Polymer science and innovative applications materials, techniques, and future developments 9780128173039, 0128173033

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Polymer Science and Innovative Applications

Polymer Science and Innovative Applications Materials, Techniques, and Future Developments Edited by

Mariam Al Ali AlMaadeed Qatar University, Doha, Qatar

Deepalekshmi Ponnamma Center for Advanced Materials, Qatar University, Doha, Qatar

Marcelo A. Carignano Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University, Doha, Qatar

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-816808-0 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

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Contents List of contributors ...................................................................................................xi

CHAPTER 1 Polymers to improve the world and lifestyle: physical, mechanical, and chemical needs ................ 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Mariam Al Ali AlMaadeed, Deepalekshmi Ponnamma and Ali Alaa El-Samak Introduction ....................................................................................1 Industrial revolutions and polymer applications ...........................2 Polymers: general classification and production ...........................3 Current lifestyle and the need of polymers .................................11 Polymers to composites ...............................................................12 Specific requirements of polymers using physical, mechanical, and chemical methods..................................................................13 Internet of Things and smart materials........................................14 Conclusions ..................................................................................16 Acknowledgments ....................................................................... 16 References.................................................................................... 16

CHAPTER 2 Morphology analysis................................................... 21 2.1 2.2 2.3 2.4 2.5

Anton Popelka, Sifani Zavahir and Salma Habib Introduction ..................................................................................21 Polymer morphology....................................................................21 Characterization methods.............................................................27 Applications..................................................................................47 Conclusion ....................................................................................58 Acknowledgments ....................................................................... 58 References.................................................................................... 58

CHAPTER 3 Chemical analysis of polymers .................................. 69 3.1 3.2 3.3 3.4 3.5 3.6

Leena Nebhani and Aanchal Jaisingh Introduction ..................................................................................69 Molecular weight determination ..................................................70 Infrared spectroscopy ...................................................................74 Nuclear magnetic resonance spectroscopy ..................................85 Mass spectrometry......................................................................101 Conclusion ..................................................................................108 Acknowledgments ..................................................................... 111 References.................................................................................. 111

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CHAPTER 4 Mechanical analysis of polymers ............................ 117 4.1 4.2 4.3 4.4 4.5

Kalim Deshmukh, Toma´sˇ Kova´r´ˇık, Aqib Muzaffar, M. Basheer Ahamed and S.K. Khadheer Pasha Introduction ................................................................................117 Mechanical properties of polymers............................................119 Dynamic mechanical thermal analysis of polymers..................136 Factors affecting the mechanical properties of polymers .........140 Conclusion ..................................................................................147 References.................................................................................. 147

CHAPTER 5 Physical and thermal analysis of polymer .............. 153 Pallabi Saikia 5.1 Introduction ................................................................................153 Techniques used for physical and thermal analysis of polymers..................................................................................... 154 5.2 Conclusion ..................................................................................202 Acknowledgment ....................................................................... 203 References.................................................................................. 203

CHAPTER 6 Theoretical simulation approaches to polymer research .................................................................... 207 Tao Wei and Chunlai Ren 6.1 Introduction ................................................................................207 6.2 Methodologies and applications.................................................208 6.3 Conclusion ..................................................................................221 References.................................................................................. 222

CHAPTER 7 An example of theoretical approaches in polymer hydrogels: insights into the behavior of pHresponsive nanofilms................................................ 229 7.1 7.2 7.3 7.4 7.5 7.6

Gabriel S. Longo Introduction ................................................................................229 Acid base equilibrium in dilute solutions: ideal behavior ......231 Protonation of weak polyacid hydrogel films ...........................233 Histidine-tag adsorption to pH-responsive hydrogels ...............237 Adsorption of proteins to pH-sensitive hydrogels.....................241 Conclusion ..................................................................................251 Acknowledgment ....................................................................... 251 References.................................................................................. 251

Contents

CHAPTER 8 Pectin as oral colon-specific nano- and microparticulate drug carriers ................................. 257 8.1 8.2 8.3 8.4 8.5

Badrul Hisyam Zainudin, Tin Wui Wong and Halimaton Hamdan Introduction ................................................................................257 Pectin as bioactive dietary fiber ................................................259 Pectin-based oral drug delivery system .....................................266 Oral colon-specific drug delivery mechanism...........................273 Conclusion ..................................................................................276 References.................................................................................. 277

CHAPTER 9 Starch as oral colon-specific nano- and microparticulate drug carriers ................................. 287 9.1 9.2 9.3 9.4 9.5 9.6 9.7

NorulNazilah Ab’lah and Tin Wui Wong Introduction ................................................................................287 Polysaccharides as anticancer drug carriers ..............................287 Colon anatomy and physiology .................................................288 Colon cancer...............................................................................296 Colon-specific drug delivery......................................................300 Starch as a drug carrier ..............................................................304 Conclusion ..................................................................................321 Acknowledgment ....................................................................... 321 References.................................................................................. 322

CHAPTER 10 Polymers in textiles .................................................. 331 10.1 10.2 10.3 10.4 10.5 10.6 10.7

Mabrouk Ouederni Introduction ................................................................................331 Brief history of manmade fibers ................................................332 Terminology and definitions......................................................332 Fiber manufacturing ...................................................................333 Characterization and testing of textile fibers.............................337 Polymers in textiles: major manmade fibers .............................342 Conclusion ..................................................................................359 References.................................................................................. 359

CHAPTER 11 Polymers in electronics............................................ 365 Kishor Kumar Sadasivuni, Sara Mohamed Hegazy, AAliah Aboubakr Moustafa Abdullah Aly, Sadiya Waseem and K. Karthik 11.1 Introduction ................................................................................365 11.2 Type of polymers .......................................................................367 11.3 Applications of semiconducting polymers ................................372

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11.4 Conclusion ..................................................................................387 Acknowledgment ....................................................................... 387 References.................................................................................. 388

CHAPTER 12 Polymers in robotics................................................. 393 12.1 12.2 12.3 12.4

Arunima Reghunadhan, Athira Krishna and Ajith James Jose Introduction ................................................................................393 Role of polymers in robotics .....................................................400 Applications of robotics .............................................................415 Conclusion ..................................................................................418 References.................................................................................. 418

CHAPTER 13 Polymers in optics .................................................... 423 13.1 13.2 13.3 13.4 13.5 13.6 13.7

Sneha Bhagyaraj, Oluwatobi Samuel Oluwafemi and Igor Krupa Introduction ................................................................................423 Properties of optical polymers ...................................................426 Characterization of optical properties of polymers ...................428 Polymer optics: the manufacturing technology .........................429 Applications of polymers in optics ............................................430 Future perspective and challenges in polymer optics ...............447 Conclusion ..................................................................................447 Acknowledgments ..................................................................... 447 References.................................................................................. 448

CHAPTER 14 Polymers in space exploration and commercialization .................................................... 457 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9

M.Z. Naser and A.I. Chehab Introduction ................................................................................457 Space environments, actions, and conditions ............................459 Effect of space environment on polymers.................................460 Use of inorganic polymers as building materials ......................465 Space resources ..........................................................................467 Use of polymers in space ...........................................................468 Research needs and future directions ........................................476 Novel polymers ..........................................................................478 Conclusion ..................................................................................480 References.................................................................................. 480

CHAPTER 15 Polymers in sports .................................................... 485 Meena Sadashiv Laad 15.1 Introduction ................................................................................485 15.2 Materials used in sports .............................................................487

Contents

15.3 Evolution of materials used in sports from traditional to composites ..................................................................................489 15.4 Common polymers in sports ......................................................491 15.5 Polymers in winter sports...........................................................504 15.6 Polymeric sports surfaces...........................................................506 15.7 Polymers in sports protection equipment ..................................507 15.8 Polymers in tennis ......................................................................510 15.9 Polymers in athletics and gymnastics........................................511 15.10 Polymers in golf .........................................................................512 15.11 Polymers in pole vaulting ..........................................................513 15.12 Polymers in water sports............................................................514 15.13 Polymers in motor sports ...........................................................515 15.14 Polymers in cycling....................................................................516 15.15 Polymers in sportswear ..............................................................517 15.16 Polymers in sports footwear.......................................................520 15.17 Conclusion ..................................................................................520 References.................................................................................. 522

CHAPTER 16 Polymers and food packaging.................................. 525 16.1 16.2 16.3 16.4 16.5 16.6

Behjat Tajeddin and Mina Arabkhedri Introduction ................................................................................525 Food packaging ..........................................................................526 Packaging materials....................................................................527 Some methods for biopolymers production...............................537 Biopolymers and active packaging ............................................537 Conclusion ..................................................................................539 References.................................................................................. 539

CHAPTER 17 Polymers in cosmetics ............................................. 545 17.1 17.2 17.3 17.4

Rohini P. Gawade, Shamal L. Chinke and Prashant S. Alegaonkar Introduction ................................................................................545 Understanding polymer/surfactant interactions .........................547 Use of polymers in cosmetics ....................................................547 Conclusion ..................................................................................560 References.................................................................................. 560 Further reading .......................................................................... 564

CHAPTER 18 Polymers in food ....................................................... 567 Pathik Shah 18.1 Introduction ................................................................................567 18.2 Classification of food polymers .................................................570

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18.3 Conclusion ..................................................................................587 References.................................................................................. 588 Further reading .......................................................................... 592

CHAPTER 19 Future needs and trends: influence of polymers on the environment ........................................................ 593 19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 19.10 19.11

Ammavasi Nagaraj and Mariappan Rajan Introduction ................................................................................593 Polymers in the environment .....................................................602 Polymer-based materials as a new direction for environmental remediations .......................................................604 Polymer-based materials for societal applications ....................610 Polymers: recent trends, strategic changes, economic and market demands..........................................................................617 Polymers: future impacts on energy and solar cells..................619 Consequences of the nonbiodegradable polymers derived from renewable resources ..........................................................619 Recyclability, biodegradability, and reusability of polymeric products ......................................................................................621 Polymeric products disposal ways and its impacts ...................623 Waste to wealth future perspectives of ecofriendly polymer materials development and usage ..............................................624 Conclusion ..................................................................................625 Acknowledgment ....................................................................... 625 References.................................................................................. 625

Index ......................................................................................................................635

List of contributors AAliah Aboubakr Moustafa Abdullah Aly College of Arts and Sciences, Qatar University, Doha, Qatar NorulNazilah Ab’lah Non-Destructive Biomedical and Pharmaceutical Research Centre, iPROMISE, Universiti Teknologi MARA, Puncak Alam, Malaysia; Particle Design Research Group, Faculty of Pharmacy, Universiti Teknologi MARA, Puncak Alam, Malaysia; Centre of Foundation Studies, Universiti Teknologi MARA, Dengkil, Malaysia Prashant S. Alegaonkar Department of Physics, School of Basic and Applied Sciences, Central University of Punjab, Bathinda, Punjab, India Mariam Al Ali AlMaadeed Qatar University, Doha, Qatar Mina Arabkhedri Applied Science in Mechanical Engineering, University of British Columbia, Vancouver, Canada M. Basheer Ahamed Department of Physics, B.S. Abdur Rahman Crescent Institute of Science and Technology, Chennai, India Sneha Bhagyaraj Center for Advanced Materials, Qatar University, Doha, Qatar; Centre for Nanomaterials Science Research, University of Johannesburg, Johannesburg, South Africa; Department of Chemical Sciences, University of Johannesburg, Johannesburg, South Africa A.I. Chehab Department of Civil and Environmental Engineering, Wayne State University, Detroit, MI, United States Shamal L. Chinke Department of Applied Physics, Defence Institute of Advanced Technology, Pune, India; Department of Electronic Science, Savitribai Phule Pune University, Pune, India Kalim Deshmukh ˇ New Technologies—Research Center, University of West Bohemia, Plzen, Czech Republic Ali Alaa El-Samak Materials Science & Technology Program (MATS), College of Arts & Sciences, Qatar University, Doha, Qatar Rohini P. Gawade Department of Applied Physics, Defence Institute of Advanced Technology, Pune, India

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Salma Habib Center for Advanced Materials, Qatar University, Doha, Qatar Halimaton Hamdan Razak School of Engineering and Advanced Technology, Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia Sara Mohamed Hegazy College of Arts and Sciences, Qatar University, Doha, Qatar Aanchal Jaisingh Department of Materials Science and Engineering, Indian Institute of Technology Delhi, New Delhi, India Ajith James Jose Department of Chemistry, St. Berchmann’s College, Kottayam, India K. Karthik School of Physics, Bharathidasan University, Tiruchirappalli, India S.K. Khadheer Pasha Department of Physics, VIT-AP University, Guntur, India Toma´sˇ Kova´rˇ´ık ˇ New Technologies—Research Center, University of West Bohemia, Plzen, Czech Republic Athira Krishna Department of Chemistry, St. Berchmann’s College, Kottayam, India Igor Krupa Center for Advanced Materials, Qatar University, Doha, Qatar Meena Sadashiv Laad Professor (Physics) Department of Applied Sciences, Symbiosis Institute of Technology, Symbiosis International University, Lavale, Pune, Maharashtra, India Gabriel S. Longo Instituto de Investigaciones Fisicoqu´ımicas Teo´ricas y Aplicadas (INIFTA), UNLP-CONICET, La Plata, Argentina Aqib Muzaffar Department of Physics, B.S. Abdur Rahman Crescent Institute of Science and Technology, Chennai, India Ammavasi Nagaraj Biomaterials in Medicinal Chemistry Laboratory, Department of Natural Products Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai, India M.Z. Naser Glenn Department of Civil Engineering, Clemson University, Clemson, SC, United States Leena Nebhani Department of Materials Science and Engineering, Indian Institute of Technology Delhi, New Delhi, India

List of contributors

Oluwatobi Samuel Oluwafemi Centre for Nanomaterials Science Research, University of Johannesburg, Johannesburg, South Africa; Department of Chemical Sciences, University of Johannesburg, Johannesburg, South Africa Mabrouk Ouederni Qatar Petrochemical Company (QAPCO), Doha, Qatar Deepalekshmi Ponnamma Center for Advanced Materials, Qatar University, Doha, Qatar Anton Popelka Center for Advanced Materials, Qatar University, Doha, Qatar Mariappan Rajan Biomaterials in Medicinal Chemistry Laboratory, Department of Natural Products Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai, India Arunima Reghunadhan International and Interuniversity Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Chunlai Ren National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, P.R. China Kishor Kumar Sadasivuni Center for Advanced Materials, Qatar University, Doha, Qatar Pallabi Saikia Department of Chemistry, School of Basic Science, Assam Kaziranga University, Jorhat, India Pathik Shah CIPET-Institute of Plastics Technology, Ahmedabad, India Behjat Tajeddin Agricultural Engineering Research Institute (AERI), Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran Sadiya Waseem Advance Carbon Products, CSIR-NPL, New Delhi, India Tao Wei Department of Chemical Engineering, Howard University, Washington, DC, United States Tin Wui Wong Non-Destructive Biomedical and Pharmaceutical Research Centre, iPROMISE, Universiti Teknologi MARA, Puncak Alam, Malaysia; Particle Design Research Group, Faculty of Pharmacy, Universiti Teknologi MARA, Puncak Alam, Malaysia

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Badrul Hisyam Zainudin Non-Destructive Biomedical and Pharmaceutical Research Centre, iPROMISE, Universiti Teknologi MARA, Puncak Alam, Malaysia; Particle Design Research Group, Faculty of Pharmacy, Universiti Teknologi MARA, Puncak Alam, Malaysia; Malaysian Cocoa Board, Cocoa Innovative and Technology Centre, Nilai, Malaysia; Razak School of Engineering and Advanced Technology, Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia Sifani Zavahir Center for Advanced Materials, Qatar University, Doha, Qatar

CHAPTER

Polymers to improve the world and lifestyle: physical, mechanical, and chemical needs

1

Mariam Al Ali AlMaadeed1, Deepalekshmi Ponnamma2 and Ali Alaa El-Samak3 1

Qatar University, Doha, Qatar Center for Advanced Materials, Qatar University, Doha, Qatar 3 Materials Science & Technology Program (MATS), College of Arts & Sciences, Qatar University, Doha, Qatar 2

1.1 Introduction The role of polymers in biological processes is significant as they are the molecular basis of life [1 5]. The relationship of polymers with biorelated fields start from macromolecular deoxyribonucleic acid to medicines and biomedical devices. Proteins, carbohydrates such as polysaccharides, enzymes, and tissues are arranged in the form of repeating structural units similar to that of polymer skeletons [6,7]. As living tissues are composed of polymers, these macromolecules are considered as natural allies of medicines. Many polymers like polyamides, polyesters, polyurethanes, polyethylene, silicones, polycarbonate, fluorocarbons, and so forth are used in medical fields [8]. However, biocompatibility, toxicity, biodegradability, among others, are major concerns when applying synthetic polymers in medical sectors. Biomimetic synthetic phospholipid membranes for coatings, cellophanes for kidney-related applications, hydroxyapatites for dental applications, etc. are examples of the numerous applicabilities of polymers in biomedical areas [3]. While polymers can be synthesized in many different ways using polymerization techniques, their final application including mechanical, structural, and functional properties highly depends on the conformation of the monomer units, molecular size and weight, monomer type and distribution, or polydispersity index. Based on the mode of synthesis, polymers vary as homopolymers and heteropolymers, whereas based on their origin they vary as natural and synthetic [9]. There are numerous classification strategies for polymers and studies have revealed specific shapes at atomic and nanometer resolutions. In the case of industrial applications, their chain flexibility and mobility are highly desirable

Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00001-9 © 2020 Elsevier Inc. All rights reserved.

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qualities that can be achieved by the selection of polymers with small side chains, less polarity, and noncrystallizing under deformation [10]. Crosslinking is another important area, as in the case of elastomers such as natural rubber, the process of crosslinking helps the molecules to strengthen the molecular skeleton [11]. As far as industrial applications are considered, energy efficiency is also significant. Lower energy loss, high toughness, and appreciable mechanical strength are necessary parameters in addition to lightweight and stretchability. Numerous studies have been performed on various polymers over the past few decades. Reinforcements such as macro-, micro-, and nanoparticles were also handled by several research groups [12 14]. Though this vast topic of polymer science is much investigated, there is a huge demand from technology on exploring the complete exploitation of polymeric properties. This chapter will discuss the significance of polymers to satisfy the global demand. Other than explaining the history of polymers and their classifications, the role of polymers and composites in regulating global requirements, and utilization of the chemical, mechanical, and physical properties for specific demands needed in the society are explained in this chapter.

1.2 Industrial revolutions and polymer applications Polymers represent an advanced class of materials, consisting of multiple repeating building blocks known as monomers that are linked together to form a much longer chain. The importance of polymers is due to their wide application range, as they resemble industrial, economical, medical, and academic interests and goods that enhance our lives on a daily basis [1]. Polymers were utilized in daily life and in industry for a long time, yet the true appearance of their importance and use was discovered in the late 20th century. Natural polymers such as cellulose were produced in 1838 from natural plants. It is composed of repeated units of glucose. The natural polymer industry started in 1818 with the production of natural rubber for different daily life commodity items such as shoes and gloves. Artificial polymers (e.g., plastic, fiberglass, nylon, and many other products) impacted the society and changed it for the better, providing a class of synthetic polymers formed to satisfy niche applications. Synthetic and natural polymers play a major role in facilitating a comfortable lifestyle due to their integration in many aspects of modern society, including transportation, medicine, communication, and fashion [2]. The first industrial revolution started in the 18th century with an emphasis on the utilization and improvement of metals (mainly steel) through steam engines. The industrial revolutions continued through other stages of electrical energy and mass production (second revolution), electronics and automation (third revolution), and now we are in the fourth revolution, which includes cyber physical systems. Future technologies will depend on the previous revolutions and

1.3 Polymers: general classification and production

combined technologies between digital and physical sectors. The new modified technologies can improve operations and be more productive. New businesses and industries can be driven by the new technologies. Polymers, as will be seen later in this chapter, are leading many sectors in the fourth industrial revolution. Polymers were mostly applied as insulators and packaging materials due to their economic manufacturing benefits, long-term stability, significant toughness, good dielectric properties, and durable mechanical strength [15]. For example, polyethylene is one of the major polymers used particularly for the cable industry, with certain functional modified versions for special applications such as less flammability [16]. Over the past two decades, polymers were explored for their electrical and energy-related properties such as applications in energy harvesting devices, solar cells, piezoelectric nanogenerators, fuel cells, optical switches, and lithography [17,18]. They are now used in many other applications like 3D printing, aerospace, water purification, and smart textiles.

1.3 Polymers: general classification and production As explained shortly in the introduction, polymers are materials that consist of many simple structural units (monomers) joined together to form giant molecules. The art of modifying and manipulating the high number of molecules in polymers allowed modern society to fabricate different types of polymers in the form of fibers, films, and adhesives. There are two main sources of polymers, natural and synthesized. (1) Natural polymers are found in nature. Examples include proteins, rubber, cellulose, and starch. Plants and trees are made of cellulose, which make it the most common polymer on Earth. Chitin is another common polymer available in the shells of shrimp, crabs, and lobsters. It has a combination of attractive properties of hardness, insolubility, and flexibility. (2) Synthesized polymers are produced from fossil fuels. They are arranged with longer chains compared to natural polymers [8,9]. Synthesized polymers are mainly derived from its low cost and highly abundant predecessor, petroleum, with highly efficient processing methods, allowing for the use of less than 5% of an oil barrel to contribute to the production of large amounts of polymer, therefore crowning petroleum as the most effective and main source of polymers for the near future. Additionally, the polymer industry experienced rapid expansion when synthetic polymers were first discovered in the 19th century, allowing them to compete and sometimes overtake older, moreestablished industries, such as the steel, aluminum, and copper industries. The value of produced polymers depends on the processing method, properties, and specifications of these materials. Polymer properties are heavily influenced by the molecular composition, such as molecular size and branching. The processing method such as the set up and flow time can add to the expenses of the produced polymer.

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The cost of polymers depends on the manufacturing procedures. For instance, polyolefins (e.g., polyethylene and polypropylene) cost less because they are derived directly from crackers streams (the plant in which saturated hydrocarbons are broken down to unsaturated smaller hydrocarbons). Treatment and modification of polymers increase the cost, but at the same time utilize the polymers for different and new applications. In 2016 the global plastic compounding market size was estimated to be 26.73 million tons. Replacement of metallic parts with polymers owing to their lightweight and flexible nature influences the production rate, cost, and consumption patterns of typical polymers. Various factors such as demand for versatile materials, availability of feedstocks, sociopolitical events, manufacturing processes, and so forth affect the global polymer production. Reports have detailed the progressive increase in the US plastic compounding market revenue for the time period 2014 26 [3]. The market for polypropylene and polyethylene is expected to increase as these materials are highly needed in packaging, automotive, and construction applications. The production of polyvinyl chloride and polyethylene terephthalate (PET) is also increasing due to technological innovations. The environmental challenges necessitate the production of biodegradable materials, therefore biobased polymers are also receiving increasing and significant attention. It is well-known that the supply and demand of polymer production depend on the applications of the polymer. These applications will be discussed in-depth later on in this chapter, but here we would like to bring it up to view its correlation with the price rate and production percentage. Polymers that are used for structural purposes had a production rate of 32.2 billion kilograms in 1992 in the United States alone, split into plastic production of 26.1 billion kilograms, fiber production of 4.1 billion kilograms, and rubber production of 1.9 billion kilograms [4]. With rough estimates, we can conclude that the production rate has tripled since the start of the 21st century. Structural polymers are usually sold in the range of US$0.50 and upwards per pound, while the price of its raw material (crude oil) is only US$0.132 per kilogram. Therefore structural polymers represent a large turnover for the manufacturer. Another important application of polymers can be derived from thermoplastics that acquire outstanding packaging ability [19]. And since these polymers can be recycled on demand, their production and importance will only increase in the future due to advancement in recycling technology and the shift in the global demand from single-use polymers to recyclable polymers. It is essential to review the case study of ethylene production to understand the drastic increase in polymers’ supply and demand. Starting in the early 1940s, England was producing the simplest form of thermoplastic, polyethylene [20]. Polyethylene backbone consists of four hydrogen and two carbon monomer blocks that are constantly repeated. Moving to the 1980s, the discovery of different processing methods allowed the production of different grades and types of polyethylene such as low-density and high-density polyethylene. Increasing the production of linear low-density polyethylene alone, it reached approximately

1.3 Polymers: general classification and production

2.2 3 109 kilograms per year. Furthermore, synthetic rubber had high importance due to its significant use, with production numbers estimated to be 1.9 billion kilograms produced in 1992 in the United States alone. Synthetic fiber production is estimated to be 4 billion kilograms yearly in the United States (Fig. 1.1). Mass production of plastics increased from 50 million tons in 1977 to 322 million tons in 2015 [21], which is a 544% increase in production. This increase is mostly in polyolefin production, which accounts for more than 55% of the plastics industry. China is the leading country in plastic production, with about 49% of the global market [22]. Alternating polymer properties made them one of the most functional materials during the past few years. Up to 2015, about 8300 million metric tons (Mt) of virgin plastics were produced, and 6300 Mt of plastic waste was generated up to 2015 as shown in Table 1.1 [23]. This huge amount of waste causes severe land and water pollution, negatively affecting the environment. For example, microplastic is currently a global concern due to its high presence in fish, mussels, salt, and water. Fig. 1.2A shows the estimated mass of mismanaged plastics waste in 2010 [22], according to which the maximum contribution is from China. Unfortunately, oceans are considered the ideal places to dispose plastic waste, where the currents make plastic waste virtually irretrievable. Many studies demonstrated that rivers carry a large amount of plastic waste, finally depositing it into the sea. Microplastics of 1 1000 µm size are generated by such processes. Several reports show the presence of microplastics in seafood products, salt, and even in animals’ bodies.

FIGURE 1.1 US production of thermoplastics by type in the year 1990.

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Table 1.1 Estimate of the generation of plastic waste, and its disposal in landfills worldwide. Type

Million metric tons (Mt)

Year

Virgin plastic production Plastic waste Expected plastic waste in landfills

8300 6300 12,000

2017 2015 2050

FIGURE 1.2 (A) Estimated mass of mismanaged plastic waste 2010, in metric tonnes; (B) chemical composition of the isolated particles from salt. (a) Pie chart of the chemical composition of the isolated particles from all salt samples and the corresponding proportion of different (b) plastic polymers and (c) pigments [24].

Karami et al. addressed the issue of microplastics by investigating 17 types of salt produced in eight countries over four continents. As represented in Fig. 1.2B, the chemical composition of the extracted materials was studied. The group found that in a given sample, 41.6% were plastic polymers, 23.6% pigments, 29.1% unidentified, and 5.5% nonplastics. The major polymers were polypropylene, polyethylene, PET, polyisoprene/polystyrene, polyacrylonitrile, and nylon-6 [23]. Though the study did not find direct health hazards by salt consumption, possibilities of gradual accumulation of microplastics in human bodies in the future can have negative results. The Middle East region contributes to about 7.3% of the global plastic production. The developed economy, huge population, and large coastal area in this region promote the generation of microplastics. Fig. 1.3 shows the microplastics isolated from the north eastern coast of Qatar’s Exclusive Economic Zone by Castillo et al. [25]. Granular particles in the size range of 125 µm 1.82 mm and fibrous particles of 150 µm 15.98 mm were obtained. Anthropogenic activities such as oil-rig installations and shipping operations in this region cause the presence of many microplastics.

1.3 Polymers: general classification and production

FIGURE 1.3 Microplastic particles isolated from seawater samples in the north eastern marine waters of the Qatar’s Exclusive Economic Zone [25].

