Cellulose Science and Technology: Chemistry, Analysis, and Applications 9781119217589

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Cellulose Science and Technology

Cellulose Science and Technology Chemistry, Analysis, and Applications

Edited by Thomas Rosenau, Antje Potthast, and Johannes Hell

U. Natural Resources and Life Sciences, Vienna (BOKU) Muthgasse 18, Vienna, AU

This edition first published 2019 © 2019 John Wiley & Sons Inc All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www .wiley.com/go/permissions. The right of Thomas Rosenau, Antje Potthast, and Johannes Hell to be identified as the editors of the editorial material in this work has been asserted in accordance with law. Registered Office John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA Editorial Office 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Wiley also publishes its books in a variety of electronic formats and by print-on-demand. Some content that appears in standard print versions of this book may not be available in other formats. Limit of Liability/Disclaimer of Warranty In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Library of Congress Cataloging-in-Publication Data Names: Rosenau, Thomas, editor. | Potthast, Antje, editor. | Hell, Johannes, 1985- editor. Title: Cellulose science and technology : chemistry, analysis, and applications / [edited by] Thomas Rosenau, Antje Potthast, Dr. Johannes Hell. Description: First edition. | Hoboken, NJ : John Wiley & Sons, 2018. | Includes bibliographical references and index. | Identifiers: LCCN 2018025507 (print) | LCCN 2018042475 (ebook) | ISBN 9781119217626 (Adobe PDF) | ISBN 9781119217633 (ePub) | ISBN 9781119217589 (hardcover) Subjects: LCSH: Cellulose–Chemistry. | Cellulose industry. Classification: LCC QD323 (ebook) | LCC QD323 .C398 2018 (print) | DDC 547.78–dc23 LC record available at https://lccn.loc.gov/2018025507 Cover Design: Wiley Cover Images: © theasis /iStockphoto; © Mark Evans / Getty Images; © Tefi /Shutterstock Set in 10/12pt WarnockPro by SPi Global, Chennai, India Printed in United States of America 10 9 8 7 6 5 4 3 2 1

Dedicated in gratitude to our teacher, mentor and fatherly friend, Professor Paul Kosma, BOKU University Vienna, on the occasion of his 65th birthday

vii

Contents Author Biography xv List of Contributors xvii Preface xxiii Acknowledgements xxv 1

Aminocelluloses – Polymers with Fascinating Properties and Application Potential 1 Thomas Heinze, Thomas Elschner, and Kristin Ganske

1.1 1.2 1.2.1

Introduction 1 Amino-/ammonium Group Containing Cellulose Esters 2 (3-Carboxypropyl)trimethylammonium Chloride Esters of Cellulose 2 Cellulose-4-(N-methylamino)butyrate (CMABC) 7 6-Deoxy-6-amino Cellulose Derivatives 9 Spontaneous Self-assembling of 6-Deoxy-6-amino Cellulose Derivatives 10 Application Potential of 6-Deoxy-6-amino Cellulose Derivatives 13 Amino Cellulose Carbamates 21 Synthesis 21 Properties 22 Acknowledgment 24 References 24

1.2.2 1.3 1.3.1 1.3.2 1.4 1.4.1 1.4.2

2

Preparation of Photosensitizer-bound Cellulose Derivatives for Photocurrent Generation System 29 Toshiyuki Takano

2.1 2.2

Introduction 29 Porphyrin-bound Cellulose Derivatives 31

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Contents

2.3 2.4 2.5 2.6 2.7 2.8

Phthalocyanine-bound Cellulose Derivatives 34 Squaraine-bound Cellulose Derivative 40 Ruthenium(II) Complex-bound Cellulose Derivative 42 Fullerene-bound Cellulose Derivative 44 Porphyrin-bound Chitosan Derivative 45 Conclusion 47 References 47

3

Synthesis of Cellulosic Bottlebrushes with Regioselectively Substituted Side Chains and Their Self-assembly 49 Keita Sakakibara, Yuji Kinose, and Yoshinobu Tsujii

3.1 3.2

Introduction 49 Strategy for Accomplishing Regioselective Grafting of Cellulose 52 Regioselective Introduction of the First Polymer Side Chain 55 Introduction of Poly(styrene) at O-2,3 Position of 6-O-p-Methoxytritylcellulose (1) 55 Introduction of Poly(ethylene oxide) at O-2,3 Position of 6-O-p-Methoxytritylcellulose (1) 57 Regioselective Introduction of the Second Polymer Side Chain 58 Introduction of Poly(styrene) at O-6 Position of 2,3-di-O-PEO Cellulose (5) via Grafting-from Approach 58 Introduction of Poly(styrene) at O-6 Position of 2,3-di-O-PEO Cellulose (5) via Grafting to Approach Combining Click Reaction 58 SEC-MALLS Study 61 Summary and Outlook 64 Acknowledgments 64 References 64

3.3 3.3.1 3.3.2 3.4 3.4.1 3.4.2

3.5 3.6

4

Recent Progress on Oxygen Delignification of Softwood Kraft Pulp 67 Adriaan R. P. van Heiningen, Yun Ji, and Vahid Jafari

4.1

Introduction and State-of-the-Art of Commercial Oxygen Delignification 67 Chemistry of Delignification and Cellulose Degradation 70 Improving the Reactivity of Residual Lignin 73 Improving Delignification/Cellulose Degradation Selectivity During Oxygen Delignification 79 Improving Pulp Yield by Using Oxygen Delignification 90 Practical Implementation of High Kappa Oxygen Delignification 92 References 93

4.2 4.3 4.4 4.5 4.6

Contents

5

Toward a Better Understanding of Cellulose Swelling, Dissolution, and Regeneration on the Molecular Level 99 Thomas Rosenau, Antje Potthast, Andreas Hofinger, Markus Bacher, Yuko Yoneda, Kurt Mereiter, Fumiaki Nakatsubo, Christian Jäger, Alfred D. French, and Kanji Kajiwara

5.1 5.2

Introduction 99 Cellulose Swelling, Dissolution and Regeneration at the Molecular Level 102 The “Viewpoint of Cellulose” 109 The “Viewpoint of Cellulose Solvents” 113 Conclusion 118 References 120

5.2.1 5.2.2 5.3

6

Interaction of Water Molecules with Carboxyalkyl Cellulose 127 Hitomi Miyamoto, Keita Sakakibara, Isao Wataoka, Yoshinobu Tsujii, Chihiro Yamane, and Kanji Kajiwara

6.1 6.2

Introduction 127 Carboxymethyl Cellulose (CMC) and Carboxyethyl Cellulose (CEC) 128 Differential Scanning Calorimetry (DSC) 131 Small-Angle X-ray Scattering (SAXS) 133 Molecular Dynamics 136 Chemical Modification and Biodegradability 138 Acknowledgments 140 References 140

6.3 6.4 6.5 6.6

7

Analysis of the Substituent Distribution in Cellulose Ethers – Recent Contributions 143 Petra Mischnick

7.1 7.2 7.2.1 7.2.2 7.2.3 7.3 7.3.1 7.3.2 7.3.3 7.4 7.5

Introduction 143 Methyl Cellulose 146 Average DS and Methyl Pattern in the Glucosyl Unit 146 Distribution Along and Over the Chain 149 Summary 153 Hydroxyalkylmethyl Celluloses 153 Hydroxyethylmethyl Celluloses 159 Hydroxypropylmethyl Celluloses 160 Summary 165 Carboxymethyl Cellulose 166 Outlook 166 Acknowledgment 167 References 167

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Contents

8

Adhesive Mixtures as Sacrificial Substrates in Paper Aging 175 Irina Sulaeva, Ute Henniges, Thomas Rosenau, and Antje Potthast

8.1 8.2 8.2.1 8.2.2

Introduction 175 Materials and Methods 177 Chemicals 177 Preparation of Adhesive Mixtures and Films from Individual Components 177 Preparation of Coated Paper Samples 177 Accelerated Heat-Induced Aging 179 GPC Analysis 179 Contact Angle Measurements 180 Analysis of Paper Brightness 180 Results and Discussion 180 GPC Analysis of Adhesive Mixtures and Individual Components 180 Molar Mass Analysis of Paper Samples 182 Contact Angle Analysis 184 UV–Vis Measurements of Paper Brightness 185 Conclusion 186 Acknowledgments 187 References 187

8.2.3 8.2.4 8.2.5 8.2.6 8.2.7 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.4

9

Solution-state NMR Analysis of Lignocellulosics in Nonderivatizing Solvents 191 Ashley J. Holding, Alistair W. T. King, and Ilkka Kilpeläinen

9.1 9.2 9.3

Introduction 191 Solution-state 2D NMR of Lignocellulose and Whole Biomass 195 Solution State 1D and 2D NMR Spectroscopy of Cellulose and Pulp 203 Solution-state NMR Spectroscopy of Modified Nanocrystalline Cellulose 211 Solution State 31 P NMR Spectroscopy and Quantification of Hydroxyl Groups 212 Conclusions and Future Prospects 218 References 219

9.4 9.5 9.6

10

Surface Chemistry and Characterization of Cellulose Nanocrystals 223 Samuel Eyley, Christina Schütz, and Wim Thielemans

10.1 10.2 10.3

Introduction 223 Cellulose Nanocrystals 225 Morphological and Structural Characterization 228

Contents

10.3.1 10.3.2 10.3.3 10.3.4 10.4 10.4.1 10.4.2 10.4.3 10.4.4 10.5

Microscopy 228 Small Angle Scattering 230 Powder X-ray Diffraction 230 Solid-State NMR Spectroscopy 234 Chemical Characterization 237 Infrared Spectroscopy 237 Elemental Analysis 238 X-ray Photoelectron Spectroscopy 240 Other Methods 243 Conclusion 245 Acknowledgments 246 References 246

11

Some Comments on Chiral Structures from Cellulose Derek G. Gray

11.1 11.2 11.3 11.4

Chirality and Cellulose Nanocrystals 253 Can CNC Form Nematic or Smectic-ordered Materials? 255 Why Do Some CNC Films Not Display Iridescent Colors? 256 Is There Any Pattern to the Observed Expressions Of Chirality At Length Scales from the Molecular to the Macroscopic? 257 Acknowledgments 259 References 259

12

Supramolecular Aspects of Native Cellulose: Fringed-fibrillar Model, Leveling-off Degree of Polymerization and Production of Cellulose Nanocrystals 263 Eero Kontturi

12.1 12.2

Introduction 263 Fringed-fibrillar Model: Crystallographic, Spectroscopic, and Microscopic Evidence 264 Leveling-off Degree of Polymerization (LODP) 267 Preparation of Cellulose Nanocrystals (CNCs) 270 Conclusion 271 References 271

12.3 12.4 12.5

253

13

Cellulose Nanofibrils: From Hydrogels to Aerogels 277 Marco Beaumont, Antje Potthast, and Thomas Rosenau

13.1 13.2 13.3 13.3.1 13.3.2 13.3.3

Introduction 277 Cellulose Nanofibrils 278 Hydrogels 282 Cellulose Nanofibrils 284 Composites 288 Modification 293

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Contents

13.4 13.4.1 13.4.2 13.4.3 13.4.4 13.4.5 13.5

Aerogels 296 Drying of Solvogels 297 Mechanical Properties 301 Conductive Aerogels 305 Hydrophobic Aerogels and Superabsorbents 307 Other Applications 315 Conclusion 317 Acknowledgments 318 References 318

14

High-performance Lignocellulosic Fibers Spun from Ionic Liquid Solution 341 Michael Hummel, Anne Michud, Yibo Ma, Annariikka Roselli, Agnes Stepan, Sanna Hellstén, Shirin Asaadi, and Herbert Sixta

14.1 14.2 14.2.1 14.2.2 14.2.3 14.2.4 14.2.5 14.2.6 14.3 14.3.1 14.3.2 14.3.3 14.3.4 14.3.5

Introduction 341 Materials and Methods 347 Pulp Dissolution and Filtration 348 Rheological Measurements 349 Chemical Composition Analysis 349 Molar Mass Distribution Analysis 349 Fiber Spinning 350 Mechanical Analysis of Fibers 351 Results and Discussion 351 Lignocellulosic Solutes 351 Rheological Properties 352 Fiber Spinning 354 Fiber Properties 355 Summary of the Influence of Noncellulosic Constituents on the Fiber Properties 360 Conclusion 361 References 362

14.4

15

Bio-based Aerogels: A New Generation of Thermal Superinsulating Materials 371 Tatiana Budtova

15.1 15.2 15.3 15.4 15.5 15.6 15.7

Introduction 371 Cellulose I Based Aerogels and Their Composites 373 Cellulose II Based Aerogels and Their Composites 378 Pectin-based Aerogels and Their Composites 380 Starch-based Aerogels 386 Alginate Aerogels 386 Conclusions and Prospects 387 References 388

Contents

16

Nanocelluloses at the Oil–Water Interface: Emulsions Toward Function and Material Development 393 Siqi Huan, Mariko Ago, Maryam Borghei, and Orlando J. Rojas

16.1

Cellulose Nanocrystal Properties in the Stabilization of O/W Interfaces 393 Surfactant-free Emulsions 395 Emulsions Stabilized with Modified Nanocelluloses 398 Surfactant-assisted Emulsions 402 Emulsions with Polymer Coemulsifiers 406 Double Emulsions 409 Emulsion or Emulsion-precursor Systems with Stimuli-responsive Behavior 413 Closing Remarks 418 Acknowledgments 418 References 418

16.2 16.3 16.4 16.5 16.6 16.7 16.8

17

Honeycomb-patterned Cellulose as a Promising Tool to Investigate Wood Cell Wall Formation and Deformation 423 Yasumitsu Uraki, Liang Zhou, Qiang Li, Teuku B. Bardant, and Keiichi Koda

17.1 17.2 17.3

Introduction 423 Theory of Honeycomb Deformation 425 HPRC with Cellulose II Polymorphism and Their Tensile Strength 426 Validity of Deformation Models 428 Deposition of Wood Cell Wall Components on the Film of HPBC Film 430 Acknowledgment 432 References 433

17.4 17.5

Index 435

xiii

xv

Author Biography Dr. Thomas Rosenau studied chemistry and received his doctorate from Dresden University of Technology in Germany. After a time as a visiting scientist at North Carolina State University in Raleigh, USA, he joined BOKU University Vienna where he did his habilitation in organic chemistry. He is currently a professor at BOKU University, holding the Chair of Wood, Pulp, and Fiber Chemistry and heading both the Division of Chemistry of Renewable Resources and the Austrian Biorefinery Center Tulln. He is an adjunct professor of Fiber Science at Shinshu University in Japan and an adjunct professor at the Johan Gadolin Process Chemistry Center at Abo Akademi in Turku, Finland. He has been elected as a fellow of the International Academy of Wood Science and the Japanese Academy of Sciences. As of 2018, Dr. Rosenau has published more than 350 peer-reviewed scientific articles and several book chapters. His research interests are in the chemistry of renewable resources, green chemistry, and biorefineries, with a focus on cellulose and lignin analysis, chemistry, and utilization. Dr. Antje Potthast studied chemistry at the University of Technology in Dresden, Germany. She has also studied pulp and paper as a visiting scientist at North Carolina State University in Raleigh, USA. She completed her PhD at Dresden University of Technology, and her habilitation was in wood chemistry at BOKU University Vienna, Austria. She is currently a professor in the Department of Chemistry and is the deputy head of both the Division of Chemistry of Renewable

xvi

Author Biography

Resources and the Austrian Biorefinery Center Tulln. As of 2017, Dr. Potthast has published more than 280 peer-reviewed scientific articles and several book chapters. Her research interests are in chemistry and the analysis of lignocelluloses. Regarding cellulose, her main focus is on the characterization, the modification (pulp, fiber, and paper), and degradation processes. A closely related field that Dr. Potthast interested in is paper conservation science, paper conservation treatments, their sustainability, and the means of stabilizing historic paper materials. Emphasis in the lignin field is placed on the purification and characterization of technical lignins. Dr. Johannes Hell studied food chemistry at both Munich University of Technology in Germany and the University of Vienna in Austria. He completed his PhD at BOKU University Vienna, at the Department of Chemistry and the Department of Food Science, working on innovative wheat bran biorefinery concepts. He turned his long-term interest in and passion for chocolate into a profession, becoming a technical manager at a Viennese chocolate factory.

xvii

List of Contributors Mariko Ago

Marco Beaumont

Bio-Based Colloids and Materials Department of Forest Products Technology School of Chemical Technology Aalto University Aalto Finland

Department of Chemistry Division of Chemistry of Renewable Resources University of Natural Resources and Life Sciences Vienna Tulln Austria

Shirin Asaadi

Department of Forest Products Technology Aalto University Aalto Finland Markus Bacher

Department of Chemistry University of Natural Resources and Life Sciences Vienna Tulln Austria Teuku B. Bardant

Graduate School of Agriculture Hokkaido University Sapporo Japan

Maryam Borghei

Bio-Based Colloids and Materials Department of Forest Products Technology School of Chemical Technology Aalto University Aalto Finland Tatiana Budtova

MINES ParisTech PSL Research University CEMEF – Centre for Materials Forming Sophia Antipolis France

xviii

List of Contributors

Thomas Elschner

Adriaan R. P. van Heiningen

Centre of Excellence for Polysaccharide Research Institute of Organic Chemistry and Macromolecular Chemistry Friedrich Schiller University of Jena Jena Germany

Department of Chemical and Biological Engineering University of Maine Orono ME USA Thomas Heinze

Department of Chemical Engineering Renewable Materials and Nanotechnology Research Group KU Leuven Kortrijk Belgium

Centre of Excellence for Polysaccharide Research Institute of Organic Chemistry and Macromolecular Chemistry Friedrich Schiller University of Jena Jena Germany

Alfred D. French

Sanna Hellstén

Southern Regional Research Center U. S. Department of Agriculture New Orleans LA USA

Department of Forest Products Technology Aalto University Aalto Finland

Kristin Ganske

Ute Henniges

Centre of Excellence for Polysaccharide Research Institute of Organic Chemistry and Macromolecular Chemistry Friedrich Schiller University of Jena Jena Germany

Department of Chemistry Division of Chemistry of Renewables Resources University of Natural Resources and Life Sciences Vienna Tulln Austria

Samuel Eyley

Derek G. Gray

Department of Chemistry McGill University Montreal QC Canada

Andreas Hofinger

Department of Chemistry Division of Chemistry of Renewable Resources University of Natural Resources and Life Sciences Vienna Tulln Austria

List of Contributors

Ashley J. Holding

Ilkka Kilpeläinen

Materials Chemistry Division Department of Chemistry University of Helsinki Helsinki Finland

Materials Chemistry Division Department of Chemistry University of Helsinki Helsinki Finland

Siqi Huan

Alistair W. T. King

Bio-Based Colloids and Materials Department of Forest Products Technology School of Chemical Technology Aalto University Aalto Finland

Materials Chemistry Division Department of Chemistry University of Helsinki Helsinki Finland

Michael Hummel

Department of Forest Products Technology Aalto University Aalto Finland Vahid Jafari

Circa Group Pty Ltd Coburg North VIC Australia

Yuji Kinose

Institute for Chemical Research Kyoto University Kyoto Japan Keiichi Koda

Division of Environmental Resources Research Faculty of Agriculture Hokkaido University Sapporo Japan Eero Kontturi

BAM Federal Institute for Materials Research and Testing Berlin Germany

Department of Bioproducts and Biosystems School of Chemical Engineering Aalto University Aalto Finland

Kanji Kajiwara

Qiang Li

Faculty of Textile Science and Technology Shinshu University Ueda Nagano Japan

Graduate School of Agriculture Hokkaido University Sapporo Japan

Christian Jäger

xix

xx

List of Contributors

Yun Ji

Fumiaki Nakatsubo

Department of Chemical Engineering University of North Dakota Grand Forks ND USA

Research Institute for Sustainable Humanosphere Kyoto University Kyoto Japan

Yibo Ma

Antje Potthast

Department of Forest Products Technology Aalto University Aalto Finland

Department of Chemistry Division of Chemistry of Renewable Resources University of Natural Resources and Life Sciences Vienna Tulln Austria

Kurt Mereiter

Department of Chemistry Vienna University of Technology Vienna Austria Anne Michud

Department of Forest Products Technology Aalto University Aalto Finland

Orlando J. Rojas

Bio-Based Colloids and Materials Department of Forest Products Technology School of Chemical Technology Aalto University Aalto Finland Annariikka Roselli

Petra Mischnick

Braunschweig University of Technology Faculty of Life Science Institute of Food Chemistry Braunschweig Germany Hitomi Miyamoto

Yokohama National University Yokohama Japan

Department of Forest Products Technology Aalto University Aalto Finland

List of Contributors

Thomas Rosenau

Agnes Stepan

Department of Chemistry Division of Chemistry of Renewable Resources University of Natural Resources and Life Sciences Vienna Tulln Austria

Department of Forest Products Technology Aalto University Aalto Finland Irina Sulaeva

Johan Gadolin Process Chemistry Centre Åbo Akademi University Turku Finland

Department of Chemistry Division of Chemistry of Renewables Resources University of Natural Resources and Life Sciences Vienna Tulln Austria

Keita Sakakibara

Toshiyuki Takano

Institute for Chemical Research Kyoto University Kyoto Japan

Division of Forest and Biomaterials Science Graduate School of Agriculture Kyoto University Kyoto Japan

and

Christina Schütz

Department of Experimental Soft Matter Physics University of Luxembourg Esch-sur-Alzette Luxembourg Herbert Sixta

Department of Forest Products Technology Aalto University Aalto Finland

Wim Thielemans

Department of Chemical Engineering Renewable Materials and Nanotechnology Research Group KU Leuven Kortrijk Belgium Yoshinobu Tsujii

Institute for Chemical Research Kyoto University Kyoto Japan

xxi

xxii

List of Contributors

Yasumitsu Uraki

Yuko Yoneda

Division of Environmental Resources Research Faculty of Agriculture Hokkaido University Sapporo Japan

Academic Institute College of Agriculture Shizuoka University Shizuoka Japan

Isao Wataoka

Liang Zhou

Faculty of Fiber Science and Engineering Kyoto Institute of Technology Kyoto Japan

Department of Material Science and Engineering Anhui Agricultural University Hefei China

Chihiro Yamane

Department of Home Economics Kobe Women’s University Kobe Japan

xxiii

Preface The pulp and paper industries have always been mainstays of national economies worldwide. This belies the general perception of cellulosic products as being conventional, relatively low-cost bulk items. Some time ago, cellulosic products were widely taken for granted as commodities that were produced in huge amounts by not-so-complicated procedures, familiar for decades if not for centuries. Cellulosics were not perceived as high-tech materials and were rarely linked in the minds of users and customers to cutting-edge research. Fancy cell phones, the newest cars, and advanced computer technologies intrigued consumers all over the world, while a new type of paper, a new tissue brand, or a novel cellulosic fiber produced only yawns. But recent developments, connected to increased environmental awareness, recognition of global climate problems, and the advent of bioeconomies and biorefineries, have brought cellulosics back into public consciousness as valuable biomaterials and chemical feedstocks. In this context, the pulp and paper industries are increasingly regarded as businesses engaged in high-tech innovation. The emergence of biorefinery techniques has also newly highlighted the advantages of celluloses in regard to conversion, recycling, biomineralization, and permanence. Today, cellulose science is one of the most rapidly advancing fields in the chemical and material sciences. Cellulose plays and will continue to play a central role in the worldwide emergence of bioeconomies, leading the way from fossil-based chemical industries to true biorefineries. Today’s well-established uses for cellulose, including paper products, tissues, fibers, and cellulose derivatives, are only the beginning. Applications will emerge of which we are not yet brave enough to dream. This book will encourage you to start dreaming about cellulose. Cellulose science today is a thriving tree with many branches, which is not surprising for the major bioresource on our planet. Organic chemistry, analytical chemistry, and material science are key subjects in cellulose research, but their interplay is not always adequate or smooth. Organic chemistry would remain a mere hunt for new compounds if a sound input from material science was missing or if the use of new derivatives in applications was blocked.

