Reactive and Functional Polymers, Volume Three: Advanced Materials 3030504565, 9783030504564

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Tomy J. Gutiérrez  Editor

Reactive and Functional Polymers Volume Three Advanced materials

Reactive and Functional Polymers Volume Three

Tomy J. Gutiérrez Editor

Reactive and Functional Polymers Volume Three Advanced Materials

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 978-3-030-50456-4    ISBN 978-3-030-50457-1 (eBook) https://doi.org/10.1007/978-3-030-50457-1 © Springer Nature Switzerland AG 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 the 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 this 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, PhD Editor

Preface

Reactive and functional polymers are essentially linked to the chemistry of the 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 are indisputable. Volume 3 of this book has been focused on advanced polymeric materials such as active, intelligent and multiple-response polymers made from reactive and functional polymers. I appreciate the valuable contribution of each of the contributing authors for this book from 11 different countries. 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 Advanced Materials Made From Reactive and Functional Polymers: Editor’s Insights��������������������������������������������������������������������    1 Tomy J. Gutiérrez 2 Active Packaging Films Based on Polyolefins Modified by Organic and Inorganic Nanoparticles����������������������������������������������    5 Yanela N. Alonso, Ana L. Grafia, Luciana A. Castillo, and Silvia E. Barbosa 3 Smart and Shape Memory Polymers ����������������������������������������������������   29 Zijian Gao and Guanghui Gao 4 Carbon Nanoparticle-Loaded Shape Memory Polyurethanes: Design and Functionalization�����������������������������������������������������������������   55 Ayesha Kausar 5 Plastic Receptors Developed by Imprinting Technology as Smart Polymers Imitating Natural Behavior������������������������������������   69 Alberto Gómez-Caballero, Nora Unceta, M. Aránzazu Goicolea, and Ramón J. Barrio 6 Circularly Polarized Luminescent Polymers: Emerging Materials for Photophysical Applications����������������������������������������������  117 Puhup Puneet, Michiya Fujiki, and Bhanu Nandan 7 3D Printing-Processed Polymers for Dental Applications��������������������  141 Corina M. Cristache and Eugenia E. Totu 8 Flame Retardancy of Reactive and Functional Polymers��������������������  165 H. Vahabi, E. Movahedifar, and M. R. Saeb Index������������������������������������������������������������������������������������������������������������������  197

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About the Editor

Tomy  J.  Gutiérrez  has a degree in chemistry (Geochemical option) from the Central University of Venezuela (UCV) (December, 2007), a degree in education (Chemical mention) from the same university (UCV, July, 2008), has a specialization in International Negotiation of Hydrocarbons from the National Polytechnic Experimental University of the National Armed Force (UNEFA), Venezuela (July, 2011), as well has a Master’s and PhD degree in Food Science and Technology obtained in October 2013 and April 2015, respectively, both from the UCV. He also has a PhD studies in Metallurgy and Materials Science from the UCV and postdoctoral studies at the Research Institute in Materials Science and Technology (INTEMA). Dr. Gutiérrez has been a professor–researcher at the UCV in both the Institute of Food Science and Technology (ICTA) and the School of Pharmacy. He is currently an adjunct researcher in the INTEMA – National Scientific and Technical Research Council (CONICET), Argentina. Dr. Gutiérrez is author of 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, Advances in Polymer Technology 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, 2018 Impact Factor 1.341). Dr. Gutiérrez today 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 in international projects between Argentina and Brazil, Colombia, France, Poland, Spain, Italy, Sweden and Venezuela.

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Chapter 1

Advanced Materials Made From Reactive and Functional Polymers: Editor’s Insights Tomy J. Gutiérrez

Abstract  Reactive and functional polymers have allowed the development of advanced materials such as active and intelligent polymers, electroactive polymers, multi-response polymers, shape memory polymers, stimuli responsive polymers, etc. for different applications. With this chapter, we open these main topics, which will be analyzed in this volume. Keywords  Active and intelligent polymers · Electroactive polymers · Multi-­ response polymers · Shape memory polymers · Stimuli responsive polymers

1.1  Editor’s Insights The future of new advanced materials will go through the development of reactive and functional polymers with the aim of obtaining polymers with outstanding and extravagant properties to be used in different domestic, industrial and extraordinary applications. With this in mind, these aspects will be analyzed exhaustively in this book, considering several fields of application (Alizadeh et al. 2019; 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, 2017, 2018a, b, 2019, 2021; Herniou--Julien et al. 2019; Khosravi et al. 2020; Merino et al. 2018a, b, 2019a, b; Tomadoni et al. 2020; Toro-Márquez et al. 2018; Valencia et al. 2019; Zarrintaj et al. 2019).

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, Mar del Plata, Buenos Aires, Argentina e-mail: [email protected] © Springer Nature Switzerland AG 2021 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume Three, https://doi.org/10.1007/978-3-030-50457-1_1

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Acknowledgements  The author would like to thank the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) (Postdoctoral fellowship internal 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). 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. (2017b). 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. (2018a). Active and intelligent films made from starchy sources/blackberry pulp. Journal of 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). 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. (2017c). 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. 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.

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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., Morales, N. J., Pérez, E., Tapia, M. S., & Famá, L. (2015a). 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., Tapia, M. S., Pérez, E., & Famá, L. (2015b). 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., Guzmán, R., Medina Jaramillo, C., & Famá, L. (2016a). 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., Suniaga, J., Monsalve, A., & García, N.  L. (2016b). 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., Guarás, M. P., & Alvarez, V. A. (2017). Chapter 9. Reactive extrusion for the production of starch-based biopackaging. In M. A. Masuelli (Ed.), Biopackaging (pp. 287–315). Miami, EE.UU. ISBN: 978-1-4987-4968-8: CRC Press Taylor & Francis Group. https://doi. org/10.1201/9781315152349-9. Gutiérrez, T.  J., Herniou-Julien, C., Álvarez, K., & Alvarez, V.  A. (2018a). 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., Ollier, R., & Alvarez, V.  A. (2018b). Chapter 5. Surface properties of thermoplastic starch materials reinforced with natural fillers. In V. K. Thakur & M. K. Thakur (Eds.), Functional biopolymers (pp. 131–158). Springer International Publishing. EE.UU. ISBN: 978-­ 3-­319-66416-3. eISBN: 978-3-319-66417-0. https://doi.org/10.1007/978-3-319-66417-0_5. 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., Gutiérrez, T. J., Mansilla, A. Y., Casalongué, C. A., & Alvarez, V. A. (2018a). 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.

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Merino, D., Mansilla, A.  Y., Gutiérrez, T.  J., Casalongué, C.  A., & Alvarez, V.  A. (2018b). 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. Merino, D., Gutiérrez, T. J., & Alvarez, V. A. (2019a). Potential agricultural mulch films based on native and phosphorylated corn starch with and without surface functionalization with chitosan. Journal of Polymers and the Environment, 27(1), 97–105. https://doi.org/10.1007/ s10924-018-1325-1. Merino, D., Gutiérrez, T. J., & Alvarez, V. A. (2019b). 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. 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. 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. 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

Active Packaging Films Based on Polyolefins Modified by Organic and Inorganic Nanoparticles Yanela N. Alonso, Ana L. Grafia, Luciana A. Castillo, and Silvia E. Barbosa

Abstract  Nowadays, the use of polymer films for flexible packaging has gained a widespread importance because of their easy processing, good final properties, light weight and low relative cost. In order to fulfill the needs of increasingly demanding consumers with respect to the quality of packaged products, additional capabilities must be incorporated into packaging. In this sense, academic and industrial efforts have focused on new technologies that provide a complementary functionality to the packaging performance. These emerging developments involve active and intelligent packaging, which can attract to consumers, improve product quality and/or balance any detrimental effect. In this context, the use of nanoparticle (NP)-modified polyolefins, either in bulk (nanocomposites) or on the surface, allows the inclusion of specific functionalities. These new capabilities enable obtaining active packaging according to the requirements of the product. The aim of this chapter was to analyze the aforementioned approaches for the development of active films by incorporating antibacterial, antifungal and/or repellent functionalities. Currently, several sustainable developments of this type of active films are based on commodity thermoplastics such as poly(ethylene) and poly(propylene). These materials, modified by the incorporation of organic and inorganic NPs, are promising candidates, since their final properties can be tailored for packaging application. Keywords  Active films · Antimicrobial activity · Food packaging Nanocomposites · Polyolefin surface modification

Y. N. Alonso · A. L. Grafia · L. A. Castillo · S. E. Barbosa (*) Departamento de Ingeniería Química, Universidad Nacional del Sur, Bahía Blanca, Argentina PLAPIQUI (UNS-CONICET), Bahía Blanca, Argentina e-mail: [email protected] © Springer Nature Switzerland AG 2021 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume Three, https://doi.org/10.1007/978-3-030-50457-1_2

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2.1  Introduction Packaging performance is a key factor throughout the food products supply chain, as they must guarantee the protection and preservation of food at different stages from the producer to the final consumer. Therefore, packaging has an important function during food production, transportation, storage, sales and end use, thus allowing a safe delivery of food until consumption. In this sense, the packaging must fulfill several main functions, such as protection (to avoid leakage or breaking and protect the product against microorganism’s attack), containment (to facilitate transport and manipulation), communication (to give information about food product) and convenience (to be useful to the consumer) (Espinosa et  al. 2017). The packaging must provide a container for the products which allows adequate transport along the whole supply chain, reducing any physical damage and protecting against harmful environmental factors, such as dust, light, moisture, oxygen (O2), pests, volatiles, chemical and microbiological contamination, as well as wear due to handling. In addition, packaging must fulfill secondary functions such as selling and sales promotion, which have a notable influence on market profits (Sharma and Ghoshal 2018). Many authors have emphasized the priority role of packaging from a marketing point of view. Packaging can be considered as a principal communication vehicle between the consumer and the product, since 70% of purchase decisions are made at the point of sale (Poturak 2014; Gómez et al. 2015). In particular, the market sectors related to personal and household care, food and beverages, show a close relationship between consumers and packaging (Ampuero and Vila 2006; Spence and Velasco 2018). On the other hand, considering that the world population is constantly growing and their food needs are increasing, packaging becomes a key factor to facilitate and guarantee the protection and storage of food products, minimizing their waste and the use of resources. Thus, in order to extend the product shelf life, a proper selection of packaging materials should be considered, as well as the package design. The quality of the packaged product is derived from different attributes, such as flavor (smell and taste), nutritive values (vitamins A and D, minerals and dietary fibers), safety (absence of chemical residues and microbial contamination), texture (crispness, juiciness, tissue integrity, toughness and turgidity) and visual appearance (color, decay, defects and freshness). Typically, several kinds of food packaging materials have been used, such as glass, metals (aluminum, steel and tin plate foils and laminates), paper, paperboards and plastics. It is worth noting that plastics are the second most used in terms of weight, and the first ones considering units sold, with than 50% of products packaged with this kind of material. In this context, a third of the plastics produced worldwide in 2018 were used for packaging applications (Citi Research 2018). Usually, plastics packaging can be associated with different shapes and sizes, being classified as flexible and rigid. In 2018, the first group represented 33% of the global packaging market, in units sold, while the rigid packaging reached 19%. Several factors, such as light weight, low cost, and good mechanical, optical, and physical properties, promote the use of plastics for different applications. In addition, the ease of production and manufacturing make these

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materials a very attractive alternative. Another advantage is the intrinsic versatility of plastics to make tailored packaging. These materials have the capability to be thermoformed or molded, allowing the integration of packaging building, loading and closing in the same production line. However, a disadvantage related to the application of plastics in packaging, mainly for food, is its relatively low barrier property (light and gas permeability). Despite the wide variety of plastics, none of them is totally impermeable to water vapor and gases, simultaneously. Changes in people’s lifestyles have greatly influenced the production and consumption of packaging, thus fostering new challenges related to materials and design. In recent years, there has been a growing trend of consumers related to free preservative, fresh, natural, nutritious and safe foods, as well as minimally processed foods or without processing. In this way, in recent decades, a notable increase has been observed in the consumption of fresh and fresh-cut products, such as exotic fruit mixtures, fresh herbs, leafy vegetables (in consumer-size package) and sprouted seeds (Oliveira et al. 2015). It is important to note that these fresh products present the best possible quality at the harvest and, although it cannot be further improved, it can be conserved to a reasonable degree during postharvest. However, extending the shelf life of fresh products is a major challenge for food processors, since fruits and vegetables are biologically active for a considerable long time after harvest, as a consequence of metabolic activity (e.g. respiration) and external adverse factors (loss of water, physical injuries, presence of microbial flora and variable storage temperature). Therefore, the development of food packaging, allowing prolonging the shelf life of fresh products is required. In this context, the passive character of conventional packaging, acting only as a physical barrier between food and surroundings, must be modified in terms of food quality preservation. As a solution to this challenge, packaging functionality must be redefined and improved with respect to the passive packaging, in order to offer differential results in food preservation. Consequently, diverse technologies have emerged such as modified atmosphere packaging (MAP) (Oliveira et al. 2015; Zhang et al. 2015; Belay et al. 2016), active packaging (AP) (Lee et  al. 2015; Barska and Wyrwa 2017; Kumar et  al. 2018; Yildirim et al. 2018), and intelligent packaging (IP) (Biji et al. 2015; Ghaani et al. 2016; Poyatos-Racionero et al. 2018). MAP allows prolonging the shelf life of fresh and minimally processed foods by removing and/or modifying the head-space atmosphere surrounding the products. The role of AP is to improve the stability and/ or quality of the packaged products by releasing or retaining substances whose absence or presence determines the extension of shelf life, while IP involves systems that monitor the state of packaged foods, thus providing information about food quality during transport and storage. Among these alternatives, AP for food applications has received great interest because of high requirements of consumer convenience, environmental aspects, price, safety and shelf-life extension (Pacholi et  al. 2017). In this sense, the incorporation of chemical, microbiological and/or physical protection to the packaging is a real need (Gutiérrez et al. 2017). The aim of this chapter is to analyze different approaches for the development of active films by including antibacterial, antifungal and/or repellent capabilities. Most of the sustainable commercial developments of this type of film are based on com-

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modity thermoplastics, such as poly(ethylene) (PE) and poly(propylene) (PP). The modification of these materials by incorporating organic and inorganic nanoparticles (NPs) is a promising route to obtain AP, since their final properties can be tailored to specific product requirements.