1.3.1 Fabrication methods There are many techniques for polymer fabrication technology. Listed below is a summary of the common technologies: 1. Casting: The polymer in this case is solidified in a mold by chemical or physical procedures. 2. Molding: In this technology, heat and pressure are used to change the shape of the polymer using different types of molds. Examples include: • compression molding, • injection molding,

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• blow molding, and • other molding techniques such as rotational molding and reaction-injection molding. 3. Extrusion: In this technology, a screw (or more) is used to process the polymer with heat and pressure. The extruder can be divided to different chambers. Examples include: • coextrusion, • film extrusion, • blow film extrusion, and • over jacketing extrusion. 4. Electrospinning: This is the most used technique for polymer fiber production. Starting with polymers as a suspension in a suitable solvent, and by regulating the concentration of the solution and applied voltage, fibers of various dimensions can be fabricated. Other techniques of polymer fabrication include, calendaring, coating, and foaming. After specific processing, the produced polymer systems and composites can be tailored according the required final application and shape. Polymer systems can include:

• Immiscible or miscible polymer blends. • Polymers composites. • Since polymers possess certain limitations that make them functionally unstable for specific areas, or fields of applications, reinforcing them with other stable particles is often necessary. Here the significance of micro- and nanoparticles come to the fore as they are largely applied to improve polymer properties and to design the polymer composites and nanocomposites. Polymer nanocomposites can be designed based on clay nanolayers, carbon nanotubes, ZnO nanoparticles, graphene, etc. [14,26]. However, the particle dispersion is rather difficult due to less compatibility of the filler polymer chemical environments, usage of surfactants, chemical reagents, etc., which can modify the filler surfaces and thus strengthen the polymer filler interfaces. This pointed out significant enhancements in nanocomposite properties such as electrical, dielectrical, mechanical, and gas permeability properties. Different types of additives can be added to improve the properties of the polymers. This can include additives that advance the mechanical properties, antifouling additives, flame retardants, plasticizers, self-healing agents, and antioxidants. One way of modifying the polymer properties is changing it into nanosized materials. In this case they will have large surface area to volume ratio. They can also be more flexible in surface functionalities and other mechanical properties. These processing techniques can be used to tailor polymers, change them into smart materials, and improve their properties. Other techniques that can be used to change the properties include chemical modifications of the structure and physical modifications (e.g., gamma irradiation).

1.3 Polymers: general classification and production

1.3.2 Classification of polymers Polymers can be classified into two main classes, (1) thermoplastics and (2) thermosets. The main differences are shown in Table 1.2.

1.3.2.1 Thermoplastics This class of polymers can be shaped through several heating and cooling steps. The materials flow through specific molds to have the required shape when it is hot. Then a cooling procedure is arranged to solidify the polymer to the required shape. This special feature allows thermoplastics to be recyclable and allows them to be considered environmentally safe and friendly. Furthermore, thermoplastics can be produced through different techniques, such as extrusion or molding. Major types of thermoplastics include styrene, propylene, and ethylene, which resemble between 80% and 90% of the overall thermoplastic monomers produced. This class of polymers is heavily used as a source for packaging bags. Another application of thermoplastics includes high heat and mechanical resistivity. Modified thermoplastics are known as engineering thermoplastics, which cost more to produce in comparison to low-quality thermoplastics, with their application consisting of structural support in transportation, such as for automobiles and aircrafts. Thermoplastics are also viable for niche markets as they can be blended with different materials to increase toughness and chemical stability, and to obtain special forms of structures. Fig. 1.4 shows some of the classical applications of thermoplastics.

1.3.2.2 Thermosets Thermosets are specific class of polymers that form well-defined, irreversible, chemical networks that tend to grow in three dimensional directions through the process of curing, which can either occur due to heating or through the addition of a curing agent [29], therefore causing a crosslinking formation between its chemical components, and giving the thermoset a strong and rigid structure that Table 1.2 The two main classes of polymers [27,28]. Thermoset

Examples

Thermoplastic

Examples

Polymers are crosslinked and curing cause irreversible bonds. They are not recyclable or reshaped

Polyisoprene

High-density polyethylene

Flexible

Polyurethane

Dimensional stability

Benzoxazines

Cost effective

Polyester resin

Highly recyclable and reshaping ability High strength resistance Chemical resistance More expensive

Polyethermide Polyphenylene oxide Polyether ketones

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CHAPTER 1 Polymers to improve the world and lifestyle

FIGURE 1.4 Categories of uses for thermoplastics in the United States, 1990.

can be added to other materials to increase strength. Further applications of thermosets include coating and epoxy adhesives. Around 69% of the application of thermosets present in the construction and building industry, other uses of thermosets are in transportation, adhesives, and electrical equipment. Thermosets are also highly used in advanced applications, especially in the aerospace and military industries due to the multiple composites that can be produced with the presence of thermosets, including reinforced carbon fibers and glass.

1.3.2.3 Elastomers (rubbers) Elastomers are typically amorphous with a low amount of crosslinkage, while also being soft and flexible, and can usually withstand a large amount of force that would normally lead to deformation, therefore are capable of attaining their original form. Synthetic elastomers were first produced in the United States due to the shortage of natural rubber supply during World War II. Elastomers have wide applications in many industrial products, such as electrical insulators and tyres. In addition, they are found in many consumer products including sporting goods and casual wear. The interest of industries in thermosets is due to their high strength, resilience, and durability.

1.4 Current lifestyle and the need of polymers

1.4 Current lifestyle and the need of polymers The possibility of modifying polymers for specific applications through functionalization made them available around us in everyday life. The common use of polymers in the modern world is mainly due to their lightweight, easy manufacturing, and the possible tailoring procedures that suit the needs of consumers. It is wellknown that polymers have improved the lifestyle of humanity significantly. Industries and governments have their own requirements for these materials, including standards, compliances, and certifications. The use of polymers depends on their physical and mechanical properties. It is important that the used material has its own failure analysis tests, have no design faults, and have relatively low costs. One important advantage of polymers is the possibility of altering their properties [30 33], to meet specific requirements and applications, such as increases their conductivity, mechanical strength, piezoelectricity, and stability. This modification opened new markets in electronics and communications. Over the past century, polymers were used mostly as insulators and packaging materials. They are mostly synthesized materials and are not biodegradable or recyclable. They are now used in cables due to their good mechanical properties and excellent dielectric behavior. Major applications of polymers include aerospace, electronics, medical, packaging, and the automotive and building industries. Another emerging field of sensors is currently using new modified polymers. New advanced techniques in improving polymers and polymer composites led to an increase in sensors sensitivity, selectivity, and response time. Other parameters in these polymers can be utilized such as their permeability, durability, and thermal and electrical properties. Polymers are widely used in other applications as structural adhesives and protective coating [34], such as epoxies. New techniques for self-healing coatings are under study and expected to be available in the market soon. In packaging application, there are many used materials with different sources. The most common packaging material is low-density polyethylene. Most packaging materials are synthesized materials and are not biodegradable or recyclable. One major disadvantage of polymers is their long-life stability, which may affect the environment. Recycling is a common procedure to reduce this negative effect. In the case of polymers, the waste should be separated to different categories and then recycled. Another way to reduce polymer waste is incineration and direct combustion. The energy required to burn plastics exceeds 40 MJ/kg. This is slightly less than natural gas (48 MJ/kg) and approximately similar to oil combustion [35]. Landfills are also used in many countries. To reduce the negative effect on the environment, biodegradable polymers are now used in many applications to reduce waste. The most common applications for biodegradable polymers are in packaging, drug delivery, and agriculture. Biodegradable plastic: Petroleum-based plastic continues to affect the environment and cause habitat destruction. There is an increased search for

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biodegradable alternatives, and an excellent example of this is polyhydroxyalkonates, biodegradable plastics that are synthesized by microbial polyesters [36]. The reason behind the importance of these polymers to the world is that the biodegradable plastic is produced from renewable materials that involves producing the biopolymer through using waste as the primary reactant under anaerobic digestion and composting. Furthermore, these polymers are excellent for their low cost, renewability, and biodegradability. Biomedical application: Another advanced usage of polymers is present in the form of biodegradable hydrophobic polymers, which are plastics (mainly polyethylene glycol) that possess biocompatibility and high mechanical strength. Additionally, these polymers can be fused with a hydrogel network that allows them to interact and obtain cell-like interaction [37]. This interaction allows for higher water absorption that is crucial in multiple biomedical devices. There are many other applications of polymers that will be covered in more details in this book. For example, in the medical field polymers are used in controlled drug released orthopedic implants, artificial, organs, dressing, and bandages. Biocompatibility of polymers with tissues is one of the main challenges in this field. Another important application of polymers is self-healing. Self-healing: New techniques are produced to improve the protective coatings by introducing self-healing functions [34]. These polymers can save costs with regards to machinery and production, and is usually a property only possessed by plants and animals. This advanced technology is currently utilized in different industries to reduce the corrosion cost especially when using low cost and green healing materials. Current classes of polymers such as dynamic polymers and polymer composites loaded with healing agents are capable of providing self-healing properties [38]. Hence the polymer can be used as a storage device in order to contain the healing agent, and if the mechanical damage was sufficient enough to break the storage device (microcapsule), the healing agent would diffuse through the mechanical slip and polymerize, therefore minimizing the mechanical damage (Fig. 1.5).

1.5 Polymers to composites Since polymers possess certain limitations, which make them functionally unstable for specific areas or fields of applications, reinforcing them with other stable particles is necessary. Here comes the significance of micro- or nanoparticles that are largely applied to improve polymer properties and to design polymer composites and nanocomposites. Our group has significant experience in designing polymer nanocomposites based on clay nanolayers, carbon nanotubes, ZnO nanoparticles, graphene, and so forth [32,40 42]. However, particle dispersion is rather difficult due to less compatibility of the filler polymer chemical

1.6 Specific requirements of polymers using physical, mechanical

FIGURE 1.5 An example of self-healing polymers: immersed coatings in 10 wt.% NaCl solution: control epoxy coating on (A) first day, (B) second day, and (C) fourth day; EP/7.5 HNT coating on (D) first day, (E) second day, and (F) fourth day [39].

environments. Different procedures can be applied to modify the filler surfaces such as usage of surfactants and chemical reagents. These procedures can strengthen the polymer filler interfaces. This pointed out significant enhancements in the nanocomposite properties such as electrical, dielectrical, mechanical, and gas permeability.

1.6 Specific requirements of polymers using physical, mechanical, and chemical methods Many of the desirable properties for particular polymer applications are wellknown, however achieving those qualities are really challenging. Polymers in general are made of multifunctional monomer units connected through nodes in the form of a network. The physical, chemical, and mechanical properties of a polymer network depend highly on the density and functionality of these

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connecting nodes. The properties can be modulated by varying the monomer molecular weight, functionality of the units, and the multifunctional monomer ratio. However, these processes can induce variations in network chemistry and can create defects and loops that directly affect the mechanical strength of the final products [43]. The significance of such primary defects and loops on the mechanical strength and elastic modulus of a typical polymeric system was the subject of study by Chan et al. [43], where they have synthesized elastomeric polymers following selective and rapid thiol-ene click chemistry. The physical properties of a polymer highly depend on the nature and arrangement of monomer units. For instance, high-density polyethylene (HDPE) is a rigid translucent solid that can withstand up to 100 C heating, but low-density polyethylene (LDPE) is soft and deforms after 75 C of heating. The large molecular chains of polymers generally pack in a nonuniform way, with ordered crystalline regions mixed up with disordered amorphous regions. Depending on this packing, the polymers can be semicrystalline and completely amorphous. The factors that determine the crystalline nature of a polymer include the chain length, branching, and interchain bonding. A few examples of crystalline polymers include LDPE, HDPE, cellulose etc. However, elastomers like natural rubber are fully amorphous in nature. In those cases, crosslinking with sulfur results in achieving desirable properties especially at 2% 3% crosslinking, whereas 25% 35% crosslinking causes rigid and hard rubber formation. The crystallinity of the polymer regulates many properties such as piezoelectric performances (for polymers such as polyvinylidene fluoride), mechanical strength, and thermal stability. The emerging area of responsive biointerfaces that are capable of mimicking natural surfaces are based on synthetic polymers, where the function and structure of these materials vary with environmental changes. A stimuli-responsive macromolecule undergoes chemical and conformational changes when its environment (such as temperature, magnetic/electrical/optical field, chemical composition or mechanical field) changes. Various dimensional stimuli-responsive polymer systems are represented in Fig. 1.6 [44]. Stimuli-responsive polymers are highly useful for satisfying various functional needs that society demands.

1.7 Internet of Things and smart materials The polymer industry is a key factor in the Internet of Things (IoT) era. In this vision, one network will be connecting all materials in a wireless and sustainable way. The base technology of IoT is the need for flexible electronics, sensors, devices, and power sources. These smart polymers change their properties and/or shape according to the environment and external effects. These external stimuli have different characteristics such as thermal, electrical, mechanical, or magnetic effect. 3D printing of polymers is developing rapidly due to high-speed production, flexibility, low cost, and specific application customization. Utilization of

1.7 Internet of Things and smart materials

FIGURE 1.6 Galaxy of nanostructured stimuli-responsive polymer materials [44].

polymers and polymer composites in traditional applications will be reflected in future applications of new smart cities where there are interactions between users and devices due to the developments in information and communications technology [45]. Application domains of IoT are shown in Fig. 1.7 where classic polymer applications play a major role. Piezoelectric polymers with self-powered capability are common smart materials used in control systems and wireless sensors. Another smart material application entails printing them on specific sources to increase the information related to these materials. This is used in different industries such as active packaging in the food industry to maintain the quality of the food and inform the buyers of their contents. Smart biomaterials are widely used to improve the healthcare system, such as smart wound monitoring systems with bandages that control the drug release in a smart system [46]. Other applications of polymers in IoT include monitoring systems, the energy sector, and storage systems. Collaboration between scientists from different disciplines can lead to more progress in this field.

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FIGURE 1.7 Smart cities in the application domain of IoT [45].

1.8 Conclusions To conclude, polymers are an essential part of our day-to-day activities as they are involved directly or indirectly to the majority of commodity items that are being handled. Studying their significance, properties, and modification possibilities can further improve their applications. Polymers are modified by the inclusion of various fillers to develop themselves to composites, and that opens up numerous electronic, mechanical, and biomedical applications in industry and technology. This chapter provides a glimpse into this information and a short note on the fundamental aspects of the whole book.

Acknowledgments This chapter was made possible by NPRP grant 10-0127-170269 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. The author Ali Alaa El-Samak would like to acknowledge Qatar University for the support granted through the Graduate Research Assistantship Program (GRA).

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[24] Karami A, Golieskardi A, Choo CK, Larat V, Galloway TS, Salamatinia B. The presence of microplastics in commercial salts from different countries. Sci Rep 2017;7:46173. [25] Castillo AB, Al-Maslamani I, Obbard JP. Prevalence of microplastics in the marine waters of Qatar. Mar Pollut Bull 2016;111(1 2):260 7. [26] Mittal G, Dhand V, Rhee KY, Park SJ, Lee WR. A review on carbon nanotubes and graphene as fillers in reinforced polymer nanocomposites. J Ind Eng Chem 2015;21:11 25. [27] Dante RC, Santamar´ıa DA, Mart´ın Gil J. Crosslinking and thermal stability of thermosets based on novolak and melamine. J Appl Polym Sci 2009;114(6):4059 65. [28] Li Y, Xie T, Yang G. Effects of polyphenylene oxide content on morphology, thermal, and mechanical properties of polyphenylene oxide/polyamide 6 blends. J Appl Polym Sci 2005;99(5):2076 81. [29] Crosky A, Soatthiyanon N, Ruys D, Meatherall S, Potter S. Thermoset matrix natural fibre-reinforced composites. Nat Fibre Compos 2014;233 70. [30] Brostow W, Lohse S, Lu X, Osmanson AT. Nano-Al(OH)3 and Mg(OH)2 as flame retardants for polypropylene used on wires and cables. Emergent Mater 2018. [31] Nagaraj A, Govindaraj D, Rajan M. Magnesium oxide entrapped polypyrrole hybrid nanocomposite as an efficient selective scavenger for fluoride ion in drinking water. Emergent Mater 2018;1(1):25 33. [32] Ponnamma D, Erturk A, Parangusan H, Deshmukh K, Ahamed MB, Al-Maadeed MA. Stretchable quaternary phasic PVDF-HFP nanocomposite films containing graphene-titania-SrTiO3 for mechanical energy harvesting. Emergent Mater 2018;1 (1 2):55 65. [33] Fadiran OO, Girouard N, Meredith JC. Pollen fillers for reinforcing and strengthening of epoxy composites. Emergent Mater 2018;1(1):95 103. [34] Vijayan P, Al Maadeed MA. Containers’ for self-healing epoxy composites and coating: trends and advances. eXPRESS Polym Lett 2016;10(6):506 24. [35] Wasilewski R. Energy recovery from waste plastics. CHRMIK 2013;67(5):435 45. [36] Urtuvia V, Villegas P, Gonza´lez M, Seeger M. Bacterial production of the biodegradable plastics polyhydroxyalkanoates. Int J Biol Macromol 2014;70208 13. [37] Wang JY, Wang K, Gu X, Luo Y. Polymerization of hydrogel network on microfiber surface: synthesis of hybrid water-absorbing matrices for biomedical applications. ACS Biomater Sci Eng 2016;2(6):887 92. [38] Gao L, He J, Hu J, Wang C. Photoresponsive self-healing polymer composite with photoabsorbing hybrid microcapsules. ACS Appl Mater Interfaces 2015;7 (45):25546 52. [39] Vijayan PP, Hany El-Gawady YM, Al-Maadeed M. Halloysite nanotube as multifunctional component in epoxy protective coating. Ind Eng Chem Res 2016;55 (42):11186 92. [40] Parangusan H, Ponnamma D, AlMaadeed MA. Flexible tri-layer piezoelectric nanogenerator based on PVDF-HFP/Ni-doped ZnO nanocomposites. RSC Adv 2017;7 (79):50156 65. [41] Parangusan H, Ponnamma D, Hassan MK, Adham S, Al-Maadeed MA. Designing carbon nanotube-based oil absorbing membranes from gamma irradiated and electrospun polystyrene nanocomposites. Materials 2019;12(5):709.

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[42] Devi KU, Ponnamma D, Causin V, Maria HJ, Thomas S. Enhanced morphology and mechanical characteristics of clay/styrene butadiene rubber nanocomposites. Appl Clay Sci 2015;114:568 76. [43] Chan D, Ding Y, Dauskardt RH, Appel EA. Engineering the mechanical properties of polymer networks with precise doping of primary defects. ACS Appl Mater Interfaces 2017;9(48):42217 24. [44] Tang Z, He C, Tian H, Ding J, Hsiao BS, Chu B, et al. Polymeric nanostructured materials for biomedical applications. Prog Polym Sci 2016;60:86 128. [45] Alavi A, Jiao P, Buttlar WG, Lajnef N. Internet of things-enabled smart cities: stateof-the-art and future trends; 2018. [46] Derakhshandeh H, Kashaf S, Aghabaglou F, Ghanavati O, Tamayol A. Smart bandages: the future of wound care. Trends Biotechnol 2018;36(12).

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CHAPTER

Morphology analysis

2

Anton Popelka, Sifani Zavahir and Salma Habib Center for Advanced Materials, Qatar University, Doha, Qatar

2.1 Introduction Surface morphology and topography represent important properties of polymeric materials (from the nano- to the macroscale), and originate from their chemical nature/structure and production processes. Polymeric materials are characterized by specific aspects of their surface morphology, which affect their final surface properties such as wettability and adhesiveness and applicability to printing, dying, lamination, water repellency, and biocompatible processes. Therefore information about the surface morphology and topography of polymers is crucial for their use in various industry sectors such as automotive, aerospace, building, textile, medical, and packaging. Different types of polymers have different natures and structural conformations that are responsible for their resulting morphologies and topographies. The overall polymeric structure such as molecular weight, crystallinity, branching, and crosslinking, affects the final morphology. An overview of the internal morphology analysis of amorphous, crystalline, and polymer blends is discussed in this chapter. Moreover, the external surface morphology/topography of the resulting polymers in the form of foils, fibers, and foams is described in this chapter as well. Last but not least, the effects of various surface modifications that affect the surface morphology in 2D and 3D (roughness) are also reviewed. This chapter also includes a description of techniques and methods commonly applied to analyze the surface morphology/topography of polymeric materials. These techniques include various scanning probe and optical/electron microscopies. The implementation of these techniques and their combination is necessary for multidisciplinary study of polymer morphology/topography.

2.2 Polymer morphology The characterization of polymers contains many aspects; this study is concerned with morphology measurements that use developed techniques. Studying the morphology of a polymer surface gives a clear identification of the changes and Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00002-0 © 2020 Elsevier Inc. All rights reserved.

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modifications that have occurred. The measurement of polymer morphology is an interesting field of study as it refers to the functional properties, modification, and applications of polymers. Over the past four decades, many different techniques have been developed to obtain a quick, clear, and accurate morphology measurement of polymers. In the case of composition studies, photon radiation such as X-ray and infrared radiation have been efficiently used to characterize the chemical composition of materials quickly. The measurements started with the study of 1D wired detectors [1,2] and expanded to 2D structures [3
5] with the ability to sense position once the orientation of beamlines was detected [6
9]. Another significant breakthrough in the first decade of the 2000s included a morphological development from the melt state that can be analyzed by atomic force microscopy (AFM) allowing images with a high resolution to be obtained [10
12]. AFM was developed as a quick method for obtaining topographical images [13
17]. AFM is a nondestructive technique; the same area can be scanned many times without any negative impact on it. Studying multiple sites on the surface of a polymer only provides information about the morphology changes for a few micrometers of material based on the applied treatments or modifications.

2.2.1 Crystalline polymers Crystalline polymers are defined by their strict composition and perfect order or translation of atoms or molecules. A perfect translation gives the shape of the lattice defining each crystal type. Although some dislocations have a limited impact, their general translation symmetry preserves the overall shape of the polymer [18]. As the chain is well defined and oriented in one direction of the lattice, the crystals form a polymer crystal [19]. The growth rate of the crystal with a certain folding affects the stability of the folding [20]. Each type of polymer crystal has a specific order and melting temperature in accordance with the entropy of the conformation of the chain folding [21,22], which is not the case for small molecules that have a precise melting point (no chain folding effect) [23]. In small molecules, the crystallization step provides the same change in entropy with the same molecule repetition. This is not the case in polymers because the molecule differs based on each unit built causing variety in the entropy (each unit causes a certain type of entropy while folding) [24]. Despite being in a metastable state, single crystals in polymeric materials have the same orientation. This is caused by the nucleation process, and polymers are integrated based on the same parameters of the unit cell [20]. Polymer chains are able to crystallize into various metastable states (different chain folding degrees) [23,25,26]. The crystallization of polymeric chains represents a difficult process in comparison to that of small molecules [27]. If all accessible sites are already occupied in a crystal, some of the chain blocks will be excluded and are responsible for the formation of lamellar bounds [21]. Further ordering can also occur after additional reorganization in the crystalline state [28
31]. It has been reported that crystallization and heterogeneous nucleation of polymeric melts in

2.2 Polymer morphology

comparison with metals (high surface free energy surfaces) lead to noticeable changes in the surface morphology. Moreover, transcrystallinity can be incurred in polypropylene (PP) or polyethylene (PE) polymers with low surface free energy (wettability) after melts contact copper or aluminum. On the contrary, surfaces with low surface free energy (Teflon, Mylar) in a combination with nitrogen gas have no nucleating activity with a spherulitic morphology [32].

2.2.2 Amorphous polymers Amorphous homopolymers such as polycarbonate, poly(methyl methacrylate) (PMMA), or polystyrene (PS) are defined by the absence of crystalline structures and may only have weak supramolecular structures [33,34]. They are associated with the idealistic model of an amorphous solid. The strong interpenetration of a great number of macromolecular segments induces the formation of a large number of topological (physical) links, and entanglements keep the macromolecules compact, which provides toughness and strength to polymeric materials. A second contribution comes from the Van der Waals forces within macromolecule segments with an entangled molecular network. Coiled macromolecules have lower packing densities in comparison with that of parallel arranged conformations of molecules, which are typical of semicrystalline polymers. Therefore the density of amorphous polymers is usually lower than that of semicrystalline polymers (with the exception of existing heavier elements in the chains as in polyvinyl chloride). The volume that is not filled by molecules, that is, the unoccupied volume between the molecular segments, is expressed by the term “free volume,” and affects the thermal and mechanical properties of polymeric materials. The macromolecules are entangled in the interlamellar (amorphous) regions or form linked lamellae, the so-called “tie molecules” [35].

2.2.3 Semicrystalline polymers Semicrystalline polymers are based on a parallel arrangement of neighboring molecular segments, possibly by folding or bundle formation (Fig. 2.1) [36]. Macromolecules are partly included in crystals and, therefore, also have reduced mobility of neighboring segments. Thus the formation of new crystals is hindered, preventing the total crystallization (as in atomic structures) of macromolecules and yielding semicrystalline structures. The characteristic elements of semicrystalline polymers are represented by the crystalline lamellae, interlamellar amorphous regions, and lamellae interfaces. Often, chain ends, short branches, or defects in the crystals inside the lamellae between crystalline blocks are responsible for the formation of defected layers. The lamella interface (lamella surface layer) contains folds, branches, and chain ends, and is often visible in electron micrographs because of its strong staining as a separate structure. The macromolecules are entangled in the amorphous regions (interlamellar) as in amorphous polymers or they can form lamellae links, so-called tie molecules. The typical thicknesses of

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CHAPTER 2 Morphology analysis

Spherulite

Spherulite Boundary Iamellae

Nucleation site

Amorphous Crystalline

FIGURE 2.1 Scheme of spherulite with a birefringent structure consisting of crystalline lamellae with amorphous interlamellar joins [36]. Copyright 2019. Reproduced with permission from Elsevier.

lamellae lie in the range of 10
30 nm (but can reach thicknesses of more than 1 μm in high-pressure crystallized ultrahigh molecular weight PE) with lengths up to 1 μm. Different types of crystalline lamellae can be produced such as sheaflike structures, parallel arrangements in stacks or bundles, spherulites with a radial arrangement of lamellae, which are typical for high-density PE or PP, and twisted lamellae that are arranged in concentric rings of banded spherulites as is typical for low-density PE (LDPE). Depending on the polymer and thermal treatment, the spherulites can reach up to hundreds of microns in diameter [35,37,38].

2.2.4 Polymer blends Mixtures of two or more polymeric materials in polymer blends represent another combination of different macromolecules. Various polymeric materials are normally incompatible, resulting in a phase separation between the dispersed phase (minor component) and the matrix (major component). Networks or interpenetrating structures are usually formed with an equal composition of both components (50:50). This phase separation corresponds to microphase separation in block copolymers due to the absence of covalent bonds between the components with coarser structures. Several methods can be utilized to improve the compatibility of polymers including mixing with compatibilizers, reactive blending, or grafting. Compatibilizers (block copolymers or graft polymers) are responsible for particle size reduction of the minor component. Between the components, separate interfaces (thin boundaries) and interphases (thicker layers often with their own structures) exist. The fine morphologies of polymers with similar chemical compositions having relatively good miscibility can be formed [39]. For example,

2.2 Polymer morphology

the network morphology of a cryogenic fractured PE/PS blend is shown in Fig. 2.2 and was obtained by scanning electron microscopy (SEM). These blends were prepared by a reactive compatibilization using a Friedel
Crafts reaction. The homogeneity and shape of the dispersed PS phase was investigated. The homogeneous distribution of PS particles within the PE matrix can be clearly seen [40]. There are different classifications of polymer blends based on the type of polymers blended together. A homogeneous blend is a blend of two miscible polymers that have only one phase. This happens when the total free energy of the blend is negative. This leads to one glass transition temperature (Tg), making it a homogeneous material. In contrast, blends that have positive free energy are not immiscible, but compatible, and have more than one phase in the material structure; the Tg is then separate for each phase. The initial information about the morphology can be obtained by a simple visual checking. The blending of two colorless (transparent) amorphous polymeric materials with different refractive indexes results in an opaque material and the size of the phases usually surpasses the visible light wavelength ( . 500 nm). The difference in the refractive indexes has to be larger than 0.003 to have opacity [41,42]. In the case of crystalline polymers, this rule is even more complicated. These polymeric materials are often opaque due to differences in the refractive indexes of the crystalline and amorphous phases. Moreover, the spherulites are large ( . 500 nm) too [43,44]. Therefore the visual inspection can be performed for semicrystalline polymeric materials, which are heated above melting temperature of crystalline component. Nowadays, polymer blends and composites comprise over 80% of all plastic materials. Market pressures force manufacturers of resins to develop economically

FIGURE 2.2 Network morphology of a PE/PS (80:20) blend with a PE matrix containing embedded PS: (A) linear LDPE type 6200 (Mw: 52,000 g/mol) and (B) linear LDPE type 6600 (Mw: 40,000 g/mol) [40]. Copyright 2019. Reproduced with permission from Elsevier.