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Preface

Both material developments and chemical modifications succeed only if they are accompanied by proper physicochemical analysis. Without an in-depth structural understanding of starting materials and products, reasonable conversion attempts are doomed. No new material will succeed permanently in the market if its chemistry and structure are not understood in depth. In cellulose science and technology, organic chemistry, physico-chemical analysis, and material research are thus dependent on one another. Still, an isolated view of cellulose from one of those viewpoints is common in the literature, rather than a unifying approach that tries to combine and encompass all three perspectives on an equal footing. That shortcoming inspired the present book, which attempts to bring together the views of organic chemists, analytical chemists, and material scientists in order to present a unified view of cellulose, a “holistic” treatment in the original sense of the word. The book’s three parts address cellulose from the viewpoints of organic chemistry, analytical chemistry, and material science, always trying not to block alternative views when looking from one perspective. Leading international figures in cellulose science introduce their current work and present their latest research findings, and readers will benefit from the interplay of organic, analytical, and material chemistry throughout the chapters. We sincerely hope that this book will not only inform and educate, but that it will be able to convey the fascination of modern cellulose science: its versatility, its applicability, its challenges, and its bright future. This book shows why a biomaterial that has been used by humanity for more than 6000 years still intrigues researchers worldwide and makes scientists stand in awe before its mysteries. January 2018, Vienna

Thomas Rosenau

xxv

Acknowledgements We would like to acknowledge the Department of Chemistry at BOKU University Vienna for providing infrastructure and support for this book project. Our thanks goes to Dr. Alfred D. French, New Orleans, Editor-in-Chief of the journal Cellulose, for his continuous support, encouragement, and teaching of editorial skills.

1

1 Aminocelluloses – Polymers with Fascinating Properties and Application Potential Thomas Heinze, Thomas Elschner, and Kristin Ganske Centre of Excellence for Polysaccharide Research, Institute of Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller University of Jena, Humboldtstraße 10, D-07743 Jena, Germany

1.1 Introduction Cellulose is a linear d-glucan containing β-1 → 4 linkages and is the world’s most abundant natural polymer with an estimated annual global production of about 1.5 × 1012 tons and, hence, a very important renewable and sustainable resource [1]. Although unmodified cellulose is used largely as paper, board, and fibers, there is huge space to design novel and advanced products based on cellulose by its chemical modification. In particular, esters and ethers of cellulose are most important [1, 2]. Due to their low-cost production, biodegradability, and low-toxicity cationized polysaccharides are promising in fields of effluent treatment, papermaking, and food, cosmetic, pharmaceutical, petroleum, and textile industries, as well as in analytical chemistry and molecular biology [3]. In particular, cationic cellulose derivatives gain increasing interest in different scientific and industrial fields, e.g. as flocculation agents [4], being an alternative to toxic polyacrylamide. In Germany, the disposal of sludge treated with polyacrylamides has been forbidden in areas under cultivation since 2014 [5]. Considering the recent literature, the huge amount of publications was summarized in reviews about cationic synthetic polyelectrolytes [6] as well as cationized polysaccharides (amino and ammonium hydroxypropyl ethers) [3]. However, in this chapter, the authors will not review the cationic ethers; the overview refers to cationic esters, 6-deoxy-6-amino cellulose derivatives, and amino carbamates of cellulose. In spite of the industrial applications that are usually associated with cationic polymers, a variety of advanced polymer coatings providing sophisticated features, e.g. biosensors or immuno assays, will be presented.

2

Cellulose Science and Technology

1.2 Amino-/ammonium Group Containing Cellulose Esters 1.2.1 (3-Carboxypropyl)trimethylammonium Chloride Esters of Cellulose An efficient approach to cationic cellulose derivatives is the esterification of the hydroxyl groups with cationic carboxylic acids. Activated carboxylic acids such as acyl chlorides or acid anhydrides are not appropriate due to their limited solubility, availability, and the formation of acidic by-products. However, the esterification applying imidazolides obtained from the corresponding carboxylic acid and N,N-carbonyldiimidazole (CDI) is a mild and efficient synthesis strategy [2]. To synthesize cationic cellulose esters (3-carboxypropyl)trimethylammonium chloride was activated with CDI in dimethylsulfoxide (DMSO) separately and allowed to react with cellulose dissolved in N,N-dimethylacetamide (DMA)/LiCl [7]. Thus, a product with a degree of substitution (DS) of 0.75 was accessible that could be characterized by 13 C NMR spectroscopy (Figure 1.1). Cellulose (3-carboxypropyl)trimethylammonium chloride esters adsorbed on cellulose films may trigger the protein adsorption, which is a key parameter in the design of advanced materials for a variety of technological fields [8]. The protein affinity to the surface can be controlled by the charge density and solubility, adjusted by the pH value, the concentration of protein and the DS of the tailored cationic cellulose derivative. To understand the influence of the cationic cellulose ester on the protein affinity, the interaction capacity with fluorescence-labeled bovine serum albumin (BSA) at different concentrations and pH values was carried out (Figure 1.2). The adsorbed material was quantified applying QCM-D (quartz crystal microbalance with dissipation monitoring, wet mass) and MP-SPR (multi-parameter surface plasmon resonance, dry mass). Thus, the amount of coupled water in the layer could be evaluated by a combination of QCM-D and surface plasmon resonance (SPR) data. According to these studies the interaction decreases in order of 8

O 6s

7 O

10 9

11 DMSO CI– 11 +N

8

O 7

RO

O OR

9

10

2, 3, 5 6s 1

4

6

Figure 1.1 13 C NMR spectrum of cellulose (3-carboxypropyl)trimethylammonium chloride ester in DMSO-d6 . Source: Vega et al. 2013 [7]. Reproduced with permission of American Chemical Society.

Aminocelluloses – Polymers with Fascinating Properties and Application Potential

Cellulose

pH 5

pH 6

A

pH 7

B

pH 5

A

B

pH 6

A

B

pH 7

Figure 1.2 Cyclic olefin polymer slides equipped with cellulose and cellulose (3-carboxypropyl)trimethylammonium chloride ester incubated with different concentrations of labeled BSA (1000, 500, 100, 10, 1, 0.1, 0.01, and 0.001 μg mL−1 ) at different pH values. A) low DS; B) high DS [8]. Reproduced with permission of American Chemical Society. (See insert for color representation of this figure.)

pH 5 > pH 6 > pH 7 and DShigh > DSlow , respectively. The adsorption of BSA may be adjusted over a range from 0.6 to 3.9 mg m−2 (dry mass). This approach is suitable to utilize BSA as blocking agent on the surface and achieve selective functionalization of cellulosic surfaces by functional proteins (e.g. antibodies). Another application of (3-carboxypropyl)trimethylammonium chloride esters of cellulose is the surface modification of pulp fibers in order to preserve the inherent bulk properties (e.g. low density, mechanical strength) and to improve the properties of the fiber surface (e.g. wetting behavior, bacteriostatic activity) [7]. In recent studies, polyelectrolyte complexes (PECs) were prepared applying the cationic cellulose ester and anionic xylan derivatives, which were subsequently adsorbed to wood fibers. The adsorption process was studied using polyelectrolyte titration and elemental analysis. The fiber surfaces modified were characterized by X-ray spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The measurements evidence the interaction between the pulp fibers and the PECs and provide useful information about the adsorption process. In addition to monofunctional cationic cellulose (3-carboxypropyl)trimethylammonium chloride esters, multifunctional photoactive derivatives provide advanced features in context with the design of smart materials. However, sufficient DS values are required to give a pronounced photochemical response and water solubility. Therefore, different cellulose 2-[(4-methyl-2-oxo-2Hchromen-7-yl)oxy]acetates were prepared applying CDI and the corresponding

3

O

O

O

O

O

O OH O HO

OH

O

CDI O

+ HO

O O

O

RO

O (DMA/LiCI) 70 °C, 20 h R = H or

O

OR

O

+ O

O

O

N + Cl–

O

O O

CDI

HO

O Cl– RO + N

(DMA/LiCI) 70 °C, 20 h

O

O O

O

Cl– + N

O According to DS and distribution

R = H or

O

O

O

or

O

O According to DS and distribution

Figure 1.3 Synthesis scheme of cellulose 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]acetates and cellulose 2-[(4-methyl-2-oxo-2H-chromen7-yl)oxy]acetate [4-(N,N,N-trimethylamonium) chloride] butyrates by in situ activation of 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]acetic acid and (3-carboxypropyl)trimethylammonium chloride with N,N-carbonyldiimidazole (CDI) in N,N-dimethylacetamide/LiCl (DMA/LiCl). Source: Wondraczek et al. 2012 [9]. Reproduced with permission of Springer Nature.

Aminocelluloses – Polymers with Fascinating Properties and Application Potential

carboxylic acid in DMA/LiCl [9]. Subsequently, (3-carboxypropyl)trimethylammonium chloride activated with CDI forming the corresponding imidazolide was allowed to react with the photoactive cellulose derivative to obtain a water-soluble product (Figure 1.3). The partial DS values could be determined by a combination of UV–Vis spectroscopy and elemental analysis. The DS is in the range from 0.05 to 0.37 for the photoactive moiety and from 0.19 to 0.34 for the cationic group. Figure 1.4 Crosslinking by [2+2] cycloaddition of the photoactive cellulose derivative on fibers (a), storage stimulus at varying relative humidity obtained by dynamic mechanical analysis (b), white/circles: unmodified fibers, light gray/triangles: fibers modified with photoactive cationic cellulose derivative, dark gray/squares: modified fibers irradiated upon sheet formation.

O

O O

O O O

O

O

O

O O

O

O

O

O O (a)

Storage modulus (GPa)

2.6 2.4 2.2 2.0 1.8 1.6 10

20

30 40 50 60 Relative humidity (%) (b)

70

80

5

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Cellulose Science and Technology

Multifunctional, i.e. photoactive and cationic, cellulose esters were used for the coating of pulp fibers to yield new fiber-based materials, whose properties could be triggered by an external stimulus [10]. The adsorption of the polymer onto the fiber was studied by UV–Vis spectroscopy and SPR. It turned out that electrostatic interaction is the main driving force of the adsorption. However, there is a contribution of hydrophobic interactions between the fibers and the cellulose derivatives and between the polymer chains themselves. Considering the adsorption behavior, UV–Vis measurements of the solutions applied for coating led to a mechanism according to the Freundlich model. ToF-SIMS imaging revealed evenly distributed derivatives on the fiber surfaces independent of the dosage and DS of the photoactive group. Moreover, UV irradiation of the modified fibers results in crosslinking by [2+2] cycloaddition of the photoactive moieties and both light adsorption and fluorescence behavior change (Figure 1.4). Moreover, there is an enhancement of the tensile strength and Z-directional tensile strength of the pulp fibers by 81% and 84% compared to the unmodified fiber network [11]. The stiffness of individual fibers is increased by 60%. It is supposed that this work opens new pathways for the development of smart bio-based materials being superior to classical pulp and paper. Recently, 6-deoxy-6-azido-carboxmethyl cellulose could be synthesized [12]. Although it is possible to adsorb this anionic polymer on cellulose mediated by multivalent metal cations [13], it is much more promising to use cellulose modified with cationic moieties for this approach due to the anionic nature of pulp and cellulose surfaces in general. Thus, conversion of 6-deoxy-6-azido cellulose with carboxypropyltrimethylammonium chloride in the presence of CDI yielded 6-deoxy-6-azido cellulose-2,3-O-[4-(N,N,Ntrimethyl-ammonium)]butyrate chloride (Figure 1.5) [14]. In a different approach, this multifunctional cellulose derivative could be applied for coating of fiber interfaces in aqueous media. According to this concept, the cationic cellulose derivative may adsorb to the surface anionic groups from the cellulose fiber and the azido moiety provides the covalent linkN3 age of various functionalities via copper(I)-catalyzed O azide-alkyne Huisgen cycloaddition (click chemistry) O HO [15]. Thus, photoactive- as well as amino-groupO containing fibers could be obtained applying 1O ethynylpyrene or propargylamine. It was shown that the N+ CI– cycloaddition between reactive fibers and alkyne groups Figure 1.5 Structure of could be carried out in aqueous medium and in organic solvents. Field emission scanning electron microscopy 6-deoxy-6-azido cellulose-2,3-O-[4-(N,N, (FE-SEM) images revealed the preservation of the fiber N-trimethylammonium)] structure during the preparation of photo-fibers. butyrate chloride.

Aminocelluloses – Polymers with Fascinating Properties and Application Potential

1.2.2

Cellulose-4-(N-methylamino)butyrate (CMABC)

An alternative synthesis path to obtain cationic cellulose esters is the ring-opening of lactams in the presence of p-toluenesulfonic acid chloride [16]. Cellulose, dissolved in N-methyl-2-pyrrolidone (NMP)/LiCl, or 1-butyl-3-methylimidazolium chloride, could be transformed into the cationic biopolymer derivative applying NMP, N-methyl-2-piperidone, ε-caprolactam and N-methyl-ε-caprolactam. The lactam, e.g. NMP, forms a reactive intermediate in the presence of tosyl chloride according to the Vilsmeier–Haack reaction (Figure 1.6). Thus, a cationic cellulose ester is formed in the second step, i.e. the iminium ion reacts with the hydroxyl groups of the biopolymer and subsequent ring opening by water occurs. The products obtained possess DS values in the range from 0.24 to 1.17.

⊕

OTos

N CH3

CI⊝

O N CH3

O + CH3

S CI O ⊕

CI N CH3 Tos⊝

CH3

R–OH

H2O

N

⊕

CI⊝

CH3

O

O

R

R

CH3 O R

CI

O

N O OH

N

R

H

⊕

N

CH3

H CI⊝

Figure 1.6 Reaction scheme of the conversion of alcohols (R–OH) with N-methyl-2-pyrrolidone in the presence of p-toluenesulfonic acid chloride.

7

Cellulose Science and Technology

With respect to applications of cellulose-4-(N-methylamino)butyrate (CMABC), the stability in aqueous solutions and the charging behavior of amino moieties was studied [17]. Samples of the cationic cellulose esters do not hydrolyze at pH values up to 7. Decomposition of the biopolymer derivatives in cellulose and carboxylate takes place at higher pH values as revealed by titration experiments, FTIR and Raman spectroscopic studies. However, the application of CMABC in fields of flocculants or thickener is promising due to the decomposition in alkaline media subsequent to its use. As mentioned, the improvement of the properties of paper and fibers by coatings of cationic cellulose derivatives gain increasing interest. Thus, it is essential to analyze interactions of positively charged polymers with cellulose surfaces. However, monitoring of the adsorption on fibers is difficult, laborious and requires a combination of analytical techniques. An elegant way to study the adsorption behavior of CMABC in aqueous solution is the use of cellulose model thin films applying a highly sensitive surface technique such as QCM-D [18]. It turned out that at high ionic strength (25–100 mM NaCl) high adsorption is observed at pH 7 (Δf = −15 to −17 Hz), while at lower ionic strength (1–10 mM) the adsorption decreases (Δf = −2 to −12 Hz) indicated by lower absolute values of the shifts in frequency (Figure 1.7). A change in pH value from 7 to 8 caused an increased adsorption. The conformation of CMABC at low electrolyte concentration is flat-like leading to a thin layer on the cellulose substrate, which was shown by atomic force microscopy (AFM). Increasing the ionic strength, the conformation of the polymer is structured like a particle (coil). This phenomenon is associated with reduced solubility of CMABC and more material is adsorbed on the surface. The irreversibility of the adsorption process is related to interactions of cellulose and CMABC possessing structural similarities. The surface wettability increases with an increasing amount of cationic polymer on the surface. CMABC does not adsorb onto cellulose at pH values of 3 and 5. The results were validated by the determination of 10 Frequency change, f3 (Hz)

8

1 mM NaCI 10 mM NaCI 100 mM NaCI

0

–10

–20 Sample NaCI

Water

–30 0

20

40

60 80 100 120 140 160 Time (min)

Figure 1.7 Changes in the third overtone frequency (Δf 3 ) for the adsorption of cellulose-4-(N-methylamino) butyrate onto cellulose surfaces at pH 7 depending on NaCl concentration.

Aminocelluloses – Polymers with Fascinating Properties and Application Potential

the nitrogen content obtained from XPS. The amount of electrolyte incorporated into the films could be determined. The adsorption on the hydrophilic and negatively charged substrate, silicon dioxide coated quartz crystals, was not observed by QCM-D measurements. The mechanism could not be explained unambiguously up to now. However, high charge, steric hindrance induced by inter- and intramolecular hydrogen bonding and reduced affinity of CMABC to the rather rigid SiO2 surfaces seem to be relevant parameters.

1.3 6-Deoxy-6-amino Cellulose Derivatives Amino group containing biopolymers are of huge interest in field of functional surface coatings due to the biocompatible environment and the accessible amino groups being useful for immobilization of enzymes or antibodies (Figure 1.8). The design of soluble amino cellulose derivatives is carried out

Enzyme Fluorescence dye

Antibody

Specific ligand

Iron oxide nanoparticle

Cells Transfection agent e.g. PEI

DNA

Figure 1.8 Examples of stabilized iron oxide nanoparticles modified with various multifunctional ligands and receptors. Source: Heinze et al. 2015 [19]. Reproduced with permission of John Wiley & Sons.

9

10

Cellulose Science and Technology

after conversion of hydroxyl groups of the polysaccharide into good leaving groups. Based on tosyl cellulose, nucleophilic displacement reactions (SN ) could be performed at primary position 6 applying di- and oligo-amines [19]. In order to prevent any crosslinking, an excess of bi- or multifunctional amine must be applied. Varying the structure of the amine, a tunable spacer length, different pK a values and charge distributions, hydrophilic/lipophilic balance and redox chromogenic properties are provided. Moreover, modification of the secondary hydroxyl groups prior to the SN reaction is appropriate to tailor the properties of the products. Usually, esterification of the secondary hydroxyl groups could be applied to adjust the solubility of the biopolymer derivatives. 1.3.1 Spontaneous Self-assembling of 6-Deoxy-6-amino Cellulose Derivatives In addition to the solubility of amino cellulose in water or organic solvents depending on the detailed structure, extraordinary solution properties of the aqueous solutions were found. Reversible association products, which typically occur for proteins, could be discovered by analytical ultracentrifugation [20]. Sedimentation coefficient distributions of water-soluble 6-deoxy-6-amino celluloses were obtained from sedimentation velocity experiments in the analytical ultracentrifuge for different solute loading concentrations. The sedimentation coefficient distributions show between four and five discrete species with a stepwise increase in sedimentation coefficient. This behavior was found for the first time for polysaccharides and changes the whole conception of carbohydrate molecular interaction phenomena. Thus, it is very interesting in context with structural modeling of interfacial material surfaces with biological recognition functions at the molecular and cellular level. The partially reversible interactions of 6-deoxy-6-amino cellulose may be adapted to other biomolecules. For example, amino cellulose has a high affinity to glycoproteins and proteoglycans decorated with sugars, which provide receptor structures for an extracellular matrix (ECM). Recently, even a fully reversible self-association of tetramers was discovered for 6-deoxy-6-(ω-aminoethyl)amino cellulose. Moreover, these tetramers of amino cellulose chains associate further into supra-molecular complexes [21] (Figure 1.9). In addition to self-association in solution, the formation of ultrathin and transparent films of amino celluloses takes place by self-assembling on planar substrates like glass, gold, and Si-wafer that could be proven by AFM. Appling 5% solutions of amino cellulose (in water or organic solvents), a nano-scaled topography is obtained by tipping, spraying, and spin coating [22–24]. The tendency of film forming decreases with decreasing basicity of the amine residue. Increasing spacer length reduces the film quality toward higher brittleness of the layers. In addition, the occurrence of gel-like particles formed in a solution of 1,4-phenylenediamine (PDA) cellulose tosylate in DMA should

Aminocelluloses – Polymers with Fascinating Properties and Application Potential

H2 N

H2N OH HO

O HO O OH

H 2N

OH O NH

NH O HO

O HO O OH

O

S

O O

OH

H2N

H2N

O NH O HO

O HO O OH

OH O

NH O HO

NH

H2 N

O HO O OH

OH NH

H2 N

O

NH O HO O

O HO O

O S

O H2 N

12 10

c (s)

8 6 4 2 0 0

2 4 6 Sedimentation coefficient (S)

OH NH

8

Figure 1.9 Reversible tetramerization and further higher-order association of the polysaccharide 6-deoxy-6-(ω-aminoethyl)amino cellulose.

