Reactive and Functional Polymers Volume One: Biopolymers, Polyesters, Polyurethanes, Resins and Silicones 3030434028, 9783030434021

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Tomy J. Gutiérrez  Editor

Reactive and Functional Polymers Volume One Biopolymers, Polyesters, Polyurethanes, Resins and Silicones

Reactive and Functional Polymers Volume One

Tomy J. Gutiérrez Editor

Reactive and Functional Polymers Volume One Biopolymers, Polyesters, Polyurethanes, Resins and Silicones

Editor Tomy J. Gutiérrez Thermoplastic Composite Materials (CoMP) Group, Faculty of Engineering Institute of Research in Materials Science and Technology (INTEMA) National University of Mar del Plata (UNMdP) and National Scientific and Technical Research Council (CONICET) Mar del Plata, Buenos Aires, Argentina

ISBN 978-3-030-43402-1    ISBN 978-3-030-43403-8 (eBook) https://doi.org/10.1007/978-3-030-43403-8 © Springer Nature Switzerland AG 2020, Corrected Publication 2021 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 Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

I would like to dedicate this book: To my God (Father, Son, and Holy Spirit), to the Virgin Mary, and to my Guardian Angel. Its energy stimulates me to enjoy the landscape we call life, and its peace encourages me to always continue towards the future, a place where we will all go and each one will be under the law of the creative father. To my Mother (Dr. Mirian Arminda Carmona Rodríguez), For forming my character and attitude towards life. To my Grandmother (Mrs. Arminda Teresa Rodríguez Romero), A person who unfortunately left world before time, but I am sure that she is up watching me and supporting me in all facets of my life. You are in my most beautiful memories.



To my firstborn daughter (Miranda V. Gutiérrez), I will give you a lot of love!!! To all anonymous people, Those who give me their love, friendship, patience, and support in various situations. To Venezuela and Argentina, The first for giving me my academic and professional training, and the second for welcoming me with love and friendship before the dictatorship that my country (Venezuela) is experiencing today.

Tomy J. Gutiérrez, Ph.D. Editor

Preface

Reactive and functional polymers are essentially linked to the chemistry of polymers and their applications. The multiple tasks that they have accomplished in our recent history, and how they will help new advances in different crucial fields such as agriculture, environmental remediation, food, medicine, among others are indisputable. Volume 1 of this book has been focused on polymers based on epoxy resins, lignin, polyurethane and silicone, as the raw materials for the manufacture of reactive and functional polymers. I thank Dr. Runcang Sun (Associate Editor for Carbohydrate Polymers), and his research group for valuable contribution to this volume. Tomy J. Gutiérrez, Ph.D. National Scientific and Technical Research Council (CONICET) Institute of Research in Materials Science and Technology (INTEMA) Thermoplastic Composite Materials (CoMP) Group Mar del Plata, Argentina

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Contents

1 Introduction to Reactive and Functional Polymers: A Note From the Editor��������������������������������������������������������������������������    1 Tomy J. Gutiérrez 2 Biodegradable and Functional Synthetic Polymers in Nanomedicine: Controlled and Targeted Bioactive Molecule Release��������������������������    5 Xiaoming Guo, Leung Chan, and Tianfeng Chen 3 Reactive Modification of Fiber Polymer Materials for Textile Applications����������������������������������������������������������������������������������������������   21 Avinash P. Manian, Tung Pham, and Thomas Bechtold 4 Reactive Processing and Functionalization of Ground Tire Rubber��   43 Łukasz Zedler, Marta Przybysz-Romatowska, Aleksander Hejna, Xavier Colom, Javier Cañavate, Mohammad Reza Saeb, and Krzysztof Formela 5 Lignin as a Natural Antioxidant: Property-­Structure Relationship and Potential Applications����������������������������������������������������������������������   65 Zhao Qin, Hua-Min Liu, Ling-Biao Gu, Run-Cang Sun, and Xue-De Wang 6 Functional Biobased Composite Polymers for Food Packaging Applications����������������������������������������������������������������������������������������������   95 Hulya Cakmak and Ece Sogut 7 Synthesis of Biobased Polyurethane Foams From Agricultural and Forestry Wastes��������������������������������������������������������������������������������  137 Hongwei Li, Zhongshun Yuan, Yongsheng Zhang, Chun Chang, and Chunbao (Charles) Xu 8 Reactive and Functional Polyesters and Polyurethanes ����������������������  157 Morteza Akbari and Reza Najjar

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9 Lignin as a Coating and Curing Agent on Biodegradable Epoxy Resins��������������������������������������������������������������������������������������������  195 Chikako Asada, Sholahuddin, and Yoshitoshi Nakamura 10 Reactive Silicones as Multifacetic Materials ����������������������������������������  207 Suranjan Sikdar and Sukanta Majumdar 11 Reactive and Functional Silicones for Special Applications����������������  235 Carmen Racles, Mihaela Dascalu, Adrian Bele, and Maria Cazacu 12 Maxillofacial Silicone Elastomers in Dentistry ������������������������������������  293 Pinar Cevik 13 Synthetic Methods and Applications of Functional and Reactive Silicone Polymers��������������������������������������������������������������  301 Kaleigh M. Ryan, Adam D. Drumm, Claire E. Martin, Anna-­­ Katharina Krumpfer, and Joseph W. Krumpfer 14 Hydrosilyl-Functional Polysiloxanes: Synthesis, Reactions and Applications��������������������������������������������������������������������������������������  329 Jerzy J. Chruściel  Correction to: Introduction to Reactive and Functional Polymers: A Note From the Editor������������������������������������������������������������������������������������ C1 Index������������������������������������������������������������������������������������������������������������������  415

About the Editor

Tomy J. Gutiérrez  received his degrees in Chemistry (Geochemical Option) and in Education (Chemical Mention) from the Central University of Venezuela (UCV) in December 2007 and July 2008, respectively. He completed his specialization in International Negotiation of Hydrocarbons from the National Polytechnic Experimental University of the National Armed Force (UNEFA), Venezuela, in July 2011, and his Master’s and PhD degrees in Food Science and Technology in October 2013 and April 2015, respectively, both from the UCV. He has also PhD studies in Metallurgy and Materials Science from the UCV and postdoctoral studies in the Research Institute in Materials Science and Technology (INTEMA). He has been a Professor/Researcher both at the Institute of Food Science and Technology (ICTA) and the School of Pharmacy at the UCV.  Currently, he is an Adjunct Researcher in the INTEMA, National Scientific and Technical Research Council (CONICET), Argentina. Dr. Gutiérrez has at least 20 book chapters, 40 publications in international journals of high-impact factor, and 5 published books. He has been a Lead Guest Editor of several international journals such as Journal of Food Quality, Polymers for Advanced Technologies and Current Pharmaceutical Design. He is also an Editorial Board Member of different international journals such as Food and Bioprocess Technology (from April 2019 to the present, 2019 impact factor 3.356), Current Nutraceuticals (from May 2019 to the present), and Journal of Renewable Materials (from June 2019 to the present, 2019 impact factor 1.341). Currently, he is developing a line of research in nanostructured materials based on polymers (composite materials), which are obtained on a pilot scale to be transferred to the agricultural, food, pharmaceutical and polymer industries. He is also a collaborator of international projects between Argentina and Brazil, Colombia, France, Poland, Spain, Italy, Sweden and Venezuela.

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Chapter 1

Introduction to Reactive and Functional Polymers: A Note From the Editor Tomy J. Gutiérrez

Abstract  Reactive and functional polymers are manufactured essentially from the chemical or physical modification of their precursors or by the addition of active fillers. These polymers have proven important for the development of advanced materials for different applications, and they are booming and will be a fundamental part in the future life for agricultural, energy efficiency, environmental remediation, medical devices, among others. With this chapter, we open the main topics that will be analyzed in this book. Keywords  Crosslinked polymers · Crosslinking · Elastomers · Fiber · Lignin · Polyesters · Polyurethanes · Rubbers · Silicone

1.1 

Fundamentals for Reactive and Functional Polymers

In general, reactive and functional polymers are obtained by chemical or physical manipulation of polymeric structures using different reactions (Gutiérrez 2017a, b, 2018a, b, 2019, 2021; Gutiérrez and Álvarez 2016; Gutiérrez and Alvarez 2017a, b, c, d, 2018; Gutiérrez and González 2016, 2017; Gutiérrez et al. 2015a, b, 2016a, b, 2018, 2019, 2021; Herniou--Julien et al. 2019; Merino et al. 2019a, Merino et al. 2018a, b, 2019a, b; Toro-Márquez et al. 2018; Zarrintaj et al. 2019; Alizadeh et al. The original version of this chapter was revised. The correction to this chapter is available at https://doi.org/10.1007/978-3-030-43403-8_15 T. J. Gutiérrez (*) Thermoplastic Composite Materials (CoMP) Group, Faculty of Engineering, Institute of Research in Materials Science and Technology (INTEMA), National University of Mar del Plata (UNMdP) and National Scientific and Technical Research Council (CONICET), Colón 10850, Mar del Plata 7600, Buenos Aires, Argentina e-mail: [email protected] © Springer Nature Switzerland AG 2020, Corrected Publication 2021 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_1

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2019; Valencia et al. 2019; Khosravi et al. 2020; Tomadoni et al. 2020). With this in mind, in this volume provides the principles and foundations for the design, development, manufacture and processing of reactive and functional polymers, with special emphasis on biopolymers, polyesters and polyurethanes (Gutiérrez and Alvarez 2017a; Herniou--Julien et al. 2019). This text also provides an in-depth review of updated sources on reactive resins and silicones. AcknowledgementsThe author would like to thank the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) (Postdoctoral internal fellowship PDTS-Resolution 2417), Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT) (grant PICT-2017-1362), Universidad Nacional de Mar del Plata (UNMdP) for financial support, and Dr. Mirian Carmona-Rodríguez. Conflicts of Interest  The author declares no conflict of interest.

References Alizadeh, R., Zarrintaj, P., Kamrava, S. K., Bagher, Z., Farhadi, M., Heidari, F., Komeili, A., Gutiérrez, T. J., & Saeb, M. R. (2019). Conductive hydrogels based on agarose/alginate/chitosan for neural disorder therapy. Carbohydrate Polymers, 224, 115161. https://doi.org/10.1016/j. carbpol.2019.115161 Gutiérrez, T. J. (2017a). Surface and nutraceutical properties of edible films made from starchy sources with and without added blackberry pulp. Carbohydrate Polymers, 165, 169–179. https://doi.org/10.1016/j.carbpol.2017.02.016. Gutiérrez, T. J. (2017b). Effects of exposure to pulsed light on molecular aspects of edible films made from cassava and taro starch. Innovative Food Science and Emerging Technologies, 41, 387–396. https://doi.org/10.1016/j.ifset.2017.04.014. Gutiérrez, T.  J. (2018a). Active and intelligent films made from starchy sources/blackberry pulp. Journal Polymers and the Environment, 26(6), 2374–2391. https://doi.org/10.1007/ s10924-017-1134-y. Gutiérrez, T.  J. (2018b). Are modified pumpkin flour/plum flour nanocomposite films biodegradable and compostable? Food Hydrocolloids, 83, 397–410. https://doi.org/10.1016/j. foodhyd.2018.05.035. Gutiérrez, T. J. (2019). Trends in polymers for agri-food applications: A note from the editor. In: Gutiérrez, T. (Ed.). Polymers for Agri-Food Applications. Springer, Cham. https://doi. org/10.1007/978-3-030-19416-1_1 Gutiérrez, T. J. (2021). In vitro and in vivo digestibility from bionanocomposite edible films based on native pumpkin flour/plum flour. Food Hydrocolloids, 106272. https://doi.org/10.1016/j. foodhyd.2020.106272 Gutiérrez, T.  J., & Álvarez, K. (2016). Physico-chemical properties and in vitro digestibility of edible films made from plantain flour with added Aloe vera gel. Journal of Functional Foods, 26, 750–762. https://doi.org/10.1016/j.jff.2016.08.054. Gutiérrez, T. J., & Alvarez, V. A. (2017a). Cellulosic materials as natural fillers in starch-­containing matrix-based films: A review. Polymer Bulletin, 74(6), 2401–2430. https://doi.org/10.1007/ s00289-016-1814-0. Gutiérrez, T.  J., & Alvarez, V.  A. (2017b). Eco-friendly films prepared from plantain flour/ PCL blends under reactive extrusion conditions using zirconium octanoate as a catalyst. Carbohydrate Polymers, 178, 260–269. https://doi.org/10.1016/j.carbpol.2017.09.026.

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Gutiérrez, T.  J., & Alvarez, V.  A. (2017c). Data on physicochemical properties of active films derived from plantain flour/PCL blends developed under reactive extrusion conditions. Data in Brief, 15, 445–448. https://doi.org/10.1016/j.dib.2017.09.071. Gutiérrez, T. J., & Alvarez, V. A. (2017d). Properties of native and oxidized corn starch/polystyrene blends under conditions of reactive extrusion using zinc octanoate as a catalyst. Reactive and Functional Polymers, 112, 33–44. https://doi.org/10.1016/j.reactfunctpolym.2017.01.002. Gutiérrez, T. J., & Alvarez, V. A. (2018). Bionanocomposite films developed from corn starch and natural and modified nano-clays with or without added blueberry extract. Food Hydrocolloids, 77, 407–420. https://doi.org/10.1016/j.foodhyd.2017.10.017. Gutiérrez, T. J., & González, G. (2016). Effects of exposure to pulsed light on surface and structural properties of edible films made from cassava and taro starch. Food and Bioprocess Technology, 9(11), 1812–1824. https://doi.org/10.1007/s11947-016-1765-3. Gutiérrez, T. J., & González, G. (2017). Effect of cross-linking with Aloe vera gel on surface and physicochemical properties of edible films made from plantain flour. Food Biophysics, 12(1), 11–22. https://doi.org/10.1007/s11483-016-9458-z. Gutiérrez, T. J., Tapia, M. S., Pérez, E., & Famá, L. (2015a). Structural and mechanical properties of native and modified cush-cush yam and cassava starch edible films. Food Hydrocolloids, 45, 211–217. https://doi.org/10.1016/j.foodhyd.2014.11.017. Gutiérrez, T. J., Morales, N. J., Pérez, E., Tapia, M. S., & Famá, L. (2015b). Physico-chemical properties of edible films derived from native and phosphated cush-cush yam and cassava starches. Food Packaging and Shelf Life, 3, 1–8. https://doi.org/10.1016/j.fpsl.2014.09.002. Gutiérrez, T. J., Suniaga, J., Monsalve, A., & García, N. L. (2016a). Influence of beet flour on the relationship surface-properties of edible and intelligent films made from native and modified plantain flour. Food Hydrocolloids, 54, 234–244. https://doi.org/10.1016/j.foodhyd.2015.10.012. Gutiérrez, T.  J., Guzmán, R., Medina Jaramillo, C., & Famá, L. (2016b). Effect of beet flour on films made from biological macromolecules: Native and modified plantain flour. International Journal of Biological Macromolecules, 82, 395–403. https://doi.org/10.1016/j. ijbiomac.2015.10.020. Gutiérrez, T.  J., Herniou-Julien, C., Álvarez, K., & Alvarez, V.  A. (2018). Structural properties and in vitro digestibility of edible and pH-sensitive films made from guinea arrowroot starch and wastes from wine manufacture. Carbohydrate Polymers, 184, 135–143. https://doi. org/10.1016/j.carbpol.2017.12.039. Gutiérrez, T. J., Toro-Márquez, L. A., Merino, D., & Mendieta, J. R. (2019). Hydrogen-bonding interactions and compostability of bionanocomposite films prepared from corn starch and nano-fillers with and without added Jamaica flower extract. Food Hydrocolloids, 89, 283–293. https://doi.org/10.1016/j.foodhyd.2018.10.058. Gutiérrez, T. J., Mendieta, J. R., & Ortega-Toro, R. (2021). In-depth study from gluten/PCL-based food packaging films obtained under reactive extrusion conditions using chrome octanoate as a potential food grade catalyst. Food Hydrocolloids, 106255. https://doi.org/10.1016/j. foodhyd.2020.106255 Herniou--Julien, C., Mendieta, J.  R., & Gutiérrez, T.  J. (2019). Characterization of biodegradable/non-compostable films made from cellulose acetate/corn starch blends processed under reactive extrusion conditions. Food Hydrocolloids, 89, 67–79. https://doi.org/10.1016/j. foodhyd.2018.10.024. Khosravi, A., Fereidoon, A., Khorasani, M. M., Naderi, G., Ganjali, M. R., Zarrintaj, P., Saeb, M. R. & Gutiérrez, T. J. (2020). Soft and hard sections from cellulose-reinforced poly(lactic acid)-based food packaging films: A critical review. Food Packaging and Shelf Life, 23, 100429. https://doi.org/10.1016/j.fpsl.2019.100429 Merino, D., Mansilla, A.  Y., Gutiérrez, T.  J., Casalongué, C.  A., & Alvarez, V.  A. (2018a). Chitosan coated-phosphorylated starch films: Water interaction, transparency and antibacterial properties. Reactive and Functional Polymers, 131, 445–453. https://doi.org/10.1016/j. reactfunctpolym.2018.08.012.

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Merino, D., Gutiérrez, T. J., Mansilla, A. Y., Casalongué, C. A., & Alvarez, V. A. (2018b). Critical evaluation of starch-based antibacterial nanocomposites as agricultural mulch films: Study on their interactions with water and light. ACS Sustainable Chemistry & Engineering, 6(11), 15662–15672. https://doi.org/10.1021/acssuschemeng.8b04162. Merino, D., Gutiérrez, T. J., & Alvarez, V. A. (2019a). Structural and thermal properties of agricultural mulch films based on native and oxidized corn starch nanocomposites. Starch-Stärke, 71(7–8), 1800341. https://doi.org/10.1002/star.201800341. Merino, D., Gutiérrez, T. J., & Alvarez, V. A. (2019b). Potential agricultural mulch films based on native and phosphorylated corn starch with and without surface functionalization with chitosan. Journal Polymers and the Environment, 27(1), 97–105. https://doi.org/10.1007/ s10924-018-1325-1. Toro-Márquez, L. A., Merino, D., & Gutiérrez, T. J. (2018). Bionanocomposite films prepared from corn starch with and without nanopackaged Jamaica (Hibiscus sabdariffa) flower extract. Food and Bioprocess Technology, 11(11), 1955–1973. https://doi.org/10.1007/s11947-018-2160-z. Tomadoni, B., Capello, C., Valencia, G. A., & Gutiérrez, T. J. (2020). Self-assembled proteins for food applications: A review. Trends in Food Science & Technology, 101, 1–16. https://doi. org/10.1016/j.tifs.2020.04.015 Valencia, G. A., Zare, E. N., Makvandi, P., & Gutiérrez, T. J. (2019). Self‐assembled carbohydrate polymers for food applications: A review. Comprehensive Reviews in Food Science and Food Safety, 18(6), 2009-2024. https://doi.org/10.1111/1541-4337.12499 Zarrintaj, P., Jouyandeh, M., Ganjali, M.  R., Hadavand, B.  S., Mozafari, M., Sheiko, S.  S., Vatankhah-Varnoosfaderani, M., Gutiérrez, T.  J., & Saeb, M.  R. (2019). Thermo-sensitive polymers in medicine: A review. European Polymer Journal, 117, 402–423. https://doi. org/10.1016/j.eurpolymj.2019.05.024.

Chapter 2

Biodegradable and Functional Synthetic Polymers in Nanomedicine: Controlled and Targeted Bioactive Molecule Release Xiaoming Guo, Leung Chan, and Tianfeng Chen

Abstract  In recent decades, there is a sustained interest in the development of drug delivery systems based on bioactive and functional polymers such as poly(propylene glycol) (PEG), poly(vinyl alcohol) (PVA), poly(lactide acid) (PLA), poly(lactic-co-­ glycolic acid) (PLGA), poly(ε-caprolactone) (PCL), poly(trimethylene carbonate) (PTMC) and poly(p-dioxanone) (PPDO). The U.S. Food and Drug Administration (FDA) has approved these polymers for biomedical applications due to their excellent biodegradability, biocompatibility and non-toxicity. The poly-hydroxy and -carboxy characteristics not only give them a supramolecular structure for drug loading, but also to incorporate drugs into the structure through chemical interactions. This chapter aims to present the molecular and physiochemical bases for the application of PEG and PLGA for functional delivery and controlled and targeted release of bioactive compounds. Keywords  Bioactive delivery · Biodegradable polymers · Poly(lactic-co-glycolic acid) (PLGA) · Poly(propylene glycol) (PEG) · Polymeric carriers

2.1  Introduction Synthetic chemicals and natural bioactive compounds are of critical importance in nanomedicine and health care (Zarrintaj et  al. 2019). However, many promising therapeutic compounds have low water solubility, rapid clearance and diverse side effects, which greatly hinders their bioavailability and therapeutic efficacy (Lukyanov and Torchilin 2004; Tyrrell et al. 2010). One solution to address these problems is to load these drugs into nanoscopic carriers (Mura et al. 2013; Bobo X. Guo · L. Chan · T. Chen (*) Department of Chemistry, Jinan University, Guangzhou, China e-mail: [email protected] © Springer Nature Switzerland AG 2020 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_2

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et al. 2016). Thus, an important trend in the fields of biomedicine and food nanoencapsulation is the development of vehicles capable of drug delivery and release (Gutiérrez and Álvarez 2017; Gutiérrez 2018a). Polymers with injectable, biocompatible and biodegradable properties emerge as promising candidates to meet these needs since they have the ability to modulate the toxicity of the target molecules, prolong residence time in the blood, improve water solubility and concomitantly control the release of target molecules at the site of interest (Soppimath et al. 2001; Rapoport, 2007; Kumari et al. 2010; Chen et al. 2011). It has been reported that the global annual sale of polymeric nanomedicine in 2010 reached 60 billion (Zhang et al. 2010), which represents almost half of the total sale of 121 billion of the entire nanomedicine industry (Almeida and Souto 2007). In this sense, the ability to relate the structure of polymers with the performance of delivery provides knowledge for scientists and technologists in the medicine and food industries to use tailored polymers for specific applications in the drug delivery, as well as the nanoencapsulation of nutraceuticals. It is therefore necessary to identify suitable polymer candidates which can be biocompatible and biodegradable. Although there are a lot of polymers as nanocarriers. However, this chapter focuses on some selected polymers approved by the U.S. Food and Drug Administration (FDA) for biomedical applications. In this chapter, the preparation methodologies and physicochemical properties of these polymers are reviewed.

2.2  Biodegradable Synthetic Polymers for Bioactive Delivery In recent decades, a wide variety of polymers have been examined for their physicochemical properties to build delivery systems, but only a few polymers have been approved by the FDA or the European Union (EU) for pharmaceutical uses. Most polymers investigated cannot enter clinical phase studies due to their non-­ biodegradability. In fact, large polymers for intravenous administration should be hydrolyzed into smaller sizes enough to enter the passage of renal clearance and spleen filtration. Otherwise, these compounds will accumulate to harmful levels. So far, poly(propylene glycol) (PEG), poly(lactide-co-glycolic acid) (PLGA), poly(vinyl alcohol) (PVA), poly(lactide acid) (PLA), poly(ε-caprolactone) (PCL), poly(trimethylene carbonate) (PTMC), among others, have been used as biodegradable synthetic polymers to form delivery systems since they have no safety and toxicity problems. The biodegradability of the polymers is a consequence of their susceptibility to gradual degradation into smaller fractions or even monomers in vivo, which can be subsequently metabolized by the body (Gutiérrez 2018b). This biodegradability helps to design drug delivery systems with controllable and targetable drug release properties, as well as to reduce the possible side effects associated with the accumulation of them. Since polymers are composed of monomers linked by covalent bonds, therefore, degradation of them requires the breaking of said bonds. Current knowledge about

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Table 2.1  Mechanism of degradation for different polymers in specific delivery systems Polymer PLA

PPDO

Product form Circular plates of 1.5 mm thickness Suture

PLGA

Nanoparticle

PVA

Hydrogel

Tricyclodecyl methacrylate (PTCM)

Rods (4 mm in length) and discs (3 mm in diameter)

PCL

Macromer

PEG

Exclusively polymer

Degradation behaviors Heterogeneous degradation both on the surface and in the inner zone Hydrolytic degradation of poly(p-dioxanone) (PPDX) occurs apparently in a two-stage process where the amorphous regions of the sample are attacked faster than the crystalline regions of the sample Complete degradation after ten weeks in polybutylene succinate (PBS) at pH 7.4 and 37 °C There is no degradation The PTCM rods suffered in vivo surface erosion; the PTCM discs underwent surface erosion in a lipase solution A 20% weight loss occurred after treating the sample with water or PBS for 60 weeks A 75% weight loss induced by reactive oxygen species within 11 days

Degradation pathway Mainly hydrolytic mechanism Hydrolytic mechanism

References Li and McCarthy (1999) Sabino et al. (2004)

Mainly hydrolytic mechanism

Zweers et al. (2004)

Inapplicable Enzymatic degradation

Kobayashi et al. (2005) Zhang et al. (2006)

Mainly hydrolytic mechanism Oxidation degradation

Castilla-­ Cortázar et al. (2012) Ulbricht et al. (2014)

the biodegradable properties of polymers in vivo suggests that their biodegradation can be driven by hydrolytic depolymerization and, alternatively, by thermal and oxidative degradation mechanisms (Table 2.1), and in any case the degradation rate may depend on particle size and shape, pH and temperature, hydrolysis mechanism, crystallinity, molecular weight (Mw), water permeability and solubility. The occurrence of hydrolysis of chemical bonds is responsible for the degradation of polyesters, while under physiological conditions such as plasma and the gastrointestinal tract, the surrounding body fluid will first interact with the polymer chains, trigger hydrolysis of the ester bonds, and probably in a form of random chain scission is given (Shih 1995; Han and Pan 2011), consequently, gradually degrading the polymer into small fragments. At the polymer level, the specified size of a polyester determines its useful life during physiological circulation. The ease of biodegradation of different polyesters depends on the chemical nature of their specific bonds, and at the device level, degradation behavior is governed by hierarchical structures of systems maintained by polyesters and co-existing polymers. For example, polyester-based nanoparticles undergo surface- or bulk-erosion during the

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Fig. 2.1 (a) Surface and bulk erosion of polyester-based delivery systems and (b) possible multi erosions of PEG-polyester amphiphilic micelles

different degradation stages, largely depending on the predominance of the surface erosion rate and the water permeability rate from outside to inside the nanoparticle (Fig. 2.1a). Since the erosion of the polymeric nanoparticles triggers the release of the charge, it is possible to achieve a controlled drug release by controlling the degradation rate or the porosity of the polymeric structure. Unlike polyesters, PEG is much less prone to hydrolytic depolymerization because its ether bonds are more stable than ester bonds. The hydrolytic conditions (predominantly temperature) required to divide the PEG chains are hard and difficult to occur spontaneously in vivo. Experimental data show that thermal treatments at 80 °C are required (Han et al. 1997) to implement the hydrolytic cleavage of the PEG main chain. PEG is often degraded via the oxidative approach, which involves biological factors such as cytochrome P450, alcohol dehydrogenase and aldehyde dehydrogenase (Caliceti 2003), thus producing oxidative degradation compounds: isolated mono- and dicarboxylate PEG (Friman et al. 1993). Interactions between the enzyme and the polymer surface may be more useful to elucidate this possible

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degradation pathway of PEG, since enzymatic hydrolysis required contact between PEG polymers and the active sites of enzymes. It is assumed that the mechanism of decumulation of high Mw PEG after intravenous administration involves two aspects: (i) hydrolytic depolymerization or chemical/enzymatic oxidation to degrade high Mw PEG into small-sized fragments and (ii) renal excretion of low Mw PEG (Carstens et al. 2008; Caliceti 2003). On the other hand, amphiphilic polymers such as PEG-b-PCL have shown that the PCL segments are easy to hydrolyze at 37 ° C and pH 5, while the PEG segments are not altered (Geng and Discher 2005). At the level of nanoparticles, e.g. self-assembled micellar polymers of PEG (corona) and PCL (core) could have several erosion sites. If the corona formed by PEG is stable, erosion will occur at the corona and core interface and then gradually extend to the core. In parallel, the loaded drug will escape from the micelles (Fig. 2.1b). In case of degradation of PEG decomposing enzymes, the micelles will undergo the bulk erosion throughout the corona and the core.

2.3  PEG as a Hydrophilic Polymer Hydrophilic polymers are a type of polymers that are easily dissolved or swollen by water. This feature largely provided the biocompatibility and biodegradability of these polymers. In pharmaceutical science, the conjugation of hydrophilic polymers to hydrophobic drugs substantially improves their water solubility, thus increasing the bioavailability of the desired drug (Kolate et al. 2014). In addition, the hydrophilicity of the nanoparticle delivery systems can be improved after introducing hydrophilic polymers on the surface of the nanoparticles (Soppimath et  al. 2001). For example, the core (hydrophobic)-shell (hydrophilic) structure nanoparticle interactions with the solvent and the biological liquid largely determine the drug circulating time within the body and, therefore, the final therapeutic efficacy (Joshy et al. 2017; Chen et al. 2018a). PEG is a homogenous and linear polymer synthesized from ethylene oxide. PEG is amphiphilic but highly water soluble. This noticeable hydrophilicity allows the circulation and stability of PEG within the bloodstream (which mainly comprises water) after intravenous administration. PEG is also biologically inert, which makes it a ‘stealthy’ polymer that avoids recognition and adsorption of opsonin by plasma proteins (Owens and Peppas 2006). By virtue of these desirable properties, PEG becomes a representative example of synthetic hydrophilic polymers for the delivery of bioactive compounds, which can be tailor-designed for different applications at the molecular level and nanoparticles. At the molecular level, PEG can be synthesized as a hydrophilic homopolymer of a conjugate, or as a hydrophilic subunit of random or block copolymers. At the nanometric level, PEG can be manufactured in a variety of physical forms, including crosslinked nanohydrogels, PEG block polymer micelles, physical mixtures or composites with other polymers and grafts on the surfaces of other biomaterials.

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Table 2.2  FDA-approved PEG drug conjugates Commercial name Adagen

Company Enzon

Oncaspar

Enzon

Doxil

Ortho/ Schering-­ Plough Roche

Pegasys

PEG molecular weight Linear 5000 Da Liner 5000 Da Not provided

Year of approval 1990

Conjugated drug Enzyme

1994

Enzyme

1995

Liposome

2001

Protein

Neulasta

Schering-­ Plough Amgen

Somavert

Pfizer

Macugen

Pfizer

Micera

Roche

Cimzia

UCB

Krystexxa

Savient

Branched 40,000 Da Linear 12,000 Da Linear 20,000 Da Linear 20,000 Da Branched 40,000 Da Linear 30,000 Da Branched 40,000 Da 10,000 Da

Sylatron

Merck

120,00 Da

2011

Omontys

Affymax/ Takeda Astra Zeneca/ Nektar Baxalta

Branched 40,000 Da < 1000 Da

2012 2014

20,000 Da

2015

PEG-Intron

Movantik Adynovate

2001 2002 2003 2004 2007 2008 2010

References Alconcel et al. (2011) Alconcel et al. (2011) Knop et al. (2010)

Alconcel et al. (2011) Protein Alconcel et al. (2011) Protein Alconcel et al. (2011) Protein Alconcel et al. (2011) Aptamer Knop et al. (2010) Protein Alconcel et al. (2011) Anti-TNF-α Alconcel et al. Fab’ (2011) Enzyme Alconcel et al. (2011) Protein Swierczewska et al. (2015) Peptide Swierczewska et al. (2015) Small molecule Swierczewska et al. (2015) Anti-hemophilic Stidl et al. (2018) factor VIII

2.3.1  PEGylated Drugs on the Market PEGylation refers to the covalent conjugation of one or more PEG polymers to a desired compound (Veronese and Pasut 2005). This methodology was initially developed to improve the therapeutic index of biopharmaceutical drugs, e.g. proteins and enzymes, and then was extended to applications in low Mw drugs poorly water soluble (Li et al. 2013). The clinical success of PEG-protein drug gives birth to six commercialized products such as Pegasys®, PEG-Intron®, Neulasta®, Somavert®, Micera® and Sylatron® (Table  2.2). PEGylated drugs have demonstrated greater hydrophilicity and biological response to the immune system, which leads to prolonged circulation time, reduced toxicity of the drug and a decrease in

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side effects. The Mw of PEG within a certain range, plays a role in the therapeutic drug efficacy. Among the PEG-protein conjugate products, the Mw of PEG is not less than 12,000 Da. Due to the considerable advances in PEGylation chemistry, existing PEG conjugation techniques allow link PEG strands to hydrophobic agents permanently or releasably. The main difference between these two patterns is based a chemical or enzymatic liable linker. The glycosylated PEG to a target molecule through a hydrolytically labile linker can thus release the drug from the conjugate via the breakdown of the linker (Pelegri-O’Day et  al. 2014). The releasable PEGylation is attractive to modulate the pharmacokinetics of bioactive agents (control of the maximal drug concentration and total drug exposure). For example, PEGylation of complement factor D via a cleavable phenylglyoxal linker was tested by Machinaga et al. (2018) on rabbits, finding that the releasable PEG drug platform maintained the drug concentration at therapeutic levels and allowed a sustained drug release for up to 7 days. Non-covalent PEGylation has received greater attention compared to covalently linked releasable-PEGylation PEGylation for the release of some bioactive compounds susceptible to loss of activity under environmental stimuli (Reichert and Borchard 2016). Non-covalent PEGylation allows to minimize the undesirable loss in the bioactivity of the drug because it does not involve any chemical modification.

2.3.2  Small PEGylated Bioactive Compounds PEG conjugation is also applicable to small bioactive compounds. Table 2.3 shows conjugates of selected small PEGylated biomolecules. The pristine biomolecules reported here share a similarity in structure regarding the hydroxyl and carboxyl groups, which offer the possibility of conjugation of PEG. For example, Medina-­ O’Donnell et al. (2016, 2018) linked the PEG polymer to hydroxyl groups of the A-ring at C2 or C3 and the carboxyl group at C28 of oleanolic acid, thus producing twelve conjugates where four of them showed significantly higher antitumor activities than their corresponding parent compounds. However, the resulting conjugate underwent a reduction in bioactivity once PEGylation occupied the key structures responsible for bioactivity. For example, Abu-Fayyad et al. (2015) found by conjugating PEG polymers to 6-OH group of a γ-tocotrienol isomer of vitamin E for the treatment of breast cancer, that the anticancer activity decreased substantially even despite the improvement in water solubility by binding PEG strands. These findings underscore the importance of the conjugation site for biomolecules when PEGylation is conducted. It should also be noted that PEGylation cannot improve bioactivity because no additional functional moiety is created, except for PEG molecules that lack biological activity. The evidence for this could be the case of PEGylated ferulic acid developed by Nicks et al. (2012) where the integration of PEG polymers into their molecular structure only improved water solubility.

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Table 2.3  Selected examples of PEGylated phytochemicals or nutraceuticals Conjugate PEGylated silybin PEGylated dimethyl-­ coumarins PEGylated curcumin

PEGylated ferulic acids PEGylated of vitamin E PEGylated pentacyclic triterpenes PEGylated triacontanol

PEG molecular weight Therapeutic result 2000 Da Improves water solubility

References Zhang et al. (2008)

600, 900 and 1500 Da

Improves the anti-inflammatory effect

Pandey et al. (2010)

454, 600, 1000, 1500 and 2000 Da

Improves anti-inflammatory, anticancer and antioxidant activities as well as the sustained release of curcumin Improves water solubility

Pandey et al. (2010), Wichitnithad et al. (2011), Cheng et al. (2018) Nicks et al. (2012)

200, 400 and 1000 Da Increases water solubility and oral 350, 1000, bioavailability 2000 and 5000 Da Not mentioned Improves antitumor activity

2000 Da

Improves water solubility and pharmacokinetics

Lu et al. (2013), Abu-Fayyad et al. (2015) Medina-O’Donnell et al. (2016, 2018) Li et al. (2018)

2.4  PLGA as a Selected Amphiphilic Polymer Amphiphilic polymers are a type of macromolecular compounds with low surface tension consisting of hydrophilic and lipophilic segments. Amphiphilic polymers due to their chemical characteristics tend to form the separation of microphases, which can be self-assembled in certain solvents, and surface and bulk structures. Therefore, amphiphilic polymers are widely used as surfactants, nanomaterials, drug carriers, coatings, adhesives, separation membranes and thermoplastic elastomers. In recent years, nanomaterials have received attention and have become the critical research points. Many types of amphiphilic polymers have been synthesized to conduct a thorough investigation of their physicochemical properties. In terms of composition, the hydrophilic segment is generally composed of non-ionic polymers such as PVA, poly(acrylamide) (PAM), poly(ethylene glycol) (PEG), poly(ethyleneimine), poly(vinyl ether) (PVE), poly(vinylpyridine) (PVP) and poly(vinylpyrrolidone). Amphiphilic polymers from the perspective of topology can simply be classified as blocks, dendrimers, grafts, ignoring gradients and star-­ shaped (Fig. 2.2). The amphiphilic block copolymers are formed by a linear arrangement of the hydrophilic and hydrophobic chains with a certain order, such as AB, ABA, ABC, ABCD, ABCBA, (AB)n. The block length of the hydrophilic and hydrophobic chains can be controlled by the preparation process, and in particular the AB block copolymer can be used as a surfactant. Interestingly, amphiphilic polymers can be self-assembled to form different morphologies, such as spherical micelles, rod-shaped micelles, large compound micelles, vesicles and various bilayer structures (Fig. 2.3).

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Fig. 2.2  The topological structure of amphiphilic polymers

PLGA is biocompatible and biodegradable, and is composed of PLA and poly(glycolic acid) (PGA). PLGA is approved by the FDA and the European Medicines Agency (EMA) as a material for drug carriers, tissue engineering and surgical equipment with excellent controlled release performance (Sharma et  al. 2016). PLGA is degraded to lactic acid and glycolic acid by hydrolysis of the ester bonds, and the compounds obtained are then metabolized to carbon dioxide and water. The degradation rate of PLGA is controlled by the PLA/PGA ratio, Mw, crystal and pH value, e.g. a PLA content slows down the degradation. Therefore, by increasing the proportion of PLA, the degradation rate of PLGA can be controlled to achieve sustained release. In addition, surface modification and compliance with polymers can increase the hydrophilicity and PLGA targeting. For such reasons, the amphiphilic PLGA polymer can potentially be used as a functional carrier for biomedical applications.

2.4.1  D  esign of PLGA-Based Systems for Delivering Micromolecular Drugs Micromolecular drugs have some limitations such as poor solubility, high toxicity and low bioavailability. This has motivated many research groups studying PLGA-­ based delivery systems. In oral administration, drugs with low solubility and slow

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Fig. 2.3  Self-assembled amphiphilic polymers to form different morphologies

dissolution rate are excreted by the digestive system without reaching the lesion. To solve these problems, nanotechnological methodologies have managed to load micromolecular drugs to improve their solubility and bioavailability. In recent years, nanoencapsulated micromolecular drugs in PLGA-based systems have been extensively studied, and these nanosystems have achieved better biocompatibility and bioavailability, e.g. curcumin is a natural anti-cancer drug, which has been encapsulated in PLGA-based nanosystems to investigate its antitumor activity (Esmaili et al. 2018). Farajzadeh et al. (2018) prepared metformin and curcumin loaded PLGA/PEG nanosystems to synergistically inhibit the growth and hTERT gene expression in human breast cancer cells. Khan et al. (2018) and Tavakoli et al. (2018) used the PLGA/PEG nanoparticles to load curcumin and chrysin to treat the C57B16 mice with B16F10 melanoma tumors, and nanosystems showed a promising and convenient approach to improve their efficiency in melanoma cancer therapy. Davoudi et  al. (2018) prepared 5-aminosalicylic acid (5-ASA) containing PLGA nanoparticles for the treatment of inflammatory bowel diseases and showed a high drug availability and treatment efficacy. Roque et  al. (2018) used several polymers such as PLGA to prepare toothpaste and an oral gel pack with Nystatin, finding a prolonged release and a high adhesion capacity to the oral mucosa when compared to free Nystatin. PLGA nanosystems have also been shown to have outstanding antitumor performance, especially for paclitaxel transfer.

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Many bibliographic reports have suggested that PLGA-based carriers improve bioavailability and reduce the side effects of paclitaxel and its derivatives (Mandal et al. 2018; Peng et al. 2018). In addition, a PLGA/PEG polymer encapsulated a perfluoroopentane perfluorocarbon (PFP) variable phase, thus significantly improving the image signal after induction of low-intensity focused ultrasound (LIFU) to improve the accuracy of clinical detection of thyroid nodules (Hu et  al. 2018). PLGA-based nanosystems also have potential for sustained intracranial delivery of therapeutic agents to treat brain tumors (Cano et  al. 2018; Sánchez-López et  al. 2018; Silva-Abreu et al. 2018). In general, PLGA-based nanosystems are beneficial for micromolecular drug delivery, resulting in better bioavailability of the drug and reduced side effects.

2.4.2  P  LGA-Based Nanosystems for the Transfer of Biomacromolecules It is well known that peptides, proteins and enzymes have large and complex structures with low capacity to cross biological barriers. These macromolecules tend to be easily degraded. The incorporation of compounds into PLGA-based composite polymer nanoparticles seems to be a promising strategy to overcome the problems mentioned above. For example, Chen et al. (2018a, b) used bovine serum albumin and lecithin to obtain PLGA-coated nanocomposites to be a promising platform for long-term protein delivery with a reduced initial burst. Wei et al. (2018) found that nanoporous PLGA-based microspheres loaded with soy lecithin showed a controlled release of the protein. Shi et al. (2018) also developed PLGA-based nanosystems for prolonged glucoregulatory action of exenatide (peptide) to be used for type 2 diabetes therapy. Therefore, PLGA delivery systems can be potential delivery systems for peptides, proteins and enzymes.

2.5  Conclusions and Perspectives Biodegradable and biocompatible synthetic polymers have allowed the development of nanomedical compounds and theranostics devices. These compounds have proven to be promising carries for the delivery and release of bioactive compounds on diseased tissues. There are two mechanisms for the delivery of bioactive compounds over the target region: (1) drugs containing reactive functional groups such as hydroxyl, carboxyl and amino groups can be conjugated directly to the polymer adopting a suitable synthetic strategy and (2) the polymers can be assembled in nano-sized particles (spheres and micelles) via hydrogen bonds, hydrophobic interaction, electrostatic bonding and multiple intermolecular interactions to load the bioactive compounds. Despite the advantages offered by synthetic polymers, only a

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few polymer-based drugs have been approved for clinical uses. Finally, these polymer composites aim to obtain stimuli-responsive (intelligent) systems to trigger the drug release in a previously designated manner. Acknowledgments This work was supported by Natural Science Foundation of China (21877049), National Program for Support of Top-notch Young Professionals (W02070191), Science and Technology Program of Guangzhou (201902020013). Conflicts of Interest  The authors declare no conflict of interest.

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Chapter 3

Reactive Modification of Fiber Polymer Materials for Textile Applications Avinash P. Manian, Tung Pham, and Thomas Bechtold

Abstract  Many reactive processes with the purpose of modifying the structure of fiber polymers are applied in textile chemistry in order to change their properties. These processes depend on the chemical nature of the polymer and the type of modification, and these aspects are discussed with respect to alkalization, chemical crosslinking with bi- and multifunctional reagents, hydrolytic processes, deposition and grafting of polymers, as well as crosslinking with urea-based reactive systems (e.g. dry cure processes). The selection of process parameters are of decisive importance for the efficient development of a desired portfolio of properties in a certain fiber-­based product. This chapter aims to analyze the recent advances in reactive modification of fiber polymer materials. Keywords  Bulk properties · Composites · Interfaces · Surface modification · Technical textiles

3.1  Introduction The manipulation of fibrous materials into structures useful for human purposes has a long history. Initially, the focus of textile science and technology was to improve aesthetics (e.g. color and appearance), comfort (e.g. softness) and protection properties (e.g. thermal insulation) of apparel and clothing, but over time, the scope has expanded to include other aspects, both within and outside the apparel and clothing sector. Within the apparel and clothing sector, the focus is increasingly on addressing the environmental impact of textile processing (Khatri et al. 2015; Holkar et al. 2016). Therefore, the reduction of the chemical load (e.g. dyes and salts), the A. P. Manian (*) · T. Pham · T. Bechtold Research Institute of Textile Chemistry and Textile Physics, University of Innsbruck, Dornbirn, Austria e-mail: [email protected] © Springer Nature Switzerland AG 2020 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_3

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increase in the use of fibers from renewable resources and the use of non-toxic chemical agents are being investigated for textile applications (Blackburn 2009). An additional area of focus is the emerging area of ‘smart’ textiles, i.e. imparting apparel and clothing the ability to record data from external stimuli, and possibly provide a response. Outside the apparel and clothing sector, textile science and technology are used in various applications, from composites for automobiles and construction, scaffolds for tissue engineering, to supports for catalysts in industrial chemical processes (Horrocks and Anand 2016a, b). The research in the areas described above encompasses both chemical processes for modifying the fibers, and changes to the  design of fiber assemblies and the modes of their construction. The chemical processing of fibrous substrates is a broad subject. For this reason, the recent chemical processing strategies selected to modify the fiber bulk and its surface will be described in this chapter.

3.2  Fiber Modifications 3.2.1  Alkali Treatments The most researched polymer regarding the effects of alkali treatments on structure and reactivity is perhaps cellulose (Klemm et  al. 2004a, b; Budtova and Navard 2016; Gutiérrez and Alvarez 2017). Alkali treatments are a common part of the processing sequence of cellulosic textiles, and are used anywhere from the cleaning of raw (or ‘greige’) materials, to modifications of the structure. The modifications of the structure occur in treatments with sodium hydroxide (NaOH) solutions of concentrations greater than ca. 12–15% (w/w), which is often called ‘mercerization’ (Klemm et al. 2004a). This causes a change in the crystalline structure of the celluloses, thus improving their sorption properties and as a consequence also their reactivity changes. Zahid et al. (2017) reported recently that mercerization pretreatments can mitigate the potential for acid hydrolysis in application of the conductive poly(3,4-ethylenedioxythiophene) (PEDT):poly(styrene sulfonate) mixture for the creation of smart textiles. Another recent report found that hot alkali treatments on jute remove lignin and hemicellulose and break larger fiber bundles into smaller sizes, which helps improve their adhesion in epoxy resin composites (Wang et al. 2019), while a treatment of cellulose acetate fibers with ethanol/NaOH solutions, progressively deacetylates them from the outside inwards and creates a shell of cellulose surrounding a hydrophobic core (Tulos et al. 2019). Alkali treatments have also been used for hydrolytic treatments of textile substrates to improve the effectiveness of subsequent coating and functionalization treatments. Alkali hydrolysis of polyester, if limited to the surface, does not significantly affect the strength and increases the surface density of the hydroxyl and carboxyl groups and the surface roughness, (Mazrouei-Sebdani and Khoddami 2011; Hashemizad et al. 2012; Han et al. 2016 Hashemizad et al. 2017; Nourbakhsh et al.

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2018). This improves the wetting characteristics of substrates and is also beneficial for subsequent coating operations, such as with fluoropolymers and titanium and zinc oxide particles. Hydrolytic treatments have also been carried out with enzymes, and there is a long history of such cellulose treatments, and now it has been expanded to include other fibers such as wool, where it is used as an anti-felting treatment (Kaur and Chakraborty 2015). The modification of the enzymatic surface of the dyed material has been used to remove part of the color and increase the abrasive effects during the garment  wash down processes. As an example, cellulases have been used to degrade the surface of cellulosic fibers surface and intensify the color release in the denim wash down, which replaces substantial amounts of oxidants, e.g. hypochlorite or potassium permanganate, and therefore reduces pollution (Schimper et  al. 2011). In recent research works, enzymatic hydrolysis of polyesters has been carried out with cutinases, esterases and lipases, as a means to generate hydroxyl and carboxyl groups on the surface and increase the roughness (Kim and Song 2010; Lee and Song 2010; Vecchiato et al. 2017). A similar approach has been used with polyamides by using acylases, amidases, peptidases and proteases (Song and Kim 2013; Kim and Seo 2013; Abo El-Ola et al. 2014; Periyasamy et al. 2017; Kanelli et al. 2017).

3.2.2  Crosslinking Unlike grafting treatments, crosslinking treatments are generally performed to change the bulk properties, e.g. to improve the mechanical properties of the fibers (resilience), impart morphological and chemical stability, as well fix functionalization agents. In the latter case, the treatment processes can be tailored to limit the effects on the fiber surfaces. 3.2.2.1  Treatments to Improve Mechanical Resilience Cellulosic fibers easily absorb moisture/water and swell, which leads to a breakdown of non-covalent interactions between polymer chains. This causes the chains to move from their original configurations under external stress. Upon removal of moisture/water, the non-covalent interactions are restored. The displaced polymer chains do not return to their original configurations, and these changes are reflected as creases in cellulosic apparel (Schindler and Hauser 2004). Crosslinking treatments are used as a means to improve crease strength, since the treatment retards the swelling of the fiber in moisture/water and, therefore, the chain displacement is also retarded. A large proportion of crosslinking treatments on cellulosic fibers to impart crease strength are performed using N-methylol reagents derived from melamine or urea reactions with formaldehyde  (e.g. N,N’-dimethylol urea (DMU), N,N’dimethylol-4,5-dihydroxyethylene urea (DMDHEU) and trimethylol melamine

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(TMM)). These treatments are carried out via a ‘pad-bake’ process, where the substrates are passed through aqueous formulations containing the crosslinker, a catalyst and other components, squeezed between pressure rolls to remove excess liquor, then dried and baked (i.e. subjected to dry heat) at about 150 °C for 0.5 to 3 min. The catalysts used are Lewis acid salts, e.g. MgCl2, which generate acidic conditions during the baking process, and the crosslinking reaction is carried out through the protonation of oxygen of N-methylol groups that lead to carbonium ions on dehydration, which react in turn with cellulosic hydroxyls forming ether bonds. A scheme of the crosslinking mechanism is available in Schindler and Hauser (2004). A disadvantage of N-methylol reactants of the type described above is the reaction of unsubstituted amine and amide groups which react with chlorine from household and laundry detergents and bleaches to form chloramines which are hydrolyzed to hypochlorous acid, thus leading to cellulose degradation. Another disadvantage is the potential of formaldehyde release during the crosslinking treatments or during wear from unreacted formaldehyde residues. One option to reduce such release is to add formaldehyde scavengers such as urea, diethylene glycol, ethylene urea to urea-formaldehyde products, and the other is not to use formaldehyde in the synthesis of crosslinkers. An example of a non-­ formaldehyde crosslinking is N,N′-dimethyl-4,5-dihydroxyethylene urea (DMeDHEU), which is synthesized from N,N′-dimethyl urea and gloxal. Others are dialdehydes, such as glutaraldehyde and glyoxal, and polycarboxylic acids, such as butanetetracarboxylic acid, citric acid, poly(itaconic) acid and poly(maleic acid) (Harifi and Montazer 2012). The polycarboxylic acids used as cellulose crosslinkers are characterized by a minimum of three side carboxylic groups attached to adjacent carbons of an aliphatic chain. Esterification reactions are produced by the formation of an intermediate cyclic anhydride in the dehydration catalyzed by agents such as sodium hypophosphite. Polyamino carboxylic acid (synthesized by carboxylation of linear polyamines) has also been investigated as potential non-formaldehyde crosslinking agents (Dehabadi et al. 2012). Another option is ‘ionic crosslinking’, where cellulose is partially carboxymethylated first to introduce anionic groups and then treated with a cationic agent such as 3-chloro-2-hydroxypropyl trimethyl ammonium chloride, where crosslinks are caused by ionic interactions between the two polyions (Hashem et al. 2003; Hashem et al. 2005; Harifi and Montazer 2012). 3.2.2.2  Treatments to Impart Mechanical and Chemical Stability Among the regenerated cellulosic materials, the fibers obtained from the lyocell process exhibit a propensity for  fibrillation, i.e. the peeling-off of thin strands (‘fibrils’) from the surface, when subjected to mechanical forces in the wet state, such as when textiles are washed. The phenomenon is undesirable since it to leads to a blurry appearance of the articles. Crosslinking is employed as a means to prevent fibrillation, and the agents used include 1,3,5-triacryloyl-hexahydro-s-triazine, dichlorohydroxytriazine and 2,4-dichloro-6-(p-β-sulphatoethylsulphonyl)anilino-1,3,5-triazine (White 2001; Bates et al. 2006). The bifunctional colorant CI

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Reactive Black 5 has also been used as a crosslinking agent to minimize lyocell fibrillation (Bui et al. 2009). Bifunctional reactive dyes can also lead to crosslinks in protein-based fibers, e.g. silk, thus leading to a reduced solubility of crosslinked fibroin in concentrated salt solutions. As an example, crosslinking of fibroin with CI Reactive Black 5 prevented the dissolution of the degummed silk in Ajisawa’s reagent, which is a concentrated mixture from CaCl2 in ethanol/water solutions (Ajisawa 1998; Ngo and Bechtold 2016). Ngo and Bechtold (2016) only observed swelling after dyeing with bifunctional dyes, while dissolution occurred with fibers that had been dyed with a monofunctional reactive dye. Crosslinking with poly(ethylene glycol) diglycidyl ether has also been shown to significantly improve the mechanical stability of wet spun sodium alginate fibers using calcium chloride solutions as a coagulation medium (Meng et  al. 2019). Transglutaminase, an enzyme, has also been used as crosslinking agents for protein fibers such as collagen and keratin to improve their stability (Cui et al. 2013; Wu et al. 2018; Wu et al. 2019). The acid stability of chitosan (Cs) fibers can be improved by crosslinking with water soluble aziridine and epoxy compounds (Li and Tang 2016b, a), and citric acid has also been employed as a crosslinking agent to improve the stability of water-electrospun zein fibers (Jiang et al. 2010). 3.2.2.3  Treatments as a Means of Fixing Functionalization Agents A variety of agents and crosslinking techniques have been developed to fix enzymatic molecules on textile substrates to act as robust and flexible carriers in industrial operations (Kiehl et  al. 2015). For example, Kiehl et  al. (2015) achieved enzymatic immobilization by photoinitiated crosslinking with cyanuric chloride for cellulosics, glutaraldehyde for polyamide 6 and polycarbodiimide for polyester. Glutaraldehyde crosslinking was also used by Lee et  al. (2005) for enzymatic immobilization as a means to fix sericin in silk fibers, while Kongdee et al. (2005) used DMDHEU to achieve the crosslinking of sericin on cellulose fibers in order to improve the comfort properties of fabrics (Kongdee et al. 2005). In line with this, Khoddami et  al. (2011) fixed a poly(ethylene glycol) coating on the surface of hydrophobic fibers such as poly(lactic acid) and polyester using DMDHEU as a crosslinking agent in order to improve their moisture management properties. β- and γ-cyclodextrins have been used for drug delivery, these materials being fixed on polyamide 66 fibers by crosslinking with citric acid (El Ghoul et al. 2008). A similar approach has been used by Martel et al. (2002) and Ducoroy et al. (2007) to fix cyclodextrins on cotton, polyester and wool, for use as an environmental remediation material (removal by sorption of pollutants from wastewater). Alonso et al. (2009) also suggested the use of citric acid for the crosslinking of cellulose with Cs to impart antimicrobial properties to the fibers. Other crosslinking agents based on cyanuric chloride have also been used for dye fixation, e.g. polyethylene polyamine dye on cellulose and silk (Tang et al. 2006). Yoshioka-Tarver et al. (2012) reacted dimethyl phosphite with the crosslinking agent 1,3,5-triacroylaminohexahydro-s-triazine, and then investigated the

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application of this compound as a means to fix organophosphonate flame retardants on fabrics. Lewis (2011) using the same crosslinker on cellulose fibers, and then reacted with polyamines, found a significant increase in the sorption and fixation of reactive dyes on the substrates, and also discovered that similar treatments are effective on wool and nylon fibers. This approach has thus been proposed as a means to reduce the load of salt and dye in wastewater, as well as to reduce the amounts of water required to remove unbound dyes. An alternative approach has been proposed by Lewis and Yao (2000) by treating the fibers with 2,4-dichloro-6-(ρ-[(2′sulphatoethyl)sulphonyl]phenylamino)-1,3,5-triazine followed by nicotinic acid for the purpose of increasing the affinity of the reactive dyes hydrolyzed by cellulose.

3.2.3  Grafting The grafting reactions can be classified into two categories: (1) ‘conventional’ reactions, where there is minimal or no control over polymer chain lengths and chain length distributions, and (2) ‘controlled/living’ reactions, where mechanisms are used to control the aforementioned aspects. According to Hubbe et al. (2015), grafting can be defined as ‘the attachment or unit-by-unit polymerization of an oligomeric or polymeric chain onto a fiber surface’. As the definition states, grafting reactions can be classified into two types: ‘grafting-to’ or ‘grafting-from’. In the first, the pre-formed polymer chains are attached onto fiber surfaces via physisorption (e.g. electrostatic interactions, forced solvent removal) or chemisorption, where the polymer chains are end-group functionalized with reactive groups such as thiols and silanols, and then are reacted covalently to fiber surfaces (Tsukruk 1997; Zhao and Brittain 2000; Barbey et  al. 2009; Carlmark 2013), while in the second, the monomers are directly polymerized ‘from’ the fiber surfaces. The grafting-from approaches are generally more difficult than grafting-to approaches, but offer the significant advantage of greater graft densities, since it is difficult to achieve close packing of pre-formed polymer chains due to steric hindrances. In grafting-from approaches, the substrate pretreatments are usually required to activate or introduce grafting sites onto polymers, which in some cases involve the use of organic solvents, thus presenting challenges in scaling-up of laboratory results to industrial scale processes (Carlmark 2013). In principle, the same polymerization mechanisms can be used for both grafting-to and grafting-from approaches. In the following section, we describe some common controlled/living polymerization techniques for fiber grafting. 3.2.3.1  Anionic and Cationic Polymerization These are essentially addition polymerizations where the chain propagation is through ionic species: carbanions and carbocations (Advincula 2006). Polar solvents are required for chain propagation, and the solvent can exert a strong influence

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on the results due to its influence on the solubility of monomers and counterions, and the propensity of charge transfer from the propagation end which leads to a termination of polymerization. Some selected examples of this method are listed below. Hui et al. (2018) grafted nylon 6 from carbon fibers in order to improve interfacial compatibility in the composites. For this, the fibers were first treated with nitric acid followed by thionyl chloride (SOCl2) to produce acyl groups on the surface, and then the fibers were immersed in a molten solution of caprolactam to initiate anionic polymerization catalyzed by the addition of NaOH. A similar approach was used by Gao et al. (2017) to treat carbon fibers containing acyl groups with ethylene glycol to produce hydroxyl groups, and then treated with glycidol to graft polyglycerol from fibers with anionic ring opening polymerization. With the same objective in mind, Raghavendran and Drzal (2002) first metallized the carbon fibers with butyl lithium in N,N,N′,N′-tetramethylethylenediamine, and then without removing the excess reagent, they were further treated with saturated cyclic oligocarbonate or methyl methacrylate solutions to graft bisphenol A polycarbonates and polymethyl methacrylate onto the fibers by anionic polymerization. Tsubokawa et  al. (1989) suggested that carbon fibers containing carboxyl groups, when treated with potassium hydroxide to produce potassium carboxylate groups can initiate grafting by anionic polymerization with epoxides and cyclic acid anhydrides. Kim et al. (1997) also observed that a premetalization of Kevlar fibers, with sodium hydride, allows the anionic polymerization of ε-carolactam from the fibers. Yoshikawa et al. (1998) introduced amino groups onto carbon fibers by nitration followed by reduction with sodium thiosulfate, and then  initiated the cationic polymerization with isobutyl vinyl ether under catalytic conditions using a mixture of trifluoroacetic acid and zinc chloride. Carbon fibers have also been grafted with polyacetal groups by acylation followed by a treatment with silver perchlorate to produce acyl perchlorate groups which initiated polymerizations with 1,3,5-trioxane and 1,3-dioxolane to produce grafted polyacetals (Tsubokawa and Yoshihara 1994). A similar procedure was used by Tsubokawa and Yoshihara (1993) for grafting styrene and N-vinyl-2pyrrolidone onto vinyl polymers. 3.2.3.2  Ring Opening Polymerization (ROP) This method is used with cyclic monomers such as ε-caprolactones and lactides with Lewis acids (e.g. Sn(II) 2-ethylhexanoate) or metal alkoxides (e.g. from aluminum) that act as catalysts or initiators (Schwach et al. 1997; Mecerreyes et al. 1999; Jérôme and Lecomte 2008; Carlmark et al. 2012). Two mechanisms have been proposed for this type of polymerization: one involves the complexation of the metal with the monomer carbonyl group, which allows the initiation of polymerization by the addition of nucleophilic agents such as alcohols or water, while the other mechanism involves a ‘coordination-insertion’ mechanism, where the metal is coordinated to the ring oxygen of the monomer, thus promoting polymerization by inserting

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monomers in the oxygen-metal bond through electron rearrangement. Below we list some selected examples of this method. Başaran and Oral (2018) functionalized gelatin nanofibers by grafting poly(ε-­ caprolactone) via ROP of the monomers using Sn(IV) isopropoxide as an initiator, and with the amine groups of gelatin acting as co-initiators. Poly(ε-caprolactone) and poly(L-lactic acid) have also been grafted from cellulose and micro-fibrillated cellulose via ROP, using Sn(II) 2-ethylhexanoate as catalyst and benzyl alcohol as co-initiator (Carlmark et al. 2012; Lönnberg et al. 2006; Lönnberg et al. 2008). This method also produced good results in grafting wheat straw fibers (Kellersztein et al. 2016). It has also been shown that greater grafting efficiencies can be obtained if cellulose is first modified to increase the density of available hydroxyl groups, by agents such as xyloglucan derivatives or 2,2-bis(hydroxymethyl)propionic acid (Lönnberg et al. 2006; Yeo and Hwang 2017). Epoxidized soybean oil, commonly used as a plasticizer, has also been grafted from cellulose fibers in n-hexane using tin chloride as a catalyst (Huang et al. 2017). Sisal fibers have also been modified by grafting L-lactide in toluene using Sn(II) 2-ethylhexanoate as a catalyst (Ye et al. 2015; Jiang et al. 2016; Ye et al. 2017). The technique has also been employed for the creation of hyperbranched polymers onto cellulose by grafting 3-methyl-3-­ oxetanoethanol in methylene chloride using boron diethyl trifluoride etherate as an initiator in a nitrogen atmosphere at 0 ° C (Yang et al. 2011). 3.2.3.3  Radical Polymerization A dynamic equilibrium is established in the controlled/living radical polymerization, between the active and dormant states of the propagating radicals, which extend their useful life to approx. 1 h, thus allowing greater control over polymerization of the added monomers (Matyjaszewski and Spanswick 2005; Braunecker and Matyjaszewski 2007; Badri et al. 2012). Some common methods for achieving equilibrium are described below (Braunecker and Matyjaszewski 2007; Barbey et al. 2009; Zoppe et al. 2017), citing some selected examples of each: 3.2.3.3.1  Atom Transfer Radical Polymerization (ATRP) This method is based on a reversible redox exchange of a halogen atom between a transition metal complex and polymers capped with an alkyl halide end-group. The transfer of the halogen atom to the metal complex produces a carbon-centered radical at the polymer chain end which activates the chain propagation, while the reverse process converts the chain end into a dormant state. The metal complexes are commonly Cu compounds, and the alkyl halides are generally bromine compounds. The advantages are that this method can be used with a variety of functional groups and is relatively robust against impurities and residual oxygen, but a disadvantage is the potential for residual amounts of transition metal in substrates, which can limit the potential applications of these products.

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Brown et al. (2016) and Chi et al. (2019) worked on the joint grafting of acrylonitrile and tert-butyl acrylate onto polyvinyl chloride fibers using a copper bromide/ tris(2-­dimethylaminoethyl)amine complex as catalyst in ethylene carbonate medium at 65 °C. Lindqvist and Malmström (2006) using the same catalyst, grafted methyl acrylate into cellulose fibers previously modified by treatment with 2-­bromisobutyryl bromide. Lu et al. (2018) functionalized cellulose first using 2-bromisobutyryl bromide, and then was grafted with diallyl dimethylammonium chloride using as a catalyst cupric bromide/N,N,N′,N′′,N′′-pentamethyldiethylenetriamine in ultrapure water at 50 °C. A similar process was used by Hansson et al. (2015), ter Schiphorst et  al. (2016), Arteta et  al. (2017) and Wang and Wei (2018), for grafting methyl methacrylate onto cellulose filter paper, N-isopropylacrylamide onto cotton fabrics, lauryl acrylate onto cellulose fibers and N-hydroxymethyl acrylamide onto vegetable loofah fibers, respectively. Cotton was grafted by Jia et al. (2018) from glycidyl methacrylate at 40 °C under an argon atmosphere, first brominating the fibers with 2-bromisobutyryl bromide, and then they were treated in a dimethylformamide bath containing the monomers and cupric bromide/2,2-bipyridine as catalyst. A similar method was used by Liu et al. (2017) to modify the cotton fibers by grafting from 3-sulfopropyl methacrylate potassium salt. Liu et  al. (2018) also functionalized polypropylene hollow fibers first by treating with dopamine to introduce hydroxyl groups, then were brominated  with 2-bromisobutyryl bromide, followed by treatment with acrylamide in DMF  using a cuprous bromide/cupric bromide/(2-(dimethylamino) ethyl) amine mixture as catalyst. 3.2.3.3.2  Nitroxide-Mediated Polymerization (NMP) This reaction proceeds under the mediation of a nitroxide radical derived from an alkoxyamine, such as 2,2,6,6-tetramethyl-1-piperidynyl-N-oxy (TEMPO). The nitroxide radicals do not react with each other or with the monomers, but they react reversibly with the propagating chain ends, thus establishing an equilibrium between the active and dormant states. The advantage of this method is that no additional catalysts are required, but a disadvantage is that the activation of chain propagation is often thermally induced and, therefore, temperature sensitive monomers cannot be used. Daly et al. (2001) treated hydroxypropylcellulose first with N-hydroxypyridine-2-­ thione esters, and then mixed with styrene and TEMPO in dimethylformamide followed by exposure to visible light to create grafts onto polystyrene chains. Polyvinylidene fluoride was functionalized by Holmberg et al. (2004) using polystyrene irradiated with electronic radiation, followed by immersion in a solution of TEMPO in toluene and then immersing the fibers in the monomer. Karaj-Abad et al. (2016) treated cellulose first with 2-bromisobutyryl bromide in tetrahydrofuran, the attached bromine moieties were then converted into 4-oxy-2,2,6,6-­ tetramethylpiperidin-­1-oxyl groups, and the substrate was then grafted with styrene and methyl methacrylate to obtain block copolymers.

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3.2.3.3.3  Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT) This method relies on the use of chain transfer agents (e.g. dithioesters, dithiocarbamates, etc.), which react with the propagating chain-end to form adducts, which return to the original reactants or fragment to produce a free radical, and thus maintain the equilibrium between the active and dormant states of the propagating radicals. The advantage of this method is its wide applicability, since conventional free radical polymerization can be turned into a controlled process with the addition of a suitable chain transfer agent. Poly(methacrylic acid) was grafted by Söylemez et al. (2018) onto polyethylene/ polypropylene fabrics under gamma radiation, using cumyl dibenzoate as the RAFT agent. Lan et al. (2017) also used a similar method to graft poly(glycidyl methacrylate) using 4-cyano-4-[(phenylcarbonothioyl)thio]pentanoic acid as RAFT agent, while poly(glycidyl methacrylate) was grafted onto carbon fibers using 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid as the RAFT agent from the monomers dissolved in dimethyl formamide after pretreating the fibers to affix the RAFT agent onto fibers with a silane. In line with this, Xiong et al. (2017) grafted poly(acrylamide) onto carbon fibers using 2-(ethoxycarbonyl)prop-2-yl dithiobenzoate as a RAFT agent, after a pretreatment to affix the RAFT agent onto the fibers with a silane. Silk fibers modified with the coupling agent 3-(trimethoxysilyl) propyl methacrylate were grafted by Buga et al. (2015) from methyl methacrylate and tributylsilyl methacrylate using 2-cyanoprop-2-yl dithiobenzoate as a RAFT agent. Yang et al. (2013) grafted  silk fibers with N,N-dimethylacrylamide using macromolecular RAFT agents synthesized from acrylic acid, styrene, n-butyl acrylate and 2-(((dodecylsulfanyl) carbonothioyl) sulfanyl) propanoicacid to allow  tuning of the radical reactivity by control of the macromolecular composition. The literature also reports that ramie fibers have been first modified with 2-­bromisobutyryl bromide and carbon disulfide to produce 2-dithiobenzoyl isobutyrate moieties and then grafted with methyl acrylate, methyl methacrylate, styrene and p-chlorostyrene using 2-(ethoxycarbonyl)prop-2-yl dithiobenzoate as a RAFT agent (Chen et  al. 2009; Yi et  al. 2010). A similar procedure for grafting  from trifluoroethyl methacrylate onto ramie fibers in supercritical carbon dioxide was carried out by Liu et al. (2010).