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CHAPTER 2 Morphology analysis

improved materials with superior properties. This is rather to replace traditional polymers than steel or wood. Therefore the scale production of multiphase/multicomponent materials, and the establishment of new processing methods has increased. Morphology represents a decisive factor, which controls various practical properties such as transparency or impact strength in multicomponent polymeric materials [45]. Cocontinuous structures, which are formed by polymeric materials, have been of interest to material oriented scientists due to their ascendancy over those with a random structural morphology [46,47]. Although many techniques for producing polymer materials with cocontinuous structures were developed such as the violent freezing of the spinoidal structures of polymer blends [48
51], an effective technique to control their regularity is missing. A technique based on periodic photocrosslinking was demonstrated allowing for the distribution of the length-scale of the spinoidal structures in binary polymer blends to be controlled. The distribution period of the resulting cocontinuous structures is narrow when it is forced. Additionally, a particular irradiation frequency is present where the periodic structures exhibit a minimum, confirming the presence of an ordering process that is driven by the modulation frequency (external). These findings led to the discovery of an effective and easy way to produce polymeric materials that are promising for applications related to biological separation (hemodialysis) and useful for optical applications as well as [52].

2.2.5 Polymer composites Composites represent those materials that consist of two or more constituents and contain some reinforced components (fibers or particles) in a polymeric matrix. Reinforced materials are distributed and arranged in the polymeric matrix in a specific pattern to achieve unique properties for specific applications. The reinforced materials are held by the matrix to sustain a relative position. Simple commodity plastics based on polymer composites are commonly used in a broad range of products such as food packaging, bottles, cups, aerospace/aviation structures, electromagnetic shielding coatings, and biomedical coatings. Understanding the morphology of polymer composites is necessary to tailor the functional properties since a hierarchical morphology is usually implemented during processing, especially in polymer-containing particles [53,54]. These properties extend from the nanometer scale (nanoparticles, crystalline lamella) to the micrometer scale (homogeneity, domains) to the macroscale (surface and mechanical properties) and are essential for the development of novel materials and the enhancement of processing conditions [55]. Based on the reinforcing material, there are two main categories of polymers, namely particle-reinforced and fiber-reinforced polymer composites. The particles are used as reinforcing agents in particle-reinforced polymer composites. The particles, which are dispersed in the polymer matrix as a reinforcing

2.3 Characterization methods

agent, include glasses, ceramics, metals as well as organics. The size of the dispersing particles in these composites may vary from a few micrometers to several tens of micrometers; the size range in nanocomposites ranges from hundreds of nanometers to 10 nm. A large particle distribution is enabled by the high filler contents because of the small space between the large particles [35]. A small particle diameter, in the range of 10
100 nm, is used for the dispersant strengthening of composites. The shape of the particles vary with application and can be layered, spherical, or irregular [56]. The particles used in these composites can have a variety of geometries, but the dimensions of all the sides should be approximately the same (equiaxed). To achieve effective reinforcement, the particles should be small is size and evenly distributed in the matrix. Mineral-based particles are usually utilized for the improvement of surface hardness, matrix modulus, and abrasion and wear resistance; for the improvement of performance at increased temperatures; and for a reduction of friction, shrinkage, and the permeability of the matrix. Metallic particles are predominantly used for the improvement of the electrical conductivities of insulating matrices of polymeric materials. Carbon nanotube (CNT)-reinforced polymers are used in numerous applications such as actuators, textiles, microfluidic devices, biomedical implants, and lightweight structures [57
61]. CNTs incorporated in polymeric composites cause improvements in thermal, electrical, and mechanical properties [60,61].

2.3 Characterization methods This section is focused on characterization methods of polymer morphologies. The methods and techniques can be divided into two main groups depending on the way the polymer morphology is observed, namely indirect (structural, nonstructural) and direct methods. For the structural analysis of polymer morphology, small angle X-ray scattering (SAXS), X-ray diffraction (XRD), or angle light scattering (SALS) can be used. Kinetic dynamic mechanical analyses (DMA) or thermodynamic differential scanning calorimetry (DSC) represent methods for the nonstructural direct observation of polymer morphology. In contrast, most microscopic techniques can be used for the direct observation of polymer morphology or topography, for example, optical microscopy (OM), SEM, TEM, scanning tunneling microscopy (STM), or AFM.

2.3.1 Indirect observation methods Some methods can be categorized as indirect testing methods because they are not visualizing tools in which the polymer morphology is imaged by some means. Rather, these techniques provide information on the polymer morphology indirectly. For instance, the shape of an XRD [62,63] peak gives an indication of the

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CHAPTER 2 Morphology analysis

phase, whether it is amorphous, crystalline, or semicrystalline. Similarly, in DSC, the appearance of a crystallizing point/temperature (Tc) reveals that the given polymer has amorphous fragments that are crystallizable. This is extremely useful information when studying blends because some polymers do not crystallize in certain polymer matrices. Techniques such as XRD, SALS [64], and SAXS [65
67] are referred to as structural techniques, while DSC [68,69] and DMA [70,71] provide nonstructural details of polymer materials. DSC is employed when the thermodynamics of the polymerization of neat polymers, copolymers, or polymer blends is studied. These techniques are described and discussed in detail here.

2.3.1.1 X-ray diffraction XRD is a nondestructive technique, which means that the sample is not destroyed during analysis. During XRD, a monochromatic X-ray beam strikes a sample at different angles, usually starting from a θ value equal to 2.5
50 degrees (or 2θ equal to 5
100 degrees). This results in either constructive or destructive interference. Constructive interference between an atom in a particular lattice plane with the incident monochromatic radiation leads to a diffracted X-ray (Fig. 2.3). This diffracted X-ray is detected and processed, and the data are recorded and displayed via data acquisition software. Constructive interference occurs when the X-ray beam is incident on a specific set of crystallographic planes with a particular lattice spacing and forms a peak in the diffractogram. The appropriate relationship among these parameters is given by Bragg’s law (Eq. (2.1)). nƛ 5 2d sin θ

(2.1)

where ƛ is the wavelength of the X-ray, d is the interlayer spacing, and θ is the incident angle. Each different lattice plane produces a constructive interference at a certain angle and technically displays a separate peak for every single plane of a crystalline sample. XRD provides an initial indication of crystallinity and also details

X-ray source

Incident ray Diffracted ray  2 Specimen stage

X-ray detector

Incident ray

Diffracted ray

Axis 

2 D

Lattice spacing

Diffractometer path

FIGURE 2.3 Scheme of (A) diffractometer apparatus and (B) diffraction by particles (atoms) in parallel lattice planes.

2.3 Characterization methods

whether it is a neat polymer or a composite blend of two phases such as polymer
polymer [72], polymer
wax, or polymer
metal/metal oxide [73
75]. The crystallinity of a polymer is different from that of a metal or metal oxide lattice material. By definition, a crystalline material maintains a regular arrangement and orientation. XRD is indicative of the unit cell type, and for a polymer, a unit cell is a repeating segment of polymer chain. Polymer chains may consist of several tens or even hundreds of carbon atoms in a single segment. XRD enables:

• Qualitative and quantitative identification of the crystalline phase; • Determination of the crystalline or amorphous ratio of the material in bulk • •

form or in thin films; Calculation of average crystallite size often using the Scherrer equation; Identification of preferred orientation of thin films and multilayer stacks.

Fig. 2.4 shows XRD peaks for neat high-density polyethylene (HDPE) and PP based on a study by Lin et al. The XRD pattern of HDPE indicates a higher degree of crystallinity than that for PP and PP/HDPE as can be understood from the narrow and intense peaks for the (110) and (200) planes. However, the PP in the study seems to be more amorphous in nature since the peaks are weak and broad. This example also helps us understand how to interpret the

(110)

Intensity (a.u.)

(200) HDPE (110)

(040)

(111) (130) (131)

PP PP/HDPE

5

10

15

20

25

30

35

2 (º)

FIGURE 2.4 X-ray diffractogram of neat HDPE, neat PP, and a HDPE/PP hybrid [72]. Copyright 2019. Reproduced with permission from Lin JH, Pan YJ, Liu CF, Huang CL, Hsieh CT, Chen CK, et al. Preparation and compatibility evaluation of polypropylene/high density polyethylene polyblends. Materials 2015;8(12):8850
59.

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CHAPTER 2 Morphology analysis

basics of polymer morphology from an X-ray diffractogram. The PP/HDPE polymer blend seems to be even more amorphous, with peaks related to PP dominating and low-intensity HDPE peaks. The polymer blend apparently has PP as the continuous phase and HDPE as the dispersed phase, and this is consistent with the observations from XRD as well. Hence this is a particular example that reveals the importance and potential of XRD in studying the morphology of polymer blends [72].

2.3.1.2 Small angle light scattering In the nondestructive SALS analytical technique, light scattering by particles is measured. This combines the concept of light scattering and OM. When light of a particular wavelength is incident on a molecule/particle, light scatters as a result of the interaction with the particle. This light scattering could take two forms, namely elastic scattering and inelastic scattering. For small molecules where the wavelength of the incident monochromatic light is much larger compared to the particle dimensions, the intensity of inelastic scattering is insignificant. In such cases, the scattering is considered to be perfectly elastic. Physicist Rayleigh developed a relationship between the intensity of incident light (Io) and the intensity of scattered light at a scattering angle θ (Iθ) due to the interaction of the incident beam with a small molecule or particle. Iθ ð8π4 α2 ð1 1 cos2 θÞÞ 5 I0 ƛ4 r 2

(2.2)

where ƛ is the wavelength of the incident beam, r is the radius of the arc across which the detector moves, and α is the polarizability of the molecule, which is also proportional to the magnitude of the electric field strength of the incident beam. This assumption is not valid for polymer molecules in their original form because the particle dimensions of polymers are comparable to the wavelength of incident light, especially if the light is in the visible region. For instance, the laser light used in usual SALS instruments is a He
Ne laser with a wavelength of 632.8 nm, and the limiting particle size for this laser beam is approximately 20 nm. Most polymer molecules studied are approximately 20 nm or higher. When the molecules are larger than 20 nm, they can no longer be considered point objects compared to the wavelength of the light. In such cases, the intensity distribution due to the larger particles is asymmetric from the point of scattering to the perimeter (circumference) along the scattering envelope. The scattering envelope is the variation of intensity of Iθ with the change in scattered angle θ. For small particles below 20 nm, scattering is symmetric in the scattering envelope, which means scattering is similar in value and opposite in inverse directions (i.e., Iθ 5 I1802θ). A scheme of the SALS apparatus is shown in Fig. 2.5.

2.3 Characterization methods

1

2

3

4

5

6

7

8

9

10

FIGURE 2.5 Schematic of SALS instrumentation. (1) Helium
neon laser light source (wavelength 633 nm), (2) pinhole, (3) neutral-density filter, (4) pinhole, (5) polarizer, (6) sample stage, (7) analyzer, (8) screen, (9) charge-coupled device camera, and (10) computer interface.

The parameters necessary in SALS are:

• Nature and wavelength of the light source; • The refractive index increment must be high at the incident wavelength, and •

the value must be known accurately such that the refractive index of the solvent must be substantially different from that of the polymer; The sample analyzed must be completely dust-free because dust particles contribute to additional scattering, leading to false results. SALS provides information on:

• Size, structure, and dynamics of a semicrystalline material; • Average spherulite diameter within a semicrystalline polymer and identity of spherulite;

• Spherulite size α 1/width of SALS pattern allows obtaining wider SALS patterns for smaller spherulites, which makes it possible to measure smaller dimensions practically that are not possible with a light microscope. For polymer analysis, SALS can be used alone to probe spherulites, study and understand the morphological development of crystalline polymers, or it can be coupled with a rheometer to study the shear-induced structure formations during rheological measurements. SALS instrumentation has improved over the years since its first implementation, and now, the technique can be used to evaluate microscale and average nanoscale morphological variables. This also allows structures that cannot be defined perfectly to be visualized. Impingement, incomplete growth, and internal disorder are several reasons for an actual pattern of a semicrystalline polymer to deviate from an ideal SALS pattern. Sample SALS patterns are given in Fig. 2.6 [76]. A Zimm plot is a versatile tool within the SALS technique to quantitatively obtain the weight-average molar mass (M˙w) of a polymer. Modern instruments are equipped with Zimm plotting software, which displays the M˙w value at the

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CHAPTER 2 Morphology analysis

FD A cut procedure

ND TD

TD analyzer FD

ND TD

ND polarizer

Skin 0.1 mm Distance from skin

0.5 mm

0.6 mm

0.2 mm

0.3 mm

0.4 mm

0.8 mm

1.1 mm

1.5 mm

FIGURE 2.6 SALS patterns for an injection molded PVDF sample; here, skin refers to the top surface and the distance is the penetration depth from the top surface [77]. Reprinted with permission from Elsevier.

end of the test, making the work of material scientists even easier. At θ 5 0, the reciprocal of the intercept gives the M˙w. The basis for the Zimm plot is given in Eq. (2.3). Here, measurements are made at different scattering angles (θ) and extrapolated to θ 5 0. At θ 5 0: Kc 1 5 ΔR Mw

q2 R2g 11 3

!

1 2A2j c

(2.3)

2.3.1.3 Small angle X-ray scattering In principle, SAXS is similar to SALS, except in SALS, visible light is used that has a wavelength in the 400
600 nm range, whereas in SAXS, the wavelength of the incident radiation is typically approximately 0.1 nm in size. Therefore the wavelength of an X-ray is much smaller than that of visible light. This enables particles to be measured on dimension scales that are otherwise impossible with light scattering measurements [78,79]. There are some concerns with SAXS instrumentation. To take measurements, the path length difference must be maintained at less than half of the wavelength

2.3 Characterization methods

q*

Intensity [cm–1]

100

B1-3EG B1-8EG B2-3EG B2-8EG

q*

10

2q*

B3-3EG B3-8EG

1/2 3 q* 3q*

R-8EG

1

0.1 4

5

6

7

8 9

0.01

2

3 –1

4

5

6

7

8 9

0.1

q [A ]

FIGURE 2.7 SAXS profiles of representative membranes. The major q peak patterns expected for lamellar, hexagonal packed cylinder, and body-centered cubic sphere are (1, 2, 3, 4), (1, 31/2, 2, 71/2), and (1, 21/2, 31/2, 2, 51/2, 61/2, 71/2) respectively [81]. Copyright 2019. Reproduced with permission from Elsevier.

of the X-ray in the medium. For this purpose, the detector is kept far from the sample and maintained at small scattering angles (θ , 2 degree). This allows for larger separations at the detector between the incident and scattered radiation. There is no limiting molecular weight and the material does not have to be crystalline, which add value to this technique. SAXS is widely applied in studying the molecular weight, structure, and folding of ribonucleic acid (RNA) [80]. RNA is a type of nucleic acid and a main cell constituent material, and it is also a polymer. However, SAXS data, in general, are complicated in nature and hard to interpret. Computerized algorithms commonly used for processing SAXS data are plotted in Fig. 2.7. The intensity is plotted against the scattering wave vector q.

2.3.1.4 Differential scanning calorimetry In DSC, the behavior of a sample upon heating is measured and compared with a reference that is maintained under the same conditions. The DSC sample chamber consists of two sample input slots, one for the sample to be measured and the other for the reference. The reference material must be of a well-defined heat capacity over the temperature range of the analysis. Usually, the reference pan is left empty, and the polymer material to be analyzed is placed in the sample pan. The underlying principle is that the sample would heat up linearly with increasing temperatures unless the sample undergoes a phase change such as melting, crystallization, or glass transition. At a phase transition, the sample either absorbs or emits heat, depending on whether the transition is endothermic or exothermic respectively. This method is widely used in studying polymerization kinetics [82
84].

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CHAPTER 2 Morphology analysis

In DSC, the rate of heating is measured against the temperature. The temperature of the sample compartment and reference compartment are maintained at the same value by two heaters working separately to heat the two pans accordingly. To achieve this, two heaters heat at different heating rates, which means that the heat flow of the heater underneath the sample slot is different from that of the reference slot. DSC measures this heat flow difference as a function of temperature, simultaneously making measurements on both compartments. The mode of output is a plot of heat flow versus temperature in the temperature range of the sample analysis. DSC is used to determine heat capacity, crystallization point, latent heat of crystallization, melting point, latent heat of melting, heat of fusion, heat of reaction, purity, degree of crystallinity, kinetics of polymer crystallization, evaluation of curing kinetics, and thermal and thermooxidative degradation of a polymer material. Each of these properties is described individually here. Heat capacity: The heat capacity of a material is defined as the heat energy required to change (increase/decrease) the temperature by one unit. Polymers have a higher heat capacity above their Tg than below the Tg. Therefore a sudden/ quick jump in the heat versus temperature graph of DSC is an indication of a phase change or transition from a glassy to a rubbery state, according to polymer jargon. The DSC plot of heat flow versus temperature can be used to calculate Tg if the sample is analyzed starting from a temperature below its Tg. Fig. 2.8 shows a typical pattern of the plot around the Tg. Crystallization point: As a polymer material is heated above its Tg, the polymer gains enough energy to transform its disordered chains or fragments into a more ordered state, and as the order of the material increases, it emits heat, meaning that this transition is exothermic. Being exothermic, the sample emits heat to its surroundings, which in turn causes a reduction in the heating rate of the sample heater; this is the case because it requires less heat to maintain the

Heat flow/(J/s)

34

Tm Tg

Tc

Temperature/ºC

FIGURE 2.8 Heat flow versus temperature graph of a polymer that has all three transitions (endothermic reactions indicated by peaks).

2.3 Characterization methods

temperature at the same value as that of the reference. In the DSC plot, this is indicated by a drop in the heat flow, and the crystallization temperature is considered to occur at the minimum point. Additionally, the area under the curve is the latent heat of crystallization, and this value is necessary to determine the degree of crystallinity of the material. Melting point: The continuous heating of a polymer above its crystallization point causes the material to melt. Melting requires a breaking or weakening of the intermolecular attractions based on Van der Waals forces as the chains transform from an ordered to a disordered nature. Therefore this transition requires additional heat. This makes the sample heater apply energy at a faster rate than normal around the melting temperature. This is displayed in the DSC plot as a spike in the heat flow. The peak maxima are said to be the melting point of the polymer. Similarly, the area under the curve is the latent heat of the melting of this polymer. This latent heat of melting together with the latent heat of crystallization are used to calculate the degree of crystallinity of a material. The ideal situation is for all three transitions (Tg, Tc, and Tm) to appear as peaks with a highly consistent heating rate (heat flow) at temperatures outside the phase change regime. In a practical DSC graph, these transitions appear according to the sample and the temperature range analyzed. If the sample is highly amorphous and does not crystallize, then there is no peak for Tc. Additionally, whether the heat flow increases or decreases at these transitions depends on the heat capacity of the reference material compared to that of the sample. If the polymer has a higher heat capacity than the reference, the graph and the transitions would look similar to those shown in Fig. 2.8 (exothermic reactions indicated by valleys). If the heat capacity of the reference is higher than the polymer material tested, the graph would take the inverse shape of the graph shown in Fig. 2.8 since the heat flow is actually the difference between the sample and reference heat flow (endothermic reactions indicated by valleys).

2.3.1.5 Dynamic mechanical analysis DMA is used to measure the nonstructural properties of polymers. In DMA, the deformation of a sample under periodic stress is measured. The stress is usually changed periodically in a sinusoidal fashion. The output parameter, deformation, and strain are measured in terms of the angle and amplitude. The strain curve takes a sinusoidal shape as well, but with or without a phase difference to that of the stress curve. If there is no phase difference between the stress and strain curves, the material is said to be purely elastic, and such a graph would be called in-phase, whereas for a purely viscous material, the phase difference is 90 degree, which is called out-of-phase [85]. Polymers are viscoelastic materials showing combinative properties of entirely elastic and viscous materials [86,87]. Each polymer is unique in that the percentage of elastic character is varied. In this regard, DMA provides valuable information about the elastic and viscous behavior of a material, which is largely responsible for deciding whether the material is suitable for a given application.

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Polymers exhibit a phase difference somewhere between 0 and 90 degrees. This phase difference, together with the amplitude of the output strain wave, can be used to calculate a number of useful material parameters such as storage and loss moduli, storage and loss compliance, creep, transition temperature, complex and dynamic viscosity, stress relaxation, performance attributes, rate and degree of cure, sound absorption, morphology, and impact resistance. Common test conditions include a single frequency, constant deformation, and a variable temperature. Other testing possibilities include using a variable amplitude of deformation (deformation force or stress applied) or multiple frequencies. A schematic illustration of a DMA instrument is given in Fig. 2.9, and it is a useful method to study the property enhancement by reinforcing or combining polymer blends or composites [88].

1

2 3

4

5

7

4 6 7

8 9

10

FIGURE 2.9 Schematic of DMA instrumentation: (1) clamp, (2) sample, (3) drive shaft, (4) flexural suspension, (5) LVDT, (6) drive motor, (7) bearings, (8) carriage, (9) load screw, and (10) stepper motor.

2.3 Characterization methods

2.3.2 Direct observation methods The surface of polymeric materials or their internal structures can be directly analyzed by various microscopic techniques (Fig. 2.10). The morphology or topography of polymeric materials and their structures can be analyzed by OM, SEM, TEM, STM, AFM, and others [35]. All of these techniques can be utilized together to get an idea about the surface morphology or topography of the investigated polymeric material as a whole. Ultrafine structures in polymers can be analyzed by TEM, STM, or AFM, while larger segments can be analyzed by OM or SEM. The measurable sizes of structural details of polymeric materials vary based on different microscopic techniques. The attainable resolutions and magnifications using some microscopic techniques are summarized in Table 2.1 [89]. Two or SEM e– STM/AFM

Cryo-ultramicrotomyv

Replica

TEM

FIGURE 2.10 Overview of microscopy techniques for polymer morphology/topography analyses.

Table 2.1 Comparison of microscopic techniques in terms of resolutions and magnifications.

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CHAPTER 2 Morphology analysis

more of the mentioned techniques are usually used together as a complex for the investigation of the surface morphology/topography properties of polymeric surfaces. The results obtained from these techniques can help to understand the nature of the given polymer and the associated processes such as a synthesis, production, treatment, or modification.

2.3.2.1 Optical microscopy The basic principle of OM is based on the illumination of an object and the collection of light that is scattered or transmitted by a system of lenses that forms a microscopic image. These microscopic images can provide details about the specimen at a magnification in the range of 2 3 to 2000 3 with a maximum resolution of approximately 0.5 μm as a limitation of the objective lenses, light wavelength, or specimen nature. OM can provide information about the shape, size, and arrangement of visible features [90]. Moreover, the refractive index and the birefringence, which represent optical constants, can be obtained as well [91,92]. Microscopy images are often captured by digital cameras of high quality that are directly connected to a computer interface, allowing for the processing and postprocessing of the microscopic images. Simple microscopes usually operate at low magnification since they consist of only one imaging lens and operate at low magnification. Compound microscopes containing more than one imaging lens are often found in laboratories. These are able to work at higher resolutions and magnifications, providing more details on smaller samples. The magnification of this type of microscope is a result of the marked magnifications on the objective and eyepiece (40 3 12.5 5 500 3 , for example) in terms of visual observation. In contrast, digitally stored images are without magnification because they can be reproduced at any size; therefore, calibrating the system by capturing objects with a known size is necessary [90]. Among compound microscopes, binocular stereo microscopes [93,94] provide two different microscopic sample images via two eyepieces. These microscopes are useful for observing a majority of samples as a 3D image. This type of microscope represents an ideal start for analysis of a material nature. This is also a useful tool for the identification of sample regions to be studied later. It is worth mentioning that common compound microscopes are binocular. However, the images in each eyepiece are identical and act only to reduce the eyestrain of the observer. Bright and dark fields [89] represent operation modes in the OM technique (Fig. 2.11). Bright field represents the standard mode for OM in which an image plane is reached by unscattered light directly in transmission mode or by specular reflection in the reflection mode. Generally, transparent flat materials in reflection mode appear bright in bright field mode. On the other hand, variations of color and optical density are responsible for a contrast in transmitted light. Therefore fillers such as carbon black or pigment particles can be clearly seen in a polymer composite in bright field mode, whereas a thin polymer matrix is usually transparent. Scattered light reaching the image plane in dark field mode is less common in transmission mode, but is responsible for high contrast. The low

2.3 Characterization methods

Eyepiece lens

Objective lens

Reflected light Unreflected light (not captured)

Stage with sample

Condenser lens Opaque disk Light source Bright field

Dark field

FIGURE 2.11 Comparison of bright field and dark field modes in OM.

reflectivity of polymer materials is responsible for the poor detail in the images. The reflection of light can be improved with a metal coating (gold sputtering), leading to better contrast of the surface roughness. If a metal coating is not applied to the polymer surface, then observing the surface features that scatter the light is possible in dark field reflection mode [90]. Polarized light microscopy represents a special type of OM that uses polarized light for the investigation of morphology. A polarized microscope consists of similar basic constituents as a transmitted light microscope. Furthermore, it contains a polarizer located in an illumination system and an analyzer between the eyepiece and objective lens. It also has a rotatable stage and a polarizer, analyzer, or both, which have to be rotatable as well. A polarized microscope can selectively transmit light that is polarized in a particular plane. The analyzer and polarizer are both polar with a crossed arrangement and are the most common in polarized microscopes [95]. Confocal scanning laser microscopy is also a type of OM and is useful for the measurement of surface morphology. A rough surface causes poor images to be obtained using conventional OM. In contrast, confocal microscopy has no limitation in terms of roughness and has good lateral resolution. A small aperture (confocal pinhole) is placed in the plane, while rays from the object plane are focused. An image is formed during scanning using the confocal aperture and the

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illumination over the xy plane [90]. This microscopy is a standard technique for biological applications; however, it is also utilized for analyses of polymeric materials [96
98]. This technique was, for example, used for the analyses of polymeric films [99], porous membranes [100], dye penetration in fibers [101], microstructures in industrial composites [102], the morphology of reactive blends [103], emulsions [104], hydrogels, and polymer solutions [105]. Optical profilometry is a special type of OM technique that can combine highdefinition confocal microscopy and interferometry with additional features for accurate surface morphology analyses. No moveable parts are present in the confocal sensor, ensuring good stability and high resolution. A high aperture, up to 0.95 in the air, is responsible for the high vertical (up to 2 nm) and lateral (up to 140 nm) resolution. This OM system offers interferometry or confocal modes that are applicable for the measurement of the surface morphology, topography, or thickness of polymeric materials (10 nm
20 μm thickness measurable range for transparent films). Moreover, true color imaging is possible using four integrated LED sources and a megapixel CCD camera [106]. This optical device has a lower lateral resolution than that of an AFM. However, the height range is much wider and the field of view is larger [107]. OM techniques are mainly used for quickly checking surface morphology/ topography properties from the relatively large surface area of polymers, for example, for the profilometry technique, the minimum optically measured surface area is about 175 3 130 μm2 and the maximum is a few tens of centimeters using mapping mode. These techniques can be used to optimize the production or modification process of polymeric materials even with rough surfaces. The detailed information about surface morphology/topography can be further analyzed by techniques such as SEM, TEM, or AFM to focus on smaller surface areas.

