Engineering of Thermoplastic Elastomer with Graphene and Other Anisotropic Nanofillers [1st ed.] 9789811590849, 9789811590856

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Engineering Materials

Abhijit Bandyopadhyay Poulomi Dasgupta Sayan Basak

Engineering of Thermoplastic Elastomer with Graphene and Other Anisotropic Nanofillers

Engineering Materials

This series provides topical information on innovative, structural and functional materials and composites with applications in optical, electrical, mechanical, civil, aeronautical, medical, bio- and nano-engineering. The individual volumes are complete, comprehensive monographs covering the structure, properties, manufacturing process and applications of these materials. This multidisciplinary series is devoted to professionals, students and all those interested in the latest developments in the Materials Science field, that look for a carefully selected collection of high quality review articles on their respective field of expertise.

More information about this series at http://www.springer.com/series/4288

Abhijit Bandyopadhyay Poulomi Dasgupta Sayan Basak •

•

Engineering of Thermoplastic Elastomer with Graphene and Other Anisotropic Nanofillers

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Abhijit Bandyopadhyay Department of Polymer Science and Technology University of Calcutta Kolkata, West Bengal, India

Poulomi Dasgupta Department of Polymer Science and Technology University of Calcutta Kolkata, West Bengal, India

Sayan Basak Department of Polymer Science and Technology University of Calcutta Kolkata, West Bengal, India

ISSN 1612-1317 ISSN 1868-1212 (electronic) Engineering Materials ISBN 978-981-15-9084-9 ISBN 978-981-15-9085-6 (eBook) https://doi.org/10.1007/978-981-15-9085-6 © Springer Nature Singapore Pte Ltd. 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Foreword

My whole career has been dedicated to pursuing the development of various types of functional nanoparticles and their nanoscale dispersion across multiple polymer matrices, from engineering to bio-based polymers. So, the content of this book is very close to my expertise. The group, leading by Prof. Abhijit Bandyopadhyay, is well known to me through their high-quality work on processing and development of new generation engineering thermoplastic elastomers. Over the years, this group used various types of advanced nanofillers to modify the inherent properties of different types of elastomers using polymer nanocomposite technology. Over the last few years, nanocarbons and related nanoparticles are becoming emerging fillers for the development of next-generation engineering polymer materials for a wide range of applications, from construction to biomedical. Therefore, this book has immediate relevance, interest, and importance owing to the trend in the plastic industry. In this book, the authors tried to cover various characteristics of nanofillers and several types of processing techniques to disperse them in thermoplastic elastomers. The key to manufacturing a useful engineering thermoplastic elastomer nanocomposite for practical applications is to achieve the desired degree of dispersion of filler particles in a polymer matrix and tune the obtained composite properties as per the product requirement. I am thrilled to say that the authors very meticulously cover this aspect in this book. Based on my knowledge in this field and going through the content of this book, I must say that this is an ideal book for postgraduate students, researchers, and polymer processing technologists who are interested in engineering thermoplastic

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elastomers in general. I also believe this book will be beneficial for industry-based scientists and engineering, including product development managers who want to bring advanced elastomer-based products in the market. Congratulations and all the best! Professor Suprakas Sinha Ray Chief Researcher and Manager Centre for Nanostructures and Advanced Materials Council for Scientific and Industrial Research Pretoria, South Africa Distinguished Visiting Professor Department of Chemical Sciences University of Johannesburg Johannesburg, South Africa

Preface

Rubber is a unique class of polymer pact with some uncanny properties like high shock absorption, compressibility resistance, resilience, recoverable deformability along with low modulus and strength. High molecular weight, high chain entanglement density, and extremely low cohesive force of attraction among the segments are the keys to form a rubber, which during processing is mixed with several other ingredients (at least 10–12) to achieve the strange combination of properties. Vulcanization or chemical crosslinking (either sulphur- or non-sulphur-based) between the molecules of a rubber is thought to be the key that confers the true rubberiness, and once that is achieved, the rubber becomes a thermoset. However, in an era of sustainable development, a thermosetting polymer with zero recyclability and complex formulation is not a preferred choice indeed. The world is obsessed for polymers with “zero waste” technology—conventional rubber, being unfit to that, makes a way for the relatively new thermoplastic elastomers or TPE which by virtue of its inimitable molecular design has got the immense potential to replace conventional rubbers in many of its applications. Believing to that, the world has seen a steep rise in consumption of TPE of late and is also predicted to hold an even stronger ground in future. The exclusive molecular design of tri- or di-blocking of homopolymers developed though special living anionic polymerization imparts the essence of both thermoplastic and elastomeric properties combining both melt recyclability and recoverable elongation once the stress is lifted. TPE, representing a unique combination of hard and soft polymer segments alluring with high and low Tg s, respectively, inherits high cohesive strength, thus could avoid nearly all additional ingredients unlike rubbers, and emerges as an ideal “zero waste” future elastomer material. The good part is this elastomer could be tailor-made as and when, driven by the application demand. Of late, the world has seen the development of many new TPEs with different monomers, block length, etc., befitting new as well as conventional applications. Alongside, nanotechnology has emerged as a promising new material technology for serving the human kind. Both isotropic and anisotropic nanomaterials have shown remarkable properties that could revolutionize the material world with advanced applications in optical, optoelectrical, and other relevant fields. The first vii

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revolutionary work on polymer nanotechnology was reported by the Toyota Research Group in Japan nearly 30 years back, and since then different nanomaterials have been explored in a variety of thermoplastics and elastomers and yielded some good to exciting results in many of the cases. However, on critical review, anisotropic nanomaterials were found more effective on thermoplastics than on elastomers largely due to the inherent viscoelasticity and presence of huge number of ingredients in the latter. TPE, on the other hand, has been able to derive greater benefit of the anisotropic nanomaterials and, thus of late, has been considered as a better matrix than the conventional elastomer for exploration. The combination of TPE and anisotropic nanomaterials like clay, carbon materials, and graphene has yielded many exciting properties, befitting conventional as well as advanced applications. Acknowledging the progress of this important hybrid material technology for the past seven to ten years, an attempt has been made to tot up important outcomes, and analyse and predict the future applications. We believe this book would serve as an important document for the readers for awareness and knowledge enhancement. Kolkata, India

Abhijit Bandyopadhyay Poulomi Dasgupta Sayan Basak

Contents

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1 1 2 4 7 9 13 14

2 Anisotropic Nanofillers in TPE . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Nanofillers and Its Advantages . . . . . . . . . . . . . . . . . . . . . . . 2.3 Layered Double Hydroxide (LDH) . . . . . . . . . . . . . . . . . . . . 2.3.1 Structure of LDH . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Organophilisation of LDH . . . . . . . . . . . . . . . . . . . . . 2.3.3 Strategies to Fabricate the Layered Double Hydroxide . 2.3.4 Synthesis of Polymer/LDH Nanocomposites . . . . . . . . 2.3.5 Properties and the Recent Trends in the Areas of Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Nanoclay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Structure of Nanoclay . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Organomodification of Nanoclay . . . . . . . . . . . . . . . . 2.4.3 Factors Affecting the Organoclay Hybrid Formed . . . . 2.4.4 Modification of Nanoclay . . . . . . . . . . . . . . . . . . . . . . 2.4.5 Properties and Applications . . . . . . . . . . . . . . . . . . . . 2.5 Carbon Nanotube (CNT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Structure and General Properties of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2 Synthesis Routes to Fabricate Carbon Nanotubes . . . . . 2.5.3 Purification and Dispersion of Carbon Nanotubes . . . .

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1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Growth and Development of TPE . . . . . . . . . . . . . . . . . . . . 1.2 Breakthrough Developments in Commercialization of TPE . . 1.3 TPE: A Sustainable Elastomer Composition . . . . . . . . . . . . . 1.4 TPE Based on Rubber—Plastic Blends . . . . . . . . . . . . . . . . 1.5 Fillers/Nanofillers for TPE: Isotropic and Anisotropic Fillers . 1.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2.5.4 Functionalization of Carbon Nanotubes . . . . . 2.5.5 Applications . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.1 Structure and General Properties of Graphene 2.6.2 Synthesis Strategy of Graphene . . . . . . . . . . 2.6.3 Application and Recent Trends . . . . . . . . . . . 2.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 Preparation of Graphene Based Nanocomposite Based on TPE 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Trends in Graphene Research . . . . . . . . . . . . . . . . . . 3.2 Different Methods of Preparation . . . . . . . . . . . . . . . . . . . . . 3.2.1 Intercalation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 In-situ Polymerization . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Shear Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Characterization of Graphene/TPE Nanocomposites . . . . . . . 3.4 Application of Graphene/TPE Nanocomposites . . . . . . . . . . . 3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Structure—Property Co-relation of Graphene/Graphene Derivative Based TPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Some Specialized Properties of Graphene and Its Derivative Relevant to New Age Application . . . . . . . . . . . . . . . . . . . . 4.2.1 A Succinct Update on the Quantum Perspective of the Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Dipping into the Chemistry . . . . . . . . . . . . . . . . . . . 4.2.3 The Hidden Beauty . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 An Overview on the Fabrication of the Graphene Sheet Derivates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Fabrication of the Graphene and Graphene Derived Elastomeric Nanocomposites . . . . . . . . . . . . . . . . . . 4.4 Characterization Techniques for the Graphene/Graphene Derivates and Elastomeric Nanocomposites . . . . . . . . . . . . . 4.4.1 Studying the Cure Behavior . . . . . . . . . . . . . . . . . . . 4.4.2 Analyzing the Antioxidant Effect of the Graphene Derivatives in Elastomeric Nanocomposites . . . . . . . . 4.4.3 Morphology and Detailing the Dispersion of the Graphene and Its Derivative in the Elastomeric Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4 The Effect of Wrinkling of the Graphene Derivates on the Elastomeric Nanocomposites . . . . . . . . . . . . .

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4.4.5 Predicting the Consequences of Modified and Hybrid Graphene Derivates on the Mechanical Properties of the Nanocomposite Elastomers . . . . . . . . . . . . . . . . . 4.4.6 Analyzing the Dynamic Mechanical Behavior Along with Barrier Properties of the Graphene Derived Elastomeric Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Potential Application of Graphene-TPE Nanocomposite . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Sensing and Actuation . . . . . . . . . . . . . . . . . . . . . . . 5.3 Shape Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Self-healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Biomedical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Conclusion and Future Outlook . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226

About the Authors

Dr. Abhijit Bandyopadhyay is presently working as Full Professor in the Department of Polymer Science and Technology, University of Calcutta, along with as Technical Director in South Asia Rubber and Polymers Park (SARPOL), West Bengal. He did his B.Sc. (Chem. Hons.) from the University of Calcutta securing first class in the year 1997 followed by B.Tech. and M.Tech. in polymer science and technology from the University of Calcutta in the years 2000 and 2002, respectively, with first class, and subsequently completed Ph.D. in the year 2005 in polymer nanocomposites from Rubber Technology Centre, IIT Kharagpur. Before joining the University of Calcutta in November 2008, he worked as Assistant Professor in Rubber Technology Centre, IIT Kharagpur, during 2007–2008. He has published 90 papers in high-impact international journals and 3 books and has filed two Indian patents so far. He has successfully handled many funded research projects and did consultancies for renowned companies like Exide Industries Ltd., Phillips Carbon Black Ltd., etc. He is Fellow of the International Congress for Environmental Research (since 2010), Associate Member of Indian Institute of Chemical Engineers and Life Member of Society for Polymer Science, Kolkata Chapter, and Indian Rubber Institute, respectively. He is Editorial Board Member of two international journals. He has more than 12 years of teaching and research experience. He has been awarded Young Scientist Award by Materials Research Society of India, Kolkata Chapter, in 2005 and Career Award for Young Teachers by All India Council for Technical Education, xiii

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Government of India, in 2010. His research areas include polymer nanocomposites, reactive blending, adhesion, polymer hydrogel in drug delivery, waste polymer composites, green polymer composites, and hyperbranched polymers. He has successfully supervised 11 research students for their doctorate degree so far, and 4 more are presently working under him. Ms. Poulomi Dasgupta completed her graduation with Chemistry (Hons.) from Vidyasagar College, Kolkata, in 2013. She subsequently received her B.Tech. (2016) and M.Tech. (2019) degrees at the Department of Polymer Science and Technology at the University of Calcutta. She was awarded gold medal from the University of Calcutta (during B.Tech.). She was a recipient of GATE fellowship, AICTE, Government of India, during M.Tech. Prior to joining M.Tech., she worked with Indag Rubber, Himachal Pradesh, as R&D Executive. Currently, she is associated with TCG Lifesciences (Chembiotek Research International) as Research Chemist. Her area of research was based on “development of thermoresponsive self-healable elastomeric compound and its characterization”. Mr. Sayan Basak has completed his B.Tech. from the Department of Polymer Science and Technology, University of Calcutta, India (2015–2019), and is currently pursuing his Ph.D. from the School of Polymer Science and Engineering, University of Akron, USA (2019–2024). His undergraduate research interest, along with his present research domain, revolves working with thermoplastic elastomers and multi-component polymer systems, thereby prospecting into new materials to develop smart polymer materials for new-age applications. Apart from being a budding technologist, he loves to spend his time creating content, which is supported by the Society of Plastic Engineers, The Times of India, and Medium on sustainability, recyclability, and green chemistry.

List of Figures

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Market Trend of TPEs since 2012 and projection until 2022. Source Asia Pacific Thermoplastic Elastomers (TPE) Market Analysis By Product (Styrenic Block Copolymers (SBC, SBS, SIS, HSBC), Thermoplastic Polyurethanes (TPU), Thermoplastic Polyolefins (TPO), Thermoplastic Vulcanizates (TPV), Copolyester Elastomers (COPE)), By Application (Automotive, Footwear, Construction, Medical, Electronics, Industrial, Advanced Materials) And Segment Forecasts To 2022 Published: April 2016|180 Pages|Format: PDF|Report ID: 978-1-68038-625-7. http://www.grandviewresearch.com/ industry-analysis/asia-pacific-thermoplastic-elastomers-tpemarket accessed on 14.02.2018 . . . . . . . . . . . . . . . . . . . . . . . . Block copolymer morphology—illustration of hard blocks crystallized into domains with soft, rubber block regions between them. Reproduced with permission from [4] . . . . . . . Chemical structure of block copolymeric TPEs; i styrenic, ii COPE, iii thermoplastic polyurethane, and iv thermoplastic polyamide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TPO rubber/plastic blend morphology. Reproduced with permission from [4] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermoplastic vulcanisate morphology with a continuous plastic phase and discrete rubber particles. Reproduced with permission from [4] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TOT structure of Nanoclay. Reproduced with permission from [25] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Layered structure of LDH. Source: http://www.scielo.br/scielo. php?script=sci_arttext&pid=S0100-06832015000100001, accessed on 24.04.2018 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Allotropes of carbon popular as nanofillers. Reproduced with permission from [51] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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List of Figures

Top: A snapshot of the articles (including patents) published in the respective subject domains till date, Bottom: The number of manuscript share with reference to the TPE based on the nanofillers, Data source- SciFinder, Chemical Abstracts Service (Plotted with the accessed data on 24/05/2020) . . . . . Illustrative representation of a Layered Double Hydroxide. Reproduced with permission from [59] . . . . . . . . . . . . . . . . . . The Ion exchangeable double layered hydroxide developed by increasing the gallery height. Reproduced with permission from [66] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Illustration of the process of anion exchange for the synthesis of double layered hydroxides. Reproduced with permission from [85] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SEM images showing the microstructure of the Zn-Al/LDH composite for an excellent photocatalytic activity a Zn–AlLDH-(3 h of reaction time), b Zn–Al-LDH-(6 h of reaction time), c Zn–Al-LDH-(9 h of reaction time), d Zn–Al-LDH-(12 h of reaction time). Reproduced with permission from [101] . A comprehensive overview of the fabricating techniques of the double-layered hydroxides. Reproduced with permission from [115] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pathway of nanocomposite preparation by a monomer exchange and in situ polymerization, b direct polymer exchange, and c restacking of the exfoliated layers over the polymer. Reproduced with permission from reference [130] . . The TEM images of thermoplastic polyester elastomer reinforced with zinc hydroxide nitrate and sodium benzoate nanoparticle (a low magnification, b enhanced magnification). Reproduced with permission from [75] . . . . . . . . . . . . . . . . . . Crystal structures of clay minerals: a Type 1:1; b Type 2:1. Reproduced with permission from [167] . . . . . . . . . . . . . . . . . Classification of silicates based on their physicochemical nature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The layered structure of kaolinite clay. Source http://jan.ucc. nau.edu/doetqp/courses/env440/env440_2/lectures/lec19/Fig. 9_3.gif, accessed on 10.05.2020 . . . . . . . . . . . . . . . . . . . . . . . The smectite clay structure. Source http://www.pslc.ws/ macrog/mpm/composit/nano/struct3_1.htm, accessed on 11.05.2020 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TEM images of the polymer/clay nanocomposites using a combination of nitroxide-mediated radical polymerization and solution are blending methods. Reproduced with permission from [202] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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List of Figures

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Fig. 2.27 Fig. 2.28

Visualization of the use of montmorillonite‐intercalated metallocene catalyst to reinforce ethylene and 10‐undecen‐1‐ol matrix. Reproduced with permission from [210] . . . . . . . . . . . Surface modification and the possible mechanism of the exfoliation for the butadiene-based rubbers and the thiolmodified attapulgite. Reproduced with permission from [225] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TEM images of the nanofiller incorporated into the thermoplastic elastomer matrix, while a represents the solid section, whereas, b–d represents the transverse sections. Reproduced with permission from [228] . . . . . . . . . . . . . . . . . Illustrative representation of the different types of carbon nanotubes, accessed from [236] on 30/01/2020. . . . . . . . . . . . Visual representation of the different conformation of carbon nanotubes, accessed from [251] on 30/01/2020. . . . . . . . . . . . The chemical vapor deposition to fabricate carbon nanotubes a tip growth model, b base growth model. Reproduced with permission from [173]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TEM profiles of typical carbon nanotubes produced by hydrothermal treatment of polyethylene at 800 °C for 2 h in the presence of 3% Ni powder. The PE/H2O ratio was 1.6. a End of the nanotubes, b the graphite fringes c,d lattice fringe images. Reproduced with permission from [263] . . . . . . . . . . Layout of the second-generation high pressure CO disproportionation producing the single walled carbon nanotubes. Reproduced with permission from [265] . . . . . . . . A concise representative image of the functionalization of carbon nanotubes. Reproduced with permission [280] . . . . . . Green Functionalization of single-walled nanotubes in ionic liquid. Reproduced with permission from [283] . . . . . . . . . . . Green functionalization of multi-walled nanotubes with poly (ɛ-caprolactone). Reproduced with permission from [284] . . . Non-covalent functionalization of carbon nanotubes (CNTs) a with a surfactant and b with a polymeric agent. Reproduced with permission from [288]. . . . . . . . . . . . . . . . . . . . . . . . . . . The thermoplastic polyurethane matrix reinforced with carbon nanotubes a wound on a reel; b sewn into the fabric; c,d ironed onto the fabric. Reproduced with permission from [292] . . . . The microscopic stricture of graphene layers. Reproduced with permission from [304, 305]. . . . . . . . . . . . . . . . . . . . . . . . . . . The AFM image of exfoliated GO sheets with three height profiles acquired in different locations, accessed from https:// www.hielscher.com/ultrasonic-graphene-preparation.htm on 30-04-2020 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Fig. 2.29

Fig. 2.30

Fig. 2.31

Fig. 2.32

Fig. 2.33

Fig. 2.34

Fig. 3.1

Fig. 3.2

Fig. 3.3

Fig. 3.4

List of Figures

The cryogenic-TEM images of graphene flakes dispersed in chlorosulphonic acid. Reproduced with permissions from [327] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The tapping-mode AFM topography image visualizing particles obtained by thermal exfoliation of graphite oxide along with the height of the derived sheets. Reproduced with permissions from [329] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HRTEM images of nanosheets grown under 40% CH4 for 20 min on a tungsten substrate. Reproduced with permission from [339] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The arc discharge method of preparing graphene sheets under various atmospheres. Reproduced with permission from [340] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The possible mechanism of developing chemically reduced graphene using moieties having sulfur groups. Reproduced with permission from [342]. . . . . . . . . . . . . . . . . . . . . . . . . . . Representation of a stress versus strain, b strain at break, and c stress at break values for pure thermoplastic elastomer, carbon black reinforced thermoplastic elastomer, modified carbon black reinforced thermoplastic elastomer, and exfoliated graphene reinforced thermoplastic elastomer obtained from tensile test results. Reproduced with permissions from [347] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Representative example of a The two-dimensional honeycomb structure of carbon atoms in graphene under high-resolution transmission electron microscopic (TEM) image. Reproduced with permission from [6] on 24.03.2020, b three-dimensional single graphite sheet consisting of a honeycomb lattice structure of sp2 bonded carbon atoms. Reproduced with permission from [7] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphene (top) and related structures: fullerene (bottom left); carbon nanotubes (bottom centre); and graphite (bottom right). Reproduced with permission from [8] . . . . . . . . . . . . . . . . . . . Graphical representation of a number of publications/year on the graphene materials. The inset is the distribution of the document type, where only 2.7% of the publications are related to review work, b Delivery of the publications by subject area. Reproduced with permission from [18] . . . . . . . . . . . . . . . . . . Graphical representation of a number of the review publications/year on the graphene materials, b distribution of the review publications per subject area. Reproduced with permission from [18] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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List of Figures

Fig. 3.5

Fig. 3.6

Fig. 3.7

Fig. 3.8

Fig. 3.9

Fig. 3.10

Fig. 3.11

Fig. 3.12

Fig. 3.13

Fig. 3.14 Fig. 3.15

Fig. 3.16

Graphene derivatives show promising results for various fields, including energy conversion [25], energy storage [26], electronic materials [27], quantum effects [28], low density structural materials [29], sensors [30], chemical screening applications [31], and thermal interface materials [32]. Reproduced with permission from [21] . . . . . . . . . . . . . . . . . . Process flow diagram (a) and Schematic representation (b) of the melt intercalation method. Reproduced with permission from [44] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FESEM images of a the fractured surface of the SEBS/xGnPs nanocomposites containing five wt% xGnPs, b the same image at higher magnification. Reproduced with permission from [47] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SEM micrographs of the fractured surface of the TPE/GNP nanocomposites containing: a 5 wt% GNP, b 7 wt% GNP. Reproduced with permission from [49] . . . . . . . . . . . . . . . . . . Process flow diagram (a) and Schematic representation (b) of the solution intercalation method. Reproduced with permission from [51] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic representation of the preparation of functionalized graphene and it’s composite with ethylene-vinyl acetate copolymer. Reproduced with permission from [53] . . . . . . . . Schematic representation of the fabrication of graphene/TPU nanocomposite by co-coagulation plus compression molding technique. Reproduced with permission from [55] . . . . . . . . . Process flow diagram (a) and Schematic representation (b) of the in-situ polymerization process. Reproduced with permission from [60] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic representation explaining the synergetic effect of SWCNT and GNP in PTT-PTMO based nanocomposites. Reproduced with permission from [61] . . . . . . . . . . . . . . . . . . Synthetic route of PU-GNS nanocomposite. Reproduced with permission from [62] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Representation of i XRD patterns of GO, Gr, AlOOH, and AlOOH−Gr. ii TGA curves of EVA and its composites in air atmospheres. iii TEM images of a GO, b Gr, c AlOOH, and d AlOOH–Gr, e SEM image of AlOOH–Gr. iv TEM micrographs of EVA composites: a 2.0 Gr/EVA, b 2.0 AlOOH/EVA, and c 2.0 AlOOH–Gr/EVA. Reproduced with permission from [66] . . . . . . . . . . . . . . . . . . . . . . . . . . . Illustration of a X-ray diffraction patterns of synthesized materials, b Fourier transform infrared spectroscopy spectra of graphene oxide (GO) and reduced graphene oxide (RGO), c attenuated total reflection Fourier transform infrared

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Fig. 3.17

Fig. 3.18

Fig. 3.19

Fig. 4.1

List of Figures

spectroscopy spectra of low-density polyethylene (LDPE)/ ethylene vinyl acetate (EVA) and LDPE/EVA/GO 5 wt%, d C 1 s spectra of GO and RGO, e Raman spectra of as-prepared materials, and f X-ray diffraction pattern of LDPE/EVA blend and its nanocomposites with 1, 3, 5, and 7-wt% RGO. Reproduced with permission from [67] . . . . . . . . . . . . . . . . . . Representation of i WAXD patterns of pure xGnPs and SEBS/xGnPs nanocomposites. ii Raman spectra of the pristine xGnPs and xGnPs/SEBS nanocomposites. iii FESEM images of fractured surface of the SEBS/xGnPs nanocomposites containing: a 1 wt% xGnPs, b 3 wt% xGnPs, c 5 wt% xGnPs, d 10 wt% xGnPs, e 20 wt% xGnPs and f 40 wt% xGnPs. iv SAXS profiles for: a neat SEBS, b SEBS with 3 wt% xGnPs, c SEBS with 5 wt% xGnPs, d SEBS with 10 wt% xGnPs, and e SEBS with 20 wt% xGnPs. Reproduced with permission from [47] . . . . . . . . . . . . . . . . . . . . . . . . . . . Depiction of i combined small- and wide-angle X-ray diffractograms of graphite, GO Ph-iGO, and TRG. Profiles were vertically shifted for clarity. Scattering reflections from the layered spacing of graphitic carbons are marked by arrows. ii Contact-mode AFM scans of GO and TRG on mica substrates, and their height profiles (insets) along the straight white lines. iii WAXD profiles of TPU composites. The inserts are WAXD patterns in 2h = 3.5–13° for melt-blended TRG, solvent-blended PhiGO, AcPh-iGO, and in situ polymerized GO composites. iv TEM micrographs of TPU with a 5 wt % (2.7 vol %) graphite, b, c melt-blended, d solvent-mixed, e, f in situ polymerized *3 wt % (1.6 vol %) TRG, g solventmixed 3 wt% (1.6 vol%) Ph-iGO, h AcPh-iGO, and i in situ polymerized 2.8 wt% (1.5 vol %) GO. Reproduced with permission from [68] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SEM images of graphene reinforced thermoplastic elastomer poly (ethylene-ter-1-hexene-ter-divinylbenzene) a pristine thermoplastic elastomer poly (ethylene-ter-1-hexene-terdivinylbenzene), b thermoplastic elastomer poly (ethylene-ter-1-hexene-ter-divinylbenzene) with 1% graphene, c thermoplastic elastomer poly (ethylene-ter-1-hexene-terdivinylbenzene) with 3% graphene and d thermoplastic elastomer poly (ethylene-ter-1-hexene-ter-divinylbenzene) with 5% graphene. Reproduced with permission from [71] . . Graphene—the breakthrough material (https://www.autocar. co.uk/car-news/industry/graphene-breakthrough-materialcould-transform-cars, accessed on 12/07/2019) . . . . . . . . . . . .

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List of Figures

Fig. 4.2

Fig. 4.3

Fig. 4.4

Fig. 4.5

Fig. 4.6 Fig. 4.7

Fig. 4.8

Fig. 4.9

Top—trend analysis of Graphene researches. Reproduced with permission from [4] and accessed on 12/07/2019. Bottom—An illustrative pie chart along with a complimentary bar graph to visualize the categories of the manuscript with the index term ‘graphene.’ (Source Science Direct, Elsevier, with a sample size of 10971, Plotted with the accessed data on 24/05/2020). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of analysis of Graphene and its derived researchers. Reproduced with permission from [4] and accessed on 12/07/2019 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison analysis of graphene and its derived researches as a function of the countries worldwide. Reproduced with permission from [7] and accessed on 12/07/2019 . . . . . . . . . . Schematic illustration of the main graphene production techniques. a Micromechanical cleavage. b Anodic bonding. c Photoexfoliation. d Liquid phase exfoliation. e Growth on SiC. f Segregation/precipitation from the carbon-containing metal substrate. g Chemical vapor deposition. h Molecular Beam epitaxy. i Chemical synthesis using benzene as a building block. Reproduced with permission from [8] . . . . . . SEM image of graphene layers on SiC. Reproduced with permission from [14] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Epitaxial growth and functionalization of a graphene monolayer. a Clean Ni surface. b Graphene monolayer was grown by CVD. The unit cell with the nonequivalent A and B atoms is indicated. c Potassium atoms intercalate between the Ni and the graphene. The corresponding XPS spectra for d a clean Ni surface, e an epitaxially grown graphene monolayer on Ni, and f a potassium intercalated graphene monolayer with K/C = 0.69. Reproduced with permission from [15] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three different stackings for trilayer graphene (Simple hexagonal, Bernal, and Rhombohedral) and the corresponding calculated electronic structures. Reproduced with permissions from [20] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quasi-particle System. a Charge carriers in condensed matter physics are normally described by the Schrödinger equation with an effective mass m* different from the free electron mass (p is the momentum operator). b Relativistic particles in the limit of zero rest mass follow the Dirac equation, where c is the speed of light, and ! s is the Pauli matrix. c Charge carriers in graphene are called massless Dirac fermions and are described by a 2D analog of the Dirac equation, with the fermi velocity vF  1  106 m/s playing the role of the speed of light and a

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Fig. 4.10

Fig. 4.11

Fig. 4.12 Fig. 4.13

Fig. 4.14

Fig. 4.15

List of Figures

2D pseudospin matrix ! s describing two sublattices of the honeycomb lattice (3). Similar to the real spin that can change its direction between, say, left, and right, the pseudospin is an index that indicates which of the two sublattices a quasiparticle is located. The pseudospin can be indicated by color (e.g., red and green). d Bilayer graphene provides us with yet another type of quasi-particles that have no analogies. They are massive Dirac fermions described by a rather bizarre Hamiltonian that combines features of both Dirac and Schrödinger equations. The pseudospin changes its color index four times as it moves among four carbon sublattices. Reproduced with permission from [1] . . . . . . . . . . . . . . . . . . . A piece of graphene aerogel, which weighs only 0.16 milligrams per cubic centimeter—is placed on a flower. Reproduced with permission from [42] . . . . . . . . . . . . . . . . . . Representative images of a Hexagonal honeycomb lattice of graphene with two carbon atoms (A and B) per unit cell. b Energy momentum dispersion in graphene. c Schematic illustration of the covalent chemistry of graphene. d Band structure change of single-layer graphene near the K point of the Brillouin zone before (left) and after (right) chemical modification. (a) and (b) Reproduced with permission from [46] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prospects of Graphene paper. Reproduced with permission from [49] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A visualization of the thermal properties of graphene and nanostructured carbon materials. Reproduced with permission from [53] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Micrographs of a Graphene nanoribbons of sub-10-nm scale exhibit the transistor action with large on-off ratios b All the fundamentals are in place to make graphene-based HEMTs. This false-color micrograph shows the source and drains contacts in yellow, two top gates in light gray, and graphene underneath in green c Graphene-based NEMS. Shown is a drum resonator made from a 10-nm-thick film of reduced graphene oxide, which covers a recess in a Si wafer d Ready to use: Graphene membranes provide ideal support for TEM. Reproduced with permission from [1] . . . . . . . . . . . . . . . . . . . Pictorial representation of the atomic structure of a carbon atom along with the Energy levels (a and b) of outer electrons in carbon atoms. c The formation of sp2 hybrids. d The crystal lattice of graphene, where A and B are carbon atoms belonging to different sub-lattices, a1 and a2 are unit-cell vectors. e Sigma

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List of Figures

Fig. 4.16

Fig. 4.17

Fig. 4.18

Fig. 4.19

Fig. 4.20

Fig. 4.21

Fig. 4.22

Fig. 4.23

Fig. 4.24

Fig. 4.25

Fig. 4.26

bond and pi bond formed by sp2 hybridization. Reproduced with permission from [66] . . . . . . . . . . . . . . . . . . . . . . . . . . . Survey of research publications on CNTs-/GSDs reinforced elastomeric matrices and their hybrid nanocomposites during the last ten years. Reproduced with permission from [78] . . . To visualize the versatility of graphene based thermoplastic elastomers—I (Subject Category-Technology). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020 . . . . . . . . . . . . . . . . . . . . . . . . . . To visualize the versatility of graphene based thermoplastic elastomers—II (Subject Category-Polymer Chemistry). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020 . . . . . . . . . . . . . . . . . . . . . . . . . . To visualize the versatility of graphene based thermoplastic elastomers—III (Subject Category-Physical chemistry). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020 . . . . . . . . . . . . . . . . . . . . . . . . . . To visualize the versatility of graphene based thermoplastic elastomers—IV (Subject Category-Synthetic chemistry). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020 . . . . . . . . . . . . . . . . . . . . . . . . . . To visualize the versatility of graphene based thermoplastic elastomers—V (Subject Category-Biotechnology). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020 . . . . . . . . . . . . . . . . . . . . . . . . . . To visualize the versatility of graphene based thermoplastic elastomers—VI (Subject Category-Environmental Chemistry). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020 . . . . . . . . . . . . . . . . . . . . . . . . . . To visualize the versatility of graphene based thermoplastic elastomers—VII (Subject Category-Biology). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020 . . . . . . . . . . . . . . . . . . . . . . . . . . Illustration of the chemical modification of graphene to reduced graphene oxide a Graphene, b Oxidized graphite c Separation of oxidized graphite to graphene oxide sheets on sonication, d Hydrazine reduction of graphene oxide and formation of reduced graphene oxide. Reproduced with permission from [78] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The most prevalent nanofiller mixing methods used in the fabrication of Graphene and its derived-elastomeric nanocomposites. Reproduced with permission from [78] . . . . SEM micrographs of CNTs: a purified CNTs; b ball-milled CNTs. Reproduced with permission from [103] . . . . . . . . . . .

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Fig. 4.27

Fig. 4.28

Fig. 4.29

Fig. 4.30

Fig. 4.31

Fig. 4.32

Fig. 4.33

Fig. 4.34

Fig. 5.1

List of Figures

TEM micrographs of Thermoplastic Polyurethane with a 5 wt% (2.7 vol.%) graphite, b, c melt-blended, d solventmixed, e, f in situ polymerized with 3 wt% (1.6 vol.%) Thermally Reduced Graphene, g solvent-mixed 3 wt% (1.6 vol.%) Ph-iGO, h AcPh-iGO, and i in situ polymerized 2.8 wt% (1.5 vol.%) Graphene Oxide. Reproduced with permission from [106]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rheological curves showing time dependant torque for NBR and its Nanocomposites at 160 °C. Reproduced with permission from [114]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rheographic profile of a NBR-OM15 at four different temperatures, b cure conversion versus time of NBR-OM15 at four different temperatures. Reproduced with permission from [114] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SEM images of a SBR, b SBR/GO-RT(1)%, c SBR/GO-RT(2)%, d SBR/GO-RT(3)%, e SBR/GO-RT(4)%, f SBR/GO(4) control. Reproduced with permission from [119] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The reported data of the modification of the dynamic mechanic properties with the oxidative thermal aging. Reproduced with permission from [120]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The reported TEM images illustrating the wrinkling phenomenon of the graphene derivates when incorporated into an elastomeric matrix. Reproduced with permission from [78] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphical representation of a Tensile and tear strength of SBR/ CNTs and SBR/rGO–CNTs composites as a function of CNTs content. b Typical stress-strain curves of blank SBR and SBR composites with different filler systems. c Typical stress-strain curves of rGO–CNTs hybrid filled SBR composites with an rGO/CNTs ratio of 2:1. d Relative Young’s modulus of SBR composites as a function of the filler volume fraction. The solid lines are fitted by Guth–Gold–Smallwood equation. Reproduced with permission from [138] . . . . . . . . . . . . . . . . . Graphical representation of a Storage modulus (E′), b loss modulus (E″), and c loss tangent (tan (d)) versus frequency for the TPU-based nanocomposites. Reproduced with permission from [145] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphical representation of i Resistance-strain behavior of composites with different graphene content, up to 5% strain at the strain rate of 0.1 min−1 during a cyclic loading and b Resistance-strain behavior of TPU-0.2G for cycles 81–100. ii Resistance-strain behavior of TPU-0.2G, up to different strain amplitude at the strain rate of 0.1 min−1, during the

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List of Figures

Fig. 5.2

Fig. 5.3

Fig. 5.4

Fig. 5.5

1st cycle (a) and cyclic loading (cycle 11–20) (b). iii Resistance-strain behavior of TPU-0.2G, up to 30% strain at different strain rates, during the 1st cycle (a) and cyclic loading (cycle 11–20) (b). iv Experimental (dots) and theoretical (solid lines) data of resistance as a function of strain. v Change of a conductive pathways (CP) and b tunneling distance (TD) as a function of strain. Reproduced with permission from [27] . . . Electrical conductivity (rc) versus filler volume fraction (u) for TPU/RGO/PVP nanocomposites. Insert is a log-log plot of the electrical conducting versus u-uc (u and uc being the filler content and percolation threshold, respectively). Reproduced with permission from [28] . . . . . . . . . . . . . . . . . . Graphical representation of i Piezoresistive behavior of a TPU/ GE D40, b TPU/GE G40, and c TPU/GE S40 under cyclic compression, d Resistance ratio and gauge factor of TPU/GE porous structures at 8% compression strain. ii Variation of gauge factor as a function of compression strain for the TPU/GE porous structures with a Diamond, b Gyroid, and c Schwarz unit cells. iii a Piezoresistive behavior of TPU/GE S40 over 50-cycle compression test, and b resistance values at 8% strain as a function of time for all TPU/GE composite structures. Reproduced with permission from [29] . . . . . . . . . Depiction of i Responsivity of TPU-based CPCs containing 0.4 wt% graphene towards saturated a cyclohexane and CCl4 and b ethylacetate and acetone as a function of time; c the maximum responsivity in saturated organic vapors and the residual responsivity in the air in a single IDR at 30 °C. ii Responsivity of TPU-based CPCs containing 0.4 wt% graphene towards saturated a cyclohexane & CCl4 and b ethylacetate and acetone vapors in five IDRs at 30 °C. iii Responsivity of TPU-based CPCs containing 0.4 wt% graphene towards saturated a cyclohexane, b CCl4, c ethyl acetate and d acetone vapors in a single IDR at different temperatures. Reproduced with permission from [30] . . . . . . . I-V curves of SEBS and rGO/SEBS composites up to 6 wt% (a) and electrical conductivity of SEBS and GO, rGO and G-NPL composites with filler content up to 6 wt% (b), Gauge factor determination for samples up to 1, 5 to 10% of strain (c), Piezoresistive measurements for 1000 cycles at 5% strain and 5 mm/min for GO/SEBS composite (d) and Gauge Factor of GO/SEBS and rGO/SEBS composites with 4 wt%

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Fig. 5.6

Fig. 5.7

Fig. 5.8

Fig. 5.9

List of Figures

filler content for 1, 5 and 10% of maximum strain at 1 and 5 mm/min deformation speed (e). Reproduced with permission from [31] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphical representation of i Histogram of conductivity of CNT, CNT/graphene and CNT/graphene/fullerene-based sensor. ii GF and maximum strain range histogram corresponding to sensors of different sensitive unit materials. iii Human monitoring applications: a blowing air, b wrist bending, and c finger bending. Reproduced with permission from [32] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Representation of i IR absorption property of the three graphene materials and their nanocomposites. a IR absorption properties of sulfonated-graphene and isocyanate-graphene solutions with a concentration of 0.05 mg/mL. Reducedgraphene being insoluble in DMF, its IR absorption spectrum is not shown here. b The TGA curves with a heating rate of 5 °C/min from room temperature to 400 °C under N2 for isocyanate-graphene, sulfonated-graphene, and reducedgraphene. c The normalized IR absorption of the films of pure TPU, isocyanate-graphene/TPU (1 wt %), sulfonatedgraphene/TPU (1 wt%), and reduced-graphene/TPU (1 wt%) across a range of wavelength from 500 to 1100 nm. d Summary of the transmittance of IR light for the sample films at 850 nm: pure TPU, isocyanate-graphene/TPU (1 wt%), sulfonated-graphene/TPU (0.1, 0.5, and 1 wt%), and reduced-graphene/TPU (1 wt%). Reproduced with permission from [33] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Illustration of i Photocurrent switching response at 5 s intervals in a 0.3 V 1 M NaOH aqueous solution under 500 W Xenon lamp illumination. a Photocurrent response of neat SEBS and Zn-PorSEBS elastomer with different porphyrin grafting ratio. b–d Photocurrent response of Zn- PorSEBS elastomer and G/Zn-PorSEBS with different graphene content at the same porphyrin grafting ratio, (e and f) Photographs giving the light on/light off process. ii a UV–vis spectra of Zn-PorSEBS matrix and G/Zn-PorSEBS composite. b Molecular orbital energy diagram of photo-induced electron transfer from porphyrin to graphene. Reproduced with permission from [37] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphical representation of i PL spectra a and corresponding cycles of heating–cooling at above and below LCST b of ZnS NPs-containing block copolymer-GO nanocomposite. ii PL spectra with concentration increase of TNT (1  10−7 mol L−1) in the DMF solution of ZnS

. . 189

. . 190

. . 191

. . 193

List of Figures

Fig. 5.10

Fig. 5.11

Fig. 5.12

Fig. 5.13

Fig. 5.14

NPs-containing block copolymer-GO (a) and Stern–Volmer plots corresponding to the above graphs (b), Fluorescence quenching efficiency obtained for ZnS NPs-containing block copolymer-GO upon addition of 10 mM of different nitro compounds (1  10−2mol L−1) (c) and metal ions (1  10−2 mol L−1) (d). iii Uv-vis absorption spectrum of MEA-TNT (blue) and PL emission spectrum of ZnS NPs-containing block copolymer-GO in DMF (red). Reproduced with permission from [41] . . . . . . . . . . . . . . . . . . Schematic illustration of i FGO and its application as a thermal sensor. ii Photoluminescence (PL) spectra of a FGO1, and b FGO2, c changes in PL intensity at 425 nm as a function of temperature. iii Reversibility test of the on–off switching behavior of FGO2 in terms of PL quenching. Reproduced with permission from [42] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Actuation performance: displacements at 0.5 V and 0.1 Hz under a square and b sine voltages; c displacements according to DC voltages; displacements according to d voltages and e frequencies, and f durability. (The free length of the actuator was 20 mm). Reproduced with permission from [43] . . . . . . . Representation of a Maximum tip displacement of IPMCs based on SSPB/sGO/IL, SSPB/GO/IL, and Nafion/GO/IL with a series of filler content. b Bending deformation of an SSPB/sGO (0.5 wt%)/IL IPMC equilibrated under an applied potential of 2 V dc. c Comparison of bending strains of SSPB/sGO (0.5 wt%)/IL with those of top-ranked bendingtype polymer actuators impregnated with ILs reported in the literature. d Charge-specific displacement of SSPB-based membranes IPMCs. Reproduced with permission from [44] . . Pictorial representation of i NIR-triggered shape memory process for the ATA-POE/ODA-GO nanocomposites with a different weight content of ODA-GO loadings (i). ii NIRcontrolled shape recovery of the APG-0.50 nanocomposite a original shape, b 100% elongated shape and gradually threestep recovered shapes: c left segment, d middle segment, and (e) right segment. iii Stress-strain curves of the APG-0.50 scratched samples a healed for various times under NIR irradiation and b comparison of NIR and thermal healed sample for 60 min. Reproduced with permission from [46] . . MW-induced shape recovery test of SMP/GNPs (2 GPU at 2.45 GHz) (i), Unconstrained MV-induced shape recovery behavior of SMP/GNPs (2 GPUat 2.45 GHz) (ii), Unconstrained MV-induced shape recovery behavior tested for 60 s (iii). Reproduced with permission from [50] . .

xxvii

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. . 196

. . 197

. . 198

. . 200

. . 201

xxviii

Fig. 5.15

Fig. 5.16

Fig. 5.17

Fig. 5.18

Fig. 5.19

Fig. 5.20

List of Figures

Recovery photos at different times of LCPU at the first cycle (a), Stress-strain curves of the pristine LCPU, and its composites at room temperature (b). Reproduced with permission from [51] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphical representation of a Shape fixity and b shape recovery ratio of PU and its nanocomposites with GO under cyclic loading at different temperatures of 298 K, 323 K and 348 K, c Correlation of shape recovery and temperature with the crosslink density (fitted with the Maier and Göritz model) for PU, PG0.5, PG1.5 and PG3 at 298 K, 323 K and 348 K. Reproduced with permission from [45] . . . . . . . . . . . . . . . . . . Shape fixation and recovery of GO/SEBS-2 (a). The dashed line represents the temporal strain achieved by deformation. Shape recovery of GO/SEBS as a function of time under the IR light (b). L0 is the initial length of the cylindrical sample, and L is the length after shape fixation or shape. Reproduced with permission from [53] . . . . . . . . . . . . . . . . . . . . . . . . . . . Electroactive shape-recovery behavior of graphene-crosslinked PU composites. The samples undergo the transition from the temporary shape (helix, left) to permanent (linear, right) within 10 s. Reproduced with permission from [54] . . . . . . . . . . . . . The healing performances of the FG-TPU samples with different FG loadings under the three healing processes. a The IR light healing efficiencies of the pure TPU and the FG-TPU samples with different FG loadings at the optimal healing time. b The electrical healing efficiencies of the pure TPU and the FG-TPU samples with different FG loadings at the optimal healing time. c The electromagnetic wave healing efficiencies of the pure TPU and the FG-TPU samples with different FG loadings at the optimal healing time. d The optimal healing time of the FG-TPU samples with different FG loadings. e The relationship between the applied voltage and the healing time for the FG-TPU samples with different FG loadings. f The optimal healing time of the FG-TPU samples with different FG loadings. Reproduced with permission from [58] . . . . . . . . . . An overview of i Recipes for the preparation of the composites with different contents of GO. ii Healing efficiency of PU-DA and iGO-PU-DAs films determined by recovery of breaking stress. iii Stress-strain curves of PU-DA and iGO-PU-DAs films before and after thermal healing: a PU-DA, b iGO-PUDA-1, c iGO-PUDA-2, and d iGO-PU-DA-3. iv Summary

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. . 206

. . 207

List of Figures

Fig. 5.21

Fig. 5.22

Fig. 5.23

Fig. 5.24

Fig. 5.25

of the mechanical properties of the composite samples after the healing test. The average values were obtained from more than 3 samples. Reproduced with permission from [59] . . . . . . . . . Tensile strength of G-TPU composite films before and after healed by electricity and IR light, respectively (i), SEM images of scratch samples healed at 130 °C for the different time using electricity (ii), SEM images of scratch samples healed at 130 °C for the different time using IR light (iii). Reproduced with permission from [60] . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphical representation of a Healing efficiency of the nanocomposites under sunlight, b repeatable healing efficiency of the nanocomposites, c digital and optical microscopic photographs of cracked and healed nanocomposite films and d representative stress-strain profiles of HPU-T1RGO2, before and after healing with different repeating cycles. Reproduced with permission from [61] . . . . . . . . . . . . . . . . . . . . . . . . . . . Healing efficiencies of HPU/Si-GO0.5 nanocomposite under MW and sunlight (i). Stress-strain profile of HPU/Si-GO0.5 before and after healing (ii). Reproduced with permission from [62] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Demonstration of i The cellular viability detected by CCK-8 assay (OD450) at each set time point. The values are shown as the means ±SD (n = 3). ii Antibacterial activity of GO/MGO-TPU composite porous membrane with the ratio between GO and MGO. a activation of E. coli; b activation of S. aureus. iii The long-lasting antibacterial activity of the GO/MGO-TPU composite porous membrane was shaken and washed in PBS buffer for 0, 7, 30 days. a Inactivation of E. coli; (b) Inactivation of S. aureus, iv a Representative macroscopic appearance of the infected wounds, blank (Wound without any treatment), Control (Sterile Vaseline gauze covered wound), PHMG0.5-TPU, GO0.5-TPU, MGO0. 5-TPU; b Representative histological image of the length of the newly formed epithelium tongue at day 9 post-surgery in the Blank (Wound without any treatment), Control (Sterile Vaseline gauze covered wound), PHMG0.5-TPU, GO0.5-TPU, MGO0.5-TPU. c Wound healing curves; d Wound closure time. The values are shown as the means ±SD (n = 5). Reproduced with permission from [66]. . . . . . . 96 h cell culture results of NIH3T3 cells on 3D printed TPU/ PLA with different GO loadings: a 0 wt % GO, b 0.5 wt % GO, c) 2 wt % GO, d 5 wt % GO. Green color indicates live cells. Reproduced with permission from [71] . . . . . . . . . . . . .

xxix

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xxx

Fig. 5.26

Fig. 5.27

Fig. 5.28

List of Figures

Images of i Day 3 3T3 fibroblast cell culture results of freeze-dried TPU (a), TPU–GO1% (b), TPU–GO5% (c): TPU–GO10% (d) scaffolds: (a–d) are fluorescence microscope pictures (scale bar 5100 lm) where green indicates living cells. ii Day 10 3T3 fibroblast cell culture results of freeze-dried TPU (a), TPU–GO1% (b), TPU–GO5% (c): TPU–GO10% (d) scaffolds: (a–d) are fluorescence microscope pictures (scale bar 5100 lm) where green indicates living cells and red indicates dead cells. Reproduced with permission from [72] . . . . . . . . . . . 216 Micrographs of i cell morphology after a direct contact test: a negative control, b positive control and c PCU/GO composite electrospun membrane. ii MTT assay representing the cell viability (%) of the L-929 fibroblast cells on electrospun membranes. iii Graphical representation of the percentage of hemolysis of the PCURF (random fiber), PCUAF, and composite electrospun membranes with 1, 1.5 and 3% loadings of GO. Reproduced with permission from [73] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 Illustration of a Antibacterial activity of the prepared dressing membranes against S. aureus, E. coli, and C. albicans. b Wound healing process for 20 days, treated with gauze (control), XSi-PU and XSi-PU/GO5%. c Representative images of MT and H&E stained histological sections on day 20 after initial wounding, arrows indicated the blood vessels. Reproduced with permission from [74] . . . . . . . . . . . . . . . . . . . . 217

List of Tables

Table 1.1 Table 1.2 Table 1.3 Table 2.1 Table 2.2 Table 2.3 Table 2.4

Table 2.5 Table 2.6 Table 2.7 Table 2.8 Table 5.1 Table 5.2

Breakthrough developments in commercialization of TPE . . . Advantages of TPE over thermoset rubber processing . . . . . . A chart work showing subcategories of block copolymers eligible as TPE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General classification of nanoclays based on the charge distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A comprehensive overview of the various techniques employed for the purification of carbon nanotubes . . . . . . . . The major functionalization strategies of carbon nanotubes using non-covalent interactions [1]. . . . . . . . . . . . . . . . . . . . . A comparative study to analyze the advantages and the disadvantages of the functionalization methods of carbon nanotubes [248–254, 256–288] . . . . . . . . . . . . . . . . . . . . . . . The comprehensive review of the properties of carbon nanotubes, reprinted with permission from [289] . . . . . . . . . . The glimpse of how graphene gradually evolved since its inception in 1947 [298] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A concise overview of the top down techniques [314–318] . . A concise overview of the bottom up techniques . . . . . . . . . . Value of shape fixity and shape recovery ratio of LCPU and GO/LCPU nanocomposites [51] . . . . . . . . . . . . . . . . . . . Average value of shape recovery rate of all the samples [51]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.. ..