3.2.4  Polymer Deposition Polymer deposition is another method for surface modifications of fibrous polymers with other polymers, and also includes ‘grafting-to’ approaches. Günay et  al. (2017b, b) found that a conjugation with cyclic polypeptides increases the deposition of polymeric treatment agents based on poly(hydroxypropyl methacrylamide), poly(styrene-co-acrylic acid) and polyurethane, onto cellulose fibers and proteins,

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while Chen et al. (2018) reported that non-woven polypropylene sheets were modified by grafting of 3-isopropenyl-α,α′-dimethylbenzene isocyanate and then deposition, followed by dopamine self-polymerization to improve hydrophilicity. The conductive PEDT/poly(4-styrenesulfonate) mixture can also be incorporated into materials such as polyester and cotton by dipping the substrates into the monomer solutions followed by chemical oxidation, or depositing a solution of the monomers in ethylene glycol and substrate heating (Opwis et al. 2012; Tessarolo et  al. 2018). Other conductive polymers such as polypyrrole and polyaniline can also be deposited on cotton and polyester by in situ self-polymerization induced by chemical oxidation with reagents such as ammonium peroxydisulfate and ferric chloride (Engin and Usta 2014; Bajgar et al. 2016; Maráková et al. 2017; Ayad et al. 2018; Lee and Park 2018; Zhao et al. 2018). Better conductivity can be obtained as long as conductive substances such as graphene oxide are added to the textile substrate (polymer) (Yaghoubidoust et al. 2014).  Kopecká et al. (2014) and Bober et al. (2015) have suggested that precipitated conductive polymers may adopt rod shapes, rather than globular shapes, if chemical oxidation is initiated in the presence of ‘structure-guiding agents’ such as methyl orange. Conductive polymers can also be deposited by electrochemical oxidation of their monomers, as demonstrated by Mao et al. (2018) using polypyrrole and polyvinyl ferrocene on carbon fiber substrates. Conductive polymers such as PEDT, poly(3,4-dimethylthiophene), poly(3-­ methylthiophene) and poly(thieno[3,2-b]thiophene), have also been deposited on textile substrates by chemical vapor deposition of the monomer followed by  its polymerization with catalysts such as bromine plasma radicals or sublimed ferric chloride (Jahan Biglari et al. 2014; Cheng et al. 2017; Pistillo et al. 2017; Zhang et al. 2017a, b). Stempien et al. (2015) deposited the conductive tracks of polyaniline and polypyrrole on textile surfaces by using an inkjet printer, using separate printheads for the monomers and the oxidant (ammonium peroxydisulfate), thus placing the monomer first and then the oxidant or vice versa. Ionic polymers can be deposited on charged substrates such as cellulosics simply by immersing substrates in polymer solutions. The plasma-assisted chemical vapor deposition from the argon atmosphere has also been used to deposit hexamethyldisiloxane and perfluorohexane on p-aramid and ultra-high molecular weight polyethylene fibers (Struszczyk et  al. 2014; Struszczyk et al. 2017). Other polymers deposited with a similar method are hexamethyldisiloxane and tetraethylorthosilicate (Kale and Palaskar 2012a, b). Polysiloxanes have also been deposited on polyester fabrics simply by exposing substrates to monomer vapors (Zheng et  al. 2017). Wu et  al. (2015) deposited sodium alginate on cotton by precipitation with calcium chloride of a solution containing an active substance, which allowed the encapsulation of the active substance in sodium alginate precipitates. Multiple layers of polymers deposited on substrates such as cellulose and polyamide can also be built up using the ‘layer-by-layer’ technique, by successive alternating immersions between polyanions and polycations. Examples of this method include treatments with Cs and poly(sodium phosphate) (Mateos et al. 2014), Cs

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and sodium alginate (Gomes et al. 2013), poly((3-acrylamidopropyl)trimethylammonium chloride and poly(2-acrylamido-2-methylpropane sulfonic acid sodium salt) (Liu et al. 2015), and poly(allylamine hydrochloride) and sodium polyphosphate (Apaydin et al. 2014).

3.3  Summary and Outlook This chapter tried to offer a wide and varied vision of the research fields on  the modification of polymeric fibers with textile objectives, using different chemical methodologies. The topics discussed here highlight the increasing scope of the application of reactive and functional polymers on textile materials, as well as ongoing efforts to reduce the environmental impact of textile garment manufacturing operations, with a view to the green and sustainable production. Finally, despite the long history of the use of clothing by mankind, active research in this area continues as researchers continue to develop and apply innovative materials and processes to address this basic human need. Acknowledgments  The authors gratefully acknowledge funding from the Austrian Research Promotion Agency (Österreichische Forschungsförderungsgesellschaft) under the COMET program (Project: Textile Competence Centre Vorarlberg). Conflicts of Interest  The authors declare no conflict of interest.

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Wu, X., Liu, A., Wang, W., & Ye, R. (2018). Improved mechanical propertis and thermal-stability of collagen fiber based film by crosslinking with casein, keratin or SPI: Effect of crosslinking process and concentrations of proteins. International Journal of Biological Macromolecules, 109, 1319–1328. https://doi.org/10.1016/j.ijbiomac.2017.11.144. Wu, X., Luo, Y., Liu, Q., Jiang, S., & Mu, G. (2019). Improved structure-stability and packing characters of crosslinked collagen fiber based film with casein, keratin and SPI. Journal of the Science of Food and Agriculture In press., 99, 4942–4951. https://doi.org/10.1002/jsfa.9726. Xiong, L., Qin, X., Liang, H., Huang, S., & Lian, Z. (2017). Covalent functionalization of carbon fiber with poly(acrylamide) by reversible addition–fragmentation chain transfer polymerization for improving carbon fiber/epoxy interface. Polymer Composites, 38(1), 27–31. https:// doi.org/10.1002/pc.23556. Yaghoubidoust, F., Wicaksono, D. H. B., Chandren, S., & Nur, H. (2014). Effect of graphene oxide on the structural and electrochemical behavior of polypyrrole deposited on cotton fabric. Journal of Molecular Structure, 1075, 486–493. https://doi.org/10.1016/j.molstruc.2014.07.025. Yang, Q., Pan, X., Huang, F., & Li, K. (2011). Synthesis and characterization of cellulose fibers grafted with hyperbranched poly(3-methyl-3-oxetanemethanol). Cellulose, 18(6), 1611–1621. https://doi.org/10.1007/s10570-011-9587-y. Yang, L., Liu, W.-w., Jiang, H., Zhou, L.-z., Sun, P., & Shen, Y.-F. (2013). Grafting of N,N-­ dimethylacrylamide onto silk fibers via a RAFT agent controlled radical transformation process. Fibers and Polymers, 14(6), 875–885. https://doi.org/10.1007/s12221-013-0875-z. Ye, C., Ma, G., Fu, W., & Wu, H. (2015). Effect of fiber treatment on thermal properties and crystallization of sisal fiber reinforced polylactide composites. Journal of Reinforced Plastics and Composites, 34(9), 718–730. https://doi.org/10.1177/0731684415579090. Ye, C., Ma, G., Zhu, Z., Fu, W., Duan, J., & Wu, H. (2017). Effect of process parameters on grafting ratio and thermal stability of lactide-grafted sisal fiber. Journal of Natural Fibers, 14(1), 86–96. https://doi.org/10.1080/15440478.2016.1146645. Yeo, J.-S., & Hwang, S.-H. (2017). The effect of dense polymer brush on the microfibrillated cellulose for the mechanical properties of poly(ε-caprolactone) biocomposites. International Journal of Adhesion and Adhesives, 78, 89–94. https://doi.org/10.1016/j.ijadhadh.2017.06.023. Yi, J., Chen, J., Liu, Z.-T., & Liu, Z.-W. (2010). Grafting of polystyrene and poly(p-chlorostyrene) from the surface of ramie fiber via RAFT polymerization. Journal of Applied Polymer Science, 117(6), 3551–3557. https://doi.org/10.1002/app.32261. Yoshikawa, S., Tsubokawa, N., Fujiki, K., & Sakamoto, M. (1998). Grafting of polymers with controlled molecular weight onto inorganic fiber surface by termination of living polymer cation with amino group on the surface. Composite Interfaces, 6(5), 395–407. https://doi.org/10.116 3/156855499x00107. Yoshioka-Tarver, M., Condon, B.  D., Santiago Cintrón, M., Chang, S., Easson, M.  W., Fortier, C.  A., Madison, C.  A., Bland, J.  M., & Nguyen, T.-M.  D. (2012). Enhanced flame retardant property of fiber reactive halogen-free organophosphonate. Industrial & Engineering Chemistry Research, 51(34), 11031–11037. https://doi.org/10.1021/ie300964g. Zahid, M., Papadopoulou, E. L., Athanassiou, A., & Bayer, I. S. (2017). Strain-responsive mercerized conductive cotton fabrics based on PEDOT:PSS/graphene. Materials & Design, 135, 213–222. https://doi.org/10.1016/j.matdes.2017.09.026. Zhang, L., Baima, M., & Andrew, T. L. (2017a). Transforming commercial textiles and threads into sewable and weavable electric heaters. ACS Applied Materials & Interfaces, 9(37), 32299–32307. https://doi.org/10.1021/acsami.7b10514. Zhang, L., Fairbanks, M., & Andrew, T. L. (2017b). Rugged textile electrodes for wearable devices obtained by vapor coating off-the-shelf, plain-woven fabrics. Advanced Functional Materials, 27(24), 1700415. https://doi.org/10.1002/adfm.201700415. Zhao, B., & Brittain, W.  J. (2000). Polymer brushes: Surface-immobilized macromolecules. Progress in Polymer Science, 25(5), 677–710. https://doi.org/10.1016/s0079-6700(00)00012-5.

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Zhao, Z., Zhou, J., Fan, T., Li, L., Liu, Z., Liu, Y., & Lu, M. (2018). An effective surface modification of polyester fabrics for improving the interfacial deposition of polypyrrole layer. Materials Chemistry and Physics, 203, 89–96. https://doi.org/10.1016/j.matchemphys.2017.09.062. Zheng, Z., Wang, H., Zhang, N., & Zhao, X. (2017). Preparation of self-cleaning polyester fabrics by chemical vapor deposition of methyltrichlorosilane/dimethyldichlorosilane. Fibres & Textiles in Eastern Europe, 25(1), 121–124. https://doi.org/10.5604/12303666.1227892. Zoppe, J.  O., Ataman, N.  C., Mocny, P., Wang, J., Moraes, J., & Klok, H.-A. (2017). Surface-­ initiated controlled radical polymerization: State-of-the-art, opportunities, and challenges in surface and interface engineering with polymer brushes. Chemical Reviews, 117(3), 1105–1318. https://doi.org/10.1021/acs.chemrev.6b00314.

Chapter 4

Reactive Processing and Functionalization of Ground Tire Rubber Łukasz Zedler, Marta Przybysz-Romatowska, Aleksander Hejna, Xavier Colom, Javier Cañavate, Mohammad Reza Saeb, and Krzysztof Formela

Abstract  The dynamic development of the automotive industry resulted in a significant increase in rubber wastes, especially end-of-life tires, which are a serious threat to the natural environment and human health. This situation has enforced the industry and academic research groups to search new and cost-effective methods for recycling waste tires. In this field of research, reactive processing and functionalization seem to be a very promising approach to extend recycling and the ‘up-cycling’ of ground tire rubber. This chapter presents recent progress in the modification of waste rubber and valorization strategies with special attention on structure-­properties relationships of the products obtained. Keywords  Recycling · Rubber wastes · Structure-properties relationships

4.1  Introduction Among the many types of industrial and agricultural wastes constantly generated, waste tires management and recycling represents the most significant challenges (Schnecko 1998; Stevenson et al. 2008). This fact results from the irreversible process of vulcanization, by which crosslinking bonds are formed. This makes rubbers problematic wastes, since they are resistant to biodegradation and many external Ł. Zedler · M. Przybysz-Romatowska · A. Hejna · K. Formela (*) Department of Polymer Technology, Faculty of Chemistry, Gdańsk University of Technology, Gdansk, Poland e-mail: [email protected] X. Colom · J. Cañavate Department of Chemical Engineering, Universitat Politècnica de Catalunya Barcelona Tech, Terrassa, Spain M. R. Saeb Department of Resin and Additives, Institute for Color Science and Technology, Teheran, Iran © Springer Nature Switzerland AG 2020 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_4

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Fig. 4.1  Number of articles considering the term ‘rubber recycling’ during the years 2005–2018. Data according to Web of Science Core Collection database

factors. In addition, the complex physical and chemical structure of waste rubber related to its unknown composition and the partial degradation that occurs during the use of rubber products could also affect the limited application of the waste generated. Environmental and economic aspects have forced scientists and industry to make efforts focused on finding practical and cost-effective methods for efficient recycling of waste tires. Figures 4.1 and 4.2 show the number of articles published and the count of citations of articles considering ‘rubber recycling’ in the period 2005–2018. The linear growth of a number of published articles (32 in 2005 and 236 in 2018) and the logarithmic growth of the number of citations (2 in 2005 and 4338 in 2018) during the thirteen years can be observed. Figure 4.3 also shows the overall growth in end-of-life tires (EU27, Norway and Switzerland), which increased from 2.48 million tons in 2004 to 2.88 million tons in 2013 (ETRMA 2015). The trend is upwards and will continue rising, due to the development of the automotive and transportation market in Eastern Europe. Waste tire data was cross-checked with articles considering ‘rubber recycling’ during the same period of time. The data presented confirm the growing development of research studies focused on finding a reasonable solution for the management and recycling of rubber wastes. Illegal landfilling of waste tires involves high risks due to the fire threat, environmental pollution and the provision of a breeding ground for disease carrying rodents

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Fig. 4.2  Count of citations of articles considering the term ‘rubber recycling’ during the years 2005–2018. According to Web of Science Core Collection database

Fig. 4.3  The number of published articles considering ‘rubber recycling’ and amount of end-of-­ life tires during the years 2005–2013. According to data from Web of Science Core Collection database and European Tyre & Rubber manufacturers’ association (ETRMA) statistics

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and insects. One of the methods commonly used to deal with a high volume of waste tires is their application as a cheap substitute for coal in cement kilns or power plants. It should be noted, however, that the energy equivalent of 1 kg of a passenger car tire is around 128 MJ, while its combustion allows recover only 30 MJ. On the other hand, about 6.8 MJ of additional energy is required to produce 1–1.5 kg of ground tire rubber (GTR) (Schulman 2002). Waste tires and other waste rubber are made from high quality materials (elastomers and reinforcement fillers), which could be considered a valuable source of secondary raw materials. Therefore, the search for new methods of recycling and application of waste tires in the industry is totally reasonable. In this chapter, the recent progress in the recovery and modification strategies of the waste rubber will be discussed, paying special attention to the structure-­ properties relationships of the products obtained in order to better understand the mechanism of waste rubber modification and/or functionalization and its impact on the efficiency of the process.

4.2  Reactive Sintering of GTR One of the possibilities for the direct application of GTR on an industrial scale is reactive sintering. This simple and environmentally friendly process allows the molding of waste rubber into simple shapes by applying high pressure and temperature. Arastoopour et al. (1999) patented the method of sintering of wasterubber followed by compression molding at a temperature of at least 200 °C by applying a compression force of at least 10 tons to the rubber powder for a period of at least one hour. According to the patent claims, this technology ‘results in the formation of a single piece of the rubber, elastomeric, or thermoset material, with no change in chemical structure’. Morin et al. (2002) indicated that reactive sintering makes it possible to convert commercially available rubber powder into new rubber products with tensile strength in the range of 4–7  MPa and an elongation at break of 150–250%. The scheme of the reactive sintering process of rubber wastes is shown in Fig. 4.4. Hrdlicka et al. (2011) indicated that rubber recycling by reactive sintering is suitable for natural rubber and styrene-butadiene rubber vulcanized with sulfur-based curing systems, while it is not recommended for vulcanizates cured with sulfur donors or peroxides. Reactive sintering can be done without additives and with the use of curing systems or adhesive binders. It should be noted that this method is limited to the production of low-cost products with simple shapes and low quality requirements. The application of additives and/or binders during reactive sintering usually improves the processing and final performance properties of the prepared products. This is related to additional physical interactions (e.g. by reinforcing fillers) and chemical reactions (e.g. crosslinking), which have a beneficial impact on interfacial adhesion between GTR particles.

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Fig. 4.4  Reactive sintering process of rubber wastes. Based on data presented by Morin et al. (2002)

Kowalska et al. (2010) patented mixtures of waste rubber (especially GTR) with sulfur in the range of 0.1–5.0 wt.% with possible addition of dithiocarbamate up to 5.0  wt.%, which was subsequently revulcanized in a temperature range between 150–210 °C, under pressure of 5–50 MPa and for 0.1–5.0 min/mm of product thickness. Products obtained with tensile strength between 0.9 and 2.7 MPa could, for example, be used as rubber floor coverings, wipers or car floor mats. Crespo et al. (2012) studied the impact of latex and polychloroprene as binders during the reactive sintering of ground ethylene-propylene-diene-monomer rubber. It was found that the increase in pressure and temperature improved mechanical properties of the material, however, the tensile strength of the prepared materials was relatively low (in the range of 0.3–0.9 MPa). The authors noted that, regardless of the compression temperature, the addition of 5% latex adhesive leads to a 50% reduction in tensile strength and elongation at break. A higher content of latex adhesive results in a more visible deterioration of mechanical properties. On the other hand, for polychloroprene adhesive an opposite tendency was observed, the mechanical properties of reactively sintered rubber wastes increase with an increase in the content of this binder. Polyurethane (PU) adhesives are commonly used as binders during waste rubber recycling (Tan et al. 2008; Sułkowski et al. 2010, 2012), which is related to their relatively simple processing and a broad spectrum of properties. Perfromance can be easily tailored by changing the chemical structure of the PU matrix or the application of a specific modifier. Balas et al. (2014) patented composition of PU waste rubber crosslinked with unsaturated compounds. As could be expected, a higher content of waste rubber into PU matrix caused a significant deterioration of mechanical properties, which is illustrated in Fig. 4.5. This is related to the low compatibility between the PU matrix and the crosslinked rubber particles. The examples presented by Balas et al. (2014) also confirmed that the isocyanate index has a significant impact on the mechanical properties of PU/waste rubber composites, which is related to the efficiency of the reactions between the isocyanate component and the active sites present in waste rubber, as presented in Fig. 4.6.

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Fig. 4.5  Effect of waste rubber on the mechanical properties of the PU matrix. Based on data presented by Balas et al. (2014) Fig. 4.6  General scheme of possible reactions between isocyanates and GTR particles

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This correlation seems to be a promising approach to tailor and improve the tensile properties of PU/waste rubber composites. Recently, Strakšys et al. (2018) studied the impact of the shape and size of the waste rubber particles, the adhesive content and the curing conditions on the PU/ GTR composites. The results showed that the tensile and compressive properties of the composites were more dependent on the adhesive content than on the curing conditions. The characteristics of rubber particles were important for the composites studied because they affect the mechanism of void formation, thus wetting of rubber particles with the adhesive. As a result, structure of these particles has a significant impact on the mechanical properties of the material. Some research groups, for a better understanding of the mechanisms of formation of holes during sintering, have investigated the morphology, shape parameters and compression behavior of GTR (Ferrer-Giménez et al. 2009; Nadal et al. 2016). Due to the high elasticity of rubber, the main technical problem for the industrial application of reactive sintering has been related to the high pressure necessary for efficient compression of crosslinked rubber particles. This problem has been solved in the Sustainable Moulding of Articles from Recycled Tires (SMART) project financed by the European Commission. The aim of the SMART project was to develop a new GTR moulding process without additives or binders (Quadrini et al. 2013). This technology allows to produce large-size tiles (1 m2) with a thickness of up to 50 mm, characterized by a tensile strength of ~0.6 MPa, an elongation at break of ~31% and a hardness of ~66 °ShA (Quadrini et al. 2019). Quadrini et al. (2019) also presented calculations that have clearly shown that the direct reactive sintering of GTR is ~50% cheaper compared to the conventional process with the application of PU adhesive as a binder. Another solution to eliminate the high pressure problems necessary for GTR compression is the application of smaller particles (Gugliemotti et al. 2012). In this sense, Shen et al. (2019) studied the effects of grinding conditions (ambient/cryogenic) and GTR particle size (in the range of 75–350 μm) for reactive sintering in the presence of sulfur curing system (N-tert-butyl 2-benzothiazole sulfenamide (TBBS) - 0.8 phr and sulfur 1.2 phr). It was observed that the tensile strength of the rubber sheets increased for GTR with a smaller size and a more developed particle surface. On the other hand, the result showed that the moulding pressure had no effect on density, tensile strength or elongation at break of the materials obtained. However, it should be noted that reducing the particle size increased the energy consumption of the grounding process, which obviously affects the final costs of GTR. Prut et al. (2015) indicated that the addition of up to 2 phr of sulfur had a beneficial impact on the tensile properties of the revulcanized ethylene-propylene-diene monomer, while a higher content of sulfur resulted in deterioration of the mechanical properties. For a better understanding of the current progress in reactive sintering of rubber waste. A summary and a comparison of the performance properties of the products prepared by different research groups is shown in Table  4.1. The data presented

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Table 4.1  Performance properties of reactive sintered rubber waste determined by different research groups Characteristic of rubber wastes Composition: Natural rubber/ styrene-­ butadiene rubber mixture (there is no information about used fillers) Particle size: 0.18 mm GTR Particle size: Below 1.6 mm

Used additive –

sulfur (2 wt.%)

Reactive sintering conditions Pressure: 8.6 MPa Time: 60 min Temperature: 100–220 °C

Pressure: 8–9 MPa Time: 1.2–8 min (3 min/mm thickness of product) Temperature: 180–190 °C Pressure: PU adhesive GTR 5 MPa binder with Particle size: Time: Below 1.0 mm variable isocyanate index 90 min Temperature: (60 wt.%) 80 °C – Pressure: GTR 10 MPa Particle size: Time: 1–2 mm 5–25 min (5 min/mm thickness of product) Temperature: 180–200 °C Pressure: sulfur (1.2 GTR phr) + TBBS (0.8 Time: Particle size: 10 min phr) 0.075– Temperature:0.25 mm Pressure: di(2-tert-butyl-­ GTR peroxyisopropyl) 4.9 MPa Particle size: Time: Below 1.5 mm benzene (1 phr) 5.4 min Temperature: 180 °C

Performance properties Tensile Elongation Hardness strength at break (°ShA) (MPa) (%) References 0.6–4.0 50–400 – Morin et al. (2002)

2.2–2.9 112–185

59–65

Kowalska et al. (2010)

3.2–6.8 139–208

65–80

Balas et al. (2014)

0.5–0.7 25–40

~66

Quadrini et al. (2019)

6–7-9.5 160–240

–

Shen et al. (2019)

3.8

65

Our results in the laboratory scale.

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(continued)

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Table 4.1 (continued) Characteristic of rubber wastes GTR Particle size: Below 0.4 mm

GTR Particle size: Below 0.4 mm

Reactive sintering conditions Used additive dicumyl peroxide Pressure: (0.8 phr) 4.9 MPa Time: 3.4 min Temperature: 180 °C – Pressure: 4.9 MPa Time: 5 min Temperature: 180 °C

Performance properties Tensile Elongation Hardness strength at break (°ShA) (MPa) (%) References 3.6 149 63 Our results in the laboratory scale

2.6

79

58

Our results in the laboratory scale

indicates that the use of curing additives significantly improves the mechanical properties of revulcanized rubber, which make this approach more promising than the application of PU adhesives. Another solution to modify the performance properties or decrease the price of revulcanized rubber is the application of fillers. Stefani et  al. (2005) and García et al. (2007) found that the reactive sintering from GTR/rice husk mixtures were dependent on the rice husk content and the particle size distribution, thus affecting the mechanical properties of the composites. For example, the addition of 5 wt.% rice husk caused deterioration of tensile properties (depending on the particle size of the rice husk - tensile strength: 0.8–2.2 MPa and elongation at break: 40–108%) compared to pure GTR (3.0 MPa and 144%). Among the samples studied, the best mechanical properties for the filler (GTR and rice husk)-based composition with a similar particle size distribution (~0.3 mm) was determined. Ubaidillah et al. (2016a) indicated that GTR after high-pressure high-­temperature sintering possesses very good insulation properties, what extent the possible applications of GTR without any additives (Ubaidillah et al. 2017). In addition, the same research group works on a new class of GTR-based magnetorheological elastomers and their mixtures with iron powders (Ubaidillah et al. 2016b, 2016c). The authors indicated that the sintering process at 25 MPa and 200 °C for one hour allows the preparation of GTR-based magnetorheological elastomers characterized by random dispersion of magnetic particles into the GTR matrix without porosity or limits of rubber particles (Ubaidillah et al. 2016b, 2016c). Jia et al. (2017) investigated flexible and low-cost electromagnetic interference shielding materials based on GTR/carbon nanotube mixtures, which were formed by sintering at high pressure and temperature. It was found that composite filled with only 5.0 wt.% of carbon nanotubes had a very good electrical conductivity of 109.3  S/m and an electromagnetic interference shielding effectiveness of 66.9 dB. The performance parameters obtained were higher than many other carbon

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nanotube-based polymer composites reported. The data presented above suggest that the application of functional fillers or the suitable modification of GTR allows the sustainable development of tire waste recycling and upcycling technologies. The functionalization and modification of GTR will be discussed in the next section.

4.3  Functionalization and Modification of GTR Most of the current applications for GTR are based on their mixtures with different matrices (polymers, bitumens and concretes). The main practical problem of this approach is the weak compatibility between the GTR and most of the matrices studied. GTR is constituted by a crosslinked network with low affinity for other materials. For mixtures or composites with GTR, the resulting structure shows two phases that are not interfacially linked, which leads to poor mechanical properties of prepared materials. This phenomenon strongly limits the potential application of GTR. In order to provide a solution to this problem, a series of modifications have been proposed involving changes in the surface and structure of GTR. The main objective is to functionalize and/or modify the GTR surface allowing the physical interfacial interactions with the other components of the composition or additional chemical reactions in the matrix-GTR filler boundary. Data from the literature has shown that efficient functionalization and modifications of GTR can be performed using different strategies, which were presented in this chapter.

4.3.1  Reclaiming/Devulcanization Many reclaiming methods of rubber wastes have been developed, which were described comprehensively in the works of Movahed et  al. (2016) and Asaroa et  al. (2018). Reclaiming process (called also  devulcanization in the literature) allows breaking of the three-dimensional crosslinked network present in GTR.  However, it should be noted that with scission of disulfide crosslinking bonds (devulcanization), the partial degradation of the polymer main chain (reclaiming) usually also occurs. The mechanism of crosslinked network scission and the adequate control of conditions allow to limit of the unwanted phenomenon of the degradation of the main chain, which affects the process selectivity (devulcanization vs. reclaiming). Recent research trends in this field are focused on combining low temperatures and a short-time of reclaiming in order to reduce energy consumption and production costs (Dobrotă and Dobrotă 2018). In addition, the application of mild conditions in GTR reclaiming limit the emission of hazardous volatile organic compounds generated during the scission of disulfide bonds and the degradation of the main chain (Gągol et al. 2015).

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Gibala et  al. (1996, 1999) and Gibala & Hamed (1994) pointed out that the selected curing additives can migrate from the GTR to the polymer matrix, as well as in the opposite direction. It is worth mentioning that this phenomenon is also affected by the degree of GTR degradation. Recent studies have shown that low-temperature reclaiming can improve carbon black migration from GTR (Formela and Haponiuk 2014, Song et al. 2018). This phenomenon has a beneficial impact on the reinforcement effects on rubber compounds filled with partially reclaimed GTR. The mechanism of carbon black migration related to rubber reclaiming is presented in Fig. 4.7. The reclaimed rubbers as compared with the GTR can be easily processed, shaped, molded and vulcanized. However, the final properties of reclaimed rubber are strongly affected by reclaiming methods and conditions. For a better understanding of the influence of these factors on the tensile properties of the reclaimed rubber, the comparison of the reclaimed GTR prepared by different methods is summarized in Table 4.2.

Fig. 4.7  The mechanism of carbon black migration during rubber reclaiming. Based on data presented by Formela and Haponiuk (2014) and Song et al. (2018) Table 4.2  Comparison of the tensile strength of the reclaimed GTR prepared by different methods Reclaiming methods Microbial desulfurization Shearing in pan mill reactor Bitumen plasticization/microwave treatment Thermo-mechanical in counter- and co-rotating twin screw extruder Auto-thermal extrusion Mechano-chemical in the presence of organic peroxides Mechano-chemical in the presence of bitumens

GTR (mm) 0.05 0.25 0.50

Tensile strength of reclaimed GTR (MPa) 3.3 4.2–8.4 5.2–6.1

1.50

3.3–6.5

0.5 0.5

4.6 2.6–4.1

0.5

1.3–2.6

Reference Li et al. (2012) Zhang et al. (2007) Zedler et al. (2018) Formela and Cysewska (2014) Zedler et al. (2019) Our results in the laboratory scale. Our results in the laboratory scale.

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4.3.2  Increasing the Polarity of the GTR Surface Other attempts to improve adhesion between the polymer matrix and the GTR have been conducted by the chemical etching of the GTR surface, creating porosity and improving the possibilities of a better interlocking between the polymer matrix and the GTR as a semi-reinforcement filler. With this in mind, Manchón et al. (2004) subjected GTR to acidic treatments (H2SO4, HNO3 and H2SO4/HNO3 mixtures) with the aim of using waste tires as a carbonaceous adsorbent. The rubber surface was modified by the action of acids in terms of pore size distribution in the range of mesopores and macropores. These authors concluded that the development of porosity caused by chemical treatments produced changes in the macropore range. The effect of HNO3 specifically was decisive for creating large pores in the material. Other researchers have also worked on the preparation of highly mesoporous activated carbons from tire wastes as a promising and effective way to treat wastewater (Li et al. 2010) or capture SO2 (Nieto-Márquez et al. 2016). Colom et al. (2009) also found that acid treatment improves tensile properties when applied to composites constituted by thermoplastic matrices such as high density polyethylene (HDPE) and GTR. Acidic treatments generate functional groups on the surface of the GTR, such as C=O, C=C, peroxides and specific groups related to the nature of the acid applied. This phenomenon improves the compatibility due to better mechanical adhesion and the elimination of moieties and residues on the surface of GTR. The treatment with sulfonitric mixtures also produces better results in terms of tensile properties, but only H2SO4 has shown a remarkable effect. The effect of the acidic pretreatment is more intense when particles of smaller sizes are used, and consequently samples with higher tensile strength and Young’s modulus are produced. The mechanism of matrix-GTR compatibilization facilitated by controlled oxidation (e.g. acid treatment) can be observed in Fig. 4.8. Although the acidic treatments can generate environmental concern, the increase in stiffness and other effects produced by the acid treatments of GTR on the

Fig. 4.8  The mechanism of matrix-GTR compatibilization facilitated by controlled oxidation. Based on data presented by Zhang et al. (2009)

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properties of the composites still remains the object of research studies (Hernández et al., 2017). Another approach to increase the polarity of the GTR surface is chlorination, which is usually performed by using of trichloroisocyanuric acid (TCl) (Naskar et al. 2001, 2002). TCl as in the case of other acids, allows controlled oxidation of the GTR surface resulting in the formation of new functional groups, such as C=O and S=O. Recently, Yao et al. (2019) indicated that chlorination of GTR resulted in a reduction in particles size (particle size >0.85 mm = 96.8 wt.% for untreated GTR and 56.3–83.1 wt.% for GTR after chlorination (as a function of the degree of chlorination)) and also a significant improvement in the tensile properties of its revulcanized. Tensile strength and elongation at break for the untreated GTR was 2.4 MPa and 143%, respectively, while for the GTR after chlorination these parameters were in the range of: 5.1–10.3 MPa and 72–94%. The creation of polar groups is also possible with other oxidizing agents. In line with this, Sonnier et al. (2006, 2007) tested several oxidation treatments on HDPE/ GTR mixtures: using a wet process with potassium permanganate (KMnO4) and a dry process using γ-irradiation. The modified GTR powder was combined in a polypropylene composite matrix containing grafted maleic anhydride. The results with KMnO4 showed an improvement in the elongation at break, which was related to the better compatibility between the dispersed GTR particles and the matrix. These authors also used γ-irradiation to induce scission of the polymer chains, which allows the creation of the free radicals suitable for crosslinking or reaction with oxygen from the air, resulting in the formulation of polar groups. GTR oxidation was also tested by Sonnier et al. (2006, 2007) through specific tests, however, the mechanical properties of the materials studied were not significantly improved. Some other alternatives for the creation of polar groups on the GTR surface can be conducted through the use of ozone (Cao et  al. 2014), high energy radiation, corona discharge, electro-beam or plasma to create oxidized groups (Ratnam et al. 2013; Xiang et al. 2018). The results obtained by these treatments have been adequate to improve compatibility with polar polymers. The main drawback of these methods seems to be their viability from an economic perspective.

4.3.3  Using Coupling Agents and Additives The use of coupling agents is a common practice in composite materials. The coupling agents act as a link between the reinforcement and the filler, in this case, the GTR and the matrix. The coupling agent must have an affinity for both components and the ability to successfully improve compatibility between them. The application of compatibilizers is easy and does not require special equipment to produce the composite. Under these assumptions, the use of coupling agents or additives could lead to a process that would be environmentally friendly, economically competitive and suitable for industrial scale application.

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In the case of mixtures and their compounds modified with GTR, silanes have also been tested. The application of γ-methacriloxypropyltrimethoxysilane (silane A-174) as a coupling agent on the surface of the GTR has been reported as able of improving the interactions between GTR and non-polar matrices (as HDPE), thus providing higher values on tensile strength than other treatment methods. As could be expected, silane development interactions with methyl and methylene groups that produce interpenetration with the matrix, and improve the bond between the GTR particles and the thermoplastic matrix (Colom et al. 2006). Figure 4.9 shows comparative results of tensile strength and elongation at break in composites made of HDPE containing between 0–40 wt.% GTR treated with silane and TCl. Other types of modifiers commonly used in the plastics industry are wetting or dispersing agents. These additives reduce interfacial tension, facilitate wetting and prevent the formation of agglomerates in polymer-filler systems, which is related to additional physical interactions at the limit of the phases (such additives usually do not participate in chemical reactions). Commercial additives and waxes as compatibilizers applied to the GTR have caused the encapsulation of the GTR by the matrix, thus achieving a better entanglement and co-crystallization phenomena with the polymer matrix, which has led to an improvement in the properties of the polymer/ GTR composites (Cañavate et al. 2010).

Fig. 4.9  Mechanical properties of HDPE/GTR composites in which GTR was untreated or pre-­ treated with TCl and silane as a coupling agent

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4.3.4  Grafting of Chemical Compounds on the Surface of GTR The increase in interfacial adhesion of the GTR matrix can be achieved by grafting monomers or other chemical compounds on the surface of GTR. This way differs essentially from the others presented above. In this case, the modification of the GTR includes a chemical bonding reaction, grafting the chosen  modifier to the GTR. This grafted component participates in the subsequent crosslinking reaction or becomes entangled with macromolecular chains of the matrix. Common monomers used for grafting on GTR are: styrene, maleic anhydride, acrylamide and derivatives from acrylic and metacrylic acids. The reaction of grafting can be carried out by: I. Decomposition of a free-radical initiators at high temperatures: has been applied to graft styrene, maleic anhydride and others compounds on the double bonds of GTR. They are generally applied in an internal mixer using an initiator such as dibenzoyl peroxide. The resulting polymer mixtures including maleic anhydride and styrene grafted GTR have shown improved mechanical performance compared to samples with untreated GTR (Coiai et  al. 2006; Zhang et al. 2012). II. High energy radiation: maleic anhydride and acrylamide have been grafted by γ-irradiation onto GTR to develop thermoplastic elastomers (Tolstov et  al. 2007). Abdel-Bary et  al. (1997) grafted GTR with acrylamide, acrylic acid, acrylonitrile by this technique. III. Photo-induced grafting: Fuhrmann and Karger-Kocsis (2003) functionalized the GTR with glycidyl methacrylate and methacrylic acid, which was subsequently treated by UV irradiation. Allylamine and bismaleimide have been also grafted onto GTR by this method (Du et al. 2005; Shanmugharaj et al. 2006). Kocevski et  al. (2012) also indicated that grafting of acrylic acid could occur under mild conditions (temperature: 80–85 °C, nitrogen atmosphere) even without initiators. The scheme of this reaction is presented in Fig. 4.10. It was found that the GTR functionalized with acrylic acid produced an increase in the viscosity and failure temperature of the modified bitumen. Some researchers also recommend pretreatment of GTR before grafting to activate the surface of GTR and make it more suitable for subsequent modification or functionalization. Fan and Lu (2011) proposed the ozonization of GTR before the grafting reaction with methyl methacrylate. The intensity of the ozonization treatment had a beneficial impact on the degree of grafting and, consequently, the efficiency of the modification. Xiaowei et  al. (2017) GTR pretreated with oxygen in the low-temperature plasma. Subsequently, the pretreated GTR was modified by a low temperature polymerization process in the presence of ethanol in order to improve the adhesion property with the oil well cement matrix and the performance properties of cement. The results showed that the modified GTR had a rougher surface than the pretreated GTR, and that hydrophilic groups, such as -COOH, C–OH and -CHO, were created

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Fig. 4.10  Grafting of acrylic acid onto GTR without initiators. Based on the mechanism proposed by Kocevski et al. (2012)

Fig. 4.11  Oxygen pretreatment and low temperature plasma modification of GTR in the presence of ethanol. Based on the mechism proposed by Xiaowei et al. (2017)

on the surface of the modified GTR, which affected their more hydrophilic behavior. The scheme of oxygen pretretment of GTR and its low-temperature plasma modification process is presented in Fig.  4.11. This phenomenon resulted in the improvement of dispersibility, which resulted in better mechanical properties of cement filled with modified GTR. In general, grafting techniques can produce subsequent reactions of grafted monomers with the corresponding matrices and an improvement in the properties of the materials investigated. The modifications can also change the surface energy of the GTR by improving the wettability of the matrix and reducing the tendency to agglomerate the GTR particles.

4.4  Conclusions and Future Trends In the past 15 years, many attempts at reactive processing and functionalization of ground tire rubbers (GTR) have been depicted, and in this chapter they were summarized. It seems that GTR sintering, functionalization and modification are very

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promising approaches to increase the level of tire waste recycling. Proper treatment of GTR has usually improved chemical reactions and physical interactions between GTR particles and the polymer matrix. However, the repeatability of the functionalization/modification process and the storage stability of the treated GTR should be investigated. In addition, the modification and functionalization of GTR by continuous methods (e.g. reactive extrusion) should be developed in the near future, which is related to greater efficiency of these methods and, consequently, an easier implementation of the laboratory results on an industrial scale. Acknowledgments  Financial support from the National Centre for Research and Development (NCBR, Poland) in the frame of LIDER/6/0035/L-8/16/NCBR/2017 project is gratefully acknowledged. Conflicts of Interest  The authors declare no conflict of interest.

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Chapter 5

Lignin as a Natural Antioxidant: Property-­Structure Relationship and Potential Applications Zhao Qin, Hua-Min Liu, Ling-Biao Gu, Run-Cang Sun, and Xue-De Wang

Abstract  Lignin, as one of the most abundant natural polymer compounds in wood and annual plants, has a complex chemical structure. It is fundamentally an aromatic polymer, composed of many aromatic rings with methoxyl and hydroxyl functional groups. Because of its multifunctional side groups, lignin can act as a free radical scavenger, thus acting as a natural antioxidant agent. However, extraction conditions, source species and structure affect the antioxidant activity of lignins. As a relatively safe and natural antioxidant, lignin can be used in foods, pharmaceuticals, cosmetics, and industrial materials. In this chapter, the correlation of lignin between structural characteristics and its radical scavenging capabilities, the relationship between antioxidant capacity and potential cytotoxicity, and the advances in commercial applications of lignin as an antioxidant were reviewed and analyzed. Keywords  Composite · Cytotoxicity · Modification · Structure-activity relationship

Z. Qin · H.-M. Liu (*) · X.-D. Wang (*) College of Food Science and Technology, Henan University of Technology, Zhengzhou, China e-mail: [email protected] L.-B. Gu School of Biological and Food Engineering, Anyang Institute of Technology, Anyang, China R.-C. Sun Center for Lignocellulose Science and Engineering, College of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian, China © Springer Nature Switzerland AG 2020 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_5

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5.1  Introduction Antioxidant agents can prevent or delay oxidation, thus improving the stability of products and prolonging their shelf life. They can be divided into natural and synthetic antioxidants. Synthetic antioxidants, such as butylated hydroxytoluene (BHT), butylhydroxyanisole (BHA), propyl gallate (PG) and tert-­butylhydroquinone (TBHQ), represent an important part of the antioxidant market (Yang et al. 2018c). However, the potential toxicity of synthetic antioxidants has been recognized (André et al. 2010). For this reason, more and more attention has been given to natural antioxidants. Natural antioxidants compared to synthetic antioxidants, in particular, polyphenols are biodegradable and non-toxic. Nonetheless, due to their low molecular weight (Mw) and instability at prolonged high temperatures, natural antioxidants cannot prevent oxidation under conditions of thermal stress. Lignin is a natural phenolic polymer with relatively higher thermal stability, thus it has relatively more applications as an antioxidant (Gutiérrez et  al. 2016; Gutiérrez and Alvarez 2017). In this sense, interest in the use of lignin is growing. Figure  5.1 clearly illustrates this trend in research publications, while Fig. 5.2 shows the many areas in which lignin research is conducted.

30000

6000

A

"antioxidant"

Number of references

Number of references

35000

25000 20000 15000 10000 5000 0

2000

0

Number of references

C

08 09 10 11 12 13 14 15 16 17 18 20 20 20 20 20 20 20 20 20 20 20

Year

Year

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"lignin"

4000

08 09 10 11 12 13 14 15 16 17 18 20 20 20 20 20 20 20 20 20 20 20

200

B

"lignin" and "antioxidant"

120 80 40 0 20

08 009 010 011 012 013 014 015 016 017 018 2 2 2 2 2 2 2 2 2 2

Year

Fig. 5.1  Number of references per year between 2008 and 2018 using the keywords ‘antioxidant’, ‘lignin’ and ‘antioxidant’

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Biochemistry and Molecular Biology 8.77% Others 35.28%

Chemistry 7.78%

Agriculture 7.71% Environmental Sciences/Ecology 7.50%

Energy/Fuels 5.26%

Engineering 7.49%

Plant Sciences Biotechnology/Applied Microbiology 7.3% 6.21% Materials Science 6.7%

Fig. 5.2  Research areas in lignin. Notes: ‘Others’ includes: biomedicine, business economics, cell biology, conservation, food science and technology, forestry, genetics, human physiology, instrumentation, life sciences, microbiology, mycology, pharmacology, physics, polymer science, and public, environmental and occupational health

In addition to cellulose and hemicellulose, lignin is the most abundant component in the plant (Álvarez et al. 2017; Gutiérrez et al. 2017). The structural representation of lignin is shown in Fig.  5.3. Lignin is a biopolymer formed by the interconnection of three phenylpropane units: p-hydroxyphenol alcohol, guaiacyl alcohol, and syringyl alcohol, linked by ether and carbon-carbon bonds (Fig. 5.4). Lignin is an abundant resource with an estimated annual production of approximately 100 billion tons (Tribot et al. 2019). Lignin can be applied in many other fields in addition to its common uses in the paper and energy industries (Table 5.1). Lignin links to cell wall polysaccharides by covalent bonds (Qin et al. 2018a) to form the lignin-carbohydrate complex (LCC), which also has antioxidant activity due to the presence of the lignin or carbohydrate constituents (Niu et  al. 2016; Huang et al. 2018). The application of lignin as an antioxidant is an effective way to increase its value.

5.2  Antioxidant Activity-Structure Relationship Understanding how lignin structure affects antioxidant activity is useful not only to design more effective antioxidants, but also to reveal the free radical scavenging mechanism for different applications. The 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•) free radical scavenging assay is a widely used method to measure the antioxidant activity of lignin. The hypothetical pathways for the reaction of lignin with DPPH• free radicals are shown in Fig. 5.5.

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OMe OH O

OMe

HO

MeO

OH

OMe

HO OMe

MeO

OMe

O

OMe

OH MeO

O

OMe

MeO

OH HO

OH

O

OH OMe

OMe

OH

O

O

OH

O

OMe

OMe

O OH

OH

HO

OH

OH

OH

O

OH

HO

O

O

MeO

OH

O

OMe

O

O

O

MeO

OH

OMe

O

OH

HO

OH

MeO

OMe

Fig. 5.3  Representation of the structure of lignin

HO HO α

R γ

O O

4' βO

HO α OMe

OMe

γ

β

α

γ

O

α β γ

OH

O

D

β

H α

OMe

OAr OMe

O

I

HO

α O

B

γ OH α

β

γ

β' γ'

γ

J

B' γ

α OMe

O

O

OMe

5' HO γ β α O

O

β'

O

MeO 4 OH

FA

O

OMe

O

S

OMe

C

OH

β

OM e

α'

O

OMe

O β

β

γ' OH

α O

O

A'

OMe

O α'

OMe

O

A

HO

O

OMe

O

O

O Me

γ β O 4'

OH

O

OMe MeO

OMe

O

S'

OH

OMe O

G

O

H

Fig. 5.4  Main substructures present in lignin: (A) β-O-4 alkyl-aryl ethers, (A′) β-O-4 alkyl-aryl ethers with acylated γ-OH, (B) β-β resinols, (B′) tetrahydrofuran, (C) phenylcoumarans, (D) spirodienones, (I) cinnamyl alcohol end-groups, (J) cinnamyl aldehyde end-groups, (FA) ferulates, (S) syringyl units, (S′) oxidized syringyl units bearing a carbonyl at Cα, (G) guaiacyl units and (H) p-hydroxyphenyl units

Much research has revealed that the antioxidant activity of lignin is correlated with its structural characteristics (Mw, polydispersity, functional groups) and purity (relative amounts of hemicellulose and other components). Dizhbite et al. (2004) assessed the antioxidant activities of lignin fractions isolated from deciduous and

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Table 5.1  Applications of lignin Fields Agroforestry

Building materials Food industry Light industry

Rubber and plastics Synthetic resins and adhesives Others

Examples Fertilizer, pesticide corrosion inhibitor, plant growth regulator, food additive, soil amendment, liquid mulch film, etc. Asphalt emulsifier, water-reducing agent of concrete, etc. Packaging materials Antioxidant, dye dispersant, adsorbent, flocculant, surface-active agent, synthetic tanning agent, activated carbon preparation, lignin-based carbon fibers, lignin-­ synthetic polymer mixtures, etc. Rubber reinforcing agent, plastic additives, etc. Lignin-phenol-formaldehyde resin, lignin polyurethane, lignin-epoxy resin, lignin-furfural-based adhesives, etc. Production of antioxidants such as vanillin

References Xiao et al. (2007), Abu-Dalo et al. (2013), Yin et al. (2014), Sipponen et al. (2016) Takahashi et al. (2014), Yuliestyan et al. (2017) Núñez-Flores et al. (2013), Aguié-­Béghin et al. (2015) Hayashi et al. (2000), Da Silva et al. (2011), Liu et al. (2015), Kai et al. (2016a), Balasubramanian et al. (2017), Wang et al. (2018a), Chen et al. (2018a), Zhang et al. (2018) Barana et al. (2016), Chen et al. (2016) Xue et al. (2014), Dongre et al. (2015), Yang et al. (2015), Li et al. (2018a)

Fache et al. (2016)

coniferous wood species by various methods, and concluded that aliphatic hydroxyl groups in side chains, non-etherified phenolic hydroxyl groups onto the lignin structure, and high heterogeneity, polydispersity and Mw were the main factors to reduce the free radical scavenging activity of lignin. In addition, low purity lignin containing many residual carbohydrates has a low antioxidant activity because carbohydrates can generate hydrogen bonds with phenolic hydroxyl groups of lignin, which interferes with the antioxidant activity of lignin. Pan et al. (2006) evaluated the free radical scavenging activities of 21 ethanol lignin fractions extracted from hybrid poplar, and concluded that lignin with less aliphatic hydroxyl groups, more phenolic hydroxyl groups, lower Mw, and a narrower Mw distribution had a greater antioxidant activity. Similar conclusions were made by Dizhbite et al. (2004), i.e. low Mw lignins having more aromatic hydroxyl groups and high Mw, showed a higher antioxidant activity. Morales and Lucas (2010) found that the length of the alkyl side chain in the phenylpropane units, and in the carboxylic and alcohol groups helped increase the antioxidant activity. A longer alkyl chain gave lignin a greater radical scavenging capacity. Ponomarenko et  al. (2015a) explored the quantitative and qualitative structure-­activity relationship of lignins. The results revealed that the content of phenolic hydroxyl groups is the most important factor to determine the antioxidant activity of lignin. Other factors, such as the content of guaiacyl and syringyl units, the number of phenylpropane units with CH2 groups in the side chains at the α-position, and the amount of methoxyl per phenylpropane unit, also had positive influences on antioxidant activity. In contrast, the number of phenylpropane units

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DPPH-H

OH

OCH3

OCH3

O

OCH3

O

DPPH

DPPH O

OCH3

H3CO

DPPH

O

OCH3

H3CO O

OH

O OCH3

OH OCH3

Fig. 5.5  Possible reaction pathways of lignin with DPPH• free radical

with oxygen-containing groups in the side chains and the  size of the conjugated systems reduced the antioxidant activity. In another study, Ponomarenko et  al. (2015b) found that the reactions of o-OCH3, α-CH2, and aliphatic carbonyl groups with free radicals were based on a proton-coupled electron transfer mechanism, while the reactions of the ether bonds in the aliphatic side chains, the carbohydrate impurities, and the OCH3 groups in C9 unit were based on a sequential proton loss electron transfer (SPLET) mechanism, and the π-conjugated systems were based on both mechanisms. The antioxidant activity of lignin is mainly determined by the formation of phenoxy radicals to scavenge free radicals (Zhao et  al. 2018). The lower bond dissociation enthalpy (BDE) leads to the formation of more phenoxy radicals. The presence of para- or ortho-substituted electron-donating groups, including OH, OCH3 and CH3 reduce the BDE (see Table  5.2  - BDE values reference from Wei et  al. (2004)) to improve the stability of the phenoxyl radical and, therefore, increase antioxidant activity (Anouar et al. 2013). However, the stronger the ability of substituents to donate electrons, the lower the radical scavenging capacity is (Son and Lewis 2002; Bendary et al. 2013; Cesari et al. 2019). For example, the OH groups at an ortho position can donate more electrons than those at an para position. In line with this, the radical scavenging capacity of o-cresol is lower than that of p-cresol (Cesari et al. 2019).

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Table 5.2  Bond dissociation enthalpy (BDE) values of various phenolic compounds Compounds 4-(1-Hydroxyethyl) phenol

Structures CH3 HC OH

BDE (kJ/mol) 87.0

Compounds p-Coumaric alcohol

Structure CH2OH HC

OH

OH

4-Hydroxybenzaldehyde

H

O

C

90.3

CH3

p-Cresol

85.3

OH

OH

4-Hydroxybenzyl alcohol

CH

BDE (kJ/mol) 84.7

CH2OH

87.2

Phenol

88.0 OH

OH CH3

4-Methylsyringol

81.7

CH2OH

Sinapic alcohol HC

CH

80.9

OCH3

H3CO OH

OCH3

H3CO OH CH3 HC OH

a-Methylsyringyl alcohol

82.6

OCH3

H3CO

CH3 HC OH

C

O

85.0

OCH3

H3CO

OH

Apocynol

H

Syringaldehyde

OH

86.3

Syringol

OCH3

H3CO

83.3

OH OCH3 OH

CH2OH

Coniferol HC

CH

83.5

CH2OH

Syringyl alcohol

82.7

OCH3

H3CO OH

OCH3 OH

Creosol

CH3

84.8

Vanillin

H

C

OCH3

89.9

OCH3

OH

Guaiacol

O

OH

OCH3

86.6

Vanillyl alcohol

CH2OH

OH

86.1

OCH3 OH

5.3  Preparation of Lignin With High Antioxidant Activity As indicated above, the high antioxidant activities of lignin are determined by several factors, such as large amounts of certain functional groups (carbonyl, carboxyl, methoxyl and phenolic hydroxyl), high homogeneity, low Mw and a small content of carbohydrate impurities. These factors depend on the origin, pretreatment,

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extraction and chemical modification of lignin (García et al. 2012; Qin et al. 2018c). Using this knowledge, several studies have been conducted to obtain lignin with high antioxidant activity. García et  al. (2010) studied the effects of different separation methods on the antioxidant activities of lignins from Miscanthus sinensis. The lignin fraction obtained by organosolv fractionating (OL) exhibited the highest free radical scavenging activity, followed by lignins obtained by autohydrolysis and alkali treatment. The alkali treatment had more carbohydrate residues than the organosolv fractionating. Hydrogen bonds between phenolic hydroxyl groups of lignin and carbohydrates lead to low antioxidant activity. Lignin with low hydroxyl contents has a high compatibility with the thermoplastic matrix, so it can be used as a good thermal stabilizer for polymers. Wen et al. (2013) analyzed the structure of lignin isolated by three different extraction processes, namely milled wood lignin (MWL), alkali lignin (AL) and dimethylsulfoxide/N-methylimidazole-dissolved lignin (DL). The contents of β-O-4 bonds in these three samples had the following order: DL  aliphatic OH groups ≈ total phenolic OH groups > condensed phenolic OH groups > polydispersity. The antioxidant activity of lignin in the KL/PP mixture was affected by its compatibility with PP (Chen et al. 2018b). To improve its solubility in PP, lignin can be modified by butylation reactions. According to Ye et al. (2016), the compatibility of KL/PP mixture increased with the degree of lignin butylation. However, extensive butylation alters the antioxidant properties (thermal-­ oxidative stability) of the mixture. Thus, the conflict between the compatibility and antioxidant capability must be resolved. Aminolysis provides an effective approach to treat this problem. As shown in Fig. 5.9, with pyrrolidine, the phenolic ester in lignin can be unblocked much faster than the aliphatic ester. Ye et al. (2018) reported that after the selective aminolysis of acetylated KL, phenolic OH group content of lignin increases and its aliphatic OH groups were strongly blocked by the acetate group. This allowed a good dispersion of lignin into PP, and therefore, improves the thermal anti-oxidation properties without affecting the mechanical properties of the mixtures (Ye et  al. 2018). Gadioli et  al. (2014) prepared PP composites using

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O O O

dioxane fast

O

N R1

R2 OH

R2 O

R1 O

O

N H OH

dioxane slow

N R1

R2

O

OH

Fig. 5.9  Fast aminolysis of phenolic and slow aminolysis of aliphatic acetoxy groups with pyrrolidine

cellulose fibers with various lignin contents. The presence of lignin not only improved the thermo-oxidation stability of the PP matrix but also improved fiber/ matrix adhesion (Gadioli et al. 2014). On the other hand, due to the large amounts of unsaturated double bonds in its molecular structure, natural rubber (NR) is very susceptible to oxidation. For this reason, the addition of lignin as a filler can effectively protect the thermal-oxidative degradation of NR. Barana et al. (2016) indicated that the solubility and diffusion of lignin into NR can affect its antioxidant effect. The higher Mw lignin also had a lower diffusion rate in the NR, which led to lower thermal stability. In the case of a good dispersion, a linear correlation between the thermooxidative stability and the ratio of phenolic OH to peak Mw was observed. In addition, the increase in lignin concentration did not always improve the thermal resistance in the lignin/NR mixture. At low concentrations, the protection time has a linear relationship with the concentration of lignin, while at high concentrations, the linear relationship was stabilized, thus indicating saturation (Barana et al. 2016).

5.6  Challenges of Integrating Lignin into Polymers The antioxidant, physical and mechanical characteristics of the polymers can be improved by the addition of lignin. However, the heterogeneous and complex structures of lignin do not lead to their interaction with the polymer matrix (Sadeghifar and Argyropoulos 2016). This problem compromises certain mechanical properties and the thermal stability of the copolymers. Two methods can be used to overcome

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this problem: 1) by chemical modifications (e.g. esterification, acetylation, and alkylation) and 2) by organic solvent fractionation, which can selectively separate the lignin fractions with low polydispersity by changing the solvent polarity. Lignin also has little compatibility with non-polar polymers such as PE, due to a large number of polar groups into lignin structure (Yeo et al. 2015). Poor compatibility between the polymer and lignin can restrict its antioxidant activity (Avelino et al. 2019). Poor compatibility can also have a negative influence on the mechanical properties of the composites (Dehne et al. 2016). Some strategies to improve the compatibility of lignin with polymers have been those previously indicated. Although other alternatives can also be highlighted such as the addition of coupling agents (e.g. maleic anhydride-grafted PP and ethylene-vinyl acetate) or lignin grafting of with non-polar chains (Ye et al. 2016). Modified lignin has been shown to have good compatibility and dispersibility with the polymers and can be used for many purposes, such as making biomedical, packaging and thermoplastic materials, as well as asphalt and cosmetics.

5.7  L  ignin as Raw Material for the Production of Antioxidants From the point of view of the chemical structure, lignin is composed of phenolic units and is mainly degraded to phenolic compounds with low Mw under hydrothermal or pyrolysis conditions (Kang et  al. 2015; Larson et  al. 2017). Some of the phenolic products derived from the lignin decomposition have high antioxidant activities and can be used for the production of commercial antioxidants (Kang et al. 2015). This means lignin can be used as a source for the production of antioxidants. Kang et  al. (2015) reported that AL products obtained after hydrothermal liquefaction at 320  °C for 30  min. showed better antioxidant capacities than the original AL. Cesari et al. (2019) found that eight phenolic compounds derived from the depolymerization of lignin, namely guaiacol, m-, o- and p-cresol, phenol, pyrocatechol, syringol and vanillin, exhibited a good radical scavenging and reducing powers. Larson et al. (2017) extracted phenols from the dichloromethane-soluble portion of the pyrolyzate. Dimeric fractions such as α- and ω-dicatechols and diguaiacols are the main products responsible for their high antioxidant properties. The high antioxidant capacities of these lignin-derived products suggest they could be potentially used as commercial antioxidants.

5.8  Conclusions and Future Perspectives A clear understanding of the structure-antioxidant activity relationship will contribute to the production of lignin with high antioxidant capacity, an effective chemical modification and a wider use. Lignin has already found many applications in a wide variety of fields, but some of its properties still undermine its use. These

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characteristics are its innate lack of homogeneity, hydrophobicity, and bioincompatibility. The inhomogeneity of lignin hinders its large-scale application in industry. Therefore, new technologies still need to be developed to obtain high-purity homogeneous lignin. Due to its hydrophobic nature, the solubility of lignin in water should be considered for its wider applications in the future. Biocompatibility is a key problem that limits the application of lignin. Measures to improve biocompatibility between lignin and the polymers should be developed without affecting the mechanism of the copolymers. The antioxidant efficacy of lignin in several phases may be different. It is thus necessary to evaluate the antioxidant properties in the corresponding systems. More than anything, in view of its potential use in the biomedical and food fields, the toxicity of lignin-based materials should be evaluated in detail. Acknowledgments  We sincerely acknowledge the financial support by Post-doctoral Research Start-up Fund of Henan University of Technology (21450009), the earmarked fund for Modern Agro-industry Technology Research System (CARS14-1-29), the earmarked fund for National Natural Science Foundation of China (U1804111) and Technological Innovation Talents of Colleges and Universities (19HASTIT012). Conflicts of Interest  The authors declare no conflict of interest.

References Abu-Dalo, M. A., Al-Rawashdeh, N. A. F., & Ababneh, A. (2013). Evaluating the performance of sulfonated Kraft lignin agent as corrosion inhibitor for iron-based materials in water distribution systems. Desalination, 313, 105–114. https://doi.org/10.1016/j.desal.2012.12.007. Aguié-Béghin, V., Foulon, L., Soto, P., Crônier, D., Corti, E., Legée, F., Cézard, L., Chabbert, B., Maillard, M.-N., Huijgen, W. J. J., & Baumberger, S. (2015). Use of food and packaging model matrices to investigate the antioxidant properties of biorefinery grass lignins. Journal of Agricultural and Food Chemistry, 63(45), 10022–10031. https://doi.org/10.1021/acs. jafc.5b03686. Álvarez, K., Famá, L., & Gutiérrez, T.  J. (2017). Chapter 12. Physicochemical, antimicrobial and mechanical properties of thermoplastic materials based on biopolymers with application in the food industry. In M.  Masuelli & D.  Renard (Eds.), Advances in physicochemical properties of biopolymers: Part 1 (pp.  358–400). EE.UU.. ISBN: 978-1-68108-454-1. eISBN: 978-1-68108-453-4, 2017: Bentham Science Publishers. https://doi.org/10.217 4/9781681084534117010015. Aminzadeh, S., Lauberts, M., Dobele, G., Ponomarenko, J., Mattsson, T., Lindström, M. E., & Sevastyanova, O. (2018). Membrane filtration of kraft lignin: Structural charactristics and ­antioxidant activity of the low-molecular-weight fraction. Industrial Crops and Products, 112, 200–209. https://doi.org/10.1016/j.indcrop.2017.11.042. An, L., Wang, G., Jia, H., Liu, C., Sui, W., & Si, C. (2017). Fractionation of enzymatic hydrolysis lignin by sequential extraction for enhancing antioxidant performance. International Journal of Biological Macromolecules, 99, 674–681. https://doi.org/10.1016/j.ijbiomac.2017.03.015. An, L., Si, C., Wang, G., Sui, W., & Tao, Z. (2019). Enhancing the solubility and antioxidant activity of high-molecular-weight lignin by moderate depolymerization via in situ ethanol/ acid catalysis. Industrial Crops and Products, 128, 177–185. https://doi.org/10.1016/j. indcrop.2018.11.009.

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Chapter 6

Functional Biobased Composite Polymers for Food Packaging Applications Hulya Cakmak and Ece Sogut

Abstract  Biobased polymers are of great interest due to the release of tension on non-renewable petroleum-based polymers for environmental concerns. However, biobased polymers usually have poor mechanical and barrier properties when used as the main component of coatings and films, but they can be improved by adding nanoscale reinforcing agents (nanoparticles  - NPs or fillers), thus forming nanocomposites. The nano-sized components have a larger surface area that favors the filler-matrix interactions and the resulting material yield. For example, natural fibers from renewable plants could be used to improve the mechanical strength of the biobased composites. In addition to the mechanical properties, the optical, thermal and barrier properties are mainly effective on the selection of type or the ratio of biobased components. Biobased nanocomposites are one of the best alternatives to conventional polymer composites due to their low density, transparency, better surface properties and biodegradability, even with low filler contents. In addition, these biomaterials are also incorporated into composite films as nano-sized bio-fillers for the reinforcement or as carriers of some bioactive compounds. Therefore, nanostructures may provide antimicrobial properties, oxygen scavenging ability, enzyme immobilization or act as a temperature or oxygen sensor. The promising result of biobased functional polymer nanocomposites is shelf life extension of foods, and continuous improvements will face the future challenges. This chapter will focus on biobased materials used in nanocomposite polymers with their functional properties for food packaging applications. Keywords  Montmorillonite · Nanocellulose · Nanocomposite · Nanoparticles · Nanoreinforcement

H. Cakmak (*) Department of Food Engineering, Faculty of Engineering, Hitit University, Corum, Turkey e-mail: [email protected] E. Sogut Department of Food Engineering, Faculty of Engineering, Suleyman Demirel University, Isparta, Turkey © Springer Nature Switzerland AG 2020 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_6

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6.1  Introduction The main purpose of food packaging is to create a protective barrier against the transfer of moisture, water vapor and gases (O2, CO2 and ethylene), protecting food against contamination from the storage environment, and in turn providing information about the ingredients, expiration date and other commercial information about the company which helps advertising for consumers, therefore, it has a communication function that allows traceability and safe disposed after use (Marsh and Bugusu 2007; Robertson 2013; Álvarez et  al. 2017). In the early times, natural materials such as plant leaves, earthen pots, gourds and baskets were used for food preservation and storage (Risch 2009; Suárez and Gutiérrez 2017). Industrial revolutions and the increases in the rates of mass food production has helped improve packaging materials and technology, such as tin cans, glass bottles and jars, paper/board boxes, flexible plastic packaging, retort pouches are evolved, and today 100% biodegradable packaging materials from natural biomasses are replacing the current alternatives from non-renewable resources and unsustainable technologies (Brody et al. 2008; Gutiérrez 2018a). According to the statistics published by European Bioplastics Organization (2018), 1200 kilotons of total bioplastics were assigned to biobased/non-­ biodegradable sources, while biodegradable plastics correspond to 912 kilotons of the total bioplastics produced. The largest market share of biobased non-­ biodegradable plastics belongs to polyethylene terephthalate (PET, 26.6%), and the largest amount of biodegradable bioplastics belongs to starch mixtures (18.2%) and polylactic acid (PLA, 10.3%). However, the current values of bioplastics only correspond to 1% of total plastic production. Their projections, which include 2023, shows that the market shares of bioplastics increase linearly and will reach to 1288 kilotons for biodegradable bioplastics and 1328 kilotons for biobased non-­ biodegradable ones. Recently, global food companies are setting targets for the conversion available to 100% recyclable plastic packaging by 2025 as a contribution to the circular economy, and even some of their products currently on shelves are already become biodegradable sources and natural biomass. The food industry has also focused on the development of biobased packaging materials that have active properties to preserve and extend the shelf life of food products (Gutiérrez et  al. 2017a; Álvarez et  al. 2018). However, biobased films have inherent disadvantages, such as poor water vapor barriers and high moisture sensitivity which also affect their mechanical properties (Gutiérrez et al. 2016a, b; Herniou--Julien et al. 2019). It is thus required to improve the performance of biopolymers in terms of their mechanical, barrier and thermal properties, as well as processability to replace traditional polymer (Gutiérrez 2018b,c,d). In general, the use of small amounts of nano-fillers has proven effective in improving the barrier and mechanical properties of biopolymers to form films and coatings with satisfactory properties (Khan et  al. 2014; Toro-­ Márquez et  al. 2018; Gutiérrez et  al. 2019). The most recent approaches include biobased plastics with nanoparticles (NPs) and functional biobased active layers

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and the use of nanocomposites as carriers of active compounds such as antioxidants and antimicrobials (Yildirim et al. 2018; Merino et al. 2018a, 2019a). Biobased composite/nanocomposite films for food packaging applications made from natural biomass (polysaccharides, proteins and lipids and waxes), and the effects of functional nanomaterials incorporated into the composites are discussed in this chapter.

