fig. 15.11) of LDHcarboxymethyl cellulose, LDH platelets were encapsulated into carboxymethyl cellulose with smaller size (layer number) of each LDH stack, which helped the uniform distribution of LDH-carboxymethyl cellulose NFs in glycerol-plasticized starch matrix. SEM images of LDH-carboxymethyl cellulose/glycerol-plasticized starch 2 wt.% NCs (Fig. 15.12A,B) indicated the good distribution of prepared NFs in the matrix

Applications of layered double hydroxide biopolymer nanocomposites

Figure 15.11 TEM image of LDH
CMC. The thickness of platelets is shown with white circles. CMC, Carboxymethyl cellulose. Source: Adapted from Wu, D., Chang, P.R., Ma, X., 2011. Preparation and properties of layered double hydroxide
carboxymethylcellulose sodium/glycerol plasticized starch nanocomposites. Carbohyd. Polym., 86(2), 877
882, with kind permission of Elsevier.

Figure 15.12 SEM micrograph of the fragile fractured surface of NCs with different LDH
CMC contents (A, B) 2 wt.% and (C, D) 8 wt.%. CMC, carboxymethyl cellulose. Source: Adapted from Wu, D., Chang, P.R., Ma, X., 2011. Preparation and properties of layered double hydroxide
carboxymethylcellulose sodium/glycerol plasticized starch nanocomposites. Carbohyd. Polym., 86(2), 877
882, with kind permission of Elsevier.

615

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Layered Double Hydroxide Polymer Nanocomposites

due to the improved hydrophilic property of LDH after stabilization, decreasing the size of LDH platelets after cellulose introduction and strong interaction between two polysaccharides, starch and carboxymethyl cellulose. Furthermore, low loading of NFs (6 wt.%) led to improvement in mechanical properties (an increase in the tensile strength and elongation) and water resistance (decrease in the water vapor permeability) compared to the pure glycerol-plasticized starch. There are two reasons for this improvement: (1) because polysaccharides could form complexes with metal ions due to the high number of coordinating functional groups (hydroxyl and glucoside groups), strong associations between the metal ions and the carboxymethyl cellulose occurred for the nucleation and initial crystal growth of the LDH, and thus the LDH was successfully encapsulated by the carboxymethyl cellulose and (2) the hydrophilic carboxymethyl cellulose component and the smaller size of each LDH stack allowed the LDH to be well dispersed in the starch matrix and good interactions between the NF and the matrix were formed because of the carboxymethyl cellulose component (Ramawat and Me´rillon, 2015). However, the bio-NCs displayed a decrease in the thermal decomposition temperature because the weak thermal stability of the carboxymethyl cellulose could weaken the interactions between the LDH filler and the starch matrix and facilitate the decomposition of the starch. Furthermore, a high LDH content (8 wt.%) could result in the agglomeration of the NF in the matrix and thus reduce the mechanical properties and water vapor permeability (Figs. 15.12C,D and 15.13). Privas et al. (2013) modified LDH with organic compound, lignosulfonate, and employed this filler (LDH-LS) with a concentration of 1 up to 4 wt.% for incorporation into thermoplastic corn starch (TCS) as matrix. Incorporation of LDH/LS in starch was done using LDH/LS slurry instead of powder in order to avoid secondary particle aggregation. This reinforced starch was used for preparing a

Figure 15.13 (Left) The effect of LDH
CMC contents on tensile yield strength and elongation at break of the composites. (Right) The effect of LDH
CMC contents on water vapor permeability of the composites. CMC, carboxymethyl cellulose; WVP, water vapor permeability. Source: Adapted from Wu, D., Chang, P.R., Ma, X., 2011. Preparation and properties of layered double hydroxide
carboxymethylcellulose sodium/glycerol plasticized starch nanocomposites. Carbohyd. Polym., 86(2), 877
882, with kind permission of Elsevier.

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617

Figure 15.14 Transmission electron microscopy images of thermoplastic corn starch mixtures containing 1 wt.% of LDH/LS at different magnifications. LS, lignosulfonate. Source: Adapted from Privas, E., Leroux, F., Navard, P., 2013. Preparation and properties of blends composed of lignosulfonated layered double hydroxide/plasticized starch and thermoplastics. Carbohyd. Polym., 96(1), 91
100, with kind permission of Elsevier. Table 15.2 Tensile properties of TCS and LDH/LS
TCS NC Sample

Young’s modulus (MPa)

Stress at break (MPa)

Elongation at break (%)

TCS LDH/LS 1%
TCS LDH/LS 2%
TCS LDH/LS 4%
TCS

7.3 (1.0) 10.2 (2.3) 2.3 (0.4) 1.5 (0.1)

2.7 (0.2) 3.2 (0.1) 1.7 (0.1) 1.4 (0.1)

61 (5) 100 (7) 101 (2) 104 (6)

TCS, thermoplastic corn starch; LS, lignosulfonate. Source: Adapted from Privas, E., Leroux, F., Navard, P., 2013. Preparation and properties of blends composed of lignosulfonated layered double hydroxide/plasticized starch and thermoplastics. Carbohyd. Polym., 96(1), 91
100, with kind permission of Elsevier.

starch
polyethylene-based polymer blend composite. For this purpose, LDH/ LS
starch NCs were mixed with Lotader 3210 [a random terpolymer of ethylene, butyl acrylate (6%) and maleic anhydride (3%)] at concentrations of 20 and 40 wt. %. Finally, the researchers evaluated the interest of composites, mainly considering gas barrier and mechanical properties. To characterize the dispersion of LDH/LS in TCS, XRD and TEM (Fig. 15.14) analysis were employed. TEM images showed a uniform distribution of LDH/LS NFs into the TCS for all NF loading. The dimensions of the LDH/LS particles (70 nm length and B3 nm) proved an exfoliated morphology. Table 15.2 shows the mechanical properties of TCS and LDH-LS/TCS NCs. Mechanical properties of LDH-LS/TCS are shown in Table 15.2. As can be

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Layered Double Hydroxide Polymer Nanocomposites

seen, starch with 1 wt.% LDH/LS showed an interesting property improvement, while higher LDH-LS concentrations exhibited degraded properties. One reason could be that the addition of hydrophilic NF enabled the building of strong polar interactions with water which may help to retain water molecules, leading to a more plasticized material. Lignosulfonate is already known to plasticize starch materials. After melt processing, thermoplastic starch undergoes retrogradation, which provides crystallization and reduces the elongation at break. Introduction of NF may stop or reduce this recrystallization process, a phenomenon already observed with starch
clay NC. Similar to the mechanical properties, when thermoplastic starch was filled with a small amount of LDH/LS, as low as 1 wt.%, an increase of oxygen and water barrier properties was observed. Lotader/(LDH-LS/ TCS) with two composition ratios 80/20 and 60/40 by weight was prepared and the mechanical properties of these materials, as well as pure Lotader and unfilled TCS, were examined (Table 15.3). As can be seen, the most interesting blends were with 20 wt.% of starch-based content. Tensile strength is around 76.9 MPa for Lotader/ (LDH/LS 1%
TCS), much higher than for Lotader/TCS (38.7 MPa). Tensile strength and elongation at break for Lotader80%/(LDH/LS
TCS) are similar to

Table 15.3 Tensile properties of Lotader, TCS, and Lotader/TCS with 0, 1, 2, and 4 wt.% LDH/LS materials Sample

Young’s modulus (MPa)

Tensile strength (MPa)

Elongation at break (%)

41.3 (1.8) 7.3 (1.0)

80.0 (2.8) 2.7 (0.2)

229 (3) 61 (5)

27.5 (0.5) 31.5 (0.8) 32.1 (1.4) 32.3 (1.3)

38.7 (4.8) 76.9 (7.6) 77.6 (9.3) 74.3 (8.2)

183 (11) 221 (7) 214 (8) 202 (8)

45.5 (2.7) 38.9 (1.2) 39.3 (1.6) 42.1 (3.4)

16.8 (0.9) 20.6 (2.8) 21.2 (8.1) 24.4 (4.8)

91 (8) 106 (13) 104 (32) 112 (14)

References Lotader 3210 TCS

80/20 Lotader/TCS Lotader/(LDH/LS 1%
TCS) Lotader/(LDH/LS 2%
TCS) Lotader/(LDH/LS 4%
TCS)

60/40 Lotader/TCS Lotader/(LDH/LS 1%
TCS) Lotader/(LDH/LS 2%
TCS) Lotader/(LDH/LS 4%
TCS)

TCS, thermoplastic corn starch; LS, lignosulfonate. Source: Adapted from Privas, E., Leroux, F., Navard, P., 2013. Preparation and properties of blends composed of lignosulfonated layered double hydroxide/plasticized starch and thermoplastics. Carbohyd. Polym., 96(1), 91
100, with kind permission of Elsevier.

Applications of layered double hydroxide biopolymer nanocomposites

619

pure Lotader. The improvement in tensile strength can be attributed to the formation of a three-dimensional network reinforcing the blend through a percolation mechanism Wilhelm et al. (2003) investigated the influence of various types of natural [kaolinite (neutral mineral clay) and hectorite (cationic exchanger mineral clay)] and synthetic [LDH (anionic exchanger) and brucite (neutral)] layered NFs on the properties of various types of starch (native starch, oxidized starch, glycerol-plasticized starch, and oxidized plasticized starch). The starch/hectorite proportions were 100/ 0, 95/05, 90/10, 85/15, 80/20, and 70/30, relative to starch mass, while the composite films of starch with other layered compounds were prepared only in 90/10 proportions. The composite films were prepared from the respective aqueous suspensions by casting. The effects of the filler type and the plasticizer were analyzed by XRD, dynamic mechanical analysis, and thermogravimetric analysis (TGA). The XRD results of plasticized starch composites revealed that only hectorite showed an increase in the amount of interplanar basal distance, which was attributed to the intercalation of means that kaolinite, LDH, and brucite were not influenced when the starch matrix was present. Substitution of plasticized starch matrix by a plasticized oxidized starch or native/oxidized starch blend gave rise to composites with higher interplanar basal distances, indicating that both short oxidized starch chains and glycerol molecules could be intercalated between clay layers. In the absence of glycerol, oxidized starch was preferentially intercalated in relation to native starch chains due its lower chain size and probable higher diffusion rate.

15.2.1.3 Chitosan/layered double hydroxide nanocomposites Chitosan is a cationic, biodegradable, biocompatible, eco-friendly, and inexpensive polysaccharide which is obtained by partial deacetylation of chitin (Zafar et al., 2016). It is composed of β-(1-4)-linked glucosamine units together with some N-acetyl-D-glucosamine units (Chen et al., 2017; Kim, 2010). Chitosan has been successfully utilized as a matrix for the preparation of organic
inorganic hybrid materials and there are some reports for the preparation of chitosan/LDH NCs where the matrix is neat chitosan (Darder et al., 2008; Depan and Singh, 2010), modified chitosan (Wang and Zhang, 2014), or a blend of chitosan with other biopolymers such as alginate and PVA (Ribeiro et al., 2014). Due to having multifunctional groups, chitosan can be used as a biosorbent with high adsorption capacity and sorbent for the removal of pollutants from wastewater. Elanchezhiyan and Meenakshi (2017) produced a chitosan/LDH NC by coprecipitation technique and used it as an adsorbent for the removal of oil particles from an oil in water emulsion. These NCs showed improved adsorption efficiency for oil adsorption at acidic PH compared to raw LDH or chitosan. This observation was a result of a high amount of LDH in chitosan, which facilitated the immobilization of oily particles. As can be seen in the SEM of NCs and oil-adsorbed NCs in Fig. 15.15, a rough coral reef heterogeneous structure of NCs improved the diffusion of oil particles onto the surface of the adsorbent and consequently enhanced adsorption efficiency.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 15.15 SEM micrograph of (A) CS-LDHCs with respective mapping images and (B) oil sorbed CS-LDHCs with respective mapping images. CS, chitosan; Cs, composite. Source: Adapted from Elanchezhiyan, S.S., Meenakshi, S., 2017. Synthesis and characterization of chitosan/Mg-Al layered double hydroxide composite for the removal of oil particles from oilin-water emulsion. Int. J. Biol. Macromol., with kind permission of Elsevier.

On the other hand, due to the occupation of oil molecules on the adsorbent surface, the coral reef structure was destroyed and a smooth heterogeneous structure was observed after oil adsorption. The mechanism for the adsorption of oil on NCs was predominantly controlled by a hydrophobic
hydrophobic interaction as well as physical forces. According to the thermodynamic results, the nature of adsorption was spontaneous and endothermic. Due to its biocompatibility and biodegradability, chitosan has displayed an outstanding potential in a variety of biomedical applications such as drug delivery and tissue engineering (Ahmed and Ikram, 2016; Gohil et al., 2016; Jayasuriya, 2016; Moura et al., 2016; Pathania et al., 2016; Rijal et al., 2016; Ryu et al., 2015; SaberSamandari and Saber-Samandari, 2017; Sionkowska, 2016; Vunain et al., 2016; Zare and Shabani, 2016; Zhang et al., 2017a). It is noteworthy that most of the applications of chitosan/LDH NCs are around biomedical applications too (Chen et al., 2017; Chi et al., 2017; Qin et al., 2015; Rezvani and Shahbaei, 2015; Ribeiro et al., 2014; Wei et al., 2012; Wei et al., 2015; Zhao et al., 2015). In 2017, Chen et al. (2017) reported the self-assembly preparation of the pifithrin-α-LDH/chitosan nanohybrid composites as drug-delivery systems for stem cell osteogenic differentiation. As can be seen in SEM and TEM images of NCs (Fig. 15.16), the LDH nanoplates self-assembled into a flower-like shape and the chitosan was covered uniformly by the nanoplates which increased the drug loading efficiency and drug release property as compared with the pure LDH (Fig. 15.17).

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Figure 15.16 (A) SEM image, (B)TEM image, (C) EDS spectrum, and (D) ED pattern of PFTα-LDH/CS nanohybrid composites. CS, chitosan; PFTα, pifithrin-α. Source: Adapted from Chen, Y.-X., Zhu, R., Xu, Z.-L., Ke, Q.-F., Zhang, C.-Q., Guo, Y.-P., 2017. Self-assembly of pifithrin-α-loaded layered double hydroxide/chitosan nanohybrid composites as a drug delivery system for bone repair materials. J. Mater. Chem. B, 5(12), 2245
2253, with kind permission of the Royal Society of Chemistry.

Figure 15.17 Schematic illustration of sustained release of PFTα from PFTα-LDH/CS. CS, chitosan; PFTα, pifithrin-α. Source: Adapted from Chen, Y.-X., Zhu, R., Xu, Z.-l., Ke, Q.-F., Zhang, C.-Q., & Guo, Y.-P. (2017). Self-assembly of pifithrin-α-loaded layered double hydroxide/chitosan nanohybrid composites as a drug delivery system for bone repair materials. Journal of Materials Chemistry B, 5(12), 2245
2253, with kind permission of the Royal Society of Chemistry.

The mesopores throughout the LDH nanoplates acted as channels for loading pifithrin-α. The results revealed that the combination design of the pifithrinα/LDH/chitosan nanohybrid composites created a reserving method for bone tissue regeneration. It is noteworthy that chitosan/LDH NCs have been employed in other applications such as preparation of biosensors (Ai et al., 2008; Ding et al., 2011; Han et al., 2007), food packaging due to possessing the remarkable oxygen barrier

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properties (Pan et al., 2015), and catalyst (Adwani et al., 2015). Adwani et al. (2015) synthesized bio-NCs of chitosan and LDH (hydrotalcite) by the coprecipitation method. The prepared NCs showed efficient catalytic activity for selective synthesis of jasminaldehyde by aldol condensation of benzaldehyde and 1-heptanal (Fig. 15.18). The characterization of the catalyst was carried out by N2 sorption, FT-IR, TGA, XRD, SEM, and TEM. The BET surface area of NCs was found to be 3.77 m2/g. Fig. 15.19 shows the XRD pattern of hydrotalcite, chitosan, and the final NC. The sharp, intense, and symmetric peaks at lower diffraction angles (2θ 5 10
25 degrees) and a broad asymmetric reflection at higher diffraction angles (2θ 5 30
50 degrees) in the XRD pattern of hydrotalcite, are characteristic of a

Figure 15.18 Schematic illustration of synthesis of jasminaldehyde. CMA, chitosan/Mg/Al hydrotalcite bio-NC. Source: Adapted from Adwani, J.H., Noor-ul, H.K., Shukla, R.S., 2015. An elegant synthesis of chitosan grafted hydrotalcite nano-bio composite material and its effective catalysis for solvent-free synthesis of jasminaldehyde. RSC Adv., 5(115), 94562
94570, with kind permission of the Royal Society of Chemistry.

Figure 15.19 XRD pattern of HT, CT, and CMA. HT, hydrotalcite; CT, chitosan; CMA, chitosan/Mg/Al hydrotalcite bio-NC. Source: Adapted from Adwani, J.H., Noor-ul, H.K., Shukla, R.S., 2015. An elegant synthesis of chitosan grafted hydrotalcite nano-bio composite material and its effective catalysis for solvent-free synthesis of jasminaldehyde. RSC Adv., 5(115), 94562
94570, with kind permission of the Royal Society of Chemistry.

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highly crystalline layered structure. In the XRD pattern of NC, characteristic peaks of semicrystalline chitosan (broad peaks at 2θ 5 10 and 21 degrees) were present and no significant change in the characteristic planes of hydrotalcite was observed, indicating retention of the structure of hydrotalcite. However, after the formation of the NC, a slight shift towards lower 2θ values for peaks corresponding to (003) and (006) planes was observed, which was due to a concurrent decrease in the positive charge of the layers of hydrotalcite. The XRD pattern was also indicative of a slight intercalation of the biopolymer into the LDH layer as observed by a marginal shift of (001) peak relative to parent LDH along with the retention of both phases (chitosan and hydrotalcite). The TEM of the NC showed that the polymer was grafted over LDH material (Fig. 15.20A). Fig. 15.20B reveals the presence of an interplanar distance of approximately 1.4 nm, indicating some exfoliation of the LDH material. The inset in Fig. 15.20B shows presence of single LDH layers in the composite sample. Fig. 15.20C shows the presence of the small particles of the layered structure whilst Fig. 15.20D shows the lattice fringes of the LDH material. The inset in Fig. 15.20D shows the electron diffraction pattern of the composite. It confirms the presence of pure composite material. The investigations were performed in detail as a function of amount of the catalyst, temperature, and molar ratio of 1-heptanal to

Figure 15.20 TEM images of CMA. CMA, chitosan/Mg/Al hydrotalcite bio-NC. Source: Adapted from Adwani, J.H., Noor-ul, H.K., Shukla, R.S., 2015. An elegant synthesis of chitosan grafted hydrotalcite nano-bio composite material and its effective catalysis for solvent-free synthesis of jasminaldehyde. RSC Adv., 5(115), 94562
94570, with kind permission of the Royal Society of Chemistry.

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benzaldehyde to observe the effect of these reaction parameters on the conversion, selectivity, and rate of the formation of the condensation products. All these parameters were found to influence the performance of the catalyst. On increasing the catalyst amount, the rate of formation of jasminaldehyde was effectively increased, while that of 2-pentyl-2-nonenal, increased first and then showed a decreasing trend. On increasing the temperature, the rate of formation of jasminaldehyde increased while the rate of formation of 2-pentyl-2-nonenal tended to decrease. The initial rates of the formation of jasminaldehyde were found to be more, while the rate of formation of 2-pentyl-2-nonenal was less with NC as compared to the individual catalysts, chitosan and hydrotalcite. The catalyst was operative under solvent-free conditions, giving the highest selectivity to jasminaldehyde 89%, with .99% conversion with 100 mg catalyst at 160 C under optimized reaction conditions. Furthermore, the catalyst could be elegantly separated and found to be effective for recycling six times without any substantial loss in its activity.

15.2.1.4 Alginate/layered double hydroxide nanocomposites Alginate is a natural anionic polysaccharide usually extracted from brown seaweed and also some special bacteria can synthesize it. It is a water-soluble, linear copolymer of guluronic acid and mannuronic acid which are presented irregularly (Darder et al., 2008). The ratio and pattern of these blocks is mainly governed by the origin of alginate. Biocompatibility, biodegradability, low toxicity, low cost, and mild and easy gelation are promising properties which are made possible with the employing of alginate and also its nanohybrid composites in a wide range of applications such as biotechnology (immobilization of biocatalysts) (Cheryl-Low et al., 2015; Dekamin et al., 2016; Saha et al., 2009), biomedical (wound healing, tissue engineering, drug delivery, and controlled release of encapsulated substrates) (Guarino et al., 2015; Lee and Mooney, 2012; Sood et al., 2016), food (thickeners, viscosifiers, emulsifiers, stabilizers and gel-formers, film-formers, or water-binding agents) (Bierhalz et al., 2012; Brownlee et al., 2009), and environmental engineering (biosorbent for removal of heavy metals or other pollutants) (Bertagnolli et al., 2015; YuLin et al., 2010). There are some reports for the preparation (Landman and Focke, 2006) of alginate/LDH NCs and application of them for catalytic performance (YanChun and Chen, 2014; Zhao et al., 2011), sensing (Darder et al., 2005; Lopez et al., 2010; Shou-Nian et al., 2009), drug delivery (Alcantara et al., 2010; Mahkam et al., 2013; Rezvani and Shahbaei, 2015), and removal of water pollutants such as phosphates, dyes, and heavy metal ions (Kim Phuong, 2014; Lee et al., 2012; Lee et al., 2013; Phuong, 2014; Sebastian et al., 2014) due to the ability of both alginate and LDH in these areas. Furthermore, Wang et al. (2010) reported alginate assistance for LDH assembling. The catalytic activity of alginate/LDH NCs was reported by YanChun and Chen (2014) for the oxidation of α-pinene (Fig. 15.21). Microwavecrystallization and low saturated state of coprecipitation were employed for the intercalation of sodium alginate into a series of binary Mg-Al, Cu-Al, and Zn-Al

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Figure 15.21 The reaction of α-pinene oxidation.

Figure 15.22 SEM images of Zn-FLDH/SA (A) and Zn-CFLDH/SA (B). FLDH, flaky layered double hydroxides; CFLDH, calcined flaky layered double hydroxides.

flaky LDH. Then the NCs were calcined and employed for the catalytic oxidation of α-pinene using 30 wt.% hydrogen peroxide as oxidant. SEM images of both Znflaky LDH/sodium alginate NC and the calcined one showed petal-like structures with many cavities (Fig. 15.22). The observation of interleaved flexible curled nanosheets was attributed to the extra expansion of LDH as a result of large ions of inserted sodium alginate between its sheets. Therefore, by such a huge increase in the interlayer spacing of LDH, the layers completely disappeared and the LDHs appeared as monolayers, which then created Zn-flaky LDH/sodium alginate nanoflakes. The calicined type of Zn-flaky LDH/sodium alginate NC maintained the flake-like morphology but more cavities were observed which were attributed to peeling out the water during the calcination procedure. According to the results, the calcined Zn-flaky LDH/sodium alginate NC had better catalytic performance for the oxidation of α-pinene compared with the other method. By using this catalyst, α-pinene conversion and the selectivity of α-pinene epoxide, verbenol and verbenone could reach 69.6% and 29.1%, 39.6% and 12.0%, respectively. Due to the order of catalytic activity (calcined Zn-flaky LDH/sodium alginate . Zn-flaky LDH/sodium alginate . calcined Zn-flaky LDH . Zn-flaky LDH) it is concluded that the existence of sodium alginate has the main role in α-pinene oxidation. Use as an adsorbent is another example of the application of alginate/LDH NC. Sebastian et al. (2014) prepared alginate/Mg-Al LDH NCs (with 3, 5.9, 11, and 20 wt.% alginate concentrations) by in situ coprecipitation method and used them for the removal of anionic dyes (Acid Green 25 and Acid Green 27) from water. XRD (Fig. 15.23) and SEM (Fig. 15.24) were used to characterize the alginate/MgAl LDH NCs. As can be seen, the XRD pattern of MgAl-Alg (20%) NC showed a

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Figure 15.23 X-ray diffraction patterns of MgAl LDH and MgAl-Alg composites with varying amount of sodium alginate. Alg, sodium alginate.

Figure 15.24 SEM images of MgAl, sodium alginate and their composites. (A) MgAl LDH; (B) sodium alginate, (C) MgAl-Alg (3%), (D) MgAl-Alg (5.9%), (E) MgAl-Alg (11%), and (F) MgAl-Alg (20%). Alg, sodium alginate.

shift in 003 and 006 diffraction peaks, from higher to lower 2θ values, indicating an increase in the interlayer space of the layered clay after composite formation. This revealed that alginate ions, when present in high concentration, get intercalated into the interlayer space of the layered clay structure. According to the SEM images, pristine MgAl crystals were flake-like with sharp edges, stacked in the form of layers (Fig. 15.24A), whereas sodium alginate (Fig. 15.24B) had a smooth surface. Small alginate particles can be seen spread over the clay surface and between layers of the clay, at low alginate concentrations (Fig. 15.24C,D). At higher concentrations (Fig. 15.24E,F), alginate started forming a smooth continuous/semicontinuous layer

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Figure 15.25 Influence of sodium alginate concentration on the adsorption capacity of MgAl-Alg composites for the dyes, AG 25 and AG 27 (wt. of adsorbent: 0.1 g; vol. of adsorbate: 50 cm3; adsorption time: 3 h; adsorption temp.: 25 C). Alg, sodium alginate; AG, Acid Green.

on the clay. The results showed that the adsorption behavior of the NCs was superior to that of the pristine clay, however, it varied with the alginate concentration in the NC. The maximum adsorption capacity of the composite was enhanced by 51% for Acid Green 25 and 160% for Acid Green 27, compared to the pristine layered clay sample. The maximum adsorption rate increased by 48% and 147% for the adsorbates AG 25 and AG 27, respectively. Fig. 15.25 shows that the adsorption capacity for both dyes was varied with the sodium alginate concentration and passed through a maximum at an alginate concentration of 5.9%. The increase in adsorption capacity may be directly correlated with the increase in specific surface area of the NCs up to an alginate concentration of 5.9%.

15.2.1.5 Other polysaccharides There are some reports for the preparation of LDH NCs with other polysaccharides. Carrageenan was incorporated with LDH and characterized by Gwak et al. (2016). Darder et al. (2005) synthesized bio-NCs by intercalation of anionic polysaccharides including alginic acid, pectin, κ-carrageenan, ι-carrageenan, and xanthan gum in [Zn2Al(OH)6]Cl,nH2O LDH. The “coprecipitation” versus the “reconstruction” method was confirmed as the best method for the intercalation of such highmolecular-weight biopolysaccharides within the LDH. The “reconstruction” procedure from the calcined LDH in the presence of the anionic polysaccharides only resulted in a partial intercalation of the organic guest. In agreement with the fact that most of the studied biopolymers interact strongly with calcium ions producing homogeneous gels, the prepared biopolymer/LDH NCs were operative as active

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Figure 15.26 (A) Potentiometric responses of the CPEs prepared with the [Zn2Al]Cl material (K) and the ι-carrageenan-LDH NC (’) in CaCl2 solutions of increasing activity. (B) Potentiometric responses of the PVC membrane-based electrodes prepared with the alginateLDH (K), ι-carrageenan-LDH (’), pectin-LDH (▼), and κ-carrageenan-LDH (▲) NCs in CaCl2 solutions of increasing activity. Source: Adapted from Darder, M., Lo´pez-Blanco, M., Aranda, P., Leroux, F., Ruiz-Hitzky, E., 2005. Bio-nanocomposites based on layered double hydroxides. Chem. Mater., 17(8), 1969
1977, with kind permission of the American Chemical Society.

phases of sensors for the recognition of calcium ions. Hence, the biopolymer/LDH NCs were incorporated in carbon paste or PVC matrices for the development of potentiometric sensors. These devices were applied to calcium determination by direct potentiometry and the best responses were obtained for the sensors based on alginate/LDH and ι-carrageenan-LDH NCs (Fig. 15.26). One of the other applications for carrageenan/LDH NCs is catalytic performance for C
C bond formation, which was reported by Mahdi et al. (2015). Huang et al. (2013) synthesized a dextran-magnetic LDH-fluorouracil NC for drug delivery. Pectin is another polysaccharide which has been incorporated with LDH for drug delivery (Darder et al., 2005; Gwak et al., 2016). Gorrasi et al. (2012) prepared LDH with intercalated active molecules: benzoate, 2,4-dichlorobenzoate, parahydroxybenzoate, and ortho-hydroxybenzoate, incorporated into apple pectin. Incorporation of these active molecules gave antimicrobial properties to the NC films, indicating the potential application of prepared NCs in the packaging field and opened new perspectives in using pectin-antimicrobials as coating agents for a wide number of packaging polymers. The prepared NCs were characterized with XRD analysis (Fig. 15.27), which showed the absence of the peak corresponding to the basal spacing of the LDH hybrids in the composite samples, suggesting the exfoliation of the filler in all cases. Thermal, mechanical, and barrier properties of the prepared NCs were investigated. TGA showed a better thermal resistance of pectin in the presence of fillers, especially para-hydroxybenzoate and ortho-hydroxybenzoate. Such an improvement could be due either to the LDH layers that created a more tortuous path, lowering the diffusion of oxygen, or with a protecting

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Intensity (a.u)

(A) d c b

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10

15

20

25

30

35

a 40

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(B)

a b c d e 5

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Figure 15.27 X-ray diffraction patterns of (A): (a) LDH-Bz, (b) LDH-DCBz, (c) LDH-oOHBz, (d) LDH-p-OHBz; and (B): (a) pectin, (b) pectin/LDH-Bz, (c) pectin/LDH DCBz, (d) pectin/LDH-o-OHBz, (e) pectin/LDH-p-OHBz. Bz, benzoate; DCBz, 2,4-dichlorobenzoate; P-OHBz, p-hydroxybenzoate; o-OHBz: o-hydroxybenzoate. Source: Adapted from Gorrasi, G., Bugatti, V., Vittoria, V., 2012. Pectins filled with LDHantimicrobial molecules: Preparation, characterization and physical properties. Carbohyd. Polym., 89(1), 132
137, with kind permission of Elsevier.

effect of LDHs increasing the thermal stability of the biopolymer. Such increasing of thermal stability was confirmation of a strong interaction between the pectin and the inorganic phase. Mechanical properties showed an improvement of elastic modulus in particular for the LDH-para-hydroxybenzoate nanohybrid, probably for stronger interactions between pectin matrix and nanohybrid layers via formation of hydrogen bonds, better favored by the para-hydroxybenzoate molecule. Barrier properties (sorption and diffusion) to water vapor showed an improvement in the dependence on the intercalated active molecule, the best improvement was achieved for NCs containing para-hydroxybenzoate molecules, suggesting that the interaction between the filler phase and the polymer plays an important role in sorption and diffusion phenomena. Antimicrobial activity of the NC films was examined by storing them at room temperature and humidity conditions, along with the control pectin films (Fig. 15.28). As can be seen, mold formation was noticed in the pectin films after 2 weeks of storage, but there was no such indication in the NC films even after 12 months. These results clearly suggested the potential of utilizing pectin films enriched with LDH intercalated antimicrobial compounds as novel packaging materials. Other polysaccharides such as xanthan gum and microbial polysaccharides such as welan gum, scleroglucan, and EPS I (a novel polysaccharide with structural

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Figure 15.28 Pictures from cast film of pectin and composites with nanohybrids after storage for 12 months at ambient temperature. Source: Adapted from Gorrasi, G., Bugatti, V., Vittoria, V., 2012. Pectins filled with LDHantimicrobial molecules: Preparation, characterization and physical properties. Carbohyd. Polym., 89(1), 132
137, with kind permission of Elsevier.

similarities to xanthan gum) have been used in the preparation of LDH NCs (Darder et al., 2005; Gwak et al., 2016; Plank et al., 2012). Plank et al. (2012) investigated the intercalation ability of three microbial polysaccharides, welan gum, scleroglucan, and EPS I into the Zn-Al-LDH (Fig. 15.29). The NCs were synthesized by direct coprecipitation of Zn(NO3)2 and Al(NO3)3 in the polysaccharide solutions at pH B 8.5. Results from XRD and TEM (Fig. 15.30), proved that welan gum was successfully intercalated into the Zn
Al
LDH structure, while neutral scleroglucan failed to be intercalated. Instead, this biopolymer was only surface-adsorbed on inorganic CaAl
OH
LDH platelets, as was evidenced by dewashing experiments. As can be seen in Fig. 15.30, well-ordered layered structures with an average d-spacing of B 0.9 nm could be observed for pure LDH, whereas more disordered layers at much higher interlayer distances were found for the sample incorporating welan gum. This despite the fact that, in the case of scleroglucan, only layered structures characteristic of LDH could be observed, confirming that no intercalation into the LDH framework occurred with this biopolymer. In contrast to regular xanthan gum, EPS I was intercalated into the LDH structure to give a sharp X-ray reflection representing a

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Figure 15.29 Chemical structure of the microbial polysaccharides, welan gum and scleroglucan.

Figure 15.30 TEM images of the pure [Zn2Al]NO3
LDH and of coprecipitates from welan gum and scleroglucan.

d-spacing of 2.77 nm. This behavior proved that slight modifications of the polysaccharide could greatly improve its intercalation ability. It was found that the intercalation ability of these biopolymers depends on two main factors; the charge and the steric position of the anionic functions in the biomolecule. Successful incorporation

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was only possible when the biopolymer possessed an anionic charge, which was sufficient to compensate a significant portion of the positive charge in the inorganic frame. Additionally, intercalation was more favored when the anionic charges were present on the backbone of the biopolymer, instead of its side chains. Therefore, when LDHs were utilized for chemical encapsulation of such biopolymers, for example, for medical applications to provide a time-controlled release, these two factors should be taken into account. Agarose and hyaluronic acid are other polysaccharides which have used for the preparation of bioNC with LDH for drug delivery (Gwak et al., 2016)

15.2.2 Protein/layered double hydroxide nanocomposites Proteins are a great choice between natural polymers for the preparation of composite materials. They exist in either globular or fibrous structures. The globular proteins are spherical in shape and coupled by an ordering of hydrogen, ionic, hydrophobic, and covalent (disulfide) bonds, while the fibrous ones are completely extended and arranged in parallel form, commonly via a hydrogen bond. Gluten, milk protein, and soy protein are employed in the production of edible films, despite the fact that keratin is employed in the fabrication of nanofiber, film, and composites for material fields. It seems that protein-based biopolymers can be employed in the areas of biomaterials, packaging materials, and in coatings industries in the future (Gupta and Nayak, 2015). Similar to polysaccharides, proteins have some disadvantages, such as poor mechanical, thermal, barrier, water resistance properties, etc., which can be improved by the incorporation of low loading of NFs into them. For example John and Thomas (2012) reviewed soy protein NCs which were reinforced with inorganic NFs such as clay, carbon nanotubes, etc. Also, there are some reports on the preparation of protein/LDH NCs (Yasutake et al., 2008) for bone tissue engineering (Fayyazbakhsh et al., 2011, 2012, 2017), drug delivery (Gwak et al., 2016), and fluorescent biosensors (Zhang et al., 2017b). As an interesting example we can point to the preparation of the gelatin/LDH and gelatin/LDH-hydroxyapatite NC for bone tissue engineering scaffolds using coprecipitation and solvent-casting techniques, as reported by Fayyazbakhsh et al. (2017). They investigated both in vitro and in vivo studies. As can be seen in Fig. 15.31, SEM images of NCs showed the likeness between the microstructures of both scaffolds with natural spongy bone and interconnected macropores. The porosities of 90% and 92%, as well as Young’s modulus of 19.8 and 12.5 GPa, were obtained for gelatin/LDH and gelatin/LDH-hydroxyapatite scaffolds, respectively. Moreover, the SEM images revealed that between two scaffolds, the gelatin/ LDH-hydroxyapatite with needle-like secondary hydroxyapatite crystals showed better bioactivity. Alkaline phosphatase activity and alizarin red staining results indicated that gelatin/LDH-hydroxyapatite scaffolds improved bone-specific activities compared to gelatin/LDH scaffolds, as well as a control sample. Finally, all the scaffolds suffered implantation (Fig. 15.32) on animals and by in vivo results indicated that both implanted groups (acellular and cell-seeded groups) did not show a serious inflammatory response and provided new bone

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Figure 15.31 Micropraphs of (A) pure LDH/GEL scaffold, (B) LDH-HA/GEL scaffold, (C) natural bone microstructure for comparison with synthetic scaffolds (high structural similarity can be seen between bone and LDH-HA/GEL interconnected scaffold) (magnification: 100 3 ). The SEM images of scaffolds after immersing in SBF. (D, E, F) Pure LDH/GEL scaffold, (G, H, I) LDH-HA/GEL scaffold with different magnification. (D, G) The nucleation of secondary HA through the pores after day 3, (E, H) the growth of secondary HA crystals and scaffold degradation after day 14, (F, I) needle-like morphology of secondary HA on the surface of scaffolds after day 21. GEL, gelatin; HA, hydroxyapatite. Source: Adapted from Fayyazbakhsh, F., Solati-Hashjin, M., Keshtkar, A., Shokrgozar, M.A., Dehghan, M.M., Larijani, B., 2017. Novel layered double hydroxides-hydroxyapatite/gelatin bone tissue engineering scaffolds: Fabrication, characterization, and in vivo study. Mater. Sci. Eng. C., with kind permission of Elsevier.

formation. Consequently, gelatin/LDH-hidroxyapatite scaffold satisfied the crucial demands of BTE and had the potential to be employed in orthopedic and reconstructive surgery.

15.2.3 PHA/layered double hydroxide nanocomposites PHAs are bacterial biopolyesters that generally consist of (R)-3-hydroxy fatty acids, with various side chains. The pendant group (R) varies from C1 to C13 and is saturated or unsaturated, or contains a substituent (Roy and Visakh, 2014). Similar to

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Figure 15.32 The general implantation process: (A) fat tissue sampling from intrascapular area, (B) OM image of cultured ASCs (magnification 40 3 ), (C) measuring and marking the defect site on radius bone, (D) cutting the radius to create the defect by surgical saws, (E) with/without ASC scaffolds were soaked in culture media and then swelled, the swollen scaffold vs. dissected bone. The size differences can be seen but the swollen scaffold showed a flexible behavior, therefore it was placed in the defect site, (F) the scaffold placed in the defect site (i.e., implanted), (G) muscle sutures after implanting, (H) dermal sutures, (I) radiography position and the aluminum graded phantom to evaluate the density of newly formed bone. Source: Adapted from Fayyazbakhsh, F., Solati-Hashjin, M., Keshtkar, A., Shokrgozar, M.A., Dehghan, M.M., Larijani, B., 2017. Novel layered double hydroxides-hydroxyapatite/gelatin bone tissue engineering scaffolds: Fabrication, characterization, and in vivo study. Mater. Sci. Eng. C., with kind permission of Elsevier.

other biopolymers, to overcome the disadvantages of PHAs, there are reports for the preparation of hybrid NCs with PHA, a blend or a copolymer of PHA (Roy and Visakh, 2014), especially with LDH and the authors have usually investigated the mechanical and thermal properties of prepared NCs (Bunea et al., 2016; Ciou et al., 2014; Dagnon et al., 2009a; DeGruson, 2014; Liau et al., 2014; Pak et al., 2013; Zhang et al., 2012). For example, Ciou et al. (2014) prepared bio-NCs based on poly(3-hydroxybutyrate) and a copolymer of poly(3-hydroxybutyrate), poly(3hydroxybutyrate-co-3-hydroxyvalerate) with poly(3-hydroxyvalerate) content of

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5 and 12 wt.%, as matrices and an oleate-modified LDH as NF. They also investigated the degradation behavior of bio-NCs by using Caldimonas manganoxidans as a microbial catalyst. The oleate intercalated LDH was synthesized by a novel one-step coprecipitation forming structure of bilayer and monolayer in which the molecules were expected to lay on or tilt at a fixed angle to enlarge the interlayer ˚ ). XRD and TEM images of both NCs 5 wt.% (Fig. 15.33) showed spacing (34.0 A that most of the hydroxide layers of LDH were exfoliated and randomly distributed in both matrices.

Figure 15.33 TEM micrographs of 5 wt.% (A) PHB/m-LDH and (B) PHBV12/m-LDH NCs. TEM micrograph obtained from the embedded and cut 5 wt.% PHB/m-LDH NC is shown in (C). PHB, poly(3-hydroxybutyrate); PHBV12, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with 12% poly(3-hydroxyvalerate); m-LDH, modified Mg-Al LDH. Source: Adapted from Ciou, C.-Y., Li, S.-Y., Wu, T.-M., 2014. Morphology and degradation behavior of poly (3-hydroxybutyrate-co-3-hydroxyvalerate)/layered double hydroxides composites. Eur. Polym J., 59, 136
143, with kind permission of Elsevier.

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Table 15.4 Dynamic storage modulus and crystallinity (Xc) of PHB/m-LDH and PHBV/ m-LDH NCs Sample code PHB 1 wt.% PHB/m-LDH 3 wt.% PHB/m-LDH 5 wt.% PHB/m-LDH PHBV5 1 wt.% PHBV5/m-LDH 3 wt.% PHBV5/m-LDH 5 wt.% PHBV5/m-LDH PHBV12 1 wt.% PHBV12/m-LDH 3 wt.% PHBV12/m-LDH 5 wt.% PHBV12/m-LDH

E0 at 250oC (MPa) % 1680 2280 2710 3540 1000 1520 1930 2060 670 960 1220 1610

Xc (%) 66 65.4 64.7 65.1 51.5 53.2 52.4 52.0 36.5 36.1 37.1 37.2

PHB, poly(3-hydroxybutyrate); PHBV: poly(3-hydroxybutyrate-co-3-hydroxyvalerate); m-LDH, modified Mg-Al LDH. Source: Adapted from Ciou, C.-Y., Li, S.-Y., Wu, T.-M., 2014. Morphology and degradation behavior of poly (3hydroxybutyrate-co-3-hydroxyvalerate)/layered double hydroxides composites. Eur. Polym J., 59, 136
143, with kind permission of Elsevier.

It was shown that the storage modulus of both poly(3-hydroxybutyrate) and poly (3-hydroxybutyrate-co-3-hydroxyvalerate) and their NCs depended on the temperature and the storage modulus of both, and were highest at 250 C and decreased with increasing temperature. As can be seen in Table 15.4, at 250 C, the storage modulus of all NCs was increased by introducing and increasing the amount of modified LDH, which was attributed to the reinforcement effect of the presence of the rigid LDH layers as well as the superior interaction between the polymers and modified LDH, leading to a prominent improvement in the stiffness of the polymer matrix. In terms of biodegradability, it was observed that although the introduction and increasing of modified LDH in all poly(3-hydroxybutyrate)-based film improved their mechanical properties, but the addition of modified LDH up to 5% showed an insignificant difference for biodegradability. Microbial degradation offered that the poly(3-hydroxybutyrate) depolymerase of C. manganoxidans was governed by the exo-type hydrolysis activity, where the degradation of poly(3hydroxybutyrate) polymer was started from both ends of polymer chains. Pak et al. (2013) used a blend of poly-3 hydroxybutyrate/poly(butyleneadipateco-terephthalate) (PHB/PBAT) as matrix for the preparation of NCs with 1, 2, 3, 4, and 5 wt.% of LDH. The Zn3Al LDH was synthesized using a coprecipitation method and modified with stearate anion surfactant via an ion exchange reaction. The stearate anions were successfully intercalated into pristine LDH confirmed by the observation of the alkyl group in the FT-IR spectrum. According to the XRD pattern of pristine LDH and modified LDH, the basal spacing of the LDH was ˚ after modification with stearate. The remaining enhanced from 8.77 to 24.94 A peaks of PHB/PBAT blend in the XRD patterns of NCs (Fig. 15.34C) suggesting

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Figure 15.34 (A and B) TEM micrographs of PHB/PBAT/2.0 wt.% stearate-Zn3Al LDH. (C) XRD pattern for (a) pure PBAT, (b) pure PHB, (c) PHB/PBAT blend, and NCs of 1.0, 2.0, 3.0, 4.0, and 5 wt.% of stearate-Zn3Al LDH (d
h). PHB, poly-3 hydroxybutyrate; PBAT, poly(butyleneadipate-co-terephthalate).

that the polymer blend crystalline lattice was not modified appreciably in the existence of LDH. TEM images (Fig. 15.34A,B) of NCs indicated that the NCs formed an intercalated structure as the modified NF percentage increased, thus providing better compatibility between the blend matrix and the galleries of LDH. It was observed that the tensile strength of PHB/PBAT was decreased by increasing the percentage of PBAT from 10% to 50% compared to the neat PHB, while the tensile modulus and elongation at break of this blend were increased by the addition of 10% PBAT and then decreased by increasing up to 50%. Therefore, the PHB/ PBAT blend with 90/10 ratio was chosen as the best ratio among others for investigation of the mechanical properties of NCs. As can be seen in Fig. 15.35, adding 2.0 wt.% modified LDH into the blend matrix enhanced the elongation at break (from 35.03% up to 54.58%), with an improvement of 56% compared to that of the unfilled blend. This was due to the presence of the long-chain hydrocarbon parts of stearate anions in the modified LDH that acted as a plasticizer. However, after more addition of 3.0
5.0 wt.% of LDH, decreasing elongation at break was observed, which may be due to the presence of large agglomerates which resulted in more brittle NCs. Zhang et al. (2012) prepared the exfoliated bio-NCs based on poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3,4)HB) and Co-Al LDH (SSLDH) (1, 3, 5 wt.%)

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Figure 15.35 Tensile strength, tensile modulus, and elongation at break of PHB/PBAT blends with different stearate-Zn3Al LDH content. PHB, poly-3 hydroxybutyrate; PBAT, poly(butyleneadipate-co-terephthalate).

via melt intercalation. Moreover, mechanical, thermal, and flame-retardant properties for these bio-NCs were systematically investigated. The TEM (Fig. 15.36) images clearly reveal that the LDH sheets were mainly exfoliated and disorderly dispersed in the P(3,4)HB matrix and confirmed that exfoliated P(3,4)HB/SS-LDH bio-NCs were successfully prepared by the melt blending process. DMA (dynamic mechanical analysis) testing, which is one of the techniques commonly used to characterize the time, frequency, and temperature dependency of the viscoelastic nature of polymers, showed that a small amount of LDH could significantly enhance the storage modulus and thermomechanical properties of bio-NCs. TGA results indicated that the thermal stability of the bio-NCs decreased with the increasing loading of LDH. The reduction of thermal stability was probably related to the degradation of the SS-LDH, which influenced the decomposition of P(3,4) HB. However, the mass loss rates for the bio-NCs were reduced as the weight percentage of LDH increased, which was possibly attributed to the barrier effect of the nanosheets of LDH. Thermal combustion properties of P(3,4)HB/SS-LDH bio-NCs were evaluated by a microscale combustion calorimeter (MCC), which is a rapid, and small-scale flammability testing instrument to research polymer combustion properties. From MCC data (Table 15.5), it was found that the flame retardancy of P(3,4)HB/SS-LDH bio-NCs was enhanced with the addition of LDH content. As can be seen, compared to neat P(3,4)HB, the PHRR (peak of heat release rate), HRC (heat release capacity), which is an important parameter of the fire hazard,

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Figure 15.36 TEM images of P(3,4)HB/5 wt.% SS-LDH bio-NC (A) at low magnification and (B) at high magnification. P(3,4)HB, poly(3-hydroxybutyrate-co-4-hydroxybutyrate); SSLDH: Co-Al LDH. Source: Adapted from Zhang, R., Huang, H., Yang, W., Xiao, X., Hu, Y., 2012. Preparation and characterization of bio-nanocomposites based on poly (3-hydroxybutyrate-co-4hydroxybutyrate) and CoAl layered double hydroxide using melt intercalation. Compos. Part A: Appl. Sci. Manuf., 43(4), 547
552, with kind permission of Elsevier.

Table 15.5 MCC results for neat P(3,4)HB and its bio-NCs Samples

PHRR (W/g)

THR (kJ/g)

HRC (J/g K)

TPHRR ( C)

P(3,4)HB P(3,4)HB/1 wt.%SS-LDH P(3,4)HB/3 wt.%SS-LDH P(3,4)HB/5 wt.%SS-LDH

761.9 518.6 468.7 435.7

13.6 13.3 12.6 11.8

743 525 472 451

298.5 300.7 289.2 282.8

HRC, heat release capacity, 6 5 J/g K; PHRR, peak of heat release rate, 6 5 W/g; THR, total heat release, 6 0.1 kJ/ g; TPHRR, temperature at PHRR, 6 2 C. Source: Adapted from Zhang, R., Huang, H., Yang, W., Xiao, X., Hu, Y., 2012. Preparation and characterization of bio-nanocomposites based on poly (3-hydroxybutyrate-co-4-hydroxybutyrate) and CoAl layered double hydroxide using melt intercalation. Compos. Part A: Appl. Sci. Manuf., 43(4), 547
552, with kind permission of Elsevier.

and THR (total heat release) values for P(3,4)HB/LDH bio-NCs were remarkably reduced, resulting from the barrier effect of exfoliated LDH layers. Bunea et al. (2016) synthesized novel biocomposites based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV) and modified LDH for tissue engineering. For this purpose, LDH was modified with SDS by a coprecipitation method. The

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Figure 15.37 SEM microphotographs reveal the morphology of: PHBHV-film (A), PHBHV/ LDH-SDS 1% (B), and PHBHV/LDH-SDS 3% (C). PHBHV, poly(3-hydroxybutyrate-co-3hydroxyvalerate).

prepared NF (LDH-SDS) (1, 2, 3 wt.%) was incorporated into PHBHV and the biocomposites were obtained by a solvent casting technique. To investigate the morphology of NCs, SEM was employed for PHBHV film and PHBHV/LDH-SDS NCs (Fig. 15.37). As can be seen, PHBHV microparticles were dispersed in a compact structure in the PHBHV film. A compact surface could be observed in the composites where the LDH-SDS particles were embedded into the polyester film. TGA curves of PHBHV/LDH-SDS (1% and 2%) revealed that the presence of the clay did not influence the thermal properties of the composite films. The composites started to decompose at lower temperatures compared to PHBHV. This may be due to the presence of dodecyl sulfate. The residue of these samples increased compared to the pure polymer. The biocompatibility of these novel biocomposites was studied in relation to human adipose-derived stem cells. Live/dead fluorescence microscopy assay [based on the simultaneous staining of live (green-labeled) and dead (redlabeled) cells] was performed to evaluate hASC viability in direct contact with PHBHV film (B) and PHBHV_LDH-SDS composites with 1% (B1), 2% (B2), and

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Figure 15.38 Confocal fluorescence microscopy micrographs revealing live and dead cells on PHBHV film (B) and PHBHV/LDH-SDS composites with 1% (B1), 2% (B2), and 3% (B3) LDH-SDS, after 24 h of culture. PHBHV, poly(3-hydroxybutyrate-co-3hydroxyvalerate).

3% (B3) LDH-SDS biomaterials (Fig. 15.38). The results showed that hASCs survived after 24 h of culture in contact with all tested biomaterials. Moreover, no significant differences were detected in the ratio between live and dead cells on the PHBHV film and PHBHV/LDH-SDS composites. Consequently, due to good biocompatibility, these materials could be promising candidates for in vivo testing for wound dressing applications. Dagnon et al. (2009a) reported that the thermomechanical performance of poly [(3-hydroxybutyrate)-co-(3-hydroxyvalerate)] (PHBV) is associated with its crystallization. They suggested enhanced nucleation using a stearate-functionalized LDH (LDH-SA) as a solution. Stearate anions were intercalated into the LDH via an ion exchange reaction. Then, the various amounts of prepared NF (1, 3, 5, and 7 wt.%) were incorporated into the PHBV matrix and the NC films were obtained via a solution casting technique. According to the XRD results, increasing the basal spacing of the LDH-SA in all hybrids indicated some degree of intercalation of PHBV chains within the LDH-SA interlayer galleries. On the other hand, the existence of sharp Bragg peaks after solution casting indicated that the dispersed LDH-SA still retains an ordered structure and proved that the LDH-SA crystals remain essentially integral within the PHBV matrix and, consequently, exfoliation of LDH-SA did not occur. TEM showed the intercalated structure of the LDH-SA and there was limited dispersion of LDH-SA in the PHBV matrix as the LDH-SA concentration increased (Fig. 15.39). The effect of LDH-SA on the crystallinity of PHBV was also evaluated with WAXD. According to WAXD, PHBV is a semicrystalline polyester (rhombic cell). The reflections of all NCs were observed at the same values as for the neat biopolymer and this indicated that, in the NCs, PHBV crystallized in its typical crystalline form and its unit cell was not changed after being incorporated in LDH-SA. However, lamella size for the (020) and (021) directions increased with increasing LDH-SA indicating that the crystalline lamella size increased in the presence of LDH-SA. The TGA results showed that the NF destabilized the matrix, leading to decreased thermal stability of the NC with increasing LDH-SA loading. This could be due to the release of water from LDH, which hydrolyzed the ester bonds of PHBV. According to Table 15.6, the thermomechanical properties were modified by the presence of LDH-SA. As can be seen from the maxima of tan δ, Tβ of the matrix polymer (c. 2101 C), which is conventionally associated with

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Figure 15.39 TEM micrographs of PHBV/SAnNCs: (A) PHBV/SA1; (B) PHBV/SA3; (C) PHBV/SA5; (D) PHBV/SA7. PHBHV, poly(3-hydroxybutyrate-co-3-hydroxyvalerate); SA, stearate anion. Source: Adapted from Dagnon, K. L., Chen, H. H., Innocentini-Mei, L. H., & D’Souza, N. A. (2009a). Poly [(3-hydroxybutyrate)-co-(3-hydroxyvalerate)]/layered double hydroxide nanocomposites. Polymer International, 58(2), 133
141, with kind permission of John Wiley and Sons. Table 15.6 Dynamic thermomechanical properties of PHBV and PHBV/SAn NCs Sample

Tg ( C)

Tβ ( C)

E0 (GPa) at 2125 C

E0 (GPa) at 25 C

PHBV PHBV/SA1 PHBV/SA3 PHBV/SA5 PHBV/SA7

13 12 10 10 10

2 101.8 2 102.5 2 102.4 2 102.6 2 102.0

6.61 ( 6 0.05) 6.92 ( 6 0.07) 7.59 ( 6 0.10) 8.13 ( 6 0.10) 7.24 ( 6 0.09)

2.45 ( 6 0.05) 2.40 ( 6 0.03) 3.21 ( 6 0.10) 3.40 ( 6 0.10) 2.87 ( 6 0.07)

PHBHV, poly(3-hydroxybutyrate-co-3-hydroxyvalerate); SA, stearate anion. Source: Adapted from Dagnon, K. L., Chen, H. H., Innocentini-Mei, L. H., & D‘Souza, N. A. (2009a). Poly [(3hydroxybutyrate)-co-(3-hydroxyvalerate)]/layered double hydroxide nanocomposites. Polymer International, 58(2), 133
141, with kind permission of John Wiley and Sons.

local crankshaft motion of the (CH2)n segment, was unaffected by the addition of LDH-SA. The Tg of the hybrids was also unaffected by the addition of LDH-SA. The PHBV-based NCs showed higher storage moduli than pure PHBV. The higher E0 values of the PHBV/SAn NCs reflected the reinforcement potential of LDH-SA in the biopolymer matrix. The increase in rubbery modulus could be ascribed to the reinforcing effect of the nanofiller. Although E0 generally increased with increasing LDH-SA content, PHBV/SA7 showed E0 lower than that of PHBV/ SA3 and PHBV/SA5. This could be attributed to increased aggregate formation, leading to a deterioration in the mechanical properties.