3 5

..

6

..

34

..

56

..

61

..

63

..

64

.. .. ..

66 76 81

. . 203 . . 203

xxxi

Chapter 1

Introduction

Thermoplastic Elastomers (TPEs) are an important class of engineering polymers [1], the emergence of which provided a new dimension to the field of polymer technology in a greater multitude. TPEs have become the most rapidly developing segment of polymer science & technology in the last four decades, and their growth has achieved a high level of commercial significance [2]. There was an apprehension that the global demand for TPEs would rise by 5.2% annually up to 6.7 million metric tons in 2019, valued a fussiness of worth greater than $24 billion [3].

1.1 Growth and Development of TPE TPEs were commercially introduced in the market with thermoplastic polyurethane (TPU) during the late 1950s by the B.F. Goodrich Co. However, previously TPU was made in Germany in 1930 but did not receive any commercial interest. During that period, extensive research was carried out in the making of novel TPEs from block copolymer and blends of thermoplastic olefins (TPO) with elastomers. Later on, the commercialization of styrene-based block copolymer styrene-butadienestyrene or SBS (Kraton) was commercialized by Shell in 1965. In 1970, DuPont patented commercial copolyester, Hytrel, and Uniroyal, prepared from the blends of polypropylene (PP) and ethylene-propylene-diene-monomer (EPDM) rubber. The scientist had already started recognizing this all-important class of polymer, and subsequently, the research and development activity took its full toll on and from 1980, which claimed to be continuing even today [2]. Following is a figure (Fig. 1.1), which clearly shows a steadily rising market trend of few TPEs since 2012 and a projected market until 2022.

© Springer Nature Singapore Pte Ltd. 2020 A. Bandyopadhyay et al., Engineering of Thermoplastic Elastomer with Graphene and Other Anisotropic Nanofillers, Engineering Materials, https://doi.org/10.1007/978-981-15-9085-6_1

1

2

1 Introduction

Fig. 1.1 Market Trend of TPEs since 2012 and projection until 2022. Source Asia Pacific Thermoplastic Elastomers (TPE) Market Analysis By Product (Styrenic Block Copolymers (SBC, SBS, SIS, HSBC), Thermoplastic Polyurethanes (TPU), Thermoplastic Polyolefins (TPO), Thermoplastic Vulcanizates (TPV), Copolyester Elastomers (COPE)), By Application (Automotive, Footwear, Construction, Medical, Electronics, Industrial, Advanced Materials) And Segment Forecasts To 2022 Published: April 2016|180 Pages|Format: PDF|Report ID: 978-1-68038-625-7. http://www.gra ndviewresearch.com/industry-analysis/asia-pacific-thermoplastic-elastomers-tpe-market accessed on 14.02.2018

1.2 Breakthrough Developments in Commercialization of TPE Table 1.1 represents generation wise progressive developments during the commercialization of TPE. TPEs offer some special advantages that conventional rubber never provides. A conventional rubber is of no use unless it is vulcanised. Being a thermoset, it cannot be reprocessed, often regarded as scrap, and has to be disposed of. It can only be ground in crumb form to be used as fillers. Huge capital investment is required for the production and fabrication of rubber goods. These limitations act as key factors for continuous acceptance and sustained growth of TPE. TPE behaves like elastic thermoset rubber at room temperature and can be reprocessed when heated above melting temperature, which addresses the recyclability limitation of thermosets, thus producing less amount of scrap. No vulcanization and very little compounding are required too. TPE can be processed using a cost-effective conventional plastic processing method (like injection molding, blow molding, thermoforming, etc.) with shorter cycle time than conventional rubber. TPE articles can be remelted and reprocessed to minimize scrap materials by grinding it and mixing it with virgin TPE. Their properties can be manipulated by changing the ratio of the components. This gives a necessary economic as well as environmental impact [4]. Table 1.2 pointed out the specific advantages of TPE over conventional rubber in regard to processing.

1.2 Breakthrough Developments in Commercialization of TPE

3

Table 1.1 Breakthrough developments in commercialization of TPE Stages

Year

Advancements

Initial stages

1933

Semon: Flexible PVC, Goodrich patent

1940

Henderson: PVC–NBR blends, Goodrich patent

1947

Commercial PVC–NBR blends, Goodrich

1952

Synder: Elastic thread from linear copolyester, DuPont patent

1st generation

2nd generation

3rd generation

4th generation

1954

PU Spandex fiber, DuPont patent

1955–1957

Schollenberger: PU TPE, patent and paper, Goodrich

1957

Bateman, Merrett: NR–PMMA grafts, BRPRA

1958–1959

Tobolosky suggestion of crystalline and amorphous polyolefin copolymers

1959

Commercial PU elastic fiber, DuPont

1960

Commercial PU TPEs

1961

Ionomeric TPE, DuPont (Surlyn)

1962

Kontons a-olefin TPE research, Uniroyal

1962

Gessler patent on PP-CIIR blends, dynamically cured

1965

Commercial triblock TPEs, Shell (Kratons)

1967

California Institute of Technology and ACS, Polymer Group, Symposium on TPE theory

1968

Radial styrenic block polymers, Phollips (Solprene)

1972

Polyolefin blends, Uniroyal (TPR)

1972

Copolyester TPEs, DuPont (Hytrel)

1972

SEBS TPEs, Shell (Kraton G)

1978

A.Y. Coran’s research on the melt–mixed blends of elastomer and thermoplastics with dynamic vulcanization

1981

Commercial melt–mixed blends of EPDM and PP, dynamically vulcanized, Monsanto (Santroprene)

1982

Polyamide TPE, Atochem (Pebax)

1985

Commercial blends of NBR and PP, dynamically vulcanized, Monsanto (Geolast)

1985

Single phase melt-processable rubber, DuPont (Alcryn)

1988

Functionalization hydrogenated styrenics TPEs, Shell (Shell FG)

1988–2006

Blends of TPEs with existing polymers for enhancement of properties (continued)

4

1 Introduction

Table 1.1 (continued) Stages

Year

Advancements Academic research in various fields of TPEs worldwide New family of TPV having heat and oil resistance based on ACM and polyamide Development of crystalline–amorphous block copolymers (Engage), mettalocene catalyzed TPEs, Polyolefin elastomer (POEs), application research on TPEs Protein-based block-copolymer

Source Modified from Legge, N.R., Rubber Chem. Technol., 62, 529 (1989) [2]

1.3 TPE: A Sustainable Elastomer Composition TPE combines the properties of both elastomer and thermoplastic within a specific temperature window. It possesses biphasic morphology that includes (i) a soft, flexible, amorphous, mobile rubbery segments with a low glass transition temperature (Tg ) responsible for elastomeric behaviour and (ii) a hard, rigid, crystalline, glassy segment with a high melting temperature (Tm ) or high Tg acting as physical crosslinks which melt at a higher temperature and the whole mass soften and flow under shearlike conventional thermoplastics. However, the two segments should be thermodynamically incompatible at service temperature in order to restrict the interpenetration and chain slippage [2, 5]. Most of the TPEs meet the standard ASTM D 1566 definition of rubber because (i) they recover quickly and forcibly from large deformations, (ii) they can be elongated by more than 100%, (iii) their tension set is less than 50%. TPEs are broadly classified into three following categories [4]: 1. Block copolymer 2. Blends and elastomeric alloys 3. Ionomers Block copolymer can be divided into four subcategories (Table 1.3). Styrene-butadiene-styrene or SBS and Styrene-isoprene-styrene triblock copolymer or SIS are commercially produced by living anionic polymerization. They are the two most important TPEs, which consist of an elastomeric, soft butadiene/isoprene block at the center that lies in between two hard styrene end blocks [6]. Rubbery block forms the continuous domains of rubbery chains that are locked together with a rigid and oriented structure of hard styrene block dispersed in the continuous matrix (Fig. 1.2). The soft blocks offer elastomeric behavior at ambient temperature, and above the melting point, the copolymer chains are no longer locked but free to flow, giving typical thermoplastic characteristics [4]. These types of ABA triblock copolymer are synthesized by three methods: 1. Sequential addition of monomer. 2. Successive polymerization of monomer using a bifunctional initiator. 3. Synthesis of AB diblock followed by coupling reaction.

Slow (min)

Thermoset rubbers

High percentage waste

Reusable

Scrap

Required

None

Curing agent

Special vulcanizing equipments

Conventional thermoplastic

Machinery

Several processing aids

Minimal or none

Additives

Impossible

Yes

Remold parts

Source Walker, B.M. and Rader, C.P. (eds.), Handbook of Thermoplastic Elastomers, Van Nostrand Reinhold, New York (1988) [2]

Rapid (s)

Fabrication

Variables

Thermoplastic elastomers

Matrices

Table 1.2 Advantages of TPE over thermoset rubber processing

No

Yes

Heating, sealing

1.3 TPE: A Sustainable Elastomer Composition 5

6

1 Introduction

Table 1.3 A chart work showing subcategories of block copolymers eligible as TPE

Block Copolymer

Styrenic block

Thermoplastic

Copolymer(SBC) coplyamides(COPA)

Thermoplastic Copolyester(COPE)

The polyurethanes(TPU

Fig. 1.2 Block copolymer morphology—illustration of hard blocks crystallized into domains with soft, rubber block regions between them. Reproduced with permission from [4]

Though anionic polymerization is the major and widely accepted synthesis route to TPEs, the recent development of living carbocationic polymerization has also been used at times. Polyisobutylene based TPEs is one of such examples that are synthesized using living carbocationic polymerization [2]. In the case of polyester/polyether based TPE, the polyester-polyether segment is produced by melt transesterification. Polyester TPEs consist of hard blocks of oligomeric polyurethanes or aromatic polyester and soft blocks of aliphatic polyether. COPE is produced by heating a mixture of phthalate ester, a low molecular weight diol, a poly(alkylene ether) diol to 160 °C in the presence of a transesterification catalyst. The first commercial producer of COPE was DuPont. They marketed the product under the trade name of “Hytrel” which has been categorized as ‘engineering thermoplastic elastomer’ [2, 7]. TPUs are linear copolymer having alternating soft and hard blocks. TPUs are prepared from three chemicals: a macroglycol, an isocyanate, and a chain extender, which produce the multiple hard/soft block structure [4]. The soft segment is generally of higher molecular weight polyester or polyether, and the hard segment is a polyurethane produced by the reaction of a diisocyanate and a low molecular weight diol. TPUs are prepared by condensation polymerization. The chemical nature of glycol determines the performance of TPU, and the soft block influence the oxidation resistance, chemical resistance, hydrolytic stability, and low temperature flexibility.

1.3 TPE: A Sustainable Elastomer Composition

7

Fig. 1.3 Chemical structure of block copolymeric TPEs; i styrenic, ii COPE, iii thermoplastic polyurethane, and iv thermoplastic polyamide

Here, the repeating units in the rubbery region are connected by the polar bonds leading to a higher oil and solvent resistance against the hydrocarbon fluids [2, 4]. Segmented COPA possesses a regular chain of rigid polyamide with flexible polyether in between, connected by amide linkages. Hard segments are generally partially aromatic polyamide or aliphatic polyamide as the case may be, and the soft segments are polyester or polyether ester, e.g., Polyester amide (PAE), Polyether ester amide (PEEA), etc. Three conventional methods are used to prepare thermoplastic polyamide elastomer; namely, (i) Dow process, (ii) ATO Chemie process, and (iii) Emser industries process [2]. Figure 1.3 nicely demonstrates the chemical structure of all four subcategories of block copolymer TPEs.

1.4 TPE Based on Rubber—Plastic Blends TPE based on rubber-plastic blends has grown dramatically in the last two decades. A polymer blend containing rubber-thermoplastic can be prepared either when rubber rich mixture produces a soft TPE or when plastic rich blends form rubber toughened thermoplastics.

8

1 Introduction

TPE consists of a combination of polyolefin rubber, and a thermoplastic can be categorised into two subclasses: (i) simple blends, commonly known as thermoplastic elastomeric olefin (TEO) and (ii) where the rubber phase is dynamically vulcanized, commonly designated as thermoplastic vulcanizate or TPV [8]. In this type of blend, rubber, and plastic both should be incompatible with forming a completely separate phase. The plastic phase generally forms the continuous phase due to low viscosity, while the rubber phase remains as a dispersed phase [4]. Figure 1.4 shows a typical morphology of a TPO, while Fig. 1.5 shows that of a TPV. The first commercially useful rubber-plastic blend was prepared from nitrile rubber (NBR) and polyvinyl chloride (PVC) way back in 1940 [4]. However, the most popular rubber/plastic blend has been made by mixing polypropylene (PP) with polyethylene propylene diene methylene rubber (EPDM). Jha et al. [9, 10] have developed TPEs from a reactive blend of nylon-6 and acrylate rubber (ACM), which are both heat and oil resistant. De Sarka et al. [11] have extensively studied a series of TPE derived from halogenated styrene-butadiene rubber (SBR) and polyethylene (PE) [2]. Ionomers are classified under TPE due to the presence of thermolabile ionic groups (meltable crosslinks). They contain hydrocarbon backbone having pendant acid groups which are partially or fully neutralized to form salts [2]. Few examples

Fig. 1.4 TPO rubber/plastic blend morphology. Reproduced with permission from [4]

Fig. 1.5 Thermoplastic vulcanisate morphology with a continuous plastic phase and discrete rubber particles. Reproduced with permission from [4]

1.4 TPE Based on Rubber—Plastic Blends

9

of this class of TPE are: zinc salt of sulphonated EPDM rubber, the zinc salt of carboxylated NBR, sodium, or zinc salt of sulphonated SEBS triblock copolymer, etc.

1.5 Fillers/Nanofillers for TPE: Isotropic and Anisotropic Fillers So far, we have discussed the origin, development, and growth of TPEs in a nutshell. Along with processing facilities like thermoplastics, TPEs possess mechanical properties, and elastic performance similar to conventional thermoset rubbers and fillers have got some importance too similar to conventional rubbers, for the amelioration of properties and performance. Fillers are materials that enhance the properties of rubber and also act as a cheapening agent. Besides reinforcing and non-reinforcing classification of fillers, it can be categorized in a different way, such as isotropic and anisotropic type. The following discussion shall give a snapshot of some commonly used fillers in this particular category. When the properties of a material are the same in all directions, it is said to be isotropic. Alternatively, when properties vary with different crystallographic orientation, it is anisotropic. Mainly particulate fillers fall under the division of isotropic fillers; most common are carbon black (CB), some mineral filler like silica, calcium carbonate (CaCO3 ), synthetic alumina tri hydrate, mica and some metal oxides and hydroxides [12]. Soon after the discovery of CB as an ‘active’ filler in rubber, it became one of the most important ingredients in rubber product manufacturing. The reinforcement of elastomers by CB is governed by the morphology of black, physicochemical interaction with polymer, and it affects tensile properties, electrical conductivity, and impact strength of the TPE matrix [5]. Bazgir et al. [13] have reported improvement of dynamic mechanical properties and rheological properties of CB filled dynamically vulcanized PP/EPDM matrix. The use of silica is also very familiar in TPEs. Ersoy et al. [14] have reported that silica can be used as a reinforcing agent for styrene-ethylene co butylenes-styrene (SEBS)/PP/oil polymeric blend for the production of soft and transparent technical goods. Nano silica improves the thermal stability of low-density polyethylene (LDPE) and ethylene-vinyl acetate (EVA) based TPE significantly [15]. The effect of several metal oxides and hydroxides on TPEs has also been studied. Apart from being an activator and a curing agent, zinc oxide (ZnO) has also acted as an improved antimicrobial additive for SEBS based TPE [16]. Titanium dioxide (TiO2 ) also performs the same task. Researchers reported that ATH improves tensile modulus and hardness of PP/EPDM matrix, but tensile strength and elongation at break are sacrificed at higher ATH content [17]. Magnesium hydroxide is used as a flame retardant agent in SEBS [18]. If the anisotropic segment, a generation wise different anisotropic fillers have been used in TPE so far. Anisotropicity or unidirectional properties come from the filler

10

1 Introduction

geometry (shape, size, aspect ratio); new generation nanofillers are mostly anisotropic having a rod-like or a platelet like structure. We have come across several nanofillers, the emergence of which occurred with nanoclay nearly two decades back in the field of polymer science and technology. Subsequently, continuous and rigorous research activity gifted us with several other nanofillers like layered double hydroxide (LDH), carbon nanotube, nanofibre, and now with graphene, graphene oxide (GO) and other graphene derivatives. In 1963, Greeland prepared poly (vinyl alcohol) (PVC)/montmorillonite (MMT) nanocomposite in aqueous medium [19]. In 1980, the Toyota research group of Japan unveiled a new research area in polymer composite by introducing nylon-6 based nanocomposite with smectite group of clay and used it as a timing belts in cars [20– 23]. Since then, researchers started using nanoclay in different polymer matrices for various applications. Clay belongs to phyllosilicate family comprising of a octahedral (O) aluminium oxide/hydroxide layer sandwiched between two tetrahedral (T) silicate layers producing a T-O-T structure [24]. The most common smectite clays are MMT and hectorite. The stacking of O and T layers are held together by van der Waals forces within almost 1 nm gap (galleries). This gallery is occupied by alkaline metal like Na+ or K+ , which counterbalance the extra positive charge, which is originated by the substitution (substitution of Al3+ by Mg2+ in case of MMT) within clay layers [25] (Fig. 1.6).

Fig. 1.6 TOT structure of Nanoclay. Reproduced with permission from [25]

1.5 Fillers/Nanofillers for TPE: Isotropic and Anisotropic Fillers

11

But pristine layered silicate is hydrophilic, and most of the polymer is hydrophobic. In order to have a compatible clay based nanocomposite, organomodification of clay is carried out by replacing interlayer cation (Na+ , K+ ) by quarternary alkylammonium or alkylphosphonium cations through a cation exchange process aided to increase hydrophobicity. With this exchange reaction, the basal plane of clay is increased favouring the diffusion of polymer chains into clay layers and leads to the successful development of nanocomposite [24]. As mentioned before, many polymers have been used so far. In the field of TPE, the successful incorporation of MMT in SEBS, leading to a significant improvement in mechanical properties, has been reported [26]. Ray and Bhowmick have reported improvement in the mechanical properties of Engage (ethylene-octene copolymer)/clay nanocomposite [27]. Several studies have been performed on EVA/clay nanocomposites [28, 29]. Besides improvement in mechanical properties, clay provides excellent barrier properties, flame retardancy, thermal stability, chemical resistance, and optical clarity. In the last few years, researchers took interest in the synthesis and application of a special class of 2D lamellar material, namely layered double hydroxide (LDH), commonly known as anionic clay [30]. Although LDH was known for over 150 years since the invention of hydrotalcite mineral [31], it had never been applied successfully in polymers before. LDHs are a particular class of ionic lamellar compound comprising of positively charged brucite like layers containing anions in the interlayer region for charge compensation along with some water molecules. The general formula of LDH is: 3+ n− [M2+ 1−x Mx (OH)2 ][A ]x/n ·zH2 O, where metal cations occupy the centre of edgesharing octahedra. The vertex of octahedra contains hydroxide ions that connect to form infinite 2D sheets. The common divalent metal cations which are present are: Mg2+ , Zn2+ , or Ni2+ , and trivalent cations that are present are Al3+ , Fe3+ , Mn3+ , or Ga3+ 2− − respectively. The nonframework negative charges present are CO2− 3 , Cl , SO4 and − RCO2 . Figure 1.7 demonstrates the layer organization in a typical LDH. Because of high charge density of LDH layers and strong interlayer electrostatic interaction and strong hydrogen bonding, LDH shows an intense hydrophilic character, and exfoliation or delamination of layers become more difficult than clay. Organomodification of LDH interlayer is done by anion exchange with anionic surfactant like dodecyl sulphate (DDS), the elongated hydrophobic tails of which enlarge the brucite interlayer distance and weaken the force of interaction. Delamination occurs by dissolving it in a highly polar solvent which solvates the tails of the intercalated anion. Butanol, carbon tetrachloride (CCl4 ), toluene, formamide, etc. have been successfully used as solvent [32]. Among the widespread application of LDH in catalysis, anion exchangers, and as CO2 absorbent, polymer/LDH nanocomposites have attracted considerable interest. Bhowmick et al. have reported the formation of a LDH based polyurethane(PU) blended nitrile butadiene rubber(NBR) nanocomposites that showed excellent improvement in tensile strength and elongation at break, improved dynamic mechanical properties, thermal stability, flame retardancy as compared to neat polymer [33]. The effect of LDH on EVA/LDPE blend results in an enhancement of mechanical properties, thermal stability, limiting oxygen index, and solvent resistance properties [34].

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1 Introduction

Fig. 1.7 Layered structure of LDH. Source: http://www.scielo.br/scielo.php?script=sci_arttext& pid=S0100-06832015000100001, accessed on 24.04.2018

Successive development and growth in the field of layered anisotropic fillers have reached such an altitude where we are having the most advanced class of carbonbased nanofillers with huge potentiality. Carbon-based nanofillers are mainly layered graphite, graphene oxide (GO), and carbon nanotube (CNT). Graphite is an allotrope of carbon (all allotropes are shown in Fig. 1.8) consists of stacked layers of graphene. Graphene is one atom thick two dimensional sp2 hybridized carbon sheets where atoms are packed in a honeycomb structure [35]. GO is prepared by modifying graphite flakes by the oxidizing agent [36]. Polymer/expanded graphite composite is prepared by heating GO to a high temperature. The exfoliation expands the layers of graphite up to several hundred times leading to a fluffy material with low density and high thermal resistance. This results in the formation of nano dimensional flakes, which provides higher surface to volume ratio for interaction with a suitable matrix [37]. Origin of fullerene chemistry gave birth to the invention of CNT in 1985. In 1991, Ijiama invented CNTs, which was an extended fullerene with hexagonal carbon in the wall, rolled up into a hollow cylinder and capped at each end [38, 39]. CNTs can be single-walled or multi-walled depending upon the number of cylinders being rolled out during synthesis. The high aspect ratio of CNT gives it some exceptional properties which distinguish it from others.

1.5 Fillers/Nanofillers for TPE: Isotropic and Anisotropic Fillers

13

Fig. 1.8 Allotropes of carbon popular as nanofillers. Reproduced with permission from [51]

Both graphene and CNT possess exceptional strength and mechanical properties, which make them an ideal choice for being used as reinforcing fillers [40]. They also have very good thermal and electrical conductivity. There are several references on the application of these fillers in the various matrix, particularly in the case of TPEs [41–48], mainly studied for improved electrical and mechanical properties. Graphene is an emerging class of nanofiller exploited in TPE owing to its unique mechanical, optical, and electrical properties. Ultra-flexible, ultra-light, stronger than steel, conductivity better than copper, notably transparent all such impressive qualities of graphene have attracted the immense interest of researchers, and continuous researches are being carried out in developing such advanced composites based on graphene and its derivatives with different polymers.

1.6 Conclusion Because of the processing ease and recyclability, TPEs are gradually replacing most of the thermoset rubbers in the application. They can be co-molded or even overmolded on a hard thermoplastic giving a “soft touch” feel [49]. Cost advantage and design flexibility have made them a proper choice for both automotive sector [50] and in exterior as well as interior applications. A major use of TPE is in electrical insulation. Besides that, it is also suitable for medical and healthcare applications. This book has been dedicated towards engineering of TPEs with nanofillers with

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1 Introduction

special reference to grapheme and its derivatives and their potential application. It should be recalled that not much literature have been published so far on TPE based nanocomposite as compared to either polymer/elastomer-based. Thus the current book could serve as bonafide literature for further reading on this subject.

References 1. Sreekanth, M.S., Bambole, V.A., Mhaske, S.T., Mahanwar, P.A.: Effect of concentration of mica on properties of polyester thermoplastic elastomer composites. J. Miner. Mater. Charact. Eng. 8, 271–282 (2009) 2. Costa, F.R., Dutta, N.K., Choudhury, N.R., Bhowmick, A.K.: Thermoplastic elastomers. In: Bhowmick, A.K. (ed.) Current Topics in Elastomers Research, pp. 101–164 Taylor & Francis, Boca Raton (2008) 3. https://www.freedoniagroup.com/industry-study/world-thermoplastic-elastomers-3326.htm. Access date 12 Jan 2018 4. Kear, K.E., Introduction. In: Humphreys, S. (ed.) Developments in thermoplastic elastomers, pp. 3–11 RAPRA, U.K. (2003) 5. Yasar, M., Bayram, G., Celebi, H.: Effect of Carbon black and/or elastomer on thermoplastic elastomer-based blends and composites. Am. Inst. Phys. 1664(120003), 1–5 (2015) 6. Ban, H.T., Kase, T., Kawabe, M., Miyazawa, A., Ishihara, T., Hagihara, H., Tsunogae, Y., Murata, M., Shiono, T.: A new approach to styrenic thermoplastic elastomers: synthesis and characterization of crystalline styrene-butadiene-styrene triblock copolymers. Macromolecules 39:171–176 (2006) 7. J.R. Wolfe, Meier D.J. (ed.): Block-Copolymers: Science and Technology, MMI Press Symposium Series, Ellis Horwood, New York, 145 (1983) 8. Rader, C.P., Rader, S., Abdou-Sabet, S.K., De, Bhowmick, A.K. (eds.): Thermoplastic Elastomer from Rubber–Plastic Blends. Ellis Horwood, London (1990) 9. Jha, A., Bhowmick, A.K.: Thermoplastic elastomeric blends of nylon 6/acrylate rubber: Influence of interaction of mechanical and dynamic mechanical thermal properties. Rubber Chem. Technol. 70, 798 (1997) 10. Jha, A., Dutta, B., Bhowmick, A.K.: Effect of fillers and plasticizers on the performance of novel heat and oil-resistant thermoplastic elastomers from nylon-6 and acrylate rubber blends. J. Appl. Polym. Sci. 74, 1490 (1999) 11. De Sarkar, M., De, P.P., Bhowmick, A.K.: New polymeric blends from hydrogenated styrene– butadiene rubber and polyethylene. Polymer 39, 1201 (1998) 12. Katz, H.S., Fillers, P, Peters, S.T. (ed.) Handbook of composites. Chapman & Hall, London (1998) 13. Katbab, A.A., Nazockdast, H., Bazgir, S.: Carbon black-reinforced dynamically cured EPDM/PP thermoplastic elastomers. I. morphology, rheology, and dynamic mechanical properties. J. Appl. Polymer Sci. 75, 1127–1137 (2000) 14. Deniz, V., Karakaya, N., Ersoy, O.G.: Effects of fillers on the properties of thermoplastic elastomers. Soc. Plast. Eng. 1–4 (2009) 15. Hui, S., Chattopadhyay, S., Chaki, T.K.: Thermal and thermo-oxidative degradation study of a model LDPE/EVA based TPE system: effect of nano silica and electron beam irradiation. Polym. Compos. 31, 1387–1397 (2010) 16. Simões, D.N., Pittolb, M., Tomacheski, D., Ribeiro, V.F., Santana, R.M.C.: Thermoplastic elastomers containing zinc oxide as antimicrobial additive under thermal accelerated ageing. Mater. Res. 1–6 (2016) 17. Farzad, R.H., Hassan, A., Jawaid, M., Afendi, M. Piah, M.: Mechanical properties of alumina trihydrate filled polypropylene/ethylene propylene diene monomer composites for cable applications. Sains Malaysiana 42(6), 801–810 (2013)

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18. Xiao, W.-D., Kibble, K.A.: Comparison of aluminium hydroxide and magnesium hydroxide as flame retardants in SEBS-based composites. Polym. Polym. Compos. 16(7), 415–422 (2008) 19. Greenland, D.J.: Adsorption of poly(vinyl alcohols) by montmorillonite. J. Colloid Sci. 18–647 (1963) 20. Kojima, Y., Usuki, A., Kawasumi, M., Okada, A., Kurauchi, T., Kamigaito, O.: J. Polym. Sci., Part A: Polym. Chem. 31, 1755 (1993) 21. Okada, A., et al.: Composite material and process for manufacturing same Kabushiki Kaisha Toyota Chou Kenkyusho, US Patent 4739007 (1988) 22. Kawasumi, M., et al.: Process for producing composite material. Kabushiki Kaisha Toyota Chuo Kenkyusho, US Patent 4810734 (1989) 23. Pavlidou, S., Papaspyrides, C.D.: A review on polymer-layered silicate nanocomposite. Prog. Polym. Sci. 33, 1119 (2008) 24. Anadão, P.: Polymer/clay nanocomposites: concepts, researches, applications and trends for the future, Nanocomposites—new trends and developments. INTECH (2012) 25. Nguyen, Q.T., Baird, D.G.: Preparation of polymer–clay nanocomposites and their properties. Adv. Polymer Technol. 25(4):270–285 (2006) 26. Ganguly, A., De Sarkar, M., Bhowmick, A.K.: Thermoplastic elastomeric nanocomposites from poly[styrene–(ethylene-co-butylene)–styrene] triblock copolymer and clay: preparation and characterization. J. Appl. Polym. Sci. 100, 2040–2052 (2006) 27. Ray, S., Bhowmick, A.K.: Characterization and properties of montmorillonite clay-polyacrylate hybrid material and its effect on the properties of engage-clay hybrid composite. Rubber Chem. Technol. 74(5), 835–845 (2001) 28. Pramanik, M., Srivastava, S.K., Samantaray, B.K., Bhowmick, A.K.: Preparation and properties of Ethylene vinyl acetate (EVA)-Clay hybrids. J. Mater. Sci. 20, 1377–1380 (2001) 29. Haurie, L., Ferna´ndez, A.I., Velasco, J.I., Chimenos, J.M., Cuesta, J.-M.L., Espiell, F.: Thermal stability and flame retardancy of LDPE/EVA blends filled with synthetic hydromagnesite/aluminium hydroxide/montmorillonite and magnesium hydroxide/aluminium hydroxide/montmorillonite mixtures. Polymer Degradation Stab. 92, 1082–1087 (2007) 30. Nalawade, P., Aware, B., Kadam, V.J., Hirlekar, R.S.: Layered double hydroxide: a review. J. Sci. Ind. Res. 68, 267–272 (2009) 31. Evans, D.G., Slade, R.C.T.: Structural aspect of layered double hydroxide. In: Duan, X., Evans, D.G. (eds.) Layered double hydroxide. Springer, vol. 119, pp. 3–34 (2005) 32. Wang, Q., O’Hare, D.: Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets. Am. Chem. Soc. 112, 4124–4155 (2012) 33. Kotal, M., Srivastava, S.K., Bhowmick, A.K.: Thermoplastic polyurethane and nitrile butadiene rubber blends with layered double hydroxide nanocomposites by solution blending. Polymer Int. 59, 2–10 (2010) 34. Kuila, T., Srivastava, S.K., Bhowmick, A.K., Saxena, A.K.: Thermoplastic polyolefin based polymer—blend-layered double hydroxide nanocomposites. Compos. Sci. Technol. 68, 3234– 3239 (2008) 35. Huang, X., Yin, Z., Wu, S., Qi, X., He, Q., Zhang, Q., Yan, Q., Boey, F., Zhang, H.: Graphenebased materials: synthesis, characterization, properties, and applications. Small 7, 1876–1902 (2011) 36. Inagaki, M., Tashiro, R., Washino, Y., Toyoda, M.: Exfoliation process of graphite via intercalation compounds with sulphuric acid. J. Phys. Chem. Solids 65, 133 (2004) 37. Chung, D.D.L.: Exfoliation of Graphite. J. Mater. Sci. 22, 4190 (1987) 38. Zhou, X.-W., Zhu, Y.-F., Liang, J.: Preparation and properties of powder styrene–butadiene rubber composites filled with carbon black and carbon nanotubes. Mater. Res. Bull. 42, 456–464 (2007) 39. Iijima, S.: Helical microtubules of graphitic carbon. Nature 354, 56–58 (1991) 40. Kim, Y.A., Hayashi, T., Endo, M., Gotoh, Y., Wada, N., Seiyama, J.: Fabrication of aligned carbon nanotube-filled rubber composite. Scr. Mater. 54, 31–35 (2006) 41. Khan, U., May, P., O’Neill, A., Coleman, J.N.: Development of stiff, strong, yet tough composites by the addition of solvent exfoliated graphene to polyurethane. Carbon 48, 4035–4041 (2010)

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42. Nawaz, K., Khan, U., Ul-Haq, N., May, P., O’Neill, A., Coleman, J.N.: Observation of mechanical percolation in functionalized graphene oxide/elastomer composites. Carbon, 50, 4489–4494 (2012) 43. Kim, H., Miura, Y., Macosko, C.W.: Graphene/polyurethane nanocomposites for improved gas barrier and electrical conductivity. Chem. Mater. 22, 3441–3450 (2010) 44. Liang, J., Yanfei, X., Huang, Y., Zhang, L., Wang, Y., Ma, Y., Li, F., Guo, T., Chen, Y.: Infraredtriggered actuators from graphene-based nanocomposites. J. Phys. Chem. C 113, 9921–9927 (2009) 45. Liu, H., Li, Y., Dai, K., Zheng, G., Liu, C., Shen, C., Yan, X., Guo, J., Guo, Z.: Electrically conductive thermoplastic elastomer nanocomposites at ultralow graphene loading levels for strain sensor applications. J. Mater. Chem. C (2015) 46. Li, H., Wu, S., Wu, J., Huang, G.: Enhanced electrical conductivity and mechanical property of SBS/graphene nanocomposite. J. Polym. Res. (2014) 47. Koerner, H., Liua, W., Alexander, M., Miraub, P., Dowty, H., Vaia, R.A.: Deformation–morphology correlations in electrically conductive carbon nanotube—thermoplastic polyurethane nanocomposites. Polymer 46, 4405–4420 (2005) 48. Hemmati, M., Narimani, A., Shariatpanahi, H., Fereidoon, A., Ahangari, M.G.: Study on morphology, rheology and mechanical properties of thermoplastic elastomer polyolefin (TPO)/carbon nanotube nanocomposites with reference to the effect of polypropylene-graftedmaleic anhydride (PP-g-MA) as a compatibilizer. Int. J. Polym. Mater. Poly. Biomater. 60, 384–397 (2011) 49. http://www.mexichemspecialtycompounds.com/blog/tpe-compounds-are-the-right-touch-formany-products/3215/. Access date: 24 Apr 2018 50. Dufton, P.W.: Thermoplastic Elastomers, RAPRA Industry Analysis Report (2001) 51. Geim, A.K., Novoselov, K.S.: The rise of graphene. Nat. Mater. 6(3), 183–191 (2007)

Chapter 2

Anisotropic Nanofillers in TPE

2.1 Introduction The year 2013 saw a rapid boom in the area of the thermoplastic Elastomers recording a market share of more than 19.6 million metric tons and has predicted that the figure should be going beyond 29 million metric tons by 2020 [1]. There are two most bolstered reasons as to why the polymer industry is continuously expanding. The primary goal is, of course, because of the reinforced properties, including toughness, chemical, and corrosion resistance, which these materials posses [2]. Secondarily, these materials inherit the characteristics of being modified to achieve the desired property with ease and efficacy [3–5]. As far as the application area is concerned, the use of both these advantages is being used in the automobile industry, aerospace application, defense-related manufacturing, commercial items, packaging materials, anti-corrosive materials, marine industry, sports sector and various other thermomechanical applications [6–12]. As an evolutionary note, Polymer nanoparticle filled composite materials are in the limelight because of the tailor-made behavior of the composite material owing to the versatility of the nanoparticle being incorporated into the system (Fig. 2.1) [13]. The advanced and efficient features are exhibited due to the high aspect ratio of the nanoparticle dispersed in the polymer matrix.

2.2 Nanofillers and Its Advantages The properties that are improvised due to the incorporation of these nanofillers are broadly categorized into mechanical behavior, electrical conductivity, optical transparency, and barrier properties [5–14]. Researchers for the past couple of decades used to work with nanofillers such as the diatomite, carbon black, and pyrogenic silica as additives in polymeric materials. The Toyota research and Developmental group, in the year 1993, headed by Dr. Usuki, was the first to analyze the roles of © Springer Nature Singapore Pte Ltd. 2020 A. Bandyopadhyay et al., Engineering of Thermoplastic Elastomer with Graphene and Other Anisotropic Nanofillers, Engineering Materials, https://doi.org/10.1007/978-981-15-9085-6_2

17

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Number of Articles published including Patents

292502

159535

6646

12654

Nanoclay

LDH

CNT

Graphene

Percenatges of Articles published including Patents TPE/Graphene based Nanocomposites 35%

TPE/Nanoclay based Nanocomposites 12%

TPE/LDH based Nanocomposites 6%

TPE/CNT based Nanocomposites 47%

Fig. 2.1 Top: A snapshot of the articles (including patents) published in the respective subject domains till date, Bottom: The number of manuscript share with reference to the TPE based on the nanofillers, Data source- SciFinder, Chemical Abstracts Service (Plotted with the accessed data on 24/05/2020)

the nanaoclays when dispersed into the polymeric Nylon-6 matrix. The more recent list of the nanofillers is comprised of the multiwalled carbon nanotubes, graphene, graphene oxides, halloysite nanotubes, nanoclays, and nanoribbons [14–19]. Owing to their bolstered performance, the researchers and the industrial communities are continually updating themselves with the progress in nanomaterials for various high-end material applications [17–19]. A classic example may be cited based on the aircraft industry, which uses low weight nanocomposite materials to minimize the fuel consumption without affecting any of the mechanical (thermal conductivity, stiffness, strength) or the chemical properties (corrosion resistance) [20]. On a similar trend, the modern polymers, especially for the ones which are employed in high energy applications, focuses a lot on fire safety. To address this,

2.2 Nanofillers and Its Advantages

19

the two-dimensional nanoclays which possess inherent fire retardancy proprieties are being used [21–23]. The platelet structure of the clay conforms itself into a protective barrier when exfoliated, enabling in the increase of the overall property efficacy of the system. Moreover, the elevated intercalation chemistry of the clay microstructure makes it prone to natural chemical and physical modifications [24]. The one dimension nanotubes are being used extensively for most of the electrical applications to incorporate the flame retardancy property within them [25–37]. The enhanced length to diameter ratio coupled with the thermal conductivity and the light weightiness makes the carbon nanotubules one of the unique materials in the family of the nanofillers [28]. With the advent of technology, nowadays, the scientific community is more interested in the hybrid behaviors of these nanofillers. The incorporation of two different filler properties on a single matrix is one of the trending researches going on from the past couple of years. Ternary nanocomposites may yield a balanced and more robust optical, mechanical, electrical, and corrosion properties [6–12, 29–31]. On a very general note, nanofillers bridge the loopholes in a pristine polymeric system and make them more robust by providing property integrity [32, 33]. A trace amount of the nanofillers modifies the physical, chemical, thermal, mechanical, optical, magnetic, and electrical properties of the pure polymer matrix [34]. Although several scientists have worked in the field of nanotechnology, an efficient and enhanced feature shall only arise when the nanofiller is well dispersed in the matrix system [35]. To date, efficient mixing without agglomeration still proves to be a challenging task in preparing polymer/nanofiller composites [36]. Apart from the issue of the blend, the incompatibility of the fillers (arising mainly due to the polarity of the materials) might also turn out to be one of the tough jobs to overcome. The effectiveness of the nanocomposites is directly related to the formation of the interfacial bonds between the reinforcement and the matrix. Scientists primarily use a chemical modifier, which acts as a dispersing agent to homogenize the matrix and the filler system. But often, these chemicals prove to be economically high, not benign to the environment, and at times may damage the model of the host polymer [37, 38]. Some other means to address the issue of the non-homogeneity is by using a high shear rate, which aids the process of deagglomeration and restricts the formation of the Van der Walls force of attraction responsible for the operation of agglomeration. However, high employing shear rates may result in the breaking of the nanofibers and the destruction of the tubules [39]. The recent development in the electromagnetic interference shielding makes use of the conductive polymer composite. The reflection and absorption phenomenon generally shield electromagnetic interference. On this note, as in the case of the single carbon-reinforced polymer composite, the conductivity initiates from the reflection phenomenon resulting in secondary electromagnetic interference pollution [40, 41]. The hybridization of the nanofillers provides enough possibilities to rule out the phenomenon of the non-homogenous mixing and henceforth provides efficient methods to enable safe and effective interaction between the matrix and the filler [41]. The combination of the nanofillers not only brings a synergistic effect within the polymer composite but also helps to minimize the disadvantages the individual

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fillers posses. The hybrid characteristics of these nanofillers indeed benefit the science with advanced yet cost-effective materials for all innovations [42, 43]. Types of Nanofillers This chapter gives an overview of the various nanofillers which are used to reinforce Thermoplastic Elastomers. On an additional note, we have tried to put an overview not only on the preparation strategies but also on the application areas so that the readers may easily relate the practicability of the nanofillers.

2.3 Layered Double Hydroxide (LDH) Double layered hydroxides are a particular class of the nanofillers having the advantage of being environmentally friendly and industrially viable, is primarily composed of hydrotalcite type anionic clays. Not only do these fillers yield a higher dispersion with the polymer matrix, but it also can be fashioned according to the needs by varying the metal-ion combinations [44–46]. The unique property enables the Double layered hydroxide modified polymers to exhibit enhanced chemical stability, stability, and biocompatibility.

2.3.1 Structure of LDH Double layered hydroxides have initially been fabricated as a chemical complex having the empirical formula of Mg6 Al2 (OH)16 ](CO3 )·4(H2 O) [47, 48]. The term hydrotalcite was given due to the content of the water in the system and the presence of the carbonaceous group [49]. As these materials have a broad scope of variation, the general empirical formula may be cited as [SII1−x SIIIx(OH)2 ]x + [An− x/n·y− ]. The S(II) and S(III) represent the divalent and trivalent cations, respectively, whereas the An− represents the n valent anion [50]. The cationic ratio between the two valences, along with the anionic counterpart, may be tuned to develop tailor-made Double layered hydroxides. For a very general understanding, we may assume that the valency range between 0.2 and 0.33 is the most common, while the ratio of S(II) and S(III) lies in the range of 2:1–4:1. While on increasing the S(III) content yield is the formation of the M(OH)3 while reducing the same generates the precipitation of M(OH)2 [51–56]. The hydroxides inherit the octahedral sheets made from the positively charged mineral of magnesium hydroxide along with the alternating intercalated carbonate anions and water molecules (Fig. 2.2) [57–59]. The ratio between the size and the charge plays an essential role in influencing the property of the composite [60]. An optimized balance must be carried out between the matrix layer and the interpenetrating layer. In the case where the anions from long-chain ionic systems, for instance, in carboxylates or sulfonates with long alkyl

2.3 Layered Double Hydroxide (LDH)

21

Fig. 2.2 Illustrative representation of a Layered Double Hydroxide. Reproduced with permission from [59]

chains, the penetrating layer may organize themselves into more than a single layer (bilayers or trilayer). The variations in the number of the layers form directly affect the volume of the system, and hence the density-which, in turn, turns out the most prime factor for tuning the nanofiller [58–60]. The composition of the Layered Double hydroxide may be made in the magnesium hydroxide composition by substituting the same with a uniform distribution of the bivalent or trivalent cation. The presence of the excess positive charge is counterpoised by the anions present in the intercalated domains. However, an excess of the positive or the negative charge shall make the filler much polar, resulting in the same to transit to hydrophilic [61–65]. The general crystalline structure of these materials generally forms the octahedral structure where the coordinated metal cations are present in the center. The hydroxyl groups are oriented along with the corners with infinite sheets giving rise to the intercalated structure. Two significant forces dominate the attraction behavior, including the columbic charge and the hydrogen bonding [62]. An ionic radius ranging in between 0.65 and 0.80 Å may be the perfect candidate for the metals present in the metal coordination center. A higher radius seems to an incompatible fit for any layered structure since the higher radius shall disrupt the octahedral coordination, and thus, it shall result in

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2 Anisotropic Nanofillers in TPE

Fig. 2.3 The Ion exchangeable double layered hydroxide developed by increasing the gallery height. Reproduced with permission from [66]

the generation of one open end within the system in which water molecules may accumulate [63, 64]. The double-layered hydroxides are generally characterized by the weak bonds present in the intercalated regions, and thus it enables us to perform an ion exchange operation to develop tailor-made structures. The stability of the lamellar region is dependent on several primary and secondary factors, such as the charge density, interlayer packing, synthesis routes, temperatures, and applied pressure [64]. Reflecting the pure layered double hydroxides, the carbonate anions are oriented in parallel to the hydroxide groups providing enough interaction between the anionic group and the positively charged hydroxide layers predominantly via the formation of the hydrogen bonds. When a new anionic group is incorporated into the system, the groups tend to interact with the hydrophilic groups present while the hydrophobic carbon chains conform to the lowest energy state (being shifted away from the interaction zone) [65, 66]. Increasing the gallery height of the system may take into account more bulky anions, which inherently increases the photocatalytic activity as compared with its counterpart with a lowered gallery height because of the facilitation of the interaction between the charge carriers and the hydroxyl groups undermining the charge recombination (Fig. 2.3) [66].

2.3.2 Organophilisation of LDH 3+ x+ The LDH structure consists of cationic brucite-like layers [M2+ in (1−x) M x(OH)2 ] 2+ 3+ which M is partially substituted by M , and anion as well as by water molecules

2.3 Layered Double Hydroxide (LDH)

23

(Ax nH2 O)x− between the brucite-like layers where the strong electrostatic attraction between the hydroxide sheets and short inter-layer gallery spacing makes LDH materials difficult for the intercalation/exfoliation by the polymer matrix as most of the polymers are hydrophobic [67]. The compatibility of hydrophobic polymers with hydrophilic LDH is improved by modifying inorganic layers through the exchange of interlayer anions with an ionic surfactant [68–70]. Accumulation of large no. of anion into LDHs causes an increase in interlayer spacing yielding modified organoLDHs with hydrophobic character, and this intercalation of LDHs with large organic anions makes their exfoliation easier in solvents or melts [71–78]. Zhao and Nagy [79] synthesized a series of organic–inorganic nanocomposite of dodecyl sulfate–LDHs. Nyambo et al. [80] performed extensive study on the compatibility of hydrophobized LDHs with various polymers. The Mg–Al LDHs intercalated with palmitate or undecenoate were added into various polymer composites prepared by melt blending, and good LDH dispersion and nanocomposite formation were observed only in the poly(methyl methacrylate). Leroux and Besse [81] reported the restacking of exfoliated LDH layers intercalated with poly(styrene sulphonate) over the polymer. Manzi-Nshuti et al. [82] synthesized MII –Al LDH/PMMA nanocomposites (MII = Co, Cu, Ni, Zn) where LDH modified with undecenoate were added to PMMA. Thus, for better dispersion of inorganic nanoparticles in the polymer matrix, such chemical treatments on pristine LDH are prerequisites.

2.3.3 Strategies to Fabricate the Layered Double Hydroxide Since the last decades of years, the researches on the Layered Double Hydroxide have climbed up rapidly. The synthesis of LDH nanosheets falls under two broad classification: “Top-down” and “Bottom-up” approaches, among which the former is the most widely developed and applied method.

2.3.3.1

Top-Down Method or Delamination

In general, Top-down approach modifies the interlamellar environment in a suitable solvent system by ion-exchange intercalation with various kinds of anionic systems that are made to be intercalated within the matrix such as NO3 − and Cl− [61, 83, 84] and some other anionic surfactants like dodecyl sulphate (DDS). The process involves the synthesis of the double-layered hydroxides first, and then co-precipitated via these externally added NO3 − and Cl− anions [83]. In case of anionic surfactants, the long hydrophobic tails of DDS occupy the interlayer spacing enlarging the in-between distances and the anions in the intercalated domains are once again exchanged with the precursor of the hydroxides with a solution loaded with anions as the exchange proceeds weakening the brucite interlayer attractive force as well. The primary dominant forces which aid in the process of this exchange is the electrostatic forces of attraction present between the layers of the hydroxide (positively charged) and

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2 Anisotropic Nanofillers in TPE

Fig. 2.4 Illustration of the process of anion exchange for the synthesis of double layered hydroxides. Reproduced with permission from [85]

the anions being incorporated (negatively charged) [84]. The presence of the weak electrostatic forces of interaction between the layers of the hydroxides provides an opportunity for the anions, which have higher electrostatic forces of interaction to replace them easily [85]. A highly polar solvent disperses the nanosheets homogeneously, causing delamination/exfoliation by solvating hydrophobic tails of intercalating anions [86]. Not only shall a definite solvent allow favorable anion exchange, but the exchange capacity also depends on the composition of the mineral form of the hydroxides present. Several pieces of research say that the process of exchange shall accelerate further on increasing the temperature and at high pH levels (decreasing the pH levels shall induce in the disruption of the hydroxyl layer) (Fig. 2.4) [85]. Complete delamination of Zn-Al-NO3 LDHs was first reported using DDS refluxed in butanol by Adachi-Pagano et al. in 1999 achieving a shelf-life of 8 months [87]. O’Leary et al. observed delamination of LDH in organic solvents, particularly in polar acrylate monomers (loadings of between 1 and 10 wt%) at 70 °C in the presence of high shear leading to the sliding of LDH layers over each other and majority of the solid remained suspended after 24 h [88]. In particular, suspension prepared in 2-hydroxyethyl methacrylate (HEMA) remained stable for several weeks at loading up to 10 wt%. The extent of exfoliation depends on the degree of the solvation of aliphatic tails of DDS− intercalated anions leading to a high degree of interdigitation to maximize guest-guest dispersive interaction. Jobbágy and Regazzoni claimed the delamination of Mg–Al–DDS in toluene and CCl4 [89]. Naik et al. also came up with the same observation that LDH-DDS (Mg–Al, Co–Al, Ni–Al, Zn–Al) can be spontaneously delaminated in toluene and subsequently associated with a tactoidal microstructure [90, 91]. It was reported that, with increasing concentration of the dispersed layered solid in toluene, a gel-like state was obtained, a preferred state of dispersion, which was the consequence of the modified cohesive dispersive interactions between DDS surfactant chains tethered to opposing inorganic sheets mediated via toluene molecules, acting as ‘molecular glue,’ leading to the gel formation.