2.3.2.2 Scanning electron microscopy Light microscopy is currently among the most common microscopy techniques because it is inexpensive and typically nondestructive during imaging. However, light microscopy has limits such as in distinguishing features smaller than 0.1 mm [89]. The resolution limitation for this type of microscopy is determined by the radiation wavelength of the visible light (400
700 nm). Radiation with shorter wavelengths is more susceptible to interaction with nanomaterials and leads to obtaining images with higher resolution [108]. The momentum of electrons affects their wavelengths, and voltage can change them by accelerating the electrons. Electrons with high energy and smaller wavelengths can be produced by higher accelerated voltages and vice versa. Accelerating voltages from 1 to 300 kV can produce electrons with 40
1 pm wavelengths [109,110]. SEM is a microscopic analytical tool that uses a probe that ejects electrons to form an electron beam to produce an image based on the reflection of these electrons during scanning (Fig. 2.12). The scanning electron microscope scans only a few micrometers of the sample surface [90,109,111
114]. For basic information, SEM uses a vacuum chamber that can contain the sample to be measured, and the

2.3 Characterization methods

Computer Electron source

Anode Scan generator

Condensor lenses

Amplifier x,y scancoils

Objective lens Back-scattered electron detector X-ray detector

Secondary electron detector Sample Motorized stage

FIGURE 2.12 Scheme of the scanning electron microscope [119]. Copyright 2019. Reproduced with permission from Elsevier.

probe is on the top of the chamber. The probe emits electrons toward the dark chamber, and the reflected electrons define the morphology of the scanned sample. The images obtained from a scanning electron microscope can be affected by the compositional and topographical (shape) properties of the measured samples [111]. The electrons focused on a small spot are scanned sequentially across the samples in the scanning electron microscope [115
117]. The electrons are produced in a thermionic, field-emission, or Schottky cathode and are subsequently accelerated based on a difference in voltage between the anode and the cathode, which can be in the range of 0.1
50 keV. The diameter at the cross-section of the electron gun is 50 μm for thermionic emission and 100 nm for field-emission or Schottky cathode, which can be demagnified by a lens system (two or three stages). Thus an electron probe of 1
10 nm can carry an electron-formed current to the sample ranging from 1029 to 10212 A, and using a probe diameter of  0.1 μm can lead to 1028 A of electron current [118]. Particular signals are

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emitted from the sample at each location, and detectors subsequently collect these signals, which form an image by their combination with dimensions depending on the scan pattern. SEM is considered to be an advanced microscopy technique in comparison with other OM techniques. The use of electrons has advantages compared with the use of light since their charge is negative, enabling extremely strong interactions with atoms that produce emission signals from the sample. Therefore SEM achieves a higher resolution than OM and enables the imaging of objects that are less than 15 nm. A high magnification range can be obtained by SEM, which is typically 10 3
500,000 3 ; it obtains images of features from the micro- to the nanoscale. The resolution of an image is responsible for distinguishing object features, while magnification is responsible for larger images. To improve the resolution, a high voltage may be required and, thus, the chamber must be in low-pressure (vacuum) to eliminate any interaction occurring between the electrons and air. The usual pressure in a scanning electron microscope is low, typically in the range 0.1
1024 Pa [119]. In general, there are no special requirements for materials except that they must be nonconductive. Polymers are nonconductive materials that are burned at high voltages due to charging by the electron beam. Therefore to deal with polymers, they have to be either coated with an electrically conductive material (gold, platinum, carbon) or analyzed using a low accelerating voltage (1
4 kV). This prevents the burning of the sample by the electron beam [120,121]. The images obtained in standard scanning electron microscopes are easily interpreted qualitatively, and the surface can be detected as a photographed image with high resolution. Cross-sectional images (fractured or microtomed surface) can also be taken using SEM and can provide an identification of the structural arrangement of a material. For polymers, it can provide an image of the crystals or polymer branches on the surface or inside the layers of the sample [122]. SEM is among unique nondestructive techniques that allow detailed investigation of the surface morphology of polymers. Using this technique, the surface roughness is not a limiting factor and, therefore, different 3D shapes of polymeric materials can be analyzed such as foils, sheets, granulates, powders, fibers, and foams.

2.3.2.3 Transmission electron microscopy TEM represents a microscopic technique for analyzing internal nanostructures in polymers and composites such as thin films, fibers, and particles. The typical magnification range for TEM is 2000 3 to 1,000,000 3 , allowing for the analysis of samples even at the atomic level. The main components of a transmission electron microscope are an electron gun, a detection system for transmitted electrons, and a system of lenses for focusing the electrons. Electrons are usually accelerated from the electron gun by the application of an accelerating voltage (80
300 kV), providing enough energy to penetrate a material up to 1 mm in depth [109]. A standard TEM image is obtained using electrons with an energy of 200
300 keV, and for the characterization of light elements such as carbon, electrons with lower energy

2.3 Characterization methods

Electron source

Anode

Condensor lenses

Condensor aperture STEM scan coils X-ray detector

Objective lens

Sample

Objective aperture Selected area aperture Projector lenses BF, ADF, HAADF detectors Viewing screen CCD camera or photographic plates

EELS detector

Magentic prism

CCD camera

FIGURE 2.13 Scheme of the transmission electron microscope [119]. Copyright 2019. Reproduced with permission from Elsevier.

(,100 keV) should be applied to avoid damaging the sample. The limitations of a conventional transmission electron microscope include the sample dimensions because the thickness of a sample should not exceed 200 μm and the diameter should not exceed 3 mm because of the lenses, which limit the space for sample insertion. This can be partially corrected using additional lenses with possible gaps of up to 5 mm that allow for the use of much larger samples, but the resolution can be less than 100 pm. A scheme of a transmission electron microscope is shown in Fig. 2.13 [119]. The samples for TEM should contain regions that electrons can pass through [123]. Moreover, the transmission electron microscope requires an antivibration environment, stable temperature, and an area free of electromagnetic fields. TEM requires more control over the electron beam in comparison with the SEM technique because of the electron scattering sensitivity [110]. The low atomic

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number of polymers is responsible for the poor contrast when using TEM because electrons are scattered poorly. The strong electron beam usually leads to mass loss, dimension changes, or loss of crystallinity in polymers. These damaging effects can be avoided using a high accelerating voltage or sample cooling. In addition, sample preparation can improve the contrast of the obtained images [90]. The image contrast obtained by the transmission electron microscope is as a result of electron scattering (elastic). However, electrons scattered by the sample at large angles are not contributing to the image in bright field mode. For amorphous polymers, the obtained results indicate a change in mass thickness. Higher scattering regions are responsible for darker areas in bright field images. A low accelerating voltage leads to better contrast. The most crucial factor for TEM analysis is the sample thickness. It must be as thin as possible to ensure the penetration of electrons without losing energy, and this is why it is even more important to use a low accelerating voltage. For crystalline polymers, orientations and the crystal thickness substantially affect the scattered intensity. When a crystal is properly oriented for diffraction, a thin crystal appears dark. Polymers usually scatter electrons weakly in bright field mode; therefore, the contrast is low. The use of dark field mode can generally increase the contrast, but the intensity is weaker than in bright field mode. However, TEM images of amorphous materials have low intensities in dark field mode, and therefore, its use is rare. This can be caused by the scattering of electrons in all directions, which means that the aperture of the objective can capture only a few scattered electrons. The dark field mode is more useful for the analyses of crystalline materials, where the intensity of scattered electrons is focused on the objective lens. Therefore dark field mode can provide information about crystalline polymer structures that is unavailable in bright field mode. On the other hand, the analyses of polymers in dark field mode can be difficult due to unstable imaging [124,125]. TEM represents a versatile technique that allows for the analysis of the morphology of polymeric materials in great detail. Using this technique, nano or even atomic structures of polymers can be obtained. However, there are difficulties related with sample preparation, for example, the use of a microtone knife is often necessary.

2.3.2.4 Scanning tunneling microscopy The STM technique represents the first developed scanning probe microscopy technique. The probe (tip) in the STM represents an electrical conductor (at a bias voltage), and the current passing between the probe and electrically conductive sample represents a signal (Fig. 2.14). The imaging of molecules present on a substrate surface is not a direct mechanism. The tip/electrically conductive sample interaction represents a mechanical tunneling current (quantum) measurable for small distances between them (,1 nm). The molecules should geometrically fit into the tunneling gap to execute direct tunneling between the tip and substrate. The local presence of molecules can be responsible for a change of electronic state that leads to contrast in the image. This phenomenon is responsible for

2.3 Characterization methods

Pieze actuator Z-piezo

Y-piezo X-piezo

Conductive tip I

FIGURE 2.14 Scheme of scanning tunneling microscope.

probing the local electron density on the surface. Therefore the STM technique can be considered as a spectroscopic technique too. Surface atomic arrangements can be determined using STM. This technique has some limitations in the analyses of polymeric materials because samples have to be electrically conductive. However, successful analyses of thicker organic samples (bulk insulators of 1
20 nm in size) were carried out by STM with some modifications. Direct STM is not applicable for obtaining stable images and an increase in electrical conductivity is necessary (B10 magnitude orders) in comparison with neutral organic samples [126,127]. Mechanisms such as resonant tunneling [128], electron hopping [129], or field-emission [130], were applied for this purpose. Intra/intermolecular relaxation of biopolymers is responsible for steady tunneling imaging due to the impact of a high electrical field in the gap (107 V/cm) [131]. Moreover, a reduction of the pressure in the gap was applied [132,133]. The Langmuir
Blodgett technique [134] that uses the deposition of monolayers onto conductive graphite was used for the analyses of poly(octadecyl acrylate) and PMMA by STM [135]. The STM technique is a suitable technique for the analyses of the surface topography in 3D scale of different polymeric materials. However, a limitation of this technique for surface topography analyses of different polymers is electrical conductivity. Therefore by this technique, electrically conductive polymers or only few nanometers thick dielectric polymers deposited on electrically conductive substrates can be measured successfully.

2.3.2.5 Atomic force microscopy The invention of STM approximately three decades ago was focused on the development of other techniques of scanning probe microscopy that are commonly used in nanotechnology [136,137]. The limits of STM for the measurement of mainly electrically conductive materials provided the impulse to develop a new type of scanning probe microscope a few years later, namely the AFM technique [138], which is useful for the morphology and topography analyses of different

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materials [139] even with an atomic resolution [140]. The initial application of AFM was for the investigation of polymeric structures on surface areas, and it provided outstanding spatial resolution [141
144] including for biological and synthetic polymers [145
147]. The AFM technique can be directly used for quantitative analyses of micro- and nanostructures of polymer morphologies and topographies [148], molecular scaled crystals [149], particular polymeric chains [150], lamellae [151], and spherulites [152]. Mizes et al. analyzed the 3D surface topography of PS, polyacetylene, and poly(3-hexylthiophene) at the atomic level using STM and AFM. These microscopic techniques provided information about the fibrillary structures and film growth of these polymers. Both of these techniques were well suited to study the fibrillary nature and growth of these films [153]. The AFM technique has an advantage over other microscopy techniques since there are no limits in terms of the environment. Samples can be measured in air, under water, and in other solutions by AFM. However, surface forces or elastic repulsion between the AFM tip and the soft surfaces can be responsible for poor quality of AFM images [154,155]. The main part of the AFM is a flexible microsized cantilever containing a sharp probe (tip) with a reflective coating, which is mounted at the end of the cantilever. This tip is positioned above the surface area of the sample that is going to be analyzed. The deflection of the cantilever during scanning over the topographic structures in the sample surface is monitored, and a 3D image is usually obtained with a high resolution. Various scanning techniques can be used in AFM for the imaging of the sample surface based on its properties and conditions. The interaction between the force sensing and the sample surface induces a deflection of the cantilever allowing a topographic image of the surface to be obtained. Surface roughness can also be quantitatively evaluated by this technique. There are three basic techniques, namely contact mode (static conditions), intermittent/tapping/AC mode (dynamic conditions), or noncontact mode. In contact mode, the AFM tip is in permanent contact with the sample surface. In contrast, the AFM tip oscillates with an amplitude (typically 100
200 nm) near a resonant frequency over the sample in tapping mode, while in the noncontact mode, the oscillated AFM tip is not in contact with the sample at all [156]. A basic scheme of the AFM method is shown in Fig. 2.15. The cantilevers used in AFM are usually v- or rectangular-shaped and contain sharp tips at their free ends that interact with the sample. The most typical geometries of the atomic force microscope tips are a pyramid with a square base or a cylindrical cone. Cantilevers with tips that are commercially available for AFM are predominantly made of silicon nitride (Si3N4) or silicon, and the upper layer is a reflective coating of aluminum, gold, or platinum to reflect light from the laser diode. These materials are used because of their excellent mechanical properties [157
159]. However, instead of using silicon-based materials, diamond tips [160
162] or a lever can be used, especially when the tip may be subjected to high pressure. Quartz cantilevers with polymer tips [163] can be used for dynamic imaging.

2.4 Applications

Position sensitive detector

Mirror Light source

Signal

Shake piezo Focusing optics

Cantilever with tip

Chip

Sample

x-y piezo stage

FIGURE 2.15 Scheme of the atomic force microscope.

Moreover, tips or cantilevers are made based on tungsten [164], lead zirconate titanate [165], and platinum/iridium [166]. A cantilever chip mounted to a piezoelectric crystal is responsible for bringing the AFM tip into contact with the sample surface. Another piezoelectric crystal can be mounted to the stage allowing for the scanning of the x
y axes of the sample. The reflected beam from the laser diode is then captured by the photodetector with high position sensitivity. The laser beam position on the photodetector is changed based on the cantilever deflection. A typical photodetector is divided into four parts by vertical and horizontal lines, allowing for the detection of the cantilever deflection in the z-direction or the bending (lateral/torsional) of the cantilever [167]. AFM is an advanced nondestructive microscopic technique, which is commonly used for the detailed analysis of the 3D surface topography of various types of polymers. Electrical conductivity is not a limiting factor for this technique. However, the limitations include rough surfaces where the AFM tip is not able to scan rugged layers of the analyzed surfaces.

2.4 Applications The techniques of XRD, light scattering, calorimetry, scanning probe microscopy, electron microscopy, and OM represent powerful tools for analyzing the surface morphology/topography of a broad spectrum of polymeric materials that require high performance and tailored properties. The structures of polymers are closely related to the process variables and have a direct impact on the mechanical, physical, and surface properties. The resulting properties of polymers can be affected by the chemical composition, surface morphology, and processing history. Two

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aspects are important for morphological analyses, namely choosing a suitable instrumental technique and sample preparation. The polymer morphology should be analyzed and then correlated with the material properties to be able to develop advanced polymeric materials. Ponnamma et al. investigated the influence of BaTiO3/hexagonal boron nitride filler on the energy harvesting of piezoelectric nanocomposites. The XRD technique was used for obtaining patterns of the particularly used fillers in polyvinylidene fluoride hexafluoropropylene [168]. In a separate study, the modification of LDPE using Fe and a second component of intumescent flame retardant (IFR) has been deeply analyzed by their respective XRD patterns as shown in Fig. 2.16. The two diffraction peaks at approximately 21 and 23 degrees can be ascribed to LDPE. The peaks indicate a fairly crystalline morphology that is not perfectly crystalline because the peaks are slightly broad with respect to the background. The peak intensities in the pattern “a” corresponding to LDPE and in “b” related to LDPE/IFR30 have decreased, and some new peaks have appeared, specifically two sharp peaks at 14.6 and 15.6 degrees. The appearance of new peaks is a clear indication of the existence of a new phase/material (second component). Patterns “c” and “d” are similar to that of “b.” This could be because the amount of Fe in the LDPE/IFR system is low; therefore, the peak intensity from Fe is negligible or the IFR peaks overlap with Fe and it is difficult to distinguish between the two [71].

LDPE LDPE/IFR30 LDPE/IFR28/Fe-OMMT2 LDPE/IFR28/Fe-MMT2 Intensity/(C)

48

d c

b a 10

20 2/(deg)

30

40

FIGURE 2.16 XRD graph of LDPE and LDPE with different forms of Fe [71]. Copyright 2019. Reproduced with permission from Elsevier.

2.4 Applications

Apart from studying the morphology of prepared or synthesized polymers, XRD is even more useful in studying the degradation of polymers and phase changes over time. In a study of the influence of melamine polyphosphate on the thermal stability of polyamide (PA), a PA/melamine polyphosphate composite was subjected to heat treatment at 350 C, and the residue samples collected in predefined time intervals were studied by XRD; the profile is given in Fig. 2.17. In the residue after 10 min at 350 C, the composite material has narrow sharp peaks responsible for the melamine/polyphosphate phase. As the time increased to 30 min, the sharp peaks completely disappeared, with one broad shoulder appearing at a 2θ value 10 degrees and a broad peak at 2θ value 20 degrees. This indicates that the system is now completely amorphous. As confirmed by other techniques such as thermogravimetric analysis and solid-state nuclear magnetic resonance, the underlying reason for this is the depolymerization of melamine

350/10

350/30

350/40

350/50

350/60

350/90 9.0

10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0 2THETA ANGLE

FIGURE 2.17 XRD of PA/melamine polyphosphate composite after heat treatment at 350 C for 10, 30, 40, 50, 60, and 90 min (patterns from top to bottom respectively) [169]. Copyright 2019. Reproduced with permission from Elsevier.

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polyphosphate in the first instance and then the formation of melamine phosphate and the transformation to melamine pyrophosphate. These forms were not crystallizable within the 6,6 PA matrix utilized in this study. The case studies described demonstrate the strength of XRD in studying an as-prepared polymer morphology after combining it with a second component and/or the aging [170] of the material over time [171]. In a study by Xianbo [172], the authors explain how using an adaptive surrogate modeling-based platform for DMA can greatly reduce the number of experiments that need to be performed to obtain the elastic modulus under high temperature and several strain rates using a single specimen of the material/test sample. In this study, the establishment of a surrogate model based on the principle of time
temperature superposition provided the basis and was the initial step. The time domain was then transformed from this surrogate model and a function of temperature-dependent relaxation was obtained from which the elastic modulus (strain rate sensitive) was extracted. The DMA results were then validated with results obtained from conventional tensile tests. DMA was shown to be extremely important in demonstrating how the presence and loading of a minute amount of silica nanoparticles can improve the thermomechanical properties of a polymeric material such as ultimate tensile strength, elongation at break, Young’s modulus, and loss and storage modulus [173]. In a study, enantiomers of lactide, L-lactide and D-lactide, were subjected to stereocomplexation [174]; the enantiomeric forms are known to produce stereocomplex crystallites. The formation of stereocomplex crystallites was confirmed with DSC by the changes in melting temperature. The melting points of both poly-D-lactide and poly-L-lactide were observed to be approximately 150 C, and the stereocomplex samples prepared in two solvents, namely acetonitrile and THF, both exhibited a melting peak in the DSC at 210 C. There was an increase in the melting point of 60 C. This indicates that in the presence of the two enantiomers, homocomplexation did not take place; only stereocomplexation occurred because there was no melting peak at approximately 150 C in the DSC graph. In this study, the DSC study played a vital role in deciding which processes took place. The DSC technique was extensively used in another study [175] for the evaluation of the effect of polytetrafluoroethylene (PTFE) nanofibrils on the PP matrix in a nanocomposite of PP/PTFE. It was found from the DSC graph that both the peak and the onset temperature increased significantly for crystallization with the addition and increased loading of PTFE nanofibrils. Based on this observation, it was deduced that PTFE nanofibrils act as a nucleating agent for the heterogeneous crystallization for PP, which can boost the crystallization of nanocomposites by accelerating the nucleation rate. The degree of crystallinity was quantified in this study with good precision, where the crystallinity of neat PP was 36.3%, while the PP/PTFE nanocomposites exhibited lower degrees of crystallinity in the order of 35.7%, 24.7%, and 33.1% for PTFE loadings of 1%, 3%, and 5% respectively. Fadiran et al. analyzed pollen-reinforced epoxy composites

2.4 Applications

using DSC. The effectiveness and strengthening of composites made of pollen filler and epoxy as a matrix was analyzed in terms of Tg, interfacial morphology, and mechanical properties obtained by DSC. Composites prepared with base-acid treated pollen showed improved load-bearing, interfacial morphology, and Tg at 10 wt.% pollen loading. Treated pollen elevated the Tg significantly because of crosslinking with the epoxy and a lack of soft interphases [176]. Ponnamma et al. analyzed the crystallinity behavior of immiscible compounds consisting of semicrystalline HDPE, elastomeric acrylonitrile butadiene rubber, fillers, and compatibilizers. The effect of these all components on the crystallinity behavior was analyzed by DSC technique. The HDPE crystallinity was strongly impacted by the acrylonitrile butadiene rubber, the concentration of compatibilizer, the blend composition, and filler addition. The presence of this rubber in HDPE resulted in a slight enhancement of the crystallinity of HDPE blends [177]. SAXS was used for studying the structure disorientations of PP composites, which were prepared at different temperatures [178]. Anisotropy investigations of SAXS allowed us to not only qualitatively recognize the disorientation, but also to quantify the disorientation crystallites. Furthermore, the disorientation/Young’s modulus correlation of the composite was established. Being a nondestructive method, the authors highly recommended this test method for the prediction of the mechanical properties of polymer composites. In a separate study, alterations in thermal stability and microstructure upon the introduction of TiO2 nanoparticles into an HDPE matrix were studied extensively by DSC and SAXS [179]. According to the DSC results, the crystallinity of the composite was maximized with a low TiO2 loading, and the crystallinity was greatly reduced when the TiO2 content was 2% or higher. The SAXS results led to the inference that a homogeneous distribution of TiO2 nanoparticles in the HDPE matrix was achieved with better surface fractal characteristics at a small scale, and the best balance between the microstructure and the mechanical properties was observed at a TiO2 loading of 2%.

FIGURE 2.18 Optical microscopy images of dewetting stages of a PS film (110 nm in thickness) on hydrophobized silicon: (A) hole nucleation, (B) hole growth and beginning of coalescence, and (C) decay into isolated droplets [180]. Copyright 2019. Reproduced with permission from Elsevier.

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0.25 D = 12.1 um Nf = 9.79x107 cells/cm3

Fraction

0.20 0.15 0.10 0.05 0.00 0

3

6

0.25

Fraction

9 12 15 18 21 24 27 30 33 Pore size (um) D = 9.8 um Nf = 2.04x108 cells/cm3

0.20 0.15 0.10 0.05 0.00 0

3

6

9 12 15 18 21 24 27 30 33 Pore size (um)

0.25 D = 6.8 um Nf = 7.02x108 cells/cm3

0.20 Fraction

52

0.15 0.10 0.05 0.00 0

3

6

9 12 15 18 21 24 27 30 33 Pore size (um)

FIGURE 2.19 SEM images of foams and distribution of pore size of (A) PLA, (B) PLA/PU, and (C) PLA/ PU/PTFE [182]. Copyright 2019. Reproduced with permission from Elsevier.

These case studies are only a few that demonstrate the strength of each of these nonstructural analytical techniques alone and/or collectively coupled with other compatible techniques for polymer morphology analyses. However, microscopy techniques can be used directly to observe polymer morphology. Telford et al. illustrated micropatterned polymer surfaces fabricated by dewetting bilayers of thin PS films. These micropatterns could be applicable for the

2.4 Applications

FIGURE 2.20 SEM images of (A) linear-LDPE foam and (B) linear-LDPE foam with paraffin wax; scale bar represents 100 μm [182]. Copyright 2019. Reproduced with permission from Springer Nature.

selective patterning/attachment of proteins/cells or for water collection of an air atmosphere (Fig. 2.18) [180]. Huang et al. investigated the morphologies of foams prepared from polylactic acid (PLA), polyurethane (PU), and PTFE (Fig. 2.19) by SEM. Small elliptical pores with large sizes were present in the neat PLA. PLA/PU showed a better morphology, while the distribution of pore size was reduced. The size of pores decreased, and their density increased in the PLA/PU/PTFE foam with a more uniform morphology [181]. Popelka et al. used SEM for the analysis of hexagonal pores in linear-LDPE foam with incorporated paraffin wax representing the phase change material (Fig. 2.20). The pore size was in the range 100
200 μm [182]. Colucci et al. analyzed the microstructure and morphology of polymer/CNT nanocomposites prepared by laser printing using SEM. This method was used for the fabrication of conductive paths on PP substrates using multiwall CNTs. They confirmed that a close relationship between the final integrity of the nanocomposites and the electrical resistance after laser treatment (high absorbed energy by polymer and high temperature) could be responsible for damaging the final materials [183]. The surface morphology of fabricated co-PA nanocomposite membranes reinforced with cellulose nanocrystals for enhanced mechanical properties using the electrospinning method was analyzed by Sobolˇciak et al. [184]. Information about the surface morphology (Fig. 2.21) and topography (Fig. 2.22) of membranes with different compositions was obtained by SEM and profilometry techniques respectively. Brostow et al. analyzed the impact of filler types, concentrations, and sizes on the fire resistibility of different PP composites with nanosized Al (OH)3 and Mg(OH)2 filler. SEM was used for the investigation of the distribution

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FIGURE 2.21 SEM images of co-PA fiber mats fabricated by electrospinning with compositions of cellulose nanocrystals of (A) 0 wt.%, (B) 0.2 wt.%, (C) 0.5 wt.%, (D) S1 wt.%, (E) 2.5 wt. %, and (F) S5 wt.% (black scale bar represents 50 μm and white scale bar represents 10 μm) [184]. Copyright 2019. Reproduced with permission from Elsevier.

2.4 Applications

FIGURE 2.22 3D profilometry images of electrospun co-PA fibers with compositions of cellulose nanocrystals of (A) 0 wt.% (Sa 5 1.6 μm), (B) 0.2 wt.% (Sa 5 1.7 μm), (C) 0.5 wt.% (Sa 5 1.7 μm), (D) S1 wt.% (Sa 5 1.8 μm), (E) 2.5 wt.% (Sa 5 2.9 μm), and (F) S5 wt.% (Sa 5 2.7 μm). Sa is roughness parameter [184]. Copyright 2019. Reproduced with permission from Elsevier.

of the filler in the polymer matrix and the resulting roughness. The surfaces of the composites containing Al(OH)3 showed slightly rougher structures than those with Mg(OH)2 [185]. Putz et al. studied aluminum
polyimide interfaces acting as a thermal insulator that are used for satellite applications. TEM was employed for studying the interfacial morphology in relation to the adhesion between these two materials, which must withstand thermal loadings originating from the sun when a satellite is in orbit. These analyses proved that the interfacial properties were unaffected for up to 200 thermal cycles carried out at 150 C in a nitrogen environment; the analyses also revealed the presence of an unchanged 3.6 nm amorphous layer at the metal/polymer interface after thermal treatment, which acts as an adhesion promoter in this system (Fig. 2.23) [186]. Ponnamma et al. investigated the distribution of filler combinations (titanium dioxide nanotubes, reduced graphene oxide, and strontium titanate) throughout a poly(vinylidene fluoride-co-hexafluoropropylene) matrix for piezoelectric

55

FIGURE 2.23 TEM images of aluminum
polyimide laminate interfaces in cross-section (A and C) asdeposited, and (B and D) after 200 thermal cycles; (B and D) at 150 C [186]. Copyright 2019. Reproduced with permission from Elsevier.

FIGURE 2.24 AFM images of poly(n-butyl acrylate) brunches prepared from poly(alkyl methacrylate) and poly(alkyl acrylate) backbones [190]. Reprinted with permission from Yu-Su SY, Sun FC, Sheiko SS, Konkolewicz D, Lee H, Matyjaszewski K. Molecular imaging and analysis of branching topology in polyacrylates by atomic force microscopy. Macromolecules 2011;44(15):5928
36. Copyright 2019 American Chemical Society.

2.4 Applications

FIGURE 2.25 AFM images (3D height, line profiles) of polyvinylidene fluoride/FeO surface using various laser pulse energies [191]. Copyright 2019. Reproduced with permission from Elsevier.

applications. TEM was used for the analysis of the filler morphologies and SEM images confirmed a uniform distribution of the used fillers in the polymer matrix showing good piezoelectric properties [187]. Parangusan et al. used SEM and TEM for analyzing of surface morphology of poly(vinylidene fluoride hexafluoropropylene) nanofibers, native and filled with ZnO, prepared by the electrospinning technique for piezoelectric applications [188].

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Sheiko and Mo¨ller showed that AFM can provide information (quantitative) about the topology of branching, which includes the distribution of branches [189]. The molecular brushes of PMMA with poly(n-butyl acrylate) side chains were analyzed by AFM using the tapping mode. The polymer chains grafted by side chains can be clearly distinguished (Fig. 2.24). The AFM technique gave direct and quantitative information regarding the topology of the branching, the distribution, and the length of particular branches, which was not obtainable by other methods [190]. Ponnamma et al. used AFM for the investigation of the laser effect on the surface structures in a semicrystalline polyvinylidene fluoride and its composites with ZnO (B100 nm in diameter) and FeO (B30 nm in diameter) nanoparticles. The AFM images confirmed the formation of periodic structures (Fig. 2.25) [191].

2.5 Conclusion The study of polymer morphology represents an interdisciplinary approach from the nanolevel (e.g., polymer structure, conformation, and crystallinity) to the macrolevel (e.g., surface morphology of final products, fibers, foils, blends, and composites). This research usually involves the characterization of morphologies and understanding their relationships with processing. Advances in the development of new sophisticated characterization techniques make it possible to analyze morphology with appropriate accuracy, to work with high magnification, and to reduce the time necessary for measurements. These techniques often allow for correlation between polymer morphology and other properties such as mechanical, chemical, adhesive, electrical, and thermal properties, which are crucial for obtaining polymer materials with desired properties.