2.2  AP Films In order to extend the product shelf-life, AP design must take into account several processes that occur in the packaged product: (1) physiological process which can be evidenced during respiration, such as in fresh fruits and vegetables, (2) chemical processes that can, for example, be detected in lipid oxidation, (3) physical processes which are present in staling of bread or dehydration, (4) microbiological processes as in the case of putrefaction by microorganisms and (5) infestation by insect attack. The purpose of AP is to preserve and protect the product against spoilage and microorganism attack. In this technology, an active component is incorporated into packaging films which, by interacting with internal or external factors, promote actions that extend the product shelf-life, and preserve its quality and safety. This component performs its activity by itself or by chemical reaction, resulting in a beneficial influence on the packaged food. Depending on the requirements of certain food products, a mixture of active components is included in the packaging materials. These components can release and/or absorb substances into/from the product or the surrounding. In this sense, AP technologies can be classified into absorbing (scavengers) and releasing (emitters) systems (Fig. 2.1). There are different active components that play a specific role in the protection and preservation of foodstuffs, such as antimicrobials, carbon dioxide (CO2) and O2 scavengers, ethanol emitters and ethylene and moisture absorbers (Bhardwaj et al. 2019).

2.3  Classification of AP AP systems can be divided into scavenging and emitters. Regarding the absorbing systems, their functions are to eliminate undesirable substances from the foodstuff or the head-space of the packaging such as CO2, O2, ethylene, moisture, odor, among others. On the other hand, the active character of the release systems consists in delivering specific compounds to the packaged foodstuff or into the atmosphere such as CO2, ethanol, ethylene, flavors, as well as antimicrobial and antioxidant compounds and preservatives. Figure  2.2 shows the active agents used for food packaging applications. When products require more than one active agent, combinations of compounds with different functionality are incorporated into the packaging system. It is important to mention that the selection of active agents depends mainly on the characteristics of the product. In this way, antioxidants are used in

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Fig. 2.1  Active food packaging

Fig. 2.2  Active agents used for food packaging

packaged lipid-rich foods (Ganiari et al. 2017) and ethylene scavengers in the case of fruits and vegetables (Álvarez-Hernández et al. 2018). Oxygen scavengers  The presence of O2 in the packaging is the main responsible for the oxidation of lipids and pigments. Furthermore, this gas negatively influences the quality and shelf-life of foodstuffs, since it promotes microbial growth (aerobic bacteria and molds), changes of color, sensorial attributes (rancidity) and texture, as well as nutritional loss (vitamins A, C and E). These alterations can lead to the for-

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mation of toxic aldehydes due to the degradation of polyunsaturated fatty acids, making the foodstuff unacceptable for human consumption (Fang et  al. 2017). Additionally, O2 has a significant effect on respiration and the rate of ethylene production of breathable foods, such as fruits and vegetables. It is important to remove the residual O2 inside the packaging when foodstuffs are very sensitive to this gas. Even though MAP technology allows excluding O2, a low O2 volumetric ­concentration (0.5–5%) remains in the headspace, which can increase along the supply chain for different reasons. In this sense, the causes can be associated with an incomplete removal during packaging process, low quality zip, O2 permeation through the packaging film or O2 released from foodstuff to reach equilibrium with the gas phase in the headspace (Ahmed et al. 2017). The mechanism involved in O2 scavenging is developed through chemical reactions. Iron-based scavengers are the most widely used and their activity is triggered by moisture, since iron can be oxidized to a stable ferric oxide trihydrate complex. In the case of cobalt, it acts as a catalyst for the oxidation of the polymer, while palladium catalyzes the oxidation of hydrogen into water. Other scavenging agents are ascorbic acid, gallic acid, photosensitive dyes and unsaturated fatty acids. Antioxidant releasers  Taking into account the detrimental effects on food quality due to high O2 concentrations inside packaging, the removal of this gas is crucial in terms of conserving the food shelf-life (Cichello 2015). In this context, the use of antioxidants prevents lipid oxidation caused by O2. These compounds can be applied directly on foodstuffs through different processing methods (dipping, mixing or spraying). However, this can alter food quality parameters, such as color or taste, and can affect consumer acceptance of the product. Furthermore, the antioxidant action ends once the active compounds are consumed in the reaction, degrading even more the food quality. Thus, the incorporation of antioxidant in the packaging film is an interesting alternative to improve the quality of oxidation susceptible foodstuffs. Despite the fact that synthetic antioxidants, such as butylated hydroxyanisole and butylated hydroxytoluene are the most widely used, there is a greater interest in incorporating natural antioxidants in food packaging. With this in mind, essential oils (EOs), plant extracts, polyphenols and tocopherols from herbs and spices are considered to develop antioxidant packaging materials (Alonso et  al. 2016; Vilela et al. 2018). Ethylene scavengers  Ethylene, is a phytohormone which is a growth stimulator, and accelerates the ripening and senescence of climacteric fruits and vegetables in postharvest. In addition, this gas contributes to the chlorophyll degradation and the softening of fruits. For this reason, the elimination of ethylene from food packaging through the incorporation of ethylene scavengers allows extending the food shelf-­ life. These active compounds can be included by dispersing them in material packaging or incorporating them in a layer of the flexible laminated packaging (Yildirim et al. 2018).

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Carbon dioxide emitters  The antimicrobial capacity of CO2 has been widely used in the food industry to prolong the food shelf-life. This gas molecule is soluble in the aqueous and fatty phases of foods, thus generating carbonic acid, which acidifies the food. In this way, CO2 is involved in complex mechanisms which inhibit the growth of many spoilage bacteria. It is worth mentioning that the antimicrobial effect of CO2 depends on the solubility rate and the dissolved amount of this gas in food products. In this sense, the activity of this gas is proportional to its partial ­pressure. For this reason, the CO2 concentration in headspace of packaging defines the antimicrobial efficiency to avoid the growth of spoilage and pathogenic bacteria, thus extending the shelf-life of fresh packaged foods. Antimicrobial releasers  These agents are one of the most investigated active compounds considering that microbial growth is the main responsible for food putrefaction (Álvarez et al. 2018). Some microorganisms that cause loss of food quality and safety are Aspergillus spp., Bacillus cereus, Candida spp., Escherichia coli O157:H7, Klebsiella spp., Lactobacillus spp., Listeria monocytogenes, Pseudomonas spp., Rhizopus spp., Salmonella spp., Staphylococcus aureus, Torulopsis spp., among others (Otoni et al. 2016; Ahmed et al. 2017). Consequently, antimicrobial compounds are one of the classes of active agents having the largest number of commercial products with reference to other packaging systems. These products comprise emitting sachets and absorbent pads containing active agents based on allyl isothiocyanate, chlorine dioxide, ethanol vapor emitting, glucose oxidase, natamycin, silver zeolite, silver (Ag), sulfur dioxide and triclosan (Fang et al. 2017). These compounds are mainly used for packaged products such as bread, cheese, dried fish, fruits, meat and vegetables (Kapetanakou and Skandamis 2016; Otoni et al. 2016; Haghighi-Manesh and Azizi 2017; Yildirim et al. 2018). Examples of these commercial AP are AgIon®, Bactiblock®, Biomaster®, Food-touch®, IonPure®, Irgaguard®, Sanic Films, SANICO®, Surfacine® and Wasaouro® (Realini and Marcos 2014; Otoni et al. 2016; Barska and Wyrwa 2017). However, sachet-based active systems are not well accepted by consumers, as they are considered as foreign devices within food packaging (Restuccia et al. 2010). The main reason of this disapproval is based on the risk of accidental breakage, leading to an involuntary consumption of sachet content. Furthermore, an additional operation is required to place sachets or pads in each packaging, which is done manually, thereby leading to a lower packaging rate and higher production cost (Otoni et al. 2016). Another disadvantage of these devices is that they are not suitable for liquid foods since their contact can lead to the spillage of content. Given these disadvantages related to active sachets or pads, there is a trend to incorporate natural antimicrobial compounds into packaging material to protect food from microorganism’s attack. Thus, much attention is paid from academic and industrial sector, resulting in extensive research in this field. In this chapter, special emphasis will be given to antimicrobial packaging films for food applications.

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2.4  AP Technologies The design of AP systems can be carried out following two methods (Fig. 2.3), by means of: (1) the inclusion of an independent device which contains the active component such as sachets and pads (Otoni et al. 2016) and (2) incorporating the active agent in packaging material or on its surface (Cerqueira et  al. 2016). The latter method is the most widely used since the independent devices are often considered as toxic because they contain inedible food contact components which are not well accepted by consumers. Thus, the preferred alternative is the incorporation of active agents in packaging materials to allow their interaction with the food. In this way, they can eliminate undesirable compounds that are responsible for spoilage, release substances that can improve the food quality or modify the atmosphere inside the packaging. The aforementioned aspects allow the promotion of adequate conditions that conserve freshness and safety in food. In addition, the incorporation of active agents into films is advantageous due to the partial amorphous character and the free volume (voids) presence in the polymer matrices. These characteristics contribute to transport phenomena through processes such as migration, permeation, scalping and sorption, which are essential for the development of active materials. Despite these advantages, some considerations should be considered during AP design. Keeping this in view, it is important to mention that the incorporation or the action of active component should not alter packaging performance along the supply chain. Furthermore, the agent should not lose its activity as a consequence of interactions or degradations during the manufacture of the film. The mechanism of action (absorption or release) of the compound must be controlled and preserved to be considered as AP. In addition, the inclusion of a triggering mechanism in the packaging system is relevant, since it avoids premature action and partial exhaustion

Fig. 2.3  AP systems

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before the presence of foodstuffs. There are several mechanisms to satisfy this function which are activated by humidity, radiation and temperature (Thomas et al. 2018). Considering the incorporation of active agents in the packaging material, there are two alternatives: into polymer film or on film surface. Depending on the type and design of packaging, as well as the foodstuff, there is a more appropriate option to incorporate active agents into the packaging material. With regard to the inclusion of the activity in bulk polymer film, material processing generally involves the combination of active agents with polymer, thus obtaining a concentrate which can later be used in the manufacture of packaging. Thus, this concentrate is fed to different processes to obtain flexible or rigid packaging via blown extrusion, casting, compression, injection molding, among others (Somade and Adegboye 2018). The proportion of concentrate used in these processes is established in terms of the required concentration of active agent in the final packaging material to protect the packaged food. The inclusion of activity in the bulk polymer film allows providing high efficiency as the active compound slowly migrates from the packaging material to the food. In this sense, the fact that active agent could be released over a long period allows to offer protection during transport, storage and shelf life of packaging. Another alternative to include active agents in the packaging system is to anchor them on the material surface (Romani et al. 2020). In this case, active component can act via direct contact with the packaged food (Lee et al. 2015). The incorporation of the active agent at the product contact side presents some benefits compared to its bulk incorporation into polymer film. This alternative allows to preserve the bulk properties of the packaging material and reduce the agent concentration. Various technologies related to the incorporation of activity on the surface of the packaging film have been reported. One of these techniques is the solution casting which involves the dispersion of an active agent in a polymer dissolution in a solvent medium which is poured on film surface (Bastarrachea et  al. 2011). This method, despite being widely used at the laboratory level, has some disadvantages because the polymers used in the packaging material are dissolved at high temperatures using organic solvents, which affects the stability and efficiency of the active compounds. Another method to incorporate active agents on the surface of packaging material is via immobilization. Due to the inert nature of the polymers, a surface modification is necessary to functionalize the material and promote the immobilization of active compounds. In this sense, surface activation can be carried out through physical (corona discharge, flame, plasma and ultraviolet (UV) radiation) and wet (use of corrosive liquids) methods (Bastarrachea et al. 2015). In general, physical methods are more widely used than wet methods on an industrial scale, since corrosive liquids are not required. The aim of these methods is to generate reactive groups on the film surface by anchoring active compounds via covalent or non-­ covalent immobilization (Mishra 2019). Another alternative to include active agents on the packaging surface is the photo-grafting. This technique allows to obtain active films due to the grafting of polymer chains onto the surface by exposure to UV light. The active agents can also be incorporated during this process or by subsequent immobilization after grafting.

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2.5  Active Film Packaging Based on Polyolefin and NP 2.5.1  A  ntimicrobial and Repellent Activity by Incorporating Organic Particles Most food preservatives, traditionally used in the food industry, are antimicrobial organic compounds marketed in powder form (Silva and Lidon 2016). There is a wide variety of these agents, including particles with intrinsic activity and those acting as antimicrobial agent carriers, mainly liquids. The first group includes organic compounds of synthetic origin such as acetic, benzoic, propionic and sorbic acids; antibiotics (e.g. triclosan); benzoates; benzoic, propionic and sorbic anhydrides; chelating agents (e.g. ethylenediaminetetraacetic acid); fungicides (e.g. benomyl and imazalil); parabens (e.g. ethyl paraben and propyl paraben); propionates; sorbates, among others. Meanwhile, naturally occurring antimicrobial agents, whether from vegetal or animal sources, are alginate and chitosan (Cs) NPs, antimicrobial peptides, encapsulated EOs, lauric acid, lysozyme, natamycin, nisin, among others (Davidson et al. 2013; Rai et al. 2016; Vishwakarma et al. 2016; Nakazato et al. 2017; Pisoschi et al. 2018; Zanetti et al. 2018). The second group includes materials of high absorption and retention capacity such as microcrystalline cellulose, wood flour, fibers, starch particles, among others (Khosravi et al. 2020). Some of them are conventionally used as excipients and can be employed as a support of active agents such as antimicrobial EOs and repellents, thus improving their active performance (Rani et al. 2013; Urbankova et al. 2015; Rozenblit et al. 2018). The incorporation of these active agents into the packaging system prolongs the food shelf-life even more than when they are added directly to it. As a result, this application form improves the agent’s efficiency and effectiveness (Malhotra et al. 2015). Polymer films containing traditional food preservatives have been used to manufacture AP. The main advantages of these active agents are their commercial availability, ease of handling, low cost, as well as being food grade (generally recognized as safe – GRAS), water soluble and effective against a broad spectrum of bacteria and fungi. However, the pH of foods becomes a limitation of the active efficiency. The antimicrobial mechanism of organic acids and their salts is based on the cell membrane penetration by the undissociated molecules. For this reason, they are suitable only for food with a specific acidity range (Espitia et al. 2016; Hauser et al. 2016). In this sense, different strategies have been studied for its incorporation as an active agent in polyolefin films. For example, Thanakkasaranee et al. (2018) proposed active sodium propionate/PP composite films obtained via extrusion for use as a bread packaging material. These authors indicated that all composite films containing 5% w/w sodium propionate showed improved antimicrobial, mechanical and thermal properties compared to pure PP films (Thanakkasaranee et al. 2018). The composite films achieved a reduction against E. coli and S. aureus by more than 90%. A similar study was developed by Fasihnia et al. (2018) via extrusion to obtain sorbic acid/PP composite films, which showed effective antimicrobial activity as food packaging against Aspergillus niger, E. coli and S. aureus. A significant