2.3 Layered Double Hydroxide (LDH)

25

Hibino and Jones investigated the delamination of a range of amino acid intercalated LDH nanosheets in formamide. They concluded Mg–Al-glycine/formamide to be the optimum combination of successful delamination due to the presence of strong hydrogen bonding between the intercalated anions and formamide, leading to the penetration of solvent molecules between the LDH layers [92, 93]. Li et al. [94] and Wu et al. [95] also observed successful delamination of Mg–Al–NO3 in formamide caused by the hydrogen-bonded interlayer water molecules by the polar carbonyl groups of formamide. In addition to that, the formation of a colloidal dispersion resulting from delamination of Mg–Al-CO3 in N,N-dimethylformamide (DMF)–ethanol solvent mixture associated with decarbonation was further reported by Gordijo et al. [96]. Various hybrid double-layered hydroxides can’t be synthesized by the conventional co-precipitation method due to hindrance provided by the large size of the anions. The anion exchange method is also advantageous in cases, especially in biomedical applications, where a drug needs to encapsulate with the layers for a potential pharmaceutical implementation [84, 85].

2.3.3.2

Bottom-Up Approach or Controlled Nucleation

Bottom-up synthesis makes use of the traditional aqueous co-precipitation system into an oil phase with specific surfactant and co-surfactant. The reverse micelles act as nanoreactors, thus providing limited space and nutrients for the formation of LDH single layers and having on effective control on the particle size both in diameter and thickness by the water to surfactant ratio as well (Fig. 2.4). In the year 2005, Hu et al. first employed a reverse microemulsion method for the facile one-step synthesis of Mg–Al LDH monolayer using traditional aqueous co-precipitation system (Mg(NO3 )2 ·6H2 O + Al(NO3 )3 ·9H2 O at pH ≥ 10) into an oil phase of isooctane with DDS as surfactant and 1-butanol as cosurfactant [97, 98]. XRD analysis of centrifuged gel-like material before drying suggested the formation of highly exfoliated LDH layers in these reverse microemulsion systems, followed by a gradual growth to gain some structural order upon drying. The AFM study showed a very small average thickness indicating a very small average thickness limited number of layers in each particle having no long-range coherence along the c dimension, which was also in agreement with XRD pattern where characteristics strong basal plane Bragg reflections were absent. The Co-precipitation route is the most widely practiced strategy, which can work with several divalent and trivalent cations along with a variety of anionic salts and macromolecules (including biomolecules) [99–101]. Recently, this ‘bottomup’ approach has also been exploited to other LDHs systems such as Ni–Al [102] and Co–Al LDHs [103], and to other reverse microemulsion systems such as cetyltrimethylammonium bromide/n-butanol/isooctane/MII and AlIII nitrate aqueous solution [104]. A solution containing the desired anion is prepared and is added gradually to the salt of the n-valent cation. The process is finally exposed to a higher pH by the addition of a primary medium to help the formed double layered hydroxide

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to precipitate out [99]. The crystallinity of the sample is generally enhanced by treating the precipitated-out sample thermally. The precipitation is mechanized by the condensation of the Hexa-aqua metal complexes in the added solution, thereby aiding in the formation of the layered structure, inheriting the matrix as the cation and the interlaced inter-lamellar zones as the anions [99]. A ubiquitous example of a Layered Double Hydroxide composite material synthesized by this technique is the Zn-Al/LDH composite, which exhibits excellent crystallinity and photocatalytic activity which arises due the decreased basal gap originated from the incorporation of the tetravalent cation into the system (Fig. 2.5) [100, 101]. Another example may be cited based on the composite prepared by Zne/Fe LDH, which generally has a fixed ratio of the zinc and the ferrous cation, but the intercalated anionic content varies. The co-precipitation method is also being employed nowadays to insert the drug moieties into the intercalated layers of positive bilayers for a probable application in the biomedicine field [105–107]. Metallic incorporated Layered Double Hydroxide also provides several advantages when compared with the synthesis strategies via some different routes. The recently developed hydroxides with the magnesium and aluminum ions were ranged in the nanoscale while the original material was in the microns [108, 109].

Fig. 2.5 SEM images showing the microstructure of the Zn-Al/LDH composite for an excellent photocatalytic activity a Zn–Al-LDH-(3 h of reaction time), b Zn–Al-LDH-(6 h of reaction time), c Zn–Al-LDH-(9 h of reaction time), d Zn–Al-LDH-(12 h of reaction time). Reproduced with permission from [101]

2.3 Layered Double Hydroxide (LDH)

27

Fig. 2.6 A comprehensive overview of the fabricating techniques of the double-layered hydroxides. Reproduced with permission from [115]

On the note of hybrids, the researchers have come up with another technique to synthesize these hydroxides layers by the process popularly known as the ‘reconstruction’ route [110–112]. The strategy follows a calcinations method coupled with rehydration to fetch the original structure of the double-layered hydroxides. The hydroxides may be transited to their metal oxides by oxidizing at (400–500 °C). The transition yields in the complete removal of the water molecules along with the hydroxyl groups of the layers to disappear [111]. The mixed metals, when rehydrated in a solution system, give back the original hydroxide structures. An inert atmosphere is generally recommended to carry out the process to prevent the percolation of any foreign matter into the system [112]. A higher temperature shall result in a more efficient calcining process, but the same shall affect the layered structure when it is subjected to reconstruction. Haraketi and his coworkers had developed a salicylic acid intercalated Zn-Al/Mg–Al layered double hydroxides by the method of reconstruction. Since then, several studies have followed, which reported the incorporation of biomolecules, such as phenylalanine between the layers made up of Zinc and Aluminum [113]. From these studies, we may conclude that there are majorly three scalable and economical routes to fabricate Hydroxides, including the Co-precipitation route, Ion exchange methods, and the Rehydration strategy [99–101, 105–107, 114]. Concluding this section, we may say that although the anion exchange is better suited to treat complexes having a large size of anions, the co-precipitation is advantageous from the perspective that it results in a better yield of the products with better control in the process parameters. As the scientific realms are emerging, we have other pilot processed propping up, for instance, the hydrothermal process, sol-gel technique, and the microemulsion strategy (Fig. 2.6).

2.3.4 Synthesis of Polymer/LDH Nanocomposites Layered double hydroxide is found to be one of the most promising nanofillers for the preparation of polymer nanocomposites with improved physic-chemical properties [116–118]. Owing to lamellar structure, ion exchange capacity, and novel

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2 Anisotropic Nanofillers in TPE

mechanical, optical, thermal properties, these hybrids are attractive for a wide range of technological applications like ion-exchangers, adsorbents, pharmaceutical stabilizers, and precursors of new catalytic materials [119], organoceramics, biomaterials, electrical and mechanical materials [120–127]. The performance of the polymer nanocomposites depends on the degree of intercalation or exfoliation of LDH layers in the polymer matrix [128], although exfoliated nanocomposite results in much enhanced properties compared to intercalated one [81]. Hence, the pivotal prerequisite is to develop a stable homogeneous dispersion of inorganic phase in polymer matrix. The possible strategies to synthesize exfoliated LDH/polymer nanocomposites can be classified into three major options: (1) intercalation of the monomer molecules and in situ polymerization, (2) direct intercalation of extended polymer chains, (3) pre-exfoliation and followed by mixing with polymer, as shown in Fig. 2.7 [129–131].

2.3.4.1

Intercalation of Monomer Molecules and in Situ Polymerization

As the name suggests, monomers can also be intercalated into the interlayer LDH gallery along with inorganic anions and organic moiety owing to the high exchangeability of interlayer anions followed by in situ polymerization of intercalated monomers. This results in the homogeneous distribution of exfoliated LDH nanosheets in the polymer substrate, and the same was observed when acrylate anions were being used by Tanaka et al. [1] for the anion exchange of Mg–Al–X-LDH (X = CO3 2− , Cl− , and NO3 − ). In situ polymerization of acrylate anions occurred at 80 °C with the aid of free radical initiator to form LDH/polyacrylate nanocomposites. Among CO3 2− , Cl− , and NO3 − , Mg–Al-NO3 − yielded better-intercalated compound. Alike organo modified exfoliated clay/epoxy nanocomposites, synthesis ofMg-Al-LDHs/epoxy nanocomposite was also reported by Hsueh et al. using amino laurate intercalated LDHs, EPON 828 resin, and Jeffamine D400 as a curing agent [126]. The reaction between amine groups of amino laurate and epoxy resin allows the epoxy group to enter into large gallery space of intercalated LDH nanosheets and diffusion of curing agents as well at elevated temperature to aid the thermal curing process. Polyimide/LDH can also be prepared following a similar method [97, 132]. In situ atom transfer radical polymerization (ATRP) reaction was exploited by Qiu et al. for the preparation of an exfoliated Zn–Al LDH/polystyrene nanocomposite where initially the LDH basal surfaces were delaminated by intercalating with DDS followed by polymerization of styrene by the α-bromobutyrate which became grafted to the surface of the Zn–Al LDH nanolayers [133, 134]. The enlarged spacing between LDH nanolayers after DDS modification resulted in facile transport of styrene monomers and catalysts into the gallery space leading to swelling. Finally, the polymerization gave rise to the LDH/PS nanocomposite exhibiting highly improved thermal stability compared to pure PS. Emulsion polymerization can also be another route for the synthesis of in situ polymerized exfoliated polymer/LDH nanocomposites, as indicated by Ding et al. [135] and Qiu et al. [136]. Emulsion polymerization of styrene occupying the large gallery spacing of exfoliated LDH

2.3 Layered Double Hydroxide (LDH)

29

Fig. 2.7 Pathway of nanocomposite preparation by a monomer exchange and in situ polymerization, b direct polymer exchange, and c restacking of the exfoliated layers over the polymer. Reproduced with permission from reference [130]

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nanosheets, modified in the presence of N-lauroyl-glutamate (LG) surfactants and long-chain n-hexadecaneleading to the formation of PS/LDH nanocomposites was reported.

2.3.4.2

Direct Intercalation of Extended Polymer Chains

Apart from the intercalation of organic monomers and subsequent in situ polymerization, direct intercalation of long-chain polymer molecules into LDHs can also be achieved assisted by solvent mixing or melt intercalation. The Qu group carried out an intensive investigation with several polymers for the preparation of exfoliated polymer/LDH nanocomposites like PE-g-MA/Mg–Al–LDH [137], LLDPE/Zn– Al–LDH [138, 139] PMMA/Mg–Al–LDH [140] by direct intercalation of polymers in xylene. TEM and XRD analysis study showed proper exfoliation of LDH, which can be improved by decreasing the LDH content, elongating the refluxing time, and rapid precipitation. Du et al. used cyclohexane for solvent intercalation poly(propylene carbonate) (PPC) in organo modified Mg-Al–LDH (OMg–Al–LDH). TEM image confirmed the exfoliation of Mg–Al nanosheets and its random dispersion in PPC matrix. Peng et al. was also able to successfully dispersed polycaprolactone (PCL) in DDS modified Co–Al–LDH using cyclohexane to fabricate highly exfoliated PCL/Co–Al–LDH nanocomposites following the previous method [141]. In addition to solvent intercalation, melt compounding is another equally important industrially feasible method for direct intercalation of polymers in LDH host matrix during extrusion, although complete exfoliation is hard to achieve by this process. Zammarano et al. reported the preparation of melt-processable exfoliated polyamide 6/LDH nanocomposite for low exchange capacity LDH whereas high anion exchange capacity LDH ended up with residual tactoid structure [142]. Du et al. investigated melt intercalation of LLDPE in partially organo modifiedMg-Al-LDH with severalfold improvement in the thermal stability of the final exfoliated nanocomposites [143]. This method is eventually found to be promising for the preparation of conventional thermoplastic polymers, e.g., polystyrene, polypropylene, polyethylene, and LDH nanocomposites.

2.3.4.3

Pre-exfoliation Followed by Mixing with Polymer

Several delamination/exfoliation methods of LDH have already been explored in our previous segment regarding Top-down strategies for the fabrication of LDH nanosheets. Multiple groups of researchers have demonstrated pre-exfoliation of LDH for the preparation of polymer/LDH nanocomposite; among them, O’Leary et al. were the pioneer group to report the delamination of LDH in acrylate monomers and subsequent polymerization thereafter. Li et al. worked on prior delamination of Mg–Al-glycine in formamide and successive mixing with PMMA solution in acetone under vigorous stirring followed by rapid evaporation of organic solvents restraining re-aggregation of dispersed LDH layers [144]. Pre-exfoliated

2.3 Layered Double Hydroxide (LDH)

31

Mg-Al-LDH/poly(vinyl alcohol) nanocomposites were also fabricated using the previous method. In another attempt, a colloidal suspension of positively charged Co-Al-NO3 LDH in formamide was mixed with negatively charged CNT-COONa dispersion in the same solvent in order to prepare exfoliated LDH/CNT hybrids with simultaneous sedimentation after mixing caused by electrostatic attraction between positively charged LDH and negatively charged CNT. Huang et al. carried out this pre-exfoliation of LDH to synthesis high-performance polyamide 6/modified LDH nanocomposites [145]. Dimethyl sulfoxide (DMSO) has also been used for partial exfoliation of LDH to prepare polymer nanocomposites. Zhao et al. selected 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid (HMBS), an organic UV absorber, for the modification of LDH in DMSO to prepare ethylene-vinyl alcohol copolymer (EVOH) composite film with improved thermal stability and transparency of this inorganic/organic nanohybrid [146]. Yuan et al. introduced a novel approach for the blending of pre-exfoliated organo LDH with acrylic resin and subsequent polymerization therein [147]. DDS intercalated expanded LDH nanolayers were grafted with γ -(2,3-epoxypropoxy)propyltrimethoxysilane (KH560), a silane coupling agent, followed by epoxidation with thiol group using trimethylolpropane thioglycolic acetate (TMPT) as a trithiol terminal A3 monomer to obtain thiol-endcapped LDH (LDH-SH) hybrid. Then the LDH-SH hybrid was blended with an acrylic resin and subjected to polymerization under UV irradiation in the presence of 1-hydroxycyclohexyl-phenyl ketone as a photoinitiator forming polymer/LDH nanocomposites.

2.3.5 Properties and the Recent Trends in the Areas of Application Polymer nanocomposites are one of the exotic areas for the researchers to study owing to its excellent mechanical properties coupled with thermal stability and enhanced physiochemical properties. Various literature infers that the class of layered double hydroxide might turn out to excellent filler for thermoplastic elastomer because of the presence of ionic species in the interlayer, which can be fine-tuned to make numerous tailors made composites [148–153]. The brucite structures, along with the exchangeable ionic interlayer, make them an excellent candidate for a realm of application areas including biomedicine, flame retarders, catalysts, ion exchangers, and pharmaceutics [152]. Thermoplastic Elastomers are one of the versatile classes of elastomer, which exhibits several brilliant characteristics such as improved tensile modulus, enhanced abrasion resistance, and high solvent/chemical resistance. Shrivastava et al. had reported the incorporation of layered double hydroxides as the nanofiller in the TPE/NBR hybrid [154]. The nanofiller reinforced elastomer fabricated via solution intercalation method exhibited an improved mechanical strength due to the induced polar interaction amongst PU, NBR, and LDH. Such an exciting

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finding can lead to an upbringing of the material properties, especially in adhesives, laminates, and fiber industries. In the same context, the authors synthesized a natural thermoplastic elastomer reinforced with an aluminum-magnesium hybrid layered double hydroxides [155]. Reported data shows that the mechanical properties obtained from this novel nanocomposites were almost 67% higher as compared to the pure polyurethane matrix. Not only has the partial exfoliation of the filler in the polyurethane matrix enhanced the heat stability by 29° centigrade, but the flame retardancy was also improved by 4% when compared with its counterpart. Hu and his coworkers used the amalgamation of zinc hydroxide nitrate and sodium benzoate nanoparticle as the modifying filler in the thermoplastic polyester elastomer matrix [156]. The developed nanocomposite via solution blending was further examined under a transmission electron microscope. The uniformly dispersed property of the filler within the matrix, along with the partially exfoliated framework, tuned the material more robust. Apart from the improvement in the storage modulus by 26%, the flame retardancy property was enhanced by 56% when compared with the natural thermoplastic polyester elastomer (Fig. 2.8) [156]. The works on stearate based LDH embedded on thermoplastic polyurethanes, as demonstrated by Bhowmick et al., concluded that functionalizing the LDH enables the basal spacing to increase. The enhancement of the spacing enables better compatibility between the polymer matrix and the layer of the hydroxide [157]. The composite showed a 45% improvement in the Tensile strength and 53% improvement in the Elongation at Break at 1 and 3% filler loading, respectively. While shifting the glass transition temperature by 15 32 °C, 20% enhancement in the modulus was observed on 8% filler loading; the thermal stability also recorded to be the highest at 32 °C [157]. With the advent of various advanced scientific tools and characterization methods, the future directions of LDH based thermoplastic Elastomers look very promising. Being a relatively new field of study, there are indeed numerous pathways to rediscover the material and its associated behavioral property. Apart from the conventional usage of LDH based elastomeric composite in the domain of adhesives, laminates, and biomedicine, the new age material shall find its prospect on a broader scope of application.

2.4 Nanoclay Clays are one of the oldest classes of fillers originating from inorganic compounds comprising of delicate crystals with high aspect ratio and improved specific surface areas (generally 750 m2 /g). Amongst these, there are some which posses’ one of its dimensions at the nanometer scale can be classified as nanofiller [157–162]. The invention of the Toyota research group of Japan in the year 1980 regarding in situ polymerization of nylon 6/montmorrilonite clay nanocomposite was one of the biggest milestones in the clay research area [163]. Afterward, nanocomposites

2.4 Nanoclay

33

Fig. 2.8 The TEM images of thermoplastic polyester elastomer reinforced with zinc hydroxide nitrate and sodium benzoate nanoparticle (a low magnification, b enhanced magnification). Reproduced with permission from [75]

based on several polymers and nano clay with different structures have been started to be synthesized. Significant enhancement in mechanical properties, thermal resistance, and gas barrier attributes have been reported by the incorporation of a small amount of clay in polymer nanocomposites along with low cytotoxicity for the use in biopolymers [164, 165]. One of the most important reasons for clay being widely exploited is its commercial availability, cost-effectiveness, and easy modification process.

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2.4.1 Structure of Nanoclay Based on its charge distribution, Nanoclays may be classified into three major categories, as described in Table 2.1. Most of the clays are elucidated based on the cationic composition and where the cations are located [166]. For instance, one of the most common clay systems based on 2:1 phyllosilicate consists of a sandwiched octahedral sheet between two tetrahedral silica sheets. The layered are positioned in a way so that the oxygen atom is shared between both the layers in the matrix of this triple sandwich (Figs. 2.9 and 2.10) [166]. In general, the clay minerals are divided into three major groups, namely (i) kaolinite, (ii) montmorillonite/smectite, (iii) illite/clay-mica group. The kaolinite group consists of three polymorphs (kaolinite, dickite, and nacrite) and is characterized by the formula of Al2 Si2 O5 (OH)4 [168] where the silicate sheets (Si2 O5 ) are bonded to aluminum oxide/hydroxide layers (Al2 (OH)4 ). The tetrahedral (T) silicate layers are bonded with octahedral (O) aluminium oxide layers via strong interlayer H-bonding (Fig. 2.11) [169] that prevents the inclusion of any molecules into the gallery spacing. Hence, it cannot be used in nanocomposite preparation. Table 2.1 General classification of nanoclays based on the charge distribution Charge distribution

Neutral

Negatively charged

Positively charged

Nature of the clay

Pyrophyllite, talc, kaolinite

Phyllosilicates

Hydrotalcites

Cationic Layers; The anionic charge is balanced by the positive charge present in the interface

Anionic layers; The cationic charge is balanced by the negative charge present in the interface

Demonstrated features Layers interact with each other via Vander Waals force or the hydrogen bond

Fig. 2.9 Crystal structures of clay minerals: a Type 1:1; b Type 2:1. Reproduced with permission from [167]

2.4 Nanoclay

35

SILICATES PHYLLOSILICATE Tecto silicates

2:1 INVERTED RIBBONS

1:1 PHYLLOSILICATE

Sepiolite

Kaolinite

Palygorskite

Serpenne

Others

2:1 PHYLLOSILICATE

Talc Mica

Fig. 2.10 Classification of silicates based on their physicochemical nature

Fig. 2.11 The layered structure of kaolinite clay. Source http://jan.ucc.nau.edu/doetqp/courses/env 440/env440_2/lectures/lec19/Fig.9_3.gif, accessed on 10.05.2020

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The montmorillonite/smectite clay minerals, which usually comprise of saponite, hectorite, beidellite, sauconite, pyrophyllite, talc, vermiculite, griffithite, and nontronite, are used extensively as the commercial clay polymer nanocomposites owing to their excellent swelling characteristics and polarity. The natural montmorillonite clay minerals have a unit negative charge of 0.67 units, which might be another advantage of using them as it tends to behave like a weak acid [167, 170– 173]. This group is characterized by general formula of (Ca, Na, H)(Al, Mg, Fe, Zn)2 (Si, Al)4 O10 (OH)2−x H2 O, where x represents the variable amount of water and a sandwich structure comprising two silicate layers having a gibbsite layer in between in a T-O-T stacking sequence (Fig. 2.12) [170]. Over the years, scientists have tried to investigate the effect of interchanging the coordination system with other inorganic moieties [86]. While in the tetrahedral coordination, silicon may be replaced by aluminum and phosphorous, the octahedral complex may be maneuvered with magnesium, chromium, and nickel. The incorporation of various other moieties enables the development of diversified montmorillonite clay minerals [173, 174]. The illite or the clay-mica group is identified with a general formula of (K, H)Al2 (Si, Al)4 O10 (OH)2−x H2 O, where x represents the variable amount of water and resembles the structure of MMT group with two silicate layers sandwiching gibbsitelike layer in between in T-O-T fashion. The variable amounts of water molecules, as

Fig. 2.12 The smectite clay structure. Source http://www.pslc.ws/macrog/mpm/composit/nano/str uct3_1.htm, accessed on 11.05.2020

2.4 Nanoclay

37

well as potassium ions, lie between the T–O–T layers. This is a common component of rock-forming minerals like shales and other argillaceous rocks. Not only is the cost-efficacy the only factor that makes these materials so versatile, but the low toxicological property also enables it to be applied on a more comprehensive scope of application, including biomedicines. The excellent material property, along with their functionalities, makes them suitable to be used with macromolecules, inks, grease, ceramics, and cosmetics [167, 171–173].

2.4.2 Organomodification of Nanoclay Naturally occurring hydrophilic clay minerals are consist of a layered structure of aluminosilicate comprising silica tetrahedral bonded to alumina octahedra in various ways. The counter cations are attracted by the net negative charges within the clay platelets and can be shared by two neighboring layers resulting in the stacking of platelets held together by strong electrostatic forces [174]. This strong force of attraction hinders the entrance of any molecule within interlamellar spacing, and hydrophilicity of charged layers makes them incompatible to interacting with hydrophobic polymer matrices [175–177]. Hence, prior to the preparation of polymer nanocomposites, the clay surface must be modified to render them compatible with organic matrix through the ion-exchange process as well as to separate the clay platelets for easy intercalation or exfoliation. The cations are not strongly bonded to the clay layers, so external small molecule cations can replace the cations present in the clay. The enhancement of the basal spacing of the modified clay depends on the chemical structure of the surfactant, the degree of cation exchange, and silicate layer thickness [178]. Ammonium surfactants containing alkylammonium cations with aliphatic chains and benzyl groups are widely used in commercially available organoclays [6–8]. Phosphonium surfactants are also in use for the same purpose [179–187]. Singla et al. investigated a series of organic surfactants, including Dodecyltrimethylammonium bromide, Hexadecyltrimethylammonium bromide, Tetradecyltrimethylammonium bromide, Tetraphenylphosphonium bromide, Zinc Stearate for sodium montmorillonite (Na-MMT) clay modification [187]. Lin et al. reported intercalation and exfoliation of Na-MMT using poly(oxypropylene) amine for the preparation of epoxy/clay nanocomposite, which led to the spatial interlayer expansion from 12 to 92 Å [188]. Acid chelating intermediate using alkyl carboxylic acid into the interlamellar spacing of clay containing divalent metal counter ions was also investigated [189]. C12–18 carboxylic acids intercalated monovalent (Na-MMT) clay showed low organic embedment with a basal spacing of 15 Å, whereas divalent M2+ MMT resulted in larger d spacing of 30 or 43 Å due to the formation of thermally stable intermediate in case of divalent M2+ . Although, organo modification of clays increases the cost, still clay remains relatively cheap feedstock with abundance on supply.

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2.4.3 Factors Affecting the Organoclay Hybrid Formed The ultimate properties of resulting modified clay/polymer nanocomposites are governed by the several key factors which affect the delamination of clay. In the previous section, we have mentioned about alkyl ammonium salts being used for the organo-modification of clay having general formula CH3− (CH2 )n−NH3 + , where n lies between 1 and 18. Lan et al. investigated the impact of chain length of ammonium ions on the resulting structure of the nanocomposites [190]. Alkyl chains having more than eight carbon atoms lead to the formation of exfoliated nanocomposites, whereas shorter chain lengths end up with intercalated morphology. It was observed that the same organic salt could not exchange with the clay containing divalent counter cations like Mg2+ or Ca2+ in MMT [191]. The cation exchange capacity of clay, chemical nature of the modifying cations, the polarity of the reaction medium are different key factors affecting organoclay hybrid’s characteristics. By modifying surface polarity of the clay, onium ions help in the insertion of polymer precursors in the interlayer gallery. The use of block or graft copolymers is another alternative route to achieve favorable interaction between clay and polymer [190], which is very similar to the method of compatibilization of polymer blends. The block copolymers are comprising of one clay compatible hydrophilic block, and another polymer compatible hydrophobic block act as a bridging component between polymer and clay. Silane coupling agents can also be used with onium-ion treated organoclay for modifying clay-polymer interaction.

2.4.4 Modification of Nanoclay Although melt blending coupled with solution blending Fabrication technique and Structural and in situ polymerization has been the primary method to synthesize nanostructured elastomeric frameworks, new age techniques such as click chemistry and controlled polymerization are also proving to an emerging synthesis route in this context [192, 193]. Every process usually aims to achieve a uniform dispersion of Nanoclay in the polymer matrix. While the solution is blending yields in a better dispersion because of the low viscosity system, melt blending is considered more industrially viable and environmentally benign. In situ polymerization, along with the other fabrication techniques, is used mostly to introduce a specific functional property into the polymer system. The following section describes the ways of preparing elastomeric nanocomposites based on Nanoclay [193, 194].

2.4.4.1

Solution Blending Method

The polymers and the prepolymers are made to be dissolved in a solvent, which induces a swelling behavior within the clay microstructures [195]. A soluble polymer

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solvent is taken, which causes the clay layers to exfoliate, and the polymer chains can then place themselves within the clay interlayer [196, 197]. Introducing entropy gained process by removing the solvent molecules aids the intercalated sheets to reassemble, resulting in the formation of clay reinforced nanocomposites. Apart from external agitation such as stirring and shear mixing, ultrasonication has turned out to be an exciting process, especially in the case of thermoplastic Elastomers. Contemplating several aspects such as the reaction time, yield, efficacy, and clay dispersion, ultrasonication proves to be a better candidate than the rest of the agitation techniques [198]. The composite prepared from the introduction of Nanoclay to synthesized polyglycidyl methacrylate (GMA) via ultrasonication proved that there was enhanced intercalation of the filler within the matrix, which resulted in the elevation of the thermal stabilities as compared with the control based on shear mixing [198, 199]. Exfoliated rubber nanocomposites are one of the prime areas where Nanoclays find their usages in modern times of scientific realms. López-Manchado et al. had inferred that solution blending sequels in a better amount of bound rubber and improves the compression set of the system [199]. On the same note, the preparation of ethylene octene copolymer based on MMT clay resulted in the increase of tensile modules by 63% and the tensile strength by 44% when processed via solution blending method. Recent advents in the area of processing have to lead to the utilization of volatile organic separator solvents, which develops a stable state composite and the melt compounding temperatures alleviating the handling issues during processing and compounding [199]. Solution mixing also allows easy incorporation of various secondary features into the polymer system, such as magnetic properties or dielectric properties. For instance, excellent distribution of graphene nanoplate structured thermoplastic Elastomer showed an amplified electrical property owing to the coherent polymer-filler interactions [198, 200]. Polystyrene and polycarbonate modified composites are one of the trended polymeric matrixes to incorporate filler because of the variety of properties it exhibits and the range of applications in which it may be employed. The experiment by Lago and his workers stemmed from a polycarbonate reinforced composite which employed solvent exchange to redisperse the fillers in 1,3-dioxolane (which is an environmentally begin solvent) for high-end 3D printing application [201]. Abbasian et al. had further investigated the processing paths of developing strengthened polymer/clay nanocomposites using a combination of nitroxide-mediated radical polymerization and solution blending methods (Fig. 2.13) [202]. Although solution blending provides several advantages, there are various drawbacks to the above system. The primary concern is the low selectivity rate of the process, which means that the intercalation works for a specific set of polymers and nanofillers [199–202]. The environmental concern is a big issue with the method. Using common solvents creates various sustainability issues. Furthermore, shifting to a more environmentally suitable solvent shall elevate the price of the process.

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Fig. 2.13 TEM images of the polymer/clay nanocomposites using a combination of nitroxidemediated radical polymerization and solution are blending methods. Reproduced with permission from [202]

At a commercial level of preparing thermoplastic elastomeric nanocomposites, melt blending is always preferred over solution blending owing to its accessible processing parameters and the excellent sustainability support which it provides.

2.4.4.2

Melt-Blending Method

The implementation of temperature above the polymers’ softening point in the presence of inert gas is one of the crucial characterizations of the melt blending process. Although the yield is not so as high as in the case of the solution blending, since it involves an infinite number of operating steps, the process is synchronous with the industrial requirements [198, 203–206]. The absence of the solvent deprives the required interactions between the polymer and the filler complex, thus effectively reducing the dispersion capacity of the filler in the host polymer matrix [204]. Moreover, an infinite number of process parameters come into the picture, including temperature profile, exit pressure, feed rate, screw speed, motor power, and oxidative environments. Three crucial factors cater to the blending process to take place-The the enthalpy of the molecules, the increased interlayer spacing between the interface, and the diffusion time for the center stacked layers [198, 205, 206]. Vassiljeva et al. processed styrene-co-acrylonitrile (SAN) reinforced with carbon nanotubes (CNTs) using the reactive melt blending method. The improved tensile strength and the modules proved that this method might be used as a potential tensile strength upbringing strategy [207, 208]. Thermoplastic polyurethanes strengthened with Nanoclay, as studied by Ercan and his team, resulting in an increase in the mechanical properties when they had

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prepared their sample using melt blending than using solution blending [208]. On an additional note, the fabricated Elastomers also exhibited creep resistance and an augmented gas permeability resistance [208]. Static melt blending is usually carried out in a vacuum coupled with an applied temperature of 50° centigrade above the transition temperature without the use of any mixing supplements. However, dynamic melt blending operates under an inert atmosphere with an internal mixing process. An ideal mixture should be one with which can control both the residence time and the temperature profile profoundly, especially in concerns involving heat-sensitive materials. Nowadays, the melt blending process involving the incorporation of the supercritical CO2 is being used to aid and accelerate the exfoliation process of the Nanoclays. This process, as employed by Quigley and his research team on a nylon 6 matrix showed to have an upgraded polymer-filler interaction influencing the mechanical properties to rise [209]. Rubber filler dispersion turns out to be an arduous job to do with melt blending as one needs to tune a high-temperature phenomenon, which may result in the unwanted oxidation of reactive species and consumption of more energy. With optimized process parameters and with the introduction of various advanced processing parameters, the industry still selects melt bending to be its ideal choice for fabricating Nanoclay reinforced thermoplastic Elastomers.

2.4.4.3

In Situ Polymerization

The disadvantages of dispersion both in solution mixing and melt bending can be overcome by using the in situ polymerization, which does not require the thermodynamic regulations associated with the polymer matrix [210]. For a tailor-made composite synthesis, which needs the exhibition of a particular feature, the in situ technique very helpful as it allows to polymerize with preloaded molecular designs (Fig. 2.14) [211]. The novel poly (2-ethyl-2-oxazoline)/Nanoclay composites developed by Ozkose et al. used a ring-opening polymerization, which initiated the exfoliation process with the clay interlayers, thus providing a reinforced intercalated composite [212]. Herrero et al. have also studied the influence of the shape and the area of Nanoclays using the in situ polymerization technique. The results indicate that the needle-shaped clay induces the higher molar mass and enriched mechanical properties with a wide range of versatility as compared to the conventional layered structure [213]. The preparation of Nanoclay modified PMMA matrix synthesized via in situ polymerization displayed a mixed intercalated morphology essential for the majority of the optoelectronic devices [214]. Further modifications involve ultrasound-assisted in situ emulsion polymerization method, which has reported providing with high mechanical properties and thermal degradation behavior. Owing to the micro-convection resulted from ultrasound and cavitation, the interaction between the functional groups of the polymer along with the activated clay particle enhanced yielding a highly exfoliated structure [212].

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Fig. 2.14 Visualization of the use of montmorillonite-intercalated metallocene catalyst to reinforce ethylene and 10-undecen-1-ol matrix. Reproduced with permission from [210]

The nickel-based catalyst system in the Nanoclay system tunes the 1, 4 bonds of Polybutadiene selectively while polymerizing. The rubber Nanoclay complex developed from the composite reinforced on cis-1, 4-Polybutadiene/MMT demonstrated excellent mechanical features such as tear resistance and tensile strength [213].

2.4.4.4

Controlled/Living Polymerization

The in situ strategy involves swelling of the clay layers before getting exfoliatedwhich at times becomes a time-consuming process [215]. The surface-initiated controlled/living radical polymerization technique uses a faster method of initiation technique aided by an external stimulus or a catalyst by which the intercalated microstructures of the polymer composite can develop [215, 216]. Not only is the layer distribution of the clay can be controlled, but the process can also maneuver several other parameters such as the molecular weight and the topology. The controlled polymerization, unlike the conventional free radical polymerization, proceeds in a more regulated way, thus canceling out the chances of the formation of unnecessary side branches [216]. The process is proven to yield products that can be used in a variety of matrices including in glass-reinforced polymer composite materials. The initiators that are used in this synthetic process are in their static states held between the clay layers by replacing the ions present within them. Modifications of this technique have opened up various allied strategies to obtain a premeditated composite such as nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization, ring-opening polymerization (ROP), ringopening metathesis polymerization (ROMP), living anionic polymerization and living cationic polymerization. The emerging trend of devolving core-shell/polymer nanocomposites along with the brush polymer nanocomposites makes use of this

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strategy, which enables us to gain several advantages with regards to the structural medications in a polymer system [215–217]. The works of Utama et al. reflects the use of inverse miniemulsion periphery RAFT for delving highly structural polymer/nanofiller hybrids principally for the watersoluble drug delivery application [218]. The review drafted by Beyazit et al. lists the possible advantages and disadvantages of using a surface-initiated controlled/living radical polymerization technique. Although the strategy has several disadvantages, the method still emerges to be one of the best methods to develop polymer composites that require a post-polymerization functionalization [219].

2.4.4.5

Controlled Radical-Mediated Photopolymerization (P-CRP) and Click Chemistry

Both the above techniques are comparatively newer strategies employed to develop application-specific polymer nanocomposites using clay as the nanofiller. Light is one of the exciting sources to synthesize new routes for nanocomposites fabrication because of its cheap cost and easy availability [220, 221]. Intramolecular photochemical processes and photoredox processes are the two such kinds of routes by which any Photopolymerization proceeds. Apart from the traditional advantages, it provides such as enabling the fabrication of photoresponsive gel, the method also allows a better yield even at a low temperature. Xie et al. had developed a nanocomposite using poly triacrylated siloxane and cetyltrimethylammonium bromide-modified MMT layers as the matrix and the filler, respectively [222]. As an evolutionary note, as the concept of click chemistry gradually startled exploding, the researchers began implementing the same in the field of polymeric nanocomposites. The wavelength of the light can be tuned accordingly to synthesize a new range of composites that are responsive to light as a stimulus [223]. The photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) process was used to fabricate polymers that had a streamlined tacticity, dispersity, and molecular weight as compared to the solution mixing or melt blending [223]. Modifications are now being made with the advent of click chemistry. For a nanocomposite system, reactions such as the cycloaddition, Diels-Alder, and thiolene reactions proceed when the polymer-filler counterpart has the reaction-active functional groups. The click chemistry enables the product to be more robust as it establishes a strong chemical bond with the functional groups and incorporates various photophysical properties into the system. Zhang and his research group developed a novel polyfluorene hybrid using halloysite Nanoclay via click chemistry, which enabled them to incorporate side chain functionality into the system [224]. The development of stimuli responsiveness composites, bio-active composites, and nanostructure composites for drug delivery are favorable using click chemistry as it uses low temperature to undergo critical reactions. The latest hybrid development of butadiene rubber (NBR)/attapulgite complex via click chemistry by Pan et al. has

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Fig. 2.15 Surface modification and the possible mechanism of the exfoliation for the butadienebased rubbers and the thiol-modified attapulgite. Reproduced with permission from [225]

broadened the scope of fabricating nanocomposites using the thiol-ene interfacial system even at an industrial scale (Fig. 2.15) [225].

2.4.5 Properties and Applications Thermoplastic Elastomers, reinforced with Nanoclay, finds a diversified application owing to its diversified property variation. The superior engineering properties, coupled with the enhanced structural integrity, make them one of the robust composite materials to be employed at an industrial scale. Approximately (75–80) % of the claybased nanomaterials market is shared amongst automobile, and aeronautical industry and the area of the application still seems to spread out as companies are investing billions of dollars every day in this sector [226, 227]. The works of Trung and Geng investigated the cyclical loading and unloading behavior of flexible thermoplastic polyurethane (TPU)/nanoclay composite foams under the ambient environment. The properties of stress softening, hysteresis and strain deformation were analyzed under a differential loading of the clay amount [228]. As the compression strain was gradually increased from 20 to 60%, the inelastic behavior of pristine thermoplastic polyurethane foams became more prominent. With the incorporation of the Nanoclay, the residual strain and the stress softening of the composite upgraded significantly. The microscopic analysis revealed that

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the filler dispersion in the polymer matrix effectively supplemented the enhancement in the permanent deformation of the cell wall. Thus, Nanoclays proves to be an excellent material to tune the inelastic behavior of thermoplastic elastomeric compounds and can be an attractive building block to fabricate parts and designs for automobile industries (Fig. 2.16) [228]. A detailed study of the mechanical properties was carried out by Crespo et al., who varied the Nanoclay contents in a thermoplastic polyurethane matrix from 0.5 to 10 wt%. The Cloisite based Nanoclay was dispersed by melt blending, and the composite showed a 28% increase in the stress and a 35% enhancement in the strain at break. Furthermore, the 3% weight induced elastomeric matrix showed the optimized results with an 88% increase in the energy value as compared with the neat elastomeric matrix without the Nanoclay [229].

Fig. 2.16 TEM images of the nanofiller incorporated into the thermoplastic elastomer matrix, while a represents the solid section, whereas, b–d represents the transverse sections. Reproduced with permission from [228]

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Thermoplastic elastomers are also being used to alleviate the impact sound in the building. In this context, Auad et al. had fabricated a laponite clay-based thermoplastic polyurethane composite, which demonstrated competing for features as an analyte in floating floor technology with its other counterpart [230]. The introduction of the cay particle mainly involves the amplification in the viscoelastic properties of the overall matrix. Further investigations suggested that (5–10) wt% of the clay simultaneously reduces the dynamic stiffness and increases the damping behavior. The results from the viscoelasticity tests were aligned with the Cremer-Ver’s model, which concluded that the impact sound insulating property at the resonance frequency was improved when the nanocomposite made from this laponite based clay was used as the viscoelastic layer supporting the floating slab [231]. Jana and his coworkers developed a reactive Nanoclay based composite material, which exhibited a 100% increase in the tensile strength along with optical clarity over pure thermoplastic polyurethanes [231]. Organically modified layered silicates can significantly improve the properties of the neat metal grade thermoplastics. A polarizing optical microscope was used to study the morphological characteristics of the matrix loaded with the nanofiller [232]. As the process was carried out via melt blending, the shear stress during the mixing resulted in the efficient rubber-filler integration. The organoclay had enhanced the complex viscosity and the dynamic storage modulus of the system due to the amalgamation of the interphase formation between the hard and the soft segment, and the excellent dispersion of the organoclay [232]. Naderi et al. used a combination of Nylon 6 and Nitrile Rubber to prepare a hybrid composite reinforced by Cloisite clay. The mixing behavior was found to be excellent when investigated under SEM, as it showed an exfoliated nature of the clay layers within the polymer chains [233]. Thermoplastic Elastomers also are one of the promising candidates in the field of shape memory polymers. The elastomers based on polycaprolactone (PCL) diol, methylene diisocyanate, and butanediol were prepared using the bulk polymerization after incorporating the same with reactive Nanoclays [233]. The melting point of the soft segment formed the transition temperature required for any memory actions. The shape recovery and the shape fixity tests were carried out, and the results yielded a 20% increase in the shape recovery stress. The factors of reduced soft segment and the clay content can be maneuvered to induce an optimum tensile strength within the composite. Although we have outstanding progress in the field of thermoplastic elastomer/Nanoclay based composite materials, a better understanding is still required for the materials which are employed in high-performance materials. The market of clay-based nanocomposites shall increase to about 3.2 million tons by the end of this year and shall continue to evolve at a cost rate of 15 billion per year [198]. Apart from the traditional application area of health, food packaging, and environmentally benign materials, these composites hold the potential to be employed in a number of different other perspectives of employment.

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2.5 Carbon Nanotube (CNT) The carbon nanotube is one of the modifications of the graphite sheets in which the sheets are rolled into a tubular structure [234]. These can be classified into singlewalled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multiwalled carbon nanotubes (MWCNTs) depending on the number of times they have been rolled (Fig. 2.17) [234, 235]. The nanotubes thus formed, can be characterized by a chiral vector and a chiral angle supplemented by the following equation Ck = na1 + ma2 where Ck is the chiral vector n, m are the integral number of steps along with the zigzag carbon bonds of the hexagonal lattice, and a1 and a2 are unit vectors. The chiral angle of 0° and 30° results in two limiting cases of the geometrical structures in the case of carbon nanotubes. Various factors define the property of carbon nanotubes, such as the atomic arrangement, surface morphology, and aspect ratio [237–239]. The blend of high flexibility, along with low mass density, has grasped the attention of these nanofillers since the time of its advent in 1991 [238]. Further modifications have enabled nanotubes to exhibit mechanical properties better than steel, electrical conductivity higher than copper, and volume to mass ratio advantageous than aluminum [236, 240–242]. Single Wall based carbon nanotubes can exhibit tensile modulus ranging from 640 GPa to 1 TPa and tensile strength within the focus of 140 GPa. The superior electrical property usually results from the large phonon mean free path lengths [243, 244]. The ability to be compatible with a variety of polymer matrix tunes them to be one of the most versatile nanofillers in the carbon-based nanofiller industry [245]. Polymer host matrix such as epoxy, polyvinyl acetate (PVA), polyvinylchloride

Fig. 2.17 Illustrative representation of the different types of carbon nanotubes, accessed from [236] on 30/01/2020

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(PVC), polyethylene (PE), polyamide (PA) has already been tried reinforcing with carbon tubes. Following the trend, the researches based on the carbon nanotubes has witnessed an exponential increase in its progress. The works based on carbon nanotubes reinforced elastomeric composites have recorded a nine folds increase in terms of both research papers and patents, taking into consideration the previous two decades. Thermoplastic elastomer, which is strengthened with carbon nanotubes are one of the trending research perspectives in modern times owing to its optimized structural, electrical, and electronic property and behavior [246, 247]. The durability and the strength thus obtained by incorporating a thermoplastic elastomer with nanotubes can be comparable with ones that use carbon black as the reinforcing agent [248]. This chapter aims to summaries the synthesis routes along with the structural medications of nanotubes considering elastomers as the matrix. Moreover, we have also discussed the latest application area where carbon nanotubes based thermoplastic elastomer are currently being employed.

2.5.1 Structure and General Properties of Carbon Nanotubes The nanotubules are rolled-up sheets of graphene composed of benzene type hexagonal rings of arranged carbon atoms [249]. As compared to graphene sheets, which are derived from honeycomb lattice abstracted to be a single layer of graphite, a carbon nanotube is set of graphene sheets rolled up to form a concentric cylindrical structure [250]. Each of the nanotubes comprises of millions of atoms along the cylindrical length, which may range up to ten micrometers and diameters as small as 0.7 nm, resulting in a high aspect ratio. Owing to this high length to diameter ratio, the nanotubes are usually classified as a one-dimensional nanomaterial [249]. Multiwalled carbon nanotubes are those who have several layers of concentric cylinders so as to their outer diameter may go up to 15 nm. Structures, which have an outer diameter than 15 nm, are generally not classified as tubes, but as carbon nanofibres [249]. Reinforced on these basic structures, the carbon nanotubes are further categorized into armchair carbon nanotubes, zigzag carbon nanotubes, and chiral carbon nanotubes according to the rolling up pattern of the graphene sheets. The relaxation of the rolling axis relative to both the hexagonal network and the inner radius of the cylinder allows the fabrication of these several models of nanotubes. As indicated, the chiral vector is responsible for the positioning orientation of the nanotubes as ‘zigzag’ or ‘armchair,’ as the vector indices determine the two directions of the honeycomb crystal lattice of graphene (Fig. 2.18) [250, 251]. The crucial reason for which these tubes can be employed as fillers is because of their strong carbon-carbon bonds. With a wide range of functionalities, the nanotubes are reported to exhibit a simultaneous enhancement in the domains of mechanical, thermal, electrical, and electronic properties [252]. Supplemented by a low density

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Fig. 2.18 Visual representation of the different conformation of carbon nanotubes, accessed from [251] on 30/01/2020

(1.3 g/cm3 ), these nanofillers exhibit young’s modulus of 1 TPa, which is approximately five times higher than that of the steel. In a more specific case, where the fillers are modified with other complementary agents, these particles exhibit tensile strength up to 63 GPa [252, 253]. With a resistive nature towards the harsh environment, the electrical properties of nanotubes may be compared with that of copper. The rolling actions break the symmetry of the original planar structure and assume new directions imbibed both by the hexagonal lattice and the axial symmetry. This allows manipulating the nanotubes into completely conducting filler or semiconductor type filler by bringing in the needed change in the symmetry. The behavior exhibited by these fillers when loaded onto a matrix ranging from metals (like copper) and semiconductors (like silicon) develops various novel pathways to systemize variants of active and smart nanocomposites employed as sensing devices, fuel cells and scanning probe microscope [254].

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2.5.2 Synthesis Routes to Fabricate Carbon Nanotubes Iijima and his group were the first to develop the multi-wall nanotubes via the arc discharge method. Since then, various scientists have investigated various ways to develop the variations of carbon nanotubes with enhanced physicochemical properties [241, 255]. In 1993, the same group made a breakthrough by discovering singlewalled carbon nanotubes using arc discharge and supplemented by a metal catalyst [256]. This section reviews the fabrication techniques of the carbon nanotubes and the characteristic features the flourished material has.

2.5.2.1

Arc-Discharge

As stated earlier, the technique has a long-standing relationship with the inception of carbon nanotubes following the trend of Iijima from the year 1991 [255]. The structures they reported were based on a needle like tubes framed upon finite carbon structures prepared by placing them on the negative end of the direct current modified arc discharge apparatus [257]. The apparatus still proves to be the basic layout of the modern arc discharge methods containing two vertical thin electrodes at the centre of the chamber. A direct current of 200 A at a potential drop of 20 V across the two electrodes was employed to initiate the process of synthesis [257, 258]. All the three chemical moieties argon, iron, and methane were critical for the synthesis of SWNT. Usually, the cathode has a shallow dip carved in it to accommodate the iron during the process of evaporation [258]. The first formed nanotubes thus had the mean diameter on 1 nm, with the range spreading between 0.7 and 1.65 nm. Bethune et al. had modified the initial set up to fabricate the anode with bored holes brimming with pure powdered metals like iron, nickel, and cobalt [254]. An implementation of the helium reactor was introduced by Journet and his group to develop carbon nanotubes on an industrial scale. The applied pressure of 660 mbar along with a current discharge of 95–105 A makes the process of arc discharge one of the most widely implemented technique to synthesize carbon nanotubes [257]

2.5.2.2

Laser Ablation

Just after the success of arc discharge, in 1998, a group of scientists headed by Smalley developed superior properties of single-walled carbon nanotubes using the laser ablation process supplemented with vaporization on graphene at 1200 °C [258]. The nanotubes continue to grow until there is a limitation initiated by the agglomeration of the catalysts at the end of the tube. Although the process consumes a higher amount of energy, the method allows framing novel nanotubes with a catalyst particle at the end of the tubes. However, the entanglement of the nanotubes amongst themselves requires one more step to separate them, resulting in the overall efficacy to go up, despite having a high overall yield.

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Fig. 2.19 The chemical vapor deposition to fabricate carbon nanotubes a tip growth model, b base growth model. Reproduced with permission from [173]

2.5.2.3

Chemical Vapor Deposition

The two crucial drawbacks popping us from the above two methods-scalability and structural reinforcement may be overcome by the more robust synthetic technique based on chemical vapor deposition [259]. The chemical vapor deposition allows a controlled and ordered growth pattern of the nanotubes, making it as one of the competing fabricating methods in this world of emerging technology. A reaction chamber is sealed with a mixture of hydrocarbon gas, acetylene, and methane/ethylene/nitrogen; and is subjected to temperature gradient ranging from 700 to 900 °C. At elevated pressure, the nanotubes are formed at a relatively lower temperature, which is a potential benefit both from the economic and safety point of view. Although the quality of the product may turn out to detrimental at times, the deposition of the catalyst on the matrix can yield the nanotubes to exhibit various novel properties (Fig. 2.19) [260, 261].

2.5.2.4

Hydrothermal Method

Although we have various methods to fabricate Carbon Nanotubes, the hydrothermal method provides an edge over the other techniques by being environmentally benign.