Acknowledgments This chapter was made possible by an Award JSREP07-022-3-010 from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors.

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2):55
65. [188] Parangusan H, Ponnamma D, Al-Maadeed MAA. Stretchable electrospun PVDFHFP/Co-ZnO nanofibers as piezoelectric nanogenerators. Sci Rep 2018;8(1):754. [189] Sheiko SS, Mo¨ller M. Visualization of macromolecules: a first step to manipulation and controlled response. Chem Rev 2001;101(12):4099
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83.

CHAPTER

Chemical analysis of polymers

3

Leena Nebhani
and Aanchal Jaisingh Department of Materials Science and Engineering, Indian Institute of Technology Delhi, New Delhi, India
[email protected]

3.1 Introduction Polymers are everywhere. Polymers have diverse applications; for example, they are commonly used in paints, coating, packaging, etc., as well as in several high-end applications such as drug encapsulation, scaffolds for biomedical engineering, etc. [14]. The method to be used for the synthesis of a polymer is selected based on the potential area of application of that particular polymer. The properties of a polymer are highly dependent on its structure, functional groups, and molecular weight, therefore, a detailed and complete characterization of a polymer sample is highly important. To determine the properties of a polymer, a thorough characterization is required for a reliable and accurate evaluation of its composition, functional groups, and structure. In general, polymers can be characterized by various techniques, for example, by chemical, mechanical, thermal, morphological as well as rheological methods of analysis [58]. A detailed characterization of a polymer sample can be performed using combination of variety of techniques because different techniques provide information about different properties of the polymer. Thermal, mechanical, electrical, and rheological techniques are utilized for analyzing the bulk properties of a polymer sample [912]. Thermal analysis techniques, for example, differential scanning calorimetry and thermogravimetric analysis are commonly performed to get information on different transition temperatures and degradation temperatures respectively. The evaluation of thermal properties is important as these transition temperatures determine the proper temperature that should be utilized for the processing of that particular polymer sample. Also, details about the thermal stability of a polymer sample under different environments are studied by thermal analysis techniques. Microscopic techniques such as transmission electron microscopy, scanning electron microscopy, and atomic force microscopy are used for studying the morphological and surface properties of a polymer [13,14]. Chemical methods provide information about all the aspects related to the functional group, structure, and composition of a polymeric system. In this chapter, the discussion is focused on the chemical analysis of polymers. The techniques discussed in detail in this Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00003-2 © 2020 Elsevier Inc. All rights reserved.

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chapter include the molecular weight determination of polymers by end group analysis. The determination of the molecular weight of polymers is possible via various techniques, for example, end group analysis is utilized to calculate the number average molecular weight (Mn) by simple titration experiment, provided the nature and number of the terminal functional groups are known [15]. Gel permeation chromatography (GPC) is widely used for calculating the number average (Mn) and weight average (Mw) molecular weight of different types of polymer samples having a wide distribution of molecular weights [16]. The other techniques include the analysis of the molecular weight by studying the colligative properties of a polymer, for example, membrane osmometry, vapor pressure measurement, etc. [17]. After the discussion on the determination of molecular weight, the characterization of polymers using Fourier-transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy has been discussed in this chapter. FT-IR spectroscopy is the most commonly used technique for the detection of functional groups in a polymer chain [18]. NMR spectroscopy is used for structure confirmation, for calculation of monomer conversion, end group analysis, study of configurational isomers (tacticity) as well as molecular weight determination [19,20]. Lastly, the characterization of polymers using mass spectrometry (MS) techniques has been discussed in this chapter. MS techniques like matrix assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) are also commonly used for the end group analysis and molecular weight determination of polymers [21,22].

3.2 Molecular weight determination Polymers or macromolecules are composed of a large number of constituent monomers that are chemically linked together. The process of polymerization results in polymer chains of varying lengths; this gives rise to polymer chains with different molecular weights and molecular weight distributions. In other words, polymer chains are polydisperse in nature unless techniques for controlling the molecular weight are utilized to synthesize polymers with low polydispersity. As polymer chains are of different lengths, the molecular weight of a polymer is described as an average molecular weight [23]. The molecular weight is an important characteristic of a polymer sample as several properties, for example, mechanical (stressstrain, impact, fatigue, creep, etc.) and thermal properties are influenced mainly by molecular weight and molecular weight distribution [24]. Electrical properties such as conductivity, dielectric constant, and dielectric loss as well as physical properties including viscosity, osmotic pressure, elevation in boiling point, depression in freezing point, etc., also depend on the molecular weight significantly [2528]. The different types of average molecular weights are number average molecular weight (Mn), weight average molecular weight (Mw), viscosity average molecular weight (Mv), and Z-average molecular weight (Mz). The molecular weight of a polymer can be determined using a variety of methods, and it is possible to classify these methods into absolute, equivalent,

3.2 Molecular weight determination

and relative methods [23]. Absolute methods include the estimation of the molecular weight of a polymer directly from measured quantities without taking into account the physical/chemical properties of a polymer sample. However, knowing the chemical structure of the polymer sample is an additional requirement for the equivalent method of analysis. Relative methods are based on the measurement of properties that are a function of the physical properties as well as the chemical structure of polymers; wherein the molecular weight of polymer standards having similar physical properties and chemical structures as that of the polymer to be characterized are required. Relative methods of analysis are more popular because they involve easy and more reproducible measurements as well as sample preparation. Absolute or primary techniques are mostly based on the colligative properties of polymers such as ebullioscopy (elevation in boiling point), cryoscopy (depression in freezing point), membrane osmometry, etc. These techniques are discussed in a comprehensive manner in many textbooks [27,29,30]. This chapter discusses in detail the determination of the molecular weight of a polymer by end group analysis using titration and by 1H NMR spectroscopy.

3.2.1 Determination of molecular weight by end group analysis The analysis of end groups has been established as an important tool for the characterization of polymers, particularly for monitoring the molecular weight of a polymer. It can also be used for studying the kinetics of polymerization and depolymerization as well as to study polymer inhibition [31,32]. There are variety of chemical and physical methods available that can be employed for the determination of end groups. The chemical techniques include methods based on halogenation, saponification, phthalation, acetylation, hydrogenation, which are performed using titration and colorimetric procedures [33]. The end group analysis performed via titration experiment can specifically be used to calculate the Mn of a polymer. Some general principles that apply for end group analysis to determine Mn are given here [34]. 1. The end groups in the polymer sample must be detectable by titration experiment and the structure of the end groups must be sufficiently different from the repeat unit. 2. The number of end groups per polymer chain must be well specified or known. For a linear polymer, there can be one functional end group or two functional end groups per chain that may or may not be distinct from each other, whereas for a branched polymer, the number of functional end groups cannot be determined precisely unless all the branching points are known. 3. This technique is generally restricted to low molecular weight polymers, that is, of the order of 20,00030,000 g/mol. The sensitivity of the procedure tends to decrease with an increase in the molecular weight as the concentration of these groups become too small for accurate measurement [35].

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4. The end group method of analysis is suitable for linear condensation polymers such as polyesters and polyamides due to the presence of unreacted terminal functional groups. The various possibilities of end groups can be understood by looking at the example of polyamides as shown in Fig. 3.1. 1. A linear polyamide containing a carboxyl and an amino end group as shown in the Fig. 3.1A. 2. There is a possibility of the presence of an amino group at both the terminal positions (Fig. 3.1B) if the synthesis of polyamide is carried out in the presence of an excess of diamine. 3. If the polyamide synthesis is carried out using an excess of dicarboxylic acid, the polymer will have carboxylic groups on both the ends (Fig. 3.1C). Similarly, in this case, only carboxylic groups will be titrated, and two groups will be counted per polymer chain. Similar structures are possible in the case of polyester; in this case, the end functionalities are carboxyl and hydroxyl groups.

3.2.1.1 Chemical analysis of amine, carboxyl and hydroxyl groups For the determination of amine groups in polyamides, the sample should be cooled in liquid nitrogen before crushing or grounding into fine pieces for fast dissolution. The standard method for determining the amine value or amine equivalent is by titration with a standardized acid solution, the procedure of which is briefly discussed here [35,36]. The reagents required for titration are phenol and methanol (both freshly distilled) and a standardized HCl solution. Procedure for titration: An accurately weighed sample of polyamide is commonly dissolved in phenol and methanol and the resulting solution is refluxed till it reaches complete dissolution. After cooling at room temperature, the solution can be titrated against the standardized HCl solution to endpoint using thymol blue as an indicator. The volume of titrant used is recorded each time. Most methods for the quantification of carboxyl groups in polymers rely on titration techniques, for example, in polyesters such as polyethylene terephthalate, end group determination entails preparing a homogenous solution, followed by

FIGURE 3.1 Examples of polyamides with (a) One amino and other carboxylic acid as terminal groups, (b) Two terminal amino groups, and (c) Two carboxylic acid groups.

3.2 Molecular weight determination

titration with sodium hydroxide [37]. The standard procedure for the determination of carboxyl equivalent is discussed briefly here [35,38,39]. An accurately weighed polymer sample is commonly dissolved in pyridine and heated at 105 C110 C until the sample is completely dissolved. This is followed by the addition of distilled water and further heating for a few minutes. Finally, n-butanol is added and the solution is titrated against NaOH to endpoint(from yellow to purple) using aqueous cresol red and aqueous thymol blue as a mixed indicator solution in a ratio of 1:3 respectively. The volume of titrant used is noted for all the solutions. Terminal hydroxyl groups are present in many polymers like polyethylene glycol (PEG), polypropylene glycol [40], and polyethylene terephthalate [37,41]. The hydroxyl groups in polymers can be determined via reaction with acetic anhydride [42], phthalic anhydride [43,44], sulfobenzoic anhydride [45], nitrophthalic anhydride [46] and 3,4-dinitrobenzoyl chloride [47], etc. The standard procedure for calculating the hydroxyl equivalent is as follows [35,38,42,48]. An accurately weighed polymer sample is dissolved in an acetylating agent and heated while stirring. Distilled water and pyridine are added and the solution is heated for another five minutes to around 100 C. After bringing it down to room temperature, the solution is titrated against NaOH to neutral endpoint using a mixed indicator solution (mentioned above). In this case also, the volume of titrant used is important in order to calculate Mn.

3.2.2 Determination of number average molecular weight by end group analysis The amount of reagent utilized in the above discussed methods for the determination of amine, carboxylic, or hydroxyl group can further be used to calculate the number average molecular weight for a polymer under consideration. The number average molecular weight can be calculated using Eq. (3.1). Mn 5

fwe a

(3.1)

In Eq. (3.1), Mn is the number average molecular weight of the polymer sample, f is the functionality or number of functional groups per polymer chain, a is the amount of reagent used in titration (obtained from the above described methods), and e is the equivalent weight of the reagent used for the titration. The Mn can also be obtained using colligative techniques, for example, membrane osmometry, vapor pressure osmometry, elevation in boiling point, and depression in freezing point. In membrane osmometry, osmotic pressure is calculated by the measurement of the height for the pure solvent and the polymer solution, which are separated by a semipermeable membrane. Vapor pressure osmometry is another technique that can be utilized to determine the Mn of a variety of polymers having an appropriate range of molecular weights. It relies on the difference in vapor pressure between the pure solvent and the polymer solution, which further depends on the number of solute

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particles present [49]. In addition, Mn can be determined using viscosity measurement by comparing the flow time of the pure solvent with respect to the polymer solution. All the colligative techniques as well as end group analysis for the determination of Mn have one common limitation, that is, they are only applicable to polymers having molecular weights less than 30,000 g/mol. The Mw and Mz can be determined using light scattering and the centrifugation technique respectively. The complete information about different average molecular weights (Mn, Mw, Mz) and molecular weight distributions can be obtained by size exclusion chromatography (SEC), commonly known as GPC. In addition to the determination of the molecular weight, it is important to perform a characterization of the functional groups that are present in a polymer chain. In order to characterize the functional groups present in a polymer sample infrared (IR) spectroscopy is utilized and is discussed in detail in the section below.

3.3 Infrared spectroscopy The absorption of IR radiation by a molecule results in the excitation of molecular vibrations to a higher energy level. It is necessary for the IR radiation to possess an energy equal to or more than the difference between the two energy levels (ΔE) for IR absorption to occur. The energy difference between two vibrational energy levels is given by Eq. (3.2). ΔE 5 hυ

(3.2)

In Eq. (3.2), ΔE is the energy difference between two given vibrational levels, h is Planck’s constant, and υ is the frequency of IR radiation. For a molecule to be IR active or to show IR absorption, an overall change in the dipole moment of the molecule is required [50,51]. In IR spectroscopy, the vibrational motion of a molecule can be understood by considering two atoms bonded via a covalent bond. Two atoms connected via a covalent bond can be considered to be similar to two masses connected by a spring. For this case, the ~ of the bond can be given by Eq. (3.3). vibrational frequency (V) 1 V~ 5 2πc

sffiffiffiffi K μ

(3.3)

In Eq. (3.3), K is the force constant and is the measure of bond strength, c is the velocity of light, and μ is the reduced mass, calculated by mm1 11mm2 2 . Here, m1 and m2 represent the masses of the two atoms linked through a particular covalent bond [52,53]. As seen in Eq. (3.3), the vibrational frequency is directly proportional to the force constant and inversely proportional to the reduced mass. Therefore molecules with a large force constant or possessing chemical bonds with a high bond strength will absorb at a higher frequency, which expressed in wavenumber (cm21), while molecules having heavier atoms and consequently large reduced mass, will absorb at a lower wavenumber (cm21).

3.3 Infrared spectroscopy

The vibrations of bonds in a molecule can be classified into stretching and bending vibrations. The stretching vibrations can be symmetric or asymmetric stretching, and the bending vibrations can be out-of-plane bending vibrations, for example, rocking and twisting vibrations, or wagging and scissoring, which are in-plane bending vibrations. The different types of stretching and bending vibrations are shown in Fig. 3.2. Stretching implies a change in the bond length between atoms, while bending refers to a change in the bond angle [5456]. The energy required to stretch a bond is always higher than the energy required to bend a bond. It is, therefore, observed in the IR spectrum that stretching vibrations appear at a higher wavenumber as compared to bending vibrations. The IR region having absorption frequencies in the range of 4004000 cm21 is the most commonly studied spectral region and is referred to as the mid IR region. The range from 4 to 400 cm21 is enveloped under far IR region. The near infrared region (NIR) extends from 4000 to 14,000 cm21 and is regarded as an efficient process monitoring region and is, therefore, relevant in various fields of research, for example, NIR can be used to monitor monomer conversion using the overtone of 5 CH stretching vibration [57,58]. An IR spectrum is commonly represented by the light absorbed or transmitted by the molecule versus the wavenumber. The absorbance of a sample is calculated using Eq. (3.4). A 5 log10

  Io I

(3.4)

In Eq. (3.4), A is the absorbance, Io is the intensity of the incident radiation, and I is the intensity of the transmitted radiation. The concentration of the sample

FIGURE 3.2 Various modes of vibration, namely symmetric stretching, asymmetric stretching, scissoring, twisting, wagging, and rocking.

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can be determined using the relationship between the absorbance and concentration as shown in the Eq. (3.5). A 5 εlc

(3.5)

In Eq. (3.5), ε is the absorption coefficient and l and c are the cell thickness and concentration of the sample respectively. Eq. (3.5) is referred to as Beer’s law. The IR spectrum can also be depicted in terms of percent transmittance instead of absorbance, which is calculated using Eq. (3.6) [58].   I %T 5 100 Io

(3.6)

IR spectroscopy is a universal technique that is fast, easy, and comes with relatively inexpensive instrumentation. A unique advantage lies in the fact that different types of samples including solid, liquid, semisolid, and even gaseous samples can be analyzed for obtaining information about functional groups. The sample preparation is crucial in IR spectroscopy as it is necessary for the IR radiations to pass through the sample. Materials that are transparent to IR radiation should, therefore, be used for preparing sample for the IR analysis. The most commonly used materials for the preparation of samples are alkali halides, for example, potassium bromide (KBr) or sodium chloride (NaCl). There are various methods that can be used in the case of solids for sample preparation. The most popular method is the pressed pellet technique, which involves grinding the sample with anhydrous KBr using a mortar and pestle followed by compressing it in the form of pellet with a hydraulic press. Liquid samples can be directly examined by placing a thin layer of the sample over polished and transparent KBr or NaCl plates. Gaseous samples require a special sample cell; in this case, the vapors of the gaseous sample are trapped in the cell and analyzed to obtain the IR spectrum. Solid and liquid samples can also be directly analyzed using an attenuated total reflectance IR (ATR-IR) spectrometer. ATR-IR spectroscopy is an extremely powerful technique. In ATR-IR, multiple internal reflections of an IR beam take places. The sample is placed over an optically dense crystal having a high refractive index, for example, diamond crystal. This method eliminates the need for sample preparation for both solids and liquids and is, therefore, being used extensively [59,60]. IR spectroscopy offers a qualitative identification of a variety of materials via distinct absorption/transmittance bands for every functional group. The bands obtained in the IR spectrum correspond to the IR absorption by different functional groups. IR spectroscopy can also provide quantitative information about the functional groups present, but in order to obtain quantitative information, it is important to establish proper calibration curve [50,55,58].

3.3.1 Infrared analysis of saturated polymers Morrent et al. [61] utilized high-resolution IR spectroscopy for the characterization of polypropylene (PP) films. Fig. 3.3 shows the IR spectrum of a PP film. In Fig. 3.3, asymmetric and symmetric stretching vibrations of a methyl group

3.3 Infrared spectroscopy

FIGURE 3.3 IR spectrum of polypropylene [61]. Reproduced with permission from John Wiley & Sons, Ltd. Copyright 2008.

(CH3) are observed at 2955 and 2873 cm21 respectively, while for the methylene group (CH2), asymmetric and symmetric stretching vibrations are observed at 2922 and 2843 cm21, respectively. Strong bands at 1460 cm21 arise due to CH3 asymmetric deformations or CH2 scissoring vibrations and a band at 1378 cm21 arises due to CH3 symmetric deformations. The band at 1167 cm21 corresponds to wagging vibrations. High-resolution IR spectra can also be used to characterize atactic and syndiotactic PP. Sevegney et al. [62] observed that as syndiotacticity increases, a change in wavenumber and intensity of bands was observed in a PP sample. As shown in Fig. 3.4, the band at 972 cm21 observed in the case of atactic PP (Fig. 3.4A) splits into 978 and 963 cm21 in the case of syndiotactic PP (Fig. 3.4B). In addition, a band at 1158 cm21 in atactic PP splits into three diffused bands in the case of syndiotactic PP. The splitting of bands is a consequence of dichroism or the existence of amorphous domains in the syndiotactic PP. FT-IR has also been extensively used for the characterization of nanocomposites. Parangusan et al. [63] studied nanocomposites consisting of poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) and nickel (Ni)-doped ZnO nanoparticles. The FT-IR spectrum of pure as well as Ni-doped ZnO based PVDF-HFP nanocomposites is shown in Fig. 3.5. The sample designations are A1 (PVDFHFP), A2 (PVDF-HFP/1 wt.% ZnO), A3 (PVDF-HFP/0.5 wt.% Ni-ZnO), A4 (PVDF-ZnO/ 1 wt.% Ni-ZnO), A5 (PVDF-ZnO/2 wt.% Ni-ZnO). These samples were stacked together in order to obtained layered films. The absorption bands at 974 and 762 cm21 can be assigned to α-phase, while at 841 cm21 it is due to β phase. It can clearly be observed in the spectrum that,

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FIGURE 3.4 IR spectrum of (A) atactic polypropylene and (B) syndiotactic polypropylene [62]. Reproduced with permission from Elsevier B. V. Copyright 2005.

3.3 Infrared spectroscopy

FIGURE 3.5 IR spectrum of pure PVDF-HFP and nanocomposite sandwiched films [63]. Reproduced with permission from Royal Society of Chemistry. Copyright 2017.

with the increase in the amount of Ni-doped ZnO, there is a decrease in the intensity of α-phase peaks and an increase in that of β phase. Similar studies have been carried out to identify different phases in polymers through FT-IR spectroscopy [64,65].

3.3.2 Infrared analysis of polymers containing unsaturation The band for 5 CH stretching appears between 3100 and 3000 cm21, while 5 CH out-of-plane bending is around 1000800 cm21. Arjunan et al. [66] performed Raman and IR analysis of trans 1,4-polyisoprene. FT-IR analysis showed a weak band at 3052 cm21, which can be attributed to 5 CH stretching vibration and a band at 884 cm21 corresponding to out-of-plane bending for 5 CH bond (as shown in Fig. 3.6). Highly intense and distinct bands were observed at 2961 cm21 due to the asymmetric stretching of the methyl group, while the bands

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FIGURE 3.6 IR spectrum of trans 1,4-polyisoprene [66]. Reproduced with permission from Elsevier Science B. V. Copyright 2001.

at 2850 and 29102930 cm21 can be attributed to symmetric and asymmetric stretching of the methylene group respectively. A strong absorption band was observed at 1667 cm21 for C 5 C stretching. The bending modes of the methyl group resulted in high intensity bands due to asymmetric deformation at 1384 cm21, symmetric deformation at 1355 cm21, and wagging vibrations at 1154 cm21. The weak absorption band at 1445 cm21 was observed to be due to the deformation vibration of the methylene group. The methylene wagging and twisting vibrations were observed around 1300 cm21 (weak intensity), while the rocking vibration was observed at 785 and 769 cm21.

3.3.3 Infrared analysis of polymers containing aromatic group Aromatic groups can be analyzed through characteristic absorption bands due to 5 CH stretching around 31003000 cm21 and C 5 C stretching (skeletal vibrations) around 16001400 cm21. Jang and Lee [67] reported the IR spectrum for polystyrene nanocapsules prepared via microemulsion polymerization. As shown in Fig. 3.7, the distinct bands observed around 758 and 698 cm21 can be assigned to out-of-plane vibrations and out-of-plane ring deformation due to the monosubstituted phenyl group.

3.3.4 Infrared analysis of polymers containing hydroxyl group The OH stretching vibration is characterized by a broad band between 3500 and 3300 cm21 in the IR spectrum. The IR stretching frequency for OH is highly influenced by the presence of H-bonding. H-bonding weakens the OH bond, which in turn leads to a decrease in the force constant of the OH bond. Hence the stretching frequency of the OH bond is lowered due to H-bonding [68]. Both intermolecular H-bonding (interaction between two or more different

3.3 Infrared spectroscopy

FIGURE 3.7 IR spectrum of polystyrene [67]. Reproduced with permission from Royal Society of Chemistry. Copyright 2002.

molecules) and intramolecular H-bonding (interaction in the same molecule) have different consequences on the shift of the OH stretching vibration. Intermolecular H-bonding is highly influenced by the concentration of the chemical species. The intensity of intermolecular H-bonding decreases on dilution considerably, while for intramolecular H-bonding, the intensity of the H-bonding remains the same even at low concentrations. The IR spectrum of PEG is shown in Fig. 3.8; in this spectrum, a broad and strong band at 3450 cm21 is observed due to OH stretching. The absorption bands in the region 30002800 cm21 correspond to methyl and methylene stretching vibrations, while those lying in the region 12001100 cm21 are assigned to CO stretching vibrations. The bending vibrations of the CH group are observed around 1380 and 1450 cm21.

3.3.5 Infrared analysis of polymers containing ester group The carbonyl stretching vibration is observed as a strong band in the IR spectrum and its wavenumber of absorption is strongly influenced by the functional group attached next to the carbonyl group. The carbonyl stretching vibration varies between aldehydes (17401690 cm21), ketones (17501680 cm21), esters (17501720 cm21), and amides (16901630 cm21). The primary reasons for the change in the frequency of the carbonyl group could be attributed to resonance and inductive effect. The inductive effect is predominant in the case of aldehydes, ketones, anhydrides, esters, and acid halides. The increase in the electron withdrawing nature of the substituent attached next to the carbonyl group shortens the C 5 O bond

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FIGURE 3.8 IR spectrum of polyethylene glycol. Reproduced from NIST webbook.

and, hence, increases the force constant. In the case of amides, resonance plays a major role. The excess electron density on the nitrogen atom leads to a decrease in the double bond character of C 5 O, which in turn leads to a decrease in its force constant and, consequently, the absorption frequency for the carbonyl group in amides shifts toward a lower wavenumber [69]. Polyesters have been well characterized using IR spectroscopy. Ramesh et al. [70] accounted the IR spectrum of poly(methyl methacrylate) (PMMA) in their studies on polyvinyl chloride and PMMA blends. Fig. 3.9 shows the IR spectrum of PMMA in which an intense stretching vibration was observed at 1721 cm21 due to the carbonyl group present in PMMA. Vibration due to CO stretching was observed at 1159 cm21. Strong absorption bands observed in the region 29272986 cm21 were due to CH stretching vibrations. Orozco et al. [71] studied blends of maleic anhydride grafted poly(lactic acid) (PLA-g-MA) and starch using IR spectroscopy (Fig 3.10). As shown in Fig. 3.10, the presence of a carbonyl group was confirmed via C 5 O stretching vibration at 1761 cm21. Other distinct bands include CO stretching vibration around 11901090 cm21 and C 5 O stretching overtones around 36603500 cm21. The stretching vibrations of methyl protons were observed in the region from 3000 to 2940 cm21.

3.3.6 Infrared analysis of polymers containing carboxylic acid group The IR stretching frequency for the carbonyl bond in carboxylic acids lies in the range of 17801700 cm21. Le et al. [72] reported the IR spectrum of poly(acrylic acid), which is shown in Fig. 3.11. The carbonyl stretching frequency for poly (acrylic acid) can be identified by the presence of a distinct band at 1730 cm21. The sharp band at 1450 cm21 represents the bending vibration of a methylene

3.3 Infrared spectroscopy

FIGURE 3.9 IR spectrum of poly(methyl methacrylate) [70]. Reproduced with permission from Elsevier B. V. Copyright 2006.

FIGURE 3.10 IR spectrum of poly(lactic acid) [71]. Reproduced with permission from Wiley Interscience. Copyright 2009.

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FIGURE 3.11 IR spectrum of poly(acrylic acid) [72]. Reproduced with permission from Elsevier Ltd. Copyright 2009.

group. The bands at 1240 and 1400 cm21 can be attributed to CO stretching and OH bending vibrations respectively.

3.3.7 Infrared analysis of polymers containing amide group The amide functional group can also be distinctly characterized through IR spectroscopy. Vasanthan carried out experiments to estimate the crystallinity in Nylon 66 by FT-IR and density measurements [73]. FT-IR experiments were carried out on Nylon 66 sample at variable temperatures. The IR spectrum was recorded at 240 C, is shown in Fig. 3.12, and shows strong bands due to an NH stretching vibration at 3300 cm21, at 3070 cm21 due to overtones of NH bending, and at 1550 cm21 due to an NH bending vibration. A distinct band at 1640 cm21 corresponds to C 5 O stretching in amide. The bands due to methylene protons include 2930 cm21 (asymmetric stretching), 2860 cm21 (symmetric stretching), 1200 cm21 (twisting and wagging), and 1373 cm21 (wagging). Kele¸s et al. [74] reported the synthesis of thiol-functionalized PEG via a reaction of amine-terminated PEG with mercaptopropionic acid. The formation of thiol-functionalized PEG containing an amide bond was studied by 1H NMR as well as FT-IR spectroscopy (Fig. 3.13). The characteristic vibration due to the

3.4 Nuclear magnetic resonance spectroscopy

FIGURE 3.12 IR spectrum of Nylon 66 at 240 C [73]. Reproduced with permission from American Chemical Society. Copyright 2012.

amide group at 1522 cm21 and the band at 722 cm21 due to CS stretching were observed in the FT-IR spectrum of the PEG terminated with a thiol group. Table 3.1 provides a compiled list of vibrational frequencies corresponding to the different functional groups.