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r­eduction in the elongation at break, transparency and water vapor permeability values of the films were also reported by Fasihnia et al. (2018) by adding 6% w/w of sorbic acid. In addition, the UV absorption of composite films was increased. Kuplennik et al. (2015) also developed antimicrobial composite films from linear low-density polyethylene (LLDPE) and ethylene vinyl acetate (EVA) containing potassium sorbate and using glycerol monooleate as a dispersant for use as acidic food packaging material, obtaining improved materials with antifungal efficiency against Saccharomyces cerevisiae. Kamalipour et al. (2016) formulated antimicrobial composite films from maleic anhydride grafted-PE, medium density polyethylene (MDPE) and triclosan blends. These authors reported that antibacterial activity against E. coli, K. pneumoniae and S. aureus was improved because of the triclosan release rate increased due to decreased crystallinity degree and increased polarity. On the other hand, the AP including natural antimicrobial agents have had a growing interest at present, since consumers are focused on healthy, organic and safe foods, avoiding the content of synthetic compounds (Bondi et  al. 2017; Khaneghah et al. 2018; Pisoschi et al. 2018; Cottaz et al. 2019). However, special care is required during plastic processing, as most natural antimicrobials are thermolabile (Figueroa-López et al. 2019), and more expensive than traditional synthetics. For this reason, the use of natural antimicrobials effectively and efficiently is of great interest. The packaging success is a result of the balance between product requirements and costs with the use of the active system. It is thus important to consider that the relevance related to the use of antimicrobial systems in a fresh food compared to nonperishable foodstuffs. In this context, dairy derived foods (e.g. cheese) are very susceptible to spoilage by fungal microorganisms and their mycotoxins (Benkerroum 2016). Therefore, antifungal agents in these foods are usually incorporated into the food bulk or on its surface. In particular, one of the most widely used natural agents in the dairy industry is the polyene natamycin (Jalilzadeh et al. 2015). Direct incorporation of natamycin particles on PE surface and/or in situ generation of natamycin crystal are advantageous approaches to improve antifungal performance. These surface modifications make it possible to avoid excess active agent, and consequently, its undesirable effects on food. In line with this, antimicrobial natamycin particles can, for example, be physisorbed or chemisorbed (linked) on the film surface through adaptable, free adhesive, low-cost and versatile methods (e.g. spraying) for film production (Grafia et al. 2018). Grafia et al. (2018) sprayed natamycin from an ethanol solution and/or a n-heptane suspension to a semi-molten PE film surface. It is worth noting that the commercially used source of natamycin is typically composed of 50% w/w of pure natamycin and 50% w/w of lactose. Considering that natamycin is soluble in ethanol but not in n-heptane, while lactose is insoluble in both solvents. Thus, the mechanism of natamycin inclusion on PE depends on the solvent (Grafia et al. 2018). According to Grafia et al. (2018), during spraying of natamycin from an ethanol solution, the lactose particles impact the heat-softened film surface and act as nucleation points for the natamycin crystals. In contrast, when the n-heptane suspension is sprayed, the particles impact the softened film surface and remain attached. In both cases, the solvents are evaporated during the

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spraying process, and finally, when polymer solidifies, the partially immersed particles are immobilized on the film surface (Fig. 2.4), resulting in superficially modified films with antifungal properties against A. niger sp. Following Grafia et  al. (2018), the natamycin particles were well distributed and adhered to the surface of the films, thus obtaining a better uniformity of coverage when the solution was sprayed. This demonstrated that the antifungal particle spray methodology was effective, even using less than 1% w/w of natamycin, and without causing polymer or antifungal degradation. The main advantage of this methodology lies in the possibility of being extended to other antifungal natural agents. Other natural antimicrobials used as active agents in flexible polyolefin packaging for fresh foods are nisin and Cs. The former is the most widely used bacteriocin due to its commercial availability, heat stability and good performance against a broad spectrum of bacteria (Santos et  al. 2018). Different approaches have been implemented in order to include nisin in the package, including absorption solution on polymer, bulk polymer incorporation (composite material) and polyolefin coating. However, there is a significant risk of nisin inactivation when added during the manufacture of composite films. Despite this, successful results were obtained by Meira et al. (2014) for active PP composite films obtained by compression molding at 180 °C. Cs is a polysaccharide obtained from crustacean shells, which have attracted industrial and scientific attention due to its high biodegradability, inherent antimicrobial activity and non-toxicity (Castillo et al. 2017). In particular, this active polymer can also be used as a carrier for other active substances such as acetic acid, cinnamaldehyde, lauric acid, among others, into polyolefin matrices. Strategies for using Cs as an active agent in polyolefin packaging include both composite material (Reesha et al. 2015; Hooda et al. 2018) and direct grafting of Cs NP onto the film surface (Buslovich et al. 2017). The encapsulated antimicrobial natural EOs can be considered as a special case of organic particles commonly used in polymer films. An enclosing technique by

Fig. 2.4  Scheme of natamycin particles included on PE film surface

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applying a polymer coating allows to protect EOs due to its high volatility. The encapsulated product improves the EO stability, which favors its controlled release, thus they can be applied as an antimicrobial agent in AP. Although this methodology is gaining relevance, there are still few reported works with application in polyolefin films (Bile et al. 2016; Ribeiro-Santos et al. 2017; Zanetti et al. 2018). Another approach to immobilize active agents in polymer matrix is through their absorption or attachment in solid inert carriers. Also, the simultaneous incorporation of an active agent and an inert particle into a polymer matrix can improve the antimicrobial efficiency by agent-controlled release. There are different alternatives to incorporate or tailor the activity in packaging films by using carrier particles and active agents. Recent reports on these methodologies applied to polyolefin matrices have been analyzed in the following systems: cellulose/nisin (Lu et al. 2018), cellulose/titanium oxide (TiO2) (Shah and Pandey 2017), starch/sorbitol (Pirooz et al. 2018), as well as wood flour and molecular sieves with biologically active substances of EOs (Urbankova et al. 2015).

2.5.2  Biocidal Activity by Incorporating Inorganic Particles The use of inorganic particles as polyolefin reinforcements to develop composite and nanocomposite materials is widely known. These fillers improve the mechanical performance of polymeric materials, as well as modify their permeability and, depending on particle nature, may include functionalities or specific activity to the polymer. In this sense, the particles can be ‘active’ themselves or act as a carrier/ support of active agents. To prevent or reduce the growth of microorganisms in foods, antimicrobial packaging made from active films can be used. Many research papers related to this topic have recently been published, which have focused on polyolefin-based nanocomposites and inorganic particles. In this regard, Ag NPs have been widely used due to its high efficiency as a toxic agent for different microorganism strains. Its high antimicrobial effectivity can be associated with the large surface area of these particles, which allows a close interaction with microorganism cells (Carbone et al. 2016). The antibacterial activity is the result of various processes that can have an adverse effect on protein synthesis and DNA, or involve cell damage (Cavaliere et al. 2015). Ag NPs also have the ability to absorb and decompose ethylene, which has a positive impact on the food shelf-life, mainly on fresh vegetables and fruits (Bratovčić et al. 2015). On the other hand, another advantage of Ag NPs is the good thermal stability, which allows a wide range of polymer processing alternatives (Beigmohammadi et al. 2016). Similarly, TiO2 NPs are commonly used in different fields because of its self-disinfecting property. TiO2 induces the inactivation of pathogenic bacteria, promoting phospholipid peroxidation of microbial cell membranes (Bodaghi et al. 2013). Meanwhile, copper (Cu) NPs have biocidal properties, which present a wide range of action against bacteria and molds, adding a good cost-effective relationship (Kalatehjari et al. 2015; Xiong et al. 2015). Although this

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metal is necessary for the metabolism of aerobic microorganisms, a high Cu concentration can inhibit cell growth. The antimicrobial property of Cu NPs is related to the ability to donate and accept electrons, thus interacting with membrane cells, nucleic acids and enzymes, which induce the death of microorganisms (Lejon et al. 2010; Beigmohammadi et  al. 2016). It is important to highlight that the biocidal efficiency of Cu NPs is influenced by concentration, morphology, particle size, production method and type of microorganism (Hoseinnejad et al. 2018). Zinc oxide (ZnO) is an inorganic NP with antimicrobial activity and UV protective capacity used to develop packaging materials (Huang et al. 2018). These attributes are mainly determined by ZnO concentration and surface area, together with shape and crystalline structure of the particles (Sirelkhatim et al. 2015). Its biocidal activity can be associated to: (1) the generation of hydrogen peroxide (H2O2) from the surface of particles and (2) the positive electricity of the ion (Zn2+), which penetrates the cell microorganism, thus causing its death (Li et al. 2010). Figure 2.5 shows a schematic representation of different proposed biocidal mechanisms induced by inorganic NPs. Many authors have reported on the use of these particles as fillers for polyolefin matrices. The most commonly used polyolefins for packaging are PE and PP. Several factors influence the biocidal performance of films such as the characteristics of the active agent, the chemical nature of the polymer, the processing method and its conditions, among others. With this in mind, active nanocomposite films made from Ag NP loaded commercial polyolefins have demonstrated antimicrobial effect of against Pseudomonas aeruginosa and S. aureus (Dehnavi et al. 2013; Oliani et al. 2017). Similar results were also reported by Emamifar and Mohammadizadeh (2015) using active ZnO NP-loaded LDPE-based films with the aim of extending the shelf life of fresh products such as strawberries by reducing the microbial growth rate. The ZnO-loaded PP-based nanocomposite films have also shown relevant antibacterial activity against E. coli, thus proving to be suitable for active food packaging (Silvestre et  al. 2016). Non-toxic active nanocomposite polymers were also developed by Beigmohammadi et  al. (2016) from the addition of copper oxide (CuO) particles to the polymer matrix, resulting in a reduction in the load of total

Fig. 2.5  Schematic representation of the biocide mechanisms of inorganic particles

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coliforms in cheese packaged with this material. The active TiO2 NP-loaded LLDPE-based films can also inactivate Pseudomonas spp. and Rhodotorula mucilaginosa, thus demonstrating that these materials can potentially be used for fruit packaging applications (Bodaghi et al. 2013). As mentioned above, active polymers can be obtained by including both types of particles: active by themselves or active agent carriers. Keeping this in mind, the use of clay minerals has also received great attention because they can be incorporated into packaging materials to include specific activities or functionalities. The common natural clays are low-cost and toxin-free materials, which are widely used in many fields. The active clay-loaded polymer nanocomposite films have also been shown to improve the barrier, mechanical, physical and thermal properties (Shankar and Rhim 2016). These clay NPs can also lead to the development of controlled volatile or active substance release systems, such as EOs, due to their large surface area and porosity (Campos Requena et al. 2016; Gul et al. 2016). Its structure includes the stacked arrangement of silicate layers (platelets) and inorganic cations which gives it its hydrophilic character (Gutiérrez et al. 2017) and efficacy as active agents. For this reason, the clay NPs have been studied for the development of AP by means of in vitro experiments or directly on foodstuff surfaces (Kuorwel et al. 2015). Concerning nanocomposites, diverse types of nanoclays (e.g. montmorillonite – Mnt and organophilic Mnt) can be used to improve the characteristics of polymers, resulting in compatible thermoplastic composite materials because of their large aspect ratio (Bumbudsanpharoke and Ko 2019). In addition, active nanocomposite films containing certain types of modified Mnts have a relevant antimicrobial activity against Gram-positive and negative bacteria (Fasihnia et  al. 2017). In this regard, nanoclay-loaded PE-based AP films have demonstrated high efficiency to prolong the shelf life of foods, such as fruits. Physiologic changes in fresh foods can be avoided by using modified nanocomposite films, having good antimicrobial and barrier properties, so these materials can lead to delayed ripening of fruits (Ebrahimi et al. 2018). This can be associated with quaternary ammonium groups present on the surface of the modified Mnts (Bumbudsanpharoke and Ko 2019). On the other hand, the application of clay mineral NPs as carriers for AP has been widely studied. Antimicrobial materials have been developed by incorporating various biocidal volatile compounds such as carvacrol, lemon and rosemary EOs, among others, to clay/polymer nanocomposites (Alonso et  al. 2016; Campos Requena et al. 2016). This type of active polymer films has shown a high efficiency against bacteria (E. coli and Listeria innocua) and fungi (Alternaria alternata). The main advantage of these active nanocomposite films is the controlled release capacity of volatile agents, thereby extending their biocidal activity (Shemesh et al. 2015). It is thus crucial to analyze the retention and desorption mechanisms related to the absorption and release of the active agent. A better understanding of this phenomenon allows the development of tailored active nanocomposite films. In this way, many authors have studied the sorption phenomena of volatile compounds from nanocomposite, considering that these materials are constituted of a permeable polymer phase and non-permeable NPs. The active agent release is thus restricted to

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Fig. 2.6  Representation of talc NPs: distribution and crystalline regions within nanocomposite film

the polymer matrix, regardless of the absorption capacity of the particles. In addition, the presence of particles modifies the crystalline structure of the polymer, changing the sorption in the polymer matrix. A phenomenological analysis on this subject has been reported by Alonso et al. (2016) for talc-loaded PP-based nanocomposite films. It is important to highlight that talc is a phyllosilicate with good absorption capacity which also acts as nucleating agent of PP. These characteristics of the particles make the phenomenological analysis more complex, since the polymer crystalline structure is heavily modified and the particles can absorb and retain the volatile agent. Many factors must thus be considered: crystalline structure, orientation, distribution and dispersion of particles, swelling, tortuosity and volatile absorption in particles. Although the particles increase the tortuosity, this is not the unique factor that modifies the agent’s release mechanism, mainly in systems constituted by a semi-crystalline matrix and particles with high absorption, such as talc (Fig. 2.6) (Alonso et al. 2016).

2.6  Future Trends Global requirements in relation to the food industry is constantly increasing in terms of quantity, quality and accessibility. On the one hand, there is a clear need to reduce food waste throughout the supply chain. On the other hand, there is great global awareness of the consumption of fresh, healthy, nutritious and safe foods. Such demands occur in a context where, in turn, the food accessibility (clearly labeled, commercial availability, cost, among others) and sustainability must be considered. Technological innovations in the packaging field are driven by both demand and consumer convenience: ease of opening, robustness, transportation and versatility (Realini and Marcos 2014). Antimicrobial packaging has an added value compared to conventional packaging, as it allows to extend the food shelf-life by preventing

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microbial attack. Progress in this field is focused on the effectiveness and efficiency of food protection in a sustainable way. In addition, these materials are intended with the aim of reducing the amount of active agents used on food, thus allowing for safer and more natural food. In this sense, active agents obtained from natural sources can induce bioactivity in packaging systems, and are gaining ground compared to traditional synthetic antimicrobials (Dıblan and Kaya 2015; Janjarasskul and Suppakul 2018). Particularly, the interest on nanoantimicrobials has gained importance since their high area/volume ratio, which makes them highly efficient active agents. For this reason, low amounts of antimicrobial agents are required to obtain the same, or even better, food protection effect compared to other non-­ nanometric substances (Malhotra et al. 2015). One of the newest and most outstanding trends in AP is the multifunctional packaging having multiple properties. The combination of active and intelligent packaging is the most imminent packaging technology (Montazer and Harifi 2017). Some authors have defined this technological approach as ‘intelligent (smart) packaging’, referring to the integration of active and intelligent functions in a system (Sharma and Ghoshal 2018). These special AP respond dynamically to any change in the quality profile of the foodstuff, and their combined functions involve the detection, control and information about the food quality. The design of AP functions must be targeted and tailored, considering the requirements of the food, as well as consumer and market demand. It is worth noting that active and intelligent packaging are of great relevance for the industry 4.0. This simplifies the production processes, facilitates distribution logistics, protects and improves the product quality, provides information about the product during marketing, among others (Glistau and Coello Machado 2018). In this sense, AP innovations attempt to optimize the packaging system and explore the potentialities in a more sustainable way. AP’s future challenges involve profitable and sustainable developments on an industrial scale. The implementation of these technologies depends largely on the cost/benefit relationship, food safety and regulations on their industrial applications (Han et al. 2018). In this way, the development of active polymer films made from low cost polyolefins and organic/inorganic particles is a promising alternative for food packaging applications. These materials combine synergistically matrix and particles properties, having biocidal activity and good mechanical properties. These characteristics promote its industrial development for the mass production of active polymer films, thus offering a wide range of possibilities, depending on the specific application and the requirements of the foodstuff. This versatility is associated with the possibility of including certain functionalities, either in bulk or by the superficial modification of the packaging material. Acknowledgments  The authors wish to express their gratitude to the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and the Universidad Nacional del Sur (UNS). Conflicts of Interest  The authors declare no conflict of interest.