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Moreover, the process proves to be cost-efficient, along with giving the scope to reduce the free energies for various equilibria [262–264]. The supercritical water having a high diffusivity in the pressured state, along with the optimal density and low viscosity allows the fabrication of various exotic chemistries. Yoshimura and his coworkers developed carbon coatings (>200 °C) from the hydrothermal process by exposing the reactants at a relatively low temperature coupled with moderately applied pressure (10–100 MPa), defining the feasibility of the process [262]. Gogotsi et al. had developed both the open-end and closed multiwall carbon nanotubes via the hydrothermal method, with the wall thickness ranging from a few to more than a hundred carbon layers. The research group used a polyethylene/water mixtures with a nickel catalyst and the reaction temperature of 700–800 °C under 60–100 MPa pressure. Their results reveal that the wall thickness of the nanotubes developed by the hydrothermal process was relatively small (about 10% of the inner diameter whose dimension was 20–800 nm) [263]. However, the Raman microspectroscopy, along with the high-resolution TEM, concluded that the nanotubes had a well-ordered structure. Although the method requires a comparatively higher reaction condition, the environmental suitability of the process along with using water as the reactant medium still makes the route competitive in the realms of current carbon nanotube fabrication methods [263, 264] (Fig. 2.20).

2.5.2.5

High Pressure CO Disproportionation

The process is an advanced gas phase synthesis for the production of single walled carbon nanotubes (SWCN), which incepted in 1998 (Fig. 2.21) [265]. The nanotubes develop in a high pressure chamber, which involves the flowing of carbon monoxide (CO) on the catalytic clusters of iron. The catalyst is generally developed in situ by thermal decomposition of precursor agents like iron pentacarbonyl. When applied with heat, the iron pentacarbonyl decomposes into atoms, which are then further made to condense, resulting in the formation of relatively larger clusters of the catalyst. A Boudouard reaction follows, which yields in the nanotubes to nucleate and grow on the clusters via the ‘CO disproportionation’ [265]. CO + CO → CO2 + CSWCN (in the presence of Fe catalyst cluster) The process is advantageous from the prospect that it does not require the catalysts which need to be fabricated previously (unlike all the CVD methods). Moreover, the process is a continuous one rather than a batch process, elevating the efficacy of the product generation rates [265]. However, the Boudouard reaction inherently possesses a low rate of conversion (1 of 15,000 CO molecules entering the reactor is converted into nanotubes, even at 30 atm pressure), which proves to be the only disadvantage of the process [265].

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Fig. 2.20 TEM profiles of typical carbon nanotubes produced by hydrothermal treatment of polyethylene at 800 °C for 2 h in the presence of 3% Ni powder. The PE/H2O ratio was 1.6. a End of the nanotubes, b the graphite fringes c, d lattice fringe images. Reproduced with permission from [263]

Fig. 2.21 Layout of the second-generation high pressure CO disproportionation producing the single walled carbon nanotubes. Reproduced with permission from [265]

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2.5.2.6

2 Anisotropic Nanofillers in TPE

Other Methods Including Gas Phase Synthesis and the Reverse Micelle Method

Gas Phase synthesis supplemented by a metal catalyst involves the embedding of the catalyst on the surface [266]. The gas phase is implemented to incorporate the catalyst along with the hydrocarbon, which then follows a catalytic reaction pathway in the same state. The nanotubes, thus formed, are free from the catalyst. Thus particularly yield two major advantages, the first one being, there is no need to refine the formed nanotubes, and secondly, the process can be operated continuously since the catalyst remains in the system [266, 267]. The sol-gel method is another promising way to establish nanotubes, which employ chemically modified dried silicon oil. The process leads to the development of highly oriented crystals, which can attain a length of 2 mm when the growth time is elongated. Not only can a variation in the length of the nanotubes be maneuvered with this system, but the usage of the same substrate also elevates the efficiency of the process [261]. The reverse micelle method uses molybdenum and cobalt catalyst nanoparticle dissolved in an organic solvent in presence of a surfactant. This quasi-colloid like structure can be injected to a furnace. The elevated temperature stimulates the solvent to vaporize immediately, leaving behind the carbonaceous remains, which then may be removed and collected by passing a hydrogen stream. The framework of the organic solvent vapor acts as the carbon source, while the metal nanoparticle acts as the catalyst, proving the process to be one of the quickest route to develop delicate nanotubes [268]. The choice of the substrate and the catalysts are one of the most important aspects which determine the structure of the nanotubes. Although in the majority of the cases, silicon wafers are used as the substrate, alumina and glass are also amongst the promising materials to be employed as the substrate [269]. The catalysts are selected according to the choice of the substrate. For instance, using silicon wafers shall obviously lead to the selection of metals like iron, nickel, or cobalt, which then may be embedded on the substrate by solution, electron beam evaporation, or by physical sputtering. In particular, in the application, which requires minute attention to catalyst particle size, the catalyst depositing technique plays a pivotal role in shaping the tube to be formed. Pieces of literature claim that porous silicon/catalyst complex is one of the excellent materials to develop self-oriented nanotubes [269]. The presence of the Van Der Waal’s force between the catalyst and the substrate along with several associated forces pushes the nanotubes to be formed either parallel or perpendicularly to the surface of the substrate

2.5.3 Purification and Dispersion of Carbon Nanotubes A majority number of the commercially viable method contains a lot of carbonaceous impurities and metal catalysts which need to removed to enhance the filler efficacy.

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Apart from the chemical impurities such as graphite sheets, carbon soot, and a metal catalyst, there are various structural defects such as the dangling bonds, and loose ends that are very often associated after the extraction of the carbon nanotubes. The widely used industrial tools to separate the impurities are provided in a tabular form below (Table 2.2). Pure mono-dispersed single walled carbon nanotubes are very challenging to fabricate even in this advanced era of science [276]. Single walled carbon nanotubes owing to their chiral coordination can exhibit a gradient behavior in their conductivity. However, the strong Van Der Waal’s force restricts the chiral vector to orient correctly, and instead lead to the formation of bundles [277]. Hence, disaggregation and uniform dispersion are two of the most potent factors responsible for the robustness of the developed nanomaterial. While the non covalent approach for improving the dispersion seeks to change the surface energy of the particles by mechanical means, the covalent process uses chemical means to increase their compatibility, which in fact induces an enhancement in the adhesion properties, thus alleviating the problem of aggregation [276–278]. Although, the modifications via the physical means are generally since the usage of a high concentration of chemicals, in case of covalent means may result in unwanted oxidization of the nanofiller; a chemical means of functionalization yield a better product in comparatively less time.

2.5.4 Functionalization of Carbon Nanotubes Even after purification of the pristine nanotubes, we often find the reinforcing properties are not achieving a significant value, primarily due to the weak interfacial interaction coupled with the limited Van der Waals interaction between the CNT and the polymer matrix [279]. Moreover, the dispersion of these nanotubes (with a small diameter in the nanometer scale and high aspect ratio) is slightly tricky when compared to the traditional nanofillers, which are spherical, majorly owing to the difference in the surface areas of the respective systems (Fig. 2.22) [279, 280]. Thus, a post-processing operation involving functionalization is always followed after the synthesis has been carried out to modify the surface behavior of the carbon nanotubes.

2.5.4.1

Covalent Functionalization

The carbon nanotubes are synthesized with high reactive end caps (highly curved fullerene-like hemispheres) as compared to the side walls. Moreover, the sidewalls comprise multiple numbers of defects and vacancies, such as Stone-Walls defects and sp3-hybridized defects [281]. Chemical functionalization focuses on using the advantages of the covalent bonds of the functional groups. It integrates with the ends caps of the carbon nanotubes or the side walls defects, as discussed. The functionalization at the sidewalls involves a transition of the hybridization state from sp2 to sp3 by making the nanotubes react with a chemical moiety, thus resulting in the

The nanotubes are placed in a suitable Separates tightly agglomerated agent such as distilled water or chemical and physical moieties from toluene and is exposed to sonication the carbon nanotubes surface for 30 min

The process is placed in a centrifugation chamber with 7000 g of samples for 30 min to 3 h depending on the sample quality and the number of cycles

Ultrasonication

Centrifugation

Have the capability to remove nanospheres, metal nanoparticle, and other carbon particles

The carbon nanotubes are exposed to The tip of the nanoparticle is cut-off The process is comparatively costly 0.2 M nitric acid with a potential drop and converted to the hydrophobic state and difficult to incubate of 2 V and at a scan rate of 50 mV s−1 to the hydrophilic state, generating optimal polarity required for the enhanced dispersion

Electrical oxidation

[270]

References

With an increase in the number of centrifugation spins, the yield goes down

The separation technique is highly dependent on a variety of factors such as the surfactant, solvent, and reagent used

(continued)

[274]

[273]

[272]

Similar to the previous case, use a [271] slightly concentrated acid solution may result in the oxidation of the carbon nanoparticle

The treatment not only removes the metal traces but also the fullerenes

The system is refluxed along with nitric acid or hydrochloric acid for (4–48) hours

At times, it oxides the carbon nanotubes introducing unwanted functional groups in the system

Major disadvantages

Acid treatment

Major advantages Carbonaceous compounds are removed effectively

Brief overview

Gas phase oxidation Chemical oxidation process in the gaseous phase with air, oxygen, chlorine or water

Process name

Table 2.2 A comprehensive overview of the various techniques employed for the purification of carbon nanotubes

56 2 Anisotropic Nanofillers in TPE

Brief overview

Major advantages

Magnetic separation The nanotubes, along with the The method has the potential to impurities, are dissolved in a remove all the catalyst materials and suspended surfactant. An external small inorganic particles agent, mainly Zirconium dioxide, is added to make the nanoparticle susceptible to a magnetic field. A sonication process s further employed to separate the impurities which get trapped in the magnetic poles

Process name

Table 2.2 (continued) The number of steps to execute the process makes the system less efficient and time-consuming

Major disadvantages [275]

References

2.5 Carbon Nanotube (CNT) 57

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Fig. 2.22 A concise representative image of the functionalization of carbon nanotubes. Reproduced with permission [280]

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loss of the p-conjugation system on the layer of graphene [279, 281]. One of the most excellent methods used to functionalize the carbon nanotubes is by fluorination [279]. As the C-F bonds are weaker in the nanotubes as compared to that in the alkyl fluorides, various functional groups, including hydroxyl, amino, and alkyl, can be used to replace the fluorine atoms in the end caps of the nanotubes. Other techniques include cycloaddition reaction using agents such as carbene, nitrene, chlorine, bromine, hydrogen, and azomethine ylide [279]. The functionalization in the sidewalls of the carbon nanotubes can be developed by oxidizing with strong acids, which shall leave holes in the structure complemented by oxygenated functional groups. For instance, on reacting these sidewalls with strong acids (HNO3 , H2 SO4 or a mixture of both) or oxidants (KMnO4 , ozone, plasma), the tubes open up and tunes to be functionalized with reactive groups such as carboxylic acid, ketone, alcohol, and ester groups [279, 282]. The functionalization via introducing multiple functional groups enhances the polarity, and the solubility of these carbon nanofillers, thus aids in the process of dispersion, especially when the matrix is polar/hydrophilic. However, during the operation of the reaction, especially with strong chemical reagents, the nanotubes often gets degraded or decomposed into smaller fragments, deteriorating not only the overall mechanical property of the filler but also the π electron and the electrical/transport properties of the system [279]. Moreover, often the chemicals and the reagents used to oxidize the sidewalls are not environmentally benign, which is also a source of concern. Recent developments have shifted to the eco-friendly methods of functionalization using ionic liquids (ILs) and poly (E-caprolactone) (Figs. 2.23 and 2.24) [283, 284].

2.5.4.2

Non-covalent Functionalization

In order to preserve the conjugated system of the carbon nanotubes’ sidewall, the research trends shifted from the covalent functionalization to non-covalent modification, which preserves the chemical property and the structural integrity of the nanotubes [279, 285]. The functionalization is usually achieved with the introduction of aromatic compounds, surfactants, and polymers via hydrophobic interactions, electrostatic interactions, and the π-π stacking. A short table is provided, which lists the major techniques to functionalize carbon nanotubes covalently (Table 2.3) [279]. The uses of surfactants to modify the surfaces of carbon nanotubes are also one of the prevalent techniques to functionalize the graphene derivative. The surfactants behave as solubilizers, which aid to disperse the nanotubes via physical interaction [286]. Moreover, the adsorption results in the lowering of the effective surface tension of the nanotubes, thus are preventing the framework from agglomeration. However, the effectiveness of the surfactant induced functionalization highly significantly depends on the structure of the surfactants along with the medium in which it is dispersed. On an additional note, although the process is economically beneficent, the surfactants are usually permeable to the plasma membrane, which limits the application of these functionalized materials in biomedical realms [286]. To address the issues of biocompatibility, modern techniques use several biomacromolecules,

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Fig. 2.23 Green Functionalization of single-walled nanotubes in ionic liquid. Reproduced with permission from [283]

Fig. 2.24 Green functionalization of multi-walled nanotubes with poly (E-caprolactone). Reproduced with permission from [284]

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Table 2.3 The major functionalization strategies of carbon nanotubes using non-covalent interactions [1] Research group

Year

Method

Dai et al.

2001

Immobilization and integration of biological molecules onto CNTs

Hecht et al.

2006

photoinduced electron transferring using CNT system having zinc porphyrin derivative

Hu et al.

2008

Self-assembling the pyrene-functionalizedCdSe nanoparticles on the surface of the Carbon Nanotubes

Zhao et al.

2009

Coating of N-succinimidyl-1- pyrenebutanoate on the surface of the Carbon Nanotubes

Barone et al.

2006

Solubilization of CNTs with a biological component

Zhao et al.

2006

π-π stacking and van der Waals interactions (polymer wrapping) between the carbon nanotubes and poly(m-phenylenevinylene)co-(2,5-dioctoxy-p phenylene) vinylene (PmPV)

including simple saccharides and polysaccharides, along with proteins, enzymes, and DNA to synthesize non-covalently functionalized carbon nanotubes (Fig. 2.25; Table 2.4) [287, 288].

2.5.5 Applications The novel attributes of the carbon nanotubes make them more reinforced to use as filler as compared to conventional ones such as carbon black. The table below lists a comprehensive review of the properties of carbon nanotubes (Table 2.5). Pieces of literature reveal that to improve the physicochemical properties of a polymer matrix, a higher amount of carbon black is needed as compared to the carbon nanotubes. Thermoplastic based nanotubes reinforced nanocomposites can be used for a variety of applications owing to its optimized functioning of the stretchability, flexibility, conductivity, and mechanics when dispersed into the matrix [290]. Chou et al. had used a nanocomposite framed with Thermoplastic polyurethane, and multiwall carbon nanotubes as the reinforcing filler for a probable application is wearable strain sensors. With high tensile strength, the hybrid material resulted in a sensitivity gauge factor of 2800 (in the strain variation of 5–100%) [290]. The chiral vectors of the nanotubes were able to monitor the shape of the matrix on the 2D mapping when applied a force. The material was further modified to fabricate into a stretchable elastic film which could detect ultra-sensitive strain tensors. Owing to the potential of the process to scale up easily, these nanotubes reinforced polymer composites can be a modern technique to synthesize highly stretchable textile-based strain sensors [290]. To address the growing environmental concerns, scientists are developing composites and technologies to fabricate environmentally benign materials. One

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Fig. 2.25 Non-covalent functionalization of carbon nanotubes (CNTs) a with a surfactant and b with a polymeric agent. Reproduced with permission from [288]

of the examples may be cited from Wang and his coworkers’ work, where multiwall carbon nanotubes were employed as a nanofiller to enhance the property of a composite made from pure thermoplastic polyurethane and recycled polyethylene terephthalate [188]. The nanofiller had no detrimental effect on the crystalline phase of the polymeric hybrids, and instead, the improvement in the interfacial adhesion between the filler and the recycled polyethylene terephthalate shifted the composite to be more elastomeric than crystalline. With high tensile strength and magnified thermal stability, the nanofiller proves to be one of the reinforcing agents used to inflate the overall properties of a semi-recycled product [291]. Prof. Lekawa-Raus’s group came up with a cheap, convenient, and scalable carbon nanotubes/Thermoplastic elastomer composite for a potential application in the smart textile industries. The carbon nanotubes may be tuned accordingly to frame a userdefined composite, which shows improvement in both the electrical conductivity and mechanical properties. Since the composite exhibited a high stretchability, the

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Table 2.4 A comparative study to analyze the advantages and the disadvantages of the functionalization methods of carbon nanotubes [248–254, 256–288] Method

Specifications

Working principle Advantages

Disasdavantes

Covalent functionalization

Side wall

The transition of the hybridization state

Enhances the polarity and thus the dispersion of the nanoparticle in the matrix

Causes damages to the pristine nanotube

Defects

The transition of the defect site and functionalization

Easy to use/Convenient along with cost efficacy

Often the method is considered not eco-friendly (making the process eco-friendly shall increase the process cost)

Polymer wrapping

π-π stacking and van der Waals interactions

No damage caused to the original nanofiller

Weak polymer filler interaction

Surfactant adsorption

Physical adsorption

Convenient along Non-biocompatible with cost efficacy

Endohedral method

Capillary method

Retains the original properties of the nanofiller

Non-covalent functionalization

High chances of agglomeration

material finds its space in the textile industry with an amalgamation of light-weight, high conductivity, and flexibility (Fig. 2.26) [292]. Li et al. had developed a highly cost-effective strategy to develop carbon nanotubes based composites for wearable heaters. The thermoplastic polyurethane matrix loaded with the nanotubes exhibited a high electrical conductivity of 142.6 Sm−1 . The composite was tested to have an excellent heat retention property for a heatingcooling cyclic operation. Even at 20% strain, the reinforced matrix attained a maximum tensile strength of 86% higher than the pristine material. The material had the ability to return to its original position once the load was removed. Even at low temperatures, the material is reported to work well, leaving various evolutionary scopes of the nanotubes to mature in wearable devices for human heating stems [293]. Kim and Tran had studied the relative comparison between incorporating carbon nanotubes in thermoplastic elastomer and in natural rubber [294]. With a 2% filler loading, both the nanofilled composites showed an elevation of the mechanical properties. The Gauge value of the thermoplastic elastomeric based composite was observed to be higher than the natural rubber-based composite on developing 100% strain in both the materials. In the case of stretchability, the thermoplastic elastomer has the edge over its rubber counterpart, and that is why it is being employed majorly in the field of stretch sensors [295].

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Table 2.5 The comprehensive review of the properties of carbon nanotubes, reprinted with permission from [289] Properties

Range of values

Average diameter (single walled nanotubes)

1.2–1.4 nm

Distance from opposite carbon atoms

2.83 Å

Analogous carbon atom separation

2.456 Å

Parallel carbon bond separation

2.45 Å

Carbon bond length

1.42 Å

C-C tight bonding overlap energy

Approximately 2.5 eV

Group symmetry (10, 10)

C5V

Lattice: bundles of ropes of nanotubes

Triangular lattice (2D)

Lattice constant

17 Å

Lattice parameter: (10, 10) Armchair

16.78 Å

Lattice parameter: (17, 0) Zigzag

16.52 Å

Density: (10, 10) Armchair

1.33 g/cm3

Density: (17, 0) Zigzag

1.33 g/cm3

Density: (12, 6) Chiral

1.40 g/cm3

Interlayer spacing: Armchair

3.38 Å

Interlayer spacing: Zigzag

3.41 Å

Interlayer spacing: Chiral

3.39 Å

Conductance quantization

12.9 k−1

Resistivity

10−4  cm

Maximum current density

1013 A/m2

Thermal conductivity

Approximately 2000 W/m/K

Phonon mean free path

Approximately 100 nm

Relaxation time

Approximately 10–11 s

Young’s modulus (SWNT)

Approximately 1 TPa

Young’s modulus (MWNT)

1.28 TPa

Maximum tensile strength

Approximately 100 GPa

On investigating on the flammability of the carbon nanotubes based thermoplastic elastomer, Tayfun et al. had used thermoplastic polyurethane composite embedded with nanotubes whose proportion varied from 0.5 to 2 wt%. The compatibility of the carbon surfaces with the elastomer matrix was modified with a mixture of sulfuric and nitric acid to get a better interactive property [295]. Not only at optimum filler loading of 1%, the flam retardancy behavior improved, but enhancements were also observed in the tensile strength and the modulus. Adding more amount of nanofiller results in the increase of the melting temperatures, thus inducing a decrease in the glass transition temperature, which shall yield in brittleness. Thus, optimized filler incorporation results in the ideal filler-elastomer interaction exhibiting various amplified behaviors as compared to the unmodified matrix [295].

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Fig. 2.26 The thermoplastic polyurethane matrix reinforced with carbon nanotubes a wound on a reel; b sewn into the fabric; c, d ironed onto the fabric. Reproduced with permission from [292]

Thus, the up-gradation in the tensile strength, flexibility, stretchability, and conductivity in the nanofilled thermoplastic elastomer composite tunes them to be used in charge storage devices, oil hoses, smart materials, and high pressure strain tensors even in jarring environments [191–295]. With the emerging progress in science, thermoplastic elastomer has a lot of insights for future study, some of it which include-functionalization, synthesis routes, characterization, and structure-property relationship. A deeper study on the chemical and physical interaction in carbon nanotubes reinforced thermoplastic elastomer may result in a better understanding of how these materials can be redesigned and refashioned further.

2.6 Graphene A Canadian physicist by the name Wallace, in the year 1947 theoretically discovered the two-dimensional graphite, which is characterized by its atomic thin single layer

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[296]. Although two decades later, two professors collaborated to visualize the same experimentally; their research groups had no perspective on the structure of the synthesized product. The material was rediscovered in 2004, with the efforts of Gem et al., which has evolved to be one of the best nanofillers available for fabricating nanocomposites [297]. The timeline below lists the gradual emergence of graphene as one of the most potent nanofiller in modern times (Table 2.6). The structure is very similar to that of the honeycomb layered network, with all the carbon atoms being supported by the 6 sigma bonds in the plane. The presence of the delocalized electron system about the pi orbital, along with the electrons’ orientation perpendicular to the plane, tunes graphene to be electrically conductive and resistive towards gas molecules [298]. The material’s interaction with the p-p, p-H, p-cation, p-metal, and p-cation system enables it to exhibit a diverse range of characteristics traits such as the electrical, mechanical, optical, and physiochemical behavior. The wide variety of their functional properties enables them to be employed in electronic products, sensors, detectors, biological devices, and optoelectronic appliances [298, 299]. However, there are various associated defects that are linked with the posttreatment procedure after the fabrication of graphene. As graphene is essentially a layer, it is susceptible to anomalies such as vacancies, edges, grain boundaries, Table 2.6 The glimpse of how graphene gradually evolved since its inception in 1947 [298] 1560 s

Simonies and Lyndiana Bernacotti developed the first pencil.

1924

The layered structure of the graphite was first identified by John Desmond Bernal

1947

Wallace’s theoretical concept titled “a single graphite sheet”

2004

Andre Geim and Konstantin Novoselov, isolated graphene from bulk graphite

2007

The emergence of graphene nanomembranes

2008

Liquid exfoliation of graphene to produce ‘graphene inks’

2009

Graphene used as transparent electrodes—ideal for displays and solar cells

2010

Commercial scale production of graphene based touch screen displays

2012

Graphene electronics aided by inject printing

2013

Launch of the Graphene Flagship program to commercialize graphene technologies within 10 years

2014

Graphene batteries are developed to increase the efficiency of the process

2015

Ultra high sensitive magnetic detectors reinforced with graphene

2016

Graphene based mobile phones

2017

Ultrafast graphene photodetectors enable high bandwidth data transmission

2018

Solar cells with a higher surface area use graphene interface engineering

2019

Magic-angle” Graphene

2020 (till now)

Development of the twisted bilayer graphene

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chemical impurities, and curvatures. The presence of the chemically active edges within the graphene allows it to align its edges in either a zigzag pattern or in an armchair pattern [300]. The conformation induces the efficacy and the robustness when used in various electronic and optical gadgets. The enhancements in their unique property infer from the fact that graphene has two different Dirac points for each valance and a conduction band along with a zero bandgap. Owing to their direct relationship with the energy and momentum, graphene layers possess a very high Fermi energy, almost similar to copper [301, 302]. As far as the optical properties are concerned, graphene has proved to absorb only 2.3% of the incident white light with a negligible amount of reflectance [303]. Interestingly, for graphene monolayer, the absorption behavior changes abruptly after a wavelength of 250 nm modulating it to be one of the bets deserving material used for nonlinear optical applications [303]. The basal plane of graphene is supplemented with strong s-bonds, which enhances Young’s modulus of graphene up to TPa coupled with an elevation of the fractural strength to 130 GPa [304]. Furthermore, when applied with a maximum amount of strain, it has been recorded that the electrical conductivity, together with the optical traits, remains the same, which makes it ideal to be applied to flexible electronics and wearable devices [304]. As the sheets exhibit high cohesive interactions, aggregations, the high p-p integrations, and the overlap of the valence band and the conduction band creates several solubility issues with graphene. With an amalgamation of exciting properties and tailor-made attributes, a suitable process to synthesize graphene sheets is the most difficult task. Graphene has been functionalized via several surface modification techniques to design a reinforced and a user defined nanofiller [302–304]. These functionalizations, majorly obtained via a covalent approach or a non covalent treatment aims to functionalize the basal plane of the graphene monolayer. The dangling bonds present at the monolayer edges of the graphene sheets are usually highly reactive than the basal plane atoms. This causes any foreign particle to bond more easily with the edges imparting the solubility of the material, and hence the dispersion behavior. However, the covalent functionalization of the graphene profoundly affects the p-p conjugation system of the nanosheet.

2.6.1 Structure and General Properties of Graphene Graphene is an allotrope of a two dimensional carbon form bound in a hexagonal lattice, in which a single atom forms the vertex with sp2 hybridization. The length of each carbon bond is found to be about 0.142 nm, and the structure has three σ bonds that stabilize the hexagonal lattice of the material [305]. The locally developed π bond located vertically to the lattice plains accounts for the dominance of electrical conductivity. The combination of the orbitals of px and py as a result of the orientation of the sp2 hybridization provides a robust framework for graphene as a potential nanofiller. The π-bonds, centered on the pz orbital, along with the px

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and py aids to form the π-π *bands, which eventually is the reason why graphene exhibits high conductivity. In a mono-layer, graphene is made up of tightly packed carbon atoms that look similar to a honeycomb lattice plane (Fig. 2.27) [304, 305]. These carbon atoms participate to bond with their nearest carbon atoms in a state of sp2 hybridization resting in the formation of a benzene ring. The hybridization allows the conformation to donate one unpaired electron to exhibit its conductive properties. Being only 0.35 nm thick, the structure of the graphene is very stable due to the strong carbon-carbon covalent inter-linkages [306]. A nanoribbon is one of the most popular conformations of the graphene sheet to allow the energy barrier to occur close to the central point. The lateral charge mobility can be tuned according to the thickness of these nanoribbons. This particular attribute gests featured in graphene based electronic devices as the lateral charge mobility along with the energy band gap may be fine-tuned by modeling the width of the graphene nanoribbons accordingly [306]. Graphene is a very light material with a planar density ranging between (0.77 and 0.79) mg/m2 . The hexagonal carbon ring present in the unit structure of graphene has a surface area of 0.052 nm2 [303]. The additional advantage of ultrathin makes

Fig. 2.27 The microscopic stricture of graphene layers. Reproduced with permission from [304, 305]

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69

graphene possess a very high optical clarify with transparency of 97.7% [303]. The numerical analysis derived from the Dirac Fermion theory may yield the number of layers graphene has using the advantages of transparency. The optical property may further be manipulated by introducing a variation in the thickness of the graphene sheets, which makes it a competing material to develop membranes with high mechanical strength. The traditionally used membranes such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) have various disadvantages such as fragility and environmental sensitivity, which can be addressed by graphene layers. With the concept of doping, graphene, which was previously used as an acceptor in optoelectronics devices, may now be used as an electrode in supercapacitors [304–309]. For graphene, generally at the near infer-red region, the adsorption of light would reach at a point of saturation, beyond which the adsorption remains constant with the increment of the intensity due to the zero band gap of graphene. The property is now used to synthesize fiber lasers in the domain of ultrafast fiber photonics [309]. The electron mobility of the graphene (2 × 105 cm2 /Vs) makes it be behaving as a conductive particle at room temperature. The hybridization allows in the liberation of one unpaired electron to the π bond, resulting in conductivity of 106 S/m and sheet resistance of 31 /sq [309, 310]. There is a minute overlap between the conduction band and the valance band of graphene sheets allowing the electrons to jump from the conduction band to the valence band without any externally added stimulus. The two bands are structured like a cone intercepting at a zone known as the Dirac Point. The relativistic electron transport phenomenon takes place in a similar strategy to that of a quantum Hall effect. The recent trends in science have allowed us to manipulate the conductivity of the pristine graphene by inserting a hydrogen atom into each of its corner zones, tuning the material to be electrically inert [310]. With the advantage of being a low noise material, graphene can also be modified accordingly to sense and investigate the presence of an external electric or magnetic field. Considering the mechanical properties of the material, the tensile strength, and the elastic modulus of graphene are 130 GPa and 1.1 TPa, respectively [311]. A typical 1 m2 of graphene sheets can withstand the effect of at least 4 kg of load inducing its strength to be approximately 100 times to that of the steel. As referred earlier, this turns out why graphene is so extensively used as a reinforcing nanofiller to develop nanocomposites. Graphene also demonstrates an excellent thermal conductivity of 5 × 103 W/m K, which is believed to be at least 10 times higher than that of copper [312]. The efficient attribute of the specific surface area of graphene is employed as micro detectors for gas molecules. The high surface area per unit weight of 2630 m2 /g, enables it to respond to any abrupt change in the conditions by using adsorption and desorption as the sensing phenomenon.

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2.6.2 Synthesis Strategy of Graphene The fabricating process of graphene has been at the crest of interest to the researchers because of the exciting properties which the material provides. The section describes the presently employed synthesis strategies to prepare graphene on a commercial scale, including the widely known top-down and bottom-up approaches along with the novel next-generation manufacturing process. • Top-Down Process 2.6.2.1

Mechanical Exfoliation

The method was deemed to be the first-ever known method to synthesize graphene flakes on a basal substrate. Stress in the longitudinal or the transverse direction is applied using the scotch tape or AFM tip. As the graphite layers are held together by weak Van Der Waals’s force of attraction with an interlayer distance of 3.3 Å and interbond energy of 2 eV/nm2 , the application of an external force results in the extraction of a graphene [313]. The method generally requires a force of 300 nNμm−2 to separate a single layer of the nanosheet from the graphite. The process may be catalyzed using a supplementary agent such as an electric field or ultrasonication. Ruoff et al. had developed graphene using adhesive tapes from a millimeter thick, highly ordered pyrolytic graphene [313, 314]. The process involved the usage of oxygen plasma to compress the dry-etched graphite layer on a wet photo resistive layer of glass. The ordered graphene was baked to attach the same to the substrate, and the scotch tape method was applied to yield the graphene layers. Although the process is easy to experiment with, scalability and viability at a commercial scale are still a problem. On an additional note, a single layer of graphene is not obtained via this method, and we end up getting a couple of sheets of graphene stacked together. Thus ultimately, the process efficacy decreases as the issue results in further processing of separating the stacks of layer [313–315].

2.6.2.2

Chemical Exfoliation

The method involved the formation of suspensions, which lead to the transitions from the graphite to graphene via the fabrication of graphene intercalated composites. The interlayer spacing is generally increased by adding an alkali metal, which induces the reduction in the Van Der Waal’s force, enabling separation of the layers [316]. The potential ionization difference between the graphite and the alkali metals enables the formation of these intercalated compounds. Potassium alkali metal is the most used metal to prepare intercalated regions at 200 °C under an inert atmosphere. When the formed material was dissolved in aqueous ethanol, the nanoparticles were precipitated.

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71

Furthermore, the dissolution process resulted in the emergence of hydrogen gas, which further pushed the graphite layers apart to yield a single-layered sheet. The process not only has the potential to synthesize a large quantity of graphene at a relatively low temperature, but it is also scalable, and a wide of functionalized graphene nanoparticles can be produced. The major drawback of the system is that the graphene tends to reallocate back to its initial form of graphite. Optimized quantities of surfactants and other chemical moieties such as ferric chloride, supercritical carbon dioxide, and nitromethane may be added to reinforce the intercalation process and hence preventing it from tracing back its original confirmation [316, 317].

2.6.2.3

Chemical Synthesis

This is perhaps the most convenient way to prepare graphene, especially in a scaledup quantity. Three basic underlying methodologies that are employed to carry out the chemical reaction (namely the Brodie method, Staudenmaier method, and Hummers method) oxidize graphite in the presence of a strong acid and an oxidant [318–320]. The Brodie method (1959) introduces the chemical oxidation by using potassium chlorate as the oxidant coupled with nitric acid carries out in a slurry phase. The process, although yielded graphene sheets, the longer time consumption turned out to be the prime drawback. The method was modified by Staudenmaier, who changed the acidic medium to sulfuric acid. Hummers et al., in the year 1958, introduced sodium nitrate as a co-agent along with potassium permanganate and sulfuric acid [318]. The oxidation essentially increases the interlayer spacing between the graphite. For instance, the interlayer spacing, when oxidized with sulfuric acid, increases from 3.34 to 7.35 Å. When exposed to physical agitation, such as ultrasonication, the spacing between the layers moves further apart. Due to the various environmental concerns, the use of harmful chemicals as the oxidants such as nitric acid or potassium chlorate had made scientists find a benign environmental design. As the Hummer’s method proved to be a quicker route to develop graphene, Marcano and his team excluded the use of sodium nitrate from the process, and instead proposed to incorporate a mixture of sulfuric acid and nitric acid in the ratio of 9:1. Not only had the refurbishment resulted in higher efficacy and yield, but the method is also completely inert to the deterioration of the environment [321–323]. The chemical method is a very powerful tool in the fabrication technique of graphene sheets as it removes most of the functionalities resent at the interface while keeping the structure inert. However, incomplete oxidation leaves traces of the alcohol or the carboxylic groups, which hinders the effective property of the nanocomposite. Care must be taken on the concentration and the ratio of the oxidizer complexes being used to yield the product with perfect physiochemical properties.

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Fig. 2.28 The AFM image of exfoliated GO sheets with three height profiles acquired in different locations, accessed from https://www.hielscher.com/ultrasonic-graphene-preparation.htm on 3004-2020

2.6.2.4

Direct Sonication

Ultrasonication is perhaps the most reliable and advantageous process to fabricate high-quality graphene with controlled sizes, especially on an industrial scale. Ultrasound allows the fabrication of graphene in organic solvents or ionic liquid, bypassing the usage of potent oxidizing or reducing agent, which prevents the deterioration of the fabricated graphene sheets. The figure shows the AFM image of the graphene oxide fabricated by Stankovich et al. via ultrasonication at a concentration of 1 mg/ml in water (Fig. 2.28) [324]. The results reveal that the fabricated sheets had a uniform thickness (approximately 1 nm), highlighting the robustness of the process. The study conducted by Stengl et al. describes the successful preparation of graphene sheets in large quantities using ultrasonication to develop nonstoichiometric TiO2 graphene nanocomposites. The natural graphite was exposed to a high-intensity cavitation field at 5 bar, and the layers that were obtained had a high surface area along with enhanced electronic properties, thereby supplementing TiO2 to improve the overall photocatalytic activity. Recently, the method of ultrasonication has harmonized as one of the extensively used methods to synthesize graphene oxide layers. An experiment conducted by Oh and his research group has shown that the development of graphene oxide layers from the original reactants does not significantly alter the functionality of the product, thereby proving the method to be advantageous when contrasted with the chemical processes [324].

2.6.2.5

Electrochemical Exfoliation

In the recent trends, the exfoliation of graphite using an electrochemical means has become a simple, yet an economical way to fabricate graphene [325, 326]. The

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graphite is used as the electrodes in the liquid electrolytes, and depending on the applied potential, graphene can be fabricated in either a negative biased process (cathodic) or a positively biased process (anodic) [325]. The potential is applied to drive the ionic species present in the electrolyte to intercalate into the graphite electrode, thereby increasing the distances between the two layers. For instance, in case of using ammonium sulfate as the electrolyte, the sulfate ions and the water migrate to the electrode, thereby generating local gas bubbles (such as SO2 , O2 ), pushing the two sheets apart. The interlayer distance can be controlled by altering the process parameters such as the applied potential, processing time, and the structure, along with the chemical composition of the electrolyte [325, 326]. However, the process only allows us to use graphite monoliths as the exfoliation material as the material has to be continuous and connected to the external power supply. Thus, materials like loose graphite powders (which are easily accessible) cannot be used to run the process [326]. Moreover, at times, the electrochemical exfoliation process results in the monolith to expand without any control resulting in the degradation of the source material (monolith). As soon as these material starts to disassemble, they turn into powder, thereby interrupting the flow of the current, and thus the exfoliation process stops resulting in the low yield of graphene [326]. The problem was addressed by Liu et al., who placed the graphite monoliths at the bottom of the process vessel. The suggested hypothesis by the authors reflected that even if the material disintegrates, gravity will tether the particles allowing the current to complete the circuit. Although the group reported a slight improvement in the graphene yield, the problem of controlling the graphite from disintegration remains a challenge to solve [326].

2.6.2.6

Exfoliation Using Superacid

The production of graphene using superacid chlorosulfonic acid (CSA) to exfoliate the pristine graphite has evolved into one of the most emerging technology in nanotechnology [327, 328]. The spontaneous protonation and the exfoliation of the graphite via the superacid yield a colloidal liquid crystal dispersion of a single graphene layer. The work by Pasquali et al. has revealed that superacid chlorosulfonic acid exfoliates graphite into isotropic dispersions of graphene at concentrations significantly higher (one order of magnitude) as compared to the other existing routes. As the acid protonates the graphene, the layers begin to exhibit repulsive characteristics and thus undergo exfoliation without losing their functional traits (Fig. 2.29) [327]. Furthermore, the dissolution of the material at a high concentration requires a certain critical degree of protonation to achieve to make the process spontaneous. The process involves neither requires any mechanical agitation nor any stabilizing agent (for instance, surfactants), thereby promoting the physical properties of the fabricated exfoliated sheets. On an additional note, the concentrated liquid crystalline phases, along with the isotropic phases, are a potent source for functionalization tuning the process to be more robust and versatile in terms of developing nanocomposites for a wide range of application.

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Fig. 2.29 The cryogenic-TEM images of graphene flakes dispersed in chlorosulphonic acid. Reproduced with permissions from [327]

However, superacid chlorosulfonic acid has poor compatibility with most solvents and metals. Thus often, it reacts violently when brought in contact with any external surfaces/chemicals (water, alcohol, polymers, and metals) [327, 328].

2.6.2.7

Thermal Exfoliation

The thermal exfoliation is one of the traditionally employed techniques involving rapid heating of the natural graphite material (Fig. 2.30) [329]. The process results in the evolution of a number of molecules such as water, carbon dioxide, and carbon monoxide, leading to an increase in the internal pressure of the system resulting in the exfoliation of the graphite sheets along with an increase in the volume and generating high surface area [329, 330]. With a decrease in the graphite peak (~26.2–26.55°) intensity, the XRD data concludes the formation of the exfoliated graphite sheets. Several studies have been focused on investigating the optimum temperature for the exfoliation process with high carbon yield and bulk density. A communication by Mochane et al. described an experiment in which expanded graphite was analyzed under a range of temperatures from 200 to 1000 °C [330]. The results show that the peak intensity decreased significantly at a temperature of 750 °C, owing to the enhanced swelling of the intercalates. However, above 750 °C, evaporation started to dominate, causing a reduction of the bulk density, and subsequently, the sample expandability decreased [330]. Thermal exfoliation has various advantages, and the predominant one being it is a swift process. Exposure to high temperatures enables the exfoliation process to

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Fig. 2.30 The tapping-mode AFM topography image visualizing particles obtained by thermal exfoliation of graphite oxide along with the height of the derived sheets. Reproduced with permissions from [329]

be completed within seconds. Furthermore, as the method takes place in a gaseous environment, the use of solvent or liquids can be avoided [329, 330]. However, as the technique relies majorly on the successful build-up of the pressure which can exceed the van der Waals interlayer attractions, the starting material has to be modified (expanded graphite, and intercalated graphite compounds) rather than pure graphite, thereby increasing the effective cost of the process [329, 330]. Table 2.7 reports a concise overview of the Top down Techniques • Bottom Up Approach 2.6.2.8

Pyrolysis of Graphene

The solvothermal synthesis of graphene had been one of the basic bottom up technique to prepare graphene involving the solved interacting with a co-agent [330]. A feed ratio of equimolar quantity of ethanol and sodium is used to process the reaction at an elevated pressure. The sodium ethoxide, hence formed, is used to detach the layers of graphite into smooth graphene sheets. The Raman Spectroscopy of these newly fabricated samples showed a broad D-band and G-Band with an intensity ratio of 1.16. The results obviously are not synchronous with pristine graphene sheets, but this method is an easy and very less expensive pathway to fabricate graphene [330].

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Table 2.7 A concise overview of the top down techniques [314–318] Techniques

Typical dimension

Advantages

Disadvantages References

Thickness Lateral Micromechanical exfoliation

(2–15) layers

μm to cm

High surface area of the fabricated graphene

Can’t be scaled up

Sonication of graphite

Single, 2–3 layers

μm or sub-μm

Cheap and convenient

The process [315] of the separation of the impurities is complicated

Electrochemical Single, exfoliation/functionalization 2–3 of graphene layers

2.6.2.9

200–500 nm Results in a High cost highly functionalized graphene

[314]

[318]

Epitaxial Growth of Graphene on Silicon Carbide Surface

The well-known method follows the principle of embedding a single crystalline film on a monocrystalline substrate. The surface-film differences are classified as homoepitaxial layers (both the film and the substrate are of the same material) and heteroepitaxial layers (both the film and the substrate are of the same different material). The process was first discovered in 1975 when Bommel et al. used silicon carbide as the substrate, on which he embedded graphite under ultrahigh pressure and a temperature gradient from 1000 and 1500 °C [331–333]. The breakthrough in this area came in 2012 when Juang and his coworkers devised a strategy to employ a catalyst along with the silicon carbide wafers. This enabled the developed graphene layer to be easily transferred and incorporated into different materials. The method is considered to be one of the best ways to produce excellent quality of graphene along with its superior electronic properties [334].

2.6.2.10

Chemical Vapor Deposition

The process allows the decomposition of precursors in the gaseous phase under elevated temperature, which is then deposited on to the desired substrate. In cases where the high temperature is not feasible, the plasma-assisted reaction is supplemented as a secondary process to reduce the temperature [314]. The thermal chemical vapor deposition employs precursor gases such as methane, hydrogen, and argon to flow through a quartz tube at an elevated temperature. The parameters like flow rate, pressure, and reaction time may be manipulated to get a variation in the number of the layers in the graphene sheets. For a compassionate application, the substrate is chosen to deliver the best quality of graphene, such as the transition metals (Nickel, palladium, and Iridium), amalgamated with benzene or

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ethylene. Owing to the interaction of these metal particles with the hydrocarbon at the high temperature, carbon saturates on the substrate, and when cooled, it recombines to form a coherent film. Over the due course of time, several types of research had investigated ways to make the process more efficient and less cost consuming [314, 335]. At the same time, Lang et al. had deposited graphene layers by sing platinum and ethylene in the gaseous phase, while Eizenberg and his team used platinum to develop the graphene sheets via chemical vapor deposition [314, 335]. However, back then, no such reinforced application area was investigated in which they could employ these nanostructures until 2004 with the advent of graphene-based electronics. In 2006, graphene was synthesized on a nickel substrate using camphor as the precursor agent. The framework, when viewed under the microscope, revealed that almost 35 layers of single-layered graphene sheets were stacked together, having the interspacing distance of 0.34 nm. To address the scalability of the process, copper layers are used to fabricate graphene [336]. The first large scale prototype of graphene synthesis was carried out on copper foils at 1000 °C, by Li et al. The cooling process aided the formation of the film, which, when investigated via the High-resolution Transmission electron microscopy exhibited excellent graphene layers with negligible chemical influences [337]. Although pieces of literature report multiple numbers of to fabricate the substrate for the process, in most of the cases, copper and nickel are used to aid the scalability of the process. Furthermore, the layers grown on the other metal substrate tend to fold and produce wrinkles that deteriorate the physicochemical property of the developed nanomaterial. The temperature-induced chemical vapor deposition process is often replaced by plasma under vacuum. Various sources, for instance, radiofrequency, microwave irradiation, and inductive coupling, are used to generate the plasma for the plasmaenhanced chemical vapor deposition. The process not only consumes less time as compared to the conventional vapor deposition technique but also uses a relatively lower temperature making the process more economical friendly [337]. The process parameters can be controlled to achieve a catalyst-free reaction making it lucrative for several industrial applications. Obraztsov et al. had demonstrated the process of plasma mediated graphene layer deposition by using a direct current discharge in a mixture of methane and hydrogen [338]. Microwave-assisted plasma layer deposition was carried out by Wang et al., who used a carrying methane concentration along with a temperature gradient to fabricate graphene at 900 W microwave wattage (Fig. 2.31) [339]. Recent developments have paved the way to introduce techniques such as the atmospheric pressure process to fabricate pure graphene layers. The methodology is still in its evolutionary stages as researchers are still investing in methods to fabricate single-layered graphene with a uniform large scale production, keeping the economic benefit in the backdrop.

2.6.2.11

Arc Discharge Method

The method of synthesizing graphene via arc discharge was first established by Krastchmer and Hoffman in 1990 as a part of their fabrication process to develop

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Fig. 2.31 HRTEM images of nanosheets grown under 40% CH4 for 20 min on a tungsten substrate. Reproduced with permission from [339]

C60. Since then, the method continued to grow, and now it is extensively used to fabricate a wide range of carbon-based materials. The arc oven usually consists of two electrodes coupled with a steel chamber, which is cooled by water (Fig. 2.32) [340]. Pure graphite rods are incorporated into the chambers as both the anode and the cathode, and a discharge current at 100–150 A is maintained throughout the process. As the rods are brought closer to each other in an environment of H2 , NH3 , He, or air (as required), the discharge originates, resulting in the generation of plasma. The soot deposited on the inner wall of the chambers is collected at ambient temperatures to purify the newly developed graphene particles. The method is still one of the

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Fig. 2.32 The arc discharge method of preparing graphene sheets under various atmospheres. Reproduced with permission from [340]

best practices to establish decagram scale graphene with high purity and in a timeefficient process. Moreover, arc-discharge is one of the most prominent technologies employed to fabricate pure B- and N-doped graphene moieties. While B-doped graphene is obtained by carrying out the discharge in an environment comprising of H2 and B2 H6 , the fabrication of N-doped graphene uses an NH3 or H2 and pyridine based environment.

2.6.2.12

Unzipping of Carbon Nanotubes

The single-layered graphite with high crystalline property along with enhanced electronic features has emerged to be one of the most exciting domains in the realms of Nanoscience. The thin elongated strips of graphene are one of the recent advancements tuning graphene not only to behave as semiconductors but also semimetals [341]. Tour et al. recently designed a strategy to obtain graphene nanoribbons using multi-walled carbon nanotubes, which were suspended in concentrated sulphuric acid. The process was followed by oxidation with 500 wt% KMnO4 for 1 h at room temperature, post which it was isolated and found to be soluble in water and ethanol [341]. The process resulted in the unzipping of the nanotubes generating straightedged ribbons, similar to the phenomena observed during the opening of graphene oxide. The authors describe that the unzipping effect may occur either in a longitudinal fashion or in a spiral manner depending upon the chiral angle of the nanotube coupled with the site where the initial attack is taking place [341]. The opening mechanism occurs by a two-step process, where the first step involves the formation of the manganate ester, which eventually determines the rate of the reaction. On further oxidation in the dehydrating medium, the ester eventually converts into a dione [341]. Moreover, the coupling of the ‘buttressing’ ketone disturbs the

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stability of the alkenes, making them susceptible to the next attack (by the permanganate), and thus the process continues. As the reaction progresses, although due to the availability of space for projecting the carbonyl group, the ‘buttressing’ strain alleviates; the bond angle strain shall eventually make the alkene highly reactive. Thus, once the unzipping process gets initiated, it is continued either by the ‘buttressing’ strain or via the bond angle strain [341]. The decrease in the bond angle strain is observed as the nanotubes gradually complete opening up and forms the graphene ribbons. Although the ‘unzipping’ process can easily be scaled up to an industrial scale, the emergence of residual oxidized defect sites reduces the electronic properties of the material when compared with the exfoliated graphene sheets [341].

2.6.2.13

Chemical Reduction of Graphene Oxide

One of the most critical methods of graphene synthesis is via the chemical reduction of graphene oxide by an efficient, reducing agent. To date, various reported chemicals result in the formation of graphene layers, including hydrazine (and its derivatives) hydroquinone, amino acid, NaBH4 , NaOH, and vitamin C [342]. Amongst the above-listed chemicals, hydrazine and its derivatives are one of the most widely used reducing agents owing to their high reducing efficacy [342]. However, the Ndoping in graphene often undermines the conductivity of the fabricated graphene when contrasted with the pure material. More importantly, not only is hydrazine is a toxic material but an explosive material which should be avoided in large scale manufacturing processes [342]. Although NaBH4 is another competitive reducing agent in fabricating graphene layers, the poor hydrolysis property often leads to an unstable aqueous solution, thus reducing the efficacy of the process. Any alternative reducing agents (amino acids and phenylenediamine) have the deoxidization ability much weaker than hydrazine or NaBH4 . The invention of Vitamin C as a reducing agent for developing graphene by Paredes et al. resulted in the reducing efficacy similar to that of hydrazine. However, the inefficient hydrolysis property still demands the need for a new reducing agent [342]. Bangal et al. had developed a novel route to reduce graphene oxide via the use of sulfur-containing compounds such as NaHSO3 , SO2 , SOCl2 , Na2 S2 O3 , and Na2 S [342]. The experimental study reveals that these sulfur-bearing compounds, especially NaHSO3 and SOCl2 , exhibit the same reducing property as hydrazine in synthesizing chemically reduced graphene (Fig. 2.33). Being less toxic and explosive, the development of sulfur-bearing agents has widened the scope and the area of employing chemical reduction to fabricate graphene, at an industrial scale [342]. The following table reports a concise overview of the Bottom Up Techniques (Table 2.8).