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15.2.4 PLA/layered double hydroxide nanocomposites PLA is a biocompatible and biodegradable polymer included in the class of aliphatic polyesters which is obtained from α-hydroxy acids. D-/L-lactic acid (D-/L-2hydroxy propionic acid) is the fundamental constituent of PLA, which is generated by carbohydrate fermentation or chemical synthesis. The properties of PLA are governed by the ratio between two enantiomers which has caused the generation of a broad range of PLA polymers to fit the performance demands. High-molecularweight PLA polymers (larger than 100,000 Da) can be synthesized by various methods, such as direct polycondensation, azeotropic dehydrative polycondensation, and ring-opening polymerization of lactide. PLA has been applied in different fields, particularly including the commodity field and industry products, such as packaging and service ware, fiber and nonwoven, engineering plastic, biodegradable hot melt adhesive, environmental remediation, paints, cigarette filters, 3D printing, and parts for space exploration, etc., as well as the field of biomedical materials including tissue engineering scaffolds, controllable drug delivery, surgical sutures, ideal fillers for soft-tissue augmentation, and mesh insertion for groin hernia repair (CastroAguirre et al., 2016; Ren, 2011). There are many reports of the synthesis, characterization, and applications of organic/inorganic hybrid materials based on PLA, many of which have utilized LDH as NF. In some of them neat LDH has been used, while in several reports to enhance the compatibility between NF and matrix, LDH has been modified with various molecules such as surfactants, antioxidants, polymer, lignosulfonate (Hennous et al., 2013), phosphinic acid derivative, ionic liquids (Ha and Xanthos, 2010; Livi et al., 2012), drugs (Dagnon et al., 2009b), etc. which are discussed below. On the other hand, copolymers of PLA with other polymers such as poly(lactic-co-glycolic acid) have also used as matrix (Chakraborti et al., 2011; Chakraborti et al., 2012). For example, Chiang et al. used both neat and modified LDH [modified with PLA-COOH by ion exchange process (Chiang et al., 2011; Chiang and Wu, 2010) and with γ-polyglutamate by melt blending process (Chiang and Wu, 2012)] and then incorporated the new NFs into the PLLA (PLA obtained from L-lactic acid). The morphology, water vapor permeability, as well as barrier, mechanical, and thermal properties of obtained NCs were investigated. LDH was modified by polylactide with carboxyl end group (PLA
COOH) using an ionexchange process and then incorporated into the PLLA matrix via solution intercalation. XRD and TEM images (Fig. 15.40) of NCs demonstrated that the modified LDH was exfoliated and randomly distributed into the PLLA matrix. As can be seen in Table 15.7, the mechanical properties of the NC with 1.2 wt.% NF showed a significant increase in the storage modulus as compared to unfilled PLLA. However, by introduction of more modified LDH into the PLLA matrix, a decrease in the storage modulus of NCs was observed, which was probably due to the excessive amount of PLA
COOH molecules with low mechanical properties. In another work, Chiang and Wu (2012) modified LDH with γ-polyglutamate via anion exchange method using LDH
NO3 as a precursor. Then, both unmodified LDH and γ-polyglutamate modified LDH (γ-LDH) (1, 3 and 5 wt.%) were incorporated into the PLLA by a melt blending process. The XRD and TEM

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Figure 15.40 TEM micrographs of: (A) 5 wt.% PLLA/P-LDH and (B) 10 wt.% PLLA/PLDH NCs. P-LDH, PLA-COOH modified LDH. Source: Adapted from Chiang, M.-F., Wu, T.-M., 2010. Synthesis and characterization of biodegradable poly (L-lactide)/layered double hydroxide nanocomposites. Compos. Sci. Technol., 70(1), 110
115, with kind permission of Elsevier.

Table 15.7 Dynamic storage modulus, degradation activation energies (Ea) and Mg and Al residues of PLLA and PLLA/P-LDH NCs Sample code

PLLA 0.4% PLLA/P-LDH 1.2% PLLA/P-LDH 2% PLLA/P-LDH 4% PLLA/P-LDH

Storage modulus (MPa) 220 C

80 C

459 750 865 718 651

105 175 231 164 158

Eaa (kJ/mol)

Rb

Mgc (wt.%)

Alc (wt.%)

158.04 140.50 129.52 105.17 102.92

0.9998 0.9993 0.9902 0.9978 0.9999


0.020 0.137 0.579 1.110


0.027 0.100 0.320 0.574

P-LDH, PLA-COOH modified LDH. a The degradation activation energies (Ea) were obtained from the Kissinger equation. b R: correlation coefficient. c Mg and Al ratio were obtained from ICP-AES experiment. Source: Adapted from Chiang, M.-F., Wu, T.-M., 2010. Synthesis and characterization of biodegradable poly (Llactide)/layered double hydroxide nanocomposites. Compos. Sci. Technol., 70(1), 110
115, with kind permission of Elsevier.

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Figure 15.41 Bright-field TEM micrographs of (A) 5 wt.% PLLA-L and (B) 5 wt.% PLLAγL. The inset in (B) is the high-magnification TEM images of 5 wt.% PLLA-γL. L, LDH; γL, γ-polyglutamate modified LDH. Source: Adapted from Chiang, M.-F., Wu, T.-M., 2010. Synthesis and characterization of biodegradable poly (L-lactide)/layered double hydroxide nanocomposites. Compos. Sci. Technol., 70(1), 110
115, with kind permission of Elsevier. Table 15.8 Mechanical properties of PLLA/LDH NCs Sample

Young’s modulus (MPa)

Tensile strength (MPa)

Elongation at break (%)

PLLA 1 wt.% PLLA-L 3 wt.% PLLA-L 5 wt.% PLLA-L 1 wt.% PLLA-γL 1 wt.% PLLA-γL 1 wt.% PLLA-γL

453 ( 6 16) 417 ( 6 46) 412 ( 6 14) 396 ( 6 13) 462 ( 6 18) 478 ( 6 25) 507 ( 6 44)

46.5 ( 6 2.8) 43.4 ( 6 3.3) 37.8 ( 6 3.1) 34.7 ( 6 2.4) 45.1 ( 6 5.6) 42.0 ( 6 1.4) 39.5 ( 6 2.5)

18.0 ( 6 2.9) 16.7 ( 6 1.6) 14.3 ( 6 1.5) 13.9 ( 6 1.0) 14.5 ( 6 0.3) 14.0 ( 6 0.6) 12.5 ( 6 1.1)

L, LDH; γL, γ-polyglutamate modified LDH. Source: Adapted from Chiang, M.-F., Wu, T.-M., 2010. Synthesis and characterization of biodegradable poly (Llactide)/layered double hydroxide nanocomposites. Compos. Sci. Technol., 70(1), 110
115 with kind permission of Elsevier.

(Fig. 15.41) results of NCs revealed that the unmodified LDH with a certain amount of aggregates was unevenly distributed throughout the matrix, while γ-LDH allowed the formation of an intercalated NC. As can be seen in tensile properties of NCs in Table 15.8, the Young’s modulus of NCs based on γ-LDH was increased by increasing the content of the γ-LDH to 5 wt.%, while, when unmodified LDH was

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Figure 15.42 TEM images of PLA/stearate-LDH NCs with (A) 3.0, (B) 5.0, (C) 7.0, and (D) 10.0 wt.% stearate-LDH content. (Magnification is 200 3 .)

used, the modulus of NCs was slightly reduced by increasing the loading of unmodified LDH and was almost independent of the LDH contents. These results indicated that the aggregated and unevenly dispersed unmodified LDH in PLLA matrix would not act as a reinforcing filler for the final NCs. On the other hand, it can be seen that either for unmodified LDH or modified LDH, the tensile strength was decreased by increasing the loading of NF, which may be due to some inevitably agglomerated unmodified and modified LDH platelets in the PLLA matrix that acted as sites of stress concentration. Furthermore, elongation at break of two types of NCs was decreased by increasing the amount of NF as compared to pure PLLA. This observation was attributed to high stiffness provided by the LDH sheets and the restraints on the mobility of the molecular chains caused by possible interactions between hydroxide layers and PLLA backbones. Eili et al. (2012) studied the preparation and characterization of PLA/stearatemodified LDH (stearate-LDH) NCs. The bio-NCs were prepared by a solution casting method and from XRD and TEM (Fig. 15.42) analysis; the stearate-LDH lost its ordered stacking structure and was greatly exfoliated in the PLA matrix. The effect of stearate-LDH loading on tensile properties and soil biodegradation was studied. Significant improvement in elongation at break was observed as a result of the addition of 1.0
3.0 wt.% of stearate-LDH to the PLA. Moreover, results showed an enhancement in the biodegradation of PLA in soil by incorporation of stearate-modified LDH. This effect can be caused by the catalytic role of the stearate groups in the biodegradation mechanism leading to much faster disintegration of NCs than pure PLA.

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Figure 15.43 TEM micrographs of PLA/LDH-Bph at low (left) and high (right) magnification. Bph, 4-biphenyl acetic acid. Source: Adapted from Oyarzabal, A., Mugica, A., Mu¨ller, A.J., Zubitur, M., 2016. Hydrolytic degradation of nanocomposites based on poly (L-lactic acid) and layered double hydroxides modified with a model drug. J. Appl. Polym. Sci., 133(28), with kind permission of John Wiley & Sons.

Figure 15.44 Visual aspect of (A) neat PLA, (B) PLA/Bph, and (C) PLA/LDH-Bph after 0, 14, 22, 28, 35, 42, and 56 days under incubation in PBS (pH 5 7.2) at 37 C. Bph, 4biphenyl acetic acid. Source: Adapted from Oyarzabal, A., Mugica, A., Mu¨ller, A.J., Zubitur, M., 2016. Hydrolytic degradation of nanocomposites based on poly (l-lactic acid) and layered double hydroxides modified with a model drug. J. Appl. Polym. Sci., 133(28), with kind permission of John Wiley & Sons.

Stearate-modified LDH could also cause better flexibility of PLA (Mahboobeh et al., 2010). Drugs also can be used for the modification of LDH. Oyarzabal et al. (2016) incorporated the LDH into the PLA after modification with 4-biphenyl acetic acid (Bph) as a drug model. NCs were prepared by solvent casting with 5 wt. % of drug-modified LDH. The obtained NC had a partially exfoliated morphology as determined by TEM (Fig. 15.43). Moreover the hydrolytic degradation was carried out in a PBS solution at pH 7.2 and 37 C and compared with neat PLA with 5 wt.% Bph (Fig. 15.44). For PLA/Bph, an acid catalytic effect, caused by the drug, accelerated the PLA mass loss. However, for PLA/LDH-Bph, the presence of LDH produced a barrier effect that initially reduced the diffusion of the oligomers produced during hydrolytic degradation. Dagnon et al. (2009b) also functionalized LDH with ibuprofen (Ibu) and prepared PLLA/LDH-Ibu NCs by 1, 3, and 5 wt.% of LDH-Ibu using the solution

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Figure 15.45 TEM micrographs of the PLLA/LDH-Ibu NCs: (A) 1LDH-Ibu, (B) 3LDH Ibu, and (C) 5LDH-Ibu. Ibu: ibuprofen. Source: Adapted from Dagnon, K.L., Chen, H.H., Innocentini-Mei, L.H., D’Souza, N.A., 2009a) Poly [(3-hydroxybutyrate)-co-(3-hydroxyvalerate)]/layered double hydroxide nanocomposites. Polym. Int., 58(2), 133
141; Dagnon, K.L., Ambadapadi, S., Shaito, A., Ogbomo, S.M., DeLeon, V., Golden, T.D., et al., 2009b. Poly (L-lactic acid) nanocomposites with layered double hydroxides functionalized with ibuprofen. J. Appl. Polym. Sci., 113(3), 1905
1915, with kind permission of John Wiley and Sons.

casting route. The TEM micrographs of the PLLA/LDH-Ibu NCs are shown in Fig. 15.45, and intercalated and exfoliated structures of the LDH-Ibu component were observed. It is clear, however, that the localized structure showed an intercalated/exfoliated dispersion and that aggregation and clustering of the intercalated/ exfoliated regions existed. Thus, the uniform dispersion of LDH-Ibu in the PLLA matrix was limited as the NF concentration increased. The authors examined the potential to decrease cell proliferation, while simultaneously increasing mechanical performance through LDH organically modified with Ibu dispersed in PLLA. For this purpose, smooth muscle cells (SMCs) were used for in vitro studies of the NCs. LDH-Ibu incorporated in PLLA inhibited the proliferation of SMCs after 5 days of exposure. By comparing Ibu, PLLA/Ibu, and LDH functionalized with Ibu incorporated into PLLA, it was concluded that the Ibu component was the dominant cause of the decreased cell proliferation. Incorporating Ibu into the LDH resulted in effective drug release, which led to a multibiofunctional NC with a significant mechanical advantage. Leng et al. (2015) studied the structure
property relationships of PLA/SDBSmodified LDH NCs which were produced by the melt blending procedure. They investigated the various properties of obtained NCs, such as the influence of the modification of LDH with SDBS on the dispersity of NF in the matrix, degradation stability, crystallinity, dielectric relaxation behavior, and thermal properties. Smallangle X-ray scattering showed the increase in the space of layers in LDH after SDBS incorporation indicated the successful modification process. This observation was also observed in terms of NC and from this it was concluded that the NCs had a partly exfoliated morphology with mixed nanostacks. It is noteworthy that size exclusion chromatography (SEC) measurements showed a small increase in the degradation of PLA due to the LDH NPs but it was too small to influence the properties of NCs significantly. This is despite the fact that Gerds et al. (2012) reported the degradation of PLLA during melt processing with LDH. They proposed that the

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state of LDH dispersion in the PLA matrix, which influenced the accessibility of PLA to the hydroxide layer, catalytically active Mg sites and release of water by LDH during melt compounding were causative factors in the PLA degradation, while the latter had the minor role in the degradation process. In addition to solution casting, melt processing (Katiyar et al., 2011) and other methods for the preparation of PLA/LDH NCs, in situ polymerization and in situ bulk polymerization of lactide monomer have also used (Katiyar et al., 2010; Nogueira et al., 2016). Although by using this method, because of chain termination via LDH hydroxyl groups and/or metal-catalyzed degradation, the significant reduction in the molecular weight of PLA was observed, this method was effective for the preparation of NC due to ensuring good distribution of LDH in the matrix (Katiyar et al., 2010). Neat and modified LDH (LDH-CO3 and laurate-modified) (Katiyar et al., 2010), salicylate intercalated and sebacate intercalated were used (Nogueira et al., 2016). PLA/LDH NCs can be used as a packaging material (Bugatti et al., 2013). For example, Demirkaya et al. (2015) fabricated PLA/LDH NC films by utilizing two different Mg:Al ratios of LDH (2:1 and 3:1) via the solution casting method. To improve the compatibility between NF and matrix, they also modified the LDH by sodium dodecyl sulfate (SDS) surfactant via intercalation between the layers of LDH and the properties of NCs with both neat and modified LDH were compared. As can be seen in the AFM images of NCs in Fig. 15.46, the intercalation of SDS in LDH caused better dispersion and exfoliation of NF into the matrix. Furthermore, using modified NF resulted in better mechanical properties and Tg. The highest oxygen and water vapor permeability values were observed for the NCs, prepared with LDH-SDS, which was suitable for packaging materials. Compared to pure PLA (497 cm3/m2.bar.day) and NCs with 3, 5, and 10 wt.% of LDH, the films containing 5% of modified NF at Mg:Al ratio of 3:1 (380 cm3/m2. bar.day) and 5% of modified NF at an Mg:Al ratio of 2:1 showed the best oxygen barrier. The oxygen barrier property for SDS-modified LDH containing NCs was found to increase by 23%, while unmodified LDH containing NCs remained almost unchanged. This was due to the high degree of dispersion and uniformity between the SDS-modified Mg-Al LDH particles and PLA matrix. The water vapor permeability of NC films decreased about 80% for PLA/SDS-Mg-A-LDH (2:1) 5% compared to neat PLA film. This is mainly attributed to the tortuous path for water vapor diffusion due to the impermeable clay layers distributed in the polymer matrix consequently increasing the effective diffusion path length. Due to the application of LDH and PLA for drug delivery and other biomedical applications, there are some reports for the application of PLA/LDH NCs for drug delivery using PLA or a copolymer of PLA. Drugs were usually used for the modification of LDH. For example, San Roma´n et al. (2013) intercalated the drugs (diclofenac, chloramphenicol, and ketoprofen) into LDH and then supported and dispersed these NFs into the PLA for drug delivery. One of the other applications of PLA/LDH NCs is the preparation of fibers by electrospinning technique, as reported by Zhao et al. (2008). Miao et al. (2012) also prepared electrospun fibers of PLA/LDH NCs for drug delivery. For this purpose, they intercalated ibuprofen

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Figure 15.46 AFM images of (A) PLA/Mg-Al LDH (2:1) 3%, (B) PLA/SDS- Mg-Al LDH (2:1) 3 %, (C) PLA/ Mg-Al LDH (3:1) 3%, (D) PLA/SDS- Mg-Al LDH (3:1) 3%, NCs and (E) pure PLA. SDS, sodium dodecyl sulfate. Source: Adapted from Demirkaya, Z.D., Sengul, B., Eroglu, M.S., Dilsiz, N., 2015. Comprehensive characterization of polylactide-layered double hydroxides nanocomposites as packaging materials. J. Polym. Res., 22(7), 124, with kind permission of Springer.

into LDH via coprecipitation and the LDH-ibuprofen NF was incorporated into the PLA. Poly(oxyethylene-b-oxypropylene-b-oxyethylene) (Pluronic) was also added to the fibers for increasing the hydrophilicity and modulation of release. As can be seen in the TEM images of PLA/LDH-ibuprofen fibers (Fig. 15.47), NFs were homogeneously distributed throughout the NC fibers. Interestingly, it was observed that the initial ibuprofen release from PLA/LDH-ibuprofen and PLA/Pluronic/LDHibuprofen fibers was faster than those fibers without LDH, which was a result of the strong interaction between alkyl groups in ibuprofen and methyl substituent groups in PLA, as well as the hydrophilicity of LDH-ibuprofen NPs leading to an easier diffusion of water with a faster release of ibuprofen. Amaro et al. (2016) reported the thermo-oxidative stabilization of PLA after incorporation with antioxidant-modified LDH. The utilized antioxidants are 3-(3,5di-tert-butyl-4-hydroxyphenyl)propionic acid (IrganoxCOOH) and 6-hydroxy2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox, a water-soluble analog of vitamin E), which have phenolic moieties. Interestingly, after modification of LDH with antioxidants, antioxidant power remained in terms of Trolox, and was even

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Figure 15.47 SEM images of electrospun fibers of (A) neat PLA, (B) 2 wt.% IBU/PLA and (C) 5 wt.% PLA/LDH-IBU and low- (D) and high-magnification (E) TEM images of 5 wt.% PLA/LDH-IBU composite fibers. IBU, ibuprofen. Source: Adapted from Miao, Y.-E., Zhu, H., Chen, D., Wang, R., Tjiu, W.W., Liu, T., 2012. Electrospun fibers of layered double hydroxide/biopolymer nanocomposites as effective drug delivery systems. Mater. Chem. Phys., 134(2), 623
630, with kind permission of Elsevier.

amplified in the case of IrganoxCOOH, so after a controlled period of aging, both LDH-antioxidants prevented the occurrence of chain scission in PLA (SEC measurements). Furthermore, antioxidants showed a low tendency to migrate away from the LDH-antioxidant incorporated in the matrix (from preliminary migration test) so retaining the antioxidant protected inside the NF layers and leading to it remaining active for a longer time. There is also a report for the preparation of PLA/LDH NCs with fire-retardant behavior and good transparency (Ding et al., 2015). For this purpose, 2-carboxyethyl-phenyl-phosphinic acid (CEPPA) was used for the modification of LDH which not only increased the flame retardancy of PLA but also improved the compatibility between PLA and LDH. The SEM and TEM images of LDH before and after modification with CEPPA are shown in Fig. 15.48, which indicates the exfoliated structure after modification. All the NC films showed good transparency even with a high content of LDH-CEPPA (up to 10 wt.%). Furthermore, the films absorbed the ultraviolet light, which alleviated the embrittlement of PLA films in the procedure. The uniform dispersion of LDH-CEPPA in PLA, coupled with the low visible light absorption of LDH-CEPPA and PLA, lead to good transparency of the PLA/LDH-CEPPA films. The flame-retardant properties of the PLA/LDH-CEPPA films were evaluated using MCC analysis and the corresponding data are summarized in Table 15.9. As can be seen, the peak heat release rate (PHR) value increased with the incorporation of LDH-CEPPA,

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Figure 15.48 Particle morphology and size of LDH and LC samples. (A) SEM and (C) TEM of LDH; (B) SEM and, (D) TEM images of LC. CEPPA:2-carboxylethyl-phenyl-phosphinic acid, LC: LDH-CEPPA. Source: Adapted from Ding, P., Kang, B., Zhang, J., Yang, J., Song, N., Tang, S., et al., 2015. Phosphorus-containing flame retardant modified layered double hydroxides and their applications on polylactide film with good transparency. J. Colloid. Interface. Sci., 440, 46
52, with kind permission of Elsevier. Table 15.9 The MCC data of the LCP films with different LC loadings Sample

LC content (wt.%)

PHR (W/g)

THR (kJ/g)

PLA LCP-1 LCP-5 LCP-8 LCP-10

0 1 5 8 10

294.9 373.8 450.9 458.5 404.2

12.0 11.9 11.2 10.7 9.7

LCP, PLA/LDH-CEPPA; LC:LDH-CEPPA. Source: Adapted from Ding, P., Kang, B., Zhang, J., Yang, J., Song, N., Tang, S., et al., 2015. Phosphoruscontaining flame retardant modified layered double hydroxides and their applications on polylactide film with good transparency. J. Colloid. Interface. Sci., 440, 46
52, with kind permission of Elsevier.

indicating that the modified NFs promoted the degradation rate of the PLA/LDHCEPPA films. However, since endothermic decomposition of the LDH during combustion is negative for the heat release rate, especially at the higher LDH loadings, thus, these two opposite effects led to a maximum PHR value when LDH-CEPPA loadings were lower than 10 wt.%. Moreover, the total heat release (THR) value was decreased by increasing the LDH-CEPPA loadings. Improved flame retardancy property of PLA/LDH-CEPPA may due to the following factors. The first is attributed to the surface property of LDH and LDH after modification with CEPPA. The zeta potential of LDH was positive, indicating the hydrophilic property of the LDH

Applications of layered double hydroxide biopolymer nanocomposites

653

surface, while after modification by CEPPA, the zeta potential of LC LDH-CEPPA changed to a negative value. This change from a hydrophilic to a hydrophobic property improved the compatibility between inorganic LDH NFs and organic PLA matrix. Second, such an exfoliated CEPPA-grafted-LDH structure into PLA improved the dispersion stability of CEPPA with low melting point and enhanced the synergistic effect of flame retardancy. Furthermore, LDH layers can protect the underlying material from further burning and reduce its heat release as a result of the physical barrier property of LDH that can decelerate heat and mass transfer between the gas and the condensed phases. On the other hand, CEPPA is an effective class of reactive flame retardants for polyesters, realized by the esterification reactions between PLA (or its degradative products) and CEPPA. The esterification reactions can be catalyzed by acid point on LDH layers or their metal oxides (degraded during combustion). This favored the improvement of PLA thermal stability.

15.2.5 PVA/layered double hydroxide nanocomposites Unique properties of PVA including chemical stability, biodegradability, biocompatibility, good mechanical and optical properties, etc., make possible its broad range of applications, such as packaging, medicine, drug delivery, bioplastics, membranes and coatings, water treatment, etc. (Goodship and Jacobs, 2009; Wang et al., 2017). Reinforcement of PVA or a blend of PVA with other polymers such as chitosan (Bercea et al., 2015a; Bercea et al., 2015b) or alginate (Kim Phuong, 2014; Lee et al., 2012; Phuong, 2014) with LDH has been reported recently. The prepared NCs have properties such as PH-sensitivity (Bercea et al., 2015a; Bercea et al., 2015b), catalytic (Shu et al., 2015), photostability (Gaume et al., 2013) (necessary for coating applications), improved mechanical and thermal properties (Dinari and Nabiyan, 2016; Huang et al., 2011; Li et al., 2003; Mallakpour and Dinari, 2014a,b, 2016; Mallakpour et al., 2015a,b; Ramaraj and Jaisankar, 2008; Ramaraj et al., 2010; Shu et al., 2014; Tian et al., 2014; Wang et al., 2017; Zhou et al., 2017), electrical conductivity (Chen et al., 2010), flame retardancy (Zhou et al., 2016, 2017), etc. and they have shown the potential for a wide range of applications such as packaging (Du et al., 2014), fibers (Qin et al., 2012; Zhao et al., 2010), water remediation by using a blend of alginate/PVA as matrix (Kim Phuong, 2014; Lee et al., 2012; Phuong, 2014), luminescent ultrathin films (Liang et al., 2014; Zhang et al., 2016), electrospinning (Lv et al., 2016), and fuel cells (Zeng et al., 2012). Researchers have used either pristine LDH (Ramaraj and Jaisankar, 2008; Ramaraj et al., 2010; Zhou et al., 2017) or a modified type as NFs. PVA hydrogels have attracted much attention due to their potential applications as biomedical materials, drug delivery, bioseparation in biotechnology, and carriers for cell immobilization. There are several methods for the preparation of PVA hydrogels. Huang et al. (2012) prepared PVA/LDH NC hydrogels using physical crosslinking by a freezing and thawing technique and investigated the mechanical properties and water swelling behavior of NCs. The freezing and thawing technique has been indicated as having great potential for various applications due to its

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simplicity, lack of toxicity, and low temperature. However, a critical barrier to their applications as load-bearing tissue replacements is a lack of sufficient mechanical properties. In this work, for the preparation of PVA/LDH NC hydrogels, PVA/LDH (0.2 and 0.5 wt.%) solutions were poured into a poly(ethylene terephthalate) (PET) mold and hydrogels were obtained by subjecting the tightly sealed aqueous solution to freezing/thawing cycles: freezing at 220 C for 20 h and thawing at 25 C for 4 h (tightly sealed PET mold guarantees that water does not evaporate and the diameter of the samples does not change with freeze
thaw cycles). Different cycle numbers (7, 9, and11) were chosen to prepare neat PVA and PVA/LDH NC hydrogels. TEM results (Fig. 15.49) showed that the LDH were well dispersed in PVA matrix, and thus increased the physical crosslinking density by inducing PVA crystallization and the formation of more uniform polymer networks. As a result, Young’s modulus (for PVA/LDH NC 5 wt.%: 25%, 22%, and 149% after 7, 9, and 11 freezing/ thawing cycles, respectively), tensile strength (for PVA/LDH NC 5 wt.%: 100%, 100%, and 300% after 7, 9, and 11 freezing/thawing cycles, respectively), and elongation at break of PVA hydrogels were greatly improved, even at very low LDH loading levels. On the other hand, the water swelling ability of PVA/LDH NC hydrogels decreased due to high crosslinking density caused by nonswollen LDH. Mallakpour et al. modified LDH with diacids (Mallakpour and Dinari, 2014a,b, 2016; Mallakpour et al., 2015a,b) via ion exchange reaction and coprecipitation. By

Figure 15.49 TEM images at low (A) and high (B) magnifications, showing a fine dispersion of LDH throughout PVA NC hydrogel containing 0.2 wt.% LDH. The dark platelets or disks represent LDH, and the light region is the polymer matrix. (C) Schematic of the formation of PVA/LDH NC hydrogels. Source: Adapted from Huang, S., Yang, Z., Zhu, H., Ren, L., Tjiu, W.W., Liu, T., 2012. Poly (vinly alcohol)/nano-sized layered double hydroxides nanocomposite hydrogels prepared by cyclic freezing and thawing. Macromol. Res., 20(6), 568-577, with kind permission of Springer.

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Figure 15.50 XRD patterns of different samples. MLDH, modified LDH. Source: Adapted from Mallakpour, S., Dinari, M., 2014a. Manufacture and characterization of biodegradable nanocomposites based on nanoscale MgAl-layered double hydroxide modified with N, N0 -(pyromellitoyl)-bis-L-isoleucine diacid and poly (vinyl alcohol). Polym. Plast. Technol. Eng., 53(9), 880-889; Mallakpour, S., Dinari, M., 2014b. Novel bionanocomposites of poly (vinyl alcohol) and modified chiral layered double hydroxides: Synthesis, properties and a morphological study. Prog. Org. Coat., 77(3), 583
589, with kind permission of Elsevier.

using N,N0 -(pyromellitoyl)-bis-l-phenylalanine diacid as modifier (Mallakpour and Dinari, 2014b), an expansion in interlayer distance was observed in the XRD pattern of this modified chiral LDH compared to the unmodified type. On the other hand, the complete disappearance of LDH peaks in the XRD patterns of NCs (4, 6, 8 wt.%) could be due to the complete exfoliated structure (Fig. 15.50). Furthermore, extensive TEM observations of NC 8 wt.% revealed the coexistence of organo-nano-LDH layers in the intercalated and partially exfoliated morphology (Fig. 15.51). In another work, Mallakpour et al. (2015a) synthesized a diacid from the reaction of tetra-bromophthalic anhydrides and L-aspartic acid and used this diacid for modifying LDH. It was observed that by incorporation of modified LDH into the PVA matrix, the mechanical behavior of NCs was improved significantly compared to pure PVA (Table 15.10). This was due to the strong interaction between modified LDH and PVA via hydrogen bonding which could cause good dispersion of LDH in the matrix. On the other hand, the increase in the stiffness of NCs by increasing the amount of LDH caused a decrease in the elongation at break of NCs compared to pure PVA. Dinari and Nabian (2016) used citric acid (CA) as a modifier for the preparation of bio-NCs based on PVA. The CA-modified ZnAl-LDH (CA-LDH) was synthesized by coprecipitation method in aqueous media and different amounts of this NF

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Figure 15.51 TEM micrographs of MLDH (A, B) and PVA hybrids containing 8 wt.% of MLDH (C, D). MLDH, modified LDH. Source: Adapted from Mallakpour, S., Dinari, M., 2014a. Manufacture and characterization of biodegradable nanocomposites based on nanoscale MgAl-layered double hydroxide modified with N, N0 -(pyromellitoyl)-bis-L-isoleucine diacid and poly (vinyl alcohol). Polym. Plast. Technol. Eng., 53(9), 880-889; Mallakpour, S., Dinari, M., 2014b. Novel bionanocomposites of poly (vinyl alcohol) and modified chiral layered double hydroxides: Synthesis, properties and a morphological study. Prog. Org. Coat., 77(3), 583
589, with kind permission of Elsevier.

Table 15.10 Mechanical properties from tensile testing of PVA and PVA/mLDH NCs Sample

Tensile strength (MPa)

Young’s modulus (MPa)

Elongation at break (%)

Pure PVA PVA/mLDH NC 2 wt.% PVA/mLDH NC 4 wt.% PVA/mLDH NC 8 wt.%

40.00 40.66 51.74 116.64

1.101 1.328 1.965 5.106

8.00 5.31 5.51 4.86

mLDH, modified LDH. Source: Adapted from Mallakpour, S., Dinari, M., Hatami, M., 2015a. Novel nanocomposites of poly (vinyl alcohol) and Mg
Al layered double hydroxide intercalated with diacid N-tetrabromophthaloyl-aspartic. J. Therm. Anal. Calor., 120(2), 1293
1302; Mallakpour, S., Dinari, M., Talebi, M., 2015b. Exfoliation and dispersion of LDH modified with N-tetrabromophthaloyl-glutamic in poly (vinyl alcohol): morphological and thermal studies. J. Chem. Sci., 127(3), 519
525, with kind permission of Elsevier.

(2, 4, and 6 wt.%) were added to the PVA solution for the preparation of PVA/CALDH NC materials under ultrasonic irradiation by solution film-casting techniques. TEM images (Fig. 15.52) of NCs revealed the good dispersion of the LDH platelets into the PVA matrix in addition to the well-organized intercalated regions. The

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Figure 15.52 TEM micrographs of the (A, B) CA-LDH and (C, D) NCs of PVA with 4% of CA-LDH. CA, citric acid. Source: Adapted from Dinari, M., Nabiyan, A., 2016. Citric acid-modified layered double hydroxides as a green reinforcing agent for improving thermal and mechanical properties of poly (vinyl alcohol)-based nanocomposite films. Polym. Compos., with kind permission of John Wiley and Sons. Table 15.11 Mechanics properties of pure PVA and different NCs of PVA and CA-LDH Sample

Tensile strength (MPa)

Ultimate strain (%)

Pure PVA NC2% NC4% NC6%

57.6 68.1 80.3 70.2

106.8 89.4 74.6 62.7

CA, citric acid. Source: Adapted from Dinari, M., Nabiyan, A., 2016. Citric acid-modified layered double hydroxides as a green reinforcing agent for improving thermal and mechanical properties of poly (vinyl alcohol)-based nanocomposite films. Polym. Compos., with kind permission of John Wiley and Sons.

effective incorporation of modified LDH into the matrix led to an increase in thermal decomposition temperature and improved mechanical properties. Tensile stress increased by increasing the CA-LDH loading (when LDH loading is less than 4 wt. %), and then decreased with incorporation of more LDH such as 6 wt.% (Table 15.11). The main explanation for the improvement in tensile modulus in PVA NCs was the good interaction between matrix and LDH layers via formation of hydrogen bonds of the clay edges. The improvement in thermal properties was attributed to the homogeneous dispersion of CA-LDH in matrix and the strong hydrogen bonding between OH groups of PVA and the oxygen atoms of LDH

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Figure 15.53 The schematic representation of the preparation procedure of the PC-LDH hybrid. PC, phosphorylated cellulose. Source: Adapted from Wang, W., Kan, Y., Pan, H., Pan, Y., Li, B., Liew, K., et al., 2017. Phosphorylated cellulose applied for the exfoliation of LDH: An advanced reinforcement for polyvinyl alcohol. Compos. Part A: Appl. Sci. Manuf., 94, 170
177, with kind permission of Elsevier.

layers or the carbonyl group of intercalated citrate anion. In addition, the UV-Vis transmission spectra exhibited that the samples retained high optical clarity, even at high clay loadings (6 wt.%). Therefore, these materials may have potential applications in the packaging industry. Some works report the flame-retardant properties of PVA/LDH NCs. Wang et al. (2017) used phosphorylated cellulose (PC) as a modifier agent for LDH and different amounts of this filler (PC-LDH) were used for the reinforcement of PVA matrix (Fig. 15.53). The results showed that incorporation of PVA with PC-LDH could cause improved mechanical and thermal properties of PVA NCs. Improvement in mechanical properties of PVA after incorporation of PC-LDH (Table 15.12) was due to the excellent compatibility between PVA and PC, which existed due to hydrogen bonds caused by the OH groups in PVA and the ring oxygen in PC. The flame retardancy of the pure PVA and its NCs was investigated by MCC test and the corresponding data are collected in Table 15.12. After the incorporation of PC-LDH, PVA-PC-LDH showed significantly improved flame retardancy, and the HRR value was also decreased with the increase in the PC-LDH content. With the same loading, PC-LDH-based PVA performed better than LDHfilled PVA, which showed lower HRR and THR values. The THR value was reduced after the addition of PC-LDH. These results were attributed to the synergistic effect, which contained a physical barrier effect of LDH layers and the catalytic charring effect of phosphorus element.

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Table 15.12 MCC and tension tests results of pure PVA and its composites Sample

HRR (W/g)

PVA0 PVA-LDH2 PVA-P-LDH0.5 PVA-P-LDH1 PVA-P-LDH2

201.5 237.3 198.4 155.0 116.9

6 6 6 6 6

16.2 13.3 9.5 11.3 7.9

THR (kJ/g)

24.0 21.7 22.8 20.1 61.4

6 6 6 6 6

1.2 0.9 0.7 0.9 0.6

Tensile strength (MPa) 38.7 36.4 45.4 52.4 61.4

6 6 6 6 6

3.9 2.8 2.7 3.3 4.2

Elongation at break (%) 178 6 19 86 6 10 151 6 16 135 6 17 98 6 8

Source: Adapted from Wang, W., Kan, Y., Pan, H., Pan, Y., Li, B., Liew, K., et al., 2017. Phosphorylated cellulose applied for the exfoliation of LDH: An advanced reinforcement for polyvinyl alcohol. Compos. Part A: Appl. Sci. Manuf., 94, 170
177, with kind permission of Elsevier.

Figure 15.54 Illustration for the preparation of MoS2-LDH nanohybrids by self-assembly method. Source: Adapted from Zhou, K., Hu, Y., Liu, J., Gui, Z., Jiang, S., Tang, G., 2016. Facile preparation of layered double hydroxide/MoS 2/poly (vinyl alcohol) composites. Mater. Chem. Phys., 178, 1
5, with kind permission of Elsevier.

Zhou et al. (2016) prepared LDH/MoS2 hybrid as a promising NF for flame retardancy by self-assembly of exfoliated MoS2 nanosheets and LDH nanoplates via electrostatic interaction (Fig. 15.54). Subsequently, the prepared hybrids (loading of 0.5, 1, and 3 wt.%) were incorporated into PVA to serve as reinforcements by a solution blending method. According to TGA thermograms of PVA and its NCs (Fig. 15.55A), the presence of LDH/MoS2 hybrids in the composites resulted in poor thermal stability. However, PVA composites with LDH/MoS2 hybrids resulted in a higher amount of residues than neat PVA, owning to the catalytic carbonization effect of the LDH/MoS2 hybrids. The combustion behavior of PVA and its NCs was evaluated by MCC and their heat release rate (HRR) curves are shown in Fig. 15.55B. It was observed that incorporation of a small amount of

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Layered Double Hydroxide Polymer Nanocomposites

Figure 15.55 TG and HRR curves of PVA and PVA composites. Source: Adapted from Zhou, K., Hu, Y., Liu, J., Gui, Z., Jiang, S., Tang, G., 2016. Facile preparation of layered double hydroxide/MoS 2/poly (vinyl alcohol) composites. Mater. Chem. Phys., 178, 1
5, with kind permission of Elsevier.

LDH/MoS2 hybrids significantly reduced the fire risk of the PVA NC. PVA is a flammable polymer with high peak heat release rate (PHRR). The PHRRs of all the PVA composites were lower than pure PVA and decreased gradually as the loading of the LDH/MoS2 hybrids increased. When the loading of LDH/MoS2 hybrids reached 3 wt.%, the PHRR value was decreased from 288 W/g for neat PVA to 151 W/g with a reduction of 48%, indicating that the LDH/MoS2 hybrids could significantly reduce the fire risk of the PVA. Moreover, the microstructure of the char residues was investigated by SEM, as shown in Fig. 15.56A,B. As can be observed in Fig. 15.56A, neat PVA showed a discontinuous char layer with a large amount of cracks and holes. However, after addition of 3 wt.% LDH/MoS2 hybrids, the char

Applications of layered double hydroxide biopolymer nanocomposites

(A)

661

(B)

G

(C)

G

(D)

ID/IG = 3.69

ID/IG = 3.07 D

400

600

800

1000

1200

1400

Raman shift (cm–1)

1600 1800 400

600

800

1000

1200

D

1400

1600 1800

Raman shift (cm–1)

Figure 15.56 SEM images of the char residues for PVA (A) and PVA composites (B), and Raman spectra of the char residues for PVA (C) and PVA composites (D). Source: Adapted from Zhou, K., Hu, Y., Liu, J., Gui, Z., Jiang, S., Tang, G., 2016. Facile preparation of layered double hydroxide/MoS 2/poly (vinyl alcohol) composites. Mater. Chem. Phys., 178, 1
5, with kind permission of Elsevier.

residues became more compact and dense, which provided a more effective protecting layer during combustion. Fig. 15.56C,D shows the Raman spectra of the residual char of neat PVA and PVA composites with 3 wt.% LDH/MoS2 hybrids. Raman spectroscopy is widely used to prove the existence of carbonaceous char in the residue and to analyze the special component of the char. It can be observed that the ID/IG ratio followed the sequence of PVA composites (3.07) , pure PVA (3.69), indicating a higher graphitization degree and the most thermally stable char structure of the PVA composites with LDH/MoS2 hybrids. The high content of graphitized carbons in the residual char could act as a barrier to mass and heat transfer and decrease the heat release rate during combustion. Qin et al. (2012) prepared PVA/LDH NC nanofibers using an electrostatic fiber spinning. In this work, either inorganic LDH carbonate (LDH-CO3) or L-lactic acid-modified LDH (Lact-LDH) were used as NF. As can be seen in Fig. 15.57, neat PVA, PVA/LDH-CO3, and PVA/Lact-LDH showed different stability behavior in aqueous solution after ultrasonication. The LDH-CO3 nanoparticles deposited at the bottom of the bottle after 1 h because of the agglomeration, whereas the LactLDH could keep stable even after a long storage time. Therefore, the dispersion of Lact-LDH improved the electrospinnability of the mixture well, which resulted in a decrease in the average diameter of fibers. TEM investigations (Fig. 15.58) also indicated that the dispersity of the LDH in PVA matrix was much improved after modification with L-lactic acid. The lactic acid in the interlayer of LDH may

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Layered Double Hydroxide Polymer Nanocomposites

Figure 15.57 Images of the appearances of LDH dispersion in PVA solutions. (A) After ultrasonication, (B) after storage for 1 h. Source: Adapted from Qin, Q., Liu, Y., Chen, S.C., Zhai, F.Y., Jing, X.K., Wang, Y.Z., 2012. Electrospinning fabrication and characterization of poly (vinyl alcohol)/layered double hydroxides composite fibers. J. Appl. Polym. Sci., 126(5), 1556
1563, with kind permission of John Wiley and Sons.

Figure 15.58 TEM images of electrospun fibers. (A, D) PVA/5% LDH-CO3, (B, E) PVA/ 3% Lact-LDH; (C, F) PVA/5% Lact-LDH. Lact, L-lactic acid. Source: Adapted from Qin, Q., Liu, Y., Chen, S.C., Zhai, F.Y., Jing, X.K., Wang, Y.Z., 2012. Electrospinning fabrication and characterization of poly (vinyl alcohol)/layered double hydroxides composite fibers. J. Appl. Polym. Sci., 126(5), 1556
1563, with kind permission of John Wiley and Sons.

weaken the strong cohesion between layers of the nanoparticles and interact with PVA chains, and therefore greatly improves the dispersibility of LDH in PVA matrix. The mechanical properties of the PVA/LDH fibers were obviously enhanced compared to those of neat PVA. For example, the tensile stress and elongation at break of the PVA/Lact-LDH electrospun fibrous mat with 5 wt.% Lact-LDH were

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Figure 15.59 Top: Schematic illustration of the synthetic strategy of artificial nacre-like Au NPs 2 LDH 2 PVA hybrid films. Bottom: Cross-sectional SEM images of Au NPs 2 LDH 2 PVA nacre-like hybrid films with different Au NPs densities. (A) WAu:WLDH 5 0.5:1 hybrid film. (B) WAu:WLDH 5 1:1 hybrid film. (c) WAu:WLDH 5 2:1 hybrid film. (D) WAu:WLDH 5 4:1 hybrid film. NPs, nanoparticles. Source: Adapted from Shu, Y., Yin, P., Liang, B., Wang, H., Guo, L., 2015. Artificial nacre-like gold nanoparticles
layered double hydroxide
poly (vinyl alcohol) hybrid film with multifunctional properties. Ind. Eng. Chem. Res., 54(36), 8940
8946, with kind permission of the American Chemical Society.

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31.7 MPa and 36.7%, respectively, which were significantly higher than those of neat PVA, and also higher than those of PVA/LDH-CO3 owing to the better dispersity of Lact-LDH nanoparticles. Shu et al. (2015) synthesized LDH-Au NPs hybrid as NF which caused nacrelike PVA/Au NPs 2 LDH NCs with multifunctional properties including Raman scattering and catalytic properties for a reduction reaction. For this purpose, the nanosheets of LDH were initially modified with (3-aminopropyl) triethoxysilane to increase the interactions between LDH and Au NPs via a coordination reaction with amino groups of silane modifier. The NC films were fabricated via bottom-up assembly of pretreated LDH-Au NPs nanosheets and subsequent spin-coating of PVA (Fig. 15.59). The reduction of 4-nitrophenol (4-NP) by NaBH4 was chosen as a model reaction to study the catalytic properties of multifunctional PVA/Au NPs 2 LDH hybrid films. The reduction was followed by UV-Vis analysis. In the absence of hybrid films, no signs of reduction were observed, even in a period of 3 days. However, an obvious change in the UV-Vis spectra was found upon the addition of a piece of PVA/Au NPs 2 LDH hybrid film (WAu:WLDH 5 4:1). As can be seen in Fig. 15.60, the absorption at 400 nm significantly decreased as the reaction proceeded. Meanwhile, a new peak appeared at 295 nm and gradually increased as the reaction went on, revealing the successful reduction of 4-NP to 4-aminophenol (4-AP). Moreover, observation of an isosbestic point (320 nm) between two absorption bands indicated that the catalytic reduction of 4-NP yielded 4-AP without any byproducts.

Figure 15.60 UV-vis spectra of the reduction of 4-NP in an aqueous solution recorded every 2 min using the Au NPs 2 LDH 2 PVA hybrid film as a catalyst. 4-NP, 4-nitrophenol; 4-Ap, 4-aminophenol; NPs, nanoparticles. Source: Adapted from Shu, Y., Yin, P., Liang, B., Wang, H., Guo, L., 2015. Artificial nacrelike gold nanoparticles
layered double hydroxide
poly (vinyl alcohol) hybrid film with multifunctional properties. Ind. Eng. Chem. Res., 54(36), 8940
8946, with kind permission of the American Chemical Society.

Applications of layered double hydroxide biopolymer nanocomposites

15.3

665

Conclusions

The focus of this chapter is the recent advances in biopolymer/LDHs NCs based on most of the biopolymer matrices including polysaccharide, protein, PHA, PLA, and PVA with LDH NFs. Researchers have used both pristine LDH and modified types as NFs, where the modifiers are various molecules such as drugs, surfactants, biomolecules, etc. Several methods such as coprecipitation, ion exchange, in situ polymerization, etc., have been used for the fabrication of biopolymer/LDH NCs. Based on the literature, incorporation of LDH into biopolymers can cause improved mechanical, optical, barrier, and thermal properties, which endows the biopolymer/ LDH NCs with possible utilization in many important applications, such as flame retardants, water treatment, drug delivery, tissue engineering, packaging, and catalysts.

Acknowledgments The authors acknowledge the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, I. R. Iran, for partial financial support. Further financial support from National Elite Foundation (NEF), Tehran, I. R. Iran, Iran Nanotechnology Initiative Council (INIC), Tehran, I. R. Iran and Center of Excellence in Sensors and Green Chemistry Research (IUT)), Isfahan, I. R. Iran, is gratefully acknowledged.

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Raji Vijayamma1, Nandakumar Kalarikkal1,2 and Sabu Thomas1,3 1 International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India, 2School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala, India, 3School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

16.1

Introduction

In recent years, there has been considerable excitement over the potential benefits associated with applying nanotechnology to biological systems. Advances in nanotechnology have pushed forward the synthesis of a variety of functional materials such as semiconductor quantum dots, metallic nanoparticles (NPs), inorganic NPs, and nanocomposites with polymers or bioactive molecules (Thanh and Green, 2010; Pankhurst et al., 2003; Kim et al., 2013; Kang et al., 2014b; Mulero et al., 2010; Shah et al., 2011; Bhattarai and Bhattarai, 2013). The unique electronic, magnetic, and optical properties exhibited by these nanometer-sized materials have enabled a broad spectrum of biomedical applications. Biomedical applications of NPs include drug carriers, labeling and tracking agent vectors for gene therapy, hyperthermia treatments, and magnetic resonance imaging (MRI) contrast agents. Among the large number of highly advanced materials, a class of inorganic NPs, namely layered double hydroxide (LDHs) NPs, has attracted considerable interest as they are potential candidates for drug release and gene delivery (Oh et al., 2009; Aware et al., 2016). LDH, also known as anion clays, can be represented by the general formula of [M211
xM31x(OH)2]x1 (Am
)x/m  nH2O, where M21 and M31 designate the diand trivalent cations, Am2 the interlayer anions, and x 5 (M31/(M21 1 M31)) the hydroxide layer charge density (Aware et al., 2016; Khan et al., 2009). They are superb anion-exchange materials, cheap and facile to prepare, and mostly biocompatible. They therefore find application in diverse areas such as heterogeneous catalysis, separation science, and optical materials. It has been recognized for some time that LDHs could be useful materials for the uptake, storage, and controlled release of industrially important (and particularly biologically active) anions (Xu et al., 2007; Wong et al., 2010). A schematic illustration of LDH structure is shown in Fig. 16.1. Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00016-1 © 2020 Elsevier Ltd. All rights reserved.

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Figure 16.1 Schematic illustration of layered double hydroxide structure and chemical components. Source: Copyright 2015. Image reproduced with permission from Royal Society of Chemistry.

Due to their unique host
guest structure, many therapeutic drugs and biomolecules, such as nonsteroidal antiinflammatory drugs (NSAIDs), anticancer agents, and cardiovascular drugs, can be readily intercalated into the LDH interlayer via anion exchange and coprecipitation routes to form drug-intercalated LDH nanohybrids (Li et al., 2011, 2014). Novel biohybrids of LDH and biomolecules are designed and organized artificially on the nanometer scale to provide opportunities for reservoirs and delivery carriers of functional molecules in gene therapy and drug delivery.

16.1.1 Layered double hydroxide nanocomposites With the development of nanotechnology and antigen-delivery systems, biomaterial-based NPs can offer several advantages over other traditional antigendelivery systems. Many structurally stable materials have been investigated for drug delivery. LDH, commonly known as hydrotalcite-like materials and anionic clays, is a family of layered nanomaterials that has been widely applied in catalysis, absorption, pharmaceutics, and photochemistry. In recent years, the synthesis of LDH-based nanocomposites has received much attention because of the potential use of nanocomposite-based engineering materials. The use of LDH materials in preparing polymer nanocomposites is a new field. LDH is a host
guest material consisting of positively charged metal hydroxide sheets with intercalated anions and water molecules. Over the past decade, LDHs have attracted much attention in drug delivery and gene therapy owing to their good biocompatibility, nontoxicity, and controlled-release property. Kang et al. (2014a) found that LDH nanosheets could be stabilized by alginate molecules in aqueous media and that LDH nanosheets in nanocomposites could be redispersed in aqueous media. Recently, nanocomposite materials with a shell
core structure produced by combining two kinds of nanometer materials have received much attention, as these materials incorporate the virtues of the individual

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materials. Zhao et al. (2015) reported in their study the immune efficacy of LDH@SiO2 NPs with a shell
core structure as a delivery carrier for Newcastle disease virus DNA vaccine. They found the advantage of LDH to protect the plasmid DNA against nuclease degradation, realizing the long-acting mechanism of DNA vaccine within the chicken’s body. Based on the results, LDH@SiO2 NPs can be used as a delivery carrier for mucosal immunity of Newcastle disease DNA vaccine, and have great application potential in the future. LDH materials, being unstable in acidic conditions, do not survive for long in the stomach. However, given a suitable enteric coating, slow release of drugs into the small intestine could be achieved, leading to effective delivery of genetic materials and drugs into cells. The development of different nanocomposites by combining various nanomaterials with other functional molecules, such as biopolymers or biomolecules, is an emerging area in the field of nanobiotechnology (Zhao et al., 2015a; Kumar et al., 2018; Xu et al., 2006). Due to the rich ionic surface
OH group and the inherent positive charge, LDH NPs or nanosheets can interact with other nanomaterials or polymeric molecules, generating functionally active nanocomposites with particular architectures. Different classes of LDH nanocomposites are described elsewhere, with their synthesis and biomedical applications in a review published in 2015. Gu et al. (2015) classified LDH nanocomposites into four classes: core@LDH, LDH@shell, dot-coated LDH, and targeting moiety functionalized LDH. In the core
shell structure, the functionality of LDH endows LDH with a dual role. LDH can be the shell component that is used to modify other NPs including silica NPs, magnesium ferrite NPs, etc. (Fig. 16.2A), and can also be fabricated as the core coated by other nanomaterials, such as the silica shell or the polymeric

Figure 16.2 Schematic illustration of LDH-based nanocomposites with different structures: (A) core@LDH; (B) LDH@shell; (C) dot-coated LDH; and (D) targeting moiety functionalized LDH. Source: Copyright 2015. Image reproduced with permission from Royal Society of Chemistry.