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Chapter 3

Smart and Shape Memory Polymers Zijian Gao and Guanghui Gao

Abstract  Smart polymers are materials that could exhibit apparent responsive changes to external environment stimulation, such as pH, temperature, light, electrical and magnetic fields, as well as enzymes and chemicals, etc. Shape memory behavior, as one of the unique intelligent characteristics, could endow polymers with recovering their shape via external stimuli. As a result, smart and shape memory polymers (SMPs) exhibit enormous potential capacity for various applications. In this chapter, the feature of SMP was firstly presented and different mechanisms were introduced. Several types of SMPs were then discussed, including gel, polyurethane, resin, etc. Finally, the application and future outlook were described for SMPs. Keywords  Composite materials · Gels · Polyurethane elastomer · Resins · Shape memory

3.1  Introduction Materials are the basis of human life and manufacture, which could be classified into novel functional materials and traditional materials. With the development of technology, intelligent materials gradually emerged, which was named by Takagi Junyi in 1989. After that, R.E. Neunham proposed the concept of smart materials. Intelligent materials could perceive and response to external environment. The smart materials are novel functional materials with perception, execution and response ability. Hereafter, the biominetic function can be introduced into the smart materials so as to complete self-test, self-decision, self-commitment and self-execution process, thus facilitating that the materials reach a higher level. Currently, the functions of smart materials can be listed as follows:

Z. Gao · G. Gao (*) Polymeric and Soft Materials Laboratory, School of Chemical Engineering, and Advanced Institute of Materials Science, Changchun University of Technology, Changchun, P. R. China e-mail: [email protected] © Springer Nature Switzerland AG 2021 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume Three, https://doi.org/10.1007/978-3-030-50457-1_3

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1. Function of sensing: this kind of smart material could sense the environment and condition, such as load, stress, strain, light, heat, electricity and so on; 2. Function of feedback: materials with feedback function could compare the output information and input information and provide information for controlling system; 3. Function of information recognition and accumulation: materials could identify all kinds of information, which can be obtained from the sensor network and accumulate them. 4. Response ability: smart materials with response ability could respond to the change of external environment and internal conditions timely and also take necessary actions dynamically. 5. Self-diagnosis ability: smart materials with self-diagnosis ability could self-­ diagnose and adjust the problems such as system failures and misjudgments by analyzing and comparing the present situation of the system with the past. 6. Self-repairing ability: smart polymers could repair local damage or destruction by means of the regeneration mechanism such as self-reproducing, self-growing, in situ healing and so on. 7. Self-adaptive ability: smart materials with self-adaptive ability could automatically adjust its structure and function in time according to the changing external environment and conditions. Then, the state and behavior are changed correspondingly in an optimized way for material system to meet external changes. As a unique kind of smart polymer material, shape memory materials could perceive change of environment (e.g. temperature, force, chemical solvent, electricity, magnetism, among others.) and respond to these changes. After completing the above procedures, the shape memory materials recover to pre-setting shape through adjusting the mechanical parameters. As a kind of smart materials, shape memory performance endows the materials with propestive potential applications. Therefore, the shape memory materials have been developed since 1963. W. J. Bueler firstly discovered the memory effect of Ni-Ti alloy by accident, implying shape memory alloy materials with engineering significance does emerge. Hereafter, the scientists developed various kinds of shape memory materials, which can be classified as follows: 1 . Shape memory alloy: Ni-Ti alloy, Cu-Ni-Mn, Fe-Mn-Si; 2. Inorganic non-metal shape memory materials: garnet, mica glass; 3. Shape memory composite materials: aluminum embedded with shape memory nickel-titanium alloy wire. 4. Shape memory polymers (SMPs): shape memory poly(urethane) (PU), shape memory crosslinked poly(ethylene) (PE) and shape memory hydrogels; SMPs have the following advantages compared to shape memory alloys and shape memory ceramics: strong memory effect, low sensing temperature, cheap, easy processing and a wide range of applications. Thus, SMPs have attracted a lot of attention from researchers. A. Charlesby firstly described memory effect of radiation-crosslinked PE.  Then National Aeronautics and Space Administration

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(NASA) proved the shape memory performance of radiation-crosslinked PE after realizing the potential application of SMPs in aerospace field in 1970s. And different kinds of polymers with shape memory property have been explored in recent years such as polynorbornene, trans-polyisoprene, styrene-butadiene copolymer, PU polyester and polymer gels. The SMPs induced with various stimulus have also been explored, including temperature, light, electricity, magnetism and chemical solvent. As a kind of smart polymers, SMPs possess remarkable application potential in many fields, such as car, electronics, chemical, packaging, toys and daily necessities. Specific application would be introduced in the follow-up of this chapter.

3.2  Mechanism of SMPs SMPs are generally composed of two-phase structures, which are made up of a stationary phase of original shape and a reversible phase which could reversibly solidify and soften with temperature changes. Generally, stationary phase comprises amorphous zone with crosslinked structure. Phase with high melting temperature (Tm) or glass transition temperature (Tg) would form molecular chain entanglement, which also can be employed as stationary phase. The process of shape memory is depicted in Fig. 3.1. First, the shape memory mechanism is introduced at the structural level. As is well known, molecular weight (Mw) of polymers varies from tens of thousands to hundreds of thousands. The macrostructure of most polymers contains a crystalline and an amorphous zone with the existence of molecular chain entanglement. Non-crosslinked polymers show flow properties temporarily when heated. When a linear polymer becomes a three-dimensional structure, the materials are not

Fig. 3.1  The exhibition of shape memory effect

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Fig. 3.2  The relationship between polymeric status and temperature

capable of melting even by heating above Tm. In this context, the crosslinked polymer materials exhibit performance of elastomer in a broad temperature range (Fig. 3.2). The shape memory phenomenon of polymeric materials is absent as long as the performance temperature of the material is lower than its Tg. Polymeric chains begin to thaw and move as temperature rises above Tg. Polymeric segments can be stretched by applying tension. Finally, polymeric materials could regain their original shape once the applied external force is removed. Therefore, the prerequisites for polymeric shape memory materials is high elastic deformation caused by movement of the molecular chain and high elastic deformation achieved by changing conformation. However, when the conformational change is not maintained with the applied stress, then hysteresis appears. Hysteresis could thus be defined as a strain (ε) falling behind the change in stress, which gives the possibility of freezing deformation in time. On the other hand, once a stress is applied to the polymeric material at a temperature higher than its Tg, then the polymeric chains can crystallize or freeze due to a decrease in temperature, maintaining the applied stress. Therefore, in this state the equilibrium is not reached. This unresolved reversible deformation must immobilize the macromolecular chains in the form of internal stress. In this sense, the polymeric material can be very elastic by overheating, since the crystalline sections are fused and the amorphous sections can get movement again. As a result, the material can return to its original shape, since the unfinished reversible deformation can be driven by internal stress, which is essentially the shape memory phenomenon. According to the above discussion, shape memory polymeric materials should possess the following characteristics:

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1. The structure of polymeric materials must contain crystalline and amorphous zones in an appropriate ratio, simultaneously; 2. High elasticity in a broad temperature range above Tg or Tm. The glassy state is also presented in broad temperature range to ensure freezing stress without releasing under storage. 3. The mechanical strength (also known as stress, σ) of SMP materials is good enough to implement deformation. Based on the rubber elasticity theory, the size of the heat shrinkability of shape memory material could be characterized by the elastic modulus (also known as Young’s modulus, E) of the material as follows:

Memory characteristics E = 3Vka 2 gT

(3.1)

where T is the absolute temperature (above Tm), g the entanglement factor, k Boltzmann constant, a the dialation factor (average length of the chains during orientation/ average length of the chains when not are oriented), V the number of chains per unit volume (V  =  ρN/Mc(1-2Mc/Mn) ρ-density; Mn-number average molecular weight; Mc- molecular weight between crosslinked bonds). Based on the above equation, the following conclusion can be given: the property of shape memory increases with the degree of crosslinking. Mw and density also have a positive effect on the shape memory property. From the point of view of thermodynamics, the shape memory phenomenon is a transition from an unstable to a stable state. For this reason, this effect can be measured by Gibbs free energy change (ΔG). The transformation of the ΔG is shown in Fig. 3.3. It should be noted that A is the polymeric material in the initial or original state, B the material under high strain conditions, which is a thermodynamic stabilization system before applying stress, C a state of high strain, in which the polymer chains can be oriented and stretched along the direction of the stress force, thus obtaining a thermodynamically unstable and reversible state, and D the storage state after stretching and cooling to room temperature. Freeze stress tends to be released

Fig. 3.3  Thermodynamic analysis of polymer shape memory process

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at any time, and can only be temporarily preserved by quenching the relatively unstable system. The thermodynamic function of the system can be defined as follows: G = U + PV - TS



(3.2)

The heating, stretching and cooling processes of shape memory materials are completed under isobaric pressure, whose volume change could be neglected. Thus, G could also be defined as follows: dG = dU - Tds - SdT



(3.3)

Depending on the nature of the thermodynamic functions, the change in function of the process for the preparation of SMPs can be shown as follows: DG = DG1 + DG2 + DG 3



(3.4)

ΔG1 is the change in the thermodynamic function caused by the temperature from T0 to T1, and ΔG3 is the change in the thermodynamic function caused by the temperature from T1 to T0, while the effect of volume change on the thermodynamic function can then be ignored:

DG1 = G 3

(3.5)

DG = DG2

(3.6)

Thus,

ΔG2 is the change in thermodynamic function induced by stretching in the high strain state. In this context, under low strain-at-break conditions, the change in internal energy caused by the stress is insignificant, which mainly causes the entropic change, due to the isothermal process, thus obtaining Eq. 3.3.

DG = DG2 = -T1dS

(3.7)

The above equation indicates: dS > 0, ΔG  0

(3.12)

Therefore, the storage state of SMPs after stretching and freezing stress is a thermodynamically unstable state. This unstable state always has resilience to the steady state, but the relatively unstable system is temporarily preserved by quenching. The shape recovery of the polymer occurs once the stretched and frozen stress is reheated to a high strain state. Lastly, the thermal movement of the polymer chains eventually makes it easier for the system to tend to be disorderly. Therefore, the degree of disorder of the system and the entropy increases, while the ΔG of the system decreases. The post-shape recovery state is a stable aggregation state. Furthermore, there is no internal stress to stabilize the state of the polymeric material. Therefore, the shape recovery process is a spontaneous thermodynamic process.

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Fig. 3.4  Schematic diagram of stress relaxation

3.3  Factors Affecting SMPs 3.3.1  Effect of Stress and Speed It is well known that an internal stress is generated in polymeric materials once an external force is applied, and this internal stress gradually decreases with prolongation of time, which is called stress relaxation. Figure 3.4 shows the conformational change of polymer materials under stress relaxation conditions. The stress relaxation phenomenon is a characteristic of the thermodynamics of highly elastic materials, which is beneficial for obtaining shape memory property, since this property is obtained by freezing a higher internal stress. In contrast, the stress relaxation phenomenon weakens internal stress. However, the stress relaxation takes a while to occur. Therefore, shortening the external stress time as much as possible can reduce the negative effect on shape memory performance. In summary, the higher tensile speed at which the stress is applied, greater the shape memory recovery that can be obtained without the materials being fractured at a given temperature.

3.3.2  Effect of Creep Performance on Shape Memory Phenomenon Creep also has an important role on SMPs, and this has a similar effect as stress relaxation on SMPs. The difference lies in the fact that creep occurs both during the storage of the polymer before its use, as well as when the elastic regime of the material is exceeded, i.e. the ε values of the materials changes over time under internal freezing stress, which results from movement of molecular chains. In general, polymeric materials exhibiting creep do not show good shape memory properties. It should also be noted that the creep behavior of polymeric materials is closely related to their chemical structure. In this regard, creep performance is weakened by

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increasing force between polymer chains and longer polymer chains, as well as the following factors: crystallinity, degree of crosslinking, Mw, polar chains, side groups, among others. Other parameters influencing creep are fillers, stress and temperature. High temperatures and pressures can, for example, accelerate creep, and this is a negative point for SMPs. In contrast, fillers can have a positive effect on SMPs, as they restrict the movement of polymer chains, thus inhibiting creep of polymer materials.