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Fig. 2.33 The possible mechanism of developing chemically reduced graphene using moieties having sulfur groups. Reproduced with permission from [342]

Table 2.8 A concise overview of the bottom up techniques Techniques

Self-assembly Chemical vapor deposition

Arc discharge

Epitaxial growth on Sic

Unzipping of carbon nanotubes

(3–10) layers

Single, (2–5) layers

Few layers

Multiple layers

100’s nm

Very large (cm)

Few 100 nm to a few μm

Up to cm few μm long size nano ribbons

Advantages

Controlled thickness

Can be Rate of scaled up at production an industrial is high scale

Pure graphene with high surface area

Tailor made if the carbon nanotubes is functionalized accordingly

Drawbacks

Escalation of the defects

Still in its Low yield evolutionary coupled phase with impurities

Very difficult to scale up

Very expensive

References

[331]

[335]

[334]

[342]

Typical Thickness Mono-layer Dimension Lateral

2.6.2.14

[340]

Other Methods

Apart from these major pathways for fabricating graphene, a several number of novel ways exists to synthesize layers from graphite. The unzipping action of carbon nanotubes using an oxidizing agent (potassium permanganate) or laser irradiation catalyzes to widen the carbon nanotubes and result in graphene nanoribbons [343]. The arc discharge is also used in special cases to generate doped graphene with boron and nitrogen to exhibit the semi conductance property. Although, there are multiple ways to develop graphene, not all methods are cost-effective and result in pure graphene. Moreover, most of these methods are specific to the products or the

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application area, thus essentially narrowing the scope of the process [344, 345]. Till now, considering an optimization in both the yield and the cost coupled with purity, chemical vapor deposition still deems to be the best technique amongst all, especially for a large scale graphene production.

2.6.3 Application and Recent Trends The research trends on graphene have metamorphosed the contemporary comprehension and applications of two-dimensional atomic crystal. The first lab-made 2D atomic crystal possesses the ability to exhibit unmatched physical and chemical properties. The integration of high strength, along with enhanced thermal and electrical conductivity, has allowed modern science to witness the penetration of graphene in realms circumscribed on physical, biological, and biological applications [346]. Focusing on some of the recent developments in the area of graphene reinforced thermoplastic elastomer, we must mention the research communicated by Lee and his coworkers addressing the current drawbacks in the scope of wear properties, especially when processing graphene/thermoplastic elastomer nanocomposites via 3D printing [347]. As the current scientific avenues are steering towards advanced additive manufacturing techniques, pieces of literature claim that two leading significant process elevates the mechanical properties of the fused deposition modeling(FDM) derived products: (a) Amalgamating various toughened polymeric resins with the pristine matrix (b) adding superior reinforcing filler into the system. Although various reports infer that carbon-based fillers, often after a post surface treatment, improves the dispensability of the filler in the polymer matrix, there was an immediate demand to study the structural aspects of these fillers for inducing superior wear properties in the polymer matrix [347]. The group 3D printed exfoliated graphenemodified polyether-based thermoplastic elastomer, and interestingly they observed that the composite exhibited high wear resistance and the 43% improvement in tensile strength with the integration of 1% graphene. Moreover, the wear resistance of the graphene-based thermoplastic elastomers was way better than the 3D printed of the pristine thermoplastic resin, owing to the high dispersibility of graphene along with its robust two-dimensional frameworks [347]. The strategy may be a leading stone in fabricating high performance electrical and mechanical devices, which demands integrated structure long with elevated wear properties (Fig. 2.34). If we follow the trends, another impeccable application based trend in the recent developments in photomechanical actuators, that can alter their dimensional stability or exhibits mechanical motion, when stimulated by plight/photos [348]. The absorption of the energy from the photons enables these smart materials to undergo programmed mechanical deformation, which makes them one of the emerging candidates in fabricating photothermal actuation. On this note, the flexible elastomeric matrices reinforced with various functional fillers are on the competent nominee to frame these smart actuation systems. To conduct an in-depth study of the influence of nanofillers on the thermoplastic elastomers aiming to generate mechanical

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Fig. 2.34 Representation of a stress versus strain, b strain at break, and c stress at break values for pure thermoplastic elastomer, carbon black reinforced thermoplastic elastomer, modified carbon black reinforced thermoplastic elastomer, and exfoliated graphene reinforced thermoplastic elastomer obtained from tensile test results. Reproduced with permissions from [347]

deformation when exposed to light, the research group synthesized graphene oxidebased poly(propylene-co-ethylene) via grafting with poly(methyl methacrylate) and poly(n-butyl methacrylate) [348]. The introduction of grafting enabled scientists to have a better understanding of the interactions between the thermoplastic elastomer and the nanofiller. With an input of only 1 vol.% of the poly(methyl methacrylate) and poly(n-butyl methacrylate) modified graphene oxide, the system exhibited actuation properties of 120 and 110 μm, when exposed to a light source of 6 mW and 12 mW respectively. Although a detailed study on the behavior of the actuation property on varying the filler concentration is needed, the research widens the scope to fabricate

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robust photothermal actuation systems for the new age photonic switches, robotics, plastic motors, and adaptive micro-mirrors [349–351]. With the advent of numerical models and simulation tools, various micromechanical models have been used to describe the reinforcement of polymers by twodimensional nanostructures. The stiffening mechanisms in elastomers reinforced with graphene are often reported using classical theories such as the Guth-Gold theory and the jamming theory. However, the recent graphene chemistry has faced difficulties in reciprocating these models to macroscopic scales with experimental consistency [352]. As an attempt to solve the challenge, Papageorgiou et al. have recently developed a novel theoretical model based on the shear-lag and the rule-ofmixtures theories, along with the mechanical percolation phenomenon, which can be used to understand the mechanisms of reinforcement of polymers by graphene nanoplatelets. The model had been tested on graphene reinforced Pebax® (a commercially available thermoplastic elastomer), which infers that the stiffness along with the yield strength is significantly higher as compared to the pristine polymer. Moreover, the model predicts that as the filler contents cross the threshold limit, the stiffening of the nanocomposites increased sharply ascribed due to the reduction of the gaps between the adjacent filer flakes [352]. The model shows excellent consistency with the experimental data, thereby giving an edge over the prevalent co-existing numerical models, especially by accurately predicting the percolation of the tensile modulus.

2.7 Conclusion Thermoplastic elastomers have seen already seen to flourish especially post 2000. However, as the mechanical properties of the pure elastomeric compounds are inferior, reinforcing nanofillers play a pivotal role in making them one of the versatile materials in the current age of nanocomposites. The latest trends of graphenebased thermoplastic elastomers project not only to boost the market for flexible electronics but also the actuator sector. With advancements percolating from both the experimental and simulational science, graphene-based thermoplastic elastomers are gradually transforming the way we used to perceive two-dimensional nanotechnology.

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Chapter 3

Preparation of Graphene Based Nanocomposite Based on TPE

3.1 Introduction In the year 1959, professor Richard Feynman prognosticated the propitious future of nanomaterials with his famous speech, saying, “There’s Plenty of Room at the Bottom. I can’t see exactly what would happen, but I can hardly doubt that when we have some control of the arrangement of things on a small scale, we will get an enormously greater range of possible properties that substances can have, and of different things that we can do.” [1]. For scaling down the macro-sized components to micro and nano levels, several pieces of research have been performed since then, envisioning the wonder of understanding and exploitation of atomic-level properties of nanomaterials. With the discovery of fullerene (C60 ) in the year 1985 by Kroto and his co-workers [2], the scientific community started to receive several carbon nanomaterials. One of the illustrious examples was the discovery of carbon nanotube (CNT) by Iijima [3] in the year 1991. The hunt for nanomaterials escalated with the breakthrough invention of graphene in late 2004 by Konstantin Novoselov and Andre Geim of Manchester University, the UK, which brought them a Nobel Prize in physics in the year 2010 [4]. The isolation of one atom thick single-layer graphite, so-called graphene, by mechanical exfoliation of graphite, unleashed the concept of single atomic components to reality. Graphene has become a rising star on the horizon of material science, and several research laboratories across the globe have been devoting significant attention to this particular field, focusing on continuous exploration of this ‘wonder material.’ Graphene is nothing but a flat sheet of graphite comprising of an ideal twodimensional monolayer of carbon atom packed in a honeycomb crystal lattice [5]. Figure 3.1 shows the crystal lattice packing of carbon atoms in graphene under high-resolution Transmission Electron Microscopy (TEM) and a three-dimensional graphite sheet of a honeycomb lattice structure. Graphene is recognized as the fundamental building block and mother of all carbon-based graphitic nanomaterials, starting from zero-dimensional (0D) fullerene to one-dimensional (1D) nanotubes © Springer Nature Singapore Pte Ltd. 2020 A. Bandyopadhyay et al., Engineering of Thermoplastic Elastomer with Graphene and Other Anisotropic Nanofillers, Engineering Materials, https://doi.org/10.1007/978-981-15-9085-6_3

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Fig. 3.1 Representative example of a The two-dimensional honeycomb structure of carbon atoms in graphene under high-resolution transmission electron microscopic (TEM) image. Reproduced with permission from [6] on 24.03.2020, b three-dimensional single graphite sheet consisting of a honeycomb lattice structure of sp2 bonded carbon atoms. Reproduced with permission from [7]

followed by stacked three-dimensional (3D) graphite [6]. Figure 3.2 demonstrates the evolution of various carbon nanomaterials from single-layer graphene.

Fig. 3.2 Graphene (top) and related structures: fullerene (bottom left); carbon nanotubes (bottom centre); and graphite (bottom right). Reproduced with permission from [8]

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3.1.1 Trends in Graphene Research Fundamental research and emerging industrial applications have made graphene a new revolutionary material towards the scientific community. The arrival of graphene in the nascent stage of the post-silicon era experienced a sharp rise triggered by the Nobel Prize-winning publication of Novoselov et al. in late 2004. The continuous increase of public as well as private investments in this field of research propels the surge of a substantial amount of scientific publications and patent filing. Worldwide emergence of several industrial ventures for the last few years aimed at the manufacturing and commercialization of graphene and graphene-based products, which further aids the apprehension of mass and low-cost production of graphene in coming days [9]. The number of publications containing research articles and patent applications related to graphene has shown a steady increase since 2004, which is illustrated in Figs. 3.3 and 3.4. In recent years, many scientific articles, theoretical and/or experimental studies regarding graphene and its derivatives, have been published. To cite a few examples, through discussions of Synthesis [10], electronic transport properties [11], bandgap engineering [12], optoelectronic technologies [13, 14], Raman spectroscopy processes of transport mechanisms [15–17], etc. and some other comprehensive reviews on recent advancement on graphene are among the listed one. Most recent updates on graphene research enlighten us with several important information and activities going on all over the world. Graphene, being the highly potent wonder material, offers numerous applications with one of the most remarkable in the energy storage market sector. Within the next decade, over 30% of the graphene market will be used in energy storage applications with multiple high-profile use cases, as received from a report.

Fig. 3.3 Graphical representation of a number of publications/year on the graphene materials. The inset is the distribution of the document type, where only 2.7% of the publications are related to review work, b Delivery of the publications by subject area. Reproduced with permission from [18]

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Fig. 3.4 Graphical representation of a number of the review publications/year on the graphene materials, b distribution of the review publications per subject area. Reproduced with permission from [18]

Significant technological research is being carried out by a group of researchers at MIT to develop a graphene-based device that can convert ambient terahertz waves (T-waves) into a direct current. The scientists explain that any device that sends out a Wi-Fi signal also emits terahertz waves, and electromagnetic waves having frequency somewhere between microwaves and infrared light. Even our bodies or any inanimate objects around us, whichever registers temperature, can produce these high-frequency radiations. To date, the energy of terahertz waves remains wasted, as there has been no device to capture and convert them into any usable form. The MIT team is set to harness the concentrated power within the waves that could potentially offer as an alternative source of energy. Another group of researchers at the University of Illinois has arrived with a superior outcome that states that that crumpled graphene is more than ten thousand times more sensitive to DNA by creating electrical “hot spots.” It is claimed that the sensor can detect the ultra-low concentration of molecules necessary for early diagnosis [19]. A team from Boston college is also engaged to capture inherent electronic signals in biological structure using a sheet of graphene to identify deadly strains of bacteria. Rice university group transformed laser-induced graphene (LIG) into self-sterilizing filters in 2019, which can track pathogens out of the air and destroy them with small pulses of electricity. Although commercial graphene-based biosensor has been marketed earlier, these captivating scientific inventions unleash new horizons to various applications safe and secure diagnosis and environmental monitoring. A research and market report from 2017 divulges that there were a total of 13,371 patent filings regarding graphene, which marked a 30.7% increase compared to 2016 and with a compound annual growth rate (CAGR) of 60.9% between 2010 and 2017. Industries are also coming ahead to commercially push the mass production of graphene and bring it to the mainstream market as it reached a market value of US$200 million in 2018 [20].

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Fig. 3.5 Graphene derivatives show promising results for various fields, including energy conversion [25], energy storage [26], electronic materials [27], quantum effects [28], low density structural materials [29], sensors [30], chemical screening applications [31], and thermal interface materials [32]. Reproduced with permission from [21]

Today, graphene is the thinnest known super material and strongest ever measured possessing unique physical properties which are not observed in nanomaterials before and have tremendous potential both in fundamental and applied research in material science for emerging demanding application, as depicted in Fig. 3.5 [21]. Some remarkable properties include huge mechanical strength, superior thermal conductivity, tunable bandgap, exceptional flexibility, ultra-high electron mobility, etc.[7]. There are multiple experimental reviews on graphene illustrating synthesis [22], transport mechanisms [16, 17], relevant applications of graphene such as transistors and the related band-gap engineering [14], and of graphene optoelectronic technologies [13], room-temperature quantum Hall effect [23], long-range ballistic transport and many more. A comprehensive synopsis published in a review article by Castro Neto et al. provides state-of-art of graphene research [24]. Our present review aims to offer a concise overview of graphene and its derivative-based polymer nanocomposite (PNC). Catering to various thermoplastic elastomer (TPE) based composite materials, we have tried to summarize recent research activities in this particular segment in terms of synthetic routes, characterization, and potential application area of those PNC with several representative examples.

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3.2 Different Methods of Preparation In the field of material science, the development of multiphase nanocomposites had drawn considerable attention to improving material characteristics when traditional microscale composites failed to achieve property optimization [33]. Although researchers have published several approaches of nanocomposite preparation, uniform and homogeneous dispersion of nanoparticles in polymer matrix still limits its complete success in each method [34]. High surface area and van der Waals attraction between nanoparticles lead to their agglomeration, thereby hindering proper dispersion as well as deterioration of properties of final products [35–37]. Anisotropic nanofillers, having a high aspect ratio, are very susceptible to severe restacking or re-entanglement. Monolayer graphene has one atomic layer thickness (~0.34 nm) but several microns long in the lateral dimension resulting in an aspect ratio of ~10–4 , which leads to strong π-π interaction between huge graphene basal planes to diminish total surface energy [38]. Therefore, often nanocomposites deliver worst properties compared to conventional polymers restricting their effective application. To subdue this significant problem, the development of new fabrication methods of nanocomposites combining higher control on both morphology and chemistry has become one of the prime focuses on the scientists. The following discussions summarize the discrete routes of preparation of polymer nanocomposites [33]. The common practice for the preparation of polymer nanocomposite follows mainly three fabrication method, namely 3.2.1 Intercalation 3.2.1.1. Melt intercalation 3.2.1.2. Solution intercalation 3.2.2. In-situ polymerization 3.2.3. Shear mixing

3.2.1 Intercalation This method is a top-down approach, mainly practiced for the incorporation of layered nanomaterial (nanoplatelets) into the polymer matrix. Polymer chains diffuse into the gallery spacing of plate-like nanofillers in intercalated morphology, which requires proper surface modification for the homogeneous dispersion of nanofillers and compatibilization between dispersed and continuous phases [39–41].

3.2.1.1

Melt Intercalation

Melt intercalation, also known as melt blending or melt compounding, is a more preferred and typically adaptable technique specifically for thermoplastic polymer

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a ThermoplasƟc

Blending

Annealing

Nano composite

Organoclay

b

Fig. 3.6 Process flow diagram (a) and Schematic representation (b) of the melt intercalation method. Reproduced with permission from [44]

nanocomposites [42]. The polymer matrix is annealed at higher temperatures with the required amount of intercalated nanofillers and finally kneading the entire mass to achieve homogeneous distribution. On another route, the polymer may be premixed with the fillers in dry condition and then heated in a mixer under sufficient shear such as injection molding or extrusion [43] (Fig. 3.6). Melt blending is a comparatively simple, cost-competent, and environmentally benign process as it doesn’t involve any usage of toxic solvents, making the process more industrially feasible for mass-scale production. The diffusion of macromolecules into the interlamellar galleries leads to a decrease in free energy. Now, the free energy governing parameters, i.e., enthalpy and entropy, have their contribution towards the process of diffusion. Enthalpy governs the chemical interaction between the nanofillers and intercalating matrix, while entropic contribution is associated with the randomization of the macromolecular segment. Insertion of fillers into matrix transforms entangled configuration to elongated chain structure, thereby reducing entropy, which becomes energetic constraints for diffusion [45]. Thus successful melt intercalation is an enthalpy driven phenomenon. Although this process possesses several industrial feasibilities, the homogeneous distribution of fillers at high filler loading becomes very difficult due to the increased viscosity of the thermoplastic polymers [46]. Also, the high temperature used in

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(b)

Strong adhesion between matrix and xGnPs Fig. 3.7 FESEM images of a the fractured surface of the SEBS/xGnPs nanocomposites containing five wt% xGnPs, b the same image at higher magnification. Reproduced with permission from [47]

this process can adversely affect the surface modification of fillers. Hence, the optimization of processing conditions is very critical, achieving proper exfoliation. Graphene and modified graphene are intercalated with the polymer matrix using this conventional method. Rath reported the preparation of a TPE/graphene nanocomposite by a melt-compounding technique based on polystyrene-b-poly(ethylener-butylene)-b-polystyrene (SEBS) and exfoliated graphite nanoplates (xGnPs) for the improvement of mechanical properties [47]. Nanocomposites were prepared by simple melt mixing with varying amounts xGnPs in an extruder at 200 °C for 5 min. Uniform dispersion and strong interfacial adhesion of xGnPs throughout the SEBS matrix, supported by the Scanning Electron Microscopy (SEM) image (Fig. 3.7), results in the enhancement of mechanical properties. Haghnegahdar et al. reported a thermoplastic elastomer nanocomposite prepared by melt mixing method based on Polypropylene (PP)/Ethylene Propylene diene Monomer(EPDM)/Graphene showing notable improvement in electrical and thermal conductivity and thermal stability of the composite [48]. The TPE nanocomposites were prepared in an internal mixer Brabender. PP, EPDM, and graphene nanoparticles were melt mixed at 180 °C for 10 min using rotor speed of 100 rpm followed by compression molding at 200 °C for 10 min as a post-processing step to fabricate suitable sample sheet for further characterization. Tarawneh and his team examined the property enhancement of a TPE based on a blend of polypropylene (PP), natural rubber (NR) and liquid natural rubber (LNR) and reinforced by graphene nanoplates (GnPs) [49] TPE/GNP nanocomposites were compounded by melt blending method using the internal mixer. In the first step, GNPs were mixed with LNR under ultrasonication for increased dispersibility of GNPs within LNR, which was then melt blended with PP and NR in internal mixer at 180 °C for 13 min. This melt processing method resulted in the homogeneous dispersion of GNPs throughout the TPE matrix (Fig. 3.8) and strong interfacial interaction remarkably improved the mechanical properties yielding a high-performance composite.

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Fig. 3.8 SEM micrographs of the fractured surface of the TPE/GNP nanocomposites containing: a 5 wt% GNP, b 7 wt% GNP. Reproduced with permission from [49]

3.2.1.2

Solution Intercalation

The solution intercalation method, also known as exfoliation adsorption, is based on a solvent system in which polymers are soluble and nanofillers swell [45]. First, the nanofillers are swollen and disperse into the solvent before mixing with the polymer. On mixing the polymer solution in this nanofiller dispersion, polymers displace the solvent and intercalate with the interlayer of nanofillers [50]. Eventually, upon solvent removal, a multilayer intercalated structure of polymer nanocomposite is formed. Sometimes ultrasound is passed through nanofiller agglomerate in the form of high-frequency waves to separate the agglomerate into smaller bundles which in turn exfoliate eventually into a monodispersed state with increasing time and frequency of ultrasound where the nanofillers are completely separated from each other in the polymer matrix resulting in an effective nanocomposite [38] (Fig. 3.9). Huang et al. prepared a novel flame-retardant functionalized graphene nanocomposite using modified graphene oxide (GO) and ethylene–vinyl acetate (EVA) as base polymer via solvent blending [52]. GO was modified with intumescent flame retardant PPSPB [poly(piperazine spirocyclic pentaerythritol bisphosphonate)] by covalent grafting of PPSPB onto the surface of GO followed by chemical reduction of GO using hydrazine, which enhances the dispersion of graphene sheets in the polymer matrix and improves the flame retardancy of the nanocomposite. Initially, GO-PPSPB powder was dispersed in tetrahydrofuran (TFH) assisted by 60 min sonication at room temperature. Afterward, the required amount of EVA was dissolved in the above suspension after mechanical stirring at 60 °C for two h. This resulting homogeneous dispersion was reduced by an ammoniacal hydrazine solution at 60 °C for four h. The solvent TFH was removed at 60 °C for 12 h under vacuum. Kuila and his co-workers described a method for the preparation of functionalized graphene/ethylene-vinyl acetate copolymer (EVA) composites by solution mixing process for improved mechanical and thermal properties [53]. The surface functionalization of graphene was executed using Octadecyl amine (ODA) in an aqueous medium. Then, the required amount of ODA-modified graphene (ODA-G)

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a

Solvent ThermoplasƟc EvaporaƟon

Swelling

IntercalaƟon

Organoclay

Polymer

Nanocomposite

b

Fig. 3.9 Process flow diagram (a) and Schematic representation (b) of the solution intercalation method. Reproduced with permission from [51]

was dispersed in toluene by 1 h ultrasonication at room temperature. EVA was also dissolved in toluene simultaneously at 100° C under stirring. The ODA-G dispersion was then poured in the EVA solution and refluxed at 100 °C for 12 h followed by the addition of curing agent and removal of solvent first at room temperature and then under vacuum at 75 °C. Figure 3.10 depicts the reaction scheme for the preparation of functionalized graphene and its composite with ethylene-vinyl acetate copolymer. Nawaz and his colleagues reported the fabrication of functionalized graphene oxide (GO)/thermoplastic polyurethane (TPU) nanocomposite via solution intercalation and observed a percolation threshold for various mechanical properties of the composite [54]. They described that functionalized graphene flakes formed a network at that percolating threshold, influencing the behavior of various mechanical quantities afterward. GO was covalently functionalized with octadecyl amine (ODA) to

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Fig. 3.10 Schematic representation of the preparation of functionalized graphene and it’s composite with ethylene-vinyl acetate copolymer. Reproduced with permission from [53]

form GO-ODA, which was dispersed in THF by sonication followed by centrifugation and filtration. TPU was dissolved in THF by stirring for 24 h at 40 °C. After that, GO-ODA was added to the TPU solution to prepare a range of ODA/GO/Polymer dispersion of required concentration. Hu Liu and his team prepared a strain sensor by fabricating electrically conductive TPU/graphene nanocomposite at ultralow graphene loading levels, which showed a wide range of strain sensitivity and superior sensing stability for different strain pattern [55]. Figure 3.11 depicts the fabrication process of nanocomposite by co-coagulation plus the compression molding technique. In short, TPU was dissolved in DMF at 40 °C by vigorous stirring around 30 min. Simultaneously, a calculated amount of aqueous graphene dispersion was mixed with DMF under ultrasonication for 10 min for homogenous distribution. Subsequently, the graphene/DMF dispersion and TPU/DMF solution were mixed with additional 30 min sonication. Methanol being the non-solvent was added to the mixture to flocculate graphene/TPU, which was filtered, dried, and finally hot pressed to form the required sheet of the sample.

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Fig. 3.11 Schematic representation of the fabrication of graphene/TPU nanocomposite by cocoagulation plus compression molding technique. Reproduced with permission from [55]

3.2.2 In-situ Polymerization Thermally unstable or insoluble polymers that cannot be processed by melt blending or solution mixing can be considered under this method [42]. In this process, the nanofillers are taken in liquid monomer or monomer solution so that the low molecular weight monomers cause swelling of nanofillers by entering in between the interlayer, which upon polymerization forms an intercalated or exfoliated nanocomposite [56]. The polymerization is initiated by using a suitable initiator, curing agent in the presence of heat or radiation [57, 58], which forms covalent bonding between functionalized nanofillers and polymer matrix. However, this process is associated with a drawback of an increase in viscosity during the advancement of polymerization that limits load fraction [59] (Fig. 3.12). Paszkiewicz et al. studied the synergistic effect of graphene nanoplatelets (GNP) and single-walled carbon nanotubes (SWCNT) on the electrical conductivity of poly(trimethylene terephthalate-block-poly(tetramethylene oxide) (PTT-PTMO) thermoplastic elastomer-based nanocomposites prepared by in-situ polymerization with varying concentration of GNP and SWCNT as conducting nanofillers [61]. PTTPTMO based nanocomposites were synthesized by melt transesterification followed by a polycondensation reaction. First, the required amount of carbon nanofillers

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a Organoclay

Curing Agent

PolymerizaƟon

Nanocomposite

Swelling Monomer

b

Fig. 3.12 Process flow diagram (a) and Schematic representation (b) of the in-situ polymerization process. Reproduced with permission from [60]

was dispersed in 1,3-propanediol(PDO) under high-speed stirring and ultrasonication. Then, the transesterification of dimethyl terephthalate (DMT) was carried out with the prepared nanofiller dispersion in PDO and tetrabutyl orthotitaniate (TBT) as the catalyst at 165 °C for 1.5 h followed by the addition of poly(tetramethylene oxide) glycol (PTMG), Irganox 1010 (used as a thermal antioxidant) and the second portion of catalyst (TBT) for polycondensation step at 250 °C under reduced pressure of ~15 Pa. Dielectric measurements and Raman spectroscopy explain the significant synergistic effect on improving electrical conductivity with 0.3 wt% SWCNT and 0.1 wt% GNP nanoparticles (which is above the percolation threshold). As the conducting network has already been formed at 0.3 wt% SWCNT alone, an additional 0.1 wt% GNP creates further electron conduction path by synergism. A schematic representation, explaining the synergetic effect of SWCNT and GNP in PTT-PTMO based nanocomposites, is depicted in Fig. 3.13. Wang and his research group evaluated the enhancement of mechanical and thermal properties of polyurethane (PU) reinforced by graphene nanosheets (GNS), derived from chemically reduced graphite oxide (GO) nanosheets [62]. For the fabrication of graphene nanosheet functionalized PU, initially, aqueous GO nanosheet

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Fig. 3.13 Schematic representation explaining the synergetic effect of SWCNT and GNP in PTTPTMO based nanocomposites. Reproduced with permission from [61]

dispersion was chemically reduced by hydrazine to obtain GNS. Then, GNS powder was dispersed in DMF assisted by room temperature sonication for proper exfoliation followed by addition of 4,40-Diphenylmethane diisocyanate (MDI) at 70 °C for 2 h. in the next step, poly(tetramethylene glycol) (PTMG) and ethylene glycol (EG) were introduced in the reaction vessel at 80° C under constant stirring for another 8 h and finally dried at 50 °C for two days. The morphological analysis depicts homogeneous dispersion and higher contact area of GNS throughout the PU matrix due to the formation of chemical bonds where residual hydroxyl functional groups on the GNS surface aided the bond formation process. Strong interaction between PU and GNS yielded superior mechanical properties, which was reflected by the elevation of the tensile strength and storage modulus of the host matrix by 239% and 202%, respectively, with the incorporation of 2.0 wt% of GNS. Alongside displaying high electrical conductivity, the nanocomposite also showed excellent thermal stability for PU. Figure 3.14 represents a synthetic route for PU-GNS nanocomposite.

3.2 Different Methods of Preparation

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Fig. 3.14 Synthetic route of PU-GNS nanocomposite. Reproduced with permission from [62]

3.2.3 Shear Mixing Shear mixing is another important route of synthesis polymer nanocomposite, which has found potential industrial applications [38]. Recent advancement in exfoliating layered nanomaterials indicates that this process is a scalable alternative for their effective dispersion in the polymer matrix [63]. Shear-induced interlayer-sliding involves minimum shear rate (although the shear rate depends on types of fillers and solvents) without severe degradation of the fillers [64]. Rotating rollers, stirrer, or rotors exert moderate to high shear force on nanofiller agglomerates to break them apart insolvent or polymer solution. It is not only suitable for dispersing some layered nanosheets but also effective for agglomerates of weakly bound nanoparticles like silica nanosphere [65].

3.3 Characterization of Graphene/TPE Nanocomposites The synergistic dispersion effect of graphene (Gr) and boehmite (AlOOH) on ethylene–vinyl acetate (EVA) copolymer composite properties was investigated by Yuan et al. using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) both for nanomaterials and nanocomposites respectively [66]. XRD pattern in Fig. 3.15i displayed sharp characteristics crystalline peak of GO at 10.7° corresponds to an interlayer spacing of 0.827 nm which shifted

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Fig. 3.15 Representation of i XRD patterns of GO, Gr, AlOOH, and AlOOH−Gr. ii TGA curves of EVA and its composites in air atmospheres. iii TEM images of a GO, b Gr, c AlOOH, and d AlOOH–Gr, e SEM image of AlOOH–Gr. iv TEM micrographs of EVA composites: a 2.0 Gr/EVA, b 2.0 AlOOH/EVA, and c 2.0 AlOOH–Gr/EVA. Reproduced with permission from [66]

to approximately 25.2° in Gr corresponding to an interlayer spacing of 0.354 nm, confirming the hydrothermal reduction of GO to Gr. Likewise, characteristic peaks of AlOOH were observed in AlOOH-Gr assembly along with diffraction peak for Gr (marked by a yellow rectangle), indicating AlOOH acts as spacers for irreversible restacking of graphene nanosheets. The morphology study by TEM (Fig. 3.15iii (a and b)) exhibited silk-like, crumpled, and transparent feature for both GO and Gr. Strong interaction between AlOOH and Gr was visible from SEM image (Fig. 3.15iii (e)) where AlOOH are densely distributed over graphene nanosheets. The binary nanofiller hybrid (AlOOH-Gr)/ethylene vinyl acetate copolymer (EVA) nanocomposite displayed a more homogeneous dispersion than those of composites containing single nanomaterial as observed from TEM micrograph (Fig. 3.15iv (a, b, c)). Being a barrier, AlOOH nanoplatelets disrupted the van der Waals attraction, and π-π interactions between graphene planes and similarly graphene disturbed some of the hydrogen bonds between AlOOH resulting mutual decrease in the cohesive force of

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both nanofillers and thereby good dispersion in EVA matrix. Thermogravimetric analysis (TGA), shown in Fig. 3.15ii, revealed that degradation temperature at 80 wt% for the nanohybrid-based nanocomposite was increased by 27 °C compared to that of neat EVA. In contrast, only a 14 °C improvement and no enhancement were observed for the AlOOH- and graphene-based composites, respectively emphasizing significant enhancement in the thermal stability of nanocomposite. Tayebi et al. investigated the crystal structure of LDPE/EVA/Graphene nanocomposite employing XRD analysis (Fig. 3.16a) where the characteristic peak of graphite at 2θ = 26 was absent in GO but reappeared in Reduced graphene oxide (RGO) indicating the successful reduction of GO to RGO [67]. The effect of RGO in the XRD spectrum of the LDPE/EVA blend merely made any significant changes owing to its amorphous nature keeping the main two peaks (21.3° and 23.7°) of LDPE/EVA blend intact (Fig. 3.16f). However, a continuous increase besides RGO after 3% loading diminished the peak intensity probably because of the FTIR spectra of GO exhibited several distinguished peaks associated with oxygen functionalities (shown in Fig. 3.16b), which were entirely or partially absent in RGO re-establishing the success of the former reduction process. ATR-FTIR analysis (Fig. 3.16c) of LDPE/EVA blend depicted that after incorporation of GO, oxygen-containing peaks (C–O and C=O) became more intense than that of in pure binary combination suggesting the amalgamation of GO with additional oxygen functionalities. The authors further investigated the efficiency of the reduction process by X-ray photoelectron microscopy analysis (XPS) study (Fig. 3.16d) Binding energies 286.7 and 288.6 eV corresponding to C–O (hydroxyl, epoxy) and C=O (carbonyl, carboxyl) bonds in GO showed a considerable decrease in RGO. The oxygen functionalities on the RGO surface reduced by 74% compared to GO, which was quantitatively evaluated by peak area calculation of C and O, and the C/O ratio was found to be highest for RGO. Raman spectra study described the shifting of G band in graphite towards higher wavenumber in GO along with an increase in ID/IG ratio in GO compared to graphite due to the oxidation process (Fig. 3.16e). Raman spectra of LDPE/EVA/GO 3% contained characteristics peaks (shown in Fig. 3.16e) corresponding to CC stretching, D-band, and CH2 bond. Rath and Li inspected the phase structure of polystyrene-b-poly(ethylener-butylene)-b-polystyrene (SEBS) and exfoliated graphite nanoplates (xGnPs) nanocomposite by Wide-angle X-ray diffraction (WAXD) studies to evaluate structure and exfoliation of graphite inside the polymer matrix [47]. WAXD pattern presented (Fig. 3.17i) an amorphous halo at 2θ = 18° in pure SEBS and a sharp characteristic peak of graphite at 2θ = 26.52 in both pure xGnPs and SEBS/xGnPs nanocomposite confirming the presence of pure graphite nanosheets in nanocomposite containing multilayer graphite sheets with a d-spacing of 3.36 Å. A slight shifting of spectral position of D and G band (1300–1400 cm−1 and 1586 cm−1 respectively for pure graphite) was noticed from Raman spectra in SEBS/xGnPs nanocomposite (Fig. 3.17ii) along with an increase in intensity and sharpness, probably due to the secondary force of interaction between polymer and graphite. The dispersion of nanoplatelets and its reinforcing mechanism was analyzed by Field Emission Scanning Electron Microscopy (FESEM) which showed (Fig. 3.17iii) proper distribution,

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Fig. 3.16 Illustration of a X-ray diffraction patterns of synthesized materials, b Fourier transform infrared spectroscopy spectra of graphene oxide (GO) and reduced graphene oxide (RGO), c attenuated total reflection Fourier transform infrared spectroscopy spectra of low-density polyethylene (LDPE)/ethylene vinyl acetate (EVA) and LDPE/EVA/GO 5 wt%, d C 1 s spectra of GO and RGO, e Raman spectra of as-prepared materials, and f X-ray diffraction pattern of LDPE/EVA blend and its nanocomposites with 1, 3, 5, and 7 wt% RGO. Reproduced with permission from [67]

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Fig. 3.17 Representation of i WAXD patterns of pure xGnPs and SEBS/xGnPs nanocomposites. ii Raman spectra of the pristine xGnPs and xGnPs/SEBS nanocomposites. iii FESEM images of fractured surface of the SEBS/xGnPs nanocomposites containing: a 1 wt% xGnPs, b 3 wt% xGnPs, c 5 wt% xGnPs, d 10 wt% xGnPs, e 20 wt% xGnPs and f 40 wt% xGnPs. iv SAXS profiles for: a neat SEBS, b SEBS with 3 wt% xGnPs, c SEBS with 5 wt% xGnPs, d SEBS with 10 wt% xGnPs, and e SEBS with 20 wt% xGnPs. Reproduced with permission from [47]

intimate contact and better interfacial interaction between nanoplatelets and SEBS by the morphology of fractured surface up to 10 wt% loading of nanofillers above which localized agglomeration started to arise. As SEBS displays microphase separation into ordered morphologies, the authors further inquired about the effect of welldistributed nanoplatelets on SEBS microstructure by performing Small-angle X-ray diffraction (SAXD) experiment (Fig. 3.17iv). Explicit multiple scattering peaks were observed in nanocomposite with almost exact peak positions in SEBS, suggesting that xGnPs does not interrupt the microphase separated morphology of SEBS. Kim et al. reported a graphene/thermoplastic polyurethane (TPU) nanocomposite with a comparative study on types and modification of graphene derivative and the processing technique of nanocomposite [68]. They modified graphene oxide (GO) by chemical approach (isocyanate treated GO, iGO) and thermal exfoliation (thermally reduced GO, TRG) with three different dispersion methods of nanofillers in TPU, namely melt compounding, solvent blending, and in-situ polymerization respectively. XPS, Combined small characterized modified graphenes- and wideangle X-ray scattering (SAXSESS) (Fig. 3.18i) and AFM (Atomic force microscopy)

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Fig. 3.18 Depiction of i combined small- and wide-angle X-ray diffractograms of graphite, GO Ph-iGO, and TRG. Profiles were vertically shifted for clarity. Scattering reflections from the layered spacing of graphitic carbons are marked by arrows. ii Contact-mode AFM scans of GO and TRG on mica substrates, and their height profiles (insets) along the straight white lines. iii WAXD profiles of TPU composites. The inserts are WAXD patterns in 2θ = 3.5–13° for melt-blended TRG, solventblended PhiGO, AcPh-iGO, and in situ polymerized GO composites. iv TEM micrographs of TPU with a 5 wt % (2.7 vol %) graphite, b, c melt-blended, d solvent-mixed, e, f in situ polymerized ~3 wt % (1.6 vol %) TRG, g solvent-mixed 3 wt% (1.6 vol%) Ph-iGO, h AcPh-iGO, and i in situ polymerized 2.8 wt% (1.5 vol %) GO. Reproduced with permission from [68]

(Fig. 3.18ii). XPS scans revealed an up to 30 mol% increase in oxygen concentration in graphite after oxidation and also a reduction in oxygen count in TRG, indicating the successful partial reduction of GO. The characteristic sharp peak of pristine graphite at 2θ = 26.4° was observed from the X-ray diffraction pattern with dspacing 0.34 nm. After functionalization, interlayer spacing increased to 0.70 nm with a shifting in peak position towards the lower side (2θ = 12.7°) as anticipated. Incorporation of phenyl isocyanate during functionalization further lowered the 2θ value with increased d-spacing due to bulky isocyanate moieties. Morphological studies of TPU composites were evaluated by TEM (Fig. 3.18iv) and WAXD (Fig. 3.18iii) where TEM micrographs showed thick tactoid like structure (Fig. 3.18a) in melt blended graphite composite, but homogeneous dispersion of then sheets were

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observed (Fig. 3.18b) in case of TRG under same processing condition. Further investigation revealed oriented, a stacked structure in melt-compounded TRG due to higher matrix viscosity in melt state (Fig. 3.18c) compared to randomly distributed then sheets in solvent blended or in-situ polymerized composites (Fig. 3.18d)and f). WAXD pattern of solvent blended or in-situ polymerized TRG composites received no reflections from layered nanoparticle confirming exfoliated morphology of the same. In contrast, a sharp peak at 2θ = 26.4° appeared in melt-blended graphite composite, suggesting no splitting of carbon layers. (Fig. 3.18g) displayed wellseparated GO sheets in isocyanate functionalized (iGO) composite but higher magnification (Fig. 3.18h) revealed few stacked layers of iGO, which was also in agreement with peak broadening at 2θ ≈ 4–7° in the WAXD.

3.4 Application of Graphene/TPE Nanocomposites Graphene, since its inception, has been in the limelight owing to these various character traits and is being used in diversified areas of application, including solar cells, hydrogen storage materials, next-generation composites, and supercapacitors. The reinforcement of graphene into the polymer matrix has permitted us to fabricate several tailor-made properties targeted for spheres in several high-end applications. Young et al. had fabricated graphene nanoplatelets reinforced thermoplastic Elastomers with a variation in the diameter. The microstructures were interpreted under the scanning electron microscope along with the quantification by the Raman Spectroscopy [69]. It was found that the graphene sheets significantly contributed to the enrichment in the overall mechanical properties of the system. The stiffening mechanism was studied by the shear-lag theory and the rule of mixtures. It was confirmed that the effective length to diameter ratio decreased with an increase in the filler loading due to agglomeration. The model is useful to study the various rheological aspects of graphene reinforced thermoplastic Elastomers [69]. Tarawneh and his co-workers had fabricated a novel blend of polypropylene and natural rubber using liquid natural rubber as the compatibilizer to study the electrical and the mechanical improvements in the system after incorporating the graphene nanoplatelets [70]. The young’s modulus and the strength of the material reached an optimum value, after which it started to depreciate. The maneuvering of the graphene’s network enables researchers to work on enhancing the electrical conductivity of the composite. The research proved that the strong interfacial adhesion attributed to the thermoplastic elastomer aids in the improvement of the mechanical property and the electrical conductivity [70]. Professor Ham’s group had used a novel polyolefin-based thermoplastic elastomer poly (ethylene-ter-1-hexene-ter-divinylbenzene) to blend it with high quality of graphene using the solution casting method. The composite with an enhanced

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mechanical and electrical property could be used in defense applications for fabricating lightweight EMI shielding materials [71] (Fig. 3.19). Nowadays, personal care applications are employing printable strain tensors modified from polymer nanocomposites owing to their exhibition of piezoelectric characteristics. The polymers used for these applications are majorly made of chemically cross-linked polydiene or polysiloxane, which ends up being a dormant waste after its consumption. Guo et al. had observed that an ultralow reinforcing percentage (0.1 wt%) of graphene in the Thermoplastic polyurethane elastomer exhibited electrical conductance. The nanocomposites were exposed to strain sensitivity testing in which it showed a gauge factor of 0.78 for the matrix where the graphene content was 0.6 wt% at a strain rate of 0.1 min−1 . On reducing the graphene content to 0.2 wt%, the sensitivity was recorded to be 17.7 at a strain rate of 0.3 min−1 [72]. The nanocomposites at the low filler loading displayed an enduring behavior when the

Fig. 3.19 SEM images of graphene reinforced thermoplastic elastomer poly (ethylene-ter-1hexene-ter-divinylbenzene) a pristine thermoplastic elastomer poly (ethylene-ter-1-hexene-terdivinylbenzene), b thermoplastic elastomer poly (ethylene-ter-1-hexene-ter-divinylbenzene) with 1% graphene, c thermoplastic elastomer poly (ethylene-ter-1-hexene-ter-divinylbenzene) with 3% graphene and d thermoplastic elastomer poly (ethylene-ter-1-hexene-ter-divinylbenzene) with 5% graphene. Reproduced with permission from [71]

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cycle was reproduced. Numerical analysis and simulation analysis with the Tunneling theory was developed to study the response of the strain under various strain rates. The resistance strain behavior was mainly addressed because of the transition of the conductive pathways and tunneling distance when improvised with a strain [72].

3.5 Conclusion The advent and the evolution of the novel 2D material have contributed significantly to the nanoelectronics industry. With the discoveries of the functionalized graphene, the application areas have broadened its scope beyond nanoelectronics, including biomedicine and energy. Although there are various issues still associated with the scaling up of high-quality graphene, we believe with an annual growth rate of 40% and the introduction of artificial intelligence; graphene can play a pivotal role in shaping the next generation nanotechnology.

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63. Backes, C., Higgins, T.M., Kelly, A., Boland, C., Harvey, A., Hanlon, D., et al.: Chem. Mater. 29, 243–255 (2017) 64. Chen, X., Dobson, J.F., Raston, C.L.: Chem. Commun. 48, 3703 (2012) 65. Wengeler, R., Nirschl, H.: J. Colloid Interface Sci. 306, 262–273 (2007) 66. Yuan, B., Bao, C., Qian, X., Jiang, S., Wen, P.,Xing, W., Song, L., Liew, K.M., Hu, Y.: Synergetic dispersion effect of graphene nanohybrid on the thermal stability and mechanical properties of ethylene vinyl acetate copolymer nanocomposite. Ind. Eng. Chem. Res. 53, 1143−1149 (2014) 67. Tayebia, M., Ahmad Ramazani, S., Hamed Mosaviana, M.T., Tayyebi, A.: LDPE/EVA/graphene nanocomposites with enhanced mechanical and gas permeability properties. Polym. Adv. Technol. 26, 1083–1090 (2015) 68. Kim, H., Miura, Y., Macosko, C.W.: Graphene/polyurethane nanocomposites for improved gas barrier and electrical conductivity. Chem. Mater. 22, 3441–3450 (2010) 69. Liu, M., Papageorgiou, D.G., Li, S., Lin, K., Kinloch, I.A., Young, R.J.: Micromechanics of reinforcement of a graphene-based thermoplastic elastomer nanocomposite. Compos. A Appl. Sci. Manuf. 110, 84–92 (2018) 70. Tarawneh, M., Yu, L.-J., Al-Tarawni, A., Ahmad, M., Al-Banawi, S., Batiha, M.O.: High performance thermoplastic elastomer (TPE) nanocomposite based on graphene nanoplates (GNPs). World J. Eng. 12, 437–442 (2015) 71. Park, N.H., Kim, D.H., Kim, K.Y., Lim, D.Y., Ham, H.: Electrical properties of novel polyolefin based thermoplastic elastomer and graphene nanocomposites. Fibers Polym. 14(12), 2117– 2121 (2013) 72. Liu, H., Li, Y., Dai, K., Zheng, G., Liu, C., Shen, C., Guo, Z.: Electrically conductive thermoplastic elastomer nanocomposites at ultralow graphene loading levels for strain sensor applications. J. Mater. Chem. C 4(1), 157–166 (2016)

Chapter 4

Structure—Property Co-relation of Graphene/Graphene Derivative Based TPE

4.1 Introduction Graphene is a splendid material with so many fascinating adjectives to its name. Just like a coin has its two sides, even graphene has, Being the world’s thinnest material, it is one of the strongest materials to be measured to date (Fig. 4.1) [1]. Apart from having a zero-effective mass, Graphene’s charge carriers can exhibit an enormous amount of intrinsic mobility and can travel up to a few micrometers without being scattered at room temperature [1]. The reason graphene and its derivatives are used as Nanofillers in most of the polymer composites is that it can withhold current densities six orders of magnitude higher even than that of copper without disturbing any of its thermal conductively or stiffness [2]. Moreover, the material has an excellent gas barrier property and harmonizes both brittleness and stiffness to an optimized condition. On an additional note, the Dirac equation using the benchtop experiments, which describes the electron transport in graphene, relates the relativistic quantum phenomena [1, 2]. This chapter aims to analyze and retrace the recent trend in researches in Graphene and its application on a perspective to polymer and polymer Nanocomposites. We have also tried to put forward our views on the same research trends and have discerned the probable path in which the field may likely resonate in the near future. The trend of the research based on Graphene and its derived nanofillers have been magnified at a perpetual track (Figs. 4.2 and 4.3). We do see every day that several research papers have been trending based on graphene, and if the metrics and the predicted data are to be traversed back, the amount of literature based on this new age material will treble in the next few years [3]. This makes the synchronization between the novel concepts developed every day and the scaling up of these prototypes in the Industry a bit difficult as far as Graphene is concerned. On an additional note, due to the large number of novel developments being synthesized, it is very difficult for the new age scientists to concord the previous researches and the new fabrications. To resolve this conflict, several books and reviews have been in its inception based © Springer Nature Singapore Pte Ltd. 2020 A. Bandyopadhyay et al., Engineering of Thermoplastic Elastomer with Graphene and Other Anisotropic Nanofillers, Engineering Materials, https://doi.org/10.1007/978-981-15-9085-6_4

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Fig. 4.1 Graphene—the breakthrough material (https://www.autocar.co.uk/car-news/industry/gra phene-breakthrough-material-could-transform-cars, accessed on 12/07/2019)

on graphene and its derived material [4]. The electronic and the charge properties of graphene were recently discussed in an extensive review by Castro [5]. On a special note, more focused properties of Graphene, such as the quantum Hall effect, Raman Spectroscopic properties, and epitaxial growth, were amalgamated by Kim [6]. Because of the large amount of data and the literature available for graphene, the research has reached a certain epitome where a comprehensive update is needed to encapsulate the past trends, the present ongoing researches, and the future graph of this spotless material [1]. As an attempt to redefine the track of this material, this chapter has been designed in such a way that it provides a comprehensive summary to the existing pieces of literature and, at the same time provide an assertion to the readers as to where the limelight on graphene as various nanofillers may lead to (Fig. 4.4).

4.2 Some Specialized Properties of Graphene and Its Derivative Relevant to New Age Application Graphene, which is essentially a single atomic plane of Graphite, is isolated to an extent from its bulk so that it has the ability to possess advanced autonomous properties. We are very familiar with atomic planes as a part of the bulk crystals, but one atom thick material such as Graphene remained undermined for a very long period. The prime reason for the assertion is that nature disallows the forth of low dimensional crystals [2]. It is implied that the growth of crystal will induce a rise in temperature, which untimely deteriorates the stability of the macroscopic one dimensional and the two-dimensional structures. It is possible to grow flat molecules and nanometersized crystallites, but as their lateral size enlarges, the phonon density summed up over the three-dimensional spaces for high energy vibration intensifies splitting on a macroscopic scale. The above phenomenon pushes the two-dimensional crystallites to transform into a stable three-dimensional structure [1, 2].

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Fig. 4.2 Top—trend analysis of Graphene researches. Reproduced with permission from [4] and accessed on 12/07/2019. Bottom—An illustrative pie chart along with a complimentary bar graph to visualize the categories of the manuscript with the index term ‘graphene.’ (Source Science Direct, Elsevier, with a sample size of 10971, Plotted with the accessed data on 24/05/2020)

In a myopic vision, it may seem that fabricating a two-dimensional layer may be impossible. Indeed one can synthesize a single layer as a part of an inherent threedimensional system (on top of a primary crystal) [8]. The bulk then can be removed at a relatively lower temperature such that the thermal and the energetic vibrations are unable to break the bonds, especially in the synthesized two-dimensional crystals, and then morph them into three-dimensional structures, if needed [5, 8]. This empirical theory leads us to two major routes for fabricating three-dimensional crystals. The

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Fig. 4.3 Comparison of analysis of Graphene and its derived researchers. Reproduced with permission from [4] and accessed on 12/07/2019

Fig. 4.4 Comparison analysis of graphene and its derived researches as a function of the countries worldwide. Reproduced with permission from [7] and accessed on 12/07/2019

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Fig. 4.5 Schematic illustration of the main graphene production techniques. a Micromechanical cleavage. b Anodic bonding. c Photoexfoliation. d Liquid phase exfoliation. e Growth on SiC. f Segregation/precipitation from the carbon-containing metal substrate. g Chemical vapor deposition. h Molecular Beam epitaxy. i Chemical synthesis using benzene as a building block. Reproduced with permission from [8]

first approach being mechanically diverging layered material into a single atomic plane. In fact, this was exactly how graphene was incepted, popularly known to us as the scotch tape technique (Fig. 4.3) [8] (Fig. 4.5). Although the process is time inefficient, it provides crystals that are tempted with high mechanical and electronic quality plunging in the order of millimeters. Since the technique yields an excellent quality of the crystals, the process remains one of the prime choices for synthesizing graphene, at least in the laboratory scale and validating the proof of concept. The process can be modified further into an automated system where the separation of the layers may be carried out by the application of an external field such as ultrasonic cleavage [8]. The ultrasonic cleavage further stabilizes the system and assists in the formation of the crystals at a submicrometer level of graphene. This newly developed crystallite can now be used to make advanced polycrystalline films and smart composite materials [9]. The ultrasonic cleavage works in a similar fashion where the chemically destabilized graphite is intercalated and, as a result of which the atomic planes get partially detached, making the overall phenomenon to be more efficient [6, 9]. The second approach is starting with the graphitic layers, which are fabricated epitaxially on top of the other crystals [10]. This is the exact 3D development during

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which the substrate, along with the bond-breaking variation, is undermined. As the epitaxial structure is cooled down, a technique known as the chemical etching is used to remove the substrate. The aforesaid technique is quite popular for making SiN membranes. However, as theory states, the stability of a single atom thick crystal was calculated to be impossible, and no one did try out this route in recent years [11– 13]. It was in recent times that the breakthrough arrived, which proved the isolation of this epitaxial monolayer, and their overlaying to a feasible binding substrate [14] (Fig. 4.6). As time advanced, the research and the scientific trends of graphene synthesis looked to be an easy job to be carried out even at an industrial scale. If we take an example of an identical system comprising of tungsten layers synthesized epitaxially on a tin Ni film on top, we shall find a similar phenomenon as it should be observed [15]. The process can be continued by the process of chemical vapor deposition of a carbon monolayer. On an additional note, the growth of the graphene on Ni can be withheld or paused by misbalancing the lattice (Fig. 4.7) [15]. The phenomenon may be carried out by depositing a polymer on the top of the surface and etching the Ni layer, which was previously synthesized. This would lead to a single graphene layer on an insulating and accepting surface, while the tungsten wafer may be used for another round of production if any [15].