3.4 Nuclear magnetic resonance spectroscopy A nucleus experiences magnetic resonance because of its inherent property of spin. Like mass or charge, spin is also a fundamental property of a nucleus. The unpaired proton, neutron, and electrons possess a spin of 1/2. Table 3.2 discusses the inherent properties, for example, mass, charge, and spin, of the electron, proton, and neutron. It is the unpaired spin that is of importance in NMR [75]. A nucleus of hydrogen (1H) contains one proton and, therefore, its spin quantum number I is 1/2. The total spin of a nucleus is determined by combining the spins of the neutrons and protons. For example, 2H contains one proton and one neutron. In the case of 2H, the nuclear spin quantum number I is 1, when the spins of a proton and a neutron are combined in a parallel configuration, or the nuclear spin quantum number I is 0, when the spins of a proton and neutron are combined in an antiparallel configuration. There is a large difference in

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FIGURE 3.13 IR spectrum of poly(ethylene glycol) [74]. Reproduced with permission from Elsevier B. V. Copyright 2012.

energy (B1011 kJ/mol) between two nuclear spin states. This energy difference largely exceeds the energy required for ordinary chemical reactions. Therefore the nuclear excited state may be ignored. The nuclear spin quantum number possessing the lowest energy is called ground state nuclear spin. For 2H, the ground state nuclear spin has a nuclear spin quantum number equal to 1. In general, isotopes possessing odd mass numbers have half-integer spins, for example, 1H, 13C, etc. Isotopes possessing even mass numbers have integer or zero spins. The general guidelines to determine I for isotopes with even mass numbers are: 1. The ground state nuclear spin quantum number I is 0 when the number of protons and neutrons are both even. For example, 12C, 16O. 2. The ground state nuclear spin is an integer larger than zero when the number of protons and neutrons are both odd. For example, 2H and 14N have I 5 1, 10 B has I 5 3. Nuclei having a ground state nuclear spin quantum as a half integer are NMR active, while nuclei having a ground state nuclear spin quantum number as an integer are also NMR active, but it is difficult to obtain and interpret their NMR spectrum. While nuclei having a ground state nuclear spin quantum number as zero are NMR inactive.

3.4 Nuclear magnetic resonance spectroscopy

Table 3.1 IR vibrational frequencies for different functional groups. Functional group Alkane Alkene

Alkyne Aromatic Alkyl halide

Alcohol

Amine

Acid

Aldehyde Amide

Anhydride Ester Ketone Ether Nitrile

Type of vibration CH stretch CH bending 5 CH stretch 5 CH bending C 5 C stretch CH stretch CC stretch 5 CH stretch C 5 C stretch CF stretch CCl stretch CBr stretch CI stretch OH (stretch, H-bonded) OH (stretch, no H-bonding) CO (stretch) NH stretch CN stretch NH bending C 5 O stretch OH stretch CO stretch C 5 O stretch O 5 CH stretch C 5 O stretch NH stretch NH bending C 5 O stretch C 5 O stretch CO stretch C 5 O stretch CO stretch CN stretch

Vibrational frequency (cm21) 28503000 13501480 30003100 6751000 16201680 3300 21002260 30003100 14001600 10001400 600800 500600 500 32003600 35003700 10501150 33003500 10801360 1600 17001725 25003300 12101320 17401720 28202850 and 27202750 16401690 31003500 15501640 18001830 and 17401775 17351750 10001300 17051725 10001300 22102260

3.4.1 Nuclear Zeeman splitting A nuclear state with ground state nuclear spin quantum number I is 2I 1 1-fold degenerate. This degeneracy is broken by the application of a magnetic field (Bo). The splitting between the nuclear spin levels is called nuclear Zeeman splitting (Fig. 3.14). Therefore 1H having a nuclear ground state spin quantum number

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Table 3.2 Inherent properties of particles present in a nucleus. Particle Electron Neutron Proton

Mass (kg)

Charge 231

9.109 3 10 1.675 3 10227 1.673 3 10227

Negative Zero Positive

Spin /2 /2 1 /2 1 1

FIGURE 3.14 Nuclear Zeeman splitting for proton when placed in an external magnetic field (Bo).

I 5 1/2, splits into two levels in the presence of a magnetic field. When a sample containing 1H is placed in a strong magnetic field, each hydrogen nuclei will take one of the two possible spin states. Spin state 11/2, in which the magnetic moment is aligned in the direction of the applied Bo and 1/2 spin state, in which the magnetic moment is aligned opposite to the direction of Bo. As the 11/2 spin state is slightly lower in energy, slightly more than half of the hydrogen nuclei will occupy a 11/2 state, while slightly less than half will occupy the 1/2 state. The difference in energy between the 11/2 and 1/2 spin states increases with the increase in the applied field strength (Bo). The energy difference between the two spin states is given by the following equation:. ΔE 5

hγ Bo 2π

(3.7)

where h is Planck’s constant, Bo represents applied magnetic field strength (in Tesla), and ϒ is the magnetogyric ratio, which is defined as the ratio of magnetic moment to angular momentum. In a stationary magnetic field of strength Bo, when two energy levels for the protons have been established, it is possible to introduce radiation in the radiofrequency range in order to cause a transition between the energy levels.

3.4 Nuclear magnetic resonance spectroscopy

In the case of a proton, having I equal to 1/2 and 2I 1 1 levels in the presence of external magnetic field Bo, it is possible to introduce radiofrequency (ϑo) to affect a transition between these energy levels in a stationary magnetic field of strength Bo. Now it is possible to correlate Eqs. (3.7) and (3.8) in order to obtain Eq. (3.9) ΔE 5 hϑo

(3.8)

γBo 2π

(3.9)

ϑo 5

where ϑo is the frequency of electromagnetic radiation that is responsible for transitions between the two states [60]. The introduced radiofrequency ϑo is normally given in MHz. Eq. (3.9) gives rise to another fundamental equation as a consequence of the precessional motion of the magnetic moment in the magnetic field. ωo 5 γBo

(3.10)

where ωo is the precessional frequency, also called the Larmor frequency [76]. All the NMR instruments that operate at a high magnetic field strength (above 100 MHz) are based on a superconducting magnet cooled using helium and are operated in Fourier-transform mode. The sample for which the NMR spectrum needs to be recorded is dissolved in a deuterated solvent. The ideal solvent for the sample dissolution should contain no protons, be inert, have a low boiling point, and should be inexpensive.

3.4.2 Chemical shift In an applied magnetic field of a given strength, different resonance frequencies are obtained due to nonidentical protons as well due to protons present in different locations in a molecule. In other words, protons in different chemical environments have different resonance frequencies, while protons in the same chemical environment have the same resonance frequency. The chemical shift of a particular proton is obtained by the difference in the absorption position of a particular proton from the absorption position of a reference proton. The most generally used reference compound in NMR is tetramethylsilane. The chemical shift (δ) can be expressed in terms of a dimensionless unit of parts per million (ppm), by dividing the resonance frequency of a proton in the sample (in Hz) by the applied frequency (in Hz) and multiplying by 106. Table 3.3 gives the range of chemical shifts observed for protons (underlined in the table) present in different chemical environments. The magnetic field actually experienced by the proton is slightly different from an applied external magnetic field owing to its electron cloud. The density of the electron cloud around protons varies with the chemical environment. The surrounding electrons generate their local magnetic moment, which tends to oppose the applied magnetic field Bo, resulting in a shielding effect. The degree

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Table 3.3 The chemical shift of protons in different chemical environments. Group

Proton

Alkyl Alkyl Alkyl Allylic Benzylic Alkyl chloride Alkyl bromide Alkyl iodide Ether Alcohol Ketone

Chemical shift (ppm)

RCH3 RCH2CH3 R3CH CH2 5 CHCH3 ArCH3 RCH2Cl RCH2Br RCH2I ROCH2R HOCH2R

R

Aldehyde

R

O

O

0.81.0 1.21.4 1.41.7 1.61.9 2.22.5 3.63.8 3.43.6 3.13.3 3.33.9 3.34.0

CH3

2.12.6

H

9.59.6

Vinylic

R2C 5 CRH

5.25.7

Aromatic Acetylenic

ArH RCCH

6.09.5 2.53.1

Carboxylic

R

O

OH

1013

of shielding will depend on the electrons circulating around the nuclei of a proton. The degree of shielding or deshielding of a proton attached to a carbon atom will depend on the inductive effect of other groups attached to the same carbon atom. If an electron donating group is bonded with the carbon, it shields the proton, whereas an electron withdrawing moiety will deshield the proton. A strongly shielded nucleus will absorb at a low frequency and the absorption is said to occur upfield. Similarly, a deshielded nucleus will absorb at a high frequency and the absorption is said to occur downfield. Other than the chemical shift, the integration of an NMR spectrum is crucial in deciphering the structure of the compound or polymer under study. The area under a specific peak obtained in an NMR spectrum is directly proportional to the number of protons due to which that peak arises [60].

3.4.3 Spinspin coupling Spinspin coupling results in the splitting of the NMR signal of one proton into a set of peaks by the influence of neighboring nonequivalent protons. The distance between

3.4 Nuclear magnetic resonance spectroscopy

FIGURE 3.15 Spinspin coupling in the case of 1,1,2-trichloroethane.

adjacent peaks in a multiplet is called the coupling constant (J) and is expressed in Hz. The applied magnetic field has no influence on the coupling constant (J). Spinspin coupling can be understood by considering the simple molecule of 1,1,2-trichloroethane as an example. As shown in Fig. 3.15, there are two equivalent protons, Hb1 and Hb2, with respect to Ha. Therefore the alignment of the Hb1 and Hb2 protons can take place in four possible ways. They can both be aligned in the same direction as the external magnetic field Bo or they can both be aligned in the opposite direction to Bo. In the third possibility, Hb1 can be aligned in the same direction as Bo, while Hb2 is aligned opposed to the direction of Bo. The exact opposite of this case will include Hb1 being aligned in the opposite direction to Bo, while Hb2 in the same direction as Bo. Hence the signal obtained for Ha is a triplet with the intensity of the middle peak being twice as large as that of the two outer peaks. The chemical shift of the equivalent protons Hb1 and Hb2 will similarly get affected by the neighboring Ha proton. The Ha proton can either be aligned in the same direction as Bo or in the opposite direction to Bo. The two equal possibilities in this case will, therefore, lead to two peaks of the same magnitude, that is, a doublet will be obtained in an NMR spectrum. In molecules with relatively free rotation about the CC sigma bond, protons bonded to the same carbon in CH3 and CH2 groups are generally equivalent. The coupling in molecules with unrestricted bond rotation follows the n 1 1 rule as discussed in the example for 1,1,2-trichloroethane. However, if there is a restricted rotation, as in alkenes and cyclic structures, protons bonded to the same carbon may not be equivalent. Nonequivalent protons on the same carbon will couple and cause signal splitting. The coupling of nuclear spins is mediated through intervening bonds. For protons that are three bonds apart, the coupling is referred to as vicinal coupling. For protons that are two bonds apart, the coupling is referred to as germinal coupling.

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Protons that are more than three bonds apart generally do not exhibit any noticeable coupling. The range of coupling constants for geminal and vicinal protons is given in Table 3.4. The complex splitting pattern can be explained by considering vinyl group in methyl acrylate as an example. As shown in Fig. 3.16, NMR signals consist of different peaks for Ha, Hb, and Hc protons. The Hc proton will couple with both Ha and Hb. The chemical shift for Hc is obtained at 6.21 ppm. The larger coupling constant for vicinal (trans) protons (Ha in this case) will first result in the coupling of Hc with Ha with a coupling constant around Jca 5 17 Hz. Each Hc doublet will

Table 3.4 The coupling constant for geminal and vicinal coupling. Types of coupling

R1 R2

R1 Hb Ha R1

C C

C C

C C

Ha

Name

Range (Hz)

Geminal coupling

03

Vicinal coupling, trans

1118

Vicinal coupling, cis

614

Hb

Ha R2 Hb R2

FIGURE 3.16 NMR splitting pattern in methyl acrylate.

3.4 Nuclear magnetic resonance spectroscopy

further split into another doublet due to the geminal coupling between Hc and Hb with a coupling constant being much smaller (JcbB2 Hz). This splitting pattern is referred to as a doublet of doublet (dd). Similarly, the peaks for Hb and Ha are obtained at 5.64 and 5.95 ppm respectively as a dd with Jac 5 17 Hz, Jbc 5 2 Hz, and Jab and Jba 5 10 Hz [60]. NMR spectroscopy is widely used for the elucidation of polymer structure, analysis of tacticity in polymers, analysis of polymer molecular weight, copolymer compositional analysis, determination of monomer reactivity ratios, and determination of monomer sequence distribution in copolymers. In this chapter, analysis of end groups, molecular weight analysis, and copolymer analysis using NMR spectroscopy are discussed in detail.

3.4.4 Analysis of end groups by 1H nuclear magnetic resonance spectroscopy The identification and quantification of end groups are extremely important for the detailed characterization of polymers. The incorporation of end groups into polymer chains is generally a consequence of initiator or chain transfer agent/terminating agent, which introduces significant changes in the properties of a polymer [7678]. 1H NMR spectroscopy is an established and powerful technique to understand the mechanism of polymerization and has facilitated the design of macromolecular structures with desirable properties [33,79]. Through NMR spectroscopy it is possible to obtain the molecular weight of a given polymer by calculating the ratio of the number of end groups to the number of repeat units in the chain [50]. NMR spectroscopy is widely used in determining the end groups in polymers having low molecular weights. The characterization of end moieties in high molecular weight polymers becomes a challenging task due to the low concentration of end groups in comparison to the main polymer chain. However, reports have claimed an accurate quantification of end groups in polymers having a higher degree of polymerization with the aid of NMR spectrometers operating at a high magnetic field strength [80,81]. The determination of end groups is extremely important in the case of polymers synthesized using controlled radical polymerization techniques. In polymers synthesized using controlled radical polymerization techniques, termination reactions are minimized, but they are not completely absent. Therefore in a polymer synthesized, for example, by atom transfer radical polymerization (ATRP), not all polymer chains are terminated by halogen [82,83]. Lutz and Matyjaszewski performed the ATRP of styrene in the presence of CuBr/4,40 -di-(5-nonyl)-2,20 -bipyridine and methyl 2-bromopropionate. The 1H NMR spectrum was analyzed to study the introduction of bromine as an end group [84]. The 1H NMR contained broad aromatic and aliphatic regions due to the repeat units present in the polymer chain (Fig. 3.17). The NMR signals due to the methoxy (-OCH3) protons (labelled as Ha) were observed around 3.35-3.6 ppm, because of the use of methyl-2-bromopropionate as an initiator. The protons

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FIGURE 3.17 1

H NMR spectrum of polystyrene prepared by ATRP [84]. Reproduced with permission from John Wiley & Sons. Copyright 2005.

in α-position to bromine functionality showed resonance around 4.3-4.6 ppm (labeled as Hf). The percentage of polymer chains capped by bromine moiety was determined using Eq. (3.11) by the comparison of the integrations of the peaks arising due to two end functionalities (Ha and Hf).    Ha Functionality% 5 100 Hf = 3

(3.11)

The results obtained in this work were in accordance with the simulation models used to analyze the cause for the loss of the halogen end group at the higher degree of conversion. The loss of the halogen end group at the high degree of conversion was found to be due to both termination and elimination reactions. Another controlled radical polymerization technique that is extremely popular and is a versatile method for the synthesis of end-functionalized polymers is reversible addition-fragmentation chain transfer (RAFT) polymerization. RAFT polymerization involves radically mediated chain transfer reactions between a molecule containing a thiocarbonyl moiety (RAFT agent) and a propagating radical. Perrier et al. [85] proposed a method for the polymerization of methyl methacrylate (MMA) using S-methoxycarbonylphenylmethyl dithiobenzoate (MCPDB) as a RAFT agent and azobisisobutyronitrile (AIBN) as an initiator and the product obtained was analyzed by 1H NMR spectroscopy. The analysis using 1H NMR spectroscopy ensured the removal of the dithiobenzoate moiety as its characteristic

3.4 Nuclear magnetic resonance spectroscopy

resonances at 7.30, 7.38, and 7.93 ppm were absent (Fig. 3.18A). These results were also confirmed by elemental analysis and UVvisible spectroscopy. The authors further optimized the reaction conditions by changing the concentration of initiator in the reaction medium. The authors reported that when the PMMA:AIBN ratio was 1:10, the characteristic peaks of vinyl protons at 5.7 and 6.2 ppm were observed due to disproportionation termination reactions. These peaks completely disappeared when the ratio of PMMA:AIBN was increased to 1:20 (Fig. 3.18B). The feasibility of the use of polymers prepared using RAFT polymerization for further modification of end functionality is also widely studied. It is easy to convert RAFT agent functionalized polymers to thiol-functionalized polymers [86]. Boyer et al. devised a suitable method for the same, rooting out the possibility of side reactions [87]. In this work, poly(N-isopropyl acrylamide) (PNIPAM) was synthesized using 3-(benzylsulfanylthiocarbonylsulfanyl)propionic acid and AIBN. The product was characterized by 1H NMR spectroscopy (Fig. 3.19A). The resonances at 2.6 and 3.5 ppm were due to methylene protons adjacent to sulfur atoms (ω-group) while between 7.1 and 7.3 ppm were due to protons of the aromatic ring (α-group). The end group functionality in this case was calculated by the ratio of integration of ω-group protons with respect to that of α-group, which is given by the following equation. ftc 5

 2:6ppm 

I 1 I 3:5ppm =4 = I phenyl =5

(3.12)

Here ftc is the end group functionality for the thiocarbonate moiety. Further, the chemical modification of the RAFT end group of PNIPAM was carried out by

FIGURE 3.18 1

H NMR spectrum of poly(methyl methacrylate) when ratio of AIBN:PMMA is (A) 10:1 and (B) 20:1 [85]. Reproduced with permission from American Chemical Society. Copyright 2005.

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FIGURE 3.19 1

H NMR spectra of PNIPAM (A) synthesized by RAFT polymerization and (B) after aminolysis in the presence of DTP [87]. Reproduced with permission from John Wiley & Sons. Copyright 2009.

reducing thiocarbonate moiety to thiol group by using ethanolamine and 2,2’dithiopyridine (DTP). This resulted in the formation of a stable pyridyl disulfide (PDS) functionalized polymer. The characteristic resonances of the pyridyl ring at 7.1, 7.37.4, and 8.4 ppm in the 1H NMR spectrum (Fig. 3.19B) confirmed the bonding of PDS to the polymer. It was further validated by the emergence of a broad signal at 3.4 ppm (associated to the thiol group) and the disappearance of the CH signal at 4.6 ppm (bonded to the thiocarbonate moiety) as well as methylene signals at 3.55 and 2.6 ppm. The end group functionality for the pyridyl moiety (fpy) was calculated by comparing the integrations of the CH signal of the

3.4 Nuclear magnetic resonance spectroscopy

pyridyl ring at 8.4 ppm and the methylene signal at 2.5 ppm using the following equation: fpy 5 I 8:4 ppm =ðI 2:5 ppm =2Þ

(3.13)

3.4.5 Determination of molecular weight by 1H nuclear magnetic resonance spectroscopy Some of the most commonly used techniques for the determination of the molecular weight of polymers include GPC, membrane osmometry, viscometry, and light scattering. In addition to these techniques, 1H NMR spectroscopy is also extensively used for the determination of molecular weight. When compared to other techniques for the determination of molecular weight, there are several advantages of using NMR spectroscopy. For example, when compared to viscometry and GPC techniques, viscometry is a secondary method as it requires knowledge of K and α for a polymersolvent pair in order to determine the molecular weight. Similarly, GPC is also a relative technique through which molecular weight is determined only after establishing a calibration curve using polymers (standards) of known molecular weight. While 1H NMR spectroscopy is a quantitative method for the determination of polymer molecular weight and it does not require any prior calibration [88]. Both 1H and 13C NMR spectroscopy techniques are widely used for the structure elucidation and for compositional analysis. 1H NMR confers some advantages over 13 C NMR because of its improved resolution of chemical shifts and superior signal to noise ratio [88]. However, certain limitations are present in 1H NMR, for example, in the case of stereoregular configurations, a small difference in chemical shifts can cause overlapping, which may lead to little or no peak resolution in the case of some polymers. It is important to note that 1H NMR spectroscopy is usually employed for the determination of the molecular weight of polymers having a molecular weight less than 25,000 g/mol because with increases in molecular weight, the resolution of the NMR peaks is compromised. However, nowadays with the use of high magnetic field NMR spectrometers, the identification of end groups and, hence, the calculation of the molecular weight of high molecular weight polymers is also possible. The basic principle for the determination of polymer molecular weight by 1H NMR spectroscopy is the fact that Mn is related to the area under the resonance peak in the 1H NMR spectrum, which is related to the number of molecules of the analyte. The Mn can be calculated using the following: P Ai Mn 5 P Ni

(3.14)

where Ai is the area under the resonance peak for the species i in the 1H NMR spectrum, Ni is the number of molecules of species i.

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Izunobi and Higginbotham highlighted the easiness and suitability of 1H NMR spectroscopy for the determination of the Mn of α-methoxy-ω-amino-PEG [89]. The peak areas corresponding to the end group and repeat unit were obtained by integration of the 1H NMR spectrum (Fig. 3.20). The number of repeat units in the polymer (nr) was determined by comparing the area under the peak for the end group (ae) having a known number of protons (me) to that of the repeatunit using the following equation: nr 5

ar :me :ne ae :mr

(3.15)

In Eq. (3.15), ar is the area under the NMR peak for the repeat unit, ae is the area under the NMR peak for the end group, nr is the number of repeat units, ne is the number of end groups, and mr and me are the number of protons in the repeat unit and end group respectively. As a result, Mn can be calculated by substituting nr from Eq. (3.15) in the equation: Mn 5 nr M0 1 Me

(3.16)

where nr is the number of repeat units, Mo is the repeat unit molecular weight, and Me is the molecular weight of the end groups. In the case of α-methoxy-ω-amino-PEG, the resonance for the methoxy (CH3O) end group was present at 3.25 ppm and resonance for the amino (NH2) end group was present at 2.71 ppm as shown in Fig. 3.20. The resonance due to oxymethylene protons ((OCH2CH2)n) was observed at 3.52 ppm. The peak areas of the repeat units and end groups, the number of end groups, and the

FIGURE 3.20 1

H NMR spectrum of end functionalized PEG (in DMSO-d6) [89]. Reproduced with permission from American Chemical Society. Copyright 2011.

3.4 Nuclear magnetic resonance spectroscopy

number of protons in the repeat unit and end groups were substituted in Eq. (3.17) to calculate nr and finally Mn. nOCH2 CH2 5 5

aOCH2 CH2 :mOMe :nOMe aOMe :mOCH2 CH2

(3.17)

133:1 3 3 3 1 D111 0:9 3 4

Mn 5 75:1 1 44:06 3 111 1 44:09 g=mol 5 5006:33 g=mol

The Mn of α-methoxy-ω-amino-PEG was determined to be 5006.33 g/mol. In a similar fashion, the Mn of a variety of polymers can be calculated by first determining the number of repeat units using 1H NMR spectroscopy.

3.4.6 Copolymer analysis by 1H nuclear magnetic resonance spectroscopy Traditional techniques like elemental analysis are not reliable for the precise quantification of monomer ratios in a copolymer. High-resolution NMR spectroscopy has been established as an extremely efficient technique for the structural analysis of copolymers, determination of copolymer composition, as well as determination of monomer reactivity ratios. Pekel et al. synthesized and studied homopolymers and a copolymer of N-vinylimidazole and acrylonitrile via radical polymerization using AIBN as an initiator [90]. 1H NMR spectroscopy (Fig. 3.21) was performed to calculate the copolymer composition and the monomer ratio was calculated using Eq. (3.18) by comparing the integration of the methine protons present in poly(acrylonitrile) and the imidazole ring protons present in poly (vinylimidazole). m1 =m2 5 n2 Am1 ðCH of imidazole ringÞ=n1 Am2 ðCH of AN unitÞ

(3.18)

In Eq. (3.18), m1 represents monomer 1, which is vinylimidazole and m2 represents monomer 2, which is acrylonitrile. Am1 is the area under the peak for the imidazole ring in poly(vinylimidazole) and Am2 is the area under the peak for the methine proton in poly(acrylonitrile). n1 is the number of protons in the imidazole ring and n2 is the number of protons in the methine. From 1H NMR, Am1 was determined to be 0.56, which is due to three protons in the imidazole ring. Am2 was determined to be 0.146 due to one proton in the acrylonitrile unit. Using Eq. (3.18), the composition of the copolymer was determined to be 56.07 mol.% of poly(vinylimidazole) and 43.93 mol.% of poly(acrylonitrile). Reddy et al. [91] studied the copolymerization of N-phenyl methacrylamide (PMA) and glycidyl methacrylate (GMA) using benzoyl peroxide as an initiator. 1 H NMR spectroscopy was utilized to calculate the average composition of PMA and GMA in the copolymer using different feed ratios. The 1H NMR spectrum of

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FIGURE 3.21 1

H NMR spectra of (A) poly(N-vinylimidazole), (B) poly(N-vinylimidazole-co-acrylonitrile), and (C) poly(acrylonitrile) in DMSO-d6 [90]. Reproduced with permission from John Wiley & Sons. Copyright 2004.

the copolymer showed a multiplet at 7.07.8 ppm corresponding to the aromatic protons. The resonance associated with the methyleneoxy group of GMA was observed at 4.2 ppm, while that corresponding to the epoxy group was at 3.7 ppm. The resonance due to the methyne protons of the epoxy group of GMA was observed at 3.2 ppm. The resonance due to backbone peaks of methylene protons and α-methyl protons for both the monomers was observed around 1.82.5 and 0.81.2 ppm respectively. The analysis indicated that there were 5 aromatic protons and 5 aliphatic protons in PMA, while GMA contained 10 aliphatic protons. The intensities of both the aromatic and aliphatic protons in PMA and GMA respectively were obtained from the 1H NMR spectrum (Fig. 3.22).

3.5 Mass spectrometry

FIGURE 3.22 1

H NMR spectrum of copolymer of PMA and GMA [91]. Reproduced with permission from Taylor & Francis. Copyright 2008.

The composition of PMA in the copolymer was estimated by comparing the intensities of the aromatic protons of PMA and the aliphatic protons of both and by Eq. (3.19): 5m1 Intensity of aromatic protons 5P 5 Intensity of aliphatic protons 5m1 1 10ð1 2 m1 Þ

(3.19)

where m1 is the mole fraction of PMA, while (1 2 m1) is the mole fraction of GMA. Eq. (3.19) can be further simplified as: m1 5

10P 5 1 5P

(3.20)

As previously discussed, NMR spectroscopy is an important and widely used technique in polymer characterization. However, for an accurate analysis through solution NMR, it should be made sure that the sample is completely soluble in the deuterated solvent chosen as well as the peaks are well-resolved in order for NMR to be used for the calculation of molecular weight as well as copolymer analysis.

3.5 Mass spectrometry Mass spectrometry (MS) has emerged as one of the most crucial and powerful analytical technique for the investigation of the molecular structure of macromolecules. In order for macromolecules to be analyzed successfully using MS, the challenging task of the ionization of polymers had to be overcome. However,

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using techniques like matrix-assisted laser desorption/ionization (MALDI) [92,93] and electrospray ionization (ESI), characterization of macromolecules by mass spectrometry has become easier [94]. In 2002, John Fenn and Koichi Tanaka were awarded the Nobel Prize in Chemistry for the development of soft desorption ionization methods. Both MALDI and ESI can be utilized for studying end group, polymerization mechanism, assessing copolymer structure, sequencing, tacticity and branching in polymers, and for obtaining molecular weight and structural information for synthetic macromolecules as well as biological macromolecules, for example, proteins, carbohydrate, nucleic acids, etc. [95,96]. The advantages of MS over other techniques, for example, NMR, FT-IR and UVvisible spectroscopy and SEC, is that MS provides an absolute method for the characterization of polymers as it does not rely on any standards, which in many cases are not available for polymers being studied. In addition, MS can provide information that is not averaged out like in other techniques. The mass spectrometer basically consists of an ionization source, mass analyzer, and detector. The principle of operation of all mass spectrometers is the use of an electric/magnetic field for the ionization of the analyte to the gaseous state. The ionization is followed by the separation of ions according to their m/z ratio, where m is the mass of ion in the atomic mass unit (Dalton) and the charge on the ion is z.