6.2  Biobased Polymers 6.2.1  Polysaccharide Biomass Biomass based on polysaccharides such as starch (native, thermoplastic etc.), cellulose and its derivatives (carboxymethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methylcellulose, methyl cellulose), alginate, chitin, chitosan (Cs), maltodextrin, agar, pectin, carrageenan, heparin, chondroitin, glucomannan, pullulan, kefiran, curdlan and gums (gellan, guar, locust bean, mesquite, tara) can be used as pure polymer, polymer matrix or filler (Miller and Krochta 1997; Tharanathan 2003; Lacroix and Le Tien 2005; Falguera et al. 2011; Bonilla et al. 2012; Cirillo et al. 2015; Zia et al. 2015; Cazón et al. 2017; Gutiérrez 2017a; Araque et al. 2018). The mostly studied polysaccharide biomasses and their properties are summarized below. Starch is composed of two macromolecules: amylose and amylopectin. Amylose is a linear polymer consists of D-glucose units linked with α-1-4 bonds, while amylopectin consist of both linear glucose segments with α-1-4 bond and α-1-6 linked glucose segments at the branching points (Pérez et  al. 2009; Bertolini 2010; Gutiérrez et al. 2014). The crystalline structure of native starch is attributed to amylopectin, while the crystalline and amorphous lamellas of amylopectin are packed in larger structures called block-lets (Bertolini 2010; Gutiérrez 2018e). However, the amorphous structure is dominant in the starch granules, since the amylose and most of the amylopectin are formed by an amorphous structure (Pérez et  al. 2009). Temperature, pressure and pH may limit the film forming capacity of native starches regarding sensitivity to high shear, decomposition with temperature, retrogradation and syneresis (Bertolini 2010; Jiménez et  al. 2012; Gutiérrez and Álvarez 2016; Gutiérrez and González 2016). Thus, the semi crystalline structure of starch is transformed into a homogeneous amorphous matrix to improve processing ability (Bertolini 2010). Applications of mechanical or thermal energy are required together with the presence of water for starch destructuring and, therefore, the production of thermoplastic starch (TPS) (Bertolini 2010; Thiré 2010; Puthussery et  al. 2015; Gutiérrez et al. 2018a; Gutiérrez and Alvarez 2018). In addition to water, polyols such as glycerol and sorbitol provide the plasticizing effect, and increase flexibility and decrease the brittleness in films (Thiré 2010; Jiménez et  al. 2012; Medina Jaramillo et  al. 2016). Water, glycerol, sorbitol, propylene glycol, polyethylene

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glycol and other polyols, oligosaccharides, fatty acids and lipids are used as plasticizers for the formation of TPS (Bertolini 2010; Campos et al. 2011; Puthussery et al. 2015; Jawaid and Swain 2017). Plasticizers improve the oxygen permeability of extruded TPS. However, it may become susceptible to moisture, which accelerates the biodegradation of films (Siracusa et al. 2008). In addition, the incorporation of nano fillers such as phyllosilicates, cellulose/Cs based NPs, carbon nanotubes, metal oxides and other materials may improve the mechanical strength, water and gas barrier properties of starch based films (Xie et  al. 2013; López et  al. 2015; Gutiérrez and Alvarez 2017a). Native and thermoplastic starch is generally used in biodegradable packaging formulations due to its low cost, abundant sources and safe functions for the environment (Thiré 2010; Lagarón 2011). Since the starch is cheap, renewable and has widely available sources, researchers are focused on functional properties of starches from different origins such as ahipa, cassava (called also tapioca), corn, elephant foot yam, palm, potato, sugar sago and wheat (Gutiérrez et  al. 2015a,b,c,d). The mechanical properties of the matrix may change depending on the origin of starch (Medeiros et al. 2010; Jiménez et al. 2012). Cellulose is composed of linear β-1,4 linked D-glucopyranose units, which is the main component of plant cell wall materials (Khan et al. 2014; Bracone et al. 2016). In addition to its extraction from plant sources such as woods, cotton, hemp, agricultural waste, aquatic plants and grasses, cellulose is also produced by Gluconacetobacter species in form of bacterial nanocellulose (Abdul Khalil et al. 2012; Khan et al. 2014; Azeredo et al. 2017). The purity, high mechanical strength, hydrophobicity and better gas barrier properties make bacterial cellulose more advantageous than plant-based alternatives (Khan et al. 2014; Cazón et al. 2017). However, the moisture sensitivity of bacterial nanocellulose impairs its oxygen barrier properties, especially at high relative moisture (Nair et al. 2014). After starch, cellulose is the second most abundant, cheap and renewable source for food packaging applications (Vilarinho et al. 2018). Nanostructures from cellulose such as cellulose nanofibrils, nanocrystals and nanowhiskers can participate as fillers of composite films due to their supreme mechanical and barrier properties due to their good dispersion into the polymer matrix (Khan et al. 2014). In addition to cellulose, hydroxypropyl methylcellulose and methylcellulose form transparent and flexible films with a good gas and lipid barrier structure (Cazón et al. 2017). Cs is a linear polysaccharide consisting of 1,4-linked 2-amino-2-deoxy-β-Dglucan, and the amino group can form interactions with anionic groups in an acidic environment (Xu et  al. 2005). It is the deacylated derivative of chitin in alkaline medium (Kumar 2000; Elsabee and Abdou 2013). The biodegradable, non-toxic, biocompatible, antimicrobial and good film-forming nature of Cs makes it favorable for food packaging applications (Campos et al. 2011; Cazón et al. 2017; Merino et al. 2018b, 2019b). Cs has selective gas permeability and mechanical properties in addition to having a wide range of antimicrobial activity against bacteria, yeast and molds (Elsabee and Abdou, 2013; Cazón et al. 2017). Cs is not thermally stable, therefore the processes that require heat applications such as extrusion, molding or heat sealing are not favorable for the production of

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Cs-based films (Cazón et  al. 2017). In addition to improving the mechanical strength, water vapor permeability (WVP) can also be improved by mixing of Cs with other polysaccharides (alginate, cellulose, pectin, starch) or proteins (gelatin, whey proteins) (Xu et al. 2005; Elsabee and Abdou, 2013; Cazón et al. 2017). Alginate is a linear copolymer of β-(1–4) linked D-mannuronic acid and α-(1–4) linked L-guluronic acid extracted from brown algae (Abdollahi et al. 2013; Cazón et  al. 2017). The alginate films have good mechanical strength but have a brittle nature with high WVP. However, the crosslinking of the alginate with polyvalent cations improve its WVP, increase its mechanical strength and improves the controlled release of incorporated functional materials (Campos et  al. 2011; Cazón et al. 2017). Pullulan is a water-soluble exopolysaccharide which is synthesized by Aureobasidium pullulans (Cirillo et  al. 2015; Chen et  al. 2017). Pullulan-based films are biodegradable and non-toxic, as well as have good oxygen permeability, transparency, oil and grease resistance (Tharanathan 2003; Chen et  al. 2017). Pullulan can be extruded as a film with good mechanical stability (Tharanathan 2003; Chen et al. 2017). Kefiran, like pullulan, is also a water-soluble exopolysaccharide produced by the microorganisms present in kefir grain flora (Motedayen et al. 2013). It has a good film-forming capacity depending on the plasticizer used (Ghasemlou et al. 2011). Due to their hydrophilic nature, the polysaccharide films are permeable to water vapor (Lacroix and Le Tien 2005). Although the WVP values of kefiran films are similar or even better than the corn starch and Cs-based films (Piermaria et al. 2009).

6.2.2  Protein Biomass The protein biomasses used in biobased composite film formulations include whey proteins, casein, gluten, soy protein, lentil, peanut, mung bean, quinoa, canola proteins, kafirin, zein, egg albumen, collagen, fibrin, fibroin, spidroin, gelatin (fish skin, other animal origin), myofibrillar protein, keratin (hair), sericin (silk) (Baldwin et al. 1995; Miller and Krochta 1997; Anyango et al. 2011; Plackett 2011; Abugoch et al. 2016; Doblhofer et al. 2016; Cazón et al. 2017; Li et al. 2017; Garrido et al. 2018; Tomadoni et al. 2020). Whey proteins have good gas barrier properties, good film-forming capacity, biodegradability, elasticity, transparency and can act as a potential carrier of antioxidant and antimicrobial agents, therefore, they are the best alternative among other protein sources (Miller and Krochta 1997; Sothornvit et al. 2009; Ramos et al. 2012; Oymaci and Altinkaya, 2016). In addition, whey proteins have a nature compatible with metal oxide NPs, nanocellulose, zein NPs, nanoclays and are suitable for crosslinking or lamination with lipid matrix (Miller and Krochta 1997; Bourtoom 2009; Sothornvit et  al. 2009; Zolfi et  al. 2014; Oymaci and Altinkaya 2016; Qazanfarzadeh and Kadivar 2016).

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The WVP and mechanical strength of protein-based films are comparably lower than the synthetic polymers such as low-density polyethylene (LDPE), ethylene vinyl alcohol (EVOH) and cellophane. This is due to the hydrophilic structure of proteins (Bourtoom 2009; Hernandez-Izquierdo and Krochta 2008). However, gas (O2, CO2, and aroma) and water permeability of films are improved when the pH of the solution is higher than the isoelectric point of the proteins (Olivas and Barbosa-­ Cánovas 2005). In addition, the inclusion of NPs such as nanocellulose, montmorillonite (Mt) and silver (Ag) NPs improves the water and oxygen barrier properties, mechanical strength and thermal stability, in addition to providing antimicrobial properties for the protein-based nanocomposite films (Bae et al. 2009; Kumar et al. 2010; Pereda et  al. 2011; Kanmani and Rhim 2014; Qazanfarzadeh and Kadivar 2016). Zein films have a strong and glossy structure, resistant to grease, and cross­linking or the addition of glycerin improves the mechanical properties of these films (Tharanathan 2003). Although the brittleness is reduced with the addition of plasticizer, the barrier properties (water vapor and gas) of zein-based films are negatively affected (Arora and Padua 2010). Similar to zein, the addition of plasticizer reduces the brittleness of whey protein films (Plackett 2011). But the type and ratio of plasticizer together with the storage environment are effective on oxygen permeability values of whey protein-based films (Miller and Krochta 1997; Ramos et al. 2012).

6.2.3  Lipid and Wax Biomass The first development of food coating was the manufacture of wax-based coating on fruits to minimize dehydration and provide a bright appearance (Ansorena et  al. 2018). Fat and oil-based biomasses such as bees, candelilla, carnauba and rice bran waxes, lecithin, shellac, mineral and vegetable oils, stearic acid, lauric acid, glycerin, triolein, fatty acid sucrose esters and resins are examples of lipids and waxes that are used in food packaging applications (Baldwin et al. 1995; Morillon et al. 2002; Rhim and Shellhammer 2005; Bonilla et  al., 2012; Robertson, 2013). Synthetic waxes such as paraffin and petroleum waxes are also used for food packaging (Robertson 2013). In addition to using the lipids as a matrix in biocomposites, essential oils from plants are used as antimicrobial agent in composite films (Debeaufort et al. 2000; Morillon et al. 2002; Bourtoom 2009). Lipids and waxes are used in the composite films for their barrier properties against water vapor (Rhim and Shellhammer 2005). The long fatty acid chains are effective in reducing WVP values. However, fatty acid chains with more than 18 carbon atoms cause a negative effect on WVP values, due to heterogeneity in the polymer structure (immiscible mixtures) (Morillon et al. 2002). Self-standing lipid films have low processability due to their low mechanical strength (Rhim and Shellhammer 2005). Although its strength may be improved with the addition of hydrocolloids, either with lamination or making emulsions (Falguera et al. 2011). Lipid-based films and waxes have low moisture permeability due to their

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Fig. 6.1  Schematic representation of tactoid (A), intercalated (B) and exfoliated (C) structures

hydrophobic nature (Morillon et al. 2002). Polymorphism and crystalline structure influences the WVP of films formed by lipids and waxes (Morillon et al. 2002). The water vapor transmission rate (WVTR) had also been comparably lower when solid fat is used as a layer in the case of lipids in bilayer films (Debeaufort et al. 2000).

6.3  Nanoreinforcement There are three types of biobased nanocomposite configurations: phase-separated (tactoid), intercalated, and exfoliated (Rhim et al. 2013). These three main configurations are presented in Fig. 6.1. Tactoid structure is formed when a polymer and a filler are immiscible due to their poor chemical interactions, and as a consequence the filler layers are not separated (Tang et  al. 2012). The tactoid structure that predominates in conventional composite materials, which results in the stacking of the filler in the matrix, thus causing poor properties for the material (Alexandre et al. 2009). The intercalated structures are obtained by direct extension of the filler to create spaces between the layers when the polymer chains enter the primary space of the filler. The intercalation arises from the permeation of the polymer chains within the filler layers. This results in a well-organized multilayer structure containing polymer/filler layers with recurrent distances (Weiss et al. 2006). The exfoliated nanocomposites are formed when the polymer chains penetrate into the arranged and randomly dispersed filler layers in the polymer matrix (Ludueña et al. 2007). These structures are obtained after filler loses its layered structure and is isolated into single sheets within the constant polymer phase due to the strong chemical interactions between the polymer and the filler (Turan et al. 2018). Exfoliation is the best way to obtain an ideal interaction between the filler and polymer matrix (Adame and Beall 2009; Azeredo 2009). The use of inorganic nano-fillers for the manufacture of nanocomposites has gained interest thanks to its distinctive properties which are suitable for numerous industrial applications. Although many nano-fillers have potential to improve the performance of polymers, the packaging industry has concentrated on using layered inorganic fillers such as clays and silicates, due to their availability, low cost and comparatively simple processability. Its presence in polymer formulations increases the tortuosity, thus a penetrating molecule is forced to diffuse through a longer

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pathway within the film, and the resulting film has excellent barrier properties (Adame and Beall 2009). The mechanical strength of biopolymer films is also improved, thus increasing the glass transition temperature (Tg) and thermal degradation temperature, while some studies have reported a decrease in transparency as minor disadvantages of fillers on polymer (Yu et al. 2003; Petersson and Oksman 2006; Weiss et al. 2006; Cyras et al. 2008). In this section, the use of inorganic fillers (e.g. clay minerals) and natural fillers (e.g. cellulose based nanostructures) in the nanocomposite films will be discussed.

6.3.1  Clays and Silicate-Based Fillers Clay minerals contain tetrahedral and octahedral sheets which are negatively charged or neutral layers of hydrated aluminum phyllosilicates (Murray 2000; Unalan et al. 2014; Gutiérrez et al. 2017b). The superimposition of tetrahedral and octahedral sheets creates layered structures, which then form a layer called platelet. The isomorphic replacement of aluminum and/or silicon with a lower-valence atom causes a negative surface charge for minerals. In some clay groups, the negative surface charge is balanced with positive inorganic ions found in the interlayer (Unalan et al. 2014). The cations have an important effect on the swelling behavior of clay when in contact with water molecules. In general, two arrangements of main sheets are observed, namely 1:1 and 2:1, in nanoclays (Unalan et  al. 2014). The nanoclays are grouped as smectite, kaolinite, halloysite, etc., according to their morphology and chemical structure (Fig. 6.2). Mt is the most commonly used filler in the formation of nanocomposites. The surface of Mt is negatively charged because the trivalent Al-cation is partially replaced with the Mg divalent cation. Sodium and calcium ions, which are hydrated between the layers, balance the charge (Manias et al. 2001). The weak forces that hold layers together allow water and other polar molecules to penetrate between the layers that expand the matrix (Chin et al. 2001). In addition to improving the tensile strength and gas barrier properties of composite films, modified Mt particles also show an antibacterial effect that increases the potential use for food packaging applications (Sothornvit et al. 2009; Souza et al. 2012; Kanmani and Rhim 2014). However, the migration of the metal from packaging film to the food due to the addition of clay should be observed. Kaolinite is a the crucial mineral among the group of kaolin clays, and is a layered silicate consisting of a tetrahedral sheet with dioctahedral sheet layers (Murray 2000). The structure of kaolinite provides great cohesive energy regarding hydrogen bonds between adjacent layers (Sanchez-Garcia et  al. 2008a). In addition to its lower aspect ratio, kaolinite has a low absorption capacity, associated with the low surface area and the minimal layer loading (Krishnamachari et al. 2009). Although it is inexpensive, abundant and environmentally friendly, kaolinite is rarely used for the preparation of nanocomposites due to the difficulties faced by intercalation of polymers. Within the kaolin group, halloysites represent a dominant form of hollow

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Fig. 6.2  Schematic representation of clay groups according to the type of layer: 1:1 (A) and 2:1 (B)

tubes formed by two-layered aluminosilicate clay (Yah et al. 2012). Halloysite has attracted the attention of researchers because they can readily be dispersed into polymers without exfoliation due to the tubular shape and less surface hydroxyl groups (Du et al. 2010). Halloysites can be used in the composites to release active agents such as antimicrobials, oxygen scavengers, etc. Double layered hydroxides are also known as non-silicate oxides and hydroxides with similar properties to those of clay minerals (Forano et al. 2006). Hydrotalcite is one of the most characteristic minerals in this group. The hydroxide layers have a positive surface charge contrary to layered silicates. In these layers, the Mg atoms are replaced by Al atoms that are counteracted by anions located between consecutive layers (Sorrentino et al. 2005). Many of biobased components are negatively charged, therefore, hydrotalcites are ideal fillers for biobased polymers (Rives 2001). Mica is a group of phyllosilicate minerals, which can be classified into two groups: flexible micas and brittle micas. In flexible micas, the negatively charged tetrahedral sheet is balanced by a monovalent cation located between layers (generally potassium) while in brittle micas, a higher negative charge of the tetrahedral sheet is replaced by a divalent cation such as calcium (Unalan et  al. 2014). The cations presented between the mica layers are anhydrous and do not have an internal surface area similar to pyrophyllite.

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The talc has a plate-like structure with magnesium-oxygen/hydroxyl octahedral layers packed in two layers of tetrahedral silica (López et al. 2015). Highly lamellar talc is associated with large single platelets, while microcrystalline talc has small platelets. Talc has naturally poor interactions with nonpolar materials and has a high surface energy (Gorrasi et al. 2018). Talc is mostly used to improve thermal resistance, mechanical properties and dimensional stability of the films (Sakthivel and Pitchumani 2011). Zeolites are aluminosilicates in the form of hydrated crystals with a highly porous structure due to the system of three-dimensional channels. Zeolites have univalent and divalent cations. Water molecules are the other components located in the system of channels that give the exchange capacity and dehydration/rehydration capacity to zeolites (Gascon et al. 2012). The underlying reason for the addition of zeolites into the polymer is to form mixed-matrix membranes and improve the mechanical and thermal properties.

6.3.2  Metallic Nanostructures Ag NPs are one of the most used metals for the production of active biobased nanocomposites, due to their antimicrobial properties against a wide variety of microorganisms (Dallas et al. 2011). The widely recognized mechanisms of antimicrobial activity are: (1) the interaction of Ag+ ions with negatively charged nucleic acids contributes to the disruption of metabolic processes, the disintegration of cell wall causing the cell death (Kanmani and Rhim 2014), and (2) the membrane binding to the surface causes morphological changes and then the structural integrity is lost (Sondi and Salopek-Sondi 2004; Álvarez et al. 2018). Ag NPs are increasingly used in the formation of nanocomposites for food packaging applications, since it provides slower release rates with lower acute antimicrobial responses due to the high aspect ratio and, thus improving its surface reactivity (Egger et al. 2009). Copper is another well-recognized nano-sized particle that exhibits a lower biocidal activity compared to Ag ions. However, the use of copper in nanocomposites is restricted because it is considered as toxic when in contact with food and increases the oxidation rate of the food product (Fernández et al. 2010). Metal oxides (e.g. MgO, TiO2, and ZnO) also show antimicrobial activity against various microorganisms, such as bacteria, yeasts and molds. Nanocomposites containing titanium dioxide (TiO2) NPs show antimicrobial properties thanks to their photocatalytic activity, related to their crystal structure. The antimicrobial effect of TiO2 NPs is associated with the generation of O2 and hydroxyl radicals after irradiation at higher energies than the band gap. The organic molecules are then oxidized by reactive oxygen species that lead to cell death (Llorens et al. 2012). In addition to their antimicrobial activity, TiO2 NPs protect food products against the oxidizing effect of ultraviolet (UV) irradiation, as well as maintain optical clarity related to their efficient short-wavelength light absorbing properties (Duncan 2011). Zinc

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oxide (ZnO) particles have an antimicrobial activity similar to that of other metal oxides, and are preferred for its low cost and its UV-blocking properties.

6.3.3  Carbon-Based Nanomaterials Graphene is the building unit of graphite. The exfoliated graphite nanoplatelet or graphite nanoplatelet is a three-dimensional layered mineral allotrope and contains a stack of more than 10 layers of carbon atoms (Gorrasi et al. 2018). A thinner version of graphene is known as graphene nanosheets and has different derivatives that are presented as oxide, reduced oxide and nanoplatelets. Among them, graphene oxide is rich in functional groups such as carboxyl, epoxide and hydroxyl, thus has a high potential to be used in biobased nanocomposite materials. Nanoplatelets and nanosheets of graphite have lower functional groups, and they are hydrophobic, therefore do not contribute to polar interactions or hydrogen bonding. In general, the exfoliation of graphite in layered graphene sheets is based on the disruption of van der Waals-like forces between the graphite layers. The use of graphene within a composite improves the mechanical and barrier performance, as well as electrical and thermal conductivity (Unalan et al. 2014). Carbon nanotubes are formed by concentric tubes entitled as single-wall or multi-wall nanotubes. These fillers show remarkably high aspect ratios and Young’s modulus (E) with a high reinforcing capacity of biobased polymers (Zhou et  al. 2004). These fillers are used not only for their performance to improve thermal/ mechanical/barrier properties of polymer composites, but also for their compatibility with chemicals, metal/metal oxide/chalcogenides NPs, and non-scattering electron transports (McEuen et al. 1999; Baur and Silverman 2007).

6.3.4  Polysaccharide Based Nanostructures Cellulose-based nano-fillers are classified into three groups: cellulose microfibers, nanocrystalline cellulose or cellulose nanowhiskers (Azizi Samir et  al. 2005). Cellulose nanofibers include a group of elongated molecules, which are stabilized by hydrogen bonds. Cellulose nanowhiskers consist of the crystalline part of the micro fibrils, which is obtained by acid hydrolysis (Gorrasi et al., 2018). The microfibrils are composed of crystalline and amorphous parts, which have nanosized diameters and micrometer size lengths (Oksman et  al. 2006). The lengths of nanowhiskers, which are also recognized as nanocrystals, nanorods or rod-like cellulose microcrystals, range between 500 nm – 2 μm, 8–20 nm or less in diameter providing high aspect ratios (de Souza Lima and Borsali 2004). The cellulose crystals had about 150  GPa of modulus and 10  GPa of strength, thus suggesting the potential use of cellulose instead of carbon nanotubes (Helbert et  al. 1996). The moisture resistance of biobased polymers has also been improved by cellulose

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nanoreinforcements (Paralikar et  al. 2008; Sanchez-Garcia et  al. 2008b; Svagan et al. 2009). The improvement in the barrier performance is related to the increase in the tortuosity that leads to slower diffusion and lower WVP values (Sanchez-­ Garcia et al. 2008b). Nanosized cellulose fibrils have also a potential to improve the thermal properties of biobased polymers (Helbert et al. 1996; Oksman et al. 2006; Petersson and Oksman 2006). Starch nanocrystals are one of the distinguished polysaccharide-based nano-­ fillers, which have been shown to improve the mechanical properties of biobased polymers when incorporated up to a 10 wt.% content (Aldao et al. 2018; Villa et al. 2019). The native starch granules are hydrolyzed below the gelatinization temperature to separate the crystalline lamellae, which are more resistant to hydrolysis (Azeredo 2009). The crystalline starch particles generally have a thickness of 6–8 nm and show a platelet morphology (Kristo and Biliaderis 2007). Chitin whiskers or Cs NPs have been used successfully to improve mechanical strength and barrier performance. Cs NPs have also improved thermo stability and reduced affinity with water when used in the nanocomposite formulations (Antoniou et al. 2015). Cs NPs can be produced by ionic gelation. In this technique, the positively charged amino groups of Cs, interact electrostatically with crosslinking polyanions such as tripolyphosphate (López-León et al. 2005).

6.4  Processing Techniques for Biobased Nanocomposites The biobased nanocomposites are obtained by incorporating nano-fillers into the biobased polymer matrices providing a considerable improvement of the mechanical, barrier, thermal and biodegradation properties of the films due to the favorable interactions between the polymer and the nano-filler (Xie et al. 2013). Three thermodynamically viable structures can be formed for these systems: phase separated (tactoid), intercalated or exfoliated (Ojijo and Sinha Ray 2013) as mentioned previously (see sect. 6.3 – Nanoreinforcement). The desired performance of the nanocomposite is influenced by many factors such as the fillers’ spatial arrangement, the morphology of the final product (polymer with nano-filler), the distribution of nano-­ filler through the polymer matrix and the interfacial interactions between filler and polymer (Unalan et  al. 2014). Although several strategies have been applied to develop biobased nanocomposites, the three most commonly used techniques are the following: (1) in-situ polymerization, (2) melt processing or (3) solution casting is mostly adopted (Alexandre and Dubois 2000). In addition, other preparation techniques, such as roll milling, high shear mixing, micro pattern approaches, using supercritical conditions and sonication have gained interest lately. In the following sections, we will focus on three main synthesis approaches and recent techniques to form biobased nanocomposites.

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6.4.1  In-Situ Polymerization In-situ  polymerization method includes the polymerization of monomers in the presence of the layered materials to begin the polymer formation (Unalan et  al. 2014). First, the nano-fillers are swollen in a monomer solution, and then the polymerization is initiated by different polymerization techniques such as a suitable catalyst or organic initiator, diffusion, heat or radiation applications before the swelling of the monomer (Koo 2006). The main disadvantage of this method is the phase separation or sedimentation that may occur depending on the incompatible structure of the organic polymer (proteins and polysaccharides) and the fillers (Chivrac et al. 2009). To overcome the problems of phase separation, the interaction between the polymer solution and the filler is improved to stabilize the nanoparticle dispersions by linking specific groups onto their surface (Althues et al. 2007).

6.4.2  Melt Processing Melt processing is commonly used for thermoplastic polymers and is considered economically viable and ‘green’ (solvent-free) techniques (Armentano et al. 2018). In the melt processing method, the nano-fillers are incorporated into the molten polymer, while being heated statically or under shear (Vaia et al. 1996; Vaia and Giannelis 1997). The solvent-free structure (especially organic solvents) and compatibility with existing industrial processes (e.g. extrusion and injection molding, etc.) make melt mixing process more advantageous compared to in-situ polymerization or solution-based techniques (Gutiérrez and Alvarez 2017b,c,d). In addition, the melt intercalation can be used for the biopolymers that are not suitable for in-­ situ polymerization. Two main factors that control the level of dispersion of the nano-fillers within the polymer matrix throughout the melt processing are: (1) enthalpy-driven interaction between the polymer and the nano-filler, and (2) processing conditions (Ojijo and Sinha Ray 2013). The polymer must be adequately compatible with nano-filler surface and there must be adequate enthalpic interaction between them to ensure a favorable dispersion. The interaction between the NPs and the polymer depends on the diffusion of polymer chains from the molten bulk through the filler layers, which in turn effects the formation of biobased nanocomposites (Vaia and Giannelis 1997). For the formation of biobased nanocomposites, the melt processing method has a disadvantage, which is the polymer degradation due to the high temperature and mechanical shear force. The processing instability results in the segmentation of polymer chains with a reduction of molecular weight due to the thermal, oxidative and hydrolytic degradations that occurs during processing. Therefore, the processing parameters should be optimized for the heat sensitive biobased polymers.

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6.4.3  Solution Based Approaches: Wet Chemistry Solution-based techniques are often preferred instead of melt mixing methods to develop biobased nanocomposites since the thermal degradation temperatures of biobased polymers are lower than their Tg (Xiong et al. 2018). These techniques are influenced by the physicochemical properties of the biomaterial solutions, and the following points must be taken into account: the selection of the solvent, the polymer concentration/solvent ratio, the external conditions such as pressure, temperature, and moisture during the solvent evaporation (Armentano et al. 2018). In this method, the selected polymer is dissolved in an appropriate solvent and then the dispersed nano-fillers (in the same or a different solvent) are mixed with the polymer solution to form a homogeneous dispersion. The choice of the suitable solvent provides a rapid exfoliation of the filler and the consecutive addition of polymer solution to the dispersed nano-fillers provides a strong interaction between them. Once the polymer chains are intercalated, the solvent within the interlayer of the filler is displaced and adsorbed onto the surface. The driving force to intercalate the biobased polymer into nano-filler layers from a solution is the entropy gained by desorption of the solvent molecules (Ojijo and Sinha Ray 2013). After evaporation of the solvent, the final intercalated structure executes a desired nanocomposite structure (Ojijo and Sinha Ray 2013). The frequently used solution based approaches include layer-by-layer (LbL) assembly, fiber spinning techniques, one-pot approaches and emulsion polymerization (Li et al. 2015; Wang et al. 2017; Xiong et al. 2018; Valencia et al. 2019). The LbL assembly is used to prepare thin multilayers and coatings that allow the control of the distribution, thickness and components of biomolecules in a single layer with the help of interactions taking place at the interface, such as hydrogen bonding, hydrophobic-hydrophobic interactions and electrostatic interactions (Ding et  al. 2017; Xiong et  al. 2018; Valencia et  al. 2019). A wide variety of biobased nanocomponents can be assembled into different biobased polymers through specific deposition techniques such as spray coating, spin coating and immersive/dip-­ coating (Richardson et al. 2015). These systems also allow the pH control to regulate the behavior of NPs into the polymer matrix. One-pot directed assembly includes cast drying and vacuum-assisted filtration techniques, which require a homogeneous mixture of the biopolymer solution and nano-filler dispersion (Wang et  al. 2014). In the vacuum-assisted filtration technique, the colloidal mixture of polymer solution and nano-filler dispersion is passed through a nano-sized pore filter while avoiding the aggregation of these nano-fillers with bonded biopolymers in the upper layer. In the solution-casting technique, the final film solution is poured into the dishes or poured on the different surfaces with different methods, such as spraying or bar coating to fabricate layered films after solvent removal (Xiong et al. 2018). Both techniques produce biobased nanocomposites with high compatibility, homogeneity and improved properties compared to LbL (Hu and Tsukruk 2015).

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Spinning techniques: wet spinning and electrospinning are conventional methods to obtain biobased nanocomposite fibers (Yan et al. 2010; García-Moreno et al. 2018). Wet spinning is suitable to produce large-scale biobased microfibers, while electrospinning produces extremely fine and specifically aligned nanofibers which can be used as a matrix or nanoreinforcement. Similar to the solution intercalation technique, the polymer nanocomposites are produced after dispersion of layered silicate in the aqueous phase (Koo 2006).

6.4.4  High Shear Mixing and Roll Milling The solid or liquid nano-fillers are mixed with the polymer solution using a high-­ shear equipment to prevent the aggregation of nano-fillers and disperse the polymer chains through the nano-filler layers. If the surface treated NPs are compatible with the selected polymer, an intercalated or exfoliated nanocomposite structure will be formed. Roll milling is another shear mixing technique which requires less shear stress compared to the high shear mixing technique. In this method, all components are mixed at room temperature based on shear to disruption of van der Waals interactions between layers (Sorrentino et al. 2005; Guo and Chen 2014). The energy transfer between the mills and the mixture of nanocomposites not only promotes the mixing and dispersion of the filler, but also maintains the intrinsic structure of the newly obtained layers.

6.4.5  Other Methods Freeze-drying is a dehydration process, which is used by freezing biobased nanocomposites in a solution or hydrogel (Rey and May 2010). This technique produces highly porous and ultralight weight bionanocomposite aerogels with respect to the preservation of the structure of nanocomposites in the wet state. The capillary force-­ induced collapse of the nanopores is prevented even when the sublimation of the surrounding small molecular solvents. The distribution and orientation of the aerogels, as well as their pore size and shape, are controlled during the freeze-drying process (Lorenzo et al. 2018; Patel 2018). Micro-patterned biobased nanocomposites produce 2D or 3D organized morphologies showing comprehensive physical properties and an unusual distribution of the components. Micro-patterns provide a way to adaptive behavior, such as self-­ rolling, self-folding, and actuation. The pattern strategies to form biobased nanocomposites include mainly mask-based patterning, ink-jet printing, and 3D/4D printing (Sun et al. 2018; Xiong et al. 2018). The sonication has gained attention to the generation of novel NPs (Gutiérrez and Álvarez 2017a; Gutiérrez 2018f). Sonication includes the deagglomeration and reduction of micro-sized particles such as tactoids by the application of sound

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waves. This is achieved due to the cavitation effect, which is the formation, growth, and the collapse of bubbles in a liquid (Hielscher 2005; Gutiérrez and Álvarez 2017a). After the collapse of the bubbles, several important local events accelerate deagglomeration of dispersed micro-sized particles. The acoustic cavitation helps form unique materials at room-temperature liquid in contrast to the extreme conditions such as high pressures and temperatures, or a longer reaction period than that required in the conventional methods. The ultrasonication was first adopted to manufacture polymer nanocomposites based on petroleum-derived polymer/inorganic clay systems, and was then gradually extended to biobased nanocomposites (e.g. polysaccharides, proteins and lipids) (Feng et  al. 2014; Soheilmoghaddam et al. 2014). As another processing method, the sol-gel process is a synthesis process that contains in the preparation of a sol, successive gelation and solvent removal. In these systems, the sol (colloidal solution) acts as a precursor and the gel (three-­ dimensional polymeric network) is formed from hydrolysis, followed by polycondensation (Vartiainen et al. 2014).

6.5  Properties of Polymer Nanocomposites The mechanical, barrier, optical, thermal and functional (i.e. antimicrobial, antioxidant) performance of nanocomposites are the most important parameters for food packaging applications. The properties of biobased nanocomposites depend on their microstructure related to their high aspect ratio. Substantial improvements in these properties are associated with the degree of crystallinity, presence of amorphous phase, polar or apolar groups into the polymer, degree of crosslinking, Tg and pretreatments (Galić and Ciković 2003). This section presents the main properties that can be improved by incorporating fillers.

6.5.1  Barrier Properties The quality of a food product is exposed to continuous change due to the transfer of water vapor and oxygen through the wall of the polymer package (Gagnard et al. 2004). The type and size of the nano-fillers and the structure of the nanocomposites influence the degree of improvement in the barrier performance of nanocomposites (Shankar and Rhim 2016a). The reason for such improvement is the presence of highly dispersed nano-fillers that form an impermeable structure to the molecules in the polymer matrix due to the their high aspect ratio (Xu et al. 2006; Choudalakis and Gotsis 2009). The permeant molecules are forced to travel through a tortuous pathway within the polymer composite, thus increasing the length of the diffusion path (Gutiérrez and Álvarez 2017b; Tapia-Blácido et al. 2018). The barrier properties are also influenced by the size, shape, and polarity of the penetrating molecule

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and crystallinity, the degree of crosslinking and polymer chain (Pillai and Sinha-­ Ray 2015). For example, a higher degree of polymer crystallinity decreases the rate of gas transmission, because the crystallites do not allow the gas molecules to permeate through the polymer, while the amorphous regions limit the permeation through the semicrystalline polymers (Siracusa 2012). In addition, platelet-shape fillers show better permeability performances than fillers more compact shaped (Duncan 2011). In contrast, some nano-fillers cause an increase in the permeability when the polymer does not wet the filler.

6.5.2  Mechanical Properties The purpose of the packaging is to protect the food from undesirable external influences and deficiencies such as random breaks and cracks in the material (Zeman 2007). The mechanical properties of a biobased polymer, such as E, maximum stress (σmax), strain at break (εb) and tenacity (T), should be improved for food packaging applications. The formation of biobased nanocomposites with fillers has the potential to obtain excellent mechanical properties when low levels of filler ( 1, then the formulation is safe to proceed. For the aforementioned formulation F  =  2/1.89  =  1.06. Therefore, the starting materials are safe to polymerize. Another method to predict the gel point of the reaction is based on the Patton’s constant, which is defined as Eq. 8.9. K=

mi e COOH

(8.9)

where K is Patton’s constant. When K > 1, the formulation is safe to polymerize. For the previous formulation K = 19/18 = 1.05, which means that the formulation is safe (Percec et al. 1989). Polyesters containing reactive carboxylic acid are prepared in the same way as reactive hydroxyl groups, with the exception when the dicarboxylic acid groups/ hydroxyl groups ratio is >1. In addition, instead of TMP for branching, a trimellitic anhydride is often used. 8.1.2.4  Final Reactions on Saturated Reactive Polyester Depending on the end-group of polyesters, they can react with various curing agents to produce the final product. When the end-group is hydroxyl, it reacts primarily with polyisocyanates to form PU and with MF. Polyesters with hydroxyl functional groups are crosslinked with MF in the presence of an acidic catalyst at temperatures about 110–130 °C. The first formaldehyde reacts with melamine to form a methylolated or alkoxy methyl melamine (Fig. 8.5) in the presence of a basic catalyst (Greunz et al. 2018). Then, the hydroxyl groups of the polyester react by transetherification with alkoxy methyl melamine or by etherification with methylolated melamine. These reactions are in equilibrium but progress because methanol or water as the by-products are vaporized. Fig. 8.5 Chemical structures of: (a) melamine and (b) methylolated melamine

a

N

ROCH2

N

ROCH2 NH2

N CH2OR

CH2OR N

N

N

N H2N

b

NH2

N

N

CH2OR

CH2OR

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The reaction rate of polyester with modified melamine depends on several factors, such as the catalyst concentration, the temperature, the volatility of formed alcohol and type of hydroxyl. Hydroxyl groups with less hindrance react faster with metholylate and alkoxy methyl melamine. Primary hydroxyls react much faster than secondary or tertiary hydroxyls. The catalyst concentration has a direct effect on the reaction rate, but high concentrations can lead to side reactions. The volatility of the alcohol formed is important to precede the reaction quickly. The reaction rate based on the type of alcohol is as follows: methanol > ethanol > n-propanol > n-butanol (Wilson and Pfohl 2000; Merline et al. 2013). The hydroxyl functionality of the polyesters can also react with polyisocyanate and form PU. Depending on the type of polyisocyanate, this reaction can take place at room temperature for aromatic polyisocyanates or at about 60 °C for aliphatic polyisocyanates. The reactivity of hydroxyl groups towards isocyanates depends on the type of hydroxyl and isocyanate, as well as the temperature. Primary hydroxyls react with isocyanates faster than secondary and tertiary hydroxyl groups: approx. 5 times faster than secondary hydroxyls and 200 times faster than tertiary hydroxyls. The aromatic isocyanates react with the hydroxyl group at about 60–90 °C without using any catalyst (Persoons et al. 2016). By adding catalyst, the reaction can be preceded at the ambient temperature. Catalysts such as tertiary amine compounds (e.g. 1,4-diazabicyclo[2.2.2]octane - DABCO) and tin compounds (e.g. dibutyl tin dilaurate) are used to facilitate the reaction (Fig. 8.6). It is believed that these compounds help the transfer of hydrogen from the hydroxyl group to the nitrogen of isocyanate to form the urethane group. Some researchers have also shown that a combination of both type of catalyst is more effective than each of them (Szycher 2012; Zhang et al. 2012). The functions of the carboxylic acid in the polyesters can react with various reactants to form the final product. They can be crosslinked with 2-­hydroxyalkylamides, epoxy resins and MF (Jones et al. 2017). Polyesters with carboxylic acid end group react with epoxy resin at elevated temperature in the presence of a catalyst to form a crosslinked polymer. This polymer is widely used in powder coatings (Jones et al. 2017). Mainly polyester is produced using NPG, terephthalic acid and trimellitic anhydride as a trifunctional monomer. Although hydroxyl functional polyesters, which are often based on application, have an acid value of less than 10 mgKOH/g, the acid value of polyesters are used for powder coatings is in the range of 50–120 mgKOH/g. Choline chloride and tetrabutylammonium bromide are used as a catalyst and temperatures in the range of 150–200 °C are applied (Belder et al. 2001; Gheno et al. 2016). Another reaction of the functional carboxylic acid polyester is with triglycidyl isocyanurate (TGIC), which is widely used as a crosslinking agent (Fig.  8.7). Fig. 8.6 Chemical structures of: (a) dibuthyl tin dilaurate and (b) DABCO catalyst

a

CH3

O

CH3(CH2)9CH2 H3C

O

Sn

CH2(CH2)9CH3

O O

b

N N

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Despite epoxy resins can show poor exterior durability because they have an aromatic ring in their structure, TGIC-cured polyesters show excellent outdoor durability. Benzyltrimethylammonium chloride is the most effective catalyst to cure the polyester by TGIC. This reaction takes place at temperatures about 120 °C (van der Linde et al. 2000; Salla et al. 2002). Hydroxyalkylamides are used to cure polyester with a carboxylic end group (Fig. 8.8). However, hydroxyalkylamides have hydroxyl groups in their structure, but their reaction toward carboxylic acid is faster than simple alcohols. The reaction of a hydroxyalkylamide with polyester takes place at temperatures above 150 °C. Despite the simple esterification reactions in which the primary hydroxyls are traced faster, the secondary hydroxyls here have a faster reaction than the primary alcohols. The acid catalysts have no effect on reaction rate and aromatic esters react faster than aliphatic ones (Zeno et al. 1998; Crapper 2012). 8.1.2.5  Applications and Uses Saturated polyesters are supplied in organic solvents such as butyl acetate, butyl glycol, ethyl acetate, toluene, xylene, etc., or without solvent as a 100% solid (Jones et al. 2017). MF-cured polyesters are the most favorable for coil coatings since they have good adhesion to metals (Jones et al. 2017). Steel, aluminum and galvanized steel are used as coil coating substrates, and using appropriate monomers, saturated polyesters show excellent outdoor stability. The construction industry, household appliances and the automotive industry are the main sections of coil coating users (Sorce et al. 2019). Another important section of the use of reactive polyesters is in the food industry. Because polyesters do not affect the taste of food, they can be used both inside and outside of cans. Melamine-cured reactive polyesters are an appropriate option in this industry (Greunz et al. 2018; Sorce et al. 2019). Fig. 8.7 Chemical structure of TGIC

Fig. 8.8 Chemical structure of hydroxyalkylamide

HO

OH

O N

N HO

O

OH

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PU foams, coatings, adhesives and elastomer are important sections for reactive polyesters. Polyisocyanates play the role of hardener or crosslinker for reactive polyesters (Septevani et al. 2015). Polyesters cured with aliphatic polyisocyanates show excellent stability and flexibility outdoors (Sahoo et al. 2018). They can be used for aircraft finishes. High Mw polyesters (3000–5000 g/mol) are used for PU elastomers as high quality shoe soles (Clemitson 2015). Two-component PU adhesives are another application of polyesters cured with polyisocyanates, which will be discussed in Sect. 8.2.5.2. Powder coating is a promising method to replace solvent-based methods that are harmful to the environment (Knudsen and Forsgren 2017). Reactive polyesters cured by epoxy resins or hydroxyalkylamides are widely used for coating appliances such as cooling systems and refrigerators, steel furniture and some agricultural machinery. The resulting coating shows good adhesion to metals, good outdoor stability and high impact strength (Knudsen and Forsgren 2017).

8.2  Polyurethanes (PUs) 8.2.1  Introduction PUs were first introduced by Otto Bayer in 1937 in Germany as a polymer to compete with polyamides, whose patents were held by DuPont Company in the United States (Bayer 1947). At first, polyester polyol was used to synthesize PUs on a large scale. But then the polyethers became commercially available and replaced the polyesters to a large extent. Since then, thousands of patents have been registered in PUs by different companies worldwide. PUs are among the most versatile polymers which are synthesized by polyaddition reactions (Vaidya and Chaudhury 2002; Jeong et  al. 2006; Tan et  al. 2011; Kreye et al. 2013). These polymers are mainly produced by reaction of diisocyanates and polyols at temperatures below 100 °C with formation of urethane group. PUs can be produced with completely different properties (Finnigan et  al. 2004; Sonnenschein et al. 2005). For example, elastomers, and soft and hard foams can be produced from almost the same starting materials. The final properties of PUs depends on many parameters, such as atmospheric humid, crosslinking, degree of branching, Mw, NCO/OH ratio, type of monomer used (e.g. aromatic or aliphatic, linear or branched, hydrophile or hydrophobe), distance between urethane groups (hard segments) in a single chain, etc. (Crawford et  al. 1998; Gorna et  al. 2002; Zlatanić et  al. 2004; Pattanayak and Jana 2005; Corcuera et  al. 2011; Saralegi et al. 2013). Mw of polyol affects the hardness and elasticity of the final PU (Gorna et  al. 2002). For example, a high Mw of polyol leads to the formation of an elastomer and a low Mw of polyol gives a hard PU. The PUs resulting from the reaction of polyols

8  Reactive and Functional Polyesters and Polyurethanes Fig. 8.9 Hydrogen bonding in the urethane groups

173

O O

O N

H

O N

H

and diisocyanates contain hard (HS) and soft segments (SS). SS include polyol chain and HS consist of urethane groups. HS show a higher Tg value than SS, and often, PUs show two Tg values (Li et al. 2017). A higher Tg of HS is attributed to the formation of hydrogen bonds between the urethane groups (Fig.  8.9). HS act as crosslinked weak points. The SS and HS formation depends largely on the nature of the polyol. When polyester is used as the polyol, the ester groups can form hydrogen bonds with the urethane group which lead to the creation of more HS, and when polyether is used as the polyol, they cannot form hydrogen bonds and the creation of HS is limited to the urethane group. Therefore, the use of polyester for producing PU leads to the formation of a hard polymer compared to polyol (Xiang et al. 2017).

8.2.2  Monomers Two types of monomers are mainly used for producing PUs: polyols such as macro-­ monomer and diisocyanates (Akindoyo et al. 2016). The polyol part introduces flexibility to the final polymer which is defined as the soft part and the diisocyanate imparts hardness to the polymer and is defined as the hard part (Akindoyo et al. 2016). Polyols are more versatile than diisocyanates and their structure has a great effect on the PUs. 8.2.2.1  Polyols The term polyol is used for long chain material that has two or more hydroxyl group at the ends of the chain or as a pendent group on the main chain. The length of the polyol determines the hardness and the softness of the PU. A high Mw of polyols results in a soft PU and vice versa. Depending on the final application, Mws from 200 to 10,000 g/mol are used generally for producing PUs. There are many types of polyols which are used on a large scale to produce PUs, such as acrylic polyols, aminic polyols, polybutadiene polyols, polyesters, polyethers and polysiloxane polyols (Ionescu 2005). Among these polyols, polyether and polyesters are more important.

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n

O

KOH H2O

HO

O n -1

OH

Fig. 8.10  Preparation of PEO from ethylene oxide

8.2.2.1.1  Polyethers Polyether polyols are produced by exothermic ring opening polymerization (ROP) of alkylene oxides (or oxiranes) such as butylene oxide, ethylene oxide and propylene oxide in the presence of KOH or sodium hydroxide (NaOH) as catalysts and glycols or water as chain initiators or primers (Fig.  8.10) (Wilms et  al. 2009). Polyethylene oxide (PEO) which is produced by polymerization of ethylene oxide is rarely used as the polyol for obtaining PU. This problem is due to the high hygroscopic properties of PEO and also tends to crystallization. For these reasons, ethylene oxide is often used with other alkylene oxides to produce polyether copolymer (Harris 1985; Hillmyer and Bates 1996). Propylene oxide is the most important monomer for producing polyether polyols (Ionescu 2005). Polypropylene oxide is predominantly produced by two processes (Ionescu 2005). The first method is to use KOH as a catalyst and glycols or water as an initiator, and the second method is to use double metal cyanide (DMC) as a catalyst (Hofmann et  al. 2001; Clement et  al. 2002). The first mentioned method is cheaper and simpler. The DMC method is more expensive but more effective than using KOH as a catalyst (Raghuraman et al. 2016). Double metal cyanide catalysts are about 1000 times more active than KOH or NaOH (Ionescu 2005). Polypropylene glycol has a lower reactivity with diisocyanates than polyethylene oxide because it has secondary hydroxyl groups. To overcome this problem, propylene oxide is end-­ capped with ethylene oxide units (Ionescu 2005). Glycols determine the functionality of the final polyether. For example, if EG is used as the initiator polyol functionality it would be 2 and if TMP is used as the initiator functionality it would be 3 (Huang et al. 2010; Meng et al. 2016). Another important polyether polyol is polytetramethylene ether glycol (PTMEG) which is produced by ROP of tetrahydrofuran (THF). The most frequent method to obtain PTMEG is by ROP of THF using acid catalysts. This method is somewhat different from the polymerization of ethylene oxide and propylene oxide (McDaniel et al. 2016). 8.2.2.1.2  Polyesters Polyesters are compounds with ester group in their structure which are produced by polycondensation reaction. These materials have a higher viscosity than polyethers, so high Mws of polyesters are hard to use compared to polyethers (Faunce 2003). Polyester polyols are produced by polycondensation reaction of dicarboxylic acids (or anhydrides) and glycols (Benítez et al. 2017) or by ROP of lactones (Labet and

8  Reactive and Functional Polyesters and Polyurethanes O

OH

OH

n

O

HO

+ n

O O

HO HO

175

O

+ n H2 O

O

H n

Fig. 8.11  Preparation of the polyethyleneterphtalate (PET)

Thielemans 2009). In the polycondensation reaction, water is produced and must be removed from the reaction medium to obtain high yield polyester since the reaction is in  equilibrium. For ROP of lactones, no by-product is produced (Labet and Thielemans 2009). Polyesters are tough, oil resistant with good adhesion properties compared to polyethers. However, they show lower hydrolysis resistance and higher viscosity than polyethers (Petrović et al. 2008). There is a wide range of commercially available monomers to produce polyester polyols. Based on the final application of the mature PU, different types of dicarboxylic acids (or anhydrides) and glycols can be used. Dicarboxylic acids such as adipic acid, isophthalic acid, terephthalic acid and phthalic anhydride are well known in the art (Fig. 8.11) (Díaz et al. 2014). Adipic acid imparts UV resistance and flexibility to PU, while isophthalic acid and terephthalic acid introduce good adhesion to metals, thermal resistance and hydrolysis resistance to the PU (Scremin et al. 2019). 1,4-cyclohexane dimethanol, BD, DEG, EG and HD are frequently used as glycols to produce polyester polyol for PU. EG is a commercially cheap monomer and imparts hardness to the polyester polyol though its hydrolysis resistance is weak. DEG has a long ether bond in its structure and introduces flexibility to the final polymer (Datta et al. 2017). Polyester polyols are produced in high capacity reactors. First, the starting liquid materials are added to the reactor, the temperature is raised to about 100 °C and then solid materials are introduced into the reactor. The temperature is raised again to about 160 °C until distillation of water begins. The temperature is then gradually increased 10 degrees per hour to 220 °C. The reactor temperature is maintained at 220 °C until 90% of water in the reaction vessel is distilled. At this time catalyst (dibutyl tin oxide, p-toluene sulfonic acid or zinc acetate) is added to facilitate the polymerization reaction. The amount of catalyst used is about 0.05 to 1% of the all starting materials. The reaction continues until an acid value of less than 2 or preferably less than 1 is reached (Witt et al. 1994; Dutta et al. 2004). One of the common problems facing polyester synthesis is water removal from the reaction medium. In the early stages of the reaction, water removal is not difficult. However, when the reaction proceeds and Mw of the polyester grows, the removal of water becomes difficult due to the high viscosity of the resulting polyester (Ullmann et al. 1985). There are several ways to remove the water in the final stages of polymerization. Lowering the pressure of the reaction medium by using a

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vacuum pump is an easy method for removing water (Ullmann et al. 1985). Another method is to use a compound which forms azeotrope with water such as toluene and xylene (Ullmann et al. 1985). Since the polyester is used for producing PU, mainly free solvent polyesters are used, therefore the second method is not as favorable as the first (Ullmann et al. 1985). Polycaprolactone (PCL) is another type of polyester which is used as the polyol for producing PUs (Babaie et al. 2019). Although this compound is more expensive than common polyester, it imparts excellent mechanical and hydrolysis resistance to PU. PCL is produced by ROP of caprolactone in the presence of a glycol as an initiator and catalyst (Fig. 8.12) (Labet and Thielemans 2009). 8.2.2.1.3  Acrylic Polyols Acrylic polyols are produced by radical polymerization of compounds having double bonds in their structure (Fig. 8.13). Monomers such as acrylic acid (AA), BA, methacrylic acid (MAA), MMA and styrene are used to produce acrylic polymers (Jones et al. 2017). The introduction of hydroxyl functional is carried out by the addition of monomers such as 2-hydroxyethyl acrylate (HEA), 2-hydroxyethyl methacrylate (HEMA) and 3-hydroxypropyl methacrylate (HPMA) to the formulation (Jones et al. 2017). The polymerization takes place in an aromatic solvent such as toluene or xylene in the presence of an initiator such as di-benzoyl peroxide at a temperature about 100 to 130 °C (Gite et al. 2010; Yang et al. 2017).

O n

O

OH

O HO

H

Catalyst

O n

OH

O

Fig. 8.12  ROP of caprolactone by using glycol as a catalyst

O

O O

O + m

n

* n

* O

O

m

O

O

OH BA

HEMA

OH

Fig. 8.13  Copolymerization of butyl acrylate (BA) with 2-hydroxyethyl methacrylate (HEMA)

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8.2.2.1.4  Polybutadiene Polyols Polybutadiene is polymerized by radical polymerization of butadiene with an initiator having a hydroxyl group such as 4,4-azobis (4-cyanopentanol) or hydrogen peroxide (Fig. 8.14) (Brosse et al. 1987). Another way to polymerize butadiene with hydroxyl end-capped is to use cationic initiators such as sodium naphthenate and the treat the resulting polymer with propylene oxide to obtain the polyol (Chen et al. 2010). Since polybutadiene shows very low Tg, the resulting PU with this polymer is very soft (Boutevin et al. 2000; Zhang et al. 2017a, b; Sikder and Jana 2018). 8.2.2.1.5  Polysiloxane Polyols Like polybutadienes, polysiloxane polyols have a very low Tg due to the long Si-O bond and show high elasticity at very low temperatures (as low as −100  °C). Polysiloxane polyols are produced in two stages. First, dichlorodimethylsilane is polymerized in the presence of water and chlorodimethylsilane as initiator. A polymer is formed with Si-H end-capped. The resulting polymer is then reacted with a chemical compound containing both a double bond and a hydroxyl functional group (such as allyl alcohol) in the presence of palladium or platinum compounds as a catalyst (Fig. 8.15) (Domschke et al. 2000; Yang et al. 2019a, b).

H2O2

n

HO

n

OH

Fig. 8.14 H2O2 initiated radical polymerization of butadiene

Cl

n

Si Cl

H

+ x

n H 2O

Si

Si

H

O

Si

O

Cl OH

Catalyst

HO

Si

O

Fig. 8.15  Preparation of polysiloxane polyols

Si

O

n

Si

OH

n

Si

H

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8.2.2.1.6  Aminic Polyols Aminic polyols are produced in the same manner as polyether polyols with the exception of initiator. Initiators such as diethylene triamine (DETA) and ethylenediamine (EDA) are used to produce polyols with amine end-capped (Ionescu 2005). This type of polyol is very reactive with aromatic diisocyanates (the reaction takes place in seconds). So, the pot-life and gel-time are very low. To overcome this problem, aliphatic diisocyanate which are less reactive than aromatics are used to produce PU. 8.2.2.2  Diisocyanates Two types of diisocyanates are used to produce PUs: aromatic and aliphatic diisocyanates. Aromatic diisocyanates are widely used as they are very cheaper than aliphatic diisocyanates. Aromatic diisocyanates such as 2,4- and 2,6-toluene diisocyanates (TDI) and 4,4′-diphenylmethane diisocyanates (MDI) are frequently used in the PU structure (Fig. 8.16) (Akindoyo et al. 2016). TDI is more reactive than MDI, but when its first isocyanate reacts with hydroxyl group, the reactivity of the second isocyanate group on the molecule drops markedly (Reardon et  al. 2019). Polymers with TDI in their structure show higher viscosity than MDI (Pocius and Dillard 2002). In addition, PUs prepared with TDI show better moisture resistance and are more flexible than their MDI counterpart. On the other hand, PUs having MDI in their structure show better microbial and chemical resistance, and better adhesion to metals at low temperatures (Malik and Kaur 2018). MDI has a higher Mw than TDI and its isocyanates have almost the same reactivity (Petrović and Ferguson 1991). There are several types of aliphatic diisocyanates which are used on a large scale. Aliphatic diisocyanates such as 1,6-hexamethylene diisocyanate (HDI), 4,4′-dicyclohexylmethane diisocyanate (H12MDI), isophorone diisocyanate (IPDI) and xylylene diisocyanate (XDI) are used to produce PUs (Fig.  8.17) (Nozaki et  al. 2017). The PUs produced from aliphatic diisocyanates have better UV resistance and impact resistance than aromatic diisocyanates (Barszczewska-Rybarek 2017). In addition, due to the lower reactivity of these compounds with water compared to the aromatic diisocyanates, they can be used for synthesizing waterborne PU (WPU) which are dispersed in aqueous medium (Najjar et al. 2018). PU compounds resulting from HDI tend to crystallize and show a higher viscosity than PUs prepared from IPDI. Fig. 8.16 Chemical structures of: (a) MDI and (b) TDI

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Fig. 8.17  Chemical structures of: (a) HDI, (b) H12MDI, (c) IPDI and (d) XDI

It should be noted that diisocyanates (especially aromatic ones) are very unhealthy and their inhalation or contact with them can cause serious problems in the human body (Prueitt et al. 2017).

8.2.3  Chemistry of PUs The high reactivity of the isocyanate group is attributed to the high electronegativity of oxygen and nitrogen which are directly linked to carbon atom. Carbon atom becomes positive and is prone to attack by  atoms with a small negative charge. Based on this, the isocyanates are very reactive and can react with almost every compound having a partial negative charge (Szycher 2012). Isocyanates can react with alcohols, amines, anhydrides, carboxylic acids, epoxides and water (Luceño-­ Sánchez et al. 2018). These compounds can also react with themselves to form a dimer or trimer (Szycher 2012). Isocyanates can react with the alcohol at room temperatures without any catalyst (Raspoet et al. 1998). This reaction is exothermic and leads to the formation of the urethane group and is the basis of PU (Ionescu 2005). Primary alcohols react faster than secondary alcohols and secondary alcohols react faster with isocyanates than tertiary (Dyer et al. 1949). The reactivity of aliphatic alcohols is also greater than aromatic alcohols such as phenol (Ionescu 2005). For example, a primary alcohol without hindrance reacts 1000 times faster with isocyanates than phenol (Dyer et al. 1949). The isocyanates can also react with amines at low temperatures of up to 0 °C without any catalyst (Szycher 2012). The reaction of isocyanates with amines is exothermic and difficult to handle (Szycher 2012). As a result of this reaction, a urea group is formed (Szycher 2012). The same as the reactivity of the alcohols of the primary amines is faster than the secondary amines (Szycher 2012). For example, the reactivity of a primary amine is almost 3 times greater than the secondary amine (Frisch and Rumao 1970). In addition, aliphatic amines are more reactive to isocyanates than aromatic ones. A primary aliphatic amine reacts about 500 times faster

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than aromatic amines (Vilar 2006). The reactivity of amines with isocyanates is also much faster than alcohols (Vilar 2006). For example, a primary amine compound reacts almost 1000 times faster than a compound containing a primary hydroxyl group (Vilar 2006). The reaction of the isocyanates with water takes place in two stages: first carbon dioxide (gaseous compound) and an amine compound are formed, and in the second stage due to the greater reactivity of the amines towards the isocyanates, it reacts spontaneously with isocyanate to form a urea group. Therefore, one molecule of water reacts with two molecules of isocyanate, this must be taken into account for the formulation dealing with water (Kent 2007). It should be noted that an unstable compound called carbamic acid is formed before the formation of amine and carbon dioxide (Ni et al. 2002). Another important reaction of the isocyanate is trimerization (Frisch and Rumao 1970). As a result of this reaction, a heterocyclic compound named isocyanurate is formed which is an intermediate in PU synthesis (Frisch and Rumao 1970). The isocyanates also react with the urethane group at high temperatures (above 100 °C) to form an allophanate group (Lapprand et al. 2005). Similarly, they react with urea above 100 °C to form biuret group (Dušek et al. 1991). The reaction of isocyanates with carboxylic acids leads to the formation of an amide compound and carbon dioxide (Sasaki and Crich 2011). The isocyanates can also react with epoxides and anhydrides (Paul et al. 2015). The reaction product of isocyanates with epoxides is oxazoline and with anhydrides is imide (Schwetlick and Noack 1995). The reactivity of isocyanates to different compounds is listed in Table 8.2.

8.2.4  Production of PUs 8.2.4.1  Solvent-Borne PU Synthesis There are three methods to produce solvent-based PU: prepolymer method, quasiprepolymer method and one-shot method (Szycher 2012). Table 8.2  The relative reactivity of different compound toward isocyanates Reactive compounds Primary aliphatic amine Secondary aliphatic amine Primary aromatic amine Primary aliphatic hydroxyl Water Carboxylic acid Secondary aliphatic hydroxyl Tertiary hydroxyl Phenolic hydroxyl

Chemical structure R-NH2 R2-NH2 Ar- NH2 R-CH2-OH H 2O R-COOH R2-CH-OH R3-C-OH Ar-OH

Reactivity rate (25 °C) 2500 500–1250 5–7.5 2.5 2.5 1 0.75 0.0125 0.0025

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In prepolymer method, a diisocyanate reacts with a polyol with specific Mw in the temperature range of 60 to 90 °C (Rogers and Long 2003). This reaction can occur in the presence of a catalyst for aliphatic diisocyanates or without using any catalyst for aromatic diisocyanates (Szycher 2012). Tin compounds such as dibutyl tin dilaurate (DBTDL) and amine compounds such as triethylene diamine (under DABCO trade name) can be used to catalyze the urethane reaction (Ashida 2006). The prepolymer method takes place in two stages: first, the molar ratio of diisocyanate to hydroxyl NCO/OH is maintained about 2 (Ashida 2006). A prepolymer is formed with a terminated isocyanate group, and second, a chain extender compound such as BD, EG or ethylene diamine is added to produce the mature PU and a high Mw PU is formed (Krol 2007). The quasiprepolymer method is similar to the prepolymer method, except that the NCO/OH ratio is greater than 2 (Szycher 2012). In this method, the free diisocyanate is present in the reaction medium together with the prepolymer end-capped with isocyanate (Ding et al. 2015). In the second stage, a chain extender is added to obtain the high Mw PU. The final NCO/OH ratio for quasi-prepolymer and prepolymer method is 1 to 1.1 (Szycher 2012). In regions with high humidity, this ratio is about 1.1 due to the reaction of some isocyanate groups with water, and in dry regions, this ratio is about 1 (Wiser-Halladay 1990). One-shot method is the simplest method. All reactants are mixed at the same time. In this method, efficient mixing is very important and should be done as quickly as possible before reaching gel-point (Szycher 2012). Although this method is simple, it needs a careful calculation of the necessary materials and temperature control must be taken into account. This method may not be applicable for bulky materials, because the large amount of heat released can cause distortion in the final product (Rausch Jr. and Mcclellan 1972; Rivera-Armenta et al. 2004). 8.2.4.2  Waterborne PU (WPU) Synthesis WPU also called PU dispersion (PUD) is produced in a different way compared to solventborne PUs. For producing WPU some internal surfactants are used such as dimethylolpropionic acid (DMPA). In a typical synthetic reaction, an aliphatic diisocyanate such as IPDI, a polyol with a Mw between 1000 and 2000 g/mol, an internal surfactant and a catalyst are charged to the reactor. The aromatic diisocyanates will hardly be used due to extremely rapid reactions with water. The temperature is raised to about 70 °C and maintained at that temperature until the required NCO number is reached. Whenever necessary, acetone is added to decrease the viscosity. Temperature is then lowered to 45 °C and a neutralizing agent such triethylamine (TEA) is added to neutralize the carboxylic acid group of surfactants. Then, the temperature is lowered to room temperature and water is added to disperse the PU in the water medium. The chain extender can be added after adding water or before adding TEA. After adding water, only diamine chain extenders are usable. Once the reaction is complete, acetone is removed by lowering the reactor pressure (Najjar et al. 2018; Mirmohseni et al. 2019).

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8.2.5  Reactive PUs Reactive PUs are categorized into two groups: One-component PUs and two-­ component PUs. 8.2.5.1  One-Component Reactive PUs One-component reactive PUs are subcategorized into air-drying urethanes (Wicks and Wicks Jr. 2005), blocked urethanes (Delebecq et  al. 2012; Pilch-Pitera et  al. 2017), moisture curable urethanes (Chattopadhyay et al. 2006) and UV cure hybrid urethanes (Dzunuzovic et al. 2005; Li et al. 2019). Air-drying urethanes which are known as uralkyds (or oil-modified urethanes) are based on alkyds such as polyol for PU synthesis. This type of reactive PUs has double bond in its structure. The uralkyds react with atmospheric oxygen to form a crosslinked polymer. This reaction is catalyzed in the presence of cobalt, magnesium or zirconium compounds as catalysts at room temperatures (Xu et al. 2002). The scratch resistance and tensile strength of uralkyds are superior to the alkyds, but cannot compete with pure PUs (Chiou and Schoen 2002; Prashantha et al. 2015), and also show better solvent resistance than alkyds. Moisture curable PUs are urethanes with isocyanate end groups. For producing moisture curable PUs, the isocyanates/polyol (NCO/OH) ratio is more than one (Karak 2016). After applying on a substrate, the reactive isocyanate groups react with atmospheric water even at the melting point of water to form the final product. After reacting the water with isocyanates, an amine end group is formed and then the amine reacts with isocyanate to form urea groups. Aliphatic isocyanates react slower with atmospheric water and higher temperatures are required. The reaction rate depends on the atmospheric moisture content, temperature and type of isocyanate (Hofacker et al. 2004). In the blocked PU, the isocyanate groups are blocked by a compound, which after being applied to the polymer is unblocked by means of a trigger such as temperature (Guo et  al. 2016). The blocking agents protect the isocyanates group in storage. Blocking agents such as 2-butanone oxime, 2-ethylhexyl alcohol, e-­caprolactam and uretdiones are used commercially (Rolph et al. 2016). The reaction of the blocking agents with isocyanates is an equilibrium reaction. The blocking reaction is exothermic, and the unblocking reaction is endothermic (Rolph et al. 2016). So, for activating the blocked polymer, the elevated temperature is necessary. The blocking agents must be evaporated from the polymer because when the temperature drops it can block the isocyanate again. The curing rate depends on several factors, such as catalyst, polymer thickness, steric hindrance of blocking agents, temperature and volatility of blocking agents (Gnanarajan et  al. 2000; Iyer et al. 2002). Another type of one-component reactive PU is UV curable PU. UV curable PUs are prepared by a prepolymer method with the exception that instead of the chain

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extender a one-functional hydroxyl monomer containing double bond in its structure such as HEMA, which is an hydroxy functional acrylate (Chattopadhyay and Raju 2007). The acrylate can have one to several double bonds in its structure. The more double bond, the more crosslink density. Other acrylic monomers such as HEA and pentaerythritol triacrylate (PETA) are used. HEA has a double bond and PETA has three double bonds, which leads to increase crosslink density (Kotha et al. 1998). The prepared urethane with acrylate end group with a photo initiator such as 2,2-dimethoxy-1,2-diphenylethan-1-one, 2-hydroxy-2-methyl-1-phenylpropanone or benzophenone is applied on the considered substrate. Then, the radiation is applied by a UV lamp to initiate the radical polymerization and curing the polymer (Kim et al. 2006). The prepared product shows a hybrid of PU and acrylate properties. No temperature is needed to cure the PU, so this type of polymer is useful for plastic and wood coatings. One-component WPU is also used as UV curable vehicles. UV curable WPUs can be prepared in almost the same way as WPUs as shown in this section. In UV curable WPUs, hydroxyl functional acrylate monomer is added just before adding TEA (Kim et al. 2006; Zhang, Xu et al. 2002). 8.2.5.2  Two-Component Reactive PUs Two component PUs are the majority of PU production and is the most important route to PU. One component is an isocyanate terminated polymer with low Mw and the other component is catalyst, polyol, solvent and other necessary additives. The curing can be performed at the room or elevated temperatures. All polyols such as acrylate polyol, polybutadiene polyol, polyester, polyether, etc. can be used in the two-component urethane formulation. The polyisocyanate/polyol ratio is usually 1.1:1. For regions with high humidity the polyisocyanate/polyol ratio is more than 1.1:1 (Yang et al. 2018). Two component WPU are also available. The first component is a prepolymer of hydroxyl or amine terminated which is dispersed in water. This prepolymer is synthesized by method discussed in Sect. 8.2.4.1, only its Mw is less. The second component is a low Mw polyisocyanate which is dispersed in water. Despite solvent-borne PU have an NCO/OH ratio of about 1.1:1, here the ratio is approx. 2:1 or even more than that, especially if end group of first component is hydroxyl. This is because some isocyanate group reacts with water and becomes an amine (Noble 1997; Melchiors et al. 2000).

8.2.6  Applications and Uses The application of PUs is divided into four main categories: coatings, elastomers, foams, and adhesive and sealants. The main application of PUs is in foam production (approx. 80%). PU foams are used for furniture, transportation, mattresses and

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bedding, packaging, refrigerators and freezers. PU elastomers are used in footwear, the wheels construction, electronics, tires, transportation and machinery with majority for footwear. PU adhesives are consumed in leather and textile industry, packaging, shoes and wood binding. More than half of PU adhesives are solvent based and high solids, waterborne and hot-melt adhesives are found in the following places, respectively. Large volume of PU coating is dedicated to the transportation industry. Wood industry, metal protection, roofing, powder coatings and radiation curable coatings are other applications of PU coatings (Sonnenschein 2014).

8.3  Conclusions and Perspectives Reactive polyesters and polyurethanes (PUs) have shown to be industrially an important group of polymers for numerous applications. There are many ways to produce reactive polyesters and PUs in laboratory, but some of them have proven to be of industrial importance. These polymers as intermediate materials should undergo more reactions to become a mature product. The final products from polyesters and PUs are used for many applications, such as adhesive, automobile, coating, construction, etc. Given these important and bulk uses of the reactive polymers, their production and use can be expected to increase significantly in the near future. Acknowledgments  The vice dean for research of the University of Tabriz is greatly acknowledged for the support. The authors also acknowledge the Koosha Resin Company for their support and encouragement. Conflicts of Interest  The authors declare no conflict of interest.

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Chapter 9

Lignin as a Coating and Curing Agent on Biodegradable Epoxy Resins Chikako Asada, Sholahuddin, and Yoshitoshi Nakamura

Abstract  Epoxy resin has been widely used as a coating agent within the food, aerospace, automotive, and electronics industries. However, these materials obtained from petrochemical industry cause problems related to environmental impact. In this sense, ecological epoxy resins from biodegradable natural polymers have been proposed as an alternative. Lignin is a biodegradable polymer obtained from unused plant biomass (agricultural waste or byproduct), and its use is less promising than cellulose for the manufacture of bioethanol. This chapter aims to analyze recent advances in studies of epoxy resin made from lignin. Keywords  Biopolymers · Films

9.1  Introduction The epoxy resin is the important thermosetting resin in the industrial sector, being used for many products such as coating agents for food and beverage cans, electronic and composite industries, automobiles as well as in the aerospace industry because they have valuable properties, such as high thermal and chemical resistance, low moisture absorption and good electrical and mechanical properties (May 1988; Ellis 1993). Since epichlorohydrin (ECH) and bisphenol A are used as epoxy resin raw materials derived from fossil resources, and considering that these substances are suspected of having an endocrine disruptive effect as well as other concerns such as carcinogenicity, mutagenicity and reprotoxicity (Markaverich et al. 1995). Therefore, the development of an alternative raw material to the epoxy resin is desired as a coating material. To support the sustainable development goals C. Asada (*) · Y. Nakamura (*) Department of Bioscience and Bioindustry, Tokushima University, Tokushima, Japan e-mail: [email protected]; [email protected] Sholahuddin Graduated School of Life and Material System Engineering, Tokushima University, Tokushima, Japan © Springer Nature Switzerland AG 2020 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_9

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(SDGs) and encourage sustainable industry (United Nations Climate Change Conference 2015), many researchers have studied in recent decades on the synthesis of biobased epoxy resins (Raquez et al. 2010). In this sense, oils have been considered as interesting multifunctional materials in the synthesis of thermosetting materials (Crivello et al. 1997; Vlcek and Petrovic 2006; Goud et al. 2007; Zeleke and Ayana 2017). Acetone soluble lignin from white poplar (Asada et al. 2018), green tea tannins (Benyahya et al. 2014), itaconic acid (Ma et al. 2013), liquefied wood (Kishi et al. 2011) and methanol soluble lignin from moso bamboo (Sasaki et al. 2013; Asada et al. 2015a) have also been used as biobased raw materials to synthesize epoxy resins. Biobased epoxy resins have shown interesting thermal and mechanical properties, comparable to those synthesized from bisphenol A. Plant biomass can thus be an excellent substitute for bisphenol A as the epoxy resin raw material.