Fig. 4.6 SEM image of graphene layers on SiC. Reproduced with permission from [14]

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Fig. 4.7 Epitaxial growth and functionalization of a graphene monolayer. a Clean Ni surface. b Graphene monolayer was grown by CVD. The unit cell with the nonequivalent A and B atoms is indicated. c Potassium atoms intercalate between the Ni and the graphene. The corresponding XPS spectra for d a clean Ni surface, e an epitaxially grown graphene monolayer on Ni, and f a potassium intercalated graphene monolayer with K/C = 0.69. Reproduced with permission from [15]

Ni proves to be a demonstrative model for the above kinetics, and there are several reported papers where researches have used copper for the formation of the accepting layer. Recently, wafers of continuous graphene monolayer have been successfully fabricated on the model Ni polycrystalline films and have been transferred to plastic and Silicon matrices [15]. To add more attributes to the phenomenon, these films display electron carrier mobility of around 4000 cm2 V−1 s−1 which is near about the literature data obtained that of a non computed cleaved graphene [15]. The question arises that where does this event facilitate the growth of graphitic layers on SiC? These technologies have been considered one of the most potent

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routes for the fabrication of graphene wafers for various applications, including in electronic devices. The primary reason behind using the same as the substrate is that SiC autonomously provides an insulating surface. As the literature suggests, SiC induced graphene are generally of two major types [16]. The first one is the blend of both single and double layer crystals grown on the Si side of the material while the other is popularly known as the “multilayer epitaxial graphene” that grows on the carbon face (Unlike Silicon in the former case). For the blend of both the multilayer’s, the carbon layers are loosely bound to the substrate. This allows the retention of the Graphene’s linear spectrum (>0.2 eV) unmatched with the charge neutrality point [16]. Nevertheless, the bimodal interaction with the substrate allows the graphene to facilitate with strong doping of approximately 1013 cm−2 along with low energy disorders [16, 17]. One of the major setbacks faced by this technique was that the crystal quality and the surface homogeneity were not up to the mark. Emtsev et al. recently improved the same, making the technique and the fabrication route more viable [18, 19]. As far as the later technique is concerned, the turbo static graphene, which is an alternate name to the epitaxial multilayer are rotationally disordered, having no Bernal stacking [20]. Moreover, the layers are separated by a distance which is slightly greater than that of graphite. These so-called turbo static graphene layers exhibit the Dirac spectrum of autonomous graphene, including minimal doping and high electronic quality, where the electron carrier mobility is around 250,000 cm2 V−1 s−1 at room temperature (Fig. 4.7) [21] (Fig. 4.8). Scientists have hypothesized that these exceptional features can be seen in this marvelous material due to the weak electronic coupling between the submerged layers, their protection from the surrounding by outer layers, and the absenteeism of microscopic corrugations. These turbo static graphene offers selective amounts if the potential for electronics because of the underlying fact that the external field is synchronized within the surface layers, thus screening the same [1, 20, 21]. But these features provide graphene an excellent material to deal with it as a nanoparticle. Whichever direction or perspective through which we may look graphene through, it is evident that the challenges which were quite impossible a couple of years back now seem possible because of the aforesaid growth, transfer, and cleavage techniques.

4.2.1 A Succinct Update on the Quantum Perspective of the Graphene One of the most trending topics in regards to graphene researches is the electronic properties of the material [2, 3, 5]. Although the area is one of the most revisited topics since the last couple of years, we have tried to amalgamate the same as a short piece of communication. Indeed there are various other condensed matter systems apart from graphene, but the electronic properties of graphene are idiosyncratic. The primary attribute for resulting in the same is Graphene’s electronic spectrum. The

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Fig. 4.8 Three different stackings for trilayer graphene (Simple hexagonal, Bernal, and Rhombohedral) and the corresponding calculated electronic structures. Reproduced with permissions from [20]

electrons which are being directed through the honeycomb-like lattice effectively lose their net mass, which facilitates the moieties to be in a quasi-particle status that is best observed by the Dirac-like equation instead of the traditional Schrödinger equation (Fig. 4.9). Although, as we know, the traditional Schrödinger equation is amongst the best empirical formula to study the quantum realms of materials doesn’t hold for graphene because of its eccentric charge material and zero rest mass. Figure 4.9 gives a glimpse of the conclusions of how our thoughts have mutated since the inception of graphene and the Dirac Like equitation. On an additional note, the propagation of the electron waves within the graphene is carried inside a monolayer whose thickness is about the size of an atom. This particular feature not only makes the material susceptible to various scanning probes but also more sensitizing towards high dielectrics, superconductivity, and ferromagnetism, making it the most tunable material when compared with the other two-dimensional electronic system [1, 22]. Furthermore, Graphene’s electronic quality is unmatched. Even at rough surfaces or substrates that are covered with adsorbents, Graphene’s electron can traverse submicrometer distance without being deflected without the induction of any thermal agitation [23, 24]. Since graphene has a rest mass of zero, the quantum domains in graphene are bolstered and can even remain static at room temperature. Researchers in the past had focused on what the current trend has cultivated form the Dirac like equation within

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Fig. 4.9 Quasi-particle System. a Charge carriers in condensed matter physics are normally described by the Schrödinger equation with an effective mass m* different from the free electron mass (p is the momentum operator). b Relativistic particles in the limit of zero rest mass follow the Dirac equation, where c is the speed of light, and → s is the Pauli matrix. c Charge carriers in graphene are called massless Dirac fermions and are described by a 2D analog of the Dirac equation, with the fermi velocity vF ≈ 1 × 106 m/s playing the role of the speed of light and a 2D pseudospin matrix → s describing two sublattices of the honeycomb lattice (3). Similar to the real spin that can change its direction between, say, left, and right, the pseudospin is an index that indicates which of the two sublattices a quasi-particle is located. The pseudospin can be indicated by color (e.g., red and green). d Bilayer graphene provides us with yet another type of quasi-particles that have no analogies. They are massive Dirac fermions described by a rather bizarre Hamiltonian that combines features of both Dirac and Schrödinger equations. The pseudospin changes its color index four times as it moves among four carbon sublattices. Reproduced with permission from [1]

the model condensed matter formalism [23, 24]. The present trend of the quantum model of graphene has led to the invention of the Hall Effect and the allied simulated models such as the Klein tunneling [25], zitterbewegung [1], the Schwingerproduction [26], supercritical atomic collapse [27], and Casimir-like interactions between substrates on graphene [28]. The past couple of years had produced multiple numbers of articles on Klein tunneling [29, 30]. Although, as theory stated, the real-time monitoring of the Graphene’s transport properties has yielded in much complex quantum electrodynamics and some basic queries relevant to the Graphene’s electronic spin properties still remain to be resolved. For instance, there is no proven fact that defines the limiting factor of the electron mobility for the scattering phenomenon, as observed in graphene. Secondly, there are no concrete shreds of evidence to prove the transport properties of Graphene as a nanoparticle [31], and finally, there are more than a hundred interaction fields that are yet to be discovered. We predict that in the near future, the research forecasted above shall continue to bulge, keeping the realms of the other low dimensional system with the aid from the recycling technologies and phenomenon. It is indeed that we strive to rediscover science every day, but various potent research moieties such as graphene-based

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carbon dots [32–34], nanoparticle-based p-n junction [19, 20], nanoribbons [35, 36] and magneto-transport phenomenon [37] have not received the sustained recognition they deserve. Interesting designs involving the super lateral lattices, magnetic tunneling, optical sciences using electrons, and several ballistic applications can redefine in a more efficient way incorporating graphene into the system (Maybe we can get a better picture of the supramolecular physics associated with the system) [1]. Scientists are also diving into the electronic and the magnetic properties coupled with the optical properties of systems, in which graphene provides several scopes of inventions and discoveries. As we know that although graphene is a structurally malleable material, its properties (such as the electronic, magnetic and optical) can be redefined using the application of strain and deformation [38, 39]. An exhibiting of strain results in the creation of the local gauge fields [3], and finally alters Graphene’s electron band conformation. This is why the innovations on graphene-based on folding actions and scrolling actions are getting more accelerated day after day. Modified graphene and turbo static graphene are a potential scope of application for scanning probe microscopy [40, 41]. Furthermore, several simulated experiments may be created using this material involving screening using supercritical conditions, wave mapping, and magmatic moment traversing detection. In the case of graphene, the split bilayers interact very differently than the other two dimensional materials, which makes the graphene physics more interesting as compared to the other competent materials present [41]. The current research frontier, while speaking about graphene, is the fractional quantum hall effect, which yields a plateau-like feature at fractional fillings in case of graphene, making the effect more viable to be reproduced for various applications [6]. The ideation may take several decades to get established and scaled up but shall highly depend on the synthesis of the graphene layers with the improvement in the electron mobility values. The authors do believe that when the wafer reduces to an inch size and the electron mobility reaches a value of 1 million, several physical phenomena may emerge, which can be highly interesting to study, and of course, the term ‘graphene dreams’ shall incept into a reality.

4.2.2 Dipping into the Chemistry Graphene is a fascinating material if we look at its surface phenomenon, having two side faces with zero bulk materials in-between the layers(Fig. 4.10). Despite the surface behavior of graphene is one of the most reviewed scientific trends; its scientific realms are still left to be conquered. Graphene do exhibits the chemical nature similar to that of graphite by adsorbing and desorbing various atomic moieties; in most cases, the weakly attached adsorbents act as charge donors or acceptors, and hence untimely varies the carrier concentration of the system. And this is the sole reason why graphene remains a superior conducting material [42, 43]. Allied adsorbates such as H+ and OH− influences the localized regions, which are very close

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Fig. 4.10 A piece of graphene aerogel, which weighs only 0.16 milligrams per cubic centimeter—is placed on a flower. Reproduced with permission from [42]

to the nano-level, giving the fabrication of graphene oxide, and one faced graphene, which is inferior when it comes to conductivity [30]. We have seen that the process of thermal annealing or a potent chemical treatment helps to synthesis graphene to its original version, which comparatively very few defects left within the system [30]. The atomic matrix of the material is too bolstered, and this is why we do see reversible oxidation and reduction of the base material keeping the properties intact and untouched. While describing the surface phenomenon, although the chemistry of the graphene looks similar to that of the graphitic chemistry, there are various induced differences too. The charge variations in graphene are much more expressive than that of graphite because of the absence of the bulk contribution [44]. Unlike the graphite surface, graphene does not show a typical flat structure but instead displays corrugations in the nanometric scale [44]. The localized reactivity, in the case of graphene, can be induced dominantly by the strain and the applied curvature. Reagents or other external chemical moieties could attach to both the graphene faces, which prompts to change the energetic facilitating chemical bonds that would not be in a stable state if the surface were to be exposed [44]. Linus Pauling suggested an alternate as to consider the surface chemistry of graphene as a bulged flat molecule [45]. It is evident that, like any other chemical moieties, graphene can take part in chemical reactions. An interesting fact to note here is that, in the case of graphene, the adsorbates are inherently assumed to adhere to the carbon matrix in a measured stoichiometric amount, which is more regular rather than random. This allows graphene to yield in various alternative new two-dimensional crystal structures with dissimilar and distinct properties such as chemical, optical, and electrical [45].

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Fig. 4.11 Representative images of a Hexagonal honeycomb lattice of graphene with two carbon atoms (A and B) per unit cell. b Energy momentum dispersion in graphene. c Schematic illustration of the covalent chemistry of graphene. d Band structure change of single-layer graphene near the K point of the Brillouin zone before (left) and after (right) chemical modification. (a) and (b) Reproduced with permission from [46]

The first known example of its kind is indeed graphene itself with a twodimensional hydrocarbon structure having one atom attached to every site of the lattice popularly known as the ‘honeycomb’(Fig. 4.11) [46]. Because the adsorbents are theoretically to arrange themselves into a regular and an ordered structure similar to the case of graphene, several graphene-based derivates can be synthesized for targeted usage. Instead of doping it with the hydrogen atom, as we see in the case of graphene, F−, OH− may be used for the functionalization in the quest for the fabrication of several novel graphene-based two-dimensional crystals. The entire chemistry of graphene is growing more interesting day by day as new concepts are being hypothesized. For instance, the stoichiometric derived moieties can tailor-make the chemical conformations, which can be applied in the field of electronics [46, 47]. This shall allow us to make the interconnecting circuitry of pure graphene, while modified graphene by its chemical conformation shall allow fabricating the semiconducting transistors. Even the disordered graphene-based compounds may be used as functionalized graphene in various specific sectors of the graphene industry (Fig. 4.12). If we cite the example of graphene paper which one of the classics examples is of how the disorder graphene can turn into a functionalized one [48, 49].

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Fig. 4.12 Prospects of Graphene paper. Reproduced with permission from [49]

The starting material is generally synthesized with the suspension on pristine flakes [49], and consequently, the resulting material turns out to be fragile and susceptible to degradation. Trends have been shifted in making the flakes of graphene oxide to provide higher mechanical properties and stiffness [49]. What happens in the latter case? The functional groups that were introduced bind the individual sheets together, making the structure more isotropic, similar to that of the nacre, which is widely popular for its mechanical strength. In case of nacre, which is bound by aragonite biopolymer glue, graphene oxide layers in reduced forms use the aid of stitching at an atomic level, making it one of the strongest known nanomaterials. The only severe disadvantage that graphene faces are that the compound does not have a specific surface, which is why graphene chemistry did not receive the enigmatic recognition, which is supposed to have. The problems are because graphene, as discussed, do not follow the traditional chemistry routes as other materiel does. However, recent discoveries have led to the synthesis of liquid-phase chemistry of

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graphene [1, 2] and perhaps, who knows this may be the secret key to unlock the mystery behind the graphene chemistry.

4.2.3 The Hidden Beauty 90% of the reported journals these days introduce their paper by letting the audience know that graphene has a unique electronic configuration [1]. Indeed it is true that that the essences of Dirac like quasi-particle phenomena were known, but the monolayer functionality did somewhat looked haywire [6]. Speaking on this context, graphene has established itself as a novel material, but unfortunately, the negligible amount of data are available on its non-electronic behavior [6]. But for the past couple of years, the situation has camouflaged into something distinct research community, which lets us, view Graphenes through a new perspective. In the year 2008, both the mechanical and the thermal properties of graphene were measured, disrupting the entire scientific community. Graphene exhibited a breaking strength of around 40 N/m, which is almost theoretically the highest [50, 51]. On an additional note, the reported thermal conductivity was 5000 Wm−1 K−1, and the Young’s Modulus displayed around 1 TPa [52]. Interestingly graphene can be stretched 20% more than any other conventional crystals [52]. The reported data was already observed in carbon nanotubes and graphite, which were structurally somewhat analog to that of graphene sheets. The phenomenon of the micromechanical cleavage in the system allows the graphene to be theoretically absent of the crystal defects. Even the samples that do not correlate s find themselves in the space of comprehension. If we take an example with the graphene, the material shall shrinks as the temperature rises because of the major dominance of the membrane phonons [50–52]. Often do we find a higher value of pliability within the graphene accompanied by brittleness (the sample breaks similar to that of glass) at high strain levels [52]. The former and the later sentences sound like an oxymoron, but on an interesting note, graphene has a property that combines both the above-described properties. It is highly anticipated that a thick monolayer film is impermeable to gases [53]. If we contrast the above phenomenon with the graphene wafers, there is a possibility of collision of the molecular and the ion vacillating through graphene with the fabricated pore. Now coming to the point of the non electronic properties, researchers still don’t know how the melting kinetics of graphene behaves. To add more, neither do we know the order of the phase transition of graphene nor the exact melting point of the material. Superthin films are expected to showcase melting temperatures that radiantly decreases with the fall in temperature [1]. To be very specific, the thermodynamic phenomenon of the two-dimensional membranes in a three dimensional space is very different when talking about the thin films, and they are more likely to resemble that of the soft membranes (Fig. 4.13). We can predict that, by the Kosterlitz-Thouless transition, melting can proceed in the material through the generation of defects

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Fig. 4.13 A visualization of the thermal properties of graphene and nanostructured carbon materials. Reproduced with permission from [53]

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and might depend on the lateral size [54]. Although the experimental and analytical chemistry researches carried out in the field of Graphene’s thermodynamic properties are being retarded by various obstacles, we hope to overcome those soon. Even the theoretical chemistry-based graphene research has remained in the slower track because of the minute size of the crystals, which is a defined problem in molecular dynamics and several other numerical approaches which fail to correlate the physics of in the size of the nanoscale [1, 54]. Till now, there are thousands of research papers that deal with the various applications of graphene, and the recent two years have experienced a quantifiable amount of progress in the field of graphene science. The substantial breakthrough amongst the technologies is the bulk production of graphene. The technique of scaling up had changed the entire scenario of graphene from a limited set of applications to a broader domain of application keeping novel inventions and developments in the backdrop. The maximum amount of Graphene’s achievement is discovered in the fields of computer science and electronic science [55, 56]. A peculiar example to describe the same is that graphene is now known as the electronic material beyond the age of silicon [55]. Although the disadvantage lies in the fact that Graphene’s property cannot be evaluated exactly, graphene finds its importance in the field of Nanoscience, where the conducting channel becomes the important part of the planned architecturebased circuit. Several models have been proposed to date to bridge Graphene’s properties with its potent application. In the recent trends, nanoribbons transistors have been proposed, with a higher ratio of the on/off values even at room temperature [57, 58]. Even after these prospects, the buzz around the phrase ‘graphene inside’ has remained something in the grey area. Scientists claim that this is not because several features of the graphene have left undiscovered, but because there are not enough precision tools to predict the properties. We feel that more efforts are needed in the upbringing of the precision tools to investigate the background properties of graphene and its derivatives. More experimental tools ad models should come up with atomic precision to narrow down the accuracy of graphene. As a counter assertion, we must state the application of graphene in the transmission electron microscopy. Although the application is mundane, graphene proves to be a potent material in the electron microscopy because of its single-crystal membranes, monoatomic thickness, and negligible atomic mass. Now that we have commercially viable micrometer sized crystals available in solution [8], for their deposition into the standardized grids coupled with films which can be transferred from metals [1, 8] onto these platforms in a cheap and a user-friendly fashion had made graphene a superior material for the transmission electron microscopy and its aided instrumentations (Fig. 4.14). Indeed, the bridging gap between the Graphene’s properties and the immediate use is framed with the applications. Let us take into consideration an example which is neither too fascinating nor boring: Mono layered ultrahigh-frequency analog transistors. Currently, his area is controlled by Gallium-Arsenic based devices, which are commonly known as the high-electron-mobility transistors (HEMTs), extensively used in communication devices. Now, what exactly graphene does to the system? A concise answer to that shall be graphene extends the overall operational coverage of the existing range into the terahertz frequency.

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Fig. 4.14 Micrographs of a Graphene nanoribbons of sub-10-nm scale exhibit the transistor action with large on-off ratios b All the fundamentals are in place to make graphene-based HEMTs. This false-color micrograph shows the source and drains contacts in yellow, two top gates in light gray, and graphene underneath in green c Graphene-based NEMS. Shown is a drum resonator made from a 10-nm-thick film of reduced graphene oxide, which covers a recess in a Si wafer d Ready to use: Graphene membranes provide ideal support for TEM. Reproduced with permission from [1]

Graphene, even at room temperature, exhibits a ballistic transport phenomenon (the charge transition between the source and the receiver takes 0.1 ps for an operational channel length of 100 nm) [59]. The gate electrons are placed high above the graphene making the transits shorter and allowing swift transport. The gapless spectrum of graphene yields a very minimal on-off ratio (10–100); they are elucidated as perfect for the analog electronic theory. The fast progression of graphene in the field of HEMT in suppurating the accession of the range in the microwave region. The first frequency test o the graphene transistors was a new event in the time course of the graphene development [59]. 30 GHz was the cutoff frequency, which was hindered by the long channels and the slow electron mobility. The only positive aspect of the phenomenon is that the scaling that was studied based on the wavelength in contrast to the channel length and electron mobility gives us a perspective that the terahertz frequency maybe accessible [59]. With the functionalization of graphene, these ultra-high frequency devices, including the electronic circuits, shall definitely evolve and have a potent ideology to scale up and seize the current electronic market.

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The current reviews on graphene suggest that graphene can theoretically be used in all composite and aided materials [60]. The statement is often asserted with the fact that carbon nanotubes have been a long-standing example and guidance for various novel applications. The graphene-based powder is considered to be excellent nanofillers for composite materials [60], while rapid progress has taken place on graphene based batteries, supercapacitors, fielded emitters, and interconnects. Even though graphene is a most buzzed new age material, it is impossible to predict whether graphene can be at par with its kin comprising of the nanotubes family. Indeed graphene and graphene-based derivatives have turned into the most viable material in the field of optoelectronics [60]. The production of graphene-based coatings aided by suspensions using the spinning and the printing technique in a cheaper aspect turns out to be a new aspect in the field of nanotechnology. An alternative to this technology is the preparation of the films that are fabricated on the nickel host. Scientists often claim these coatings as a competitor to the existing designed coatings such as the Indium Tin Oxide, which are used in applications like solar cells, liquid crystals displays, and light-harvesting materials [1]. However, there exists a fundamental flaw with graphene is the fact that graphene reports resistivity of more than five hundred ohms against the standard calibrations of nearly eight percent, which makes the material inferior (as compared to the Indium Tin Oxide) when it comes to the application of solar cells. We all have to wait and see whether the current prospects on graphene can improvise on the required property based on specific applications. Now, as there always exists a flip over the flop, graphene does exhibit several advantages over Indium Tin Oxide [1]. Being chemically stable complemented with a reinforced and flexible structure, graphene provides an edge over the other traditional material in touch screen and flexible electronics applications. Nano-mechanical systems are developing rapidly fortified upon the advantages of the graphene-based materials [61] owing to the two side advantage which graphene provides being light and stiff. If we compare nanotubes in this context, graphenebased resonators are advantageous in the sense that they provide not only low inertial mass but also ultra-high frequencies that perfectly synchronizes with the tempo of the electronic circuits [59, 61]. As far as reported data are concerned, graphene-based membranes tend to display a quality factor of 100 at even 100 MHz frequency range [57–59]. This has eventually led to the fabrication of the drum reactor resonator using films synthesized from reduced graphene oxide. The single-layered nanometric thick polycrystalline nano-electromechanical systems show a fascinating Young’s modulus along with a superior quality factor of more than 4000 even at room temperature [52]. Microfabrication techniques are generally employed to develop these films from the pre-developed wafers. On an additional note, it is expected that the functionalization and the modification of the graphene shall facilitate the graphene nanoelectromechanical systems to supplement the inertial sensing of atoms and recording the zero-point oscillations. Lab on chips or the so-called electronic noses is one of the prime areas which are being complemented by the graphene research for various resistive memories. It is all due to the fact that graphene has one of the condescending sensitive to its chemical

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environment capable of detective gas molecules even at any ambient temperatures [62]. Furthermore, let us imagine a set of graphene systems, with each array functionalized differently and uniquely, so that they may react to different chemicals or higher organic molecules. The systems have been well tested with carbon nanotubes, and graphene contributes to the mass production and scaling up of these pilot projects of chemosensors [60]. Various references demonstrate the reversible switching resistance of graphenebased wires, especially in the area in the application of the sequencing current pulses [63]. Well, the deep mechanisms underlying the above phenomena are still to be discovered, but the scale switches in the dimensions of nanoscale provide an insight into the phase change behavior of the material. Trends have also shifted towards the graphene derived ferromagnetic phase-changing materials which definitely needs a wider perspective of vision than it is now in its nascent stage [63]. Graphene has metamorphosed quickly from a position of uncertainty to a new age material having unexpected superior properties- gravitating into a material of high standards. The initial debate which the people held regarding the inferiority of graphene is gradually diminishing owing to the materials’ recent discoveries in its property values. Still, graphene does remain as one of the veered materials in the development of the chemical sciences. For instance, the researches on Graphene’s electronic property have been unfolding, which is letting us revisit graphene once again with several perspectives on the backdrop. This is possible since the advancements in the quantum transport phenomenon of graphene by strain engineering and other structural modification, which were something very gloomy in the past decades. We do believe that graphene shall remain the most retraced material in the field of functional material and condensed matter. The frontier developments in the Graphene’s nonelectronic properties have led to the disruption of the entire graphene chemistry phenomenon. We do hope that these developments just not grow but erupt, with the introduction of the advanced characterization and high precision tools coupled with machine learning and artificial intelligence making the statement “There’s Plenty of Room at the Bottom” more remarkable. Enhancing the polymeric properties and reduction of the material cost using various kinds of fillers such as talc, carbon black, glass fiber, layered silicates, and calcium carbonate has been one of the prime research topics for over the years [64]. Researches claim that fillers below 100 nm of size provide the optimized level of physical reinforcement along with the increased mechanical efficacy as far composite materials are concerned [64]. Amongst all the various kinds of nanofillers that have been introduced into the scientific community, the carbon-based nanofillers comprising of the carbon nanotubes and graphene nanosheets have been one of the trending interest in the field of polymer chemistry due to their high aspect ratio coupled with superior mechanical support [65]. The polymeric Nanocomposites have thus found a realm of science because of the fascinating properties of graphene (including mechanical, electrical, and permeability), which allows it to disperse in various matrices [64, 65]. The graphene is primarily composed of tightly packed flat

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Fig. 4.15 Pictorial representation of the atomic structure of a carbon atom along with the Energy levels (a and b) of outer electrons in carbon atoms. c The formation of sp2 hybrids. d The crystal lattice of graphene, where A and B are carbon atoms belonging to different sub-lattices, a1 and a2 are unit-cell vectors. e Sigma bond and pi bond formed by sp2 hybridization. Reproduced with permission from [66]

single-layer carbon atoms where the lattice conforms to a honeycomb-like structure having sp2 conjugated planar orientation (Fig. 4.15) [66]. When the first discovery of graphene was made in 2004 by Novoselov and his teammates, they thought the structure to be unstable at even ambient temperatures [67]. But with the advancements in science, researchers are striving to fabricate graphene with all the possible synthetic routes to increase the stability of this new age material. Presently there are more than twenty prominent methods for the synthesis of graphene popularly classifies as the bottom up or top don synthesis, Processes like chemical vapor deposition, arc discharge [68]. Chemical conversion, reduction of CO, microemulsions, unzipping of carbon nanotubes, and epitaxial growth are commonly categorized under the bottom-up technique [68]. The top-down synthesis method of graphene is fabricated mainly by the exfoliation of graphite or graphitic derivates [69]. Recent studies have shown that monolayered graphene exhibits a young’s modulus of around 1TPa, an ultimate tensile strength of 130 Pa, a recorded thermal conductivity of 5000 Wm−1 K−1 and a very superior electrical conductively of 6000 Scm−1 [70]. Rolling of one thick atomic graphene is termed as carbon nanotubes attributed by one-dimensional tubular structure [70, 71]. The high surface area with a theoretical limit of 2630 m2 /g makes graphene one of the most useful and apt nanomaterials used in various Nanocomposites for improving different degrees of the properties [71]. Scientists are now trying to incorporate graphene and graphene sheet derivatives into the polymer, aiming to improve the overall material processing characteristics. Like the carbon nanotubes, graphene reinforced nanocomposites are employed for instance as in the case of Graphene Sheet Derivates/polystyrene [72], Graphene Sheet Derivates/epoxy [73], and Graphene Sheet Derivates/natural rubber (NR) [74] to bolster both the physical and the chemical properties as compared

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to the virgin materials. To date, the prime challenge which scientists face when employing the technology of the Nanocomposites is the synthesis of the homogenized dispossession of the nanofillers, especially graphene derivates into the matrices. This nimble distortion in the processing of these composites occurs primarily due to the differences in the interfacial interactions. The interaction with the matrices conducts weaker interface interactions, while the graphene sheet surfaces exhibit higher attractions between the interfaces [75]. There have been several pathways which the technologists have paved to address the problem of the homogeneity [75]. But even after several trials and efforts, there is a lack of perfect method which may yield in a perfect homogenous Nanocompositeyielding the maximum benefits for its subjected applications. The authors believe that there is a scope to explore more on the scientific domains keeping the Polymers/Nanocomposites dispersion on the subject line. The past couple of years have experienced various amounts of potential works in the field of the carbon nanotubes [76] and the polymer Nanocomposites [77] based on graphene and graphene sheet derivates, but most of the reported experiments are not yet addressed properly and with concordance. This summary aims to encapsulate the idea of the various issues linked to the Nanocomposites enhanced elastomeric compounds, including various new-age smart elastomers such as thermoplastic elastomer and thermoplastic vulcanizates for the synthesis of the industrial and small scale devices for aerospace, surface engineering, medial and energy applications. For the past ten years, the number of journal papers and patents published concerning the elastomeric compounds mended with nanofillers has elevated, proving the rising importance of the area in the fields of science and technology (Figs. 4.16, 4.17, 4.18, 4.19, 4.20, 4.21, 4.22, 4.23, 4.24) [78]. The trend also reflects the number of trending researches that has been up brought in the domains of carbon nanotubes, graphene, and graphene sheet derivates along with various anisotropic nanofillers keeping the elastomer as a base material [78]. The prime drawback of the entire research that has been carried out to date is the fact that the researches have been scattered and not consistent with their previous trails. On an additional note, the recent explored sub realms of nanotechnology like the chirality, length to diameter ratio accompanied by the advanced graphene derivates like the expanded graphene have failed to get into the picture of the recent developments in the field of nanofillers considering elastomer processing, morphology and the dispersion states [78]. In this compilation and perspective, we have tried to tie and frame the current research trends on the graphene and the aided nanofillers based on elastomeric Nanocomposites and their hybrids by highlighting both the new age developments and the advanced processing and characterization techniques. The elastomeric matrixes, which are bolstered with the various types of nanofillers, have been a prime trait to predict the final properties of the Nanocomposites. We have tried to dig deep into the structure and relationship property of the modified Nanocomposites to give the readers a better view of the influence of the nanofillers into the elastomeric matrices. To summarize, we have also presented the current challenges for the budding technologists and scientists revolving around

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Fig. 4.16 Survey of research publications on CNTs-/GSDs reinforced elastomeric matrices and their hybrid nanocomposites during the last ten years. Reproduced with permission from [78] Number of Articles published including Patents Metallurgy Process and Apparatus

ConstrucƟon

Ceramics

Power and Fuel Imaging and Recordings

Materials and Products

Substances in Technology

Fig. 4.17 To visualize the versatility of graphene based thermoplastic elastomers—I (Subject Category-Technology). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020

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Number of Articles published including Patents ApplicaƟon and Phenomenon

Miscellaneous Substances

Process and Apparatus

Polymers

Modifiers and AddiƟves

Fig. 4.18 To visualize the versatility of graphene based thermoplastic elastomers—II (Subject Category-Polymer Chemistry). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020

Number of Articles published including Patents Surface Phenomenon

Subatomics

ParƟcle Thermodyanamics phenomenon

Spectra and Spectroscopy

Substances in Property Studies

Electric and MagneƟc Phenomenon

Miscellaneous Substances

Mechanics

Fluids Phenomenon

Substances in Process

Fig. 4.19 To visualize the versatility of graphene based thermoplastic elastomers—III (Subject Category-Physical chemistry). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020

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Number of Articles published including Patents Purified Substances Electric and MagneƟc Phenomenon

ReacƟons Prepared Substances

Reactants and Reagents

Manufactured Substances

Fig. 4.20 To visualize the versatility of graphene based thermoplastic elastomers—IV (Subject Category-Synthetic chemistry). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020

Number of Articles published including Patents Substances in Agriculture

Subsatnces in medical Medicine

Substances in Biological Use

Agriculture

Food

Fig. 4.21 To visualize the versatility of graphene based thermoplastic elastomers—V (Subject Category-Biotechnology). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020

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4 Structure—Property Co-relation … Number of Articles published including Patents Formed, removed and Other substances

Environment

Geology and soil Chemistry

Fig. 4.22 To visualize the versatility of graphene based thermoplastic elastomers—VI (Subject Category-Environmental Chemistry). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020 Number of Articles published including Patents

Substances in Biology

Endocrinology

Process and Systems

Immunology

Anatomy

Fig. 4.23 To visualize the versatility of graphene based thermoplastic elastomers—VII (Subject Category-Biology). Source SciFinder, Chemical Abstracts Service, Plotted with the accessed data on 24/05/2020

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Fig. 4.24 Illustration of the chemical modification of graphene to reduced graphene oxide a Graphene, b Oxidized graphite c Separation of oxidized graphite to graphene oxide sheets on sonication, d Hydrazine reduction of graphene oxide and formation of reduced graphene oxide. Reproduced with permission from [78]

the Nanocomposites. The authors believe that this review shall bridge the existing gap between the blanks related to Nanocomposites and the elastomeric composites together with their processing properties, material properties, physical and chemical attributes.

4.3 An Overview on the Fabrication of the Graphene Sheet Derivates The first reported synthesis of graphene was way back in the year 1970, but the fabrication of the sole single-layered graphene sheet was pioneered in the year by the process of the micromechanical cleavage [78, 79]. As science grew older, scientists discovered that various other fabrication methods could yield a better property of the graphene/graphene derivates over the traditional micromechanical cleavage (especially for the rudimentary studies) [79]. Over the years, researchers have found the dispersion of the graphene into a polymer matrix to depend on the sheet size and the functionality [80]. Surface modification, especially in these cases, plays a major role in increasing the dispersion of the graphene into the target polymer [80].

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Graphene, if not functionalized, do interact with the polymer moieties aided by the π − π stacking, hydrogen bonding, and the Van der Waals forces- which in turn are very weak to produce an optimized composite [81]. Coming to the area of the modified graphene sheets (RGO), the pristine graphene flakes are exfoliated by either chemical, mechanical, or thermal induction. As the literature goes, both the widely known techniques of Hummer’s Method (Fig. 4.24) [82, 83] and Brodie’s method [84] are used to produce graphene oxide and the reduction of the same via hydrazine to yield the final product of reduced graphene oxides. The functionalization induced within the system (for instance, oxygen) shall develop an electrostatic repulsion between the moieties and, in turn, reduce the Van der Waals forces, which previously existed between the layers of the graphene sheets allowing us to separate the same (exfoliation) [80]. The product which s generated by further oxidation and the reduction (usually by hydrazine) have various functional groups that can even be a potential candidate to get further oxidized if needed [80]. The exact chemical structure of the reduced graphene oxide is often ambiguous because the final structure depends on the amount of the reduction and the type of the reduction agent used, but as a very raw prototype researcher’s claim that carboxylic acid (−COOH) groups are one of the persistent groups in the structure attached to the tail ends of the reduced Graphene Oxide. Apart from the carboxylic groups, the functional groups such as hydroxyl or amine are located generally at the base plane (as reflected from Fig. 4.17). These functional groups have two significant roles to play, the primary being to help in the process of dispersion, and the second is to prevent the layers from getting agglomerated [78]. As far as the polymer composites are concerned, surface modification of the graphene/graphene derivates is a must. The virgin graphene has negligible efficiency for the process of intercalation and hence gets agglomerated even after the process of reduction [85]. On an additional note, we know that if we synthesize graphene oxide directly from by the process of oxidation of the graphene sheet, the product shall turn into an insulating and unstable mass (both mechanically and thermally) because of the absence of the reinforced functional groups and the aided bonds [85]. Hence to avoid the defects, graphene is exposed to the process of exfoliation employing thermal agitation or sonication to yield the final product of graphene oxide, which eventually gets reduced with a strong reducing agent to fabricate the reduced graphene oxide [85]. These graphene oxides have the property to disperse well in-between the polymer chains to give rise to the intercalated structure [85]. The other advantage of this method is that the generated reduced graphene oxide does have electrical conductivity (which was absent in graphene oxide) and the dilution of the π − π stacking in graphene [85]. To increase the stability of the formed reduced graphene oxide, it is generally stabilized by a stabilizing polymer or surfactants to prevent the agglomeration of the reduced graphene oxide irreversibly in an aqueous medium [86, 87].

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4.3.1 Fabrication of the Graphene and Graphene Derived Elastomeric Nanocomposites The dispersion of the nanofillers in the Elastomers has been a recent interest to the scientists in the ongoing research era. The hybrid materials with Elastomers and nanofillers have proven to be an edge over the conventional rubber composites [88]. Similar to the nanofillers, the enhancement of the property of the graphene derived Nanocomposites depend primarily on the extent of the homogenous dispersion of the nanomaterial in the elastomer [80, 88]. The composites having low nanofillers loading shows a higher interfacial interaction at the matrix interfaces because the net stress is found to distribute homogenously. Although aromatic groups containing polymers have shown that they interact more with the nanofillers, fabricating them with an additional functionalization provides a wider scope of application in the area of the electrical, chemical, and mechanical [80, 88]. On an additional note, the method of mixing also is a determining factor for the achievement in the homogeneity of the final composite. Typically there are three major processes in order to enhance the homogeneity of the composite- solution mixing, melt blending, and in situ polymerization (Fig. 4.25) [89]. Solution mixing is a very popular technique to prepare Nanocomposites comprising of graphene sheets/functionalized graphene sheets and natural rubber/styrene-butadiene rubber [90]. The process is, at times, modified by using agents such as bis(triethoxysilylpropyl)tetrasulfide functionalized graphene oxide with acrylonitrile-butadiene rubbers [91]. Several reports have claimed that the solution mixing technique may be useful to fabricate composites using graphene oxide

Fig. 4.25 The most prevalent nanofiller mixing methods used in the fabrication of Graphene and its derived-elastomeric nanocomposites. Reproduced with permission from [78]

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and a wide range of polymer matrices ranging from fluoroelastomer, nitrile butadiene rubber till ethylene propylene diene monomer rubbers [91–93]. The graphene oxide bolstered styrene butadiene-vinyl pyridine rubber demonstrated much better dispersion of graphene oxide as compared to the virgin polymer when processed via the solution method [94]. In the above-discussed experiment, the graphene oxide was derived by the process of exfoliation of natural graphite flakes using the technique we commonly referred to as the modified Hummers method [94]. Further references lead us to the conclusion that the solution mixing is often used to synthesize Styrene-Butadiene Rubbers modified with graphene nanoplatelets to increase the overall composite properties [95, 96]. In a recent study, researchers have shown that the solution mixing method is one of the best methods to synthesize functionalized graphene oxide using poly (dimethylsiloxane) as the base matrix material [96]. Through a mechanism of self-assembly in latex and curing methods, these composites do find their application in the field of stretchable and electromagnetic shielding Nanocomposites [97]. Infect, the latex mixing method has been found very useful to develop the graphene sheets based on the typical matrix of natural rubbers [98, 99]. Potts and his coworkers used the same latex method to fabricate natural rubber composites using the framework of thermally treated graphene oxide, which definitely resulted in a higher mechanical yield compared to the products developed from the sold mixing method [98]. Even in the case of graphene nanoparticle, the nanoparticle enhanced elastomeric matrices resulted in a better adhesion between the filler and the elastomer, which made the homogeneity of the sample more transparent [99]. Melt mixing is the next trending route for the preparation of Nanocomposites based on Elastomers. This technique is employed to synthesize systems such as nitrile butadiene rubbers reinforced with layered graphene [100] and graphene nanoparticle with thermoplastic Elastomers [101] primarily to improvise on the heat stability and the mechanical behavior of the polymer system for high-temperature end-use. The exfoliated graphene, when intercalated with the styrene-butadiene-styrene rubber matrix, displayed not only high heat resistivity but also an improved curing characteristic [102, 103]. In the case of thermoplastic Elastomers, to be specific, a special technique can be used known as the latex hetero-coagulation to develop a composite system shoeing minimal agglomeration and heterogeneity in the system [104] (Fig. 4.26). The in situ polymerization is not a very popular method for the preparation of the large scale Nanocomposites derived from Elastomers [105–109]. Kim and his research group had focused on this technique to correlate the properties of the Elastomers when incorporated with graphene and its allied derivative [106]. They induced the thermoplastic Elastomers with thermally reduced graphene oxide and isocyanate modified graphite oxide using the technique of melt mixing, solution blending, and in situ polymerization [106]. Their results concluded that in situ, polymerization was able to enhance the tensile modulus up to ten times the usual values reported via melt mixing and solution blending methods [106]. Apart from the modulus, the physical properties were concluded to enhance by 80% of the original value. These improvements have titillated the fact that in situ polymerization may be used as an

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Fig. 4.26 SEM micrographs of CNTs: a purified CNTs; b ball-milled CNTs. Reproduced with permission from [103]

effective technique to synthesize elastomeric Nanocomposites, especially having the thermoplastic polyurethane as the base polymer [106] (Fig. 4.27). As a concluding note to this section, we may conclude, although we have three major techniques to fabricate elastomeric Nanocomposites, the choice of the process depends on the system that we intend to choose along with the desired properties of the final product.

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Fig. 4.27 TEM micrographs of Thermoplastic Polyurethane with a 5 wt% (2.7 vol.%) graphite, b, c melt-blended, d solvent-mixed, e, f in situ polymerized with 3 wt% (1.6 vol.%) Thermally Reduced Graphene, g solvent-mixed 3 wt% (1.6 vol.%) Ph-iGO, h AcPh-iGO, and i in situ polymerized 2.8 wt% (1.5 vol.%) Graphene Oxide. Reproduced with permission from [106]

4.4 Characterization Techniques for the Graphene/Graphene Derivates and Elastomeric Nanocomposites 4.4.1 Studying the Cure Behavior Goodyear’s discovery of the cure characteristics of the natural rubber was the initiation of the development of the compact rheological and cures behavior of the polymer materials. Composites have been developed with optimized reinforcing agents to fabricate enhanced properties of the virgin material [110]. As a result of the same,

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the reinforcement of the rubber moieties by the filler has turned out to be an intensive part of the processing and compounding industry [110, 111]. The last decade saw the invention of various types of reinforcing agents, namely classified as clays, layered clays, silicates, carbon-based nanofillers, and various other anisotropic Nanofiller [112]. On a trending note, the graphene sheet derivates are found to be the most enhanced carbon-based nanofillers making its significance in most of the elastomeric materials [113]. Researchers have found that there has been a significant amount of the differences in the cure characteristic comparing the graphene sheet derivates and its closest counterpart, such as the Carbon black and Carbon Nanotubes [112, 113]. The Graphene derivate rubber Nanocomposites are able to exhibit various scorch time (TS2 ), cure rate index, and optimum cure time. For instance, Nitrile Butadiene Rubber, when mixed with organo-modified (OM15) and the unmodified clay (M15), shows different cure curves when measured via rheometers (Fig. 4.28) [114]. NBR-OM15 shows a lower scorch time and torque differences than the rest of the tested samples because OM15 behaves as a vulcanization accelerator, as it posses the quaternary ammonium salt which participates in the vulcanization process [114]. It is often reported by the researchers that a reverse trend of the optimum cure time and the scorch time is investigated for graphene and its derivatives. The curing characteristics of the NBR composite framed with layered graphite structure provided an improvement in the net torque generated, but the curing properties were poor even having carbon black as the secondary filler. The higher value of the scorch time is attributed to the fact that the filler particle had a high surface area to volume

Fig. 4.28 Rheological curves showing time dependant torque for NBR and its Nanocomposites at 160 °C. Reproduced with permission from [114]

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ratio turning the same to be a barrier for the curing reaction as well as for the zinc accelerator coupling [114]. The cure behavior of the RGO/NBR composite was seen to improve as compared to its control of NBR and GO/NBR (Fig. 4.29). This can be attributed to the act that the polar groups in the RGO helped in the dispersion of the matrix by improving the crosslinking via hydrogen bonding and associated secondary forces [114, 115]. A similar analogical phenomenon can be designated to the rubber vulcanizates synthesized with Carbon nanotubes and Carbon Blacks [101]. Various recent theories hold that the improvement in the cure characteristic is dominant because of the absorption of the cure enhancer along with the accelerator during the process of the vulcanization [116]. We attribute the shift of the cure properties of the elastomer to various factors such as the chemistry of the reinforcing agent, properties of the matrix, additives, method of processing, and experimental conditions. Typically for nanofillers and with graphene derivatives, we have additional factors which volume into play such as the surface to volume ratio along with the mode of the functionalization and the wrinkling criteria when dispersed into the elastomeric matrices [107, 108]. The fillers such as those based on graphene and their derivative have initially shown an inferior behavior while contrasting cure behavior but eventually shows an efficient reinforcement with optimized conditions even at very low concentrations of less than 1 phr. (in comparison to the traditional fillers which requires twenty times the motioned amount to develop the same chemical and physical properties) [107, 108].

4.4.2 Analyzing the Antioxidant Effect of the Graphene Derivatives in Elastomeric Nanocomposites Elastomeric materials are widely used in an application that uses end-use projections as aerospace, automotive, and healthcare sectors due to materials high extensibility and deformation rate [117]. The most severe disadvantage which Elastomers faces, especially with the traditional ones such as the Natural Rubbers and the Styrene Butadiene Rubbers, is that they are vulnerable to oxidation and aging effect because of the presence of the unsaturated bonds [78, 117]. Although the incorporation of the oxidation enhances the aging resistance of the system, the migration along with the volatilities of antioxidants accelerates the aging effect and ultimately turns out to be one of the major sources of environmental pollution [78]. Thus the need for the development of highly immobilized antioxidants are being researched upon lately. Several trials have been carried out with N-1,3-dimethylbutyl-N’-phenyl-pphenylenediamine functionalized graphene oxide, which is being used in conductive Elastomers for solar cells [118]. The synthesis process aims to eliminate the oxygen from the graphene oxide and, at the same time, functionalization of the same by N-1,3-dimethylbutyl-N’-phenyl-p-phenylenediamine [118]. These immobilized antioxidants did exhibit not only a superior anti-migratory property but

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Fig. 4.29 Rheographic profile of a NBR-OM15 at four different temperatures, b cure conversion versus time of NBR-OM15 at four different temperatures. Reproduced with permission from [114]

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also anti-agglomeration property for the SBR elastomer matrices. Furthermore, the thermal conductivity of the matrix loaded with the nanofiller showed a higher thermal resistance due to the reduced interfacial thermal resistance (Fig. 4.30) [118, 119]. Apart from the above, researchers have also used 4-aminodiphenylamine grafted Graphene Oxide as an advanced antioxidant. While testing the oxidation induction time by the differential scanning calorimeter, the report justified that the elastomer having the 4-aminodiphenylamine grafted Graphene Oxide showed a better oxygen induction time of around 40 min compared to its control. We may conclude that the novel antioxidant not only enhanced the aging resistance of the matrix

Fig. 4.30 SEM images of a SBR, b SBR/GO-RT(1)%, c SBR/GO-RT(2)%, d SBR/GO-RT(3)%, e SBR/GO-RT(4)%, f SBR/GO(4) control. Reproduced with permission from [119]

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but also the thermal-oxidative stability of the Elastomer [78]. It was also found that the 4-aminodiphenylamine grafted Graphene Oxide displayed a better antioxidative behavior than the traditionally used N-1,3-dimethylbutyl-N’-phenyl-pphenylenediamine. An accelerated aging test was carried out for 12 days at 85 degrees centigrade, and it was found that the contemporary antioxidant resulted in a better mechanical property with a 2.79 MPa higher tensile strength [78]. Graphene, in its pristine state, is used to improve the aging properties of rubber composites due to its inherent negative oxidative behavior. This can be accounted for the presence of the free radical scavenging abilities coupled with the gas barrier properties [120], which consecutively reduces the free radical density and the oxygen permeability in the elastomeric Nanocomposites. Newer inventions have led to the fact that layered graphene structure proves to be a better radical scavenger than pure graphene, although graphene having a very high surface area to volume ratio. Although there is a competing factor between the chemical structures of graphene over layered graphene, which turns out to be a disadvantage, scientists generally tend to use graphene taking advantage of the high surface area to volume ratio [120]. The graphene oxides act as a protective barrier for the Elastomers by destroying the reactive oxygen species formed by the haywire reduction of oxygen by surface modifications catalyzed by the heteroatom [120–122]. Although we desire to have a low-temperature lifetime of the graphene/elastomer composites, no bolstered procedures or reports have been published focusing on the thermal oxidation process of the nanofilled Elastomers in the long run [122]. Therefore the industries still follow the conventional method of homogeneously dispersing the nanofillers in the elastomer matrix, usually by a latex mixing technique and then post-process the same for an accelerated aging test [120]. To prevent stacking up of the layers during the reduction process, two roll mill and vulcanization are applied to process both the matrix and the nanofiller. Reports claim that the nanofillers loading up to 7 phr exhibits the most homogenous system, which in turn enhances the gas permeability and significantly reduces the oxygen penetration into the matrix. The above system displayed oxygen permeability of 9.5 * 105 m3 cms−1 m−2 Pa, which is about 10% of the virgin matrix [78, 120]. The method of latex mixing method using hydrazine as a co chemical agent synthesis reduced graphene oxide, which turns out to be more effective than graphene oxide [123]. Unfilled SBR at 90° centigrade ruptured on the 13th day while the nanofilled SBR remained flexible without any brittleness even at the room temperature [120]. The moving die rheometers determined the storage modulus of the material, and the unfilled matrix showed a leathery plateau at ninety degrees centigrade while contrasted with the graphene oxide loaded elastomeric matrix (Fig. 4.31). The loss modulus, too, showed a negligible dependence on the aging time of the 7 phr loaded matrix as compared to the control set up [120]. The loss tangent shifted towards the higher temperature, and the intensity of the same was undermined during the process of the thermal-oxidative aging of the SBR matrix [120]. On an additional note, the SBR loaded with the nanocomposite exhibited a notable reduction in the

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Fig. 4.31 The reported data of the modification of the dynamic mechanic properties with the oxidative thermal aging. Reproduced with permission from [120]

extensibility from 53.2 to 17.1% after aging the material under oxygen for 13 days (Fig. 4.31) [120].