3.4  Types and Applications of SMPs 3.4.1  Shape Memory Gels (SMGs) In the 1950s, researchers began to explore the sensitivity of gels. Flory (1953) was the first to study the swelling sensitivity of gels, while Scarpa et al. (1967) were the first to report thermo-sensitive hydrogels based on poly(N-isopropyl acrylamide) (PNIPAAm). Dušek and Patterson (1968) also predicted the volume change of the gels, and ten years later, Tanaka (1978, 1981) and Tanaka et al. (1982) reported pHsensitive hydrogels made from poly(acrylamide) (PAAm). Thereafter, research on the volume phase transition of gels and the critical phenomena associated with them have been actively studied. Since then, a new field of research has been explored, such as hydrogels sensitive to different environmental conditions (also called stimulating sensitive hydrogel). SMGs are a novel type of smart polymeric materials, whose network is made up of polymeric chains and solvent. As smart materials, SMGs could perceive small changes from the external environment, such as electric field, light intensity, pH, temperature, etc (Zarrintaj et al. 2019). After the change of shape, the physicochemical properties of these materials are altered, mainly volume phase transition (VPT). SMGs could return to their original shape after the stimulus is gone. SMGs can also have the ability to exchange energy and information, as a result, they can be used as controlled drug release devices, material separation, sensors, among others. 3.4.1.1  Thermo-Responsive SMGs The themo-sensitive SMGs have been studied since 1956. In this sense, Scarpa et al. (1967) found that NIPAAm-based hydrogels have a phase transition between 30 and 40 ° C, thus indicating a lower critical solution temperature (LCST). Tanaka et al. (1978) also prepared PNIPAAm gels and found their sensitivity to temperature and concentration of solvents. Since then, themo-sensitive SMGs have been an intensively studied research field. Hirai et al. (1992) reported a thermo-sensitive poly(vinyl alcohol) (PVA) hydrogel system obtained chemically via crosslinking, resulting in an elastic material in

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boiling water. After this, these hydrogels were immersed in different solvents in order to fix the temporary shape during the cooling process, thus obtaining thermosensitive hydrogels. Skrzeszewska et  al. (2011) also found that the chemically crosslinked telechelic peptide hydrogel showed excellent thermo-sensitive shape memory performance, as a result, the hydrogel could be stretched 2 times its initial length by increasing temperature. After cooling, the collagen-like segments at different molecular chain ends were combined by winding to form a triple screwrotating node. Thus, the hydrogel sample could fix the shape temporarily, which could be maintained for a long time at room temperature, as well as the initial shape could be recovered in a few minutes at a temperature of 50 °C or more. However, the mechanical properties of this material were so poor that the E values varied between 102–104 Pa, which greatly limited its potential application. In order to overcome this disadvantage, the Osada’s group. prepared a chemically crosslinked copolymer of acrylic acid (AAc) and octadecyl acrylate (OA) by solution polymerization using ethanol as solvent (Osada 1995; Matsuda et al. 1994; Miyazaki et al. 2002). These authors reported that the copolymer gel showed excellent thermosensitive shape memory performance, due to the ordered-disorder transition of the microdomain formed by the hydrophobic octadecyl side chain of the copolymer, resulting in a E value of 1.7 × l07 Pa at 30 °C, and dropped to 2.2 × 105 Pa when heated above 49 °C. Bilici and Okay (2013) took advantage of sodium dodecyl sulfate (SDS) as a surfactant to be used in the development of micelles in aqueous solution to build a chemically crosslinked hydrogel from a OA-co-AA copolymer through polymerization. Thereby, the resulting thermo-sensitive hydrogel was positively affected in terms of their E values. According to Candau and Selb (1999), the water solubility of the OA monomer is so poor, that the direct obtaining of the copolymer SA-co-AA in aqueous solution is difficult. For this reason, a certain amount of sodium chloride (NaCl) plus the surfactant could dissolve the hydrophobic monomer (OA) as much as possible. These authors reported that spherical micelles were formed once the NaCl concentration in aqueous solution exceeded the critical micelle concentration (CMC). As a result, interactions between the Na+ cations and the negatively charged external sulfate groups of the micelles were given, thus allowing to obtain worm-like capsules, which could tailor the mechanical strength and thermosensitivity of the hydrogel to a certain extent. 3.4.1.2  Light-Induced SMGs As a clean and controllable environmental stimulus, illumination is indispensable for the development and application of new smart polymers. In recent years, scientists’ attention has focused on light-induced SMPs. This type of polymer has mainly pho-

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tosensitive groups into the gel network, such as azobenzenes, photoionization and decomposition of monomers (such as triphenylmethane derived hydrazine) or other photosensitive molecules (such as cinnamic acid) (Fig. 3.5). In this sense, the photosensitive groups in the gel network are crosslinked, photoisomerized or photodissociated, due to ultraviolet (UV) radiation, which leads to an increase in local temperature or a configuration change in the polymer chains. The gel is then swollen, thus exhibiting the shape memory phenomenon (Lendlein et al. 2005; Song et al. 2009; Sun et al. 2009; Tomatsu et al. 2005). Light-induced SMGs as compared to thermo-sensitive SMGs, the light stimulation signal could not only be transmitted immediately to the detection site, but also the amount of signal can be well controlled. Azobenzene is a typical compound with photoisomerization property. Peng et al. (2010) reacted functionalized dextran (Dex) with functionalized azobenzene (FA) and cyclodextrin (CD) to produce two products, which were named FA-Dex and CD-Dex, thus obtaining a light-induced SMG via a ‘click’ chemistry reaction. Thereby, azobenzene in the molecular chain would change from trans structure to cis structure under UV irradiation. From the macroscopic view, the hydrogel changed from a gel state to a sol state in order to release the drug. Yamaguchi et al. (2012) also reported a photocontrolled hydrogel based on PAAm chemically crosslinked with azobenzene, which was able to be assembled or disassembled using CD under UV irradiation conditions. 3.4.1.3  Ultrasound-Induced SMGs Ultrasound is another important stimulant for SMPs, as high intensity ultrasound can facilitate the shape recovery of SMPs through special energy converters. Li et  al. (2015) prepared PVA/melamine-based SMGs by means of the freeze-thaw method, which could respond to the ultrasound produced by the therapeutic ultrasound system. The introduction of melamine hardened the resulting PVA hydrogel, due to hydrogen (H)-bonding interactions between melamine and PVA, thus providing a stable polymer skeleton for the shape memory performance of the hydrogel system. This type of hydrogels could be implanted in the blood vessels, being controlled by ultrasound harmless to humans, as well as they could also be used for aerospace applications. 3.4.1.4  Redox-Induced SMGs Redox reactions can also be used as an external driving force to obtain SMGs. Redox-induced SMGs show excellent cyclical shape memory behavior under ambient conditions. In addition, a triggering stimulus could induce multi-responsive SMGs, since reversible interactions are multi-response (Le et al. 2016; Xiao et al. 2013, 2016). As for driving force, host-guest interactions are vital for constructing redox induced SMGs. With this in mind, Miyamae et al. (2015) used two different types

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Fig. 3.5  The main types of photosensitive groups and monomers in light-responsive SMPs

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of host-guest interactions (β-CD-ferrocene (Fc) and β-CD-adamantane (Ad)) to develop a series of redox-induced SMGs under oxidation conditions. In this way, two temporary shapes could be controlled by tuning the redox stimuli. Thereby, the self-healing and expansion-contraction performance were integrated into this hostguest hydrogel. Tomatsu et  al. (2006) also separately manufactured two redox-induced SMGs via host-guest interactions from CD-chitosan (CD-Cs) and poly(ethyleneimine)-Fc (Fc-PEI), and they were then mixed in an aqueous solution of glutaraldehyde (chemical crosslinker) with the aim of reacting the amino groups of the two polymers, thus forming a chemically and physically crosslinked hydrogel. In particular, the chemical crosslinking sites were used as a stationary phase, while the redox sensitive physical crosslinking sites acted as a reversible phase. The gel could thus be directly oxidized and reduced using ammonium cerium nitrate and sodium hydrogen sulfite, respectively. These redox-induced SMPs could also be indirectly oxidized by glucose. 3.4.1.5  Applications of SMGs SMGs have several applications (Table 3.1). However, one of the most interesting is the transformation of energy. This feature could be used in mechanical and power systems. The example typically used is bionic artificial muscle. Keeping this in view, Le et al. (2019) prepared a resistant hydrogel based on PVA/PAA using the freeze-thaw method. The hydrogels obtained exhibited excellent σ values, while showing reversible chemical and mechanical performance, and high persistence. In addition, the speed of response to the stimulus decreased with increasing thickness of the gel fiber. The high-level structure such as the coordination direction of the polymer chains, the degree of crosslinking and the porous property of the gel fiber, could be designed to control the speed and contraction force of the gel fiber. Jiang et al. (2019) also developed a resistant and electro-responsive hydrogel based on

Table 3.1  Applications of SMGs Field Sensor Actuator Drug release Biotechnology Smart dimming material Smart fabric Smart control Material separation

Application Light, heat, and ion selective sensor, biological sensor Artificial muscle micromachine Signal controlled release, disease location release and release on-off control Enzyme immobilization, immunoassay, biochemical extraction Transparent, turbid dimming material under strong light at room temperature Automatic color changing clothing and automatic temperature control clothing made of SMG fiber Light control, temperature control switch Inductive gel films, chemical valves

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Fig. 3.6  Double layer SMGs (a), double layer gel strip (b) and gel hand (c)

Fig. 3.7  Smart drug release system

semi-interpenetrated PNIPAAm/PAAm networks, which could function as artificial muscle instead of a robot powered by electricity. The resulting hydrogel was sensitive to temperature and acetone concentration, since PNIPAAm and PAAm are respectively sensitive to these parameters. The hydrogel hand could lift or drop an object controlling the rise or fall of temperature (Fig. 3.6). Smart drug delivery systems (DDS) can be defined as those materials of controlled and sustained drug release upon the application of an external stimulus. As shown in Fig. 3.7, this type of system could perceive a signal generated by injury,

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the drug could then be released after judging the intensity of the signal. The polymer gel can usually be used as carries for drug delivery, because the gel could respond to a specific signal, such as chemicals, electric field, pH, temperature, among others. Santini et al. (1999) synthesized from poly(N,N-diethyl acrylamide) norfloxacinloaded thermo-sensitive SMGs for drug release above its LCST, due to the contraction of the network. This type of material could thus be designed to respond to a specific stimulus to treat some disease. With this in mind, Freiberg and Zhu (2004) prepared a dual sensitive hydrogel (pH and temperature) for controlled release of papain (enzyme). This hydrogel would restrict the release of the enzyme by diffusion at pH = 1.4 and temperatures lower than 37 °C, while the enzyme would be released at pH = 7.4 and 37 °C, due to the swelling of the gel. As for the intelligent control system, the controlled drug release could be given according to the intensity of the signal or the external stimulus. An electric field sensitive polyelectrolyte could for example serve as a vehicle for an electric field sensitive drug delivery system. In this same line, Cao and Jana (2007). synthesized from 2-acrylamido-2-methylpropanesulfonic acid and N-butyl methacrylate chlortetracycline-loaded electric field-responsive hydrogels for the controlled release of this antibiotic. In this sense, SMGs have great potential in the field of emerging technologies, such as artificial muscles, chemical valves and smart materials.

3.4.2  Shape Memory PU (SMPUs) In particular, SMPUs have attracted the attention of many researchers, due to their biodegradability, easy processing, good mechanical properties, light weight and low cost, which makes them potential candidates for the manufacture of synthetic medical materials, as well as aerospace textiles (Cao and Jana 2007; Lendlein 2002; Ni et al. 2007a). 3.4.2.1  Thermo-Responsive SMPUs PUs is a thermoplastic polymer made up of two types of polymeric materials with different Tg values, which can be used as SMPs (Chen et al. 2014; Gu et al. 2015; Wang et al. 2013a; Yang et al. 2014). Microseparation of phases in the internal structure of SMPUs, due to thermodynamic incompatibility, can be thermally tuned. In this context, the PU segments with the lowest Tg value are referred to as the soft segment, and play an important role in the strain of the material, while the hard segments are those with the highest Tg value, and are associated with the restrictions on the movements of the amorphous polymer chains. Therefore, upon reaching the Tg of the soft segments, the immobile hard segments act as crystallization sites, thus generating the shape memory phenomenon, as long as the temperature does not exceed the Tg of the hard segments. In this sense, higher Mws and hard segment contents, the shape memory phenomenon is positively affected (Ahmad et al. 2011; Chen et al. 2009a, 2010a, b; Hu et al. 2005a; Merline et al. 2008; Ping et al. 2005;

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Fig. 3.8  Schematic diagram of hard and soft segment in SMPUs

Wang et al. 2012; Xu et al. 2009; Xue et al. 2009, 2010, 2012; Yang et al. 2005; Zhuohong et al. 2006) (Fig. 3.8). 3.4.2.2  Photo-Sensitive SMPUs Photo-sensitive SMPs are an emerging type of SMPs, which are convenient and safe (Yakacki et  al. 2007). Photo-sensitive SMPs have potential applications for the manufacture of biomedical devices such as optical switches and remote optical drives (Yamada et al. 2008, 2009). Photo-induced SMPs are usually determined by the nature of the photosensitive groups that the polymer contains. For this purpose, the cinnamic acid and the azo compounds are frequently used. The former is mainly based on reversible crosslinking and de-crosslinking under different wavelengths of UV, while the latter is mainly based on its reversible cis-trans isomerism under different wavelengths of light. In particular, cinnamic acid and its derivatives are very safe for biomedical materials. With this in mind, Wang et al. (2013b) manufactured a photo-induced SMPUs based on cinnamyl groups, and poly(caprolactone) (PCL) being introduced into the SMP system to act as a soft segment, resulting in more biodegradable materials. As a result, the Tg of the SMPUs varied around body temperature. On the other hand, hydrogel containing cinnamic acid after being irradiated with UV light at λ > 260, a cycloaddition reaction was given, while at λ < 260 nm a reversible de-crosslinking reaction occurred (Wang et al. 2013b). It should be noted that the crosslinking reactions can be used to prepare photo-sensitive polymers (Andreopoulos et  al. 1998; Dušek and Patterson 1968b; Garle et  al. 2012; Rochette and Ashby 2013; Snyder and Tong 2005; Wu et al. 2011). 3.4.2.3  Water-Induced SMPUs Water-induced SMPs are mostly thermogenic SMPs, which indirectly achieve the shape memory phenomenon, because water molecules can enter the polymer matrix, thus lowering the Tg of the polymer, due to its role as a plasticizer. These materials have the advantage of their potential implantation in body fluids without the need for additional stimuli, since their Tg is also around body temperature. Keeping this in view, Wang et al. (2014) proposed a novel strategy for the manufacture of PVA/ PU-based water-induced SMPUs with comb structure. Hu’s group has also studied the water-induced shape memory phenomenon from PU containing pyridine units (Chen et al. 2009b, 2012). In this sense, the hydrophilic pyridyl groups contained in

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the resulting materials, allow the easy entry of water molecules to the polymer matrix, thus destroying the pyridinium ring-PU H-bond interactions, as a result, the Tg is reduced. Zhang’s research group also reported the water-induced shape memory phenomenon from crosslinked PVA hydrogels, thus obtaining materials that can recorver their original shape after 45 min at room temperature (Du and Zhang 2010). 3.4.2.4  Electrical and Magnetic Induced SMPUs Electrical and magnetic SMPs are indirect thermo-responsive SMPs. The electroand magneto-type SMPs are constructed from conductive or magnetic materials such as conductive carbon black, metal powders, conductive polymers and ­ferromagnetic inorganic particles. The composite materials increase the temperature of the system by electric current or the heat generated by the alternating magnetic field (AMF), which leads to the shape recovery. Therefore, composite materials possess electrical conductivity, magnetic responsiveness and excellent shape memory function (Cai et al. 2013a, b; Gong et al. 2012; Niu and Cohn 2013; Razzaq et al. 2007b; Zhang et al. 2013). Compared to traditional thermo-sensitive SMPs, composite materials could be continuously heated by electric and magnetic fields, which is of great value to design complex devices and realize localized heating (Razzaq et al. 2007a; Weigel et al. 2009; Xiao et al. 2010; Yu et al. 2009). For example, Xiao et al. (2010) obtained shape memory composite materials from crosslinked PCL and carbon nanotubes by the application of a DC alternating electric field. These authors obtained a sperial-shape material as the initial shape and after the application of an DC alternating electric field for 22 s, the material had a straight shape. Today, most of the developed SMPs studied are stimulated by direct contact stimuli. However, non-contact stimulation methods, such as AMF, electric field, infrared light, laser and ultrasound can be safely used for medical applications. 3.4.2.5  Applications of SMPUs Based on the above research process, SMPUs could be applied in many fields due to the advantage of permeability, mechanical properties and temperature adjustability. Until now, SMPUs are mainly used for textile applications, as breathability and water resistance can effectively improve shape memory performance. Keeping this in mind, Mondal and Hu (2007) investigated the water vapor permeability of SMPUcoated textiles, noting that SMPUs can be adjusted with body temperature. However, SMPUs generally cannot tightly control shape memory performance for different practical applications because the recovery temperature is not accurate enough. In addition, the excellent mechanical properties, high blood compatibility and temperature adjustability from SMPUs broaden the prospects of SMPUs for medical applications. The biodegradability of the PU materials could be imparted by using biodegradable polymers as the soft segment, while the Tg of the PUs could be

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tailored around body temperature by controlling the soft segment/hard segment ratio in the system. Currently, SMPU-based materials are used as catheters, drug delivery systems, fracture fixation, orthopedic surgery, etc (Burke and Hasirci 2004; Grapski and Cooper 2001; Lendlein et al. 2010; Yakacki and Gall 2009) SMPUs could also be used for aerospace and automotive applications (Paik et al. 2006).