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molecule shell (Fig. 16.2B). Dot-coated LDH is a new group of composites with smaller silica or metal NPs being deposited on the surface of LDH (Fig. 16.2C). Conjugating antibodies, other biopolymers, or folic acid on LDH provides an opportunity for LDH targeting to the site of disease (Fig. 16.2D). These LDH nanocomposites with different functionalities can be used mainly for various biomedical applications such as drug delivery, gene delivery, bioimaging, and photothermal therapy applications in cancer therapy (Gu et al., 2015). It is possible to develop a nanohybrid structure akin to Au NP-decorated matrices by conjugating gold NPs on the surface of LDH and demonstrate its use for in vitro photothermal therapy and fluorescence imaging (Komarala et al., 2017).

16.1.2 Layered double hydroxide nanocomposites in the medical field The human body is exposed to diverse types of NPs directly or unconsciously by inhalation, ingestion, dermal penetration, and injection. The transferring of drugs and biomolecules across the cell membrane to the cytoplasm is an interesting area of biological research. Conventional drug delivery is not very effective as it suffers from many issues, including enzymatic degradation, poor circulation, noncontrolled dose delivery, and variations in blood drug levels; these may lead to weakening of the immune response and many other problems (Oh et al., 2006a; Zhao et al., 2015b; Wong et al., 2010). To overcome the drawbacks of conventional therapy, new drug-delivery systems (DDSs) have been developed according to multidisciplinary approaches, and several drug-delivery vehicles have been developed— recently research has focused on the excellent properties of NPs (Moger et al., 2008; Rajan and Raj, 2013; Reis and Neufeld, 2006). The great advances in the 20th century included the development of nanomaterials, a miracle for medicine, however the surface charge of NPs became a serious problem. LDH has wide applications in detection, imaging, diagnosis, gene transport, and drug delivery. In modern medicine, drug-delivery molecules have encapsulated small molecule-based drugs, such as proteins and nucleic acid, into the inner layer, increasing the stability and solubility of the drug molecule. LDH has a high ion-exchange affinity to negatively charged drugs and nucleic acid; when it enters into the layer the molecules acquire extra energy by an electrostatic interaction between the cationic and anionic layers within the system (Bi et al., 2014; Zhang et al., 2009; Xu et al., 2006), which makes for effective delivery of impermeable molecules. Among the different LDH NPs, MgAl
LDHs are very often used in drug and gene delivery owing to their low cytotoxicity, high anion exchange capacity, pHcontrolled release, good biocompatibility, tunable particle size, and protection of drugs and genes in their interlayers (Xu et al., 2006; Sydney, 2016). Hydrotalcitederived antacid and antipeptic have had a great impact on the pharmaceutical industry. MgAl has structural materials for the production of antipyretic analgesic and antiinflammatory medicines. Recent research has shown that various biofunctional

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molecules, including DNA, siRNA, drugs, and vitamins, have been successfully preserved by incorporation into LDHs with high delivery efficiency and bioactivity.

16.1.2.1 Cellular uptake mechanism and biodistribution The mechanism of cellular uptake using NPs can be regarded as one of the crucial factors that could determine the rate of cytotoxicity in a biological environment which facilitates greater control in predicting the cytotoxic effects (Oh et al., 2006a; Cooperstein and Canavan, 2013). In earlier studies, passive diffusion was shown to be the main mechanism in drug transport across biological barriers; it is a key determinant in pharmacokinetics. Later studies showed that carrier-mediated drug transport across the biological membrane played major roles, and these two processes are vital in drug transport to areas of interest irrespective of admission route (Shukla et al., 2005; Li et al., 2013; Prakash et al., 2011; Xu et al., 2007). A graphical representation of the proposed cellular uptake of LDH NPs is shown in Fig. 16.3. In the case of LDH, the positively charged outer sheet of the delivery system is attracted by a negatively charged cell membrane, enabling a facile penetration of LDH into cells (Xu et al., 2007; Lu et al., 2011, 2013). A previous study by Kura

Figure 16.3 Proposed cellular uptake mechanism of layered material, LDH. (A) Drug
LDH nanohybrids approach cell membrane; (B) drug
LDH nanohybrids are internalized via clathrin-mediated endocytosis; (C) nanohybrids are transported inside the cell through early endosome; (D) in late endosome, LDHs are partially dissolved due to the slight acidity; (E) in lysosome, drugs are released; and (F) LDHs are externalized via exocytosis. Source: Copyright permission obtained.

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et al. showed the impact of surface charge on LDH and the mechanism of its cellular uptake (Kura et al., 2014; Xu et al., 2006). They proved that LDH cellular uptake and penetration are via endocytosis and mainly through clathrin-mediated pathways. A further influence in LDH particle uptake is particle size and shapes, with sizes between 50 and 200 nm showing concentration-dependent uptake, while sizes 350 nm and above are not concentration-dependent (Mulero et al., 2010; Oh et al., 2006a; Wong et al., 2010). Both hexagonal and rod-shaped LDH nanocomposites entered the cells through the same endocytotic pathway (Liu et al., 2015), however the hexagonal-shaped particles were found to be distributed within the nucleus of the cells. These results suggested further the promising drug-delivery potential of LDH at the cellular level without necessarily damaging cell structure. Oh et al. (2006, 2009) evaluated the role of LDH particle size in cellular uptake. They synthesized four types of hexagonal LDH NPs with various sizes and studied the uptake in osteosarcoma cells, the confocal microscopic images for the cellular uptake of FITC-conjugated LDH NPs is shown in Fig. 16.4.

Figure 16.4 (A) Fluorescence microscopic images of FITC
LDH-treated MNNG/HOS cells depending on incubation time. (B) Immunofluorescence microscopic images showing the colocalization of clathrin and LDH in MNNG/HOS cells. (C) Confocal microscopy: colocalization of FITC
LDH and clathrin in MNNG/HOS cells. Localization of (a) nucleus, (b) clathrin, and (c) FITC
LDH, and the merged image (d) in MNNG/HOS cells. Cells were incubated with FITC
LDH for 2 h, treated with clathrin antibodies, and stained by TR and DAPI. Scale bar represents 10 μm. (D) Magnified images of the white boxes in (C) are also given. The images identify areas showing colocalization of FITC
LDH and clathrin, as shown merged in yellow.

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Fluorescence microscopic study showed size dependant uptake of particles and increasingly strong fluorescent signal inside the cell as the incubation time increased to 2 h. Another interesting finding in this study is that only particles that ranged within a size of 200 nm are uptaken by the cells via clathrin-mediated endocytosis, whereas the larger counterparts entered the cells without any specific cellular entry pathway, providing a new perspective to design LDH for biological purposes. It was further demonstrated that particles of all dimensions were taken up by the cells within a time span of 15 min, irrespective of their size. Interestingly, 50-nm particles showed a high uptake efficacy in the initial stages (up to 8 h), whereas particles with higher dimensions reached a flattened state after 2 h of incubation (Kim et al., 2014b; Oh et al., 2006a). It is interesting to find that the intracellular concentration of all particles increased in a concentration-dependent manner, suggesting possible scope for receptor-mediated endocytotic uptake. The particle retention studies showed that particles with larger dimensions (100
350 nm) tend to have a longer retention time than particles with smaller dimensions, suggesting possible scope for the design of drug-intercalated LDH according to the required retention time (Oh et al., 2006a). Molecules internalized by a process called clatharin-mediated endocytosis and possess several properties such as low cytotoxicity, pH-controllable release, good biocompatibility, tunable particle size, high affinity to anionic molecules, and large layer density protection of molecules during the delivery time (Oh et al., 2006a; Xu et al., 2007; Hasan et al., 2013; Shukla et al., 2005). The targeted DDS helps in reducing the concentration of drug, with long-term distribution of drug in the desired area, thereby reducing any side effects. Drug concentration and frequency of therapy also reduce the treatment expense and duration time. Cellular targeting of the molecule depends on the morphology and size of the particle. Hexagonal NPs were located in the perinuclear cytoplasm, while rod-shaped structures were present in the nucleus. Recent studies have revealed that NP size is very important in drug delivery, with particle sizes of about 50 nm being effectively targeted by groups of cell lines such as HeLa, Caco-2, and HT-29 cells (Zhang et al., 2013; Wong et al., 2010). SiRNA and small molecule drug delivery have different biological, physical, and chemical characteristics. SiRNA has the ability to turn off or on different genes by inhibiting or activating the relevant cellular pathways (Ladewig et al., 2010; Zhang et al., 2014). Nanocomposites are a marvelous material in the era of medical imaging but need modification, otherwise they can degrade in the cellular cytoplasm. Medical imaging can be a very useful tool in understanding and monitoring the functional and biologic events inside the human body (Swarnalatha et al., 2013; Ali and Ahmed, 2018). At higher concentrations of NPs, cellular uptake found increases and the major drawbacks of NPs is the difficulty in structural modifications. Positively charged molecules can easily enter the cytoplasm because of the negatively charged cell membrane. However, penetration to the cell membrane and nuclear membrane is difficult due to the presence of small nuclear pores on the surface (Li et al., 2013; Oh et al., 2006a; Wong et al., 2012). Recently Li Yan et al. reported that the drug-

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delivery efficiency of LDH is increased by conjugating modified folic acid to its structure. This molecule shows a high loading capacity, and high membrane and nuclear penetration ability (Kim et al., 2014a; Chen et al., 2012). Oh et al. (2006) reported the mechanism for the cellular uptake of LDH NPs that are internalized into MNNG/HOS cells, principally via clathrin-mediated endocytosis. The intracellular LDHs are highly colocalized with not only typical endocytic proteins, such as clathrin heavy chain, dynamin, and eps15, but also transferrin, a marker of the clathrin-mediated process, suggesting their specific internalization pathway (Oh et al., 2006a). They demonstrated that the positively or neutrally charged LDHs, which encapsulate anionic molecules, are easily attached to the surface of a negatively charged plasma membrane, and in such a way their internalization into cells could be facilitated. Thus LDHs may prove useful as gene- or drug-delivery vectors (Shafiei et al., 2008; Xu et al., 2007).

16.1.2.2 Tissue distribution of layered double hydroxide nanoparticles The distribution of LDH nanocomposite is not much different from other NPs—the reticular endothelial system (RES) takes a larger amount, especially the liver, kidney, spleen, and lungs (Mulero et al., 2010; Xu et al., 2006). The sizes of the particles play a significant role in determining the particle distribution (Mulero et al., 2010). Particles less than 5 nm are easily removed by renal clearance; sizes greater than 100 nm are mostly sequestrated by the RES of the liver, lungs, kidney, and spleen (Mulero et al., 2010). Intraperitoneal administration of LDH to mice for 5 days of sizes 100
200 nm showed a higher distribution to the liver, lung, spleen, and kidney, but not the brain and/or heart (Xu et al., 2006). Both zinc- and aluminum-based LDH revealed good plasma distribution of NSAIDs after oral ingestion and the analgesic effectiveness is very similar to its counterpart (pure NSAID), with the added advantage of increase gastric tolerability (Gu et al., 2015). LDH containing antibiotics as a local DDS also showed promising results (Komarala et al., 2017). This was made possible due to the likelihood of sustained release and low-toxicity properties of this noble carrier, leading to good drug delivery to the infected middle ear in the presence of a prosthesis. More recently, an in vivo study conducted by a group of scientists showed the distribution of LDH to the liver, lung, and spleen following an intravenous tail vein injection (Aware et al., 2016). This same LDH carrier showed preference for the lung when coated with chitosan. A further increase in chitosan concentration (higher concentration) led to accumulation in the liver and avoided the lung (Aware et al., 2016). This preferential in vivo biodistribution of LDH following modification of the coating substance opened a high potential for developing organspecific DDSs using this noble carrier both in diagnosis and therapy. In general, findings of wider distribution via different routes of administration within the range of 50
250 nm are further adding to the merits of the potential of LDH in drug delivery. However, more needs to be done, especially, the role of different coating materials and active targeting in delivering LDH to specific areas of interest.

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16.2

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Biomedical applications of layered double hydroxide nanocomposites

16.2.1 Layered double hydroxide nanocomposites in drug-delivery applications DDSs are designed to either alter the pharmacokinetics and biodistribution of their associated drugs, or to function as drug reservoirs and should have the ability to enhance several crucial properties of “free” drugs, such as improving their solubility, in vivo stability and specificity, reducing or eliminating tissue damage, protecting the drug, or enhancing its efficacy (Bi et al., 2014). LDH nanocomposites are very interesting biomedical materials owing to their unique properties such as high surface area and porous structure, and have been examined for their application in drug delivery (Xu et al., 2006; Gaikwad et al., 2008). A particularly interesting application of NPs in general and LDH in particular includes as an alternative DDS. This resulted from their local sustained release and high intrinsic pharmacological activity compared with conventional drugs. They also have improved the delivery of poorly water-soluble drugs, targeted delivery of drugs in a cell- or tissue-specific manner, transcytosis of drugs across tight epithelial and endothelial barriers (blood
brain barrier inclusive), and delivery of large macromolecule drugs to intracellular sites of action. Thus, the synthesis of LDH for drug delivery covered a wide range of drugs, including but not limited to CNS diseases, cancers, antiinflammatories and antibiotics, and imaging agents (Kura et al., 2014). Li et al. reported that codelivery of 5-FU and siRNA by MgAl
LDH NPs significantly inhibited the growth of various cancer cells including breast cancer cells (MCF-7), osteosarcoma cancer cells (U2OS), and colon cancer cells (HCT-116) (Fig. 16.5) (Xu et al., 2007; Li et al., 2014). Zhang et al. (2015) synthesized an antiinflammatory drug-loaded MgAl
LDH shell on a magnesium ferrite core via the coprecipitation method and claimed prolonged controlled release of drugs from MgAl
LDH
magnesium ferrite nanohybrids (Zhang et al., 2013). Chen et al. (2012) developed multifunctional upconversion luminescent LDH nanovehicles for tumor optical imaging and therapy via in situ growth of the LDH
5-FU on the surface of silica-modified Y2O3: Er31, Yb31 NIR NPs and demonstrated that these nanohybrids exhibited strong red upconversion fluorescence and better anticancer efficiency (Zheng and Chen, 2016). Another report showed a novel uniform core/shell nanostructure that has a hexagonal MgAl
LDH nanoplate as a core and mesoporous silica as a shell with perpendicularly oriented channels having uniform accessible mesopores, high surface area, and large pore volume. This functional nanocomposite has a slow-release rate of ibuprofen and shows good biocompatibility in living cells, and thus can be used as an efficient material in drug delivery (Links, 2011). It is found that the encapsulation of anticancer drugs within the layers of LDH NPs can improve cellular uptake through clathrin-mediated endocytosis, which consequently suppresses the proliferation of cancer cells. The intercalation of the anticancer drug methotrexate (MTX) into LDHs has been found to overcome drug

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Figure 16.5 Schematic diagram of the LDH codelivery system coloading 5-FU and siRNA (A) and a schematic illustration of the horizontal positioning of 5-FU in the interlayer (B).

resistance in cancer cells (Xu et al., 2006). Another study showed that lowmolecular-weight heparin (LMWH) incorporated in LDHs has 10 times greater cellular uptake than LMWH alone and it exhibits significantly enhanced ability to inhibit vascular smooth muscle cell proliferation. Core
shell nanocomposites composed of a mesoporous silica shell surrounding an LDH core possess an adjustable slow-release property. Li et al. demonstrated that LDHs can regulate DNA vaccine delivery which can enhance the antimelanoma immune response. Kura et al. reported the development of a controlled-release antiparkinsonian nanodelivery system using levodopa as the active agent (Kura et al., 2014; Hasan et al., 2013). A new layered organic
inorganic nanocomposite material with an antiparkinsonian active compound, L-3-(3,4-dihydroxyphenyl) alanine (levodopa), intercalated into the inorganic interlayers of a Zn/Al
LDH was synthesized using a direct coprecipitation method. The resulting nanocomposite was composed of the organic moiety, levodopa, sandwiched between Zn/Al
LDH inorganic interlayers. The intercalated guest molecules of levodopa were found to be arranged in a monolayer manner between the inorganic interlayers. Release of levodopa from the nanocomposite was found to be governed by pseudosecond-order kinetics, and the release time of levodopa from the nanocomposite at pH 7.4 was longer than that at pH 4.8. The cytotoxicity study also demonstrated a decrease in the toxicity potential of levodopa in a normal cell line following its intercalation into Zn/Al
LDH.

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Hence, successful intercalation of levodopa may improve DDSs for the treatment of parkinsonism as well as decrease their potential toxicity to cells. Liu et al. (2015) reported the intercalation of MTX into LDHs via an exfoliation-reassembly process. MTX, one of the antifolate drugs, can effectively deactivate the metabolism of diseased cells through programmed cell death or apoptosis, and has been applied to certain human cancers such as osteosarcoma (bone cancer) and leukemia, etc. Unfortunately, the serious side effects, short plasma halflife, and high efflux rate of MTX have significantly restricted its application. Intercalation of MTX into LDH interlayers results in controlled release of the drug in cells to maintain drug efficacy and also protect MTX from deterioration during transportation (Fig. 16.6). Saifullah et al. reported the development of a controlled-release formulation of 4-amino salicylic acid, an antituberculosis agent, by forming zinc
layered hydroxide (4-ASA-ZLH) nanocomposite (Fig. 16.7). It was also reported that the

Figure 16.6 Illustration of the possible exfoliation-reassembly mechanism for MTX/LDH hybrids and arrangement of MTX/LDH hybrids.

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Figure 16.7 Spatial orientation of 4-ASA three-dimensional structure of 4-amino salicylic acid (A) and molecular structural models of 4-amino salicylic acid intercalated between interlamellae of ZLH (B). Source: Copied with permission.

controlled-release formulation of an antihistamine was obtained by forming a cetirizine
ZLH nanocomposite (Saifullah et al., 2013; Al Ali et al., 2012).

16.2.2 Layered double hydroxide nanocomposites in genedelivery applications In the field of gene delivery using LDH, remarkable work was done as early as 1999 and has continued. A schematic diagram for the genetic molecular code system is shown in Fig. 16.8, it clearly demonstrates the application of layered nanomaterials for genetic coding and decoding. The ideal properties of LDH such as anion exchangeable property and acid decomposable properties that aid in the interaction of LDHs with various negatively charged biomolecules have been well documented (Oh et al., 2006b). LDHs possess a large ion-exchange capacity for negatively charged drugs or DNA molecules and so have the potential for efficient intracellular delivery of membrane-impermeable compounds (Oh et al., 2006b). The use of LDH NPs as gene-delivery vectors can not only enhance the thermal stability of DNA but also improve its cellular uptake efficiency. Xu et al. observed that LDH morphology influenced the targeting of different subcellular compartments in living cells (Xu et al., 2007). Hexagonal LDH NPs with a size of 100 nm mainly localize in the perinuclear area of the cytoplasm, while their rod-like counterparts are concentrated in the nucleus, which offers the possibility of targeted delivery. Hexagonal LDH NPs can efficiently deliver siRNA molecules to the perinuclear cytoplasm of mammalian cells dependent upon the nucleotide sequence, where siRNA-initiated mRNA degradation takes place. Positively charged LDHs, in

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Figure 16.8 Schematic diagram of the genetic molecular code system. The system consists of four steps: (A) encoding, (B) encrypting, (C) decrypting, and (D) decoding. In the encrypting step, the artificially modified genetic codes are incorporated into layered nanomaterials to form stable code units and can be encrypted in various media.

common with other nonviral cellular delivery vectors such as mesoporous silica and polymers (Huang et al., 2009), can easily enter the cell cytoplasm because the cell membranes are negatively charged but they cannot normally penetrate the nucleus due to the impermeable nature of the nuclear membrane and the small size of the nucleus pore. When NPs are used as delivery vectors they are required to target the cell nucleus, where the genetic information is stored and the cell’s function is regulated through the expression of genes. The diagnosis and treatment of disease, such as targeted therapy and gene therapy, could be greatly enhanced by the delivery of drugs into the nucleus. Thus substantial efforts have been undertaken in order to try to develop methods to enable the uptake of NPs into the nucleus. LDH@SiO2 NPs with shell
core structures were developed to encapsulate the F gene of NDV into a eukaryotic expression plasmid pVAX1-F(o) by a coprecipitation method for enhancing the efficacy of a DNA vaccine against NDV (Li et al., 2011). The bioactivity and safety of the resulting NPs (pFDNA
LDH@SiO2 NPs) were studied by in vitro transfection and cytotoxicity analyses. In addition, we assessed the ability of the pFDNA
LDH@SiO2 NPs to induce immune responses and protect specific pathogen-free chickens from NDV infection after intranasal administration. We found that the carrier has the advantage of LDH to protect the plasmid DNA against nuclease degradation, realizing the long-acting mechanism of DNA vaccine within the chicken’s body (Zhao et al., 2015b).

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Wong et al. (2012) reported the cellular delivery efficiency of siRNAs or their mimic double-stranded (ds) DNA using LDH NPs dependent upon the nucleotide sequence. The efficacy of LDH-mediated delivery of four different siRNAs into cortical neurons and NIH3T3 cells was found to vary widely. In another study, they reported the LDH-mediated delivery of siRNAs to primary cultured neurons and described the internalization by neurons as rapid, dose-dependent, and saturable, and markedly more efficient. They demonstrated that siRNA LDH complexes are internalized by clathrin-dependent endocytosis at the cell body and in neurites, with subsequent retrograde transport to the cell body followed by efficient release into the cytoplasm. This study confirms the potential of LDH NPs as a DDS for patients suffering from neurodegenerative disease (Wong et al., 2010). LDHs can intercalate many important biomolecules with negative charge such as oligomers, single- or double-stranded DNA, and simple molecules like nucleotides. In particular, single- or double-stranded DNAs have a great deal of application potentials in various fields, from gene therapy to biosensing, and even high-density information storage. However, DNA strands are very susceptible to degradation and denaturation occurring during manufacture processes and storage. Choy et al. (1999) clearly demonstrated that biomolecules such as CMP, AMP, GMP, and even DNA are bound to LDHs by anion exchange, yielding heterostructured nanohybrids (Fig. 16.9). The intercalated DNA was safely protected from harsh conditions

Figure 16.9 Various biomolecule
LDH hybrids obtained by intercalation reaction: (A) pristine MgAl
LDH, (B) CMP
LDH hybrid, (C) AMP
LDH hybrid, (D) GMP
LDH hybrid, and (E) DNA
LDH hybrid.

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including strong alkaline or weak acidic environments, and DNase attack. It could also be recovered very easily by exposing DNA
LDH hybrids to an acidic condition due to the solubility of LDHs in acid, implying promising potential of LDHs in biological applications. Choy et al. (2000) also developed LDHs as nonviral vectors for delivery of antisense oligonucleotides. The antisense of the myc cancer gene was intercalated into LDH and transferred into HL-60 cells, the human promyelocytic leukemia cell line. The myc gene encodes a transcription factor, which plays a prominent role in a variety of commonly occurring human cancers. The effect of As-myc
LDH hybrids on the growth of HL-60 cells was considerably higher than that of As-myc alone (about 65% inhibition vs 88% after 4 days) in a time-dependent manner. It is worth noting that LDH itself did not affect cell growth, suggesting its good biocompatibility. It is, therefore, concluded that the suppression effect of cancer cell growth only results from As-myc
LDH (Scheme 16.1). Molecules intercalated into LDHs can be easily released by carbonate ions, which possess extremely high affinity with LDH layers. Furthermore, LDHs can be prepared from biocompatible compositions with arbitrarily tailored physical and chemical properties, which is a great advantage over other inorganic matrices and clay minerals. LDHs are also completely decomposed by acidic body fluids. A schematic illustration is shown in Fig. 16.10. Wong et al. reported in a study that there are three possible modes of interaction between LDHs and siRNA/dsDNA: full intercalation, partial intercalation, where the strand of siRNA/dsDNA protrudes from the interlayer, and adsorption onto the LDH surface (Fig. 16.10). Similar

Scheme 16.1 Schematic diagram for cellular uptake of As-myc
LDH nanohybrids compared to As-myc alone.

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Figure 16.10 Three interaction modes of siRNA/dsDNA
LDHs. (A) Adsorption onto the LDH basal plane; (B) edge adsorption and partial intercalation with part of siRNA/dsDNA protruding; and (C) full intercalation. Source: Copyright 2012. Reproduced with permission from Elsevier.

interaction modes have previously been reported in the context of organic dye
LDH interactions, where they occur as a consequence of the anion exchange process. An investigation into in situ kinetics demonstrated that during anion exchange the anionic dye (300
500 Da) interacted with the LDH via three distinct modes: edge adsorption, external basal plane adsorption, and intercalation (Wong et al., 2010; Zhang et al., 2014; Wei et al., 2015). Ladewig et al. (2010) reported an efficient approach to deliver siRNA into mammalian cells using LDH. Their results showed an efficient downregulation of protein expression upon LDH-mediated siRNA transfection of HEK293T cells with high biocompatibility. In this unique strategy the negative repellant charge of cellular membranes for the delivery of nucleic acids was overcome by protecting the positively charged unique layered structure of LDHs, which facilitates easy permeation into the cellular membranes. It was shown that naked siRNA cannot enter cells, whereas siRNA intercalated with LDH NPs was taken up efficiently. Relatively recent research has suggested that the particle size of NPs determines the efficiency of siRNA delivery. They reported that LDHs with a smaller particle size (45 nm) are more efficient in delivering siRNA in HEK293T cells than larger particles (114 nm) (Chen et al., 2012). Kuo et al. (2015) demonstrated that core
shell quantum dots (CdSe
ZnS) capped with various nucleus localization signaling (NLS) peptides can enter the nucleus of CHO, NIH3T3, CV-1, and HepG2 cells. Kuo also found that some very small quantum dots without NLS coating could enter the nucleus, suggesting that

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size may play an important role. Gold NPs bioconjugated with nucleus-targeting peptides facilitate nucleus transportation, which can selectively disturb the division of cancer cells. However, these studies have shown that for efficient nucleus delivery, NPs must be decorated by proteins or peptides with protein transduction domains. Another impediment is that the quantum dots or gold NPs are not biodegraded as well as LDHs.

16.2.3 Bioimaging applications Noninvasive fluorescent probes have gained the interest of many researchers for the toxicological assessment and tracking the fate of NPs in biological systems. Diagnostic applications of LDHs are rarely found, in contrast to other inorganic nanocarriers. However LDHs are ideal candidates for imaging (Kuthati et al., 2015). The unique layered structure of LDH NPs can aid the loading and safe transportation of contrast agents to the targeted site. One of the important reasons for the retarded development of LDHs as imaging tools is the acid decomposable property of the LDH framework, which can leach out the imaging contents, which is highly undesirable. However, various approaches have been developed to overcome this drawback. Lee and group synthesized chitosan-coated NIR LDH NPs for in vivo optical imaging (Wei et al., 2015). In this unique organic
inorganic nanohybrid the adsorption of indocyanine green (ICG) in the amine-modified layered structure rendered a high NIR contrast efficacy in in vivo conditions (Kuthati et al., 2015). Chen et al. (2013) synthesized novel multifunctional nanoconstructs that exhibit tumor optical imaging by monodispersed Y2O3: Er31, Yb31 NPs as NIR fluorescent nanophosphors and therapy by structured Mg21/Al31 LDH nanosheets used for cellular delivery intercalated with an antitumor agent fluorouracil (5-FU), deposited hierarchically on the surface of Y2O3: Er31, Yb31@SiO2 (Xu et al., 2006). This nanovehicle exhibited strong red upconversion fluorescence at 980 nm laser after localization in cancer cells. In addition, novel construct gadolinium-doped LDH/Au clay metal nanocomposites were synthesized not only for imaging as a diagnostic agent but also as a pH-responsive drug-delivery vehicle (Gasperowicz and Otto, 2008). High loaded doxorubicin release was monitored at specific tumor site pHs or within the cell. Moreover, this nanohybrid exhibited better in vitro MRI capabilities than commercially available MRI and CT agents, with favorable in vivo imaging performance. These nanovehicles could be used in the diagnostic and theranostic fields (Wang et al., 2013). Many metals have been proven to have excellent contrast efficacy and are widely used for MRI but their clinical application is limited due to their serious side effects. Gadolinium and terbium belong to such a class of toxic metals that were approved by the United States Food and Drug Administration (US FDA) for MRI imaging, however recent studies have shown the cytotoxic side effects caused by these contrasting agents. The use of LDHs for the delivery of these cytotoxic metal contrasting agents can be regarded as an innovative step in the field of disease diagnosis as these efficient strategies were shown to decrease the severe cytotoxicity associated with metal contrast agents, along with sharp emissions and large

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Stokes shifts improving the sensitivity. This novel finding may have applicability for other cytotoxic MRI contrast agents which may open up new possibilities in synthesizing highly efficient contrast agents with reduced cytotoxicity.

16.2.4 Biosensor A biosensor is a small device used for monitoring of biochemical changes in a system that converts the variation to analog signals (Hegner and Arntz, 2004). A biosensor consists of three parts, the sensor, transducer, and a signal converter, and works on changes in the medium to electrical signals. Biosensors can be classified into five types according to the transducer: electrochemical, optical, thermometric, piezoelectric, and magnetic. Electrochemical signals may be subdivided into potentiometric, amperometric, or conductometric types. Previously the amperometric sensor matrix was manipulated with conducting polymers, but more recently bionanohybrids have been used for the biosensor construction. The major event in the fabrication of biosensors has been immobilization of the targeted molecule to the surface of the transducer; the immobilization process was achieved by covalent bonding, crosslinking of proteins, chemiabsorption, or physical entrapment. Dan Shan et al. reported that three major factors depend on the activity of biosensors: diffusion of substrate or product through the membrane; enzyme activity in the immobilization matrix; and the efficiency of the electrotransducer connection (Nemirovsky, 2010). They possess a group of properties which include a net positive charge, hydrophilicity, swelling, and biomolecule absorption, features which make a good candidate for immobilization matrix enzymes for biosensors. The positive charge was suitable for the immobilization of nucleic acid, enzymes, hormones, antibodies, proteins, amino acids (Press, 2013), cell organelles, tissues, lectins, and microorganisms. Shan et al. (2003) developed a simple glucose amperometric biosensor, where negatively charged glucose oxidase enzymes were immobilized and entrapped into LDH by electrostatic intercalation. The level of glucose is determined by the amperometric oxidation of hydrogen peroxide (Kumar et al., 2018). Zn-AlLDH was used as the inorganic matrix for the encapsulation and immobilization of urease enzyme by the ion-exchange and coprecipitation method. High anion exchange capacity, high specific surface area, and good charge density of ZnAl LDH enable simple and low-cost production of urea biosensors. Shan et al. (2003) developed two phenol-based biosensors as an immobilization matrix for ZnAlCl. PPO/[ZnAlCl] has good properties including high sensitivity and good storage stability (Shafiei et al., 2008). Different biosensors were available depending on the immobilized enzyme: horseradish peroxidase, polyphenol oxidase, lactase, acid phosphatase, nitrate reductase, and alkaline phosphates.

16.2.5 Layered double hydroxide nanocomposites for tissue engineering applications Tissue engineering (TE) is a promising field of regenerative medicine that offers outstanding opportunities to repair damaged tissues and organs. The main concept

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underlying TE is combining a scaffold or matrix, living cells, bioreactors, and signaling factors to form a 3D tissue-engineered construction to promote the process of repair and regeneration of tissues (Gaikwad et al., 2008). The scaffold is a 3D structure that supports cell attachment, proliferation, growth, and differentiation and also guides the development of the required tissue and acts as a drug-delivery vehicle (Bhattarai and Bhattarai, 2013; Shah et al., 2011). A scaffold that can enable drug delivery can enhance the regeneration of tissue. Ca
LDHs and Mg
LDHs, because of the presence of calcium and magnesium in their primary structures (natural body elements) and also their strong biocompatibility, have the potential for bone TE applications (Gaikwad et al., 2008).

16.3

Layered double hydroxide polymer nanocomposites for biomedical applications

16.3.1 Alginate
layered double hydroxide nanocomposites Alginate is an anionic polysaccharide and consists of β-D-mannuronate (M) and α-L-guluronate (G) units with different M/G ratios depending on their origin. The abundant hydroxyl and carboxyl groups along alginate chains (Fig. 16.11) can lead to strong interactions with the positively charged surface of LDH nanosheets (Kang et al., 2014a). Alginate/LDH composites can be prepared by the incorporation of alginate chains into the interlayers of LDHs via direct coprecipitation. Mesoporous alginate/LDH composite beads, with high specific surface area and high activity, have the potential for controlled drug-release applications. Kang et al. reported in their work that LDH nanosheets can be stabilized by alginate molecules in aqueous media and the LDH nanosheets in the nanocomposites can be redispersed in aqueous media. The structure formation of LDH nanosheets in an alginate matrix is shown in Fig. 16.12. LDH nanosheet surfaces are coated on both sides with alginate when those nanosheets are dispersed in formamide and mixed with an aqueous alginate solution. This arises through electrostatic interactions that form between the positive and negative charges of the alginate chains and the LDH surface. The alginate coating prevents restacking of LDH nanosheets after removal of the formamide. Therefore the LDH nanosheets coated by the alginate chains are kinetically stable in water. When both the formamide solvent and the free alginate chains are removed by water washing and centrifugation, the colloidal nanosheets with adsorbed alginate chains flocculate and restack into layered nanocomposites (Fig. 16.12) after the water has been evaporated (Kang et al., 2014a). NO3
LDH was exfoliated in formamide into nanosheets with positive charges. These positively charged nanosheets could be stabilized in an aqueous solution through complexation with alginate molecules that possess negative charges. Then alginate chains were adsorbed on the LDH nanosheets to form a “sandwich” structure by static interaction. When formamide and free alginate in the mixed solution of LDH and alginate were removed by water washing and ultracentrifugation, a colloidal

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Figure 16.11 Polymer chains of alginate.

Figure 16.12 A schematic of the control process for converting LDH nanosheet dispersions into alginate/LDH nanocomposites.

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Figure 16.13 Scheme of the general procedure employed for the preparation of alginate
zein beads entrapping the LDH
IBU hybrid.

alginate/LDH mixture was obtained. The thermal stability of alginate in a nanocomposite was increased by LDH. The addition of LDH into the alginate matrix led to an improvement in the mechanical properties. In another reported study by Alcantara et al. (2010), new hybrid materials were developed based on the combination of LDH and two biopolymers (a protein and a polysaccharide) to produce LDH
biopolymer nanocomposites (Ribeiro et al., 2014), able to act as effective DDS in comparison to the LDH or the biopolymers alone. Ibuprofen (IBU) has been chosen as a model drug, being intercalated in an MgAl
LDH matrix. First they prepared MgAl
LDH by a coprecipitation reaction at constant pH following the intercalation of ibuprofen to Mg
Al LDH by an ion exchange method. Then, after centrifugation and thorough washing with distilled water, the material was dried overnight at 60 C. The resulting material was LDH
IBU hybrid. This hybrid material was used for composite preparations with alginate and zein (Fig. 16.13).

16.3.1.1 Release of ibuprofen from alginate
zein bionanocomposite beads IBU release from polymeric matrices, such as alginate, influences two main factors in the kinetics of the process: (1) the presence of ions in solution, for instance phosphate, can determine the passage of the calcium ions used to crosslink alginate to

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Figure 16.14 XRD patterns of [Mg2Al]Cl LDH before and after ion-exchange reaction with IBU for 3 and 7 days. Some of the most relevant reflection peaks are marked and a schematic representation of the phases present in each material is indicated on the right side.

the solution, facilitating the solubilization of the beads in aqueous media and (2) the solubility of IBU increases as the pH of the media increases so that the use of more basic media may increase the release of IBU. This result shows again that DDS based on alginate are very sensitive to pH changes and quickly release the drug loaded in conditions simulating pH conditions analogous to those of the intestinal fluid (pH 6.8
7.4). Fig. 16.14 shows the X-ray diffraction patterns of MgAl
LDH before and after the ion-exchange reaction with IBU. However, for systems that contain a higher amount of zein, the release profile is almost linear when the pH changes from that of the stomach to that of the intestine. The combination of alginate and zein biopolymers gives rise to new matrices for DDS that can be used for direct encapsulation of a drug (e.g., IBU) and also to develop bionanocomposite materials with drug-intercalated LDH systems. These DDSs were tested for controlled release of IBU, used here as a model drug (Fig. 16.15) in conditions that simulate the passage through the gastrointestinal tract, showing that the speed for delivering the encapsulated drug decreases with zein content, probably due to the hydrophobic character of this biopolymer (Aranda and Darder, 2010). These new bionanocomposite systems represent DDSs of special interest for release of drugs incorporated in LDHs as they procure a practically complete protection during the passage through the stomach. In comparison to other DDSs, the novel system proposed here has the advantage of easy processing as well as use of abundant, cheap, and safe biopolymers. The reported approach for preparation of DDSs can be applied to other oral drugs that also require controlled release.

16.3.2 Chitosan
layered double hydroxide nanocomposites Chitosan, the second most abundant naturally occurring amino polysaccharide next to cellulose, is derived as a deacetylated form of chitin. Its nontoxic, biocompatible,

Layered double hydroxide based nanocomposites for biomedical applications

699

Figure 16.15 Schematic illustration showing the differences in the IBU release from AZ/ IBU (A) and AZ/LDH
IBU (B) bionanocomposite beads.

antibacterial, and biodegradable properties have led to significant research toward its biomedical and pharmaceutical applications, such as drug delivery, TE, woundhealing dressings, etc. (Gao et al., 2012). Chitosan nanocomposite generally refers to chitosan polymer containing dispersed nanofillers with an average particles size of less than 100 nm. Thus this composite retains exceptionally enhanced properties pertaining to both polymers and NPs. Chitosan
LDH nanocomposites possess characteristic properties advantageous in various biomedical applications such as drug delivery, bioimaging, TE, etc. (Bercea et al., 2015; Wei et al., 2015; Ribeiro et al., 2014).

16.3.2.1 Drug-delivery applications Recently Xu et al. reported a study on nanocomposites of chitosan
glutathione
glycylsarcosine and LDH for topical ocular drug delivery (Wang and Zhang, 2014; Xu et al., 2018). Glycylsarcosine (GS), as an active target ligand of the peptide transporter-1 (PepT-1), could specifically interact with the PepT-1 on the cornea and guide the NPs to the treatment site. Active targeting intercalated nanocomposites could have great potential for topical ocular drug delivery due to their capacity for prolonging the retention on the ocular surface, enhancing the drug permeability through the cornea, and efficiently delivering the drug to the targeted site. LDHs can be efficient ocular delivery vehicles for a series of active compounds, such as diclofenac sodium, phacolysin, and brimonidine (Xu et al., 2018). Ribeiro et al. reported a study on pectin-coated chitosan
LDH bionanocomposite in colon cancer targeted drug delivery. 5-Aminosalicylic acid (5ASA), the most commonly used NSAID in the treatment of ulcerative colitis and Crohn’s disease, was chosen as a model drug aiming at controlled and selective delivery in the colon (Ribeiro et al., 2014). The pure 5ASA drug and the hybrid material prepared by intercalation in an LDH of Mg2Al using the coprecipitation method were incorporated in a chitosan matrix in order to profit from its mucoadhesiveness. These compounds processed as beads were further treated with the polysaccharide pectin to

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Layered Double Hydroxide Polymer Nanocomposites

Figure 16.16 Pectin-coated chitosan bead incorporating 5-aminosalicylic acid (5ASA) intercalated in Mg2Al
LDH as a new drug-delivery system.

create a protective coating that ensures the stability of both chitosan and LDH at the acid pH of the gastric fluid. The resulting composite beads presented at the pectin coating were stable to water swelling and enabled controlled release of the drug along their passage through the simulated gastrointestinal tract in in vitro experiments, due to their resistance to pH changes. Based on these results, the pectin@chitosan/LDH
5ASA bionanocomposite beads are promising candidates for the colon-targeted delivery of 5ASA, with the aim of acting only at the focus of the disease and minimizing side effects (Fig. 16.16). This study suggests that PCT@CHT/LDH systems can be proposed as promising DDS systems in view of the delivery of drugs at the intestinal tract in a controlled manner. These systems will profit from the stability of pectin at acid pH to avoid drug release in the stomach, as well as from the resistance of chitosan at basic pH to enable slow liberation of 5ASA in the intestinal tract. In addition, the mucoadhesive properties of chitosan could be especially relevant for colon-targeted delivery of the encapsulated drug.

16.3.2.2 Carboxymethyl chitosan
layered double hydroxide nanocomposite Carboxymethyl chitosan (CMC) is an important chitosan derivative extensively used in a wide range of biomedical applications because of its good water solubility, unique chemical, physical, and biological properties, and excellent biocompatibility and biodegradability (Wang and Zhang, 2014). A large number of carboxyl

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701

Figure 16.17 Schematic illustration of the bioinspired assembly process for the fabrication of a C-LDH/CMC bionanocomposite film.

groups and amide groups within the skeletal framework of CMC can interact with chelating metal cations in alkaline medium. Thus CMC may act as an organic template to induce the formation of special nanostructural materials. In addition, as an anionic biopolymer, CMC may be intercalated into the LDH interlayer as a guest molecule. Zhang et al. (2015) reported a study on a two-step approach to fabricate LDH/CMC bionanocomposite hydrogel films. CMC intercalated MgAl
LDHs (C-LDHs) were synthesized by the hydrothermal method, and the obtained C-LDHs were well dispersed in the CMC matrix to form a stable colloidal suspension. A transparent, flexible, and glossy self-supporting film with a highly oriented lamellar microstructure was then easily bioinspired and assembled during the solvent evaporation process. This bionanocomposite film, with a high swelling ratio and stability, had excellent mechanical properties and was used to immobilize lysozyme (LSZ). This strategy is an alternative approach for fabricating LDH/CMC bionanocomposite hydrogel films (Fig. 16.17), which could be potentially used in the fields of enzyme immobilization, underwater antibacterial coatings, drug storage and delivery, biosystems, etc.

16.3.2.3 Bioimaging applications Functionalized chitosan
LDH nanocomposites are potential candidates for organspecific drug delivery and bioimaging applications. Wei et al. demonstrated the feasibility of LDH NPs for developing organ-specific targeting by incorporation of an NIR fluorescent dye (ICG), a contrast agent with US FDA approval for clinical use,

702

Layered Double Hydroxide Polymer Nanocomposites

in the layered structures of amine-modified LDHs (Wei et al., 2015). The produced LDH
NH2
ICG complexes that were further coated with different amounts of chitosan molecules by covalent crosslinking of amino groups between the LDH
NH2 and chitosan molecules using glutaraldehyde as a crosslinking agent. We verify that the combination of LDHs and ICG can apply to in vivo optical imaging because the excitation and emission at long wavelengths (excitation: 780 nm; emission: 820 nm) in the NIR window minimizes the intrinsic background interference since blood and tissue are relatively transparent in the range of 700
1000 nm. To verify that the chitosan-coated LDH
NH2
ICG can be developed as an in vivo contrast agent, the noninvasive imaging capability of chitosancoated LDH
NH2
ICG was demonstrated by imaging its biodistribution in anesthetized nude mice. We demonstrate that the chitosan-coated LDH
NH2
ICG complexes have high biocompatibility, high fluorescence intensity, highly organspecific targeting, and low cytotoxicity. The chitosan-coated LDH surfaces can increase the stability of the nanocomposites; furthermore, the targeting molecules can be conjugated in the external surfaces of the chitosan molecules through a further reaction with the amino groups.

16.3.2.4 Ex vivo fluorescence image of rabbit ocular tissues The fluorescence signals of FITC
LDH eye drops were determined by in vivo FX Pro (Kodak, New Haven, CT, USA) and found to be primarily localized in cornea and sclera, suggesting that the precorneal retention could be enhanced in the presence of LDH compared with untreated ocular tissues (Fig. 16.18). For CG
GS
FITC
LDH eye drops, the signals were stronger in all ocular tissues than with FITC
LDH, especially in the iris, ciliary, and crystalline lens (Xu et al., 2018). This implied CG
GS could facilitate more FITC to arrive at the target ocular tissue of the crystalline lens. Additionally, more fluorescence signals of

Figure 16.18 Ex vivo fluorescence imaging of rabbit ocular tissues from rabbit treated with FITC: blank (A), FITC
LDH (B), CG
GS
FITC
LDH (1:0.5)-50 (C), CG
GS
FITC
LDH (1:1)-50 (D), and a physical mixture of CG
GS and FITC solution (E). CG
GS, chitosan
glutathione
glycylsarcosine; FITC, fluorescein isothiocyanate isomer I; LDH, layered double hydroxides.

Layered double hydroxide based nanocomposites for biomedical applications

703

CG
GS
FITC
LDH (1:1) were located in ocular tissues than those of CG
GS
FITC
LDH (1:0.5), but the fluorescence intensity of the physical mixture was similar to that of CG
GS
FITC
LDH (1:0.5). The improved penetration confirmed that the higher amount of GS in CG
GS after hybridizing with LDH might enhance the ocular bioavailability by increasing the affinity to PepT-1 of the corneal epithelium (Komarala et al., 2017). The produced LDH
NH2
ICG complexes were further coated with different amounts of chitosan molecules by covalent crosslinking of an amino chitosan-based delivery system for PDT based on chitosan-coated LDH NPs loaded with an FDAapproved near-infrared (NIR) fluorescent dye, ICG with photoactive properties (Fig. 16.19). The surface coating of LDH with chitosan layers may improve the biological properties such as biocompatibility, immune responses, cell internalization, and therapeutic effects (Wang et al., 2013). In addition, the chitosan coating over LDH NPs may enhance the photosensitizer excitation efficiency, which is highly beneficial for the application of LDHs in biologicals. The efficient positively charged polymer (chitosan) coating was achieved by crosslinkage using surface amine groups modified on the LDH NP surfaces. Another study reported the potential application of chitosan
LDH NPs for optical imaging. They incorporated an NIR fluorescent dye, ICG, in amine-modified LDH with glutaraldehyde as a spacer and the photodynamic efficacy of ICG was demonstrated.

Figure 16.19 A representation of the incorporation of ICG fluorescent dyes in the layer structures of LDH
NH2 through electrostatic attraction of opposite charges. The external surfaces of LDH
NH2 can be further encapsulated with different layers of chitosan using glutaraldehyde as a crosslinking agent.

704

Layered Double Hydroxide Polymer Nanocomposites

They also investigated the in vivo biodistribution of different amounts of chitosan-coated LDH
NH2
ICG samples in the animal model using an NIR fluorescent dye (ICG) as an optical contrast agent. To confirm the in vivo imaging results, ex vivo optical imaging of the organs from the sacrificed mice was performed. Fig. 16.20 shows the ex vivo optical images of dissected organs 3 h after an i.v. injection of LDH
NH2
ICG, LDH
NH2
ICG
CS-1, and LDH
NH2
ICG
CS-2 samples. They reported that the intense fluorescence of the LDH
NH2
ICG samples remained mainly in the liver and spleen. The large accumulation of LDH
NH2
ICG NPs in the liver and spleen may be due to clearance from the uptake by the RES and the mononuclear phagocytic system which were largely present in the liver, spleen, and bone marrow. Furthermore, the in vivo biodistribution of the LDH
NH2
ICG
CS-1 samples showed a major accumulation of NPs in the lungs. A possible reason for the change in the biodistribution pattern (from the liver and spleen to lungs) may come from the increase in the particle size of the mono-chitosan-coated LDH
NH2
ICG samples. Indeed, the first organs for NP influx by i.v. injection were the lungs, and the capillary circulation usually causes slower blood flow in the lungs; therefore, the large size of the particles means that they accumulate in the lungs. When the chitosan coating amount is increased using the LDH
NH2
ICG
CS-2 sample, biodistribution displayed a partial shift of fluorescence intensity from the lungs to the liver. A preliminary validation of the noninvasive imaging capability of the chitosancoated LDH
NH2
ICG was also performed by imaging its biodistribution in anesthetized nude mice (Fig. 16.21). A similar biodistribution was observed to that of the dissected organs from a sacrificed mouse. This indicated that nanofabrication by the combination of organic ICG fluorescent dyes and chitosan molecules with inorganic LDHs was indeed competent as a contrast agent with high efficiency for optical imaging of the tissue in depth. The noninvasive imaging studies of LDH
NH2
ICG NPs with different amounts of chitosan coating (administered by i.v. injection) in anesthetized nude mice also showed visible fluorescence in the target organs (the liver and the lungs). The combination of chitosan and

Figure 16.20 NIR fluorescent images of dissected organs from a mouse sacrificed 3 h after i.v. injection of (A) LDH
NH2
ICG, (B) LDH
NH2
ICG
CS-1, and (C) LDH
NH2
ICG
CS-2.

Layered double hydroxide based nanocomposites for biomedical applications

705

Figure 16.21 Biodistribution of LDH
NH2
ICG
CS-2 samples in anesthetized nude mice (A) before and (B) after (1.5 h) i.v. injection. The fluorescent imaging shows a large accumulation of LDH
NH2
ICG
CS-2 in the liver and lungs by observation of the noninvasively nude mice model.

LDH
NH2
ICG demonstrated a high potential to develop both organ-specific DDSs and in vivo contrast agents for clinical cancer diagnosis and chemotherapy.

16.3.2.5 Chitosan
layered double hydroxide nanocomposites in photodynamic therapy Highly beneficial qualities of photodynamic therapy (PDT) include its selective and minimally invasive nature, with preferential localization of photosensitizer at the desired site along with concomitant photoactivation to restrict damage. PDT has been gaining significant attention as a very good alternative to surgery and to counter drug resistance. To date, the use of LDH materials for PDT applications has barely been studied. Very recently, Wang et al. (2013) provided a nanohybrid of coloading Pt(IV) prodrugs and photosensitizers (Chlorin e6) into LDH for synergistic killing effects in cisplatin-resistant cancer cells (Links, 2011). A pH-responsive release of hydrophilic zinc(II) phthalocyanine based on an electrostatic interaction with cationic LDH to generate high photocytotoxicity against HepG2 cells was reported by Saifullah et al. (2013). Stefanakis et al. (2013) reported the synthesis of aminomodified Gd2(OH)5NO3 nanosheets with a photosensitizer (rose bengal). This nanocomposite makes PDT possible (Al Ali et al., 2012). Very interestingly, Liang et al. incorporated zinc phthalocyanines (ZnPc) into the gallery of LDHs by coprecipitation. The supramolecular photosensitizers showed high photocytotoxicity, high stability, good biocompatibility, as well as low cytotoxicity in comparison with pristine ZnPc (Oh et al., 2006b). In addition, many studies also demonstrated that the incorporation of a porphyrin-based photosensitizer in the LDH host can enhance

706

Layered Double Hydroxide Polymer Nanocomposites

the stability of photosensitizers against the photobleaching and quenching aggregation effects (Huang et al., 2009; Wei et al., 2015; Kuthati et al., 2015; Gasperowicz and Otto, 2008). Herein, we designed a chitosan-based delivery system for PDT based on chitosan-coated LDH NPs loaded with an FDA-approved NIR fluorescent dye, ICG with photoactive properties (Scheme 16.2). The surface coating of LDH with chitosan layers may improve the biological properties such as biocompatibility, immune responses, cell internalization, and therapeutic effects (Wang et al., 2013). In addition, the chitosan coating over LDH NPs may enhance the photosensitizer excitation efficiency, which is highly beneficial for the application of LDHs in biologicals. The efficient positively charged polymer (chitosan) coating was achieved by crosslinkage using surface amine groups modified on the LDH NP surface with glutaraldehyde as a spacer, and the photodynamic efficacy of ICG is demonstrated in Scheme 16.2. Li et al. reported a pH-responsive LDH
phthalocyanine nanohybrid for efficient PDT. A pH-responsive nanohybrid (LDH
ZnPcPS4), in which a highly hydrophilic zinc(II) phthalocyanine tetra-α substituted with 4-sulfonatophenoxy groups (ZnPcPS4) is incorporated with a cationic LDH has been especially designed and prepared through a facile coprecipitation approach (Li et al., 2015). The nanohybrid shows high photocytotoxicity against HepG2 cells as a result of much more efficient cellular uptake and preferential accumulation in lysosomes, whereby the acidic environment leads to the release of ZnPcPS4. This work provides a facile approach for the fabrication of photosensitizers with high photocytotoxicity, potential tumor selectivity, and rapid clearance character (Prakash et al., 2011). In this work, a new nanohybrid, ZnPc [i.e., 1,8(11),15(18),22(25)-tetrakis(4-sulfonatophenoxyl) phthalocyaninato zinc (ZnPcPS4; see Scheme 16.3)] is prepared. ZnPcPS4, which bears four sulfo groups, shows high hydrophilicity. This could facilitate its rapid excretion from the body after the PDT effect to minimize its side

Scheme 16.2 Representation of the cellular uptake of chitosan-coated LDH
NH2
ICG nanohybrids and resultant apoptosis with PDT.