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4.4.3 Morphology and Detailing the Dispersion of the Graphene and Its Derivative in the Elastomeric Nanocomposites Over the years, scientists have shown that the dispersion of the nanofillers in elastomeric matrices depends on factors such as the size of the filler, the surface area of the filler, polymer matrix, and the experimental processing during the synthesis of the nanocomposites [124, 125]. In general, graphene is difficult to disperse in solvent and the elastomer without its proper processing such as exfoliation because of the presence of secondary forces such as Van der Waals forces and electrostatic forces that provides a reinforced attractive force between the graphene layers or the modified graphene layers. For a better dispersing effect, the exfoliation process is often carried out by sonication and stirring in addition to various mixing techniques. Apart from dispersing, the other thing that is required to form a homogenously even nanocomposite is the stability of the Nanofiller loaded in the elastomeric complex [124, 125]. Researchers are preparing multi-layered graphene dispersed in the SBR matrix with high homogeneity and dispersion stability [126]. The process-induced to prepare these elastomer nanocomposites was using a controller hetero-coagulation (negatively charged SBR particles was mixed with positively charged Nanofiller in the presence of Polyaluminium chloride which behaved as a flocculent). Consequently, it delivered the composite particle size in the range of 10–20 μm at room temperature [126]. SEM micrographs confirmed that the elastomeric nanocomposite was well distributed throughout the spatial space. Furthermore, these homogenously mixed nanocomposites also exhibited higher thermal stability and an enhanced electrical conductivity while varying the loaded filler from (0.5 to 5) weight % [126]. On a recent note, Lian and his coworkers used the Hummers method to fabricate multilayered graphene, which was eventually dispersed beautifully in a butyl rubber matrix [127]. The morphological and crystalline characteristics were investigated by the SEM images and the XRD data, respectively [127]. The results highlighted that the nanocomposites based on butyl rubber had a significant number of rough surfaces compared to the nanocomposites obtained from natural rubber and was retaining the stacking order of the overall system. We assign the improvement of the characteristics in natural rubber due to the presence of organic modifiers on the topography of the natural rubber surfaces that elevated the dispersion of the nanofiller in the matrix [127]. The XRD correlation showed a peak at 2θ = 26.5°, indicating that the final composites had retained their stacking order. The XRD pattern, as assigned to the graphene oxide, was untraced, which induced a complete exfoliation of the layered graphene with overall properties improvement [127]. Nanosheet elastomer nanocomposites are one of the recent discoveries that have been synthesized to model the effects of the filler type along with the hybridization of the fillers and the processing details [128]. The expanded graphite and modified expanded graphene are generally used to develop the elastomeric composites with the aid of the solution and melt mixing. The rubber matrices of SBR and XSBR were

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used to develop the nanocomposites using the expanded graphite in the presence and absence of carbon black. TEM images of the composites were instrumented, and the modified expanded graphite displayed a homogenous dispersion in the XSBR matrix and developed superior interaction between the matrix and the graphite layers [128, 129]. The pristine expanded graphene was found to be intercalated and disordered in the SBR matrix even in the presence of carbon black. The van der Waals forces acting between the graphite sheets might disrupt the process of homogenization and thus accelerated agglomeration in the SBR matrix [128, 129]. Graphene sheet derivatives are considered to be one of the potential agents in the field of polymer nanocomposites. In fact, the graphene sheet derivates have turned out to be one of the best fillers to complement the dispersion of the hybrid elastomeric system. A majority proportion of the properly dispersed elastomeric nanocomposites take advantage of the graphene sheet derived nanoparticle [130]. Hu and his research team fabricated Elastomers based on incorporating thermally reduced graphene oxide, followed by surface and bulk characterization [130]. Using techniques such as SEM and TEM, the research team proved that the thermally reduced graphene oxide was successfully dispersed homogeneously in the silicone rubber matrix. They extended their research to incorporating carbon nanotubes in a similar system and found an enhanced dispersion of the matrix. This may be attributed because of the comparatively smaller size of the graphene moiety when superposed with the carbon nanotubes [130]. Moreover, the interaction between the silicone elastomer and the carbon nanotubes was excellent, which might catalyze the process of the superior dispersion. In the current scientific ages, researchers are trying to synthesize hybrid elastomeric composites using Graphene nanoplatelets, which have theoretically shown to have much better dispersion stability and enhancement of both the physical and chemical properties of the final composite material [78, 130].

4.4.4 The Effect of Wrinkling of the Graphene Derivates on the Elastomeric Nanocomposites Although the majority of the widely used graphene sheet derivates are known to be present in the form of flat sheets [131], they might wrinkle when exposed in an elastomeric matrix (Fig. 4.32) [131]. The above figure represents the graphene derivative fabricated by the technology of Brodie’s method reveals the scrolling on the surface of the filler [78, 132]. Several hypotheses are provided as to why this phenomenon occurs, and the most referred reason stands out that it might be a result of the thermal shocks experienced by the nanomaterials while its dispersion [132]. However, there is another popular theory which states that it is a natural phenomenon for the scrolling to appear in order to provide the nanocomposite with the optimized thermal stability [132, 133]. Interestingly these wrinkling phenomenons are indeed advantageous to the nanocomposite

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Fig. 4.32 The reported TEM images illustrating the wrinkling phenomenon of the graphene derivates when incorporated into an elastomeric matrix. Reproduced with permission from [78]

systems [133]. In a recent study, the researchers have confirmed that these wrinkles not only provided the nanocomposites with a better exfoliation behavior but also enhanced mechanical properties. Li and his research group were fascinated when they investigated the tribological properties of the composites framed with graphene oxide and Nitrile Butadiene Rubber (NBR) [132]. The morphology revealed the NBR matrix was coated with the scrolled graphene oxide sheets that were being stretched further by the polarity of the NBR matrix, eventually making the composite thicker than the virgin graphene oxide sheets (Fig. 14.22b) [132]. The further detailing

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led to the conclusion that these nanocomposite Elastomers had high wear and tear resistance than the control. An analogical characteristic detailing was found when the group had chained the matrix to Natural Rubber, proving that these wrinkled graphene derivates provide better interfacial interactions [132]. At times, thermal exposure may cause the formation of the scrolled surface in graphene nanoplatelets [133], especially in thermoplastic elastomeric matrices. The wrinkling provided with a better interlocking capacity between the TPE matrices and the nanoplatelets to yield a nanocomposite with superior mechanical properties [133]. The results discussed that on incorporating the matrix with 16.7 vol.% of the nanoplatelets, the resulting composite’s Young’s modulus, Tensile Strength and Elongation at Break was improved by 782%, 413%, and 24% respectively. The nanoplatelets provided room for the matrix to develop symmetrical structure, thus improving the dispersion and the inherent strength of the graphene moiety (Fig. 14.22d) [133]. The scrolling effect in the graphene derived nanocomposites can be further enhanced by involving the hybrid reinforcing materials in the polymeric system. Hu et a had conducted a similar kind of study where he prepared moieties of MWCNT/TRG/PDMS taking the solution mixing route [134]. TEM analysis revealed the synergistic effect of the fillers, and it was further viewed that the surface of the graphene was like ripple instead of flat and smooth. The sp2 carbon structure of the two-dimensional graphene was metamorphosed into the nonplaner three dimensional sp3 hybridized carbon binding. This led to the rupture of the original twodimensional structure of the graphene and thus pushed the system to form wrinkles. In this particular case, the ripple structure is very compatible with the silicone rubber matrix, which indeed restricts the rotation of the silicone rubber chains elevating the mechanical properties [134]. EPDM rubber matrices have shown an improved dampening property when induced with wrinkled nanoplatelets [132–134]. But the Pyroshock test concluded that the EPDM nanocomposites fabricated with the graphene nanoplatelets displayed pretty weak interfacial interactions [133, 134]. Considering the benefit of the scrolling nature of the nanofiller, it is indeed that these structures provide a lot of opportunities to participate in the active interactions with the elastomeric matrices via various processes such as mechanical locking which effectively lead to the formation of an enhanced nanocomposite while compared to its control [134]. Hence, to observe the highest efficiency in the nanocomposite, we must develop a potential matrix along with an optimized process for specific and superior applications. The same can be achieved by using the advantages of the crinkled and rippling structures of the modified graphene and its derivative compounds [134].

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4.4.5 Predicting the Consequences of Modified and Hybrid Graphene Derivates on the Mechanical Properties of the Nanocomposite Elastomers To develop polymer materials into a diversified material, two of the most researched topic on this context is the study of the reinforcement and the functionalization [135, 136]. To amalgamate the properties of various fillers, researchers have tried to investigate the effect of hybrid fillers in the elastomeric system. These fillers with various isotropic properties and geometries are designed to get the best output by optimizing both the advantages of the nanofillers. Reports show that the interacting hybrid fillers show a positive synergistic effect, thus proving a bump to the overall mechanical properties of the system [135, 136]. To elaborate on the exact phenomenon and thermodynamic behavior behind this synergistic effect, a model based on the framework of Carbon nanotubes and reduced graphene oxide was studied with a special highlight in the geometry and the aspect ratio of the fillers [137, 138]. A unique spectroscopic technique known by the name of broadband dielectric relaxation spectroscopy was used by the research group to evaluate the chain dynamics of the elastomeric system [137, 138]. A slower relaxation mode was introduced into the system as compared to the segmental mode of the chain, providing a more in-depth insight into the hybridization of the nanofillers that were being studied [138]. It was concluded that the noncovalent interactions present in the matrix dissipated energy during the process of the deformation and eventually toughens the nanocomposite by the synergistic effect [138]. The data interprets that the tensile strength of the hybrid matrix was 400 and 70% higher than the original matrix of carbon nanotubes and reduced graphene oxide, respectively. Further investigations have led to the conclusions that the synergistic effect is related to the concentration of the nanofillers that are being introduced (Fig. 4.33) [138]. For the above system, it was found that the maximum synergistic effect was observed when the mole fraction of the reduced graphene oxide was twice that of the carbon nanotubes. The nanocomposites with adjusted mole fractions exhibited a 140.3% increase in the tensile strength along with a 74% enhancement in the tear resistance (Fig. 4.33) [138]. At times, the Guth-Gold-Smallwood equation is employed to dive into the details of the hybridized nanofillers. The equation is robust in the sense that it takes into account the filler volume fraction and the shape fitting parameter, which indirectly estimates the accelerated stiffening of the nanofiller based Elastomers [138, 139]. The theoretical value of the shape fitting parameter of the hybrid nanofillers was calculated to be 46.4, which was much higher than the single filler system (reduced graphene oxide has a shape fitting value of 37.8 while carbon nanotubes have a value of 18) [140]. The enhanced value of the accelerated stiffening predicts that the hybridized system forms better networks accompanied by a superior aspect ratio. Various recent theories state that the development of the glassy phase plays a pivotal role in driving the synergistic effect in the given elastomeric material [140]. The glassy phases which were developed at the interface as a result of the adsorption of the polymer

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Fig. 4.33 Graphical representation of a Tensile and tear strength of SBR/CNTs and SBR/rGO– CNTs composites as a function of CNTs content. b Typical stress-strain curves of blank SBR and SBR composites with different filler systems. c Typical stress-strain curves of rGO–CNTs hybrid filled SBR composites with an rGO/CNTs ratio of 2:1. d Relative Young’s modulus of SBR composites as a function of the filler volume fraction. The solid lines are fitted by Guth–Gold– Smallwood equation. Reproduced with permission from [138]

segments have a secondary relaxation spectrum, thus having an effect on the behavior of the nanofiller. Thus, unlike a single filler system, a multi filler system induces the development of various relaxation dynamics in the polymer chains. Dynamic mechanical analysis proved the loss modules to decrease, suggesting an enhanced reinforcing behavior and proving the context of the synergisms [140]. In the field of self-healing elastomeric matrices, the hybrid fillers have shown to play an auspicious role. The carbon nanotubes hybridized graphene nanosheets are extensively used in the Polyurethane and Polyurethane modified matrices [141]. The carbon nanofillers did show an increase in the modulus initially, but eventually, the product lost its stability both thermally and mechanically. The equation empirically states that as the volume fraction of the carbon nanofillers increases, the stress at a particular strain shows a gradually increasing trend with a fall of the strain at the break (because of the gradually increased stiffness of the material). This disadvantage using single filled nanofillers was addressed using the hybrid filler system, which provided a 70.8% increase in the strains along with the modulus [141]. It is indeed true that since the hybrid filler system induces the system with several unique boosters, these

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elastomeric nanocomposites are a highlight to the new-age technologies such as the artificial muscle, micro, and mini actuators.

4.4.6 Analyzing the Dynamic Mechanical Behavior Along with Barrier Properties of the Graphene Derived Elastomeric Nanocomposites The effect of the filler and its reinforcement can be best explained by analyzing the mechanical behavior of the materials while it is in the state of dynamic loading [142]. As a broad spectrum, we can relate the storage modulus obtained from the Dynamic mechanical analyzed graph to the amount of energy that can be stored in the nanocomposite while the material is subjected to oscillatory forces [142]. Similarly, the loss modulus may be related to the decay of the energy stored in the material. A classic reference can be cited by stating that the storage modulus of the carboxylated acrylonitrile-butadiene rubber filler with graphene oxide (58.4%) was much higher than the pristine SBR modified with graphene oxide (9.8%) and the elastomeric matrix without any nanofiller [143]. Further revision of the experiment provided various insights into the thermal and conducting properties of the composites. The glass transition temperature of the SBR matrix with graphene oxide and the SBR matrix with the modified acrylonitrile butadiene rubber shifted to a lower side than the original values. On a precise note, the glass transition temperature of the SBR modified with the NBR derivate was higher than its counterpart. The observed variation in the glass transition temperature influenced the presence of an interaction between the interfaces of the elastomer and the nanofiller [143]. The thermal conductivity of the Nitrile rubber modified with graphene oxide reinforced SBR was found to be almost three times (31.7%) higher than the neat SBR reinforced graphene oxide (11.2%). The addition of the fillers decreased the photon scattering and the acoustic indifferences at the interface of the filler and the matrix [143]. Several investigations had been carried out using the matrices of Thermoplastic Polyurethane and its modified composites. The storage and the loss modulus of the neat Thermoplastic Polyurethane matrix, along with the amine-functionalized Thermoplastic Polyurethane matrix were studied extensively (Fig. 4.34) [144, 145]. At every loading level, the amine-functionalized nanofiller system exhibited higher dispersion primarily due to the higher molecular interaction between the urethane linkages and the functional groups of the nanofiller. The loss tangent showed a higher response even at a frequency of 1 Hz(Fig. 4.34). This might be because of the enhanced surface interaction between the matrix and the filler system, which eventually accelerates the decrease of the viscous motion of the system making the matrix to dissipate higher viscous energy when applied stress. An additional feature was observed in the nanocomposites, which were loaded with the modified nanofiller- the dielectric constant was much higher than the control. The phenomenon

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Fig. 4.34 Graphical representation of a Storage modulus (E ), b loss modulus (E ), and c loss tangent (tan (δ)) versus frequency for the TPU-based nanocomposites. Reproduced with permission from [145]

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was recurrent with the loading levels since the dipole polarization of the functional groups on the surface of the elastomer was enhanced throughout the matrix surfaces. Another example may be cited in the context, where the polyurethane elastomer was reinforced with graphene functionalized with CuPc [146]. The non-covalent coating of the copper phthalocyanine oligomers on the surface of the nanofiller led to a formation of the superior interaction between the matrix and the filler [146]. The stress-strain curved generated from the tested samples inferred that the matrixinduced with the copper phthalocyanine showed a higher strain at Break but recorded an inferior value when refereed to Young’s modulus and the tensile strength, which further leads to the observation that the matrix influences an increase in the stiffness [146]. To extend our discussion, the reinforcing effect of the exfoliated graphite is used as a diffusion barrier in various thermoplastic Elastomers to provide the composite with a better gas permeability resistance [146, 147]. The decreased permeation proves the formation of the aligned dispersion of the graphene flakes in the matrix, acting as an inhibitor to gas, similar to the Lape model [146, 147]. The elastomer filled with the nanofillers based on isocyanate modified graphene oxide displayed a 90% reduction in the nitrogen barrier properties. The follow-up tests confirmed the presence of the unidirectional aligned delaminated graphite of high aspect ratio in the Polyurethane matrix. The gas barrier properties, in general, are determined by the particle aspect ratio instead of micromechanics, the isocyanate based nanofiller proved to be having a better gas barrier property [146, 147].

4.5 Conclusions The discovery of graphene and its gradual progress towards the derived materials are tuning to be the leading frontiers in the fields of Nanoscience and nanomaterials as a part of the material science and technology [148]. Graphene-based Elastomers have been employed to fabricate several beautiful application raging from stimuliresponsive polymers to polymers used in the electronic areas and devices. The prime advantage of using graphene and its derived materials is in the fact that they are cheap and low weight along with various added multifunctional features as compared to its counterpart synthesized on the basis carbon nanotubes [78, 148]. These discussed properties of graphene derived nanocomposites based on elastomer have opened up a new domain of scientific trend in the field of composite science. Now, graphene and its functionalized derivates are used to fabricate actuators [149], thermal resistors [149], conductive seals, and packaging materials [150], stimuli-responsive materials [114], structural aids [151] and materials used in electromagnetic fields [4]. Through the development of this chapter, we tried to portray the recent developments in the field of graphene derived materials which have potential use as nanofiller in elastomeric matrix possibly outrunning the advantages of conventional nanofillers such as carbon black or organoclays. In the coming decade, as researchers shall continue to invent various advanced nanofillers based on the context of graphene sheet derivates,

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these nanofillers can give an edge over the traditional nanofillers by incorporating functional groups as required for the application- making the elastomeric matrix more tailor-made. The major drawback in the existing methods in fabricating nanofillers based on Elastomers is subjected to the process control and the properties which, we believe can be compensated with the new age of graphene-based anisotropic nanofillers. The chapter explained the various instances where nanocomposites based on pure graphene showed a variation in the mechanical, thermal, and reinforcing properties much lower than that of the Elastomers derived from functionalized and modified nanofillers based on graphene. But several challenges are to be subjected in the context of developing more advanced nanofillers suited for a particular elastomeric system. Instead of a challenge, the scientific community takes this as an opportunity to revisit science via various perspectives aiming to develop materials providing hybridized and optimized property behavior. The current nanocomposites based on graphene derived materials have shown on maximum tensile strength of 32.8 MPa using Nitrile rubber as the matrix. Additionally, the same matrix has shown Young’s modulus of 19.7 MPa when loaded with 5% of functionalized layered graphene [78]. These data merely suggest that the technologists and scientists need to develop some way to evolve these figures by incorporating more interacting groups with the matrix in the nanofillers. Defining the present scenario and considering the industrial and scientific trends, there is definitely a need to converge more on the models to develop superior synthetic routes to fabricate enhanced nanofillers to bolster the elastomeric matrix [152]. The synergistic effect is one of the breakthroughs to develop a triple hybrid nanocomposite to incorporate properties aligned to three different orientations. This gives a broad spectrum to the researchers to study the interfacial chemistry of interaction present in the hybrid graphene derivates and the Elastomers that is being used. On an additional note, high precision elastomeric Nanocomposites should be a potent research working area, which tries to consider the similar properties between the modified graphene and the pristine nanofiller. The new generation nanofillers, for example, carbon nanowires, nanoribbons, boron nitride nanotubes, can be synthesis on the track of elastomeric nanocomposites. The advancement shall not only put the technology to a crest but also try to address the issues of industrial and cost friendliness along with addressing the issues of the environmental concerns.

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137. Araby, S., Zhang, L., Kuan, H.-C., Dai, J.-B., Majewski, P., Ma, J.: A novel approach to electrically and thermally conductive elastomers using graphene. Polymer 54(14), 3663–3670 (2013). https://doi.org/10.1016/j.polymer.2013.05.014 138. Wu, S., Zhang, L., Weng, P., Yang, Z., Tang, Z., Guo, B.: Correlating synergistic reinforcement with chain motion in elastomer/nanocarbon hybrids composites. Soft Matter 12(33), 6893– 6901 (2016). https://doi.org/10.1039/c6sm01116k 139. Guglielmi, M., Martucci, A.: Sol-gel nanocomposites (2018). https://doi.org/10.1007/978-3319-32101-1_100 140. Li, H., Yang, L., Weng, G., Xing, W., Wu, J., Huang, G.: Toughening rubbers with a hybrid filler network of graphene and carbon nanotubes. J. Mater. Chem. A 3(44), 22385–22392 (2015). https://doi.org/10.1039/c5ta05836h 141. Chen, T., Zhu, K., Lin, M., Wang, B., Liu, L., Li, Y., Pan, L.: Dielectric, mechanical and electro-stimulus response properties studies of polyurethane dielectric elastomer modified by carbon nanotube-graphene nanosheet hybrid fillers. Polym. Test. 47, 4–11 (2015). https://doi. org/10.1016/j.polymertesting.2015.08.001 142. Potts, J.R., Shankar, O., Du, L., Ruoff, R.S.: Processing–morphology–property relationships and composite theory analysis of reduced graphene oxide/natural rubber nanocomposites. Macromolecules 45(15), 6045–6055 (2012). https://doi.org/10.1021/ma300706k 143. Liu, P., Zhang, X., Jia, H., Yin, Q., Wang, J., Yin, B., Xu, Z.: High mechanical properties, thermal conductivity and solvent resistance in graphene oxide/styrene-butadiene rubber nanocomposites by engineering carboxylated acrylonitrile-butadiene rubber. Compos. B Eng. 130, 257–266 (2017). https://doi.org/10.1016/j.compositesb.2017.07.048 144. Al-Hartomy, O., Al-Ghamdi, A., Al Said, S.F., Dishovsky, N., Mihaylov, M., Ivanov, M., Zaimova, D.: Fullerenes, Nanotub. Carbon Nanostruct. 23(12), 1001–1007 (2015) 145. A. Nasr Esfahani, A. Katbab, A. Taeb, L. Simon, M.A. Pope, Eur. Polym. J. 95 (2017) 520–538 146. Chen, T., Qiu, J., Zhu, K., Wang, J., Li, J.: Soft Mater. 13(4), 210–218 (2015) 147. Lape, N.K., Nuxoll, E.E., Cussler, E.L.: J. Membr. Sci. 236, 29–37 (2004) 148. Uskokovi´c, V.: Entering the era of nanoscience: time to be so small. J. Biomed. Nanotechnol. 9(9), 1441–1470 (2013). https://doi.org/10.1166/jbn.2013.1642 149. Sadasivuni, K.K., Ponnamma, D., Kim, J., Thomas, S.: Graphene-based polymer nanocomposites in electronics (2014). https://doi.org/10.1007/978-3-319-13875-6 150. Banerjee, A.N.: Graphene and its derivatives as biomedical materials: future prospects and challenges. Interface Focus 8(3), 20170056 (2018). https://doi.org/10.1098/rsfs.2017.0056 151. Li, B.L., Li, R., Zou, H.L., Ariga, K., Li, N., Leong, D.T.: Engineered functionalized 2D nanoarchitectures for stimuli-responsive drug delivery. Mater. Horiz. (2019). https://doi.org/ 10.1039/c9mh01300h 152. Tiwari, S.K., Sahoo, S., Wang, N., Huczko, A.: Graphene research and their outputs: status and prospect. J. Sci. Adv. Mater. Dev. (2020). https://doi.org/10.1016/j.jsamd.2020.01.006

Chapter 5

Potential Application of Graphene-TPE Nanocomposite

5.1 Introduction The journey of graphene remains quite interesting from the very beginning of its unexpected and accidental invention as 2D nanomaterial by Novoselov and Geim in the year of 2004 [1]. Several remarkably advantageous properties of graphene have unveiled a new era in nanotechnology in terms of their scientific and technological impact on the scientific community. Enormous research on graphene across the globe has promised potential applications including high-speed electronics [2, 3], data storage [4–6], LCD, and OLED smart display [7, 8], supercapacitor [9–11], solar cell [12–14], electrochemical sensing [15, 16] and many more. The most direct application of graphene has been demonstrated by its use in composite materials using several polymers to design tough, lightweight, high performance, and low-cost materials [17]. Polymer nanocomposites are the hotspot of today’s research perceiving multiple uses in day-to-day applications because of the combined benefit of nanosized reinforcement as well as polymer matrix to improve material characteristics. A good selection of the basic components and designing of fabrication procedure synergies the desired properties of produced nanocomposites [18]. In our present study, we have ventured into a specialty grade polymeric material, the Thermoplastic Elastomer (TPE), to deal with as it has achieved widespread recognition for a broad range of applications. The principal success of TPE regarding its processing facility, design flexibility, replacement of conventional thermoset elastomers has all been elucidated in preceding chapters. Graphene and its derivative/TPE nanocomposite meet highly diversified needs of various application ranging from unprecedented mechanical reinforcement in aerospace, the automotive industry to electrical, optoelectronics, high thermal goods, ‘smart’ device fabrication, piezoresistive nanocomposites, conducting polymer composites, self-healing, and shape memory objects, even in biomedical appliances. TPE market is rapidly gaining techno-commercial importance, and its dynamism is illustrated by the growth in the intra-material competition where TPE composites are challenging higher-rated © Springer Nature Singapore Pte Ltd. 2020 A. Bandyopadhyay et al., Engineering of Thermoplastic Elastomer with Graphene and Other Anisotropic Nanofillers, Engineering Materials, https://doi.org/10.1007/978-981-15-9085-6_5

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engineering thermoplastics in many applications. In the subsequent section, we have tried to accumulate some emerging applications in the field of graphene and its functionalized derivative-based TPE nanocomposite to have a quick view on trending research activities in this specialty domain of polymer.

5.2 Sensing and Actuation With the advent of smart technology, flexible electronic devices have become vital engineering tools, widely used in various fields including flexible photo electronics [19], artificial muscle design [20], human motion monitoring [21], intelligent weaving [22] and many more. Flexible substrate with high stretchability [23–25] and hybrid sensitive nanomaterials are gaining immense importance to be used in electrically conductive external stimuli induced controllable sensors and actuators [26]. The extraordinary mechanical compliance, excellent electrical, and optical properties of graphene have been exploited in this regard, and well defined hard-soft phase-separated morphology of TPE offers flexibility. Liu et al. reported an electrically conductive thermoplastic polyurethane (TPU)/graphene nanocomposite with an ultralow percolation threshold of 0.1 wt% of graphene [27]. The nanocomposite endows wide range of strain sensitivity at ultralow graphene loading level and excellent sensing stability for various strain pattern along with good recoverability and reproducibility after stabilization during cyclic loading at different strain amplitudes (Fig. 5.1ii (b)) and strain rate (Fig. 5.1iii (b)). The fluctuation during unloading (termed as ‘shoulder peak’) was observed from the very first cycle for 0.2 wt% graphene, followed by the 5th cycle for 0.4 wt%. It disappeared for 0.6 wt% due to the formation of an indestructible, robust conductive network with an easy reconstructible conductive path at higher graphene percentage (Fig. 5.1i (a)). The nanocomposite exhibited good recoverability and reproducibility even after 100 cyclic loadings without any fracture or electrical shot (Fig. 5.1i (b)). The conductive polymer composite (CPC) was found to record a strain gauge factor (GF) of 5.2, 2.0, and 0.78 for 0.2,0.4 and 0.6 wt% of graphene, respectively, at the strain of 5% and strain rate of 0.1 min−1 indicating higher sensitivity at lower loading. Even the GF increased up to 17.7 for TPU with 0.2 wt% graphene at the strain rate of 0.3 min−1 . The observation regarding the gauge factor showed a steady increase from 5.2 to 12.1 on increasing strain amplitude from 5 to 30% as higher strain amplitude caused rapid destruction of the conductive network (Fig. 5.1ii (a)). The strain rate also affected the sensing behavior having GF ranging from 12.1 to 17.7 with increasing strain rate from 0.1 to 0.5 min−1 (Fig. 5.1iii (a)). An analytical model based on the tunneling effect was constructed to simulate the resistance response to strain at different strain rates, and the theoretical data were almost found to superimpose with experimental one (Fig. 5.1iv). The change of the number of conductive pathways and tunneling distance under strain held accountable for the observed resistance-strain behaviors Fig. 5.1v (a) and (b)).

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Fig. 5.1 Graphical representation of i Resistance-strain behavior of composites with different graphene content, up to 5% strain at the strain rate of 0.1 min−1 during a cyclic loading and b Resistance-strain behavior of TPU-0.2G for cycles 81–100. ii Resistance-strain behavior of TPU0.2G, up to different strain amplitude at the strain rate of 0.1 min−1 , during the 1st cycle (a) and cyclic loading (cycle 11–20) (b). iii Resistance-strain behavior of TPU-0.2G, up to 30% strain at different strain rates, during the 1st cycle (a) and cyclic loading (cycle 11–20) (b). iv Experimental (dots) and theoretical (solid lines) data of resistance as a function of strain. v Change of a conductive pathways (CP) and b tunneling distance (TD) as a function of strain. Reproduced with permission from [27]

Li and his team developed a surface-modified graphene/thermoplastic polyurethane (TPU) nanocomposite where surface modification of graphene oxide (GO) was done by utilizing a polyvinyl pyrrolidone (PVP) coating on it followed by reduction (RGO) for proper dispersion of PVP absorbed RGO in TPU [28]. Beside significant enhancement in mechanical properties, the composite was characterized by good electrical conductivity with a percolation threshold (calculated based on the power-law equation) of 0.35 wt% of filler. From Fig. 5.2, an exponential increase in conductivity was observed with a low concentration of graphene, followed by a slow growth with increasing concentration. Compared to other surface-modified graphene/TPU composites, this study exhibited relatively higher conductivity at the same filler content suggesting that PVP coating on RGO improved the polymer-filler interaction without hampering filler’s inherent conductivity and formation of the conductive network.

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Fig. 5.2 Electrical conductivity (σc ) versus filler volume fraction (ϕ) for TPU/RGO/PVP nanocomposites. Insert is a log-log plot of the electrical conducting versus ϕ-ϕc (ϕ and ϕc being the filler content and percolation threshold, respectively). Reproduced with permission from [28]

Ronca et al. recently published an article about the successful fabrication of an electrically conductive and flexible thermoplastic polyurethane/graphene (TPU/GE) porous structures by selective laser sintering (SLS) technique from graphene (GE)wrapped thermoplastic polyurethane (TPU) powders comprising of three types of cellular architecture (designed by using triply periodic minimal surfaces (TMPS) equations) corresponding to Schwarz (S), Diamond (D), and Gyroid (G) unit cells with porosities ranging from 20 to 80% [29]. All the samples displayed sound electrical conductivity regardless of geometry and porosity with negative piezoresistive behavior in all cases when subjected to cyclic compression test due to the increasing contact of the GE nanoplatelet upon compression, creating more conductive pathways. Figure 5.3 depicts the piezoresistive behavior of the D, G, and S-based structures with 40% porosity and strain up to 8%. Apart from geometry, porosity played an important role in controlling the electrical conductivity. It had been observed that there was a significant variation in R0 /R8 values (resistance at zero and 8% deformation respectively) from 2.21 ± 0.20 to 4.95 ± 0.50 as porosity changed from 80 to 40% for the system with ‘G’ geometry (Fig. 5.3i (c and d)). Measurement of gauge factor (GF) for evaluating strain sensitivity displayed a higher value (>12) for S geometry than others owing to the formation of a more effective conductive path originating from its peculiar shape of the unit cell. Even GF values were found to be higher for deformation lower than 8%, and then tend to plateau towards maximum strain value (Fig. 5.3ii) To determine electromechanical cycling stability, the samples were subjected to 50 consecutive cyclic compression process (at 8% of strain), which exhibited excellent stability and signaled reversibility by measuring both mechanical and electrical response (Fig. 5.3iii (a and b)). the excellent strain sensitivity made them an effective piezoresistors as well as dielectric elastomer actuators for the detection of very small deformations (i.e., strain less than 5%).

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Fig. 5.3 Graphical representation of i Piezoresistive behavior of a TPU/GE D40, b TPU/GE G40, and c TPU/GE S40 under cyclic compression, d Resistance ratio and gauge factor of TPU/GE porous structures at 8% compression strain. ii Variation of gauge factor as a function of compression strain for the TPU/GE porous structures with a Diamond, b Gyroid, and c Schwarz unit cells. iii a Piezoresistive behavior of TPU/GE S40 over 50-cycle compression test, and b resistance values at 8% strain as a function of time for all TPU/GE composite structures. Reproduced with permission from [29]

Liu et al. developed a thermoplastic polyurethane (TPU) based conductive polymer composite (CPC) filled with finely dispersed graphene for organic vapor sensing towards four common organic vapors (cyclohexane, CCl4, ethyl acetate, and acetone) having different polarities [30]. The non-polar and low-polar vapors (cyclohexane and CCl4 ) caused swelling of the non-polar soft segment of TPU and rearrangement of the macromolecular chains with overlapping with graphene sheets leading to the formation of the conductive network. That eventually decreased the resistance rapidly, illustrating novel negative vapor co-efficient (NVC) phenomenon for the superior discriminating ability of this CPC as an organic vapor sensor due to the microphase separated structure of TPU. On exposure to air, a highly elastic soft segment returned to its initial state after desorption of vapor molecule displaying good reproducibility and fast response (Fig. 5.4i (a)). On the other hand, NVC was noticed for polar organic solvents (ethyl acetate and acetone) due to the release of the soft segment from swollen hard segment causing partial rearrangement of conducting path and hence, the slight reduction in resistance was observed, However,

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Fig. 5.4 Depiction of i Responsivity of TPU-based CPCs containing 0.4 wt% graphene towards saturated a cyclohexane and CCl4 and b ethylacetate and acetone as a function of time; c the maximum responsivity in saturated organic vapors and the residual responsivity in the air in a single IDR at 30 °C. ii Responsivity of TPU-based CPCs containing 0.4 wt% graphene towards saturated a cyclohexane & CCl4 and b ethylacetate and acetone vapors in five IDRs at 30 °C. iii Responsivity of TPU-based CPCs containing 0.4 wt% graphene towards saturated a cyclohexane, b CCl4, c ethyl acetate and d acetone vapors in a single IDR at different temperatures. Reproduced with permission from [30]

during desorption of vapor, poor mobility of macromolecular chains in hard segment caused permanent destruction of conductive pathways resulting residual resistance (Fig. 5.4i (b)). All responsibilities of this sensor corresponding to individual solvent are presented in Fig. 5.4i (c). The responsivity of CPC was investigated by five successive immersion-drying run (IDR) at 30 °C where non-polar solvent exhibited good reversibility and reproducibility along with stable electrical signal and quick response rate but polar solvent again displayed residual resistance due to the abovementioned reason (Fig. 5.4ii (a and b). organic vapor sensing responsivity of CPCs at different temperatures was found to increase with increasing temperature due to accelerated vapour absorption at higher temperatures causing higher interaction between organic vapour and TPU’s segmented microphase structure (Fig. 5.4iii). Costa et al. reported a high-performance styrene-ethylene/butylenesstyrene(SEBS) based piezoresistive sensor reinforced with different nanostructure of graphene, including graphene Graphene oxide (GO), reduced graphene oxide

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(rGO), and graphene nanoplatelets (G-NPL) [31]. The electrical conductivity of GO and RGO based composites displayed the formation of an interconnected conductive network at a percolation threshold around 2 wt% but remained indifferent with filler content in case of G-NPL up to 6 wt% (Fig. 5.5a, b). The rGO/SEBS composite achieved two orders of magnitude larger electrical conductivity than GO/SEBS due to lower oxygen amount present in rGO than GO, analyzed by XPS study. Piezoresistive measurements were performed by monitoring electromechanical response for strains from 1 up to 10%, in composites with 4 wt% GO. rGo filler content which showed good stability over 1000 repeated cycles (Fig. 5.5d) having gauge factor (GF) values ranging from 15 to 120 in GO/SEBS and 10 to 90 in rGO/SEBS composites (Fig. 5.5c and e) that increased with strain at different deformation speeds (1 and 5 mm/min). Both the composites recorded piezoelectric sensibilities larger than 100 for strain up to 10%, which was much higher than

Fig. 5.5 I-V curves of SEBS and rGO/SEBS composites up to 6 wt% (a) and electrical conductivity of SEBS and GO, rGO and G-NPL composites with filler content up to 6 wt% (b), Gauge factor determination for samples up to 1, 5 to 10% of strain (c), Piezoresistive measurements for 1000 cycles at 5% strain and 5 mm/min for GO/SEBS composite (d) and Gauge Factor of GO/SEBS and rGO/SEBS composites with 4 wt% filler content for 1, 5 and 10% of maximum strain at 1 and 5 mm/min deformation speed (e). Reproduced with permission from [31]

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that of most reported materials. Excellent electrical conductivity and piezoresistive behavior enabled the material to function as a simple circuit for data acquisition. Moreover, the suitability of the material for actual device implementation had been successfully demonstrated by fabricating a prototype hang glove with sensing fingers. Another SEBS derived highly stretchable strain sensor was developed by Pan et al. containing a trinary hybrid carbonaceous filler, including carbon nanotubes (CNTs), graphene, and fullerene [32]. The electrical conductivity of the sensors based on CNTs, CNT/graphene, and CNT/graphene/fullerene was calculated to be 1.684, 2.932, 5.179 S/m, respectively (Fig. 5.6i). The presence of graphene boosted the conductivity compared to pristine CNT and fullerene, owing to its inherent lubricity, reduced the crack propagation in sensitive unit materials acting as an electrical bridge between CNT and graphene to develop better-conducting paths, thus enhancing the conductivity. Fullerene aided the formation of dense interconnection between sensitive unit materials that resulted in a maximum strain of 203% and a moderate gauge factor of 15 with fitted linearity of 136% (R2 = 0.998) to be observed in CNT/graphene/fullerene-based sensor (Fig. 5.6ii). Effective reduction in interlaminar friction, as well as the crack generation in graphene and ensuring a conductive path between CNT and graphene due to the presence of fullerene, yielded large stretching range with low resistance change rate under 120% strain condition in case of CNT/graphene/fullerene-based sensor compared to CNT and CNT/graphene-based materials. The comparative analysis confirmed the long-term endurance of the cyclic test with strain rate-independent repeatability and reproducibility. The CNT/graphene/fullerene-based sensing probe was further utilized for

Fig. 5.6 Graphical representation of i Histogram of conductivity of CNT, CNT/graphene and CNT/graphene/fullerene-based sensor. ii GF and maximum strain range histogram corresponding to sensors of different sensitive unit materials. iii Human monitoring applications: a blowing air, b wrist bending, and c finger bending. Reproduced with permission from [32]

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real-time application by monitoring human movement where three different bending curvatures were tested (Fig. 5.6iii), resulting in three different relative resistance, thus inspiring its potential in human motion monitoring and scalable application. Liang et al. reported a TPU/functionalized graphene-based nanocomposite where graphene, being an ‘energy transfer’ unit, imparted additional infrared triggered actuation property to the nanocomposite apart from reinforcing its mechanical performance [33]. Three types of functionalized graphene, namely isocyanate (iG), sulphonated (sG), and reduced (rG) graphene-based TPU were developed to characterize the actuation behavior where sG/TPU showed maximum IR radiation absorption and outperformed the other two (Fig. 5.7i (c)). Moreover, The sulfonatedgraphene/TPU with 1 wt% and 0.5 wt% loading transmitted only 0.3% and 1% of light respectively at the wavelength of 850 nm that was noticeably lower than that of reduced-graphene/TPU (9.5%) and isocyanate-graphene/TPU nanocomposites (13.2%) as observed from Fig. 5.7i (d). the greater extent of restoration of sp2 carbon network along with homogeneous dispersion of functionalized graphene in the host polymer matrix in case of sG/TPU nanocomposite was responsible for its effective IR absorption which was also confirmed from thermogravimetric analysis (TGA) (Fig. 5.7i (b)) showing the smaller amount of mass loss for sG w.r.t iG

Fig. 5.7 Representation of i IR absorption property of the three graphene materials and their nanocomposites. a IR absorption properties of sulfonated-graphene and isocyanate-graphene solutions with a concentration of 0.05 mg/mL. Reduced-graphene being insoluble in DMF, its IR absorption spectrum is not shown here. b The TGA curves with a heating rate of 5 °C/min from room temperature to 400 °C under N2 for isocyanate-graphene, sulfonated-graphene, and reducedgraphene. c The normalized IR absorption of the films of pure TPU, isocyanate-graphene/TPU (1 wt %), sulfonated-graphene/TPU (1 wt%), and reduced-graphene/TPU (1 wt%) across a range of wavelength from 500 to 1100 nm. d Summary of the transmittance of IR light for the sample films at 850 nm: pure TPU, isocyanate-graphene/TPU (1 wt%), sulfonated-graphene/TPU (0.1, 0.5, and 1 wt%), and reduced-graphene/TPU (1 wt%). Reproduced with permission from [33]

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nanocomposite due to less amount of functionality attached to sG sheets. rG/TPU also showed very insignificant mass loss over the entire temperature range but its poor dispersion in TPU made it an inferior IR absorbent. IR induced actuation characteristics was investigated by exposing a pure TPU film and a 0.1 wt% sG/TPU film to IR source with a given deformation about 200–250%. While IR transparent TPU remained unaffected, sG/TPU contracted and returned almost to its original shape within 10 s (Fig. 5.7ii (a)). Furthermore, sG/TPU with 1 wt% sG lifted a 21.6 g weight 3.1 cm with striking 0.21 N of force and illustrated an estimated energy density of over 0.33 J/g (Fig. 5.7iii (b)). In some cycling tests, the energy density was recorded as high as 0.40 J/g, thus exemplified sG/TPU composite as a nanoscale heater and an effective energy transfer unit (Fig. 5.7(ii) b). Repeatability and reproducibility of actuation behavior were studied by performing cycling tests on sG/TPU with 1 wt% loading, which depicted significant energy densities and recovery rate to sustain stabilization after 10 cycles without noticeable attenuation indicating a repeatable response to IR radiation. Additionally, the energy densities and the recovery rate fluctuated between 0.33–0.40 J/g and 88–95%, respectively, in all cases demonstrating good reproducibility (Fig. 5.7iii (a and b)). Organic photoelectronic materials are highly demanding for the development of artificial intelligence products used in solar cells, light-emitting diode, photoelectronic sensors, etc. Photoresponsive polymers cause enabling programmable and reversible mechanical functionalities in optical switches [34], active optics [35], photomechanical actuators [36]. However, conventional photoelectronic materials have always been too stiff to be used in flexible devices like electronic skin, soft sensors, wearable devices, and so on. To circumvent this issue, Tang et al. synthesized a highly photoelectric sensitive, stretchable poly(styrene-b-ethylene/butylene-bstyrene)-zinc porphyrin–graphene hybrid composite (G/Zn-PorSEBS) with remarkable deformability [37]. The composite was efficient in capturing light changes with illumination on or off (Fig. 5.8i (e and f)) due to the presence of porphyrin chromophore that generated photocurrent under illumination (Fig. 5.8i (a–d)) by the dissociation of photon-generated excitons into free electrons and holes, but they are blocked by tunneling effect in the dark resulting low current state. Eventually, the photocurrent intensity kept on increasing with increasing photoconductive porphyrin, achieving the highest photocurrent of 0.06 μA cm−2 with 13.6% porphyrinization. The increasing mass fraction of graphene in G/Zn-PorSEBS composite made it more photosensitive compared to Zn-PorSEBS due to π-π interaction between electron abundant aromatic rings and conjugated structure of graphene resulting maximum photocurrent value of 0.13 μA cm−2 with 0.9% graphene. The UV absorption spectra clearly display (Fig. 5.8ii (a and b)) the π-π* transition (band around 420 and 500– 700 nm) of porphyrin and redshift (426.6 nm) due to π-π conjugation between highly conductive graphene and porphyrin that constituted a donor–acceptor system rapidly converting light signals to electrical signals to improve charge-carrier mobilities. This attenuation of electron–hole recombination was solely responsible for the remarkable enhancement in the photocurrent intensity after the introduction of graphene. Apart from conventional TPEs, some stimuli-responsive new-age smart polymers are gradually attracting appreciable interest in material science and technology. They

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Fig. 5.8 Illustration of i Photocurrent switching response at 5 s intervals in a 0.3 V 1 M NaOH aqueous solution under 500 W Xenon lamp illumination. a Photocurrent response of neat SEBS and Zn-PorSEBS elastomer with different porphyrin grafting ratio. b–d Photocurrent response of Zn- PorSEBS elastomer and G/Zn-PorSEBS with different graphene content at the same porphyrin grafting ratio, (e and f) Photographs giving the light on/light off process. ii a UV–vis spectra of Zn-PorSEBS matrix and G/Zn-PorSEBS composite. b Molecular orbital energy diagram of photo-induced electron transfer from porphyrin to graphene. Reproduced with permission from [37]

undergo reversible changes in chain conformation in response to external stimuli such as temperature, pH, redox, mechanical and electrical stress, optical, chemical, and biological signals. Several research activities have reported some specialty block copolymers and their composites that are being used for the development of stimuliresponsive smart polymers to offer a sensing platform for chemical and biological applications. In addition, GO displays long-distance fluorescent quenching efficiency with a high signal-to-background ratio [38–40] and hence can function as an efficient Förster resonance energy transfer (FRET) acceptor in an optical sensor when forms nanocomposites with such block copolymers. Song et al. reported the synthesis of temperature-responsive PNIPAM-bP(OEGMA-co-MQ block copolymer functionalized graphene oxide (GO) luminescent nanocomposite for the detection of 2,4,6-trinitrotoluene (TNT) where the response to the external stimuli was monitored by observing the optical changes [41]. Reversible addition-fragmentation chain transfer (RAFT) polymerization technique was adapted for the synthesis of pyrene- terminated block copolymer using a pyrene functional RAFT agent and the monomers N-isopropyl acrylamide (NIPAM), oligo(ethylene glycol) monomethyl ether methacrylate (OEGMA) and 5-(2-methacryloyl-ethyloxymethyl)-8-quinolinol (MQ) followed by the attachment of that copolymer on GO surface via non-convent π-π stacking interaction. ZnS nanoparticles (ZnS NPs) were also anchored into the block copolymer decorated GO

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via the coordination of MQ ligand of the block copolymers with amine-functionalized ZnS NPs to form metaloquinolate complexes which became the fluorescent emitting center. Investigation of thermo-responsive behavior of ZnS NPscontaining block copolymer-GO nanocomposite in water showed quenching of fluorescent emission of the composite when temperature increased from 9 to 45 °C (Fig. 5.9i (a)) due to the conformation change of PNIPAM in response to the temperature change leading to the change in FRET (fluorescence resonance energy Transfer) efficiency from the fluorescent emission center (metaloquinolate complexes in ZnS copolymer) to GO. Figure 5.9i (b) depicts the continuous heating-cooling cycle of the efficient on-off switching behavior, confirming the presence of a strong non-covalent linkage between block copolymer chains and GO sheet, making the composite highly stable. The detection of TNT was carried out by measuring photoluminescence (PL) intensity, which drastically decreased with increasing TNT concentration with a detection

Fig. 5.9 Graphical representation of i PL spectra a and corresponding cycles of heating–cooling at above and below LCST b of ZnS NPs-containing block copolymer-GO nanocomposite. ii PL spectra with concentration increase of TNT (1 × 10−7 mol L−1 ) in the DMF solution of ZnS NPscontaining block copolymer-GO (a) and Stern–Volmer plots corresponding to the above graphs (b), Fluorescence quenching efficiency obtained for ZnS NPs-containing block copolymer-GO upon addition of 10 mM of different nitro compounds (1 × 10−2 mol L−1 ) (c) and metal ions (1 × 10−2 mol L−1 ) (d). iii Uv-vis absorption spectrum of MEA-TNT (blue) and PL emission spectrum of ZnS NPs-containing block copolymer-GO in DMF (red). Reproduced with permission from [41]

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limit of 4.4 nM (Fig. 5.9ii (a)). The electron-rich amino groups on the ZnS NPs interacted with the electron-deficient TNT to form the Meisenheimer complex (MEATNT). The detection of TNT was further explained by the UV absorption spectra (Fig. 5.9iii) showing a spectral overlap between the fluorescence sensing nanocomposite (at 526 nm) and MEA-TNT (at 519 nm) that led to FRET process between the emission center in the GO nanocomposite and TNT. Figure 5.9ii (c) represents the fluorescence emission response of the fluorescent sensing probe based on ZnS NPs-containing block copolymer-GO nanocomposite to the other nitro containing compounds that showed a little quenching of emission at 526 nm compared to high fluorescence quenching efficiency (about 60%) of TNT. Another observation revealed that the presence of different metal ions did not influence the fluorescence of the sensing probe except TNT (Fig. 5.9ii (c)). Quite a similar demonstration was also communicated by Yang et al. while developing a temperature-sensing probe based on fluorescent P7AC-b-PNIPAMb-PSN3 block copolymer-functionalized graphene oxide sheets (FGO) [42]. The triblock copolymer had been synthesized by RAFT polymerization using poly(7(4-(acryloyloxy)butoxy)coumarin) (P7AC) as the fluorescent component, poly(Nisopropyl acrylamide) (PNIPAM) as the thermally responsive polymer, and a short poly(azidostyrene) (PSN3) block anchoring covalently to the GO surface. The composite exhibited extraordinary stability in the water while investigating the thermal response of FGO by measuring change in photoluminescence (PL) intensity as a function of temperature using an aqueous dispersion of FGO. Figure 5.10i (a and b) display drastic reduction in PL intensity for both FGO1 and FGO2 (with two different Mn values of PNIPAM) when the temperature was above 32 °C (LCST of PNIPAM) compared to pristine P7AC-b-PNIPAM-b-PSN3 that showed no intensity change at different temperatures. Change in the conformation of PNIPAM above 32 °C led to the shortening of the distance between P7AC and GO, thus solely responsible for the change in FRET efficiency from P7AC block to GO. Pictorial representation of the FGO as a thermal Sensor is illustrated in Fig. 5.10i. Figure 5.10iii represents cyclic on-off switching behavior of a continuous heating-cooling process that illustrates efficient fluorescence recovery on cooling and regaining of the same PL quenching efficiency on re-heating, thus enabling an efficient and robust temperature-sensing platform for optical, electronic and environmental applications. Nguyen and his team carried out an intensive investigation on the development of electroactive ionic sot actuators, a kind of artificial muscle, containing polymer electrolyte membrane based on polystyrene-b-poly(1-ethyl-3-methylimidazolium-4styrene sulfonate) (PS-b-PSS-EMIm) block copolymer having functionally antagonistic core-shell architecture [43]. Specifically designed ionic exchangeable polymer electrolyte membrane was sandwiched between two electrodes based on nitrogen and sulfur codoped (NS codoped) graphene and poly(3,4-ethylene dioxythiophene)poly(styrene sulfonate) (PEDOT: PSS) by hot pressing to fabricate highly bendable soft ionic actuators. Under the ultralow voltage of 0.5 V at 0.1 Hz of frequency, excellent actuation was observed with a high tip displacement, about 8.2 mm (0.37% strain) for the square input and 6.8 mm (0.32% strain) for the harmonic input (Fig. 5.11a,

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Fig. 5.10 Schematic illustration of i FGO and its application as a thermal sensor. ii Photoluminescence (PL) spectra of a FGO1, and b FGO2, c changes in PL intensity at 425 nm as a function of temperature. iii Reversibility test of the on–off switching behavior of FGO2 in terms of PL quenching. Reproduced with permission from [42]

b) along with a very fast rise time within 5 s (Fig. 5.11c) independent on stimulus amplitudes. Compared to Naphion and other reported block copolymers. Very good performance was observed for the corresponding actuators showing a significant deformation over a wide range of frequencies from 0.1 to 5.0 Hz at ultralow input voltages below 0.5 V (Fig. 5.11d, e). The actuator retained 94% of its original performance after 14 000 cycles V (Fig. 5.11f) when subjected to the continuous stimulation of 0.5 V and 1 Hz over 4 h, indicating excellent stability and durability and offered potential application in soft robotics, wearable electronics, and active biomedical devices.