3.4.3  Shape Memory Resins (SMRs) SMRs are another important type of SMP. These materials have been continuously developed, due to their excellent electrical insulation and mechanical performance, and thermal and corrosion resistance. Charles (1953) first reported the development of radiation-crosslinked poly(vinyl chloride) (PVC)-based SMRs as a packaging material. Following Hu et al. (2005b), these radiation-crosslinked PVC-based retractable packaging membranes have the advantages of a relatively high transparency and good shrinkage properties. Poly(norbornene)-based SMRs are one of the first industrialized products as SMPs, and are obtained via Diels-Alder reaction from cyclopentane and ethylene. These SMRs are marketed under the name NORSOREX, and their Mw is around 3 × 106 of Da, i.e. they have a Mw of 100 times greater than the convectional polymers, and their Tg is 35 °C. In these SMRs, the crosslinking sites act as a stationary phase, while the amorphous polymer chains are used as a reversible phase (和郎中 山 1990). Poly (norbornene)-based SMRs compared to other types of SMPs, have the following characteristics (Bogue 2009; Serrano and Ameer 2012): 1. They can be processed by calendering, injection extrusion and vacuum forming. However, the processing is difficult due to their high Mw. 2. Poly(norbornene)’s Tg is close to human body temperature. Therefore, it can be used as raw material for biomaterial manufacturing. However, this temperature is difficult to control. 3. Poly(norbornene) has excellent oil absorption capacity, which is from 5 to 100 times its own weight. It could thus be used as a reusable device for oil spills. After oil absorption, elastomeric resins could be obtained from them, as well as they could be turned into low hardness rubber (15 JIS grade, hard), which results in excellent materials after being properly treated. Trans-poly(isoprene) (TPI) is a crystalline polymer under ambient conditions, which can be vulcanized/crosslinked like any ordinary rubber, thus resulting in SMRs. The stationary phase is the network structure after the crosslinking reaction, while the reversible phase is made up of crystalline structures, which can be melted (Ni and Sun 2006). TPI-based SMRs have high deformation and recovery rates between 46 and 76 °C. As a result, at low temperatures could be applied as thermal control switches (Ni and Sun 2006). However, TPI-based SMRs have poor thermal resistance and weatherability, and low shape memory temperature (30–50 °C), thus restricting their potential applications. Radiation-crosslinked poly(ethylene) (PE)-

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based SMRs can also be used as SMPs. However, its difficult processing, high Tg value (around 110 °C) and small shape variables restrict thier potential applications. To overcome this disadvantage, high-density PE (HDPE) can be adequately mixed with TPI at 150 °C, thus resulting in vulcanized HDPE/TPI-based SMRs with excellent mechanical properties. In 1988, Asahi Kasei Corporation successfully developed a poly(styrene)-bpoly(butadiene) (PS-b-PB)-based SMR, in which PS (melting point (Tm) around 120 ° C) acted as phase stationary, while PB (Tm approx. 60 °C) fulfilled the role of reversible phase (Xu et al. 2010). This SMR could be processed by extrusion, injection molding and shrinkage at 120 °C, and its deformation and shape recovery temperature for both was around 60 °C (Nji and Li 2010). According to Fu et al. (2009) ε values of up to 400% were obtained, as well as these PS-b-PB-based SMRs could be reused at least 200 times, and due to their excellent colorability and acid/alkali resistance, they can extend their range of applications. PS-b-PB-based SMRs could also form a colorless transparent viscous solution in the toluene solution for casting and coating processing, the viscosity of which could be tailored.

3.4.4  SMP Composites (SMPCs) The low σ (between 5 and 10 MPa) and E values, and the relatively small recovery force make the applications from SMPs difficult (Hornbogen 2006). However, the strain recovery force of SMPs generally ranges from a few megapascals to tens of megapascals. A low σ value indicates that the strain recovery force generated under the restraint is small (Zhu et al. 2008). Reinforcement materials such as fibers, different nanoparticles (e.g. carbon nanotubes, silica nanoparticles, etc.) and other materials could be added to the SMPs to overcome the defects, and obtain SMPCs (Jung et al. 2010; Koerner et al. 2004; Miramirkhani and Navarchian 2017; Ni et al. 2007b; Robinson et al. 2017; Zhang et al. 2007). It is worth noting that the content of reinforcement material improves the mechanical properties of the SMPs up to a threshold, since once this point is exceeded, the mechanical properties decline (Liu et al. 2006; Ohki et al. 2004; Gutiérrez and Alvarez 2017d; Gutiérrez et al. 2017, 2018, 2019). SMPCs have also been frequently obtained by blending polymers such as EVA/ PVC, PCL/PE, PCL/TPI and PE/PU (Mishra et al. 2003).

3.5  Outlook In this chapter, the remarkable process in the field of SMPs was presented. SMPs have diverse applications, and others can be expected, since they can respond to external stimuli under different conditions. However, there are still challenges to be solved. In particular, multifunctional conductive SMPs will constitute a new genera-

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tion of smart material, and specifically SMGs are potential candidates for this, as they can be used for controlled and sustained delivery of drugs, biosensors, tissue engineering, and this list can be increased, due to their biocompatibility and biodegradability. This field of research can thus be very useful for the development of multifaceted materials. Acknowledgements  This research was supported by a grant from National Natural Science Foundation of China (NSFC) (No. 51873024). Conflicts of Interest  The authors declare no conflict of interest.

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Chapter 4

Carbon Nanoparticle-Loaded Shape Memory Polyurethanes: Design and Functionalization Ayesha Kausar

Abstract  Shape memory polyurethanes (PUs) have the ability to response to external stimuli such as electricity, heat, light, moisture, pH, etc. Among PUs, segmented PU possesses high recoverable strain, tunable physical properties, wide range of glass transition temperature and an easily controllable softening phenomenon. Shape memory nanocomposites have been synthesized from PU containing carbon nanoparticles (C NPs) such as fullerene, graphene, multi-walled carbon nanotube, nanodiamond, etc. The surface modification of C NPs can improve the mechanical, shape memory, strength and thermal properties of the nanocomposite. Shape memory PU and nanocomposite have recently been aroused by research interest due their versatile applications. This chapter basically highlights the design of shape memory PU and their C NP-loaded nanocomposites, the principle of shape memory behavior of these polymers and their applications with future directions. Keywords  Fullerene · Nanocomposite · Polymer · Stimulus

4.1  Introduction Shape memory polymers are able to recover their initial shape (mechanical properties) after being subjected to external stimuli such as electricity, heat, light, pH (Yan et al. 2012; Yu et al. 2013; Kanu et al. 2019), humidity (Meng and Hu 2010) and pressure (Thakur and Karak 2014). Thermoplastic poly(urethane) (TPU) and segmented poly(urethane) (PU) block copolymers have gained significant importance in this field. These materials have soft and hard segments, which have been shown to be related to shape memory properties (Khosravi et al. 2020). The soft segments (also referred to as reversible phase) are responsible for the plastic characteristics of the mechanical behavior of the material, while the hard segments (also called fixed

A. Kausar (*) National Center for Physics, Quaid-i-Azam University Campus, Islamabad, Pakistan © Springer Nature Switzerland AG 2021 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume Three, https://doi.org/10.1007/978-3-030-50457-1_4

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phase) are associated with the elastic regime. In this context, the hard segments act as regions of molecular switches, which are responsible for the recovery of the original shape of the material. Thermo-sensitive shape memory polymers show shape change by thermal stimulation above the glass transition temperature (Tg) or melting temperature (Tm). Thermo-sensitive PU has been used successfully for biomedical devices, coatings and sporting goods (Sun et al. 2011). The shape recovery effect can also be activated by electrovoltage (Liu et al. 2016) With this in mind, electroactive shape memory PUs have been used to manufacture actuators, electronics, sensors and textiles. In addition, PU-based nanocomposites loaded with conducting nanofillers such as carbon black, carbon nanotubes (CNTs), fullerene, nanodiamonds, etc., have demonstrated improved performance compared to the pristine PUs (Meng and Li 2013; Li et al. 2012). These nanofillers have also been used as reinforcements in PU matrices in order to improve shape recovery properties. These composite materials have found applications like electroactive and (moisture-, pH- and thermo-) sensitive materials for different industries such as aerospace, biomedical engineering, electronics and energy appliances (Roy et al. 2010; Hu et al. 2012; Mura et al. 2013). Despite the patented design and development of shape recovery PU materials, there are still several challenges to overcome. In this chapter, shape memory PU and their nanocomposites will be explored as novel materials for advanced applications.

4.2  PU PU is a polymer with unique properties (Thakur and Hu 2017), which have rigid and soft thermoplastic forms, as well as thermoset forms (Hepburn 2012; Liao et  al. 2019). The urethane bond in this polymer can offer hydrogen (H)-bonds between the polymer chains. The thermal and non-flammability properties of PUs have been explored for various technical uses (Kausar 2016b), as well as these materials have been used as anticorrosive coatings (Montemor 2014; Mo et al. 2015). Other unique and useful properties of PU is that it can conduct electromagnetic interference shielding (Zeng et al. 2016). High performance PU has also been used for multiple engineering applications (Caracciolo et al. 2019). PU characteristics can be modified for a desirable end application using different synthesis approaches (Prisacariu 2011). For example, segmented PU chains may be crosslinked, as a result, crosslinked PU segments may have reduced mobility, thus promoting shape recovery property.

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4.3  Shape Memory PU Shape memory PUs are an evolving class of smart polymers (Pal 2019). Tailoring the molecular design and functionality can alter the recovery processes involved. Initially, shape memory PUs have thermally induced effects, and then, moisture-, pH- and pressure-sensitive PUs have been studied. Keeping this in mind, Chen et al. (2010) prepared pyridine containing supramolecular PUs. These authors synthesized PU using 1,4-butanediol, hexamethylene diisocyanate and N,N-bis(2-­ hydroxylethyl)isonicotinamine (Fig.  4.1). The low N,N-bis(2-hydroxylethyl) isonicotinamine content resulted in a fine shape memory property of PU materials. The soft and hard PU segments were formed by H-bonding between the urethane and pyridine groups. H-bonds between urethane groups acted as physical net points for the shape memory effect, while molecular switches were formed by interactions between pyridine rings (Jiang et al. 2019). Usually, the hard segments are related to N

HO

O

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HO

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O N,N-bis(2-hydroxylethyl) isonicotinamine

N

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hexamethylene diisocyanate

O C O

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O

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OH

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1,4-butanediol

O O

O

N

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N H

H N

C

O

O

O

O C

N H

H N

C O

O N

Fig. 4.1  PU synthesis

n

m

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Fig. 4.2  A schematic representation of shape memory effect from thermo-sensitive PU

a high thermal transition temperature, while the soft segments have a low thermal transition temperature, which can act as switching segments. In general, the performance of the shape memory depends on the thermal transition of the soft segments. However, the memory effect is generally based on Tg, Tm and transition temperature (Ttrans) (Fig.  4.2). Thermo-responsive shape memory PUs have gained interest in research due to the easy synthesis, processing and shape memory tractability (Calvo Correas et al. 2019). Zhou et  al. (2011) prepared pH-sensitive PU from poly(ε-caprolactone)hydrazone-poly(ethylene glycol)-hydrazone-poly(ε-caprolactone) diol. The pH-­ sensitive PU was used as a carrier for the micellar drug. Such PUs could work in a wide pH range and possess significant applications for drug delivery and other biomedical fields (Lamba 2017). The PUs with cleavage under acidic conditions (pH ∼ 4–6) are preferred for the shape memory effect. According to Leng et  al. (2011), light-induced shape memory PUs have also been used for different purposes. The photo-responsive PUs having cinnamic groups have shown fixation in complex shapes, such as fiber, film or spiral forms (Salgado et al. 2018). Electroactive shape memory PUs have also been researched (Biswas et al. 2018). In particular, conducting nanofillers such as carbon black, CNTs, graphene and nickel nanorod have been introduced into PUs to induce conductivity and electroactive effect, thus obtaining electronic and medical devices.

4.4  Shape Memory PU/Nanocarbon Composite 4.4.1  CNT-Loaded Shape Memory PU CNT is a nanocarbon material with outstanding electrical, mechanical and thermal properties (Mittal et al. 2015; Zaporotskova et al. 2016). In line with this, Raja et al. (2011) developed functional metal NP-loaded CNT/PU nanocomposites via melt

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mixing, thus obtaining improved electrical conductivity and shape memory properties for nanocomposites compared to pure PU. Furthermore, the Tg values of the functional metal NP-loaded CNT/PU nanocomposites were higher than the pristine CNT/PU nanocomposites. This possibly caused by an increased interaction between nanofiller (CNT) and PU. Chen et al. (2015) also designed CNT-loaded TPU nanocomposites, varying the nanofiller (CNT) content from 0.5 to 5 wt.%, as a result, nanocomposites with better electrical and mechanical properties than pure TPU were obtained. This possibly due to the formation of the percolation network, which allowed to increase the conductivity properties of composite materials containing more than 2 wt.% of CNT. Furthermore, the strain sensitivity and shape memory properties of the nanocomposite were also improved using 5 wt.% of CNT. Figure 4.3 shows dynamic mechanical properties of pure TPU and their nanocomposites. The nanocomposite containing less than ≤1 wt.% filler did not show a significant reinforcing effect. However, a higher load resulted in an increase of the Tg values in the composite materials.