O

O

11

12

Cellulose Science and Technology

be considered, which became visible after about one week of storage at 4 ∘ C. Similar effects of gel formation were observed for other amino celluloses [25–27]. The aggregation behavior is influenced by the spacer structure, the basicity of the amino groups, substituents at position 2 and 3, support material and the conditions such as pH value and temperature [22, 26]. The support material possesses a very low roughness, preferably. Gold or glass coated with SiO2 and subsequently with an organopolysiloxane are suitable surfaces for amino cellulose coatings [22, 26]. Alkylene diamine and oligoamino cellulose solutions form topographically flat films (topographies 4. When a pectin–silicic acid mixture was prepared at pH 1.5 the gelation of both components was very slow; at pH 3.0 HM pectin gelled within 10 minutes followed by slower gelation of the silicic acid and at pH 5.0 there was no gelation of pectin but rapid gelation of the silica sol within 10 minutes. The morphology of pectin–silica aerogels at various pH is shown in Figure 15.11 [54]. It reveals a 3D open-pore network consisting of particle

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Cellulose Science and Technology

134°

(b)

40°

(c)

25°

20 wt%

(a)

130°

(e)

74°

(f)

73°

10 wt%

(d)

(g)

135°

(h)

146°

(i)

131°

5 wt%

Finer microstructure

(j)

148°

(k)

148°

(l)

142°

0 wt%

384

500 nm pH 1.5

pH 3.0

pH 5.0

Increasing phase separation

Figure 15.11 SEM images of the reference silica (without pectin) and pectin–silica aerogels prepared at pH 1.5, 3, and 5 and various pectin concentrations. Scale bar corresponds to 500 nm. The numbers in insets are water contact angles. Source: Zhao et al. 2015 [54]. Reprinted with permission of John Wiley & Sons.

aggregates of tens of nanometers in size; pectin “fibers” can also be detected for some formulations. Neat silica aerogel is composed of aggregates of colloidal particles linked together in a pearl-necklace-type network structure [55]. Pure pectin aerogel displays a network of polymer “nanofibers” with diameters of a few tens of nanometers primarily with mesopores and small macropores [29]. Pectin–silica aerogels gelled at pH 1.5 do not show any visible pectin

Bio-based Aerogels: A New Generation of Thermal Superinsulating Materials

Figure 15.12 Thermal conductivity (measured with a custom-built guarded hot plate device designed for small samples (guarded zone: 50 × 50 mm2 , measuring zone: 25 × 25 mm2 ) as a function of pectin concentration in composite aerogel. The “pH 2.5 silica aerogel” was used as a reference because the silica aerogel prepared at pH 1.5 is very fragile. Source: Zhao et al. 2015 [54]. Reprinted with permission of John Wiley & Sons.

Thermal conductivity (mW m−1 K−1)

“fibers” at all pectin concentrations studied (Figure 15.11a,d,g) [54]. At this pH, pectin most probably did not gel but simply coagulated when water was replaced by ethanol. Aerogels prepared at pH 3 and pH 5 show a coarser microstructure than those prepared at pH 1.5 for the same pectin concentration. Aerogels with pectin content above 10 wt% at pH 3 and 5 contain pectin “nanofibers.” At the highest pectin concentrations and pH values, the mixture segregates into pectin-rich and silica-rich domains (compare Figure 15.11b,c). For all investigated pH, an increase of pectin concentration in the mixture leads to finer microstructures with smaller silica secondary particles and pores. To summarize the structural data, the morphology of pectin–silica aerogels is controlled by both pH value and pectin content. Slow gelation of both silica and pectin at pH 1.5 results in a homogenous spatial distribution of both components and a finer microstructure. A quick gelation of at least one of the components at pH > 2 results in heterogeneous morphology with pectin-rich and silica-rich domains with larger pores. Pectin–silica aerogel density slightly increases with the increase of pectin concentration in the mixture and varies, in general, from 0.13 to 0.19 g cm−3 [54]. Specific surface area slightly decreases with the increase of pH and varies for all samples from 750 to 850 m2 g−1 . The mechanical properties of pectin–silica aerogels are improved as compared to the reference silica aerogels [54]. For example, aerogels prepared at pH 1.5 do not break up to a strain of 80%. The compressive strength and Young modulus increase with the increase of pectin content. It should be noted that dust release from pectin–silica aerogels is two to ten times lower than that of the reference silica samples. The thermal conductivity of pectin–silica aerogels as a function of pectin concentration in the mixture for all pH used is shown in Figure 15.12 [54]. All 19

pH 1.5 pH 3 pH 5 pH 2.5

18 17 16 15 14 0

5

10 15 Pectin (wt%)

20

385

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Cellulose Science and Technology

composite aerogels are thermal superinsulating materials, with the best performance corresponding to those prepared at pH 1.5.

15.5 Starch-based Aerogels Only one article and one patent coming from the same group mention the thermal conductivity of starch aerogels [56, 57]. Starch aerogels were prepared via gelatinization from room temperature to 95 ∘ C with a heating rate around 2 ∘ C min−1 under mixing, aging at 5 ∘ C overnight followed by coagulation in ethanol and drying with sc CO2 . The results are shown in Table 15.2. The most promising starch type in terms of the lowest thermal conductivity is high amylose corn starch [48, 49]. It has the lowest density and the highest specific surface area indicating the presence of mesopores. Our recent results [58] confirm that at a certain amylose/amylopectin ratio starch aerogels (or Aerostarches) can possess thermal superinsulating properties. For pea starch aerogels, the lowest conductivity values were 0.021–0.022 W m−1 K−1 .

15.6 Alginate Aerogels There is only one publication reporting thermal conductivity of alginate aerogel: 0.018–0.022 W m−1 K−1 [25]. Alginate solutions were mixed with calcium carbonate and gelled under CO2 at 5 MPa and 298 K. Water was then replaced by ethanol and coagulated alginate dried with sc CO2 . The density varied from 0.06 to 0.24 g cm−3 and specific surface area from 260 to 545 m2 g−1 . Table 15.2 Characteristics of starch aerogels.

Starch type

Starch Specific concentration surface in solution Density area (wt%) (g cm3 ) (m2 g)

Thermal conductivity (W m−1 K−1 ) References

Wheat

8

0.26

116

0.036

[48]

Corn

8

0.29

50

0.033

[48]

High amylose corn 8

0.15

145

0.024

[48]

Pea starch

0.14

200–250 0.021–0.022 [58]

5

Source: Data taken from Ref. [48] and our recent results.

Bio-based Aerogels: A New Generation of Thermal Superinsulating Materials

15.7 Conclusions and Prospects Thermal properties of bio-aerogels have been presented and discussed. The focus was to review their potential as thermal superinsulating materials. To date, most of the aerogels reported in literature are based on cellulose, either on cellulose I or cellulose II. However, except for a particular case of NFC aerogel with the lowest thermal conductivity reported around 0.018 W m−1 K−1 [32, 33], cellulose-II based aerogels do no show thermal superinsulating properties. The reasons could be the morphology of cellulose II aerogels: the presence of too large macropores that excludes the Knudsen effect and too thick pore walls, which increases the conduction of the solid phase. Cellulose in both forms, either nanofibrillated or coagulated, was also used to reinforce silica aerogels that are thermal superinsulating materials but mechanically fragile. Two main ways are possible, either dispersing NFC in silica sol, or impregnating coagulated cellulose with silica sol. In the former case, the thermal conductivity was as low as 0.014–0.015 W m−1 K−1 that are typical values for silica aerogel. In the latter case, the conductivity below that of air (0.021–0.022 W m−1 K−1 ) was obtained when cellulose was hydrophobized. However, in the case of composite aerogels, it is not cellulose but silica phase that is “responsible” for thermal superinsulating properties. Very little is known about the thermal conductivity of other bio-aerogels. Pectin aerogels turned out to be extremely promising materials in terms of thermal insulation. Both acid-gelled and calcium-gelled pectin aerogels have thermal conductivity from 0.016 to 0.020 W m−1 K−1 . Starch and alginate also seem to be promising polysaccharides: the lowest values of conductivity obtained are 0.021–0.022 and 0.018 W m−1 K−1 , respectively. Because bio-aerogels are very “young” materials, the correlations between polysaccharide type, aerogel preparation conditions, morphology, and properties are not yet well understood. It should also be taken into account that thermal conductivity is not easy to measure in the region of values below the conductivity of air. If focusing on aerogel thermal properties, one of the fundamental questions is “why are cellulose II based aerogels not thermally superinsulating materials while starch is?” To put it differently: why do pectin, alginate, and some starches show fine morphology as aerogels, with small pores and thin pore walls, and cellulose II does not? More systematic studies and screenings of the thermal properties of aerogels based on other polysaccharides are needed. When bio-aerogels from nonmodified polysaccharides are hydrophilic, moisture adsorption leads to aging and an increase of thermal conductivity. This is a serious drawback if these materials are considered for engineering applications. Protection from water vapors or chemical modification of the polysaccharides is needed to preserve their initial properties. The measurements of conductivity can be biased by aging.

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Finally, because bio-aerogels are based on polysaccharides that are “humanfriendly” (nontoxic, biocompatible, and biodegradable), they can be used in many areas that are “forbidden” for synthetic aerogels. Scaffolds for bio-medical applications, matrices for controlled release of drugs, wound dressings, etc., are the areas widely open for the application of bio-aerogels. Thermal conductivity can be used not only as a property per se, but as a measure of bio-aerogel “finesse”.

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ica gels and aerogels prepared with new polymeric precursors. J. Non-Cryst. Solids 186: 1–8. Pekala, R.W. (1989). Organic aerogels from polycondensation of resorcinol with formaldehyde. J. Mater. Sci. 24: 3221–3227. Koebel, M., Rigacci, A., and Achard, P. (2012). Aerogel-based thermal superinsulation: an overview. J. Sol-Gel Sci. Technol. 63: 315–339. Rao, V., Bhagat, S.D., Hirashima, H., and Pajonk, G.M. (2006). Synthesis of flexible silica aerogels using methyltrimethoxysilane (MTMS) precursor. J. Colloid Interface Sci. 300: 279–285. Meador, M.A.B., Weber, A.S., Hindi, A. et al. (2009). Structure-property relationships in porous 3D nanostructures: epoxy-cross-linked silica aerogels produced using ethanol as the solvent. ACS Appl. Mater. Interfaces 1: 894–906. Wang, X. and Jana, S.C. (2013). Synergistic hybrid organic−inorganic aerogels. ACS Appl. Mater. Interfaces 5: 6423–6429. Li, X., Wang, Q., Li, H. et al. (2013). Effect of sepiolite fiber on the structure and properties of the sepiolite/silica aerogel composite. J. Sol-Gel Sci. Technol. 67: 646–653. Markevicius, G., Ladj, R., Niemeyer, P. et al. (2017). Ambient-dried thermal superinsulating monolithic silica-based aerogels with short cellulosic fibers. J. Mater. Sci. 52: 2210–2221. Gavillon, R. and Budtova, T. (2008). Aerocellulose: new highly porous cellulose prepared from cellulose–NaOH aqueous solutions. Biomacromolecules 9: 269–277. Liebner, F., Haimer, E., Potthast, A. et al. (2009). Cellulosic aerogels as ultra-lightweight materials part 2: synthesis and proper-ties. Holzforschung 63: 3–11. Tsioptsias, C., Stefopoulos, A., Kokkinomalis, I. et al. (2008). Development of micro- and nano-porous composite materials by processing cellulose with ionic liquids and supercritical CO2 . Green Chem. 10: 965–971.

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aerogelsfrom ionic liquid solutions. Carbohydr. Polym. 75: 125–129. 13 Sescousse, R., Gavillon, R., and Budtova, T. (2011). Aerocellulose from

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cellulose–ionic liquid solutions: preparation, properties and comparison with cellulose–NaOH and cellulose–NMMO routes. Carbohydr. Polym. 83: 1766–1774. Sescousse, R., Gavillon, R., and Budtova, T. (2011). Wet and dry highly porous cellulose beads from cellulose–NaOH–water solutions: influence of the preparation conditions on beads shape and encapsulation of inorganic particles. J. Mater. Sci. 46: 759–765. Tan, C., Fung, B., Newman, J.K., and Vu, C. (2001). Organic aerogels with very high impact strength. Adv. Mater. 13: 644–646. Fischer, F., Rigacci, A., Pirard, R. et al. (2006). Cellulose-based aerogels. Polymer 47: 7636–7645. White, R.J., Budarin, V., Luque, R. et al. (2009). Tuneable porous carbonaceous materials from renewable resources. J. Chem. Soc. Rev. 38: 3401–3418. García-González, C.A., Uy, J.J., Alnaief, M., and Smirnova, I. (2012). Preparation of tailor-made starch-based aerogel microspheres by the emulsion-gelation method. Carbohydr. Polym. 88: 1378–1386. García-González, C.A., Camino-Rey, M.C., Alnaief, M. et al. (2012). Supercritical drying of aerogels using CO2 : effect of extraction time on the end material textural properties. J. Supercrit. Fluids 66: 297–306. Kenar, J.A., Eller, F.J., Felker, F.C. et al. (2014). Starch aerogel beads obtained from inclusion complexes prepared from high amylose starch and sodium palmitate. Green Chem. 16: 1921–1930. Ubeyitogullari, A. and Ciftci, O.N. (2016). Formation of nanoporous aerogels from wheat starch. Carbohydr. Polym. 147: 125–132. Robitzer, M., David, L., Rochas, C. et al. (2008). Nanostructure of calcium alginate aerogels obtained from multistep solvent exchange route. Langmuir 24: 12547–12552. Chtchigrovsky, M., Primo, A., Gonzalez, P. et al. (2009). Functionalized chitosan as a green, recyclable, biopolymer supported catalyst for the [3+2] Huisgen cycloaddition. Angew. Chem. Int. Ed. 48: 5916–5920. Silva, S.S., Duarte, A.R.C., Carvalho, A.P. et al. (2011). Green processing of porous chitin structures for biomedical applications combining ionic liquids and supercritical fluid technology. Acta Biomater. 7: 1166–1172. Gurikov, P., Raman, S.P., Weinrich, D. et al. (2015). A novel approach to alginate aerogels: carbon dioxide induced gelation. RSC Adv. 5: 7812–7818. Ganesan, K. and Ratke, L. (2014). Facile preparation of monolithic 𝜅-carrageenan aero-gels. Soft Matter 10: 3218–3224. White, R.J., Budarin, V.L., and Clark, J.H. (2010). Pectin-derived porous materials. Chem. Eur. J. 16: 1326–1335.

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28 García-González, C.A., Carenza, E., Zeng, M. et al. (2012). Design of bio-

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compatible magnetic pectin aerogel monoliths and microspheres. RSC Adv. 2: 9816–9823. Rudaz, C., Courson, R., Bonnet, L. et al. (2014). Aeropectin: fully biomass-based mechanically strong and thermal super-insulating aerogel. Biomacromolecules 15: 2188–2195. Veronovski, A., Tkalec, G., Knez, Z., and Novak, Z. (2014). Characterisation of biodegradable pectin aerogels and their potential use as drug carriers. Carbohydr. Polym. 113: 272–278. Liebner, F., Haimer, E., Wendland, M. et al. (2010). Aerogels from unaltered bacterial cellulose: application of sc CO2 drying for the preparation of shaped, ultra-lightweight cellulosic aerogels. Macromol. Biosci. 10: 349–352. Kobayashi, Y., Saito, T., and Isogai, A. (2014). Aerogels with 3D ordered nanofiber skeletons of liquid-crystalline nanocellulose derivatives as tough and transparent insulators. Angew. Chem. Int. Ed. 53: 10394–10397. Jiménez-Saelicesa, C., Seantier, B., Cathala, B., and Grohens, Y. (2017). Spray freeze-dried nanofibrillated cellulose aerogels with thermal superinsulating properties. Carbohydr. Polym. 157: 105–113. García-González, C.A., Alnaief, M., and Smirnova, I. (2011). Polysaccharide-based aerogels-promising biodegradable carriers for drug delivery systems. Carbohydr. Polym. 86: 1425–1438. Budarin, B., Clark, J., Luque, R. et al. (2008). Palladium nanoparticles on polysaccharide-derived mesoporous materials and their catalytic performance in C–C coupling reactions. Green Chem. 10: 382–387. Guilminot, E., Gavillon, R., Chatenet, M. et al. (2008). New nanostructured carbons based on porous cellulose: elaboration, pyrolysis and subsequent use as substrate for proton exchange membrane fuel cell electrocatalyst particles. J. Power Sources 185: 717–726. Sai, H., Xing, L., Xiang, J. et al. (2014). Flexible aerogels with interpenetrating network structure of bacterial cellulose–silica composite from sodium silicate precursor via freeze drying process. RSC Adv. 4: 30453–30461. Hayase, G., Kanamori, K., Abe, K. et al. (2014). Polymethylsilsesquioxane–cellulose nanofiber biocomposite aerogels with high thermal insulation, bendability, and superhydrophobicity. ACS Appl. Mater. Interfaces 6: 9466–9461. Wong, J.C.H., Kaymak, H., Tingaut, P. et al. (2015). Mechanical and thermal properties of nanofibrillated cellulose reinforced silica aerogel composites. Microporous Mesoporous Mater. 217: 150–158. Zhao, S., Zhang, Z., Sèbe, G. et al. (2015). Multiscale assembly of superinsulating silica aerogels within silylated nanocellulosic scaffolds: improved

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mechanical properties promoted by nanoscale chemical compatibilization. Adv. Funct. Mater. 25: 2326–2334. Bendahou, D., Bendahou, A., Seantier, B. et al. (2015). Nano-fibrillated cellulose–zeolites based new hybrid composites aerogels with super thermal insulating properties. Ind. Crops Prod. 65: 374–382. Demilecamps, A., Reichenauer, G., Rigacci, A., and Budtova, T. (2014). Cellulose–silica composite aerogels from “one-pot” synthesis. Cellulose 21: 2625–2636. Rudaz, C. (2014). Cellulose and pectin aerogels: towards their nano-structuration. PhD thesis. MINES ParisTech. Cai, J., Liu, J., Feng, J. et al. (2012). Cellulose–silica nanocomposite aerogels by in situ formation of silica in cellulose gel. Angew. Chem. Int. Ed. 51: 2076–2080. Liu, S., Yu, T., Hu, N. et al. (2013). High strength cellulose aerogels prepared by spatially confined synthesis of silica in bioscaffolds. Colloids Surf., A 439: 159–166. Demilecamps, A., Beauger, C., Hildenbrand, C. et al. (2015). Cellulose–silica aerogels. Carbohydr. Polym. 122: 293–300. Demilecamps, A., Alves, M., Rigacci, A. et al. (2016). Nanostructured interpenetrated organic–inorganic aerogels with thermal superinsulating properties. J. Non-Cryst. Solids 452: 259–265. Grant, G.T., Morris, E.R., Rees, D.A. et al. (1973). Biological interactions between polysaccharides and divalent cations: the egg-box model. FEBS Lett. 32: 195–198. Gavillon, R. (2007). Preparation and characterisation of utra-porous cellulose materials. PhD thesis. Paris, France: MINES ParisTech. Sescousse, R. (2010). Nouveaux matériaux cellulosiques ultra-poreux et leurs carbones à partir de solvants verts. PhD thesis. Paris, France: MINES ParisTech. Alaoui, A.H., Woignier, T., Scherer, G.W., and Phalippou, J.J. (2008). Comparison between flexural and uniaxial compression tests to measure the elastic modulus of silica aerogel. J. Non-Cryst. Solids 354: 4556–4561. Bisson, A., Rigacci, A., Lecomte, D., and Achard, P. (2004). Effective thermal conductivity of divided silica xerogel beds. J. Non-Cryst. Solids 350: 379–384. Lu, X., Caps, R., Fricke, J. et al. (1995). Correlation between structure and thermal conductivity of organic aerogels. J. Non-Cryst. Solids 188: 226–234. Zhao, S., Malfait, W.J., Demilecamps, A. et al. (2015). Strong, thermally superinsulating biopolymer–silica aerogel hybrids by cogelation of silicic acid with pectin. Angew. Chem. Int. Ed. 54: 14282–14286.

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gels synthesis and different mechanical reinforcing strategies. J. Non-Cryst. Solids 385: 55–74. 56 Glenn, G.M. and Irving, D.W. Starch-based microcellular foams. Cereal Chem. 72: 155–161. 57 Glenn, G.M. and Stern, D.J. (1999). Starch-based microcellular foams. US Patent 5,958,589. 58 Druel, L., Bardl, R., Vorwerg, W., and Budtova, T. (2017). Starch aerogels: a member of the family of thermal superinsulating materials. Biomacromolecules 18: 4232–4239.

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16 Nanocelluloses at the Oil–Water Interface: Emulsions Toward Function and Material Development Siqi Huan, Mariko Ago, Maryam Borghei, and Orlando J. Rojas Bio-Based Colloids and Materials, Department of Forest Products Technology, School of Chemical Technology, Aalto University, 00076 Aalto, Finland

16.1 Cellulose Nanocrystal Properties in the Stabilization of O/W Interfaces The ability of cellulose nanocrystals (CNC) to stabilize an oil/water (O/W) interface has been ascribed to their amphiphilic character [1, 2]. CNC surface charge density can be modulated by the process used for their production (sulfuric versus hydrochloric acid hydrolysis) or by various post-treatments. Interfacial stabilization by CNC owes clearly to its amphiphilic character, which has been explained by molecular organization at the crystal surfaces. It has been postulated that regardless of the crystalline origin of the cellulose material, electrostatic interactions play a major role in the control of the interface. Interestingly, it was reported that CNCs with a surface charge density above ∼0.03 e/nm2 were not able to efficiently stabilize an O/W interface, whereas a reduced surface charge density led to stable emulsions. Irrespective of all the parameters investigated, the (200)β/(220)α hydrophobic edge plane of CNC appears responsible for the adsorption of CNCs at the interface, and therefore, its stabilizing ability in emulsified systems (Figure 16.1a) [5]. Changes in the wetting behavior can be achieved by tuning the ionic character of the system, by removal of CNC charged groups (for example, via desulfation of CNC derived from sulfuric acid hydrolysis) or by their screening (for example, by increasing the ionic strength of the medium) (Figure 16.1b). The typical rigid, rod-like CNCs can be isolated to yield particles with varying aspect ratios. This was cleverly accomplished by Kalashnikova et al., who isolated and compared CNCs from plants (cotton CCN), microorganisms (bacterial cellulose nanocrystals, BCN), and algae (Cladophora ClaCN). These nanocelluloses were then used to stabilize oil droplets

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Figure 16.1 (a) Schematic representation of the stabilization of the (Iβ CNC from cotton) at the oil/water interface, exposing the hydrophobic edge (200) to the oil phase. (b) Phase separation by centrifugation of oil (hexadecane)-in-water emulsions stabilized with native BCN, BCN from sulfuric acid hydrolysis (s-BCN) and BCN from HCl hydrolysis (ds-BCN). (c) Images showing a representative droplet distribution one week after the preparation of hexadecane-in-water Pickering emulsions stabilized by 2 g l−1 CNCs from cotton (CCN), bacteria (BCN), and algae (Cladophora ClaCN), with aspect ratios ranging from 13 to 160, respectively. (d) Schematic representation of emulsions stabilized by the same CNCs in (c) according to the length under diluted (top) and concentrated (bottom) conditions. The shorter nanocrystals are shown to stabilize individual droplets, whereas longer nanocrystals promote highly networked systems. (e) CNCs form emulsion droplets that can selfassembled into highly mesoporous structures upon freeze-drying. Source: Panels (a), (b), and (e): Jiang et al. 2015 [3]. Copyright 2015. Reprinted with permission of American Chemical Society. Panels (c) and (d): Kalashnikova et al. 2013 [4]. Adapted with permission of Royal Society of Chemistry.

via interfacial adsorption (Figure 16.1c,d). The macroscopic properties of nanocellulose-stabilized emulsions were modulated as the droplets were covered by adsorbed, short nanocrystals, or entangled networked systems, for example, if the longer nanocrystals are used (Figure 16.1d) [4]. It is of great interest to take advantage of the high degree of sustainability of cellulose nanoparticles, in addition to their morphology (size and aspect) to enable emulsions with given stability and from there to synthesize other structures, such as foams and aerogels (Figure 16.1e) [3]. As an example, O/W emulsions were freeze-dried to yield porous solids with a morphology that depended on

Nanocelluloses at the Oil–Water Interface

CNC hydrophobicity or their ability to self-assemble at the oil–water interface in the precursor emulsion [3].