9.2  Lignin Epoxy Resin For a long time, oils obtained from food crops have been considered as attractive multifunctional materials in the synthesis of thermosetting materials (Crivello et al. 1997; Vlcek and Petrovic 2006). However, the use of food crops as a source of epoxy resins resulted in competition for food needs. To change the dependence of the industry on products derived from fossil resources, the manufacture of biomaterials such as ecological epoxy resins made of biomass cannot compete with the food needs of humans, i.e. lignocellulosic biomass can be a solution to the problem of non-renewable products, the depletion of products derived from petroleum and environmental problems (Scott et  al. 2013; Jeon et  al. 2014; Silalertruksa and Gheewala 2010). Lignin is one of the main components of lignocellulosic biomass, which constitutes between 13–35% of plant matter, and is a three-dimensional phenylpropanoid polymer, mainly linked by ether bonds between monomeric phenylpropane units, which are not readily hydrolyzable (Gutiérrez and Alvarez 2017). In recent years, many researchers have used lignin not only as a biobased epoxy resin, but also as a natural curing agent and adhesives as shown in Table  9.1 (Nakamura et al. 2001; Kong et al. 2014; Asada et al. 2015a; Ferdosian et al. 2015). Hard segments of lignin can provide stiffness as a lignin-cured copolymer, which can significantly change or affect the properties of the copolymer. In addition, the mechanical strength and level of crosslinking can be changed in lignin-cured copolymers (Basnet et al. 2015). The use of lignocellulosic biomass such as beech (Fagus crenata Blume) (Fache et  al. 2016), corn straw (Kong et  al. 2014), eucalyptus (Eucalyptus globulus) (Nakamura et  al. 2001; Asada et  al. 2015a; Basnet et  al. 2015), German spruce (Picea abies) (Kishi et  al. 2006), Japanese cedar (Cryptomeria japonica) (Asada et al. 2012), moso bamboo (Phyllostachys pubescense) (Sasaki et al. 2013; Asada et al. 2015a), rice straw (Muranaka, et al. 2015) and white poplar (Populus tremuloides) (Asada et al. 2018) has been reported in the literary as epoxy resin materials.

Extraction Water and methanol

Water and methanol N/A

Water and methanol

Steam explosion

N/A

Steam explosion

Moso bamboo (Phyllostachys pubescens) de-polymerized organosolv lignin (DOL) de-polymerized kraft lignin (DKL) Cedar (Cryptomeria japonica) Eucalyptus (Eucalyptus globulus) Moso bamboo (Phyllostachys pubecens)

Supercritical methanol

N/A

Kraft lignin

Grinding

Pretreatments Steam explosion

2 step liquifaction (Nanocatalyzed and acid-catalyzed) Oil palm empty fruit N/A Diethyl ether followed bunches (EFB) black liquor by cyclohexane-ethanol mixture Corn straw Enzymatic hydrolysis N/A

Raw materials Eucalyptus (Eucalyptus globulus) German spruce (Picea abies) Lignin

Polyamine (TY-200)

Epoxy 331(a diglycidyl ether of biphenol A) Bisphenol epoxy resin with lignin DER353 (Dow Chemicals)

Epoxized lignin EP828 (a diglycidyl ether bisphenol A; Japan Epoxy Resins Co. Ltd.)

Epoxized DOL Epoxidized DKL

Epoxized lignin

DDM

Epoxized lignin

4,4-diaminodiphenylmethane (DDM, an aromatic amine) diethlyenetriamine (DETA, an aliphatic amine) Lignin TD2131 (a phenol novolac; DIC corp.)

Linin-derived polycarboxylic acid Succinate monoester Hexahydrophthalic anhydride Lignin 2E4MZ-CN

Curing agents

Cured epoxy resin Epoxy resin Epoxized lignin

Table 9.1  Epoxy resins synthesized from various types of lignin as raw materials

(continued)

Asada et al. (2015a)

Sasaki et al. (2013)

Yin et al. (2012) Qin et al. (2013)

Abdul Khalil et al. (2011)

References Nakamura et al. (2001) Kishi et al. (2006)

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Base-catalyzed depolymerized (BCD) lignin

Soda lignin (SL) from Japanese cedar Klason lignin (KL) from Japanese cedar. Acacia mangium pulp

de-polymerized hydrolysis lignin (DHL, commercial lignin of aspen wood hydrolysis) (FPInnovation) Residue of enzymatic hydrolysis of Douglas fir White poplar (Populus tremuloides) Softwood kraft lignin

Raw materials Cedar (Cryptomeria japonica) Rice straw Beech (Fagus crenata Blume) DOL DKL

Table 9.1 (continued)

Epoxized lignin

Water and acetone Acetone

Organic onium hydroxide aqueous solutions (OHAS)

Steam explosion

LignoBoost process and/or glycidylation in DMSO N/A

N/A

Ethanol

Epoxized lignin

N/A

Mild hydrogenolysis

Ionic liquid (IL) treatment N/A

Partially depolymerized lignin-based epoxy Epoxized lignin

Low pressure de-polymerization (disclosed patent fiiling)

N/A

Ferdosian et al. (2016b)

DDM

4,4-methylenedianiline (MDA)

Lignin

Nisha et al. (2019) Ortiz et al. (2019)

Nagatani et al. (2019)

Xin et al. (2016) Asada et al. (2018) Jablonskis et al. (2018)

Ferdosian et al. (2016a)

DDM

Maleic anhydride

References Muranaka et al. (2015)

Curing agents Hexamine Triphenylphosphine

IL-Lignin and epoxy 1,2,3,6-tetrahydromethyl-3,6-­ prepolymer methanopthalic anhydride BCD lignin Epoxized lignin with glycerol diglycidyl ether (GDE)

Bisphenol A-based epoxy resin with 25 to 100% DOL or DKL content Bisphenol A-based epoxy resin with 25 to 100% DHL content

N/A

N/A

Cured epoxy resin Epoxy resin Epoxized lignin

Extraction Water/aceton binary solution

Pretreatments Milling

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Since the lignin-carbohydrate complex of lignocellulosic biomass has a recalcitrant character that is not easy to break down into components, it is necessary to degrade and/or separate the components by a pretreatment method before used as a source of lignin or cellulose (Agbor et al. 2011). The pretreatment process to break up the lignin-carbohydrate complex is an important first step for the effective biochemical conversion of lignocellulosic biomass into biomaterial or biofuel (Mosier et  al. 2005). For the effective pretreatment of lignocellulosic biomass, the increased surface area and porosity, the total or partial modification of the lignin structure, the depolymerization and removal of hemicellulose, the reduction of cellulose crystallinity, among others, should be performed (Potumarthi et  al. 2013; Asada et  al. 2015b). In general, pretreatment methods are classified into biological, chemical, physical and multiple or combinatorial pretreatments (Kurosumi et  al. 2008; Yamashita et al. 2010; Asada et al. 2011; Asada et al. 2015c; Asakawa et al. 2016; Zhang et al. 2016). Kishi et al. (2006) studied liquefaction as a wood treatment to recover the lignin content of the German spruce (Picea abies) for the manufacture of epoxy resins. First the liquefaction of the wood was carried out, and then the synthesis of epoxy resins from liquefied wood was made. The authors evaluated the resorcinol (catalyst)/wood ratio and the time of liquefaction at various treatment times, observing that the liquefaction time did not provide any significant effect on the flexural strength and resin modulus. However, the average molecular weight (Mw) after the liquefaction process was increased, and also the flexural strength and the resin modulus was increased using the catalyst. These authors concluded that flexural strength and modulus of wood-based epoxy resins is equivalent to bisphenol-A epoxy resin at room temperature. Kishi et al. (2006) also indicated that the epoxy function of the wood-based resin is controlled by the concentration of phenolic OH groups in wood liquids, which is the dominant factor for crosslinking density and the properties of cured epoxy resins. Muranaka et  al. (2015) studied the process of extracting lignin from different lignocellulosic plants such as cedar, rice straw and beech (with particle sizes of 150 μm), using a water acetone mixture as a medium of extraction for the development of cured epoxy resins. These authors determined that lignin was depolymerized using a flow reaction system, and the Mw of these materials was low and had many active sites for the curing process, resulting in a uniform curing activity. Muranaka et al. (2015) also confirmed that this resinification method of lignin is adaptable to some types of biomass, and the resins obtained are competitive with a conventional phenolic novolac resin. Ferdosian et al. (2015) studied the synthesis of lignin epoxy resin from DOL and DKL, which were depolymerized by ECH under alkaline conditions in the presence of a phase transfer catalyst. The Mw of the DOL/epoxy mixture (≈ 790–2800 g/ mol) was greater than the DKL/epoxy mixture (Mw ≈ 500–1400 g/mol). The lignin epoxy resin was also cured with DDM and DETA to produce a three-dimensional crosslinking structure with the following formulations: DKL-DETA, DOL-DETA, DKL-DDM and DOL-DDM.  The results obtained from this study showed that lignin-­based epoxy samples cured with DDM curing agent had greater thermal

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stability than those cured with DETA. However, the thermal stability of all lignin-­ based samples was comparable to the thermal properties of petroleum-based epoxy resins. In addition, the char yield at 800 °C for the DKL-DDM formulation (38%) was much higher than conventional bisphenol A-based epoxy resins. This suggests that the DKL-DDM formulation could be a promising substitute for petroleum-­ based epoxy resins. Ferdosian et al. (2016a) continued studying bisphenol A-based epoxy resins containing DOL and DKL as composites. These results showed that maximum degradation temperature (Tmax) was almost stable between 397 and 404 °C, and this was very similar to bisphenol A-epoxy resin (405 °C). In addition, the oxygen limiting index (LOI) of all lignin-based epoxy composites was higher than that of conventional bisphenol A-based epoxy resin, and they also showed greater thermal resistance even when compared to the diglycidylether bisphenol A (DGEBA)-cured epoxy resin. In particular, the 100% DKL-DDM formulation was the most thermally resistant system, and 75% DKL-DDM, 100% DKL-DDM, and 100% DOL-­ DDM formulations can be classified as self-extinguishing materials. Ferdosian et al. (2016b) also studied the synthesis of biobased epoxy resin using DHL obtained from low pressure de-polymerization. They found that Mw of DHL varied from 2100 g mol to 5530 g/mol after the epoxidation reaction, and the thermal degradation activation energy of the DHL-epoxy-DDM system was greater than the DGEBA-DDM system during the curing process, which can be associated with low molecular mobility of the DHL-epoxy resin and the highest Mw and viscosity. In addition, mechanical properties such as tensile strength, flexural strength, Young’s modulus and flexural modulus decreased significantly when the percentage of lignin-based epoxy resin was higher. This suggests the potential of using lignin-­ rich wastes and by-products to obtain greater added-value biobased materials. The study of epoxy resins cured with Ligno Boost™ softwood kraft lignin (KL) isolated from the original black liquor was reported by Jablonskis et al. (2018). For this, lignin-based epoxy resins were obtained in two ways: extraction with acetone of glycidylated lignin and glycidylation of the acetone-soluble lignin fraction. They found that the addition of 2–10% of commercial bisphenol A-based Araldite LY1564 decreased the Tmax and increased the following mechanical parameters: stress at break, Young’s modulus, and strain at break, and also a tendency to increase the Young’s modulus of lignin-containing cured epoxy resins was observed. The lignin incorporated into the cured commercial epoxy resin matrix acts as a charcoal forming promoter in the high temperature treatment. Sasaki et al. (2013) extracted lignin from bamboo moso using the steam explosion methodology followed by water and methanol extractions for the synthesis of epoxy resins of lignin cured with DGEBA or 1-(2-cyanoethyl)-2-ethyl-4-­ methylimidazole (2E4MZ-CN). These authors determined that thermal decomposition properties of the lignin-based epoxy resins exceeded the dip-solder resistance (250–280 °C), and the mechanical properties indicated the potential usefulness of manufactured resins. Asada et al. (2015a) studied the synthesis of the epoxy resin containing lignin extracted from bamboo moso (Phyllostachys pubescens), cedar (Cryptomeria

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japonica) and eucalyptus (Eucalyptus globulus) using steam-exploded methodology, and these materials were then functionalized by reaction with ECH, and catalyzed by a water-soluble phase transfer catalyst (tetramethylammonium chloride) under alkaline conditions (30% aqueous NaOH) for ring closure using methyl ethyl ketone as a solvent. The authors found good yields for biomass-derived epoxy resins (63.4–68.2%), and these were in line with the yield from bisphenol A (70%), thus suggesting that biomass-based epoxy resins could be synthesized using the same synthetic route that bisphenol A-based resins. In addition, the higher lignin content in the cured epoxy resins reduced the Tmax. Benyahya et al. (2014) reported that biobased epoxy resin cured with green tea extract (catechin with isophorone diamine) had a Tmax (299 °C) much lower than those cured low-Mw lignin epoxy resins (298–336  °C). Asada et  al. (2018) also extracted high pure lignin (99%) (Mw ≈ 5100 g/mol and hydroxyl equivalent) from white poplar (Populus tremuloides) wastes using a steam explosion methodology followed by water and acetone extractions for the development of epoxy resins. In line with this, Asada et  al. (2015a) determined that high Mw and the hydroxyl equivalent lignin was extracted with acetone compared to those extracted in methanol. This means that not only low Mw lignin was extracted with acetone, but also high Mw lignin. In addition, the tensile strength of the cured epoxy resin obtained by Asada et al. (2015a) was (~30 MPa) similar to those obtained from epoxy resins derived from conventional fossils (~27–80 MPa). Yin et al. (2012) studied the preparation and properties of the lignin-epoxy resin composite using enzymatic hydrolysis from corn straw lignin and bisphenol. The epoxy resin could be cured together with the lignin at high temperature, but the cure temperature for the lignin-epoxy resin mixtures could be reduced by the introduction of a polyamide curing agent. The lignin-epoxy resin composite with a lignin content of up to 60% was prepared by mixing lignin with epoxy resin and polyamine using a hot-pressing molding process. The optimum performance of composite materials was achieved at the molding temperature of 130 °C. Ortiz et  al. (2019) studied the biobased epoxy resins from biorefinary by-­ products, i.e. BCD lignin containing GDE at different mass ratios: 1:1, 1:2, and 1:4 (BCD:GDE). A direct correlation was observed between the glass transition temperature (Tg) of the resin and the lignin content of the samples manufactured under vigorous mechanical stirring. They found that resins containing a higher BCD lignin content resulted in brittle samples. The direct use of BCD lignin for the synthesis of epoxy resin was also explored by Ortiz et al. (2019), since they are carried out in an aqueous medium, i.e. the use of organic solvents is not considered. However, the presence of high levels of inorganic salts with the lignin (27%) negatively affected the mechanical properties of the resins. Qin et al. (2013) reported that Kraft lignin (KL) was partially depolymerized by base catalyzed depolymerization (BCD) in supercritical methanol to increase its solubility in organic acids, and the resulting partially depolymerized lignin (PDL) was then converted to lignin-based polycarboxylic acid (LPCA) when reacting with succinic anhydride. These authors have indicated that LPCA could act as a curing or cocuring agent in the preparation of epoxy resins (Qin et al. 2013). It was also

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noted that LPCA could act as a curing agent to cure a commercial epoxy (DER 353) in a temperature range similar to commercial anhydride curing agents. The LPCA-­ cured DER 353 resin exhibited a moderate Tg, i.e. 62.3 °C, and a storage modulus comparable to curing with a commercial anhydride curing agent. The solid LPCA could be used together with other liquid curing agents such as glycerol tris (succinate monoester) (GTA) and commercial hexahydrophthalic anhydride (HHPA) to cure epoxies. The results found by these authors suggest that the use of a mixture of LPCA and a liquid curing agent not only adjusted the viscosity of the resin system but also largely regulated the dynamic mechanical properties and thermal stability of cured resins (Qin et al. 2013). This study also demonstrated that PDL can serve as a raw material for the preparation of the curing agent and be used for epoxy applications. Wang et  al. (2019) reported a new use of lignin to produce graphene-based waterborne epoxy (WEP) nanocomposites from pristine graphite in a water-only route. The interactions between graphene particles and OH groups from lignin were confirmed. Composite WEP coatings containing 0.5% graphene showed excellent corrosion resistance compared to pure WEP coating. This was explained due to the good dispersion and interactions between graphene and lignin particles, resulting in improved thermal and mechanical properties, as well as ultra-high barrier properties. This was also attributed to the ‘labyrinth effect’ caused by the addition of lignin. The prepared nanocomposite had the potential application as an anticorrosive coating of the metal substrate, which provided a new strategy for the use of lignin of high added value. Abdul Khalil et  al. (2011) reported that lignin was successfully isolated from EFB obtained from black liquor, and was used as a curing agent for epoxy resin matrices. These authors concluded that the mechanical properties of these materials were optimal when the composites were made from epoxy resins containing 25% EFB (Abdul Khalil et al. 2011). Liu et  al. (2014) reported about a novel approach to harden epoxy resin with lignin obtained from common waste materials from the pulp and paper industry. Carboxylic acid-functionalized alkali lignin (AL-COOH) was prepared and subsequently incorporated into anhydride-cured epoxy networks via a one-pot method. The addition of 1.0 wt.% AL-COOH produced increases of 68% and 164% in the critical stress intensity factor and critical strain energy release rate, respectively, compared to the pure epoxy. It is noteworthy that the tensile strength of the epoxy-­ lignin resin was almost the same as that of the pure epoxy, while Young’s modulus of the epoxy-lignin resin increased moderately as the ALCOOH load increased. The effectively hardened epoxy resins with low-cost biorenewable lignin have considerable potential for use as durable coating materials or matrix materials for high performance composites. Xin et al. (2016) synthesized lignin-based epoxy resins from the reaction of ECH and PDL. These authors found that the synthesized resins had a viscoelastic yield greater than the DER332 epoxy resin, especially at elevated temperatures. Nisha et al. (2019) compared different properties of epoxy composites containing lignin modified with triethylammonium hydrogen sulphate and pure epoxy

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resin. This work showed that the high functionalization of lignin led to an 80% increase in flexural strength (99.10  MPa), a 57% increase in flexural modulus (2.84 GPa), a 52% increase in tensile strength (40.90 MPa) and a 23% increase in toughness (86.08 kJ/m3), compared to pure epoxy matrix. Nagatani et al. (2019) studied the simple preparation of lignin-based epoxy resin using aqueous solutions of organic onium hydroxide as a solvent. They observed that the use of a crosslinking agent managed to control the properties of the developed coatings.

9.3  Conclusions and Remarks Lignin extracted from different sources of biomass and agricultural wastes has great potential as a raw material of epoxy resin and curing agent, because they have thermal and mechanical properties comparable to commercial petroleum-based epoxy resins mainly derived from bisphenol A. This chapter demonstrates the potential of lignin as a green material in the field of development of polymeric coatings (Allen et al. 2019). Acknowledgements  The authors would like to thank the partially support by a Grant-in-Aid in Young Scientists (A) (No.17H04717) and a Grant-in-Aid for Scientific Research (A) (No.16H01790) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Conflicts of Interest  The authors declare no conflict of interest.

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Sasaki, C., Wanaka, M., Takagi, H., Tamura, S., Asada, C., & Nakamura, Y. (2013). Evaluation of epoxy resins synthesized from steam-exploded bamboo lignin. Industrial Crops and Products, 43, 757–761. https://doi.org/10.1016/j.indcrop.2012.08.018. Scott, F., Quintero, J., Morales, M., Conejeros, R., Cardona, C., & Aroca, G. (2013). Process design and sustainability in the production of bioethanol from lignocellulosic materials. Electronic Journal of Biotechnology, 16(3), 1–15. https://doi.org/10.2225/vol16-issue3-fulltext-7. Silalertruksa, T., & Gheewala, S.  H. (2010). Security of feedstocks supply for future bio-­ ethanol production in Thailand. Energy Policy, 38(11), 7476–7486. https://doi.org/10.1016/j. enpol.2010.08.034. United Nations Climate Change Conference. (2015). Paris Agreement. Vlcek, T., & Petrovic, Z. S. (2006). Optimization of the chemoenzymatic epoxidation of soybean oil. Journal of the American Oil Chemists’ Society, 83(3), 247–252. https://doi.org/10.1007/ s11746-006-1200-4. Wang, S., Hu, Z., Shi, J., Chen, G., Zhang, Q., Weng, Z., Wu, K., & Lu, M. (2019). Green synthesis of graphene with the assistance of modified lignin and its application in anticorrosive waterborne epoxy coatings. Applied Surface Science, 484, 759–770. https://doi.org/10.1016/j. apsusc.2019.03.229. Xin, J., Li, M., Li, R., Wolcott, M. P., & Zhang, J. (2016). Green epoxy resin system based on lignin and tung oil and its application in epoxy asphalt. ACS Sustainable Chemistry & Engineering, 4(5), 2754–2761. https://doi.org/10.1021/acssuschemeng.6b00256. Yamashita, Y., Sasaki, C., & Nakamura, Y. (2010). Effective enzyme saccharification and ethanol production from Japanese cedar using various pretreatment methods. Journal of Bioscience and Bioengineering, 110(1), 79–86. https://doi.org/10.1016/j.jbiosc.2009.12.009. Yin, Q., Yang, W., Sun, C., & Di, M. (2012). Preparation and properties of lignin-epoxy resin composite. BioResources, 7(4), 5737–5748. https://doi.org/10.15376/biores.7.4.5737-5748. Zeleke, T. D., & Ayana, Y. M. (2017). Epoxidation of vernonia oil in acidic ion exchange resin. American Journal of Applied Chemistry, 5(1), 1–6. https://doi.org/10.11648/j.ajac.20170501.11. Zhang, C.  W., Zia, S.  Q., & Ma, P.  S. (2016). Facile pretreatment of lignocellulosic biomass using deep eutectic solvents. Bioresource Technology, 219, 1–5. https://doi.org/10.1016/j. biortech.2016.07.026.

Chapter 10

Reactive Silicones as Multifacetic Materials Suranjan Sikdar and Sukanta Majumdar

Abstract  Silicones are very useful materials that vary in their structure, reactivity and physicochemical properties, although they contain a covalent bond between silicon and the carbon atom of an organic group. Most silicone polymers are artificial because the organo-silicon bond is not found in nature. The study of the silane coupling agent has also revealed that it plays an important role in improving the durability and performance of silicone as softeners, especially the type of linear reaction. The improvements in wrinkle recovery are mainly due to the formation of an elastic silicone polymer network, which traps fibers within its matrix, which improves the fabric’s ability to recover from deformation. The results have indicated several areas of technical application for the modified fabric, such as barrier textiles with permeability control, localized modification of the mechanical properties of the fabric. The enormous opportunities in the design, synthesis and modification of the physical and chemical properties of polymers have made them the fastest growing group of materials, having great importance and possibilities for applications in cosmetology, medicine and pharmacy. In this chapter, we focus on a general description of the silicone polymers used, including polydimethylsiloxane (PDMS), with respect to their physicochemical properties and factors affecting their current applications. Finally, the use of silicone polymers as excipients in the technology of various products, e.g. skin adhesive patches and controlled drug delivery systems, are presented. The synthesis of a new class of reactive silicone resins containing vinyl and methacryloxypropyl substituents has also been discussed below. Organo-­ functional silanes, their chemistry, properties, uses and the main laboratory experiments that may also be of interest to the food and beverage industry.

S. Sikdar Department of Chemistry, Government General Degree College at Kushmandi, Dakshin Dinajpur, West Bengal, India S. Majumdar (*) Microbiology and Microbial Biotechnology Laboratory, Department of Botany, University of Gour Banga, Malda, West Bengal, India © Springer Nature Switzerland AG 2020 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_10

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Keywords  Poly(diphenylsiloxane) · Polymerization · Polysiloxanes · Silicone fluids · Silicone rubber · Thixotropy

10.1  Introduction Today, the term silicone is used principally in conjunction with the technical applications of polysiloxanes (Hill 2005). Silicones have specific properties compared to other similar types of polymers due to their bonding pattern with organic groups and inorganic atoms. The Si-O-Si link is best represented by the term ‘siloxane’ (Mojsiewicz-Pieńkowska et al. 2016). Strictly speaking, all silicones should be correctly referred as ‘polysiloxanes’ (Hill 2005). Silicones are a diverse family of special high-performance materials that include reactive silanes, silicone fluids and silicone polymers, which are widely used in many industries, including construction, electronics, food and paints (Longenberger et al. 2017). They are also used in those materials that provide essential benefits in key segments of our economy, including aerospace, construction, electronics, health and personal care, and transportation. Several silicone polymer products used daily (polydimethylsiloxane  PDMS, Fig. 10.1) allow them to enter different wastewater treatment plants (Rajai et al. 2016). This is attributed to the fact that the PDMS polymer is water insoluble and does not cause any effect in different treatment processes (McDonald et  al. 2000). The sludge formation in the process is then decomposed by biochemical treatment and spread in the agricultural fields as a good fertilizer. This latest removal technique allows PDMS to enter the soil environment (Aleklett et al. 2018).

10.2  Structures and Properties of Silicones Silicones are synthetic polymers with a silicon-oxygen bond and with organic groups linked to the silicon atoms by C-Si bonds (Fig.  10.2). Organofunctional group-containing silanes are hybrid compounds that combine the function of a reactive organic group with the inorganic characteristic of an alkyl silicate, in a single molecule (Jackson et al. 1963). This allows them to be used as molecular bridges

Fig. 10.1  Three-dimensional (3D) structure of PDMS

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Fig. 10.2  Structure of tetrahedral alkylsiloxane chain link

between organic polymers and inorganic materials. The most widespread organofunctional silanes are trialkoxysilanes (Blackledge and McDonald 1999). The most important functional groups are amino, glycidoxy, methacryloxy, sulfur and vinyl. In recent years, α-silanes have gained increasing importance compared to other silanes (Waerder et al. 2015). Instead of the usual propylene spacers, these α-silanes have a methylene bridge between the Si atom and the functional group, which makes them much more reactive than conventional g-silanes (Abdelmouleh et al. 2002). The increase in reactivity of the alkoxy has several advantages. Not only does it open new applications for α-silanes (such as fast-setting adhesives), but it also makes them much more interesting building blocks for use in established applications (Waerder et al. 2015).

10.2.1  Physical Properties of Silicones Polymers Silicone is the heaviest congener of carbon on a periodic table and less electronegative than carbon. But it was found that there were large number of analogous silicone compounds that are formed like carbon. So, the carbon atom is replaced by silicon to form compounds similar to those of methane (Table 10.1). Among these silicone compounds, not all behave similarly with carbon compounds. Some similarities between the Si-X bond in silicones and the C-X bond in carbon compounds (Seyferth 1984; Mark et al. 1986; Morse 1999): Between any given element and Si, the bond lengths are longer than for C with this element. The lower Si electronegativity (1.8) vs. C (2.5) leads to a much-polarized Si-O bond, highly ionic and with a high bond energy: 452 kJ/mole (108 kcal/ mol). The Si-C bond has a bond energy of 318 kJ/mole (76 kcal/mol), slightly lower than a C-C bond, while the Si-Si bond is weak: 193 kJ/mole (46.4 kcal/mole). These values partially explain the stability of silicones: the Si-O bond is highly resistant to homolytic fission. On the other hand, heterolytic fissions are easy for reactions that occur during polymerizations, which are catalyzed by acids or bases. Si atoms do not form stable double or triple bonds of the sp2 or sp. type with other elements, but the proximity of the d-orbitals allows dπ-pπ back bonding. Because of this back bonding, trialkylsilanols are more acid than the corresponding alcohols. However,

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Table 10.1  Comparison of bond lengths and ionic characters of various carbon and silicone bonds Element (X) Si C H O

Bond length (Å) Si-X 2.34 1.88 1.47 1.63

C-X 1.88 1.54 1.07 1.42

Ionic character (%) Si-X C-X – 12 12 – 2 4 50 22

Fig. 10.3  Structure of tetradecamethylhexasiloxane: Me3SiO(SiMe2O)4SiMe3 or MD4M

the involvement of back bonding is questioned (Kim et al. 2018). Another example of the difference between analogues is the tetravalent diphenyldisilanol (Gilman and Diehl 1961), silicone compound such as (C6H5)2Si(OH)2, which is more stable than a gem-diol of carbon compounds (Tacke et al. 2000). The Si-H bond in silicone compounds are weakly polarized, but the bond is more reactive than the C-H bond in the carbon compounds. In general, there are few similarities between a silicone polymer and a hydrocarbon polymer (Kim et al. 2018). Silicones show the unusual combination of an inorganic chain similar to silicates and often associated with high surface energy but with side methyl groups that are, on the contrary, very organic and often associated with low surface energy (Kim et al. 2018). The Si-O bonds are strongly polarized and without protection should lead to strong intermolecular interactions (Seyferth 1984). However, the methyl groups, only weakly interact with each other, protect the main chain (Fig. 10.3). The high flexibility of siloxane chain due to the very low rotational energy barrier of the Si-O bond can adopt different conformations. The free rotation of C-C bonds of -CH2-CH2- in polyethylene is about13.8  kJ/mol, while the free rotation energy of Si-O bonds is only 3.3 kJ/mol (Stark 1982). In the siloxane chain, the methyl groups are exposed in space, while hydrocarbon polymers, the methyl groups are less exposed due their rigid back bones. The interaction between the silicone chains is of a weak nature and the distance between the chains is greater in silicone compounds. Despite its polar chains in silicone compounds, it is compared with paraffin, with a low critical surface tension of wetting (Kim et al. 2018). Different conformations of the silicone compounds have been confirmed by monolayer adsorption assays in water (Corning 2001). Two structures have been proposed, one open where the Si-O-Si bonds are oriented towards the water and a more compact one where the chain adopts a helicoidal structure. The important point is that the energy difference between two structures is very low, which suggest the flexibility nature of siloxane chain (Lewis 1962). However, the Si-O- framework of silicone provides the stability of polymer such as silica. For this reason, silicone compounds can be used instead of organic compounds where they would decompose or melt.

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Fig. 10.4 Structural similarity of silane and methane

Fig. 10.5  Structure of poly(diphenylsiloxane-co-­ diphenylsiloxane), dihydroxy terminated

To distinguish between different silicon compounds, a systematic name is used based on its monomer. The simplest silicone compound is silane (SiH4), which belongs to a homologous series of silanes, corresponds to alkanes whose simplest member is methane (Fig. 10.4). The term siloxane is used as it contains a silicon atom, an oxygen atom and a saturated organic moiety. If the organic groups linked to the siloxane chain are aryl groups or phenyl groups, the resulting silicone compounds are called poly(diphenylsiloxane) (Grassiek et  al. 1980) and have repetitive units along the chain (Fig. 10.5). The surface characteristics of silicones show different properties (Kim et  al. 2018), such as: (i) PDMSs have a low surface tension, is about 20.4 mN/m and can moisten most surfaces. With the methyl groups pointing to the outside, this gives very hydrophobic films and a surface with good release properties, particularly if the film is cured after application. The silicone surface tension range (20 to 30 mN/m) is also the most promising range considered for biocompatible elastomers. (ii) Glass transition temperatures are very low (e.g. 146 K for a PDMS compared to 200 K for polyisobutylene) (Kim et al. 2018). (iii) Due to the high volume of silicones compared to hydrocarbons, the high solubility and diffusion of gas in it is explained. They have a high permeability to O2, N2 and H2O although liquid water is not able to moisten a silicone surface (Seyferth 1984). (iv) The activation energy of viscous silicone for movement is very low and has less temperature effect than hydrocarbon polymers. In addition, the chain entanglements are involved at high temperature and contribute to limit the viscosity reduction (Abdelmouleh et al. 2002). (v) A small amount of phenyl groups introduced into their chains affects crystallization and the polymer to remain more flexible at low temperature and increase the refractive index.

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(vi) The solubility parameter of the polymer containing trifluoropropyl groups has changed from 7.5 to 9.5 cal/cm3. These polymers are also used to prepare elastomers with little swelling in aromatic solvents (Hamciuc et  al. 2014; Meador 2015).

10.3  Manufacture of Silicones The development of silicone chemistry includes several main steps (Hamciuc et al. 2014; Meador 2015). In 1824 Berzelius had prepared silicone by reducing potassium fluorosilicate with potassium as follows: (10.1) 4K + K 2 SiF6 → Si + 6KF Silicon atom reacts with chlorine, giving a volatile compound known as tetrachlorosilane (SiCl4) Si + 2Cl 2 → SiCl 4



(10.2)

Later, Friedel and Craft (Seyferth 2001) synthesized the first organosilicon compound (tetraethylsilane): 2 Zn ( C2 H 5 )2 + SiCl 4 → Si ( C2 H 5 )4 + 2 ZnCl 2





(10.3)

In 1871, Ladenburg observed that the presence of a diluted acid (diethyldiethoxysilane - (C2H5)2Si(OC2H5)2), gave an oil that decomposed only at a ‘very high temperature’ (Fig. 10.6). Kipping (Januszewski et al. 2017) laid the foundation of silicone chemistry and made several silanes by means of Grignard reactions and hydrolysis of chlorosilanes to produce large molecules. The polymeric nature of silicone compounds was confirmed by Stock’s work (Barroso et al. 2019). In 1940s, silicones become commercial materials after demonstrating the thermal stability and high electrical resistance of silicone resins by the Rochow (Wake 1988) method to prepare silicones from Si and MeCl.

10.3.1  Synthesis of Different Chlorosilanes Different chlorosilanes can be formed, such as RSiCl3, R2SiCl2 and R3SiCl (where R is an organic group), by the reaction of silicon and chloromethane at a 550  K temperature and under a moderate pressure in presence of a copper-based catalyst (Masaoka et al. 2006). For example:

Si ( g ) + 2CH 3 Cl ( g ) → ( CH 3 )2 SiCl 2 ( g )



(10.4)

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Fig. 10.6  Structure of diethyldiethoxysilane

Fig. 10.7  Structures of different methylchlorosilanes

The mixture of liquids contains these three compounds (Fig. 10.7): Pure fractions of each product can be obtained by careful distillation of the liquid chlorosilane mixture. Among them, dimethylchlorosilane is the main product (ca. 70–90%) depending on the reaction conditions.

10.3.2  Nucleophilic Substitution of Chlorosilanes Dialkyldichlorosilane is hydrolyzed to a molecule with two hydroxyl groups due to the nucleophilic attack by water: R

R

(10.5)

Cl − Si − Cl + 2H 2 O → HO − Si − OH + 2HCl

R R The product obtained is disilanol. The suffix-ol in silanol is to show that the molecule contains at least one hydroxyl group bonded to the silicon atom and the simplest example of dimethyldisilanol (Fig.  10.8) (Shaffer 1957; Andrianov et al. 1964). This nomenclature is similar to that of alcohols, the simplest alcohol with two hydroxyl groups is ethane-1,2-diol or ethylene glycol (Fig. 10.8): The hydroxyl groups of silanols react spontaneously with each other to give a siloxane:



10.6

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Fig. 10.8 Structural similarity from dimethyldisilanol (left) and ethane-1,2-diol (right) Fig. 10.9  Structure of cyclic PDMSs

When R is a methyl group, it is a PDMS (McDonald and George 2002) (where n = 20–50) which is not a useful silicones because of its small chains. Such types of polymers are known as oligomers. In this reaction, some cyclic polymers ((CH3)2SiO)4 (Fig. 10.9) are also produced and easily separated from this reaction medium (Flaningam 1986). All resulting oligomers are washed and dried. Hydrochloric acid is produced when chlorosilanes are hydrolyzed, recycled and reacted with methanol to regenerate chloromethane.

CH 3 OH + HCl → CH 3 Cl + H 2 O

(10.7)

10.3.2.1  C  ondensation Polymerization for the Formation of Silicone Polymers The oligomers can be rapidly condensed in the presence of an acid catalyst to form the long chain silicone polymer shown below:

10.8



The value of (m + n) is usually between 2000 and 4000. The production of longer chains is favored if water is removed and this can be achieved under vacuum condition. Phenylchlorosilanes can also be prepared by the chemical reaction between a Grignard reagent and methyltrichlorosilane as follows (Gross 2015):

MeSiCl3 + C6 H 5 MgBr → Me ( C6 H 5 ) SiCl 2 + MgClBr



(10.9)

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Other chlorosilanes are prepared from an existing silane, e.g. methylvinyldichlorosilane is obtained by the addition of methyldichlorosilane in acetylene using a platinum complex as a catalyst (Pohanish 2017).

MeHSiCl 2 + C2 H 2 → Me ( CH 2 = CH ) SiCl 2

(10.10)



It is also possible to replace the chlorine groups by means of alcoholysis:

R 3SiCl + ROH → R 3SiOR + HCl

(10.11)

In this way, several silanes with different functional groups (e.g. alkoxy and vinyl) can be prepared (Matinlinna et  al. 2004). This allows coupling reactions between inorganic surfaces and polymers in the manufacture of composite materials (Xie et al. 2010). In this sense, PDMSs are obtained by the hydrolysis of dimethydichlorosilane in the presence of excess water as follows: xMe 2SiCl 2 + H 2 O → x′′Me 2Si ( OH )2 ( disilanol ) → yHO ( Me 2SiO )n H ( linear ) + z ( Me 2SiO )m ( cyclic ) N = 20 − 50 and m = 3, 4, 5 ′′



(10.12)

This heterogeneous and exothermic reaction gives mainly disilanol (Me2Si(OH)2) (Plueddeman 1982), which is readily condensed by intra or inter molecular fashion in the presence of HCl acting as a catalyst to give linear or cyclic polymer (oligomers) (Flaningam et al. 1986; Forbes et al. 2014). The product mixture is separated from the aqueous acid phase and the ratio of two oligomers depending on the reaction condition, i.e. concentrations, pH, solvents, etc. The resulting HCl is recycled and reacted with methanol (MeOH) to give methyl chloride which is used for further reaction. The linear and cyclic oligomers are obtained by hydrolysis of dimethyldichlorosilane to produce short chain polymer for most applications (Forbes et al. 2014; Flaningam et al. 1986). They should be polymerized to give macromolecules of enough length (Seyferth 1984). The cyclics ((R2SiO)m) can be opened and polymerized to form long chains linear, thus catalyzing the reaction by many acid or base compounds (Plueddeman 1982) and giving at equilibrium a mixture of cyclic oligomers plus a distribution of polymers. The length of the polymer chain depends on the presence of substances able to give chain ends, e.g. the average chain length in the polymerization of (Me2SiO)4 depends on the KOH concentration (10.13).

x ( Me 2 SiO )4 + KOH → ( Me 2 SiO )y + KO ( Me 2 SiO )z H



(10.13)

A stable polymer containing OH-terminated groups (HO(Me2SiO)ZH) can be isolated after neutralization under vacuum at elevated temperature. In fact, a distribution of chains with different lengths is achieved.

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The reaction can also be performed in the presence of Me3SiOSiMe3, which acts as a chain end blocker as follows:

∼∼∼ Me 2 SiOK + Me 3SiOSiMe 3 →∼∼∼ Me 2 SiOSiMe3 + Me 2 SiOK (10.14)

where~~~ is the main chain. The resulting Me3SiOK attacks another chain of the polymer to reduce the average molecular weight (Mw) of the linear polymer formed. The copolymerization of (Me2SiO)4 in the presence of Me3SiOSiMe3 with Me4N+OH- as catalyst shows a surprising viscosity change over time (Seyferth 1984). A maximum viscosity is observed at the beginning of the reaction (Poerschmann et al. 2005). In the cyclic polymer, the presence of two oxygen atoms in each silicon atom makes them more susceptible to nucleophilic attack with a base catalyst than the silicon of the endblocker, which is substituted by a single oxygen atom. The cyclics are polymerized to form viscous chain quickly, which is subsequently reduced in length by the addition of terminal groups provided by the endblocker at a slower rate. This reaction can be represented as follows:

Me 3SiOSiMe 3 + x ( Me 2 SiO )4 → Me 3SiO ( Me 2 SiO )n SiMe 3



(10.15)

The removal of the catalyst for the preparation of the silicone polymer is always an important factor, because most of the catalysts that are used to prepare silicones can also catalyze the depolymerization process, which attacks along the chain, particularly in the presence of traces of water at elevated temperature. ∼∼∼ ( Me 2 SiO )n ∼∼∼ +H 2 O →∼∼∼ ( Me 2 SiO )y H + HO ( Me 2 SiO )z ∼∼∼



(10.16)

The removal of all remaining traces of the catalyst is essential to obtain the thermal stability of the silicone polymer as much as possible. Labile catalysts have been developed, which decompose above their optimum temperature. These can be removed by overheating to prevent catalyst neutralization (Seyferth 1984). The cyclic trimer (Me2SiO)3 is characterized by an internal ring tension and can be polymerized without re-equilibration of the resulting polymers. Low Mw cyclic polymers can be prepared containing only one terminal reactive function (living polymerization). The mixing of different cyclic polymers also allows the preparation of block or sequential polymers (Seyferth 1984). Linear polymers can be condensed in the presence of acids or bases (Plueddeman 1982; Seyferth 1984) in order to give long chains by intermolecular condensation of terminal SiOH ((10.17). ∼∼∼ O − Si ( Me )2 − OH + HO − Si ( Me )2 − O ∼∼∼



→∼∼∼ O − Si ( Me )2 − O − Si ( Me )2 − O ∼∼∼ +H 2 O

(10.17)

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A long-distributed chain is favored when working under vacuum or at elevated temperatures to reduce the wastewater concentration (Maguire and Fulweiler 2017). Acid catalysts are more efficient when organosilanol carries electron-donating groups (EDG), base catalysts are efficient when it carries electron-withdrawing groups (EWG). This will be important when a mixture of linear oligomers such as dimethyl and methylphenyl-polysiloxanes are condensed (Eq.(10.18) (McCoy et al. 2017). Me 3SiOSiMe 3 + x ( Me 2 SiO )4 + Me 3SiO ( MeHSiO ) y SiMe3

→ cyclic + Me 3SiO ( Me 2 SiO )z ( MeHSiO )w SiMe3

(10.18)

Apart from the above polymer (all methyl), a reactive polymer can also be prepared and obtained from polydimethyl-­methylhydrogenosiloxane (MDZDHWM). This polymer can be further functionalized through addition reaction as follows: Me 3SiO ( Me 2 SiO )z ( MeHSiO )w SiMe3 + H 2 C = CHR



→ Me 3SiO ( Me 2 SiO )z ( Me ( CH 2 CH 2 R ) SiO )w SiMe3

(10.19)

where R = alkyl, polyglycol. The above is all linear and made up of difunctional units instead of cyclics one. When methyltrichlorosilane is hydrolyzed in the presence of trimethylchlorosilane it leads to the formation of (Me3SiO1/2)x (MeSiO3/2)y or MxTy (Colas 1990). Its physical form and uses depend on the structure of the polymer. To form silicone elastomers, gels and resins, the long siloxane chains are induced the crosslinking the polymer (Gunatillake and Adhikari 2016). There are four ways to crosslink the polymers: (i) Crosslinking can be carried out by the functional group present in the silanes that reacts most. For example, a vinyl group such as vinylmethyldichlorosilane into silanes added to dimethyldichlorosilane induces crosslinking (De Cooke and De Bruyn 1975). However, the vinyl groups into a chain can also undergo a radical initiation reaction similar to the free radical polymerization reaction of chloroethene (Emblem 2012). In this polymerization reaction, the radical initiator is supplied from an organic peroxide such as dicumyl peroxide (Eq. 10.20) (Spetz et al. 2002).



10.20

(ii) Crosslinking can also be achieved by vinyl group containing siloxanes and other siloxanes containing Si-H groups in presence of a platinum catalyst (Eq. 10.21) (Nyczyk et al. 2012).

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10.21 (iii) Another way to produce crosslinking in the polymer is by introducing an ethanoyl group into the silane, and once it is exposed to air, moisture reacts with the functional group to give a crosslinked polymer, and also an organometallic tin compound can catalyze the reaction (Cervantes et al. 2012) These systems are often used as sealants and can be used in the home. Other formed products are recognized for their smell of vinegar. (iv) If methyltrichlorosilane is added to dimethyldichlorosilane three chlorine atoms are hydrolyzed, thus producing a 3D network: CH 3 CH 3SiCl3 + 3H 2 O → HO − Si − OH + 3HCl

CH 3

(10.22)

All the four previous crosslinking methods can be controlled in order to modify the physical properties of the silicone to obtain a rubber-like product. Dimethyldichlorosilane is separated from the solution for PDMS preparation by hydrolysis (Forbes et al. 2014). Its physical form and uses depend on the structure of the polymer. (i) Silicone fluids are typically linear chains of PDMS (Fig.  10.10), with the repetitive structure (Kavanagh et al. 2003): They usually have trimethylsilyl groups, -Si(CH3)3 at each end of the chain (Fig. 10.11): The silicones with short chains are fluids, which have a more or less constant viscosity over a wide range of temperatures (200 to 450  K) compared to

Fig. 10.10 Structural formula monomeric unit of trimethylsilyl group

Fig. 10.11  Structure of PDMS

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hydrocarbons, as well as they have very low vapor pressures (Barca et al. 2014). The low surface tension of silicone fluids gives them unique surface properties. They also have antifoaming properties and have been used to suppress the foaming of detergents in wastewater disposal plants. The low enthalpy of vaporization becomes more attractive for personal care products, such as perspirants and skin care lotions (Barca et al. 2014). Most fluids are low Mws containing polysiloxanes mixed with higher Mw. Some cyclic silicones are formed during the preparation of the linear polysiloxanes. (ii) Silicone gels are the PDMS chain and some cross-links are also present in the chain to give an open 3D network. After performing the crosslinking in silicone fluid, it is poured together with a reactive group into a mold and heated in the presence of a small amount of catalyst to form crosslinking between the polymer chains. This is a very effective technique for protecting sensitive electronic equipment from vibrational damage and the polymer also acts as an electrical insulator. Silicone gels are also used as shock absorbers in shoes, particularly in high-performance trainers and running shoes (Puri and Talwar 2009). (iii) Silicone elastomers are easily transformed into a 3D network and an elastomer via crosslinking reaction which allows the formation of chemical bonds between adjacent chains (Shit and Shah 2013). The structure is somewhat similar to natural rubber and they behave like elastomers. Its structure is determined by the amount of crosslinking and the length of the chains. The efficient crosslinking in the presence of radical initiator is achieved when there are some vinyl groups in the polymer chains (Baquey et al. 2005). Although its strength is lower than that of natural rubber at ordinary and high temperatures (200–600 K), and silicone rubbers are more stable than natural rubbers (Wu and Zhang 2019). They are generally more resistant to chemical attack. Silica is added as a filler (unit 53) to make the elastomer stronger mechanically and chemically (Pavlov and Khalatur 2016). (iv) Silicone resins have a 3D structure (Fig. 10.12) with the tetrahedrally surrounded atoms around the silicon atoms (Robeyns et  al. 2018). The resins are

Fig. 10.12 Network structure of silicone resins. R = CH3, H or OH

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usually applied as a solution in organic solvents and are used as an electrical insulating varnish or for paints where water repellence is desired to protect the masonry (Righetti et al. 2016). The -OH groups present in resin react to the hydroxyl groups which are present in the various inorganic surfaces, such as glass and silica gel, and making the surface water repellent (Gurav et al. 2011). A large number of silanes, known as coupling agents, have been developed to allow chemists to bond an inorganic substrate (such as glass, metals and minerals) to organic materials (e.g. organic polymers, such as acrylics, polyamides, polyalkenes and urethanes) (Goyal 2006).

10.4  Uses and Benefits Silicones impart a number of benefits to the products in which they are used, including greater flexibility and moisture, heat, cold and ultraviolet (UV) radiation resistance (Fischer et al. 2013). Silicones can be manufactured in many forms, such as greases, liquids, oils, rubbers, semi-viscous pastes and solids (Fig.  10.13) (Rey et al. 2013).

10.4.1  Personal Care Products Silicones used in personal care products reduce the white residues and the sticky feeling of antiperspirants in deodorants. They are also ‘durable’ and help retain the color and luster associated with cosmetics, conditioners and shampoos, as well as impart better shine and allow skin care products to be made with a stronger sun protection factor (SPF). The wetting and spreading qualities provide a smooth and uniform application of cleansers, cosmetics, lotions and sunscreens (Azeem et al. 2008).

10.4.2  Energy Silicone Silicones are ideal materials for solar panel and photovoltaic applications due to their high durability, efficiency, profitability and can withstand the sun for years (Alaa et al. 2018). Most solar panels are made up of a series of silicone crystalline cells sandwiched between a front glass plate and a rear polymer plastic back-sheet supported within an aluminum frame. Fig. 10.13 Chemical structure of silicone rubber

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10.4.3  Electronics Keypads, keyboards and copier rollers are made from sturdy and durable silicones, as are many components of computers, mobile electronics and home entertainment equipment. Silicones also play an essential role in enabling LED lighting technology (Fig. 10.14). Silicones with high thermal stability and excellent dielectric properties allow their use in a variety of electrical transmission applications.

10.4.4  Aviation Because silicones can withstand extreme stresses and temperatures, silicone adhesives and sealants are used to seal and protect black boxes, doors, electric landing gear devices, electrical cables, engine gaskets, fuel tanks, hydraulic switches, upper compartments, ventilation ducts, windows, wing edges and wings (Rey et al. 2013; Dias et al. 2014).

10.4.5  Thickening and Thixotropy Pyrogenic silica is particularly important for controlling the flow and propagation of paints and coatings, adhesives and sealants, unsaturated polyester resins, cosmetics and pharmaceuticals (Brunner and Wakili 2014). Fig. 10.14 Moldable optical silicones allow LED lamps

Secondary optics (diffuser)

LED chip encapsulants

Adhesives for environmental seals

Thermal pottants for heat control

Optics (remote phosphor) Reflector materials

Conformal coating/ white reflecion Thermal interface materials

Conformal coating

Thermally conducive encapsulants for power components

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10.4.6  Reinforcement Natural and synthetic rubbers, and silicone elastomers should be reinforced with active fillers to give them the desired mechanical properties, such as elongation at break, hardness, tear resistance (notch resistance) and tensile strength. The required system properties can be individually tailored by means of the active filler such as carbon black and silica, which can effectively improve the mechanical properties (Sattayanurak et al. 2019).

10.4.7  Free Flow Agent Pyrogenic silica significantly improves the flow properties of powdered substances. In many areas, such as animal feed, bulk goods, fire extinguishers, foods, pharmaceuticals, powders for cosmetics, toners for photocopiers and powder paints in industrial and automotive coatings (Patel et al. 2009).

10.4.8  Thermal Isolation Pyrogenic silica has excellent thermal insulation properties, up to more than 1000  °C at room temperature. Typical applications include vacuum insulation panels (building thermal insulation, insulation of refrigerators), radiant heaters in stoves, car exhaust systems or fire safety systems for buildings (Peng and Yang, 2016).

10.4.9  Thermal Aging Resistance of the Silicone Polymer Silicone polymers are used in various fields of applications requiring high temperature resistance (Shit and Shah 2013). The PDMS degradation starts only at around 400  °C in the absence of impurities by thermogravimetry analysis. Its thermal stability must ensure its widespread use in depolymerization, oxidation and fire resistance. They are used for furnace seals, for insulation of electric cables, pump fluids and heat transfer fluids. In case of fire, silicone elastomer insulating an electrical cable decomposes to give insulating silica instead of electrically conductive carbon which is produced by the combustion of a hydrocarbon elastomer.

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10.4.10  C  hemical Aging and Weather Resistance of Silicone Polymers Silicones can be degraded by the substance acting as a catalyst at their elevated temperature (Plueddeman 1982). However, the hydrophobic nature of silicones limits their extent of interactions with many aqueous solutions and are used in the presence of many chemical compounds. The swelling properties of elastomers can be reduced by using copolymers in common solvents. Silicone polymers are used in the construction industry as sealants and behaves as a good weather resistance. Silicone polymers are less reactive towards water and UV light and their hydrophobic nature increases their efficiency. The low chain-to-chain interaction present in silicone polymers reduces the permanent bonding stresses and increases longevity.

10.4.11  Release Properties The transfer paper is fabricated by silicone paper coatings for self-adhesive labels (Kim et al. 2018). Silicones are generally applied as a liquid via an addition reaction to give film-coated paper. The adhesive surface tension (30–40 mN/m) does not allow the silicone to get sufficiently wet and reduced the actual surface, which allow the label to be easily removed from the silicone-coated paper (Corning 2001). The silicone elastomer can also be used to prepare a mold which is covered with a twopart elastomer. The silicone perfectly moisturizes the original surface and give a perfect negative with low surface tension. The low surface energy silicone allows to easily unmold this copy.

10.4.12  Silicone Rubber Nanocomposites Nanocomposites offer the potential for the diversification of applications for polymers due to their excellent properties, such as dimensional stability, flame retardation, high temperature resistance, and improved barrier and thermo mechanical properties. Special attention is given to the modification of the property of silicone rubber caused by the incorporation of filler for exterior insulation applications (Momen and Farzaneh 2011). For example, nanocomposite membranes based on PDMS and nanoscale SiO2 particles were prepared by Rao et al. (2012) via a convenient and mild sol-gel copolymerization of tetraethoxysilane, as well as a crosslinking reaction. The siloxane rubber/carbon black mixture also gives the benefit of a light weight compared to ferrite/rubber mixture (Al-Hartomy et  al. 2011). Following Kim et al. (2008) the nanometer-sized multiwalled carbon nanotubes (MWCNTs) were dispersed into a silicone matrix which leads to a marked improvement in the properties of the silicone-based composites. Silicone rubber/MWCNTs

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Fig. 10.15 Moldable optical silicones enable LED lamps

Superball

Silicate polymer (siliconoxygen it will bounce!!

nanocomposite was successfully prepared by functionalizing MWCNTs with silane compound (Kim et al. 2008). This allowed a homogeneous dispersion of functionalized MWCNTs in the silicone matrix. The silicone rubber/functionalized MWCNTs (1 wt.%) composites showed that the tensile strength and modulus of the composites improved dramatically by about 50 and 28%, respectively, compared to silicone rubber (Kim et al. 2008).

10.4.13  Super Ball Show The liquids solution of sodium silicate is already in the form polymer. The silicate is alternating silicon and oxygen atoms in long chains. When the ethanol is added, it bridges and connects the chains by crosslinking them. The analogy of a chain-link fence is a good picture of the idea of chains that are crosslinked. That is what the ethanol and silicate do to form this super ball (Fig. 10.15) (Meijer et al. 2017).

10.5  Silicones and Bio-Performance All categories of silicones can be used for direct contact with healthy skin (oxygen masks, babies’ bottles), temporarily contact with body fluids (dialysis, drains and catheters) and permanent contact with the body (joint prostheses, contact lens and protective cases for pacemakers) (Mojsiewicz-Pieńkowska et al. 2016).

10.5.1  The Notion of Biocompatibility The safety of silicones explains its numerous applications in food, human body, medical products, textiles, etc. Due to the complex and high nature of silicone polymers, there are various interactions. There are four phases of order of magnitude of

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the interactions, each phase correspond to a biological response of an intensity which correlates directly with the degree of biocompatibility of the material in question: (i) Initial physical, chemical and biochemical reaction between the surface of the biomaterial and the biological system; (ii) Modification by the biological environment of certain physicochemical properties of the surface of the material; (iii) Development of a local biological response in the immediate vicinity of the biomaterial and (iv) Dissemination of this local response throughout the body.

10.5.2  Biocompatibility of Silicones The biocompatibility of silicones is a direct consequence of the molecular structure of dimethylsiloxane (Table 10.2). From a toxicological point of view, the dimethylsiloxane chain is non-cytotoxic, which is a great advantage of silicones, since its purity is not compromised by using additives (e.g. antioxidants, plasticizers). Its excellent biocompatibility is partly due to low chemical reactivity and surface hydrophobicity, and high stability (Lee et al. 2003). The elastic nature of polymers reduces tissue stress. The low surface energy of the polymers reduces molecular and cellular adhesion, while their hydrophobic nature limits absorption of water molecules (Gokaltun et al. 2017).

Table 10.2  Correlations between silicone materials, performance and applications Silicone materials PDMS Organofunctional siloxane Silica + PDMSs Crosslinked PDMSs

Crosslinked PDMSs Reinforced with silica

Silicate resin in PDMSs

Key physical characteristics and performance Difusibility and film-forming Controlled occlusivity and hydrophobicity Antifoam Diluent, dispersing property Adjustable cure conditions: from low to high temperatures Foamable Mechanical resistance Adjustable modulus Adjustable cure conditions: from ambient to elevated temperature Adhesion to skin and various substrates (e.g. plastic films)

Medical and pharmaceutical applications Siliconization of needles and syringes Lubrication of medical devices Excipients for topical formulations Antiflatulent (APIs) Soft matrix for drug release

Recognized biocompatibility for human implantation (e.g. pacemaker) Medical adhesive (sealant) Film-former Film-former Transdermal drug delivery system

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10.5.3  Pharmaceutical Applications The diversity of the existing silicone products allows them to be used in most areas of pharmacology such as: active pharmaceutical excipient (e.g. development of sustained release formulations), active principle (e.g. antiflatulent fluids or emulsions, both with and without antacids), manufacturing aids (e.g. lubricants and tubes for transferring of liquids) and inert pharmaceutical excipient (e.g. glidants for powders and soft capsule shells). Silicone rubbers have been extensively used in medical. Silicone rubber tubes used in medical practice can cause some problems. The gas permeability of dimethylsilicone is high enough to cause the formation of bubbles in silicone tubes which are often used in pumps for delivery drug to patients. Some drugs are sensitive to oxygen permeate. A simple way to demonstrate this effect is to fill a section of silicone tube with water and then both ends, excluding any air bag. The water filled tube can be placed on any suitable ledge for several hours. It should be noted that air cavities are formed in tube lumen (Bradley et al. 1967). A good deal of controversy has involved the use of silicone in polyurethane bags as breast implants (Duscher et al. 2017). Again, they were used because they were thought to be very inert and resistant to dissolution or other reactions. Reports have said increased cancer risk and severe immune responses from possible leakage of the silicone from the implants. Some scientists dispute these findings (Grigg et al. 2000).

10.5.4  Epidemiology With the resolution of the legal controversy over silicone gel-filled breast implants underway, these medical devices remain available with some restriction in the U.S., where they have been used since the early 1960s. However, outside of the U.S. and Canada, access to these devices has no restrictions. Epidemiology studies have consistently found no association between breast implants and breast cancer (Hansson et al. 1996; Holmich et al. 2003). In fact, some studies suggest that women with implants may have a decreased risk of breast cancer (Brinton et al. 1996; David et al. 2019). Reports of cancer at sites other than the breast are inconsistent or attributed to lifestyle factors (Herdman and Fahey 2001). The research on autoimmune tissue disease has also been remarkably uniform and concludes that there is no causal association between breast implants and connective tissue disease (Mayesh and Vicari 2013).

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10.6  The Impact of Silicones on The Environment 10.6.1  Impact on Air, Soil and Water A large number of studies have been carried out to measure the fate and effect of silicones in the environment throughout their life cycle (Graiver et al. 2003). The synthesis of PDMSs is strictly controlled and environment emissions from this phase are very small. The environmental effects of silicones depend largely on the physical form of the material and the method of disposal. PDMSs are used as the component in consumer products, in many industrial processes. Solid and liquid silicones enter the environment as a component of waste materials, such as air conditioners, domestic, municipal waste, shampoos, silicone antifoams, etc. Silicone is also used as antiflatulents in pharmaceuticals (Schalau and Aliyar 2015). Most common silicones are very poorly soluble in water and are retained in mud that settles quickly (Pellenbarg 1979; Watts et al. 1995). In small-scale field studies, Tolle et al. (1995) showed that the application of sludge bound PDMSs to soil did not cause adverse effects on crops growth. Lehmann et al. (1994) and Griessbach and Lehmann (1999) also observed that high Mw PDMSs are too large to pass through the biological membrane of plants or animals. Extensive studies have shown that sewagesludge bound PDMSs degrade in soils contact with clay minerals (Lehmann et al. 1996). The clay acts as a catalyst to depolymerize the siloxanes backbone (Griessbach and Lehmann 1999). The main degradation product of PDMSs is dimethyldisilanol (Me2Si(OH)2) (Lehmann et  al. 1994). PDMS is even more degraded in the soil via biodegradation (Lehmann et al. 1998) or evaporated into the atmosphere via reaction with hydroxyl radicals (Lehmann et al. 1996). Due to the chemical structure of silicone, it does not cause any depletion of the ozone layer or nor does it contribute significantly to global warming (Verbiese 1993).

10.6.2  Recycling Most silicones are easily disposed and recycled. Silicones elastomers are used as fillers in new elastomers when ground. Depolymerized silicones in the presence of a catalyst to give cyclic silicones (Weidauer et al. 2015). In this way, we can reduce the amount of silicone polymer that is fed to the environment.

10.7  Conclusions By virtue of its structure, the polymethylhydrosiloxane is most commonly used because it exhibits unique properties, especially at the surface and the interface levels. Polyethylhydrosiloxane imparts not only water repellent but also shows

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greater organic compatibility. Silanol is terminated into polydimethylsiloxanes, reacts with siloxanes to produce foam like silicone materials. In the presence of oxygen and moisture influences the crosslink density and foam structure. The intrinsic purity and favorable toxicological profile make them a suitable candidate for different medical applications. Currently available data indicates that the silicones do not accumulate in the environment and their decomposition does not result in toxic waste. Finally, silicones offer substantial benefits for the study of tubes during cardiac surgery. Acknowledgments  Authors would like to acknowledge the Government of West Bengal for research facilities provided in both the education institute. Dr. Majumdar is thankful to the Hon’ble Vice Chancellor of the University of Gour Banga for providing research fund. Conflicts of Interest  The authors declare no conflict of interest.

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Chapter 11

Reactive and Functional Silicones for Special Applications Carmen Racles, Mihaela Dascalu, Adrian Bele, and Maria Cazacu

Abstract  With a history of almost 100 years, silicones -polymeric or oligomeric siloxanes- are well known as reliable materials with a wide range of applications, from home to the aerospace sector. The constant interest for these polymers is explained by their unique combination of properties, including chemical and physiological inertness, extreme resistance to ozone and corona discharge, film forming capacity, good thermal-oxidative and ultraviolet (UV) stability, high flexibility of the macromolecular chain, low dielectric constant, surface energy and transition temperatures, permeability to various gases, stability towards atomic oxygen and oxygen plasma, and UV-visible radiation transparency. The chemical modification of silicones is an active research area, since the attachment of several organic functional groups to the silicon atom imparts specific properties and triggers new applications. Functional siloxanes represent a bridge between siloxane and organic chemistry and combine valuable properties of silicones with reactivity and specific functions of organic moieties. In this chapter, the recent research progress in the field of functional organic siloxanes and their materials are reviewed, focusing on their applications in science and technology. Keywords  Functional siloxanes · Polysiloxanes · Silicone materials

11.1  Introduction Silicones, also known as polysiloxanes, are synthetic polymers consisting of an inorganic backbone chain made of alternating silicon and oxygen atoms, with organic side groups attached to each silicon atom, with general formula [–OSiR2–]n. This class of materials has a unique combination of properties (Kuo 1999), derived from the particularities of the siloxane bond. Its length (1.64 Å) indicates a partial double bond character and a lower basicity than that of ethers, while the very low C. Racles (*) · M. Dascalu · A. Bele · M. Cazacu Department of Inorganic Polymers, “Petru Poni” Institute of Macromolecular Chemistry, Iasi, Romania e-mail: [email protected] © Springer Nature Switzerland AG 2020 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_11

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energy barriers for the rotation around Si-O axis (2.5 kJ mol-1 in dimethylsiloxane) and the linearization of Si-O-Si angle (1.3 kJ mol-1) explain the unusual flexibility of the polysiloxane chain. The Si-O-Si angle (140–180°) is much wider than the tetrahedral angle, while the siloxane bond is one of the most stable bonds formed by silicon, being estimated in the range 422–494  kJ  mol-1 (Voronkov et  al. 1978; Chojnowski and Cypryk 2000). The extreme flexibility of the siloxane backbone is reflected in the low glass transition temperature (Tg) value which is around -125 °C in poly(dimethylsiloxane) (PDMS), the best-known representative of this class of materials. By modifying the organic groups attached to the silicon atoms or by replacing them with bulkier or more polar ones, the Tg of polysiloxanes can be increased as follows: C3H7 -120  °C, C3H4F3 -70  °C, C6H5 -90…-75 °C. Poly(diphenylsiloxane)s (PDPhS) show Tg between 50–100 °C depending on the molecular mass (Cazacu 2008). In PDMS, the weak intermolecular interactions and the methyl groups oriented toward the air interface lead to a significantly lower surface tension than in organic polymers (Özçam et al. 2014) and a marked hydrophobic character. In addition, silicones have excellent release properties and dielectric strength, good resistance to UV radiation and performance at extreme (high/low) temperatures, high thermal stability and oxygen permeability, minimum temperature effect on the properties and physiological inertness (Kuo 1999; Cazacu 2008). The chemical versatility of the siloxane precursors allows modifications with functional and reactive groups, thus extending the area of properties and applications. Therefore, the chemistry of organic silicon compounds is one of the fastest growing fields in synthetic organic chemistry. The development of new methodologies continues to be actively investigated (Panek 1991). The differently substituted polysiloxanes on the silicon atoms can be obtained by direct synthesis from suitable monomers (co-condensation or copolymerization) or by subsequent chemical modification (post-functionalization). Polysiloxanes containing Si-vinyl and Si-H groups constitute valuable substrates for functionalization, allowing a wide range of organic groups or functions such as alcohols, alkyls, amines, haloalkyls, thiols, etc. to be linked by hydrosilylation, which is the most accepted approach towards functional silicones, while the thiol-­ ene addition has been studied extensively to introduce thioether functions (Shankar et al. 2013). In fact, almost any other synthetic approach from organic chemistry can be easily adapted to modify the organic group attached to the silicon atom resulting in new compounds. End-functionalized polysiloxanes are very useful in the preparation of block and segmented copolymers by joining PDMS as a flexible moiety with organic polymeric sequences such as polyamides, polycarbonates, polyimides, polysulphones, polyureas, polyurethanes (PUs), etc. in various combinations and sequences within the backbone, which can induce a wide range of properties (Cazacu 2008). These systems generally exhibit biphasic morphology, due to the pronounced incompatibility between polysiloxanes and most organic polymers (Yilgör and McGrath 1988; Mark 2000; Racles et  al. 2004; Bronnikov et  al. 2005; Zha et  al. 2016). Elastomeric networks, functional telechelic oligomers, hyperbranched compounds,

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oligomeric polyhedral silsequioxanes or functional silica are materials with a diverse architecture (1D, 2D, 3D) which can be considered as functional silicones. Such materials are commercially available, but their diversity is innumerable and almost any organic group can be attached to a polysiloxane chain. The best-known applications of functional silicones are summarized here, beginning with a brief description of their synthetic methods and focusing on selected examples in the field of biomedical materials, electromechanical transducers, ligands, liquid crystals (LCs) and surfactants. Other applications in catalysis and material science are also briefly summarized. Functional silicone resins are not discussed, as these have been recently reviewed by Robeyns et al. (2018).

11.2  S  ynthesis of Functional Silicones: Classic and Modern Approaches There are different approaches that can be followed in order to chemically modify polysiloxanes, thus obtaining materials with the desired properties. The most commonly used methods are described, focusing on: condensation of functional silanes, ring opening polymerization (ROP) of functional cyclosiloxanes and post-­ polymerization chemical modification of polysiloxanes containing reactive groups.

11.2.1  S  ynthesis of Functional Polysiloxanes From Silane Monomers In general, the polymerization of silane monomers occurs through polycondensation, which is carried out by dehydration between silanols, dehydrochlorination of silanol and chlorosilanes, or dealcoholization of silanol and alkoxysilane (Ren and Yan 2016) (Fig. 11.1). Functional groups attached on the monomers are introduced laterally to the polysiloxane chain, while telechelic end-functionalization can be achieved using functional blockers, either disiloxanes or silanes. When no end-­ blocker is used, terminal hydroxyl groups are produced after hydrolysis. The acidic, basic or organometallic compounds (organic titanium and tin) are used as catalysts for the condensation reaction (Sun et al. 2014, 2003; Zhang et al. 2006; Gunji et al.