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Fig. 5.11 Actuation performance: displacements at 0.5 V and 0.1 Hz under a square and b sine voltages; c displacements according to DC voltages; displacements according to d voltages and e frequencies, and f durability. (The free length of the actuator was 20 mm). Reproduced with permission from [43]

Lee et al. developed another excellent ionic polymer-metal composite (IPMC) actuators with large ion concentration gradient derived from Styrenic block copolymer/sulfonated graphene oxide (sGO) composite [44]. The nanostructured polymer electrolyte membranes composed of a poly((t-butyl-styrene)b-(ethylene-r-propylene)-b-(styrene-r-styrene sulfonate)-b-(ethylene-rpropylene)-b(t-butyl-styrene)) (tBS-EP-SS-EP-tBS; SSPB) pentablock copolymer and an ionic liquid (IL) as inner solvent where sGO was incorporated as highly ion conductionactivating carbonaceous filler displayed superior performance compared to conventional Nafion membrane. SSPB/sGO/IL IPMC exhibited remarkably larger and faster bending displacement 90% and 1920% larger than SSPB/IL IPMC and Nafion/IL IPMC respectively (Fig. 5.12a) Maximum bending strain of 0.88% with initial strain rate 0.312% min−1 were recorded for. SSPB/sGO/IL IPMC, way higher than SSPB/IL IPMC and Nafion/IL IPMC, without any evidence of back-relaxation over the actuation period. The maximum tip displacement of 12.1 mmat 2 V dc current within 10 min was observed for SSPB/sGO (0.5 wt%)/IL IPMC as shown in Fig. 5.12b. A comparative analysis of actuation performance of the corresponding actuation under discussion with that of the high- performance ionic polymer/IL actuators can be extracted from Fig. 5.12c. Large and fast bending actuation performance of SSPB/sGO/IL was attributed to the synergistic effect of fast and efficient mass transport of bulky IL’s ions through a large well-defined phase-separated ionic domain of SSPB and strong interactions between mobile ions and sulfonic groups in sGO promoting ion transport over the adjacent ionic channels. This strong interaction between a sulphonic group of GO and mobile ions, that was absent in pristine GO caused larger mass transport per unit mobile charge leading to maximum charge specific displacement which was 30% and

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Fig. 5.12 Representation of a Maximum tip displacement of IPMCs based on SSPB/sGO/IL, SSPB/GO/IL, and Nafion/GO/IL with a series of filler content. b Bending deformation of an SSPB/sGO (0.5 wt%)/IL IPMC equilibrated under an applied potential of 2 V dc. c Comparison of bending strains of SSPB/sGO (0.5 wt%)/IL with those of top-ranked bending-type polymer actuators impregnated with ILs reported in the literature. d Charge-specific displacement of SSPB-based membranes IPMCs. Reproduced with permission from [44]

20% larger than SSPB/GO/IL and SSPB/IL actuators respectively showing the higher energy efficiency of actuation, as depicted from Fig. 5.12d. Moreover, The chargespecific displacement of the SSPB/sGO/IL IPMC was elevated with increasing sGO content; whereas, those of SSPB/GO/IL IPMCs with various GO contents remained unaffected, almost identical. The migration of IL in the nanostructured polymer electrolyte, responsible for large and fast bending actuation, was investigated through tracking one element of the IL’s anion through energy-dispersive X-ray spectroscopy (EDS) analysis in the thickness direction of the actuators immediately after the actuation under an applied voltage of 2 V along-with the pumping effect of solvated ion complex to push the loosely bound solvent molecules in the large ion channel structure of IPMC.

5.3 Shape Memory

199

5.3 Shape Memory Shape memory effect is the ability of a substance to maintain a temporary shape for a long time keeping its original shape in ‘memory’ and again going back to its actual shape when stimulated. It is well known that this effect originates from distinct ‘hard-soft’ phase-separated morphology in the polymer nanocomposite— one reversible ‘soft’ phase related to the transition temperature (glass transition Tg or melting Tm ) and another fixed ‘hard’ phase-out of crystallites crosslinking points. A reversible phase transition occurs near Tg or Tm upon heating followed by cooling making the polymers bound to take a temporary shape by accumulating strain energy, and again the polymers fix the chains back to their original random conformation by the release of stress under application of stimuli in the various forms of heat or light [45]. Since TPEs consist of well-segmented phase structure and filled TPE nanocomposites possess crystallinity, crosslinking effect on shape recovery, shape memory behavior is a noteworthy topic to be discussed in graphene-TPE based nanocomposites. 3-amino-1,2,4-triazole (ATA) crosslinked maleated polyethylene-octene elastomer (ATA-POE) reinforced with octadecylamine modified graphene oxide (ODAGO) were fabricated by Kashif et al. to prepare a structurally dynamic supramolecular hydrogen-bonded thermoplastic elastomer (TPE) nanocomposite (ATA-POE/ODAGO) that endowed both NIR-triggered shape memory as well as a self-healing effect [46]. For demonstrating NIR-triggered shape memory effect, the samples were stretched to 250% and kept for shape fixity which upon exposure to NIR radiation returned to almost its original shape owing to the presence of ODAGO nanosheets, the photo-thermal nano heater, converting IR radiation to thermal energy (Fig. 5.13i). With increasing wt% of ODA-GO, the recovery rate was also found to increase proportionally due to enhanced photo-thermal effect. To investigate the local triggering of shape recovery under NIR, three distinct segments of a deformed sample were selectively exposed to NIR light in succession, as depicted in Fig. 5.13ii. which demonstrated that the shape recovery was induced only in the NIR exposed segment due to the local photo-thermal effect. NIR actuated healing effect was observed in all ATA-POE/ODA-GO scratched samples due to the aforementioned photo-thermal effect of GO that led to re-arrangement of the supramolecular hydrogen-bonded structure on melting of polyolefin crystals causing interfacial chain diffusion and re-entanglements across the scratched surfaces. The infrared transparent ATA-POE sample did not undergo any healing. The healing efficiency of the composite containing 0.5 wt% of graphene was calculated based on strain recovery of each sample under NIR exposure, which showed maximum efficiency of 45% for 0.5 wt% loading (Fig. 5.13iii (a)) beyond which agglomeration of graphene adversely affect the material properties. A comparative study between NIR-healed and thermally-healed samples was carried out, which revealed that strain recovery of 395% with healing efficiency 45% was achieved for NIR triggered healing. In comparison, 490% recovery in strain with 55% healing efficiency was shown by

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5 Potential Application of Graphene-TPE Nanocomposite

Fig. 5.13 Pictorial representation of i NIR-triggered shape memory process for the ATAPOE/ODA-GO nanocomposites with a different weight content of ODA-GO loadings (i). ii NIRcontrolled shape recovery of the APG-0.50 nanocomposite a original shape, b 100% elongated shape and gradually three-step recovered shapes: c left segment, d middle segment, and (e) right segment. iii Stress-strain curves of the APG-0.50 scratched samples a healed for various times under NIR irradiation and b comparison of NIR and thermal healed sample for 60 min. Reproduced with permission from [46]

thermally healed samples (Fig. 5.13iii (b)). The outcome established that NIR irradiation was quite a good choice for effective healing and shape recovery that made POE based TPE to be a potential applicant in several industrial applications. Thermoresponsive PUnanocomposites have been vividly analyzed for their excellent shape memory behavior, having term activity improved by carbonaceous nanofillers. The elastomeric PU prepared by in-situ polymerization of diisocyanate with functional GNs and dispersion in polyether polyols displayed 97% shape recovery at 1wt% graphene loading [47]. Thakur and Karak developed castor oilmodified hyperbranched polyurethane (HBPU) and went nanocomposites where the shape memory effect was studied in relation to thermal stability, melting temperature (Tm ), enthalpy, degree of crystallinity, and glass transition temperature. The thermoresponsive shape memory effect showed ~ 90% shape fixity and ~99.5% shape recovery [48]. Han and Chun prepared functionalized nanolayered GNs/PU nanocomposite by covalent interaction recovery [49]. The surface functionalization of graphene was designed using diazonium salts containing phenethyl alcohol groups, and that functionalized graphene was crosslinked with the urethane linkage of NCO terminated PU. The nano-layered GN was oxidized (GNO), partially reduced

5.3 Shape Memory

201

(pRGN), and surface functionalized (FGN) for enhanced interaction between matrix and fillers. The modified PU/graphene samples exhibited high-performance shape memory behavior having 98% shape fixity and 94% shape recovery at 0.5 wt% loading compared to 56% shape fixity and 68% shape recovery of pure PU indicating homogeneous dispersion of graphene in PU matrix as well as superior mechanical, thermal and shape memory property. Patel and Purohit recently published an article on enhanced shape memory and mechanical properties of thermoplastic polyurethane (TPU) filled with different amounts of graphene nanoplatelets (GNP) under microwave (MV) irradiation [50]. The nanocomposite with 2 wt% loading of graphene (2 GPU) exhibited almost 90% of original shape recovery in 30 s compared to conventional heating, which took 90 s for the same (Fig. 5.14i). Pure TPU did not undergo any shape recovery due to the absence of graphene, the nanoscale heater, converting the MV irradiation into thermal energy. When 2 GPU sample was tested with 50% MV + convection heating, yielded superior result than only convection mode (Fig. 5.14ii). It was also observed that with

Fig. 5.14 MW-induced shape recovery test of SMP/GNPs (2 GPU at 2.45 GHz) (i), Unconstrained MV-induced shape recovery behavior of SMP/GNPs (2 GPUat 2.45 GHz) (ii), Unconstrained MVinduced shape recovery behavior tested for 60 s (iii). Reproduced with permission from [50]

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5 Potential Application of Graphene-TPE Nanocomposite

increasing both GNP concentration and incident frequency of MV irradiation, the shape recovery also increased proportionately (Fig. 5.14iii). Li et al. reported the synthesis of novel liquid crystalline polyurethane (LCPU) containing polar groups that delivered enhanced mechanical properties and shape memory behavior after being reinforced by graphene oxide (GO) [51]. Apart from imparting stiffness to the composite, the main chain rigid structure of LCPU benefited shape memory performance in terms of shape fixity and shape recovery due to the unique interaction between GO and LCPU. To investigate shape memory performance, the rectangular specimens of LCPU/GO composite were folded at 90° C followed by rapid quenching in water that exhibited no apparent recovery for 2 h in the air. On further heating to 90° C, they started to regain their original shape (Fig. 5.15a). The nanocomposite was characterized by decreasing shape fixity ratio (Rf ) with increasing GO content, having a value dropped from 98% of pure LCPU to 92% of GO/LCPU 20 containing 20 wt% loading of GO in the first cycle, but the respective values were maintained in the subsequent three cycles. Shape recovery ratio (Rr ) did not show any downfall in their values in the first cycle, but the slight decrease was noticed in the consecutive three cycles probably due to the formation of the frozen-in crystals by the re-orientation of the molecular segment in the direction of external force during the repeating cycle (Table 5.1). The remarkable improvement in shape recovery response with increasing GO content (Table 5.2)was attributed to the increased thermal conductivity as well as heat exchanged during phase transition. An interesting observation was noticed from the stress–strain curve of LCPU/GO samples that co-related a directly proportional relationship between the high-speed shape recovery and area under the curve. Larger the area of the curve, higher would be the strain energy stimulating strain recovery upon release of stress, which was also in agreement with much higher shape recovery in case of LCPU 20 having the largest area under the curve compared with pristine LCPU (Fig. 5.15b).

Fig. 5.15 Recovery photos at different times of LCPU at the first cycle (a), Stress-strain curves of the pristine LCPU, and its composites at room temperature (b). Reproduced with permission from [51]

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Table 5.1 Value of shape fixity and shape recovery ratio of LCPU and GO/LCPU nanocomposites [51] Samples

Cycle 1

Cycle 2 Rf

Cycle 3 Rr

Rf

Cycle 4

Rf

Rr

Rr

Rf

Rr

LCPU

98

99

98

99

97

98

98

97

LCPU 5

96

100

95

99

95

96

96

96

LCPU 10

95

99

95

98

95

97

94

97

LCPU 15

92

99

93

98

93

98

93

98

LCPU 20

92

100

93

99

93

99

93

98

Table 5.2 Average value of shape recovery rate of all the samples [51] Samples

Cycle 1

Cycle 2

Cycle 3

Cycle 4

LCPU (°/s)

2.6

2.6

2.4

2.3

LCPU 5 (°/s)

5.6

4.5

4.9

4.6

LCPU 10 (°/s)

6.3

5.9

5.5

5.3

LCPU 15 (°/s)

8.6

8.1

6.8

6

LCPU 20 (°/s)

9.2

8.9

7.8

7.1

In another attempt, Yoo et al. described high-speed actuation of poly(εcaprolactone) (PCL)-based shape memory polyurethane (PU) nanofibers comprising three kinds of graphene, that is, graphene oxide (GO), PCL-functionalized graphene with PCL (f-GO), and reduced graphene (r-GO) [52]. All the nanofiber samples, including pure PU, retained more than 95% shape retention even in increasing cycle. The thermoresponsive shape recovery force and recovery rate were compared and analyzed for the pure PU, GO, f-GO, and r-GO nanofibers where PU/f-GO and PU/rGO nanofiber webs showed a shape recovery time of 8 s with 1 wt% loading while the pure PU and PU/GO nanofiber webs showed the same recovery in 27 and 13 s respectively. The combined effect of good interaction between graphene and polymer matrix, the excellent thermal conductivity of graphene, and higher surface area of graphene nanofibers compared to that of the film resulted in fast shape recovery time as well as improved mechanical properties in f-GO containing composites. Ponnamma et al. addressed an elaborative explanation on the structure–property relationship of PU/graphene oxide (GO) nanocomposite affecting shape memory behavior and a novel and innovative co-relation between shape memory and Payne effect (crosslink density) [45]. The shape recovery percentage calculated from the cyclic stress–strain curve (Fig. 5.16a) showed a gradual increase of ~96.5%, ~97.7%, ~98.3%, ~99.2% on moving from pure PU to PU/GO nanocomposites with increasing GO content. It was analyzed that ionic groups on GO surface were responsible for improved inter-phase interaction of PU matrix-forming large number of crosslinks that lead to the higher amount of stored elastic strain energy, which in turn aided

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Fig. 5.16 Graphical representation of a Shape fixity and b shape recovery ratio of PU and its nanocomposites with GO under cyclic loading at different temperatures of 298 K, 323 K and 348 K, c Correlation of shape recovery and temperature with the crosslink density (fitted with the Maier and Göritz model) for PU, PG0.5, PG1.5 and PG3 at 298 K, 323 K and 348 K. Reproduced with permission from [45]

the process of regaining higher recovery stress by releasing stored elastic strain on re-heating. On the other hand, shape fixity value showed a noticeable decrease (~95.8%, ~91.8%, ~75.1%, and ~69.7%) with increasing GO content in the composite compared to pristine PU (Fig. 5.16b). As already mentioned that increasing GO concentration increased interfacial interaction forming a higher number of crosslink points, simultaneously that a large number of crosslinks were also during re-heating as compared to pure PU, thereby decreasing the composite’s capability of fixing mechanical deformation. Moreover, GO might disturb the crystallization process that governed the shape fixity process. Temperature also affected the shape fixity phenomenon negatively due to the enhanced mobility of chains at higher temperatures hindering the fixation of bonds. The authors established a linear co-relation between crosslink density (calculated from dynamic mechanical analysis, i.e., DMA, a direct consequence of the Payne effect) and shaped the memory of PU/GO nanocomposites (Fig. 5.16c). Observation of the Payne effect from DMA and quantitative estimation of crosslink density yielded the highest value for PU/GO (PG3, 3 wt% GO)

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205

composite, which can be correlated with the increasing shape recovery trend of the same sample, as mentioned earlier. The increasing amount of GO also interfered with the soft segment crystallization, causing interphase mixing, thereby affecting shape recovery. The polymer–filler interactions were strongly affected by the temperature variations. At higher temperatures, physical crosslinks got melted, and most of the bonds on the filler surface are converted into a flexible polymer chain contributing to the improved shape recovery process. Wu et al. fabricated a maleic anhydride-g-SEBS/amine-functionalized GO integrated hybrid shape memory aerogel with a triple network—(i) physical network of graphene oxide (GO) aerogel (Network I), (ii) infusion of GO in SEBS solution forming a GO crosslinked triblock copolymer network (Network II), (iii) formation of a physical network in SEBS after removal of solvent (Network III) [53]. When a cylindrical sample of SEBS/GO was cooled under a constant compression strain of 51.3% to room temperature from 90 °C, it gave rise to a shape fixity (Rf ) value of 94.9%, suggesting near-complete shape fixing of the aerogels. On re-heating, the sample regained its original shape showing a shape recovery (Rr ) of 94.4% with a residual strain as small as 4.8% indicating the high efficiency of shape memory. Consecutive cycling tests of at least five cycles displayed good repeatable and reproducible data without much deterioration (Fig. 5.17a). Almost identical results of shape memory triggered by IR illumination had been reproduced instead of direct heating because of GO’s excellent IR absorptivity (Fig. 5.17b). Resistive heating actuated shape memory behavior has been observed in many polymers incorporating conducting fillers. Rana et al. designed highly flexible, conductive, shape-memory PU prepolymer from 4,49-methylene bis(phenyl isocyanate) and poly(e-caprolactone)-diol incorporating functionalized graphene as

Fig. 5.17 Shape fixation and recovery of GO/SEBS-2 (a). The dashed line represents the temporal strain achieved by deformation. Shape recovery of GO/SEBS as a function of time under the IR light (b). L0 is the initial length of the cylindrical sample, and L is the length after shape fixation or shape. Reproduced with permission from [53]

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Fig. 5.18 Electroactive shape-recovery behavior of graphene-crosslinked PU composites. The samples undergo the transition from the temporary shape (helix, left) to permanent (linear, right) within 10 s. Reproduced with permission from [54]

crosslinkers [54]. Resistive heating triggered actuation was shown by the composite when an applied 60 V heated the sample above the transition temperature of 45 °C in 6–7 min. the electroactive shape recovery of ~97% with 95% shape retention was observed due to excellent electrical and thermal conductivity of graphene crosslinked composites (Fig. 5.18). Valentini et al. observed another electroactive shape memory effect and mechanical deformation of a graphene nanoplatelet (GNPs) coated PU block copolymer film when an electrical bias was applied to the coating generating heat by the Joule effect, referred as ohmic or resistive heating [55]. Under an applied constant voltage of 10 V, the sample started deforming and recovered almost original shape within ~20 s. It was the GNP’s electrical conductivity that generated heat and transferred it in an otherwise insulating PU substrate, making it softened, thus facilitating shape deformation. Tan et al. reported shape memory polyurethane (SMPU) nanofibers filled with GO that significantly improved mechanical properties, thermal stability, and shape memory effect of the composite [56]. The average shape fixity (Rf ) and shape recovery ratio (Rr ) was almost as high as 92.1% and 96.5%, respectively, at 4 wt% loading of GO with much better tensile stress in the course of the cyclic tensile test, compared to pure SMPU. The deformation force of that 4 wt% GO containing composite at the maximum in each cycle was nearly double of that pristine SMPU.

5.4 Self-healing Material’s ability to mimic self-healing attributes of living organisms is anticipated to hold many attractive merits, including extended lifespan, improved safety, capacity to substantially recover, and resume their functions and restoration of material’s performance as well [57]. Self-healing polymers, a representative example of ‘smart’ material’ have emerged as a potential next-generation material aiming to develop autonomic/intrinsic healing or an external stimulus aided healing process, and thermoplastic block copolymers are one of the classic examples to exhibit

5.4 Self-healing

207

self-healing attributes. These polymers are comprised of an innate combination of ‘hard-soft’ segments that contribute to the healing process through supramolecular assembly involving wetting of the surface, diffusion and equilibration, and subsequent rearrangement or recombination of polymer chains around damaged site. The soft segments undergo rapid Brownian motion on achieving Tg or Tm , leading to diffusion and rearrangement of polymer chains, while hard segments provide the structural rigidity. Incorporation of graphene-like nanomaterial in polymers expedites the healing process for being a good absorber of heat and equally efficient conductivity and dissipation of energy throughout the matrix. The previous observation on NIR induced healing of TPU [46] was further corroborated by Huang et al.’s finding on multichannel and repeatable self-healing of fewlayer graphene-TPU nanocomposite (FG-TPU) under IR, electricity and electromagnetic wave (microwave) [58]. Under IR irradiation, the heating efficiency of all FG-TPU samples with different FG loading reached almost 99% (Fig. 5.19a) within several minutes with shortest healing time at 5 wt% loading (Fig. 5.19d) whereas pure TPU sample showed zero healing efficiency implying that FG significantly improved the healing attributes of TPU composites by inducing strong IR absorptivity in composite compared to IR transparent pure TPU and by excellent thermal conduction along-with efficient transfer of Joule energy from IR radiation. Electrical

Fig. 5.19 The healing performances of the FG-TPU samples with different FG loadings under the three healing processes. a The IR light healing efficiencies of the pure TPU and the FG-TPU samples with different FG loadings at the optimal healing time. b The electrical healing efficiencies of the pure TPU and the FG-TPU samples with different FG loadings at the optimal healing time. c The electromagnetic wave healing efficiencies of the pure TPU and the FG-TPU samples with different FG loadings at the optimal healing time. d The optimal healing time of the FG-TPU samples with different FG loadings. e The relationship between the applied voltage and the healing time for the FG-TPU samples with different FG loadings. f The optimal healing time of the FG-TPU samples with different FG loadings. Reproduced with permission from [58]

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5 Potential Application of Graphene-TPE Nanocomposite

self-healing performance was investigated by applying different voltages to the fractured FG-TPU samples where samples with more than 5 wt% loading were effectively healed within 3 min with >98% efficiency (Fig. 5.19b), but samples with loading below than that along-with pure TPU exhibited zero healing due to poor electrical conductivity. This can be attributed to the formation of conducting networks above the percolation threshold due to the efficient dispersion of FG in the matrix. From Fig. 5.19e, it is evident that the lowest voltage for each sample gradually decreased with increasing FG loading for identical healing time (3 min). Also, to achieve identical healing efficiency with the same FG loading, healing time became shorter with increasing applied voltage. Under electromagnetic waves, all the FG-TPU samples with various FG loading came up with almost 98% healing efficiency compared to pure TPU that could only be partly healed even after long exposure (Fig. 5.19c). It is speculated that conjugated π—the structure of graphene formed a giant electrical dipole upon microwave absorption and transferred throughout the matrix in the form of thermal energy. Figure 5.19f displays the reduction of optimum healing time of FG-TPU samples with increasing FG loading due to the strong absorption capacity of microwave that led to diffusion and re-entanglement of polymer chains across the interfacial region. The repeatability of healing performance was also investigated by all three methods under cycling test, which recorded 20, 2, and 5 successful cycles of repetition under IR, electricity, and electromagnetic wave, respectively, with 99% healing efficiency. Li et al. reported thermally healable covalently cross-linked graphene oxide/polyurethane (PU/GO) nanocomposite based on Diels-Alder (DA) chemistry [59]. PU pre-polymer was synthesized by in-situ polymerization of 4,4diphenylmethane diisocyanate (MDI), poly(tetramethylene glycol) along with GO blocked by furfuryl alcohol (denoted by iGO-PU_FA) followed by cross-linking with N N -(4,4 diphenylmethane)bismaleimide (BMI) (denoted ad iGO-PU-DA). All the formulations have been summarized in Fig. 5.20i. To investigate the healing behavior, the original samples were torn by a tensile testing machine, and the broken surface was immediately reunited together with gentle pressure and stored at 150 °C for 4 h, followed by 24 h at 65 °C without any continuous pressure. Evaluation of mechanical properties of PU-DA and iGO-PU-DAs films from tensile testing displayed a healing efficiency of 98.16%, 71.12%, 69.54%, and 73.48%, respectively (Fig. 5.20ii). Reunion of the broken surface at 150 °C caused the recombination of the 3D network formed by reversible DA reaction between the furan-maleimide group that was broken during stretching. However, PU-DA showed the highest healing efficiency compared to GO containing samples. The plausible reason might be the hindrance in polymer chain mobility due to the addition of GO forming several anchoring points, served as physical crosslinks, throughout the matrix that disturbed the recombination of the furan–maleimide groups. In PU-DA, the molecular motion was much efficient and inter-diffusion of polymer chains resulted in better recovery of mechanical properties. But the incorporation of GO triggered the significant recovery

5.4 Self-healing

209

Fig. 5.20 An overview of i Recipes for the preparation of the composites with different contents of GO. ii Healing efficiency of PU-DA and iGO-PU-DAs films determined by recovery of breaking stress. iii Stress-strain curves of PU-DA and iGO-PU-DAs films before and after thermal healing: a PU-DA, b iGO-PU-DA-1, c iGO-PUDA-2, and d iGO-PU-DA-3. iv Summary of the mechanical properties of the composite samples after the healing test. The average values were obtained from more than 3 samples. Reproduced with permission from [59]

in tensile properties after healing (Fig. 5.20iii and (iv)) with the highest measured healing efficiency of 78% in terms of breaking stress. Wang et al. investigated the self-healing phenomenon in a graphene/thermopolyurethane (G-TPU) flexible conductive film under the influence of IR light and electrical heating, respectively [60]. Self-healing efficiency was determined by the measurement of mechanical properties of the healed sample where it showed that tensile strength of healed composite films was higher than that of unhealed samples, with IR illumination showing better efficiency than electrical healing (Fig. 5.21i) with shorter healing time. The composite film with the mass content of graphene less than 3% was not used for electrical healing because it was too resistive to show any change in temperature. Due to the excellent absorption of IR radiation of by graphene, conversion of energy, rapid and uniform heat distribution and direct irradiation to the crack raising the temperature nearly to its softening point, IR heating was more conducive towards healing process by proper wetting, diffusion, rearrangement and cross-linking of TPU chains, and curing processes compared to electrical heating where dispersed heat distribution and initiation of heating from the bottom of the crack made the complete healing process more time-consuming. Comparing SEM images of IR healed and electrically healed

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Fig. 5.21 Tensile strength of G-TPU composite films before and after healed by electricity and IR light, respectively (i), SEM images of scratch samples healed at 130 °C for the different time using electricity (ii), SEM images of scratch samples healed at 130 °C for the different time using IR light (iii). Reproduced with permission from [60]

sample (Fig. 5.21ii and iii) at 130 °C, it was observed that crack in the film gradually decreased with increasing time in both cases but a faint traces of crack was noticed in the electrically healed sample after 15 min healing which was almost disappeared in IR triggered healing indicating better performance. Regardless of the healing method, the temperature also played an important role in the complete healing process. Thakur and Karak reported a sunlight-induced tunable self-healing hyperbranched polyurethane (HPU)-TiO2 -reduced graphene oxide (rGO) nanocomposite by judicious compositional variation of TiO2 -rGO nanohybrid [61]. The experiment displayed that HPU/T1RGO (1:1 weight ratio of TiO2 /rGO) nanocomposites with different loadings were only effectively healed within 7.5–10 min under direct sunlight (Fig. 5.22a). rGO, being a good absorber of sunlight, transferred the energy to the soft segment of TPU, facilitating molecular diffusion towards the crack. The tunable healing attributes were achieved by appropriate variation in nanohybrid composition. Even after the 4th cycle of repeated healing, the composite retained almost identical healing ability (Fig. 5.22b) and significant tensile properties without much deterioration (Fig. 5.22c). Bayan and Karak fabricated poly(ε-caprolactone)diol derived bio-resource based self-healable aliphatic hyperbranched polyurethane nanocomposites (HPU/Si-GO) with different wt% of the 3-aminopropyltriethoxysilane-modified graphene oxide sheets (Si-GO) [62]. The composite displayed efficient self-healing behavior triggered by microwave (within 50–60 s at 450 W) and sunlight (within 4–6 min under 106 lx) by effecting heating of the HPU matrix, although microwave was found to be more expeditious than sunlight towards the healing process. The external stimuli caused the melting transition of soft segments leading to the diffusion and subsequent

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Fig. 5.22 Graphical representation of a Healing efficiency of the nanocomposites under sunlight, b repeatable healing efficiency of the nanocomposites, c digital and optical microscopic photographs of cracked and healed nanocomposite films and d representative stress-strain profiles of HPUT1RGO2, before and after healing with different repeating cycles. Reproduced with permission from [61]

rearrangement of polymer chains around the wounded site. The presence of Si-GO nanosheets, being a nanoscale heater, provided the extra impetus for augmenting the healing process by absorbing sufficient amount of energy and dissipating effectively throughout the entire matrix, which was absent in pristine HPU. One pertinent observation was reported that pure HPU exhibited shape memory effect (SME) but not self-healing attributes, the reason being the absorbed energy from stimuli was sufficient to activate the polymer chains for rearranging its orientation and regaining its original shape but was inadequate for melting a polymer segment and diffusion of polymer chains to the damaged area for healing. The nanocomposite manifested repeatable healing ability by retaining almost analogous healing efficiency up to the fourth cycle of healing (Fig. 5.23i) and comparable tensile strength before and after healing (Fig. 5.23ii).

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Fig. 5.23 Healing efficiencies of HPU/Si-GO0.5 nanocomposite under MW and sunlight (i). Stressstrain profile of HPU/Si-GO0.5 before and after healing (ii). Reproduced with permission from [62]

5.5 Biomedical Carbon materials are well known for environmentally and biologically benign substances than inorganic materials, as carbon is one of the most abundant elements in our ecosystem. Graphite, a very common allotrope of carbon, has been widely used without any toxicity effect, and thus graphene, single-layer graphite, are expected to be nontoxic for biological purposes [63]. Intensive research is being carried out on the biomedical application of graphene and its derivatives based nanocomposites with rapid expansion, exciting and encouraging advancement. Owing to several unique and fascinating properties, graphene and its derivative are becoming promising materials for bio-imaging, fluorescence biosensing, tissue engineering, drug delivery, and therapeutics [64, 65]. Despite these progressive developments, long term cytotoxicity and biosafety of graphene and its derivatives need to be investigated before clinical application. Jian et al. recently published the synthesis of a novel multifunctional wound dressing skin-like thermoplastic polyurethane (TPU) bilayer membrane modified by polyhexamethylene guanidine hydrochloride (PHMG) grafted graphene oxide (MGO) imparting long-lasting antibacterial properties [66]. The MGO-TPU membrane recorded excellent WVTR (water vapor transmission rate), higher than GO-TPU, having effective prevention of wound exudation that caused the infection. The cellular viability detected by CCK-8 assay displayed more cell growth on MGO 0.5-TPU compared to other tested samples (Fig. 5.24i) for 5 consecutive days observation indicating MGO-TPU to be the least cytotoxic. MGO-TPU and PHMGTPU were found to have good antibacterial activity against both Gram-negative E. coli and Gram-positive S. aureus (Fig. 5.24ii). Long-lasting antibacterial activity study revealed no significant change in bacterial viability in MGO-TPU (with 0.5 wt% loading) bilayer membrane compared to individual PHMG-TPU and GO-TPU

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Fig. 5.24 Demonstration of i The cellular viability detected by CCK-8 assay (OD450) at each set time point. The values are shown as the means ±SD (n = 3). ii Antibacterial activity of GO/MGOTPU composite porous membrane with the ratio between GO and MGO. a activation of E. coli; b activation of S. aureus. iii The long-lasting antibacterial activity of the GO/MGO-TPU composite porous membrane was shaken and washed in PBS buffer for 0, 7, 30 days. a Inactivation of E. coli; (b) Inactivation of S. aureus, iv a Representative macroscopic appearance of the infected wounds, blank (Wound without any treatment), Control (Sterile Vaseline gauze covered wound), PHMG0.5TPU, GO0.5-TPU, MGO0.5-TPU; b Representative histological image of the length of the newly formed epithelium tongue at day 9 post-surgery in the Blank (Wound without any treatment), Control (Sterile Vaseline gauze covered wound), PHMG0.5-TPU, GO0.5-TPU, MGO0.5-TPU. c Wound healing curves; d Wound closure time. The values are shown as the means ±SD (n = 5). Reproduced with permission from [66]

even after 30 days of continuous shock washing in PBS buffer probably due to the good compatibility of GO and MGO with TPU matrix leading to uniform dispersion and enabling them to maintain their constant antibacterial properties (Fig. 5.24iii). Combination of favorable test results on cytotoxicity and antibacterial activity of MGO-TPU led to further investigation of in vivo wound healing ability of the same on infected mice skin (Fig. 5.24iv (a)) where it was again proved to be superior to PHMG-TPU and GO-TPU in terms of wound healing rate (Fig. 5.24iv (c), highest for MGO-TPU, 95.45 ± 1.2%) and wound closure time (Fig. 5.24iv (d), lowest for MGO-TPU, 7.8 ± 0.44 days) along-with effective regeneration of epithelium tongue (Fig. 5.24ivb). Moreover, the MGO-TPU membrane was able to accelerate the re-epithelialization process in the infection wound, fostering rapid wound healing. Graphene oxide (GO) is well known to improve cellular growth of various cell types [67, 68] while biocompatible thermoplastic polyurethane (TPU)/poly lactic acid (PLA) are well studied for cellular culture [69] and cell scaffold material [70]. Based on this two combined observation, Chen et al. fabricated 3D printed TPU/PLA/GO nanocomposite by fused deposition modeling (FDM) technique and

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characterized its biocompatibility as seeding scaffolds using NIH3T3 mouse embryonic fibroblast cells with a LIVE/DEAD viability/cytotoxicity assay to find a promising application in tissue engineering as biomaterials scaffold [71]. NIH3T3 cells were deposited on the 3D printed monolayer of nanocomposite under 96 h incubation followed by washing of scaffold and staining the cells with a combination of calcein-AM and ethidium homodimer-1 (EthD-1) for the detection of live and dead cells respectively by fluorescent imaging. LIVE/DEAD assay detected excellent cell viability of only live cells in the scaffold supported system without any presence of dead cell (shown in green in Fig. 5.25) indicated that all the scaffolds were nontoxic towards embryonic cells. Another observation stated that scaffold supported cell grew with the highest density at 0.5% GO in TPU/PLA blend compared to all other loadings as well as TPU/PLA control, thus making it evident that GO caused no obstruction towards cell growth and presence of an optimum amount of GO was advantageous for the proliferation of cells. Jing and his group did emphasize the same observation about cell viability in the presence of graphene oxide (GO) while investigating biomedical thermoplastic polyurethane/graphene oxide (TPU/GO) composite scaffolds prepared by thermally induced phase separation (TIPS) technique for NIH3T3 fibroblast cell culture [72]. Figure 5.26i depicts fluorescent images of Day 3 cell culture detecting successful attachments of cells on composite scaffold along-with more no. of live cells TPUGO1% compared to pristine TPU that re-established the non-cytotoxicity of GO. The presence of the small amount of GO was favorable for the cell proliferation on the scaffold, which was also confirmed from the fluorescent image of Day 10 cell culture (Fig. 5.26ii), showing extensive spreading of multiple cell layers on TPUGO1% specimen. However, the higher concentration of GO delayed cell proliferation, evident from a sharp fall in the number of cells in TPU-GO5% and TPU-GO10% scaffold w.r.t that of optimal loading (1%), shown in Fig. 5.26i. Thampi et al. addressed the potential biomedical application of graphene oxide (GO) reinforced aligned fibroporous poly (carbonate urethane) membrane by conducting an in vitro cytotoxicity test with L-929 mouse fibroblast cells and percentage hemolysis tests with fresh venous blood [73]. The L-929 cells showed complete cytocompatibility when exposed to fibroporous membranes for the direct contact test and maintained their spindle-shaped morphology After incubation at 37 ± 1 °C for 24–26 h (Fig. 5.27i). A comparative MMT assay [3-(4,5-dimethylthiazol2-yl)-2,5-diphenyltetrazolium bromide] study was performed to measure metabolic activity and percentage cell viability of the individual electrospun membranes which displayed good cell viability for PCU-AF(aligned fiber) making it to be the cytocompatible but poor result for PCU-3%GO composite membrane (Fig. 5.27ii). The percentage of hemolysis was calculated by quantifying hemolysis essay, which needs to be within the recommended range prescribed by ISO 10993-4 during the period of blood contact with implanted material. Figure 5.27iii depicts that hemolysis percentage of all fibroporous composite membranes were within permissible

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Fig. 5.25 96 h cell culture results of NIH3T3 cells on 3D printed TPU/PLA with different GO loadings: a 0 wt % GO, b 0.5 wt % GO, c) 2 wt % GO, d 5 wt % GO. Green color indicates live cells. Reproduced with permission from [71]

range (less than 5%), which was in accordance with ISO guideline, thus indicating anti-hemolysis properties among all PCU and PCU/GO membranes. Shams et al. investigated another interesting antimicrobial activity in graphene oxide (GO) nanoplatelets incorporated novel polyurethane/siloxane (XSi-PU/GO) wound dressing membranes by in-vitro and in-vivo analysis [74]. Human dermal fibroblast cells in contact with either dressings or leachates extracted from dressing continued as usual cell growth and proliferation by analysis of MTT assay without being affected by the incorporation of GO and the studied range of concentration indicating peerless cytocompatibility of the dressing membranes.

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Fig. 5.26 Images of i Day 3 3T3 fibroblast cell culture results of freeze-dried TPU (a), TPU–GO1% (b), TPU–GO5% (c): TPU–GO10% (d) scaffolds: (a–d) are fluorescence microscope pictures (scale bar 5100 lm) where green indicates living cells. ii Day 10 3T3 fibroblast cell culture results of freeze-dried TPU (a), TPU–GO1% (b), TPU–GO5% (c): TPU–GO10% (d) scaffolds: (a–d) are fluorescence microscope pictures (scale bar 5100 lm) where green indicates living cells and red indicates dead cells. Reproduced with permission from [72]

Fig. 5.27 Micrographs of i cell morphology after a direct contact test: a negative control, b positive control and c PCU/GO composite electrospun membrane. ii MTT assay representing the cell viability (%) of the L-929 fibroblast cells on electrospun membranes. iii Graphical representation of the percentage of hemolysis of the PCURF (random fiber), PCUAF, and composite electrospun membranes with 1, 1.5 and 3% loadings of GO. Reproduced with permission from [73]

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Fig. 5.28 Illustration of a Antibacterial activity of the prepared dressing membranes against S. aureus, E. coli, and C. albicans. b Wound healing process for 20 days, treated with gauze (control), XSi-PU and XSi-PU/GO5%. c Representative images of MT and H&E stained histological sections on day 20 after initial wounding, arrows indicated the blood vessels. Reproduced with permission from [74]

Concentration-dependent antimicrobial activity was recorded against E. coli and S. aureus as gram-negative, and gram-positive bacteria, as well as C. albicans as a fungal strain and with increasing GO nanoplatelets, almost complete killing activity, was detected for XSi-PU/GO5% and XSi-PU/GO7.5% samples due to presence of GO acting as ‘cutters’ to damage the bacteria membrane (Fig. 5.28a). The invivo evaluation was carried out on wound created in the dorsal area of Wister, showing very efficient wound healing of 98%, enhanced epithelization of wounded tissue on the 20th day by XSi-PU/GO5% compared to XSi-PU along-with generation of good quality new skin covering wound (Fig. 5.28b). The H&E stained sections displayed (Fig. 5.28c) complete re-epithelization, pronounced neovascularization at newly formed tissue, and reconstruction of epidermis and dermis with XSi-PU/GO5% dressings on Day 20 attributed to enhanced migration of epithelial cells resulting from the preservation of the moist environment at the infected region. Malory Trichrome (MT) stained area detected parallel aligned highly matured level of collagen deposition at the wound site.

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5.6 Conclusion and Future Outlook The innovative breakthrough in fundamental material research was triggered worldwide at various theoretical and experimental aspects with the invention of graphene, and within few years of discovery, the graphene-based nanomaterials and composites were being recognized as ‘supermaterial’ in material science [17]. The development of graphene-based TPE nanocomposites has achieved considerable technological attention cum commercial exposure due to their combined physic-chemical properties that enable the realization of its wide range applications. Considering the present and ongoing achievements, the smart applications of graphene/TPE nanocomposite systems with more functionality are expected to penetrate the conventional market more deeply as new complements and/or even replacements to existing products, especially in fields of electronics, energy, soft robotics and automotive [75]. More focus is being turned to graphene than carbon nanotubes due to some of its unparalleled properties, e.g., outstanding electronic properties of graphene have promised to replace silicon of all consumer electronics offering the prospect of quantum computers operating at terahertz speed although the job is far from complete. Successful integration of graphene directly into the device is facing a lot of challenging obstacles that need to be overcome gradually along-with addressing the issues of establishing a cost-effective process for the mass production of graphene [76] and its functional derivatives with an acceptable range of reproducibility for pragmatic applications.

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Chapter 6

Conclusion

The thermoplastic elastomers incepted in the scientific realm in the early 1950s when polyurethane fibers started to mark its presence in the polymer market with the discovery of the fundamental diisocyanate addition [1]. Two years later, Snyder fabricated linear copolyester fibers whose stress–strain characteristics were analogous to natural rubber, which is believed to mark the beginning of an era of a material inheriting the traits of both the plastics and the elastomers. These multiblock thermoplastic polyurethanes evolved rapidly by 1960 with renowned companies like Bayer, Dupont, and Goodyear starting to commercialize polyurethane-based thermoplastic elastomers [1]. Over the years, the avenue of the thermoplastic elastomers did not circumscribe only along polyurethanes but also co-evolved with the advent of diverse classes of block copolymer such as polystyrene-polybutadiene-polystyrene, polystyrenepolyisoprene-polystyrene, and poly(ethylene-co-propylene) [1]. The trend of the commercially available thermoplastic elastomers under the trade name Santoprene® (Monsanto Company), Hytrel® ( DuPont), Estsmid (Dow Chemical Company) in the 1980s eventually harmonize thermoplastic elastomers to be an emerging frontier to tether the multi-disciplines of material sciences, biomedical sciences, and composite technology in taking science a leap ahead in the area of processing, manufacturing and synthesizing novel products for advanced tailor-made applications [1]. The market for the thermoplastic elastomers probably unboxed during the 1990s with researchers and companies beginning to address specific market demands with tailor-made properties and bridging the gap between innovation and sustainability. Interestingly, around the same time polymer syntheses strategies such as living/controlled polymerization techniques, including living anionic polymerization, cationic polymerization, ring-opening metathesis polymerization, reversible addition-fragmentation chain transfer polymerization, nitroxide-mediated radical polymerization, and atomic transfer radical polymerization began to spike enabling the fabrication of complex molecular architecture with fascinating functionalities, thereby supplementing to expand the limits of the contemporary market of the © Springer Nature Singapore Pte Ltd. 2020 A. Bandyopadhyay et al., Engineering of Thermoplastic Elastomer with Graphene and Other Anisotropic Nanofillers, Engineering Materials, https://doi.org/10.1007/978-981-15-9085-6_6

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thermoplastic elastomers [1–4]. Two of the most classic examples of these evolutionary trendlines are in the inception of the acrylic-based thermoplastic elastomers, commercially known as Nanostrength® (Arkema) and Kuraray™ Kurara) [4]. In light of the above considerations, a pristine thermoplastic polyurethane posses various drawbacks in terms of mechanical and chemical stability along with processability, which proves to be a hurdle in implementing them for practical applications. Therefore, to tune the hierarchical structure of the elastomers coupled with their properties, the pure elastomeric matrix is often modified with an interpenetrating polymeric network, blending process, copolymerization strategies, and various nanostructured fillers [5]. The rapid increase in the need for fabricating robust polymer matrices, especially for light-weight architectures and multifunctional characteristics, has eventually steered the scientific community to amalgamate the thermoplastic elastomers with nanofillers as reinforcement [6]. The progress in the area of nanotechnology has gifted us with superior nanomaterials ranging from nanoclays, cellulose nanowhiskers, carbon nanofibers, carbon nanotubes (CNTs), graphenes, nano-oxides like nanosilica, nanoalumina, titanium dioxide; which aids the pure polymer matric to inherit exciting multifunctional features such as electrical, thermal, mechanical, optical, fire-retardant, barrier, anti-bacterial and scratch-resistant properties, thereby not only addressing the versatility of the product but also the effective cost of the material [6]. The manufacturing industry witnessed a major game-changing event when composites started to consume the market of the popular aluminum market, especially in the production of commercial aircraft. The Boeing’s 787, along with the Airbus A350 XWB was redesigned with structures based on epoxy matric reinforced with carbon fiber, enabling the evolution of one of the most advanced twin-aisle passenger aircraft [7]. Similarly, the advent of thermoplastic composite enabled the manufacturing world to target both the aircraft and non-aircraft applications to improve the fatigue resistance, toughness, and durability. Moreover, the advantage of not having these materials crosslinked widened the avenue to develop reinforced recyclable matrices, inscribing sustainability [7]. This book serves to provide the readers with a fundamental understanding of the nanofillers reinforced thermoplastic elastomers. The authors have tried to project their perception, tethering both the synthesis techniques along with the application area of these newly trended materials. Our first chapter serves as an evolutionary token on how the current trends on thermoplastic elastomers developed along with providing insight to the ever-growing demand of the address the drawbacks which the currently used thermoplastic nanocomposites suffer. Since the advent of synthetic polymers in the 1990s, the nanofillers have always been proven to be a versatile route for fabricating robust composite materials possessing superior mechanical properties along with the optimized cost to performance ratio. In this context, anisotropic nanofillers are reported to enhance the balance between the toughness and the stiffness when integrated with polymer matrices, thereby drawing the spotlight to manufacture nanocomposites for highperformance applications [8]. The second chapter of the book specifically given a precise understanding of the anisotropic nanofillers along with their properties,

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which eventually leads to the fabrication of toughened thermoplastic elastomers for a variety of commercial applications [8]. The past three decades have witnessed a sudden peak in the research of graphene and graphene-based nanocomposites. Quantitatively speaking, there were more than 100,000 research articles that contained the term graphene in the title or the research articles, tuning the material to be tagged as “graphene possesses everything’s what you want.” [2] The global market for graphene was estimated at USD 78.7 million in 2019 by The Grandviewresearch and is expected to bulge out at the Compound average growth rate of 38.7% from 2020 to 2027 [9]. We dedicate the third chapter to emphasize developments of thermoplastic elastomers which complements graphene and graphene derived fillers for fabricating renewable, light-weight and flexible materials for usage in high-performance nanocomposites, sensors, actuators, and energy storage devices. Graphene’s superior electron mobility and high permeability enable the pristine thermoplastic elastomer matrix aids to widens the application window of these novel nanocomposites, thereby driving the product demand [9]. The last two chapters are primarily focused to critically analyze the properties of the graphene-based nanofillers when integrated with the pure polymer matrix along with their potential areas of application [10]. The incorporation of graphene and graphene derived materials have proved to enhance not only the mechanical properties of the nanocomposites but also the thermal and the barrier properties, which we illustrate while explaining the structure–property relationship of the nanocomposites. In fact, graphene proves to the missing link between the conventional nanocomposites and the low deformation extensibility, thereby addressing the drawback which the traditional thermoplastic elastomers possessed [10]. With all these fascinating properties, graphene-based thermoplastic elastomers shall positively evolve further to percolate in both commercial and high-end applications tires, automotive parts, seals of all kinds, hydraulic hoses, footwear, water-proof clothing, tribological, biomedical, aerospace, sensing devices and nanoelectronics [10]. As we say that the human race can’t evolve independently, instead it has to coevolve with the technology, we are yet to witness the crest of the graphene-based thermoplastic elastomer’s evolution. The advent of simulation and numerical modeling has opened up several opportunities to mimic an experimental design, therefore driving towards automation. The virtual models of these graphene reinforced thermoplastic elastomers shall improvise the manufacturing models as the companies shall now be simulating the process before committing a huge sum of money, thus elevating the cost-efficacy of the process and the time required to metamorphose a product from the laboratory to the market [11]. With the advent of precise instrumentations and processing tools, dispersion of the nanofillers remains a significant puzzle amongst the scientific community to solve. Most of the recent works of literature use nanofiller dispersion techniques such as sonication, surface modification, and the application of a coating layer of graphene on the polymer surface; the use of ionic liquids is reported to gain the spotlight in the recent times and thus demands more extensive research [12]. A pivotal factor that influences the characteristics of the nanofillers is the sizes of the nanofiller and the optimum filler concentration [12]. Recent researches claim to

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have observed a decrease in the mechanical but an elevation in the other functional properties associated with the larger sizes of the nanofillers [12]. However, the effect of graphene concentration of these properties is still not deciphered completely, which serves to a future direction where the graphene-based nanocomposites trend may branch [13]. The authors believe that the present research on the graphene-based polymers shall traverse towards a broader scope of application, thereby redefining the versatility of the nanocomposites. The emergence of new host matrices and novel functionalized graphene nanofillers with variation in the aspect ratio and the shape coupled with organic/inorganic/metallic additives kindles Dr. Richard Feynman’s saying “There’s Plenty of Room at the Bottom.”

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