16.2 Surfactant-free Emulsions It is important to consider alternatives to conventional surfactants in the preparation of emulsions for pharmaceutical, food, cosmetic, and coating applications, among many others. Nanocelluloses, as bio-based materials, offer a unique opportunity in such endeavors. As for typical colloidal particles, they are effective in stabilizing (Pickering) emulsions by a strong and irreversible adsorption at the O/W interface. To investigate the influence of CNC in stabilizing an emulsion, BCN were considered on account of their high purity, aspect ratio, and ribbon-like structure [6]. After hydrolysis with hydrochloric acid, BCN (855 nm in length, 17 nm in width, and 7 nm in thickness) were obtained with very low density of charged surface groups (10−3 /nm2 ). Oil-in-water emulsions were prepared using hexadecane and BCN aqueous suspension for a water-to-oil ratio (WOR) of 70 : 30. Figure 16.2a shows that BCNs were evenly distributed at the droplet surface and bent to follow its curvature, meaning that the tension required for droplet formation was high enough to force these nanocelluloses to align without desorption. The droplets showed a tendency to coalesce at low concentrations. As shown in Figure 16.2b, below 2 g l−1 , an increased amount of BCN resulted in a reduction of the average drop size, which led to a larger interfacial area and thus to a larger emulsion volume. Above 2 g l−1 , stable emulsions (∼4 μm drop size) were obtained, irrespective of the amount of BCNs added to the system. The analysis of the surface coverage indicated that the size of the droplets formed from hexadecane was controlled by the amount of stabilizing particles until a critical amount was reached. Further increase of BCN concentration to above this critical value would not change the oil droplet size. A surface coverage of about 60% was indicated for the emulsions with low BCN concentration, below the critical value, above which the surface coverage reached 100%. When the emulsions were characterized over time, and despite the typical creaming process, a stable drop size was observed for several months. A densely packed 2D network that was formed at the surface of the droplets provided a highly stable interface due to steric hindrance and the high adsorption energy prevented removal of the particles from the interface [9]. CNC produced by sulfuric or phosphoric acid hydrolysis contains charged groups that promote electrostatic interactions and provide good colloidal stability [10] and the influence of the surface charge at the O/W interface has been evaluated. Cherhal et al. [7] found that at low CNC concentration, the droplet size was controlled by the CNC concentration, regardless of the surface charge. As was noted before, the increase in CNC concentration above a critical point

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Figure 16.2 (a) Scanning electron micrographs of styrene-based Pickering emulsion stabilized by BCN and polymerized into polystyrene [6]. (b) Transmission optical micrographs indicating droplet size dependence on BCN content in the water phase in an emulsion containing hexadecane with a WOR = 70 : 30 [6]. (c) Schematic representation illustrating a flat interface at the oil surface with totally wetted CNCs in the aqueous phase leading to a rough external interface and a thicker interface with desulfated CNCs. It is proposed that CNCs are in contact with the oil phase only via their surface and not immersed in oil, supporting the possibility that the (2 0 0) crystalline plane of the CNC directly interacts with the interface [7]. (d) CLSM images of emulsions stabilized by CNCs containing increasing amounts of stained oil (hexadecane). From left to right, the frames include Pickering emulsions with 10%, 65%, and 85.6% of the internal phase. The frame in the far right is the previous emulsion using a stacking of 2D 1-μm thick optical cross section images to form a 3D reconstruction [8]. (e) Schematic illustration of the formation process of MIPE and HIPE structures from CNC-stabilized Pickering emulsions: step 1 for formation; step 2 for increased droplet volume and reduced interfacial coverage; step 3 for droplet coalescence, close packing, and formation of MIPE and step 4 for droplet deformation and HIPE formation [8]. Source: Kalashnikova et al. 2011 [6], Cherhal et al. 2015 [7], and Capron and Cathala 2013 [8]. Copyright 2011, 2015, and 2013. Reprinted with permission of American Chemical Society.

leads to a stabilization of the drop size. It was also noticed that compared to sulfated CNCs, a slightly larger drop diameter was measured when desulfated CNCs were used, likely due to the more aggregated and compact structures that were formed [11]. Overall, the results indicate similar trends as those observed for BCN-stabilized Pickering emulsions. Small angle neutron scattering (SANS) revealed that charged CNCs form a monolayer at the O/W interface (Figure 16.2c) with a thickness of about 7 nm (close to the values of the lateral cross section of an individual crystal).

Nanocelluloses at the Oil–Water Interface

Uncharged CNCs formed a thicker layer (18 nm), indicating CNC aggregation at the interface. The thinner interfacial layer of the charged CNCs was ascribed to the local orientation/alignment of the nanoparticles, resulting in a more densified interface, with higher surface coverage compared to that from neutral CNCs. It was also postulated that in both cases, the (2 0 0) crystalline plane of CNCs was directly in contact with the oil phase without deformation of the oil interface, leading to a smooth surface. While the BCN aspect ratio is significantly larger than that of wood or cotton nanocrystals, the reported results showed that stable emulsions can be produced by using nanocrystals regardless of their shape (length and aspect ratio), origin, or surface functional groups. CNCs have been used to produce high internal phase emulsions (HIPEs). HIPEs, in some cases also known as emulsion gels, contain an internal phase volume fraction (the volume of the oil relative to the total emulsion volume) > 0.74, which is the maximum packing density of monodispersed, hard spheres [12]. Notably, it was not possible to obtain, in one step, HIPEs, i.e. to stabilize high oil content with CNCs. However, HIPEs were successfully obtained by Capron et al. by adding the oil phase to an already prepared Pickering emulsion [8]. This means that the adsorption of CNC at the interface is an essential factor in the first step for further obtaining a stable gel-like system. As shown in Figure 16.2d1 , the droplets were stabilized in the initial Pickering emulsion with an average size of 4 μm. Adding oil to the Pickering emulsion resulted in an increase of the average drop size and polydispersity (Figure 16.2d2 ). When the oil volume exceeded the close packing condition, deformation of the droplets occurred due to the reorganization of the interface. As a result, a gel-like structure was formed (Figure 16.2d3 ). Figure 16.2d4 shows a sample observed at varying focal planes, forming a 3D image. It is inferred that the drops merged, some continuous walls were shared by neighboring cells, and the interfaces were reorganized in the final HIPE. The mechanism of HIPE formation is also illustrated in Figure 16.2e. At the initial stage, a large amount of CNC was used compared to the oil volume, leading to CNC multilayers at the interface (step 1). It should be noted that the amount of CNC remained the same throughout the steps. Therefore, the addition of oil increased the volume of the droplets without coalescence until the surface coverage reached a minimum value of 84% (step 2). When the coverage reached a value below the limit of medium internal phase emulsion (MIPE), it was not possible to stabilize the interface and a reorganization of the interface, together with coalescence, occurred to form larger and more stable droplets (step 3). By further increasing the oil content, the droplets grew larger, to the close packing limit, they deformed, and the walls merged to form an optimally stabilized interface (step 4). After all, through this method, CNC was shown to efficiently produce gel-like HIPEs by using less than 0.1 wt%. The results highlight the possible impact of CNC in developing environmentally friendly products based on emulsions/gels.

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16.3 Emulsions Stabilized with Modified Nanocelluloses So far, we have presented the case of oil-in-water emulsions, which are the most likely type resulting from the use of unmodified nanocelluloses. The case of oil-continuous emulsions is reported rarely. This is expected since the hydrophilic nature of cellulose favors stabilization of a water continuous phase (oil-in-water, O/W) emulsions. However, cellulosic surfaces can be hydrophobized by applying agents such as silanes or alkylamines [13, 14] so that stabilization of water-in-oil (W/O) emulsions can be made possible [15, 16]. The degree of surface modification affects the wettability and therefore has an impact on drop size and emulsion stability. Network-like aggregates of nanocelluloses at the O/W interface have been proposed as the main factor preventing coalescence of emulsion droplets [13, 17]. Fusion proteins, having both the ability to bind to cellulose and to be surface active to functionalize CNF, have been reported [18]. The interaction between the CNF fibrils and the protein-coated oil droplets lead to highly stable emulsions. Such protein-functionalized CNF can assemble into tightly packed thin films at the air/water and O/W interfaces, resulting in a synergistic improvement in the formation and stability of O/W emulsions. W/O emulsions with c. 50% of acrylated soybean oil as continuous phase were stabilized solely by bacterial cellulose nanofibrils hydrophobized by silylation or esterification. Upon polymerization and water removal, polymer foams were obtained. Related techniques can expand the application and processing options available for renewable foams, for example, to produce large composite structures and sandwich cores for composites that can be formed in situ [19]. Likewise, CNC-stabilized W/O emulsions having a polylactic acid (PLA) solution as continuous phase was found to be an effective dispersion and alignment system to control CNC assembly in continuous ultrafine composite fibers produced by electrospinning [20]. Such CNC/PLA fibers have potential as a precursor for electrodes in supercapacitors, battery separator for lithium-ion batteries, separation filters, etc. [20]. Poly(NIPAM)-g-CNC was produced by surface-initiated single-electron transfer living radical polymerization [21] and characterized for thermal responsiveness [22], as will be discussed in a later section (Figure 16.3a). Heptane-in-water Pickering emulsions were successfully prepared with the poly(NIPAM)-g-CNC [23]. Emulsions prepared with 0.05–0.5 wt% poly(NIPAM)-g-CNC were observed to be stable during more than four months. The anisotropy of the grafted nanoparticles at the oil–water interface was observed via freeze-fracture TEM. In addition, larger layered sheet assemblies were observed as an indication of grafted nanoparticle aggregation during emulsification at the oil–water interface (Figure 16.3b). Depending on the concentration of grafted nanoparticles, heptane droplets obtained

Nanocelluloses at the Oil–Water Interface

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Figure 16.3 (a) Illustrative schematics for the production of poly(NIPAM)-g-CNC. (b) TEM image of the replica oil–water interface of emulsion stabilized by poly(NIPAM)-g-CNC after freeze-fracture [23]. (c) Optical microscopy images of emulsions prepared with 0.05% and 0.5% are shown in (c) and (d) by using poly(NIPAM)-g-CNCs in emulsions with WOR = 1 [23]. Source: Panels (b)–(d): Zoppe et al. 2012 [23]. Copyright (2012). Reprinted with permission of Elsevier.

ranged from 30 to >100 μm and showed polydisperse drop size distributions (Figure 16.3c,d). It is expected that the development of thermally responsive Pickering emulsions utilizing naturally abundant substrates with controllable stability will allow significant advances in biomedical and cosmetic applications [23]. A chemical pretreatment for producing functionalized CNC via periodate oxidation and reductive amination has been reported (Figure 16.4a). CNC was produced via sequential oxidation and reductive amination followed by mechanical homogenization to liberate uniform nanocrystals [24]. This functionalization of cellulose fibers offers an alternative to the widely used acid hydrolysis process for fabricating individualized CNC. CNCs can be directly modified during the pretreatment step, and no additional posttreatments are required to tune the surface properties. Different butylamine isomers were tested to fabricate CNCs with amphiphilic features. The modified CNCs were used for stabilizing Pickering emulsions, which resulted in different stability, as measured by transmission after centrifugation (Figure 16.4b) and depending on the butylamine isomers and number of alkyl groups on the CNCs used in the reaction (iso-, n-, and tert-butylamine). By linking alkyl groups, the

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Figure 16.4 (a) Illustrative scheme for the production of CNC obtained by periodate oxidation of cellulose followed by reductive amination. Different butylamine isomers were used in the reaction [24]. (b) The modified CNCs were used for stabilizing Pickering emulsions, which resulted in different stability, as measured by transmission after centrifugation (1200 rpm, 1 h) depending on the butylamine isomers used in the reaction: iso- and n-butylamine and tert-butylamine (tert) [24]. (c) Sodium periodate and chlorite oxidation followed by reductive amination of cellulose; n-butylamines have been introduced to partially oxidized cellulose (PO-DAC). Bifunctionalized cellulose nanocrystals (But-CNCs) were obtained, which contained both carboxylic and n-butylamino groups. This was achieved by using partial, sequential periodate–chlorite oxidation and reductive amination, followed by a homogenization treatment to liberate individualized nanocrystals with amphiphilic characteristics [25]. (d) Marine diesel oil-in-water (O/W) emulsion stabilized by bifunctionalized cellulose nanocrystals (But-CNCs) is shown schematically [25]. Source: Panels (a) and (b): Visanko et al. 2014 [24]. Copyright 2014. Reprinted with permission of American Chemical Society. Panels (c) and (d): Ojala et al. 2016 [25]. Copyright 2016. Reprinted with permission of Elsevier.

Nanocelluloses at the Oil–Water Interface

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Figure 16.4 (Continued)

CNC became hydrophobic (water contact angles from self-standing films up to ∼111∘ ) and together with the hydrophilic backbone of cellulose afforded amphiphilic particles, which were utilized in stabilizing O/W emulsions [24]. Bifunctionalized CNC (But-CNCs) were obtained with both carboxylic and n-butylamino groups after using partial, sequential periodate-chlorite oxidation and reductive amination, followed by a homogenization treatment to liberate individualized nanocrystals (Figure 16.4c) [25]. These bifunctionalized CNCs were used at low concentrations (up to 0.45 wt%) as surface-active stabilizers in O/W emulsions (Figure 16.4d). The emulsion stabilized by But-CNCs can be attributed to the differences in charge density and the content of the hydrophobic amino groups and their ratio. The anionic carboxyl groups of the CNCs on the interface of oil-droplets promote stability through repulsive electrostatic interactions (Figure 16.4d) [25]. Hydrophobized BCN (bacterial cellulose nanocrystals) were obtained after esterification with acetic

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(C2-), hexanoic (C6-), and dodecanoic (C12-) acids to render hydrophobic the otherwise hydrophilic surface of BCN. The organic acid-modified BCN proved to be an excellent emulsifier to produce stable concentrated W/O emulsions [16]. The C6- and C12-BC stabilized emulsions exhibited a pH-triggered, reversible, and transitional phase separation [26]. The phase separation at high pH was associated with the increase in electrostatic repulsion between modified BC fibrils. As a result, the surface coverage of the droplets by the particles decreased, which led to the coalescence of the dispersed droplets [26].

16.4 Surfactant-assisted Emulsions Despite their amphiphilicity and the intermediate wettability, which is key to stabilizing emulsions, especially those by CNC, nanocelluloses are not significantly surface-active. Several recent studies have reported on resolving this issue. Here, we introduce in situ surfactant-assisted adsorption approaches to tailor the hydrophilicity/hydrophobicity of nanocellulose. Nonionic surfactants have been successfully used with CNF to stabilize different O/W emulsions [27]. The influence of the interplay of dilute CNF aqueous and surfactants on the rheological behavior of the formed CNF mixtures has also been investigated [28]. It was found that the micelles resulted from nonionic surfactants that associate with the CNF through the interactions between the ethoxylated hydrophilic head groups of the surfactants and CNF. The bridged micelle-nanofibril association could further promote gel formation (Figure 16.5a). It was pointed out that the adsorption rate of micelles could be effectively increased with the surfactant concentration. For ionic surfactants and with increasing concentration (at concentrations below the critical value), a high-modulus gel, with high optical clarity, was formed. Apparent fibril aggregation, however, was produced at higher surfactant concentrations, leading to a detrimental effect on suspension stability and optical clarity. These results demonstrate that the interactions between surfactants and CNF could significantly affect the rheological behavior of corresponding mixtures, providing a new approach to tune the rheology of emulsions [28]. Since the surface of CNC is covered with, for example, negatively charged sulfate ester groups, electrostatic interactions are considered in order to tune its properties. Recently, Brinatti et al. investigated the binding capability between a homologous series of cationic surfactants, Cn TAB (n = 12, 14, 16) and CNCs [29]. Electrostatic interactions took place at low surfactant concentration for all mixtures. However, the interactions between CNCs and surfactants were significantly different with varying chain length n in Cn TABs. The less hydrophobic C12 TAB tended to predominantly form hemimicelles (alkyl chains exposed to water), whereas the more hydrophobic C14 TAB and C16 TAB tended to form admicelles. With increased surfactant concentration,

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Figure 16.5 (a) Schematic illustration of CNFs bridged by surfactant micelles. Free micelles are represented in green, and the micelles establishing CNF junctions are depicted with a red corona as seen in the inset [28]. (b) Binding mechanism for both Cn TAB-CNC with an increase in the surfactant concentration. Shown is the formation of hemimicelles for C12 TAB (left) followed by the formation of admicelles (bottom and surfactant-CNC coacervates). C16 TAB only forms admicelles (bottom) [29]. (c) Water–dodecane emulsions (WOR = 50 : 50) stabilized by 0.25 wt% CNCs and DMAB surfactant with concentrations from 0 to 16 mM (left to right, as indicated). Two phase inversions are seen in emulsions with DMAB; from O/W to W/O and back to O/W, with increasing surfactant concentration [30]. (d) Confocal laser scanning micrographs of 1 : 1 water–dodecane emulsions stabilized by 0.25 wt% CNCs with DMAB. The dodecane (oil) phase is dyed and appears green in the images showing the double phase inversion in the DMAB-CNC-stabilized emulsions [30]. Source: Panel (a): Quennouz et al. 2016 [28]. Adapted with permission of Royal Society of Chemistry. Panel (b): Brinatti et al. 2016 [29]. Copyright 2016. Reprinted with permission of American Chemical Society. Panels (c) and (d): Hu et al. 2015 [30]. Copyright 2015. Reprinted with permission of Elsevier.

the micelles of C12 TAB further grew and aggregated onto the CNC surface to form admicelles (Figure 16.5b). The interaction of surface desulfated CNC and cationic surfactants was also studied. Isothermal titration calorimetry measurements showed that the Cn TAB has lower interaction with CNCs at higher desulfation degree. Moreover, the binding impact on C12 TAB with desulfated CNCs was much more obvious than those with C14 TAB and

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C16 TAB. Electrophoretic mobility measurements indicated that this binding interaction between cationic surfactants and CNCs was not controlled by the absolute surfactant concentration, demonstrating that the surfactant binding was mainly dependent on the hydrophobicity. According to the results, the authors also proposed coacervate structures within which the CNCs were associated and cross-linked by the adsorbed cationic surfactant aggregates, rather than simply electrostatic interactions to form and stabilize the mixtures [29]. As was pointed out above, CNC surface wettability needs to be modified to increase its hydrophobicity and enhance its emulsifying ability. Recently, Pickering emulsions stabilized by cetyltrimethylammonium bromide (CTAB)or didecyldimethylammonium bromide (DMAB)-modified CNCs were formulated by Hu et al. [30]. The continuous phase of the Pickering emulsions could be tuned by changing hydrophobic–hydrophilic balance (HLB) and wettability of CNCs by controllably absorbing the different surfactant types. As was illustrated nicely by the same authors, Pickering emulsions stabilized by CNCs exhibited a phase inversion process with increasing the concentration of DMAB, namely oil-in-water (W/O) to water-in-oil (W/O) followed by O/W; all CTAB-modified CNCs emulsions were O/W (Figure 16.5c). Figure 16.5d shows CLSM images of this phase transformation process upon increasing the DMAB concentration. It was indicated that the DMAB molecules containing two hydrophobic tails could decrease CNC wettability, yielding more hydrophobic CNCs that favored the formation of W/O emulsions. Moreover, it was noted that the presence of free surfactant was essential for the phase inversion. The droplet size was in the micron range, and the smallest droplets were obtained when the concentration of DMAB was above its critical micelle concentration. The emulsions stabilized by both CTAB- and DMAB-modified CNCs displayed long-term stability, minor creaming, but no visible phase separation after one year. The results indicated an efficient way to utilize CNCs as a Pickering emulsion stabilizer by surfactant modification, extending the potential applications of CNCs in new liquid formulations. Nanocellulose has also been utilized as a co-stabilizer for emulsion polymerization. Nanocomposite dispersions containing CNC as costabilizing constituent and polybutylmethacrylate (PBMA) as polymer phase were synthesized in one-step mini-emulsion polymerization [31]. The colloidal nanocomposite dispersions with 25 wt% solid content and up to 5 wt% CNC loading levels were successfully prepared with long-term stability. In the presence of cationic surfactant, dodecylpyridinium chloride, the negatively charged CNCs were electrostatically anchored around the polymer particles. The particle size continuously increased by increasing the CNC content. This was attributed

Nanocelluloses at the Oil–Water Interface

to partial aggregation of the monomer droplets by adding CNC. The surface charge of the droplets decreased after the CNC addition, leading to a spontaneous emergence of monomer droplets to decrease the surface-to-volume ratio and achieve a stable colloidal suspension system. Subsequently, film casting was used to produce the corresponding nanocomposite films. The optical transparency of the nanocomposite film remained higher than 70% even after adding 4 wt% CNC. This phenomenon indicated that the CNCs could be well dispersed throughout the nanocomposite film since irregular dispersion of nanofillers could conversely impair the optical transparency. The research provided a novel approach to prepare a simple ready-to-use formulation of colloidally stable nanocomposite material by one-step mini-emulsion polymerization, which expands the applications of surfactant-assisted nanocellulose in the field of waterborne adhesives or coatings [31]. Fabrication of functional composite microspheres constructed by multidimensional blocks has increasingly attracted interest. Recently, an easy, viable, and straightforward Pickering emulsion method was implemented to fabricate the model drug methyl red-loaded polymethyl methacrylate/CNF multifunctional composite microspheres [32]. In this study, the water-dispersible rare-earth upconversion nanoparticles (UCNPs) and CNF were used as stabilizers of Pickering emulsion. The results showed that the nanoparticles assembled evenly at the oil/water interface. The organized arrangements of CNFs and UCPNs provided sufficient inhibition against destabilization of Pickering droplets. Furthermore, the uniformly distributed composite microspheres exhibited good luminescence and an excellent drug release profile. This facile and novel method may offer an opportunity to efficiently fabricate multifunctional composite microspheres for diverse applications. Lightweight and highly porous foams with well-designed microstructure and high mechanical stability were fabricated by the synergetic combination of TEMPO-mediated oxidized CNF and a surfactant (Pluronic P123), as well as the postcross-linking agent of CaCO3 nanoparticles [33]. It was proposed that even though the foamability of CNF was not as good as that of surfactants, the strength of TEMPO-CNF-stabilized foams might be enhanced by multivalent ion-induced cross-linking, if the cross-linking interaction occurred after the foam has been formed. Wet foams were successfully generated via the composite-stabilizing interaction of CNF and P123. Meanwhile, the wet foams were cross-linked by Ca2+ ions, which were released by gluconic acid-triggered dissolution of the CaCO3 , significantly improving the storage stability. Mechanical characterization showed that the elastic modulus of the cross-linked foams with a density of 9–15 kg m−3 was higher than that of commercial polyurethane foams. Another important feature of the

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foams was their high thermal stability. Drying the formed foam at elevated temperature resulted in moderate shrinkage but was much more limited than for noncross-linked systems. The structure of the CNF/P123-stabilized foams reconstructed by 3D tomography showed that oblate-like pores were formed during the drying process; however, the internal configuration was well preserved. The utilization of CNF as a foam stabilizer highlights the possibility to synthesize high-performance materials.