Layered double hydroxide based nanocomposites for biomedical applications

707

Scheme 16.3 Schematic illustration of the preparation of the pH-responsive nanohybrid (LDH
ZnPcPS4) and its proposed mechanism for targeted PDT through pH-triggered release of deactivated ZnPcPS4 from LDH
ZnPcPS4, thus leading to the restoration of its photoactivities.

effects. By introducing phenoxyl groups at the a positions of ZnPc, the ZnPcPS4 possesses a significantly red-shifted Q band relative to known ZnPcS4 (approximately 692 vs 670 nm in aqueous solution), which is desirable for PDT as it allows deeper light penetration into tissue. To enhance the cellular uptake of the polyanionic ZnPcPS4, as shown in Scheme 16.3, we further fabricated a pH-responsive nanohybrid (LDH
ZnPcPS4) based on the electrostatic interaction between negatively charged ZnPcPS4 and cationic LDH through a coprecipitation approach. The in vitro tests revealed that the nanohybrid could extraordinarily enhance the photodynamic activity of ZnPcPS4 against HepG2 cells up to 24-fold as a result of more efficient cellular uptake. More interestingly, the nanohybrid has essentially no photoactivity, but the deactivated photoactivity can be switched on efficiently in a slightly acidic tumor extracellular environment (pH 6.5) through the release of phthalocyanine from the nanohybrid. It is well documented that the pH in tumor tissues is more acidic (pH 6.5
6.8) than that in blood and normal tissues (pH 7.4). Therefore, the nanocomposite might serve as a photosensitizer of dual selectivity for targeted PDT that results from the enhanced permeation and retention effect of the nanocarrier and tumor-pH response. This work provides a facile approach for fabrication of photosensitizers with high cellular photocytotoxicity, potential tumor selectivity, and rapid clearance.

16.3.2.6 Chitosan
layered double hydroxide nanocomposites in tissue engineering applications Chitosan has been widely used by researchers as an important and promising biomaterial in TE, wound healing, drug delivery (Chen et al., 2012), and other biomedical applications. Modification of chitosan either by substituting the surface

708

Layered Double Hydroxide Polymer Nanocomposites

functional groups or crosslinking chitosan with different layers of polymers can alter its mechanical and biological properties. This will be very useful in TE applications. The compositions of glycosamino glycans and chitosan derivative nanocomposites were shown to be promising candidates in bone TE applications (Gao et al., 2012). Chitosan has also been employed to reinforce calcium phosphate cement (CPC) and CPC
chitosan scaffold may be useful for TE research in stem cell-based bone regeneration Chen et al. (2012) reported self-assembly of Pifithrin-α-loaded LDH
chitosan nanohybrids for bone TE (Fig. 16.22). This work aimed at studying the potential

Figure 16.22 (A) Schematic of the sustained release of PFTα from PFTα
LDH
CS and (B) schematic of the osteogenesis differentiation of hBMSCs by PFTα.

Layered double hydroxide based nanocomposites for biomedical applications

709

role of LDH
CS nanohybrid composites loaded with the novel osteogenic drug PFTα in bone tissue regeneration and engineering as a therapeutic strategy.

16.3.3 Other polymer
layered double hydroxide nanocomposites Polyhydroxyalkanoates (PHAs) are a family of natural biodegradable polyesters of 3-, 4-, 5-, and 6-hydroxyacids, obtained by numerous bacteria through the fermentation of sugars, lipids, alkanes, alkenes, and alkanoic acid, as unique intracellular carbon and energy storage compounds and accumulated as granules in the cytoplasm of cells. Poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3hydroxyvalerate) (PHBV) are the most studied natural polyesters and are the first commercial thermoplastics from a bacterial source. Bunea et al. (2016) reported the synthesis and characterization of novel biocomposites based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and modified LDHs with good biocompatibility for potential in TE applications. Bagher Larijani et al. (2016) reported a study on another LDH
polymer composition. They developed porous biodegradable scaffolds through simple methods as one of the main approaches of bone tissue engineering (BTE). In this work, a novel BTE composite containing LDH, hydroxyapatite (HA), and gelatin (GEL) was fabricated using coprecipitation and solvent-casting methods. Physiochemical characterizations showed that the chemical composition and microstructure of the scaffolds were similar to the natural spongy bone and the results showed that the regeneration of defects was accelerated by scaffold implantation but ASC-seeding significantly improved the quality of new bone formation (P , .05). The results

Scheme 16.4 Schematic illustration of LDH nanocomposites in tissue engineering applications.

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Layered Double Hydroxide Polymer Nanocomposites

confirmed the good performance of LDH
HA/GEL scaffolds in inducing bone regeneration. The presence of LDHs improved the bioactive interface and controlled the degradation behavior. A schematic representation of this study is shown in Scheme 16.4.

16.4

Summary

In this chapter we have briefly summarized the diverse healthcare applications of LDH nanocomposites. Hybridization of a drug or biomolecule with LDH results in remarkable transfer efficiency and stability. Therefore LDH hybrids can be useful as reservoirs and carriers for genes, drugs, and other functional molecules. LDH
polymer nanocomposites offer numerous opportunities in diverse applications, including nanocarriers, biomedical applications such as drug delivery, gene delivery, TE, development of biosensors, etc. A wide variety of polymer nanocomposites are currently under investigation for their potential applications in the biomedical field and some are summarized in this chapter.

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59. Oh, J.-M., Biswick, T.T., Choy, J.-H., 2009. Layered nanomaterials for green materials. J. Mater. Chem. 19 (17), 2553. Oh, J.M., Choi, S.J., Kim, S.T., Choy, J.H., 2006a. Cellular uptake mechanism of an inorganic nanovehicle and its drug conjugates: enhanced efficacy due to clathrin-mediated endocytosis. Bioconjug. Chem. 17 (6), 1411
1417. Oh, J.M., Kwak, S.Y., Choy, J.H., 2006b. Intracrystalline structure of DNA molecules stabilized in the layered double hydroxide. J. Phys. Chem. Solids 67 (5
6), 1028
1031. Pankhurst, Q., Connolly, J., Jones, S., Dobson, J., 2003. Applications of magnetic nanoparticles in biomedicine. J. Phys. D. Appl. Phys. 36 (13), R167
R181. Prakash, S., Malhotra, M., Shao, W., Tomaro-Duchesneau, C., Abbasi, S., 2011. Polymeric nanohybrids and functionalized carbon nanotubes as drug delivery carriers for cancer therapy. Adv. Drug Deliv. Rev. 63 (14
15), 1340
1351. Press, D., 2013. Advances in drug delivery via electrospun and electrosprayed nanomaterials. Int. J. Nanomed. 8, 2997
3017. Rajan, M., Raj, V., 2013. Potential drug delivery applications of chitosan based nanomaterials. Int. Rev. Chem. Eng. 5, 145
155. March. Reis, C.P., Neufeld, R.J., 2006. Nanoencapsulation I. Methods for preparation of drug-loaded polymeric nanoparticles. Nanomedicine 2, 8
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Ribeiro, L.N.M., Alcˆantara, A.C.S., Darder, M., Aranda, P., Arau´jo-moreira, F.M., Ruizhitzky, E., 2014. Pectin-coated chitosan
LDH bionanocomposite beads as potential systems for colon-targeted drug delivery. Int. J. Pharm. 463 (1), 1
9. Saifullah, B., Hussein, M.Z., Hussein-al-ali, S.H., Arulselvan, P., Fakurazi, S., 2013. Sustained release formulation of an anti-tuberculosis drug based on para-amino salicylic acid-zinc layered hydroxide nanocomposite. Chem. Cent. J. 7, 1
11. Shafiei, S., Birgani, Z.T., Darvish, A., Azimi, M.S., Solati-Hashjin, M., 2008. Layered double hydroxides for diagnostic applications. Int. Congr. Eval. Med. Diagnosis Mod. Technol. 1
16. Shah, N., Mewada, R.K., Shah, T., 2011. Application of biodegradable polymers in controlled drug delivery. Proc. 2nd Int. Conf. Curr. Trends Technol. NUiCONE 8
10. Shukla, R., Bansal, V., Chaudhary, M., Basu, A., Bhonde, R.R., Sastry, M., 2005. Biocompatibility of gold nanoparticles and their endocytotic fate inside the cellular compartment: a microscopic overview. Langmuir ACS J. Surfaces Colloids 21 (23), 10644
10654. Swarnalatha, B., Nair, S.L., Shalumon, K.T., Milbauer, L.C., Jayakumar, R., 2013. Poly (lactic acid)
chitosan
collagen composite nanofibers as substrates for blood outgrowth endothelial cells. Int. J. Biol. Macromol. 58, 220
224. Thanh, N.T.K., Green, L.A.W., 2010. Functionalisation of nanoparticles for biomedical applications. Nanotoday 5, 213
230. Wang, J., et al., 2013. In vivo self-bio-imaging of tumors through in situ biosynthesized fluorescent gold nanoclusters. Sci. Rep. 3, 1
6. Wang, Y., Zhang, D., 2014. Bioinspired assembly of layered double hydroxide/carboxymethyl chitosan bionanocomposite hydrogel films. J. Mater. Chem. B 2, 1024
1030. Wei, P., Kuthati, Y., Kankala, R.K., Lee, C., 2015. Synthesis and characterization of chitosan-coated near-infrared (NIR) layered double hydroxide-indocyanine green nanocomposites for potential applications in photodynamic therapy. Int. J. Mol. Sci. 16, 20943
20968. Wong, Y., et al., 2010. Efficient delivery of siRNA to cortical neurons using layered double hydroxide nanoparticles. Biomaterials 31 (33), 8770
8779. Wong, Y., Cooper, H.M., Zhang, K., Chen, M., Bartlett, P., Xu, Z.P., 2012. Efficiency of layered double hydroxide nanoparticle-mediated delivery of siRNA is determined by nucleotide sequence. J. Colloid Interface Sci. 369 (1), 453
459. Xu, T., Xu, X., Gu, Y., Fang, L., Cao, F., 2018. Functional intercalated nanocomposites with chitosan-glutathione-glycylsarcosine and layered double hydroxides for topical ocular drug delivery. Int. J. Nanomed. 13, 917
937. Xu, Z.P., Stevenson, G., Lu, C.-Q., Lu, G.Q.M., 2006. Dispersion and size control of layered double hydroxide nanoparticles in aqueous solutions. J. Phys. Chem. B 110, 16923
16929. Xu, Z.P., Walker, T.L., Liu, K., Cooper, H.M., Lu, G.Q.M., Bartlett, P.F., 2007. Layered double hydroxide nanoparticles as cellular delivery vectors of supercoiled plasmid DNA. Int. J. Nanomed. 2 (2), 163
174. Zhang, H., Pan, D., Duan, X., 2009. Synthesis, characterization, and magnetically controlled release behavior of novel core
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12148. Zhang, K., Xu, Z.P., Lu, J., Tang, Z.Y., Zhao, H.J., Good, D.A., 2014. Potential for layered double hydroxides-based, innovative drug delivery systems. Int. J. Mol. Sci. 15, 7409
7428.

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Zhang, Y., Chan, H.F., Leong, K.W., 2013. Advanced materials and processing for drug delivery: the past and the future. Adv. Drug Deliv. Rev. 65 (1), 104
120. Zhao, F., Yao, D., Guo, R., Deng, L., Dong, A., Zhang, J., 2015a. Composites of polymer hydrogels and nanoparticulate systems for biomedical and pharmaceutical applications. Nanomaterials 5, 2054
2130. Zhao, K., et al., 2015b. Synthesis, characterization, and immune efficacy of layered double hydroxide@SiO nanoparticles with shell-core structure as a delivery carrier for Newcastle disease virus DNA vaccine. Int. J. Nanomed. 10, 2895
2911. Zheng, Y., Chen, Y., 2016. Preparation of polypropylene/Mg
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1530. Gu, Z., Atherton, J.J., Xu, Z.P., 2015. Hierarchial layered double hydroxide nanocomposites: structure, synthesis and applications. Chem. Commun. (Camb.) 51, 3024
3036.

Further reading Hoyo, C.D., 2007. Layered double hydroxides and human health: an overview. Appl. Clay Sci. 36, 103
121. Rezvani, Z., Arjomandi Rad, F., Khodam, F., 2014. Synthesis and characterization of Mg-Allayered double hydroxides intercalated with cubane-1,4-dicarboxylate anions. Dalton Trans. 988
996.

Layered double hydroxide nanocomposites for agricultural applications

17

Luı´z Paulo Figueredo Benı´cio1, Frederico Garcia Pinto2 and Jairo Tronto2 1 Soil Department, Federal University of Vic¸osa, Brazil, 2Institute of Exact and Technological Sciences, Federal University of Vic¸osa - Rio Paranaı´ba Campus, Brazil

17.1

Introduction

For many years, agriculture linked to crop production has been essential for human nutrition. It has since become a highly diversified activity and now includes other production systems such as forestry, fruticulture, and activities related to animal production, such as beekeeping, poultry farming, and animal husbandry. As a result, agriculture involves not only food production, but also includes the production, transformation, promotion, and distribution of agricultural products. Agriculture plays a fundamental role in economic activities, as it is the base of the economic system in many countries, particularly developing nations. In addition to providing food and raw materials, agriculture also offers employment for large segments of the population. As such, we highlight the importance of agriculture in terms of: (1) subsistence sources; (2) food supply; (3) relevance to international markets; (4) sources of raw materials; (5) employment opportunities; (6) economic development; and (7) food security. Among these, food security has been of greatest concern in the last few years. The Food and Agriculture Organization of the United Nations (FAO) defines food security as food availability, food access, and food use. In the next few years, population growth will cause food scarcity if food production does not keep up, especially in underdeveloped countries. According to the FAO (2009), by 2050 it will be necessary to increase food production by 70% to meet international demands. In addition to the need for a rapid increase in food production, there are also concerns over environmental sustainability, to produce more food without damaging the environment. This makes the food production process more difficult, especially since agricultural land is limited. As such, it is necessary to develop specific agricultural production techniques to turn highly degraded areas into productive systems. To do so it is necessary to develop new technologies for management, as well as new materials for improving production and sustainability. Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00017-3 © 2020 Elsevier Ltd. All rights reserved.

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Over the last few years, studies involving nanomaterials have increased because of the unique chemical, physical, thermal, and mechanical properties of these materials. These materials have been used in various areas of research such as informatics, medicine, robotics, agriculture, etc. Among nanomaterials, layered double hydroxides (LDHs) show potential for use in agriculture. LDHs have a well-defined structure, which gives them properties of great interest for nutrient storage, as well as the sustained release or removal of agrochemicals. These nanomaterials can be synthetized by simple methods and at a relatively low cost. This chapter will address the potential uses of LDHs in agriculture, particularly their applications as matrices for storage and slow release of nutrients (fertilizers), herbicide carriers, plant growth regulators, as well as adsorbents used to remove pesticides from water and soil.

17.2

The history and evolution of chemical use in agriculture

Agriculture is one of humanity’s most ancient activities, having its history written side-by-side with that of human evolution, and it is considered one of the principal foundations of all civilizations. Contrary to popular belief, the use of chemical substances and compounds in agriculture is not a recent practice. The earliest recorded uses pre-date Christ, with gas produced by burning elemental sulfur used to control pests in grain stores in ancient Egypt and the Roman Empire (Kutney, 2007). Throughout history, more chemical products have been discovered and optimized for agricultural use. Another important milestone in the history of agrochemical compounds was the discovery of superphosphate. In the 1840s, the German chemist Justus V. Liebig discovered that “attacking” the ashes of plants or bones with sulfuric acid produced phosphoric acid, which had a major impact on plant nutrition. Later, in England, John B. Lawes used the phosphoric acid production bases proposed by Liebig to patent the process of making superphosphate, thereby giving rise to the fertilization industry (Russeli, 1942). During the Irish Potato Famine at the end of the 19th century, the European scientific community was mobilized to research agrochemicals to solve the problem caused by the fungus Phytophtora infestans. In 1883 the Doctor Francˆes Pierre Millardet developed a copper-based fungicide called Bordeaux Mixture (Cibim, 2004). In the mid-19th century, Liebig had already demonstrated the importance of nitrogen in plant growth, development, and production. The greatest milestone for the agricultural use of nitrogen however occurred at the beginning of the 20th century with the transformation of atmospheric air into liquid ammonia, a process developed by the chemists Fritz Haber and Carl Bosch (Smil, 2004). This discovery led to the fabrication of current nitrogen fertilizers available on the market and without this discovery it would be much more difficult to feed the currently more

Layered double hydroxide nanocomposites for agricultural applications

717

than 7 billion people on Earth. As a result, the Haber
Bosch process is considered by many to be the most important discovery of the 20th century. In the years that followed, many organochlorine and organophosphorus chemicals have been developed, thus leveraging the chemical production industry. Large companies arose during this period, including Dow Agroscience, DuPont, and Monsanto, among others. During the time between World Wars I and II, the production of chemical compounds was directed almost exclusively toward military use. At the end of World War II, the United States directed much of the chemical production technology developed during the two World Wars for use in agriculture, thus creating a technological model of agriculture that encompassed the use of agrochemicals (fertilizers and pesticides), improved seeds, and agricultural mechanization. In 1950s and 1960s, this technological agricultural model spread to developing nations such as Mexico, India, and Brazil, becoming known as the Green Revolution (Brum, 1988; Beaud, 1994). From 1940 to 1960, the agrochemical industry went through its golden phase, with intense development of active ingredients aimed at increasing productivity and improving crop management via insect, fungi, and weed control (Hartnell, 1996; Joly and Lemarie, 2002). After this period, an intermediate phase started within the industry, where the rate of new active ingredients being introduced declined rapidly. Competitive strategies then began to be geared toward cost reduction and product differentiation. Cost competition favored the introduction of active ingredients with lower dosage rates, while differentiation through new formulations and packaging led to products that were easier to apply and less damaging to human health and the environment (Hartnell, 1996). Within a short time, agrochemicals had become widely used but had their efficiency questioned, since production gains were decreasing and the harmful effects on the environment becoming apparent. Fertilizers began to show large losses in other ecosystems through different processes. Pesticides became less effective through the selection of biological organisms resistant to the utilized active ingredients, while additionally causing contamination as a result of their indiscriminate use. In order to increase productivity, producers ended up increasing the amounts of chemical fertilizers and pesticides used, which in turn had harmful effects on the environment. As such, achieving sustainable food security will be one of the largest challenges for agriculture in the near future (Foley et al., 2011). With agriculture becoming increasingly more intense and technological, pesticide use has increased significantly over the last two decades (Fig. 17.1). This increase in the amount of agrochemicals has also caused an increase in residues, which have been identified in various environments. Pesticide and fertilizer contamination of soil and water has become a large concern at local, regional, and global scales (Planas et al., 1997; U.S. Geological Survey, 1999; Huber et al., 2000; Cerejeira et al., 2003; Konstantinou et al., 2006). With this, a new era of research has begun in agrochemical products. Products with elevated efficiency and minimal environmental harm are being developed to overcome the main problems caused by antiquated agricultural technologies.

718

Layered Double Hydroxide Polymer Nanocomposites

Chemical fertilizer consumption Pesticide use in agriculture

190 M

5.0 4.5

180 M 4.0 170 M 3.5 160 M 3.0

Pesticide use (kg/ha)

Fertilizer consumption (tonnes)

200 M

150 M 2.5 140 M 2.0 1990

1995

2000

2005

2010

2015

Year

Figure 17.1 Global chemical fertilizer consumption since 2002 and agricultural pesticide use between 1990 and 2010. Data referring to pesticides are quantities of herbicides, fungicides, insecticides, and seed pesticides in kg of active ingredient per ha. Fertilizer data refer to agricultural nutrient consumption as tonnes consumed per year. Source: Data used in the figure are available at http://faoestat.fao.org/.

17.3

Principal agricultural problems to resolve with new technologies

Through the use of pesticides and fertilizers, agriculture has severely impacted water quality. Some pesticides and fertilizers can easily leach into water supplies and the water table, affecting human and animal health. Fertilizer leaching causes eutrophication by altering the amount of potable water and promoting excess algal growth, which decreases the amount of oxygen in the water and leads to the death of fish and other organisms (Ongley, 1996). Studies conducted by the US Geological Survey showed that 90% of wells examined in the United States had some kind of pesticide contamination (Gilliom et al., 2006). In the United Kingdom, studies conducted by environmental organizations show that various water samples from rivers and underwater sources presented pesticide quantities that exceeded the amounts allowed by local legislation (Bingham, 2007). In Asia, agriculture grew 62% between 1990 and 2002, causing a 15% increase in fertilizer use, which lead to increased chemicals in regional waters, increasing by 50% levels considered moderate and 25% levels considered serious. According to the Economic and Social Commission for Asia and the Pacific (ESCAP, 2005), this increase in water chemical levels has caused many eutrophication problems. In addition, fertilizer leaching contaminating water, some nitrogen fertilizers volatize, causing atmospheric pollution. Urea is the most widely used nitrogen

Layered double hydroxide nanocomposites for agricultural applications

719

fertilizer in the world because of its high concentration of nitrogen (46%), low cost, and ease of use (Engelstad and Hauck, 1974; Melgar et al., 1999; Zheng et al., 2009; Trenkel, 2010). This fertilizer, however, undergoes considerable losses through high NH3 volatilization. Various factors influence the losses, including pH, humidity, and soil temperature (Hargrove, 1988; Terman, 1979). Under severe conditions, N losses from volatilization can reach up to 60% of the total fertilizer applied (Lara Cabezas et al., 1997). In tropical regions, phosphorus (P) is lost by Fe and Al oxyhydroxide fixation in the soil. This is a problem that can be solved by adopting new technologies. Agriculture consumes between 80% and 90% of globally extracted P (Childers et al., 2011). Of all P applied via fertilization, between 5% and 30% is consumed by plants and the rest is lost by adsorption/fixation in the soil mineral fraction (Novais and Smyth, 1999; Malhi et al., 2002; Johnston et al., 2014). In addition to the inefficient use of P in agriculture, phosphate rock (the main source of P for agriculture) is a limited resource, so it is necessary to improve efficiency in using this resource in order to extend the lifespan of global P reserves. In part, some of these problems can be minimized through adequate pesticide and fertilizer management. It is also necessary, however, to look for new technologies and products to help reduce these problems.

17.4

Layered double hydroxide applications in agriculture

LDHs, also known as anionic clays, or naturally occurring or synthetic hydrotalcitelike compounds, have various compositions that allow their use in a variety of areas, including agriculture. LDHs are formed by inorganic layers composed of bivalent cations (Mg21, Zn21, Cu21, Ca21, Ni21, etc.) and trivalents (Al31, Fe31, Cr31, Co31, Mn31, etc.). These layers possess a residual positive charge, and to maintain the electroneutrality of the system, anions and water molecules are intercalated between the layers within the so-called interlamellar dominion (Fig. 17.2). In agriculture, LDHs have potential for use as carriers of fertilizers, herbicides, and growth regulators, as well as to remove contaminant anions found in water and soil (Benı´cio et al., 2015). When LDHs are used as matrices for fertilizers or pesticides, slow-release tests should be performed to verify the release kinetics of the intercalated anions to solution. These tests are usually carried out in H2O, buffer solutions, or solutions containing anions able to perform the exchange reaction with intercalated species, 2 commonly CO22 (Hashim et al., 2014). Slow-release tests can verify the 3 or Cl release of the intercalated species as a function of time. Some papers described in this chapter present slow-release test results for fertilizers and pesticides intercalated in LDHs (Bruna et al., 2009; Silva et al., 2014a, 2014b). When LDHs are applicate as adsorbent for the removal of contaminant anions in solution, adsorption tests must be conducted. These tests are carried out in solutions

720

Layered Double Hydroxide Polymer Nanocomposites

Figure 17.2 Schematic representation of the structure of a layered double hydroxide.

containing the contaminant ions, and isothermal curves are constructed to verify the maximum capacity of the adsorption ions by LDHs. For more information about the experimental and characterization parts of applications of LDHs in agriculture, readers can consult the papers by Crepaldi and Valim (1998), Wang and O’Hare (2012), and Benı´cio et al. (2015).

17.4.1 Layered double hydroxide matrices of slow-release fertilizers Table 17.1 shows the principal studies found in the literature on LDHs as slowrelease carriers of fertilizers. Obtaining high agricultural yields promotes increased fertilizer use, especially those that contain N and P. Maintaining P and N in the soil for their availability to plants without losing these elements to other ecosystems has been a major problem for producers. Principally in tropical regions, P is easily lost by fixation in the mineral phase of the soil (Rajan et al., 1994). On the other hand, N in the form of NO2 3 can easily be leached, reaching layers that plants cannot access (Foster et al., 1982; Isermann, 1990), or denitrified and volatilized, especially when urea is the N source used (McInnes et al., 1986). Torres-Dorante et al. (2009) investigated the use of LDHs as a nitrate buffer system in the soil. The experiment lasted for 18 months and evaluated the performance of different plants (wheat, spinach, and ryegrass) in two types of soil (loamy sand and sand clay loam), dosed with different amounts of LDHs. The objectives of this work were to: (1) evaluate the adsorption capacity of NO2 3 by LDHs during crop growth; (2) study the ability of LDHs to adsorb mineralized NO2 3 during soil fallow periods and their influence on leaching; (3) evaluate the reversibility of the exchange of NO2 3 from LDHs under cultivation conditions; and (4) determine the

Layered double hydroxide nanocomposites for agricultural applications

721

Table 17.1 Principal studies found in the literature on LDHs as matrices for sustainedrelease fertilizers Authors

Layer cations

Nutrient

Torres-Dorante et al. (2008) Torres-Dorante et al. (2009) Woo et al. (2011) Halajnia et al. (2012) Shimamura et al. (2012) Halajnia et al. (2013) Koilraj et al. (2013) Berber et al. (2014) Silva et al. (2014a) Silva et al. (2014b) Everaert et al. (2016) Bernardo et al. (2016) Halajnia et al. (2016) Bendinelli et al. (2016) Imran et al. (2016) Moraes et al. (2016) Benı´cio et al. (2017) Wan et al. (2017) Bernardo et al. (2017)

Mg-Al Mg-Al Ca-Fe Mg-Fe/Mg-Al Mg-Al Mg-Al Ni-Al Mg-Al Mg-Al Mg-Al/Mg-Fe Mg-Al Mg-Al Mg-Al Mg-Al Mg-Fe-Zn Mg-Fe Mg-Al Mg-Al/Mg-Fe Ca-Al

Nitrogen Nitrogen Phosphorus Nitrogen Phosphorus Nitrogen and phosphorus Phosphorus Nitrogen Nitrogen Nitrogen Phosphorus Phosphorus Nitrogen Molybdenum Zinc Nitrogen and phosphorus Phosphorus Phosphorus Phosphorus

buffer capacity of NO2 3 in the soil after the application of LDHs. The results showed that during the growth period, treatments with LDHs did not affect N adsorption in any of the tested species. In treatments where higher amounts of 2 LDHs were added as a function of NO2 3 in LDHs, the concentration of NO3 in the soil solution at harvest was reduced 10-fold compared to soil without LDHs, thus reducing the chances of losses to other environments. LDHs efficiently adsorbed 2 mineralized NO2 3 during fallow periods, keeping NO3 available for later cultiva2 tion. After 15 months, the buffer capacity of NO3 in the soil increased by 0.3 for soil without LDHs and by 2.7 with the application of 10 g/kg of LDHs in the soil. The researchers concluded that LDHs have potential for use as long-term regulators of NO2 3 movement in the soil, thus reducing the leaching risk of this anion in crop production. Berber et al. (2014) evaluated MgAl-LDH as a potential controlled-release nitrogen fertilizer, simulating two solution conditions: acidic soil (pH 5 4) and neutral (pH 5 7) at three different temperatures: 15 C, 25 C, and 35 C. The authors observed that temperature and pH directly influenced the liberation rate of NO2 3. The release occurred more rapidly with higher temperature and lower pH. In acid solution conditions, 15%, 22%, and 30% of NO2 3 was released on the first day at temperatures of 15 C, 25 C, and 35 C. The total release of NO2 3 from LDHs occurred on the 16th, 14th, and 12th days of the experiment at temperatures of

722

Layered Double Hydroxide Polymer Nanocomposites

15 C, 25 C, and 35 C. In the neutral solution, total NO2 3 release occurred on the 20th, 18th, and 16th days of the experiment at temperatures of 15 C, 25 C, and 35 C. Halajnia et al. (2016) carried out experiments to observe NO2 3 losses through leaching and the effect of different quantities of LDHs on nitrogen uptake by corn plants grown on sandy soil. The application of different amounts of MgAl-Cl-LDH in the soil was studied (2.5, 5, and 10 g/kg), along with the application of nitrogen, using urea and ammonium nitrate as sources. During corn cultivation, seven percolations with distilled water were done to observe how much NO2 3 was lost in each leaching simulation. The results showed that when compared to the control, the application of 10 g/kg of LDHs reduced NO2 3 loss through leaching by 59% when urea was used as the source of N and a 66% reduction in leaching when the source used was ammonium nitrate. In accordance with the authors, the application of LDHs functioned as an NO2 3 buffer, controlling the movement of this ion in the soil solution without interfering with plant N uptake, as shown in the cultivation results (Fig. 17.3). The application of LDHs at all doses significantly increased plant weight in comparison to the control. The authors reported that the application of a 10 g/kg dose of LDHs increased plant height by approximately 8%. In addition, the authors also observed increased N uptake by plants in treatments that included the presence of LDHs. Optimal crop performance may be obtained by supplying nutrients in synchrony with plant demands, because this practice allows better use of nutrients, improved fertilizer efficiency, and reduced nutrient losses (Aita and Giacomini, 2003; Giacomini et al., 2003; Argenta et al., 2003). The results obtained by TorresDorante et al. (2009) and Halajnia et al. (2016) show that LDHs functioned as an ion exchanger by absorbing NO2 3 in soil solution and releasing the molecule according to the needs of the plants, constituting a good alternative for improving crop yields by reducing leaching losses. In the literature, some works have reported the potential of LDHs as phosphate fertilizers (Koilraj et al., 2013; Benı´cio et al., 2015; Bernardo et al., 2016; Moraes et al., 2016). Other studies report, however, that obtaining LDHs with phosphate is more difficult than with nitrate, because, depending on the composition of the LDHs, during the incorporation of the phosphate in the material, insoluble salts can form and hinder the release of phosphate into solution (Radha et al., 2005; Watanabe et al., 2010). Shimamura et al. (2012), with the objective of evaluating the release of HPO22 4 ions intercalated in LDHs, carried out phosphate release kinetic tests in different solutions (Na2CO3, NaOH 1 NaCl, and octasulfonate). The results showed that 96 h into the reaction, the release rates were 93.3%, 58.8%, and 46.4% for octasulfonate, Na2CO3, and NaOH 1 NaCl, respectively. The authors observed a gradual release of phosphate ions contained within LDHs and that this release occurred by anion exchange and depended on the composition of the environment where it is found. The phenomenon of P fixation in the mineral phase of tropical soils causes readily soluble sources, such as triple superphosphate, simple superphosphate, and

Layered double hydroxide nanocomposites for agricultural applications

Wet weight (g/pot)

(A)

140 135 130 125 120 115 110 105 100

Urea

0.0

723

Ammonium nitrate

2.5

5.0

10.0

Shoot N concentration (%)

(B) 2.5 Urea

Ammonium nitrate

2.0 1.5 1.0 0.5 0.0 0.0

2.5

5.0

10.0

LDH dosage (g/kg)

Figure 17.3 Effect of N sources and different doses of LDHs applied to soil: (A) fresh weight of plants, (B) N concentration in the aerial part of the corn plants. Source: Adapted from Halajnia, A., Oustan, S., Najafi, N., Khataee, A.R., Lakzian, A., 2012. The adsorption characteristics of nitrate on Mg
Fe and Mg
Al layered double hydroxides in a simulated soil solution. Appl. Clay Sci. 70, 28
36.

monoammonium phosphate to be minimally used by plants. The works of Everaert et al. (2016) and Benı´cio et al. (2017) showed that when LDHs are placed in conditions with the highest possibility of P fixation (clay soils), it results in better performance than when compared with soluble sources (commercial), both in the production of dry material and P content absorbed by plants. Agronomic efficiency (AE) is an index that helps in understanding the effectiveness of applied fertilizers, since it shows plant production (dry matter or grain), for each unit of supplied nutrient, is calculated by the relation between difference of productivity with and without fertilizer by rate fertilizer applied (Doberman, 2005). Benı´cio et al. (2017) compared the agronomic efficiency of LDHs intercalated with phosphate and triple superphosphate in two soils with different phosphate retention capacities (Fig. 17.4). The results showed that, in sandy soil, the application of a 15 mg/kg dose of phosphate, using LDHs as a source, resulted in an agronomic efficiency of approximately 18 g of dry material for each gram of P applied, whereas triple superphosphate had an efficiency of around 12 g of dry material for each gram of P. Comparing sources showed that increasing P retention capacity (clay soil), the agronomic efficiency of LDHs at the same dosage (15 mg/kg) remained constant at

724

Layered Double Hydroxide Polymer Nanocomposites

LDH-P

Triple superphosphate 20

20

AE (g/g)

(A)

(B)

16

16

12

12

8

8 4

4

0

0 15

30 45 Dose of P (mg/kg)

60

15

30 45 Dose of P (mg/kg)

60

Figure 17.4 Agronomic efficiency (AE) of LDH-P and triple-superphosphate sources in two soils: (A) sandy soil, (B) clay soil with different doses of P. Source: Adapted from Benı´cio, L.P.F., Constantino, V.R.L., Pinto, F.G., Vergu¨tz, L., Tronto, J., Da Costa, L.M., 2017. Layered Double Hydroxides: New technology in phosphate fertilizers based on nanostructured materials. ACS Sustain. Chem. Eng. 5(1), 399
409.

approximately 18 g/g, while the AE of triple superphosphate was reduced from 12 to 4 g/g. While in contact with the soil solution, LDHs undergo an acid “attack,” releasing part of its OH2 ions and increasing environmental pH. This change in pH value controls the reactions of adsorption and precipitation, improving the availability of some elements, for example, P. In addition to improving the availability of nutrients, the remaining LDHs give physical protection to intercalated P ions, which are released through exchange reactions with anions found in the soil (Cl2, NO2 3, SO22 , etc.) (Torres-Dorante et al., 2008; Silva et al., 2014a), gradually replacing 4 the soil solution at the rate absorbed by plants. In addition to the use of LDHs as carriers for nutrients such as N and P, some studies have been done with the objective of evaluating the potential of these materials as micronutrient sources. Imran et al. (2016) synthesized different Mg-Fe-LDH doped with Zn, with the objective of observing the Zn release in solutions with different pH values (4, 5, and 7). The results showed that under conditions with lower pH, a better release of Zn occurred, which indicates that this material can be utilized as a potential source of Zn for plants. The authors also reported that these results need further investigation. Bendinelli et al. (2016) intercalated molybdenum anions in MgAl-LDH by different methods of synthesis (reconstruction of the calcined precursor in different solutions followed by anion exchange and regeneration in molybdenum solution). The authors verified the releasing behavior of the molybdenum anion contained

Layered double hydroxide nanocomposites for agricultural applications

725

within the produced materials. The results showed that the release kinetics varied with the adopted method of synthesis, and that molybdate intercalated LDHs exhibit a slow-release profile for the anion. In the soil, molybdenum is found in extremely low concentrations (Gupta and Lipsett, 1981), which together with the lack of the adequate replacement of Mo in the soil has lead to its depletion. The deficiency of this element is common, particularly in tropical soils (Sfredo et al., 1994, 1997). Even though the objective of Bendinelli et al. (2016) was not to produce a fertilizer containing Mo, this technology can be adapted for agricultural use, because besides serving as a nutrient source it would provide protection for soil and allow for Mg to remain longer within the environment. In various studies involving LDH fertilizer technologies, several authors have reported a rise in the pH of the environment. The elevation of pH in soil above certain limits ( 6 7) causes a reduction of the availability of various nutrients, principally micronutrients (Lutz et al., 1972). In studies carried out by our research group, for which the results have not yet been published, LDHs were used as a source of P for corn. The increase in the dose of applied LHD promoted micronutrient deficiency, limiting plant development. Most micronutrients are mainly responsible for enzymatic activation within biosynthesis pathways. Zn acts on the synthesis of hormones involved in growth (Dechen and Nachtigall, 2006). In corn plants, Zn deficiencies can be demonstrated by the presence of chlorotic and necrotic zones in new leaves, small leafy plants, and short internodes (Figs. 17.5 and 17.6).

Figure 17.5 Maize plants 20 days after sowing, using MgAl-LDH as the P source. From left to right: P doses of 0, 100, 200, 300 and 600 mg. Source: Benı´cio, L.P.F., Constantino, V.R.L., Pinto, F.G., Vergu¨tz, L., Tronto, J., Da Costa, L.M., 2017. Layered double hydroxides: new technology in phosphate fertilizers based on nanostructured materials. ACS Sustain. Chem. Eng. 5(1), 399
409.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 17.6 Zinc deficiency in corn plants at a dose of 300 mg of P using LDHs as a source. The deficiency can be observed by the presence of chlorosis in the younger leaves, short internodes, and small plant size. Source: Benı´cio, L.P.F., Constantino, V.R.L., Pinto, F.G., Vergu¨tz, L., Tronto, J., Da Costa, L.M., 2017. Layered double hydroxides: new technology in phosphate fertilizers based on nanostructured materials. ACS Sustain. Chem. Eng. 5(1), 399
409.

17.4.2 Layered double hydroxides for storage and gradual herbicide release To increase herbicide effectiveness, herbicides are often applied in quantities above the recommended doses. Elevated doses of these compounds can generate many environmental problems, particularly by contaminating soil and water. The use of inorganic carriers for herbicide encapsulation can reduce the quantity of these formulations for effective weed control, thus reducing environmental risks. Table 17.2 shows the principal works found in the literature on the use of layered double hydroxides for storage and slow release of herbicides. bin Hussein et al. (2005), with the objective of producing a slow-release herbicide, intercalated 2,4-D in Zn-Al-LDH. The release experiment was carried out in solutions containing sodium carbonate and sodium chloride at concentrations of 0.05, 0.005, 0.0005 mol/L, and distilled water. The results showed that the material allows for slow release of the 2,4-D molecule. The release rate depends on the composition of the medium and the presence of CO22 3 increased the release rate because of the higher affinity of the anion for lamellar structure (Fig. 17.7). Cardoso et al. (2006) intercalated molecules of the herbicide acids 2,4-D, MCPA, and Picloram in LDHs to obtain a slow-release herbicide. After intercalation, the authors conducted release tests in leaching columns. Columns 30 cm in height and 5 cm in diameter were filled with soil and treated with the herbicides intercalated in LDHs and underwent daily leaching of 50 mL of water for 8 days. Bioassays were also performed using Lepidium sativum as the target plant. Comparing LDHs with pure herbicides, leaching test results showed that when the molecule is released slowly, release percentages relative to pure herbicides were 79%, 60%, and 18% for Picloram, MCPA, and 2,4-D. These results highlight advantages such as lower

Layered double hydroxide nanocomposites for agricultural applications

727

Table 17.2 Principal works found in the literature on the use of LDHs for storage and slow release of herbicides Author

Layer cations

Molecule

Li et al. (2004) bin Hussein et al. (2005) Meng et al. (2005) Cardoso et al. (2006) Khan et al. (2007) Bruna et al. (2008) Bruna et al. (2009) Hussein et al. (2010a) Sarijo et al. (2010) Touloupakis et al. (2011) Bashi et al. (2016) Pavlovic et al. (2013) Hashim et al. (2014) Sarijo et al. (2015a) Sarijo et al. (2015b)

Mg-Al Zn-Al Mg-Al Mg-Al Mg-Al Mg-Al Mg-Al and Mg-Fe Zn-Al Zn-Al Mg-Al Zn-Al Mg-Al Zn-Al Zn-Al Zn-Al

Glyphosate 2,4-D Glyphosate 2,4-D; MCPA and Picloram Mecoprop Terbuthylazine MCPA (2-chlorophenoxy) propionate 2CPA and TCPA Atrazine 2,4-D Linuron, 2,4-DB and Metamitron Cloprop TBA MCPA and 3,4-D

herbicide leaching and consequently lower groundwater contamination, and disadvantages, for example, longer herbicide permanence in the system due to LDH protection may increase the possibility of degrading the molecule. For the bioassays, results showed that when intercalated in LDH, molecules of the herbicides Picloram, MCPA, and 2,4-D, showed the same pre-emergence control efficiency on Lepidium sativum as pure herbicide molecules. The advantage of intercalated molecules would be the absence of immediate losses shortly after application because of the slow-release character of LDH.

17.4.3 Layered double hydroxides for storage and slow release of plant growth regulators Growth regulators are synthetic or natural substances that promote physiological changes by altering the hormonal balance of plants, promoting defoliation, accelerated maturation, reduced growth, etc. They are used in many crops to balance the vegetative and reproductive stages of plants, to facilitate crop treatment, and increase productivity. These substances can be applied either through foliar application or directly in the soil (de Souza and MacAdam, 2001). These compounds, however, are highly susceptible to losses and in some cases may have undesirable effects, including plant death (Tsatsakis and Shtilman, 1994). The gradual release of these substances can mitigate various problems and reduce losses. Works have been published in the literature on studies of the intercalation of growth regulators in LDHs with the aim of obtaining slow-release compounds (Table 17.3).

728

Layered Double Hydroxide Polymer Nanocomposites

Figure 17.7 Release profile of intercalated 2,4-D in Zn-Al-LDH in solutions containing different anions and concentrations: (A) NaCl, (B) Na2CO3, and (C) distilled water. Insert shows the beginning of the release process. Source: bin Hussein, M.Z., Yahaya, A.H., Zainal, Z., Kian, L.H., 2005. Nanocompositebased controlled release formulation of an herbicide, 2, 4-dichlorophenoxyacetate incapsulated in zinc
aluminium-layered double hydroxide. Sci. Technol. Adv. Mater. 6(8), 956
962.

Layered double hydroxide nanocomposites for agricultural applications

729

Table 17.3 Principal works found in the literature using LDHs as carriers for storage and slow release of growth regulators Author

Layer cations

Growth regulator

Synthesis method

Hussein et al. (2002) Williams and O’Hare (2006) Hussein et al. (2007) Qiu and Hou (2009) Hafez et al. (2010) Hussein et al. (2010b) Li et al. (2014) Liu et al. (2014)

Zn-Al

α-Naphthaleneacetate

Direct: coprecipitation

Li-Al; Ca-Al

1-Naphthaleneacetic acid; 3indoleacetic acid; and 2-methyl4- chlorophenoxyacetic acid

Zn-Al

4- Chlorophenoxyacetate

Direct: coprecipitation Indirect: anion exchange Direct: coprecipitation

Mg-Al

Indole-3-butyric acid

Mg-Al

Gibberellic acid

Zn-Al

2-Chloroethylphosphonic acid

Direct: coprecipitation

Mg-Al Mg-Al

1-Naphthylacetic acid and indole3-butyric acid α-Naphthaleneacetate

Li et al. (2015)

Mg-Al

ß-Naphthoxyacetic acid

Direct: coprecipitation Direct: coprecipitation Indirect: anion exchange Direct: coprecipitation

Indirect: anion exchange and regeneration Indirect: anion exchange

It is possible to obtain LDHs containing different plant growth regulators through direct and indirect methods. The intercalation of these regulators in the interlamellar space allows a sustained release and longer molecule longevity in environments such as soil. Hafez et al. (2010) intercalated gibberellic acid (GA) in MgAl-LDH (GA-LDH). The study aimed to test the release of GA in different solutions (pH 5 3 and pH 5 7) and the breakdown of the substance in two soils (sandy and medium texture). GA-LDH results were compared with pure GA. The authors showed that GA-LDH is more effective than pure GA, since pure GA crystallizes rapidly and after 24 h of reaction only 4.5% and 10.4% is released in the pH 5 3 and pH 5 7 solutions. After this period, the hormone becomes insoluble and does not react further in the medium. After 24 h of reaction, 80% of GA-LDH was released into the pH 5 3 solution and 60% into the pH 5 7 solution. Additionally, the slow-release property

730

Layered Double Hydroxide Polymer Nanocomposites

Figure 17.8 Gibberellic acid decay in different soils. Sandy soil (dashed line) and clayey soil (solid line). (&) Before intercalcation and (’) after intercalation. Source: Hafez, I.H., Berber, M.R., Minagawa, K., Mori, T., Tanaka, M., 2010. Design of a multifunctional nanohybrid system of the phytohormone gibberellic acid using an inorganic layered double-hydroxide material. J. Agric. Food Chem. 58(18), 10118
10123.

was maintained for 6 days after the start of the reaction. Soil degradation tests showed that intercalation caused an increase in GA duration in the soil when compared to pure GA. The pure GA remained for 6 days in the medium-texture soil and for 10 days in the sandy soil. After the intercalation, the period of GA duration in the soil increased to 20 and 28 days for the medium and sandy texture soils, respectively (Fig. 17.8).

17.4.4 Use of layered double hydroxides for pesticide removal LDHs and their thermal decomposition products (oxides and mixed oxyhydroxides) can be used to remove contaminant anions from water and soil. Once anionic species are adsorbed, the versatility of LDH allows for the anionic species to be used again in the desorption processes through gradual release. In this way, anionic species that are commonly used in agriculture, such as nitrates, phosphates, and agrochemicals in general, can be removed from the environment by means of adsorption processes and later recycled for plant growth or in pest control via a slow, gradual release (Benı´cio et al., 2015). Table 17.4 lists some key publications regarding the use of LDHs as removers of anion contaminants of water and soil. The adsorbent character of LHDs depends on the anion present in the interlamellar domain, since anion exchange processes can occur between the intercalated anions and the anions in solution (Miyata, 1983; Sato et al., 1993). When calcined LDH is used, it is possible to have adsorption and regeneration of the lamellar structure, by an effect known to researchers as the memory effect (Miyata, 1980; Schaper et al., 1989; Sato et al., 1988).

Layered double hydroxide nanocomposites for agricultural applications

731

Table 17.4 Principal works found in the literature using LDH as contaminant anion adsorbents Author

Layer cations

Polluting ion

Ulibarri et al. (1995)

Mg-Al

Shin et al. (1996) Goswamee et al. (1998) Inacio et al. (2001) Seida and Nakano (2002) You et al. (2002) Park et al. (2004) Legrouri et al. (2005) Yang et al. (2005) Li et al. (2005) Pavlovic et al. (2005)

Mg-Al Mg-Al; Ni-Al, and Zn-Cr

Trichlorophenol and trinitrophenol Phosphate Cr(VI)

Mg-Al Mg-Fe and Ca-Fe

MCPA Phosphate

Mg-Al Mg-Fe and Ca-Fe Zn-Al Mg-Al Mg-Al Mg-Al

Lv et al. (2007) Xu et al. (2010)

Mg-Al Mg-Al, Mg-Fe, Ca-Al, and Ca-Fe Ni-Al Mg-Al Mg-Al 1 Carbon nanosphere Mg-Al

Dicamba Endosulfan 2,4-D Arsenate and selenate Glyphosate 2,4-D; Clopyralid and Picloram Fluoride Arsenate and phosphate Glyfosate and Glufosinate Congo Red Cu, Cd, and Pb AO10

Zn-Al

Arsenite and arsenate

Mg-Al

Atrazine

Mg-Al

Atrazine

Khenifi et al. (2010) Chen et al. (2011) Gong et al. (2011) Extremera et al. (2012) Bagherifam et al. (2014) Alekseeva et al. (2011) Halma et al. (2015)

You et al. (2002) evaluated the adsorption of the pesticide Dicamba in aqueous solution. The authors synthesized an LDH from the MgAl-CO3 system and tested the adsorption of calcined and noncalcined LDHs at different concentrations of the pesticide, with different contact times. Desorption was also tested in solutions con2 22 22 2 taining different anions such as CO22 3 , SO4 , HPO4 , NO3 , Cl , and distilled water. The authors noted that LDH-CO3 was not capable of adsorbing the pesticide, a function of the high affinity of the CO22 3 anion and the LDH layers. The calcined LDH, however, was efficient in removing the pesticide from the solution, increasing adsorption as a function of the increased concentration of the pesticide in solution (Fig. 17.9). Regarding the pesticide adsorption time, the authors showed that it is a rapid process, where 80% of the pesticide was removed in only 10 min. Over the next 20 min, the adsorption rate fell slowly until reaching equilibrium at 30 min.

732

Layered Double Hydroxide Polymer Nanocomposites

Adsorbed dicamba (mmol/g)

1.2 Calcined-LDH

Stage 2

1 LDH-CO32– 0.8 0.6 0.4 Stage 1 0.2 0 0

0.1

0.2

0.3

0.4

0.5

Equilibrium concentration (mmol/dm3)

Figure 17.9 Adsorption isotherm for dicamba retention by LDH-CO3 and calcined LDH. Source: You, Y., Zhao, H., Vance, G.F., 2002. Adsorption of dicamba (3, 6-dichloro-2methoxy benzoic acid) in aqueous solution by calcined
layered double hydroxide. Appl. Clay Sci. 21(5), 217
226.

Desorbed dicamba (mmol/g)

0.3 0.25 0.2 0.15

SO42– HPO42– CO32–

0.1

NO3– CI– Distilled water

0.05 0 0

2

4 6 8 Desorption treatments

10

Figure 17.10 Release of dicamba adsorbed on calcined LDH by various anionic solutions. Source: You, Y., Zhao, H., Vance, G.F., 2002. Adsorption of dicamba (3, 6-dichloro-2methoxy benzoic acid) in aqueous solution by calcined
layered double hydroxide. Appl. Clay Sci. 21(5), 217
226.

For desorption with different anions, the results showed that pesticide adsorption is reversible, being released in solution by ion exchange. For the anions, the desorp2 22 22 2 tion rates occurred in the following order SO22 4 . HPO4 . CO3 . NO3 5 Cl (Fig. 17.10).

Layered double hydroxide nanocomposites for agricultural applications

733

In studies that evaluated the ability to remove glyphosate in aqueous solution, Li 22 et al. (2005) prepared MgAl-LDH with different intercalated anions (NO2 3 , CO3 , 2 and Cl ). The experiments showed that glyphosate removal in solution can happen in two ways, adsorption on the outer surface and interlamellar anion exchange. In solutions with low glyphosate concentration, adsorption occurs on the external surface of the LDH crystals, but in solutions with high concentrations of the herbicide, the anionic exchange occurs between the glyphosate and the anion of the initial LDH structure. The authors also showed that the adsorption capacity varies according to the molar ratio of the lamella metals, with a higher charge density and consequently greater adsorption capacity resulting from lower molar ratios. Regarding LDH interlamellar anions, the amount of pesticide adsorbed on MgAl-LDH 22 decreased in the following order: Cl2 .NO2 3 . CO3 . In addition to studies involving direct adsorption of pesticides by LDHs, some researchers have developed studies encapsulating microorganisms in LDHs, aiming to increase the biodegradation of the pesticide, this technology extends the range of LDH uses and allows in situ remediation treatments. Halma et al. (2015) made biohybrids of MgAl-LDH and Pseudomonas sp. bacteria, with the objective of observing the increase in the degradation rate of the herbicide Atrazine. The authors produced five different materials, varying the amount of LDH (0.1, 0.2, 0.5, 1.0, and 2.0 g) for a fixed amount of bacteria (4 3 1011 colony-forming units). The results showed that the degradation rate varied according to the ratio of bacteria/LDH. After 8 h of reaction, free bacteria had a 71% degradation rate and only treatments at quantities of 0.1, 0.2, and 0.5 g of LDHs were more efficient with degradation rates of 94%, 82%, and 94%. The authors repeated the degradation process over four cycles and observed that encapsulating the bacterium maintains cell stability, with the material able to be reused without losing efficiency, although free cells lose efficiency with each reuse.

17.5

Final considerations

Since the beginning of agriculture, man has sought solutions to improve the productivity of cultivated species. Advances in science and technology have enabled productivity levels unimaginable a few years ago. Some of these advancements, however, have improved productivity at the expense of environmental harm and damage to human health. Thus, the creation of “green chemistry” technologies is essential for producing the least amount of toxic residues possible while also maintaining or even surpassing the productivity levels that have already been achieved. Among options that have materialized over the last few years within studies of this area, LDHs show promise. The application of LDHs, whether in plants or seeds, to test nutrient release, pesticides, and even growth regulators has not been frequently used in research. Most research available in the literature is limited to studies of synthesis, characterization, and release kinetics of solutions (aqueous or buffered medium). The results obtained are encouraging for the most part and show these

734

Layered Double Hydroxide Polymer Nanocomposites

materials have great potential for use in agricultural production. As such, it is necessary to expand studies in this field by carrying out bioassays that are capable of reproducing real conditions and that can thus validate the technologies proposed for the use of LDHs as matrices for the storage and release of compounds of agricultural interest. For these tests, it is necessary to bring together interdisciplinary teams that include not only chemists but also agricultural and administrative professionals, thus contributing to the development of research for new agricultural technologies involving LDHs.

List of abbreviations 2,4-D dichlorophenoxyacetic acid 2,4-DB 4-(2,4-dichlorophenoxy) butanoic acid 2CPA 2-chloro-2-methylpropane 3,4-D 3,4-dichlorophenoxyacetate AO10 Acid Orange 10 DICAMBA 3,6-dichloro-2-methoxy benzoic acid ESCAP Economic and Social Commission for Asia and the Pacific FAO Food and Agriculture Organization of the United Nations LDH layered double hydroxide MCPA 2-methyl-4-chlorophenoxyacetic acid TBA 2,4,5-tricholorophenoxy butyric acid TCPA 2,4,5-trichlorophenoxyacetate

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Shimamura, A., Kanezaki, E., Jones, M.I., Metson, J.B., 2012. Direct observation of grafting interlayer phosphate in Mg/Al layered double hydroxides. J. Solid State Chem. 186, 116
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145. Wan, S., Wang, S., Li, Y., Gao, B., 2017. Functionalizing biochar with Mg
Al and Mg
Fe layered double hydroxides for removal of phosphate from aqueous solutions. J. Ind. Eng. Chem. 47, 246
253. Wang, Q., O’Hare, D., 2012. Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets. Chem. Rev. 112 (7), 4124
4155. Watanabe, Y., Ikoma, T., Yamada, H., Stevens, G.W., Moriyoshi, Y., Tanaka, J., et al., 2010. Formation of hydroxyapatite nanocrystals on the surface of Ca
Al-layered double hydroxide. J. Am. Ceram. Soc. 93 (4), 1195
1200. Williams, G.R., O’Hare, D., 2006. Towards understanding, control and application of layered double hydroxide chemistry. J. Mater. Chem. 16 (30), 3065
3074. Woo, M.A., Kim, T.W., Paek, M.J., Ha, H.W., Choy, J.H., Hwang, S.J., 2011. Phosphateintercalated Ca
Fe-layered double hydroxides: crystal structure, bonding character, and release kinetics of phosphate. J. Solid State Chem. 184 (1), 171
176. Xu, Y., Dai, Y., Zhou, J., Xu, Z.P., Qian, G., Lu, G.M., 2010. Removal efficiency of arsenate and phosphate from aqueous solution using layered double hydroxide materials: intercalation vs. precipitation. J. Mater. Chem. 20 (22), 4684
4691.