3.5.1 Electrospray ionization mass spectrometry In the ESI process, a dilute polymer solution is passed through a needle (small diameter capillary at a rate of 110 μL/min) at a high potential (B4 kV). Due to the presence of a high electric field, highly charged solvent droplets are produced. A Taylor cone (an elliptically shaped fluid cone) is generated due to high electrostatic force sufficient enough to pull liquid from the capillary, which leads to the generation of charged aerosol droplets of the solution. Further, the droplets shrinks due to the evaporation of the solvent. The droplet shrinkage continues until the Rayleigh limit, which is reached when the droplet surface charge density exceeds the liquid surface tension. Finally, the droplet bursts into smaller droplets in a series of columbic explosions or fissions [97]. The droplet size is reduced continuously by a series of explosions and this continues until the solvent is completely removed from the charged polymer chains. Finally, the charged polymer chains are introduced into the mass analyzer.

3.5.2 Matrix-assisted laser desorption/ionization mass spectrometry MALDI is a soft ionization technique that was reported simultaneously by Karas and Hillenkamp and Tanaka and coworkers. The basic principle for operation of MALDI is that it utilizes a large amount of organic matrix and a small quantity

3.5 Mass spectrometry

of analyte (sample to be analyzed). MALDI is a soft ionization technique as most of the energy from the laser beam is absorbed by the matrix instead of the analyte. There are two critical roles performed by the organic matrix; first it absorbs energy from the laser due to its high molar absorptivity and then it transfers energy to the analyte, and second, it dilutes the analyte, thus, preventing any aggregation of analyte molecules, which in turn prevents the degradation of analyte molecules [98]. For sample preparation in the MALDI technique, one of the most commonly used methods is the dried droplet method. Other methods for sample preparation are the fast evaporation and smashed crystal methods. In the dried droplet method, the matrix (e.g., 2,5-dihydroxybenzoic acid) is mixed with a dilute polymer solution. The matrix is in a much higher concentration than the analyte. In addition, a cationizing agent is also added to ionize the polymer chain. The most common cationizing agents used are silver trifluoroacetate, sodium iodide, etc. A small quantity of this mixture is applied onto the sample plate followed by the solvent evaporation, which results in the cocrystallization of the polymer and the matrix. This mixture of matrix and analyte is then exposed to the laser beam. The most common lasers used are N2 gas and neodymium-yttrium garnet where the fundamental frequency is 337 nm. When the laser is exposed to the mixture of analyte, matrix, and cationizing agent, the matrix molecules absorb the laser energy and are vaporized, while the analyte is ionized. The polymer chain usually carries charge provided by the cationizing agent. The most common mass detectors used in MS are the time-of-flight analyzer, quadrupole mass analyzer, sector mass analyzer, linear ion traps, and 3D ion traps.

3.5.3 Applications of electrospray ionization and matrix-assisted laser desorption/ionization spectrometry Buback et al. [99] utilized ESI for the end group analysis of PMMA synthesized using free radical polymerization of MMA initiated by different peroxypivalates. The different peroxypivalates used in this work were t-butyl peroxypivalate, tamyl peroxypivalate, 1,1,3,3-tetramethylbutyl peroxypivalate, and 1,1,2,2-tetramethyl propyl peroxypivalate. The structures of these peroxypivalates are shown in Scheme 3.1. The ESI-MS of PMMA synthesized via the polymerization of MMA in toluene at 90 C using t-amyl peroxypivalate as an initiator is shown in Fig. 3.23. Fig. 3.23A shows the complete mass distribution and Fig. 3.23B shows the mass spectrum covering a mass range of one monomeric unit. As shown in Fig. 3.23B, there are four major peaks per repeat unit. As shown in Scheme 3.2, following the decomposition of the initiator, five radical species can be generated. In the obtained mass spectrum, polymeric chains carrying one or two acyloxy radicals (A) could not be identified. The radical (C) can be present in PMMA chains in two ways, namely (1) PMMA with one t-butyl group plus one sodium cation,

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SCHEME 3.1 Peroxypivalates utilized for free radical polymerization of methyl methacrylate. Adapted from Buback M, Frauendorf H, Vana P. Initiation of free radical polymerization by peroxypivalates studied by electrospray ionization mass spectrometry. J Polym Sci A Polym Chem 2004;42(17):426675.

FIGURE 3.23 ESI-MS of poly(methyl methacrylate) synthesized using methyl methacrylate and tert-amyl peroxypivalate as an initiator in toluene at 90 C. (A) Complete ESI-MS spectrum, (B) ESIMS spectrum for one monomer repeat unit, (C) ESI-MS spectrum for disproportionation peak, and (D) ESI-MS spectrum for combination peak (cc) [99]. Reproduced with permission from John Wiley & Sons. Copyright 2004.

3.5 Mass spectrometry

SCHEME 3.2 A general scheme for the decomposition of peroxypivalate containing tertiary ester group [99]. Reproduced with permission from John Wiley & Sons. Copyright 2004.

Table 3.5 Peak assignment table for poly(methyl methacrylate) synthesized using methyl methacrylate using tert-amyl peroxypivalate as an initiator in toluene at 90 C [99]. Peak

End groups

c cc cd d dd

tert-Butyl tert-Butyl tert-Butyl Ethyl Ethyl

 tert-Butyl Ethyl  Ethyl

m/ztheo

m/zexp

1079.5/1081.5 1037.6 1009.6 1051.5/1053.5 1081.6

1079.6/1081.6 1037.7 1009.6 1051.7/1053.7 1081.6

Reproduced with permission from John Wiley & Sons. Copyright 2004.

which corresponds to the peak c; the m/z theoretical and experimental values for this peak are shown in Table 3.5, which corresponds to m/z experimental 1079.6/ 1081.6 and m/z theoretical 1079.5/1081.5. (2) PMMA with two t-butyl groups, which corresponds to the peak (cc). Fig. 3.23C shows an enlarged version of peak d from Fig. 3.23B, which was formed by disproportionation having ethyl unit (d) as an initiator. Fig. 3.23D shows an enlarged version of the combination peak (cc) from the spectrum shown in Fig. 3.23B. Inglis et al. [100] reported the conversion of bromide (Br) end-functionalized polymers to cyclopentadiene (Cp) end-functionalized polymers using nickelocene (NiCp2). The conversion was followed by 1H NMR spectroscopy and ESI-MS. As shown in Fig. 3.24, the ESI-MS of PMMA terminated with Br and PMMA terminated with Cp are depicted. The ESI-MS of PMMA-Br shows the presence of a small amount of impurities that are due to side products formed during ATRP.

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FIGURE 3.24 ESI-MS spectra for bromide-terminated PMMA and cyclopentadiene-terminated PMMA [100]. Reproduced with permission from American Chemical Society. Copyright 2009.

However, the conversion of Br end-terminated PMMA to Cp end-terminated PMMA can be clearly identified by a change in m/z values after the reaction. Similarly, NiCp2 was utilized for the end group transformation of poly(isobornyl acrylate) terminated with bromide (PiBoA-Br) to poly(isobornyl acrylate) terminated with Cp (PiBoA-Cp). In this case, the reaction was also followed using ESI-MS. The ESI for PiBoA-Br and PiBoA-Cp are shown in Fig. 3.25, which clearly indicated a change in the m/z values after the end group transformation. Dietrich et al. [101] investigated the one-pot conversion of acrylate and methacrylate polymers synthesized using RAFT polymerization into hydroxylfunctionalized polymers. The conversion of the RAFT end group to a hydroxyl end group proceeds in air and tetrahydrofuran at a temperature of 60 C in the presence of an azo-initiator; this step leads to the formation of a hydroperoxide end-functionalized polymer. The hydroperoxide end-functionalized polymer can be converted into a hydroxyl-functionalized polymer via the use of triphenylphosphine. Fig. 3.26 shows the ESI-MS of RAFT end-functionalized poly(methyl acrylate), hydroperoxide-functionalized poly(methyl acrylate), and hydroxyl-functionalized poly (methyl acrylate). Nebhani et al. [102] reported the use of ESI-MS to monitor the hetero DielsAlder reaction between butadiene- or Cp-functionalized PEG and RAFT agents. The RAFT agents used for this study possess a high electron-deficient C 5 S group. Two different RAFT agents were synthesized in this work, which

3.5 Mass spectrometry

FIGURE 3.25 ESI-MS spectra for bromide-terminated and cyclopentadiene-terminated PiBoA [100]. Reproduced with permission from American Chemical Society. Copyright 2009.

are benzylmethylsulfonyldithioformate and benzylphenylsulfonyldithioformate. Using ESI-MS, it was observed that the reaction between the butadienefunctionalized PEG and the electron-deficient RAFT agent can take place under ambient conditions within 24 hours without the use of any catalysts. While the reaction between Cp-functionalized PEG and the electron-deficient RAFT agent can take place under ambient conditions within 1 hour. Scheme 3.3 shows the reaction between Cp-functionalized PEG (1) and benzylmethylsulfonyldithioformate RAFT agent (2). As shown in Scheme 3.3, the main product, a stable cycloadduct (3), is formed by hetero DielsAlder reaction between (1) and (2). However, in addition to the main product (3), there is a minor amount of (1) as well some side products formed by the loss of sulfinic acid (4) as well as the loss of benzyl mercaptan (5) and a side product formed by the oxidative ring opening of the thiopyran ring followed by the loss of benzyl mercaptan. As shown in Fig. 3.27, the ESI-MS for the reaction of (1) and (2) was recorded for the starting polymer (1) and for the reaction between (1) and (2) after 1 and 24 hours of reaction time without any added catalyst. As shown in Fig. 3.27, after a reaction period of 1 h, the main peak in ESIMS corresponds to the cycloadduct formed via a reaction between (1) and (2). In addition, only minor peaks are present corresponding to (1), (3), (4), and (5). There is no substantial change between the ESI-MS recorded after 24 h of reaction time, suggesting a faster reaction between Cp-functionalized PEG and benzylmethyldithioformate. The limitations of ESI-MS is that only low molecular weight polymers can be analyzed and polymers need to be soluble in an appropriate solvent, while the

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FIGURE 3.26 ESI-MS spectra of poly(methyl acrylate) carrying dithiobenzoate end group, PMA carrying to OOH end group after the addition of AIBN/THF in the presence of oxygen at 70 C and PMA carrying to OH end group after the addition of PPh3 as a reducing agent at 40 C [101]. Reproduced with permission from Royal Society of Chemistry. Copyright 2009.

limitation of MALDI-MS is related to trials involved in the choice of matrix, cationizing agent, and common solvent for dissolving the polymer and matrix.

3.6 Conclusion In this chapter, different techniques that can be utilized for the chemical analysis of polymers have been discussed. The chemical analysis of polymers is crucial for the elucidation of the structures of polymers and, therefore, their properties. The molecular weight of a polymer can be determined by simple titration

SCHEME 3.3 Formation of a stable cycloadduct (3) after the reaction of cyclopentadiene-functionalized PEG (1) and benzylmethyldithioformate (2). Minor side products formed after the loss of sulfinic acid (4), loss of benzyl mercaptan (5), and oxidative ring opening of thiopyran ring followed by the loss of benzyl mercaptan (6) [102]. Reproduced with permission from John Wiley & Sons. Copyright 2009.

FIGURE 3.27 ESI-MS spectra of reaction between cyclopentadiene-functionalized PEG (1) and benzylmethyldithioformate (2) [102]. Reproduced with permission from John Wiley & Sons. Copyright 2009.

References

experiment as well using solution 1H NMR spectroscopy. The functional groups and the structure of a polymer can be characterized via IR and NMR spectroscopy respectively. In addition, NMR spectroscopy can be utilized for the characterization of end groups, for copolymer concentration determination, and for the determination of tacticity. Other than the above mentioned techniques, MS can be utilized for the characterization of end groups as well as for the characterization of the mechanism of polymerization. In order to characterize a polymer sample completely, a combination of different techniques can be utilized. In addition, for the complete characterization of a polymer sample, it is possible to combine two techniques. One example is, the combination of SEC with FT-IR; in this case, SEC helps in the fractionation of polymers and IR helps in the identification of the functional groups present in the fractionated polymer sample. It is common to combine SEC with ESI-MS; here again, ESI-MS is performed on a sample that first undergoes separation using SEC. To summarize, in order to completely characterize a polymer, a combination of different techniques is required, which can provide information on the chemical, mechanical, rheological, morphological, and thermal properties of a polymer.

Acknowledgments The authors would like to acknowledge Indian Institute of Technology Delhi

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CHAPTER

Mechanical analysis of polymers

4

Kalim Deshmukh1, Toma´sˇ Kova´rˇ´ık1, Aqib Muzaffar2, M. Basheer Ahamed2 and S.K. Khadheer Pasha3 1

ˇ Czech Republic New Technologies—Research Center, University of West Bohemia, Plzen, Department of Physics, B.S. Abdur Rahman Crescent Institute of Science and Technology, Chennai, India 3 Department of Physics, VIT-AP University, Guntur, India

2

4.1 Introduction Knowledge of the different materials used in our daily lives plays a pivotal role in making our lives much easier. This highlights the prominent growth and improvements of materials and polymer science fields in upcoming years. [1,2]. It is essential to recognize the association and structure of the current materials; thus, by uniting suitable materials, the preferred properties are attainable in the creation of the next generation of these materials. Polymers are the most appropriate class of materials with advantages like cost feasibility, easy processing, reproducibility, etc. [3
5]. Polymers based on the increasing environmental consciousness are of tremendous interest and have been inspiring the enlightened replacement of nonbiodegradable polymers or synthetic polymers with engineered environmentfriendly biodegradable or natural polymers [6,7]. These smartly engineered materials tender a number of benefits including their environmental biodegradability based on their renewable and harmless nature. In the 20th and 21st centuries, organic polymers have emerged as a widely accepted class of material owing to their technological advancements. The wide range applicability of polymeric systems requires the proper study of the structures and properties associated with these materials to ascertain the correlation between their manufacturing, structure, and the resulting characteristics [8,9]. The mechanical properties of polymers are of primary and fundamental importance in determining their applicability in any technological field. The mechanical properties of polymers involve their behavior under the application of stress and include the extent of stretching, bending, hardness, softness, etc. These features makes polymer different from small molecules [10]. The mechanical properties of polymers are different to those of metals or ceramics and are reliant on the polymer type used, molecular structure, molecular weight (MW), and degree of crystallinity, etc. These properties in turn depends on the Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00004-4 © 2020 Elsevier Inc. All rights reserved.

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fabrication method and testing procedure utilized for reporting the values of particular property [11]. Polymeric materials unveil various useful mechanical characteristics like high modulus, tensile strength, impact strength, and elongation at break, and as such, provide cost-efficient materials as compared to ceramics and metals [12,13]. The ease of fabricating polymers in different forms like extrudates, films, fibers, membranes, and moldings also adds to their significance in everyday applications. The basic mechanical properties of polymeric materials are tensile strength, elongation at break, Young’s modulus, toughness, and viscoelasticity. The tensile strength of a polymeric material is generally defined in terms of the stress required for breaking the material. The strength of polymers is measured in terms of tensile, flexural, compressional, impact, and torsional strengths. The tensile strength of polymers is associated with polymer stretching, flexural strength with polymer bending, compressional strength with polymer compressing, impact with hammering, and torsional strength with polymer twisting. The factors that affect the strength of polymeric materials are MW, crystallinity, and crosslinking. The elongation at break of a polymer yields the amount of strain at its breakage, provides the change in material length prior to its fracture, and measures ductility. Contrary to this, Young’s modulus pertains to the stress
strain ratio to enable measurements regarding polymer stiffness, while the toughness of a polymer is measured from the area under the stress
strain curve and is defined in terms of the energy absorbed by the polymer prior to its breakage. In addition, the viscoelasticity of a polymer enables the study of material deformations. The mechanical behavior of polymeric materials is highly affected by temperature, pressure, polymer processing, and designing. For instance, the stress and stiffness vary with temperature and pressure. The processing and designing of a polymer involving shape and size produce an impact on the toughness of the polymer. Both toughness and fracture toughness are affected by chemical attack and environmental conditions like oxidative, thermal, and ultraviolet aging [14,15]. The change in a specific parameter of a polymer affects its processing and properties and can govern any undesired behavior of the material. Therefore it is essential to comprehensively analyze the experimental data for an efficient understanding of the material behavior to enable the dependable and practical selection of polymer and grading [16,17]. Generally, polymers behave like viscoelastic materials and reveal intermediate behavior between elastic solids and viscous liquids. Elastic solids follow linear proportionality of stress and strain in accordance with Hooke’s law [18]. Polymers at high frequency or low temperature behave like glassy substances, while at low frequency or high temperature, polymers behave like rubbery materials. In the former case, the modulus values are lower, while in the latter case, they are of a higher order without undergoing permanent deformations [19]. Thus, this chapter highlights the basic mechanical properties of polymeric materials and the factors influencing them.

4.2 Mechanical properties of polymers

4.2 Mechanical properties of polymers 4.2.1 Stress
strain behavior The mechanical properties continuously attracts the significant attention during the selection of a material for designing and manufacturing of a product which can withstand mechanical loadings as it is an essential criterion in almost all applications. Before designing a polymeric material, it is essential to set a region within which the mechanical properties of the polymer should be attained (generally within the elastic limits of the material), which is termed as a stress
strain curve. Fig. 4.1 demonstrates that polymers exhibit a wide variety of stress
strain behaviors ranging from hard and brittle to ductile [20]. The stress
strain curve obeys Hooke’s law, which tells us about the linear relationship between the stress and strain of particular elastic materials, that is, within the proportionality limit, stress is directly proportional to strain. The internal forces acting on a material per unit cross-section yield the measurement of stress. Fig. 4.2A
C shows the stress
strain behavior of polyethylene (PE)-like polymer systems based on various degrees of polymerization, strain rates, and temperatures. The stimulated stress
strain curves revealed elastic deformations at low temperatures with elastic yielding as well as stress-based hardening and softening [21]. On the other hand, stress yield and modulus showed an inverse

FIGURE 4.1 Stress
strain curve for different polymers. Adapted from https://polymerdatabase.com/polymer%20physics/Stress-Strain%20Behavior.html.

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FIGURE 4.2 The factors influencing the mechanical properties of polymeric systems, namely (A) degree of polymerization, (B) temperature, and (C) the strain rate. Reproduced with permission from Shang Y, Zhang X, Xu H, Li J, Jiang S. Microscopic study of structure/ property interrelation of amorphous polymers during uniaxial deformation: a molecular dynamics approach. Polymer 77;2015:254
65. Copyright 2015, Elsevier. https://s100.copyright.com/CustomerAdmin/PLF.jsp? ref 5 58872fb8-68c9-40a3-a0e8-046dbc8dd153.

relation with temperature. The polymeric systems at 400 K (above the glass transition temperature (Tg)) display viscoelastic behavior with no obvious yielding. The viscoelastic behavior can be attributed to the higher levels of particle kinetic energy and higher particle velocity fluctuations. In addition to this, the activation

4.2 Mechanical properties of polymers

of intramolecular and intermolecular movements at higher temperatures result in lower yield stress and modulus. Contrary to that, at increasing strain rates, the shape of strain
strain curves are affected by low temperatures as shown in Fig. 4.2C. At the same time, the yielding point disappeared when the strain rates were lowered. In modeling studies, the pressure was measured as a tensor of six element vectors as a function of kinetic energy. The relocation of atoms takes place quickly at higher strain rates, and as such, the pressure tensor varies directly with the stress tensor demonstrating the typical properties of polymer systems.

4.2.2 Viscoelasticity The viscoelastic behavior of polymeric materials is one of the key parameters to be considered while dealing with their mechanical properties. This parameter yields the elasticity of polymers on the application of stress, resulting in a polymer recovery or creep. The reorganization of molecules in a polymer after the stress
strain cycle is often termed as the viscoelasticity of the polymer [22]. Polymers are important viscoelastic materials with excellent features possessing the properties of both an elastic solid and a viscous liquid. These properties are dependent on the temperature as well as on the time scale. In viscoelastic materials, when stress is applied, strain responses are immediately observed [23]. An elastic material instantly deforms on the application of a load and returns back to its original configurations once the load is removed [24]. In viscous materials, deformations observed on the application of stress are linearly dependent on time. Elastic solids store all the energy obtained from external forces during deformation and the same energy is later used for restoring the original shape on the removal of the forces. On the other hand, viscous liquids do not possess definite shape, but flow irreversibly under the influence of an external force [25]. There are several parameters that are considered to be accountable for influencing the viscoelastic behavior of polymeric systems. These inherent parameters include chemical structure, molecular orientation, MW, copolymers, polymer blends, crosslinking, and the effect of plasticizers, etc. [24]. Several types of molecular motions are involved in response to the mechanical deformation of polymeric materials above their Tg. For example, flexible polymeric chains may rearrange quickly on the repeating unit length scale. After a longer duration, the polymeric chains get disentangled. The relaxation time, which is analogous with this process, depends strongly on the MW and the molecular structure of the polymeric system. Due to the complexity of their molecular structure, polymeric chains exhibit a wide distribution of relaxation time, which is extended over several decades in the time or frequency domain [24]. After a short time (long frequency), mainly elastic behavior is observed, while after a longer time (short frequency), mainly viscous behavior is observed as demonstrated in Fig. 4.3. The elastic part of the mechanical deformation is recoverable and it is time dependent due to its entropic nature.

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FIGURE 4.3 Deformation behavior of viscoelastic polymer system with respect to temperature and frequency. Adapted from Cerada ML. Introduction to the viscoelastic response in polymers; 2005, p. 167
82. ISBN: 84-9749-100-9.

The viscoelastic properties of polymeric materials can be evaluated using the creep test. The time-dependent changes occurring in the position of polymeric chains on the application of stress is called creep. In other words, creep describes the ability of the material to deform temporarily or permanently on the application of stress. Creep measurements are of practical importance because they provide information about the dimensional stability of a polymer. For engineers, creep tests are of tremendous interest for applications where polymeric materials need to sustain loads for long periods of time. It was demonstrated that the MW of polymeric materials can affect their creep behavior. An increase in the MW of

4.2 Mechanical properties of polymers

a polymer promotes secondary bonds between the polymeric chains and makes the polymeric material more creep resistant. Aromatic polymers are even more creep resistant due to their stiffness, which is a result of the presence of benzene rings in their structure. Thus both MW and aromatic rings increase the creep resistance and improve the thermal stability of polymers [26].

4.2.3 Time
temperature dependence The temperature-dependent mechanical behavior of polymeric systems is supremely crucial because polymers such as rubber and plastics demonstrate large changes in their mechanical behavior with changes in temperature [27,28]. Time
temperature dependence is a well-recognized procedure often used to study the temperaturedependent rheological properties of liquid polymers or to increase the time or frequency region at a particular temperature at which polymer behavior is studied [29]. Thermal motion is one of the factors that contribute to the deformation of polymers. Due to thermal motion, the secondary bonds of the polymeric chains regularly break and reform. Thus the viscoelastic properties of polymeric materials change with increasing or decreasing temperature. In general, the behavior of polymeric materials changes from the glassy state to the rubbery state as the temperature is increased or the time scale of an experiment is prolonged. At low temperatures in the glassy state, high stiffness is observed due to changes in the elastic energy caused by deformations related to the small movements of molecules from their equilibrium position [25]. On the other hand, at high temperature in the rubbery state, the molecular chains are flexible enough to adopt conformations, leading to maximum entropy in the undeformed state. Thus elastic deformations occurring in the glassy state are associated with molecular conformations [25,30]. Fig. 4.4

FIGURE 4.4 Influence of temperature on the modulus of amorphous and semicrystalline polymers. Adapted from https://omnexus.specialchem.com/polymer-properties/properties/continuous-servicetemperature-of-plastics.

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demonstrates the comparison of a typical modulus versus temperature curve of a completely amorphous polymer and a semicrystalline polymer [31]. It can be seen that for the semicrystalline polymer, a minimum loss in the modulus was observed at the Tg and the maximum loss in the modulus was observed near melting temperature (Tm). On the other hand, the amorphous polymer showed contrasting behavior with a maximum loss in modulus at the Tg [31].

4.2.4 Tensile strength The tensile strength at break or ultimate tensile strength is defined as the force per unit area required for breaking a material [32]. For the tensile test, specimens are prepared in stock shapes using injection molding and are placed in between the two jaws of the universal tensile testing machine (Fig. 4.5) where a given specimen is pulled from both ends by the tensile testing machine and the force required to pull the specimen back is measured in order to discern the extent to

FIGURE 4.5 Representation of tensile strength test of polymers. Adapted from http://www.matweb.com/reference/tensilestrength.aspx.

4.2 Mechanical properties of polymers

which the specimen stretches before it breaks [32]. Tensile strength tests are of prime significance in elaborating the mechanical properties of polymeric systems. For critical applications, polymers with high tensile strength are required that can supplement the information about their operating conditions. For various applications, a balance of stiffness, toughness, processability cost, and MW are the key factors that decide the choice of materials. Generally, the rigidity of polymers increases on the basis of impact strength and the processing requirements, which in turn are based on the limits of the MW of polymers, thus, producing a significant impact on the tensile strength.

4.2.5 Flexural modulus (modulus of elasticity) During the bending process, the measurements pertaining to stiffness yield the flexural modulus of the material. Generally, it is defined as the ability of a material to tolerate the applied forces and comprises of tensile and compressive stresses. The flexural modulus is also known as the bending modulus and is computed as the tendency of a material to resist bending. Flexural modulus is commonly measured by ASM-D-790 and it is often called the modulus of elasticity in bending [33]. The flexural modulus is obtained from the slope of the stress
strain plot produced by a flexural test and is measured as a consequence of changes in stress with respect to strain. In general, the flexural modulus or the modulus of elasticity is equivalent to the tensile modulus or Young’s modulus, but these values may differ in the case of polymeric materials. While designing products, it is essential to know the flexural modulus of the materials used. Flexural strength is defined as the capability of a material to resist bending forces, which are applied perpendicular to the longitudinal axis as shown in Fig. 4.6 [34]. A load is applied at the center of a specimen and the load at the yield is measured as the flexural strength of the specimen. This method of flexural strength testing is called the three-point bending test.

FIGURE 4.6 Representation of flexural strength test according to ASTM-D-790. Adapted from https://en.wikipedia.org/wiki/Flexural_modulus.

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Rodrigues et al. [35] reported the flexural strength and modulus of elasticity for various resin-based composites. The flexural strength and the elastic modulus were obtained via a three-point bending test. The author observed different modulus values for different resin-based polymer composites, revealing the compositional differences between the different manufacturers (monomer type, size, and shape of nanofiller, etc.), and influencing the mechanical behavior of polymeric materials. Polymers exhibit the possibility of viscous flow with a rise in temperature due to interconnected networks. The modulus of polymers reaches a constant value similar to rubber-like polymers at a temperature higher than Tg. At lower temperatures, polymers are in vitreous state and behave like solids with low strain. The flexural modulus of polymers varies with temperature in the vicinity of Tg.

4.2.6 Elongation at break There are several mechanical properties that define the application potential of a polymeric material. Among them, elongation at break is one of the most essential and widely studied polymer properties that are frequently used in structural applications [36]. Elongation at break is the property of a material that represents its ability to resist changes in shape without crack formation. The elongation at break value indicates the ductility of a polymer. It actually yields the amount of stress required to break the material. Elongation can also be defined as a type of deformation under the application of stress. Usually, the elongation at break is measured in percentage (%) as the rate of stretched or final length (L) to the original or initial length (L0) of the sample multiplied by 100. L=L0 3 100 5 % elongation

(4.1)

The final values of elongation at break not only depend on the crosshead speed, but also on the ambient temperature. Commonly, ceramics possess low (,1%), metals possess moderate (1%
50%), thermoplastic have .100%, and thermosets have ,5% values of elongation at break. The values of the elongation at break for brittle materials can be small and are typically assumed to be zero [37]. In particular, fibers have a low elongation at break and elastomeric polymers exhibit elongation up to greater lengths (50%
100%), while retaining their original shape [38]. The two most essential parameters concerning the elongation of a material are elastic and ultimate elongation. While the former represents the elongation without going through permanent deformation and the later represents the extent of up to which a material can be stretched until it breaks.