16.5 Emulsions with Polymer Coemulsifiers Bacterial cellulose (BC) produced in an agitated culture showed excellent emulsion-stabilizing effects. A mechanical barrier and a scaffolding structure composed of fine fibrils of BC interrupted the coalescence of oil droplets in BC-based emulsions and, in comparison with stabilizers such as xanthan gum (XG) and sorbitan monolaurate, have been noted to be stable against the addition of salt and changes in pH and temperature [34]. This stability is due to the mechanical barrier and a scaffolding structure composed of stable crystalline cellulose since the fine BC fibrils cover a larger surface area of the oil droplet [34]. O/W emulsions (10 wt% olive oil) were prepared by different types of cellulose hydrocolloids (HPMC, CMC) and compared with BC using formulations that included concentrations varying from 0.1 to 1 wt% and by using two emulsification methods (high shear mixer and ultrasound) (Figure 16.6a) [35]. BC showed better emulsifying capability compared to HPMC and CMC as its emulsions separated at a slower rate. The observed high stability, as discussed in other sections, was explained by the formation of BC fibril flocs that adsorb on the surface of the oil droplets and form a strong network. The BC emulsions were not affected by external factors such as pH, temperature, or ionic strength, unlike the emulsions prepared with the other types of celluloses [35]. Emulsions containing extra virgin olive oil were prepared at pH 3.8 stabilized with whey protein isolate (WPI) as an emulsifier and BC as a thickener. Stable emulsions were observed due to either steric stabilization or network formation of BC fibrils. Emulsions stabilized with either BC, locust bean gum (LBG), and XG were compared. The emulsions were destabilized by creaming, flocculation, and coalescence, which limited emulsion stability. Thickeners may increase the shelf-life by either providing a steric stabilization or by increasing the viscosity, or decreasing the shelf-life due to bridging flocculation of the oil droplets or depletion effects [38]. Specifically, fast creaming usually occurs in emulsions containing a nonadsorbing polymer, such as XG, due to the mechanism of depletion flocculation. Creaming would be completely retarded at a specific critical viscosity concentration, at which the droplets lose their mobility and remain separated from each other [38]. Their

Nanocelluloses at the Oil–Water Interface

rheological profile indicated that BC has a similar shear thinning behavior as XG, but smaller amounts of BC were needed to obtain the same low shear viscosity (yield stress). These results show that BC is a good alternative for commonly used thickeners [38]. The question that emerges from the previous discussion is what is the effect of hydrosoluble polymers on emulsion properties if they are used in combination with nanocellulose? Peddireddy et al. [36] addressed this question by preparing water-in-water (W/W) emulsions from two incompatible water-soluble polymers (dextran and poly(ethylene oxide)) that cannot be stabilized with molecular surfactants but can be stabilized by CNCs [36]. Static light scattering and confocal microscopy techniques were used to determine the surface coverage by CNCs. Due to their high anisotropy, less material was needed to stabilize W/W emulsions than when homogeneous spherical colloids were used [36]. In the presence of 50 mM NaCl, very weak gels were formed by excess CNC in the continuous phase. In this way creaming of the dispersed phase could be arrested (Figure 16.6b). This result indicates the viability of CNC for large-scale applications, especially for biologically sensitive emulsions and in the fields of packaging and health care where biodegradability and nontoxicity are critical [36]. The combination of CNCs and adsorbing water-soluble cellulose derivatives has led to a synergistic stabilization of dodecane-water emulsions and emulsion gels. The emulsion stability was substantially enhanced when CNCs were first mixed with surface active, cellulose-adsorbing polymers (HEC or MC) followed by oil addition and emulsification [30]. CNC–HEC and CNC–MC emulsions were more stable than emulsions stabilized by CNCs or polymers alone. The combination at the oil/water interface of polymer-coated CNCs (Pickering stabilization) and free polymer (high surface activity and fast interfacial partitioning) is responsible for the long-term emulsion stability [30]. Hu et al. [37] showed that when CNCs were mixed with excess adsorbing polymer, either methyl cellulose or hydroxyethyl cellulose, followed by emulsification with corn oil, O/W emulsions were transformed without oil leakage into solid dry emulsions, via freeze-drying (Figure 16.6c). These dry emulsions exhibited droplet coalescence within the solid matrix and thus could not be redispersed. However, addition of tannic acid (TA) (after emulsification) imparted dispersibility to the dried emulsions due to complexation between the cellulose derivatives and TA, which condensed the “shell” around the oil droplets. When dried emulsions with TA were placed in water, the emulsion droplets redispersed readily without the need for high energy mixing, and minimal changes in emulsion droplet size was observed [37]. Therefore, it was demonstrated quite elegantly that the simple addition of two sustainable components to CNC Pickering emulsions (i.e. TA and methyl cellulose or hydroxyethyl cellulose) lead to dried and redispersible CNC-based emulsions with oil content as high as 94 wt%. These processing abilities will likely extend the use

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Figure 16.6 (a) Micrographs of O/W emulsions stabilized with 1 wt% of various types of celluloses (from left to right HPMC L, HPMC H, CMC, and BC) prepared by high shear mixer (top) and ultrasound (bottom) [35]. (b) Water-in-water (W/W) emulsions formed by mixing incompatible water-soluble polymers cannot be stabilized with molecular surfactants. However, CNC can efficiently stabilize them [36]. (c) The effect of freeze-drying and re-dispersion on the appearance of emulsions (20 vol% corn oil) stabilized by CNCs (0.25 wt%), MC (0.25 wt%), and tannic acid, TA (0.5 wt%). The figures include a CLSM image of the emulsion and illustrate the possibility for freeze-drying and redispersion in water [37]. Source: Panel (a): Paximada et al. 2016 [35]. Copyright 2016. Reprinted with permission of Elsevier. Panels (b) and (c): Peddireddy et al. 2016 [36] and Hu et al. 2016 [37]. Copyright 2016. Reprinted with permission of American Chemical Society.

Nanocelluloses at the Oil–Water Interface

of these surfactant-free, “green,” and potentially edible emulsions to new food, cosmetic, and pharmaceutical applications [37].

16.6 Double Emulsions Surface chemical modification of CNF and CNC by hydrophobic molecules was considered as a useful tool to tailor the hydrophobic–hydrophilic properties that can in turn increasing the emulsification ability of CNF and CNC [39]. Utilizing the native and lauroyl chloride-modified nanocellulose as stabilizers, double Pickering emulsions were prepared by a two-step emulsification process. The unmodified native nanocellulose was used to stabilize the inner interface (O/W), and the hydrophobic-modified nanocellulose was used to stabilize the outer interface to finally form oil-in-water-in-oil (O/W/O) emulsions. The formed surfactant-free O/W/O double emulsions are shown in Figure 16.7a. The results showed that the double emulsions stabilized by modified CNF could form larger globules and their ability to stabilize the emulsion was much better than that of native nanocellulose and modified CNC. It was concluded that the length of nanofibers was also an important factor for making stable double Pickering emulsions [39]. The incorporation of CNF into oil–water system to form, in one step, stable water-in-oil-in-water (W/O/W) multiple emulsions was also reported [40]. A pseudo-ternary phase diagram of surfactant, oil, and water system was used for better understanding the emulsion formulations with different CNF concentrations, oil types, and compositions. The phase diagrams are illustrated in Figure 16.7b1 ,b2 . By using soybean oil, it was found that only W/O emulsions were formed in the absence of CNF (Figure 16.7b1 ). However, W/O/W double emulsion were observed in the presence of CNF (0.5 wt%) (Figure 16.7b2 ). Figure 16.7c includes confocal images of the double W/O/W emulsion formed. In the phase diagrams, it is possible to observe an area corresponding to a W/O microemulsion (ME), which was kept almost the same regardless of CNF addition. However, by adding CNF, the composition zone to attain W/O/W emulsions was enlarged. The rheological behavior indicated that CNF could significantly increase the viscosity and achieve emulsions with strong shear-thinning behavior. Smaller droplet size and enhanced stability were obtained at high CNF concentration due to the formation of swollen fibrillar gel networks that prevented droplets collision. Overall, a single-step procedure was introduced to facilitate double emulsion preparation in the presence of nanocellulose. The investigation of double emulsion formulation variables in this study also provides a basic knowledge to design custom-made emulsions stabilized by CNF [40].

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Figure 16.7 (a) Micrographs of O/W/O double emulsions stabilized by hydrophobized nanocelluloses (water was stained with fluorescein) [39]. (b) Pseudo-ternary phase diagrams of surfactant–oil–water (SOW) systems consisting of a surfactant mixture (S), soybean oil (O), and water (W) containing CNF. The different diagrams were constructed from SOW composition scans with systems that did not contain CNF (b1 ) and with CNF present in the aqueous phase (b2 ). The “ME” regions indicate thermodynamically stable water-in-oil microemulsions. The “W/O” region represents kinetically stable water-in-oil emulsions and the “W/O/W” region represents the kinetically stable water-in-oil-in-water multiple emulsions. This condition is triggered by the presence of CNF [40]. (c) Fluorescent microscope image of emulsions having a water-to-oil ratio (by weight) of 50 : 50 with the oil phase dyed with Nile red and water phase containing CNF [40]. (d) SEM images with section plane of the inner area of large droplets containing smaller droplets obtained from precursor multiple emulsions [41]. (e) Schematic representation of the preparation process of beads in W/O/W systems: (e1 ) emulsification of water in pre-polymerized styrene/PS mixture using CNF/polystyrene composite nanospheres, and (e2 ) suspension polymerization of styrene/PS droplets containing emulsified water [42]. (f ) Cross-section FE-SEM micrographs of PS beads at the location of a water droplet with CNF/PS nanospheres [42]. Source: Panel (a): Cunha et al. 2014 [39]. Copyright 2014. Reprinted with permission of American Chemical Society. Panels (b) and (c): Carrillo et al. 2015 [40], (d): Zhu et al. 2015 [41], (e) and (f ): Nikfarjam et al. 2015 [42]. Copyright 2015. Reprinted with permission of Elsevier.

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Figure 16.7 (Continued)

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Due to the large difference in surface energy, the fabrication of stable suspensions or emulsions with dispersed CNF and nonpolar polymers is a major challenge for preparation and utilization of CNF-enhanced materials. An alternative approach based on the surfactant compatibilization mechanism was proposed [27]. A model emulsion composition consisted of an aqueous phase containing CNF and an organic phase containing polystyrene. As a result, stable W/O/W double emulsions were prepared by using various formulation variables under the synergistic effect of nonionic surfactants and CNF. Notably, this approach was further utilized to form double emulsions as a starting system to manufacture polystyrene fibers loaded with CNF. Thus, an effective and facile method to integrate CNF aqueous suspensions and nonpolar polymer dispersions offers a novel manufacturing method to design and prepare CNF-based materials [27]. Recently, novel multihollow magnetic imprinted microspheres (HM-IMs) were successfully synthesized by double Pickering emulsion polymerization stabilized by Fe3 O4 nanoparticles in the inner interface and CNCs in the outer interface [41]. The prepared W1 /O/W2 double emulsion was thermally initiated to induce polymerization and cross-linking of the monomers. The microsphere section plane in the SEM image showed that the internal holes made from the precursor multiple emulsions were distributed uniformly (Figure 16.7d). It was also demonstrated that the multihollow structure could be designed and controlled by adjusting the amounts of stabilizers and the degree of emulsification. The hydrophilicity and biocompatibility of CNC stabilizer in the outer surface provided a possibility to utilize the obtained microspheres in the field of biomaterials. Surface grafting of nonpolar polymer chains is an efficient way to increase the compatibility of nanocellulose and polymer matrices, thereby also increasing the emulsifying ability of nanocellulose. Novel polystyrene beads containing water microdroplets were fabricated by W/O/W double Pickering emulsion polymerization using CNF as a stabilizer [42]. Two steps were used to prepare these multistructured polystyrene beads. First, the water droplets were emulsified in prepolymerized styrene/PS mixture using CNF/polystyrene composite nanospheres as a stabilizer to form W/O/W double emulsion (Figure 16.7e1 ). The CNF/polystyrene nanospheres were enabled in situ by styrene/maleic anhydride, which improved the stabilizing ability of CNFs. Subsequently, the suspension polymerization of styrene/PS droplets containing emulsified water was initiated to form polystyrene beads (Figure 16.7e2 ). The water microdroplets within polystyrene beads were found to be well distributed and entrapped by the closely packed layer of CNF/polystyrene composite nanospheres (Figure 16.7f ). Furthermore, increasing the CNF content was shown to enhance markedly water encapsulation efficiency as well as the stability of water microdroplets.

Nanocelluloses at the Oil–Water Interface

16.7 Emulsion or Emulsion-precursor Systems with Stimuli-responsive Behavior As has been discussed so far, the irreversible adsorption of CNC at the oil–water interface is an important contributing factor in the stabilization of Pickering emulsions. However, subsequent emulsion collapse or destabilization may be necessary in some specific applications such as oil transport, fossil fuel production, or emulsion polymerization. Therefore, development of modified cellulose-based nanoparticles with switchable interfacial properties is essential. Recently, grafting stimuli-responsive polymers onto the nanoparticles was demonstrated to control the Pickering emulsion stability by changing their environment such as pH, heat, ionic strength, and light [43]. Poly(N-isopropylacrylamide) (Poly(NIPAAM)), a thermo-responsive polymer with a lower critical solution temperature (LCST) between 30 and 35 ∘ C can form an aqueous solution that phase-separates above LCST, due to thermal-driven chain dehydration [44]. Such a response near physiological temperatures (37 ∘ C) is of interest for applications such as drug-release. Poly(NIPAAM) brushes grafted from CNCs synthesized via surface-initiated single-electron transfer living radical polymerization (SI-SET-LRP) were discussed in light of Figure 16.3a [21]. Figure 16.8a1 depicts the thermoresponsiveness of unmodified CNC and such poly(NIPAAM)-g-CNC in aqueous suspensions by light transmittance, showing that unmodified CNCs were not responsive to temperature [22]. However, a large shift in light transmittance with the onset of 32 ∘ C, close to the LCST of Poly(NIPAAM) was observed for poly(NIPAAM)-g-CNCs. In addition, the viscosity of the grafted polymer increased as the temperature approached the LCST (32 ∘ C). It was reported that the ionic strength also influenced the responsiveness of poly(NIPAAM)-g-CNCs. The interaction forces versus the separation of films of poly(NIPAAM)-g-CNCs in 10 mM NaCl (Figure 16.8a2 ) showed a nonlinear trend due to the loss of conformational degree of freedom. The onset of 25 nm steric repulsion implied a partially swollen polymer layer on the CNCs. Overall, the increased ionic strength in aqueous media leads to a partial collapse of poly(NIPAAM) brushes and a reduction of the adhesion. When an emulsion was made using poly(NIPAAM)-g-CNCs, the oil-in-water emulsion were noted to be stable for longer than four months; however, the emulsion broke after heating above LCST of poly(NIPAAM) for one minutes [23]. This phenomenon was explained by the increased viscosity of emulsions when approaching LCST. Poly[(2-dimethylamino)ethyl methacrylate] (PDMAEMA), another interesting weak polyelectrolyte (pK a about 7.4), shows both pH and thermoresponsive properties [47]. Under increased pH, PDMAEMA undergoes stretch-to-collapse transition due to the deprotonation of amine functional groups [48]. Tang et al. grafted PDMAEMA onto the surface of CNCs via free

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Nanocelluloses at the Oil–Water Interface

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radical polymerization and studied the influence of PDMAEMA-g-CNC on the stability of Pickering emulsion prepared from heptane and toluene [45]. Increasing the pH in both cases made the emulsions more stable, since deprotonation of PDMAEMA in alkaline condition increased the hydrophobicity of the polymer chain, while CNC nanoparticles were preferentially wetted by water (Figure 16.8b1 ,b2 ). When the acidity increased, PDMAEMA-g-CNCs were more hydrophilic and desorbed from the oil–water interface resulting in phase separation. Nile red, as a solvatochromic dye, displayed a color change from yellow to red due to the shift of its adsorption wavelength, when the polarity of its corresponding solvent decreased. The presence of Nile red in the emulsions demonstrated more clearly the pH-responsive property of modified CNC. In the case of toluene emulsions, the color range did not vary significantly since the polarity of toluene is almost 10 times higher than that

Figure 16.8 (a1 ) Normalized light transmittance versus temperature of aqueous dispersions of unmodified CNCs and poly(NiPAAm)-g-CNCs with different grafting densities (low, LD; medium, MD/LDP; high, HD; very high, VHD) [22]. (a2 ) Interaction forces (colloidal probe microscopy) versus separation between a silica sphere against a flat layer spin-coated on silica wafers consisting of unmodified CNCs and MD poly(NiPAAm)-g-CNCs) [22]. (b1 ) A weak polyelectrolyte, poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA), was grafted onto the surface of cellulose nanocrystals via free radical polymerization. The resultant suspension of PDMAE-MA-grafted-cellulose nanocrystals (PDMAEMA-g-CNC) possessed pH-responsive properties. Stable heptane-in-water and toluene-in-water emulsions were prepared with PDMAEMA-g-CNC. (b2 ) Using Nile red as a fluorescence probe, the stability of the emulsions as a function of pH and temperature was elucidated. It was deduced that PDMAEMA chains promoted the stability of emulsion droplets and their chain conformation varied with pH and temperature to trigger the emulsification and demulsification of oil droplets. Interestingly, for the heptane system, the macroscopic colors varied depending on the pH condition, while the color of the toluene system remained the same. Reversible emulsion systems that responded to pH were observed [45]. (c1 ) SEM images of polystyrene microbeads encapsulated by hybrid CNC-CoFe2 O4 shells. The microbeads can be separated by magnetic manipulation as shown in (c2 and c3 ) [46]. (d) Magnetic hysteresis loops of Fe3 O4 (a) and HM-IMs-5 (b) and photographs of HM-IMs-5 suspended in water and in the presence of an externally placed magnet (c) [41]. (e) The transitional phase behavior of C2–BC (e1 ), C6–BC (e2 ), and C12–BC (e3 ) stabilized emulsion of water and toluene. The arrow shows flocculated modified BC. Note that the slightly yellowish color of C2–BC stabilized emulsion is a result of the color of C2–BC, which is slightly yellowish after the esterification of BC with acetic acid [26]. Source: Panels (a)–(c): Zoppe et al. 2011 [22], Tang et al. 2014 [45], and Nypelö et al. 2014 [46]. Copyright 2011 and 2014. Reprinted with permission of American Chemical Society. Panel (d): Zhu et al. 2015 [41]. Copyright 2015. Reprinted with permission of Elsevier. Panel (e): Lee et al. 2014 [26]. Copyright 2014. Reprinted with permission of Elsevier. (See insert for color representation of this figure.)

Nanocelluloses at the Oil–Water Interface

of heptane. Under UV light, Nile red displayed fluorescence of pale green and yellow for heptane and toluene, respectively. Therefore, unstable emulsions at low pH were clearly distinguished as the oil phase on the upper part of the emulsion became fluorescent (Figure 16.8b2 ). Magneto-responsive CNC materials have also been reported as a promising approach for selective adsorption, in situ polymerization or separation of toxic dyes in stimuli-responsive Pickering emulsions. CNC–CoFe2 O4 hybrid nanoparticles were synthesized by precipitation of cobalt ferric salts and used to stabilize the O/W interface forming magneto-responsive Pickering emulsions [49]. Subsequently, these emulsions were used to prepare polystyrene microbeads via in situ polymerization of styrene droplets enclosed in self-assembled CNC–CoFe2 O4 shells [46]. This method resulted in microbeads with small size (about 8 μm) and entangled CNC shells covering the polystyrene core together with the magnetic particles, which were extended from the surface, as 3D surface structures (Figure 16.8c1 ). These microbeads could successfully be attracted and collected from the dispersing media by using a magnet (Figure 16.8c2 ). Also, they were effective in the removal of methylene blue dye from a solution; thus, demonstrating their potential for material delivery and wastewater purification. Magnetic nanoparticles and CNCs were used to form double water-in-oil-in-water (W1 /O/W2 ) emulsion polymerization to obtain the HM-IMs introduced in previous sections [41]. The hydrophobic Fe3 O4 nanoparticles were used to stabilize the inner W/O droplets, CNCs were applied for stabilizing the outer O/W interface. HM-IMs were used as sorbents to selectively adsorb template bifenthrin (BF) and demonstrated superparamagnetic properties as shown in magnetic hysteresis loop (Figure 16.8d). BC has been also modified to obtain pH-triggered phase inversion properties [26]. First, BC was hydrophobized via esterification using alkanoic acids of increasing aliphatic chain length; namely acetic acid (C2 ), hexanoic acid (C6 ), and dodecanoic acid (C12 ). As a result, such hydrophilic nature of modified BC enabled the formation of W/O emulsions with toluene stabilized at pH 5. When the pH was increased from 1 to 14, the Pickering emulsion was phase separated in C2 -BC and showed irreversible phase inversion (O/W) as the pH returned back to acidic (Figure 16.8e1 ). However, C6 - and C12 -BC presented slightly different behavior than C2 -BC. These emulsions were also phase separated when the pH was raised to 14, but they showed reversible reformation of W/O emulsions (Figure 16.8e2 ,e3 ). The phase separation at high pH was ascribed to the increased electrostatic repulsion between the modified BC. The phase inversion seen in C2 -BC emulsion when the pH was readjusted to 1 was hypothesized to be the result of hydrolysis of ester bonds of cellulose acetate, causing a decrease in the degree of surface substitution and, thus, C2 -BC turned more hydrophilic and a O/W emulsion was formed.