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Yang, L., Shahrivari, Z., Liu, P.K., Sahimi, M., Tsotsis, T.T., 2005. Removal of trace levels of arsenic and selenium from aqueous solutions by calcined and uncalcined layered double hydroxides (LDH). Ind. Eng. Chem. Res. 44 (17), 6804
6815. You, Y., Zhao, H., Vance, G.F., 2002. Adsorption of dicamba (3, 6-dichloro-2-methoxy benzoic acid) in aqueous solution by calcined
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226. Zheng, T., Liang, Y., Ye, S., He, Z., 2009. Superabsorbent hydrogels as carriers for the controlled-release of urea: experiments and a mathematical model describing the release rate. Biosyst. Eng. 102 (1), 44
50.

Further reading Fageria, N.K., Baligar, V.C., Clark, R.B., 2002. Micronutrients in crop production. Adv. Agron. 77, 185
268. Sarijo, S.H., Ghazali, S.A.I.S.M., Hussein, M.Z., Sidek, N.J., 2013. Synthesis of nanocomposite 2-methyl-4-chlorophenoxyacetic acid with layered double hydroxide: physicochemical characterization and controlled release properties. J. Nanopart. Res. 15 (1), 1356. Singh, Z., Khan, A.S., 2012. Surfactant and nutrient uptake in citrus. Advances in Citrus Nutrition. Springer, Netherlands. Williams, G.R., O’Hare, D., 2006. Towards understanding, control and application of layered double hydroxide chemistry. J. Mater. Chem. 16 (30), 3065
3074.

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Layered double hydroxide polymer nanocomposites for food-packaging applications

18

Giuliana Gorrasi1 and Andrea Sorrentino2 1 Department of Industrial Engineering, University of Salerno-via Giovanni Paolo II 132, Fisciano, Italy, 2Institute for Polymers, Composites and Biomaterials (IPCB), National Research Council (CNR), Lecco, Italy

18.1

Introduction

The strategic evolution of the modern manufacturing industry is leading to a profound change, due to an increasing interest in new eco-friendly technologies (Gorrasi and Sorrentino, 2015; Delogu et al., 2017) and materials with integrated multifunctionalities (Gorrasi et al., 2011; Costantino et al., 2011). This is particularly true for the food-packaging industry (Coles et al., 2003; Silvestre et al., 2011). As a matter of fact, the increasing requirement for foods ready to be consumed, the globalization of the trade and the distribution from centralized processing, superimpose a big challenge to ensure and increase the safety and quality of foods (Joseph Kerry, 2008; Raheem, 2012). The concept of “total quality” of products has become more and more important, considering not only the raw material and the process technology, but also the food packaging, strictly intercorrelated (Quintavalla and Vicini, 2002; Renato Souza et al., 2012). The packaging must possess several characteristics, often difficult to obtain from a single standard material (Debeaufort et al., 1998; Sharma et al., 2015). Hygienic, safety, organoleptic, and esthetic qualities must be coupled with the more traditional mechanical and barrier properties (Sorrentino, 2011; Azeredo, 2009). Packaging and food are now seen as a single element that can interact and improve the consumer’s perception and acceptability (Brody, 1997). The most common food-packaging materials are metal, polymer, glass, wood, and paper. Obviously, each of these materials has different performances and interactions with foods. Of these, polymers are more numerous and adaptable to the specific food requirements (Robertson, 2016; Garofalo et al., 2013). Polymer materials can be formed into a variety of useful products. They are moldable, heat-sealable, easy to print, and can be integrated into the food production line. Each polymer has its own unique properties, based on its chemical structure/composition. Modern chemical synthesis techniques make available hundreds of synthetic polymers but, in practice, only a few polymers are regularly used for food packaging. A list of these polymers with their main properties and common uses is outlined in Table 18.1. Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00018-5 © 2020 Elsevier Ltd. All rights reserved.

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Layered Double Hydroxide Polymer Nanocomposites

Table 18.1 Polymer materials used in food packaging (Robertson, 2016) Polymer type

Properties

Common uses

Code

Polyamides

Polar and sensitive to humidity, but in turn, they offer a good barrier against gas permeation. Good wear resistance and excellent resistance to puncture High barrier to gases including oxygen, high strength when dry, high water vapor permeability, sensitive to moisture Transparent, hard, and tough. Good mechanical modulus and excellent dimensional stability Excellent chemical resistance, good resistance to organic solvents, good fatigue and wear resistance, higher impact strength, lower working temperatures, resistant to staining, low moisture absorption rates Hard, stiff, good dimensional and excellent wear resistance. Highly transparent and colorless. Good chemical resistance and resistant to microbial. High/good gas barrier properties, low water absorbability Good adhesive properties and heat-sealing functionality Excellent resistance to stress, highly resistant to cracking, high operational temperatures, highly resistant to most alkalis and acid and organic solvents, nonstaining. Has moderate permeability to moisture, gases, and odors, which is not affected by changes in humidity

Kitchen utensils and gas barrier layers in flexible packaging

PA

High barrier films or coatings

EVOH

Plastic containers and bottles

PC

Plastic bag, films, containers including bottles, etc.

PE

Bottles, tray

PET

Hot-melt manufacturing Bottle closures, bottle, tubs, films

EVA

Ethylvinylalcohol

Polycarbonates

Polyethylenes

Polyethylene terephthalates

Poly(ethylene vinyl acetates) Polypropylenes

PP

(Continued)

Layered double hydroxide polymer nanocomposites for food-packaging applications

745

Table 18.1 (Continued) Polymer type

Properties

Common uses

Code

Polystyrenes

Clear, transparent, hard, and brittle. Relatively low melting point. High level of humidity resistance. Poor barrier to oxygen and water vapor Water-soluble and completely biodegradable. Very high oxygen barrier Fire resistant but not environmentally friendly. PVC can release its monomer vinyl chloride, which has high toxicity. Also, plasticizers, essential to soften PVC, can migrate to food. These reasons have caused a decline in PVC use over time, especially for food applications Good heat sealability. Excellent barrier properties against water vapor, odors/flavors, and gases Excellent sealing and formability. Good toughness as well as transparency Produced by a previous depolymerization followed by chemical transformations and repolymerization of materials coming from natural, renewable sources. Properties similar or slightly lower than those shown by synthetic oil-based polymers Short and simple chemical transformations, with a low environmental impact and low final costGenerally, not excellent mechanical properties and the final product may vary slightly depending on different raw material batches

Tubs, protective packaging, containers, lids, and disposable cutlery High barrier films or coatings

PS

Polyvinyl alcohols Polyvinyl chlorides

Polyvinyldene chlorides

Ionomers

Bioplastics

Natural polymers

PVOH

Gaskets for jar lids, pipes, and tubes

PVC

Multilayer films and container coating

PVDC

Sealing in flexible packaging Films, containers, and bottles

I

Flexible packaging

PLA, PBS, PHB, PCL, etc.

Cellulose, starch, proteins, fibers, etc.

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Layered Double Hydroxide Polymer Nanocomposites

As shown in Table 18.1, each polymer presents specific properties but also limitations that do not allow meeting the multiple performance requests for a packaging material. For that, multilayer structures are generally used to overcome some specific disadvantages, such as high cost, low barrier, and mechanical properties. Polymers such as polyethylene and polypropylene can provide mechanical properties at low cost, while barrier polymers such as polyamides and ethylene vinyl alcohol can provide protection against gases, flavors, and odors through the packaging. The main drawback of these multilayer packages is represented by their difficult recycling and reprocessing into new products (Coccorullo et al., 2009). The production and use of packages around the world have increased enormously, worsening the problem of waste disposal. The growing interest in the environmental impact of discarded materials requires thermoplastic polymers that can be melted and reused as raw materials for new applications. An alternative strategy to improve the physical properties (i.e., mechanical, thermal, degradation, barrier, etc.) of a traditional polymeric system is the incorporation of inorganic materials at the nanometer level (Sorrentino et al., 2007; Mihindukulasuriya and Lim, 2014; Wyser et al., 2016). The chemical functionalization of such materials, using active molecules with specific properties, can also impart new functionalities to the polymers. Other interesting prospects concern some specific applications, such as controlled release (Gorrasi et al., 2016a; Lvov and Abdullayev, 2013). High-quality and microbiologically safer foods, as well as longer product shelf-life, represent a continuous stimulus for the basic and applied research to develop new food preservative strategies (Sonkaria et al., 2012). It is well known that the characteristics of food packaging, which aimed at reducing bacterial spoilage and protecting from the pollutant environment, are quite different from the packaging of other products and require more specific properties (Sung et al., 2013). Nanoscience and nanotechnologies are opening new opportunities for producing materials with surprisingly unusual properties. The possibility to manipulate on an atomic or molecular scale leads to structures that have unique characteristics and completely new functionalities. In addition, the possibility to have several functionalities in one layer is highly desirable (Caseri, 2007). For the realization of new polymeric nanocomposites, in which an inorganic component is dispersed, it is necessary for the analysis of the structural organization of the polymer and the resulting physical properties (Sorrentino et al., 2006a, 2012). In fact, the properties of the material depend on the properties of the components and on the interfacial interactions between the different phases (Gorrasi et al., 2013). A crucial point for understanding the many properties of the composites and the provision of their behavior is the analysis of the structure and the polymer/inorganic compound interface (Sorrentino et al., 2006b). In this sense, analysis of the structural organization of the polymer and correlation with the physical properties is of fundamental importance (Verdolotti et al., 2014; Gorrasi and Sorrentino, 2013). Moreover, the release of the active molecules will be dependent on the interaction between the organic molecule and inorganic lamella, composition, type of dispersion of the modified inorganic material into the polymer, and contact with food (Gorrasi et al., 2014; Bugatti et al., 2013). The new approach of introducing inorganic fillers containing

Layered double hydroxide polymer nanocomposites for food-packaging applications

747

active molecules into polymeric matrices can lead to a synergism of properties typical of organic/inorganic composites (Sung et al., 2013; Olah, 2000). Several factors influence the release of active molecules, the most important are: electronic structure, interaction with the polymer matrix, concentration, and morphology of the resulting composites (Sorrentino et al., 2006c; Lvov et al., 2008). The application of nanocomposites will help to expand the use of both oil-based polymers and biobased plastics in food-packaging applications. In this optic, nanotechnology promises several stimulating revolutions to enhance wealth, health, and quality of life (Mihindukulasuriya and Lim, 2014; Dingman, n.d.). In this chapter, we report several examples of nanocomposite systems based on different polymer matrices (either from fossil or renewable resources) and LDH-based nanofillers. Special emphasis will be dedicated to their application in food packaging as active functional nontoxic systems. Some considerations on regulatory issues for the use of nanotechnology in the food-packaging field are also briefly reported.

18.1.1 Characterization and analytical techniques of polymer nanocomposites for food-packaging applications The performance of food packaging mainly depends on the possible interactions between the food and the packaging material. Poor packaging performances can be responsible for detrimental chemical and organoleptic changes that occur during storage (Noonan et al., 2014; Scarfato et al., 2015). These changes can be associated with: G

G

G

sorption of food constituents, especially aroma compounds, by polymeric packaging materials; permeation of gases and vapors from the outside into the packaging; and migration of additives, fillers, and low-molecular-weight molecules from the packaging material into the food.

In order to decide whether a plastics-packaging material complies with the storage requirements, two sets of questions should be considered. Concerning the plastics materials, one must decide if the type to be used in contact with food has the barrier properties necessary to ensure the correct protection of the food stored and whether it releases substances or additives in the allowed concentrations (Ahvenainen, 2003). Food quality may be lost if flavor and aroma are absorbed into the packaging material. Also, loss or gain of moisture vapor may be responsible for food changing as it can cause a loss of texture, dehydration, or loss of weight. Hence, the permeability of the packaging material to moisture vapor and common gases such as oxygen, carbon dioxide, and nitrogen is an important property of the packaging material. It has been measured by standard and well-established test methods. The results provide guidance with respect to the choice of material, additives, and thickness necessary for the correct packaging of specific food products. In any case, complementary shelf-life tests are required to establish performance with the food under consideration. The toxicity of plastic materials, particularly those kept in contact with food for a prolonged period or heated during the particular food processing, is first

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Layered Double Hydroxide Polymer Nanocomposites

determined by the extent to which the additives migrate into the packed food during filling and storage under normal conditions (Crompton, 2007). To ensure that adequate product safety procedures are carried out, many countries have enacted extensive legislation to maintain safety with respect to plastics in contact with food. Similarly, a large amount of research regarding the migration of volatiles, additives, monomers, and oligomers from plastic packaging materials into food was conducted. The main objectives of these studies are: G

G

G

to identify the potential migrants or contaminants in the food contact
packaging materials; to determine their toxicological potency; and to identify the factors affecting the level and time necessary for migration of such contaminants.

The analytical procedures typically involve the contacting of the packaging material for a selected time and temperature with either the foodstuff or the beverage to be packed. Sometimes, quantitative determination of the migrated additives in the heterogeneous foodstuff is extremely difficult. Therefore, migration must be done in a variety of oily, alcoholic, and aqueous solutions that simulate the food. Several studies have been published on the migration of nanoparticles out of polymers (Bart, 2006). Most are concentrated on the migration of antimicrobial nano-silver particles (Bott et al., 2014). It is generally concluded that little or no migration of inorganic nanoparticles can be detected from the nanocomposites. However, for many nanomaterials, the evaluation of their migration from a plastic matrix requires very sensitive technique. Nano-clay migration from PLA nanocomposites was not detectable (Schmidt et al., 2011; Petersen et al., 1999). The same authors determined the migration of organo-modified LDH platelets from PLA nanocomposites according to a European standard method (Schmidt et al., 2011). In that case, LDH particle migration was indirectly determined via migration of magnesium. The results indicate that the material properties are in compliance with the migration limits for total migration as set down by EU legislation (Souza and Fernando, 2016). Modeling migration of nanoparticles any detectable migration of nanoparticle from packaging to food could take place only in the case of very small nanoparticles (lower 1 nm range), not bound in the polymer matrices. However, more migration studies needed to draw general conclusions on potential risks to consumers, especially if mechanical stress or strong interactions with foods are expected to cause physical release. At the moment, in accordance with current EU regulation on substances, the particular combination polymer nanoparticles should be risk assessed on a case-by-case basis (EC, 2009).

18.2

Layered double hydroxides as hosts of active molecules for potential in food-packaging applications

Several active molecules have been analyzed for food-packaging applications for their antimicrobial effects on molds. Table 18.2 is a list of the most studied active

Layered double hydroxide polymer nanocomposites for food-packaging applications

749

Table 18.2 Active molecules tested for food-packaging applications Active molecules tested

Organism

Medium

Reference

Allyl isothiocyanate

Aspergillus flavus Monilinia fructicola Penicillium italicum

Peanuts

Otoni et al. (2014) Yan et al. (2012)

Berberine Bergamot essential oil

Bergamot, thyme, and tea

Penicillium italicum Penicillium italicum

Caraway essential oil

Candida krusei

Carbendazim Carvacrol and montmorillonite Carvacrol, cinnamaldehyde Cinnamaldehyde

A. fumigatus Botrytis cinerea Botrytis cinerea P. expansum A. niger

Cinnamaldehyde and eugenol Cinnamaldehyde, natamycin

Peach

Oranges Citrus (murcott tangar) Baby carrot

Bread and cheese Bread

Candida albicans A. solani

Bread

Alternaria sp.

Cut apples

Cinnamomom zeylanicum

P. digitatum

Citrus

Cinnamon

Aspergillus flavus Aspergillus flavus Botrytis cinerea

Table grapes

Botrytis cinerea

Peaches

P. expansum

Cinnamon, clove, anise, turmeric, guava leaf, nutmeg, and lime oils

Rhizopus stolonifer Aspergillus flavus

Sa´nchezGonza´lez et al. (2010) Cha´fer et al. (2012) Chien et al. (2007) Gniewosz et al. (2013) Park et al. (2001) Mascheroni et al. (2011) Ben et al. (2007) Balaguer et al. (2013 ) Balaguer et al. (2013b) Sanla-Ead et al. (2012) Balaguer et al. (2014) Garrido Assis and de Britto (2011) Kouassi et al. (2012) Manso et al. (2013) Manso et al. (2014) Melgarejo-Flores et al. (2013) Montero-Prado et al. (2011) Manso et al. (2015) Rodrı´guez et al. (2008) Matan (2012)

(Continued)

750

Layered Double Hydroxide Polymer Nanocomposites

Table 18.2 (Continued) Active molecules tested

Organism

Cinnamon, clove, oregano

Eurotium repens

Clove, cinnamon, and oregano

Fusarium concentricum Eurotium repens

Clove, cinnamon, and star anise Clove, cinnamon, oregano, cinnamaldehyde

Medium

Green asparagus

P. digitatum A. alternata

Cherry tomatoes

A. solani Ethanolic extract of propolis

A. niger A. niger

Eugenol and thymol

Botrytis cinerea

Table grapes

Grafted quaternary amino groups Grapefruit seed extract L. scaberrima

Aspergillus flavus Botrytis cinerea P. digitatum

Table grapes Oranges

Lactoferrin, lactoferrin hydrolysate, and lactoperoxidase Lemon

P. commune

Botrytis cinerea

Strawberries

Lemongrass

C. capsici C. gloesporioides C. gloesporioides

Bell pepper Papaya Avocado

Lemongrass and cinnamon

C. gloesporioides

Papaya

Limonene and peppermint Linalool, carvacrol, and thymol

Botrytis cinerea A. niger

Strawberries

Lippia scaberrima Malic acid, nisin and natamycin

Saccharomyces cerevisiae C. gloesporioides P. chrysogenum

Avocado

Reference Lo´pez et al. (2007) Qiu et al. (2014) Rodrı´guez et al. (2007) Wang et al. (2011) RodriguezLafuente et al. (2010) Younes et al. (2014) Pastor et al. (2010) Sebti et al. (2005) Valero et al. (2006) de Oliveira Pedro et al. (2013) Xu et al. (2007) du Plooy et al. (2009) Min and Krochta (2005) Perdones et al. (2012) Ali et al. (2015) Ali et al. (2010) Mpho et al. (2013) Maqbool et al. (2011) Vu et al. (2011) Kuorwel et al. (2014) Kuorwel et al. (2011) Regnier et al. (2010) Pintado et al. (2010) (Continued)

Layered double hydroxide polymer nanocomposites for food-packaging applications

751

Table 18.2 (Continued) Active molecules tested

Organism

Medium

Reference

Mineral salts, organic acid salts, parabens

Botrytis cinerea

Cherry tomatoes Plums

Fagundes et al. (2013) Karaca et al. (2014) ValenciaChamorro et al. (2008) ValenciaChamorro et al. (2009b) Ziani et al. (2009) Cong et al. (2007) Ture et al. (2011)

Mineral salts, organic acid salts, parabens, EDTA, 2-deoxy-D-glucose

Monilinia fructicola P. digitatum and P. italicum P. digitatum

Natamycin

Oranges

A. alternata A. alternata A. niger and P. roquefortii A. ochraceus

Fresh kashar cheese

P. roqueforti

Gorgonzola cheese

Natamycin and rosemary extracts Oregano

A. niger and P. roquefortii P. digitatum

Oregano, cinnamaldehyde, hydrocinnamaldehyde, thymol, carvacrol Oregano, cinnamon, lemongrass

P. islandicum

Avila-Sosa et al. (2010) Gutie´rrez et al. (2009)

P. digitatum A. niger

Potassium sorbate

Aspergillus flavus A. niger A. niger

Pistachios

Apples

Botrytis cinerea

Potassium sorbate and natamycin

Cladosporium sp. P. commune

Se´bastien et al. (2006) De Oliveira et al. (2007) Tu¨re et al. (2008)

Strawberries

Avila-Sosa et al. (2012) Martı´nezCamacho et al. (2010) Sayanjali et al. (2011) Tu¨re et al. (2012) Mehyar et al. (2011) JunqueiraGonc¸alves et al. (2013) Park et al. (2006) da Silva et al. (2013) (Continued)

752

Layered Double Hydroxide Polymer Nanocomposites

Table 18.2 (Continued) Active molecules tested

Organism

Medium

Reference

Potassium sorbate, sodium benzoate, sodium propionate Sodium benzoate and potassium sorbate Sodium benzoate, sodium propionate, and mixtures

P. digitatum

Mandarins

P. notatum P. digitatum

Orange

Soybean trypsin inhibitor extract Sweet basil extract

Aspergillus flavus A. niger

Peanuts Apples

Thyme oil

C. gloesporioides

Avocado

Thymol

Candida lusitaniae Botrytis cinerea

Strawberries

Botrytis cinerea

Strawberries

ValenciaChamorro et al. (2011) Chen et al. (1996) ValenciaChamorro et al. (2009a) Zhang et al. (2009) Synowiec et al. (2014) Sellamuthu et al. (2013) Del Nobile et al. (2008) Cagnon et al. (2013) Eroglu et al. (2014)

Trans-2-hexanal, and 2-nonanone

molecules. Several works have reported on the use of organically modified smectite clays, in particular, montmorillonites, as fillers of polymeric nanocomposites, while much less attention has been dedicated to anionic hydrotalcite-type clays (Rives, 2001; Costantino et al., 1998). These latter materials compare favorably with natural clays in terms of purity, well-known stoichiometry, higher ion exchange capacity, and a wider possibility of functionalization with a variety of organic anions (Mohapatra and Parida, 2016). The last are generally much more numerous than organic cations commonly involved in the modification of smectite clays. In recent times, research into layered double hydroxides (LDHs) has been developed in different directions, on both fundamental and applied aspects of these materials (Theiss et al., 2016; Yan et al., 2016). As a result of their high compositional variability and large range of processing parameters, LDH materials have shown great potential in healthcare, environmental remediation, energy conversion, and photocatalysis (Mohapatra and Parida, 2016). The constant improvements in the methods of assessing the structure and the physicochemical properties of solids continually enhance understanding of LDHs materials (Li et al., 2014). Significant advances have been made on the “tailor-made” preparation of LDHs with specific compositions, particle size, and functionalities (Forano et al., 2013). Natural LDHs, or hydrotalcite-like compounds (HTlc) are generally represented by the empirical formula [M(II)1
xM(III)x(OH)2]x1[An
x/n]x
 mH2O, where M(II) and M(III) are

Layered double hydroxide polymer nanocomposites for food-packaging applications

753

Figure 18.1 Schematic of a layered double hydroxide (LDH) structure.

bi- and trivalent metal cations with suitable ionic radius, A is the interlayer exchangeable anion with charge
n, x is the molar ratio M(III)/[M(III) 1 M(II)] which ranges between 0.2 and 0.4, and m is the mole of cointercalated water (Costantino et al., 1998, 2011; Theiss et al., 2016) (Fig. 18.1). It has been reported that a large variety of materials with different properties can be obtained by changing the nature of the divalent and trivalent cations, and the type of interlayer molecular anions, opening the way for a wide range of applications. LDHs have found applications as heterogeneous catalysts, support of catalysts (Forano et al., 2013; Busca et al., 2006), adsorbents, anion exchangers, anion scavengers (Newman and Jones, 1998; Pre´vot, 2001), components, and/or active principles in pharmaceutical and cosmetic formulations (Forano et al., 2013; Tammaro et al., 2007) and additives of polymeric blends (Rives, 2001; Forano et al., 2013). LDHs have been modified with several organic molecules used in the foodpackaging field (Sorrentino, 2011). When organically modified LDHs, possessing specific activity, are exfoliated and homogeneously dispersed into the polymeric matrices, an interesting new class of nanocomposites can be obtained. In these systems, the active molecules, fixed by ionic bonds to the inorganic lamellae, not only improve the compatibility with the polymeric matrix but also carry out the biological activity (i.e., antimicrobial or antioxidant) (Gorrasi et al., 2012). The modified LDH nanofillers thus provide active release systems, simultaneously improving the mechanical and barrier properties of the polymer matrix. The possibility to substitute organic active anions, by a simple ionic exchange procedure, makes hydrotalcites ideal solids to be used as a host of potentially active molecules with a negative charge. LDH can be prepared with simple procedures and high level of purity, they are also economic and eco-compatible; in conclusion, they are excellent candidates for active food packaging (Sorrentino et al., 2007; Forano et al., 2013).

754

18.3

Layered Double Hydroxide Polymer Nanocomposites

Polymeric nanocomposites based on layered double hydroxide-active molecules

The successful preparation of nanocomposites with lamellar nanosized fillers depends on the disruption of their primary structure (exfoliated morphology) or the intercalation of polymeric macromolecules between the platelets (intercalated morphology), as well as on their homogeneous dispersion in the polymer matrix (Sinha Ray and Bousmina, 2005; Raka et al., 2010). Moreover, it is crucial to achieve good interaction (compatibilization) between the polymer matrix and the layers (LeBaron et al., 1999; Jeon and Baek, 2010). Several strategies may be used to prepare polymer
LDH nanocomposites. The most commonly used are in situ polymerization, melt mixing, and solution blending (Sorrentino et al., 2012; Sinha Ray and Okamoto, 2003). In the following we briefly outline their description. G

G

The in situ polymerization method involves LDH hybrid mixing with a liquid monomer to allow intercalation into the interlayer gallery (Fig. 18.2). A catalyst is generally added to the solution to promote the monomer polymerization in situ. The polymerization of the monomer in the interlayer gallery leads to rupture of the lamellar structure and promotes the homogeneous dispersion of the layers. The main advantage of this method is that it is able to promote efficient exfoliation of the layered materials. However, it is a complex process and rather industrially unfeasible for commodity polymers, it is also difficult to apply to natural polymers such as pectins and starch. The melt-mixing method for the preparation of polymer
LDH nanocomposites consists of the dispersion of an LDH hybrid in a polymer matrix by applying high local shear stresses and rather high temperatures (Fig. 18.3). Using this procedure it is possible to prepare a wide range of nanocomposites with exfoliated or intercalated structures. It has important advantages over other methods. From an environmental point of view, it does not require the use of organic solvents, and it is compatible with common plasticprocessing technologies such as extrusion or injection. The disadvantages are related to the fact that it is not applicable to polymers sensible to high temperatures or intense mechanical stress, such as materials from renewable sources.

Figure 18.2 In situ polymerization.

Layered double hydroxide polymer nanocomposites for food-packaging applications

755

Figure 18.3 Melt mixing.

Figure 18.4 Solution blending. G

G

Solution blending is a method in which the pre-expanded layered particles are dispersed in a polymer solution to promote the entry of the polymeric macromolecules into the interlayer gallery (Fig. 18.4). The remaining solvent is afterward evaporated, resulting in the precipitation of the polymer incorporated between the inorganic composite platelets. Even if this procedure appears quite simple, in practice it could be very difficult to find a solvent that is able to dissolve the polymer and allow dispersion of the layered silicate. In addition, this procedure has the inconvenience of using solvents, which have a generally high environmental impact. Recently an alternative and efficient method of mixing polymers and fillers that is based on mechanical energy has been proposed: mechanical milling (MM). This represents an interesting sustainable alternative to achieve homogeneous dispersion of nanofillers into polymers (Gorrasi and Sorrentino, 2015; Vertuccio et al., 2009). The mechanical forces imposed by the grinding medium allow the dispersion and compatibilization of the “active” phase (filler) into the polymer matrix (Fig. 18.5). At the same time, the thermal and mechanical degradation can be strongly limited by controlling the processing conditions such as the temperature, milling time, and grinding media.

756

Layered Double Hydroxide Polymer Nanocomposites

Figure 18.5 Mechanical milling.

18.3.1 Nanocomposites from oil-derived polymers Tammaro et al. (2014) reported the preparation and characterization of seven modified hydrotalcites via the coprecipitation method. The intercalated organic molecules, in anionic form, were: salicylic acid, para-hydroxybenzoic acid, aleuritic acid, citric acid, glycolic acid, serine, and 2.2-bis-hydroxymethyl-propionic acid. The chemical formulas are given in Fig. 18.6. These molecules were chosen for their oxygen scavenger properties. The obtained organic
inorganic hybrids were dispersed at 3 wt.% in poly(ethylene terephthalate) (PET) by high-energy ball milling (HEBM). Good delamination and dispersion of the organic
inorganic hybrids in the polymer phase were achieved for all composites. Structural, thermal, and oxygen barrier properties were analyzed for all samples. The PET
LDH composites showed a similar thermal degradation trend to pure PET, indicating a higher thermal stability. It was also demonstrated that the LDH has a protective effect on the organic molecules intercalated. The thermal properties of the organic molecules after intercalation in the LDH lamellae and incorporation in PET matrix have been studied for all the composites, showing a thermal stability of the organic molecule inside the inorganic phase of about 200 C higher, in comparison with the pristine molecules. All the composites showed oxygen diffusion and permeability coefficients lower than that for a pure PET, due to the presence of highly dispersed clay platelets. The data are reported in Table 18.3 (the number related to the LDH refers to the type of molecule reported in Fig. 18.6). These composites have been considered very promising candidates for foodpackaging applications. Tsai et al. (2015) reported the preparation of poly(ethylene terephthalate)/clay or Li-Al LDH nanocomposites, using a thermally stable modifier. The PET nanocomposites were prepared by a twin-screw microcompounder. The dispersion morphology of inorganic layered materials was analyzed by wide-angle X-ray diffraction (XRD) and transmission electron microscopy (TEM). Nanocomposite also showed

Layered double hydroxide polymer nanocomposites for food-packaging applications

757

Figure 18.6 Active molecules used in Tammaro et al. (2014).

Table 18.3 Oxygen sorption (S), diffusion (D), and permeability (P) coefficients for all PET-LDHn, where n is the type of molecule shown in Fig. 18.6 (Tammaro et al., 2014) Samples

S (cc(STP)/cm3 atm)

D (cm2/s) 3 109

P (cc(STP)/cm3 atm 3 cm2/s) 3 109

PET PET-LDH1 PET-LDH2 PET-LDH3 PET-LDH4 PET-LDH5 PET-LDH6 PET-LDH7

0.27 0.40 0.32 0.39 0.40 0.36 0.40 0.37

9.05 2.90 3.60 2.80 2.95 3.00 3.00 2.90

2.48 1.16 1.15 1.09 1.18 1.08 1.20 1.07

better mechanical, UV resistance, and gas barrier properties. The resulting mechanical, optical, and thermal properties allowed such nanofillers to be considered able to improve the physical properties of the analyzed materials, widely used in food packaging. Gorrasi and Bugatti (2016a) reported the intercalation of several organic anions with a potential antimicrobial effect. The dispersion of the nanohybrids at 3 wt.% into a PE matrix was obtained using HEBM technology with no solvents and at ambient temperature. Table 18.4 reports the chemical formulas of the LDH

758

Layered Double Hydroxide Polymer Nanocomposites

Table 18.4 Chemical formulas of the LDH nanohybrids with the different intercalated molecules, the active molecule wt.% into the nanohybrids, and the active molecule wt.% in the PE matrix (Gorrasi and Bugatti, 2016a) Guest anion

Composition

Active molecule into LDH (wt%)

Active molecule into PE
LDH composites (wt%)

Aleuritic carboxylate Citrate

[Zn0.65Al0.35(OH)2] (NO3)0.03(C16H31O5)0.32  2.4H2O [Zn0.65Al0.35(OH)2](C6H7O7)0.35  2.5H2O [Zn0.65Al0.35(OH)2] (NO3)0.11(C2H3O3)0.24  1.25H2O [Zn0.65Al0.35(OH)2](C7H5O3)0.35  1.86H2O [Zn0.65Al0.35(OH)2](C7H5O3)0.35  0.7H2O [Zn0.65Al0.35(OH)2](C3H6NO3)0.35  1.57 H2O

42.5

1.27

33.8

1.01

13.5

0.40

28.6

0.86

32.7

0.98

24.2

0.72

Glycolate Parahydroxybenzoate Salicylate Serine carboxylate

nanohybrids with the different intercalated molecules, the active molecule wt.% into the nanohybrids, and the active molecule wt.% into the PE matrix. XRD analysis showed a good degree of intercalation of PE macromolecules into LDH lamellae, in almost all cases. TGA analysis showed that the LDH hosts constitute efficient protection for organic molecules, allowing their degradation at higher temperatures compared to the unanchored ones. Sorption (S) and diffusion (D) to oxygen were also evaluated for all samples (Table 18.5). All the composites showed a higher sorption with respect to the unfilled PE. This was attributed to the capability of the modified LDHs to act as oxygen absorbers. The diffusion (D) coefficients were lower than that of PE for the presence of highly dispersed LDH platelets. As a consequence, the permeability of the composites was lower than that of PE, reaching a minimum for the LDH modified with p-OH-benzoate and salicylate. Overall migration tests were performed on PE and LDH-salicylate/PE films, using 3% acetic acid and 10% ethanol (Table 18.6). Such solvents were used to simulate foods and beverages with alcoholic characteristics and dairy products under swelling conditions, and acidic and hydrophilic character in nonswelling conditions, respectively. The overall migration, in both cases, resulted in compliance with the migration limits (10 mg/dm2) as set down by the EU legislation for foodcontact plastics. Gorrasi et al. (2016b) formulated a novel antimicrobial coating based on an LDH intercalated with antimicrobial salicylate anions (listed in EC-Directive 10/ 2011/EC of 14 January 2011). The polymer used for coating was a commercial

Layered double hydroxide polymer nanocomposites for food-packaging applications

759

Table 18.5 Sorption (S), diffusion (D), and permeability (P 5 S 3 D) of oxygen for PE and composites at P 5 1 atm and T 5 25 C (Gorrasi and Bugatti, 2016a) Sample

S [cm3O2(STP)/cm3/ atm]

D 3 107 [cm2/s]

P 3 108 [cm3O2(STP)/ cm3/atm] [cm2/s]

LDPE PE/LDH (aleuritic carboxylate) PE/LDH (citrate) PE/LDH (glycolate) PE/LDH (parahydroxybenzoate) PE/LDH (salicylate)

0.086 6 0.012 0.172 6 0.023

7.00 6 0.59 1.95 6 0.15

6.02 6 0.13 3.35 6 0.07

0.127 6 0.012 0.185 6 0.028 0.130 6 0.037

2.35 6 0.42 1.80 6 0.24 1.30 6 0.18

2.98 6 0.08 3.33 6 0.09 1.69 6 0.07

0.152 6 0.041

1.20 6 0.20

1.82 6 0.08

Table 18.6 Overall migration tests in acetic acid 3% and ethanol 10% for PE and PE/ LDH-salicylate (Gorrasi and Bugatti, 2016a) Sample

PE PE/LDH-salicylate

Overall migration (mg/dm2)

Overall migration (mg/dm2)

Acetic acid 3%

Ethanol 10%

0.4 6 0.2 0.8 6 0.2

0.5 6 0.2 0.8 6 0.1

PET. The samples were tested in vitro against the spoilage bacteria of Mozzarella cheese. The population of spoilage microorganisms (total coliforms, Pseudomonas, fungi) along with the functional microbiota of Mozzarella cheese (lactic acid bacteria) was characterized. Figs. 18.7 and 18.8 report the number of lactobacilli and streptococci found in samples of Mozzarella cheese packaged in a PET-uncoated film (control) and the PET film coated with LDH-salicylate, respectively. Microbial shelf-life was evaluated at 18 C, to simulate thermal abuse. The number of lactic acid bacteria was found to remain constant throughout the duration of the experiment. In contrast, significant differences were found for other microbial groups, both in population successions and in the final biomass. PET film coated with LDH-salicylate hindered the growth of the main microorganisms responsible for spoilage of the cheeses (coliforms, yeasts, and Pseudomonas spp.). In summary, the experimental results show an increase in the microbial shelf-life of the packaged Mozzarella cheese of about 20 days, confirming that the investigated active coating may exert an inhibitory effect on the microorganisms responsible for spoilage phenomena (Fig. 18.9).

760

Layered Double Hydroxide Polymer Nanocomposites

Figure 18.7 Microbial counts of Mozzarella cheese stored at 18 C for: (A) 48 h and (B) 72 h (Mascheroni et al., 2011).

18.3.2 Nanocomposites of bioplastics from fossil-based resources Costantino et al. (2009) and Bugatti et al. (2010, 2011) reported the preparation and characterization of polycaprolactone (PCL)-based biodegradable active polymer films using MM as the mixing technology. The composites were developed as models for biodegradable “active packaging” systems. Four antimicrobial molecules were intercalated into LDH: benzoate (Bz), 2,4-dichlorobenzoate (BzDC), and para- and ortho-hydroxybenzoate (p- and o-BzOH). Polymeric films were obtained by hot-pressing the milled powders. The chemical and stereochemical structure of

Layered double hydroxide polymer nanocomposites for food-packaging applications

761

Figure 18.8 Microbial counts of Mozzarella cheese stored at 18 C from 72 h to 22 days (Mascheroni et al., 2011).

the active molecule, as well as the milling process, developed different composite morphologies. Antimicrobial tests, evaluated on the films, indicated that the composites were able to inhibit Saccharomyces cerevisiae growth by up to 40%. The release profile of the benzoate molecules and of the p-OH-benzoate molecules, dispersed in PCL either bonded to LDH or free-mixed to PCL, is shown in Fig. 18.10. The release kinetics of active molecules chemically bonded to the LDHs and incorporated into the polymeric matrix resulted in much slower than the release if the molecule is free dispersed into the PCL. The type of active molecule, its percentage, and the milling conditions can be used for tuning the kinetic release to the specific applications in the packaging field.

762

Layered Double Hydroxide Polymer Nanocomposites

5.5 Control sample Active film Gompertz model

Log (cfu/g)

5.0 4.5 4.0 3.5 3.0 0.25

1

100

10 Time (days)

Non-released fraction (wt/wt)

Figure 18.9 Time course during storage of coliform concentration in Mozzarella cheese stored at 18 C in PET (control) and PET_active film (Mascheroni et al., 2011). 100 80 60 PCL/LDH-p-BzOH/3 PCL/LDH-BzDC/3

40

PCL/Na-p-BzOH/1.1 PCL/Na-BzDC/1.3

20 0 0.0

0.5

1.0 1.5 2.0 2.5 Contact time (days)

3.0

Figure 18.10 Release profile as a function of the time of PCL/LDH-bonded molecules and PCL/free dispersed molecules at the same molecule percentage (wt%). Source: Adapted from Bugatti, V., Costantino, U., Gorrasi, G., Nocchetti, M., Tammaro, L., Vittoria, V., 2010. Nano-hybrids incorporation into poly(ε-caprolactone) for multifunctional applications: mechanical and barrier properties. Eur. Polym. J. 46, 418
427; and Bugatti, V., Gorrasi, G., Montanari, F., Nocchetti, M., Tammaro, L., Vittoria, V., 2011. Modified layered double hydroxides in polycaprolactone as a tunable delivery system: in vitro release of antimicrobial benzoate derivatives. Appl. Clay Sci. 52, 34
40.

Schmidt et al. (2011) reported the preparation of nanocomposite films of a commercial poly(L-lactide) (PLA) and laurate-modified Mg-Al LDH-C12. In order to test the suitability of the produced composites as materials to be used for food contact, the films were tested for total migration (according to a European standard method), and specific migration of tin, laurate, and LDH. Films showed the migration of nanosized LDH, which was quantified using acid digestion followed by

Layered double hydroxide polymer nanocomposites for food-packaging applications

763

Table 18.7 PLA and PLA/LDH-laurate samples analyzed in Schmidt et al. (2011) Sample

LDH-laurate loading (%)

Description

PLA-PF PLA-PF1


1.8

PLA-PF2

5.5

PLA-PF3

1.8

Film from extruded pellets Nature Workss Film from extruded pellets based on PLA and 5.3% of LDH-laurate diluted three times with PLA Film from extruded pellets based on PLA and 10% of a 50% PLA/50% LDH-laurate masterbatch Film from extruded pellets based on PLA and 10% of a 50% PLA/50% LDH-laurate masterbatch diluted three times with PLA

Table 18.8 Total and specific migration of components from samples of Table 18.7 (Schmidt et al., 2011) Sample

Total migration (mg/dm2)

PLA-PF PLA-PF1 PLA-PF2 PLA-PF3

2.5 9.6 31.9 8.3

6 6 6 6

0.6 1.9 7.4 0.8

Laurate migration (mg/dm2)

LDH migration (mg/dm2)

Tin migration (mg/dm2)

, 0.2 3.4 6 0.3 8.8 6 1.0 2.6 6 0.2

, 0.1 0.2 6 0.1 2.2 6 0.1 1.0 6 0.1

, 0.0001 , 0.0001 0.0025 6 0.0002 0.0008 6 0.0001

inductively coupled plasma mass spectrometric (ICP-MS) detection of 26Mg. Migration of LDH from the film was also confirmed by examining migrates using TEM and was attributed indirectly to the significant PLA molecular weight reduction observed in extruded PLA-LDH-C12 films. Migration of tin was detected in two of the film samples prepared by dispersion of LDH-C12 using a masterbatch technique. Migration of the laurate organo-modifier took place from all film types. The results indicate that the material properties are in compliance with the migration limits for total migration and specific lauric acid migration as set down by the EU legislation for food-contact materials, at least if a reduction factor for fresh meat is taken into consideration. The tin detected arises from the use of organotin catalysts in the manufacture of PLA (see Tables 18.7 and 18.8). Demirkaya et al. (2015) reported a comprehensive characterization of polylactide-LDH nanocomposites as packaging materials. Two different Mg-Al LDH nanofillers at Mg:Al mol ratios of 3:1 and 2:1 were produced by coprecipitation method to improve the interfacial interaction. The nanoparticles were modified by intercalating sodium dodecyl sulfate (SDS) into the LDH galleries. The PLA nanocomposite films were prepared using unmodified (Mg-Al LDH) and modified (SDS-Mg-Al LDH) nanofillers. Morphological characterization showed an increasing dispersion and improved exfoliation of SDS-Mg-Al LDH into the PLA matrix.

764

Layered Double Hydroxide Polymer Nanocomposites

Dynamic mechanical properties indicated that, at each composition, filler modification resulted in higher storage modulus and Tg. The composite films showed an increasing oxygen barrier property of up to 23.5%, compared to PLA. The films containing 5% of SDS Mg-Al LDH at an Mg:Al ratio of 3:1 exhibited a significant increase in oxygen barrier property. The materials were promising as packaging with a high oxygen barrier.

18.3.3 Nanocomposites of bioplastics from renewable sources Gorrasi et al. (2012) reported biocomposites from renewable sources based on pectins and active nanohybrids of LDH with intercalated active molecules: benzoate, 2,4-dichlorobenzoate, p-OH-benzoate, and o-OH-benzoate. The incorporation of the nanohybrids into the pectin matrix was carried out using MM, in the presence of water. Cast films were obtained and analyzed. Structural analysis showed a complete destructuration of all nanohybrids in the pectin matrix. Thermogravimetric analysis showed a better thermal resistance of pectin in the presence of fillers, especially p-OH-benzoate and o-OH-benzoate. Mechanical properties showed an improvement of elastic modulus in particular for LDH-p-OH-benzoate nanohybrid, due probably to a better interaction between pectin matrix and modified LDH layers. The diffusion to water vapor showed improvement in the dependence on the intercalated active molecule (Table 18.9). The best improvement was achieved for composites containing the intercalated p-OH-benzoate, suggesting that the interaction between the filler phase and the polymer plays an important role in the kinetic diffusion phenomenon. Antimicrobial activity of the nanocomposite films was evaluated by storing them at room temperature and humidity conditions along with the control pectin films. Fig. 18.11 shows pictures taken on pectin and nanocomposite films after 12 months of storage at room temperature and at different humidities (i.e., 30%
60%). Mold formation can be noticed in the pectin films after 2 weeks of storage, but there was no such indication in the nanocomposite films even after 12 months. These results clearly suggest the potential of utilizing pectin films enriched with LDH intercalated antimicrobial compounds as novel packaging materials from renewable sources. Table 18.9 Barrier properties of composites based on pectins and LDH-antimicrobial molecules (Gorrasi et al., 2012) Sample

Do (m2/s)

Pure Pectin Pectin/LDH-Bz 5 wt% Pectin/LDH-DCBz 5 wt% Pectin/LDH-o-OHBz 5 wt% Pectin/LDH-p-OHBz 5 wt%

2.30 1.97 3.10 3.20 1.35

3 3 3 3 3

10212 10212 10213 10213 10213

Layered double hydroxide polymer nanocomposites for food-packaging applications

765

Figure 18.11 Film of pectin and composites with nanohybrids after storage 12 months at ambient temperature (Garrido Assis and de Britto, 2011).

Dou et al. (2014) produced transparent and flexible films based on LDH
cellulose acetate (CA) with highly improved oxygen barrier properties. The preparation of the films was conducted on an alternating assembly of CA and LDH nanoplatelets followed by thermal annealing treatment. The films exhibited greatly enhanced oxygen barrier properties. The oxygen transmission rate (OTR) of the resulting (CA/LDH)n multilayer films was tuned by changing the aspect ratio of high-crystalline LDH nanoplatelets from 20 to 560. Molecular dynamics simulations revealed that a hydrogen bonding network occurred at the interface of highly oriented LDH nanoplatelets and CA molecules, accounting for the suppression of oxygen transport and the resulting largely improved barrier behavior. Therefore, this work demonstrated a facile and cost-effective strategy for the fabrication of an LDH-based oxygen barrier material, which can be potentially used in flexible displays and drug and food packaging. Pan et al. (2015) produced composites based on chitosan (CTS) and hierarchical LDH (H-LDH). The synthesis was conducted by a continuous calcination
rehydration treatment of plate-like LDH (P-LDH), which was then used as a building block to fabricate multilayer films with CTS by an alternate spin-coating technique. The resulting (H-LDH/CTS)n film exhibited ultra-high gas barrier behavior with an oxygen transmission rate (OTR). In addition, the (H-LDH/CTS)n film displayed excellent storage and thermal stabilities, which would guarantee promising practical applications in food/pharmaceutical packaging. Gorrasi and Bugatti (2016b) reported the preparation and testing of novel edible coatings based on pectin and an LDH modified with salicylate anion, as an antimicrobial agent. Composites filled with 5 wt.% of LDH-salicylate (salicylate content:

766

Layered Double Hydroxide Polymer Nanocomposites

Table 18.10 Permeability to water vapor for samples described in Gorrasi and Bugatti (2016b) Sample

P (g/100 g/mmHg) 3 (cm2/s) 3 108

Pure pectin Pectin/LDH-salicylate Pectin/LDH-salicylate/4% glycerol Pectin/LDH-salicylate/8% glycerol Pectin/LDH-salicylate/16% glycerol

50.30 8.07 0.14 0.81 0.22

6 6 6 6 6

3.39 0.51 0.08 0.09 0.06

Figure 18.12 (A) Uncoated apricots, (B) apricots coated with pectin/LDH-salycilate 1 4% glycerol, after 10 days of storage at ambient temperature and 60% RH (Manso et al., 2014).

1.6 wt.%) were plasticized with different glycerol loading (4, 8, and 16 vol.%). The presence of glycerol improved the mechanical properties of the pectin, in terms of elongation at break (flexibility). Permeability to water vapor showed an improvement in such parameters for the composites plasticized with glycerol (Table 18.10). The authors suggested that the weak bonds, which determine the interaction of pectin-active filler glycerol, play an important role in determining the barrier phenomena. Coating tests on fresh apricots at 60% relative humidity (RH) (Fig. 18.12), using the composite plasticized with 4 vol.% of glycerol, showed the significant capability of extending the preservation of such fruits. The experimental results indicated the potential application of prepared systems in the food-packaging field and open new perspectives in using natural pectin antimicrobials as coating agents, either directly on selected foods, or for a wide number of packaging polymers.

18.4

Regulation issues

The application of nanotechnology to the food sector is already well-documented and implemented around the world. In contrast, only a few initiatives have been taken by legislators in regulating the use of these materials (Wyser et al., 2016). These initiatives are sporadic and generally have different approaches in different countries (Sonkaria et al., 2012; Magnuson et al., 2013; Chaudhry et al., 2008). For the United States, the Food and Drug Administration (FDA) is mainly responsible for the control

Layered double hydroxide polymer nanocomposites for food-packaging applications

767

and regulation of all drugs, cosmetics, medical devices, and food supply on the market. Any new food-contact materials must be subject to a specific authorization. In the case of nanomaterials, the particle size is important as it impacts on their functionality or technical effects. The FDA addresses the issue of properties that are specific to particle size (FDA, 2014). More specific information can be obtained informally, case-by-case, by the FDA during the evaluation of these materials. In any case, the manufacturer has the responsibility to ensure that the product proposed on the market must be safe and lawful. In any case, the FDA encourages food manufacturers to follow the technical indications reported in their specific guidance documents (FDA, 2014, 2016). In Europe, the use of food-contact materials is regulated by (EC) No. 1935/2004 (Benckiser, 2012; EC, 2004). This regulation sets the general rules for food-contact materials with regard to their safety, labeling, and traceability. However, for all cases which are not covered by the European regulations, the specific legislation of the various members generally supplies the guidance for producers. In any case, the regulations that take into account the possible use of nanomaterials are already very limited and frequently inadequate for the actual number of new materials proposed every year. The definition of nanomaterials generally accepted is that present in Recommendation 696/2011 (EC, 2011). This definition identifies a nanomaterial as a “natural, incidental or manufactured” product that presents at least “50% of the particles with one or more external dimensions in the size range 1
100 nm.” Several studies have been carried out with the aim to revise this definition with a more specific and reliable classification of the nanomaterials (Wyser et al., 2016). The use of additives to modify polymer material properties is not covered by Regulation 10/2011 and its amendments (EC, 2011). Only inorganic fillers specifically mentioned in Annex I of the above regulation are allowed in the plastic packaging. This Annex has been revised several times by specific initiatives of the European Food Safety Authority (EFSA, 2012a, 2012b; Scientific, 2015; EFSA, 2014). Substances released by active packaging into food to become a component of the food must comply with food legislation (EC, 2008a, 2008b). Currently, no specific nanoparticles have been authorized as food preservatives or antioxidants in the EU. The use of nanoparticles has to be specifically authorized as food additives. If the substance is not intentionally released, Regulation (EU) No. 450/2009 provides a specific authorization scheme (EC, 2009).

18.5

Conclusions and future perspectives

The packaging industry is always on the lookout for new technologies and products that can meet the growing demand for food safety, environmental responsibility, and resource optimization. Active nanofillers, as polymeric additives, can meet the expectations of consumers by offering themselves as the vanguard of a new family of composites based on low-cost and already-existing polymer materials. In this vision, the addition of active layered engineered silicates complying with food-contact regulations to polymeric materials, through innovative technology, is

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Layered Double Hydroxide Polymer Nanocomposites

now available as a formidable tool for improving the properties of polymers and biopolymers and, therefore, enhancing packaged food quality and safety aspects. With the advent of a new generation of nanomaterials providing multiple functionalities, that is, combined physical reinforcement and active performance, to plastics, the plastic packaging field becomes consolidated in its own right as a high-tech area of development. Hydrotalcite-like compounds (LDHs) are an extremely versatile class of materials that can be produced at low cost and can be easily modified with simple procedures. Organically modified LDHs represent an excellent class of hosts of active molecules (i.e., antimicrobials, antioxidants, oxygen-scavenging, food preservatives, etc.) that can be easily produced and dispersed into the polymeric matrices, either biodegradable or not. The organic molecule not only can make both phases compatible (organic polymer and inorganic filler), but can also act as an active agent that, released in a controlled way, is able to prevent microbial growth, hence extending the shelf-life of the packaged food. Traditional systems for food conservation include a series of more or less forced chemical-physical treatments and in some cases even the input of additives. These are treatments that destroy or drastically reduce valuable components of the foodstuff, often changing its nutritional and taste properties. It is no surprise that there is strong interest in innovative technologies and/or treatments that can minimize these unwanted effects. The LDHs modified with active molecules, used in food-packaging applications, which are obtained from substances recognized by current regulations in the field as permitted for contact with foodstuffs, are perfectly biocompatible and, consequently, the fillers produced are completely recyclable. It is also possible to design fillers that incorporate more functions, based on the characteristics of the foodstuff. In fact, in addition to improving mechanical, thermal, and barrier properties, modified LDHs can also allow the incorporation of active components to obtain packaging able to (1) maintain or improve the quality of foodstuffs (active packaging), and (2) monitor the condition of the foodstuff or enable its traceability (intelligent packaging)

Acknowledgment A. Sorrentino acknowledges financial support from the “FHfFC” (Future Home for Future Communities) project, D.R. Lombardia n. 7784 del 05/08/2016, CUP:B16J16001430002.

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63. Available from: https://doi.org/10.1016/j. ijfoodmicro.2014.04.029. Zhang, B., Wang, D.-F., Li, H.-Y., Xu, Y., Zhang, L., 2009. Preparation and properties of chitosan
soybean trypsin inhibitor blend film with anti-Aspergillus flavus activity. Ind. https://doi.org/10.1016/j. Crops Prod. 29, 541
548. Available from: indcrop.2008.10.007. Ziani, K., Ferna´ndez-Pan, I., Royo, M., Mate´, J.I., 2009. Antifungal activity of films and solutions based on chitosan against typical seed fungi. Food Hydrocoll. 23, 2309
2314. Available from: https://doi.org/10.1016/j.foodhyd.2009.06.005.