4.4.2  Fullerene-Loaded Shape Memory PU Fullerene is a nanocarbon nanofiller with prominent electronic and optical properties (Nicolaidis et al. 2019). Fullerene applications are often limited due to its insolubility and difficult processing. The conducting carbonaceous nanofiller-loaded PU has shown shape memory performance (Yang et al. 2012; Du et al. 2015). In this context, Zhang et al. (2003) manufactured self-crosslinked fullerene/urea/PU nanocomposites from the synthesis of 4,4ˈ-diphenylmethane diisocyanate, aminoethylaminopropyltrimethoxysilane and poly(tetramethylene oxide), resulting in materials with a fine dispersion and optical limiting properties. Tayfun et al. (2015) also studied TPU and their fullerene-loaded nanocomposites (0.5–2  wt.%). These authors reported that the silane-modified fullerene (0.5%)-loaded nanocomposite had a sigTPU TPU0.5C TPU2C TPU5C

6000 Storage Modulus (MPa)

Fig. 4.3 Dynamic-­ mechanical thermal analysis (DMTA) from pure TPU and their CNT/ TPU nanocomposites. Reproduced with permission from Chen et al. (2015)

5000 4000 3000 2000 1000 0 –70

–60

–40 –30 –20 –50 Temperature (°C)

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Table 4.1  Mechanical properties from TPU and their fullerene-loaded composites Samples TPU TPU/0.5% C60 TPU/1% C60 TPU/1.5% C60

σm (MPa) 24.8 ± 1.7 38.9 ± 2.4 35.7 ± 2.0 32.4 ± 1.9

εb (%) 331.8 ± 4.3 574.0 ± 5.5 573.3 ± 6.9 419.7 ± 5.8

E (MPa) 33.4 ± 3.0 65.3 ± 2.8 65.1 ± 3.2 64.1 ± 3.5

Reproduced with permission from Tayfun et al. (2015)

nificant improvement in mechanical performance in terms of maximum stress (σm), strain at break (εb) and Young’s modulus (E) compared to pure PU (Table  4.1) (Tayfun et al. 2015). Furthermore, surface modification of fullerene resulted in fine dispersion of the filler within the matrix, as well as improved mechanical properties were obtained in the surface-modified nanocomposite compared to non-surfacemodified filler-loaded nanocomposite (Tayfun et al. 2015). Following Dobashi et al. (2014), fullerene/PU nanocomposites also have potential applications as actuators, electronic packaging and gas barrier applications (Dobashi et al. 2014).

4.4.3  Graphene-Loaded Shape Memory PU Graphene is two-dimensional nanomaterial with a high electrical conductivity, mechanical properties, surface area (700–1500 m2/g), surface polarity and thermal resistance (Kausar 2018). Recently, thermally reduced graphene (TRG) with improved physical properties has also been developed by Son et al. (2016). In this regard, Kim et al. (2014) developed allyl isocyanate-modified graphene-loaded PU nanocomposites (0–2.5 phr), and observed a shape recovery effect in the nanocomposites containing a filler concentration above 1.5 phr. Kim et al. (2015) also prepared electroactive shape memory PU nanocomposites from the synthesis of 1,3-butandiol, 4,4-methylenebis(phenyl isocyanate) and poly(tetramethylene ether) glycol, and used allyl isocyanate-modified TRG (iTRG) as a nanofiller. Figure 4.4 shows the shape recovery behavior of the nanocomposites. The electric current did not show any shape change at low iTRG contents. However, the high temperature and current induced a shape recovery (Table 4.2). It should be noted that the nanocomposites obtained by Kim et al. (2015) must still be optimized.

4.4.4  Nanodiamond-Loaded Shape Memory PU Nanodiamond is a chemically inert and mechanically stable material, which has active surface groups such as aldehyde, carboxylic acid, epoxide, hydroxyl, nitro, etc., and have a size of ~5 nm (Zou et al. 2010). The use of nanodiamond as a nanofiller has been appreciated due to its biocompatibility, mechanical properties, scal-

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Fig. 4.4  Electroactive shape memory behavior of EP00 (a), EP10 (b), EP15 (c), EP20 (d), EP25 (e). The straight casting line was deformed (left) and recovered (right) partially with EP20 and almost completely with EP25. EP00 and EP10 did not respond to the stimulation of the electric current. Reproduced with permission from Kim et al. (2015) Table 4.2  Electroactive shape memory performance from iTRG/PU nanocomposites at 50 V for 2 min PU sample code EP00 EP10 EP15 EP20 EP25

iTRG (Phr) – 1.0 1.5 2.0 2.5

Temperature (°C) 17 17 18 46 64

Shape recovery (%) 0 0 0 44.4 96.7

Reproduced with permission from Kim et al. (2015)

able synthesis and thermal conductivity. Nanodiamond has also found importance for biomedical, composite and electrochemical applications (Gong et  al. 2012; Yadav et al. 2013). In particular, the functional nanodiamond has a better opportunity to interact with PU matrix compared to the non-functional nanodiamond (Williams et al. 2010). With this in mind, Kausar (2016a) manufactured blends from diglycidyl 1,2-cyclohexanedicarboxylate epoxy and PU, which were reinforced with nanodiamonds (0.1–5  wt.%). These authors reported that the inclusion of 5  wt.% of nanofiller led to a 47% and 80% increase in the σm and E values, ­respectively, compared to the pure PU matrix, while the heat-induced shape memory effect of the nanocomposite showed a 95% recovery in the original shape, due to the self-­assembled interpenetrating network formed by the epoxy/PU blend, which had a unique morphology (Kausar 2016a). On the other hand, Yoo et  al. (2017) prepared poly(ε-caprolactone)diol-functionalized nanodiamonds to be used

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as a filler in PU matrices. As a result of the inclusion of functional nanofillers in the PU matrix, a high-performance shape recovery of more than 95% was obtained in the nanodiamond-loaded composites (0.5–2%), due to the better interactions between the functional nanodiamonds and the PU, thus leading to better mechanical properties and shape recovery. Thus, the development and integration of functional nanodiamonds in the PU matrix can result in increased mechanical strength and shape memory properties.

4.5  A  pplication of Shape Memory PUs and Their Nanocomposites Conductive nanofiller-loaded shape memory PUs have shown good electrical, mechanical, shape recovery, and thermal properties (Zhao et  al. 2015). Fibrous material-reinforced shape memory PUs have been used in the aerospace industry in deployable solar panels, extendible tubular booms, hinges, morphing wings, reflective antennas, roll-up booms and truss booms (Lan et al. 2009; Liu et al. 2014). PU and their nanocomposites also have a thermo-sensitive shape recovery effect. However, external heating can sometimes be derogatory to stimulate the conventional shape memory effect. These materials have also been used to avoid the shock effect and heavy weight. The electroactivate shape memory PUs and their nanocomposites have been used in actuators, and their actuation performance has been used for space vehicles (Hager et al. 2015). Conductive nanofiller-loaded shape memory PUs can cause spontaneous electric triggering of the PU nanocomposites due to technological relevance. The shape recovery rate generally depends on the magnitude of the applied voltage in electrically-induced shape memory materials. The carbon black NP-loaded PU nanocomposites have shown an improved electroactive memory effect compared to the micro-sized conductive filler-loaded PU nanocomposites (Włoch et al. 2019). Furthermore, the nanofiller can form a conductive network interconnected with homogeneously dispersed NPs. The high nanofiller load can also cause aggregation and low conductivity properties. In addition, the poor dispersion of nanofiller in the matrix can lead to low σm and E values in shape memory PUs. Shape memory PUs have also found application for biomedical devices and systems (Leng et al. 2011). For example, Neffe et al. (2009) studied the biodegradability and the effect of drug release in a shape memory PU. In this context, controlled drug release can be designed from appropriate switching segments and network structures, which can be activated by mechanical or thermal stimuli to allow drug diffusion (Small IV et al. 2005). Shape memory PUs have also been suggested for removal of the blood clots (Small IV et al. 2005). This is possible due to the shape recovery effect of the PU microactuator (Fig. 4.5). These materials can also be used for dental applications such as orthodontia (Jung and Cho 2010).

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Fig. 4.5  Scheme illustrating the performance of a shape memory PU microactuator for removing a blood clot: (a) formation of temporary straight rod, (b) heating to form permanent corkscrew and (c) retraction of microactuator to capture the clot

4.6  Summary Poly(urethane) (PU) and their nanocomposite have been used as versatile materials responsive to several external stimuli such as electricity, humidity, light, mechanical force, pH, among others. In particular, conducting carbon nanoparticle-loaded PU nanocomposites such as carbon nanotubes, fullerene, graphene and nanodiamonds have been of great interest, as these materials have multiple improved properties compared to the pure PU matrices. The development in the field of shape memory PUs and their nanocomposites have had various technological applications in the aerospace, biomedical and electronics industries. Finally, the design of shape memory PUs and their nanocomposites must be focused in order to obtain materials with good mechanical properties, high recovery strain, and low cost and density of these materials. These materials could also lead to several advanced high-performance applications for the automotive and electronics industries. Acknowledgments  The author has no acknowledgements to be made. Conflicts of Interest  The author declares no conflict of interest.

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Chapter 5

Plastic Receptors Developed by Imprinting Technology as Smart Polymers Imitating Natural Behavior Alberto Gómez-Caballero, Nora Unceta, M. Aránzazu Goicolea, and Ramón J. Barrio

Abstract Molecularly imprinted polymers (MIP) have attracted considerable attention as smart materials so far. They are known to be capable of imitating the recognition event that happens biologically in living organisms and, in this light, they have noteworthy been used as substitutes of natural receptors. Traditionally, imprinted polymers have been developed by bulk free radical polymerization (FRP), obtaining rigid polymers, which are lately ground to get fine particles. Bulk synthesis of imprinted polymers was substituted by other polymerization approaches in order to overcome typical drawbacks associated with bulk MIPs. Recently, new polymer syntheses have emerged such as solid phase imprinting which have allowed for obtaining smart materials with higher affinity for the target ligand, achieving binding affinities similar or even higher than natural receptors. Materials obtained this way are known as plastic antibodies. This chapter focuses on recent advances on imprinted polymers as potential substitutes of natural receptors, emphasizing on new synthesis strategies and novel imprinted nanomaterials. Keywords  Molecularly imprinted polymers · Nanoparticles · Plastic antibodies · Smart materials

5.1  Introduction The molecular recognition event is a fundamental biochemical process for the functions of living systems, and therefore, changes in this receptor-ligand interaction lead to relevant modifications in cellular behaviors. The ability of biological hosts

A. Gómez-Caballero (*) · N. Unceta · M. A. Goicolea · R. J. Barrio MetaboloMIPs Research Group, Department of Analytical Chemistry, University of the Basque Country UPV/EHU, Vitoria-Gasteiz (Alava), Spain e-mail: [email protected] © Springer Nature Switzerland AG 2021 T. J. Gutiérrez (ed.), Reactive and Functional Polymers Volume Three, https://doi.org/10.1007/978-3-030-50457-1_5

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to specifically bind to a particular molecular structure is a key factor in molecular recognition. Many of the current and future diagnostic tests are based on the use of biological receptors as biochemical recognition elements, representing an important economic impact. In 2016, the global market for in vitro diagnostics was worth $55 billion and is projected to reach $89,86 billion by the end of 2025 (Transparency Research Market 2018). Although biological receptors can show excellent recognition capabilities and good selectivity, their low stability and high cost, limit their applications (Baggiani et al. 2013). These problems pose serious challenges in the transfer of a large number of bio-diagnostic platforms to Point of Care Testing systems. The mechanisms involved in molecular recognition provide valuable information about natural receptor-ligand interactions, which allows to mimic or even modulate cellular functions by chemical means. A promising approach to create artificial receptors is molecular imprinting technology (MIT). This technique allows to create three-dimensional (3D) polymer networks that present shape memory and functionality of a certain target molecule (ligand). It involves the copolymerization of the mixture of monomers (functional(s) and crosslinker) in the presence of a template molecule (ligand) and a solvent, known as porogen. After polymerization, the template is removed to obtain molecularly imprinted polymers (MIP) having complementary binding sites to the template. MIPs are capable of selectively rebinding template molecules. Therefore, molecular imprinting technology is based on the molecular recognition of two molecules (MIP-template) with a complementary ‘guest-host’ relationship. The molecular recognition mechanism in MIPs is reminiscent of natural interactions and constitutes an induced molecular memory, which makes them capable of selective recognition. In addition, they have certain advantages such as chemical stability, easy preparation, long shelf life, low cost, mechanical robustness and resistance to high pressure and temperature. Furthermore, it does not require any preclinical development involving animals. They can also behave like smart polymers that respond to single or multiple stimuli such as biomolecules, electric and magnetic fields, ionic factors, light intensity, pH, temperature, etc (Ge et al. 2013; Zarrintaj et al. 2019). In this chapter, we present the most commonly used polymerization mechanisms to synthesize MIPs with well-defined molecular structures, in several various formats and with improved binding properties. In order to determine the aspects related to the binding behavior of imprinted polymers, the main methods used in their morphological and physicochemical characterizations, as well as the binding experiments performed with these materials, are also presented here. Although much still remains to be done, the progress made in this field has undoubtedly contributed to the use of MIP as substitutes for antibodies and other natural receptors in many applications. For example, it is impossible to ignore the great potential of molecular imprinting to build smart materials potentially applicable in the healthcare market.