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16.8 Closing Remarks In this review, we discussed nanocelluloses (cellulose nanofibrils, CNF, and CNC as well as BC) as materials that are readily available, renewable, and sustainable and thus represent interesting opportunities, especially in the field of emulsion formulation. Many options for deployment of these nanomaterials can be anticipated if used in the design of multiphase systems for papermaking, coatings, paints, composites, and with relevance in cosmetics, pharma, food, and oil-and-gas industries, among others. Such consideration comes from the ability of nanocelluloses to network and self-assemble in aqueous dispersions and at interfaces. The combination with surfactants and surface functionalization further expands the possibilities, for example, in converting emulsions into high order, solid structures, and stimuli-responsive and active materials. Nanocellulose-based emulsions should be counted on to advance the knowledge in the field. The recent developments found in the literature and summarized here provide many promises in these directions.

Acknowledgments We acknowledge the Academy of Finland for their support of the Center of Excellence “Molecular Engineering of Biosynthetic Hybrid Materials Research” (HYBER).

References 1 Oza, K.P. and Frank, S.G. (1986). Microcrystalline cellulose stabilized emul-

sions. J. Dispersion Sci. Technol. 7: 543–561. 2 Tingaut, P., Zimmermann, T., and Sèbe, G. (2012). Cellulose nanocrystals

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and microfibrillated cellulose as building blocks for the design of hierarchical functional materials. J. Mater. Chem. 22 (38): 20105–20111. Jiang, F. and Hsieh, Y.-L. (2015). Holocellulose nanocrystals: amphiphilicity, oil/water emulsion, and self-assembly. Biomacromolecules 16 (4): 1433–1441. Kalashnikova, I., Bizot, H., Bertoncini, P. et al. (2013). Cellulosic nanorods of various aspect ratios for oil in water Pickering emulsions. Soft Matter 9 (3): 952–959. Kalashnikova, I., Bizot, H., Cathala, B. et al. (2011). Modulation of cellulose nanocrystals amphiphilic properties to stabilize oil/water interface. Biomacromolecules 13 (1): 267–275. Kalashnikova, I., Bizot, H., Cathala, B. et al. (2011). New Pickering emulsions stabilized by bacterial cellulose nanocrystals. Langmuir 27 (12): 7471–7479.

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7 Cherhal, F., Cousin, F., and Capron, I. (2015). Structural description of

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the interface of Pickering emulsions stabilized by cellulose nanocrystals. Biomacromolecules 17: 496–502. Capron, I. and Cathala, B. (2013). Surfactant-free high internal phase emulsions stabilized by cellulose nanocrystals. Biomacromolecules 14 (2): 291–296. Binks, B.P. and Horozov, T.S. (2006). Colloidal Particles at Liquid Interfaces. Cambridge University Press. Araki, J. (2013). Electrostatic or steric?–Preparations and characterizations of well-dispersed systems containing rod-like nanowhiskers of crystalline polysaccharides. Soft Matter 9 (16): 4125–4141. Cherhal, F., Cathala, B., and Capron, I. (2015). Surface charge density variation to promote structural orientation of cellulose nanocrystals. Nord. Pulp Pap. Res. J. 30 (1): 126–131. Cameron, N., Sherrington, D., Albiston, L. et al. (1996). Study of the formation of the open-cellular morphology of poly (styrene/divinylbenzene) polyHIPE materials by cryo-SEM. Colloid. Polym. Sci. 274 (6): 592–595. Xhanari, K., Syverud, K., and Stenius, P. (2011). Emulsions stabilized by microfibrillated cellulose: the effect of hydrophobization, concentration and o/w ratio. J. Dispersion Sci. Technol. 32 (3): 447–452. Lif, A., Stenstad, P., Syverud, K. et al. (2010). Fischer–Tropsch diesel emulsions stabilised by microfibrillated cellulose and nonionic surfactants. J. Colloid Interface Sci. 352 (2): 585–592. Andresen, M. and Stenius, P. (2007). Water-in-oil emulsions stabilized by hydrophobized microfibrillated cellulose. J. Dispersion Sci. Technol. 28 (6): 837–844. Lee, K.-Y., Blaker, J.J., Murakami, R. et al. (2014). Phase behavior of medium and high internal phase water-in-oil emulsions stabilized solely by hydrophobized bacterial cellulose nanofibrils. Langmuir 30 (2): 452–460. Xhanari, K., Syverud, K., Chinga-Carrasco, G. et al. (2011). Structure of nanofibrillated cellulose layers at the o/w interface. J. Colloid Interface Sci. 356 (1): 58–62. Varjonen, S., Laaksonen, P., Paananen, A. et al. (2011). Self-assembly of cellulose nanofibrils by genetically engineered fusion proteins. Soft Matter 7 (6): 2402–2411. Blaker, J.J., Lee, K.-Y., Li, X. et al. (2009). Renewable nanocomposite polymer foams synthesized from Pickering emulsion templates. Green Chem. 11 (9): 1321–1326. Li, Y., Ko, F.K., and Hamad, W.Y. (2013). Effects of emulsion droplet size on the structure of electrospun ultrafine biocomposite fibers with cellulose nanocrystals. Biomacromolecules 14 (11): 3801–3807. Zoppe, J.O., Habibi, Y., Rojas, O.J. et al. (2010). Poly (N-isopropylacrylamide) brushes grafted from cellulose nanocrystals via surface-initiated

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single-electron transfer living radical polymerization. Biomacromolecules 11 (10): 2683–2691. Zoppe, J.O., Osterberg, M., Venditti, R. et al. (2011). Surface interaction forces of cellulose nanocrystals grafted with thermoresponsive polymer brushes. Biomacromolecules 12 (7): 2788–2796. Zoppe, J.O., Venditti, R.A., and Rojas, O.J. (2012). Pickering emulsions stabilized by cellulose nanocrystals grafted with thermo-responsive polymer brushes. J. Colloid Interface Sci. 369 (1): 202–209. Visanko, M., Liimatainen, H., Sirviö, J.A. et al. (2014). Amphiphilic cellulose nanocrystals from acid-free oxidative treatment: physicochemical characteristics and use as an oil–water stabilizer. Biomacromolecules 15 (7): 2769–2775. Ojala, J., Sirviö, J.A., and Liimatainen, H. (2016). Nanoparticle emulsifiers based on bifunctionalized cellulose nanocrystals as marine diesel oil–water emulsion stabilizers. Chem. Eng. J. 288: 312–320. Lee, K.-Y., Blaker, J.J., Heng, J.Y. et al. (2014). pH-triggered phase inversion and separation of hydrophobised bacterial cellulose stabilised Pickering emulsions. React. Funct. Polym. 85: 208–213. Carrillo, C.A., Nypelö, T., and Rojas, O.J. (2016). Double emulsions for the compatibilization of hydrophilic nanocellulose with non-polar polymers and validation in the synthesis of composite fibers. Soft Matter 12: 2721–2728. Quennouz, N., Hashmi, S.M., Choi, H.S. et al. (2016). Rheology of cellulose nanofibrils in the presence of surfactants. Soft Matter 12 (1): 157–164. Brinatti, C., Huang, J., Berry, R.M. et al. (2016). Structural and energetic studies on the interaction of cationic surfactants and cellulose nanocrystals. Langmuir 32: 689–698. Hu, Z., Ballinger, S., Pelton, R. et al. (2015). Surfactant-enhanced cellulose nanocrystal Pickering emulsions. J. Colloid Interface Sci. 439: 139–148. Mabrouk, A.B., Vilar, M.R., Magnin, A. et al. (2011). Synthesis and characterization of cellulose whiskers/polymer nanocomposite dispersion by mini-emulsion polymerization. J. Colloid Interface Sci. 363 (1): 129–136. Liu, H., Geng, S., Xu, Y. et al. (2016). Facile fabrication of versatile PMMA/CNF–NaYF 4: Yb/Er composite microspheres by Pickering emulsion system. Mater. Lett. 166: 55–58. Gordeyeva, K.S., Fall, A.B., Hall, S. et al. (2016). Stabilizing nanocellulose-nonionic surfactant composite foams by delayed Ca-induced gelation. J. Colloid Interface Sci. 472: 44–51. Ougiya, H., Watanabe, K., Morinaga, Y. et al. (1997). Emulsion-stabilizing effect of bacterial cellulose. Biosci. Biotechnol., Biochem. 61 (9): 1541–1545. Paximada, P., Tsouko, E., Kopsahelis, N. et al. (2016). Bacterial cellulose as stabilizer of o/w emulsions. Food Hydrocolloids 53: 225–232. Peddireddy, K.R., Nicolai, T., Benyahia, L. et al. (2016). Stabilization of water-in-water emulsions by nanorods. ACS Macro Lett. 5: 283–286.

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37 Hu, Z., Marway, H.S., Kasem, H. et al. (2016). Dried and redispersible cellu-

lose nanocrystal pickering emulsions. ACS Macro Lett. 5: 185–189. 38 Paximada, P., Koutinas, A.A., Scholten, E. et al. (2016). Effect of bacterial

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cellulose addition on physical properties of WPI emulsions. Comparison with common thickeners. Food Hydrocolloids 54: 245–254. Cunha, A.G., Mougel, J.-B., Cathala, B. et al. (2014). Preparation of double Pickering emulsions stabilized by chemically tailored nanocelluloses. Langmuir 30 (31): 9327–9335. Carrillo, C.A., Nypelö, T.E., and Rojas, O.J. (2015). Cellulose nanofibrils for one-step stabilization of multiple emulsions (W/O/W) based on soybean oil. J. Colloid Interf. Sci. 445: 166–173. Zhu, W., Ma, W., Li, C. et al. (2015). Well-designed multihollow magnetic imprinted microspheres based on cellulose nanocrystals (CNCs) stabilized Pickering double emulsion polymerization for selective adsorption of bifenthrin. Chem. Eng. J. 276: 249–260. Nikfarjam, N., Qazvini, N.T., and Deng, Y. (2015). Surfactant free Pickering emulsion polymerization of styrene in W/O/W system using cellulose nanofibrils. Eur. Polym. J. 64: 179–188. Saigal, T., Dong, H., Matyjaszewski, K. et al. (2010). Pickering emulsions stabilized by nanoparticles with thermally responsive grafted polymer brushes. Langmuir 26 (19): 15200–15209. Schild, H.G. (1992). Poly(N-isopropylacrylamide): experiment, theory and application. Prog. Polym. Sci. 17 (2): 163–249. Tang, J., Lee, M.F.X., Zhang, W. et al. (2014). Dual responsive pickering emulsion stabilized by poly[2-(dimethylamino) ethyl methacrylate] grafted cellulose nanocrystals. Biomacromolecules 15 (8): 3052–3060. Nypelö, T., Rodriguez-Abreu, C., Kolen’ko, Y. et al. (2014). Microbeads and hollow microcapsules obtained by self-assembly of pickering magneto-responsive cellulose nanocrystals. ACS Appl. Mater. Interf. 6 (19): 16851–16858. Yao, Z. and Tam, K. (2011). Synthesis and self-assembly of stimuli-responsive poly(2-(dimethylamino) ethyl methacrylate)-block-fullerene (PDMAEMA-b-C60) and the demicellization induced by free PDMAEMA chains. Langmuir 27 (11): 6668–6673. Rinkenauer, A.C., Schallon, A., Günther, U. et al. (2013). A paradigm change: efficient transfection of human leukemia cells by stimuli-responsive multicompartment micelles. ACS Nano 7 (11): 9621–9631. Nypelö, T., Rodriguez-Abreu, C., Rivas, J. et al. (2014). Magneto-responsive hybrid materials based on cellulose nanocrystals. Cellulose 21 (4): 2557–2566.

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17 Honeycomb-patterned Cellulose as a Promising Tool to Investigate Wood Cell Wall Formation and Deformation Yasumitsu Uraki 1 , Liang Zhou 2 , Qiang Li 3 , Teuku B. Bardant 3 , and Keiichi Koda 1 1 Division of Environmental Resources, Research Faculty of Agriculture, Hokkaido University, Sapporo 060-8589, Japan 2 Department of Material Science and Engineering, Anhui Agricultural University, Hefei 230036, China 3 Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Japan

17.1 Introduction Some natural materials, such as compound multifaceted eyes of insects and human spongy bone, have a structure of hexagonal array like a honeycomb, as do some artificial materials with stiff and lightweight characteristics [1]. Wood cell walls also arrange like a honeycomb [2]. To elucidate the deformation mechanism of such materials under external load, several theories for honeycomb deformation were proposed. In 1982, Gibson et al. proposed the first theory on “the mechanics of two-dimensional cellular materials,” [3] which will be referred to as “the bending model” in this article. In 1996, Masters and Evans proposed “the general model” in their paper “Models for elastic deformation of honeycomb.” Their paper also included “the stretching model” to elucidate the stretching deformation and “the combined model” for consideration of bending and stretching deformation [4]. Afterward, Mod˙en and Berglund proposed a modified model for the combined model, which was designated as “two-phase annual ring model” for elucidating transverse anisotropy in softwood [5]. This theory was constructed by considering the contribution of earlywood and latewood to the combined model. However, such theories were not necessarily proven empirically by using an actual honeycomb material. In this study, the validity of the deformation theories was investigated by using honeycomb-patterned cellulose as a basic framework of artificial wood cell wall. We have already successfully demonstrated the fabrication of honeycombpatterned cellulosic films with cellulose I and II polymorphisms, as shown in Figure 17.1 [6]. These films were developed as a basic framework of artificial

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Polydimethyl siloxane (PDMS)

Transcription

Transcription

Third template Second template

First template Self-organization (breath-figure method)

Transcription

Cellulose acetate (CA)

Honeycomb-patterned CA film Deacetylation

Transcription to agar medium Honeycomb-patterned agar film (HPA)

with NaOCH3

Honeycomb-patterned regenerated cellulose film (HPRC) with cellulose II polymorphism)

Incubation of G.xylinus on the agar under 90% CO2 Honeycomb-patterned bacterial cellulose film (HPBC) with cellulose l polymorphism)

Figure 17.1 A schematic diagram of preparation of HPBC with cellulose I polymorphism (right) and HPRC with cellulose II polymorphism (middle)

wood cell wall. Fabrication of both types of honeycomb-films started with the preparation of the first template on the basis of self-organization of synthetic polymers including an amphiphilic polymer, generally termed as breath-figure method [7]. The second template, with opposite surface morphology to the first template, was prepared by a transcription process with polydimethyl siloxane (PDMS). The second template was stamped on an acetone solution of cellulose acetate (CA) followed by evaporation of the solvent to give honeycomb-patterned CA films. Finally, the CA film was saponified to give a honeycomb-patterned regenerated cellulose (HPRC) film with cellulose II polymorphism. The third template of PDMS was to be prepared for the fabrication of a honeycomb with cellulose I polymorphism. It was done in the same way as the second template, but the resulting template was stamped on a hot agar solution that included nutrients for cellulose-producing bacteria. Finally, the bacteria were cultured on the agar film with honeycomb-patterned surface under very unique conditions, ca. 90% CO2 atmosphere and saturated relative humidity, to yield a honeycomb-patterned bacterial cellulose (HPBC) film with cellulose I polymorphism [6, 8].

Honeycomb-patterned Cellulose as a Promising Tool

We considere such honeycomb-patterned cellulosic films to be a promising model material to investigate the validity of the deformation theories. In addition, investigation of deposition of other wood cell wall components, such as hemicellulose and lignin, onto the cellulosic films would give us new insights into the mechanical functions of the components of wood cell walls.

17.2 Theory of Honeycomb Deformation Briefly, the deformation concepts are introduced in this section. In the bending model, the elastic deformation of the honeycomb is caused by bending of inclined beams in the hexagon under in-plane uniaxial force (X 2 direction) as shown in Figure 17.2, A [3]. In contrast to that, the stretching model is assumed to expand the two vertical beams and decrease the 𝜃 2 angle between the inclined beams as shown in Figure 17.2, B. The combined model constitutes the bending and stretching deformation based on the Voigt model to elucidate viscoelastic properties of a polymer. This model was considered to reflect a more accurate deformation than the other theories of the bending model and the stretching model in two academic publications [4, 5]. However, Modén and Berglund introduced a new idea to the combined model, considering the contribution of bending and stretching deformation to the hexagonal unit which depends on the pore size of the honeycomb [5]. The small pore size of the honeycomb, like latewood cells, is influenced by the stretching deformation, while the large pore size of the honeycomb, like earlywood cells, is influenced by the bending deformation. Therefore, they proposed A σ2 Bending X1 θ1

B

h

σ2

σ2

Stretching θ2 t

C

Hinging σ2

※b: height of beam in vertical direction X2

Figure 17.2 Fundamental concepts of deformation of a honeycomb cell under in-plane compressive stress in X 2 direction (𝜎 2 ). l, length of inclined beam; h, length of vertical beam; t, width of beams; 𝜃 1 , angle of the inclined beam to X 2 axis; and 𝜃 2 , angle between two neighboring inclined beams.

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a “two-phase annual ring model” by using an import factor 𝛼, taking into consideration the ratio of contribution of bending to stretching. Masters and Evans proposed a general model as a comprehensive mathematical model for predicting the honeycomb deformation [4]. They stated that deformation of the connecting area, so-called “hinging,” constructed by three adjacent, hexagonal units (Figure 17.2, C), could not be neglected when t/l is larger than 1/4 [4]. Therefore, they proposed the general model comprised of bending, stretching, and hinging deformation. In the above five theories, the geometry of the honeycomb is related to the relative density (density of honeycomb construction material/density of honeycomb). The relative density can be expressed using parameters such as the thickness (t) of the hexagonal beam, beam length (l), and angle (𝜃 1 ), which are shown in Figure 17.2. Finally, the modulus of the honeycomb in the theories can be expressed by using the parameters and modulus of the construction material, as demonstrated for the modulus of cellulose (Ec ) in the following: ( )3 E cos 𝜃1 t 1) b = Bending model Ec l (1 + sin 𝜃1 )sin2 𝜃1 ( ) E 1 t Stretching model 2) s = Ec l (1 + sin 𝜃1 ) cos 𝜃1 )]−1 [ (( )3 2 Ebs t sin 𝜃1 t 3) Bending and Stretching = Ec (1 + sin 𝜃1 ) + cos 𝜃1 Ec l cos 𝜃1 l model E 1 𝛼 1−𝛼 − b 4) = + 𝛼 = e Es Two − phase annual ring model Ebs−two Eb Es ) ( Eg 5(1 + sin 𝜃1 ) t 3 5) = [ ] General model ( )2 Ec l cos 𝜃 8cos2 𝜃1 + 5 tl (2 + sin2 𝜃1 ) where Eb is the modulus of the honeycomb in the bending model, Es in the strething model, Ebs in the combined model, and Eg in the general model. Of course, honeycomb moduli are also defined for tangential stress (X 1 direction in Figure 17.2) [3], but the equations are identical to radial stress (X 2 direction in Figure 17.1) when 𝜃 1 is 30∘ . Therefore, only the equations for radial modulus are shown in this chapter.

17.3 HPRC with Cellulose II Polymorphism and Their Tensile Strength We have developed two types of honeycomb-patterned cellulosic films, HPBC films, and HPRC films. The fabrication of HPBC was carried out by the control

Honeycomb-patterned Cellulose as a Promising Tool

(a)

(b)

10 µm 5 kV

10 µm ×1, 500

10 µm 42 mm

(c)

5 kV

10 µm ×1, 500

39 mm

5 kV

10 µm ×1, 500

39 mm

(d)

10 µm 5 kV

10 µm ×1, 500

10 µm 42 mm

Figure 17.3 SEM images of HPRC with four different pore diameters. (a) 5 μm; (b) 10 μm; (c) 15 μm; and (d) 20 μm.

of the movement of cellulose-producing bacteria (Gluconacetobacter xylinus; body size, ca. 1 × 5 μm) on a honeycomb-patterned agar medium. Thereby, it was very difficult to fabricate HPBC with a pore sizes smaller than 15 μm. To investigate the validity of the five deformation theories, we used HPRC instead, because its hexagonal pore size was easily controlled with the first template with different pore sizes. Here, HPRCs with four different pore sizes (5, 10, 15, and 20 μm 𝜙) were prepared as probes for tensile tests, as shown in Figure 17.3. The tensile test was carried out by using a hand-made testing machine (Figure 17.4) [9]. A rectangular specimen (1.5 × 20 mm) was attached on a paper support, and then the support was fixed on the machine by using a screw nat. The stress was measured by a load cell, and the displacement of the specimen was monitored with a displacement transducer. Poisson’s ratio was calculated from images of specimens taken under a 3-D laser microscope (model VK-9500, Keyence Co., Osaka, Japan) during the tensile test. Table 17.1 shows the geometrical parameters, t and l, mechanical properties, tensile strength and modulus of elasticity (MOE) of the resultant films. The tensile strength and MOE decreased with increasing pore size. This result suggests that the mechanical strength of the honeycomb to in-plane stress depends on the density and the amount of the honeycomb-shaped, cellulosic

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Figure 17.4 Tensile testing apparatus. DT is a displacement transducer. Source: Uraki et al. 2010 [9]. Reproduced with permission of Taylor & Francis.

Cut after specimen is fixed on the support Fixed to one of a couple of supports Specimen

Sample holder

Load cell

Sample Displacement

DT

Micrometer

Table 17.1 Geometrical dimensions of cell and mechanical properties of HPRC.

t/l

Poisson’s ratio

Tensile strength of HPRC (MPa)

Elastic modulus of HPRC (MPa)

2.89

0.17

0.97

66.2

956

0.89

5.77

0.15

0.96

44.2

885

1.21

8.66

0.14

0.98

26.6

693

1.37

11.5

0.12

0.93

19.8

477

Pore size (𝛍m)

Width of beam, t (𝛍m)

Length of beam, l (𝛍m)

5.00

0.49

10.0 15.0 20.0

material. Actually, it is noteworthy that the stiffness of the honeycomb structure is essentially the ability to resist deformation from stress vertical to the honeycomb surface, but not from in-plane stress. The Poisson’s ratio for all the films was approximately 1, suggesting that the cellulosic honeycomb films were anisotropic [10].