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Layered double hydroxide polymer nanocomposites for water purification

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Shadpour Mallakpour1,2 and Vajiheh Behranvand1 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

19.1

Introduction

The extensive and important influence of water on all parts of human life, containing food, health, energy, and economy, is completely perspicuous for all of us. Therefore, in recent years, contamination of water resources caused by numerous pollutants such as dyes or oils, organic materials, and inorganic pollutants (e.g., heavy metal cations or anions) has been a global and environmental concern. Hence, the elimination of these pollutants before the release of wastewater is essential (Amin et al., 2014; Sharma and Das, 2013; Kang et al., 2017; Zhao et al., 2012). Novel handling techniques to obtain high-quality drinking water are needed, and these should be stable, cost-effective, and more efficient in comparison to conventional methods (Amin et al., 2014). A number of approaches for the removal of pollutants from wastewater including adsorption, ion exchange, sedimentation, coagulation, photodegradation, and filtration have been used (Mallakpour and Behranvand, 2017b; Abdolmaleki et al., 2015). Amongst these, adsorption technology is commonly employed due to the ease of the procedure, high efficiency, economical aspects, and capability to eliminate wide-ranging contaminants by different sorbents (Jin et al., 2011; Duan et al., 2016; Shao et al., 2014; Pal et al., 2016). Recently, various kinds of nanoadsorbents have been introduced to water industry that have shown favorable results. Different types of nanoadsorbents, such as activated carbon (Djahed et al., 2015), carbon nanotubes (CNTs), and manganese dioxide (MnO2) polymer nanocomposites (Mallakpour et al., 2016c; Mallakpour and Behranvand, 2017a; Mallakpour and Motirasoul, 2016; Ma et al., 2012), graphene (Hu et al., 2016) and layered double hydroxide (LDH) nanocomposites (Bruna et al., 2012) etc., have outstanding sorption properties and were used for the removal of various pollutants. In 2014, Unuabonah and Taubert (2014) highlighted different clay polymer nanocomposites that were employed as sorbents for contaminant elimination. Zubair et al. (2017) have more recently reviewed the sorption potential of hybrids Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00019-7 © 2020 Elsevier Ltd. All rights reserved.

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based on LDH for removing different contaminants such as heavy metals (Pb21, Cd21, Cu21, Cr61, etc.) and dyes (methyl orange, methylene blue, etc.). In this study, the potential application of LDH nanomaterials in water purification are reviewed. The structural properties of LDHs related to their adsorption ability are explained in the first section. Then, recent works related to LDHs and their modified forms with particular focus on their capability to treat pollutants from water and their sorption mechanisms are reviewed. Due to the shortcomings of using LDHs or modified LDHs in adsorption processes, the following and most important section of this chapter will provide an overview of the adsorption behavior of LDH polymer nanocomposites based on the works accessible to date.

19.2

Pollutant elimination from water: why layered double hydroxides?

19.2.1 Structural properties of layered double hydroxides LDHs, anionic equivalents of cationic clays, are signified with the general formula 21 31 n2 31 of [M21 12x Mx ðOHÞ2 ] [Ax=n  yH2 O] [M : divalent metal ions, M : trivalent metal ions and positive charges on the sheets are neutralized with interlayer anions (An2)] (Mallakpour et al., 2016b; Jiang et al., 2015). LDHs can create host
guest inclusion complexes by intercalation of organic and inorganic ions which have directed them for use in many applications, especially as pollutant adsorbents in water treatment (Mallakpour et al., 2016a,b). LDHs have specific characteristics such as layered construction, high surface area and porosity, and important amounts of exchangeable anions. Thus, they show high adsorption capacities for organic anions such as pesticides, aromatic carboxylic acids, and phenols, as well as inorganic anions including fluorides, sulfates, chromates, and so on (Yang et al., 2016; Zhao et al., 2012).

19.2.2 Layered double hydroxide modification Different anionic organic surfactants can be incorporated into LDH interlayers to obtain organo-LDHs owing to their unique anion interchangeability. This organic modification procedure causes the following outcomes: corresponding to high hydrophilic nature, high charge density of layers, and strong attractions between the two LDH layers, combination of these materials with organic and hydrophobic matrices such as polymers is difficult. Therefore, modification changes the hydrophilic surface of LDHs to the hydrophobic nature which not only makes them favorable candidates for water remediation but also creates uniform distributions of them in the organic phase (Mallakpour et al., 2014, 2015; Mallakpour and Dinari, 2015; Zhao et al., 2011; Basu et al., 2014). Several methods, for instance, reconstruction, coprecipitation, anion exchange, and thermal reaction can be employed for LDH modifications (Mallakpour and Hatami, 2017; Kumar et al., 2012).

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19.2.3 Water pollutants A significant subject for human health, industry, and agriculture is the accessibility of freshwater without noxious substances and pathogens (Dichiara et al., 2015). With the rapid human progress and industrialization, as well as urbanization, various pollutants are entering wastewater that are hazardous to human health and can be divided into the following groups: G

G

G

Heavy metal cations: These are fairly high-density pollutants which are toxic at low concentrations, and contain cadmium (Cd), mercury (Hg), nickel (Ni), arsenic (As), thallium (Tl), chromium (Cr), copper (Cu), lead (Pb), and zinc (Zn). The sources of these pollutants are mining, chemical and battery manufacturing, metal plating, agricultural resources, and so on (Gautam et al., 2015; Wang et al., 2012, 2013; Kadirvelu et al., 2000). Inorganic anions: Oxyanions such as arsenate, phosphate, chromate, selenate, nitrate, etc., as well as monoatomic anions like iodide, chloride, bromide, and fluoride also exist in water, causing water pollution at high levels (Goh et al., 2008; Wang et al., 2013). Organic dyes: Nowadays, numerous industries, for example, paints, cosmetics, leather, textile, and rubber use over 10,000 commercially obtainable dyes. The low biodegradability, deep color, and complex aromatic constructions of dyes create toxicity, esthetic contamination, and dangers to aquatic life (Peng et al., 2016; Long et al., 2016; Wang et al., 2013; Pal et al., 2016).

19.2.4 Mechanisms of adsorption Different mechanisms have been suggested for the elimination of pollutants by LDH materials from wastewater, including physical adsorption (electrostatic attraction, van der Waals force, hydrogen bonding, π
π interactions, etc.), ion exchange, and chemical bonding (Qin et al., 2016; Zhang et al., 2014). Shan et al. studied the mechanism of Cd21 adsorption on the surfaces of Mg
Al
CO3 and magnetic Fe3O4/Mg
Al
CO3 LDHs (Shan et al., 2015). Easy separation of this sorbent from the suspension was obtained by the combination of Fe3O4 with LDH through an external magnetic field. According to their results, they proposed that the sorption of Cd21 could be related to (1) the precipitation of CdCO3, (2) substitution of Cd21 by Mg21 in the LDH layers, (3) interaction between Cd21 and deprotonated hydroxyl groups (Sur
O2) and generation of outer-sphere complexes, or (4) surface sorption resulting from the cation nature (Fig. 19.1). Deng et al. (2015) synthesized a magnetic CoFe2O4/MgAl-LDH hybrid for the separation of Cr61 from water, which showed operative Cr61 adsorption with rapid separation. They concluded that the adsorption of Cr61 onto the prepared sorbent can occur through three pathways: adsorption of heavy metals into the LDH pores; by electrostatic attraction between positively charged LDH surface and creation of anions containing Cr in the 16 state; or exchanging of NO2 3 present in the sorbent interlayer with anions containing Cr in the 16 state. (Fig. 19.2). The behavior of synthesized CoMo-LDH was studied on Pb21 removal from a water solution by Mostafa et al. (2016). According to a scanning electron

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Figure 19.1 Schematic representation of the adsorption mechanisms of Cd21 onto Mg
Al
CO3
LDH. Source: Adapted from Shan, R.R., Yan, L.G., Yang, K., Hao, Y.F., Du, B., 2015. Adsorption of Cd(II) by Mg
Al
CO32 and magnetic Fe3O4/Mg
Al
CO32 layered double hydroxides: kinetic, isothermal, thermodynamic and mechanistic studies. J. Hazard. Mater. 299, 42
49, with kind permission of Elsevier.

OH2+/HCrO4–

OH2+ Cr(VI) solution

OH2+ OH

OH

OH2+/CrO42–

Equation (a): electrostatic attraction

OH2+/NO3–

OH2+/HCrO4– Cr(VI) solution

CoFe2O4/MgAl-LDH OH2+/NO3–

OH2+/CrO42–

Equation (b): ion-exchange

Figure 19.2 Schematic illustration of the adsorption mechanism of Cr61 on CoFe2O4/MgAlLDH. Source: Adapted from Deng, L., Shi, Z., Peng, X., 2015. Adsorption of Cr(VI) onto a magnetic CoFe2O4/MgAl-LDH composite and mechanism study. RSC Adv. 5, 49791
49801, with kind permission of The Royal Society of Chemistry.

microscopy (SEM) micrograph (Fig. 19.3B), the black parts showed that some Pb21 was adsorbed on the LDH surface, while the main structure was preserved. It is one of their outstanding features that can motivate the use of LDHs in different fields.

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Figure 19.3 SEM images of CoMo-LDH before (A) and after (B) Pb21 sorption. Source: Adapted from Mostafa, M.S., Bakr, A.S.A., Naggar, A.M.A.E, Sultan, E.S.A., 2016. Water decontamination via the removal of Pb(II) using a new generation of highly energetic surface nano-material: Co12Mo16LDH. J. Colloid Interface Sci. 461, 261
272, with kind permission of Elsevier.

Figure 19.4 Proposed mechanism for surface adsorption of Pb21 onto CoMo-LDH. Source: Adapted from Mostafa, M.S., Bakr, A.S.A., Naggar, A.M.A.E, Sultan, E.S.A., 2016. Water decontamination via the removal of Pb(II) using a new generation of highly energetic surface nano-material: Co12 Mo16 LDH. J. Colloid Interface Sci. 461, 261
272, with kind permission of Elsevier.

They suggested that the Pb21 sorption onto the CoMo-LDH happens via coordination between the fairly negative oxygen atoms of the nitrate anions and the Mo61 of the brucite layers (Fig. 19.4). An interesting and new technique for effective elimination of acid yellow as anionic dyes by LDHs was proposed by Sansuk and coworkers (2016). This elimination process concurrently happened with LDH construction and traditional LDH synthetic stages were not essential. The process for collection remediation of acid yellow through LDH construction is shown in Fig. 19.5. UV-vis absorption spectra showed the simultaneous elimination of dyes over the addition of metal cation precursors for LDH formation (Fig. 19.6).

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Layered Double Hydroxide Polymer Nanocomposites

Figure 19.5 Schematic illustration for the removal of anionic organic dyes by electrostatic assembly and their recovery by anionic exchange. Source: Adapted from Sansuk, S., Srijaranai, S., Srijaranai, S., 2016. A new approach for removing anionic organic dyes from wastewater based on electrostatically driven assembly. Environ. Sci. Technol. 50, 6477
6484, with kind permission of the American Chemical Society.

Figure 19.6 UV-vis absorption spectra with corresponding photo image (inset) of AY25 solution at an initial concentration of 100 mg/L before (A) and after 5 min removal time (B). Source: Adapted from Sansuk, S., Srijaranai, S., Srijaranai, S., 2016. A new approach for removing anionic organic dyes from wastewater based on electrostatically driven assembly. Environ. Sci. Technol. 50, 6477
6484, with kind permission of the American Chemical Society.

In fact, the suggested method was simple, fast, and low cost in comparison to other removal techniques, since the elimination can be done without the necessity for adsorbent construction. Mg3Al-LDH has been modified with dodecyl sulfate (DS) anion (DS-LDH) by Zhang et al. and the results showed that this hybrid is a promising sorbent for pesticide removal from an aqueous solution (Zhang et al., 2015a,b). In 2017, Sun’s group synthesized CaAl-LDH for the elimination of aqueous fluoride (Sun et al., 2017). Intercalated ZnAl-LDHs with DS organic molecules were employed for methylene blue (MB) removal by Starukh et al. (2016). The existence of DS significantly enhanced the attraction of the obtained organo-LDHs

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for MB owing to hydrophobic attractions between the two components. The MgAlLDH exhibited suitable capacity for selenium sanitization from extreme amounts of sulfate-polluted water (Li et al., 2017). Zhang et al. (2017) intercalated lactate molecules into the MgAl-LDH interlayer space to obtain LCT-LDH. Then, it was exfoliated into single-layer LDH nanoflakes in aqueous media for the preparation of LDHNSs. This composite was applied as sorbent for Cr61 removal, which exhibited a rapid sorption speed along with high sorption efficacy. Chebli et al. (2016) examined the anionic dye sorption behavior of both noncalcined LDHs (LDH)s and calcined LDHs (CLDH)s. “Memory effect” is a significant feature of CLDHs which can be employed for the removal of contaminants and helps LDHs to be interchanged with favorable anions (Chebli et al., 2016; Zubair et al., 2017). The CLDHs exposed larger sorption capacities, so they seemed to be more effective than LDH. One of the interesting ways to improve adsorption properties of LDHs is their combination with other compounds, for example, graphene oxide (GO), carbon nanotubes (CNTs), carbon quantum dots (CQDs), etc. In this way, a composite will be obtained which has the mixed capabilities and properties of both components (Zubair et al., 2017). Wu and coworkers used this idea and manufactured magnetite-graphene-LDH (MGL) for arsenate removal (Wu et al., 2011). SEM (Fig. 19.7A) and transmission electron microscopy (TEM) (Fig. 19.7C) micrographs of MG confirmed the presence of uniformly dispersed magnetite nanoparticles on the surface of graphene sheets. Throughout the production of the MGL hybrids, the

Figure 19.7 SEM images of (A) MG and (B) MGL, and TEM images of (C) MG, and (D) MGL. Source: Adapted from Wu, X.L., Wang, L., Chen, C.L., Xu, A.W., Wang, X.K., 2011. Water-dispersible magnetite-graphene-LDH composites for efficient arsenate removal. J. Mater. Chem. 21, 17353
17359, with kind permission of The Royal Society of Chemistry.

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flakes of LDH were inserted in the MG regions and caused a reduction in MG particle aggregation (Fig. 19.7A and B). Firm attachment of graphene sheets on the LDH plates was also observed in the TEM micrograph (Fig. 19.7D). The results showed that the arsenate adsorption capacity for the MGL composite was higher than LDHs because of the existence of iron oxide and graphene, which increased the surface area of the hybrids and gave more active sites for the sorption of arsenate. Zhang et al. prepared MGL composites and applied them for the elimination of 2,4-dichlorophenoxyacetic acid (2,4-D) and Pb21 from aquatic media (Zhang et al., 2015a,b). The SEM and TEM micrographs of the MGL (with 7.92 wt.% GO) are depicted in Fig. 19.8A,B. The presence of Fe3O4 and LDH connected to GO sheets was observed in these images. Fig. 19.9 shows an easy and fast (5 min) separation process of well-dispersed MGL composites via magnet after water treatment which is one of the advantages of using Fe3O4 in pollutant removal as described above. Meanwhile, clear solutions were attained after about 1 h by LDH, Fe3O4, and GO, alone. It was proposed that Pb21 could be adsorbed via formation of Pb3(CO3)2(OH)2 precipitates and 2,4-D sorption was through ion exchange as well as π
π and hydrophobic attractions. In another work, which was reported by Tan et al., NiAlLDH/GO composite was applied for the adsorption of uranium (VI) (Tan et al., 2015). The agglomeration of pure NiAl-LDH is illustrated in Fig. 19.10A. In Fig. 19.10B it can be seen that the GO sheets were placed between two LDH plates, which prevented the graphene sheet aggregation. The presence of elements of both GO and LDH was detected in an energy-dispersive X-ray spectrometry analyzer (EDS) (Fig. 19.10C). Fig. 19.10D and E show LDH growth on the graphene surface. The obtained composite with mesoporous properties showed more sorption capacity in uranium (VI) removal from water media. The mechanism of complexation played a key role in uranium (VI) sorption on NiAl-LDH/GO. Zhang et al. (2014) tried to increase the sorption sites of LDH and so designed a composite based on LDHs and CQDs and employed it for the elimination of MB.

Figure 19.8 (A) SEM and (B) TEM micrographs of MGL. Source: Adapted from Zhang, F., Song, Y., Song, S., Zhang, R., Hou, W., 2015. Synthesis of magnetite 2 graphene oxide-layered double hydroxide composites and applications for the removal of Pb(II) and 2,4-dichlorophenoxyacetic acid from aqueous solutions. ACS Appl. Mater. Interf. 7, 7251
7263, with kind permission of the American Chemical Society.

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Figure 19.9 Photographs of MGL aqueous dispersion (A) before and (B) after magnetic separation for 5 min. Source: Adapted from Zhang, F., Song, Y., Song, S., Zhang, R., Hou, W., 2015. Synthesis of magnetite 2 graphene oxide-layered double hydroxide composites and applications for the removal of Pb(II) and 2,4-dichlorophenoxyacetic acid from aqueous solutions. ACS Appl. Mater. Interf. 7, 7251
7263, with kind permission of the American Chemical Society.

Figure 19.10 SEM images of pure LDH (A), the GO/LDH composite (B), the EDS spectrum of the GO/LDH composite (C), and TEM images at different magnifications of the GO/LDH composite (D and E). Source: Adapted from Tan, L., Wang, Y., Liu, Q., Wang, J., Jing, X., Liu, L., Liu, J., Song, D., 2015. Enhanced adsorption of uranium (VI) using a three-dimensional layered double hydroxide/graphene hybrid material. Chem. Eng. J. 259, 752
760, with kind permission of the American Chemical Society.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 19.11 TEM image of (A) LDH, (B) HRTEM images of CQDs and (C) LDH
CQDs, (D) normalized fluorescence spectra of LDH and LDH
CQDs (λex 5 360 nm). Source: Adapted from Zhang, M., Yao, Q., Lu, C., Li, Z., Wang, W., 2014. Layered double hydroxide
carbon dot composite: high-performance adsorbent for removal of anionic organic dye. ACS Appl. Mater. Interf. 6, 20225
20233, with kind permission of the American Chemical Society.

The platelet structure of the raw LDH, CQDs with spherical shapes, and uniform dispersion of CQDs on LDH sheets were observed in TEM (Fig. 19.11A) and highresolution TEM micrographs (HRTEM) (Fig. 19.11B,C). Moreover, the fluorescence feature of LDH was enhanced by the addition of CQDs (Fig. 19.11D). As can be observed in Fig. 19.12A, the removal efficiencies of 96%, 45%, and 0% of MB was detected for LDH
CQD, raw LDH, and CQDs within 20 min, which showed that the CQDs attachment on the LDH surface simplified the creation of H-bonding between CQDs and dye (Fig. 19.12B).

19.3

Pollutant elimination by polymer/layered double hydroxide nanocomposites

19.3.1 Importance of using polymer/layered double hydroxide nanocomposites in water purification The main drawbacks of using adsorbents such as LDHs, activated carbon, zeolites, and agricultural residues are that they have weak interactions with adsorbates, show

Layered double hydroxide polymer nanocomposites for water purification

791

Figure 19.12 (A) Percentages of removal of methyl blue from an aqueous solution over different adsorbents including CQDs, LDH, and LDH
CQDs (80 mg/L MB, 0.5 g/L adsorbent dose, T 5 298 K). (B) Schematic proposed view of MB adsorption on LDH
CQDs. Source: Adapted from Zhang, M., Yao, Q., Lu, C., Li, Z., Wang, W., 2014. Layered double hydroxide
carbon dot composite: high-performance adsorbent for removal of anionic organic dye. ACS Appl. Mater. Interf. 6, 20225
20233, with kind permission of the American Chemical Society.

low adsorption capacities as well as separation, and their reuse from aqueous solutions in some cases is problematic. In order to solve these issues, favorable hybrids of inorganic
organic polymers have been employed for noxious contaminant elimination from aqueous media in more recent years. In such hybrids, the contact of polymer and nanofiller leads to considerable enhancements in the properties of the system. Actually, the functional variation of polymers is linked with the benefits of robust inorganic parts with thermal stability, which cause strong binding attractions to the pollutants and fairly high sorption capacities (Samiey et al., 2014; Herna´ndez-herna´ndez et al., 2016). In addition, the choice of polymer nanocomposite as sorbent for water purification will be cost-effective because just by addition of a very low quantity of nanofiller in the polymer matrix, high removal efficiency is obtained.

19.3.2 Polymer/layered double hydroxide-based adsorbents Zhu et al. (2017) fabricated a core
shell system based on polydopamine as cores and MgAl-LDHs as shells, and obtained PDA@MgAl-LDHs. High removal efficiency was attained for uranium and europium sorption from radionuclide wastewater by this composite. They concluded that the main mechanism for these radionuclides sorption could be physical sorption, such as electrostatic attraction. Polyaniline/LDHs were synthesized by Zhu et al. and were then used to eliminate Cr(VI) from water media (Zhu et al., 2016). A key factor for sorption performance is the surface area. The surface areas of LDHs, polyaniline, and polyaniline/LDHs is obtained from BET analysis (Fig. 19.13). They were calculated and found to be

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Layered Double Hydroxide Polymer Nanocomposites

Figure 19.13 Nitrogen adsorption
desorption isotherms of polyaniline/LDHs, polyaniline, and LDHs [inset is a digital photograph of Cr(VI) adsorption on polyaniline/LDHs (C) compared with Cr(VI) adsorption on polyaniline (B) and LDHs (A) after reacting for 2 days]. Source: Adapted from Zhu, K., Gao, Y., Tan, X., Chen, C., 2016. Polyaniline-modified Mg/ Al layered double hydroxide composites and their application in efficient removal of Cr(VI). ACS Sustain. Chem. Eng. 4, 4361
4369, with kind permission of the American Chemical Society.

1.80, 15.66, and 42.64 m2/g, respectively. The inset digital photo in Fig. 19.13 shows the higher performance in Cr(VI) sorption on the polyaniline/LDHs compared to the pure LDHs and polyaniline. This was due to their higher surface area and more active sites. The sorption mechanism for Cr(VI) removal by means of polyaniline/LDHs was proposed based on a mutual influence of chemical sorption of Cr(VI) and Cr(III) and reduction Cr(VI) to Cr(III). Shami et al. reported the production of MgAl-LDH arrays which were grown on polyacrylonitrile porous nanomembranes with the aim of oil remediation (Shami et al., 2016). They examined the water drop repellency ability of the attained nanofabrics for both hot water and cool water drops (Fig. 19.14). Their outcomes showed that both hot and cold water drops were repelled on the surface of membrane while the oil (n-heptane) drops penetrated into the membrane quickly, due to the great surface roughness of the hierarchical LDHs. This behavior was tested for pure polyacrylonitrile and it was observed that oil drops as well as water drops were rapidly spread out. They used this property for the separation/harvest of oil selectively from an oil-contaminated aquatic environment. Beyki et al. (2017) fabricated magnetic CaAl-LDH-cellulose ionomers to adsorb diclofenac sodium (DF) as an antiinflammatory drug. Ionomer had a hydrophilic feature and anion exchange ability, which were combined with LDH anion exchange ability and created an effective platform for anion exchange to uptake DF. The combination of cellulose and LDH constituents in drug sorption caused maximum sorption capacity compared to the cellulose ionomer and LDH ionomer. In fact, incorporation of LDH in ionomer matrices causes its subsequent exfoliation;

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Figure 19.14 Surface wetting behavior of (A) the porous polyacrylonitrile/LDHs nanomembranes [(a
h) water repellency and (g, h) oil-removing], and (B) water adsorption and (C) oil (n-heptane) adsorption process by porous pure polyacrylonitrile nanomembranes. Source: Adapted from Shami, Z., Amininasab, S.M., Shakeri, P., 2016. Structure-property relationships of nano-sheeted 3D hierarchical roughness MgAl-layered double hydroxide branched to electrospun porous nanomembrane: a superior oil-removing nanofabric. ACS Appl. Mater. Interf. 8, 28964
28973, with kind permission of the American Chemical Society.

its active sites are more available, which increases the DF sorption efficiency of the final ionomer. To decrease the cost of pollutant adsorption as well as to make the process ecofriendly, the interest of investigators has moved to applying biomaterials as adsorbents (Malik et al., 2016; Pal et al., 2016). A safe and biodegradable polymer extracted from algae is alginate, which has received attention in water purification in recent years. For example, alginate was entrapped by MgAl-LDH to remove phosphate according to a report by Han and coworkers (Han et al., 2011). The adsorption results showed that the elimination capacity related to alginate/LDH was two orders of magnitude greater than pure alginate. Sebastian et al. (2014) used sodium alginate/LDH nanocomposites for dye adsorption and obtained superior sorption behavior compared to virgin LDH. In another work, Lee and Kim (2013)

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fabricated magnetic alginate/LDH composites for the elimination of phosphate from water. In fact, the preparation of composites with a magnetism property allows for an enhanced effective sanitization technology (Ambashta and Sillanp¨aa¨ , 2010). Their results showed the capability of phosphate removal by both magnetic alginate/LDH composites and magnetic iron oxide, but the first presented more adsorption capacity and easy separation. Magnetic alginate beads containing MgAl-LDH (magnetic alginate beads/LDH) were synthesized and used for fluoride remediation from aqueous media (Gao et al., 2014). Due to the presence of LDH in this composite, a higher adsorption capacity was obtained. In addition, this composite had biodegradability potential because of the alginate and owing to the Fe3O4 nanoparticles, its separation was easy and fast by an external magnetic field. The fluoride sorption mechanism by magnetic alginate beads/LDH, which was a complex process, was studied and is shown in Fig. 19.15. At the first step, fluoride entered inside the magnetic alginate beads/LDH from aqueous media. Next,

Figure 19.15 Schematic adsorption process of F2 onto magnetic alginate beads/LDH. Source: Adapted from Gao, C., Yu, X.Y., Luo, T., Jia, Y., Sun, B., Liu, J.H., Huang, X.J., 2014. Millimeter-sized Mg
Al-LDH nanoflake impregnated magnetic alginate beads (LDHn- MABs): a novel bio-based sorbent for the removal of fluoride in water. J. Mater. Chem. A. 2, 2119
2128, with kind permission of The Royal Society of Chemistry.

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electrostatic attractions were created between the edges and external surfaces of the LDHs and fluoride. Then, fluoride pollutants could be replaced by the hydroxyl groups on the metal cation surfaces or interlayer nitrates. Some could create Hbonds with the interlayer water. LDHs in powder form may not be appropriate for pollutant removal from wastewater. A magnetism approach is an effective way to improve water purification, due to the effects and advantages discussed earlier. Preparation of LDHs in granular forms is another way for easy separation of sorbents based on LDH from water. With this aim, Ha and coworkers, and also Phuong, used an easy and low-cost method for reinforcement of LDHs into blending poly(vinyl alcohol) (PVA) with calcium alginate to remove chromate and phosphate, respectively, from water solutions (Ha et al., 2016; Phuong, 2014). Han et al. prepared PVA-alginate/LDH beads for phosphate remediation and compared its deformation with alginate beads/LDH during adsorption, and concluded that blending PVA with calcium alginate causes stability improvement during phosphate remediation (Han et al., 2011). Another kind of cost-effective, safe, and biodegradable polymer is chitosan, which contains amino and hydroxyl groups that make it a good sorbent for contaminants (Mallakpour and Madani, 2016a,b). Chitosan/MgAl-LDH composite was designed by Elanchezhiyan’s group for the adsorption of oil from oil-in-water emulsion at pH 3.0 (Elanchezhiyan and Meenakshi, 2017). The oil-removal mechanism using manufactured chitosan/MgAl-LDH is displayed in Fig. 19.16. The results showed that the oil sorption was higher at pH 3.0 (acidic media) than sorption in an alkaline environment. They explained that at acidic pH, the surface of the sorbent becomes more positive, which enhances the electrostatic interactions among the oil

Figure 19.16 Feasible mechanism of oil recovery using chitosan/MgAl-LDH composite. Source: Adapted from Elanchezhiyan, S.S.D., Meenakshi, S., 2017. Synthesis and characterization of chitosan/Mg-Al layered double hydroxide composite for the removal of oil particles from oil-in-water emulsion. Int. J. Biol. Macromol. 104, 1586
1595, with kind permission of Elsevier.

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pollutants and sorbent. On the other hand, electron lone pairs exist in the active functional groups of chitosan which can be coordinated with LDH and cause the hydrophobicity and increase the tendency for the adsorption of hydrophobic oil particles. Pandi et al. (2017) synthesized and characterized a chitosan/Fe3O4/MgAl-LDH composite for the fluoride elimination from aqueous solution. The prepared composite exhibited good separation capability and adsorption capacity in comparison to the pure LDH, chitosan, and Fe3O4. They proposed the following mechanisms for the fluoride removal by the obtained composite: (1) Electrostatic interaction can be formed between the surfaces of LDH with positive charges and fluoride ions. (2) Higher valance metal ions with positive charges such as Fe31 and Al31 could remove negative fluoride ions via both electrostatic interaction and complexation. (3) The ion-exchange mechanism was through the replacement of more electronegative fluoride with the interlayer OH2 ions (Fig. 19.17). Mallakpour et al. fabricated reinforced recycled poly(ethylene terephthalate) (RPET) nanocomposites with CNT/MgAl-LDH (CNT/LDH) to obtain an R-PET/ CNT/LDH nanocomposite and which can eliminate Cd21 from a water solution (Mallakpour and Behranvand, 2017c). TEM images showed a strong interaction among the nanotubes and LDH flakes, with sizes around 40 nm and nice dispersion in the R-PET matrix (Fig. 19.18).

Figure 19.17 Defluoridation mechanism of Fe3O4/MgAl-LDH/chitosan composite. Source: Adapted from Pandi, K., Periyasamy, S., Viswanathan, N., 2017. Remediation of fluoride from drinking water using magnetic iron oxide coated hydrotalcite/chitosan composite. Int. J. Biol. Macromol. 104, 1569
1577, with kind permission of Elsevier.

Layered double hydroxide polymer nanocomposites for water purification

Figure 19.18 TEM images corresponding to R-PET/CNT/LDH nanocomposites at three different magnifications (A
C). Source: Adapted from Mallakpour, S., Behranvand, V., 2017c. Water sanitization by the elimination of Cd21 using recycled PET/MWNT/LDH composite: morphology, thermal, kinetic and isotherm studies. ACS Sustain. Chem. Eng. 5, 5746
5757.

Figure 19.19 Possible sorption mechanisms of Cd21 onto the R-PET/CNT/LDH nanocomposite surface. Source: Adapted from Mallakpour, S., Behranvand, V., 2017c. Water sanitization by the elimination of Cd21 using recycled PET/MWNT/LDH composite: morphology, thermal, kinetic and isotherm studies. ACS Sustain. Chem. Eng. 5, 5746
5757.

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They stated that the incorporation of CNT/LDH into the R-PET matrix leads to easy and fast separation. Also, the obtained polymer composite showed higher sorption capacity compared to pure R-PET. Also, they proposed two mechanisms for this sorption including electrostatic interactions mainly and chemical sorption (such as bidentate and monodentate complexes) as well (Fig. 19.19).

19.4

Conclusions

The wastewater treatment field is fortunate to have nanotechnology as one of the most progressive procedures in this area. It is important to mention that, when the size of materials decreases, the surface area increases, which improves the adsorption capacity of adsorbents for pollutant removal. One of the most promising nanomaterials which could be a candidate as sorbents in water treatment is LDHs. These nanomaterials have particular features such as high surface area, layered construction, and porosity. Different pathways for the sorption of pollutants on the LDH surface have been described: The cation in the main backbone of LDH as well as the interlayer anion between layers could be exchanged by cationic and anionic pollutants. Also, π
π interactions, electrostatic attractions, van der Waals forces, and hydrogen bonding participate in the sorption of contaminants. In spite of the good performance of LDHs in water treatment, there are some limitations, such as weak attractions with contaminants, low sorption capacities, as well as difficult reusability. To solve these problems, several approaches have been proposed and performed as follows: G

G

G

Surface functionalization of LDHs; Preparation of LDH hybrids, for example, CNT/LDH, QDs/LDH, GO/LDH, etc., and their magnetization; Fabrication of polymer/LDH nanocomposites and their magnetization.

Researchers should concentrate more on the last item, because employing polymer nanocomposites as adsorbents for water sanitization would not only be a costeffective process with high removal efficiency, but also can decrease the toxicity of nanomaterials. In fact, there are two reasons for lower toxicity: (1) combination of polymer with nanomaterials leads to the use of a lower amount of them; also (2) the toxicity will decrease by using biopolymers such as chitosan, PVA, alginate, etc. This area of research is very attractive, which is why there are many research work currently being done in this area and this will guarantee more achievements in the near future in making this technology more versatile.

Acknowledgments The authors appreciatively acknowledge the Research Affairs Division of Isfahan University of Technology (IUT), Iran. Thanks are also given to Iran Nanotechnology Initiative Council (INIC), Tehran, Iran, National Elite Foundation (NEF), Tehran, Iran, and Center of Excellence in Sensors and Green Chemistry Research (IUT).

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Layered double hydroxide polymer nanocomposites for catalysis

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Shadpour Mallakpour1,2 and Hashem Tabebordbar1 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

20.1 Introduction In recent years, the fabrication and applications of polymer nanocomposites (PNCs) have been placed in the spotlight of researchers and industrialists due to impressive improvements in their mechanical, electrical, thermal, gas permeability, flammability, and UV stability properties (Mallakpour and Javadpour, 2016c; Kotal and Bhowmick, 2015; Liu et al., 2017; de Leon et al., 2016; Mallakpour and Behranvand, 2016a; Mallakpour and Khadem, 2016d). This success can be achieved in the presence of a low content of fillers, which is a special feature in the industry. Hence, researchers are enthusiastic to create, identify, and develop various types of PNCs and have published numerous and useful reviews in this context. Various fillers are employed in the preparation of PNCs based on the types of requests and applications (Mallakpour and Khadem, 2015; Tan and Thomas, 2016; Mallakpour and Behranvand, 2016b; Mallakpour et al., 2016). Layered double hydroxide (LDH) is one of the fillers that with appropriate structure could find a special position in the field of PNC manufacturing (Maheskumar et al., 2014; Velasco et al., 2012; Mallakpour et al., 2015a; Mallakpour and Behranvand, 2017a). LDH is a class of anionic layered mineral with a brucite-like sheet structure with natural and sources. common chemical formula of LDH is defined as synthetic x1 The 21 n
31 M21 M : ð OH Þ ð A Þ signifies a divalent metal ion, 2 x=n : yH2 O, in which M 1
x x 31 n
M signifies a trivalent metal ion, and A signifies an anion. Since the interlayer anions (Cl2, F2, CO322, NO32, OH2, SO422, etc.) and employed metal ions (Cu, Zn, Mg, Ni, Co, Fe, Mn, Cr, Al, Ga, etc.) are variable, diversified chemical compositions of this anionic layered inorganic filler can be produced. Different ranges of x value have been reported for organized LDH, which generally changes in range from 0.2 to 0.33 (Han et al., 2014). To gain a better understanding, a schematic drawing of the LDH structure is shown in Fig. 20.1. The high water capacity, crystallinity, high reactivity, tunable assembly consistency, nontoxicity, and especially catalytic activities of LDHs make them appropriate for energy storage, Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00020-3 © 2020 Elsevier Ltd. All rights reserved.

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Figure 20.1 Schematic drawing of the structure of LDH compounds: (A) side view; (B) top view. (C) Octahedral units of LDH compounds assembled through hydrogen bonding of water molecules, electrostatic force of anions between the interlayer and hydroxyl group of 21 21 21 21 21 21 the sheets. M21 or Ga21. M231: Al31, Cr31, Mn31, 1 : Mn , Fe , Co , Ni , Cu , Zn 2 31 31 31 31 n2 2 2 22 22 Fe , Co , Ni or La . An : OH , NO3 , Cl , ClO2 4 , CO3 or SO4 . Source: Adapted from Mao, N., Zhou, C.H., Tong, D.S., Yu, W.H., Lin, C.X.C., 2017. Exfoliation of layered double hydroxide solids into functional nanosheet. Appl. Clay Sci. 144, 60
78. With kind permission of Elsevier.

heterogeneous and homogeneous catalysts, environmental safekeeping, additives for fabrication of nanocomposites (NCs), medicinal, makeup materials, and UV preservative applications. The identification and development of catalysts are essential because they play an effective role in the progress of chemical technology (Wang and O’Hare, 2012; Li and Duan, 2006; Costantino et al., 2013; Mallakpour et al., 2015b). The majority of chemical reactions take place in the presence of catalysts, including the production of epoxides, acids, ketones, aldehydes, and alcohols through oxidation, hydrogenation of saturated compounds, and polymerization. Today, industries rely heavily on catalysts and extensive researches are carried out to produce stable and active catalysts that act selectively (Sudha and Sivakumar, 2015; Robinson et al., 2016). The layered structure has provided appropriate conditions for the design of practical catalysts with high performance in the synthesis of organic molecules, water splitting, pollutant degradation, air conditioners, and energy storage (Barrado, 2015). Catalytic applications of LDH have various aspects that have been developed over time. The layered structure, cation-exchangeability, uniform dispersion of metal cations, adjustable basicity, high surface area, and ability to combine with metal oxides (MOs) and polymers are advantages of LDH for catalytic activity (Fig. 20.2) (Feng et al., 2015; Li et al., 2015).

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Figure 20.2 Properties and applications of supported catalysts fabricated using LDHs as supports/precursors for catalytic oxidation and hydrogenation. Source: Adapted from Feng, J., He, Y., Liu, Y., Du, Y., Li, D., 2015. Supported catalysts based on layered double hydroxides for catalytic oxidation and hydrogenation: general functionality and promising application prospects. Chem. Soc. Rev. 44 (15), 5291
5319. With kind permission of RSC.

The objective of this chapter is to investige the catalytic behavior of LDH and polymer/LDH NCs. Accordingly, some important catalytic applications of polymersupported LDHs are discussed separately. In this regard, the synthesis methods of polymer/LDH NCs have also been debated. It is hoped that this chapter will be useful and create a pathway for progress in the field of catalysts.

20.2 Applications of layered double hydroxides in catalysis The high potential of LDH for surface adsorption of diverse metal ions makes it conducive for utilization in redox catalysis. On the other hand, incorporation of functionalized anions between LDH layers creates an active basic surface that facilitates the redox reactions. LDH with mixed MO has been employed as ecological heterogeneous base catalysts for the synthesis of organic molecules through oxidation processes. Supported Au-LDH was used as a heterogeneous catalyst for synthesis of lactones from diols with a high turnover number (1400) (Mitsudome et al., 2009b). The procurement process of Au-LDH catalyst with the corroborant analysis is displayed in Fig. 20.3. With small Au-LDH loading without any other additive, the lactonization was performed at relatively low temperatures, which is a good advantage compared to previously reported catalysts. The catalytic activity of Au-LDH was compared with other Au-MO catalysts for aerobic oxidation of 1-phenylethanol. The Au-LDH showed excellent turnover number, as large as 200,000, with three optimum recyclings without any change in selectivity and activity (Mitsudome et al., 2009a).

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Figure 20.3 A schematic of the AuNCs/LDH catalyst and (A) UV
vis spectra of the original solution of GS-AuNCs (a) and the supernatant after impregnation over LDH (using Mg3Al-LDH as the example) (b); (B) HRTEM image of GS-AuNCs (inset shows the crystalline structure of an individual NC and the histogram of the size distribution). Source: Adapted from Feng, J., He, Y., Liu, Y., Du, Y., Li, D., 2015. Supported catalysts based on layered double hydroxides for catalytic oxidation and hydrogenation: general functionality and promising application prospects. Chem. Soc. Rev. 44 (15), 5291
5319. With kind permission of RSC.

This distinction is due to the stability of adsorbed Au negative charge in the presence of LDH positive charges that promote the oxidation process. Studies showed that Mg/Al atomic ratio, calcination temperature, and using different M21 and M31 cations for preparation of Au-LDH catalysts affected the catalytic performance in oxidation reactions. Generally, the catalytic efficiency was enhanced with increasing calcination temperature and Mg/Al ratio. Also, replacement of cations in AuLDH structure with transition metal cations improved the efficiency of the catalyst by increasing the synergetic effect (Liu et al., 2012; Takagaki et al., 2011; Li et al., 2014). High yields of aldehydes and ketones were synthesized without any isomerization through oxidation of alcohols, with the help of heterogeneous Pd-LDH catalysts (Kakiuchi et al., 2001). The results exhibited that the presence of Bronsted basic sites on the LDH structure increases the activity of catalysts. The functionalized Pd-LDH as a recyclable catalyst was used for oxidation of benzyl alcohols. Diamine as nitrogen donor ligand stabilized the Pd sites. The benzaldehyde was produced selectively, with 94% conversion by this catalyst. Functionalized Pd-LDH

Layered double hydroxide polymer nanocomposites for catalysis

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can be simply recycled for several periods with the previous selectivity and activity (Sahoo and Parida, 2013). The Ru-LDH catalyst was applied for the tandem synthesis of quinolone. Ru species facilitate the aerobic oxidation and, in the following, basic sites of LDH manage the aldol reaction (Motokura et al., 2004). The hydrogenation reaction is another approach for the synthesis of organic species. In these reactions, catalysts play an important role in yield and selectivity. The LDH catalyst with excellent properties is also a useful candidate for hydrogenation of unsaturated bonds. The Pd-LDH catalysts with diverse mixed MO showed outstanding catalytic activity for hydrogenation of the disposed compounds. The CH2F2 was obtained selectively by hydrodechlorination of CCl2F2 with the help of Pd-LDH catalyst (Padmasri et al., 2004). The Pd-LDH was modified with arginine and was used as an active catalyst for the formation of alcohols from hydrogenation of ketones (Tao et al., 2010). The modified catalyst showed higher activity than unmodified catalyst by converting more than 97%. This advantage is originated from arginine which, with its amino group, creates a durable synergistic effect between LDH and Pd NPs. The decline in the activity of the catalyst was not observed after being reused five times, representing the stability of the prepared catalyst. Besides the electrostatic interactions between the arginine and Pd NPs, the guanidyl group contributes to the stability of the catalyst by establishing coordination with Pd21 ions. Formation of α-alkylated nitriles as vital structures for different biologically active substances was catalyzed with Pd-LDH catalyst (Motokura et al., 2005). This reaction was carried out in two phases, the basic site on LDH contributed in aldol condensation, and Pd NPs supported the hydrogenation process. This prepared catalyst was more active than Pd/C as a commercially existing catalyst in the hydrogenation reaction. Hydrogenation of colophony was performed over Ni-LDH catalyst under an optimized operating state (Huang et al., 2016). The results indicated a correlation between the reaction temperature and the rate of conversion. The greatest amount of conversion (99.73%) was obtained under applied pressure 5.5 MPa of H2 at 178 C for a duration of 90 min. The catalytic activity of the heterogeneous Co-LDH catalyst was investigated for hydrogenation of CO under adjusted conditions (Tsai et al., 2011). The Co-LDH catalyst revealed a remarkably progressive activity compared to the other prepared catalysts in the absence of a reduction supporter. Comparing the activity of prepared catalysts suggests that the support has a significant influence on the performance of the catalyst. According to the performed analysis, it was found that high surface area and thermal stability of LDH, good interaction between Co particles and LDH, and uniform dispersion of Co particles on the surface of LDH were all significant factors for the superiority of the Co-LDH catalyst. The reforming of CO and CO2 to CH4, which is called methanation, is very important due to the worthwhile environmental effects and production of a useful and applicable reactant. Also, methanation is applied to remove traces of CO and CO2 during the ammonia synthesis process. Several metal catalysts, such as Ru, Co, Fe, Rh, and Ni, with different performances have been introduced for the formation of CH4 from CO and CO2. Ni-based catalyst has practical advantages such as low cost and high activity (Li et al., 2016; Zhang et al., 2014). However, the

810

Layered Double Hydroxide Polymer Nanocomposites

exothermic phenomenon in high temperature makes it deactivate quickly. The LDH with high adsorption capacity and anchoring effect has a good potential for adsorption of CO2. Hence, the flower-like Ni-LDH-Al2O3 with a high degree of stability and dispersion was used for methanation of CO/CO2 (Fig. 20.4). Due to the strong interaction between the Ni particles and LDH as a support, the new catalyst showed an extraordinary activity and after 252 h, only 7% reduction was observed in CO2 conversion (He et al., 2013). Nitrogen and sulfur oxides are known as dangerous pollutants with most environmental problems, including some diseases, acid rain, demolition of the ozone layer, and changing climate, originating from them. These threats and damages have forced researchers to develop ways for controlling these

Figure 20.4 Illustration of the formation of Ni NPs with high dispersion and high density on a hierarchical flower-like Al2O3 matrix via an in situ reduction process of a NiIIAlIII-LDH precursor; (A, B) SEM images of the flower-like NiAl-LDH; (C, D) HRTEM images of the flower-like NiAl-LDH; (E) high-angle annular dark field (HAADF) STEM images of Ni nanoparticles in the sample of flower-like NiAl-LDH; (F) profiles of CO2 conversion vs. temperature for CO2 methanation in the presence of flower-like NiAl-LDH. Source: Adapted from Feng, J., He, Y., Liu, Y., Du, Y., Li, D., 2015. Supported catalysts based on layered double hydroxides for catalytic oxidation and hydrogenation: general functionality and promising application prospects. Chem. Soc. Rev. 44 (15), 5291
5319. With kind permission of RSC.

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pollutants. Among different approaches, the NOx storage and reduction (NSR) strategy is the best technique for NOx removal due to user convenience, cheapness, and high efficiency. The NSR catalyst oxidized firstly the NO to NO2, formerly the produced NO2 is stored on the surface of the catalyst and finally, the adsorbed NOx is reduced to inert N2 gas. Furthermore, several NSR catalysts with various transition metals were reported for the exclusion of NOx (Dai et al., 2012; Obalova et al., 2009; Wang et al., 2012). Palomares and coworkers produced several types of LDHs with diverse approaches for the exclusion of NOx at high temperature (Palomares et al., 2012). Among them, CoAl-LDH showed the best potential for elimination of NOx in the presence of water and oxygen at 750 C. The results proposed that the catalytic activity is independent of the fabrication method but is totally dependent on the constituent metals. LDH with middle basicity is a promising catalyst for SO2 removal. For example, The MgAl-LDH showed an outstanding contribution to the eradication of SO2 in the air flow (Kameda et al., 2011; Kameda et al., 2012). The resulting SO3 from the oxidation of SO2 is dissolved in aqueous solution and then removed by instauration of LDH intercalated with detached sulfate. The removal percentage of SO2 increased well by enhancing the LDH dosage and temperature. Epoxides due to high chemical reactivity have special trade status in the chemical industry in which it is extensively employed as crude and versatile materials for surfactants, paints, epoxy resins, and synthesis of commonly used chemicals. This imperative intermediate is generally prepared by the chlorohydrin process and straight oxidation of olefins by means of molecular oxygen or organic peroxides and peracids. Straight oxidation with O2 is preferred for epoxide production on an industrial scale. Intercalation of polyoxometalates (POMs) into the LDHs produces highly stable heterogeneous catalysts for epoxidation of olefins. The selectivity of LDHPOM catalysts was compared with homogeneous Na-POM catalyst for epoxidation of several allylic alcohols in the presence of dilute H2O2 as an oxidant. The results showed that the LDH-POM catalysts without pH being regulatory are the absolute winner; this advantage can be attributed to the fine hydrothermal permanency of LDH and sustained interaction between the LDH and POM. Moreover, catalyst activity remained unchanged after continuous recycling (Liu et al., 2008, 2009).

20.3 Polymer/layered double hydroxide nanocomposites Incorporation of LDHs into the polymers produces multifunctional NCs with a wide range of applications. LDHs, due to their layered structure, high thermal stability, adjustable composition, and catalytic activity promote the optical, flame retardancy, mechanical, rheological, and thermal properties of the host polymeric medium. To date, various types of polymers as a matrix have been reported for the preparation of LDH NCs. In general, the dispersion index of LDH in the polymer environment directly affects the structure and properties of resultant NCs. Hence, the development of methods for the preparation of homogeneous polymer/LDH NCs is important.

812

Layered Double Hydroxide Polymer Nanocomposites

20.3.1 Preparation of polymer/layered double hydroxide nanocomposites Various procedures have been reported for the fabrication of polymer/LDH NCs which are reviewed briefly in four principal routes: in situ LDH synthesis in the polymer solution, in situ polymerization, melt mixing, and solution blending.

20.3.1.1 In situ layered double hydroxide synthesis in polymer solution This method is a good strategy for enhancing the interfacial connections between LDHs and host polymers. The target is the gradual formation of LDHs within the predetermined polymer solution. The NCs are obtained with the implantation of the polymer chains into layers during the assembling LDH construction. The LDH formation process is accomplished by coprecipitation of two fundamental metal salts in an appropriate polymer solution. The polyester and poly(vinyl pyrrolidone) (PVP) were interposed to ZnAl-LDH by quantized precipitating respective metal salts in the presence of fixed polymer solution under fixed pH 9 at room temperature (Stimpfling et al., 2016). The achieved PVP-LDH NCs were more homogeneous due to their ability to create a gel-like suspension. The photostable poly(vinyl alcohol) (PVA)/Zn2Al-LDH NCs were fabricated based on steady coprecipitation of LDH plaques in aqueous PVA solution under constant pH 9 (Gaume et al., 2013). The Zn(NO3)2 and Al(NO3)2 were applied as constructive metal salts of LDH with a 2:1 ratio of Zn to Al. This applicable technique provides a well-organized approach to generate exfoliated NCs with ideal structural properties, such as photostability and a gas barrier. Leroux and coworkers applied this method for the production of exfoliated PVP/Zn2Al-LDH NCs accompanied by alginate. Alginate as an effective separating agent prevents the accumulation of LDH plaques. The results from different analysis techniques indicated the improvement in ionic conductivity and dielectric properties of the obtained NCs (Leroux et al., 2012). Highly dispersed Ni NPs over carbon nanotubes (CNTs) were synthesized from NiAl-LDH/poly(acrylic acid) (PAA) functionalized CNTs (PAA-CNTs) hybrid for selective hydrogenation of o-chloronitrobenzene. The NiAl-LDH/PAA-CNTs NC was assembled by coprecipitation of Ni(NO3)2  6H2O and Al(NO3)2  9H2O as constructive metal salts of LDH with a 3:1 ratio. The supported Ni catalyst presented excellent catalytic activity in the selective hydrogenation of o-chloronitrobenzene to o-chloroaniline with a yield of 98.1% in 150 min (Wang et al., 2013).

23.3.1.2 In situ polymerization This solution-based process has been extensively utilized for fabrication of LDHbased polymer NCs. The procedure begins with the LDH dispersing in pristine monomer or solution monomer. Subsequently, in situ polymerization is carried out into the LDH layers using the proper initiator. Some polymer/LDH NCs have been successfully produced for different purposes by this technique (Zhu et al., 2016; Cui et al., 2012). The antimicrobial polyacrylonitrile/ZnAl-LDH NCs were synthesized by the in situ polymerization method (Barik et al., 2017). The thermal

Layered double hydroxide polymer nanocomposites for catalysis

813

stability and antibacterial activity of the obtained NCs improved, which is presumably justified with a fine dispersion of LDH NPs and electrostatic contact between them and charged surface of bacterial cells. The multifunctional polydopamine (PDA)/LDH NCs were successfully synthesized by interlayer polymerization of dopamine in the gallery space of the CoAl-LDH and MgAl-LDH NPs (Nam et al., 2016). This synthetic strategy was used in order to obtain homogeneous and welldispersed LDHs within the PDA in resultant NCs, which ultimately leads to improvements in its catalytic activity and electrochemical properties. The PDA/CoAl-LDH and PDA/MgAl-LDH NCs exhibited high catalytic activity for reduction of p-nitrophenol into p-aminophenol by NaBH4 with a conversion efficiency greater than 97% and 87%, respectively. Also, the resultant NCs showed excellent capacitance value and structural sustainability without any noticeable drop during testing.

20.3.1.3 Solution blending This technique is used for straight intercalation of qualified polymers with suitable functional groups and various molecular weights into interlayered sections of LDHs. The intended LDH NPs are dispersed directly into the prepared polymer solution and then, the NCs are achieved by solvent evaporating. There are shortcomings that make it difficult to form uniform NCs. For example, incompatibility between LDH NPs and the polymer environment, narrow interlayer space, which reduces the intercalation of polymer chains, and aggregation of LDH NPs due to high charge density. Surface modification with suitable organic species is the most important proceeding that has been done to improve the intercalation conditions. Different compositions of pristine and modified LDH were incorporated within the various polymers. The proton conductivity of sulfonated poly(ether ether ketone) (SPEEK) was enhanced with the embedding diverse quantities of MgAl-LDH. Solution blending as a common technique was successfully applied for the preparation of these NCs. The results showed that the obtained SPEEK/MgAl-LDH NCs can be used as a good candidate for a polymer electrolyte membrane fuel cell (Kim et al., 2015). The new polyphenol oxidase immobilization hybrid based on Zn2Al-LDH/alginate was produced by a solution-blending process for fabricating phenol biosensors. This biosensor showed very sensitive performance for detection of phenol in water and also chloroform (Lopez et al., 2010).