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5.2  Polymerization Mechanisms 5.2.1  Free Radical Polymerization (FRP) FRP has been the main method for synthesizing MIPs so far and is the simplest technique for large-scale preparation of imprinted polymers using acrylic or vinyl monomers (Haupt et al. 2012). Here, the functional monomers and the target compound, called template, are dissolved in a given amount of solvent resulting in a more or less stable complex governed more frequently by non-covalent interactions, but not limited to. Non-covalent imprinting has been, by far, the most widely used imprinting approach for MIP development (Yilmaz et al. 2004), mainly due to its versatility, the large amount of available monomers and its faster binding kinetics compared to others, such as the covalent approach (Sellergren and Hall 2001). The prepolymerization complex mentioned above is later immobilized in a highly crosslinked 3D polymer network due to the addition of a crosslinking monomer to the polymerization mixture. This reaction starts thanks to an initiator, which decomposes into two active radicals that react with double bonds to initiate chain polymerization (Belbruno 2019). FRP consists of three principal steps: initiation, propagation and termination (Fig. 5.1). During the initiation, a small amount of an initiator is cleaved into two active radicals under ultraviolet (UV) radiation or temperature and reacts with acrylic or vinyl monomers giving rise to intermediate radical species. Subsequently, propagation starts and intermediate free radical species react with single monomers allowing chain lengthening. FRP initiation is usually slow, slower than chain

Fig. 5.1  FRP mechanism

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p­ ropagation, and some initiators are not unconsumed at the end of polymerization (Matyjaszewski 2009). The termination of the growing chains usually arises through the coupling of two radical species or disproportionation, obtaining in the latter a double bond and a C-H bond at the end of the chain. In addition, termination can also occur by chain transfer to monomers, polymers, solvents, template or transfer agents, by reaction of an active chain end with the initiator or even by reaction of radical species with impurities, oxygen or polymerization inhibitors. In FRP, the concentration of radicals is relatively high and they are generated continuously from the beginning to the end of the polymerization process. In light of this, propagation and termination reactions are uncontrollable, since radical species with very high reactivity can interact to give dead polymer chains. In this context, FRP provides heterogeneous polymers with a wide molecular weight distribution (MWD or polydispersity index − PDI = weight-average apparent molecular weight (Mw)/number-average apparent molecular weight (Mn)), and the MIPs developed under these conditions have a more heterogeneous distribution of binding sites (Ye 2015; DiPasquale and Byrne 2016), also known as polyclonality of cavities (Wulff 2013), which may be responsible for cross-reactivity. Consequently, this type of MIPs are considered analogous to polyclonal antibodies being able to recognize not only the target compound (template) but also other structural analogues which may belong to the same family (Haupt et  al. 2012; Garcia et  al. 2015). To overcome the main drawbacks typically associated with MIPs prepared under FRP conditions, MIP technology adopted new polymerization strategies based on controlled/living mechanisms, which can be implemented simply by replacing the conventional initiator used in FRP synthesis (Zhang 2013).

5.2.2  Reversible Deactivation Radical Polymerization (RDRP) Controlled/living Controlled radical polymerization (CRP), namely RDRP as recommended by the International Union of Pure and Applied Chemistry (IUPAC) (Jenkins et al. 1996), emerged in the 1980s with the objective of minimizing the bimolecular termination, thereby increasing the shelf life of growing polymer chains (Beyazit et al. 2016). In RDRP, fast initiation and slow propagation are implicit and polymerization continues until total monomer consumption (Otsu and Matsumoto 1998), or until the UV/heat source is switched off, being possible to restart it again at any time. This polymerization approach, unlike FRP, allows for control over architecture, molecular weight (Mw), polymer size and tacticity, which results in more efficient polymer networks with more homogeneous structures (Salian and Byrne 2013). In this polymerization strategy, a dynamic equilibrium is established during polymerization between active propagation chains and dormant species, which are not capable of propagation or termination. Since dormant species are predominate over propagating chains, the equilibrium is pushed towards the deactivation of active chains, which leads to an excess of dormant species. This increases the

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l­ifetime of the growing chains from about 1 s in FRP to more than 1 h in RDRP allowing controlled propagation where all polymer chains grow at almost the same rate (Braunecker and Matyjaszewski 2007). RDRP is often slower than FRP, which favors the generation of much more homogeneous imprinted polymers, more likely to have a homogeneous distribution of the binding site. This makes MIP resemble to monoclonal antibodies, which have (pseudo) monoclonal binding properties (Canfarotta et  al. 2016a) and less cross-reactivity than the imprinted polymers developed by FRP (Fig. 5.2). Currently, different subclasses of RDRP approaches can be found in the literature according to their polymerization mechanism, namely, iniferter polymerization, nitroxide-mediated radical polymerization (NMP), atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization. Iniferter polymerization and NMP are also known as stable free radical polymerization (SFRP) where reversible cleavage of weak bonds in a covalent species results in a growing radical and a less reactive species (dormant radical) which cannot promote polymerization. This dormant compound reacts reversibly with growing radicals, resulting in deactivated species that can spontaneously return (by UV radiation or thermally) to the active state (Braunecker and Matyjaszewski 2007).

Fig. 5.2  Schematic illustration of imprinted sites created in a polymer matrix by free radical and controlled/living radical polymerizations

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5.2.2.1  Iniferter Polymerization Iniferters are compounds that promote initiation, transfer and termination (ini-tiator, trans-fer agent, ter-minator) in radical polymerizations. Since the bimolecular termination and other transfer reactions are insignificant, they allow the introduction of monomers in the iniferter bond, obtaining polymers with two iniferter fragments at the chain ends (Otsu 2000). The concept of iniferter was first proposed by Otsu et  al. (1982) to allow the controlled synthesis and molecular design of polymers through radical polymerization (Otsu and Matsumoto 1998). They described the synthesis of styrene and methyl methacrylate using phenylazotriphenylmethane and benzyl-N,N-diethyldithiocarbamate as thermal and photochemical iniferters, respectively (Tasdelen and Yagci 2013). Photoiniferters such as dithiocarbamate esters are dissociated under UV radiation into a reactive carbon-centered radical and a relatively stable diethylcarbamylthiyl radical, known as dormant radical (Fig. 5.3), which is less- or non-reactive and cannot promote initiation. If the energy source (in form of UV radiation or even heat) is stopped, both radicals are recombined again and chain growth stops, being it possible to restart it again using the same or other monomer mixtures (Haupt et al. 2012). The produced carbon radicals are extremely reactive and initiate polymerization by reacting with mononomers having double bonds, which leads to a p­ ropagating

Fig. 5.3  Iniferter polymerization mechanism

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macroradical, which can then be combined with the dormant radical, giving rise to a dormant polymer chain. Alternatively, propagation macroradicals can also react with iniferter molecules or dormant polymer chains, thus promoting chain transfer (Otsu and Matsumoto 1998). MIP materials synthesized using iniferter polymerization have been developed almost exclusively through photochemical initiation (Beyazit et al. 2016). Table 5.1 includes different iniferters used so far in imprinting technology, as well as their main features. 5.2.2.2  Nitroxide Mediated Polymerization Georges et al. (1993) reported an FRP process conducted by benzoyl peroxide but in the presence of a stable radical, namely 2,2,6,6-tetramethyl-1-piperidynyloxy (TEMPO), to synthesize poly(styrene) (PS). The polymers were synthesized by thermal initiation at 123 °C and presented polydispersities below 1.3 (Georges et al. 1993). This publication made by NMP has gained considerable attention. NMP or aminoxyl-mediated radical polymerization (as recommended by the IUPAC) is a stable free radical mediated polymerization in which the deactivation of free radicals involves a reversible coupling with aminoxyl radicals (Jenkins et al. 1996), also known as nitroxides. These are capable of reversibly deactivating active propagation (macro)radicals, to produce a (macro)alkoxyamine as the predominant species (Nicolas et al. 2013), which allows the establishment of a rapid equilibrium between the active and dormant species. This equilibrium is responsible for providing the polymerization with a living character since, otherwise, radicals can undergo a natural termination. Initially, NMP involved a bicomponent pathway, in which a conventional thermal initiator such as 2,2′-azobis isobutyronitrile (AIBN) or benzoyl peroxide (BPO) is used in the presence of a nitroxide such as TEMPO. Alternatively, unimolecular initiators, such as alkylated TEMPO (Hawker et al. 2002), which decompose under temperature can also be used, resulting in an initiator radical and a nitroxide (1:1). These compounds are known as alkoxyamine initiators and allow better control over Mw and architecture of resulting polymers compared to those mentioned above (Benoit et al. 1999; Nicolas et al. 2013). A summarized polymerization mechanism for NMP is illustrated in Fig. 5.4. Despite its potential benefits, NMP has been little used for imprinting so far. To the authors’ knowledge, only one article describes NMP in imprinting technology. In light of this, Boonpangrak et al. (2006) developed a MIP for cholesterol by NMP and demonstrated that it had higher selectivity than MIPs prepared by traditional FRP, probably due to a better ordered polymer structure. In any case, the MIP synthesis by NMP has not attracted much attention, which can be attributed to the high temperatures required for activation-deactivation of alkoxyamines, which can be counterproductive when MIPs are synthesized using the non-covalent approach (Haupt et al. 2012). Usually, temperatures above 100 °C are employed in this strategy. However, the latest trends on NMP point towards alkoxyamines capable of

4-((3-(trimethoxysilyl)propyl)carbamoyl) benzyl diethylcarbamodithioate

3-(trimethoxysilyl)propyl diethylcarbamodithioate

3-​mercaptopropyl diethylcarbamodithioate

Iniferter

An iniferter also useful for developing MIP layers on ceramic carbon electrodes in sensor technology. Silane iniferter can be attached to glass substrates for surface-initiated polyerizations, giving rise to thin MIP films grafted into glass.

This silane iniferter can be covalently bonded to silica or glass surfaces to promote controlled surface-grafted polymers.

Prasad et al. (2013)

Prasad et al. (2011)

Bossi et al. (2010) Gutierrez-­Climente et al. (2016) Kushwaha et al. (2018)

Features References García-Mutio et al. (2018) An iniferter capable of self-­assembling on gold (Au) surfaces. It can be useful for polymer grafting from Au substrates for the construction of sensors.

Table 5.1  Description of different photochemical iniferters employed for molecular imprinting

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Benzyl diethyldithiocarbamate

4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid

4-(trimethoxysilyl)benzyl diethylcarbamodithioate

Iniferter

References Rueckert et al. (2002) Perez-Moral and Mayes (2007) Barahona et al. (2010) Halhalli et al. (2012)

(continued)

Subrahmanyam et al. (2013) Wang et al. (2016) Gutierrez-­Climente et al. (2017)

Marchyk et al. (2014) Synthesis of core-shell nanoparticles capable of grafting onto various types of shell. Polymerization is conducted under hybrid UV/thermal initiation.

It is used to develop MIP micro- and nanoparticles (NPs) by precipitation polymerization or the post-dilution method.

Lee and Kim (2009) Carbon nanotubes can be modified on the surface with this iniferter to graft MIP thin films if nanotubes are previously hydroxylated with Tween 20. Garcia-Soto et al. (2017) Iniferter can undergo thermal or photoinitiated polymerization with visible blue (435 nm) or green (525 nm) light.

Features An iniferter capable of being covalently bonded to silica particles for grafting MIP thin films in a straightforward way. It has also been used for the development of core-shell particles.

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4-(((diethylcarbamothioyl)thio)methyl) benzenediazonium tetrafluoroborate

Water-soluble dithiocarbamate iniferter platform

Iniferter

Table 5.1 (continued)

Iniferter containing a diazonium end-group for surface anchoring on magnetic iron oxide NPs.

Griffete et al. (2012)

Features References Des Azevedo et al. (2013) It is used in sensor technology to graft MIP thin layers onto a polyaniline-­derived film to build hybrid sensors. A water-soluble iniferter platform useful to develop Bonomi et al. (2016) imprinted particles and gels in aqueous media through photochemical initiation.

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Fig. 5.4  Nitroxide mediated polymerization mechanism

being activated at lower temperatures than usual (Ye 2015; Edeleva et  al. 2019), light-sensitive alkoxyamines, which have led to nitroxide-mediated photopolymerization (Guillaneuf et al. 2010; Telitel et al. 2014), or even alkoxyamines capable of being activated by a combination of thermal and photochemical mechanisms (Morris et  al. 2015). These findings may contribute to the expansion of NMP in imprinting technology in the near future. 5.2.2.3  Atom Transfer Radical Polymerization Based on atom transfer radical addition (ATRA), a modified version of Kharash addition, was reported by Wang and Matyjaszewski (1995a,b) in order to obtain the controlled synthesis of PS using 1% mol Copper (Cu) as catalyst. The process was called ATRP and was studied simultaneously by Kato et al. (1995) and Percec and Barboiu (1995). Since 1995, ATRP has experienced a remarkable expansion in polymer chemistry and the number of papers on this topic has grown up exponentially. In ATRP, control over polymerization happens thanks to fast initiation and catalyzed intermittent activation of dormant species to form propagation radicals. The first allows for the concurrent growth of chains, while the second avoids premature termination (Ribelli et al. 2019). The mechanism of traditional ATRP lies in the cleavage of an alkyl halide bond by a transition metal complex. This reaction involves the addition of the halide to the metal center and the formation of an oxidized complex and a radical species (Matyjaszewski 2009, 2018) (Fig.  5.5). The radical is then propagated until it

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Fig. 5.5  Mechanisms of conventional and reverse ATRPs

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reacts reversibly with the oxidized metal complex. At this point, the halide is transferred back again from the oxidized metal, giving rise to a dormant species (Pn−X) and the transition metal complex, which will be ready for reactivating any dormant species. Apart from alkyl halides traditionally used, there are other ways to conduct ATRP. Consequently, a conventional free radical initiator such as AIBN is used in reverse ATRP (Fig. 5.5), which decomposes into two active radicals, which in turn react with the transition metal complex at highest oxidation state. As a consequence, the halogen is transferred from the complex to the radicals coming from the decomposition of the initiator, giving rise to a dormant halide species (Matyjaszewski 2018). Propagation takes place through the addition of monomers to active (macro) radicals before being deactivated by the oxidized metal complex. The fast initiation and reversible deactivation of active species in the ATRP reactions favor the uniform growth of all polymer chains, obtaining polymers with narrow MWDs (Banerjee et al. 2014). The normal and reverse ATRP aforementioned was combined by Gromada and Matyjaszewski (2001) in order to overcome common inconveniences associated with these systems, which were limited to less active catalysts, to avoid too fast termination reactions, which may arise using too reducing catalysts, i.e. more active. In addition, combined ATRP was found to be compatible with low polymerization temperatures, since the system tolerated the slow decomposition of the initiator (Gromada and Matyjaszewski 2001). Likewise, other ATRP systems were also reported later, based on initiation mechanisms rooted in non-radical generating reducing agents, which resulted in activators generated by electron transfer ATRP (AGET) and activators regenerated by electron transfer ATRP (ARGET). More recently, other ATRP initiation mechanisms have emerged, such as electrochemically, mechanically/ultrasonically and photochemically initiated ATRP (Ribelli et al. 2019). In general terms, ATRP has been quite likely the most studied CRP technique, which can be attributed to the high availability of initiators, its applicability to a large number of monomers such as acrylonitriles, (meth)acrylates and styrenes, and mild reaction conditions (Wang et al. 2016). In any event, ATRP has not been widely used in imprinting technology. One reason for this may revolve around the incompatibility of many acidic or hydrogen bonding monomers and/or templates with ATRP, which are responsible for hindering proper polymerization, since they can negatively influence the metal-ligand complex (Ramakers et al. 2019). The first MIPs developed under ATRP were mainly confined to surface imprinting (Wei et al. 2005; Wang et al. 2006; Lu et al. 2009). MIPs with adjustable pores and uniform structures were obtained through ATRP using 4-vinylpyridine and ethylene glycol dimethacrylate (EDMA) as momoners. Zu et al. (2009) combined for the first-time precipitation polymerization and ATRP. Based on previous results on surface imprinting, they demonstrated that both normal and reverse ATRP could also be used to obtain tailor-made microspheres with improved bonding properties, compared to MIPs developed under traditional precipitation polymerization by FRP (Zu et al. 2009). The potential benefits of ATRP for small templates (