17.4 Validity of Deformation Models The MOE of cellulosic honeycomb was plotted against t/l in Figure 17.5. Elasticity of cellulose (Ec ) was calculated, based on the five deformation models at the highest decision coefficient (R2 ), which was also calculated from the deviation between theoretical value and experimental value. The stretching model gave the lowest Ec (7 GPa) and R2 . The MOE of cellulose II structures was reported to

Modulus of elasticity of honycomb film (MPa)

Figure 17.5 Modulus of elasticity of honeycomb as a function of t/l from experiment and predicting of model. Star is experimental value. (a) This line with decision coefficient (R2 ) = 0.80 was plotted based on the stretching model at E c = 7 GPa; this line with R2 = 0.90 was plotted based on the this bending model at E c = 97 GPa; line with R2 = 0.92 was plotted based on the combined model at E c = 105 GPa. This line with R2 = 0.97 was (b) plotted based on the two-phase annual ring model at E c = 112 GPa; this line with R2 = 0.91 was plotted based on the general model at E c = 163 GPa.

Modulus of elasticity of honycomb film (MPa)

Honeycomb-patterned Cellulose as a Promising Tool

1000

800

600

400 0.10

0.12

0.14 t/l (a)

0.16

0.18

0.12

0.14 t/l (b)

0.16

0.18

1000

800

600

400 0.10

be 78 GPa [11], 98 GPa [12], 106 GPa [13], and 163 GPa [14, 15]. If calculated Ec and literature values are compared, the calculated value seems extremely low. Consequently, our experiment reveals that the stretching model has a considerably low validity to explain the MOE of cellulose. Nonetheless, other models including the concept of the bending model gave reasonable values of Ec , which was within the range of literature values, with a remarkably high R2 . The combined model (bending and stretching model) revealed a higher R2 (0.92) than the bending model (0.90). The two-phase annual ring model showed even higher R2 (0.97) than the combined model. This tendency suggests that the stretching model should be taken into consideration to understand the mechanism of honeycomb deformation and the contribution of the stretching model, which is dependent on the pore size or honeycomb diameter, is also an important parameter. However, the general model gave the lower R2 (0.91) at an Ec of 163 GPa than the combined and two-phase annual ring models. The latter value of Ec was almost identical with the maximum literature value, which was estimated from the crystalline state of cellulose II polymorphism [14, 15]. Since HPRC is a semitransparent film, which suggests it is partially crystalized film, the HPRC films cannot possibly show such a high Ec value. Therefore, the validity of the general

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model must be low. This suggests that a hinging effect on the honeycomb deformation can be ignored.

17.5 Deposition of Wood Cell Wall Components on the Film of HPBC Film To elucidate functions of cell wall components other than cellulose, that is, hemicellulose and lignin, we attempted to apply models of hemicellulose and lignin to the cellulosic honeycomb as a basic framework of an artificial wood cell wall. Here, we used HPBC as the frame, because its morphology is similar to that of wood cellulose. First, commercial beech xylan (Xyl, 86.0%; Gal, 1.2%; Man, 0.5%; uronic acid, 11.8%; lignin, 1.3%) was deposited onto/into the cellulosic film by immersing the film in aqueous xylan solution. As a control, we used flat bacterial cellulose (FBC), which was obtained as a flat pellicle after a short-time incubation of the bacteria in liquid Hestrin–Schramm medium. The adsorbed amount of xylan on cellulose was determined in the xylan concentration range of 0.1–1.0 g l−1 on an HPLC system. Xylan was adsorbed on HPBC in much larger amount than FBC [16]. This result showed that HPBC had a larger surface area accessible to xylan than FBC did, because most of the microfibrils are exposed to their external circumstances. Furthermore, deposition of xylan not only on both types of films but also inside the films was confirmed by fluorescent microscopic images of the films immunolabeled with xylan-specific antibodies [16]. Second, an endwise polymerization (Zutropfverfahren) of coniferyl alcohol (CFA) as a monolignol in the presence of bacterial cellulose was attempted by dropwise addition of CFA and hydrogen peroxide in phosphate buffered saline (PBS) (0.01 M, pH 6) to horseradish peroxidase in PBS including HPBC. Distribution of the resultant dehydrogenation polymer (DHP) as an artificial lignin was mapped by scanning electron microscopy–energy dispersive X-ray analysis (SEM–EDX) after bromination [17]. The content of DHP in the films was determined by the acetyl bromide method [18] and the content of aryl-ether bonds was estimated by the alkaline nitrobenzene oxidation method [19]. From the SEM observations (Figure 17.6), DHP was generated inside the cellulosic films in both xylan-adsorbed HPBC and FBC. However, in the absence of xylan, DHP was generated and deposited on the surface of FBC and a smaller amount of DHP than in the xylan-adsorbed films was generated inside HPBC. In addition, the DHP content in both types of films was increased by the presence of xylan [16]. These phenomena imply that xylan has an ability to induce CFA into the polysaccharide matrix comprised of cellulose and hemicellulose.

Honeycomb-patterned Cellulose as a Promising Tool

HPBC

DHP-HPBC

DHP-Xyl-HPBC

20 µm

20 µm

(a)

(b)

20 µm

(c)

Figure 17.6 Representative SEM images of (a) HPBC, (b) DHP-HPBC, and (c) DHP-Xyl-HPBC. White spots show the deposition of DHP.

The yield of the nitrobenzene oxidation was also increased by the presence of xylan [16]. This result suggests that xylan regulates the frequency of interunitary linkages, or even the whole structure of DHP. Consequently, xylan is likely to act as a scaffold for monolignol deposition and as a structure-regulator for lignin in living trees [16, 20, 21]. Finally, the influences of the deposition of xylan and DHP on the mechanical properties of both types of cellulosic films were investigated as shown in Figure 17.7. It should be noted that the results of tensile testing are discussed in this section based only on the thickness, but not on both thickness and weight, because it was very difficult to measure the small weight gain by the deposition of xylan and DHP precisely. Both tensile strength and MOE were enhanced by the stepwise deposition of xylan and DHP for both HPBC and FBC. The increment ratio (xylan and DHP-adsorbed film/intact film) of tensile strength for HPBC and FBC was 1.9 and 1.6 times, respectively. Likewise, the increment ratio of MOE was found to be 3.0 and 2.5 times, respectively. Thus, MOE enhancement by the deposition of the two components was larger than that of the tensile strength. The increased mechanical properties were either caused by xylan deposition or DHP deposition, but a combination of both gave higher results that each one used individually. These results demonstrated that xylan and DHP contributed to strengthening cellulose, MOE in particular [16, 20, 21]. When MOE variation between HPBC and FBC was compared, MOE of FBC was found to increase integrally by the stepwise deposition. Meanwhile, MOE of HPBC was synergistically increased by the final deposition of DHP. Considering the morphological similarity between cellulosic films and cross-sections of wood, we conclude that lignin must give high MOE or the rigidity to wood cell walls with the aid of xylan. In conclusion, the validity of honeycomb deformation theories can be evaluated by using honeycomb-patterned cellulose. Successive deposition of xylan as a hemicellulose component and DHP as an artificial lignin onto/into the

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140 HPBC

FBC

Tensile strength (MPa)

120 100

90.4

80 60

72.7

71.6

55.1 31.9

40 19.4

36.6

23.6

20 0

BC

Xyl-BC

DHP-BC

DHP-Xyl-BC

(a) 2000 Modulus of elastictiy (MPa)

432

HPBC

FBC

1600

1434

1200

1058

965 833

800 574 400 0

278

BC

486 352

Xyl-BC

DHP-BC

DHP-Xyl-BC

(b)

Figure 17.7 Tensile strength (a) and modulus of elasticity (b) of HPBC and FBC after deposition of xylan and/or DHP.

cellulosic honeycomb was performed to estimate their effect on the mechanical strength of cellulose material. This fabrication process mimicked the formation of wood cell walls. Therefore, the resultant composite can be considered to be an artificial wood cell wall. In this study, functions of those wood components were also significantly elucidated. Thus, we concluded that our cellulosic honeycomb model serves as a good tool to investigate wood cell wall in vitro.

Acknowledgment This study was financially supported by JSPS KAKENHI (Grant-in-Aid for Scientific Research (A), Grant No. 2625202204).

Honeycomb-patterned Cellulose as a Promising Tool

References 1 Zhang, Q., Yang, X., Li, P. et al. (2015). Bioinspired engineering of honey-

2

3

4 5

6

7 8

9

10 11

12

13

14

comb structure – using nature to inspire human innovation. Prog. Mater Sci. 74: 332–400. Ando, K. and Onda, H. (1999). Mechanism for deformation of wood as a honeycomb structure I: effect of anatomy on the initial deformation process during radial compression. J. Wood Sci. 45 (2): 120–126. Gibson, L.J., Ashby, M.F., Schajer, G.S. et al. (1982). The mechanics of two-dimensional cellular materials. Proc. R. Soc. London, Ser. A 382 (1782): 25–42. Masters, I.G. and Evans, K.E. (1996). Models for the elastic deformation of honeycombs. Compos. Struct. 35 (4): 403–422. Moden, C.S. and Berglund, L. (2008). A two-phase annual ring model of transverse anisotropy in softwoods. Compos. Sci. Technol. 68 (14): 3020–3026. Uraki, Y., Tamai, Y., Hirai, T. et al. (2011). Fabrication of honeycomb-patterned cellulose material that mimics wood cell wall formation processes. Mater. Sci. Eng., C 31 (6): 1201–1208. Zhang, A., Bai, H., and Li, L. (2015). Breath figure: a nature-inspired preparation method for ordered porous films. Chem. Rev. 115 (18): 9801–9868. Uraki, Y., Nemoto, J., Otsuka, H. et al. (2007). Honeycomb-like architecture produced by living bacteria, Gluconacetobacter xylinus. Carbohydr. Polym. 69 (1): 1–6. Uraki, Y., Matsumoto, C., Hirai, T. et al. (2010). Mechanical effect of acetic acid lignin adsorption on honeycomb-patterned cellulosic films. J. Wood Chem. Technol. 30 (4): 348–359. Almgren, R.F. (1985). An isotropic three-dimensional structure with Poisson’s ratio = −1. J. Elast. 15 (4): 427–430. Langan, P., Nishiyama, Y., and Chanzy, H. (1999). A revised structure and hydrogen-bonding system in cellulose II from a neutron fiber diffraction analysis. J. Am. Chem. Soc. 121 (43): 9940–9946. Eichhorn, S.J., Young, R.J., and Davies, G.R. (2005). Modeling crystal and molecular deformation in regenerated cellulose fibers. Biomacromolecules 6 (1): 507–513. Matsuo, M., Sawatari, C., Iwai, Y. et al. (1990). Effect of orientation distribution and crystallinity on the measurement by X-ray diffraction of the crystal lattice moduli of cellulose I and II. Macromolecules 23 (13): 3266–3275. Kroon-Batenburg, L.M.J., Kroon, J., and Northolt, M.G. (1986). Chain modulus and intramolecular hydrogen-bonding in native and regenerated cellulose fibers. Polym. Commun. 27 (10): 290–292.

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15 Tashiro, K. and Kobayashi, M. (1991). Theoretical evaluation of

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three-dimensional elastic constants of native and regenerated celluloses: role of hydrogen bonds. Polymer 32 (8): 1516–1530. Li, Q., Koda, K., Yoshinaga, A. et al. (2015). Dehydrogenative polymerization of coniferyl alcohol in artificial polysaccharides matrices: effects of xylan on the polymerization. J. Agric. Food. Chem. 63 (18): 4613–4620. Saka, S., Thomas, R.J., and Gratzl, J.S. (1978). Lignin distribution-determination by energy-dispersive analysis of X-rays. Tappi 61 (1): 73–76. Hatfield, R.D., Grabber, J., Ralph, J. et al. (1999). Using the acetyl bromide assay to determine lignin concentrations in herbaceous plants: some cautionary notes. J. Agric. Food. Chem. 47 (2): 628–632. Chen, C.L. (1992). Nitrobenzene and cupric oxide oxidations. In: Methods in Lignin Chemistry (ed. S.Y. Lin and C.W. Dence), 301–321. Springer-Verlag. Burgert, I. (2006). Exploring the micromechanical design of plant cell walls. Am. J. Bot. 93 (10): 1391–1401. Salmén, L. (2004). Micromechanical understanding of the cell-wall structure. C.R. Biol. 327 (9–10): 873–880.

435

Index a acrylic-SAP 131 activated carboxylic acids 2 adhesive mixtures in paper conservation GPC analysis of 180–182 preparation of 177, 178 Aerocellulose 380, 381 density 378 shapes 378 thermal conductivity 378, 379 aerogels 296–297 alginate 386 applications 315–317, 373 cellulose I based 373–378 cellulose II based 378–380 conductive 305–307 density and specific surface areas 372 description 371 drying of solvogels 297–301 hydrophobic aerogels and superabsorbents 307–315 mechanical properties 301–305 pectin-based 380–386 starch-based 386 aeropectins mechanical properties 381, 382 morphology 381, 382

thermal conductivity vs. density 382, 383 aging see also paper preservation and restoration approach brightness of paper before and after 185, 186 contact angle before and after 184, 185 alginate aerogels 386 alkyne-modified CMC 295 ambient-dried aerogel membranes 306 amino-/ammonium group containing cellulose esters cellulose-4-(N-methylamino) butyrate 7–9 (3-carboxypropyl) trimethylammonium chloride esters 2–6 amino cellulose carbamates properties 22–23 synthesis 21–22 ω-aminoethyl cellulose carbamate 23 ω-aminoethyl-p-aminobenzyl cellulose carbamate 23 amorphous cellulose 264 anhydroglucose units (AGUs) 113, 115, 116, 118, 119, 227 attenuation length 240 azido-modified CMC 295

436

Index

b bacterial cellulose (BC) 254 emulsions stabilized with 406–407 BC sheets 430–432 ball milling 194 Bemberg fiber 343 bifunctional cellulose nanocrystals 401 bio-aerogels 387, 388 see also aerogels Biocelsol process 343 biocompatibility 128 biodegradation 128 bovine serum albumin (BSA) 2, 3 1,2,3,4-butanetetracarboxylic acid 307 TM

c Cagliotti parameters 233 carboxyethyl cellobiose structure 136 radial distribution function on O6 137 contour density maps 138 carboxyethyl cellulose (CEC) 128 biodegradability 129 bound and free water contents 131, 132 chemical modification and biodegradability 138–140 degree of polymerization 129 degree of substitution 129 DSC thermograms of water adsorbed on 131, 132 exo-type cellulose treatment of 129 molecular dynamics simulation 136–138 SAXS 133–135 super-absorbent polymer 130 carboxylate-based ionic liquids (ILs) 346 carboxymethylated cellulose nanofibrils (cm-CNF) 282 carboxymethylation of 280

hydrogel formation 311 carboxymethyl cellobiose average structure 136 contour density maps for 138 radial distribution function on O6 137 (3-carboxypropyl)trimethylammonium chloride esters of cellulose 2–6 cationic ring opening polymerization (CROP) 102 cellobiose average structure 136 radial distribution function on O6 137 contour density maps for 138 cellulose 1 aerogels 296–297 see also aerogels allomorphs 101 cellulose dissolution see cellulose swelling, dissolution and regeneration cellulose-dissolving ionic liquids (ILs) 345 cellulose ethers, history 143, 144 cellulose hydrogels 283 cellulose I based aerogels 373–378 cellulose II aerogels see Aerocellulose cellulose microfibrils (CMF) 235, 265 “amorphous” regions 264 36 (6 x 6) chain model 263 crystallinity 263 fringed-fibrillar model 264–267 high aspect ratio 263 leveling-off degree of polymerization 267–269 longitudinal order of 263 preparation of cellulose nanocrystals 270–271 schematic representation of 264 small angle neutron scattering 264 cellulose nanocrystals (CNCs) 193 from bacterial cellulose 224

Index

bifunctionalized 401 binding capability of cationic surfactants 402–404 chemical characterization elemental analysis 238–239 infrared spectroscopy 237–238 proton NMR 244 pulsed-field-gradient spin-echo NMR 244 solid-state NMR spectroscopy 234–237 X-ray photoelectron spectroscopy 240–243 chirality and 253–255 CNC–CoFe2 O4 hybrid nanoparticles 417 degree of substitution 228 degree of sustainability 394 expressions of chirality 257–259 flat ribbon 226 high internal phase emulsions 397 level-off degree of polymerization (LODP) 225 model representation 227 and molecular structure of cellulose 223–224 morphological and structural characterization microscopy 228–230 powder X-ray diffraction 230, 232–234 small angle scattering 230–231 nematic/smectic-ordered materials 255–256 periodate oxidation and reductive amination 400, 401 preparation of 270–271 protofibrils 223 films without iridescent colors 256–257 size 225–226 small angle neutron scattering 396

sulfuric/phosphoric acid hydrolysis 226, 395 surface charge density 393 surface wettability 404 terminal complexes 223 unit cell 224–225 Valonia, 224 XPS scans on 243–244 cellulose nanofibrils (CNFs) aerogels 296–297 applications 315–317 conductive aerogels 305–307 drying of solvogels 297–301 hydrophobic aerogels and superabsorbents 307–315 mechanical properties 301–305 AFM and TEM micrographs of 280–281 alginate composite 291 carboxymethylation 280 CNF/PVA composite 304 degree of crystallinity 279 enzymatic pretreatment 280 fibrillation 280 formation of microfibril bundles 277–278 high-intensity ultrasonication 280 high-pressure homogenization 279 hornification 281 hydrogels biocompatibility 286 biomedical application 283 composite 288–293 dark field microscopy micrograph 288 definition 282 from enzymatically pretreated pulp 286 fibrillar nanostructure 286 food applications 283 and hepatocellular carcinoma cells 284–285 mechanical properties 286

437

438

Index

cellulose nanofibrils (CNFs) (contd.) modification 293–296 Pickering emulsions 288 preparation of 284–285 properties and high-value applications of 317–318 stimuli–responsive properties 283 tempo-oxidized gyroidal hydrogels 287 inter-and intra-chain hydrogen bonds 277 interfiber bondings 281 mechanical pretreatment 280 mechanical properties and viscosity of 282 nanocellulose 278 nitrocellulose 277 tensile strengths and elastic moduli 279 tempo-oxidized hydrogels 280, 287 cellulose nanofibrils (CNF)/polystyrene composite nanospheres 412 cellulose-4-(N-methylamino)butyrate (CMABC) 7–9 cellulose–silica composite aerogels 375, 379, 380 cellulose swelling, dissolution and regeneration cellulose solvents 113–118 13 C-perlabeled cellulose model compounds 102–108 DMAc/LiCl solvent system 110, 112 effects on anhydroglucose unit 113, 115, 116, 118, 119 EMIm-OAc solvent 113, 114 H-bond cleavage 109 nonclassical CH-hydrogen bonds 117 swelling phases 109 cellulosic bottlebrushes “grafting-from”approach 50

“grafting-to”approach 51 chiral nematic arrangement 253–254 2-chloro-4,4,5,5-tetramethyl-1,3,2dioxaphospholane 237 click reaction 61 CMF see cellulose microfibril (CMF) CNCs see cellulose nanocrystals (CNCs) composite hydrogels 307 conductive aerogels 305–307 conductive hydrogels 292 coniferyl alcohol (CA) 430 conventional freeze drying (CFD) 373, 374 13 C-perlabeled cellulose 103 13 C-perlabeled cellulose model compounds 102, 103 C,C-couplings 104 NMR experiments 104–107 NMR intensity gain 105–106 specific hydrogen bonds, detection of 107, 108 cross-linked hydrogel 317 crystallinity index 232 Cuoxam 341

d [DBNH]OAc preparation chemical composition analysis 349 fiber spinning 350 mechanical analysis of fibers 351 molar mass distribution analysis 349–350 pulp dissolution and filtration 348–349 rheological measurements 349 stepwise purification 348 dehydrogenation polymer (DHP) 430–431 delignification/cellulose degradation selectivity in batch reactor 82 for Kraft pulp 86–90

Index

definition 81 HexA content 83 hydroxyl/oxyl radical generation 82 NaOH charge effect 79, 80 NaOH concentration 84, 85 number of cellulose chain scissions per cellulose polymer 81 oxygen pressure effect 84, 85 pulp properties 84 temperature effect 84, 85 6-deoxy-6-amino cellulose derivatives 9–21 application potential 13–21 spontaneous self-assembling 10–13 1,5-diazabicyclo[4.3.0]non-5-ene1-ium acetate ([DBNH]OAc) 347 differential scanning calorimetry (DSC) 131–133 direct hydrophobization method 295 dissolution see cellulose swelling, dissolution and regeneration double emulsions 409–412 drying of solvogels 297–301 dye-sensitized solar cells (DSSCs) 29, 30 dynamic light scattering (DLS) 228

surfactant-assisted 402–406 surfactant-free 395–397 enzymatically pretreated pulp 286 enzyme immobilization on amino cellulose 13, 14

f fiber spinning 350, 354–355 fibrillar-structured CNF cryogel 303 flat bacterial cellulose (FBC) 430–432 fluorescence/UV-Vis spectroscopy 243 342 Fortisan 4D biomimetic printing of CNF 291–292 free-standing hydrogels 373 free water 127 freeze-dried hydrogels 314 fringed-fibrillar model 264–267 fullerene-bound cellulose derivative 44–45

®

g gel permeation chromatography (GPC) see size exclusion chromatography (SEC) gyroid CNF scaffold 287

e

h

elasticity of cellulose 428 electron microscopy 228 elemental analysis 238–239 [EMIm]OAc 346 emulsions double 409–412 emulsion-precursor systems with stimuli-responsive behavior 413–417 with polymer coemulsifiers 406–409 stabilized with modified nanocelluloses 398–402

heptane-in-water Pickering emulsions 398 hexeneuronic acid content of pulp 76 highest decision coefficient 428 high-intensity ultrasonication 280 high internal phase emulsions (HIPEs) 397 high-kappa oxygen delignification 92–93 highly crystalline algal cellulose 254 highly transparent aerogels 303 honeycomb deformation 425–426