Fig. 11.1  Polycondensation of functional silane monomers

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2013; Kupareva et  al. 2013; Chen et  al. 2014; Hanamura and Nemoto 2014; Hanamura et al. 2014; Kaneko et al. 2014; Lee et al. 2014; Liu et al. 2014; Tokunaga et al. 2015; Kowalewska and Nowacka 2015). It has been established from the literature that: (i) acid catalysts are appropriate for the condensation of silanols with electron-donating groups, while the basic catalysts for the condensation of silanols with electron-withdrawing groups (Williams et  al. 1976), (ii) the highly diluted solution favors formation of cyclic oligosiloxanes, (iii) the microstructure of the co-­condensing products of the differently substituted silanes is difficult to control. However, polycondensation is an important method for the synthesis of high molecular weight (Mw) siloxane homopolymers and block-copolymers from preformed siloxane oligomers properly end-functionalized with hydroxyl, halogen or alkoxy groups. This reaction is also important in the synthesis of modified siloxane polymers when organic sequences are inserted into the polymer chain (Noll 1968; Dvornic and Lenz 1990). An example illustrating this reaction pathway is found in Stepp et  al. (2015) who obtained copolymers with chloromethyl side- and vinyl end-groups by polycondensation of dichloro(chloromethyl)methylsilane and dimethyldichlorosilane, using vinyldimethylchlorosilane as end-blocker. A particular case of silane condensation is the Piers-Rubinsztajn reaction, which occurs between alkoxy and H-silanes, using tris(pentafluorophenyl)borane (B(C6F5)3) as a catalyst (Fig. 11.2) (Grande et al. 2010; Hong et al. 2018). This reaction allows for better structural control in functional polysiloxanes (Madsen et al. 2014b). The latest research results on the morphological structure control and optimized synthesis of silicone materials using the Piers-Rubinsztajn reaction were reviewed by Chen et al. (2019). The advantages of using this reaction are presented instead of the classic approaches to silicone materials, such as their mild reaction conditions, high efficiency, high selectivity of catalysis and non-­ polluting characteristics.

11.2.2  ROP of Functional Cyclosiloxanes The ROP of functional cyclosiloxane monomers (Ren and Yan 2016) (Fig. 11.3) has the advantage of obtaining high Mw siloxane homopolymers and copolymers (González-Pluma et al. 2013). Functional silicones with tuneable content of functional groups into polymer can be obtained using a different ratio between the

Fig. 11.2  Silane condensation by Piers-Rubinsztajn reaction

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Fig. 11.3  ROP of functional cyclosiloxane

functional and non-functional monomers. Table 11.1 comprises several examples of functional groups linked to the silicone cycles, conditions for ROP and their influence on the resulting crosslinked polymers or materials. Specially designed cyclosiloxanes can be obtained by chemical modification of commercial ones, the most common being 1,3,5,7-tetramethylcyclotetrasiloxane (D4H), which is usually subjected to modifications by hydrosilylation. When an end-blocker is not used, the terminal groups are –OH, which can be used later for crosslinking by condensation with silanes. A high Mw poly[methyl(3,3,3-trifluoropropyl)siloxane] was obtained by Li et al. (2012) and Fei et  al. (2014) via anionic ROP of 1,3,5-tris(trifluoropropylmethyl) cyclotrisiloxane (F3) in bulk using dilithium diphenylsilanediolate or tetramethylammonium hydroxide ((CH3)4NOH, TMAH) as initiators (Table 11.1, entry 1a,b). Functional silicones containing trifluoropropyl side-groups in various proportions and vinyl end-groups (Table 11.1, entry 1c) were also synthesized by Dascalu et al. (2015) via anionic copolymerization of F3 using octamethylcyclotetrasiloxane (D4) in presence of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane (DVTMDS) as end-­ blocker. Racles et al. (2013a) and Dünki et al. (2016b) modified polysiloxanes with nitrile groups via ROP of the appropriate cyclosiloxane previously functionalized by hydrosilylation (Table  11.1, entry 2). Different approaches were compared: anionic and cationic ROP, as well as equilibration-redistribution reactions, in order to obtain high Mw copolymers with variable content of functional groups and homogeneous elastomer materials (Racles et al. 2013a, 2014a, b). Perju et al. (2018) modified monofunctional heptamethylcyclotetrasiloxane monomers with nitroaniline (NA) or Disperse Red 1 (DR1) push-pull group, using hydrosilylation reactions, and attempted a subsequent ROP reaction in the presence of TMAH (Table 11.1, entry 3).

11.2.3  Post-Functionalization of Silicones The most versatile method to prepare a wide variety of functional silicones is the postfunctionalization (Kolb et al. 1996; Moukarzel et al. 2013; Anger et al. 2014; Racles et al. 2014a, 2016). The content of the functional groups can be tuned by modifying the amount of active sites in polysiloxane substrates. Different end-­groups such as

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Table 11.1  Examples of functional silicones obtained by ROP No. Functional groups 1a

1b

1c

2a

2b

3

Type of reaction conditions Anionic ROP using initiator and different promoters (P)

Observation The nature and concentration of P influence the reaction mechanism Anionic ROP (TMAH) Increased Tg and viscosity, and improved oil resistance of the silicone rubber The F groups % influence the dielectric permittivity (ε’) and Tg values of the elastomers ε’ increased by Cationic ROP of D4 and D4CN with a combination of increasing the % of cyanopropyl wet and dry cation repeat units exchangers Anionic ROP of D4, redistribution/equilibration reaction of a high Mw PDMS with D4CN hydrosilylation, D4H, Pt Relatively low cat.; anionic ROP, TMAH Mw elastomers with good elastic properties Variations in Tg, ROP of [D4 + monofunctional-D4] ε’ and α, β transitions in (TMAH) dielectric spectra, depending on substituent

References Fei et al. (2014)

Li et al. (2012)

Dascalu et al. (2015)

Racles et al. (2013a)

Dünki et al. (2016b)

Perju et al. (2018)

allyl, amino, epoxy or vinyl can be introduced via end-blockers, which are also available for additional chemical modification to obtain other terminally modified silicones. Like any polymer modification reaction, the on-chain post-­polymerization modification of polysiloxanes can lead to incomplete functionalization. The reactions chosen should have an extremely high yield and should not cause any side reactions (Gunay et al. 2011), considering that polymers with different degrees of substitution are difficult to separate. In the case of silicone-­based materials, the post-modification is often a challenge, due to very different solubility of the starting silicones and organic reagents. Care should also be taken to avoid strong acidic or basic media, as these would affect the integrity of the siloxane backbone.

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Fig. 11.4  Hydrosilylation reaction for modification of polysiloxanes

Polysiloxanes with terminal or pendant hydrosilyl or vinyl groups are the most commonly used as siloxane substrates for post-polymerization modifications using hydrosilylation (Fig.  11.4) and thiol-ene addition reactions, respectively (Tables 11.2 and 11.3). These can be obtained with the composition tuned, by ROP of D4 using 1,1,3,3-tetramethyldisiloxane (TMDS) or DVTMDS as end-blockers (for telechelic oligomers) and by copolymerization of D4 and D4H or 1,3,5,7-tetramethyl-­1,3,5,7-tetravinylcyclotetrasiloxane (D4V), when poly(dimethylmethylhydro)siloxane (PDMHS) and poly(dimethyl-methylvinyl)siloxane (PDMVS) substrates, respectively, are obtained. Generally, the hydrosilylation reaction, which is the most used and versatile tool for the modification of silicones, occurs under an inert atmosphere in dry solvents, with noble metal catalysts (Marciniec 1992, 2008; McGrath et al. 1995; Burger and Kreuzer 1996; Marciniec et al. 1997; Gigler et al. 2012; Igarashi et al. 2014). The most efficient and widely used are Karstedt’s (Platinum(0)-divinylsiloxane complex) and Speier’s (H2PtCl6 in isopropyl alcohol), although other catalysts have been proposed for specific reactions, such as: cationic copper(I) complexes (Díez-González et  al. 2006), Pt(II) complexes with general formula LL’PtCl2, Wilkinson’s rhodium catalyst (Ph3P)3RhCl (Zuev et  al. 2004), H2PtCl6-­cyclohexanone, Pt(PPh3)2(CH2  =  CH2), PtCl2(PPh3)2, ruthenium complexes (Ru3(CO)12) (Marciniec et al. 1997), isocyanideN-­ hetrocyclic carbene-platinum(II) complexes (Hubbert et  al. 2014), PtO2 (Sabourault et al. 2002), silver triflate (Wile and Stradiotto 2006), styrene-divinylbenzene copolymer-supported platinum (Wawrzyńczak et  al. 2011), platinum on titania (Alonso et al. 2011), thin gold films and microwave irradiation (Shore and Organ 2008), zinc complexes (Junge et al. 2012) and Zn(OAc)2 (Ozasa et al. 2010). The hydrosilylation is a reaction of great interest in silicon and organic chemistry considering the balance between the effort/cost and benefits. In presence of the same catalysts, the dehydrocoupling between Si-H and proton-donating groups is promoted, which explains the need for anhydrous conditions in the hydrosilylation and the preliminary introduction of protective groups in such double bond reagents. On the other hand, this is a method for the synthesis of modified silicones with labile element-element bonds (such as silyl-ethers or silazanes). The dehydrocoupling between hydrosilane and alcohols/phenols leads to a rapid synthesis of functional silicones that carry Si-O-C groups (sometimes unwanted, prone to hydrolysis). For example, Szawiola et  al. (2017) prepared a series of phenoxylated siloxane polymers starting from poly(dimethylsiloxane-co-methylhydrosiloxane) and different phenol derivatives such as 3-pentadecylphenol, 4-tert-octylphenol and phenol,

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Table 11.2  Post-functionalization of polysiloxanes by hydrosilylation

No. Double bond reagent 1

2

3

4 5

6 7

Polymer/ elastomer Type of reaction substrate properties Hydrosilylation on high Elastomers with Mw PDMHS. poor mechanical properties and ε’ improved. co-hydrosilylation on Copolymers PMHS. with tunable composition; ε’ increases linearly with polar content. Hydrosilylation on Elastomers with PDMHS. improved ε’ and decreased elastic modulus. Hydrosilylation on low A combined Mw PDMHS. effect of dipole moment, molar polarizability, molar volume and water sorption capacity on ε’. Hydrosilylation on PMHS Improved dielectric properties.

8

Hydrosilylation on PMHS; subsequent modification with

9

Hydrosilylation on H-PDMS-H of different Mw

ε’ improved for up to 70% of functional groups; at >90% of functional groups, ε’ decreased (decrease in conformational mobility of the polymer, which was also reflected in Tg) The modified telechelic-PDMS assembles into a 2D + 1D structure; the complex viscosity was improved.

References Racles et al. (2013a)

Racles et al. (2014a, 2015a)

Kussmaul et al. (2011)

Racles et al. (2016)

Putzien et al. (2010; Lowe 2014)

Yang and Wnek (1992)

Ishiwari et al. (2018)

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Table 11.2 (Continued)

No. Double bond reagent 10

Polymer/ elastomer References Type of reaction substrate properties Boehm et al. Hydrosilylation on The block PDMS-b-PMVS copolymers with (2012) tris(ethoxysilyl) units are of interest for surface modification and passivation.

Table 11.3  Thiol-ene click coupling used for polysiloxane modification No. Thiol 1

Substrate and subsequent modification PMVS; HO-PDMS-OH

2

PMVS

3

PDMVS

4a 4b

5

PDMVS; PMVS; HO-PDMS-OH

Elastomer properties Materials with improved ε’ and Tg below RT

References Dünki et al. (2015b, a, 2016b, 2017)

ε’ improved depending on polar group

Racles et al. (2016)

Silicone dual network with improved actuated + strain, tensile strength, tensile toughness and ε’ PDMVS/vinyl end PDMS; Intense Crosslinking via thiol-ene photoluminescence addition with under UV light, very high coloric purity; complexing lanthanide ions reduce the contact angles

Sun et al. (2019b)

Zuo et al. (2014a, b)

which influenced the physical and thermal properties of the polymers. The Disperse Red 1 (DR1) has also been chemically liked to siloxane oligomers, either cyclic or linear, via silyl-ether bonds, in the presence of Pt catalyst (Racles 2009; Racles et al. 2016). Liu and Zhao (2008) also reported that a ruthenium complex is an efficient catalyst for the preparation of silyl esters by dehydrocoupling of carboxylic acids and silanes.

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From another perspective, in the post-functionalization of the siloxane copolymers by hydrosilylation there is the risk of incomplete conversion, depending on the reactivity of the double bond reagent or steric factors, and an un-controlled crosslinking can be produced by the subsequent condensation involving hydrolysed Si-H groups. For example, Racles et al. (2014a) only achieved an 89% conversion in the modification of poly(methylhydrosiloxane) (PMHS) with allyl cyanide (Table 11.2, entries 1, 2). The residual Si-H bonds are slowly hydrolyzed to Si-OH groups which can be exploited for subsequent crosslinking by condensation (Racles et al. 2015a). With this in mind, Kussmaul et al. (2011) partially grafted the Si-H groups of a poly(dimethyl-co-methylhydro)siloxane (PDMHS) using polar N-allyl-­ N-methylaniline (Table 11.2, entry 3), and simultaneously crosslinked with vinyl-­ terminated PDMS, which leads to dielectric elastomers. Racles et  al. (2016) also obtained functionalized silicones containing pendant epoxy, benzaldehyde or DR1 groups from PDMHS (Table 11.2, entries 4–6). The carbonate functional groups can also be introduced by hydrosilylation (Putzien et  al. 2010; Lowe 2014) (Table  11.2, entry 7). The epoxy groups can be subsequently modified by reaction with aldehydes or amines (Table 11.2, entry 4) (Racles 2010; Racles and Cozan 2012; Racles et al. 2016, 2018). For example, Yang and Wnek (1992) introduced silyl ketene acetals as pendant groups into PMHS (Table 11.2, entry 8) and the silicones obtained were further reacted using 4-nitrobenzenesulphenyl chloride. Ishiwari et al. (2018) showed that end functionalization of PDMS of different Mw using a triptycene moiety with a 1,8-substitution pattern leads to significant changes in the rheological, structural and thermal properties of PDMS. The excellent self-assembly capacity of the 1,8-substituted triptycene moiety, which is powerful enough to assemble even an amorphous polymer into a specific ‘2D  +  1D’ periodic structure is responsible for this. The physical and structural properties of PDMS also did not change with the end functionalization with another triptycene derivate (1,4-Trip, Table 11.2, entry 9), which differs only in the substitution pattern. The hydrogen terminated PDMS (PDMS-H2) oligomers synthesized through the cationic ROP of D4 in the presence of TMDS were subjected to a hydrosilylation reaction with tert-butyl methacrylate (tBMA) by Yang et  al. (2014). PDMS(COOH)2 was obtained after acidic hydrolysis of the t-butyl ester terminated PDMS (PDMS-tBMA2) intermediate product (Yang et al. 2014). A particular case is exemplified in the last entry in Table 11.2 where the vinyl group belongs to the polysiloxane substrate (poly(methylvinyl)siloxane, PMVS) and the Si-H group is attached to the functional molecule (Fig.  11.4b). A controlled  two-step  one-pot synthesis was described by Boehm et  al. (2012), who obtained well-defined polysiloxane block copolymers by the sequential anionic ROP of D3 and D4V. Substituents bearing ethoxysilanes or epoxides, as well as long aliphatic chains, have subsequently been linked to the resulting PDMS-b-PMVS via hydrosilylation (Table 11.2, entry 10). The thiol-ene addition (Fig. 11.5) has been used as an alternative pathway for the modification of polysiloxanes with polar groups (Le Neindre and Nicolaÿ 2014; Dünki et al. 2015a; Opris 2018) because it is easy to handle and is produced with

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Fig. 11.5 Functionalization of poly(methylvinylsiloxane) via thiol-ene addition

high performance in a short time, under UV irradiation, which is included in the category of ‘click’ reactions. The thiol-ene addition can be initiated by Lewis acids, metals, organometallics or strong bases. The chemistry of thiols, whether mediated by radicals or catalysts, requires special attention, being influenced by the basic structure of the thiol (Hoyle and Bowman 2010). Through this method different functional groups have been introduced, such as butyl thioether (Dünki et al. 2016b), nitrile (Dünki et al. 2015b, a, 2016b), sulfolane or methylsulfone (Dünki et al. 2017) (Table 11.3). The content of the functional groups can also be modified by using two different thiols (Table  11.3, entries 1–2). Dünki et  al. (2015a, 2016b) obtained a series of elastomers containing nitrile groups, using 3-mercaptopropionitrile/butyl thiol and a bifunctional thiol for simultaneous crosslinking (Table 11.3, entry 1). Following the same strategy, carboxyl and chloride groups have been chemically linked into PDMVS using 3-chloro-1-propanethiol (Table  11.3, entry 3) (Opris et al. 2015; Racles et al. 2016) and 3-mercaptopropionic acid (Table 11.3, entry 4) (Racles et al. 2016; Lei et al. 2019). Methylvinylcyclosiloxanes with three and four siloxane units have been functionalized with carboxyl groups by the thiol-ene addition of thioglycolic and 3-mercaptopropionic acid (Turcan-Trofin et al. 2019). The carboxypropyl-functionalized polysiloxanes (Table  11.3, entry 4a) can be further modified with Disperse Red 1 forming stimuli responsive free-standing thin film (Racles et  al. 2019-unpublished results). Sun et  al. (2019a) introduced carboxyl, ester and hydroxyl as pendant groups onto poly(dimethylmethylvinyl)siloxanes using photochemical thiol-ene click reaction. The carboxyl-modified silicones were subsequently used by Sun et al. (2019b) for crosslinking of PDMS containing amino groups (Table 11.3, entry 4b), in combination with a condensation-driven silicone network. Thiol-ene click reaction has also been used to obtain polysiloxane-based luminescent elastomers complexed by lanthanide ions. Keeping this in view, end or pendant vinyl–functional polysiloxanes were partially functionalized Zuo et al. (2014b, a) using N-acetyl-L-cysteine, with residual vinyl groups being used as crosslinking sites (Table 11.3, entry 5). The functionalization of polysiloxanes via alkyne-azide cycloaddition click chemistry has also been intensively studied (Gonzaga et  al. 2009; Pandey et  al. 2012; Yang et al. 2015). Some examples in this regard can be highlighted below:

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The pendant chloride groups may be replaced  by azide groups, which can be used to introduce polar groups via aza-click chemistry (Madsen et al. 2014c). In this sense,  Madsen et  al. (2014b) synthesized functional silicones using chloropropyl side-groups and allyl or vinyl end-groups via a Piers-Rubinsztajn reaction, where the siloxane substrates were used to incorporate other functional groups via coppercatalyzed cycloaddition reactions between the azides and a variety of alkynes, while the vinyl or allyl end-groups allow crosslinking via a hydrosilylation reaction. Rambarran et  al. (2013) reported a synthetic strategy that allows the sequential functionalization of polysiloxanes with hydrophilic agents including carboxylic acids, bromoalkylesters and oligo(ethylene oxides) using a simple metal-­free click reaction (Table 11.4, entry 2). Wang et al. (2015) also synthesized a graft side-chain liquid-crystalline polymer (Table 11.4, entry 3) using chloropropyl substituted polysiloxane as a backbone and poly(6-(4′-octyloxyphenyl-4″-benzoyl)hexylacrylate) (a LC polymer -LCP) as the  side chain via atom transfer radical polymerization (ATRP) technique. Similarly, Lu et  al. (2013) prepared polymers consisting of oligo(dimethylsiloxane) and oligothiophene units arranged alternately from α,ωdibromooligo(dimethylsiloxane). The latter was obtained by Pd-catalyzed dehydrobromination of α,ω-dihydrooligo(dimethylsiloxane) with isopropyl bromide (Table 11.4, entry 4). Miyauchi et al. (2015) obtained a polysilsesquioxane (PSQ) containing phthalimido side-chain groups with good solubility and high thermal stability by a two-step reaction (Table 11.4, entry 5). Other less common synthetic routes have also been explored for functional silicones. Gretton et al. (2012) used Piers-Rubinsztajn reaction to obtain hydrolytically stable silicone polymers with pendant triarylamine functional groups. De Bruycker et al. (2019) linked thiolactone units to polysiloxanes to prepare bivalent or a multivalent thiolactone-functional silicone as the hydrophobic component of an amphiphilic network (Table  11.4, entry 6). They used thiolactone-based conjugation chemistry as a new crosslinking strategy for the preparation of PDMS based amphiphilic co-networks, where commercial polyethylene glycol (PEG) diacrylates of various chain lengths were selected as hydrophilic cross-linkers, which should react quickly with the thiols that are generated by the aminolysis of the thiolactone units, thus by adding of an amine in the polymer mixtures, the thiolactone segment generated thiol groups via aminolysis, which reacted further with the acrylates in the mixtures. The crosslinking reactions were quick and efficient at room temperature (RT), which led to the formation of transparent elastomers. Zhao et al. (2019) reported the reaction of 3-aminopropyl di- or multi-functional silicones with 2-furoyl chloride or maleic anhydride (Table 11.4, entry 7). These authors reported that silicone oligomers obtained with reactive motifs were crosslinked by dynamic Diels–Alder bonds. By incorporating graphene nanosheets into polysiloxane elastomer, self-curable nanocomposites were obtained with a significantly improved maximum stress (σm) and excellent electromechanical properties to be used as pressure sensor. Dendrimers and dendrons are monodisperse macromolecules with precise structure and self-assembly, comparable with those of biological molecules and macromolecules. Percec’s group discussed and reviewed the hyperbranched functional polysiloxanes with various structures in Rosen et al. (2009).

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Table 11.4  Miscellaneous chemical modification reactions No. Functional reagents 1 NaN3, Bu4NN3

Type of reaction and conditions 1) click chemistry a) on PDMS with Cl-propyl pendant groups, b) with 1-ethynyl-4nitrobenzene. 2) hydrosilylation

2

Metal-free click chemistry on poly(azidopropyl methyl)-co(dimethylsiloxane)

3

ATRP on PDMS with Cl-propyl pendant groups Dibromooligomer synthesis, H-PDMS-H, PdCl2; n-BuLi, hydrolysis

4

1) 2)

5

Ammonium-groupcontaining PSQ, Et3N

6

Condensation: HO-propyl end PDMS or NH2 propylmethyl-­ dimethylsiloxane copolymer, DBTDL

7

Elastomer properties ε’ improvement for polymers with the highest azide, chloride and nitrobenzene content PEG-based silicone surfactants

Grafted liquid crystalline silicone The polymer electronic states can be tuned by changing the oligothiophene chain length Polymers with good solubility and high thermal stability

Transparent PDMS-based amphiphilic co-networks with a low soluble fraction NH2 propyl end/pendant Self-heating polysiloxanes -PDMS, acetic acid, TEA (two step curing) with tunable NH2 propyl end-PDMS, mechanical properties DCM, DMAP, TEA (stretching (two step curing) capacity)

References Madsen et al. (2014c, b, 2015a)

Rambarran et al. (2013)

Wang et al. (2015) Lu et al. (2013)

Miyauchi et al. (2015)

De Bruycker et al. (2019)

Zhao et al. (2019)

11.3  Silicones for Electromechanical Applications Crosslinked polysiloxanes (elastomers) have found a special place among ‘smart’ materials and electromechanical devices such as actuators, generators or sensors. Silicone elastomers (soft insulating materials) are part of the category of dielectric elastomers, a subgroup of electroactive polymers, which can change their shape and/or size under the influence of an external stimulus, therefore fitting into the

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smart category. When a dielectric elastomer film is between two electrodes to which an electric field is applied, the Coulomb forces that are developed compress the elastomer in the section causing its lateral expansion due to its incompressibility (Carpi et al. 2011). This response known as actuation is directly proportional to the dielectric permittivity of the material and inversely proportional to the Young’s modulus (Y), according to Eq. (11.1) (Kornbluh and Pelrine 2008).



Sz = −

ε ′ε 0 (V / z ) Y

2

(11.1)



where Sz is the mechanical response of the actuator, V is the applied voltage, z is the thickness and ε0 is the vacuum permittivity (8.8 × 10−12 F/m). In the generation mode, a similar system is subjected to a mechanical stimulus under which the elastomeric film is deformed. By removing the force, with the return of the highly elastic film to the original size, electrical energy is harvested. In this case, the mechanical and electrical properties also have a great influence on energy conversion yield (Kornbluh and Pelrine 2008). For example, one of the most determining properties is the maximum area expansion of the generator which is determined by the strain at break (Smax) and Y (in correlation with the stiffness), and the maximum applied voltage which is determined by the dielectric breakdown strength (EBD) of the dielectric membrane (Eqs. 11.2 and 11.3).

E = 0.5C1V12 ( C1 / C2 − 1) C = ε ′ε 0

A V = ε ′ε0 2 z z



(11.2) (11.3)

where A is the area of the capacitor, E is the harvested energy, V1 is the initial applied voltage, C1 and C2 are the electric capacitances in stretched and relaxed state, respectively. Silicones have high elasticity and elongations, and tunable Y values. However, the low dielectric permittivity of unmodified silicones is a drawback for such applications. Improving this property has been the main objective of researchers in recent years in this field. Thus, specially designed silicone elastomers are developed, in order to optimize their electromechanical properties. Comprehensive reviews on silicones as dielectric elastomer transducers have been published, including the performance of commercially available materials, as well as chemically modified silicones (Madsen et al. 2016a; Opris 2018). The scientific community has developed several strategies to optimize the actuation properties or to maximize the energy conversion efficiency of PDMS-based transducers, such as chemically designed silicone elastomers (Opris 2018), interpenetrating polymer networks (IPNs) (Madsen et  al. 2016a) and silicone-based composites having different type of incorporated fillers (Madsen et  al. 2016a). Functional silicones can be used in dielectric silicone elastomers in two main designs: (a) chemical modification of polysiloxanes by laterally attaching polar

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groups or by inserting polar segments in the main chain followed by crosslinking, alone or in combination (blends, interpenetrating networks) with unmodified PDMS and (b) crosslinking with polar trifunctional silanes and reinforcing with modified silica fillers.

11.3.1  Polysiloxanes With Polar Groups in Dielectric Elastomers Attaching polar groups to the polysiloxane backbone is one of the many strategies studied, which mainly focuses on improving dielectric permittivity and energy densities by intrinsically increasing the polarizability of silicone elastomers. For example, polar silicones containing cyanopropyl groups were synthesized by Racles et al. (2013a) via ROP followed by crosslinking with alkyltriacethoxysilane and tetra(2-­ ethylhexoxide) as a catalyst, finding that the dielectric permittivity increased twice, but the conductivity also increased along with the poor mechanical properties. Under another approach, Racles et al. (2014a, 2015a) attached cyanopropyl groups to the main backbone by hydrosilylation together with non-polar hexyl pendant groups and mixed with a PDMS polymer in order to obtain elastomers. These authors reported that together with a higher permittivity, good EBD values were obtained (~56 V/μm) and actuation strains were achieved 13-times higher compared to pure PDMS. Hydrosilylation has also been used by Yang and Wnek (1992) for attaching silyl ketene acetal onto the main backbone followed by reaction with electrophile moieties such as 4-nitrobenzenesulphenyl chloride. The resulting functional polymer with 70% p-nitrothiophenoxy groups led to a dielectric permittivity of 8.96, compared to 4.83 for the silyl ketene acetal functional polymer. Stepp et al. (2015) obtained polysiloxanes having chloromethyl groups via polycondensation of dichloro(chloromethyl)methylsilane and dimethyldichlorosilane, with vinyldimethylchlorosilane as end-blocker. The copolymers were crosslinked via hydrosilylation between the vinyl end-groups and a multifunctional hydrosilane and for about 60% mol of chloromethyl side-groups, the dielectric permittivity increased up to around 6–6.8 at 105 Hz and the values of the dielectric EBD were from 50 to 60 V/μm (Stepp et al. 2015). Madsen et al. (2014b, 2015b) synthesized polar copolymers via Piers-Rubinsztajn reaction between 3-chloropropyl-methyldimethoxysilane and several hydride-­ terminated dimethylsiloxane pre-polymers with different Mws, thus creating copolymers with well-distributed azido groups which also offered the possibility to add dipole molecules (e.g. 1-ethynyl-4-nitrobenzene) via alkyne-azide cycloaddition. The resulting elastomers had increased breakdown strengths. At high concentrations of nitrobenzene polar groups, high dielectric losses and increased conductivities were registered, while chloropropyl-functional siloxane copolymers (Madsen et al. 2015a, c) gave elastomers with low modulus of elasticity (0.4–1 MPa, approx. 75–90% lower than the reference silicone elastomer), moderate dielectric permittivity (4.7 at 106 Hz), high dielectric breakdown strengths (94.4 V/μm) and seven- to ten-times higher achievable actuation values.

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Dascalu et al. (2015) reported the anionic ROP of F3 and D4 catalyzed by TMAH, in presence of DVTMDS, which yielded siloxane copolymers end-capped with vinyl groups. Different F3:D4 ratios led to a range of polymers with trifuoropropyl side-groups of 28 to 58 mol% that were crosslinked with tetrakis(dimethylsiloxy) silane by Pt-catalyzed hydrosilylation reaction. The dielectric permittivity increased up to 6.4 with the trifuoropropyl content, apart with increased Tg values (up to -88.6 °C) and low Y (0.019 MPa), while the maximum lateral actuation strain was 5.4% at 7.8 V/μm of applied electric field. In line with this, Dünki et al. (2015a, b) chemically modified a high Mw polymethylvinylsiloxane-α,ω-diol using 3-­ mercaptopropionitrile and subsequently crosslinked with a bifunctional thiol (2,2′-(ethylenedioxy)diethanethiol). A noticeable increase in dielectric permittivity (10 at 106 Hz), a significant actuation strain (20.5% at 10.8 V/μm) and self-healing capacity of the elastomer were observed. However, dielectric losses and conductivities increased, while the value of maximum EBD was low. Sun et al. (2019a) studied the influence of the dipole structure and its interaction on the electromechanical and actuation performance of homogeneous silicone dielectric elastomers. Poly(dimethylmethylvinyl)siloxane with different molar content of pendant vinyl groups synthesized by anionic polymerization were reacted with three different dipoles (carboxyl, ester and hydroxyl) using photochemical thiol-ene click reaction to prepare homogenous dielectric elastomers. The combined effects of dipolar moment, mobility and interactions of carboxyl, ester and hydroxyl dipoles had a significant influence on actuated, dielectric and mechanical properties. The simultaneous improvement of the dielectric and mechanical properties of silicone rubber was further optimized by Sun et al. (2019b). First, a carboxyl modified polysiloxane was obtained by a photochemical thiol-ene click reaction. The functional silicone was further mixed with a silanol terminated PDMS followed by the crosslinking of PDMS-OH using an amino support crosslinking agent. Thus, the dual crosslinking network obtained includes the chemically crosslinked PDMS-NH2 network and the network formed by hydrogen (H)-bonds between PMVS-COOH and PDMS-NH2. The modification of polysiloxanes with sulfonyl groups in various proportions via thiol-ene chemistry was reported by Dünki et al. (2017) as a viable way to tune the dielectric permittivity within a wide range (5–22.7), while keeping the Tg below RT, which opens up possible applications in actuators capacitors and flexible electronics. An alternatively efficient method to increase the electromechanical response is mixing two polymers, one of them with high dielectric permittivity which can be accompanied by a decrease in the Y values. The high ε’ component is in many cases of an organic nature, such as a PU elastomer with permittivity of 15.5 at 10  Hz (Gallone et al. 2010) or a un-doped poly(3-hexylthiophene) with permittivity >2000 at 1 Hz (Carpi et al. 2008b, a). When the PEGs (PEG600 and PEG1500) were mixed by Liu et al. (2012) using a liquid silicone (Silastic DC3481), immiscibility between PEG and PDMS was observed despite good electromechanical performance. Trying to overcome this drawback, Razak et  al. (2015) synthesized PEG-PDMS block copolymers and mixed them with a commercial silicone (MJK 4/13). The

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immiscibility was reduced on a microscopic scale, and for 5  wt.% PEG-PDMS block copolymer, the permittivity was increased up to 60% with low dielectric losses and high dielectric breakdown strengths (~100  V/μm). Meanwhile, Risse et al. (2012a) used cyanopropyl-functional silicone as a high permittivity filler and as a plasticizer in mixtures with PDMS matrix. The dielectric permittivity increased considerably, but the dielectric EBD decreased by 75% (~20 V/μm). Fluorosilicones are also very interesting polymers to use in mixtures, which give good results in some cases (Pelrine et al. 1998; Böse et al. 2012; Biggs et al. 2013). For a trifluoropropyl-functional divinylpolydimethylsiloxane crosslinked into an elastomer and mixed with 40 or 45  wt.% fluorinated silicone oil, the actuation strains were higher by a factor of five, the dielectric permittivity values were around 5.4 and lower Y values were achieved (0.07 MPa) (Böse et al. 2012). Chloropropyl dipole attached onto the main backbone or on crosslinking nodes, was also used in silicone oil (commercially available as LMS-152) as additives for a commercial silicone elastomer (LR3043/50) (Madsen et al. 2015c, 2016b). For a 30 phr additive content, a modest value of dielectric permittivity was obtained (4.4) with a decreasing loss tangent and a plasticizing effect (reducing the Y from 2 to 1.35 MPa). Even if a high percentage of polar additive was used and the Y decreased, the dielectric EBD values were higher (130 V/μm). In addition, this chloropropyl-based additive provided a low dielectric loss when used in TiO2-filled silicone elastomer. Racles et al. (2015a) recognized phase separation, when cyanopropyl-modified polysiloxanes were used as a high ε’ component in mixtures or co-networks with PDMS, resulting in negative effects on the final properties. As an alternative, the same materials were mixed in polymer-polymer composites, where submicron particles were first obtained from the polar material with a specially designed siloxane surfactant and then used as the disperse phase into PDMS matrix. The mechanical properties were improved as a result of a more controlled sub-microscopic phase separation. Asandulesa et al. (2018) reported an in-depth dielectric investigation of these materials focused on interfacial phenomena in co-networks and mixtures with a similar composition and a different morphology in terms of domain size and nature of the interface. Following the same strategy, Racles et al. (2017a) processed silicones with CN or Cl-propyl groups as submicron particles, stabilized with a hydrophobic commercial surfactant and crosslinked, then the particles were mixed into a high Mw PDMS matrix. These authors obtained EBD up to 63 V/μm, dielectric permittivity up to 4.7 at 104 Hz and soft elastomeric materials with low Y, which also showed an atypically high piezoelectric coefficient (d33) at ambient temperature without poling. Similarly, the polyazomethine (PAZ) submicron particles were obtained by Racles et  al. (2015b) via polycondensation reactions occurring in reverse micelles of an amphiphilic siloxane oligomer, and then dispersed into a high Mw PDMS.  The dielectric and mechanical properties can thus be largely tuned depending on the structure of the dispersed phase (containing or not siloxane units) and their proportion. The high dielectric constant of metal complexes of bis-azomethines containing dimethyl siloxane segments makes them candidates to be considered as fillers for

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dielectric elastomers. The self-assembling of such metal complexes in aggregates with hydrophobic shell ensures a good compatibility and dispersion of the filler within the hydrophobic environment created by the polymer matrix, without requiring laborious operations (Stiubianu et al. 2016). In general, different simultaneous or sequential crosslinking pathways to form silicone based IPNs have been used. IPNs are expected to have an improved combination of properties from both networks, such as better mechanical properties or degradation stability. There are reports in the literature about silicon-based IPNs (Mark and Ning 1985; Hamurcu and Baysal 1993; Shah 2004; Brochu et al. 2013; Madsen et al. 2014a; Yu et al. 2014), many of which are made by combining kits commercially available (Shah 2004; Brochu et al. 2013; Yu et al. 2014). With respect to electromechanical applications, most studies include networks formed of PDMS with different Mws or crosslinking mechanism (sometimes based on reaction of functional groups), and only a few of them refer to functional silicones in IPNs. Brochu et al. (2013) chose hydroxyl-terminated PDMS crosslinked at RT with tetrapropylorthosilicate (TPOS) to form the first network, and two low Mw polysiloxanes (vinyl terminated PDMS and PMHS) crosslinked together via hydrosilylation at 160 °C forming the second network. The partially crosslinked RT network was pre-strained, then heated to allow full curing while retaining some previously applied strain. This strategy led to high area strains (45% at 60 V/μm in diaphragm mode and 15% in linear mode). Tugui et al. (2015a) applied the same crosslinking systems to combinations of PDMS and chemically modified silicones carrying cyanopropyl, phenyl or trifluoropropyl moieties, which were used in IPNs without preserved pre-strains. In all cases, dielectric EBD decreased drastically; the IPN with cyanopropyl moieties were the most promising (EBD = 29 V/μm, ε’ = 9.27 at 5 Hz). Tugui et al. (2015b) also used two PDMS of different Mw in several ratios, sequentially crosslinked, to obtain full silicone bimodal IPNs, thus highlighting the beneficial role that pre-straining applied to the first network had on the electromechanical properties. In another approach, ionic crosslinking between amine-functional PDMS (AMS-162) and carboxydecyl-ended PDMS in different ratios were used by Yu et al. (2015) for the first network, while a two-component commercial mixture was crosslinked by hydrosilylation, forming the second network. Overall, the presence of amine-functional PDMS increased the σm and decreased the Smax, but an optimized ratio of 50:50 wt.% between networks gave the highest dielectric permittivity (1.7 × 102 at 0.1 Hz and 11.7 at 106 Hz) and moderate to high dielectric EBD (78 V/ μm). A very important feature was that ionic IPNs presented self-healing properties. Tugui et  al. (2016, 2017) combined PDMS  with poly(siloxane-urethanes) and poly(siloxane-imides) as polar components in interpenetrated networks to obtain dielectric elastomers with high dielectric permittivity. The organic components in the siloxane-organic copolymers act as enhancers of the dielectric permittivity, while the siloxane segment was thought to ensure compatibility and good dispersion of the copolymer within the silicone matrix. Although phase separation has not been completely avoided, with moderate polar component contents it did not have

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significant effects on the properties of interest. For example, Tugui et  al. (2016) obtained a 7.1% lateral strain at an applied field of 20 V/μm for a network containing 10 wt.% PU. The PDMS interpenetration with polyimides resulted in an increase of dielectric permittivity and breakdown strength, thus, despite the unwanted growth of the Y, actuation values up to 8.7% were recorded for a polyimide content of 10 wt.%. In addition, Tugui et al. (2017) reported the piezoelectric coefficient d33 of 26 pm/V, attributed to the presence of the polyimide component. This dual response recommends these materials as active elements for both actuators and sensors.

11.3.2  Polar Crosslinking Centers Another way to increase the polarization of the silicone elastomers is by using polar crosslinking centers. Kussmaul et al. (2012) and Risse et al. (2012b) used a hydride-­ functional polysiloxane as a crosslinking agent, as well as substrate for the chemical binding of a dipolar molecule (N-allyl-N-methyl-p-nitroaniline) by hydrosilylation. The chemical modification and crosslinking of a PDMS matrix was produced by a one-step process. An increase in dielectric permittivity was observed (ε’ = 3.7 for 13 wt.% dipole content) along with a decrease in dielectric breakdown strength. In addition, competitive reactions resulting in network imperfections, dangling substructures and large sol-fractions can be produced. The optimization of the materials led to a low Y (0.142 MPa), greater permittivity (ε’ = 5.56) and a maximum actuation strain of 2% at 10 V/μm for 10.7 wt.% polar content (Kussmaul et al. 2011). A similar strategy was used by Zhang et al. (2015) from a dihydroxyl-functional polar molecule (DR19) together with TEOS and DBTDL to cross-link a hydroxyl-ended PDMS. Although DR19 participates in the condensation cure, it can act as a polar molecule in crosslinking centers or as a chain extender, thus the control of dipole content in the matrix cannot be easily managed. The dielectric permittivity (a maximum of 4.9 at 103 Hz), Y and the Smax increase linearly with the dipole molecule content, while the EBD decreases linearly. Madsen et al. (2013a, b, 2014a) obtained an azido-functional cross-linker which was compatible with silicones and allowed high dipole moments at the crosslinking centers. The curing reactions were made using azide-alkyne 1,3-dipolar cycloaddition, whereby the polar entities (nitrobenzene and nitroazobenzene) were well distributed within the matrix. For a limited number of polar groups present only at crosslinking centers, a modest increase of dielectric permittivity was registered by 20%. Functional trialkoxysilanes have been used as cross-linkers to obtain silicone elastomers via Sn-catalyzed condensation route of hydroxyl-ended PDMS chains (Bele et al. 2015; Dünki et al. 2016a). Due to the low content of polar groups, rather modest dielectric permittivity values have been obtained when chloride (ε’ = 3.7) or nitrile (ε’ = 3.9) were used as crosslinking centers. However, in some cases due to the low Y, quite high lateral actuations were achieved at a very low voltage (1.68% at 0.83 V/μm).

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11.4  Functional Silicones in Liquid Crystalline Materials The LCs represent a state of matter discovered in 1888 by Reinitzer and Lehmann, defined as an intermediary state between ordinary liquids and three-dimensional (3D) solids (Demus et  al. 1998). LCs are used intensively on LCD screens (displays), but other applications also include the scaffolds for biosystems and nanoparticle (NP) assemblies, as well as actuators and sensors (Ohm et al. 2010; Carlton et al. 2013). LCs gained a privileged place among the organo-siloxane compounds, as a consequence of the hallmark of the siloxane segments on the overall properties and mesomorphic behavior. Siloxane moieties are used as flexible segments into main chain or side chain LCPs and as elastomeric matrix in LC elastomers (LCEs). Usually, a significant decrease in Tg values is obtained, as well as a decrease in the other transition temperatures, due to the low rotational energy of the Si-O bond (Aguilera et  al. 1983). The role of siloxane segments has been demonstrated as compared to aliphatic ones, thus reducing transition temperatures in LCs, due to their flexibility and bulkiness, which is used in order to obtain high mobility of the mesophases and mesogenic properties at moderate temperatures. Another characteristic of the siloxane-containing LCs is the microphase segregation which generally occurs, due to the incompatibility of the siloxane moieties with most of the organic structures. Siloxane backbone materials tend to show weaker anisotropy due to the flexibility of the backbone which opposes the perfect confinement between the layers (Demus et al. 1998). Siloxane-containing LCs, their chemistry, physical properties and applications have been previously reviewed (Teyssier and Boileau 2000; Racles 2008; Zhang et al. 2018). The search for better performance in areas such as actuators and sensors, medical science, microelectronics, optoelectronics, etc. maintains the interest of the scientific community for this area. Keeping this in view, the functional siloxanes are briefly described by selected examples of structures and application-related properties, emphasizing the role of the siloxane units in LC materials. Like any other functional moiety, the binding of mesogenic groups to the (poly) siloxane backbone can be carried out in three main routes: hydrosilylation, thiol-ene addition and chemical modification of non-mesogenic functional groups (by esterification, etherification, free radical polymerization etc.). The LC behavior (temperatures, nature of mesophases, phase transitions) of polysiloxanes substituted with mesogenic groups depends largely on their structure and is influenced by the microphase separation in copolymers (Rössle et  al. 2005). In addition, the mechanical properties of the LCEs obtained therefrom, especially the smectic layer compression, are also a consequence of the chain microstructure. For example, the phase behavior of side chain LC polysiloxanes containing cholesteryl groups depends on the length of the mesogenic core, the flexible spacer and the copolymer composition. Interestingly, copolymers with both cholesteryl and biphenyl mesogenic groups have shown cholesteric phases, while the homopolymers with cholesteryl groups have exhibited smectic phases (Hu et  al. 2004). Recently, Katsuki et  al. (2019) investigated siloxane-based monomeric and dimeric LCs which carry

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cholesteryl mesogenic groups, compared to the corresponding LCs that have normal alkyl chains spacers. The nature of the LC phases was associated with the nanosegregation. It was found that all compounds showed a smectic A phase in a wide temperature range, and only the dimeric LC with the shortest siloxane was found to exhibit a chiral nematic phase and a blue phase in addition to the smectic A phase.

11.4.1  Low Mw and Polymeric Siloxane-Containing LCs Li et al. (2007) reported that trisiloxane-terminated LCs with 2-phenylpyrimidine cores formed partially bilayered SmA and SmC phases, exhibiting a more ‘de Vries-­ like’ character (minimal layer shrinkage after SmA-SmC transition) compared to a non-siloxane analogue. This particular behavior is explained in correlation with nanophase segregation. In ferroelectric SmC∗ phase, the consequence of de Vries’ behavior is the minimization of chevrons and zigzag defects that severely degrade the quality of electro-optical devices. Siloxane-based ferroelectric LCs have revealed chevron-free long-term bistable behavior and an easy regeneration of bistable performance after damage (Xu et al. 2011). Trisiloxane-containing polymers with pendant azobenzene moieties were obtained by Pandey et al. (2012) via click chemical reaction and showed LC behavior and photoactuation. The order of the mesophase depended on the nature of the substituent at the para- position of the azobenzene mesogen, i.e. nematic for -H, smectic for -CN and no mesophase for –COOCH3. LC cyclosiloxanes have exhibited properties intermediate between low Mw LCs and polymeric ones, combining the low melt viscosity of the former with the glass-­ forming capacity of the latter, and providing the advantage of molecular monodispersity (Soltysiak et al. 2004). Supramolecular polymers are based on chemical recognition, e.g. between bipyridine and carboxylic acids (Racles and Cazacu 2008). Such new materials may exhibit an LC behavior, due to the H-bonds, while depending on the structure of the siloxane diacids (chain mobility, free volume and molecular polarizability), the mesophase range can be tailored. One of them, based on 1,3-bis(carboxypropyl)tetramethyldisiloxane (CX) and 4,4′-bipyridine (BPy) was subsequently analyzed by Racles et al. (2013b) using single crystal X-ray diffraction, which confirmed the strong H-bonding between the carboxyl and pyridyl groups with the formation of the supramolecular structure. Recently, Nickmans et  al. (2018a, b) reported a very nice development in the supramolecular approach of LC materials. These authors synthesized liquid crystalline H-bonding hepta(dimethylsiloxane) carboxylic acid dimers, which formed sub-5  nm lamellar characteristics in the bulk and at the liquid solid interface or hexagonal columnar phase, depending on the coil-to-rod volume fraction. These compounds were mixed with a homopolymer or a block copolymer containing H bond-accepting moieties (poly(4-vinylpyridine) or poly(styrene)-b-poly(4-­ vinylpyridine)), thus obtaining nanostructured hierarchical materials with

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Fig. 11.6  Examples of side-chain LCPs with siloxane segments and azobenzene mesogens

columnar-­in lamellar or smectic-in lamellar arrangement with vertical alignment, useful for manufacturing dense line spaces and contact holes or for vertical orientation of low surface energy oligodimethylsiloxane. In the side-chain LCPs, the flexible polysiloxane backbones can improve the decoupling of the movements of the side chain and main chain, giving rise to higher thermal stability of the mesophases. This is further increased for the longer polysiloxane backbone, which is also associated with more ordered phase structures and a higher Tg and LC-isotropic transition temperature (Ti) (Zhang et al. 2018). Side-­ chain LCPs can be obtained by attaching the mesogenic group onto a polysiloxane chain or by reacting mesogenic and siloxane dimers. Different structures containing azobenzene moieties are exemplified in Fig. 11.6. Steglich esterification was used by Racles et  al. (2006a) to obtain side chain azo-polyesters with different

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siloxane-­organic backbones (Fig.  11.6a). The thermotropic behavior had a direct correlation with the content of mesogenic group, the length of siloxane segment and the chemical structure of the starting diacids. Petr and Hammond (2011) described a RT side-chain liquid crystalline polymer with a rapid photoresponsive behavior, which combined a side-on azobenzene LC with a low Tg polysiloxane backbone (Fig. 11.6b) and exhibited a nematic LC phase at RT. It was argued that the side-on arrangement (with mesogens parallel with the polymer’s backbone) is more suitable for highly responsive chain conformations compared to the end-on side chain LC functionalization. The relatively short response times are relevant for actuator and responsive elastomer applications. Siloxane polymers possessing both photoactive and liquid crystalline properties have been prepared using click chemistry between azo-diazides and a trisiloxanediyne (Pandey et al. 2012) (Fig. 11.6c). Apart from LC behavior, the trans-cis isomerization has led to a photomechanical effect. Side-chain-end-on and side-on LCPs with polysiloxane backbones have also been synthesized by grafting mesogenic monomers to poly(3-­ mercaptopropylmethylsiloxane) via thiol-ene click chemistry (Yang et  al. 2013). According to Yang et  al. (2013) when mercapto- groups were deliberately left unmodified, these were additionally used as crosslinking sites to prepare LCE fibers and films, but the thermal actuation effects of these films were modest. Another method for the synthesis of side-chain LCPs is the ATRP. Zhang et al. (2014) synthesized a series of LC block copolymers using a functionalized PDMS-Br as a macroinitiator (same PDMS block length) and 11-(4-cyano-4′biphenyloxy)undecyl polymethacrylate LC with different block lengths. The transition temperatures increased with increasing the LC block length. These authors, using transmission electron microscopy (TEM), investigated the morphology of the diblock copolymers after thermal annealing, thus observing that the microphaseseparated nanostructure exhibits a transition from the lamellar phase to dispersed cylinder phase with the increase of the LC block content. In addition, higher values of dielectric permittivity for block-copolymers than for pure LCP were observed, which was attributed to the increase of orientation order parameter in confined microdomains. Likewise, the induced orientation of the side-chain mesogenic groups (perpendicular or parallel to surface) tunes the permittivity. Similarly, diblock copolymers PDMS-b-poly(6-(4-(40-cyano-phenylazo)phenoxy)-hexyl acrylate) with well-defined structures and relatively narrow Mw distributions were prepared by Wei et al. (2014) and exhibited smectic mesophase (Fig. 11.6d). These authors indicated that the mesomorphic structures of LC subphase of the diblock copolymers were identical to those of the homopolymer, and the morphology of these annealed samples also revealed microphase separation, with lamellar or spherical morphology, depending on composition. The photoisomerization behavior of the annealed block copolymer film depends on the microphase-­ separated morphologies. On the other hand, Racles and Cozan (2014) reported that siloxane copolymers containing pendant azomethine mesogenic units bound in ~50% of the structural units (Fig.  11.7) showed single Tg, so there was no phase separation. These

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CH3 Si CH3

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Fig. 11.7  Side-chain LCPs with azomethine mesogens

copolymers showed lower melting temperatures (Tm), similarly high LC-isotropic transition temperatures and more ordered mesophase compared to previously reported telechelic siloxane oligomers containing similar mesogenic groups (Racles and Cozan 2002), which was attributed to differences in chain microstructure.

11.4.2  Polysiloxane-Based Liquid Crystalline Elastomers LCEs are obtained by crosslinking of liquid crystalline polymer systems. They combine the properties of an LC phase (order and mobility) with rubber-like elasticity and have unique mechanical, optical and piezoelectric properties. LCEs exhibit a high-stroke reversible mechanical actuation when triggered by external stimuli (electric, heat, light and magnetic field). Their actuator and sensor applications have been reviewed, including shape memory materials, in particular for artificial muscles, industrial manufacturing, health and microelectromechanical systems (MEMS) and usable devices on their basis (Ohm et al. 2010, 2012; Jiang et al. 2013). LCEs have potential applications as auxetic (negative Poisson’s ratio) materials (Ren 2007). Normally, initially formed LCEs are in a polydomain state, i.e. contain many domains whose local orientation is misaligned with each other. Thus, a polydomain LCE does not have a uniform global orientation in its mesophase (Fridrikh and Terentjev 1999). Networks with programmed and complex director profiles in 3D can be obtained by aligning the LCs before polymerization (De Haan et al. 2014). In this sense, the actuation behavior of LCEs is induced by the anisotropic-isotropic deformation of polymer chains. To achieve actuation effects, the mesogens and backbone chains of LCEs must be aligned uniaxially by electric or magnetic fields, mechanical forces or command surfaces. The most commonly used method in the case of siloxane-­ based LCEs is the two-step crosslinking process, which was introduced by Finkelmann and Rehage (1980), where the polymers are partially crosslinked, then mechanical stress is applied to produce an anisotropic order, followed by a full crosslinking step. With this in mind, Ren (2007) studied the mechanical behavior of LCEs during alignment. The shape change caused by the phase transition or the electric field in aligned LCEs and actuation applications have been linked to the chemical structure and sample geometries. Polysiloxane-based LCEs were

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Fig. 11.8  Difunctional monomer with ionic and mesogenic groups for LCE

analyzed by Ohm et al. (2012) in terms of specific aspects, compared to organic networks. A series of LCEs with smectic mesophase, obtained by linking a divinyl monomer containing biphenyl benzoate mesogenic units and brilliant yellow moieties (Fig. 11.8) to polysiloxane was reported by Meng et al. (2005). This combination of ionic and mesogenic groups allowed tuning of Tg and Tm, as well as stability of the mesophase region. Monodomain LCEs can be obtained by prealigning parent LCPs in a magnetic field (since the aromatic units are diamagnetic) followed by crosslinking. The alignment process is reversible, after heating the LCE monodomain to its isotropic phase and cooling it back to its nematic phase, the originally macroscopic orientation of the nematic director is completely recovered (Legge et  al. 1991). Li and Keller (2006) recognized the rapid relaxation of the nematic orientation and the importance of in-situ crosslinking to lock in the nematic order after alignment of triblock copolymers using a magnetic field. An interesting application of LCEs was to generate oriented, periodic and reversible surface patterns in response to moderate temperature changes (Agrawal et al. 2012). These surface wrinkles in LCEs can be used to measure the modules of nanoscale thin polystyrene films. The photoactuation of LCE can be induced with specific photo-responsive molecules, either chemically linked or dissolved in the elastomer (Camacho-Lopez et al. 2004). According to Camacho-Lopez et al. (2004) the last method gave even better results than the previous one when only 0.1 wt.% of the azo dye Disperse Orange I was dissolved into monodomain LCE consisting of poly(methylsiloxane) backbone with laterally linked biphenyl mesogens, weakly crosslinked and mechanically aligned before full crosslinking. The dye-doped nematic LCEs undergo large and rapid position-dependent shape changes on illumination, being able to bend and move on water. Main chain polydomain LCEs are less studied than side chain LCEs and monodomain materials (networks). Keeping this in view, Burke et  al. (2014) proposed a two-step hydrosilylation process to achieve more uniform crosslinking. In the first step, the mesogen was attached into the main chain of a Si-H terminated prepolymer, then a tetrafunctional cross-linker was used in stoichiometric amount to obtain high gel fraction materials with a tunable cross-link length. These polydomain LCEs exhibited a high actuation strain of ca. 30% under thermal cycling, which was explained by stress-induced orientation of the mesogens and polymer chains along the strain axis, in the isotropic state. Papadopoulos et  al. (2010) induced the

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formation of domains with permanent electric dipole moment in smectic C systems by doping with chiral mesogens, which exhibited piezoelectric properties. Shear processing is an effective way to produce electromechanically active LCEs from uniaxially oriented films. Another simple synthetic approach to tune the dynamic mechanical response of polydomain smectic elastomers for possible applications in vibration isolation or impact absorption is introduction of methyl substituents into the mesogen (Patil et al. 2007).

11.4.3  P  olymer-Dispersed LCs (PDLCs) and Hybrid LC Materials PDLCs represent a mixture of LCs and a monomer/polymer in which phase separation is induced by thermal/UV treatment (crosslinking or polymerization of the matrix) or by solvent evaporation. Another method for the preparation of PDLCs is the encapsulation of LCs, usually in water soluble polymers. The nematic texture within the LC domains is randomly oriented, while the application of the external field (electromagnetic, mechanical, thermal, etc.) causes the orientation of domains in the preferred direction, ensuring PDLC transparency when the ordinary refractive index coincides with that of the polymer matrix (Li et al. 2005). PDLCs are widely used for manufacturing flat-panel displays, holographic films, switchable windows, etc. PDMS is not commonly used as a matrix in PDLCs, probably due to the chemical incompatibility with the organic LCs, which may lead to uncontrolled morphology. PDLCs composed of PDMS, crosslinked PDMS or polysulfone as matrices, and an azomethine compound as an embedded mesogen (5–80 wt.%) have been prepared by Bronnikov et al. (2009) via solvent-induced phase separation as model systems to study the nematic phase growth kinetics through the isotropic-nematic phase transition. This transition was described analytically with the universal law for the cluster growth, which revealed the influence of both flexibility of the polymer matrix and the mesogen content. For the LC/PDMS systems where the components were immiscible through the I-N phase transition, the soft un-crosslinked polymer matrix did not prevent the nematic phase growth (the mean droplet size was independent of the LC phase concentration), while in semi-flexible crosslinked PDMS matrix, the growth of the nematic droplets was restricted (the mean droplet size decreased significantly with the LC phase concentration). Racles et al. (2015b) also reported the preparation of PDLC materials by encapsulation in PDMS. These authors first prepared LC siloxane-containing polyazomethine NPs, which were stabilized in toluene using an amphiphilic siloxane oligomer, then mixed with PDMS, and the matrix was crosslinked. This approach provided highly flexible films with Smax of 600–800%, low Y and increased dielectric permittivity up to 300%, which were primarily intended for electromechanical applications.

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A surfactant was specifically designed by Loudet and Poulin (2002), suitable for stabilizing silicone oil/LC nonaqueous emulsions, composed of PDMS and cyanobiphenyl groups. Depending on the size of the PDMS segment, silicone in LC emulsions or LC in silicone emulsions were stabilized. The materials obtained were not optimal for electrooptical applications because of the poor mismatch between the optical indexes of the LC and silicones used, but they were presented as an alternative for chemically bonding LC onto siloxane chain, and coatings, cosmetics or thermal sensors applications were proposed. Organic-inorganic LC materials containing siloxane moieties have also been investigated. NPs (magnetic NPs, nanotubes, photo-isomerisable dyes) incorporated into LCE networks have been shown to increase their sensitivity to external stimuli (i.e. electrical, electro-thermal, magnetic, optical, strain or stress). For example, carbon black NPs integrated at surfaces can be used for electrothermal heating of LCE systems (Chambers et al. 2009). Fujisawa et al. (2018) reported LC gold complexes containing siloxane units in the flexible segment and phenyl-­ isocyanide complexing groups. These materials exhibited a special photoluminescence behavior in condensed phase, i.e. different luminescence colors were observed depending on the aggregate structure due to the effect of intermolecular interactions, while said effect was not observed in the solution. LC-grafted silica NPs have also been tested by Rachet et al. (2007) in photoinduced phase separation experiments to prepare PDLC with droplets of submicron size. Micrometre-sized ‘artificial muscles’ are desirable for applications involving surface-responsive materials, such as microfluidics, and present an alternative to more classic responsive materials. The Zentel’s group reported a procedure for producing LCE actuator microparticles based on a microfluidic device (Ohm et  al. 2011; Hessberger et al. 2018). To avoid the need for mechanical manipulation during curing conventional LCEs, another original method was proposed for the preparation of PDLC elastomers (Rešetič et  al. 2016), which offers the advantage of imprinting the thermal shape memory anisotropy by curing in external magnetic field. The method is based on crushing the starting bulk LCE into microparticles, dispersing these in uncured PDMS elastomer, aligning of LCE microparticles in a magnetic field and subsequent thermal curing of the silicone matrix.

11.5  Functional Silicones as Surfactants The colloidal phenomena associated with the silicon element have been observed for a long time being considered a science in itself (Hauser 1955). In this context, the ability of the polysiloxane-based materials for the formation of nano-aggregates and their utility in practical applications have been extensively studied (Hill 1998, 1999, 2002; Schlachter and Feldmann-Krane 1998; Iwakiri et al. 2009). Silicone surfactants are used for agricultural adjuvants, cosmetic formulations, paint additives, PU foams, textile conditioning and (more recently) in nanotechnology and drug delivery systems. They are unique because of a series of

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characteristics, partly derived from the specificity of siloxanes, others come from the combination of those with a variety of hydrophilic molecules. For example, silicone surfactants exhibit both hydrophobic and oleophobic properties, thus considerably reducing the surface tension of water and organic media, which is due to the high flexibility and low cohesive energy of the siloxane bond (Noll 1968; Voronkov et al. 1978). The effect of siloxane-based compounds as surface active materials is comparable to that of polyfluorinated compounds, but the latter are potentially bio-­ accumulative and toxic. Chen et  al. (2018) estimated that more than one billion pounds of organosilicon surfactants are produced worldwide per year, and those containing hydrophilic polyalkoxylate (ethoxylate and propoxylate) chain are the best known. In agriculture, siloxane surfactants are used as adjuvants to improve herbicide absorption (https://brandt.co/), being the most super-spreading and penetrating adjuvants available. A wide range of organic moieties, namely nonionic, anionic, cationic and zwitterionic groups, have been linked to polysiloxane backbones, as previously reviewed (Hill 2002; Racles et  al. 2010b). A characteristic of siloxane surfactants is their lower critical aggregation concentration and the corresponding surface tension compared to traditional hydrocarbon surfactants. The flexibility of the siloxane bond favors the chain conformations exposing the methylene groups towards the interface or surface, thus reducing the surface tension to values close to the pure PDMS. Another rather general aspect of siloxane surfactants is their propensity for self-assembly in vesicles (Guoyong et al. 2010; Racles 2010; Racles et al. 2014d; Zhou and Zhang 2016). The formation of aggregates with less curvature has been attributed to the bulkiness of the hydrophobic dimethylsiloxane tails, which has an important impact on the critical packing parameter. Self-assembled spherical vesicles open potential applications as drug delivery systems in microspheres and as biomembrane models (Guoyong et al. 2010). The surface properties and aggregation behavior of tetrasiloxane Gemini imidazolium surfactants were studied by Zhao et al. (2016) compared with the corresponding monomer and the hydrocarbon-based Gemini imidazolium surfactant. These authors observed that the former has the greatest capacity to form micelles, but less efficiency to reduce surface tension, while the CMC value was about an order of magnitude less than that of the monomer. On the other hand, the tetrasiloxane-­based surfactants have a greater capacity to reduce the surface tension than the hydrocarbon-based surfactant. Carbohydrate-modified siloxane surfactants have been synthesized and their surfactant behavior has been extensively investigated (Wagner et al. 1996a, b; Racles and Hamaide 2005; Racles et  al. 2006b; Berson et  al. 2008; Halila et  al. 2008; Guoyong et al. 2010; Racles et al. 2010b; Wang et al. 2011; Zhou and Zhang 2016). The polysiloxane functionalization with carbohydrates is a challenge, mainly because of the completely different solubility of the reagents. Several methods have been reported, which may or may not require protection-deprotection steps of carbohydrates (Fitremann et al. 2008). Selected examples refer to the hydrosilylation of allylglycosides (Jonas and Stadler 1994; Wagner et  al. 1996a; Akimoto et  al. 2000; Racles and Hamaide 2005; Racles et  al. 2006b; Berson et  al. 2008), ring opening reaction between an amino-functional siloxane and a carbohydrate lactone

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(Braunmuhl et al. 1995; Wagner et al. 1996b, 1997), transacetalation (Ogawa 2003), glycosidation with epoxy-siloxanes catalyzed by a cation exchanger (Racles and Cozan 2012), click chemistry (Halila et al. 2008), or glucamine reaction using polysiloxanes modified with activated ester (Moukarzel et al. 2013). Lipase catalyzed esterification between ethylglucoside and diacid terminated siloxanes was also reported by Sahoo et al. (2005). The regioselective formation of ester bonds involving the primary hydroxyl group of ethylglucoside is produced in a one-step reaction carried out under mild conditions without protection-deprotection steps or activation of the acid groups, with the integrity of the siloxane bonds. The reaction of poly(3-aminopropyl)siloxane with fatty acid chlorides, followed by the reaction with gluconolactone gave amphiphilic polysiloxanes with an aggregation behavior tuned by the structures and functionalities of the hydrophobic parts (Iwakiri et al. 2009). Several recent and atypical applications of amphiphilic siloxanes have been developed. Hetzer et al. (2014) designed a siloxane surfactant as fluorine-free aqueous film-forming foams for fire extinguishing agents for pool fires. Silicones of different architecture modified by addition reactions onto epoxide groups have also been explored as surface active materials with applicative potential in various directions. These contain tris(hydroxymethyl)aminomethane (known as tromethamol or Tris -buffer) (Racles 2010) or p-aminopyridine (Racles et al. 2018) as hydrophilic groups, which act also as ligands for metals (Racles et al. 2012, 2014e). Excellent surfactant skills have been demonstrated for the first set of materials, in stabilization of polymer or metal oxide NPs (Racles et al. 2011, 2014c; Racles 2013) and micellar solubilization of a hydrophobic drug (Racles et  al. 2014d), while the latter showed phase transfer properties (Racles et al. 2018), based on the encapsulation of hydrophilic molecules in their vesicles. The variation of chemical structure, in particular the length of the polysiloxane chain, allows to tune the solubility and surfactant properties in organic solvents, with applications in polymer-polymer composites (Racles et al. 2015a, b). Carbon nanotubes have also been efficiently dispersed into nonpolar organic solvents using amphiphilic molecules composed of long siloxane tails and pyrene or porphyrin, which were chosen based on their affinity for the surface of graphene nanotubes (Ji et al. 2009). Following Ji et al. (2009) these compounds offered superior stabilization than those containing C18 carbon tails and imparted compatibility with the polysiloxane elastomer matrix. Carbohydrate-­ containing cyclosiloxane surfactants have also been used by Racles et al. (2010a) for the preparation of silver NPs with a tunable size. Gold nanochains can also be obtained with a siloxane surfactant (Jia et al. 2011); the 1D assembly of gold NPs depends on the surfactant concentration and ionic strength.