20.3.1.4 Melt mixing Direct melt mixing is the typical traditional technique for the formation of PNCs with the excellent distribution of fillers on an industrial scale. In this process, the fluid polymers are diffused into the interlayer of LDH NPs under high local shear stress by heating them above the melting temperature or glass transition temperature of polymers. This convenient technology can manufacture the polymer/LDH NCs with uniform morphology without the need for complicated reactions. Melt mixing always entices the particular interest of industrialists and researchers due to advantages including availability, simplicity, solvent-free, and environmentally friendly. A brief content concerning the production of polymer/LDH NCs through melt

814

Layered Double Hydroxide Polymer Nanocomposites

mixing is described in the following. The PS/NiAl-LDH NCs were prepared using a melt-mixing system by means of a twin-screw extruder. The significant intermolecular relations between the LDH NPs and neat PS improved the thermal decomposition, flexural strength, and tensile strength (Suresh et al., 2017). Donato and coworkers produced polypropylene (PP) NCs based on modified MgAl-LDH through melt blending (Donato et al., 2012). The results showed that this procedure can improve the properties of PP. The crystallinity, stiffness, and thermal degradation of PP were increased in the presence of modified LDH. These developments could be described by the role of modified LDH as a plasticizer and nucleating agent into the PP background. In another project, the properties of pristine ethylene vinyl acetate (EVA) copolymer and one filled with ZnAl-LDH were investigated. The EVA/LDH NCs were achieved by both melt mixing and solution intercalation methods with various quantities of LDHs. The amounts of loaded LDHs for the melt mixing were twice as high as the solution method, which can be attributed to the robust shear force interplay. The properties were completely dependent on the level of diffusion and LDH-loaded value. The obtained EVA/LDH NCs showed higher mechanical and thermal properties than pristine EVA and their values were ceaselessly enhanced with the proliferate LDH loading (Zhang et al., 2008). Also, the prepared polymer/LDH NCs by melt-mixing method can also have catalytic applications. For example, the modified LDH/high-density polyethylene (HDPE) NCs showed good antimicrobially properties over different bacteria (Kutlu et al., 2014). At first, the MgAl LDHs were modified with camphorsulfonic acid (CSA) and ciprofloxacin. The thermal stability of CSA was developed over 160 C under LDH shielding. Then, the functionalized LDHs were melt-compounded with HDPE. The antimicrobial testing showed that the prepared modified LDH/HDPE NCs have good susceptibility against the tested bacteria, while the pristine HDPE indicates no inhibitory effects. In addition to these general methods, layer-by-layer assembly (LBL) and reversible addition fragmentation chain transfer (RAFT) polymerization are also used commonly as fabrication methods of polymer/LDH NCs for catalytical applications. LBL is the wet chemical technique for the synthesis of ultrathin film onto a solid surface which has attracted enormous interest as a powerful methodology for producing photoactive and catalytic surfaces. Its advantages are the experimental simplicity and cheap price, ease in controlling the thickness of the layers, and possibility of using numerous materials as building units, containing metal oxide NPs, carbon-based nanomaterials, and polyelectrolytes. In this method, the films are prepared by the consecutive transfer of supplementary ingredients to a proper solid substrate through immersing, spraying, and spinning procedures. The adsorption process on the surface of substrates can be derived through electrostatic or nonelectrostatic interactions. This simple and flexible technique can be applied in the development of NC-based thin films (Nunes et al., 2017). RAFT polymerization is one such methodology which has been successfully employed for encapsulation of LDHs. RAFT as a kind of living radical polymerization is an ideal technique for fabrication of NCs with prechosen molecular weight and architecture. By this method, the LDH surface can be easily covered with

Layered double hydroxide polymer nanocomposites for catalysis

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various polymers that are susceptible to catalytic applications. The thiocarbonylthio compounds such as xanthates, dithioesters, and thiocarbamates were used as chain transfer agents in RAFT polymerization (O’Donnell, 2012; Perreira et al., 2017). Accordingly, the surfaces of LDHs should be modified initially with proper thiocarbonylthio compounds. In the next step, the intended polymer grows on LDH surfaces by providing reaction conditions such as monomer and initiator.

20.4 Applications of polymer/layered double hydroxide nanocomposites in catalysis As already mentioned, today catalytic processes are frequently applied in the majority of reactions, especially in industry. Many of these practical catalysts are inherently homogeneous, suffering from low chemical and thermal resistance. Moreover, their separation is difficult, costly, and may contaminate the final products and the environment. The use of supports for the preparation of heterogeneous catalysts is an ideal procedure to overcome the drawbacks. The supports not only facilitate the separation of the catalyst from the reaction mixture but also expand the catalytic activity. Therefore, discussion about the heterogeneous catalysts has become a hot subject in the chemical industry. Polymer NCs can be used as a standard support for the formation of catalysts (Pessoa and Maurya, 2017). The combination of polymers with catalyst active materials is a good method to develop their catalytic performances. This process, in addition to enhancing the properties of polymers in terms of mechanical, optical, electrical, thermal, and morphological properties, can promote the catalytic efficiency by increasing the active sites. Among the various materials, the ability of LDHs has stood the test of time in increasing the efficiency of polymers (Maheskumar et al., 2014). Therefore, many hybrids can be produced with the penetration of LDHs within the diverse polymers which have wide applications in different aspects (Mallakpour and Hatami, 2017b). Among the numerous applications of polymer/LDH NCs, their catalytic activity has been less evaluated, while their effectiveness has been reported in various fields such as the synthesis of chemical compounds, fuel cells, and sensors. The PVA/Au-LDH composite films were prepared by the bottom-up LBL assembly and successfully catalyzed the reduction of 4-nitrophenol (4-NP) by sodium borohydride (Shu et al., 2015). For this purpose, the prepared CoAl-LDH NPs were firstly modified with (3-aminopropyl)triethoxysilane to obtain monodispersed AuLDH hybrids. Then, the PVA was repeatedly coated on a glass substrate by spinning and exposed to a mixture of Au-LDH hybrids. The final hybrid films were achieved by transfer of the Au-LDH NPs to the coated PVA layers. The catalytic performance of the obtained hybrids was detected by UV
visible spectroscopy. Without the presence of synthesized hybrids, no changes were observed in reduction of 4-NP even after 3 days. As is clear in Fig. 20.5, with the progress of the reaction in the presence of PVA/Au-LDH composites, the absorption band related to 4-NP at 400 nm decreased expressively but the absorption band associated with

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Layered Double Hydroxide Polymer Nanocomposites

Figure 20.5 (A) UV
vis spectra of the reduction of 4-NP in an aqueous solution recorded every 2 min using the Au NPs 2 LDH 2 PVA hybrid film as a catalyst. (B) Relationship between ln(Ct/C0) and the reaction time (t), wherein the ratios of the 4-NP concentration (Ct at time t) to its initial value C0 (t 5 0) were directly given by the relative intensity of the respective absorbance At/A0 and, therefore, the reduction process could be directly reflected by these absorption curves. Source: Adapted from Shu, Y., Yin, P., Liang, B., Wang, H., Guo, L., 2015. Artificial nacrelike gold nanoparticles
layered double hydroxide
poly (vinyl alcohol) hybrid film with multifunctional properties. Ind. Eng. Chem. Res. 54 (36), 8940
8946. With kind permission of ACS.

the produced 4-aminophenol (4-AP) at 295 nm was enhanced gradually. This means that 4-AP has been properly synthesized without any derivatives. Also, the kinetic experiment indicated that the reduction process is consistent with a pseudo-firstorder model. The efficiency of PVA/Au-LDH catalyst remained stable after recycling 10 times in the same conditions. Polymer-supported LDHs have been widely used in the synthesis of organic molecules with remarkable efficiency due to low cost, environmental compatibility, experimental simplicity, high stability, and simple separation. For example, the innovative PAA-grafted LDH hybrid as a high thermal stable catalyst was prepared by RAFT polymerization for promoting the synthesis of benzo[4,5]imidazo[1,2-α] pyrimidines (BIPs) (Reddy et al., 2017). Fig. 20.6 illustrates well the process of grafting PAA on MgAl-LDH, which was modified with S-(3-trimethoxysilyl) propyltrithiocarbonate (BTPT). For the synthesis of BIPs, the 1H-benzo[d]imidazole-2amine was coupled easily with diverse α,β-unsaturated carbonyl compounds with the help of effective PAA-LDH heterogeneous catalyst at 80 C (Fig. 20.7). In a catalyst-free reaction, the BIPs were slowly synthesized with low yield after 8 h but by putting the catalyst into the reaction, a superior yield (92%) was obtained within 20 min. The reaction conditions, such as the content of catalyst, solvent, time, and temperature, were optimized and the derived data are depicted in Table 20.1. Furthermore, the performance of PAA-LDH was compared with other catalysts. The results suggested that the highest yield was obtained by 10 mg of the newly fabricated catalyst under neat reaction at 80 C. According to the mechanism, the PAA-LDH catalyst with its acidic character conducted the formation of an imine intermediate at the initial step and in the final period produced the suitable products

Layered double hydroxide polymer nanocomposites for catalysis

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Figure 20.6 Synthesis of PAA-g-LDHs. Source: Adapted from Reddy, M.V., Reddy, G.C.S., Lien, N.T.K., Kim, D.W., Jeong, Y.T., 2017. An efficient and green synthesis of benzo [4, 5] imidazo [1, 2-a] pyrimidines using highly active and stable poly acrylic acid-supported layered double hydroxides. Tetrahedron 73 (10), 1317
1323. With kind permission of Elsevier.

Figure 20.7 Synthesis of benzo[4,5]imidazo[1,2-α]pyrimidines. Source: Adapted from Reddy, M.V., Reddy, G.C.S., Lien, N.T.K., Kim, D.W., Jeong, Y.T., 2017. An efficient and green synthesis of benzo [4, 5] imidazo [1, 2-a] pyrimidines using highly active and stable poly acrylic acid-supported layered double hydroxides. Tetrahedron 73 (10), 1317
1323. With kind permission of Elsevier.

by removing the H2 molecule. Mild conditions, low cost, high yield, short time, simplicity, and sequential reusability are the advantages of this procedure. In another similar research, the poly(oligoethylene glycol methacrylate)-gsupported CaAl-LDH (LDH-g-POEGMA) as a green catalyst was applied for the synthesis of chromene merged dihydroquinoline derivatives by a one-pot threecomponent condensation of aromatic amines, 4-hydroxy-2H-chromen-2-one, and different aldehydes through solvent-free conditions (Fig. 20.8) (Reddy et al., 2016). The RAFT polymerization was employed for the growing POEGMA on the surface of LDH-BTPT. The catalytic activity of LDH-g-POEGMA was compared with

Table 20.1 Optimization of reaction conditions for the synthesis of 3aa

Entry

Solvent

Catalyst

Temperature ( C)

Time (min)

Yieldb (%)

1 2c

Neat Neat

Neat PAA-g-LDHs (10 mg)

80 80

480 20

3 4c 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Neat Neat Neat Acetonitrile DMF THF Water Toluene Dioxane Benzene Neat Neat Neat Neat Neat Neat Neat Neat

PAA-g-LDHs (5 mg) PAA-g-LDHs (15 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA LDHs LDHs-BTPT PSPTSA PTSA Proline InCl3 FeCl3

80 80 100 80 120 80 100 100 80 80 80 80 80 80 80 80 80 80

20 65 20 55 85 85 110 75 70 125 80 60 60 60 120 86 110 90

25 92, 91, 90, 89 92 70 92 75 79 65 45 65 80 74 70 80 77 80 65 74 75 70

a

Reaction 1H-benzo[d]imidazol-2-amine (1, 1 mmol) and (E)-3-(4-isopropylphenyl)-1 phenylprop-2-en-1-one (2a, 1 mmol). Isolated yield. c Catalyst was reused four times. Source: Adapted from Reddy, M.V., Reddy, G.C.S., Lien, N.T.K., Kim, D.W., Jeong, Y.T., 2017. An efficient and green synthesis of benzo [4, 5] imidazo [1, 2-a] pyrimidines using highly active and stable poly acrylic acidsupported layered double hydroxides. Tetrahedron 73 (10), 1317
1323. With kind permission of Elsevier. b

Figure 20.8 Synthesis of chromene-incorporated dihydroquinoline derivatives (4a-z, 4a0 -d0 ). Source: Adapted from Reddy, M.V., Lien, N.T.K., Reddy, G.C.S., Lim, K.T., Jeong, Y.T., 2016. Polymer grafted layered double hydroxides (LDHs-g-POEGMA): a highly efficient reusable solid catalyst for the synthesis of chromene incorporated dihydroquinoline derivatives under solvent-free conditions. Green Chem. 18 (15), 4228
4239. With kind permission of RSC.

Layered double hydroxide polymer nanocomposites for catalysis

819

other existing catalysts for catalyzing the multicomponent mixture of 4-hydroxy2H-chromen-2-one (1), 4-methoxybenzenamine (2a), and 2-methylbenzaldehyde (3a) as a typical reaction. In addition, to achieve the optimal setting, the performance of LDH-g-POEGMA was considered in different conditions of temperature, time, solvent, and the amount of catalyst (Table 20.2). Based on overall results, the LDH-g-POEGMA showed the greatest potential as the best intended chromene-integrated dihydroquinoline with excellent yield and no Table 20.2 Optimization of reaction conditions for the synthesis of 4aa

Entry

Solvent

1 2

Neat Neat

3

Neat

c

Neat

5

Neat

6

Neat

7

Neat

8

Ethanol

9

THF

10

Toluene

11

DMF

12

DCM

13 14 15 16

Ethanol Ethanol DMF CH3CN

4

Catalyst (%)

Neat LDHs-g-POEGMA (2 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (8 mg) LDHs-g-POEGMA (3 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (5 mg) PTSA (10 mol%) Cu(OTf)3 (10 mol%) Zn(OTf)3 (10 mol%) InCl3 (10 mol%)

Temperature ( C)

Yield (%)b

Time (min) 4a

5a

6a

RT RT

300 60


70

92 10





RT

60

73

10




60

10







60

10

95, 94, 92, 90, 89 95







60

25

83







80

15

95







60

45

65




20

60

60

55

30




60

65

53




41

60

48

69




22

60

60

55

40




80 80 120 80

30 300 240 240


30 35 29

85





52 55 51

(Continued)

820

Layered Double Hydroxide Polymer Nanocomposites

Table 20.2 (Continued) Entry

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Solvent

H2O Neat H2O DMF Ethanol CH3CN Water Neat Ethanol Neat CH3CN Ethanol Neat Neat Neat Neat Neat Neat

Catalyst (%)

FeCl3 (10 mol%) H3BO3 (10 mol%) Cu(OAc)2 (10 mol%) DBA (10 mol%) TMG (10 mol%) L-proline (10 mol%) L-proline (10 mol%) DBA (10 mol%) DBA (10 mol%) TMG (10 mol%) TMG (10 mol%) Et3N (10 mol%) PS/PTSA (10 mg) PS/ AlCl3 (10 mg) PS/ GaCl3 (10 mg) POEGMA (10 mg) LDH (10 mg) POEGMA 1 LDH (10 mg)

Temperature ( C) 70 60 100 120 80 80 100 60 80 60 80 80 60 60 60 60 60 60

Yield (%)b

Time (min) 180 300 180 240 300 240 300 240 300 255 240 270 90 88 100 120 55

4a

5a

6a

28 33 37 25 20 32 15 22 20 21 22 23 62 58 63 25 55 60




35 50 60
62 45 18 23 55 25 32 30 65
30

43 57 42 20 22
70
18 40 35 10



30


a

Reaction of 4-hydroxy-2H-chromen-2-one1 (1, 1 mmol), 4-methoxybenzenamine (2a, 1 mmol), and 2methylbenzaldehyde (3a, 1 mmol). Isolated yield. c Catalyst was reused five times. Adapted from Reddy, M.V., Lien, N.T.K., Reddy, G.C.S., Lim, K.T., Jeong, Y.T., 2016. Polymer grafted layered double hydroxides (LDHs-g-POEGMA): a highly efficient reusable solid catalyst for the synthesis of chromene incorporated dihydroquinoline derivatives under solvent-free conditions. Green Chem. 18 (15), 4228
4239. With kind permission of RSC. b

lateral product in a shorter reaction time. Also, catalyst stability, as an important factor for use in industrial applications, was considered on a model reaction (Table 20.2, entry 4). The results indicated that LDH-g-POEGMA can be reused for several sequential cycles without significantly reducing catalytic activity. The mechanism study revealed that the LDH-g-POEGMA with an active surface and inner core has an important role in the organization of enamine intermediate (Fig. 20.9). The LDH-g-POEGMA catalyzes the reactions through interactive, chelating, and binding with reacting component. Being environmentally friendly, with an easy work-up procedure, simplicity in catalyst separation from the reaction mixture, high product yields, and shorter reaction time are advantages of LDH-gPOEGMA as an inexpensive catalyst. In another study, the prepared LDHs-g-POEGMA was used as an efficient catalyst for synthesis of benzo[g]chromene-5,10-diones (4a-aa) by multicomponent reaction of a 2-hydroxy-1,4-naphthoquinone, different aldehydes, and (E)-N-methyl1-(methylthio)-2-nitroethenamine (Fig. 20.10) (Krishnammagari et al., 2017).

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Figure 20.9 Schematic illustration of LDHs-g-POEGMA catalyzed synthesis of titled compounds (4a-z, 4a0 -d0 ). Source: Adapted from Reddy, M.V., Lien, N.T.K., Reddy, G.C.S., Lim, K.T., Jeong, Y.T., 2016. Polymer grafted layered double hydroxides (LDHs-g-POEGMA): a highly efficient reusable solid catalyst for the synthesis of chromene incorporated dihydroquinoline derivatives under solvent-free conditions. Green Chem. 18(15), 4228
4239. With kind permission of RSC.

To reach the optimum conditions in order to obtaining the highest yield, a model reaction was investigated under different conditions in terms of temperature, time, solvent, and catalyst consumption. The best result was achieved when the model

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Figure 20.10 Synthesis of benzo[g]chromene-5,10-diones catalyzed by LDHs-g-POEGMA in solvent-free conditions. Source: Adapted from Krishnammagari, S.K., Lee, S.M., Jeong, Y.T., 2017. Solvent-free synthesis of 4H-pyranonaphthoquinones using highly active and stable polymer-grafted layered double hydroxides (LDHs-g-POEGMA) as an efficient and reusable heterogeneous catalyst. Res. Chem. Intermed. 1
17. doi: 10.1007/s1116. With kind permission of Springer.

reaction was run with 10 mg of the catalyst at 80 C within 20 min under solventfree conditions. Also, in comparison with another applied catalysts, LDHs-gPOEGMA gives the highest product yields in the shortest time possible. Recycling studies revealed that the catalyst could be reused for four consecutive runs without loss of activity. According to mechanism sequences, the LDHs-g-POEGMA with appropriate functional groups activates the intermediates for subsequent reactions (Fig. 20.11). The alkaline direct ethanol fuel cells (DEFCs) due to high energy density, availability, and eco-friendly features are applied broadly in energy storage applications. Anion exchange membrane (AEM) has an important role in the performance of DEFCs. As a result, the design and assembly of AEMs with high heat resistance and high ion conductivity is particularly important for the development of fuel cells. The studies showed that PVA can be used as an AEM and its properties are improved by loading the qualified inorganic additives. Hence, the crosslinked PVA/ MgAl-LDH membranes were fabricated through a solution-casting system (Zeng et al., 2012). The extracted data from analyses indicated that among the different percentages of membranes, the PVA/LDH 20 wt.% showed low ethanol permeability and high ionic conductivity. The penetration of PVA within the interlayer galleries of LDHs makes a meandering route for transmission of ethanol and, naturally, permeability will be promoted with increasing values of LDHs. Considering the hydrophilic nature of the LDHs, the adsorbed water enhanced with increasing quantity of loaded LDHs, which will improve the ionic conductivity based on the Grotthuss mechanism. The cell performance of commercial A301 and PVA/LDH 20 wt.% membrane was compared in the same conditions at variable temperatures (Fig. 20.12). The graph clearly showed that synthesized membrane was best at both 60 C and 80 C. The A301 membrane is certainly destroyed at temperatures above 60 C, while the PVA/ LDH hybrid is completely stable and increases the power density. The smart polymers are determined to be stimuli-responsive materials that answer to slight external physical or chemical alterations such as pH, light, biological molecules, humidity, electric or magnetic field, and temperature. Their unique properties make them mainly suitable for drug delivery, bioseparation, and sensor applications. Unpleasant sensitivity, long response time, and insignificant stability restrict their applications (Ghizal et al., 2014; Aguilar et al., 2007). According to

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Figure 20.11 Schematic mechanism for the catalytic activity of LDHs-g-POEGMA in the synthesis of title compounds (4a
aa). Source: Adapted from Krishnammagari, S.K., Lee, S.M., Jeong, Y.T., 2017. Solvent-free synthesis of 4H-pyranonaphthoquinones using highly active and stable polymer-grafted layered double hydroxides (LDHs-g-POEGMA) as an efficient and reusable heterogeneous catalyst. Res. Chem. Intermed. 1
17. doi: 10.1007/s1116. With kind permission of Springer.

recent studies, the incorporation of inorganic NPs within smart polymers is an ideal approach to the development of their properties like flexibility and reversibility. LDH NPs with special layered structures have been excellently applied in electrochemical, biology, and optical fields. These suggest that LDH NPs are a good candidate for combination with smart polymers in order to build electrochemical sensors. The temperature-responsive poly(N-isopropyl acrylamide) (PNIPAA)/ CoAl-LDH ultrathin films (UTFs) were employed as switchable electrocatalysts (Dou et al., 2012). The PNIPAA/LDH UTFs were produced by an LBL assembly procedure based on periodic deposition of PNIPAA and LDH NPs which exhibit the reversible on
off property by moderating the temperature between 20 C and 40 C. The XRD and scanning electron microscopy (SEM) analysis indicated that LDH nanoplates in the form of C were oriented on the substrate plane (Fig. 20.13). Also, the PNIPAA/LDH UTFs showed a monotonous layered arrangement with a thickness of about 132 nm on quartz substrate. The atomic force microscopy indicated a decline of about 43 nm in root-mean-square when the temperature increased

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Figure 20.12 Polarization and power density curves of AEM DEFC employing PVA/20LDH composite polymer membrane and commercial A301 at different temperatures. Closed symbols represent cell voltage, open symbols represent power density (anode: 3 M ethanol 1 1 M KOH, 1 mL/min; cathode: dry oxygen, 100 sccm). Source: Adapted from Zeng, L., Zhao, T.S., Li, Y.S., 2012. Synthesis and characterization of crosslinked poly (vinyl alcohol)/layered double hydroxide composite polymer membranes for alkaline direct ethanol fuel cells. Intern. J. Hydr. Energy 37 (23), 18425
18432. With kind permission of Elsevier.

from 20 C to 40 C, and then showed an increase in the previous value with cooling temperature to 20 C. Also, the ellipsometry evaluations indicated that the thickness of UTFs was approximately 136 nm at 20 C and then was reduced to almost 66 nm by increasing the temperature to 40 C. These processes were successfully repeated for several consecutives, indicating excellent reversibility for surface topography and thickness. The cyclic voltammetry and electrochemical impedance spectroscopy were applied to check the electrochemical on
off behavior of the PNIPAA/LDHmodified indium tin oxide (ITO) electrodes at 20 C and 40 C. The results revealed fine electrochemical on
off performance at both temperatures due to the fast
slow interfacial charge conduction brought by temperature-adjusted shape conversion of PNIPAA. Finally, the (PNIPAA/LDH)10/ITO electrodes demonstrated good temperature-responsive performance for electrocatalytic oxidation of glucose as a typical reaction (Fig. 20.14). The electrocatalytic current and sensitivity of electrodes were increased at 40 C, which is related to high electric communication between the shrunk polymer and LDH layers. The long-term service and nice stability of electrodes are attributed to well-organized assembly of PNIPAA into the qualified LDH.

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Figure 20.13 (A) XRD pattern. (B, C) Top view of the SEM image with (B) low magnification and (C) high magnification. (D) Side view of the SEM image for the (LDH/ PNIPAA)10 UTF on an ITO substrate. Source: Adapted from Dou, Y., Han, J., Wang, T., Wei, M., Evans, D.G., Duan, X., 2012. Temperature-controlled electrochemical switch based on layered double hydroxide/poly (N-isopropylacrylamide) ultrathin films fabricated via layer-by-layer assembly. Langmuir 28 (25), 9535
9542. With kind permission of ACS.

Figure 20.14 (A) Current 2 time curves measured at 0.5 V for the (LDH/pNIPAM)10/ITO electrode with successive addition of glucose in 0.1 M NaOH at 20 C and 40 C. (B) Calibration curves at 20 C and 40 C. Source: Adapted from Dou, Y., Han, J., Wang, T., Wei, M., Evans, D.G., Duan, X., 2012. Temperature-controlled electrochemical switch based on layered double hydroxide/poly (N-isopropylacrylamide) ultrathin films fabricated via layer-by-layer assembly. Langmuir 28 (25), 9535
9542. With kind permission of ACS.

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Fuel cells are very hopeful options for clean energy conversion in the search for replacing the usual combustion-based technologies. The oxygen reduction reaction (ORR) is the most significant reaction for generating clean electrical energy in fuel cells. The development of stable, active, and inexpensive electrocatalysts for ORR in fuel cells is a substantial challenge. Recently, the LDHs due to the unique structure and versatile composition have shown significant potential in the field of electrocatalysis. LDHs with positive surface charges provide suitable conditions for oxygen uptake and subsequent ORR (Huo et al., 2014; Indra et al., 2014). Accordingly, in a research paper, the hybrids of polydopamine spheres (PDAS) with CoFe-LDHs were fabricated and their catalytic activity toward the ORR was examined (Zhang et al., 2015). The

Figure 20.15 SEM images of (A) PDAS and (B) CoFe-LDHs/PDAS. (C) TEM images of CoFe-LDHs/PDAS. (D) XRD patterns of (a) CoFe-LDHs/PDAS and (b) pure CoFe-LDHs. (E) SEM image of pure CoFe-LDHs. Source: Adapted from Zhang, X., Wang, Y., Dong, S., Li, M., 2015. Dual-site polydopamine spheres/CoFe layered double hydroxides for electrocatalytic oxygen reduction reaction. Electrochim. Acta 170, 248
255. With kind permission of Elsevier.

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Figure 20.16 (A) CVs of (a) PDAS, (b) HT-PDAS, (c) CoFe-DHs, and (d) CoFe-LDHs/ PDAS in an O2- or N2-saturated 0.1 M KOH solution at a scan rate of 50 mV/s. (B) Linearsweep voltammetry (LSV) curves of PDAS, HT-PDAS, CoFe-LDHs, and CoFe-LDHs/PDAS in 0.1 M KOH solution at a scan rate of 10 mV/s at 1600 rpm. Source: Adapted from Zhang, X., Wang, Y., Dong, S., Li, M., 2015. Dual-site polydopamine spheres/CoFe layered double hydroxides for electrocatalytic oxygen reduction reaction. Electrochim. Acta 170, 248
255. With kind permission of Elsevier.

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Figure 20.17 Chronoamperometric responses of (a) CoFe-LDHs/PDAS and (b) 5% Pt/C in an O2-saturated 0.1 M KOH solution at
0.3 V. Source: Adapted from Zhang, X., Wang, Y., Dong, S., Li, M., 2015. Dual-site polydopamine spheres/CoFe layered double hydroxides for electrocatalytic oxygen reduction reaction. Electrochim. Acta 170, 248
255. With kind permission of Elsevier.

CoFe-LDHs/PDAS hybrids were synthesized by in situ growing CoFe-LDHs on the surface of PDAS. PDAS with nitrogen and oxygen functional groups provides ideal conditions for chelation of metal ions and subsequently construction of ORR catalyst. The SEM and TEM images and XRD analysis confirmed that the surface of PDAS has been covered by CoFe-LDHs (Fig. 20.15). The SEM images indicated that the smooth surface of PDAS has turned into a rough and dark surface after adsorption of the CoFe-LDHs. Furthermore, the TEM images confirmed the composite structure of CoFe-LDHs/PDAS hybrids and showed that the thickness of coated CoFe-LDHs is about 20 nm. The series peaks of LDHs are easily visible in the XRD analysis of prepared hybrid, which indicates the presence of CoFe-LDHs. The electrocatalytic activity of PDAS, CoFe-LDHs, CoFe-LDHs/PDAS hybrids, and PDAS treated hydrothermally (HT-PDAS) for fair comparison was evaluated by cyclic voltammetry (CV) (Fig. 20.16). The results indicated that among the samples, the CoFe-LDHs/PDAS hybrid showed the most positive onset potential and highest cathodic current. This may be related to the synergistic effect of PDAS and CoFe-LDHs. Linear-sweep voltammetry at rotating disk electrode was also studied for the assessment of ORR catalytic activity of PDAS, CoFe-LDHs, HT-PDAS, and CoFe-LDHs/PDAS. As shown in Fig. 20.16B, the CoFe-LDHs/PDAS hybrid showed the highest catalytic activity among the considered samples, which was absolutely compatible with the CV results. The durability of CoFe-LDHs/PDAS hybrid was compared with commercial 5% Pt/C ORR catalysts by chronoamperometry. As shown in Fig. 20.17, the current of CoFe-LDHs/PDAS hybrid is almost steady over 10,000 s of the test, while the current of Pt/C ORR catalysts is accompanied by a noticeable drop, indicating that the CoFe-LDHs/PDAS hybrid has a more favorable stability than 5% Pt/C. Due to the great importance of polymer/LDH NCs in improving catalytic activity in various fields, the design and construction of this type of catalyst, with different

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types of materials, should be studied more and further researches need to be done in this area.

20.5 Conclusions Today, technology is aimed at catalysts, and LDHs with layered and flexible compositions can be used effectively in this direction. The high potential of LDH for surface adsorption of diverse metal ions, as well as the integration of functionalized anions between LDH layers, generates an active basic surface that makes it suitable for redox reactions. Characteristics like high adsorption capacity, high thermal stability, and anchoring effect, create promising LDHs for adsorption of CO, CO2, SO2, and NOx. The combination of polymers with LDHs is a good method in order to develop their catalytic performances. In general, the dispersion index of LDH in the polymer background directly affects the structure and properties of the resultant NCs. Hence, the development of methods for the preparation of consistent polymer/LDH NCs is essential. In situ LDH synthesis in the polymer solution and in situ polymerization are good policies for improving the interfacial relations between LDHs and host polymer. Solution blending is accompanied by defects such as aggregation and melt mixing which is introduced as a traditional technique for the formation of PNCs on an industrial scale. LDHs not only improve the performances of polymers by promoting mechanical, optical, electrical, and thermal properties but also enhance the catalytic efficiency by a synergistic effect and increase the active sites. The obtained results from applications of polymer/LDH hybrids in important fields, such as the synthesis of organic molecules, fuel cells, and sensors, indicate that they can be used as effective catalysts due to high chemical and thermal resistance and good interfacial relations. The mild conditions, low cost, high yield, short time, simplicity, and sequential reusability are advantages of polymer/LDH catalysts.

Acknowledgments The research group acknowledges the Research Affairs Division of Isfahan University of Technology (IUT), Isfahan, I. R. Iran. Thanks are also given to National Elite Foundation (NEF), Tehran, I. R. Iran, Iran Nanotechnology Initiative Council (INIC), Tehran, I. R. Iran, and Center of Excellence in Sensors and Green Chemistry Research (IUT), Isfahan, I. R. Iran.

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51. Perreira, A.C., Pearson, S., Kostadinova, D., Leroux, F., D’Agosto, F., Lansalot, M., et al., 2017. Nanocomposite latexes containing layered double hydroxides via RAFT-assisted encapsulating emulsion polymerization. Polym. Chem. 8 (7), 1233
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4239. Reddy, M.V., Reddy, G.C.S., Lien, N.T.K., Kim, D.W., Jeong, Y.T., 2017. An efficient and green synthesis of benzo [4, 5] imidazo [1, 2-a] pyrimidines using highly active and stable poly acrylic acid-supported layered double hydroxides. Tetrahedron 73 (10), 1317
1323. Robinson, A.M., Hensley, J.E., Medlin, J.W., 2016. Bifunctional catalysts for upgrading of biomass-derived oxygenates: a review. ACS Catal. 6 (8), 5026
5043. Sahoo, M., Parida, K.M., 2013. Pd (II) loaded on diamine functionalized LDH for oxidation of primary alcohol using water as solvent. Appl. Catal. A: Gen. 460, 36
45. Shu, Y., Yin, P., Liang, B., Wang, H., Guo, L., 2015. Artificial nacre-like gold nanoparticles
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4369.

Index

Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively. A AA. See Acrylic acid (AA) ABS. See Acrylonitrile-butadiene-styrene (ABS) ABTS. See 2,2’-Azinobis-3ethylbenzothiazoline-6-sulfonate (ABTS) Acid red 97 (AC97), 171
172 Acid yellow, 785 Acid-modified corn starch (AMS), 612
614 Acrylic acid (AA), 472
474 Acrylonitrile butadiene rubber (NBR), 347
349 Acrylonitrile-butadiene-styrene (ABS), 315 Active molecules, 748
753, 749t, 757f, 761 Adsorption, 485
486, 781 AE. See Agronomic efficiency (AE) AEM. See Anion exchange membrane (AEM) AEM DEFCs. See Anion exchange membrane-directed ethanol fuel cells (AEM DEFCs) AEMFCs. See Anion-exchanged membrane fuel cells (AEMFCs) AFM. See Atomic force microscopy (AFM) Aging process, spectroscopic characterization for, 270
272 Agriculture, LDH nanocomposites for, 54
55, 715, 719
733 Agrochemical(s), 716
717 Agronomic efficiency (AE), 723 AH. See Aluminum hydroxide (AH) 27 Al MAS NMR spectrum of LDH-AB, 260, 260f Alginate (ALG), 588
591, 695, 793
795 alginate/LDH nanocomposites, 624
627, 695
698

alginate
zein bionanocomposite beads, 697
698 polymer chains, 696f Alpha rays, 206 α-relaxation, 253
254 Aluminum hydroxide (AH), 312 4-Amino salicylic acid-zinc
layered hydroxide (4-ASA-ZLDH), 687
688 2-Aminoethanesulfonic acid. See Taurine 4-Aminophenol (4-AP), 664, 815
816 5-Aminosalicylic acid (5ASA), 117, 120f, 699
700, 700f AMLR. See Average mass loss rate (AMLR) Ammonium polyphosphate (APP), 320
322, 330
331, 334
335, 483 AMS. See Acid-modified corn starch (AMS) Amylopectin, 612
614 Amylose, 612
614 Angular position, 209 Anion exchange membrane (AEM), 582
585, 822 Anion exchange membrane-directed ethanol fuel cells (AEM DEFCs), 582
585 Anion-exchanged membrane fuel cells (AEMFCs), 582
585 Anion(ic) clays, 461
462, 719 coupling agents, 231
232 exchange, 6
7 reaction, 12 exchange, 105 organic surfactants, 782 surfactants, 319 Annealing temperature, 422 Anti corrosion materials, 55 Antimicrobial tests, 760
761 4-AP. See 4-Aminophenol (4-AP)

836

APP. See Ammonium polyphosphate (APP) APPP. See Poly(p-phenylene) anionic derivate (APPP) APTS. See γ-aminopropyltriethoxysilane (APTS) Aqueous miscible organic solvent treatment, 338 Arsenic (As), 783 4-ASA-ZLDH. See 4-Amino salicylic acidzinc
layered hydroxide (4-ASAZLDH) 5ASA. See 5-Aminosalicylic acid (5ASA) Aspartic acid, 90
91, 91f aspartic acid-modified Li
Al LDH, 90 Assembly of LDH/CP nanocomposite, 497
515 Atom transfer radical polymerization (ATRP), 47, 472
474 Atomic force microscopy (AFM), 136
137, 138f, 158, 164
165, 465, 500, 538 analysis, 40
41 ATRP. See Atom transfer radical polymerization (ATRP) Average mass loss rate (AMLR), 315 2,2’-Azinobis-3-ethylbenzothiazoline-6sulfonate (ABTS), 587 B BA. See Butyl acrylate (BA) Barrier properties of composites, 764, 764t Basal reflections, 212 spacing, 213, 215, 215t Base material, 217 Batteries, 438
439, 574
579 application of LDH nanocomposites in, 575
577 application of LDH/polymer in, 577
579 BCZC. See 2-Hydroxy benzo[a]carbazole-3carboxylate (BCZC) Benzo[4,5]imidazo[1,2-α]pyrimidines (BIPs), 816
817 Benzoate (Bz), 628
629, 760
761, 764 Benzoyl peroxide (BPO), 469 Beta rays, 206 β-relaxation, 254 BHET. See Bis-hydroxy ethylene terephthalate (BHET) Bingham plastic models, 304

Index

Bio-based materials, developing, 600
601 Biobased silicone modifier, 183
186 Biodistribution, 681
684 Bioimaging applications, 693
694, 701
702 Biomacromolecules, 499
500 Biomedical applications, 32
35, 44
48 Bionanocomposite (BNC), 134, 178
181, 614
616, 633
635, 646 Bioplastics nanocomposites from fossil-based resources, 760
764 from renewable sources, 764
766 Biopolymers, 600
601 PHA/layered double hydroxide nanocomposites, 633
642 PLA/layered double hydroxide nanocomposites, 643
653 polysaccharide/layered double hydroxide nanocomposites, 604
632 protein/layered double hydroxide nanocomposites, 632
633 PVA/layered double hydroxide nanocomposites, 653
664 Biosensor, 694 4-Biphenyl acetic acid (Bph), 647 BIPs. See Benzo[4,5]imidazo[1,2-α] pyrimidines (BIPs) Bis-hydroxy ethylene terephthalate (BHET), 259
260 Bis(2-sulfonatostyryl)biphenyl (BSB), 251 Bis(N-methylacridinium) (BNMA), 504, 505f Block copolymer, 505
507 BMA. See Butyl methacrylate (BMA) BNC. See Bionanocomposite (BNC) BNMA. See Bis(N-methylacridinium) (BNMA) Bone tissue engineering (BTE), 709
710 Bordeaux Mixture, 716 Bovine serum albumin (BSA), 604
605 Bph. See 4-Biphenyl acetic acid (Bph) BPO. See Benzoyl peroxide (BPO) Bragg diffraction, 165, 208 Bragg’s law, 207
208 Brucite, 210 BSA. See Bovine serum albumin (BSA) BSB. See Bis(2-sulfonatostyryl)biphenyl (BSB) BTE. See Bone tissue engineering (BTE)

Index

BTPT. See S-(3-Trimethoxysilyl) propyltrithiocarbonate (BTPT) Butyl acrylate (BA), 124
125 Butyl methacrylate (BMA), 470
471 Bz. See Benzoate (Bz) BzDC. See 2,4-Dichlorobenzoate (BzDC) c-methacryloxypropyltrimethoxysilane (MPTS), 174
175 C CA. See Cellulose acetate (CA); Citric acid (CA) Cadmium (Cd), 783 Calcined hydrotalcite (CHT), 300 Calcined LDHs (CLDH), 786
787 Calcium phosphate cement (CPC), 707
708 Caldimonas manganoxidans, 633
636 Camphorsulfonic acid (CSA), 170
171, 813
814 Carbon dots, 546 fibres, 25 (nano)fibers/LDH nanocomposites, 432
436 nanorings/LDH, 433
434 Carbon cloth (CC), 535
536 Carbon nano-onions (CNOs), 433
434 Carbon nanofibers (CNFs), 25, 550
551 Carbon nanoforms (CNFs), LDH nanocomposites based on, 411
415, 431
436 applications, 436
448, 437t carbon (nano)fibers/LDH nanocomposites, 432
436 carbon spheres/LDH nanocomposites, 431
432 CNTs/LDH nanocomposites, 425
430 CQDs/LDH nanocomposites, 431 fullerene/LDH nanocomposites, 431 graphene and GO/LDH nanocomposites, 415
425 graphene/single-walled CNT/LDH nanocomposites, 436 nanocomposites between LDH and CNFs, 415f properties, 414t Carbon nanotubes (CNTs), 1
2, 25, 157
158, 330
331, 411
412, 531, 569
571, 781, 787
788, 812

837

CNTs/LDH nanocomposites, 425
430 1D, 411
412 synthesis, 427
430 Carbon quantum dots (CQDs), 431, 541, 787
788 CQD/NiFe-LDH nanoplate hybrid, 546
548 CQDs/LDH nanocomposites, 431 Carbon sphere (CS), 542 carbon spheres/LDH nanocomposites, 431
432 Carbonaceous materials, 439
440 Carbonaceous nanomaterials, 538
541, 543 Carbonate anions, 461
462 Carboxylated acrylonitrile
butadiene rubber composites (XNBR composites), 166 2-Carboxylethyl-phenyl-phosphinic acid (CEPPA), 327
329, 650
653 Carboxymethyl cellulose (CMC), 115, 264
265, 700
701. See also Alginate (ALG) carboxymethyl cellulose/LDH-cephalexin NC beads, 611 CMC
layered double hydroxide nanocomposite, 700
701 Carboxymethylcellulose/layered double hydroxide nanocomposites (CMC/ LDH NCs), 115
116 Carrageenan/LDH NCs, 628
629 Carreau model, 305 Casson model, 304 Catalysis, 25, 486
487 applications of polymer/LDH nanocomposites in, 815
829 CNF/LDH nanocomposites in, 443
445 LDHs applications in, 807
811 polymer/layered double hydroxide nanocomposites, 811
815 Catalysts, 546
548, 805
806 Cathode rays, 206 CC. See Carbon cloth (CC) CCD camera. See Charge-coupled device camera (CCD camera) CCSs. See Colloidal carbonaceous spheres (CCSs) CD. See Circular dichroism (CD) Cellular uptake mechanism, 681
684, 681f Cellulose acetate (CA), 765

838

Cellulose/layered double hydroxide nanocomposites, 604
611 Cephalexin, 611 CEPPA. See 2-Carboxylethyl-phenylphosphinic acid (CEPPA) Charge-coupled device camera (CCD camera), 160
161 Chemical compounds, 717 Chemical vapor deposition (CVD), 413 Chemical use in agriculture history and evolution of, 716
717 Chiral diacid intercalated LDH, 135
136 Chiral organic modifier, 94f Chitosan (CTS), 698
699, 765 chitosan/LDH NCs, 117
118, 619
624 chitosan/MgAl-LDH composite, 795
796 chitosan
layered double hydroxide nanocomposites, 698
709 in drug-delivery applications, 699
700 functionalized, 701
702 in PDT, 705
707 in tissue engineering applications, 707
709 Chloride-containing polymers, 323 Chromium (Cr), 783 CHT. See Calcined hydrotalcite (CHT); Conventional hydrothermal treatment (CHT) Ciprofloxacin, 170
171 Circular dichroism (CD), 500 Citric acid (CA), 655
658 Clatharin-mediated endocytosis, 683, 686 Clay(s), 482
483 exfoliation, 461
462 particles, 461
462 polymer nanocomposites, 781
782 CLDH. See Calcined LDHs (CLDH) CMC. See Carboxymethyl cellulose (CMC) CMC/LDH NCs. See Carboxymethylcellulose/layered double hydroxide nanocomposites (CMC/LDH NCs) CNFs. See Carbon nanofibers (CNFs); Carbon nanoforms (CNFs) CNOs. See Carbon nano-onions (CNOs) CNTs. See Carbon nanotubes (CNTs) Co-precipitation, 5, 532 method, 627
628 one step, 11

Index

synthesis, 536
537 “Coprecipitation” versus the “reconstruction” method, 627
628 Coassembly method, 502
504 Cocoamphodipropionate (K2), 123 CoFe-LDHs/PDAS hybrids, 587, 826
828 Cole
Cole plots, 291
292 Colloidal carbonaceous spheres (CCSs), 545
546 Colloidal crystal template array, 480
481 Composite materials, 205 CONE. See Cone calorimeter (CONE) Cone calorimeter (CONE), 313 CoNi LDH nanoflakes, 545
546 Conjugated polymers (CPs), 497
498 Controlled/living radical polymerization (CRLP). See Reversible deactivation radical polymerization (RDRP) Conventional emulsion polymerization, 468
471 Conventional hydrothermal treatment (CHT), 360 Copper (Cu), 783 Core
shell nanocomposites, 686 quantum dots, 692
693 CPC. See Calcium phosphate cement (CPC) CPs. See Conjugated polymers (CPs) CQDs. See Carbon quantum dots (CQDs) CS. See Carbon sphere (CS) CSA. See Camphorsulfonic acid (CSA) CTS. See Chitosan (CTS) CV. See Cyclic voltammogram (CV) CVD. See Chemical vapor deposition (CVD) Cyclic voltammogram (CV), 575
576, 828 D D-/L-lactic acid, 643 D-LDH. See Decavanadate
intercalated LDH (D-LDH) DABCO. See 1,4-Diazabicyclo[2.2.2]octane (DABCO) Data acquisition in X-ray diffractometer, 209 DBS. See Dodecyl benzene sulfonate (DBS) DDA. See Dodecanedioic acid (DDA) DDSs. See Drug-delivery systems (DDSs) Debye
Scherrer equation, 208 Decavanadate
intercalated LDH (D-LDH), 267
270

Index

DEFCs. See Direct ethanol fuel cells (DEFCs) Delamination, 219 delamination/restacking method, 13
14 Derjaguin, Landau, Verwey, and Overbeek theory (DLVO theory), 468 Detectors, 208
209 DF sodium. See Diclofenac sodium (DF sodium) DGEBA. See Diglycidyl ether of bisphenol A (DGEBA) 3-(3,5-Di-tert-butyl-4-hydroxyphenyl) propionic acid, 650
653 Diacid, 655 Diacid-diimide modified Mg-Al LDH (DLDH), 263
264 1,4-Diazabicyclo[2.2.2]octane (DABCO), 582
585 Dicamba, 731
732 2,4-Dichlorobenzoate (BzDC), 628
629, 760
761, 764 2,4-Dichlorophenoxyacetic acid (2,4-D), 726
727, 788 Diclofenac sodium (DF sodium), 792
793 Dielectric spectroscopy, 232
234 of LDHs polymer NCs, 251
258 Diffraction theory, 207 Diffractometer, 209 Diffusion (D), 758, 759t Difluoromethane (CH2F2), 809 Diglycidyl ether of bisphenol A (DGEBA), 300 Dimethicones, 298
300 2-(Dimethylamino)-ethyl methacrylate (DMAEMA), 47 Direct ethanol fuel cells (DEFCs), 822 Direct mechanochemical synthesis, 105
106 Direct melt mixing, 813
814 Direct synthesis, 105 Dispersion centers, 207 Diverse mathematical correlations, 304 DLDH. See Diacid-diimide modified Mg-Al LDH (DLDH) DLVO theory. See Derjaguin, Landau, Verwey, and Overbeek theory (DLVO theory) DMAc. See N,N-dimethylacetamide (DMAc) DMAEMA. See 2-(Dimethylamino)-ethyl methacrylate (DMAEMA)

839

DMTA. See Dynamic mechanical thermal analysis (DMTA) Dodecanedioic acid (DDA), 236
239 Dodecyl benzene sulfonate (DBS), 213, 291
292, 327
328, 546 Dodecyl sulfate (DS), 140, 291
292, 319, 328, 469
471, 588
591, 786
787 Dodecyl sulfonate (DSO), 423
424 3-DOM. See Three-dimensionally ordered macroporous structures (3-DOM) Dot-coated LDH, 679
680 Double in-situ method, 39 Drug delivery chitosan
layered double hydroxide nanocomposites’ applications in, 699
700 CNF/LDH nanocomposites in, 447 Drug-delivery systems (DDSs), 680, 684
685 LDH-based nanocomposites in, 685
688 DS. See Dodecyl sulfate (DS) DSO. See Dodecyl sulfonate (DSO) Dye-sensitized solar cells (DSSC), 51 Dyes, 781
782 Dynamic mechanical thermal analysis (DMTA), 139
140 E ECM. See Extracellular matrix (ECM) Economic and Social Commission for Asia and Pacific (ESCAP), 718 EDLCs. See Electrical double-layer capacitors (EDLCs) EDX spectroscopy. See Energy-dispersive X-ray spectroscopy (EDX spectroscopy) Effective heat of combustion (EHC), 558 EIS. See Electrochemical impedance spectroscopy (EIS) Elastomer-LDH nanocomposites dynamical mechanical properties, 388
398 mechanical properties, 372 EPDM/LDH nanocomposites, 380
382 EVA/LDH nanocomposites, 377
379 NBR-LDH and XNBR/LDH nanocomposites, 382
383 PU-LDH nanocomposites, 372
376 SBR/LDH nanocomposites, 382

840

Elastomer-LDH nanocomposites (Continued) SR/LDH nanocomposites, 379
380 morphology EPDM/LDH nanocomposites, 363
364 EVA-LDH nanocomposites, 358
360 NBR-LDH and XNBR-LDH nanocomposites, 365
367 NR/LDH nanocomposites, 367 PU-LDH nanocomposites, 350
358 SBR/LDH nanocomposites, 364
365 SR-LDH nanocomposites, 361
363 Elastomeric blend-LDH nanocomposites dynamical mechanical properties, 388, 398
402 mechanical properties, 372 EVA blend-LDH nanocomposites, 386
387 PU blend-LDH nanocomposites, 383
385 morphology EVA blend-LDH and EPDM blendLDH nanocomposites, 369
372 PU blend-LDH nanocomposites, 367
369 Elastomers, 347
349 LDH fillers types in elastomer fabrication, 350, 351t in elastomeric blend nanocomposites fabrication, 350, 351t Electrical and electronic applications of LDH PNCs, 565, 587
591 batteries, 574
579 fuel cells, 579
587 SCs, 568
574 Electrical double-layer capacitors (EDLCs), 439
440, 568 Electrocatalyst, 822
824, 826
828 Electrocatalytic WS, 441 Electrochemical capacitors, 565 cell, 574
575 deposition, 6 properties of LDH macroporous structures, 485
488 signals, 694 supercapacitors, 439
440 Electrochemical impedance spectroscopy (EIS), 575
576

Index

Electrode materials, 568 for super capacitor, 30
31 Electrolyte membranes, 580
582 Electron-transfer rate, 567 Electronic microenvironment (EME), 504
505 Electrostatic interaction, 796 LbL assembly method based on, 500
507 LbL assembly method, 502 valency principle, 211 EME. See Electronic microenvironment (EME) Emulsion, 461
462 polymerization, 468
469 Energy applications, 50
51 CNF/LDH nanocomposites in energy storage and conversion, 436
443 batteries, 438
439 supercapacitors, 439
441 WS, 441
443 Energy-dispersive X-ray spectroscopy (EDX spectroscopy), 14
15, 232
234, 536
537 analyzer, 788 of LDHs polymer NCs, 243
246 Environment protection, CNF/LDH nanocomposites in, 445
447 Environmental remediation, 27
30 EPDM. See Ethylene propylene diene monomer (EPDM) Epoxides, 811 Epoxy resin (ER), 186
189, 552, 554 Epoxy resin/MoS2/layered double hydroxide nanocomposites (ER/MoS2/LDH NCs), 141
143, 142f EPS I, 629
632 ε-caprolactam, 550 ER. See Epoxy resin (ER) ESCAP. See Economic and Social Commission for Asia and Pacific (ESCAP) Escherichia coli, 120 Ethylene propylene diene monomer (EPDM), 245
246, 347
349

Index

EPDM blend-LDH nanocomposites dynamical mechanical properties, 401
402 morphology of, 369
372 EPDM/LDH nanocomposites dynamical mechanical properties, 395
397 mechanical properties, 380
382 TEM of, 364 XRD of, 363 Ethylene vinyl acetate (EVA), 139
140, 192
195, 266
267, 297, 313, 347
349, 813
814 EVA blend-LDH nanocomposites dynamical mechanical properties, 401
402 mechanical properties of, 386
387 morphology of, 369
372 EVA-LDH nanocomposites dynamical mechanical properties, 390
394 mechanical properties, 377
379 TEM of, 359
360 XRD of, 358 Ethylene vinyl alcohol, 743
746 EVA. See Ethylene vinyl acetate (EVA) Ewald sphere, 208 Ex vivo fluorescence image of rabbit ocular tissues, 702
705, 702f Exfoliated/exfoliation bio-NCs, 637
639 exfoliation-restacking synthesis, 537 exfoliation/adsorption, 206, 218 morphology, 223 nanocomposites, 219 Extracellular matrix (ECM), 191 Extraction process, 485
486 F Fabrication technologies of LDH PNCs, 497
515. See also Microscopic characterization for LDH PNCs future perspectives, 148
152 natural polymer/LDH NCs, 115
120 polymer/LDH NCs, 106
114 synthetic polymer/LDH NCs, 120
148 FE-SEM. See Field emission scanning electron microscopy (FE-SEM)

841

Fertilizers, 717
718 FGN. See Fluorinated graphene (FGN) Field emission scanning electron microscopy (FE-SEM), 122
123, 122f Field ion microscopes (FIMs), 158, 162 Filler, 217, 219, 805
806 FIMs. See Field ion microscopes (FIMs) Fingerprint, 209 Fire retardancy. See Flame retardancy Flame retardancy, 143, 323, 483
485, 555 applications, 23
24, 43
44 mechanism using LDH, 338
339 performance of LDH-based nanocomposites, 324
337 of polymers, 313
317 posttreatment of LDHs as, 337
338 Fluorescence resonance energy transfer (FRET), 519 of LDH/CP nanocomposites, 519
521 Fluorescence spectroscopy, 232
234 of LDHs polymer NCs, 247
251 Fluorinated graphene (FGN), 544 Fluorine doped tin oxide (FTO), 480 Fluorouracil (5-FU), 693 Food packaging applications, 51
52, 743 characterization and analytical techniques of PNCs, 747
748 LDHs as hosts of active molecules for potential in, 748
753 polymer materials used in, 744t PNCs based on LDH-active molecules, 754
766 regulation issues, 766
767 Food security, 715 Fossil-based resources, 760
764 Fourier transform infrared spectroscopy (FTIR spectroscopy), 77, 128, 232
233, 532
533. See also Nuclear magnetic resonance spectroscopy (NMR spectroscopy) absorption bands, 77 bands, 20t for LDHs, 79
87 characteristic absorption bands, 79
85 polymer NCs, 236
239 Freeze-drying, 110