439

440

Index

honeycomb-patterned bacterial cellulose (HPBC) breath-figure method 424 cellulose I and II polymorphisms 423–424 cellulose II polymorphism and tensile strength 426–428 deposition of wood cell wall components 430–432 schematic diagram 423–424 validity of deformation models 428–430 honeycomb-patterned regenerated cellulose (HPRC) 424 hornification 281 HPBC see honeycomb-patterned bacterial cellulose (HPBC) hydrogels biocompatibility 286 biomedical application 283 composite 288–293 dark field microscopy micrograph 288 definition 282 enzymatically pretreated pulp 286 fibrillar nanostructure 286 food applications 283 and hepatocellular carcinoma cells 284–285 mechanical properties 286 modification 293–296 Pickering emulsions 288 preparation of 284–285 properties and high-value applications of 317–318 protonation 287 stimuli–responsive properties 283 tempo-oxidized gyroidal hydrogels 287 hydrogen bond network in cellulose 100, 101 hydrophilic polymer 307 hydrophobic aerogels 307–315

hydrophobized bacterial cellulose nanocrystals 401–402 hydroxyalkylmethyl celluloses 153–159 hydroxyethylmethyl celluloses (HEMCs) 159–161 hydroxypropylmethyl celluloses (HPMCs) 160, 162–166 MALDI-ToF-MS 160, 162 methyl subpattern 162–164

i imidazolium-based ionic liquids (ILs) 345 infrared spectroscopy 237–238 inorganic aerogels 296 in situ surfactant-assisted adsorption approach 402 intermediate water 127 ionic liquids (ILs) 344–345

j jamming process 254 Janus-type cellulosic bottlebrush 49, 50 regioselective formation 51, 52 size-exclusion chromatography 61–63

l Langmuir–Blodgett (LB) film photosensitizer-bound cellulose derivatives 30, 31 phthalocyanine-bound cellulose derivatives 36, 39 squaraine-bound cellulose derivative 42 leveling-off degree of polymerization (LODP) 225, 267–269 light silica aerogels 296 liquid chromatography under critical conditions (LCCC) 149 living radical polymerization (LRP) 49

Index

low-density silica aerogels Lyocell process 343 Lyocell-type fibers 344

296

m magnetic hydrogels 292 magneto-responsive CNC materials 417 man-made cellulosic fibers (MMCFs) Bemberg fiber 343 Bocel/Fibre B 344 carbamate process 342 with carboxylate-based ILs 346 direct solvents 341 direct solvent systems 344 fiber analysis chemical composition 349 spinnability 350 mechanical analysis 351 molar mass distribution 349–350 pulp dissolution and filtration 348–349 rheological measurements 349 stepwise purification 348 fiber properties 355–360 fiber spinning 354–355 Fortisan 342 with halide-containing ILs 346 influence of noncellulosic constituents 360–361 intermediate cellulose derivatives 342 with ionic liquids 344–346 Lyocell process 343 Lyocell-type fibers 344 nitrocellulose 341 from NMMO monohydrate 343 polyacrylonitrile 345 rheological properties 352–354 from solution in superbase-based ILs 347 steam pretreatment 343 TM

®

®

Tencel 344 for textile applications 343 viscose production 342 manmade hydrogels 283 medium internal phase emulsion (MIPE) 397 mesoporous hydrogels 300 methyl cellulose (MC) average degree of substitution 146–149 distribution along and over polymer chains 149–153 fractionation 152 methylation pattern 146–149 regioselectivity influence on methylation patterns 148 methyl 4′ -O-methyl-β-D-cellobioside13 C 12 in NMMO solvent 110, 111 swelling phases of 109 microcontact printing (μCP) 16, 17 microscopy 228–230 MMCFs see man-made cellulosic fibers (MMCFs) modified nanocelluloses, emulsions stabilized with 398–402 modulus of elasticity 428–429 molar mass analysis, of paper samples 182–184 molecular bottlebrush 49, 50 multi-angle light scattering (MALLS) 228 multihollow magnetic imprinted microspheres (HM-IMs) 412 multistructured polystyrene beads 412

n nanocellulose 253, 278 nanofibrillated cellulose (NFC) see cellulose nanofibrils (CNFs) nanozeolite (NZ) 377, 378 native cellulose 264 nematic-ordered cellulose 255

441

442

Index

for commercial Kraft pulp 86–90 HexA content 83 hydroxyl/oxyl radicals generation 82 NaOH charge effect 79, 80 NaOH concentration 84, 85 number of cellulose chain scissions per cellulose polymer 81 oxygen pressure effect 84, 85 pulp properties 84 temperature effect 84, 85 disadvantage 68 extending the range of 69–70 improving pulp yield 90–92 initial reaction 70, 71 mass transfer resistance 69 orthoquinone and muconic acid structure formation 71 reactivity of residual lignin 73–79 stepwise reduction of oxygen to water 71, 72 two-stage conditions 70

nematic/smectic-ordered materials 255–256 nitrocellulose 341 N-methylmorpholine N-oxide monohydrate (NMMO) 107–108, 343 “No-D” (no deuterium) NMR technique 209 nonfreezing water 127 nonmodified single-walled carbon nanotubes (SWCNT) 305

o 6-O-bromoisobutyryl-2,3-di-Omethylcellulose synthesis 52–53 oil–water interfacial stabilization, by CNC see cellulose nanocrystals (CNCs) one-dimensional (1D) solution-state NMR spectroscopy see solution-state NMR spectroscopy 6-O-p-methoxytritylcellulose poly(ethylene oxide) introduction at O-2,3 position 57–58 poly(styrene) introduction at O-2,3 position 55–57 organic electrolyte solutions 194 organo-soluble amino cellulose derivatives 20 organo-soluble 6-deoxy-6-(ω-aminoalkyl) amino cellulose carbamates 18 oxygen delignification Berty CSTR experiments 83, 84, 90 oxygen delignification of softwood Kraft pulp 67 advantages 68 carbohydrate loss during 91 delignification/cellulose degradation selectivity in batch reactor 82

p palladium (II) phthalocyanine-bound cellulose derivatives 40 paper-based heritage materials 175 paper brightness 185–186 paper destruction, exogenic factors of 175 paper preservation and restoration approach 175 accelerated heat-induced aging 179 adhesive mixtures and films 177, 178 adhesives/surface consolidants 176 paper sample preparation 177–179 contact angle measurements 180, 184–185 GPC analysis 179–182 paper brightness analysis 180, 185–186

Index

pectin aerogels 387 pectin-based aerogels 380–386 pectin–silica aerogels mechanical properties 385 morphology of 383, 384 thermal conductivity 385 perdeuterated cellulose solvents 103, 104 phosphorus (31 P) NMR 214, 237 autohydrolyzed birch samples, solubility of 218, 219 ionic liquid-based media 212 phosphitylated MCC 215, 216 wood solubility in ionic liquids 215, 216 photosensitizer-bound cellulose derivatives 29, 31 Langmuir–Blodgett film of 30, 31 photocurrent generation from 46 phthalocyanine-bound cellulose derivatives 34 chemical structure 36 LB monolayer film 36, 39 palladium (II) containing 40 polyacrylonitrile (PAN) 345 polyelectrolyte complexes (PECs) 3 polyethyleneimine-grafted CNF aerogels 317 polymer analog reaction 149 polymer coemulsifiers 406–409 poly(N-isopropylacrylamide) grafted CNC 398, 399 porphyrin-bound cellulose derivatives 31–34 porphyrin-bound chitosan derivative 45 powder X-ray diffraction 230, 232–234 protofibrils 223 pulsed-field-gradient spin-echo (PFGSE) NMR 244 pure form factor 230 PVA/CNF nanocomposites 289–290

r rag paper samples molar mass changes 183 water contact angles 184 regenerated cellulose fibers 99 regeneration see cellulose swelling, dissolution and regeneration regioselective grafting 51 of cellulose 52–54 of polyNIPAM 53 Rietveld refinement 233 rosette terminal complexes 277 ruthenium(II) complex-bound cellulose derivative 42–44

s sampling depth 240 scanning electron microscopy (SEM) 228 scattered intensity 230 self-assembled monolayers (SAM), of amino celluloses 12 self-assembling of 6-deoxy-6-amino cellulose derivatives 10–13 self-healing nanocomposite 289 silica aerogels 296 cellulose nanofibril reinforcement 375 drawbacks 296 improving mechanical properties 372 siloxane CNF composite 310 size exclusion chromatography (SEC) of Janus-type bottlebrush 61–63 of paper samples 179–182 small-angle X-ray scattering (SAXS) 133–135, 230–231 smart hydrogels 283 softwood Kraft pulp see oxygen delignification of softwood Kraft pulp solid state NMR spectroscopy 234–237

443

444

Index

solution state NMR spectroscopy application in biofuels and biorefineries 193 comparison with solid-state 13 C CP-MAS NMR 191 of cellulose and pulp 203–211 of lignocellulosics 191, 192 of modified nanocrystalline cellulose 211–212 ((poly) methylmethacrylate)-grafted cellulose nanocrystals 211, 212 solvents used 191, 192 solvent-suppression technique 209 wheat bran extracts 210, 211 whole lignocelluosic biomass 198 anomeric region of 2D HSQC spectrum 197 DMSO-d6 /HMPA-d18 solvent system 202 DMSO “gel-state” method 195 HSQC spectra, pretreatment methods 199–201 HSQC spectrum of pulps 205-207 ionic liquid-based organic electrolyte solutions 199 NMI-d6 /DMSO-d6 solvent system 195, 196 perdeuterated pyridinium chloride electrolyte solution 197, 199 solvogels, drying of 297–301 spray freeze drying (SFD) 373, 374 Spurlin model 145 squaraine-bound cellulose derivative 42 stable CNF hydrogels 288 starch-based aerogels 386 Steglich esterification 37 styrene-based Pickering emulsions 395, 396 superabsorbents 130, 307–315

superamphiphobic CNF aerogel 312–313 superbase-based IL 1,5-diazabicyclo[4.3.0]non-5ene-1-ium acetate ([DBNH]OAc) 347 superhydrophobic aerogel 311 surface grafting 412 surfactant-assisted emulsions 402–406 surfactant compatibilization mechanism 412 surfactant-free emulsions 395–397 surfactant-free O/W/O double emulsions 409, 410 swelling see cellulose swelling, dissolution and regeneration synthetic strategies, of phthalocyanine-bound cellulose derivatives 35, 38

t tannic acid (TA) 407 TEMPO-CNF-stabilized foams 405–406 TEMPO-oxidized cellulose nanofibrils (TO-CNF) aerogels 296 and graphene oxide nanosheets 293 gyroidal hydrogels 287 hydrogels 280, 287 344 Tencel 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation 266 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized wood 254 thermal conductivity acid-gelled pectin aerogels 387 calcium-gelled pectin aerogels 387

®

Index

cellulose–silica composite aerogels 379 vs. density, aeropectins 382, 383 nanocellulose aerogels 373, 374 of pectin–silica aerogels 385 thermoreversible gelation 145 transmission electron microscopy (TEM) 228–229 tritylcellulose aerogel 380 two-dimensional (2D) solution-state NMR spectroscopy see solution-state NMR spectroscopy two-phase annual ring model 426 two-stage oxygen delignification conditions 70

u

®

Ultra-Turrax disperser 282 UV–Vis measurements, of paper brightness 185–186

v viscose fibers

342

w water absorbency of SAP 130 water-dispersible rare-earth upconversion nanoparticles 405 Whatman paper samples molar mass changes 183 water contact angles 184 wheat bran extracts 210, 211

x X-ray photoelectron spectroscopy (XPS) 240–243

y Young modulus of acid-gelled aeropectins 381, 382

z Zeisel method

146

445

Cellulose

pH 5

pH 6

A

pH 7

B

pH 5

A

B

pH 6

A

B

pH 7

Figure 1.2 Cyclic olefin polymer slides equipped with cellulose and cellulose (3-carboxypropyl)trimethylammonium chloride ester incubated with different concentrations of labeled BSA (1000, 500, 100, 10, 1, 0.1, 0.01, and 0.001 μg mL−1 ) at different pH values. A) low DS; B) high DS [8].

(a)

(b)

Figure 1.16 Images of polystyrene solutions after ATRP using MNP@AC50 (a) and N,N,N′ ,N′′ ,N′′ pentamethyldiethylenetriamine (PMDETA) (b) as ligands.

Paper modified with the dye

Paper coated with TEAE Cellulose and modified with the dye Dye

Coomassie brilliant blue R-250

Congo red

Thymol blue

Eriochrome black T

Figure 1.17 Images of printing paper coated with amino cellulose and subsequent coloring.

Cellobiose

CMCbiose

CECbiose

Residue 2

Residue 2

Residue 2 Residue 1 Residue 1

Residue 1

Figure 6.6 Average structure of cellobiose, its carboxymethyl- and carboxyethyl derivatives during 100 ns.

Figure 6.9 Contour density maps for water oxygen around cellobiose, carboxymethyl cellobiose, and carboxyethyl cellobiose. The contours enclose the regions with a water density 1.35 times larger than that of bulk water.

Poplar

Pine

(a)

Corn

(b)

Arabidopsis

(c)

(d)

Figure 9.3 Lignin aromatic region of different whole biomass samples in DMSO-d6 /pyridine-d5 . (a) 2-year-old greenhouse-grown poplar wood, (b) mature pine wood, (c) senesced corn stalks, and (d) senesced Arabidopsis inflorescence stems. Source: Mansfield et al. 2012 [4]. Reproduced with permission of Springer Nature.

20 Untreated poplar

Acetyl CH3

30 40

Bβ OMe

2-O-Ac-Xylp

50 60

Aα

80 Cα

S2/6

90

Aβ

f1 (ppm)

70 3-O-Ac-Xylp

100

Sʹ2/6 G2

110

(1-4)-β-D-Xylp(1-4)-β-D-Glcp

120

G5 G6

130 PB2/6

140 150

9.0

8.0

7.0

6.0 5.0 f2 (ppm)

4.0

3.0

2.0 20

Acetyl CH3

30 Steam pretreated poplar

40

OMe

50 2-O-Ac-β-D-Xylp Aα

60

80 Aβ

90

S2/6

f1 (ppm)

70

3-O-Ac-β-D-Xylp

100 G2

110

(1-4)-β-D-Glcp

G5

120

G6

130 PB2/6

140 150

9.0

8.0

7.0

6.0 5.0 f2 (ppm)

4.0

3.0

2.0

Figure 9.5 Effect of pretreatment methods on the HSQC spectra of poplar in perdeuterated [Hpyr]Cl-d6 /DMSO-d6 . Source: Samuel et al. 2011 [9]. Reproduced with permission of Elsevier.

20

Acetyl CH3

30

Lime pretreated poplar

40

OMe

50 Aα

70 80 Aβ

S2/6

90

f1 (ppm)

60

100 G2 G5 G6

110 (1-4)-β-D-Glcp

120 130

PB2/6

140 150 9.0

8.0

7.0

6.0 5.0 f2 (ppm)

4.0

3.0

2.0 20

Acetyl CH3

❦

30

Acid pretreated poplar

40

OMe

50

70 80 90 (1-4)-β-D-Glcp

100 110 120

Lignin aromatic region

130 140 150 9.0

8.0

7.0

Figure 9.5 (Continued)

6.0 5.0 f2 (ppm)

4.0

3.0

2.0

f1 (ppm)

60

Anomeric region Glcp int

Glcp RE/NRE

Aliphatic region Xylp

Manp

Solvent/Unassigned

Figure 9.8 2D HSQC NMR spectrum of softwood sulfite-dissolving pulp in [P8881 ][OAc]/DMSO-d6 (50 : 50 w/w). Source: Holding et al. 2016 [8]. Reproduced with permission of John Wiley & Sons.

Anomeric region

Glcp int

Glcp RE/NRE

Aliphatic region

Xylp

Manp

Solvent/Unassigned

Figure 9.9 2D HSQC NMR spectrum of hardwood prehydrolysis kraft-dissolving pulp in [P8881 ][OAc]/DMSO-d6 (50 : 50 w/w). Source: Holding et al. 2016 [8]. Reproduced with permission of John Wiley & Sons.

Stacked DOSY 1H gradient array

DOSY pseudo 2D spectra

PMMA-CNC [P4444][OAc]

Similar diffusion constant (interconnected polymer)

Figure 9.15 DOSY pseudo 2D spectra and normalized 1 H NMR traces for the diffusion gradients, showing reduced solvent peaks compared to the slow diffusing polymeric PMMA-g-CNC peaks. Source: King et al. 2018 [36]. Reproduced with permission of American Chemical Society.

(a)

(b)

1 μm

500 nm

(d)

(c)

1 μm

500 nm

Figure 13.3 AFM and TEM micrographs of CNF (a, b) and TO-CNF (c, d). CNF was prepared from pulp with an enzymatic pretreatment and subsequent homogenization in a microfluidizer. TO-CNF was produced from pulp under alkaline conditions at pH 10. Source: Sacui et al. 2014 [76]. Copyright 2014. Reprinted with permission of American Chemical Society.

(a)

HepG2 in CNF 18 Gʹ before shearing

16 Shear modulus (Pa)

Figure 13.5 CNF as a cell culture medium for hepatocellular carcinoma cells (a). The CNF cell medium with the seeded cells is injectable (b). This is based on the shear thinning effect of CNF shown in (c); in the syringe, a shear stress is applied on the suspension, and the shear storage and loss moduli decrease, thereby the injection of the cell CNF suspension. (a, c) Source: Bhattacharya et al. 2012 [94]. Copyright 2012. Reprinted with permission of Elsevier.

14

Gʹ after shearing

12 10 8 6 4

Gʺ before shearing

2

Gʺ after shearing

0 0

(b)

50

100 150 200 250 300 Time (s)

(c)

(b)

(a)

15 mm

(c)

15 mm

10 mm

(e)

(d)

20 μm

10 mm

Figure 13.6 Gyroid CNF scaffold (b) obtained from an acrylate template (a) after alkaline hydrolysis of the acrylate. Shape recovery of the scaffold after rehydration (b, c, d). Human mesenchymal stem cells adhere to the CaPO4 -coated scaffold, and the cell centers are marked with a dot (e). Source: Torres-Rendon et al. 2015 [140]. Copyright 2015. Reprinted with permission of John Wiley & Sons.

(a)

(b)

(c)

Figure 13.7 Dark field microscopy micrograph of CNF stabilized emulsions, O/W emulsion with CNF (a), W/O emulsion with lauric acid-modified CNF and O/W/O double emulsion with a mixture of CNF and the modified CNF. Hexadecane represents the oil, and the water was stained with fluorescein. The scale bar is 50 𝜇m. Source: Cunha et al. 2014 [163]. Copyright 2014. Reprinted with permission of American Chemical Society.

(a)

(b) Water

Dodecane

(c)

(d)

Me

Me

Si O Si OH

OH

O

OH

CNF

O

Me O

Si O O H H

O

CNF MTMS

Figure 13.23 Comparison of the wettability of a CNF cryogel and a siloxane CNF composite. The CNF cryogel absorbs water and dodecane (a, c), whereas the CNF/MTMS (methyltrimethoxysilane) composite is hydrophobic and absorbs selectively the dodecane of an aqueous mixture (d). Source: Zhang et al. 2014 [268]. Copyright 2014. Reprinted from with permission of American Chemical Society.

Unmodified CNC

Poly(NiPAAm)-g-CNCs

1000

85

LD

80

MD

75

HD

70

VHD

65

MDP

F/R (μN m−1)

Normalized % transmittance

90

100

Unmodified CNCs

10

60 55

HDP

50 20 22 24 26 28 30 32 34 36 38 40 Temperature (°C) (a1)

1 0

10 20 Separation (nm)

30

(a2)

Figure 16.8 (a1 ) Normalized light transmittance versus temperature of aqueous dispersions of unmodified CNCs and poly(NiPAAm)-g-CNCs with different grafting densities (low, LD; medium, MD/LDP; high, HD; very high, VHD) [22]. (a2 ) Interaction forces (colloidal probe microscopy) versus separation between a silica sphere against a flat layer spin-coated on silica wafers consisting of unmodified CNCs and MD poly(NiPAAm)-g-CNCs) [22]. (b1 ) A weak polyelectrolyte, poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA), was grafted onto the surface of cellulose nanocrystals via free radical polymerization. The resultant suspension of PDMAE-MA-grafted-cellulose nanocrystals (PDMAEMA-g-CNC) possessed pH-responsive properties. Stable heptane-in-water and toluene-in-water emulsions were prepared with PDMAEMA-g-CNC. (b2 ) Using Nile red as a fluorescence probe, the stability of the emulsions as a function of pH and temperature was elucidated. It was deduced that PDMAEMA chains promoted the stability of emulsion droplets and their chain conformation varied with pH and temperature to trigger the emulsification and demulsification of oil droplets. Interestingly, for the heptane system, the macroscopic colors varied depending on the pH condition, while the color of the toluene system remained the same. Reversible emulsion systems that responded to pH were observed [45]. (c1 ) SEM images of polystyrene microbeads encapsulated by hybrid CNC-CoFe2 O4 shells. The microbeads can be separated by magnetic manipulation as shown in (c2 and c3 ) [46]. (d) Magnetic hysteresis loops of Fe3 O4 (a) and HM-IMs-5 (b) and photographs of HM-IMs-5 suspended in water and in the presence of an externally placed magnet (c) [41]. (e) The transitional phase behavior of C2–BC (e1 ), C6–BC (e2 ), and C12–BC (e3 ) stabilized emulsion of water and toluene. The arrow shows flocculated modified BC. Note that the slightly yellowish color of C2–BC stabilized emulsion is a result of the color of C2–BC, which is slightly yellowish after the esterification of BC with acetic acid [26]. Source: Panels (a)–(c): Zoppe et al. 2011 [22], Tang et al. 2014 [45], and Nypelö et al. 2014 [46]. Copyright 2011 and 2014. Reprinted with permission of American Chemical Society. Panel (d): Zhu et al. 2015 [41]. Copyright 2015. Reprinted with permission of Elsevier. Panel (e): Lee et al. 2014 [26]. Copyright 2014. Reprinted with permission of Elsevier.

pH = 11

Toluene

pH increase

pH increase

pH = 3 Oil droplet Nile red CNC

Heptane

Heptane

PDMAEMA

Heptane

Water

Water

Under UV

(b1)

(b2)

Magnet

Magnet

100 μm (c1)

(c2)

(c3) b

0.4 0.2 0.0

c

–0.2

Magnet

Magnetization (emu g–1)

0.6

–0.4 –0.6 –2000

–1000

0

1000

Magnetic field (Oe) (d)

Figure 16.8 (Continued)

2000

Figure 16.8 (Continued)

(e1)

@ pH = 5 w/o

@ pH = 1 w/o

@ pH = 14 No emulsion

@ pH = 1 (again) o/w

@ pH = 1

@ pH = 14

@ pH = 1 (again)

@ pH = 1 w/o

@ pH = 14 No emulsion

@ pH = 1 (again) w/o

(e2)

@ pH = 5

(e3)

❦

@ pH = 5 w/o