11.6  Functional Silicones for Biomedical Applications Siloxanes have been used in many consumer products for a long time. However, there is little information available about its toxicity, and the existing one is mainly limited to widely used, unmodified cyclosiloxanes (mainly D4 and D5),

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hexamethyldisiloxane (HMDS) and PDMS. Siloxanes have been generally recognized as safe (GRAS) in consumer products. According to Lassen et  al. (2005), siloxanes have low general toxicity and no reported genotoxic potential, apart from some data showing inflammatory effects of silicones found mainly in cosmetics. The International Agency for Research on Cancer (IARC) has assessed that there is evidence suggesting lack of carcinogenicity in humans of breast implants made of silicone, for female breast carcinoma (IARC 1999). Certain studies stated that siloxane surfactants used as adjuvants in agriculture showed a negative impact on honey bees when ingested in high concentrations (Chen et al. 2018). Nonetheless, biocompatibility is a complex problem which depends on a number of factors, and must be evaluated for each particular case (Mojsiewicz-Pieńkowska et al. 2016). In biological research, PDMS is used as a support in microfluidic devices. Kuo et al. (2016) observed a long-range signal delivery on 10- and 20-μm PDMS barriers in microfluidically tetracycline inducible expression systems, thus suggesting that this communication depends largely on the PDMS barrier for the delivery signal, since it is a ‘wired’ cell communication instead of wireless. In another example, the siloxane coating was able to significantly reduce bacterial adherence, and this effect was assumed to be mainly a result of the high hydrophobicity of the coating substrate (Pfeiffer et al. 2016). On the other hand, for many biomedical applications the hydrophobic behavior is a drawback. The low surface energy of PDMS elastomers used in prostheses can cause abrasion, irritation and ulceration of tissues, as well as bacterial colonization. It is known that an appropriate hydrophobic/hydrophilic balance of a drug molecule is essential for cell uptake. Biofouling (adsorption of contaminating matter, e.g. nonspecific proteins or organisms) is another serious problem in relation to biomedical implants and devices. Polymers with antifouling properties must be hydrophilic and electrically neutral, and must have H-bond acceptors but no H-bond donors (Chapman et al. 2001). Here comes the role of the special functionalization of siloxanes, in order to modify the polarity and make the final material more hydrophilic. The strategy to improve clinical performance by modifying the surface of silicone elastomers is based on chemical, physical and combinations of both methods (Abbasi et al. 2001; Hron 2003). In general, physical methods produce Si-OH groups, and therefore temporary hydrophilic surfaces, which can be chemically modified with a permanent hydrophilic effect. The antifouling coatings for PDMS were already reviewed by Zhang and Chiao (2015). Usually, these are attached either by physical adsorption or by covalent bonding of appropriately designed PEG/poly(ethylene oxide) (PEO) derivatives, polyzwitterions or other compounds on the PDMS surface with or without preliminary UV/plasma treatment. Goncalves et  al. (2013) superficially polymerized PDMS using poly(ethylene glycol)methacrylate by surface-initiated ATRP, preceded by UV/ozone exposure and covalent bonding of the initiator (1-trichlorosilyl-2-(chloromethylphenyl)ethane) onto the hydroxylated PDMS via chemical vapor deposition. These authors succeeded to obtain hydrophilic surfaces (with water contact angles around 60°), thus resulting in the inhibition of bacterial adhesion and the non-cytotoxic effect on human skin fibroblasts. In this same line, the surface modification of PDMS substrates by using ABA-type block copolymers which comprise poly(2-­methacryloyl

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oxyethylphosphorylcholine) and PDMS segments were proposed by Seo et  al. (2008) as a procedure for manufacturing biocompatible surfaces on PDMS substrates. The hydrophobic surface of PDMS became a hydrophilic surface, which was beneficial for reducing cell and protein adhesion compared to those of the untreated PDMS substrate. Gottenbos et al. (2002) also modified the surface of silicone rubber by covalent bonding of 3-(trimethoxysilyl)-propyldimethyloctadecylammonium chloride after argon plasma treatment resulting in materials with antimicrobial properties against adhering bacteria. Egli et  al. (2011) reported PDMS-b-poly(2-methyloxazoline) diblock copolymers with hydroxyl or piperazyl functionalities at the hydrophilic terminus and their polymersomes. In this regard, the subsequent covalent binding of biological ligands by bis-aryl hydrazone conjugation chemistry was a promising approach to make immunoassays more sensitive and drug delivery more effective. Surface modification of PDMS is useful for lab-on-chip applications. For example, Escutia-Guadarrama et  al. (2017) achieved surface oxidation of PDMS substrates and silanization with (3-aminopropyl)triethoxysilane, followed by crosslinking with glutaraldehyde and incubation with type I collagen, for the biomimetic confinement of cells which do not adhere to unmodified PDMS. Arginine-modified oligoalkylaminocyclosiloxane-conjugated poly(ethyleneimine) (PEI) was synthesized by Morris and Sharma (2010). These derivatives were complexed with plasmid DNA and the resulted complexes showed a very good cell viability (opposed to cytotoxicity of branched PEI) and greater transfection compared with branched PEI. In this same line, Marangoci et al. (2008, 2014) observed a significant reduction in the toxicity of viologens when viologen-­ polysiloxanes were used in cyclodextrine based polyrotaxanes. Hurduc et al. (2013) synthesized polysiloxanes with chlorobenzyl groups, which were subsequently modified with several azobenzene molecules. Enea et al. (2007) demonstrated that this type of materials resulting based on azo-polysiloxanes can support cell adhesion and growth. The further modification of such polymers with nucleobases opens possible applications for the immobilization of biomolecules and nano-manipulation. Siloxane surfactants have also been tested for encapsulation of a model drug in PDMS submicron particles (Racles 2013). Solubilization of a hydrophobic drug in siloxane-containing polymersomes formed by a non-ionic polysiloxane surfactant or a metalo-surfactant has been reported by Racles et al. (2014d, e). Power-Billard et  al. (2004) also proposed stimuli responsive polymersomes obtained from the water-soluble PDMS-b-polyferrocenylsilane diblock copolymer for redox-tunable encapsulation. Carboxybetaine functionalized PDMSs were proposed by Lin et  al. (2016) as antifouling materials. These materials exhibited good resistance against proteins, which was attributed to the zwitterions side groups’ binding ability between the PDMS chain and the protein. Fitremann et  al. (2008) reported saccharide-modified siloxanes as potential or proven biocompatible compounds. Feher et al. (1998) showed that carbohydrate-­ functionalized silsesquioxanes exhibit selective and reversible complexation to

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carbohydrate-binding proteins, based on amidation reaction with amino-polyhedral oligomeric silsesquioxane (POSS), while a more efficient synthetic route for a variety of POSS-based glycoclusters, by photoaddition of thiols to vinyl was reported by Gao et al. (2004). The biomaterial applications of POSS have been reviewed by Wu and Mather (2009). The relatively easy preparation of POSS functionalized with several organic groups, such as -COOH, -NH2, -OH or -SH opens up promising perspectives for application in DNA/gene delivery, DNA/protein detection and drug delivery. Majumdar et al. (2010) investigated quaternary ammonium functionalized-POSS with antimicrobial activity in PDMS coatings. These types of materials have been designed as molecular platforms for biomedical applications such as a bifunctional luminescent POSS carrying fluorescein and carboxylic groups, suitable to anchor different organic compounds (contrast agents, drugs, dyes, markers) (Olivero et al. 2012). Interestingly, Olivero et al. (2012) reported that dye emission increased four times after the reaction with POSS, which made this compound very promising as a contrast agent. In addition, efficient and selective cell uptake of these materials was demonstrated. In general, sol-gel materials have multiple biomedical applications (Avnir et al. 2006). In particular, functionalized silica can show very interesting properties for this purpose. Sui et al. (2005) reported that a sol-gel matrix containing biocompatible silane precursors and covalently bound sugar moieties showed high biocompatibility for protein entrapment. Liang et al. (2005) also demonstrated a sol-gel based immunosensor for the amperometric detection of hepatitis B with a gold/silica/antibodies composite. This latter was achieved due to the self-assembly of gold NPs into a thiol containing sol-gel networks. The gelatin-siloxane NPs were obtained by Tian et  al. (2012) via reaction of epoxy functional alkoxysilane with the amino groups in gelatin, followed by further functionalization with PEG and modification with cell permeable peptides. These authors discovered that these hybrid NP systems improve the plasmid DNA transfection efficiency, being membrane-penetrable and crossing the blood-brain barrier for the delivery of a drug to its target site in the brain. In addition to ensuring optimal oxygen permeability, silicone hydrogel contact lenses have demonstrated good efficacy and safety as therapeutic lenses (Sindt 2007). Silicone lenses must meet a series of comfort requirements in terms of mechanical and surface properties, oxygen transmissibility and hydration. Silicone hydrogels as contact lenses are based on silicone rubber combined with conventional hydrogel hydrophilic monomers, and special care has to be taken as to minimize the consequences of inherent phase separation, in order to obtain transparent materials with target properties (Jones and Tighe 2004). In addition, they can incorporate a permanent biocompatible plasma surface treatment for a smooth and continuous surface that resists deposits (Young and Tapper 2007). The equilibrium water content (EWC), internal morphologies, light transmittance, mechanical properties and oxygen permeability coefficient (Dk) were investigated by Tao et  al. (2017) on silicone hydrogels obtained from mixtures of hydrophilic monomers (hydroxypropyl methacrylate, N,N-dimethylacrylamide and N-vinylpyrrolidone),

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methacrylate-terminated siloxane macromonomer (MTSM) and tris(trimethyl­ siloxy)-3-methacryloxpropylsilane. These authors reported that the properties of interest for contact lens applications were influenced and adjusted by modifying the length of the siloxane chain in MTSM. Iron oxide NPs (IO NPs) are increasingly used in various biomedical applications, as was recently reviewed by Iacob et al. (2019). The non-toxicity and biocompatibility of magnetic NPs can be further improved by a special coating with organic or inorganic molecules. One of the most used techniques is based on covering the IO NPs with a functional silica shell (Santra et al. 2001; Kyeong et al. 2015; Wu et al. 2015). Encapsulation of hydrophobic IO NPs or a combination of these with a hydrophobic drug (nystatin) in biocompatible siloxane surfactants made magnetic NPs promising for biomedical applications (Racles et al. 2014c, e).

11.7  R  eactive and Functional Siloxanes as Ligands for Metals Short siloxanes, disiloxanes or cyclosiloxanes generally commercially available, have been chemically modified by attaching electron-donating groups able to coordinate metal ions. Both ligands as such and their complexes are of great interest in biology, chemistry and materials science (catalysis, gas separation and storage, luminescence, etc.). Original and valuable siloxanes containing suitable carboxyl groups as ligands have been prepared (Table 11.5, entries 1–6), which constitute alternatives to classic organic dicarboxylic acids. One of them is 1,3-bis(carboxypropyl)tetramethyldisiloxane (CX, Table 11.5, entry 1). Although initially only synthesized in the laboratory (Cazacu et al. 1999, 2019), it has now become commercial. Being a ligand with multiple binding sites, it generally leads to coordination polymers with single metal ions or clusters as secondary building units (SBUs) or even to their subclass of porous materials known as metal-organic frameworks (MOFs) (Cazacu et al. 2019) which are of great interest for both research and industry with a wide range of applications: catalysis, drug release, gas sorption, luminescence, magnetism, etc. (Zhu et al. 2017). The high length of the siloxane bond and the flexibility of the siloxane angle reducing the conformational stiffness lead to the formation of coordination polymers with new structural and topological forms (Davies et al. 2008). However, since it is very flexible, in general, to obtain a well-defined porous crystalline structure, it is necessary to add a co-ligand to increase stiffness and stabilize the network. It has been found that the acid itself forms supramolecular structures by means of H-bonds that can be isolated in the crystalline state with 1,2-(4-bipyridyl)ethylene, 4,4′-bipyridine (Racles et al. 2013b) and 4,4′-azopiridine (Vlad et al. 2013a). The architecture of the metal coordination structures formed is influenced by the co-­ ligand, the particularities of the metal ion and the reaction conditions. By reacting this diacid with Zn(II) perchlorate in the presence of 4,4′-azopyridine, a 2D MOF is

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Table 11.5  Examples of functional siloxane derivatives useful as ligands (carboxylic acids) or as substrates (diamines) for the development of new ligands No. 1

Structure

2

References Cazacu et al. (1999, 2019), Vlad et al. (2013b, a), Turcan-Trofin et al. (2018) Zaltariov et al. (2014b)

3

Cazacu et al. (2012)

4

Turcan-Trofin et al. (2019)

5

Turcan-Trofin et al. (2019)

6

Delmas et al. (2017, 2019)

7

Commercially available

8

Zaltariov et al. (2013)

formed in which dinuclear zinc units supported by a pair of bidentate bridging carboxylate groups are extended in two dimensions by the connection of 4,4′-azopyridine and flexible tetramethyldisiloxane linkers (Vlad et al. 2014). Two-dimensional (2D) networks are also formed if, in a similar system, the Zn(II) is replaced with Co(II) or 4,4′-azopyridine with imidazole (Vlad et al. 2013a). CX and copper salt in the presence of imidazole form a MOF containing paddle-wheel dicopper(II) moieties, where the two copper atoms are bridged by one carboxylate group of CX dianion, while the coordination sphere is supplemented by two imidazole molecules. A oxygen atom of the second carboxylate group of a neighboring CX dianion occupies an apical position. The 2D covalent MOF is extended to a 3D structure by H-bonds (Davies et al. 2008). By using 1,2-(4-bipyridyl)ethylene as a co-ligand and Zn(II) acetate, a 2D coordination polymer is formed in which the Zn atom having a strongly distorted N2O4 octahedral coordination provided by two 1,2-(4-bipyridyl) ethylene ligands and two bidentate carboxylate groups from acetate anions and CX dianions (Racles et  al. 2013b). When a preformed hexamanganese(III)

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salicylaldoximate ([Mn6]) cluster has been used as SBU, the CX diacid was able to connect the hexamanganese clusters alone into a 1D crystalline coordination polymer (Zaltariov et al. 2016). Suitable tests have revealed the self-organization of this polymer in solution or in cast films as micelles or inverse micelles and vesicles depending on the polarity of the solvent, while magnetic measurements showed the presence of slow relaxation of magnetization due to the presence of [Mn(III)6] clusters (Zaltariov et  al. 2016). In contrast, another dicarboxylic acid, 3-bis(p-­ carboxyphenylene-­ester-methylene)tetramethyldisiloxane (Table  11.5, entry 2) forms with Cu(II), in presence of imidazole, an H-bonded supramolecular structure containing complex metal units (Zaltariov et al. 2014b). Another siloxane-­containing acid, 1,3-bis(3-trimellitylimidopropyl)tetramethyldisiloxane (Table 11.5, entry 3), was obtained, characterized and used by Cazacu et al. (2012) to prepare heterotrinuclear carboxylate {Fe2CoOSin} cluster. In addition, by thiol-ene addition of 3-mercaptopropionic acid and thioglycolic acid onto disiloxane or cyclosiloxane bearing vinyl groups, appropriate acids (Table 11.5, entries 4,5) have formed, which are suitable as ligands for metals (Turcan-Trofin et al. 2019). Recently, Delmas et al. (2017, 2019) prepared a very interesting siloxane-based hexacarboxylic acid (Table 11.5, entry 6) which was applied in MOF construction obtaining a 3D triply interpenetrated H-bonded organic framework of Zn(II) accommodating 1D H-bonded chain polymers in their pores. One of the substrates that most frequently undergo chemical modifications to become ligands is 1,3-bis(3-aminopropyl)tetramethyldisiloxane commercially available (Table 11.5, entry 7), which readily reacts with carbonyl compounds forming bis-azomethines. Schiff bases derived from this siloxane compound have thus been reported with: 2,4-dihydroxybenzaldehyde, 2-chloro-2-hydroxybenzaldehyde, 2-hydroxy-5-nitrobenzaldehyde, 2-hydroxybenzaldehyde, 3,5-dibromo-2-­ hydroxybenzaldehyde (Soroceanu et al. 2013; Vlad et al. 2018), 2,6-diformyl-­4-methyl­ phenol (DFF) (Zaltariov et  al. 2014a), 3,5-di-tert-butyl-2-hydroxyben­zaldehyde (Cazacu et al. 2015; Soroceanu et al. 2015; Shova et al. 2017b, 2019), 3-carboxylsalicylaldehyde (Shova et  al. 2017a), o-vanillin (Vlad et  al. 2016), pyrrole-2-­ carbaldehyde (Vlad et al. 2012b) or ferrocenecarboxaldehyde (Vlad et al. 2012a). In the latter case, an oligomeric siloxane diamine was also used. This amine has also been used to obtain polyazomethine structures with alternating siloxane and organic segments such as: 1,4-­dihydroxyanthraquinone and 8-dihydroxy-1,4-naphthoquinone (Georén and Lindbergh 2003; Vlad et al. 2012a), which exhibit electrochemical activity and self-­assembling capacity (Soroceanu et al. 2015). In addition to the commercial diamine, another amine derivative was prepared by Zaltariov et  al. (2013) by reacting 1,3-bis(chloromethyl)tetramethyldisiloxane with sodium salt of p-aminobenzoic acid, thus linking the p-aminobenzene to the siloxane through the ester group (Table 11.5, entry 8). The bonding or coupling of polar groups/segments to the highly hydrophobic di-, oligo- or PDMSs imparts amphiphilic character to the functional silicones. Given the high flexibility of the siloxane chain/segment, amphiphilic silicones are easily self-assembled in selective solvents (Racles et  al. 2008; Cazacu et  al. 2009a, b; Soroceanu et al. 2016; Zaltariov et al. 2016; Turcan-Trofin et al. 2019). This has

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been well emphasized by dynamic light scattering (DLS), small-angle X-ray scattering (SAXS) and UV-Vis spectroscopy (Soroceanu et al. 2015). This characteristic can improve specific behaviors or properties (such as biological or catalytic) depending on the environment. Thus, the bis(2-hydroxyazomethine)s derived from the siloxane diamine 1,3-bis(amino-phenylene-ester-methylene)tetramethyldisiloxane, with salicylaldehyde and especially with 5-chlorosalicylaldehyde showed a remarkable biocidal activity (Zaltariov et al. 2013). Copper complexes of Schiff bases derived from 1,3-bis(3-aminopropyl)tetramethyldisiloxane with differently substituted (2-hydroxy, 3,5-di-bromo, 5-nitro, 2-chloro) salicylaldehydes have been shown to have potential as antimicrobial agents, as well as improvers for antioxidant activity of the spirulina extract, thus being of interest for biotechnologies (Soroceanu et  al. 2016). In particular, those with electron withdrawing substituents (bromo, chlorine and nitro) to the aromatic ring show high catalytic activity and selectivity in the aerobic oxidation of benzyl alcohol to benzaldehyde mediated by the TEMPO radical, under mild conditions (Soroceanu et al. 2013). The tetranuclear copper complex of the siloxane derivative obtained by [2 + 2] condensation of 1,3-bis(aminopropyl)tetramethyldisiloxane and 2,6-diformyl-4-methylphenol have proved suitable as catalyst precursors for the hydrocarboxylation of a linear C5-C8 variety (n-pentane, n-hexane, n-heptane and n-octane) and cyclic (cyclopentane, cyclohexane, cycloheptane and cyclooctane) alkanes to give the corresponding C6-C9 carboxylic acids (Zaltariov et al. 2014a). The good biological and catalytic activities of these complexes have been attributed to their amphiphilic nature conferred by the co-existence of the bis-ethylene tetramethyldisiloxane segment and the polar coordination block, which could facilitate molecular interactions favorable to the biological or catalytic action (Ng et al. 2007; Soroceanu et al. 2013). Cobalt, copper and zinc, coordination polymers derived from CX and bipyridyl ligands have shown very good catalytic activity in the H2O2 decomposition (Racles et al. 2013b) and as heterogeneous catalysts in photodegradation of organic pollutants such as chemotherapy drug doxorubicin (DOX) (Racles et al. 2019) and Congo red (CR) dye (Racles et al. 2017b) in ambient light and temperature. It is assumed that the pronounced hydrophobic nature of these MOFs is partly responsible for their performance.

11.8  M  iscellaneous: Special Properties and Applications of Functional Silicones Multifunctional UV-reactive silicones bearing acrylate or epoxy groups and polyethyleneoxide segments were combined by Ruckle and Cheung (2013) with organic resins having the same reactive groups and the properties of the final films (appearance, cure time, flexibility, gloss, impact resistance, mar resistance, slip and stain release) were evaluated. These authors concluded that most of the properties were

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improved when only 1% functional silicone was used, while for others, such as stain resistance, a remarkable effect was observed with around 20% functional silicone. The architecture and Mw of the modified silicones had an important effect, the linear difunctional materials being often better than pendants for these properties. The printing of textile materials with reactive silicone pastes was explored by Turalija and Bechtold (2015) as an alternative to introduce water repellence without fluorocarbon finish. The reactive silicones were mixtures from cyclic and end- functional commercial materials having vinyl groups, which were crosslinked by hydrosilylation with PDMHS. These authors were able to obtain materials with a reduction in water retention capacity, and proposed several technical application areas for the modified fabric, such as barrier textiles with permeability control or localized modification of the mechanical properties of fabric and garment. Functional arylsilanes and siloxanes, such as functional POSS, networks and polymers have also been studied as organic light emitting diode (OLED) components: fluorophore emitters, hosts for phosphor emitters, hole and exciton blocking materials, electron and hole transporting materials (Sun et al. 2015). The chemical modification of siloxanes is a tool useful to tune their surface properties throughout the range, from hydrophilic materials to super-hydrophobic materials. In this sense, Wang et al. (2017) reported that poly(siloxane-ether-urethane) copolymers, containing poly(siloxane-ether) copolymers as soft segments have a superhydrophilic behavior, with water contact angle of 2.1°, possibly useful for surfactant and coating applications (Wang et  al. 2017), while Zuo et  al. (2014a) showed that silicone elastomers modified with N-acetyl-L-cysteine and complexed with lanthanide ions have very intense photoluminescence under UV radiation and reduced the contact angles. On the other hand, superhydrophobic coatings were manufactured by Huang et al. (2016) using two silica sols and fluorinated acrylate copolymers through the organic-inorganic hybrid method. The Lotus effect was easily obtained with a water contact angle of 156.2° and a hysteresis of the contact angle of 2.4°, which indicated a good self-cleaning capability, in addition to good mechanical properties and thermostability (Huang et al. 2016). Surface-modified NPs with hydrophilic and hydrophobic surface groups are environmentally responsive, acting as colloidal surfactants. The PEG chains as a hydrophilic agent and the propyl chains as a hydrophobic agent were grafted by Behzadi and Mohammadi (2016) on silica NPs in various proportions to study their ability to modulate oil-water interface properties. The functionalized silica NPs were thus proposed for improving oil recovery (Behzadi and Mohammadi 2016). Inorganic-organic hybrid materials were also synthesized by Wan Ibrahim et al. (2011) via hydrolysis and condensation of cyanopropyltriethoxysilane and PDMS, to be used as coatings in stir bar sorptive extraction (SBSE). These materials demonstrated that their physical properties and extraction capacity were largely dependent on the solvent used for their preparation and applications for analysis of non-steroidal anti-inflammatory drugs ketoprofen and diclofenac sodium in SBSE were proposed.

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Azo-modified silicones have also been intensively studied with respect to their photoresponsive behavior for biology, solar energy conversion or plasmonic applications (Hurduc et  al. 2013, 2014; Damian et  al. 2014). These materials exhibit surface relief grating (SRG) generation ability, as a consequence of athermal photofluidisation (Hurduc et al. 2014), a phenomenon based on the concept of conformational instability, thus explaining the sharp modification of the surface properties. The proposed mechanism for the SRG formation follows three steps: (1) photo-­ fluidization of polymers in illuminated regions, (2) the mass displacement from illuminated regions to dark regions and (3) the inverse mass displacement from dark to illuminated regions. Organosilicones are also interesting as solvent-free liquid electrolytes for lithium batteries, being a promising alternative to commercial carbonyl-based electrolytes due to their electrochemical and thermal stability, as well as their low flammability and vapor pressure and relatively low viscosity (Zhang et al. 2008; Rossi and West 2009). The siloxane segment may contain functionalities capable of coordinating lithium such as PEO moieties, and its highly flexible chain provides a complete amorphous state of the polymer (Zhang et  al. 2005). Recently, cyclic and linear methylsiloxanes having carboxyl groups attached to each of the silicon atoms which behave as thermally stable liquids over a wide temperature range have shown a higher conductivity and dielectric permittivity compared to the original unmodified compounds (Turcan-Trofin et al. 2019). Following Turcan-Trofin et al. (2019) these increases were even more significant when the products were doped with lithium salt, creating the premise for use as solvent-free liquid electrolytes for ionic batteries. A photo-crosslinked polymer based on the siloxane backbone containing cyanide pendant groups showed an increased conductivity (up to 3.38  ×  10−5  S/cm) attributed to the dissociation and increased mobility of the ion pairs, promoted by the polarity and solvation power of the cyano group and the flexibility of the siloxane chain. This is of high potential as a solid electrolyte for Li batteries (Lee et al. 2003). Pyridyl-siloxanes have also been tested as phase transfer agents (PTAs), being able to transfer hydro-soluble organic molecules from water into chloroform or toluene. The proposed applications are cold-dying of polymers (e.g. transparent colored silicones were obtained) or the preparation of removable and reusable pH indicators (Racles et al. 2018).

11.9  Conclusions The introduction of functional groups onto the silicon atom is continuously explored, which generates an unlimited variety of structures, improves the already special properties of silicones and adapts them to specific requirements. Several synthetic pathways have been well established and used for a long time, such as hydrolysis-­ condensation of silane precursors, hydrosilylation and ROP of cyclosiloxanes. Other methods have been less addressed but gained more recently attention, such as

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click reactions (alkyne-azide coupling, thiol-ene addition) and Piers-Rubinstajn reaction or more complex combined chemical modifications based on reaction schemes in organic chemistry, thus showing that the field of functional silicone materials is still in development. Functional groups (electron donor, hydrophilic, mesogenic, photo-responsive, polar, etc.) linked to low or high molecular weight, linear, cyclic or crosslinked siloxane compounds, strongly influence their properties (electric, mechanical, surface, thermal properties, solubility and so on). Such compounds generally exhibit self-assembly in selective solvents, or can act as biological-active compounds, ligands, liquid crystals, surfactants, etc. In relation to the wide variety of chemical structures and properties derived therefrom, the applications of modified siloxanes are also increasingly explored and often more than one application is found for a given compound. Acknowledgements  This work was supported by a grant of the Romanian Ministry of Research and Innovation, CCCDI  – UEFISCDI, project number PN-III-P1-1.2-­ PCCDI-­ 2017-0185/ 76PCCDI/2018, within PNCDI III. Conflicts of Interest  The authors declare no conflict of interest.

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Chapter 12

Maxillofacial Silicone Elastomers in Dentistry Pinar Cevik

Abstract  Maxillofacial prostheses are used for patients with deformities in the maxillofacial area induced by cancer, trauma or congenital defects. Different types of polymers are used in the manufacture of maxillofacial prosthesis. However, silicone elastomers are the polymers most commonly used in the maxillofacial prosthodontics because of their acceptable properties. Since maxillofacial defects create profound psychological and social difficulties in these patients, recent research focused on improving the mechanical and physical properties of silicone elastomers by using various incorporations of nano-oxide particles into the silicones. Nevertheless, the use of conventional techniques in the manufacture of maxillofacial silicones still consumes a lot of time. Silicone prostheses manufactured conventionally are also still susceptible to color degradation and change in the mechanical properties. Thus, the three-dimensional (3D) printing techniques of the maxillofacial prosthesis and the development of the printable maxillofacial prosthetic materials are of great interest. This chapter aims to analyze the novel approaches in maxillofacial prosthodontic materials, especially silicone elastomers, and recent advances in 3D printing technologies in the fabrication of maxillofacial prostheses. Keywords  3D modeling · Maxillofacial prosthodontics · Nanoparticles

12.1  Introduction According to the American Academy of Maxillofacial Prosthetics (AAMP 2019), maxillofacial prosthetics is a subspecialty of Prosthodontics that involves the rehabilitation of patients with defects or deformities that were present at birth or developing due to disease or trauma. Maxillofacial prosthodontists are accustomed to working cooperatively with anaplastologists, general dentists, neurologists, plastic surgeons, radiation oncologists, speech pathologists and auxiliary staff. The general P. Cevik (*) Department of Prosthodontics, Gazi University Faculty of Dentistry, Ankara, Turkey e-mail: [email protected] © Springer Nature Switzerland AG 2020 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_12

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objective of all maxillofacial prosthetic treatment is to improve the quality of life (AAMP 2019). Maxillofacial prostheses are mainly used in patients with congenital defects or acquired defects induced by cancer, trauma or systemic disease (D’Souza 2015). Patients with maxillofacial deformities have aesthetic discomfort due to the site of the defect. These defects could be localized at the sites of the cranial plate, ear, nose or palatal and create psychological problems and social disabilities in such patients. A maxillofacial prosthesis must thus be biocompatible and functional, as well as have mechanical stability, allowing the long-term use. In this sense, maxillofacial prosthodontists should consider the prostheses’ possible mechanical and physical deficiencies after long-term use (Cevik and Yildirim-Bicer 2017) and take precautions against possible failures. In addition, prosthetic treatment may include extraoral or intraoral management of patients with deformities. Extraoral prostheses are referred to auricular, midfacial, nasal, ocular, orbital, somatic or radiation shield prostheses. On the other hand, intraoral prostheses are referred to surgical obturator prosthesis, definitive obturator prosthesis, palatal lift prosthesis, mandibular resection prosthesis and palatal augmentation prosthesis or fluoride carriers as protective apparatus (AAMP 2019). A suitable personalized prosthesis should restore the patient’s deficiency by referring to the current anatomy. In this regard, duplication of the remaining part of the face or craniofacial tissues may result in an accurate treatment for the patients with maxillofacial defects. The duplication of the healthy tissues with the help of computerized technology would give more precise results than the construction of the prosthesis individually. Various materials have been used in the manufacture of maxillofacial prostheses, such as chlorinated polyethylene, poly(methyl methacrylate), poly(urethane), poly(vinyl chloride) and poly(dimethylsiloxane) (PDMS  - silicone elastomers) (Eleni et al. 2009). An ideal maxillofacial prosthetic material is expected to have high tear resistance, tensile strength and elongation percentage with low hardness (Veres et al. 1990; Cevik and Eraslan 2017). Silicone elastomers have been materials used for many years in the process of manufacturing dental prostheses due to their advanced mechanical and physical properties (Jindal et al. 2018). In addition, they are inert, chemically resistant and biocompatible. The most preferable silicone rubber consists of PDMS (Bellamy et al. 2003; Jindal et al. 2016). Different factors such as the PDMS, the filler, the cross-linker and the catalyst concentrations are the main components of silicone elastomer, and they can modify the mechanical properties of silicone elastomer (Bellamy et al. 2003; Jindal et al. 2016). Recent researches are based on the addition of several filler nanoparticles (NPs) to improve the physical and mechanical properties of silicone rubbers (Ratner et al. 2006; Kiat-amnuay et al. 2006; Kiat-amnuay et al. 2009; Pesqueira et al. 2012; Akay et al. 2016; Cevik and Eraslan 2017). The silica NPs are most commonly used fillers to reinforce the silicone matrix (Zayed et al. 2014; Cevik and Eraslan 2017). Different concentrations in silica NPs would result in different mechanical and physical properties of silicone elastomers (Goiato et al. 2011; Pesqueira et al. 2012; Cevik and Eraslan 2017).

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The term of ‘silicone’ is used in the medical field to introduce the PDMS derivatives and PDMS based materials. However, the modern silicone is known as silane: crystallin silicone and organosiloxanes (Ratner et al. 2006; Jindal et al. 2016, 2018). While, silica is the second most common element that exists in nature after oxygen. Three-dimensional (3D) printing is known as a rapid prototyping or additive manufacturing, and is a revolution in all fabrication technologies. The cost of materials is reduced by using 3D printing and the accessibility of various types of materials in addition to rapid prototyping domestic applications has recently attracted attention (Gutiérrez 2019). However, the variety of 3D printing materials is limited compared to conventional industrial materials in terms of material color, material cost and surface textures (Sherman 2009). 3D medical printing is more used in medicine and dentistry. Maxillofacial and craniofacial reconstructions and the planning of the surgeries and future prostheses are the most common applications in 3D printing technology. Surgical guides, custom implants and surgical guides can be easily manufactured through 3D printers. 3D medical printing consists of five different groups of technologies: binder jetting, material extrusion, material jetting, selective laser sintering and stereolithography (Mitsouras et al. 2015). However, stereolithographic or selective laser sintering are the most common used technologies due to the greater hardness of the materials which help easy application during surgical planning or application (Mitsouras et  al. 2015). Most applications in maxillofacial prosthodontics have consisted of clinical case reports used in 3D printing for surgical planning, or manufacturing molds for future prostheses (Sykes et  al. 2004; Watson and Hatamleh 2014; Shankaran et  al. 2016; Federspil 2018; Ferreira and Vives 2019; Cevik and Kocacikli 2019). The disadvantages of most 3D printing materials are that they lack mechanical strength because the bonding of the layers in the 3D manufacturing process is weak. In addition, color, fracture strength and mechanical stabilities must be verified on a 3D printed object (Sherman 2009; Berman 2012). As an advantage, it is a process that does not generate waste of material unlike milling processes. According to Berman (2012), waste material is reduced by 40% in metal applications in 3D printing technologies compared to cutting/milling machines. In addition, 96% of waste material could be recycled into 3D printing technologies, so it could also be considered as an economic process (Petrovic et al. 2011). 3D printing technologies could also save time since a production material, especially an initial product, could be manufactured by 3D printers faster than any other milling machineries (Berman 2012), as well as it could improve the manufacturing process of the maxillofacial prosthesis compared to the conventional manufacturing process (Mine et al. 2019). 3D printing applications are used in the manufacture of small and medium-scale applications, such as dental crowns, initial products, medical prostheses, prototypes and replacement parts (Berman 2012; Stansbury and Idacavage 2016). In addition, 3D applications use automated manufacturing without the need for expensive molds or tools (Berman 2012). Negatively, 3D applications have some critical limitations in the manufacturing process, such as limited heat, moisture and strength resistance, reduced surface finishes and material options, as well as reduced color diversity (Berman 2012).

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Silicone could be considered as one of those reduced options for materials. Plastics and metal structures with complex features could be fabricated by 3D manufacturing by extrusion process. The thermoplastic polymers could also be manufactured by initial hot melt extrusion, followed by cooling, and final solidification process, thus leading to target shape of the structure (Roh et al. 2017). 3D printing of elastomeric polymers such as PDMS compared to thermoplastic materials is more difficult since the precursors of elastomeric polymers are liquid. In addition, silicone printing is technically complex since it needs to be heated or a special curing process must be applied. Moreover, post-curing may require UV curing agents (Roh et al. 2017; Jindal et al. 2018; Unkovskiy et al. 2018). The conventional fabrication process of a maxillofacial prosthesis is challenging and requires complex laboratory procedures and the final step depends on the prosthodontist’s artistic abilities (Cevik and Yildirim-Bicer 2017). The first step in the manufacturing process is to make the conventional impression of the defect site, then hand sculpt with dental wax, thus creating molds, and the final step is silicone curing and intrinsic and extrinsic coloration (Jindal et al. 2016). Recently, new technologies are used to capture digital images of the non-defect site and the duplication of this site that leads to design a prosthesis digitally without the use of conventional printing technique. In this regard, optical surface laser scanners or computerize tomography (CT) images are commercially available to obtain a digital impression of the defect site (Karayazgan-Saracoglu et al. 2009; Watson and Hatamleh 2014). In this regard, 3D surface data could be design by computer-­ aided design (CAD) software and the data can be printed on metal, plastic, resin or other hard materials which are not suitable for direct fabrication of a maxillofacial prosthesis (Jindal et al. 2018). In addition, the coloration of the maxillofacial prosthesis is the most difficult problem to obtain acceptable aesthetic results. With this in mind, an intrinsic coloring kit has been used to obtain the basic color of the prosthesis (Hu et al. 2014). Then, the extrinsic coloration has been adjusted to obtain the final color of the silicone. On the negative side, intrinsic coloring agents can jeopardize the mechanical strength of the silicone rubber, while the extrinsic color is more likely to change in response to moisture, heat or UV radiation (Hu et  al. 2014). Therefore, the coloring process is one of the most challenging processes in the manufacture of a maxillofacial prosthesis. Anatomic molds, such as negative and the positive molds, can be manufactured by 3D printing to be used as a mold for additional silicone prostheses (Jindal et al. 2016, 2018). In the current digital workflow of a maxillofacial prosthesis it consists of digital printing of the healthy site and obtaining the image reflected for the defect site and the manufacturing of the molds. Other processes consist of the conventional manufacturing of the silicone prosthesis. The most challenging problem is the manufacture of direct printing of maxillofacial silicone prostheses. Therefore, the recent research relies on the direct silicone printing by eliminating the conventional laboratory processes (Jindal et al. 2016, 2018; Roh et al. 2017; Unkovskiy et al. 2018). Unkovskiy et al. (2018) reported an early application of a directly printed a silicone nasal prosthesis. The authors showed that the marginal integrity of the printed silicone prosthesis was lacking in some areas because of the scantiness in the

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printing thin layers below 0.1 mm thicknesses. In addition, the biocompatibility of the silicone materials used in the study was unknown. The authors also reported that direct printing can only be adjusted for an interim post-surgical prosthesis. Likewise, medical grade silicone elastomers are needed which are not available in today’s additive manufacturing technologies. Most, if not all, studies have used 3D printing for the mold making or duplication in the field of maxillofacial prosthodontics (Karayazgan-Saracoglu et  al. 2009; Watson and Hatamleh 2014). Few studies have demonstrated the direct silicone printing, since the silicone printing technology must be developed due to complex polymerization process (Jindal et al. 2016, 2018). Two types of silicone elastomers have been used in maxillofacial prosthodontics with respect to the type of polymerization: (1) the room temperature vulcanizing (RTV) silicones, which are the most common due to their easy to use in the laboratory applications, and (2) high temperature vulcanizing (HTV) silicones, which have excellent mechanical and physical properties compared to RTV silicones, in addition to have high translucency (Polyzois et al. 1994). However, the main disadvantage of HTV silicones is their complex coloring process. Nonetheless, they have the advantage of a working time of more than 30 min (Jindal et al. 2016). The commercial RTV silicones such as one- or two-part silicones, also known as one- or two-component silicones, which are commercially available (Lontz 1990; Huber and Studer 2002). Recently, platinum catalyzed silicone elastomers were introduced and platinum based silicone elastomers are rapidly polymerized without a need of heat, besides platinum catalyst could be react with other additives in the silicones which might slow down the polymerization process (Lai and Hodges 1999; Jindal et  al. 2016). While 3D printable silicones are one-component RTV types, two-­ component RTV silicones are more common in use due to their biocompatibility (Symes et al. 2012). Jindal et al. (2016) reported that early production of a biocompatible maxillofacial silicones by 3D printing process. Although HTV silicones are not suitable for 3D printing, an experimental HTV silicone was used by Jindal et al. (2016) in the presence of a heat catalyst (platinum carbonyl cyclovinylmethylsiloxane complex), which was then replaced by a platinum catalyst to develop an ideal maxillofacial silicone. They found that the mechanical properties varied with the different concentrations in the crosslinkers (catalysts) and the different length of the PDMS chains (Jindal et al. 2016), as well as they reported that a concentration of the crosslinking agent (catalyst) at 5% was optimal for production of two-component printable maxillofacial RTV (room temperature vulcanizing) silicone elastomers. Keeping this in view, Jindal et al. (2018) reported the properties of a novel two-­ component 3D-printable maxillofacial RTV-silicone elastomer containing a thixotropic agent. However, the addition of the thixotropic agent decreased the mechanical strength of the 3D printable silicone compared to the commercial maxillofacial silicone elastomer (Jindal et al. 2018). The most notable results from Jindal et al. (2016, 2018) were: (1) the addition of a hear catalyst for the manufacture of a 3D printable silicone must be replaced with

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the RTV catalyst, and (2) the thixotropic agent can provide control of the viscosity of the silicone. Recently, Roh et al. (2017) reported a simple technique to manufacture an extrudable silicone material that could be placed directly in a 3D printer. The authors reported that the silicone rubber could be used in the biomedical applications since it is biocompatible. In addition, they reported that the printable ink consisting of water and porous biocompatible silicone components could easily be applied to direct printing of bioscaffolds on living tissues (Roh et al. 2017).

12.2  Conclusions Maxillofacial silicones are the most used materials in the field of dentistry. The latest developments include the feasibility of 3D printing technologies for direct printing of maxillofacial silicone prostheses. However, there is limited information on the direct silicone printing and its viability for maxillofacial prosthodontics. The removal of the conventional laboratory processes by using 3D printing technology is a challenging factor for doctors. Although promising results have been obtained in the few studies, future research may open new opportunities in direct silicone printing. Since time and the cost are the most important factors for both doctors and patients in the medical field, manufacturing silicone prostheses through 3D printers could reduce the costs of medical care and working time in manufacturing processes. Acknowledgments  The author acknowledges help provided by Dr. Tomy J. Gutiérrez during the editorial process. The author also acknowledges Gazi University Faculty of Dentistry Department of Prosthodontics. Conflicts of Interest  The author declares no conflict of interest.

References AAMP. (2019). American Academy of Maxillofacial Prosthetics. What is a Maxillofacial Prosthodontist? https://www.maxillofacialprosthetics.org/referringphysicians/whatismp.html. Accessed 4 May 2019. Akay, C., Cevik, P., Karakis, D., & Sevim, H. (2016). In vitro cytotoxicity of maxillofacial silicone elastomers: Effect of nano-particles. Journal of Prosthodontics, 27(6), 584–587. https://doi. org/10.1111/jopr.12533. Bellamy, K., Limbert, G., Waters, M.  G., & Middleton, J. (2003). An elastomeric material for facial prostheses: Synthesis, experimental and numerical testing aspects. Biomaterials, 24(27), 5061–5066. https://doi.org/10.1016/s0142-9612(03)00412-5. Berman, B. (2012). 3-D printing: The new industrial revolution. Business Horizons, 55(2), 155–162. https://doi.org/10.1016/j.bushor.2011.11.003. Cevik, P., & Eraslan, O. (2017). Effects of the addition of titanium dioxide and silaned silica nanoparticles on the mechanical properties of maxillofacial silicones. Journal of Prosthodontics, 26(7), 611–615. https://doi.org/10.1111/jopr.12438.

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Cevik, P., & Yildirim-Bicer, A. Z. (2017). Effect of different types of disinfection solution and aging on the hardness and colour stability of maxillofacial silicone elastomers. The International Journal of Artificial Organs, 41(2), 108–114. https://doi.org/10.5301/ijao.5000659. Cevik, P., & Kocacikli, M. (2019) Three-dimensional printing technologies in the fabrication of maxillofacial prosthesis: A case report. The International Journal of Artificial Organs. https:// doi.org/10.1177/0391398819887401 D’Souza, D. (2015). Role of implants in maxillofacial prosthodontic rehabilitation. In T.  Ilser (Ed.), Current concepts in dental implantology (pp.  179–209). Intech open. https://doi. org/10.5772/59578. Eleni, P. N., Katsavou, I., Krokida, M. K., Polyzois, G. L., & Gettleman, L. (2009). Mechanical behavior of facial prosthetic elastomers after outdoor weathering. Dental Materials, 25(12), 1493–1502. https://doi.org/10.1016/j.dental.2009.06.018. Federspil, P.  A. (2018). Auricular prostheses in microtia. Facial Plastic Surgery Clinics, 26(1), 97–104. https://doi.org/10.1016/j.fsc.2017.09.007. Ferreira, R., & Vives, P. (2019). Two auricular epithesis surgical cases retained by a custom titanium implant: Result at four years. Journal of Stomatology, Oral and Maxillofacial Surgery, 120(2), 147–151. https://doi.org/10.1016/j.jormas.2018.10.014. Goiato, M.  C., Haddad, M.  F., Pesqueira, A.  A., Moreno, A., dos Santos, D.  M., & Bannwart, L. C. (2011). Effect of chemical disinfection and accelerated aging on color stability of maxillofacial silicone with opacifiers. Journal of Prosthodontics: Implant, Esthetic and Reconstructive Dentistry, 20(7), 566–569. https://doi.org/10.1111/j.1532-849x.2011.00755.x. Gutiérrez, T.  J. (2019). Chapter 3. 3D printing of biopolymers: Trends and opportunities for medical applications. In S.  Ahmed, S.  Kanchi, & G.  Kumar (Eds.), Handbook of biopolymers: Advances and multifaceted applications (pp. 45–73). Singapore: Editorial Pan Stanford Publishing. ISBN: 978-981-4800-17-4. eISBN: 978-0-429-02475-7. Hu, X., Pan, X., & Johnston, W. M. (2014). Effects of pigments on dynamic mechanical properties of a maxillofacial prosthetic elastomer. The Journal of Prosthetic Dentistry, 112(5), 1298–1303. https://doi.org/10.1016/j.prosdent.2014.04.004. Huber, H., & Studer, S. P. (2002). Materials and techniques in maxillofacial prosthodontic rehabilitation. Oral and Maxillofacial Surgery Clinics, 14(1), 73–93. https://doi.org/10.1016/ S1042-3699(02)00018-3. Jindal, S.  K., Sherriff, M., Waters, M.  G., Smay, J.  E., & Coward, T.  J. (2018). Development of a 3D printable maxillofacial silicone: Part II.  Optimization of moderator and thixotropic agent. The Journal of Prosthetic Dentistry, 119(2), 299–304. https://doi.org/10.1016/j. prosdent.2017.04.028. Jindal, S. K., Sherriff, M., Waters, M. G., & Coward, T. J. (2016). Development of a 3D printable maxillofacial silicone: Part I.  Optimization of polydimethylsiloxane chains and cross-linker concentration. The Journal of Prosthetic Dentistry, 116(4), 617–622. https://doi.org/10.1016/j. prosdent.2016.02.020. Karayazgan-Saracoglu, B., Gunay, Y., & Atay, A. (2009). Fabrication of an auricular prosthesis using computed tomography and rapid prototyping technique. Journal of Craniofacial Surgery, 20(4), 1169–1172. https://doi.org/10.1097/scs.0b013e3181acdb95. Kiat-amnuay, S., Beerbower, M., Powers, J. M., & Paravina, R. D. (2009). Influence of pigments and opacifiers on color stability of silicone maxillofacial elastomer. Journal of Dentistry, 37, e45-e50. https://doi.org/10.1016/j.jdent.2009.05.004. Kiat-amnuay, S., Mekayarajjananonth, T., Powers, J. M., Chambers, M. S., & Lemon, J. C. (2006). Interactions of pigments and opacifiers on color stability of MDX4-4210/type A maxillofacial elastomers subjected to artificial aging. The Journal of Prosthetic Dentistry, 95(3), 249–257. https://doi.org/10.1016/j.prosdent.2005.12.006. Lai, J.  H., & Hodges, J.  S. (1999). Effects of processing parameters on physical properties of the silicone maxillofacial prosthetic materials. Dental Materials, 15(6), 450–455. https://doi. org/10.1016/s0109-5641(99)00074-3. Lontz, J.  F. (1990). State-of-the-art materials used for maxillofacial prosthetic reconstruction. Dental Clinics of North America, 34(2), 307–325.

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Mine, Y., Suzuki, S., Eguchi, T., & Murayama, T. (2019). Applying deep artificial neural network approach to maxillofacial prostheses coloration. Journal of Prosthodontic Research. In press. https://doi.org/10.1016/j.jpor.2019.08.006. Mitsouras, D., Liacouras, P., Imanzadeh, A., Giannopoulos, A.  A., Cai, T., Kumamaru, K.  K., George, E., Wake, N., Caterson, E.  J., Pomahac, B., Ho, V.  B., Grant, G.  T., & Rybicki, F. J. (2015). Medical 3D printing for the radiologist. Radiographics, 35(7), 1965–1988. https:// doi.org/10.1148/rg.2015140320. Pesqueira, A. A., Goiato, M. C., Dos Santos, D. M., Haddad, M. F., & Moreno, A. (2012). Effect of disinfection and accelerated ageing on dimensional stability and detail reproduction of a facial silicone with nanoparticles. Journal of Medical Engineering & Technology, 36(4), 217–221. https://doi.org/10.3109/03091902.2012.666321. Petrovic, V., Vicente Haro Gonzalez, J., Jordá Ferrando, O., Delgado Gordillo, J., Ramón Blasco Puchades, J., & Portolés Griñan, L. (2011). Additive layered manufacturing: Sectors of industrial application shown through case studies. International Journal of Production Research, 49(4), 1061–1079. https://doi.org/10.1080/00207540903479786. Polyzois, G.  L., Hensten-Pettersen, A., & Kullmann, A. (1994). An assessment of the physical properties and biocompatibility of three silicone elastomers. Journal of Prosthetic Dentistry, 71(5), 500–504. https://doi.org/10.1016/0022-3913(94)90190-2. Ratner, B. D., Hoffman, A. S., Schoen, F. J., & Lemons, J. E. (2006). Biomaterials science: An introduction to materials in medicine. MRS Bulletin, 31(1), 58–60. https://doi.org/10.1557/ mrs2006.17. Roh, S., Parekh, D. P., Bharti, B., Stoyanov, S. D., & Velev, O. D. (2017). 3D printing by multiphase silicone/water capillary inks. Advanced Materials, 29(30), 1701554. https://doi.org/10.1002/ adma.201701554. Shankaran, G., Deogade, S. C., & Dhirawani, R. (2016). Fabrication of a cranial prosthesis combined with an ocular prosthesis using rapid prototyping: A case report. Journal of Dentistry (Tehran, Iran), 13(1), 68–72. Sherman, L.  M. (2009). Additive manufacturing-new capabilities in materials and equipment advance rapid prototyping and production of short-run parts by "direct digital manufacturing". Plastics Technology, 55(3), 35. Stansbury, J.  W., & Idacavage, M.  J. (2016). 3D printing with polymers: Challenges among expanding options and opportunities. Dental Materials, 32(1), 54–64. https://doi.org/10.1016/j. dental.2015.09.018. Sykes, L. M., Parrott, A. M., Owen, C. P., & Snaddon, D. R. (2004). Applications of rapid prototyping technology in maxillofacial prosthetics. International Journal of Prosthodontics, 17(4), 456–459. Symes, M. D., Kitson, P. J., Yan, J., Richmond, C. J., Cooper, G. J., Bowman, R. W., Vilbrandt, T., & Cronin, L. (2012). Integrated 3D-printed reactionware for chemical synthesis and analysis. Nature Chemistry, 4(5), 349–354. https://doi.org/10.1038/nchem.1313. Unkovskiy, A., Spintzyk, S., Brom, J., Huettig, F., & Keutel, C. (2018). Direct 3D printing of silicone facial prostheses: A preliminary experience in digital workflow. The Journal of Prosthetic Dentistry, 120(2), 303–308. https://doi.org/10.1016/j.prosdent.2017.11.007. Veres, E. M., Wolfaardt, J. F., & Becker, P. J. (1990). An evaluation of the surface characteristics of a facial prosthetic elastomer. Part I: Review of the literature on the surface characteristics of dental materials with maxillofacial prosthetic application. The Journal of Prosthetic Dentistry, 63(2), 193–197. https://doi.org/10.1016/0022-3913(90)90105-l. Watson, J., & Hatamleh, M. M. (2014). Complete integration of technology for improved reproduction of auricular prostheses. The Journal of Prosthetic Dentistry, 111(5), 430–436. https:// doi.org/10.1016/j.prosdent.2013.07.018. Zayed, S. M., Alshimy, A. M., & Fahmy, A. E. (2014). Effect of surface treated silicon dioxide nanoparticles on some mechanical properties of maxillofacial silicone elastomer. International Journal of Biomaterials, 2014, 750398. https://doi.org/10.1155/2014/750398.

Chapter 13

Synthetic Methods and Applications of Functional and Reactive Silicone Polymers Kaleigh M. Ryan, Adam D. Drumm, Claire E. Martin, Anna-­Katharina Krumpfer, and Joseph W. Krumpfer

Abstract  Siloxane polymers (widely known as silicones) are ubiquitous materials with a wide range of applications, from pharmaceuticals and medical devices to nautical sealants and high temperature lubricants. This is due to its robust and advantageous properties as an inorganic-organic polymer, which differ widely from traditional polyolefin materials. In this chapter, the unique and remarkable properties of siloxane polymers will be analyzed, as well as the synthetic strategies for the preparation of traditional and functional silicones. An examination of the functions of siloxane polymers and copolymers in various industries, such as polyurethane foams and fluorosilicone lubricants, is presented. Traditional methods for crosslinking of siloxane polymers and the resulting coatings and bulk materials will be compared with recent advances in silicone coupling reactions, such as the Piers-Rubinsztajn and ‘click’ reactions. Finally, we examine the new emerging approach on the siloxane bond as a reactive functional group in its own right for the preparation of advanced non-stick resins and surfaces. Keywords  Crosslinking · Hydrophobic · Silane · Siloxane · Surfactant

13.1  Introduction Siloxane polymers, colloquially called ‘silicones’, are among the most commonplace and ubiquitous polymers in commercial and daily use (Rochow 1946; Post 1949; Brook 2000; Clarson and Semlyen 1993). Their diverse properties make them excellent candidates for a large number of applications, including astronaut boots,

K. M. Ryan · A. D. Drumm · C. E. Martin · J. W. Krumpfer (*) Department of Chemistry and Physical Sciences, Pace University, Pleasantville, NY, USA A.-K. Krumpfer IBM Corporation, Yorktown Heights, NY, USA © Springer Nature Switzerland AG 2020 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_13

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coatings (Zhang and Seeger 2011), cosmetics (Horii and Kannan 2008), foaming and anti-foaming agents (Baferani et  al. 2018), medical implants (Brook 2012), pharmaceuticals (Rodriguez-Lopez et al. 2019) and sealants. While siloxane polymers have been initially prepared since the second half of the nineteenth century (1800s), it was not until the mid-1940s that they became industrially and commercially relevant materials. Since then, they have become an important part of our daily routines in some way or another. Despite this, they are often overlooked as compared to their carbon-based counterparts. This chapter highlights many of the fundamental aspects of siloxane polymers and what gives them such unique functionality. A brief introduction on the silicon nomenclature is presented to dispel the confusion about the terminology of siloxane polymers present throughout the chapter. The basic properties and molecular characteristics which differentiate this class of polymer from traditional carbon-based polymers are also highlighted. A fundamental understanding of these properties better explains the behavior of advanced silicone materials. In addition, we will give an overview of the main methods in which monomers can be prepared with desired functional groups and the subsequent polymerization techniques for preparing linear silicone oils. The reactions of functional siloxanes for the preparation of crosslinked materials and elastomers will also be explored. In the second half of the chapter, recent advances in various applications of industrial importance will be covered. First, new emerging synthetic techniques for preparing and reacting siloxanes will be explained. Second, the importance of silicone-­based surfactants in a number of applications will be discussed. Finally, reactions of siloxanes with inorganic oxide surfaces and other substrates highlighting the siloxane bond as a functional group in its own right will be detailed.

13.2  Silicon Nomenclature One aspect of siloxane polymers which typically gives people a certain degree of problems is simply the terminology of the field. With many of the terms appearing and sounding similar, it is not entirely surprising that many people struggle at the beginning to find the right use. Figure  13.1A highlights many common silicon-­ containing compounds. Of these compounds, silanes are a typical monomer for siloxane polymers and will be discussed throughout the text. In addition, the terms siloxanes and silicones are commonly used interchangeably. However, ‘silicone’ is a more general term for siloxane-containing polymers or siloxane materials, while siloxane can refer to small molecules, side groups or polymers. An interesting element of the siloxane nomenclature is found in Fig.  13.1B, which does not often appear outside specific silicone texts. These terms, developed by industrial silicone chemists at General Electric (Hurd 1946), describe the amount of oxygens in a silicon atom in a material. In this case, a silicon atom with one, two, three or four oxygen atoms would be referred as a M, D, T or Q unit, respectively. These terms are useful for preparing the abbreviated nomenclature of compounds and resins. For

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A. Common silicon-containing compounds 14

Si

SiO2

SiO42

silica (mineral)

silicate (polyatomic anion)

28.0855

silicon (element)

Si

Si O Si

N Si

Cl

Cl

silane (compound)

(O

H

Si

Si ( O

siloxane silazane silicone (Si-O-Si containing (Si-N-Si containing (siloxane polymer or compound) compound) material) B. Silicone Chemistry Nomenclature CH3

H3C Si O CH3

M

CH3

O Si O CH3 D

CH3

O

O Si O

O Si O

O

O

T

Q

Fig. 13.1  Nomenclature for (A) common silicon-­containing compounds and (B) silicone polymers and materials. Reprinted with permission from Longenberger et al. (2017)

example, M2 would refer to hexamethyldisiloxane (shown as the example of ‘siloxane’ in Fig. 13.1A) and D3 represents the cyclic monomer, hexamethylcyclotrisiloxane. Resins comprised of these units are typically referred to using this nomenclature, e.g. MQ-resins. These materials are discussed later in this chapter.

13.3  Properties of Siloxane Polymers To better understand the behavior of siloxane polymers, a brief introduction to their unique properties and a comparison with more traditional carbon-based polymers are needed. Figure  13.2 shows the most common siloxane polymer, poly(dimethylsiloxane) (PDMS) and its closest carbon-analogue, poly(isobutylene) (PIB). The two structures are drawn to scale with appropriate relative bond lengths and angles to facilitate comparison, and a cursory examination of these structures will denote marked differences. First, the bond angles and bond lengths of siloxane polymers are typically much larger than those of carbon-based polymers. In fact, although the Si-O-Si bond is traditionally reported to be 143°, it is well known that these polymers can oscillate through 180°, much like a ball-and-socket joint. This is due to the high electronegativity difference between silicon (1.9) and oxygen (3.5), which gives the bond a highly ionic characteristic, and therefore, decreases directionality. In this sense, all bonds in PDMS are ionic to some degree, while PIB has bonds with a highly covalent nature.

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Fig. 13.2  Structural comparison of poly(dimethylsiloxane) (PDMS) and poly(isobutylene) (PIB). Reprinted with permission from Longenberger et al. (2017)

These differences in chemical architecture give rise to many of the unique properties that siloxane polymers possess, particularly with respect to thermal behavior. While PIB is well known to have an extremely low glass transition temperature (Tg, −71  °C), the greater flexibility of siloxane bonds produces an even lower Tg (−120 °C) in PDMS and an increase in its free volume. Another interesting behavior of many silicones is the lack of melting or crystallization when cooled. Thereby, many silicones remain liquid until they cool to their Tg. However, when heating from the glass transition, PDMS undergoes cold crystallization (Tc∗) and melting transitions (Tm) at - 80 and - 40 °C, respectively. The thermal stability of siloxane polymers is also affected by the ionic nature of the polymer skeleton, with degradation temperatures (Td) of 250  °C and 350  °C in air and nitrogen atmospheres, respectively. Viewing these thermal properties, silicones have a wide operating temperature range (~ −100 to 300 °C), which indicates that many linear siloxanes are liquid through these temperatures if crosslinking is not performed. Its thermal stability is mainly limited by a backbiting degradation mechanism which results in the reformation of the cyclic monomers, mainly the six-membered (D3) and eight-­ membered (D4) structures (Camino et al. 2002). It is also known that traditional siloxanes, such as PDMS, have remarkably low surface tensions (γLV ~ 20 dyn/cm) due to weak and even slightly repulsive intermolecular forces, so the ionic nature of the Si-C bonds provides repulsive forces between the chains. This makes siloxane polymers ideal materials for sealants, surfactants and water repellent coatings. Although silicones are widely used as hydrophobic materials, it should also be noted that materials made from these polymers often have high gas permeability, particularly for water, making them ‘pseudo-­ breathable’ (Mark 1999). Interestingly, siloxane polymers will form monomeric structures on water (Bernardini et  al. 2011), and they can have intrinsically

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Table 13.1  Other physical properties of poly(dimethylsiloxane) (PDMS) k (kW/m·K) 0.15

Cp (kJ/kg·°C) 1.5

α (ppm/°C) 30–300

η (cSt) 3–2.5×107

‘surfactant-­like’ properties where the siloxane skeleton is arranged on polar surfaces, although they are not particularly effective surfactants unless they are present as copolymers. The viscosities of the siloxane polymers depend significantly on the molecular weight (Mw) of the polymer and can range from easily pourable liquids to more viscous than honey, while the densities typically range between 0.90–0.98 g/ mL.  Bulky substituents on the polymer chain, such as fluoroalkyl groups, can increase density up to 1.30 g/mL. Other important physical properties of PDMS are given in Table 13.1, including thermal conductivity (k), specific heat (Cp), thermal expansion coefficient (α) and viscosity (η). Finally, siloxane polymers are surprisingly biocompatible and non-toxic materials both in vitro and within the environment. This quality has only increased the use of silicone materials in medical devices, such as catheters and stents, as well as the consumption of them in biomaterials, food products and pharmaceuticals (Zelisko, 2014). For example, silicones are widely used as ‘zero-calorie oil substitutes’ in the food industry. While many reasons for the biocompatibility of silicones within the body can be proposed, an evolutionary explanation seems easily satisfactory. Human life evolved on a planet surface which consists of 50% oxygen and 25% silicon (the earth’s crust). With this mind, it is likely that our bodies see these polymers simply as harmless and environmental debris, such as sand, for which our body does not readily break down and simply allows to pass through its system. In the environment, silicones are equally benign, where they eventually decompose directly into oxidation products: carbon dioxide, silica and water, or via hydrolysis in the soil to organosilanediols, which eventually oxidize to the aforementioned products (Graiver et al. 2003; Lin et al. 2013).

13.4  Traditional Preparations of Siloxane Polymers The first synthesis of siloxanes was attributed to Ladenburg (1872) through the polycondensation of multifunctional chlorosilanes. Ladenburg continued work on siloxanes and produced the first linear siloxane polymer via the condensation of diethyldichlorosilane to form a ‘syrupy and odorless liquid’. However, Ladenburg’s work was limited by the ability to synthesize functional monomers, and it was not until several decades after that Frederick Kipping (Thomas 2010) presented what is considered the foundational work on linear siloxane polymers. On the other hand, Kipping is responsible for the misnomer ‘silicone’, as his original synthetic goal was to prepare the silicon analogue of a ketone (Kipping and Lloyd 1901; Kipping 1937). However, silicon does not form stable double bonds with oxygen, but rather polymerizes in linear structures. Despite this fact, the term ‘silicone’ remains in the common vernacular and is widely known by the public.

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The success of Kipping was the ability to prepare functional monomers via traditional Grignard reagents (Scheme 13.1), which could then undergo polycondensation reactions. Although this approach led to the first systematic study of siloxane polymers, the use of Grignard reagents to produce functional silanes was not commercially viable, since the materials are expensive, and the process takes a long time. It should also be noted that Scheme 13.1 is greatly simplified, and in fact, a mixture of products is typically achieved in this method which must then be separated by distillation. However, while this approach may not have found wide-range commercial use, it is still used to make a series of functional monomers, which cannot be prepared through more conventional methods. For example, Missaghi et al. (2008) used Grignard reactions to produce functional hydridomethylsilane monomers, because silicon hydrides are not good leaving groups for Grignard reactions. This class of monomers cannot be prepared through the Direct Process (Fig. 13.3), which is discussed below. Otomo et al. (2005) also used this approach to prepare bis(dimethylsilyl)naphthalenes, which cannot be easily purified via distillation. While the Grignard reaction is still used to make many specialty monomers. In this context, the copper-catalyzed Direct Process is used as an industrially relevant technique to produce silane monomers. Although a ‘direct process’ was known since 1857 (Buff and Wöhler, 1857), the method developed in 1940 by Eugene Rochow at General Electric revolutionized the silicone industry and is used almost unchanged to this day (Rochow 1945). It should also be noted that this process was developed independently only a few weeks after Rochow by Richard Müller (1942), and therefore, it is also called the Rochow-Müller process. The most commonly produced silane monomer is dimethyldichlorosilane, which is used for the preparation of PDMS, in the presence of methyl chloride. However, by adding hydrogen or hydrochloric acid gas to this reaction, the precursors to most functional siloxanes can be prepared. The resulting hydridosilanes can then be subjected to hydrosilylation with a terminal alkene using a platinum catalyst, such as Karstedt’s catalyst (divinylhexamethyldisiloxane platinum catalyst) or Speier’s catalyst (H2PtCl6), to add a series of reactive functionalities. Some examples of these are shown in Scheme 13.2. These hydridosilanes can also be polymerized to produce poly(hydridomethylsiloxane)s, which are important in crosslinking materials, which will be discussed later in the chapter. Other reactive monomers can be prepared by adding other organic chlorides, such as phenyl chloride to produce phenyl-containing monomers. While vinylsilanes can be manufactured using vinyl chloride in a similar fashion, it is often more efficient to produce vinylsilanes through hydrosilylation with acetylene. Similarly, the use of alcohols in this process leads to another class of reactive silane, the alkyoxysilanes, which show a slower reactivity than chlorosilanes but are also beneficial as protecting groups. It is worth mentioning that there are other techniques that can be used to make functional silane monomers, such as electrochemical reduction (Picard et al. 1993), equilibration of hydridosilanes (Lewis 1990) or Wurtz-Fitting 2 CH3MgBr + SiCl4

(CH3)2SiCl2 + 2 MgBrCl

Scheme 13.1  Preparation of functional silanes via Grignard reagents

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Fig. 13.3  The direct method for the preparation of chlorosilane (top left), hydridochlorosilane (bottom left) and alkoxysilane monomers (right)

Scheme 13.2 Hydosilylation of hydridosilanes with a terminal alkene to incorporate functional moieties into monomers

(H3CO)3Si

H +

R

R=

Pt* (cat)

O

NH2

(

OCH2CH2

(H3CO)3Si

)n

R

C16H33

coupling (Calas and Dunoguès 1976), but the two methodologies presented here are the most significant means in use today. Commercially available siloxane polymers became a viable option through the step-growth polycondensation of reactive dichlorodialkylsilanes (Fig. 13.4) (Odian 2004). This reaction easily occurs in the presence of water which forms the silanediol intermediate via hydrolysis. It should be noted that the byproduct of this first step is hydrochloric acid, a corrosive material, but it can be easily distilled off from reaction . Other reactive silanes, such as diethoxysilanes, dimethylaminosilanes and methoxysilanes, undergo a similar hydrolysis in the first step, but different byproducts are produced (Plueddemann 1991). In this way, careful control of the byproduct can be achieved. The second step follows a polycondensation of silanediol to produce a linear polymer. While this process occurs quite easily, it is limited in the achievable Mws, which can be produced, and therefore, is only practical for producing lower Mw polymers. The limitations in Mw arise from the formation of an equilibrium with cyclic monomer species, mainly the six- (D3) and eight-membered (D4) rings (Fig. 13.5). This equilibrium is caused by a backbiting mechanism from the silanol chain ends, which begin after several chlorosilane monomers have been reacted (Camino et  al. 2002). A common method to prevent this backbiting mechanism is by adding monofunctional chain-stoppers, such as trimethylchlorosilane, or by

308 Fig. 13.4 Polycondensation of dimethyldichlorosilane to produce PDMS: (A) formation of silanediols, (B) condensation reactions and (C) equilibration with disiloxane

K. M. Ryan et al. A. Formation of Silanediol Cl

Si

Cl

+ 2 H 2O

HO

Si

OH

B. Polycondensation n

HO

Si

OH

HO

( O Si) OH

Si

H 3C

Si

O

Si

(

)

+ Si Si CH3 HO O OH

HO Si O Si O Si O Si O Si

+ (n-1) H2O

n-1

C. Equilibration with end-capper

Fig. 13.5 Backbiting degradation of siloxane polymers and formation of cyclic monomers

+ 2 HCl

n-1

H 3C

+ Si O Si OH

( O Si) CH

Si

m

3

Si O Si O O Si

equilibrating the polymerization with a disiloxane (Jones et al. 2000). In the step-­ growth polymerizations, Mw can be thermodynamically controlled in this manner, but the final Mw reached are still limited due to this equilibrium. Incidentally, the ability to produce high Mw siloxanes comes from the aforementioned cyclic siloxanes which are formed from the thermal degradation of linear step-growth polymers. By performing a chain-growth and ring-opening polymerization (ROP) of cyclic monomers, a high Mw can be quickly obtained, as shown in Fig. 13.6 (Hammouch et al. 1995). Typically, the six-­membered D3 is commonly used, since it has a significant ring-strain to drive the polymerization. Interestingly, D4 exhibits almost no ring-strain and is a rare example of an  entropically driven polymerization, due to the higher degree of freedom associated with the linear structure of the polymer. This polymerization can be performed cationicly using a strong acid or anionically using a strong base (Clarson and Semlyen 1993). Baseinitiated ROP typically produces higher Mw polymers with narrower polydispersity, and therefore, is the preferred method par excellence (Hölle and Lehnen 1975). The use of a monofunctional silanolate initiator allows control of the chain-end group. In order to prevent the backbiting degradation from occurring, monofunctional silane is added to the reaction mixture, effectively terminating the polymerization (Piccoli et al. 1960). This allows direct kinetic control of molecular weight, which leads to higher Mw polymers. While previous discussions on the siloxane polymerization techniques have largely focused on the commercially relevant PDMS, other functional siloxanes can be prepared in a similar manner. For example, diphenyldichlorosilane or phenylmethyldichlorosilane can be used to prepare poly(diphenylsiloxane) (PDPS) and poly(phenylmethylsiloxane) (PPMS), respectively (Zlatanic et al. 2018). These two polymers are important as oils and lubricants stable at high temperatures, due to the steric hindering of the backbiting mechanism (Gädda and Weber 2006). Another

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A. Initiation (H3C)3Si

−

.. O

Si

+

K

+

O

O Si

Si O

(H3C)3Si

O

( Si O ) Si

O

2

−

K

+

B. Propogation (H3C)3Si

O

(

) ..

Si O Si O −

K

Si

+

+n

O

2

O Si

Si O

(H3C)3Si

O

( Si O ) Si

3n+2

O

−

K

+

C. Termination (H3C)3Si

O

( Si O ) Si

3n+2

O

−

K

+

+

Cl

Si

(H3C)3Si

O

( Si O ) Si

3n+2

O

Si

Fig. 13.6  Anionic ring-opening polymerization of cyclic siloxanes. (A) initiation via potassium trimethylsilanolate, (B) chain propagation, (C) termination via addition of the monofunctional trimethylchlorosilane

important functional polymer is poly(hydridomethylsiloxane) (PHMS), which is widely used in crosslinking reactions. This polymer can be prepared by polycondensation of dichloromethylsilane or by the ROP of 1,3,5,7-­tetramethylcyclotetrasi loxane (D4H) to produce different Mw of the same polymer. Other functional siloxanes can be imagined by polymerization of difunctional monomers containing reactive handles, such as (ethylene oxide)methylsilanes, 3-aminopropyl­ ­ methyl(diethoxy)silane or 3-propylthiolmethyl(diethoxy)silane, which impart groups capable of ­further reaction, as well as the manufacture of water-soluble siloxanes (Dewasthale et al. 2017). End-group functionalized PDMS’s are also an important and widely used class of reactive siloxane polymers. These are prepared by including monofunctional silanes into the polymerization or being equilibrated with a functionalized disiloxane (Jones et al. 2000). These act as end-cappers to the polymer chains in the same manner as described above. The ROP of cyclic monomers provides a greater degree of tailorability with respect to the nature of the end-groups. The choice of initiator can lead to a polymer with a single functional end-group or one that has two different functionalities on each end (Peters et al. 1995). This is particularly important for the preparation of PDMS-co-poly(ethylene glycol) (PDMS-co-PEG) surfactants (Fig.  13.7), which have a wide variety of applications, including cosmetics and polyurethane (PU) foaming agents (Zhang et al. 1999).