842

FRET. See Fluorescence resonance energy transfer (FRET) FTIR spectroscopy. See Fourier transform infrared spectroscopy (FTIR spectroscopy) FTO. See Fluorine doped tin oxide (FTO) 5-FU. See Fluorouracil (5-FU) Fuel cells, 565, 579
587, 826
828 application of LDH/polymer in, 580
587 Full width at half maximum (FWHM), 208 Fullerene/LDH nanocomposites, 431 FWHM. See Full width at half maximum (FWHM) G GA. See Gibberellic acid (GA) Gamma rays, 206 γ-aminopropyltriethoxysilane (APTS), 183, 267
270 γ-poly(glutamic acid) (γ-PGA), 532
533 γ-polyglutamate modified LDH (γ-LDH), 643
646 Gas barrier materials, 53
54 sensing applications, 49
50 Gelatin (GEL), 709
710 gelatin/LDH, 632 gelatin/LDH-hydroxyapatite NC, 632 Gene-delivery applications, LDH-based nanocomposites in, 688
693 Gibberellic acid (GA), 729
730 Glass, in food packaging, 743 Glass transition temperature (Tg), 218 Glucose oxidase (GOx), 587 Glycylsarcosine (GS), 699 Glyphosate removal, 733 GNSs. See Graphene nanosheets (GNSs) GOx. See Glucose oxidase (GOx) GPPY. See Graphene/polypyrrole (GPPY) Granules, 612
614 Graphdiyne, 26
27 Graphene, 25, 411
412, 415
425, 531, 788 direct growth of LDH on, 420
423 formation in LDH layers, 423
425 graphene/single-walled CNT/LDH nanocomposites, 436 reassembly, 417
420 synthesis, 417
425

Index

Graphene nanosheets (GNSs), 554 Graphene oxide (GO), 27, 330, 412, 787
788 GO/LDH nanocomposites, 415
425 Graphene/polypyrrole (GPPY), 571
573 “Green chemistry” technologies, 733
734 Green Revolution, 717 GS. See Glycylsarcosine (GS) H H-LDH. See Hierarchical LDH (H-LDH) HA. See Hydroxyapatite (HA) Halogen-free flame-retardant (HFFR), 337 Halogenated flame retardants, 311
312 HDPE. See High-density polyethylene (HDPE) Heat release capacity (HRC), 314
315, 637
639 Heat release rate (HRR), 314
315, 659
661 Heavy metal cations, 783 HEBM. See High-energy ball milling (HEBM) HEK293T cells, 692 Helical CNTs. See Nanocoils Herbicide release, LDHs for storage and gradual of, 726
727 Herschel
Bulkley model, 304 HFFR. See Halogen-free flame-retardant (HFFR) HFMH. See Hyperfine magnesium hydroxide (HFMH) Hierarchical LDH (H-LDH), 765 Hierarchical Ni-Co LDH nanosheets, 573 High energy ball milling, 43
44 High-density polyethylene (HDPE), 813
814 High-energy ball milling (HEBM), 756 High-resolution transmission electron microscopic images (HRTEM), 464
465, 550
551, 788
790 Homogeneous polymer/LDH NCs, 811 HRC. See Heat release capacity (HRC) HRR. See Heat release rate (HRR) HRTEM. See High-resolution transmission electron microscopic images (HRTEM)

Index

HT-PDAS. See Hydrothermally polydopamine spheres (HT-PDAS) HTlc. See Hydrotalcite-like compounds (HTlc) Hydrogen bond interactions, LbL assembly method based on, 507
511 Hydrotalcite, 210, 461
462, 535, 602
603, 607
609 Hydrotalcite-like compounds (HTlc), 748
753 Hydrotalcite-like materials. See Layered double hydroxide (LDH) Hydrothermal approach, 420
421 Hydrothermal crystallization, 6 Hydrothermally polydopamine spheres (HTPDAS), 828 2-Hydroxy benzo[a]carbazole-3-carboxylate (BCZC), 248
249 6-Hydroxy-2,5,7,8-tetramethylchroman-2carboxylic acid, 650
653 2-Hydroxy-4-methoxybenzophenone-5sulfonic acid, 320 Hydroxyapatite (HA), 709
710 Hyperfine magnesium hydroxide (HFMH), 320
322, 358, 365 I Ibuprofen (Ibu), 647
648, 697 release from alginate
zein bionanocomposite beads, 697
698 ICG. See Indocyanine green (ICG) ICP-MS detection. See Inductively coupled plasma mass spectrometric detection (ICP-MS detection) IFR. See Intumescent flame retardant (IFR) In situ emulsion, LDH-based nanocomposites by, 468
475 In situ formation of polymer latexes, 463 In situ layered double hydroxide synthesis in polymer solution, 812 In situ nanoparticle synthesis, 206, 218 In situ polymerization method, 107, 109f, 206, 218, 468, 754, 754f, 812
813 In situ synthesis, 538 In-situ film growth, 6 In-situ LDH synthesis, 38 In-situ polymerization, 38 Incident waves, 207

843

Indium tin oxide (ITO), 480, 822
824 Indocyanine green (ICG), 46
47, 693 Inductively coupled plasma mass spectrometric detection (ICP-MS detection), 762
763 Infrared spectroscopy (IR spectroscopy), 77 “Innocent” metals, 424
425 Inorganic anions, 106, 783 Inorganic fillers, 347
349 Inorganic LDHs, 330. See also Organic LDHs effect on flame-retardant performance, 318 effect on thermal stability properties, 318 Inorganic NMs, 601
603 Inorganic NPs, 677 Inorganic reinforcements in polymer nanocomposites, 217 In
situ methods, 38
39 Integrated intensity, 209 Intensity distribution, 209 Intercalated nanocomposite, 219 Interlamellar anions, 4 Interlamellar dominion, 719 Interlayer domain in LDHs, 211 Interplanar basal distance, 619 Intumescent flame retardant (IFR), 334
335, 369
370, 555 Inverse opal method, 478
480 Ion-exchange technique, 10f, 532, 796 Ionomer, 792
793 iPP. See Isotactic polypropylene (iPP) iPP/LDH NCs. See Isotactic polypropylene/ layered double hydroxide nanocomposites (iPP/LDH NCs) IR spectroscopy. See Infrared spectroscopy (IR spectroscopy) IrganoxCOOH, 650
653 Iron oxide, 788 Isotactic polypropylene (iPP), 145
146 Isotactic polypropylene/layered double hydroxide nanocomposites (iPP/LDH NCs), 145
148 ITO. See Indium tin oxide (ITO) J Jasminaldehyde synthesis, 621
624 Jeffreys model, 302

844

K K2. See Cocoamphodipropionate (K2) Keratin, 632 KPS. See Potassium persulfate (KPS) L L-lactic acid-modified LDH (Lact-LDH), 661
664 LA. See Lauric acid (LA) Laccase (Lac), 587 Lact-LDH. See L-lactic acid-modified LDH (Lact-LDH) Lactate-intercalated LDHs, 498
499 Latex technology for LDH
based composite production, 464
481 assembly of preformed LDH and latex particles layer-by-layer assembly, 464
465 physical blending, 466
468 by in situ emulsion and suspension polymerizations, 468
475 Latex(es), 468
469 latex-templating approaches, 475
481 Laurate-modified Mg-Al LDH-C12, 762
763 Lauric acid (LA), 91, 91f, 319 Lauryl alcohol phosphoric acid ester potassium, 92 Lauryl phosphate (LP), 319 Layer charge densities (LCD), 248
249 Layer-by-layer assembly (LbL assembly), 39
41, 111
112, 112f, 420, 464
465, 498, 538, 814 based on electrostatic interaction, 500
507 based on hydrogen bond interactions, 507
511 based on miscellaneous interaction, 512
515 based on van der Waals forces, 511
512 Layered compounds, 231 Layered double hydroxide (LDH), 1
7, 103
104, 157, 205, 231, 284
285, 295
296, 312, 347
349, 411
412, 461
462, 531, 566
567, 601
603, 677
678, 678f, 690
691, 716, 781, 805
806 applications, 21
35, 36t in agriculture, 719
733

Index

in catalysis, 807
811 based polymer hybrid nanocomposites, 56
62 carbonate, 661
664 characterization, 14
20 differences with ordinary clays, 4 electrode for super capacitor, 30
31 fabrication routes to PNCs, 64t FTIR of, 20t, 79
87 as hosts of active molecules in foodpackaging applications, 748
753 LDH-amino benzoate, 260 LDH-based flame-retardant materials and applications, 323
339 flame retardancy mechanism using LDH, 338
339 flame retardant, 323 flame-retardant performance of LDHbased nanocomposites, 324
337 posttreatment of LDHs as flame retardants, 337
338 LDH-based NCs, 678
680, 679f bioimaging applications, 693
694 for biomedical applications, 695
710 biosensor, 694 in drug-delivery applications, 685
688 in gene-delivery applications, 688
693 in medical field, 680
685 for tissue engineering applications, 694
695 LDH-based thermal stabilizer materials and applications thermal stability properties of LDHbased nanocomposites, 318
322 thermal stabilizer, 317
318 thermostability mechanism using LDHs, 322
323 LDH-elastomer, preparative methods of, 349
350 LDH/carbonaceous nanofiller hybrids, 532, 535
536 applications, 538
548 synthesis, 536
538 LDH/CP nano platelet array, 51 LDH/CP nanocomposite, 498
499 fabrication and assembly of, 497
515 FRET of, 519
521 luminescence properties, 517
518 photocatalysis, 523
525

Index

photodetectors, 522
523 photostability of LDH/CP nanocomposites, 515
517 LDH/elastomeric blend nanocomposites, 347
349 elastomer-LDH morphology, 350
372 elastomeric blend-LDH nanocomposites morphology, 350
372 LDH fillers types in fabrication, 350, 351t LDH/MoS2 hybrids, 556
557, 659
661 LDH/MWCNT hybrids, 59
61 LDH/PEDOT NPA electrode, 571
573 LDH/polymer latex nanocomposites latex technology for LDH
based composite production, 464
481 LDH macroporous structures properties, 485
488 LDH-based nanocomposites properties, 481
485 LDH@PEDOT NPA electrode, 51 LDH@SiO2 NPs, 678
679, 689 modification, 104
106 nanocomposites for energy applications, 33t nanosheets preparation, 40
41 NCs, 26
27, 44
46, 103 applications, 21
35, 36t organic modification, 7
14, 87
95 organic modifiers used for modifications, 10t particle migration, 748 polymer NCs, 35 applications, 43
55 based on LDH-active molecules, 754
766 spectroscopic characterization of LDHs polymer NCs, 236
270 structure, 2
4 and chemical components, 232f synthesis routes, 4
7, 8t ternary metal LDHs, 85
87 X-ray diffraction analysis, 210
216 Layered double hydroxide polymer nanocomposites (LDH PNCs) electrical and electronic applications, 565, 587
591 batteries, 574
579 fuel cells, 579
587

845

SCs, 568
574 fabrication technologies future perspectives, 148
152 natural polymer/LDH NCs, 115
120 polymer/LDH NCs, 106
114 synthetic polymer/LDH NCs, 120
148 microscopic characterization of elastomer/LDH NCs, 166
170 of polymer blend/LDH NCs, 192
197 of thermoplastic polymer/LDH NCs, 170
183 of thermosetting polymer/LDH NCs, 183
191 Layered double hydroxide wrapped carbon nanotubes (LDH-w-CNTs), 552 Layered double hydroxide-nanocrystals (LDH-NCs), 538
541 Layered materials, 210 LbL assembly. See Layer-by-layer assembly (LbL assembly) LCD. See Layer charge densities (LCD) LDH. See Layered double hydroxide (LDH) LDH-g-POEGMA. See Poly(oligoethylene glycol methacrylate)-g-supported CaAl-LDH (LDH-g-POEGMA) LDH-NCs. See Layered double hydroxidenanocrystals (LDH-NCs) LDH-SA. See Stearate-functionalized LDH (LDH-SA) LDH-SSS. See Modified LDH complex (LDH-SSS) LDH-w-CNTs. See Layered double hydroxide wrapped carbon nanotubes (LDH-w-CNTs) (LDH/HB/LDH/HRP)n UTF modified electrode, 518 LDPE. See Low density polyethylene (LDPE) LDPE/LDH NCs. See Low-density polyethylene/layered double hydroxide nanocomposites (LDPE/ LDH NCs) Lead (Pb), 783 lead-acid batteries, 577 Lepidium sativum, 726
727 LGA. See N-lauroyl-glutamate (LGA) Light microscope. See Optical microscopes Lignosulfonate, 616
619 Limiting oxygen index (LOI), 122
123, 313

846

Liquid-phase exfoliation (LPE), 417
418 Lithium
sulfur batteries (Li
S batteries), 577
579 Live/dead fluorescence microscopy assay, 639
641 LM. See Loss modulus (LM) LMWH. See Low-molecular-weight heparin (LMWH) LOI. See Limiting oxygen index (LOI) Loss modulus (LM), 139
140, 285 frequency dependence of, 292f as function of frequency, 293f of PMMA, 289f pristine PS angular frequency vs., 286f of virgin PP, 285
288 Low density polyethylene (LDPE), 54
55, 131
132 Low-density polyethylene/layered double hydroxide nanocomposites (LDPE/ LDH NCs), 131
134 Low-molecular-weight heparin (LMWH), 686 LP. See Lauryl phosphate (LP) LPE. See Liquid-phase exfoliation (LPE) LSZ. See Lysozyme (LSZ) Luminescence properties, 517
518 Lysozyme (LSZ), 700
701 M M-LDH. See Phenylalanine-modified LDH (M-LDH) MA. See Melamine (MA); Methyl acrylate (MA) mADSCs. See Mouse adipose-derived stem cells (mADSCs) Magnesium (Mg) Mg
Al LDH
Cl
, 81 Mg
Al LDH
CO32
, 79
81, 80f Mg
Al LDH
HPO42
, 81 Mg
Al LDH
NO3
, 81, 82f Mg
Al LDH
PO43
, 81 Mg
Al LDH
SO42
, 81 MgAl-LDH/CQD hybrid, 542 Magnesium hydroxide (MH), 210, 312, 334 Magnetic alginate beads, 793
795 Magnetic CaAl-LDH-cellulose ionomers, 792
793 Magnetic CoFe2O4/MgAl-LDH hybrid, 783

Index

Magnetic properties of LDH macroporous structures, 485
488 Magnetism approach, 795 Magnetite-graphene-LDH (MGL), 787
788 MAHRE. See Maximum average heat rate emission (MAHRE) Maleic anhydride modified polyethylene (PE-g-MA), 292 Manganese dioxide (MnO2), 781 MAPK. See Mitogen activated protein kinase (MAPK) Materials science, CNF/LDH nanocomposites in, 447
448 Maximum average heat rate emission (MAHRE), 558 Maxwell
Wagner
Sillars polarization, 253
254 MB. See Methyl blue (MB) MCC. See Microscale combustion calorimeter (MCC) Mechanical milling (MM), 755, 756f Mechanochemical approaches, 105
106, 106t Mechanohydrothermal process, 105
106 Medical field, LDH-based nanocomposites in, 680
685 Melamine (MA), 330 Melt compounding, 35
37 extrusion, 110, 112f intercalation, 549 melt-mixing method, 754, 755f melt-spinning, 110, 111f mixing, 107, 109f, 206, 218, 813
815 rheological properties of LDHs polymer NCs modeling of rheological properties, 301
305 of polymer NCs, 281
282 of thermoplastic polymer NCs, 284
297 Melting temperature (Tm), 218 Memory effect, 12
13, 105, 412, 786
787 Mercury (Hg), 783 Mesoporous silica (m-SiO2), 330
332 dark-field STEM image and elemental mapping of, 333f

Index

Metal cations in layers, 4 for food packaging, 743 hydroxides, 311
312 ions, 602
603 salts, 610
611 Metal oxides (MOs), 806 Metallic nanoparticles (Metallic NPs), 677 Methanation, 809
811 Methotrexate (MTX), 686
687 Methyl acrylate (MA), 260
261 Methyl blue (MB), 542, 786
787 Methyl methacrylate (MMA), 123, 423
424 MgAl-LDH-loaded graphene hybrid (RGOLDH), 167
169 MGL. See Magnetite-graphene-LDH (MGL) MH. See Magnesium hydroxide (MH) MHT. See Microwave hydrothermal treatment (MHT) Microbial polysaccharides, 629
630 Microcapsulated red phosphorus (MRP), 334 Microcomposite, 219 Micronutrients, 725 Microscale combustion calorimeter (MCC), 313, 637
639 Microscopic characterization for LDH PNCs, 165
197. See also Fabrication technologies of LDH PNCs of elastomer/LDH NCs, 166
170 of polymer blend/LDH NCs, 192
197 of thermoplastic polymer/LDH NCs, 170
183 of thermosetting polymer/LDH NCs, 183
191 Microscopic techniques, 500 Microvoids, 141 Microwave hydrothermal treatment (MHT), 360 “Mid-IR” region, 77 Milling conditions, 761 Mitogen activated protein kinase (MAPK), 7
11 arrangement of MAPK anions, 11f MM. See Mechanical milling (MM) MMA. See Methyl methacrylate (MMA) MMT. See Montmorillonite (MMT) Modified layered double hydroxides (Modified LDH), 95f

847

applications, 36t characterization, 14
20 nanofillers, 753 X-ray diffraction analysis of, 210
216 Modified LDH complex (LDH-SSS), 169
170 Modified SiO2 (m-SiO2), 558 Molecular weight distribution (MWD), 295
296 Molybdenum anions, 724
725 Monte Carlo simulations, 301
302 Montmorillonite (MMT), 504 Montmorillonite (MT), 461
462 MOs. See Metal oxides (MOs) Mouse adipose-derived stem cells (mADSCs), 191 MPTS. See cmethacryloxypropyltrimethoxysilane (MPTS) MRP. See Microcapsulated red phosphorus (MRP) MT. See Montmorillonite (MT) MTX. See Methotrexate (MTX) Multiwalled carbon nanotube (MWCNTs), 30
31, 239, 433
434, 536
537, 550 MWCNTs. See Multiwalled carbon nanotube (MWCNTs) MWD. See Molecular weight distribution (MWD) N N,N’-(pyromellitoyl)-bis-L-amino acids, 533 N,N-dimethylacetamide (DMAc), 580 N-isopropylacrylamide (NIPAm), 174
175 N-lauroyl-glutamate (LGA), 124 N-trimellitylimido-L-amino acids, 533 Nafion, 580
582 Nano-onions/LDH, 433
434 Nanoadsorbents, 781 Nanobelts/LDH, 435
436 Nanocoils, 435
436 Nanocomposites (NCs), 142
143, 205, 216
217, 231
232, 566
567, 600, 677, 805
806 of bioplastics from fossil-based resources, 760
764 from renewable sources, 764
766 from oil-derived polymers, 756
759

848

Nanocomposites (NCs) (Continued) thin films based on LDHs, 498
499 Nanofillers (NFs), 216, 600 Nanofiltration (NF), 181 Nanomaterials (NMs), 601
602, 602f, 716 Nanoparticles (NPs), 601
602 Nanoplatelet array (NPA), 571
573 Nanotechnology, 599
600 2,6-Naphthalene disulfonate, 93 2-Naphthalene sulfonate, 93 Narrow interlayer space, 813 Native corn starch (NCS), 612
614 Natural cellulose, 609 Natural layered NFs, 619 Natural LDHs, 212
213, 748
753 Natural polymer/LDH NCs, 115
120 chitosan/LDH NCs, 117
118 CMC/LDH NCs, 115
116 natural rubber/LDH NCs, 118
119 pectin/LDH NCs, 116
117, 119f Natural polymers, 115, 601 Natural rubber (NR), 347
349 NR/LDH nanocomposites, 367 dynamical mechanical properties, 397
398 mechanical properties, 383 Natural rubber/layered double hydroxide nanocomposites (NR/LDH NCs), 118
119 NBR. See Nitrile butadiene rubber (NBR) NCs. See Nanocomposites (NCs) NCS. See Native corn starch (NCS) Near-infrared spectroscopy (NIR spectroscopy), 78
79 Neat LDH, 95f Neat polymers, 205, 220 Newtonian fluids, 288
289 NF. See Nanofiltration (NF) NFs. See Nanofillers (NFs) Ni-based catalyst, 809
811 NiAl LDH nanowires (NiAl-NWs), 571
573 Ni
Al LDHs/CNT hybrids, 543
544 NiAl-NWs. See NiAl LDH nanowires (NiAlNWs) Nickel (Ni), 783 NIPAm. See N-isopropylacrylamide (NIPAm)

Index

NIR spectroscopy. See Near-infrared spectroscopy (NIR spectroscopy) Nitrile butadiene rubber (NBR), 143
144 NBR-LDH nanocomposites mechanical properties, 382
383 TEM, 366
367 XRD of, 365
366 Nitrogen, 716
717, 809
811 4-Nitrophenol (4-NP), 664, 815
816 Nitroxide-mediated polymerization (NMP), 472
474 NMR spectroscopy. See Nuclear magnetic resonance spectroscopy (NMR spectroscopy) NMs. See Nanomaterials (NMs) Nonbiodegradable petro-based synthetic polymers, 600
601 Noncalcined LDHs, 786
787 Noninvasive fluorescent probes, 693 Nonsteroidal antiinflammatory drugs (NSAIDs), 677
678, 684
685 Novel antimicrobial coating, 758
759 Novel biocomposites, 639
641 NOx storage and reduction strategy (NSR strategy), 809
811 NP. See Phosphorus nitrogen (NP) 4-NP. See 4-Nitrophenol (4-NP) NPA. See Nanoplatelet array (NPA) NPs. See Nanoparticles (NPs) NR. See Natural rubber (NR) NSAIDs. See Nonsteroidal antiinflammatory drugs (NSAIDs) NSR strategy. See NOx storage and reduction strategy (NSR strategy) Nuclear magnetic resonance spectroscopy (NMR spectroscopy), 232
235 of LDHs polymer NCs, 258
262 Nucleic acids, 513 Nylon-6, 550 O O-LDH. See Organically modified LDH platelets (O-LDH) Octylsulfate (OS), 291
292 OER. See Oxygen evolution reactions (OER) Oil-derived polymers, nanocomposites from, 756
759 OLDH. See Organomodifed LDH (OLDH)

Index

Oleate-modified LDH, 290 Oleic acid, 92, 92f OMMT. See Organomodifed MMT (OMMT) One-pot hydrothermal synthesis, 538 Optical microscopes, 158
159 Order of diffraction, 207
208 Organic dyes, 783 Organic LDHs. See also Inorganic LDHs effect on flame-retardant performance, 327
329 effect on thermal stability properties, 319
320 Organic modification of LDH, 7
14, 337 Organic NiFe-LDH/CNT nanofiller hybrids, 552 Organic phosphorus compounds, 311
312 Organic polyanion, 502 Organically modified layered double hydroxides applications, 21
35, 36t Organically modified LDH platelets (OLDH), 483 Organic
inorganic hybrids, 756 Organized LDH, 805
806 Organo-LDHs, 782, 786
787 Organo-modified CoAl-LDHs, 550 Organo-modified layered double hydroxides, 87
95 Organochlorine, 717 Organomodifed LDH (OLDH), 365 Organomodifed MMT (OMMT), 365 Organophosphorus chemicals, 717 ORR. See Oxygen reduction reaction (ORR) Ortho-hydroxybenzoate (o-BzOH), 628
629, 760
761, 764 OS. See Octylsulfate (OS) OTR. See Oxygen transmission rate (OTR) Oxyanions, 783 Oxygen evolution reactions (OER), 441, 587
588 Oxygen reduction reaction (ORR), 587, 826
828 Oxygen transmission rate (OTR), 765 P P fixation, 722
723 p-BzOH. See Para-hydroxybenzoate (p-BzOH) P-LDH. See Plate-like LDH (P-LDH)

849

P(MMA-co-BA)/layered double hydroxide nanocomposites (P(MMA-co-BA)/ LDH NCs), 124
126 PA. See Palmitic acid (PA); Polyamide (PA) PAA. See Poly(acrylic acid) (PAA) PAA-CNTs. See Poly(acrylic acid) functionalized CNTs (PAA-CNTs) PAI. See Poly(amide-imide) (PAI) PAI/LDH NCs. See Poly(amide-imide)/ layered double hydroxide nanocomposites (PAI/LDH NCs) Palmitic acid (PA), 319 PAN. See Polyacrylonitrile (PAN) PANI. See Polyaniline (PANI) Paper, in food packaging, 743 Para-hydroxybenzoate (p-BzOH), 628
629, 760
761, 764 Particle morphology, 282
284 PBAT. See Poly(butylene adipate-coterephthalate) (PBAT) PBAT/OLDH film, 51
52 PBS. See Polybutylene succinate (PBS) PC. See Phosphorylated cellulose (PC) PCL. See Polycaprolactone (PCL) PCT@CHT/LDH systems, 700 Pd-LDH catalysts, 807
809 PDA. See Polydopamine (PDA) PDAS. See Polydopamine spheres (PDAS) pDNA. See Plasmid DNA (pDNA) PDT. See Photodynamic therapy (PDT) PE. See Polyethylene (PE) PE-g-MA. See Maleic anhydride modified polyethylene (PE-g-MA) Peak heat release rate (PHRR), 315, 552, 637
639, 650
653, 659
661 Pectin, 628
629, 630f Pectin-coated chitosan bead, 120f, 700f Pectin/layered double hydroxide nanocomposites (Pectin/LDH NCs), 116
117, 119f PEDOT. See Poly(3,4ethylenedioxythiophene) (PEDOT) PEDOT:PSS. See Poly(3,4-ethylene dioxythiophene) and poly (styrenesulfonate) (PEDOT:PSS) PEI/PSS/LDH NCs. See Polyethyleneimine/ poly(sodium 4-styrene sulfonate) hybrid/layered double hydroxide nanocomposites (PEI/PSS/LDH NCs)

850

PEFCs. See Polymer electrolyte fuel cells (PEFCs) PEG. See Polyethylene glycol (PEG) PEGDA. See Poly(ethylene glycol diacrylate) (PEGDA) PEI. See Polyetherimide (PEI); Polyethyleneimine (PEI) PEM. See Proton exchange membrane (PEM) PEMFCs. See Polymer electrolyte membrane fuel cells (PEMFCs) Penetration retention rate (PRR), 270
272 PEO-PPO-PEO. See Poly(ethylene oxide copropylene oxide-co-ethylene oxide) (PEO-PPO-PEO) Peptide transporter-1 (PepT-1), 699 Permeability, 758, 759t Pesticides, 717
718 LDHs use for pesticide removal, 730
733 PET. See Poly(ethylene terephthalate) (PET) pH-responsive nanohybrid (LDH
ZnPcPS4), 706
707 PHA/layered double hydroxide nanocomposites, 633
642 PHAs. See Polyhydroxyalkanoates (PHAs) PHB. See Poly(3-hydroxybutyrate) (PHB) PHB/PBAT. See Poly-3 hydroxybutyrate/ poly(butyleneadipate-coterephthalate) (PHB/PBAT) PHBHV. See Poly(3-hydroxybutyrate-co-3hydroxyvalerate) (PHBHV) PHBV. See Poly(3-hydroxybutyrate-co-3hydroxyvalerate) (PHBHV) Phenylalanine-modified LDH (M-LDH), 134 Phosphoric acid, 716 Phosphorus (P), 719 Phosphorus nitrogen (NP), 369
370 Phosphorylated cellulose (PC), 658 Photocatalysis, 444
445, 486
487, 523
525 Photodetectors, 522
523 Photodynamic therapy (PDT), 46
47, 705
707 Photographic techniques, 208 Photosensitizer excitation efficiency, 703 Photostability of LDH/CP nanocomposites, 515
517 PHRR. See Peak heat release rate (PHRR)

Index

Physical blending of polymer latex, 466
468 Phytic acid (Ph acid), 558 Phytophtora infestans, 716 PI. See Polyimide (PI) PI/LDH NCs. See Polyimide/layered double hydroxide nanocomposites (PI/LDH NCs) Picloram, 726
727 Pifithrin-α/LDH/chitosan nanohybrid composites, 621
624 PISA. See Polymerization-induced selfassembly (PISA) PLA. See Polylactic acid (PLA) PLA/layered double hydroxide nanocomposites, 643
653 Plant growth regulators, 727
730 Plasmid DNA (pDNA), 47 PlasticsaEurope statistics, 311 Plate-like LDH (P-LDH), 765 Pluronic F-127, 468 PMA. See Poly(methyl acrylate) (PMA) PMDA. See Pyromellitic dianhydride (PMDA) PMEI. See Poly(methyl-ether-imide) (PMEI) PMMA. See Poly(methyl methacrylate) (PMMA) PMMA/LDH NCs. See Poly(methyl methacrylate)/layered double hydroxide nanocomposites (PMMA/ LDH NCs) PMMA/Ni/Al LDH blends, 173 PNCs. See Polymer nanocomposites (PNCs) PNIPAA. See Poly(N-isopropyl acrylamide) (PNIPAA) pNIPAM. See Poly(N-isopropylacrylamide) (pNIPAM) Pollutant elimination by polymer/layered double hydroxide nanocomposites, 790
798 from water, 782
790 LDH modification, 782 mechanisms of adsorption, 783
790 structural properties of LDH, 782 water pollutants, 783 Pollution removal, 538
543 Poly-3 hydroxybutyrate/poly (butyleneadipate-co-terephthalate) (PHB/PBAT), 636
637

Index

Poly(3-hydroxybutyrate-co-3hydroxyvalerate) (PHBHV), 636, 639
642, 709 Poly(3-hydroxybutyrate-co-4hydroxybutyrate) (P(3,4)HB), 637
639 Poly(3-hydroxybutyrate) (PHB), 636, 709 Poly(3,4-ethylene dioxythiophene) and poly (styrenesulfonate) (PEDOT:PSS), 573 Poly(3,4-ethylenedioxythiophene) (PEDOT), 571
573 Poly(acrylic acid) (PAA), 812 Poly(acrylic acid) functionalized CNTs (PAA-CNTs), 812 Poly(amide-imide) (PAI), 127, 176
177, 239 Poly(amide-imide)/layered double hydroxide nanocomposites (PAI/LDH NCs), 127
130 Poly(amide-imide)/LDH-MWCNT NCs, 550 Poly(butylene adipate-co-terephthalate) (PBAT), 51
52 Poly(ethylene glycol diacrylate) (PEGDA), 588
591 Poly(ethylene oxide co-propylene oxide-coethylene oxide) (PEO-PPO-PEO), 295 Poly(ethylene terephthalate) (PET), 653
654, 756 nanocomposites, 291
292 Poly(methyl acrylate) (PMA), 260
261 Poly(methyl methacrylate) (PMMA), 123, 173, 220, 236, 320, 555 Poly(methyl methacrylate)/layered double hydroxide nanocomposites (PMMA/ LDH NCs), 123 Poly(methyl-ether-imide) (PMEI), 181
183 Poly(N-isopropyl acrylamide) (PNIPAA), 822
824 Poly(N-isopropylacrylamide) (pNIPAM), 587
588 Poly(oligoethylene glycol methacrylate)-gsupported CaAl-LDH (LDH-gPOEGMA), 817, 820, 821f Poly(oxyethylene-b-oxypropylene-boxyethylene), 649
650 Poly(p-phenylene) anionic derivate (APPP), 250

851

Poly(sodium 4-styrene sulfonate) (PSS), 111
112, 144, 464
465, 500
501 Poly(vinyl alcohol) (PVA), 134, 173
174, 320
322, 419
420, 504, 556
557, 566
567, 601, 795, 812 PVA-alginate/LDH beads, 795 PVA/Au-LDH composite films, 815
816 PVA/layered double hydroxide nanocomposites, 653
664 Poly(vinyl pyrrolidone) (PVP), 588
591, 812 Poly(vinylidene fluoride) (PVDF), 178, 588
591 Polyacrylonitrile (PAN), 270 Polyamide (PA), 181, 743
746 PA6, 220 Polyaniline (PANI), 183, 267
270, 538, 588
591 Polybutylene succinate (PBS), 290, 601 Polycaprolactone (PCL), 44
46, 181, 246, 601, 760
761 Polydopamine (PDA), 240
241, 812
813 Polydopamine spheres (PDAS), 587, 826
828 Polyester, 812 Polyester/layered double hydroxide nanocomposites (Polyester/LDH NCs), 136
137 Polyetherimide (PEI), 500
501, 538 Polyethylene (PE), 302, 743
746 Polyethylene glycol (PEG), 533 Polyethyleneimine (PEI), 144 Polyethyleneimine/poly(sodium 4-styrene sulfonate) hybrid/layered double hydroxide nanocomposites (PEI/PSS/ LDH NCs), 144
145 Polyhydroxyalkanoates (PHAs), 709 Polyimide (PI), 588 Polyimide/layered double hydroxide nanocomposites (PI/LDH NCs), 122
123 Polylactic acid (PLA), 601, 762
763 Polymer electrolyte fuel cells (PEFCs), 174
175 Polymer electrolyte membrane fuel cells (PEMFCs), 50
51, 580 Polymer layered double hydroxide hybrid nanocomposites

852

Polymer layered double hydroxide hybrid nanocomposites (Continued) layered double hydroxide/carbonaceous nanofiller hybrids, 535
536 modification of LDHs with organic compounds, 532
534 polymer/LDH/carbonaceous nanofiller hybrid nanocomposites, 548
558 polymer/LDH/CNT hybrid nanocomposites, 550
553 polymer/LDH/graphene hybrid nanocomposites, 554
556 polymer/LDH/other nanofiller hybrids, 556
558 Polymer LDH nanocomposites, rheology of, 282
284 Polymer nanocomposites (PNCs), 157
158, 217
218, 231
232, 281
282, 350, 461
462, 531, 566
567, 600, 603
604, 805
806, 815 AFMs, 158, 164
165 fabrication routes to, 64t FIMs, 158, 162 for food-packaging applications, 747
748 LDH, 35 applications, 43
55 microscopic characterization techniques, 158
165 optical microscopes, 158
159 rheological studies of, 281
282 rheology of polymer LDH NCs, 282
284 SEMs, 158
160 spectroscopic characterization of LDHs, 236
270 spectroscopy of, 232
235 SPMs, 158, 162
163 STMs, 158, 163
164 synthesis and characterization, 205
206 TEMs, 158, 160
161 XRT, 158, 165 Polymer/layered double hydroxide nanocomposites (Polymer/LDH NCs), 106
114, 349
350, 532, 811
815 applications in catalysis, 815
829 pollutant elimination by, 790
798 importance, 790
791 polymer/layered double hydroxidebased adsorbents, 791
798

Index

preparation, 812
815 Polymer(s), 205, 327
328, 347
349, 743
746 composites, 205 electrolytes, 574
575, 577 flame retardancy, 311 for food packaging, 743 gel electrolyte, 588
591 matrix nanocomposite, 217 nanofiller hybrid composites, 531 polymer-based materials, 311 polymer-supported LDHs, 816
817 polymer
layered double hydroxide nanocomposites, 709
710 production, 312f Polymeric melts, 281
282 Polymeric nanocomposites based on LDHactive molecules, 754
766 of bioplastics from fossil-based resources, 760
764 of bioplastics from renewable sources, 764
766 from oil-derived polymers, 756
759 Polymerization-induced self-assembly (PISA), 472
474 Polyoxometalates (POMs), 811 Polypropylene (PP), 139
140, 285
288, 552, 743
746, 813
814 Polypropylene-ethylene vinyl acetate/layered double hydroxide nanocomposites (PP-EVA/LDH NCs), 139
140 Polypyrrole (PPy), 241, 571
573 Polysaccharide/LDH NCs, 604
632 alginate/LDH NCs, 624
627 cellulose/LDH NCs, 604
611 chitosan/LDH NCs, 619
624 starch/LDH NCs, 612
619 Polystyrene (PS), 220, 284
285, 464
465 Polysulfone (PSU), 580 Polyurethane (PU), 143
144, 347
349 Polyurethane elastomer (PUE), 555 Polyurethane foam (PUF), 166
167 Polyurethane/nitrile butadiene rubber blend/ layered double hydroxide nanocomposites (PU/NBR blend/ LDH NCs), 143
144 Polyvinyl acetate (PVAc), 124 Polyvinyl acetate/layered double hydroxide nanocomposites (PVAc/LDH NCs), 124

Index

Polyvinyl alcohol/layered double hydroxide nanocomposites (PVA/LDH NCs), 134
136 Polyvinyl carbazole (PVK), 508
509, 509f Polyvinyl chloride (PVC), 137
138, 317
318 Polyvinyl chloride/layered double hydroxide nanocomposites (PVC/LDH NCs), 137
139 Polyvinyl sulfonate (PVS), 505 POMs. See Polyoxometalates (POMs) Position-sensitive photo diode (PSPD), 164
165 Posttreatment of LDHs as flame retardants, 337
338 aqueous miscible organic solvent treatment, 338 organic modification of LDHs, 337 Potassium persulfate (KPS), 125, 469
470 Power density, 568 Power law, 304 PP. See Polypropylene (PP) PP-EVA/LDH NCs. See Polypropyleneethylene vinyl acetate/layered double hydroxide nanocomposites (PP-EVA/ LDH NCs) PPy. See Polypyrrole (PPy) Preexfoliation, 110 Principal agricultural problems, resolving, 718
719 Pristine cellulose fibers, 610
611 Pristine LDH, 347
349 Protein/layered double hydroxide nanocomposites, 632
633 Proton exchange membrane (PEM), 580 PRR. See Penetration retention rate (PRR) PS. See Polystyrene (PS) Pseudocapacitors, 439
440, 568 PSPD. See Position-sensitive photo diode (PSPD) PSS. See Poly(sodium 4-styrene sulfonate) (PSS) PSU. See Polysulfone (PSU) PU. See Polyurethane (PU) PU blend-LDH nanocomposites dynamical mechanical properties, 398
401 mechanical properties of, 383
385

853

TEM of, 368
369 XRD of, 367
368 PU-LDH nanocomposites dynamical mechanical properties, 388
390 mechanical properties, 372
376 TEM of, 354
358 XRD of, 350
354 PUE. See Polyurethane elastomer (PUE) PUF. See Polyurethane foam (PUF) PVA. See Poly(vinyl alcohol) (PVA) PVA/LDH NCs. See Polyvinyl alcohol/ layered double hydroxide nanocomposites (PVA/LDH NCs) PVAc. See Polyvinyl acetate (PVAc) PVAc/LDH NCs. See Polyvinyl acetate/ layered double hydroxide nanocomposites (PVAc/LDH NCs) PVC. See Polyvinyl chloride (PVC) PVC/LDH NCs. See Polyvinyl chloride/ layered double hydroxide nanocomposites (PVC/LDH NCs) PVDF. See Poly(vinylidene fluoride) (PVDF) PVK. See Polyvinyl carbazole (PVK) PVP. See Poly(vinyl pyrrolidone) (PVP) PVS. See Polyvinyl sulfonate (PVS) Pyrolysis combustion flow calorimeter (PCFC). See Microscale combustion calorimeter (MCC) Pyromellitic dianhydride (PMDA), 122
123 Q Quaternized polysulfone (QPSF), 585
586 R R-PET. See Recycled poly(ethylene terephthalate) (R-PET) RAFT. See Reversible addition fragmentation chain transfer (RAFT) RAFT-assisted encapsulating emulsion polymerization (REEP), 472
474 Raman spectroscopy, 232
233 of LDHs polymer NCs, 239
243 RDRP. See Reversible deactivation radical polymerization (RDRP) Rechargeable metal
air batteries, 576
577 Reconstitution/rehydration, 105

854

Reconstruction/rehydration method, 7 Recycled poly(ethylene terephthalate) (RPET), 550, 796, 798 Red phosphorus (RP), 330 Reduced graphene oxide (RGO), 571
573 REEP. See RAFT-assisted encapsulating emulsion polymerization (REEP) Regeneration method, 12
13, 532 Regulation issues in food-packaging applications, 766
767 Reinforcement, 217, 219, 599
600 nanoparticles, 217 Relative humidity (RH), 766 Relaxation modulus, 281
282 Renewable sources, nanocomposites of bioplastics from, 764
766 Reticular endothelial system (RES), 684
685 Reversible addition fragmentation chain transfer (RAFT), 472
474 polymerization, 814, 817
819 Reversible deactivation radical polymerization (RDRP), 472
475 RGO. See Reduced graphene oxide (RGO) RGO-LDH. See MgAl-LDH-loaded graphene hybrid (RGO-LDH) RH. See Relative humidity (RH) Rheological models, 302 Rheological percolation threshold, 284
285 RP. See Red phosphorus (RP) Ru-LDH catalyst, 807
809 Rubber nanocomposites, 349
350 S SA. See Stearate (SA) Saccharomyces cerevisiae, 760
761 Salt-base method, 5 Salt-oxide method, 6 SAOS. See Small-amplitude oscillatory shear (SAOS) SAXRD. See Small-angle X-ray diffraction patterns (SAXRD) SAXS. See Small angle X-ray scattering (SAXS) SBR. See Styrene butadiene rubber (SBR) SBR/LDH nanocomposites mechanical properties, 382 TEM, 364
365

Index

Scanning electron microscopy (SEM), 158
160, 350, 500, 533, 783
784, 822
824. See also Transmission electron microscopy (TEM) Scanning microscopes, 162
163 Scanning probe microscopes (SPMs), 158, 162
163 Scanning tunneling microscopes (STMs), 158, 163
164 Scherrer equation, 14
15, 208 Scleroglucan, 629
630, 631f SCs. See Supercapacitors (SCs) SDBS. See Sodium dodecylbenzene sulfonate (SDBS) SDS. See Sodium dodecyl sulfate (SDS) SEC. See Size exclusion chromatography (SEC) Self-assembly process, 463 SEM. See Scanning electron microscopy (SEM) Semiconductor quantum dots, 677 Shear thinning behavior, 285, 291
292 Shear thinning exponent (STE), 288
289, 292 SiLDHs. See Silylanized Mg/Al LDH (SiLDHs) Silicone rubber (SR), 347
349 Silicone rubber/layered double hydroxide nanocomposites (SR/LDH NCs), 140
141 dynamical mechanical properties, 394
395 mechanical properties, 379
380 TEM of, 362
363 XRD of, 361
362 Silylanized Mg/Al LDH (SiLDHs), 174
175 Single-walled carbon nanotubes (SWCNTs), 538 Single-walled CNT/LDH nanocomposites, 436 SiRNA, 683 Sisko model, 304 Size exclusion chromatography (SEC), 648
649 Sjo¨grenite-hydrotalcite, 210 Slow-release fertilizers, layered double hydroxide matrices of, 720
725 Slow-release tests, 719

Index

SLS. See Sodium lignosulfonate (SLS) SM. See Storage modulus (SM) Small angle X-ray scattering (SAXS), 470
471 Small-amplitude oscillatory shear (SAOS), 297 Small-angle X-ray diffraction patterns (SAXRD), 500 Smart polymers, 822
824 SMCs. See Smooth muscle cells (SMCs) Smoke suppression characteristics, 555 Smooth muscle cells (SMCs), 648 Sodium dodecyl sulfate (SDS), 90, 91f, 123, 173, 213, 236, 347
349, 469, 649, 763
764 Sodium dodecylbenzene sulfonate (SDBS), 88
89, 88f, 89f, 139
140, 172
173, 253
254, 263
264, 347
349, 365 Sodium lignosulfonate (SLS), 366
367 Sodium styrene sulfonate (SSS), 169
170, 365 Softening point increment (SPI), 272 Sol-gel method, 6 Sol
gel synthesis, 347
349 Solid-state PEO/LDH NC electrolyte systems, 577 Solid-state SC system, 569
571 Solution blending, 37, 110f, 755, 755f, 813 Solution-induced intercalation, 549 Solvent blending, 112
113 Sonication, 43
55 Sorption (S), 758, 759t Soy protein NCs, 632 SPDP. See Spirocyclic pentaerythritol bisphosphorate diphosphoryl sodium (SPDP) Spectroscopic characterization techniques of LDHs PNCs dielectric spectroscopy, 251
258 EDX spectroscopy, 243
246 fluorescence spectroscopy, 247
251 FTIR spectroscopy, 236
239 LDH structure and chemical components, 232f NMR spectroscopy, 258
262 Raman spectroscopy, 239
243 spectroscopic characterization for aging process, 270
272 spectroscopy of PNCs, 232
235

855

UV
vis spectroscopy, 262
265 XPS, 265
270 Spectroscopic technique, 208 SPEEK. See Sulfonated poly(ether ether ketone) (SPEEK) SPEK. See Sulfonated poly (ether ketone) (SPEK) SPI. See Softening point increment (SPI) Spin-spray LbL method, 113, 114f Spirocyclic pentaerythritol bisphosphorate diphosphoryl sodium (SPDP), 136
137 SPMs. See Scanning probe microscopes (SPMs) SPSU. See Sulfonated polysulfone (SPSU) SR. See Silicone rubber (SR) SR/LDH NCs. See Silicone rubber/layered double hydroxide nanocomposites (SR/LDH NCs) SSS. See Sodium styrene sulfonate (SSS) Starch/layered double hydroxide nanocomposites, 612
619 STE. See Shear thinning exponent (STE) Stearate (SA), 319 Stearate-functionalized LDH (LDH-SA), 641
642 Stearate-modified LDH, 647 STMs. See Scanning tunneling microscopes (STMs) Storage modulus (SM), 139
140, 281
285 angular frequency vs., 285f frequency dependence of, 292f as function of frequency, 293f of PMMA, 289f Styrene butadiene rubber (SBR), 347
349 Sulfonated poly (ether ketone) (SPEK), 50
51 Sulfonated poly(ether ether ketone) (SPEEK), 50
51, 582, 813 Sulfonated polysulfone (SPSU), 50
51, 580 Sulfur oxides, 809
811 Super-hydrophobic SA/LDH/cellulose NC, 609, 609f Supercapacitors (SCs), 439
441, 543
546, 565, 568
574 application of LDH nanocomposites in, 568
571 application of LDH/polymer in, 571
574 Superphosphate, 716

856

Surface plasmon resonance microscopy, 465 Suspension polymerization, 461
462 LDH-based nanocomposites by, 471
472 SWCNTs. See Single-walled carbon nanotubes (SWCNTs) Synchrotron radiation sources, 208
209 Synergistic fire retardants, LDHs Effect with, 330
337 Synergistic thermal stabilizers, LDHs effect with, 320
322 Synthesis routes of LDH, 4
7 Synthetic hydrotalcite-like compounds, 719 Synthetic layered NFs, 619 Synthetic LDHs, 411
412 Synthetic polymer/LDH NCs, 120
148 ER/MoS2/LDH NCs, 141
143, 142f iPP/LDH NCs, 145
148 LDPE/LDH NCs, 131
134 P(MMA-co-BA)/LDH NCs, 124
126 PAI/LDH NCs, 127
130 PEE/PSS/LDH NCs, 144
145 PI/LDH NCs, 122
123 PMMA/LDH NCs, 123 polyester/LDH NCs, 136
137 PP-EVA/LDH NCs, 139
140 PU/NBR blend/LDH NCs, 143
144 PVA/LDH NCs, 134
136 PVAc/LDH NCs, 124 PVC/LDH NCs, 137
139 SR/LDH NCs, 140
141 WF/PP/LDH NCs, 127 Synthetic polymers, 601 T “Tailor-made” preparation of LDHs, 748
753 Taurine, 92, 92f TBAB. See Tetra-n-butylammonium bromide (TBAB) TCS. See Thermoplastic corn starch (TCS) TE. See Tissue engineering (TE) TEM. See Transmission electron microscopy (TEM) Templating approach, 463 Tensile strength, 612
614 Ternary CoNiMn-LDH/PPY/RGO composite, 587
588 Ternary metal layered double hydroxides, 85
87

Index

Tetra-n-butylammonium bromide (TBAB), 129
130 Tetrahydrofuran (THF), 143
144 TGA. See Thermogravimetric analysis (TGA) Thallium (Tl), 783 Thermal stability properties of LDH-based nanocomposites, 318
322 techniques for determination, 313
317 Thermal stabilizer, 317
318 Thermogravimetric analysis (TGA), 116, 117f, 313, 533, 619 Thermoplastic corn starch (TCS), 616
619 Thermoplastic polymer NCs, rheology of, 284
297 rheology of thermosetting polymer LDH NCs, 297
300 Thermoplastic polyurethane/nitrile butadiene (TPU/NBR), 367 Thermosetting polymer LDH NCs, rheology of, 297
300 Thermostability mechanism using LDHs, 322
323 THF. See Tetrahydrofuran (THF) THR. See Total heat release (THR) Three-dimensionally ordered macroporous structures (3-DOM), 475 Tin-doped indium oxide (ITO). See Indium tin oxide (ITO) Tissue distribution of LDH nanoparticles, 684
685 Tissue engineering (TE), 694
695 LDH-based nanocomposites for TE applications, 694
695, 709f TMA. See Trimethylamine (TMA) TN rubber blends. See Thermoplastic polyurethane/nitrile butadiene (TPU/ NBR) Total heat release (THR), 314
315, 637
639, 650
653 “Total quality” of products, 743 Total smoke production (TSP), 328
329 Total smoke release (TSR), 558 TPP. See Triphenol phosphate (TPP) TPU/NBR. See Thermoplastic polyurethane/ nitrile butadiene (TPU/NBR) Transition metal LDHs, 499 Transmission electron microscopy (TEM), 158, 160
161, 225, 244, 350, 500, 533, 756
757, 787
788

Index

EPDM/LDH nanocomposites, 364 EVA blend-LDH and EPDM blend-LDH nanocomposites, 370
372 EVA-LDH nanocomposites, 359
360 NBR-LDH and XNBR-LDH nanocomposites, 366
367 NR/LDH nanocomposites, 367 PU blend-LDH nanocomposites, 368
369 of PU-LDH nanocomposites, 354
358 SBR/LDH nanocomposites, 364
365 SR-LDH nanocomposites, 362
363 S-(3-Trimethoxysilyl) propyltrithiocarbonate (BTPT), 816
817 Trimethylamine (TMA), 582
585 Triphenol phosphate (TPP), 336
337 Tris (8-hydroxyquinolate-5-sulfonate) aluminum (III) (AQS3
), 504 Trolox, 650
653 TSP. See Total smoke production (TSP) TSR. See Total smoke release (TSR) Two roll mill mixing, 41
42, 109
110 Two-dimensional layered solids, 566
567 U UL-94. See Underwriters Laboratories (UL-94) Ultracapacitors, 439
440, 565 Ultrathin films (UTFs), 250, 498, 587
588, 822
824 Ultrathin NiCo-LDH, 535
536 Ultraviolet-light (UV light), 270 Ultraviolet-visible spectroscopy (UV-vis spectroscopy), 123, 232
233, 235 of LDHs polymer NCs, 262
265 Underwriters Laboratories (UL-94), 313, 315
317 United States Food and Drug Administration (US FDA), 693
694 Urea, 718
719 hydrolysis, 5 urea-hydrolyzed method, 538 US FDA. See United States Food and Drug Administration (US FDA) UTFs. See Ultrathin films (UTFs) UV light. See Ultraviolet-light (UV light) UV-vis spectroscopy. See Ultraviolet-visible spectroscopy (UV-vis spectroscopy)

857

V VAI. See Viscosity aging index (VAI) van der Waals forces, LbL assembly method based on, 511
512 VCM. See Vinyl chloride monomer (VCM) Vinyl chloride monomer (VCM), 472 Viscosity aging index (VAI), 270 Volatile compounds, 447
448 W Wagner model, 302 Water pollution, 783 Water purification, 52
53, 781
782 pollutant elimination by polymer/layered double hydroxide nanocomposites, 790
798 from water, 782
790 Water splitting (WS), 26
27, 441
443 Water vapor permeability (WVP), 116 Waterborne hyperbranched polyurethane acrylate (WHPUA), 358 WAX. See Wide-angle mode X-ray diffraction patterns (WAX) WAXS. See Wide-angle X-ray scattering (WAXS) Weak bonds, 766 Welan gum, 629
630, 631f WF-PP. See Wood flour/polypropylene (WF-PP) WF/PP/LDH NCs. See Wood flour/ polypropylene/layered double hydroxide nanocomposites (WF/PP/ LDH NCs) “White beam” topography, 165 WHPUA. See Waterborne hyperbranched polyurethane acrylate (WHPUA) Wide-angle mode X-ray diffraction patterns (WAX), 116
117 Wide-angle X-ray diffraction (WXRD), 123 diffraction patterns, 123f, 124f Wide-angle X-ray scattering (WAXS), 118
119, 121f Wood, in food packaging, 743 Wood flour/polypropylene (WF-PP), 127 Wood flour/polypropylene/layered double hydroxide nanocomposites (WF/PP/ LDH NCs), 127 Working ion, 574
575

858

WS. See Water splitting (WS) WVP. See Water vapor permeability (WVP) WXRD. See Wide-angle X-ray diffraction (WXRD) X X-ray diffraction (XRD), 167
169, 281
282, 350, 464
465, 533, 756
758 analysis, 206
210 of LDH and modified LDH, 210
216 of LDH polymer nanocomposites, 216
225 EPDM/LDH nanocomposites, 363 EVA blend-LDH and EPDM blend-LDH nanocomposites, 369
370 of EVA-LDH nanocomposites, 358 NBR-LDH and XNBR-LDH nanocomposites, 365
366 NR/LDH nanocomposites, 367 patterns, 500 PU blend-LDH nanocomposites, 367
368 of PU-LDH nanocomposites, 350
354 of SR-LDH nanocomposites, 361
362 topography, 158, 165 X-ray photoelectron spectroscopy (XPS), 232
233, 235 of LDHs polymer NCs, 265
270

Index

X-rays, 206 diffractometer, 208 diffractometry, 208 powder diffractometry, 208 scattering, 208 sources, 208
209 spectrometer, 207 Xanthan gum, 629
630 XNBR-LDH nanocomposites dynamical mechanical properties, 397
398 mechanical properties, 382
383 TEM, 366
367 XRD of, 365
366 XPS. See X-ray photoelectron spectroscopy (XPS) XRD. See X-ray diffraction (XRD) Y Yield stress, 282
284 Young’s modulus, 612
614 Z Zinc (Zn), 783 Zn-Al-LDH/PPY composites, 579 Zwitter ionic imidazolium-based ionic liquid (ZIL), 300