13.5  Crosslinking of Siloxane Polymers Several reactive and functional siloxanes can be prepared by means of different well-known polymerization techniques. However, linear siloxanes are oils in a wide spectrum of temperatures. For this reason, siloxane polymers often require

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A. Formation of hydrido-termination PDMS (H3C)3Si

O

( Si O ) Si n

.. − O

+

Cl

Si

(H3C)3Si

H

O

( Si O ) Si n

O Si

H

B. Hydrosilylation with allyl-PEG (H3C)3Si

O

( Si O ) Si n

O Si

Pt* (cat)

H

(O

+

(H3C)3Si

O

CH2CH2

( Si O ) Si n

) OH m

O Si

(

O

CH2CH2

) OH m

Fig. 13.7  Preparation of poly(dimethylsiloxane)-co-poly(ethylene glycol) (PDMS-co-PEG) surfactants: (A) termination of polymerization using chlorodimethylsilane and (B) hydrosilylation with allyl-terminated PEG

Fig. 13.8  Preparation of classic silicone ‘putty’ by adding boric acid to the polymerization of dimethyldichlorosilane 

crosslinking to become significant materials. This can be achieved through several methods. Perhaps the simplest method for preparing crosslinked siloxane materials is by the addition of a tri- or tetra-functional monomer during step-growth polymerization (Fink 2018). The classic silicone ‘putty’ is achieved by the addition of boric acid into the polymerization of dimethyldichlorosilane (Fig. 13.8) whereby the trifunctional boric acid acts as a crosslinking agent (Brook et al. 2010). As expected, the amount of crosslinker added to the reaction affects the resulting mechanical properties and flowability. While many people are familiar with the silicone putty which has been sold for decades at local toy stores, a similar crosslinked siloxanes is less well-known but far more important: DT- and MQ-resins (Sun et al. 2012). These materials are prepared industrially for use in coatings, cosmetics, sunscreens and even as reinforcing agents due to their high ‘silica’ content (Horii and Kannan 2008). In this sense, they ­typically behave similarly to solid silica particles. There is a surprising amount of  methods to prepare such resins, but for simplicity, it can be inferred that

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DT-resins  are derived from difunctional and trifunctional reactive silanes, and MQ-resins are derived from monofunctional and tetrafunctional reactive silanes. These resins are, therefore, theoretically structural isomers of each other. Recently, Flagg and McCarthy (2016) reported a new reproducible method for preparing MQ-resins and observed the interesting properties of these ‘phantom nanoparticles’ made by the addition of monomers containing vinyl or hydride, they were able to prepare resins of this type which can undergo further reaction. These materials showed extremely high thermal stability with only ~20% weight loss up to 900 °C. Another report by Gao and McCarthy (2006a) noted that the reaction of azeotropes of tetrachlorosilane (Q) and trimethylchlorosilane (M) on the surfaces resulted in the formation of superhydrophobic ‘nanofilaments’. These can be viewed as surface-bound MQ structures in their own right. Room-temperature vulcanization (RTV) of functional silicones is perhaps the most widely known of all crosslinking processes for silicones (Yoshimura et  al. 1999; Kumar and Lee 2017), as it is used extensively in bathroom caulking and nautical sealants. However, instead of starting directly from monomers, a difunctional siloxane is often mixed with a multifunctional crosslinking unit after polymerization. This provides more control over the properties of the resulting crosslinked materials and also reduces the amount of byproduct that is produced during curing. As RTV-silicone kits are widely available to the public, the use of chlorosilanes is prohibitive, due to the dangerous byproduct of hydrochloric acid. In contrast, alkyoxysilanes are often used as resulting byproducts, e.g. acetic acid, ethanol or methanol, are largely considered benign. One of the most commonly used RTV silicones is the reaction of silanol-terminated PDMS and methyltriacetoxysilane (MTAS) (Fig.  13.9). MTAS acts as a trifunctional crosslinker which is hydrolyzed in the presence of water to produce acetic acid as a byproduct. For this reason, many silicone kits emit the smell of vinegar during use. Beyond co-condensation of functional silanes, there are other methods for the preparation of silicone materials through carbon-carbon bonds. One of the oldest

)

Si O

O

( Si O ) Si

O Si O

O

OH H2O

+

O

Si O

(

Si

Si O Si O Si O Si Si O O

(

Si O

O

Si O

)

O

) O

(

HO

+ OH

Si

Fig. 13.9  Room-temperature vulcanization of silanol-terminated poly(dimethylsiloxane) (PDMS) and methyltriacetoxysilane (MTAS)

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Fig. 13.10 Self-healing silicones: (a) before cutting, (b) after cutting, (c) after thermal healing and (d) after cracking again. Reprinted with permission from Zheng and McCarthy (2012)

crosslinking methods is high temperature vulcanization (HTV) via radical-initiated crosslinking, typically with the use of peroxides, such as benzoylperoxide (Buining et al. 1997). High temperatures are required for the formation of the radical species, which then, in turn, can extract a hydrogen from a silicone methyl group. With the methyl radical now present, this can react with any adjacent methyl radicals to produce carbon-carbon bonds, thus linking the siloxane chains. The incorporation of vinyl units, which are more reactive to radicals, into the siloxane structure reduces the temperature necessary for these reactions (Dunham et al. 1957). Generally speaking, HTV curing of siloxanes is less common than other methods, but it is still useful in the preparation of silicone elastomers. In addition, the inclusion of radiation- or UV-induced radical initiators can reduce the temperatures, even to room temperature, necessary to produce this crosslinking (Okamoto et al. 1988). Despite this, some interesting materials can be made using radical crosslinking silicones. One of these materials is the self-healing silicone (Zheng and McCarthy 2012). By controlling the amount of radical crosslinks in a silicone containing a difunctional anionic initiator, a silicone material can be achieved, which in turn can be cured again. This is due to the equilibration of siloxane chains from the anionic initiator still present. Unlike other self-healing networks, no additional material is necessary, in addition to silicone, and there are no odorous byproducts (Fig. 13.10). Although radical coupling of siloxane polymers was one of the first methods for creating solid silicone materials (Heiner et al. 2003), cross-coupling via hydrosilylation of vinyl- and hydrido-functionalized siloxane polymers is currently much more common (Marciniec and Gulinski 1993). This method is widely used for the preparation of medical implants and kitchenware. Despite that platinum-based catalysts are typically used, the hydrosilylation reactions are incredibly efficient, requiring very little catalyst, and therefore, cost savings in this method exceed many other considerations. Likewise, in most cases, the effort to try to recover the platinum catalyst is more expensive than simply leaving it present in the material. This has led to an examination of the nature of platinum in silicones, particularly for medical devices, although no substantial risks have been observed (Lambert 2006).

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The preparation of crosslinked silicones via  hydrosilylation is essentially the same as the preparation of functional monomers, although multifunctional ­siloxane polymers are used instead. Figure 13.11 provides a general reaction scheme for the crosslinking of these materials. Typically, vinyl-terminated PDMS and hydridomethylsiloxanes are used, and depending on the ratio, either elastomeric and bouncy materials can be made hard or brittle glasses (Fig. 13.11). This is the same chemistry found in many two-base silicone elastomer kits that are commercially available. The processing of silicone materials has numerous advantages. First, given the low surface tensions associated with silicones, they easily conform to the shape of their molds with high resolution, and can be used in a number of techniques, including capillary force lithography, contact imprinting and injection molding (Eduok et al. 2017). This low surface tension also makes them excellent coating materials (Owen and Dvornic, 2012), as they will readily and easily spread across most substrates. In fact, they can be processed through many different coating techniques, including blade coating, draw-down coating, drop-coating and spin-coating, to produce uniform and smooth films (Zheng and McCarthy 2010). Another benefit of these materials is their high degree of transparency, which can be seen in Fig. 13.11. Finally, as many of these crosslinking reactions require some additional stimulus, such as temperature, they can be thermoset when desired. In recent years, several interesting materials have been produced specifically using hydrosilylation-crosslinking reactions. Zheng and McCarthy (2010) reported a ‘molecularly smooth, low surface energy, unfilled, UV/Vis-transparent, extremely crosslinked, thermally stable, hard, elastic PDMS’ through the crosslinking of 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (D4V) and 1,3,5,7-­tetrameth ylcyclotetrasiloxane (D4H) in a 1:2 molar ratio. This material also proved to have a nanoscale replication capacity due to the low surface tensions of silicone oligomers. Longenberger et al. (2017) used the same chemistry as shown in Fig. 13.11 in a 1:1 molar ratio, but included traditional inorganic pigments to prepare an oil-based paint which could be thermally cured (Fig. 13.12). This silicone paint showed similar properties to traditional oil paints, but could simply be cured in an oven in a matter of hours, instead of having to sit for several days.

Fig. 13.11 (A) Hydrosilylation of vinyl-terminated poly(dimethylsiloxane) (PDMS) and hydridomethylsiloxanes to form crosslinked networks. (B) Image of a virgin silicone crosslinked using the adjacent chemistry under compression. Reprinted with permission from Longenberger et al. (2017)

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Fig. 13.12 (A-C) Preparation of a silicone oil paint and (D) a comparison with an acrylic oil paint of the same composition. Reprinted with permission from Longenberger and Krumpfer (n.d.)

A brief comment on the mechanical properties of silicones should be made before closing this section. As stated earlier, most linear siloxane polymers are oils and are not present as physical solids without additional crosslinking. However, with some exceptions, the mechanical properties of virgin silicones are considered largely poor, with a ‘tofu-like’ quality due to the low cohesive forces of silicone (Song et al. 2017). For this reason, most commercially available silicones are not virgin materials, but typically exist as composites with silica toughening agents (Ansorge et al. 2012). It is the additional silica particles which help impart many of the elastomeric and tensile properties associated with silicone materials.

13.6  Recent Advances in Siloxane Chemistry Although a vast majority of siloxane chemistry still uses the same  reactions that were developed a century ago, in recent years, new siloxane chemistries have emerged along with unique ways to incorporate organic reactions not traditionally associated with this class of polymer. One of the more recent advances in siloxane chemistry that has generated great interest is the Piers-Rubinsztajn reaction (Brook 2018).

13  Synthetic Methods and Applications of Functional and Reactive Silicone Polymers

H

Si

O

Si

H

+ R

315

B(C6F5)3 O

Si

R'

H

Si

O

Si

O

Si

R'

R = -H, -alkyl

Fig. 13.13  The Piers-Rubinsztajn reaction

Initially reported in 1996 by Piers using small molecules (Parks and Piers 1996), it was Rubinsztajn and Cella (2005) who later used this to produce polymeric materials in 2005. The Piers-Rubinsztajn reaction involves the coupling of an alcohol, alkoxysilane, ether or silanol, with a hydridosilane catalyzed by the  Lewis acid, tri(pentafluorophenyl)boron, to produce an alkoxysilane bond or a siloxane bond (Fig.  13.13). The reaction is driven by the production of the thermodynamically strong Si-O bond (bis(N,N´-dimethylaminoethyl)ether (BDE) ~ 443 kJ/mol). Since its introduction, the Piers-Rubinsztajn reaction has resulted in many new materials with specific properties. In addition to simple crosslinked elastomers (Fawcett et  al. 2013), the Piers-­ Rubinsztajn (PR) reaction has made it possible to form well-defined siloxane dendrimers and hyperbranched silicones (Morgan et  al. 2017;  Grande et  al. 2014; Uchida et al. 1990). Previous attempts often produced low yields, poorly defined structures and certainly difficult synthetic processes (Kawahara et al. 2011). With this newly emerged coupling reaction, more well-defined structures can be achieved with limited work-up following the synthesis. The work of Grande et  al. (2014) managed to synthesize uniform monodisperse G3 dendrimers, using alternating hydrosilylation reactions and the PR reaction, with Mw’s of approximately 13,000 g/ mol. However, Grande et al. (2014) recognized that the complexity of the synthesis increases dramatically as the dendrimer Mw increases above 7000 g/mol. Wu et al. (2017) also used the PR reaction to prepare the fundamentally interesting siloxane macrocycles from a highly strained monomer (QD4). More recently, Flagg and McCarthy (2017) used this reaction to directly bind monofunctional siloxanes to silica surfaces. In this way, uniform brush structures could be prepared, as well as a better understanding of the surface chemistry could be determined. These are only a few examples of newly emerging structures which can be prepared with this new technique. Another methodology for making functional siloxane polymers is the incorporation of traditional organic functional groups such as side-chains or end-groups (Ryan et al. 2004). Although this may seem an obvious route to functional siloxanes, this is not as simple as it seems, since siloxanes are generally degraded in acidic or basic media. For this reason, the reaction conditions must be carefully considered. Again, Grande et al. (2014) opened many synthetic pathways that were previously unavailable through an intelligent consideration of reaction conditions and available monomer units. An important synthetic technique introduced to siloxane chemistry in the past decade has been azide-alkyne 1,3-cycloadditions, better known as the ‘click’

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Fig. 13.14 Preparation of 3-azidopropyl moieties on siloxane polymers and subsequent 1,3-­cycloaddition with terminal alkynes (‘click’ reaction)

reaction (Fig. 13.14) (Zarrintaj et al. 2019). This reaction is highly efficient with almost stoichiometric yields, making it an extremely powerful synthetic strategy for the development of new materials. Gonzaga et al. (2009) achieved this by converting 3-chloropropyl side-chains on siloxane polymers into the reactive azide. These azides readily react with terminal alkynes in the presence of Cu (I) catalyst to create a wide range of possible polymer structures or crosslinked networks. This technique was further extended to metal-free ‘click’ reactions for the preparation of amphiphilic structures (Rambarran et al. 2013; 2015). Thiol-ene ‘click’ reaction was also introduced in silicone chemistry in the last several years (Zhang et al. 2013; Liu et al. 2017). This was possible by incorporating thiol functionalities into silicone materials using 3-mercaptopropylsilanes either by copolymerization or by equilibrating an established siloxane material. These thiol groups can react with carbon-carbon double bonds in the presence of a radical initiator. In this way, PEG-modified PDMS can be prepared, as well as maleic anhydride surfaces, which could then be further reacted. In these cases, the typically hydrophobic surfaces of the silicones can be converted to hydrophilic (Liu et al. 2017). The ability to control the wetting properties of silicone materials is one of the driving forces to develop new silicone chemistries. The main desire to advance in this area is the ability to develop silicone materials which are suitable for aqueous conditions, particularly with respect to medical implants. The hydrophobic nature of silicones often leads to non-specific protein absorption (Makamba et al. 2003), which is highly undesirable in vitro and can cause infection. However, silicone hydrophilization is a non-trivial problem, since it is known to undergo a ‘hydrophobic recovery’ whereby the siloxane chains migrate to the interface, due to their lower surface energy (Fritz and Owen 1995). Several attempts to prepare hydrophilic silicones through oxygen plasma treatments have shown some promise (Zhou et  al. 2012; Zhou et  al. 2010), but their long stability is currently not understood. Recently, the Chen’s group and coworkers (Karki et al. 2016; Yan et al. 2018) have investigated poly(vinyl alcohol) (PVOH)-modified PDMS surfaces. Surprisingly, several previous reports (Carneiro et al. 2011; He et al. 2011) had concluded that

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PDMS could not be modified using PVOH, based on their results. However, Yan et al. (2018) realized that the morphology of PVOH on silicone substrates depends largely on the coating conditions and the nature of the silicone used, and also noted that the PDMS layer thickness increases, the resulting PVOH layer becomes unstable and noncontinuous coatings are formed. They were also able to increase the PVOH layer stability by a light oxygen plasma treatment of the silicone before the coating procedures. This is potentially due to the covalent bonding of PVOH with the resulting silanol species present after reacting with oxygen, although this is never explicitly determined. The resulting films did not show the typical ‘hydrophobic recovery’ associated with silicones, and therefore, were effectively hydrophilic. A final approach that has had some promising initial applications is the use of silicone macroinitiators. One of the previously reported attempts to make a siloxane-­ based macroinitiator was carried out by Baysal et  al. (1996) through end-chain functionalization using a functional urethane-based initiating group. This was later expanded  by Taskiran (2006) for the copolymerization of polystyrene particles. Taskiran (2006) found that the PDMS macroinitiator was preferentially directed to the particle interface, providing an in-situ coating method of the particles. More recently, there is some work on the preparation of siloxane-based atom transfer radical polymerization (ATRP) initiators for controlled polymerization techniques. Semsarzadeh and Amiri (2013) reported the development of an ATRP silicone-macroinitiator for the copolymerization of styrene and vinyl acetate to make pentablock copolymers. Although the concept of silicone macroinitiators is promising, more work is still needed in this field.

13.7  Silicone Surfactants One of the most important applications of siloxane polymers is as surfactants (Yilgör and Yilgör 2014). The most common siloxane surfactants are by far PDMS-­ polyether surfactants, such as the PDMS-b-PEG copolymer shown in Fig.  13.7. Given the biocompatibility and non-toxicity of both polymers, it is not surprising that they are used in a wide range of applications. Another extremely important class of surfactants are oligo(ethylene oxide)trisiloxanes (Wagner et  al. 1999a; Wagner et al. 1999b; Kanner et al. 1967) (Fig. 13.15), which are used as ‘superspreaders’ (Sankaran et  al. 2019), and which reduce the surface tension of water to values between 20–25 mN/m (as compared to organic based surfactants, which reduce the surface tension of water to 30–45 mN/m) (Kendrick et al. 1967). These surfactants show extremely fast spreading dynamics across many substrates. For this reason, these surfactants are commonly used in the spraying of pesticides in agriculture, as they disperse pesticides quickly and evenly without damaging the environment. Given the importance of oligosiloxane surfactants, many works have been carried out to control and improve their surface activities. Recently, investigations on gemini surfactants and their performances have been undertaken. Lin et al. (2017)

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Fig. 13.15  Common silicone surfactant structures

described the microwave synthesis of several superspreading gemini surfactants and found that the inclusion of spacer groups, such as alkyl chains, decreases critical aggregation concentrations (CAC), but also decreases the dynamic behavior. On the other hand, increasing the hydrophilic ethylene oxide sections increases the CAC, but also improves the dynamic behavior. Similarly, Chen et al. (2018) developed gemini cationic surfactants, whereby the rigidity of the spacer unit also affected the dynamic performance of the surfactants. Even though oligosiloxane surfactants are important for several applications, one of the drawbacks of these materials is their skin irritation on direct contact. Therefore, their use in cosmetics and pharmaceutical formulations is not viable (O’Lenick Jr. 2000). While an increase in the copolymer Mw slightly decreases the efficiency of the surfactant by decreasing the dynamics of wetting, it also becomes less irritating on contact (Cornwell 2018). For this reason, diblock copolymers of polyethers and siloxanes are more traditionally used in everyday applications. Many of the effects

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of Mw on surfactant have been studied. Gentle and Snow (1995) showed that changes in the length of the polyether block do not have a significant impact on the surfactant performance when ethylene oxide block repeat units were less than sixteen. However, surface adsorption decreased as the length of the ethylene oxide block increased. This is probably due to an increase in water solubility of the surfactants. In fact, water soluble siloxanes are often highly desirable depending on the application. Such diblock surfactants (polyethers and siloxanes) are widely used in cosmetics, such as facial moisturizers, pharmaceuticals and shampoos (Kalasho et  al. 2019; Guidi et al. 2014). In many of these cases, silicones are listed under commercial names, such as dimethicone or methicone. The use of the commercial names is due largely in part to the backlash against silicone materials from the breast augmentation lawsuits of the late 1990s. Interestingly, there is no scientific evidence that silicones were responsible for any of the health problems observed related with them, and eventually the U. S. Institute of Medicine published a report exonerating these materials (Bondurant et al. 1999). Another important use of silicone surfactants is the preparation of PU foams. Without the surfactants, the foams collapse providing an amorphous and generally unusable product (Baferani et  al. 2018; Zhang et  al. 1999;  Hasan et  al. 2016). Typically, instead of using traditional block-copolymers, PDMS with poly(ethylene oxide)-co-poly(propylene oxide) (PEO-co-PPO) grafted chains are used. In the preparation of PU materials, carbon dioxide is generated during polymerization. In foams, the silicone surfactant stabilizes the gaseous bubbles produced within the material and kinetically traps them in the PU. However, the activity of the silicone surfactants plays an important role in the resulting bubble size, with a high silicone content resulting in smaller bubble sizes (Zhang et al. 1999). A new class of silicone surfactants made from silicone-polyol copolymers has attracted the attention of many researchers, due to its interesting surface activities. These copolymers are prepared very similarly to traditional PDMS-co-PEG copolymers, i.e. via hydrosilylation. Wang et  al. (2017) demonstrated that one of these surfactants (PDMS-b-poly(glycerol)) had comparable surface tensions (γLV  ~  21 mN/m) to traditional siloxane surfactants, but with lower CACs. Another advantage of these surfactants is that they can be derived from biorenewable sources, such as polysaccharides or castor oil (Furtwengler and Avérous 2018; Mathew et al. 2018). Hyaluronan-derived silicone surfactants have been studied by Paterson et al. (2014) for their use as wetting agents for contact lens. The palm oil derived surfactants have also been tested for the preparation of PU foams as a potential replacement for traditional PDMS-co-PEG surfactants (Nasir et al. 2016). The list of potential silicone surfactants seems innumerable, and we note that the few examples selected only provide a wide range of surfactants currently in use and development.

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13.8  Inherent Reactivity of the Siloxane Bond Finally, this chapter will be closed with an examination of all siloxane polymers as inherently reactive materials. It had been generally thought for many years that siloxanes are ‘unreactive’ materials. In fact, although this view is slowly fading, there is still a lot of literature that supports this idea, due to the high thermodynamic stability of the siloxane bonds. However, we have previously shown that siloxane bonds are inherently reactive to acids and bases. In retrospect, this should have been an obvious feature of siloxanes, as they are prepared via acid/base-catalyzed ROPs. Despite that many authors consider this reaction as a ‘weak point’ of siloxane polymers, under appropriate circumstances, siloxane bond reactions can be extremely beneficial. One of the first examples of using this reactivity was the chemical modification of inorganic oxide surfaces (Krumpfer and McCarthy 2011). This was achieved by Krumpfer and McCarthy (2010) by a simple procedure: wetting an inorganic oxide surface with silicone oil at 100 °C for 24 hours. Astonishingly, the hydrophobic surfaces that were prepared from this simple procedure exhibited insignificant contact angle hysteresis (θA/θR = 104°/103°), i.e. there was no kinetic barrier to sliding (Gao and McCarthy 2006b). The surprise to this finding was that most surfaces exhibit some contact angle hysteresis, and previous attempts to prepare such surfaces involved complicated procedures and low reproducibility. The lack of a contact angle hysteresis is attributed to a ‘liquid-like’ monolayer which is formed from grafted siloxane chains (Gao and McCarthy 2006b). Despite being covalently attached to the interface, the high degree of flexibility of the siloxane chain creates a dynamic interface which prevents liquids from being fixed to the surface. In fact, since this procedure, other methodologies for preparing similar surfaces have been reported (Hozumi et al. 2011; Hastings et al. 2020). In each case, low hysteresis surfaces are created, which suggests that this is an innate characteristic of siloxanes which had not been exploited so far. The reaction of silicones on inorganic surfaces is attributed to the cleavage of the siloxane bond at the interface (Fig. 13.16) (Krumpfer and McCarthy 2011). This is probably due to hydrolysis via surface-bound water, since inorganic surfaces are typically quite hydrated (they act as desiccants due their high surface energies) or direct silanolysis from acid/base moieties on the inorganic substrate. This ultimately results in the covalent attachment of these siloxane chains to the surface. We found that this reaction is not specific to any one inorganic oxide, but can be applied to any inorganic oxide (the initial study used silica, titania, alumina and nickel oxide to provide a range of acidic-to-basic substrates). However, in each case, the reaction was found to be thermally activated, requiring temperatures above ambient conditions to occur. The hydrophobic coatings were also found to be quite robust, with no change in characteristics, even when immersed in a good solvent. In addition, the use of increasing Mw siloxanes resulted in thicker coatings on the surface. Finally, by using functional siloxanes, such as poly(3-­aminopropylsiloxane) copolymers, reactive groups can be added to any inorganic oxide surface from commercially available polymers.

13  Synthetic Methods and Applications of Functional and Reactive Silicone Polymers Fig. 13.16  Reaction of siloxane polymers on inorganic oxide substrates. Reprinted with permission from Krumpfer and McCarthy (2011)

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13.9  Outlook and Conclusion Siloxanes and silicones are truly unique materials with many advantageous properties. From their wide thermal stability to biocompatibility, siloxanes are already heavily incorporated into a number of industries. In particular, the last two decades have been an exciting time for silicone polymers with new discoveries and rediscoveries that open more and more paths to complex structures and offer a greater degree of control over reactivity and properties of materials. Given the importance of silicones in a number of applications, siloxane polymers will continue to have significant leaps at the scientific and technological level. Many misperceptions of this class of polymers have been dissipated, and the incorporation of new technologies within the easily established techniques is opening previously unthought paths. Its importance as coating materials and biomedical devices will probably increase in the coming years as more precise surface properties can be prepared. Acknowledgments  The authors thank the Pace University Office of the Provost for financial support. Conflicts of interest  The authors declare no conflict of interest.

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Chapter 14

Hydrosilyl-Functional Polysiloxanes: Synthesis, Reactions and Applications Jerzy J. Chruściel

Abstract  A comprehensive review on preparation methods of linear, branched, star and dendritic poly(hydrosiloxane)s (PHS), mainly poly(methylhydrosiloxanes) (PMHS) and Si-H functional silsesquioxanes (spherosilicates) is presented in this chapter. The most important applications of PMHS in technology of silicones, modifications of different polymers and in materials science were reviewed. Keywords  Hybrid silicone materials · Hydrolytic polycondensation · Hydrosilylation · Poly(methylhydrosiloxanes) (PMHS) · Ring-opening polymerization (ROP)

Abbreviations ((HMe2SiO)2SiO)3) Hexakis(dimethylsiloxy)cyclotrisiloxane ((HMe2SiO)2SiO)4 Octakis(dimethylsiloxy)cyclotetrasiloxane ((HMe2SiO)SiO1.5)8) Octakis(dimethylsiloxy)octasilsesquioxane - Q8MH8 ((Me3SiO)2SiO)4) Octakis(trimethylsiloxy)cyclotetrasiloxane (H2SiO)n (n = 4–23) Oligodihydrocyclosiloxanes (HMeSiO)5 1,3,5,7,9-pentamethylcyclopentasiloxane (Me2PhSi)2O 1,1,3,3-tetramethyl-1,3-diphenyldisiloxane (MVi)2SiMeH 1,5-divinyl-1,1,3,5,5-pentamethyltrisiloxane (NH4)2SO4 Ammonium sulphate 3D Three dimensional AB2 Polymerized methylvinylbis(dimethylsiloxy)silane AB3-II Vinyltris(dimethylsiloxysilane) AB3-III Tris(vinyldimethylsiloxysilane) AB4 Methylvinylbis[methylbis(dimethylsiloxy)siloxy]silane AB6 Vinyltris(methylbis(dimethylsiloxy)siloxy)silane APTES (3-aminopropyl)triethoxysilane J. J. Chruściel (*) ŁUKASIEWICZ Research Network, Textile Research Institute, Łódź, Poland e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2020 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_14

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ATRP Atom transfer radical polymerization B(C6F5)3 Tris(pentafluorophenyl)borane B(OSiMe2Cl)3 Tris(chlorodimethylsiloxy)borane br-PMHBS Branched poly(methylhydroborosiloxane) BTC 1,3,5-benzenetricarboxylic acid C5H5N Pyridine CA Cellulose acetate Ca3Si3O9Cl6 Pseudowollastonite c-D2DH Pentamethylcyclotrisiloxane - (Me2SiO)2(MeHSiO) ClSiMe2H Chlorodimethylsilane CNF Carbon nanofiber CPA Hexachloroplatinic acid - H2PtCl6 - Speier’s catalyst D3 Hexamethylcyclotrisiloxane D4 Octamethylcyclotetrasiloxane - (Me2SiO)4 D5 Decamethylcyclopentasiloxane - (Me2SiO)5 DBSA Dodecylbenzenesulphonic acid DBTDL Dibutyltin dilaurate DCP Dicumyl peroxide DDS Dimethyldichlorosilane - Me2SiCl2 DH4 1,3,5,7-tetramethyltetrahydrocyclotetrasiloxane DH5 1,3,5,7,9-pentamethyl-1,3,5,7,9-pentahydrocyclopentasiloxane DHn Methylhydrocyclosiloxanes DMAP 4-(N,N-dimethylamino)pyridine DMCS Chlorodimethylsilane - HMe2SiCl DmDHn Cyclic dimethylsiloxane-co-methylhydrosiloxane DMF N,N-dimethylformamide Dn, n ≥ 3 Dimethylcyclosiloxanes DSC Differential scanning calorimetry DVB 1,4-divinylbenzene DVi4 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcycletetrasiloxane - (MeViSiO)4 Et3B Triethylborane Et3N Triethylamine Et3SiH Triethylsilane FR Flame retardants FRs Fire retardants FTIR Fourier transform infrared H2SiCl2 Dichlorosilane H2SO4 Sulfuric acid HCl Hydrochloric acid HMe2SiC6H4SiMe2H 1,4-bis(dimethylsilyl)benzene HMeSi(O(CH2)9CH=CH2)2 Bis(undecenyloxy)methylsilane H MM Pentamethyldisiloxane - HMe2SiOSiMe3 HNBR Hydrogenated butadiene-styrene rubber

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HO(Me2SiO)mH Oligo(dimethylsiloxane-α,ω-diols) HSi(OMe)3 Trimethoxysilane - HSi(OCH3)3 HSiCl3 Trichlorosilane INEPTRD Insensitive nuclei enhanced by polarisation transfer refocussed and decoupled INVGATE Inverse-gated broadband decoupled technique IPNs Interpenetrating polymer networks KOH Potassium hydroxide LCEs Liquid-crystalline silicone elastomers LCs Liquid crystals LEDs Light-emitting diodes Lithium aluminohydride LiAlH4 LOI Limiting oxygen index LPNC [Cl3P(NPCl2)2PCl3]+PCl6− M2 Hexamethyldisiloxane - Me3SiOSiMe3 MDES Methyldiethoxysilane - MeHSi(OEt)2 MDnM Permethyloligosiloxanes or permethylpolysiloxanes MDS Methyldichloro(hydro)silane - MeHSiCl2 Me2SiH2 Dimethylsilane Me3SiO0.5 Trimethylsiloxane end group Me4NOH Tetramethylammonium hydroxide MeSi(OSiMe2H)3 Methyltris(dimethylsiloxy)silane MeSiCl3 Methyltrichlorosilane MeSiH3 Methylsilane MH HMe2SiO0.5 MH2 1,1,3,3-tetramethyldisiloxane - HMe2SiOSiMe2H MH2Dn Linear α,ω-dihydrooligodimethylsiloxanes HMe2Si(OSiMe2)nOSiHMe2 MMA Methyl methacrylate Mn Number-average molecular weight MTES Methyltriethoxysilane - MeSi(OEt)3 m-TMI m-isopropenyl-α,α-dimethylbenzylisocyanate Mw Weight-average molecular weight MW Molecular weight MWD Molecular weight distribution NaHB(s-Bu)3 Sodium tri(s-butyl)borohydride NaHBEt3 Sodium triethylborohydride NaN3 Sodium azide NaOH Sodium hydroxide n-BA n-butyl acrylate n-BuLi n-butyllithium NCs Nanocomposites NHC N-heterocyclic carbenes NMR Nuclear magnetic resonance

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NPs Nanoparticles PB Poly(butadiene) Pd(OAc)2 Palladium(II) acetate Pd2(dba)3 Tris(dibenzylideneacetone)dipalladium(0) PD5 Poly(pentmethylcyclopentasiloxane) PDI Polydispersity PDMS Poly(dimethylsiloxane) PDMS-X2 α,ω-bis(γ-hydroxypropyl)polydimethylsiloxanes PE Poly(ethylene) PEG Poly(ethylene glycol) PEO Poly(ethylene oxide) Ph2Si(OH)2 Diphenylsilanediol PhMe2SiH Phenyldimethylsilane PhMeSiH2 Methylphenylsilane phr Parts per hundred (by weight) PHS Poly(hydrosiloxane) PhSi(OEt)3 Phenyltriethoxysilane PhSi(OMe)3 Phenyltrimethoxysilane PhSiCl3 Phenyltrichlorosilane PhSiH3 Phenylsilane PHSSQ Poly(hydrosilsesquioxane) PLA Poly(lactic acid) PMDI Poly(diphenylmethane 4,4′-diisocyanate) PMHBS Poly(methylhydroborosiloxane) PMHOS Polymethylhydroxysiloxane PMHS Poly(methylhydrosiloxane) PMHS-T Branched PMHSs PMVS Poly(methylvinylsiloxane)s POM Polarizing optical microscopy POSS Poly(silsesquioxane) PPO Poly(propylene oxide) PSCS Poly(siloxysilane) poly(carbosiloxane) PSt Poly(styrene) Pt2[(ViSiMe2)2O]3 Pt2(sym-tetramethyldivinyldisiloxane)3 - Karstedt’s catalyst PU Poly(urethane) PVA Poly(vinyl alcohol) Q8MH8 1,3,5,7,9,11,13,15-octakis(dimethylsiloxy)pentacyclo-[9.5.1.13,9.15,15.17,13]octasiloxane - (HMe2SiO)SiO1.5)8 RhCl(PPh3)3 Wilkinson’s catalyst ROP Ring-opening polymerization RT Room temperature SBR Styrene-butadiene rubber scCO2 Supercritical carbon dioxide SFs Silicone foams

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Si((OSiMe2)nOSiMe2H)4) Tetraarm star H-terminated polydimethylsiloxanes Si(OSiMe2H)4 Tetrakis(dimethylsiloxy)silane SiOC Silicon oxycarbide SRs Silicone rubbers St Styrene TEA Triethylamine TEMPO 2,2,6,6-tetramethylpiperidine-N-oxyl TEOS Tetraethoxysilane - Si(OC2H5)4 - Si(OEt)4 TES Triethoxysilane - HSi(OC2H5)3 - HSi(OEt)3 TFA Trifluoromethanesulfonic acid - triflic acid - CF3SO3H Tg Glass transition temperature TGA Thermogravimetric analysis TH Hydrosilsesquioxane unit - HSiO1.5 TH8 Octahydrooctasilsesquioxane - (HSiO1.5)8 - H8Si8O12 THF Tetrahydrofuran Ti Isotropic transition temperatures Tm Melting temperature TMCS Trimethylchlorosilane - Me3SiCl TMOS Tetramethoxysilane - Si(OMe)4 TMSD Tetramethyldisiloxane-1,3-diol ViSi(OEt)3 Vinyltriethoxysilane VMDMS Vinylmethyldimethoxysilane δ Chemical shifts

14.1  Introduction Poly(hydrosiloxane)s (PHS) are inorganic-organic hybrid polymers with inorganic backbone, compounds of alternatively connected silicon and oxygen atoms. In PHS as substituents at silicon atoms there are hydrogen atoms, methyl or other organic groups. PHS are an important group of silicones, which contain pendant or/and end hydrosilane functional groups (White 1995; Kricheldorf 1996; Chruściel 1999a; Brook 2000; Rościszewski and Zielecka 2002). A Si-H bond is a very attractive functional group used for the crosslinking of silicone elastomers and preceramic polymers in order to protect their shape (e.g. fibers) (Chojnowski 1987; Laine and Babonneau 1993; Rościszewski and Zielecka 2002). They are especially used for addition of different organic groups to silicon atom, e.g. via hydrosilylation (Sauer 1952) or nucleophilic substitution reactions (White 1995; Kricheldorf 1996; Chruściel 1999a; Brook 2000; Rościszewski and Zielecka 2002), which lead to the manufacture of materials with better properties. The Si-H group was also very often linked to conventional organic polymers in order to modify their properties.

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PHS and poly(methylhydrosiloxane)s (PMHS) are colourless liquids. A chemical structure of linear PMHS is described by a general formula:

RMe 2 SiO ( Me 2 SiO )m ( MeHSiO )  SiMe 2 R n



where Me  =  CH3; R  =  CH3 (most often) or another organic group or H, m ≥ 0, n ≥ 1. The PMHS structures are often expressed by abbreviated formulas, e.g. MDmDHnM, in which appropriate molecular units are abbreviated as follows: Me D = 0.5O-Si-O0.5

Me DH = 0.5O-Si-O0.5

Me M = 0.5O-Si-Me

Me

H

Me

Me2SiO (D)

Me(H)SiO (DH)

Me3SiO0.5 (M)

Cyclic (Stock and Somieski 1919; Wilcock and Hurt 1951; Campbell-Ferguson 1965; Glidewell et al. 1970; Seyferth et al. 1983) and linear PHS of the structure Me3SiO(H2SiO)nSiMe3 with a kinematic viscosity 10 cSt (Seyferth et al. 1983) have been synthesized by a hydrolytic polycondensation of dichlorosilane (H2SiCl2) and trimethylchlorosilane (Me3SiCl, TMCS) in a cold, concentrated hydrochloric acid (Wilcock 1949). Volatile cyclosiloxanes (H2SiO)n (n = 4–23) have been distilled off from hydrolysis products of H2SiCl2 (Seyferth et al. 1983). PMHS of the structure Me3SiO[HSi(OSiMe3)O]nSiMe3 have been  prepared by the hydrolysis of trichlorosilane (HSiCl3) with 80% sulfuric acid (H2SO4). As a side product, octahydrosiloctasequioxane (TH8, (HSiO1.5)8; H8Si8O12) is obtained (Müller et al. 1959).

14.2  Synthesis of PHS and PMHS Many types of polymerization methods have been useful for the synthesis of PHS and PMHS: dehydrocondensation, ionic polymerization, polyaddition, polycondensation, redistribution (rearrangement, equilibration polymerization), dehydrocarbocondensation, thermal and catalytic depolymerization. The Si-H bond is quite stable under acidic conditions and very unstable in the presence of oxygen, bases and moisture (Lee 1966).

14.2.1  Synthesis of PMHS With Linear and Ring Structures Linear and monocyclic PMHS are prepared by the following methods (White 1995; Kricheldorf 1996; Chruściel 1999a; Brook 2000; Rościszewski and Zielecka 2002):

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1. The hydrolytic polycondensation of methyldichloro(hydro)silane (MeHSiCl2, MDS) or copolycondensation with dimethyldichlorosilane (Me2SiCl2, DDS) and TMCS (Brit. Pat. 632,954, 1949; Sauer 1952; Sokołow 1961; Rościszewski et al. 1974, 1975; Richards et al. 1993); 2. The cationic polymerization (and copolymerization) of methylhydrocyclosiloxanes (DHn) (Reikhsfeld and Ivanova 1962; Graczyk and Lasocki 1978, 1979a, b; Richards et al. 1993; Williams 1994) or their catalytic equilibration (linearization, repolymerization) with hexamethyldisiloxane (Me3SiOSiMe3, M2) or 1,1,3,3-tetramethyldisiloxane (HMe2SiOSiMe2H, MH2), in acidic medium - with protonic acids (e.g. CCl3COOH, CF3COOH, H2SO4, sulphonic acids) (Wilcock and Hurt 1951; Husemann and Greber 1962; Stober et  al. 1965; Lefort and Parasko 1966; Gallagher 1966; Harris and Kimber 1974; Jap. Pat. 60,163,809, 1985; Crivello et al. 1986; Yamamoto and Matsuda 1991; Weiss 1993; Sellinger and Laine 1996a, b; Hager 1996; Razzano et al. 1997; Nennendal et al. 1999; Ganicz et al. 2004), with acidic salts (e.g. ammonium sulphate, (NH4)2SO4), cationits, protonic acid activated-clays (Husemann and Greber 1962; Reikhsfeld and Ivanova 1962; Graczyk and Lasocki 1978; Crivello et  al. 1986; Richards et  al. 1993; Sellinger and  Laine 1996b; Nennendal et  al. 1999), dimethylsilyl sulphate (Razzano et al. 1997), Lewis acids (e.g. Al2(SO4)3, CuSO4 or Fe2(SO4)3), and also with phosphonitrile halides (Nye et al. 1997, 1999); 3. The heteropolycondensation of siloxanediolates of alkali metals (Andrianov et al. 1962; Saxena et al. 2007; Zhang et al. 2007b) or siloxanediols (Chruściel and Lasocki 1979) with hydrochlorosilanes, e.g. using chlorodimethylsilane (HMe2SiCl, DMCS) in the presence of pyridine (C5H5N), as a hydrochloric acid (HCl) acceptor, which leads to the production of PMHS with Si-H end groups. The hydrolytic polycondensation of organic chlorosilanes is a multistep process, including hydrolysis reactions of ≡Si-Cl bonds:

≡ Si − Cl

+

H2 O

→

≡ Si − OH + HCl

(14.1)

and series of consecutive reactions: heterocondensation of ≡Si-Cl groups with silanol groups:

≡ Si − OH

+

Cl − Si ≡

→

≡ Si − O − Si ≡ + HCl

(14.2)

≡ Si − O − Si ≡ + H 2 O

(14.3)

and homocondensation of silanol groups:

≡ Si − OH

+

HO − Si ≡

→

Hydrolysis reactions of hydrochlorosilanes are usually carried out at temperatures below 0  °C. α,ω-dichlorooligo(methylhydro)siloxane (Table  14.1) have been synthesized at a much lower temperature (from −78 °C to room temperature RT) (Manami and Nishizaki 1958; Chruściel et  al. 1997,  Chruściel 1999b). α,ω-­ dichlorooligo(methylhydro)siloxane are used in synthesis of PMHS with a regular block structure. The side products of syntheses of PMHS (by polycondensation and

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Table 14.1  Boiling points and 29Si-nuclear magnetic resonance (NMR) data of (α,ω-dichloro) oligo(methylhydro)siloxanes B.p. [°C/mm Hg] 29Si-NMR (in C6D6) [δ, ppm] 103/748 −9.95, −9.98; -OSiMe(H)Cl 85–86/100 −12.08; -OSiMe(H)Cl −30.67; -OSiMe(H)OClMeHSiO(SiHMeO)2SiHMeCl 70–71/12 −12.45; -OSiMe(H)Cl −32.23; -OSiMe(H)OClMeHSiOSiHMeOSiHMeOSiHMeOSiHMeCl 97–98/15 −12.53; -OSiMe(H)Cl −32.48; -OSiMe(H)O- −33.78; -OSiMe(H)OClMeHSiOSiHMeO(SiHMeO)2SiHMeOSiHM 119/5 −12.56; -OSiMe(H)Cl −32.52; eCl -OSiMe(H)O- −34.02; -OSiMe(H)OSiloxane ClMeHSiOSiHMeCl ClMeHSiOSiHMeOSiHMeCl

Source: Chruściel et al. (1997)

polymerization methods) are low molecular weight (MW) cyclic and linear oligomers. PMHS homopolymers of random structures are prepared by the hydrolytic polycondensation of MDS, while cohydrolysis of MDS and DDS leads to copolysiloxane-­ containing units: MeHSiO (DH) and Me2SiO (D). An addition of the monofunctional TMCS (as an end capping agent) introduces end trimethylsiloxane (Me3SiO0.5) groups and limits MWs of PMHS. DHn (n = 4–8) are also products of the hydrolytic polycondensation of MDS (Noll 1968; Hardman and Torkelson 1989; Crivello and Malik 1997; Tsuchida et al. 1997; Zhang et  al. 1998; Zhang et  al. 2000; Rościszewski and Zielecka 2002; Constantopoulos et al. 2004) or products of vacuum pyrolysis (at 400–450 °C) of their hydrolizates (Lee 1966; Rościszewski et al. 1975; Finkelman 1987). Cyclic dimethylsiloxane-co-PMHS (DmDHn) have been prepared by heterocondensation of dimethylsilanediol or oligodimethylsiloxane-α,ω-diols with MDS (Reikhsfeld and Ivanova 1962; Graczyk and Lasocki 1978, 1979a) or by pyrolysis of linear PDMS-­ co-­ PMHS in the presence of trifluoromethanesulfonic acid (CF3SO3H, TFA) (Chang  and  Buese 1993). The low MW oligo(methylhydro-co-diphenyl)siloxane resins are synthesized by a non-hydrolytic sol-gel heterocondensation of methyldiethoxysilane (MeHSi(OEt)2, MDES) with diphenylsilanediol (Ph2Si(OH)2), which can be catalyzed by cationic exchange resins at 100 °C (Kim et al. 2012). A very good control of MWs of PMHS has been achieved in the cationic polymerization of cyclosiloxanes: DHn (n = 4–6) or DmDHn or via copolymerization of DHn with dimethylcyclosiloxanes (Dn, n = 3–5) (Scheme 14.1) (Reikhsfeld and Ivanova 1962; Graczyk and Lasocki 1978; Richards et al. 1993), usually with the addition of M2 (Wilcock and Hurt 1951; Gallagher 1966; Weiss 1993). The copolymerization of DHn with Dn and M2 occurs in the presence of diffe­ rent acid catalysts: cationic exchange resins, Lewis acids, protonic acids, (Husemann and Greber 1962; Reikhsfeld and Ivanova 1962; Graczyk and Lasocki

14  Hydrosilyl-Functional Polysiloxanes: Synthesis Reactions and Applications Me Si Me

337

Me O

Si

+

m

O

n

H RMe 2SiO

Si Me

O

m

Si H

Si Me

Me

Me

Me

Me

O

n

O

x

Si H

O

y

SiMe2R

R = Me, Vi, H;

Scheme 14.1  Polymerization routes for obtaining PMHS copolymers. Reprinted with permission from Chruściel and Leśniak (2012)

1978; Chujo et al. 1985, Richards et al. 1993; Sellinger and Laine 1996a, Klok et al. 1997, Shermann and Kennedy 1997, Servaty et al. 1998, Feng et al. 2000), acidic salts (e.g. ammonium sulphate or dimethylsilyl sulphate (Razzano et al. 1997)), or in the presence of protonic acid-activated clays (Crivello et al. 1986; Lewis et al. 1993). The polymerization rates and the microstructure of the polysiloxane chain in PMHS depend strongly on the monomer structures and reaction conditions (Müller et al. 1959; Reikhsfeld and Ivanova 1962; Rościszewski et al. 1974, 1975; Graczyk and Lasocki 1978, 1979a; Richards et al. 1993). An equilibration of poly(dimethylsiloxane)s (PDMS) with linear PMHS and M2, towards the phosphonitrile catalyst [Cl3P(NPCl2)2PCl3]+PCl6− (LPNC) at elevated temperature has been a very useful synthetic method of PDMS-co-PMHS (Razzano et al. 1997; Nye et al. 1997, 1999; Liao and Nye 1998, 2008). Linear and branched PHS, containing different substituents at silicon, e.g. vinylsilyl groups, Me(CH2 = CH)SiO (MeViSiO, DVi) or/and ViMe2SiO (MVi), have been synthesized by the  equilibration of siloxane substrates with LPNC at temperature 80–90  °C (Liao and Nye 1998, 2008). PMHS end-capped with HMe2SiO0.5 (MH) groups have also been prepared by the catalytic equilibration of octamethylcyclotetrasiloxane (D4) with MH2 and 50% H2SO4 or in the presence of acid-activated clays, or by reducing telechelic dichlorosiloxanes with metal hydrides (e.g. lithium aluminohydride (LiAlH4), LiH or NaH towards triethylborane (Et3B)) (Greber and Metzinger 1960; Chruściel and Lasocki 1979; Chruściel 2000) or by the anionic ring-opening polymerization (ROP) of D4 in the presence of n-butyllithium (n-BuLi), followed by the termination reaction with DMCS (Zhang et al. 2010; Chen et al. 2010), by the heterocondensation of DMCS with appropriate siloxanediolates of alkali metals (Andrianov et al. 1971a, 1971b) or oligosiloxane-α.ω-diols (in the presence of an amine acceptor of HCl) (Chruściel and Lasocki 1979). Yoshino et al. (1990) prepared MHDnMH via ROP of Dn (n = 3–6) with DMCS, which was catalyzed with a silica gel. PDMS-co-PMHS-polymethylphenylsiloxane random copolymers were synthesized by Yang et al. (2003) through ROP of cyclosiloxanes (DH4.5 + D4 + DPh3 + DPh4)

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catalyzed by TFA, while phenyl modified PDMS-PMHS copolymers containing both internal Si-H and end SiH2 and MeSiO3/2 (T) units were prepared by Yang et al. (2004) via ROP of DH4.5 + hexamethylcyclotrisiloxane (D3) + DPh3 with n-BuLi, and the sequential microstructures of these copolymers were determined by 29Si-NMR.

14.2.2  S  ynthesis of PHS and PMHS with Branched, Cage, Dendritic, Ladder and Star Structures An increasing interest in the preparation and applications of PMHS of branched, dendritic and star structures has been especially observed in the last three decades. A star tetrakis(dimethylsiloxy)silane (Si(OSiMe2H)4), cage TH8, and octakis (dimethylsiloxy)octasilsesquioxane (((HMe2SiO)SiO1.5)8) are commercial products (Hoebbel and Wieker 1971; Hasegawa  and Motojima 1992, Hasegawa  1993, Hasegawa et al. 2003; Moran et al. 1993; Majoros et al. 1997; Provatas et al. 1998; Constantopoulos et al. 2004; Laine 2005; Soh et al. 2007; Hessel et al. 2006). Uchida et al. (1990) elaborated a multistep synthesis of polysiloxane dendrimer from siloxane oligomers containing different functional groups: Si-Br, Si-Cl, Si-H and Si-OH. Through many consecutive condensation reactions, the liquid dendritic polysiloxane of third generation, (MeSiO1.5)22(Me2SiO)162(MH)24, having 24 Si-H end functional groups on the globular-like surface was obtained (Uchida et al. 1990). A branched H-siloxane, {-OSi[(OSiMe2O)nSiMe2H]2-}6, was prepared by Sargent and Weber (2000) through the equilibration of hexakis(dimethylsiloxy) cyclotrisiloxane (((HMe2SiO)2SiO)3) with D4, towards TFA.  Six- and eight-­ membered ring-star silicates were synthesized by Cai et al. (2004) with low yields: [(HMe2SiO)2SiO]3 was obtained in reaction of DMCS with pseudowollastonite (Ca3Si3O9Cl6), while octakis(dimethylsiloxy)cyclotetrasiloxane (((HMe2SiO)2SiO)4) was  obtained by Sargent and Weber (2000) from octakis(trimethylsiloxy)cyclotetrasiloxane (((Me3SiO)2SiO)4) and MH2 in the presence of TFA. The equilibration of star octamethylpentasiloxane  (Si(OSiMe2H)4) with D4 towards TFA gave liquid tetraarm star polysiloxanes (Si((OSiMe2)nOSiMe2H)4) with Si-H end groups (and a number-average molecular weight (Mn) = 2,800–10,940 g/ mol). Similarly, tetraarm star oligofluorosiloxanes have been synthesized by the equilibration of 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane with Si(OSiMe2H)4 in the acidic medium (Cai and Weber 2004; Grunlan et al. 2006). The ROP of 2,4,6,8-tetramethyltetrahydrocyclotetrasiloxane (DH4) with tris(pentafluorophenyl)borane (B(C6F5)3), in toluene, was reported by Chojnowski et al. (2012). The Si-H multifunctional branched polysiloxane, composed of rings and macrocycles of different size, was soluble. A gaseous side product, methylsilane (MeSiH3), was formed, and in the bulk process gelation was observed. A hydride transfer led to a rearrangement of linear MeHSiO units into branching MeSiO1.5 units (Chojnowski et al. 2012). The poly(hydrosilsesquioxane) (PHSSQ) precursors were synthesized from triethoxysilane (HSi(OC2H5)3, HSi(OEt)3, TES) at pH 1–4. The molecular structures

14  Hydrosilyl-Functional Polysiloxanes: Synthesis Reactions and Applications

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of the PHSSQ precursors prepared (the cage/network ratio, the content of Si-OH end groups, and the MW) depended on the water:TES molar ratio and the reaction conditions. The PHSSQ films with a higher refractive index can find applications such as low dielectric constant materials (Liu et al. 2004).

14.2.3  S  ynthesis and Characterization of Random Branched PMHS New branched PMHSs (PMHS-T) with random structures of the siloxane chains and general formulas A, B, and C (Chruściel et al. 2008, 2011a, b):

Tk Dm DH n M p ( A ) , Tk T H t Dm DH n M p ( B)



k = 3, 6, 9,12, 22; m ≥ 1; n ≥ 10; t = 11 − 12; p = k + 2;



Q y Dm DH n M p ( C ) ( y = 1 − 3;, m = 49 − 52;, n = 48 − 52;, p = 2 k + 2 ) ,



[T = CH 3SiO1.5 , Q = SiO 4 /2 , D = ( CH 3 )2 SiO, D H = CH 3 ( H ) SiO, M = ( CH 3 )3 SiO 0.5 ]





were prepared by the hydrolytic polycondensation of ether solutions of chlorosilanes: DDS, HSiCl3, MDS, methyltrichlorosilane (MeSiCl3) and TMCS from −8 °C to RT. By changing stoichiometry of the monomers, 12 liquid PMHS of a type (A) and (B) were prepared with yields 57–67  wt.%. They had a different degree of branching, different content of Si-H groups, and the following molecular compositions:

T3 DH102 M 5 , T6 DH104 M8 , T9 DH112 M11 , T12 DH100 M14 , T22 DH135 M 24 ,



T3 D51 DH 51 M 5 , T6 D52 DH 52 M8 , T9 D56 DH 56 M11 , T12 D50 DH 50 M14 , T22 D90 DH 90 M 24 ,



T H12 DH 50 D50 M 24 and T11 T H11 DH 90 D90 M 24 .

Similarly, from DDS, MDS, tetraethoxysilane (Si(OC2H5)4; Si(OEt)4, TEOS) and TMCS, in diethyl (or petroleum) ether, at a temperature   6) were greater than in the case of ring-chain equilibration process with D4 and decamethylcyclopentasiloxane ((Me2SiO)5, D5) (Foston and Beckham 2006). In the presence of B(C6F5)3, dehydrocondensation reactions of hydrosilanes with alcohols and silanols (Deforth and Mignani 2001, 2003; Shinke et  al. 2007) and reduction of alcohols (Gevorgyan et al. 2000), ketones (Parks et al. 2000), aliphatic aldehydes, aryl chlorides and carboxyl group have been easily occured (Gevorgyan et al. 2001). Hydrosilanes with functional groups: chloropropyl, carboxylic, hydroxyl, among others, have been immobilized on silica surface via the dehydrocondensation reaction between silanol group of silica and Si-H group, which was catalyzed by B(C6F5)3 at RT (Moitra et al. 2014). A catalytic dehydrogenative coupling of PMHS with PDMS-α,ω-diols is one of the crosslinking methods of silicone elastomers (Borisov et  al. 1966; Lasocki et  al. 1974; Chruściel and Lasocki 1983a) (see Sect. 14.3).

14.6  Applications of PMHS A number of practical applications of Si-H functional polysiloxanes (and also other organosilicon polymers) is huge and is continuously growing.

14.6.1  General Applications of PMHS The presence of the Si-H functional group into organosilicon polymers affects their interesting chemical and physical properties and allows their numerous chemical modifications. Thus, PMHS find many practical applications, such as: 1. In dehydrocondensation reactions with telechelic silanol (Si-OH) groups of poly(dimethylsiloxanediol)s, as one of the crosslinking methods of RT vulcanized (RTV) silicone elastomers (Rościszewski 1964; Noll 1968; Hardman and Torkelson 1989; White 1995; Kricheldorf 1996; Chruściel 1999c; Brook 2000; Rościszewski and Zielecka 2002; Fejdyś-Kaczmarek et al. 2006); and 2. In synthesis and modifications of properties of different types of polymers: organosilicon, organic and inorganic, mainly in addition reactions to unsaturated

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bonds (C=C, etc.) (Marciniec et al. 1992) for the preparation of comb-like polysiloxanes with different pendant groups, used as liquid-crystalline organosilicon polymers (Finkelman 1987; Boileau and Teyssié 1991; Percec and Wang 1991, 1992; Madec and Maréchal 1993; Śledzińska et al.,1994; Białecka-Florjańczyk et al. 1995; Caminade and Majoral 1995; Ruder et al. 1998), cosmetic ingredients, surfactants (Noll 1968; Hardman and Torkelson 1989), etc. Other more important uses of PMHS are the following: 1. Synthesis of highly branched organosilicon polymers (dendrimers (Uchida et al. 1990; Morikawa et al. 1991, 1992; Van der Made and van Leeven 1992; Zhou and Roovers 1993; Ruder et al. 1998; Yoon and Son 1999) and star polymers (Zhou and Roovers 1993; Shim et al. 1998); 2. Grafting of macroinitiators (Kollefrath et al. 1996; Kickelbick et al. 1998), medicines (Bachrach and Zilkha 1984) and stabilizers (Rühlmann and Jansen 1985) onto PMHS, preparation of silicone release coatings (Chruściel and Graczyk 2006), and interpenetrating polymer networks (IPNs) with good antiadhesive (Wilczek 1993; Wilczek and Sun 1994; Dobkowski and Zielecka 1999; Zielecka et  al. 2000) and conductive properties (after dopping with salts) (Boileau et al. 1998); 3. Preparation of solid polyelectrolytes (Oh et  al. 2003) and biomaterials (Hron et al. 1997; Chekina et al. 2006), 4. Synthesis of carbofunctional polysiloxanes from MH2Dn (Greber and Jäger 1962); 5. Oxidation or hydrolysis of pendant Si-H groups and consecutive crosslinking of preceramic polymers (West 1986; Chojnowski 1987; Laine and Babonneau 1993; Matyjaszewski 1998) and hydrophobic finishes on textiles (Rościszewski 1964; Noll 1968; Hardman and Torkelson 1989; Rościszewski and Zielecka 2002); 6. Surface modification of inorganic fillers and supports (Cosgrove et  al. 1990; Reihs et al. 1995; Voliotis et al. 2003; Tertykh et al. 2003); 7. Modification of polymer properties (Bik et  al. 2003; Chruściel et  al. 2006; Bieliński et al. 2007a, b; Chruściel and Leśniak 2012); 8. Reduction of organic compounds, e.g. aldehydes and ketones (Lopez and Fu 1997; Kobayashi 1998), alkenes (Miravet and Frechet 1997), nitro-compounds (Lipowitz and Bowman 1973) or organic phosphorous esters (Fritzsche et  al. 1964, 1965; Polmanteer et al. 1971; Segall et al. 1974). PMHS are widely used as hydrophobic and antiadhesive, antifoaming (Noll 1968; Hardman and Torkelson 1989; Rościszewski and Zielecka 2002) and crosslinking agents (Noll 1968; McAfee 1985; Hardman and Torkelson 1989; Chang et al. 1993; Michalczyk et al. 1993; Matsushita et al. 1994; Hara et al. 1994; White 1995; Chruściel 1999b; Rościszewski and Zielecka 2002; Bik et al. 2003; Chruściel et al. 2006; Bieliński et al. 2007a, b), components for cosmetic materials and dental composites (Hashimoto et  al., 1994a). They are considered physiologically inert (Ger. Pat. 3,416,694, 1985; Jap. Pat. 60,163,809; 1985, Hashimoto et  al. 1994b; Zielecka and Rościszewski 1994).

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Papers and parchments coated with silicone compositions crosslinked with PMHS are most important antiadhesive materials for production of silicone release coatings and for a manufacture of packages for sticky food and industrial articles (Eckberg 1981; Reutsch 1993; Stein et al. 1993; Chruściel et al. 2006). A polyaddition of fluoroolefins to the Si-H bonds into PMHS have given materials with a consistensy of highly viscous oils, waxes and solids, and with a very low surface energy and profitable surface properties (Wilczek 1993; Wilczek and Sun 1994). Very good antiadhesive properties of emulsions of the IPN type, composed of perfluroethylene and PMHS, have been applied for the protection of building surfaces against undesirable graffiti inscriptions (Dobkowski and Zielecka 1999; Zielecka et al. 2000). The properties of PMHS have been modified very often by hydrosilylation reactions (see Sect. 14.1). The polyaddition of allyl derivatives (phenylallyl- and allylglycidyl ether, allyl methacrylate, allyl chloride and allylamine), 1-octene and other olefins into PMHS, catalyzed by Pt and Rh complexes and Ru(CO)12, is a very method useful for chemical modification of their properties (Marciniec et al. 1990, 1997). In this sense, the addition of allylglycidyl ether to PMHS, catalyzed by Rh-siloxyl complexes, provides epoxyfunctional polysiloxanes, which are further transformed into appropriate silicone surfactants. Epoxy-functional polysiloxanes upon reaction with Na2S2O3 or secondary amines and sodium acetate give thiosulphate or betaine derivatives of polysiloxanes (Maciejewski et al. 2006). PDMS-co-PMHS (MDmDHnM, m:n  =  45–50  mol.%) and poly(methylvinylsiloxane)s (PMVS) were used for fabrication of antifouling silicone materials containing pendant biocidal or fouling release groups (Boudjouk et  al. 2009). Novel PDMS with vinyl ether end groups  have been prepared by the hydrosilylation of triethyleneglycol divinyl ether (a 10-fold excess was used) or 1,4-bis-(vinyloxymethyl)cyclohexane with the Si-H functional telechelic PDMS.  The hydrosilylation reactions have been carried out  at 70  °C in toluene using Cp2PtCl2 as a catalyst at a ratio [Si-H]0/[Pt] = 5000. The modified polymers were purified by precipitation of toluene solutions into a large excess of methanol. Vinyl ether functional PDMS have also been used as additives in cationically cured photosensitive compositions (Cazaux and Coqueret 1995). Water-repellent organicpolysiloxane materials have been obtained by hydrosilylation of 1-hexadecene, i-butyl methacrylate and ViSi(OEt)3 with PMHS (Herrwerth et  al. 2008). Modification of the methacrylate ester of amine light stabilizer hindered with methyltris(dimethylsiloxy)silane (MeSi(OSiMe2H)3) gave a new siloxane light stabilizer (Pan et al. 1994). PMHS modified by the hydrosilylation treatment or plasma roughening can be used for microfluidic electrophoresis chips. The chemicallly modified PMHS exhibited UV transparency and was stable in extremely acidic media, as well as being relatively insensitive to sample thickness, thus indicating good potential for sensors based on UV absorption. High-resolution complex patterns have been  formed in PMHS by soft lithography casting techniques or by laser ablation. PMHS has also been bonded by plasma treatment and arrayed with fluidic interface ports during casting (Lee et al. 2008).

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Liquid silicone resins containing SiO4/2 silicate branching units and reactive MH groups have been used as reinforcing agents for silicone rubbers (SRs) (Muskus and Ganicz 2007). M2DH and H-silsesquioxane Q8MH8 were functionalized with aminopropyl, (hydroxyl)polyether, polyether, glicydyl and -CH2CH2Me2SiCl pendant groups by hydrosilylation of alkene derivatives in the presence of siloxyl Rh(I) complexes (Łęszczak et al. 2007; Pancer et al. 2007). Vinyldiphenylsiloxane resins which were cured with oligo(methylhydro-co-diphenyl)siloxane resins were shrinkless and showed high transparency (~90%) at the visible range, a high refractive index (n = 1.58) and excellent thermal stability against yellowing. They can be used as encapsulants for light-emitting diodes (LEDs) (Kim et  al. 2012). A low MW polymethylphenylhydrosiloxane oil, with a refractive index of 1.45 was used as a component of vinyl-MQ silicone adhesive for encapsulation of LEDs. These cured materials showed excellent optical transparency, mechanical and optoelectronic properties, and a hardness of 48 Shore A, as well as good thermal properties at temperature around 180–250 °C was observed. The thermostability of the cured silicone resins with low MW phenyl silicone crosslinker was higher than that of PMHS oil crosslinker with higher MW (Li et al. 2017). By curing of prehydrolyzed MTES solution (in acidic medium) with DBTDL catalyst and PMHS or PDMS-co-PMHS copolymers, new composite coatings were prepared at RT.  The gelation was carried out in the presence of (3-aminopropyl) triethoxysilane (APTES) or triethylamine (TEA). The coatings obtained from a 2:1 (w/w) ratio of prehydrolyzed MTES solution with equimolar amounts of PMHS homopolymer and water, catalyzed with APTES, showed high pencil hardness and excellent corrosion resistance (Huang et al. 2010). Poly(methylhydrovinylsiloxane)s, such as MD350DPh10DVi10DH10M, MD350DVi10 H D 10M and MViDH3D219MVi, have been  crosslinked by  the  hydrosilylation using Pt2[(ViSiMe2)2O]3 as a catalyst at 150 °C, which provided strong films, especially useful for the manufacture of airbag coatings (Liao and Nye 1998). New modified polymeric materials have also been prepared by adding C60 molecules into PMHS and further crosslinking (West et  al. 1994). The 10–20% solutions of Me3Si(OSiHMe)n(OSiMe2)mOSiMe3 [n/m  =  1: (5–10)] and/or Me3Si(OSiHMe)n OSiMe3 (n  =  5–50) in i-propanol or CCl4 and 0.4–0.6% β-aminoethylsilane, α-aminopropyl(methoxy)silane or HSi(OMe)3 and a catalyst were used for hydrophobization and removal ice from metal surfaces (Giurgiu et al. 1994). Polysiloxane networks obtained by crosslinking of PMVS with PMHS were further modified by hydrosilylation using Pt2[(ViSiMe2)2O]3 as a catalyst with allyl alcohol or allyl derivatives of hydrofilic substrates: allyl-polyol ethers, allylamine derivatives of polyols and analogous derivatives from simple sugars. This modification changed the surface properties, increased surface tension and decreased wetting angle of these materials (Sellinger and Laine 1996a, b; Nennendal et  al. 2000). Carbohydrate-­modified silicones, prepared by adding PMHS into allyl derivatives of mono-, di-, and oligosaccharides (in the presence of PtCp2Cl2), were surface active biocompatible materials (Loos and Stadler 2001). A new polysiloxane with pendant sugar units (glucosyl-thioureylene groups) has also been prepared by

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hydrosilylation of allylamine with PMHS, catalyzed by PtO2, followed by reaction of the copolysiloxane with glucosyl isothiocyanate (Zhou et al. 2004). New crosslinked poly[(methylsiloxane)-co-(oxymethylene)] copolymers were synthesized by the cationic ROP of DH4 and 1,3,5-trioxane with anhydrous perchloric acid at 70 °C. These new copolymers were white or glassy crystals. Their degree of crosslinking increased with a higher concentration of the MeHSiO units in the copolymers (Rodríguez-Baeza et al. 1997). The properties of PMHS were also modified by IPN formation or self-organizing ultra-thin layers (Sun et al. 1996; Boileau et al. 1998). Biodegradable multiblock copolymers were prepared by the hydrosilylation of poly(lactic acid) (PLA) or allyl-end poly(α-hydroxyacid)s with MH2Dn (Bachari et al. 1995). Two new silicone derivatives bearing a 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical moiety were synthesized by Pt or Rh-catalyzed hydrosilylation from PMHS with 4-allyl-2,2,6,6-tetramethylpiperidine-N-oxyl ether or alcoholysis reaction with 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl, and (Bu4N)2PtCl6, [Rh(cod)2]BF4 (cod  =  1,5-cyclooctadiene, nbd  =  norbornadiene), [Rh(nbd)2]BF4, [RhCl(cod)]2, [RhCl(nbd)]2, K2PtCl6, Pt2[(ViSiMe2)2O]3 and RhCl(PPh3) were used as catalysts. Due to the charge-discharge properties these new silicone materials could be applied as the next-generation active material for rechargeable batteries (Suguro et al. 2009). Linear polysiloxanes with cyclohexyl-substituted octasilsesquioxane and octyl side groups were prepared by the subsequent hydrosilylation of poly(ethylhy­ drosiloxane) with 1-allyl-2,3,4,5,6,7,8-heptacyclohexyl(octaasilsesquioxane) (TallylTR7, R = C6H11) and 1-octene using the catalyst Pt2[(ViSiMe2)2O]3. They were used for the manufacture of free-standing films by the casting method (Ryu et al. 2010). Octasilsesquioxanes with linear and branched alkyl substituents were prepared in high yield by Pt-catalyzed hydrosilylation of alkenes with Q8MH8. The derivatives with n-alkyl chains from C3 to C6 crystallized below 0 °C, while the derivatives with longer n-alkyl chains (C7 and C8) formed amorphous glasses with a Tg around −100 °C. The melting points for the iso-hexyl and iso-heptyl POSS derivatives were found above RT, and the iso-pentyl POSS derivative was liquid at 25 °C. Thermogravimetric analysis (TGA) showed a negative effect of the branching of the alkyl chain on the thermostability in air atmosphere (Perrin et al. 2011).

14.6.2  Liquid-Crystalline Derivatives from PMHS Liquid crystals (LCs) are a very important class of modern materials. They exhibit unique and extraordinary properties due to regions of highly ordered structures in both solid and liquid phases (Shibaev and Boiko 2009, Shibaev and Bobrovsky, 2017, Shibaev 2016). LC polysiloxanes are frequently prepared by hydrosilylation routes from H-siloxanes of different structures (cyclic, linear, star and silsesquioxane (cubic, ladder or double-decker)) and a variety of mesogens having end olefinic

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double bonds (Finkelman 1987; Percec and Hahn 1989; Boileau and Teyssié 1991; Percec and Wang 1992; Białecka-Florjańczyk et al. 1995; Zhang et al. 1996; Dai et al. 1997; Wan et al. 1998; Mery et al. 1999; Ganicz and Stańczyk 2003; Hu et al. 2003; Yao et al. 2005; Zheng et al. 2005; Zhang et al. 2006; Ahsan et al. 2007; Pan et al. 2007; Kim et al. 2008; Han et al. 2008; Shibaev and Boiko 2009). The silicone-based LC polymers with variable isotropic  phase transition temperatures (Ti) have been synthesized from PMHS, 10-undecenoic acid-based crosslinking agent and cholesterol-based side chain mesogen (Jana et al. 2011). Side-chain LC polysiloxanes were synthesized by cohydrosilylation of two mesogenes (a dye-­ monomer and LC monomer) with PMHS (Jia et al. 2003) or hydrosilylation of five chiral LC monomers with PMHS. A flexible siloxane backbone and a long flexible spacer led to a low Tg, high thermostability and wide mesophase temperature range (Zhang et al. 2007a). Other LCs were obtained from reaction of a cholesteric LC monomer or a fluorinated nematic LC monomer with PMHS. These LC polymers showed a smectic LC phase with very wide temperature ranges in the heating and cooling cycles (Han et  al. 2008). LC cyclosiloxanes with a very wide smectic-A range were prepared by adding spacers containing fluorine atoms, with different lengths, to pentamethylcyclopentasiloxane (DH5) (Kaneko et al. 2007). New side chain LC polysiloxanes were synthesized by adding two vinyl-­ functional LC monomers into PMHS. The LCs with fluorinated mesogen showed improved thermal stability (T5 > 300 °C) (Meng et al. 2009). A LC side chain polysiloxane with 5-(pentyloxy)-3-methyloxy-9,10,16,17,23,24-hexakis(octenyloxy) phthalocyanine groups was prepared by the hydrosilylation reaction of PMHS with phthalocyanine having a single end alkenyloxy group, in toluene, and using the catalyst Pt2[(ViSiMe2)2O]3 (Ganicz et  al. 2012). The  LC polysiloxanes obtained from the reaction of PMHS with novel chiral LC monomers showed a high thermostability and wide mesophase temperature ranges (Wang and Zhang 2014). The LC silsesquioxanes were also prepared by the hydrosilylation reaction of Q8MH8 with alkene (Zhang et al. 2001) or allyloxy (Ba et al. 2003) functionalized mesogens. The LC cubes had an average of five linked LC groups. Some LC-cube derivatives showed LC transitions, with a tendency to form spinal muscular atrophy (SmA) phase (Ba et al. 2003). A LC discotic side chain ladder-like polysiloxane derivative was obtained by the hydrosilylation of vinyl functional ladder silsesquioxane with Si-H end triphenylene-containing mesogen (Białecka-Florjańczyk and Sołtysiak 2010). The LC dendritic polymers of regular structure were prepared from the reaction of the allyl functional carbosilane dendritic matrix of a first generation with Si-H end cyanobiphenyl, methoxy-phenyl benzoate and cholesteryl mesogens (Białecka-Florjańczyk and Sołtysiak 2010). Multimesogen LC materials were obtained from the Si-H end (disiloxane)alkyl-substituted mesogen with a star octa[(diallylmethyl)propyl]silane (Ponomarenko et al. 1996). Carbosilane LC dendrimers from first to fifth generations with 8, 16, 32, 64 and 128, end chiral mesogenic groups, were prepared by coupling Si-H end mesogens with carbosilane matrices containing end allylic groups, exhibiting Tg values around −5 °C. The LC dendrimers of the generations G-1-G-3 formed a ferroelectric smectic (SmC∗) phase up to ~180 °C, while the LC dendrimers of the generations G-4 and G-5 showed a rectangular columnar mesophase (Zhu et al. 2001).

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Novel oligomers with promesogenic groups at both ends were obtained by the hydrosilylation reaction of well-defined telechelic oligocarbosilazanes terminated with vinylsilyl groups and Si-H functional promesogenic compounds. The correlations between the structure of the resulting mesogen end-capped oligomers and their phase behavior were studied by Grande et al. (2009) using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). 14.6.2.1  Synthesis of Liquid Crystalline Elastomers and Thermosets Liquid-crystalline silicone elastomers (LCEs) with a polysiloxane backbone can be covalently crosslinked into three dimensional (3D) networks (Finkelmann et  al. 1981; Küpfer and Finkelmann 1991). New side chain LC polysiloxanes were synthesized by substitution of low MW PMHS of average composition MDH7M (Mn = 700–800 g/mol) with different cholesteric and smectic monomers in the presence of Pt catalyst. Some crosslinked LCs showed very wide mesophase temperature ranges and a high thermal stability (He et al. 2005a; Wang et al. 2009). A first mono-domain polysiloxane elastomer was prepared by Küpfer and Finkelmann (1991). Other monodomain LC polysiloxane elastomers were synthesized with one or two crosslinking agents from PMHS and different mesogens in toluene, in the presence of Pt catalysts and a mechanical field (Lacey et al. 1998). New network LC polymers were synthesized by graft copolymerization onto PMHS of the difunctional mesogenic difunctional monomer (4-allyloxy) benzoyloxy-4′-allyloxy-biphenyl, which served as a mesogenic unit and a crosslinker. The crosslinked polymers rarely exhibited thermal behavior of the elastomer, while densely cured samples behaved like LC thermoset polymers (Lacey et  al. 1998). The Tg of LCEs increased with the increasing concentration of the crosslinker, but the Ti and LC range decreased slightly (Jia et al. 2002). The newer side chain smectic LCEs were synthesized from PMHS (Mn  =  700–800  g/mol) and monomer with nematic properties (4,4′-allyloxy-biphenyl-4-yl 4-­hep­tyloxybenzoate) and the chiral crosslinker (isosorbide-undecanoyloxybisate), in the presence of Pt catalyst at 60–70  °C for three days. The LCEs were precipitated with methanol and dried under vacuum at RT. These elastomers were insoluble in chloroform, DMF, toluene and xylene, but could be swollen in these solvents, thus exhibiting very wide mesophase temperature ranges and high thermal stability. The LC phase of the homopolymer and elastomers changed from smectic B (SmB) phase to SmA phase and then to SmC∗ phase with an increasing content of the chiral crosslinker. The Tgs first increased, then decreased and finally increased again with increasing content of the crosslinker, meanwhile the isotropization temperatures and mesophase temperature ranges decreased. The side chain LCEs containing a rigid mesogenic crosslinker and a nematic monomer, prepared by the H-silylation reaction and containing  5 MPa and Eb = 500%) (Kurian et al. 2003; Kennedy et al. 2004). Its additional reinforcement was achieved with Q8MH8 (Isayeva and Kennedy 2004). The crosslinking of DH4 with divinylbenzene gave hybrid coating materials with very good termal and mechanical properties (Pinho et al. 2004). Phenylsilsesquioxane resin containing vinyl groups were crosslinked with H-siloxane oligomers: linear MH2D and star Si(OSiMe2H)4 (Li et al. 2001). New polysiloxanes crosslinked with photoinitiators were prepared by the hydrosilylation reactions of PMHS and PDMS-co-PMHS of the statistical structures (MDxDH100-xM, x  =  0 or 50) with 6,6,6-tris-(trimethylsilyl)-hex-1-ene and allyl-­ glicydyl ether, which had high thermal stability (Kowalewska and Stańczyk 2003). PMHS and PDMS-b-PMHS of random structures (MDxDH100-xM, x = 30, 50, 85) or block structures (M(D2DH)7D2M and M(D2DH2)11D2M)) were modified by hydro­ silylation with vinyl-carbosilanes having sterically hindred groups (CH2  = CHRC(SiMe2R’)3, R = -CH2, −(CH2)3 or -SiMe2; R’ = −Me or -Ph). The thermal resistance of polycarbosiloxanes obtained increased due to the decrease of flexibility of the siloxane chains and a free energy change (Kowalewska et al. 2009). A new material ((C5H4Sc)8Si8O12) was obtained by Sun et al. (2007) from reaction of TH8 with a scandium-cyclopentadienyl derivative, which is very useful for hydrogen storage. Methacrylate and epoxy-functionalized silsesquioxane nanocomposites based on Q8MH8 cores were used as dental materials (Soh et al. 2007). Self-hydrosilylation of Si-H and Si-Vi functional macrocyclic siloxanes: 12- or 24-membered (dimethylsiloxy)vinylsilsesquioxanes with the formula: (MHViSiO)n [n = 6 (A), 12 (B)] (Scheme 14.12) gave highly porous crosslinked poly(silsesquio xanecarbosilanes) (Han and Zheng 2008). The inorganic-organic hybrid glasses synthesized by the hydrosilylation of DVi4 with (MeHSiO)4 and catalyzed by Pt were thermally stable up to 400 °C (Michalczyk et al. 1993). The preceramic polycarbosiloxanes were also prepared by the hydrosilylation of DVi4 with linear dihydrosiloxanes (MH2Dn, n = 0–2). Their pyrolysis at 1000 °C gave 86–89% ceramic yields (Nyczyk et al. 2011).

14.6.5  M  odification of the Properties of Polyolefins and Polydienes by the Hydrosilylation Method High MW copolymers were obtained from 1-(9-decenyl)-3,5,7,9,11,13,15-­heptaeth ylpentacyclo[9.5.1.13,9.15,15.17,13]octasilsesquioxane with ethylene or propylene in the presence of metallocene-alumoxane catalysts. They contained up to 25 wt.% (1.2 mol.%) of pendant silsesquioxane units and showed lower Tm values (by 18 K) and better thermostability in air atmosphere compared to poly(ethylene) (PE) (Tsuchida et  al. 1997). The hydrosilylation of end double bonds is an important

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Scheme 14.12.  Structures of 12- and 24-membered methylvinylhydrocyclosiloxanes. Adapted with permission from Han et al. (2008)

modification method of these thermoplastics, e.g. with a Si-H end-capped PDMS. The hydrosilylation reaction was catalyzed by the catalyst Pt2[(ViSiMe2)2O]3 in the melt phase (Malz and Tzoganakis 1998; Long et al. 2003, 2004). A positive effect of a PDMS-modified polyolefin was observed on the processing and surface properties of linear low-density polyethylene (LLDPE) (Zhu et al. 2007). The surface properties of hydrosilylated polyolefins were further modified by annealing under supercritical CO2 atmosphere conditions (Zhu and Tzoganakis 2008). Poly(propylene) has been modified in solution or in the melt with mono- or dihydrosiloxanes and a transition metal catalyst or by free radicals (Malz and Tzoganakis 1998; Long et al. 2003). Monohydrosilylation was achieved with a large excess of H-siloxane, using Pt2[(ViSiMe2)2O]3 as a catalyst and a peroxide as the co-­catalyst. Silylated PEs were also prepared (Lipponen et  al. 2007). Poly(butadiene) (PB), poly(isobutylene) and poly(isoprene) were also modified via hydrosilylation reactions (Chauhan and Balagam 2006; Wurm et al. 2008). Silylated PB were obtained by the hydrosilylation of PB with H-silanes and Pt and Rh catalysts or Pt-nanoclusters (Guo et al. 1990; Iraqi et al. 1992; Chauhan and Balagam 2006; Chauhan et al. 2008).

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Silane-modified unsaturated polymers, especially with functional groups, can find a potential application as adhesives, drug delivery agents and rubber materials (Marciniec et  al. 1992; Marciniec 2002, 2009). The properties of PB containing 93% of pendant vinyl groups were modified with MH2Dn of MW = 400 g/mol, in the presence of a catalyst RhCl(PPh3)3. With the increase of a mole ratio [Si-H]/ [CH2 = CH], the properties of modified polymers changed from a strong rubber to a viscous material, but in longer reaction times gelation was observed (Baum et al. 1998). A ‘living’ linear PB was terminated in reaction with chlorodimethylsilane (ClSiMe2H). Further addition reaction of the Si-H group to unsaturated PB moieties, with the use of the  catalyst Pt2[(ViSiMe2)2O]3, led  to the production of branched PB, which was then modified with monohydrosilanes (R3SiH) (Wurm et al. 2008). The PE-g-PDMS copolymers were obtained by catalytic hydrogenation of PB-g-PDMS. The latter copolymer was prepared by the hydrosilylation of PB with ω-HMe2Si-PDMS in solution catalyzed by cis-dichloro-bis(diethylsufide) Pt(II) salt (Ciolino et al. 2004). By crosslinking of non-conjugated α,ω-dienes: 1,5-hexadiene, 1,7-octadiene and 1,9-decadiene (or 1,5-hexadiene/1,9-decadiene mixtures) with DH4, Q8MH8 or Si(OSiMe2H)4, in the presence of the catalyst Pt2[(ViSiMe2)2O]3 at RT, organic-­ inorganic hybrid gels with particle sizes in the range of 0.5–5 nm were prepared, depending on the stoichiometry of the reagents (Naga et al. 2006, 2007). Similar hybrid gels with a homogeneous network structure (with 1.4 to 1.6 nm size of mesh) were prepared by photohydrosilylation of DH4 or Q8MH8 with 1,5-hexadiene and 1,9-decadiene, catalyzed by bis(acetylacetonato)platinum (Naga et al. 2009). The PB networks were crosslinked with p-bis(dimethylsilyl)benzene catalyzed by Pt(II) compounds (Aranguren and Macosko 1988). The hydrosilylation of α,ω-dienes, 1,5-hexadiene or 1,7-octadiene with MH2, carried out using Pt2[(ViSiMe2)2O]3 as a catalyst, led to the formation of poly(carbosilanesiloxane)s with a limited MW, due to an isomerization of end C-C double bonds to unreactive terminal 2-octenyl or 2-hexenyl end groups (Sargent and Weber 1999).

14.6.6  M  odification of Elastomers Properties with Linear PMHS PB or other poly(diene)s were crosslinked with PMHS using CPA or Pt2[(ViSiMe2)2O]3 as catalysts (Leibfried 1995). Crosslinked St-isoprene-siloxane copolymers were prepared by hydrosilylation of St-isoprene prepolymer with H-siloxanes. They were useful for the preparation of membranes for pervaporative removal of volatile organic compounds from water (Kerres and Strathmann 1993; Kujawski et al. 2003). Their properties were much better than properties of the commercial hydrophobic membranes. Diblock PSt-PB or triblock PSt-PB-PSt copolymers were functionalized via hydrosilylation with methyldiundec-10-enylsilane (AB2 monomer) (Marcos et  al. 2006). Chain-end and in-chain cyano-functionalized PSts were obtained by

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the  hydrosilylation of Si-H functionalized PSt  with allyl cyanide, as well as trialkoxysilyl-­functionalized polymers, using Pt2[(ViSiMe2)2O]3 as a catalyst (Quirk et  al. 2008). Butadiene-St rubber was crosslinked with 1,1,5,5-tetramethyl-3,3-­ diphenyltrisiloxane using also CPA as a catalyst. The vulcanizates obtained showed greater elasticity, good dynamic properties and increased resistance (Doncov 1978). Olefinic bond-end poly(styrene-b-isobutylene) was grafted onto DH6, by hydrosilylation with Pt catalyst at 150–180 °C. Multiarm (5 or 18) star copolymers were obained, depending on the stoichiometry Si-H/C=C (1.1–1.2 or 2–4, respectively) (Shim et al. 1998). The well-defined octa-PIB-arm radial star polymers with polysiloxane cores (octa(PIB-dimethylsiloxy)octasilsesquioxane), composed of eight PIB arms bound to Q8MH8,  were prepared from allyl-end PIB by means of hydrosilylation reaction with Q8MH8 in the presence of the catalyst Pt2[(ViSiMe2)2O]3 at RT (Majoros et al. 1997). By the  hydrosilylation of polydienes with PDMS-co-PMHS or MH2Dn using [Pt(R2S)2Cl2] as a  catalyst were prepared thermoplastic elastomers based on 3D clusters through hydrogen bonding (Schadebrodt et al. 1999). PMHS with different linear statistical structures were applied for the modification of properties of chosen elastomers having carbon-chains: 1 . Hydrogenated butadiene-St rubber (HNBR) (Bik et al. 2003), 2. Butadiene-acrylonitryl rubber (NBR) (Chruściel et al. 2006), 3. St-butadiene rubber (SBR) (Bieliński et al. 2007a, b). The chemical modification of organic elastomers with PMHS occurred through many radical reactions. A profitable effect of the PMHS on some properties of modified elastomers was observed, e.g. an improvement of their tribological properties and flame resistance (Bik et al., 2003; Chruściel et al. 2006; Bieliński et al. 2007a, b). HNBR was modified with Me3SiO(Me2SiO)22(MeH SiO)10SiMe3 (Mw = 8,220 g/ mol, Mn = 3,620 g/mol, PDI = 2.38). A qualitative addition of PMHS to HNBR was made by Bik et  al. (2003). The improvement of the TS, a slight increase of the Eb and rigidity for vulcanized HNBR-­polysiloxane elastomer were observed. The improvement of tribological properties was also noticed: a decrease of the friction coefficient between crosslinked HNBR and metal surface was found. By modifying HNBR with 12 wt.% of PMHS, the friction coefficient of the crosslinked vulcanizate using dicumyl peroxide (DCP) did not depend on a force of the normal loading (Bik et al. 2003). For the modification of surface properties of NBR were used the following PDMS-co-PMHS copolymers containing Si-H or Si-CH=CH2 as functional groups:

Me 3 SiO ( Me 2 SiO )x ( MeHSiO )y OSiMe 3 ,



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( A ) x = 66, y = 10; M w = 12200, M n = 4260; ( B ) x = 22, y = 10; M w = 9100, M n = 2750; ( C ) α , ω − divinyl ( polydimethylsiloxane ) : ( CH 2 = CH )( Me 2 SiO )x SiMe 2 ( CH = CH 2 ) ,

where



where = : x 45 = , M w 9640, M n = 3, 280.



For NBR-polysiloxane vulcanizates were observed (Chruściel et al. 2006): • Decreased decomposition rate and increased residue after decomposition, • Decrease of flammability (increase of LOI from 28.5% up to 36.5%):

thermal

1. As a result of the formation of insulation silica layer on the surface (during burning), thus increasing thermal resistance, 2. Due to elimination of liquid products of thermal decomposition, 3. As a result of the surface modification (by swelling with silicones in the presence of AIBN), giving the possibility for preparing self-extinguishing samples, • Increased resistance aging, as a result of the migration of polysiloxane molecules towards the surface and the formation of a protective layer, • Decrease of Tg. Properties of SBR were modified with PDMS-co-PMHS:

(

)

(

)



M 2 D22 DH10  DH  : [ D ] = 0.45; M n = 2750; M w = 9100;PDI = 3.31 ,



M 2 D66 DH10  DH  : [ D ] = 0.15; M n = 4260; M w = 12200;PDI = 2.86 ,



M 2 D110 DH10  DH  : [ D ] = 0.09; M n = 4360; M w = 16900;PDI = 3.88 and



PDMS oil M 2 D n ( M n = 5000 ) .

(

)



The SBR vulcanizates modified with above silicones showed an increase of rate and yield of crosslinking in the presence of DCP with increasing reactivity of the Si-H groups of used PMHS (i.e. with increasing content of Si-H groups in PMHS) and the increase of crosslinking density with increasing yield of the crosslinking reaction using DCP. The crosslinking was probably also accompanied by grafting reactions of PMHS onto elastomer, as a result of addition reactions of Si-H groups to double bonds of SBR. The highest rate of the crosslinking reaction showed sulfur vulcanizates modified with less reactive PMHS (M2D110DH10). The improvement of tribological properties, i.e. the increase of the friction coefficient was also observed on a macroscopic scale. The decrease of TS of the crosslinked elastomer was

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observed, especially for sulfur vulcanizates. PDMS and PMHS also showed tendency to migrate to  the surface of the elastomer (which was growing with the decrease in PMHS reactivity) and formed a protective layer on the surface, which caused an increase in aging resistance of the vulcanized SBR. The improvement of the dispersibility of silica (and DCP) into the SBR with the increase in the PMHS reactivity (the content of Si-H groups) took place (Bieliński et al. 2007a, b).

14.6.7  M  odification of Properties of Other Polymers with PMHS Transparent polysiloxane-polycarbonate copolymers (PMHS-PC) were prepared by cohydrosilylation of butenylphenylcarbonate with Me3SiO(MeHSiO)35SiMe3 and ViSi(OEt)3, followed by hydrolysis catalyzed by DBTDL at RT, and by a radical polymerization of poly(diethyleneglycol biscarbonate) with benzoil peroxide. These IPNs and analogous PMHS-PC semi-IPNs were useful as drug delivery systems (Boileau et al. 1998). Poly((4-vinylphenyl)dimethylsilane) of narrow MWDs (PDI = 1.08–1.19) was prepared in quantitative yields by means of the anionic living polymerization solution from (4-vinylphenyl)dimethylsilane 4-(CH2=CH) C6H4SiMe2H with oligo-(α-methylstyryl)Mt2 (Mt = Li, Na, K) at −78 °C. By the living polymerization with St a new block copolymer (poly(St-b-(4-vinylphenyl) dimethylsilane-b-St)) was obtained (Hirao et al. 1987). Biodegradable multiblock copolymers were prepared by the hydrosilylation of poly(α-hydroxy acid)s or PLA terminated with alkyl groups, with (α,ω-dihydro)PDMS (MH2Dn) of Mws Mn = 1700–9000 g/mol (Bachari et al. 1995). PMHS having perfluoroalkyl side chains and alkyl disulfide anchoring chains formed self-organized ultrathin layers on a surface (Sun et al. 1996). A ‘living’ PSt was functionalized with Si-H groups at end chains (by reaction with ClSiMe2H) or in-chain (by reaction with Cl2SiMeH). The Si-H end-capped PSt was reacted with allyl(N,N-bistrimethylsilyl)amine, allylamine or allyl alcohol, and using Pt2[(ViSiMe2)2O]3 or Bu4NPtCl6 as catalysts (Quirk et al. 2005, 2009; Berger-­ Nicoletti et al. 2007). The PSt functionalized with end or pendant Si-H groups, and PSt-b-PDMS were used for modification by the hydrosilylation with hexadiene or allyl- and vinyl-POSS derivatives (Chakraborty et al. 2008; Quirk et al. 2008; Zhang et al. 2009). Alkyd resins containing pendant unsaturated chains derived from a linseed oil were modified with α,ω-dihydro-PDMS in the presence of a catalyst RhCl(PPh3)3, which caused a decrease in Tg of modified resins by tens °C (Hou et al. 2000). A siloxane-modified epoxy resin (SG copolymer), with pendant epoxide rings, was prepared by the hydrosilylation reaction of PMHS with allyl glycidyl ether. The SG resin mixed with a commercial epoxy resin (DGEBA) was cured with dicyandiamide. The mobility and the thermostability of the cured resins were thus increased (Zhang et al. 2012b). Silicone resins having Si-H, Si-CH=CH2 and Si-OH groups were cured by the Pt-catalyzed hydrosilylation and the Al(acac)3-catalyzed polymerization.

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Table 14.4  Chosen properties of elastic PU-polysiloxane foams modified with PDMS, PMHS, PMHBS and carbofuntional polysiloxanes and FR Types of PUF Non-modified, without silicones and FR Modfied with ~5 wt.% of silicones without FR. Modfied with 40 wt.% of FR. Modfied with ~5 wt.% of silicones and 40 wt.% of FR.

Density (g/cm3) 0.05 0.05–0.10

LOI (%) 18.2 18.7–19.8

TS (MPa) 0.10 0.09

Eb (%) 61 48–51

0.08–0.12 0.08–0.17

28–31 33.6–39.1

0.06–0.10 0.09–0.13

25–34 39–51

Source: Chruściel et al. (2012)

Crosslinked optically clear hybrids can be applied for high-brightness LED packaging (Pavlyuchenko et al. 2009). New silicone hydrogels such as IPN with PVA were prepared by crosslinking of α,ω-divinylpoly(dimethyl-co-trifluoropropyl)siloxanes, and photopolymerization of vinyl acetate (in situ), followed by hydrolysis of acetate groups, thus showing good transparency (90–92%) and better water absorption (20–39%), which may be useful for the manufacture of soft contact lenses (Boileau et al. 1998). The properties of PU foams (PUF) have also been modified with silicones and flame retardants (FR). With this in mind, mixtures of commercial poly(dimethylsiloxane-­α,ω-diols) and commercial polyols (containing some water) and carbofuntional α,ω-bis-(γ-hydroxypropyl)-PDMS were crosslinked and foamed with PMDI and branched PMHS, or PMHBS of random structures. The elastic PU-siloxane foams prepared with the addition of 40  wt.% of FR exhibited selfextinguishing properties: similar TS compared to PU foams containing the same FR content, but without the addition of silicones (Table 14.4) (Chruściel et al. 2012).

14.6.8  Functionalization of Nanosilica with the Si-H Groups The TH8 has often been used as catalyst supports (Tour et al. 1990; Nadkarni and Fry 1993). H

H HSi(OEt)3

H2O

H

H Si

O

O

Si

O Si O H Si O O

O O Si H O Si O O

Si

Si

O

Pd, Ag, or Pt

H

catalysts on supports

H

Keeping this in view, the Si-H functionalized silica was used for the selective reduction of alkynes to Z-alkenes, in the presence of acetic acid and using tetrakis(triphenylphosphine)-palladium(0) as a catalyst (Kini et  al. 1994) and for

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deposition of metals (Ag, Pd) by reducing metal ions from solution (Fry and Nadkarni 1994; Reed-Mundell et al. 1995). Ketelson et al. (1995a; 1996a, b, 1998) and Brook et al. (1997) also modified a Stöber colloidal silica with HMe2Si(OEt) or HSi(OEt)3 (TES) in THF or acetone, thus introducing reactive TH or MH groups on the silica surface. The Si-H groups on the  silica surface (SiO2-TES) were stable within few years. The silica-supported Pt NPs (2–3.5 nm in size) were prepared by adding the catalyst Pt2[(ViSiMe2)2O]3 to SiO2-TES or alumina-TES, dispersed into THF.  The supported Pt NPs were catalically active in hydrosilylation reactions (Ketelson et al. 1995a, 1996a, b, c, 1998; Brook et al. 1997). The radical polymerization of MMA gave poly(methyl methacrylate) grafted onto silica surface, which was functionalized by the reaction of colloidal silica sols with H2C=HSi(OMe)3 or H2C=CMeCO2(CH2)3Si(OMe)3. The vinyl functionalized silica was reacted with HSiMe2-end silicones by the Pt-catalyzed hydrosilylation. These particles were sterically stabilized in hexane (Ketelson et al. 1995b). The synthesis of an ordered mesoporous silica with functional Si-H groups was achieved by hydrolytic condensation of TEOS and TES in the presence of a nonionic surfactant under acidic conditions. A very high content of Si-H groups was obtained (2.4 meq/g). The new functional micelle-templated silicates can be used for the preparation of mesoporous hybrid silica/metal nanocomposites (Mehdi and Mutin 2006). The silica gel functionalized with –CH2CH2SiMe2OSiMe2H groups was further modified with dendritic and comb PMVS, which led to the formation the branched vinyl-functionalized silica/polysiloxane hybrids useful for the immobilization of Pt and Rh catalysts, which were applied for the hydrosilylation reactions of end olefins (Rózga-Wijas et al. 2003; Michalska et al. 2004).

14.6.9  M  odification of Surface Properties of Other Inorganic Supports and Fillers Thin layers of a crosslinked PMHS were grafted onto the silica and silicon surfaces by the sol-gel method. The degree of crosslinking and elasticity of thin nano- and micro-coatings with hydrofobic properties was controlled by changing molar ratio of monomers: MDES and TES (DH:TH from 50:50 to 95:5) (Thami et al. 2007). The high reactivity of the Si-H bond allows the application of its reactions with hydroxyl groups of fillers for surface modification through a physical absorption on the surface of γ-Al2O3, followed by the chemisorption process (Cosgrove et  al. 1990; Tertykh et al. 2003). PMHS have also been used for hydrophobic modification of the surface properties of inorganic supports and fillers, e.g. alumina (Tertykh et al. 2003), silica or other metal oxides (Cosgrove et  al. 1990), metal hydroxides: Al(OH)3, Mg(OH)2 (Laine and Babonneau 1993), CaCO3, gypsum, glass and inorganic phosphate, which can be ingredients of extiguishing powder (Thomas 2001). In line with this, PMHS-modified Mg(OH)2 showed a good dispersibility into polyolefins (Cosgrove et al. 1990; Reihs et al. 1995).

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Fig. 14.1  PMHS grafted onto the surface of TiO2 pigment. Adapted with permission from Guo et al. (2018)

A zirconium catalyst ((n-BuCp)2ZrCl2) supported on PMHS-modified silica was used for the polymerization of ethylene. Its catalytic activity was three times higher than for the same catalyst supported on nonmodified silica (Greco et al. 2003). The surface of TiO2-based pigment was modified by grafting a PMHS coating via amine-catalyzed condensation reaction and using a polyanionic dispersant (Fig. 14. 1). This surface exhibited improved water-barrier properties and was hydrophobic even after drying. The PMHS-modified titania was formed by stable aqueous dispersions with a solid content greater than 75 wt.%. The surface-­modified pigments were characterized by solid-state 29Si and 13C MAS NMR and FTIR spectroscopy, TGA, elemental analyses, and ζ potential measurements (Guo et al. 2018). Porous St-DVB copolymer modified with Me3SiO and Si-H groups showed an increase of swelling resistance in benzene (Bolbukh et al. 2008). By cohydrolysis of PMHS with TEOS were prepared micro- and mesoporous hybrid materials with a very high hydrofobicity (Yang et al. 2006). In the hydrosilylation reactions of ladder spherosilicates from PHSSQ and poly(allylsilsesquioxane), many soluble products in polar organic solvents were prepared (Zhang et al. 2002). TH8 has also been used for the deposition of thin silica layers by chemical vapor deposition method (Nyman et al. 1993). A mesoporous silica was prepared (without using any template or surfactant) from TH8 precursor by the hydrolytic polycondensation, which was carried out in a THF solution using tetrabutylammonium hydroxide as a catalyst (Handke and Kowalewska 2011).

14.7  Summary A literature cited in this chapter does not cover all examples related to syntheses, and especially practical applications of linear, branched, cube and dendritic poly(methylhydrosiloxanes) (PMHS), described in the world literature. The Si-H functional polysilanes, polycarbosilanes and polysilazanes will be described in a separate publication, which is in progress.

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14.8  Conclusions The Si-H bond is much more reactive than the C-H bond. Thus, polysiloxanes containing Si-H functionality are a very important group of silicones. They are most frequently applied as crosslinking agents (not only for silicone elastomers and rubbers) and reducing agents, superhydrophobic coating materials and reagents useful for modifying different polymers and polymeric materials, often via hydrosilylation reactions. Other hydrosilyl functional organosilicon polymers and especially polysilanes, polycarbosilanes and polysilazanes also find many practical applications, e.g. as preceramic and photoresist materials. Acknowledgments  Author is very grateful to Management Staff of ŁUKASIEWICZ Research Network - Textile Research Institute in Łódź (Poland) for possibility to prepare the manuscript of this Chapter, a financial support, and encouraging, enthusiastic atmosphere at workplace. This Chapter is dedicated to a memory of my excellent teachers and outstanding, world-famous organosilicon chemists: Prof. Zygmunt Lasocki (1921-1993) who had worked at the Lodz University of Technology in Łódź (Poland)  and Prof. Adrian G.  Brook (1924-2013)  who had worked at the University of Toronto (Ontario, Canada). Conflicts of Interest  The author declares no conflict of interest.

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Correction to: Introduction to Reactive and Functional Polymers: A Note From the Editor Tomy J. Gutiérrez

Correction to: Chapter 1 in: T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_1 The format of the citations Gutiérrez et  al. 2018; Gutiérrez and Alvarez 2018; Gutiérrez et al. 2016b; Gutiérrez et al. 2015b; Gutiérrez et al. 2016a; Gutiérrez et al. 2015a; Gutiérrez et al. 2019; Merino et al. 2019a, Merino et al. 2019b; Merino et al. 2018b; Merino et al. 2018a; have been corrected to only include the year. The affiliation of T. J. Gutiérrez has been changed to: Thermoplastic Composite Materials (CoMP) Group, Faculty of Engineering, Institute of Research in Materials Science and Technology (INTEMA), National University of Mar del Plata (UNMdP) and National Scientific and Technical Research Council (CONICET), Colón 10850, Mar del Plata 7600, Buenos Aires, Argentina

The updated online version of this chapter can be found at https://doi.org/10.1007/978-3-030-43403-8_1

© Springer Nature Switzerland AG 2021 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8_15

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C2

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The below references have been added to the chapter references: Please include the following references: Gutiérrez, T.  J. (2021). In vitro and in  vivo digestibility from bionanocomposite edible films based on native pumpkin flour/plum flour. Food Hydrocolloids, 106272. https://doi.org/10.1016/j.foodhyd.2020.106272. Gutiérrez, T.  J., Mendieta, J.  R., & Ortega-Toro, R. (2021). In-depth study from gluten/PCL-based food packaging films obtained under reactive extrusion conditions using chrome octanoate as a potential food grade catalyst. Food Hydrocolloids, 106255. https://doi.org/10.1016/j.foodhyd.2020.106255. Tomadoni, B., Capello, C., Valencia, G. A., & Gutiérrez, T. J. (2020). Self-­assembled proteins for food applications: A review. Trends in Food Science & Technology, 101, 1–16. https://doi.org/10.1016/j.tifs.2020.04.015. Khosravi, A., Fereidoon, A., Khorasani, M. M., Naderi, G., Ganjali, M. R., Zarrintaj, P., Saeb, M. R. & Gutiérrez, T.  J. (2020). Soft and hard sections from cellulosereinforced poly(lactic acid)-based food packaging films: A critical review. Food Packaging and Shelf Life, 23, 100429. https://doi.org/10.1016/j.fpsl.2019.100429. Alizadeh, R., Zarrintaj, P., Kamrava, S.  K., Bagher, Z., Farhadi, M., Heidari, F., Komeili, A., Gutiérrez, T. J., & Saeb, M. R. (2019). Conductive hydrogels based on agarose/alginate/chitosan for neural disorder therapy. Carbohydrate Polymers, 224, 115161. https://doi.org/10.1016/j.carbpol.2019.115161. Valencia, G. A., Zare, E. N., Makvandi, P., & Gutiérrez, T. J. (2019). Self‐assembled carbohydrate polymers for food applications: A review. Comprehensive Reviews in Food Science and Food Safety, 18(6), 2009–2024. https://doi. org/10.1111/1541-4337.12499. Gutiérrez, T. J. (2019). Trends in polymers for agri-food applications: A note from the editor. In: Gutiérrez, T. (Ed.). Polymers for Agri-Food Applications. Springer, Cham. https://doi.org/10.1007/978-3-030-19416-1_1.

Index

A Acid catalysts, 217 Acrylic polyols, 176 Actuation, 248 Adenosine triphosphate (ATP), 80 Air-drying urethanes, 182 Alcohol soluble lignin (ASL)/starch films, 81 Alginate, 99 Aliphatic diisocyanates, 178 Alkali treatments composites, 22 hydrolytic treatments, 22, 23 lignin and hemicellulose, 22 mercerization, 22 NaOH solutions, 22 structure and reactivity, 22 Aminic polyols, 178 5-Aminosalicylic acid (5-ASA), 14 Amphiphilic polymers hydrophobic chains, 12 macromolecular compounds, 12 nanomaterials, 12 non-ionic polymers, 12 self-assembled, 12, 14 topological structures, 12, 13 Anatomic molds, 296 Antiflatulents, 227 Antimicrobial activity, 121 Antioxidant agents, 66 anti-UV agents and photosensitive materials, 75–77 asphalt binders, 77 biomaterials cellulose fibers, 78 DRG, 78 ethanolamine, 78

LNPs, 79 LCSN NPs, 80 nanoscale, 78 PCL/lignin-PCL, 78 PLA, 79 RBC exposure, 77 ROSs, 77 SESC NPs, 80 Trans-resveratrol, 78 conductive materials PANI, 80 POMA, 80 packaging materials ASL/starch films, 81 gelatin, 82 LNPs, 81 mechanisms, 81 microbial growth, 81 PLA-lignin, 81 thermal oxidation stabilizer, 82–83 Anti-UV agents/photosensitive materials photolabile substances, 77 plastics, 76 PP/lignin composite, 76 pyrethrins, 76 quercetin, 76 ROSs, 75 sunscreen agents, 75 TiO2, 75 UV-A and UV-B light, 75 Aromatic diisocyanates, 178 Artificial muscles, 261 Asphalt binders, 77 Aviation, 221 Atom transfer radical polymerization (ATRP), 28, 29, 75, 317

© Springer Nature Switzerland AG 2020 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume One, https://doi.org/10.1007/978-3-030-43403-8

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416 B Benzyltrimethylammonium chloride, 171 Biobased nanocomposite polymers configurations, 101 exfoliated, 101 fillers, 106 food packaging applications, 114 intercalated structure, 101 lipid and wax biomass, 100, 101 nano-fillers, 101 natural/inorganic-fillers, 102 polymer matrices, 106 polysaccharides biomass, 97–99 Processing techniques (see Processing techniques, biobased nanocomposites) properties (see Polymer nanocomposites properties) protein biomasses, 99–100 tactoid structure, 101 techniques, 106 Biobased polyurethane (BPU) alkaline environment, 141 industrialized foamed products, 149 lignocellulosic biomass, 149 liquefaction process, 147 organosolv biomass fractionation, 148, 149 tensile strength, 146 Biodegradable synthetic polymers bioactive delivery systems, 6 degradation mechanisms, 7 monomers in vivo, 6 PEG (see Poly(propylene glycol) (PEG)) physiological conditions, 7 polyester, 7 polymeric nanoparticles, 8 Biomedical applications antifouling coatings, PDMS, 264 consumer products, 263 gelatin-siloxane NPs, 266 hydrophobic behavior, 264 IARC, 264 IO NPs, 267 PDMS, 264, 265 physical methods, 264 polysiloxanes, 265 POSS, 266 saccharide-modified siloxanes, 265 silicone elastomers, 264 silicone hydrogel contact lenses, 266 siloxane surfactants, 265 sol-gel materials, 266 surfactants, 264 Bio-oils, see Biopolyols

Index Bioplastics, 96 Biopolyols fast pyrolysis oil, 145 preparation, 145, 146 liquefaction biomass, 146 catalysts, 146, 147 copolymerization and condensation reactions, 146 hydrothermal liquefaction processes, 146 isocyanate-polyol reactions, 147 NCO/OH ratio, 147 residence time, 146 substitutions, 147 temperature, 146 organosolv fractionation hemicellulose and lignin, 147, 148 one-shot foaming method, 148 petroleum-based polyols, 149 re-polymerization reactions, 148 residence time, 148 temperature, 148 Bisphenol A, 195 Bond dissociation enthalpy (BDE), 70, 71 1,4-Butane diol (BD), 159 C Carbon black migration, 53 Carbon nanotubes, 105, 263 Carboxylic acid-functionalized alkali lignin (AL-COOH), 202 Catalyst, 147 Cellulose-based nano-fillers, 105, 106 Cellulose-containing composite films agglomeration, 115 bamboo-derived cellulose nanofibers, 115 and characterization methods, 115–117 film-forming capacity, 115 HPMC, 118 mechanical strength, 115 microcrystalline cellulose, 115 nano-fillers, 117 SEM images, 118 WPI, 118 WVP and WVTR, 117, 118 Cellulose fibers, 78 Chitosan (Cs) fibers, 25 Chlorination, 55 Clay nanoparticle-containing composite films Ag NPs, 121 bacteriostatic effect, 121

417

Index glycerol, 119 nanocomposite transparency, 119 PET and EVOH, 119 properties, 119, 120 spidroin-hectorite, 119 Clays/silicate-based fillers halloysite, 103 hydrotalcite, 103 kaolinite, 102 Mt, 102 nanoclays, 102 phyllosilicate, 103 platelet, 102 talc, 104 zeolites, 104 Click’ reaction, 315–316 Coating material, 195 Coloring process, 296 Computerize tomography (CT), 296 Condensation polymerization acid catalysts, 217 alcoholysis, 215 catalyst, 216 chlorosilanes, 215 cyclic trimer (Me2SiO)3, 216 fluids, 218 linear and cyclic oligomers, 215 linear polymers, 216 Me2Si(OH)2, 215 Me3SiOK, 216 Me3SiOSiMe3, 216 methyltrichlorosilane, 217 oligomers, 214 PDMSs, 215 phenylchlorosilanes, 214 product mixture, 215 silanes, 215 silicone elastomers, 219 silicone fluids, 219 silicone gels, 219 silicone resins, 219 siloxane chains, crosslinking, 217, 218 structure, PDMS, 218 trimethylsilyl groups, 218 Conductive polymers, 31 Conversion technologies, ligocellulosic biomass biopolyol, 138, 139 fast pyrolysis, 138 liquefaction, 139–141 organic solvents, 141 organosolv fractionation processes, 141, 142 Coupling agents, 220

Crosslinking bulk properties, 23 functionalization agents β- and γ-cyclodextrins, 25 dimethyl phosphite, 25 enzymatic immobilization, 25 hydrophobic fibers, 25 mechanical resilience catalysts, 24 cationic agent, 24 formaldehyde scavengers, 24 mechanical resilience, 23 N-methylol reagents, 23, 24 polyamino carboxylic acid, 24 polycarboxylic acids, 24 treatments, 23 mechanical/chemical stability Ajisawas reagent, 25 Cs fibers, 25 fibrillation agents, 24 lyocell process, 24 monofunctional reactive dye, 25 poly(ethylene glycol) diglycidyl ether, 25 Crosslinking of siloxane polymers coating techniques, 313 DT-resins, 311 HTV, 312 hydrosilylation, 313 low surface tensions, 313 mechanical properties, 314 MQ-resins, 311 polymerization techniques, 309 radiation/UV-induced radical initiators, 312 radical coupling, 312 RTV-silicones, 311 self-healing silicone, 312 silicone paint, 313, 314 silicone putty, 310 temperature, 313 Curan 100 lignin (CU), 74 Cyclic trimer (Me2SiO)3, 216 Cyclohexanedimethanol (CHDM), 159 D Dialdehydes, 24 Dialkyldichlorosilane, 213 1,4-Diazabicyclo[2.2.2] octane (DABCO), 170 Diblock surfactants, 319 Dibutyl tin dilaurate (DBTDL), 181 4,4’-Dicyclohexylmethane diisocyanate (H12MDI), 178

Index

418 Dielectric silicone elastomers carboxyl modified polysiloxane, 250 chloropropyl, 251 copolymers, 249 cross-linking pathways, 252 hydrosilylation, 249 mechanical properties, 251 metal complexes, 251 modification, polysiloxanes, 250 PDMS, 250, 252, 253 Piers-Rubinsztajn reaction, 249 polar silicones, 249 ROP, 250 Diethylene glycol (DEG), 159 Diethylene triamine (DETA), 178 Diisocyanates aliphatic, 178 aromatic, 178 risks, 179 Dimethyldisilanol, 213 Dimethylolpropionic acid (DMPA), 181 Dimethylsulfoxide/N-methylimidazole-­ dissolved lignin (DL), 72 Diphenylmethane diisocyanate (MDI), 143, 178 2,2-Diphenyl-1-picrylhydrazyl radical (DPPH•), 67 Disilanol (Me2Si(OH)2), 215 Dorsal root ganglion (DRG), 78 Double metal cyanide (DMC), 174 E Electromechanical applications actuation, 248 cross-linked polysiloxanes, 247 dielectric elastomers (see Dielectric silicone elastomers) elastomeric film, 248 elastomers, 247 polar cross-linking centers, 253 scientific community, 248 Energy silicone, 220 Enzymatic hydrolysis lignin (EML), 73 Enzymatic hydrolysis pretreatment (EHP), 73 Enzymatic immobilization, 25 Epichlorohydrin (ECH), 195 Epoxy resin AL-COOH, 202 bamboo moso, 200 biobased raw materials, 196–198 biorefinary by-products, 201 bisphenol A, 195, 200 coating material, 195

DDM, 199 DETA, 199, 200 DHL, 200 ECH, 195 EFB, 202 enzymatic hydrolysis, 201 extracting lignin process, 199 industrial sector, 195 KL, 201 lignin, 196, 200 lignocellulosic biomass, 196 linin, 202, 203 liquefaction, 199 LOI, 200 LPCA, 201, 202 mechanical properties, 200 oils, 196 plant biomass, 196 pretreatment methods, 199 synthesis, 199 WEP, 202 Equilibrium water content (EWC), 266 Ethylene glycol (EG), 159 Ethylene vinyl alcohol (EVOH), 100 Ethylenediamine (EDA), 178 European Medicines Agency (EMA), 13 European Tyre & Rubber manufacturers’ association (ETRMA) statistics, 45 European Union (EU), 6 Extraoral prostheses, 294 Extrinsic coloration, 296 F Fiber polymers modification alkali treatments, 22–23 chemical processes, 22 composites, 22 crosslinking, 23–26 grafting, 26 polymer deposition, 30–32 ‘smart’ textiles, 22 textile science and technology, 21 Food and Drug Administration (FDA), 6 Food packaging biobased packaging materials, 96 bioplastics, 96 nanocomposites, 97 nano-fillers, 96 natural biomass, 97 natural materials, 96 NPs, 96 protective barrier, 96

Index recyclable plastic, 96 technology, 96 Free flow agent, 222 Freeze-drying, 109 Functional nanocomposite, 113 Functional siloxanes Azo-modified silicones, 272 biomedical applications (see Biomedical applications) chemical modification, 271 electromechanical applications (see Electromechanical applications) inorganic-organic hybrid materials, 271 LC materials (see Liquid crystalline (LC) materials) ligands (see Ligands) organosilicones, 272 PDMS, 236 photocross-linked polymer, 272 POSS, 271 post-functionalization (see Post-­ functionalization of silicones) printing, textile materials, 271 pyridyl-siloxanes, 272 ROP, 238–240 silane monomers, 237, 238 siloxane segment, 272 stain resistance, 271 superhydrophobic coatings, 271 surface-modified NPs, 271 surfactants, 261–263 G Gelatin, 82 Gelatin-siloxane NPs, 266 Gemini surfactants, 318 Grafting approaches, 26 definition, 26 polymerization (see Polymerization techniques, grafting) Ground tire rubber (GTR) applications, 52 cross-linked network, 52 functionalization/modification, 52 continuous methods, 59 grafting acrylic acid, 57, 58 chemical compounds, 57 free-radical initiators decomposition, 57 high energy radiation, 57 monomers, 57 photo-induced grafting, 57

419 hydrophilic groups, 57 oxygen pretreatment, 58 ozonization treatment, 57 polymer matrix, 59 reactive sintering (see Reactive sintering) GTR-based magnetorheological elastomers, 51 GTR functionalization/modification coupling agents, 55, 56 objective, 52 reclaiming/devulcanization, 52, 53 structure-properties relationships, 46 surface polarity improvement, 54–55 GTR surface polarity improvement acidic treatment, 54 chemical etching, 54 chlorination, 55 matrix-GTR compatibilization, 54 mesopores and macropores, 54 oxidizing agents, 55 H Halloysites, 103 Hard segments (HS), 173 Heterolytic fissions, 209 Hexamethyldisiloxane (HMDS), 264 1,6-Hexamethylene diisocyanate (HDI), 178 1,6-Hexane diol (HD), 159 HDPE/GTR mixtures, 55 High antioxidant activities chemical modification, 73 DL, 72 EHP, 73 EML, 73 enzymatic treatment, 72 extraction processes, 72 factors, 71 heterogeneity, 73 Miscanthus sinensis, 72 solubility, 73 High density polyethylene (HDPE), 54 High temperature vulcanization (HTV), 312 HTV silicones, 297 Human hepatic stellate cells (HHSteCs), 74 Human keratinocyte (HaCaT), 74 Hybrid LC materials, 260, 261 Hydridosilanes, 306 2-Hydroxyethyl acrylate (HEA), 176 2-Hydroxyethyl methacrylate (HEMA), 176, 183 3-Hydroxypropyl methacrylate (HPMA), 176 Hydrolytic treatments, 23 Hydrophilic polymers, 9

Index

420 Hydrophobic coatings, 320 materials, 304 silicones, 316 surfaces, 316, 320 Hydrosilylation, 241, 249 Hydroxyalkylamides, 171 I Induction oxidation temperature (OIT), 82 In-situ polymerization, 107 International Agency for Research on Cancer (IARC), 264 Intraoral prostheses, 294 Ionic crosslinking, 24 Iron oxide NPs (IO NPs), 267 Isocyanate alcohol, 179 amines, 179, 180 carbon atom, 179 compounds, 180 trimerization, 180 water reaction, 180 Isophorone diisocyanate (IPDI), 178 Isophthalic acid, 164, 165 K Kaolinite, 102 Kappa-carrageenan films, 118 Kefiran, 99 Kraft lignin (KL), 201 L Layer-by-layer (LbL) assembly, 108 LbL spinning techniques, 109 LC elastomers (LCEs), 254, 257–261 LED lighting technology, 221 Ligands amphiphilic silicones, 269 cobalt, 270 copper complexes, 270 functional siloxane, 267–269 MOFs, 267 porous crystalline structure, 267 siloxane chain/segment, 269 2D covalent MOF, 268 2D networks, 268 zinc, 270 Lignin antioxidant (see Antioxidant agents) applications, 67, 69 biocompatibility, 85

biopolymer, 67 high antioxidant activities, 71–73 inhomogeneity, 85 natural phenolic polymer, 66 poilymer integration, 83, 84 raw material, antioxidants production, 84 research, 66, 67 structure-activity relationship, 67, 84 structures, 68 toxicity and cell damage, 73–75 Lignin-based polycarboxylic acid (LPCA), 201 Lignin-capped Ag NPs (LCSN), 80 Lignin-carbohydrate complex (LCC), 67, 74 Lignin nanoparticles (LNPs), 79 Lignin-PCL, 78 Lignin-poly(ethylene glycol) methacrylate (PEGMA), 75, 76 LignoBoost Kraft lignin (KL), 73 Lignocellulosic biomass, 196 hydroxyl groups, 138 PUs, 141–144 Lignosulfonate (LS), 74 Ligocellulosic biomass conversion technology (see Conversion technologies, ligocellulosic biomass) Liquefaction, 139, 199 Liquid crystalline (LC) materials ATRP, 257 azomethine mesogens, 257, 258 cyclosiloxanes, 255 hybrid LC materials, 260, 261 mechanical properties, 254 mesogenic groups, 254, 255 organo-siloxane compounds, 254 PDLCs, 260, 261 polysiloxane-based liquid crystalline elastomers, 258–260 RT side-chain liquid crystalline polymer, 257 siloxane moieties, 254 siloxane segments, 254 Steglich esterification, 256 supramolecular polymers, 255 trisiloxane-containing polymers, 255 Low-density polyethylene (LDPE), 100 Low-intensity focused ultrasound (LIFU), 15 M Maleic anhydride (MA), 158 Manufacture of silicones condensation polymerization (see Condensation polymerization) diethyldiethoxysilane, 212

Index nucleophilic substitution, cholrosilanes, 213 potassium fluorosilicate, potassium, 212 SiCl4, 212 silanes, 212 Maxillofacial prosthodontics coloration, 296 congenital defects/acquired defects, 294 conventional fabrication process, 296 digital printing, 296 direct printing, 296 extraoral prostheses, 294 intraoral prostheses, 294 manufacturing process, 296 materials, 294 mechanical stability, 294 non-defect site, 296 personalized prosthesis, 294 rehabilitation of patients, 293 silica NPs, 294 silicone, 295 silicone elastomers, 294 3D surface data, 296 Melt processing, 107 Metal-organic frameworks (MOFs), 267–270 Methyltriacetoxysilane (MTAS), 311 Methylvinylcyclosiloxanes, 245 Milled wood lignin (MWL), 72 Molecular weight (Mw), 238, 239, 244, 249–252, 271 Monomers, UPs dicarboxylic acids/anhydrides, 160 glycols MA double bond, 159 TMP, 159 types, 159 reactive monomers, 161 Montmorillonite (Mt), 100 Multiwalled carbon nanotubes (MWCNTs), 223 N N,N’-Dimethyl-4,5-dihydroxyethylene urea (DMeDHEU), 23, 24 N,N’-Dimethylol urea (DMU), 23 Nanoclays, 102, 122 Nanocomposites, 223, 224 Nanoencapsulation, 6 Nanofibrillated cellulose (NFC), 118 Nano-fillers, 96, 122 Nanomedicine, 5, 6 Nanoparticles (NPs), 96, 294 Nanoreinforcement, see Processing techniques, biobased nanocomposites

421 Nanoscopic carriers, 5 Natural antioxidants, 66 Natural/inorganic-fillers carbon-based nanomaterials carbon nanotubes, 105 graphene, 105 clays/silicate-based fillers, 99–100 metallic nanostructures Ag NPs, 104 copper, 104 metal oxides, 104 polysaccharide based nanostructures, 105, 106 Natural rubber (NR), 83 Neopentyl glycol (NPG), 159 N‐tert‐butyl 2-benzothiazole sulfenamide (TBBS), 49 O Oligo(ethylene oxide)trisiloxanes, 317 Oligosiloxane surfactants, 317, 318 One-component reactive PUs acrylate, 183 air-drying urethanes, 182 blocked PU, 182 moisture curable, 182 UV curable, 182 WPU, 183 One-pot directed assembly, 108 Organofunctional group-containing silanes, 208 Organosilicones, 272 Organosolv fractionation (OL), 72, 141, 142 Oxygen limiting index (LOI), 200 P Pad-bake’ process, 24 PDMS-co-poly(ethylene glycol) (PDMS-co-­ PEG), 309 PEGylation definition, 10 non-covalent, 11 PEG conjugation, 11 pharmacokinetics, 11 Pentaerythritol triacrylate (PETA), 183 Personal care products, 220 Petroleum-based polymers, 122 Phantom nanoparticles, 311 Phenylchlorosilanes, 214 Phyllosilicate, 103 Piers-Rubinsztajn reaction, 238, 246, 315 Plant biomass, 196 Plasma-assisted chemical vapor deposition, 31

422 Plastic materials and articles, 121 PLGA-based delivery systems 5-ASA, 14 bioavailability, 15 LIFU, 15 micromolecular drugs delivery, 13 nanotechnological methodologies, 14 Polar cross-linking centers, 253 Poly(acrylamide) (PAM), 12 Polyaniline (PANI), 80 Polybutadiene, 177 Polycaprolactone (PCL), 6, 176 Polycondensation, 161, 237, 238 Poly(dimethyl-co-methylhydro)siloxane (PDMHS), 241, 244, 271 Poly(dimethylmethylvinyl)siloxane, 250 Polydimethylsiloxane (PDMS) functional siloxanes (see Functional siloxanes) silicones (see Silcones) siloxane (see Siloxane) structure, 208, 214 Poly(diphenylsiloxane) (PDPS), 211, 308 Polyester-based delivery systems, 8 Polyesterification carboxylic acid group, 166 Carother’s equation, 168 catalysts, 165 factors, 166 glycols/dicarboxylic acid, 165 hydroxyl groups, 166, 168 Patton’s constant, 168, 169 polyester formulation, 166 PU coatings, 167 reaction temperature, 165 steel reactors, 165 step-growth equilibrium reaction, 165 theoretical yield, 167 Polyether polyols, 174 Poly(ethylene glycol) (PEG), 12 Poly(ethyleneimine), 12 Polyethylene (PE), 82 Polyethylene oxide (PEO), 174 Polyethyleneterphtalate (PET), 175 Poly(glycidyl methacrylate)-co-PEGMA (PGMA-co-PEGMA), 78 Polyhedral oligomeric silsesquioxane (POSS), 266, 271 Poly(hydridomethylsiloxane) (PHMS), 309 Polyisocyanate/polyol ratio, 183 Poly(isobutylene) (PIB), 303 Poly(lactic acid) (PLA), 6, 79 Poly(lactide-co-glycolic acid) (PLGA) amphiphilic polymers, 12 lactic acid and glycolic acid, 13

Index modification and compliance, 13 PLA and PGA, 13 PLGA-based delivery systems, 13 PLGA-based nanosystems, 15 Poly(L-lactic acid) (PLLA)/PLA-lignin nanofibers, 79 Polymer deposition conductive, 31 dopamine self-polymerization, 31 in situ self-polymerization, 31 ionic polymers, 31 ‘layer-by-layer’ technique, 31 PEDT/poly(4-styrenesulfonate) mixture, 31 plasma-assisted chemical vapor, 31 polymeric treatment agents, 30 polysiloxanes, 31 surface modifications, 30 Polymer-dispersed LCs (PDLCs), 260, 261 Polymer nanocomposites properties barrier, 110, 111 functional, 113, 114 mechanical, 111 optical, 111, 112 surface, 112, 113 thermal, 112 Polymeric carriers, 13 Polymerization techniques, grafting anionic and cationic polymerizations, 26, 27 ATRP, 28, 29 NMP, 29 radical polymerization, 28 RAFT, 30 ROP, 27, 28 Poly(methacrylic acid), 30 Polymethylene polyphenyl isocyanate (PM200), 143 Poly(methyl methacrylate) (PMMA), 77 Poly(methylhydrosiloxanes) (PMHS), 244, 252 Poly(methylvinyl)siloxane (PMVS), 244 Poly(o-methoxyaniline) (POMA), 80 Polyols acrylic, 176 aminic, 178 hydroxyl group, 173 polybutadiene, 177 polyesters, 174 polyether, 174 polysiloxane, 177 Poly(phenylmethylsiloxane) (PPMS), 308 Poly(propylene glycol) (PEG) amphiphilic polymers, 9 bioactive compounds, 11

Index conjugates, 11, 12 enzymatic hydrolysis, 9 hydrolytic depolymerization, 8 hydrophilic polymers, 9 oxidative approach, 8 PEGylated drugs, 10, 11 self-assembled micellar polymers, 9 Polypropylene (PP), 76 Polysaccharides biomass alginate, 99 cellulose cell wall materials, 98 Gluconacetobacter species, 98 nanocellulose impairs, 98 nanostructures, 98 Cs, 98 kefiran, 99 starch barrier properties, 98 crystalline structure, 97 film forming capacity, 97 macromolecules, 97 plasticizers, 98 renewable, 98 TPS, 97 types, 97 Polysiloxane-based liquid crystalline elastomers, 258–260 Polysiloxane polyols, 177 Polysiloxanes alkyne-azide cycloaddition click chemistry, 245 chemical modification, 248 chlorobenzyl groups, 265 dielectric silicone elastomers (see Dielectric silicone elastomers) hydrophilic agents, 246 hydrosilylation, 241–243 PDMS, 236 ROP, 239 Si-H groups, 236 silane monomers, 237, 238 silicon atoms, 236 (see also Silicones) silicone fluids, 219 siloxane, 208 Si-vinyl, 236 thiol-ene click reaction, 244 Polytetramethylene ether glycol (PTMEG), 174 Poly(trimethylene carbonate) (PTMC), 6 Polyurethane (PU) adhesives, 47 application and uses, 183, 184 catalysts, 143 definition, 141

423 diisocyanates, 172 hydrogen bonds, 173 industrial applications, 138, 141 isocyanates, 179–180 parameters, 172 polyaddition reactions, 172 polyester polyol, 172 polyisocyanates, 143 polyols, 142 preparation categories, 143 production, 180–181 reactive PUs, 182–183 surfactants, 143 Poly(vinyl alcohol) (PVA), 6 Polyvinyl alcohol/Cs (PVA/Cs), 80 Poly(vinyl ether) (PVE), 12 Poly(vinylpyridine) (PVP), 12 Poly(vinylpyrrolidone), 12 Post-functionalization of silicones chemical modification reactions, 247 chloropropyl, 245, 246 cross-linking reactions, 246 dendrimers and dendrons, 246 hydrosilylation, 241–244 methylvinylcyclosiloxanes, 245 PDMHS, 244 PDMS, 244 PDMVS, 245 Piers-Rubinsztajn reaction, 246 PMVS, 244 polysiloxanes, 239, 241 thiol-ene click reaction, 243–245 versatile method, 239 Powder coating, 172 Pretreatment methods, 199 Processing techniques, biobased nanocomposite polymers freeze-drying, 109 high shear mixing, 109 in-situ polymerization, 107 melt processing, 107 micro-patterned, 109 roll mixing, 109 sol-gel process, 110 solution-based techniques, 108–109 sonication, 109 1,2-Propylene glycol (PG), 159 Propylene oxide, 174 Protein biomasses biobased composite film formulations, 99 NPs nanocellulose, 100 protein-based films, 100 whey proteins, 99 Zein films, 100

Index

424 PU dispersion (PUD), 181 PU foams biopolyols (see Biopolyols) crosslinked network structures, 144 foaming agents, 143 mechanical strength, 144 preparation, 138 systhesis method one-shot foaming, 144 preparation materials, 144 semi-prepolymer, 144 two-step foaming, 144 Pullulan, 99 Pyrethrins, 76 Pyridyl-siloxanes, 272 Pyrogenic silica, 221, 222 Pyrolysis oil, 145 Q Quercetin, 76 R Radical polymerization, 28 Reactive and functional polymers, 1 Reactive oxygen species (ROSs), 75 Reactive polyesters 1,4-cyclohexane dimethanol, 175 dicarboxylic acids, 175 glycols and dicarboxylic acids, 157 PCL, 176 polycondensation, 174 polyols, 175 ROP, 175 saturated (see Saturated polyesters) synthesis, 175 UPs (see Unsaturated polyesters (UPs)) Reactive sintering additives/binders, 46 composites, 49 compression molding, 46, 47 environmentally friendly process, 46 fillers, 51 GTR-based magnetorheological elastomers, 51 GTR/carbon nanotube mixtures, 51 isocyanate index, 47, 48 latex and polychloroprene, 47 morphology, 49 patented mixtures, 47 performance properties, 49–51 PU adhesives, 47 PU matrix, 47, 48

rubber wastes, 46 SMART project, 49 sulfur-based curing systems, 46 sulfur curing system, 49 Reclaiming/devulcanization, 52 carbon black migration, 53 curing additives, 53 research, 52 tensile properties, 53 three-dimensional crosslinked network, 52 Red blood cells (RBC), 77 Relative humidity (RH), 117 Reversible addition-fragmentation chain transfer polymerization (RAFT), 30 Ring opening polymerization (ROP), 27, 28, 174, 237–241, 244, 249, 250, 272 Room temperature vulcanizing (RTV) silicones, 297, 308, 309, 311 Rubber recycling, 44 Rubber wastes ETRMA statistics, 45 GTR, 46 resistant, biodegradation, 43 rubber recycling, 44, 45 tires, 44 S Saturated polyesters applications, 171, 172 carboxylic acid groups Benzyltrimethylammonium chloride, 171 crosslinked polymer, 170 hydroxyalkylamides, 171 TGIC, 170 trifunctional monomer, 170 curing agent, 169 hydroxyl/carboxylic acid groups, 163 hydroxyl functional groups aromatic isocyanates, 170 catalyst concentration, 170 methylolated melamine, 169, 170 polyisocyanate, 170 transetherification, 169 monomers curing agent, 163 dicarboxylic acids/anhydrides, 164, 165 glycols, 163, 164 polyesterification, 165 trifunctional glycol, 163 Scientific community, 248

Index Self-healing silicone, 312 Sequential proton loss electron transfer (SPLET) mechanism, 70 Silane monomers, 237, 238 Silane (SiH4), 211 Silica NPs, 294 Silicate, 224 Silicones antiflatulents, 227 aviation, 221 biocompatibility, 224, 225 chemical aging, 223 chemical structure, silicone rubber, 220 electronics, 221 energy silicone, 220 environmental effects, 227 epidemiology, 226 elastomers, 294 free flow agent, 222 hydrogel contact lenses, 266 hydrophilization, 316 manufacturing (see Manufacture of silicones) materials, 238, 273 medical field, 295 nanocomposites, 223, 224 PDMS, 227 personal care products, 220 pharmaceutical applications, 226 physical properties, 209–212 polysiloxanes, 208 printing, 296 putty, 310 recycling, 227 reinforcement, 222 Si-O-Si link, 208 sludge formation, 208 soils, 227 thermal aging resistance, 222 thermal isolation, 222 thickening and thixotropy, 221 weather resistance, 223 Silicone fluids, 208, 218, 219 Silicone rubbers (SRs), 219, 223, 224 Silicon nomenclature, 302, 303 Siloxane backbiting degradation, 308 biocompatibility, 305 carbon-based polymers, 303 commercial use, 301, 306 crosslinking (see Crosslinking of siloxane polymers) daily routines, 301 diethyldichlorosilane, 305

425 difunctional monomers, polymerization, 309 direct method, 306, 307 environment, 305 functional monomers, 306 Grignard reaction, 306 hydridosilanes, 306 hydrochloric acid, 307 hydrophobic materials, 304 inherently reactive materials, 320, 321 ketone, 305 melting/crystallization, 304 monofunctional silanolate initiator, 308 Mw, 307, 308 PDMS, 303, 304, 308, 309 PDMS-co-PEG, 309, 310 PHMS, 309 physical properties, PDMS, 305 PIB, 303, 304 polycondensation dichlorodialkylsilanes, 307, 308 silanediol, 307 polymerization techniques, 302 ROP, 308, 309 silane monomers, 306 silicon nomenclature, 302, 303 (see also Silicone) thermal stability, 304 vinylsilanes, 306 viscosities, 305 zero-calorie oil substitutes, 305 Siloxane chemistry ‘click’ reaction, 316 hydrophobic, 316 Piers-Rubinsztajn reaction, 315 PVOH, 317 side-chains/end-groups, 315 silicone macroinitiators, 317 small molecules, 315 Siloxane surfactants, 265 Simultaneous enzymatic saccharification and comminution (SESC), 80 Sludge formation, 208 Smart textiles, 22 Sodium hydroxide (NaOH), 174 Soft segments (SS), 173 Sol-gel materials, 266 Sol-gel process, 110 Solution-based techniques LbL assembly, 108 LbL spinning techniques, 109 one-pot directed assembly, 108 rapid exfoliation, 108 thermal degradation temperatures, 108

426 Solvent-borne PU synthesis prepolymer method DBTDL, 181 diisocyanate, 181 quasiprepolymer method NCO/OH ratio, 181 one-shot method, 181 Sonication, 109 Starch nanocrystals, 106 Steam explosion lignin (SE), 74 Stereolithographic/selective laser sintering, 295 Structure-activity relationship BDE, 70, 71 carboxylic and alcohol groups, 69 DPPH• free radicals, 67, 70 phenolic hydroxyl groups, 69 phenylpropane units, 70 radical scavenging mechanism, 67 SPLET mechanism, 70 structural characteristics, 68 Sugarcane bagasse lignin (BG), 74 Sunscreen agents, 75 Super Ball Show, 224 Superspreaders, 317 Supramolecular polymers, 255 Surface relief grating (SRG), 272 Surfactants agriculture, 262 amphiphilic siloxanes, 263 carbohydrate, 262 carbohydrate-containing cyclosiloxane, 263 carbon nanotubes, 263 characteristics, 262 diblock, 319 gemini, 318 hyaluronan, 319 hydrophobic, 262 Mw, 318 nano-aggregates, 261 oleophobic properties, 262 oligo(ethylene oxide)trisiloxanes, 317 oligosiloxane, 317, 318 organic moieties, 262 polysiloxane functionalization, 262 preparation, PU foams, 319 self-assembly, vesicles, 262 silicone-polyol copolymers, 319 structures, 317 surface active materials, 262, 263 tetrasiloxane Gemini imidazolium, 262 Sustainable Moulding of Articles from Recycled Tires (SMART) project, 49 Synthetic antioxidants, 66

Index T Terephthalic acid, 164 Tetrachlorosilane (SiCl4), 212 Tetrahydrofuran (THF), 174 2,2,6,6-Tetramethyl-1-piperidynyl-N-oxy (TEMPO), 29 Thermal oxidation stabilizer aminolysis, 82, 83 KL structures, 82 lignin/polymer mixtures, 82 NR, 83 PE/lignin mixtures, 82 Thermal stability, 304 Thermoplastic polymers, 296 Thermoplastic starch (TPS), 97 Thiol-ene click reaction, 243, 245 Thixotropy, 221 Three-dimensional (3D) printing additive manufacturing, 295 advantage, 295 anatomic molds, 296 automated manufacturing, 295 conventional industrial materials, 295 craniofacial reconstructions, 295 disadvantages, 295 elastomeric polymers, 296 extrudable silicone, 298 HTV silicones, 297 limitations, 295 maxillofacial, 295 medicine and dentistry, 295 mold making/duplication, 297 production material, 295 rapid prototyping, 295 RTV-silicone, 297 silicone, 296 silicone elastomers, 297 small and medium-scale applications, 295 SRs, 298 stereolithographic/selective laser sintering, 295 thermoplastic polymers, 296 waste material, 295 Tissue-engineered nerve transplantation, 78 Titanium dioxide (TiO2), 75 2,6-Toluene diisocyanates (TDI), 178 Toxicity and cell damage, lignin antioxidant, 74 carbohydrates, 75 cell cytotoxicity assay, 73 eye and skin irritation test, 74 fractions, 74 HHSteCs, 74 hMSCs, 74

Index LCC fractions, 74 MCF-7, 74 Transmission electron microscopy (TEM), 257 Trichloroisocyanuric acid (TCl), 55 Triethylamine (TEA), 181 Trifunctional glycol monomers, 163, 164 Triglycidyl isocyanurate (TGIC), 170 Trimethylol melamine (TMM), 23–24 Trimethylol propane (TMP), 164 Two-component reactive PUs polyisocyanate, 183 polyols, 183 prepolymer, 183 production, 183 Two-dimensional (2D) networks, 268 U Ultrafiltration, 73 Unsaturated polyesters (UPs) anhydrides/dicarboxylic acids, 158 applications, 162 double bond, 158 monomers, 159 peroxides, 162

427 polycondensation, 158 production, 161 properties, 158 radicals, 162 tertiary amine compounds, 162 V Vinylsilanes, 306 W Water vapor permeability (WVP), 99, 106, 117, 118 Water vapor transmission rate (WVTR), 101 Waterborne epoxy (WEP), 202 Waterborne PU (WPU) synthesis PUD, 181 TEA, 181 Whey proteins, 99 X Xylylene diisocyanate (XDI), 178