4.2.7 Crazing and shear yielding In general, there are mainly two types of plastic deformations in polymeric systems, which are crazing and shear yielding [39]. Crazing and shear yielding are the cause of brittle and ductile fractures in all kinds of polymeric materials. These

4.2 Mechanical properties of polymers

types of deformations generally depend on the molecular characteristics of a polymer material such as flexibility of polymer chain, the density of chain entanglement as well as the testing conditions such as deformation speed, temperature, and specimen geometry. Also, the specimen loading type, for instance, tensile flexural and compression loading can result in different deformation mechanisms. Microscopic techniques have played a vital role in studying and identifying these mechanisms. Polarized optical microscopy has been the most effective technique for observing crazing and studying shear yielding in polymeric materials [40]. As such electron microscopy and scanning probe microscopy techniques are employed to determine the morphology and structure of materials, but they are also important and useful in the investigation of different processes involving changes in material properties by interaction with heat, electric or magnetic fields, irradiation, and the liquids or gases present in the environment. The main aim of using microstructure analysis is to obtain information about the failure mechanisms caused by deformations at the microscopic level in multiphase polymeric materials. In particular, the interest is devoted to the study of micromechanical processes of deformation [41]. This method helps in determining the specific deformation process and in the identification of structural defects in a specimen caused by straining. Any kind of microscope is suitable to study such microscopic changes and processes when the specimen is under load. Crazing is nothing but a cavitation process in which the initial step requires the presence of dilatation components to the stress tensor [42]. Crazing is antecedent to failure in a wide range of glassy amorphous polymers and it results in fibril formation between two layers of undeformed polymer. Subsequently, the elongation and failure of the polymer occur, leading to the formation of a crack. Hence crazes act as a load-bearing structure, which creates a delay in fracture and contributes significantly to improving the fracture toughness of a polymeric material. The nucleation, growth, and failure analyses of crazes can help to prevent crazing or to improve the fracture toughness of a polymeric material. Polymers exhibiting craze have lower densities (20%) than that of bulk polymers (80%). However, polymeric materials with craze can still bear significant loads due to polymer fibril orientation. The load bearing capability of crazed polymers can be realized in some transparent polymers wherein the craze extends along with the sample while retaining most of its strength during tensile testing. Crazes break down in the form of cracks and the growth of cracks to critical size ultimately leads to sample failure. Despite the disadvantage of crazes leading to material failure, they can absorb energy as local yielding before failure. The overall impact strength of polymers is enhanced due to the existence of crazes. Fig. 4.7 demonstrates crazing in highly crystalline polypropylene (PP) [43]. These craze-like attributes are always seen in deformed as well as highly crystalline PP at lower temperatures. However, the appearance of these crazes is less evident due to the lack of transparency, but their characteristics are in good agreement with that of glassy polymers, which are amorphous in nature [43]. The authors reported that the thickness of the polymer fibril was more than 10-times

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FIGURE 4.7 Crazing behavior of PP. Reproduced with permission from Narisawa I. Crazing and shear yielding in polypropylene. In: Karger-Koksis J, editor. Polypropylene polymer science and technology series, vol. 2; 1999, p. 124
7, Springer, Dordrecht, The Netherlands. Copyright 1999, Springer. https://s100.copyright.com/CustomerAdmin/PLF.jsp? ref 5 ec6eee0c-81e9-4dd4-9642-ad2dbd12f96b.

larger and more randomly oriented than those observed in the amorphous glassy polymers, which contain stretched microfibrils within the space between the walls of microcracks. The craze structure boundary and the uncrazed surrounding are very sharp. Moreover, crazing can also be useful in improving the toughness of PP alloys such as block copolymers of propylene-ethylene [43]. In multiphase polymeric systems, it is advantageous over homogeneous glassy polymers to combine a soft rubber-like toughening phase with the rigid glassy or highly crystalline component. But these advantages depend on the properties of the two phases, their morphology, and the interface between them. Fig. 4.8 shows a section of deformed high-impact polystyrene (HIPS) demonstrating the occurrence of fibrillation in the rubber phase and crazing in the polystyrene (PS) matrix [40]. The specimens were stained with osmium tetroxide. It was reported that the dominant deformation mechanism in HIPS is not microcracking, but multiple crazing initiated by the rubber particles. Also, multiple crazing was considered to be responsible for stress whitening, volumetric expansion under tensile strain and recovery thereafter, and the yielding and absorption of energy [40]. Shear yielding is a kind of plastic deformation in polymers that are ductile in nature and in which shear bands are tied intimately to the material softening, which takes place after yield. The difference between crazing and shear yielding

4.2 Mechanical properties of polymers

FIGURE 4.8 Crazing behavior in deformed HIPS. Adapted from Bucknail CB. Applications of microscopy to the deformation and fracture of rubber-toughened polymers. J Microsc 201;2001:221
9.

is that crazing appears due to an increase in volume, whereas shear yielding takes place at a constant volume [42]. In general, the deformations in polymeric materials tend to be concentrated in localized bands with shear strains of B1 [40]. The shear yielding mechanism requires the lowest stress domination mode for the material deformation leading to material collapse. In shear yielding, the molecules and atoms slide past each other under deformation. Generally, shear yielding is related to a critical value of the effective stress ðσe Þ, which is defined as: σe 

 1=2 ðσ1 2σ2 Þ2 1 ðσ2 2σ3 Þ2 1 ðσ3 2σ1 Þ2 $ σ0 2

(4.2)

where σ1 , σ2 , and σ3 are the principal stresses. If σ1 is taken as the tensile yield stress σty andσ1 5 σ2 5 0, then σ0 is constant and equal to σty . This equation is valid for metal, but for polymers, suitable modifications are required as the critical effective stress is not constant due to its inconsistency. It is difficult to explain shear yielding at the molecular level, although many attempts have been made over the years. For example, a homopolymer of PS and styrene-acrylonitrile copolymer possess a high chain rigidity that results in high initial stress for shear yielding, which can prematurely deform by crazing. The yield stress, which strongly depends on temperature, can be given as:
   σy 5 σyield =δ2 Tg 2 T

(4.3)

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where (Tg 2 T) is the difference between the Tg of a specimen and the test temperature and δ2 is the cohesive energy density. *N 5 lim R0 2 =nl2 n-N




(4.4)

*N is the characteristic ratio, which quantifies the chain rigidity, and is considered as the predominant controlling molecular parameter for shear yielding behavior. The main reason behind polymers exhibiting brittle fractures is, however, greatly due to the crazing mechanism and partially by shear yielding mechanisms. The dominance of crazing is due to the cross-linkage of macromolecules by means of the localized shear yielding mechanism; since in a crosslinked polymer, the linkage of macromolecules is primarily due to covalent bonds to form thermosetting polymers like epoxies, etc. The cross-linkage in such materials takes place during fabrication while curing leads to a high modulus and melting point. On the other hand, ductile fractures in polymers are mainly linked with extensive shear yielding instead of crazing. The physical and chemical structure greatly contribute in materials ductility in contrast to brittleness. A ductile fracture is, however, favored by plane stress subsequent to plane strain in a brittle fracture with prime considerations of micromechanics.

4.2.8 Fracture and fracture mechanics In polymers, fractures are considered to be only for brittle polymers, which get fractured below Tg. This statement is applicable for polymers having high density, crosslinks, or bulky side groups. The molecular chains sometimes become more rigid and as such slip or it becomes difficult for chain dislocations to occur. Examples of these polymers include thermosetting polymers such as epoxy, polyester, and PS. The stress
strain behavior of these polymeric materials is directly proportional to fracture where less than 1% strain to failure occurs. Generally, microscopy is used to study the fracture and plastic flow of polymers and SEM is commonly used in that regard, enabling fracture morphologies at polymer surfaces to be obtained. Conventional fracture analyses are conducted by means of optical microscopy techniques with the drawback of not providing depth analysis of fractures. The advent of SEM enables superior depth analysis of polymers with rough fracture surfaces even at low magnifications. Fine fracture surface structures are studied by means of TEM, which is also used to study local plastic deformation occurring in shear zones or crazes during the early stages of fracture. Fracture mechanics deals with the natural undefined microflaws in materials simulated by macroscopic notches in a sample. The notch instability is analyzed during deformation as demonstrated in Fig. 4.9 [44]. The starting point of the notch failure is equivalently described as the stress at failure or by deformation energy. Fracture mechanics methods are often used to study the interfacial strengths of polymer interfaces after welding. A benefit of fracture mechanics is

4.2 Mechanical properties of polymers

FIGURE 4.9 Fracture mechanics outline. Reproduced with permission from Ramsteiner F, Schuster W, Forster S. Concepts of fracture mechanics for polymers. In: Deformation and fracture behaviour of polymers. Springer; 2001, p. 27
50. ISBN: 978-3-64207453-0. Copyright 2001, Springer. https://s100.copyright.com/CustomerAdmin/PLF.jsp?ref 5 b03abff36aaa-485d-b393-126834e3804c.

the possibility of analyzing crack instability without depending on the specimen shape giving information about real material properties. The main purpose of fracture mechanics is to develop methods for understanding the initial unreliability of macroscopic cracks and to define quantities that are dependent only on the properties of the material and not on the sample geometry [44]. There are two types of fracture mechanics present, namely linear elastic fracture mechanics (LEFM) and nonlinear fracture mechanics (NLFM). In LEFM, the tensile stress increases with an increase in strain when the load is suddenly dropped at the beginning of crack growth [44]. In NLFM, nonlinear deformation behavior arises due to an extension of the plastic deformation of polymers at the tip of the crack before crack propagation takes place causing viscoelasticity in the material. Polymer failure can be described in terms of stress failure or due to deformations taking place at the linear fracture limit. The study of fracture mechanics begins with an initial notch with a depth equal to half the width of the specimen. The instability of cracks by stress and the energy absorbed is equal in LEFM. For NLFM, the crack instability is determined at the tip of the crack due

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to deformation. The stress intensity factor in LEFM is replaced by strain criterion in NLFM. The concept of deformation energy can be defined using Griffith’s idea, according to which, a sample can only sustain an amount of deformation energy that does not expand the notch due to energy release [45]. The critical crack energy release rate, Gc, at the crack instability can be calculated using the equation: Gc 5 2

1 dU Fi2 dC U 5 5 B da dWφ 2B da

(4.5)

where U is the deformation energy, B is the specimen thickness, a is the notch length, W is the specimen width, C is the specimen compliance, Fi is the force at failure, and φ is the geometrical factor [46].

4.2.9 Coefficient of friction The main aim while engineering materials is friction reduction. However, without friction, many daily functions are not possible. The two blocks shown in Fig. 4.10 can be used to explain the concept of friction [47]. The blocks used in the explanation have equal mass and the same surface finishing with one block having double the contact area of the other. Let Fn be the applied vertical force and F be the horizontal force applied on the blocks to make them move or slide, then a frictional force (Fs) is the resisting force. Surprisingly, the Fs on both blocks are the same and are related to other forces as: Fs 5 μ 3 Fn

(4.6)

μ 5 Fs =Fn

(4.7)

FIGURE 4.10 Frictional test with different blocks by the application of horizontal force (Fn). Adapted from Zeus Technical whitepaper. Friction and wear of polymers. Copyright 2005, Zeus Industrial Products, Inc., http://www.appstate.edu/Bclementsjs/polymerproperties/$p$lastics_$f$riction$5f$w$ear.pdf.

4.2 Mechanical properties of polymers

where μ is the “coefficient of friction” for the material combination of block/plate and is approximately 0.5 for various materials or combinations of materials. It is worth mentioning that the frictional force resisting the sliding or other types of motion does not rely on the apparent contact area due to incomplete surface contact.

4.2.10 Fatigue and fatigue crack propagation Failure in a material can occur due to a variety of reasons, but in most cases, failure occurs when the applied stress is greater than the material strength. Failure can also occur due to deficient material properties, which result from improper processing conditions, leading to defects in the material. Fatigue failures refer to failures that occur due to the mechanically induced cyclic application of stress below fracture [48]. Fatigue in materials usually occurs due to a continuous and cyclic increase in stress. The cyclic stress is generally a sinusoidal, triangular, or square wave type as shown in Fig. 4.11, where parameters such as amplitude, mean stress, and frequency can be adjustable. In general, fatigue failure occurs in two steps. In the first step, the initiation of microscopic cracks occurs due to inhomogeneity (additives, grain boundaries, pores, etc.) or a defect in the specimen. In the second step, these microscopic cracks propagate throughout the material, forming large cracks and eventually leading to material failure. This step of failure analysis is called the crack propagation stage in the material [48]. For various polymeric materials, the time of initiation is greater by several orders of magnitude than the time of propagation. Crack propagation in materials is most often studied on the basis of a constant amplitude load in terms of stress or force [49]. The fatigue load and the stress at the

FIGURE 4.11 Key parameters of fatigue testing. Adapted from Lesser AJ. Fatigue behaviour of polymers. In: Encyclopedia of polymer science and technology, vol. 6, John Wiley & Sons; 2002, p. 197
251.

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crack tip are determined by the stress intensity factor KI. The maximum value of KI represents the applied load and the load ratio “R” can be given by the relation: R5

Kmin Kmax

(4.8)

where Kmin is the minimum stress intensity factor. The load amplitude (ΔK) can be given as: ΔK 5 Kmax 2 Kmin

(4.9)

Several models were proposed to understand the relation between the crack propagation rate (da/dN) and Kmax [50,51]. In some models, the fatigue crack propagation rate is related to load amplitude (ΔK), whereas load ratio “R” and the mean load “Kmean” are incorporated in some models to achieve better results [49]. To understand crack propagation in polymeric systems, the LEFM approach along with the Paris law are mainly used [52,53]. The Paris law correlates crack propagation rate to Kmax using a preexponential factor “A” and an exponent “m” as given by; da 5 AðKmax Þm dN

(4.10)

Fig. 4.12 represents the crack propagation curves with respect to the Paris law. There is always a threshold intensity factor (Kth) below which no crack propagation occurs and above which the fatigue crack propagation rate increases exponentially. The crack propagation rate increases near the end of the lifetime of a material until the occurrence of failure.

log(da/dN)

134

da = A(K)m dN Very slow crack propagation

Kth

Stable crack propagation

Unstable crack propagation

log(Kmax)

FIGURE 4.12 Crack propagation curve (solid line) with respect to the Paris law (diagonal dashed line). The boundaries of the exponential growth region are shown via vertical dashed lines. Adapted from Hawinkles RJH. Fatigue crack propagation in polycarbonate. Eindhoven University of Technology, Eindhoven, The Netherlands; 2011, p. 3
35.

4.2 Mechanical properties of polymers

4.2.11 Toughness Polymeric materials are often characterized in terms of their toughness and it is the most desired property of a material. A high degree of toughness represents the ability of a polymeric material to display large plastic deformations and high resistance to mechanical impact without failing. The toughness of a material is significantly affected by the type of load in the form of shear, bending, compression, etc., and by the experimental conditions used like pressure, temperature, and rate of loading. Generally, crystalline polymers show greater toughness above Tg as compared with amorphous polymers as the toughness of such polymers is manifested by their ductile behavior. Toughness in polymers can be enhanced by different mechanisms or methods carried out at either low or room temperature. Some of these methods are widely applied in the plastic industry, while many of them are still based on theories. These methods include the modification of polymer morphology to absorb plastic deformations mainly at micro- or nanoscale. The toughness in polymers can be enhanced without altering any of their mechanical properties by the production of heterogeneous polymeric material through morphology modifications. Toughness in polymers can be attained via a three-step mechanism, that is, particle toughening, the inclusion of homogeneous core
shell particles, and phase transformation [54]. Loyens and Groeninck [55] reported on the toughness and mechanical properties of rubber-modified polyethylene terephthalate (PET). Ethelene-co-propylene rubber (EPR) with and without reactive functional groups was melt blended with PET. The added reactive modifiers in the blend were maleic anhydride-grafted EPR (EPR-g-MA), glycidyl methacrylate-grafted EPR (EPR-g-GMAx), and ethylene-glycidyl methacrylate copolymers (E-GMAx). The dispersion of pre-blends of EPR and E-GMAx were used to enhance the toughness of the polymer and to induce transitions from a brittle to a ductile material.

4.2.12 Abrasion resistance Abrasion resistance is referred to as the propensity of a polymeric material to resist continuous rubbing actions without deteriorating. In different manufacturing industries, polymers are extensively exposed to high abrasive wear and tear. Abrasion is generated in a polymer by sliding it over another material (hard material over soft material and vice-versa) under a load [56]. The abrasive wear in a polymer can be classified as either two-body abrasion or threebody abrasion on the basis of the generation mechanism. The former occurs during direct contact between two counter surfaces where one surface is softer than the other and the later takes place during trapping of hard particles between two sliding surfaces [57]. As estimated, abrasive wear contributes up to 60% of total cost due to wear [58]. Thus the abrasion depends on the wear mechanism involved and the surface and bulk properties of the materials under testing [57]. The physical processes involved in the abrasive wear of polymers

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FIGURE 4.13 Cohesive and interfacial wear processes. Reproduced with permission from Dasari A, Yu ZZ, Mai YW. Fundamental aspects and recent progress on wear/scratch damage in polymer nanocomposites. Mater Sci Eng R 2009;63:31
80. Copyright 2009, Elsevier. https://s100.copyright.com/CustomerAdmin/PLF.jsp?ref 5 adbe4b81-d471-47b0-901db552354df49b.

are categorized into two groups, namely cohesive wear processes and interfacial wear processes as shown in Fig. 4.13 [59]. In the cohesive wear processes, the frictional work involves large volumes close to the interface either due to the interaction of surface forces or simply due to the geometric interlocking of material contacts that are interpenetrated. The two parameters that define the extent of this zone are contacting geometry and contact stress generated on the surface. In general, cohesive wear processes are dominated by the mechanical properties of the interacting materials. Examples of cohesive wear processes are abrasive wear, fatigue wear, and fretting. On the other hand, interfacial wear processes involves in a frictional work in a much thinner region (smaller volumes) and at larger energy densities resulting in an increase in temperature [59]. In order to determine the extent of wear damage, the surface chemistry and the forces originating from them should also be considered in addition to the mechanical properties of the interacting materials. Examples of interfacial wear processes are transferred wear and chemical/corrosive wear [59].

4.3 Dynamic mechanical thermal analysis of polymers Dynamic mechanical thermal analysis (DMTA) or most commonly known as dynamic mechanical analysis (DMA) is a versatile thermal analysis technique that

4.3 Dynamic mechanical thermal analysis of polymers

gives information about the thermomechanical properties of the specimen. DMA is popular and an important technique in modern polymer science. This technique is dedicated to evaluating the viscoelastic behavior of polymeric materials under small oscillating forces as a function of time and temperature [60]. DMA is a highly sensitive method for detecting secondary transitions at the molecular level and for the evaluation of macromolecular relaxations in polymeric systems [61]. Considering the applied mechanical forces to be sinusoidal in nature as depicted in Fig. 4.14, the corresponding strain and its amplitude and the phase shift can be determined [62]. The primary data that can be obtained from DMA measurements are storage modulus, loss modulus, and loss tangent. The storage modulus can be acquired from the stored energy and the loss modulus can be acquired from the loss of energy resulting from the dissipation of heat [62]. The ratio of loss modulus and storage modulus is referred to the loss tangent (tan δ) or the damping factor of the material. The values of dynamic modulus for polymeric materials are typically in the range of 101 to 107 MPa depending upon the type of polymer, frequency, and temperature [63]. The storage modulus is related to the Young’s modulus and is often associated with material stiffness. On the other hand, the loss modulus is highly sensitive to structural heterogeneity, molecular motions, morphology, thermal transitions, and relaxation processes, and it is often associated with internal friction. Thus DMA provides useful information that can be used to understand the mechanical behavior of polymeric materials at the molecular level [63].

FIGURE 4.14 Response of a linear viscoelastic material subjected to sinusoidal oscillations. Reproduced with permission from Jayanarayanan K, Rasana N, Mishra RK. Dynamic mechanical thermal analysis of polymer nanocomposites. In: Thomas S, Thomas R, Zachariah AK, Mishra RK, editors. Thermal and rheological measurement techniques for nanomaterial characterization. Amsterdam, The Netherlands: Elsevier; 2017. Copyright 2017, Elsevier. https://s100.copyright.com/CustomerAdmin/PLF.jsp? ref 5 09b40e06-dc9e-4010-8d23-b96ef8e6a192

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The primary components of DMTA instruments include a displacement sensor (also known as linear variable differential transformer (LVDT)), a temperature furnace, a linear drive motor, a drive shaft support with control system, and clamps for holding the specimens being tested as depicted in Fig. 4.15 [61]. LVDT measures the voltage change that occurs due to the movement of the instrument probe through a magnetic core. The linear drive motor is used for loading the probe, which gives a load for the applied force. The drive shaft support and the control system are used for guiding the force from the motor to the specimen. In addition, different analyzers are used for controlling both strain (displacement) and stress (force). There are different deformation testing modes in DMTA as shown in Fig. 4.16 [62]. DMTA experiments can be performed in compression or tension mode on the application of a load along the axis. Specimens can be exposed to bending or torsion on the application of a load perpendicular to the axis. The bending test consists of three modes, namely single cantilever, dual cantilever, and three-point

FIGURE 4.15 Schematic representation of DMTA instrument. Adapted from Badia JD, Santonja-Blasco L, Martinez-Filipe A, Ribes-Greus A. Dynamic mechanical thermal analysis of polymer blends. In: Thomas S, Grohens Y, Jyotishkumar P, editors. Characterization of polymer blends: miscibility, morphology and interfaces. Germany: Wiley-VCH Verlag GmBH & Co; 2014.

4.3 Dynamic mechanical thermal analysis of polymers

FIGURE 4.16 Different modes of DMTA experiments. Reproduced with permission from Jayanarayanan K, Rasana N, Mishra RK. Dynamic mechanical thermal analysis of polymer nanocomposites. In: Thomas S, Thomas R, Zachariah AK, Mishra RK, editors. Thermal and rheological measurement techniques for nanomaterial characterization. Amsterdam, The Netherlands: Elsevier; 2017. Copyright 2017, Elsevier. https://s100.copyright.com/CustomerAdmin/PLF.jsp? ref 5 1176eee1-4bcf-4c6a-9716-fac0b36d26c5.

bending. In parallel planes, tangential forces are applied in shear mode. Generally, two approaches are employed for the characterization and evaluation of macromolecular relaxation time and temperature in DMTA [64], namely (1) forced frequency mode in which the input is provided at a set frequency and (2) free resonance mode in which the material is distressed and enables free resonance decay to be shown. The modulus and the internal damping are measures of the stiffness and the energy dissipation ability of a polymeric material respectively. Damping is influenced by the microstructure, morphology, molecular motion, transitions, and other structural diversities that can have significant effects on the mechanical behavior of polymeric materials. Fig. 4.17 demonstrates the variation in the storage modulus, loss modulus, and loss tangent of a specific polymer [62]. The Tg obtained from DMA is generally considered as the temperature at which the maximum loss tangent is observed. In some cases, it is considered as the

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FIGURE 4.17 Storage modulus, loss modulus, and tan δ of a polymeric material with respect to temperature and at a particular frequency. Reproduced with permission from Jayanarayanan K, Rasana N, Mishra RK. Dynamic mechanical thermal analysis of polymer nanocomposites. In: Thomas S, Thomas R, Zachariah AK, Mishra RK, editors. Thermal and rheological measurement techniques for nanomaterial characterization. Amsterdam, The Netherlands: Elsevier; 2017. Copyright 2017, Elsevier. https://s100.copyright.com/CustomerAdmin/PLF.jsp? ref 5 1176eee1-4bcf-4c6a-9716-fac0b36d26c5

temperature at which the maximum loss modulus is observed, and in certain cases, it is considered as the temperature at which the maximum change in storage modulus is manifested [65].

4.4 Factors affecting the mechanical properties of polymers Polymeric materials, especially plastics, are often used for mechanical applications at an economical cost and as such the mechanical properties of polymers are essential for most applications. Therefore it is essential to know the basic and fundamental aspects of such materials pertaining to their mechanical behavior and how this behavior can be altered due to their innumerable structural factors. Polymers reveal the widest range and variety of mechanical properties and occur in numerous forms from liquids and soft rubbers to hard and rigid solids. The

4.4 Factors affecting the mechanical properties of polymers

interplay between properties and these structural features is important because of the need to understand the achievement of the desired properties on the basis of structural modification. There are various structural factors that ascertain the nature of the mechanical behavior of polymeric materials. Thus the primary aim of this chapter is to highlight the influence of various parameters such as MW, crystallinity, temperature, viscoelasticity, and processing methods as well as chemical composition on the mechanical properties of polymers.

4.4.1 Molecular weight In general, almost all the properties of polymeric systems are highly dependent on MW and MW distribution, that is, low or high MW polymers will have a broad range of physicochemical, thermal, and mechanical properties. For example, polymers comprising of small repetitive units will be soft and possess little or no strength, whereas polymers with higher MW will possess much improved mechanical properties [66]. Generally, the mechanical properties of polymeric materials increase with an increase in MW up to certain limit, below which the mechanical properties are independent of polymer chain length. Other properties that are dependent on MW include solubility, Tg, melting point, viscosity (solution and melt), and modulus. A high MW is generally required to get good mechanical properties and high thermal stability. On the other hand, for efficient polymer processing, a low MW is required since the viscosity increases with increases in MW, making it difficult for polymer processing. A higher MW also means that there is increased chain entanglement, which leads to an increase in the tensile strength and elastic modulus of a polymer. Fig. 4.18 schematically demonstrates the influence of MW on the viscosity, tensile strength, and impact strength of polymeric materials [67]. The exact shape of the curve may vary for different properties, as does the critical minimum value of MW. The limiting value at a high MW will vary depending on the specific polymer. Generally, polar polymeric systems and those having hydrogen bonding chains reach the maximal property values at lower MW values than those of nonpolar polymers. For most thermoplastic materials, small changes in their MW do not substantially influence their mechanical properties (yield stress or modulus), whereas properties relating to ruptures such as ultimate elongation, impact strength, and ultimate strength are influenced by a moderate increase in MW. Thus MW and MW distribution are among the most significant microstructural features that influence the deformation of polymeric materials. Boesel et al. [68] studied the effect of MW on the mechanical properties of poly(4-hydroxybutyrate) (P4HB). It was reported that a decrease in MW resulted in an increase in the crystallinity of P4HB. In addition, it was also observed that the tensile strength and the tensile modulus were decreased with a decrease in MW. Thus simply by controlling the MW, a strong yet ductile polymer with improved mechanical properties was obtained for potential biomedical applications [68].

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FIGURE 4.18 Relationship between molecular weight and properties of polymer. Adapted from Gotro J. Polymer innovation blog. In: Polymer composites, Part 2, Polymer resins. https:// polymerinnovationblog.com/polymer-composites-part-2-introduction-polymer-resins/. Copyright © 2019 Innocentrix, LLC.

4.4.2 Degree of crystallinity The presence of a crystalline structure can have a significant influence on the physical, thermal, and mechanical behavior of polymeric materials. Crystallinity depends on the molecular structure of the polymer and no polymer can be completely crystalline. The degree of crystallinity is the measure of the fraction of ordered molecules in a polymer and generally, it ranges between 10% and 80% [69]. Higher values for the degree of crystallinity can be achieved in polymers having small molecules, making them brittle in nature. Crystallized polymers are usually known as “semicrystalline” in which regular crystalline units are linked by randomly unoriented conformation chains that constitute amorphous regions. The properties of semicrystalline polymers are ascertained by their degree of crystallinity as well as the size and the orientation of molecular chains. Therefore it is important to understand polymer crystallinity knowing that the mechanical properties of semicrystalline polymers are different than those of amorphous polymers.

4.4 Factors affecting the mechanical properties of polymers

Okada and Hikosaka [70] studied the influence of crystallinity on the mechanical properties of PP and observed an increase in the tensile strength of PP with an increase in crystallinity. Katogi and Takemura [71] investigated the effect of crystallinity on the mechanical properties of carbon fiber (CF)-reinforced PP composites. The authors reported that the flexural strength and the modulus of PP/CF composites were linearly increased with an increase in the crystallinity of PP. Also, an improvement in the Izod impact value was observed with an increase in the crystallinity of PP. In another study, Stern et al. [72] reported an improvement in the tensile strength and stiffness with an increase in the crystallinity of PP homopolymer. Similarly, Chivers and Moor [73] reported the effect of crystallinity on the mechanical properties of poly(ether ether ketone) (PEEK) resin and they observed enhancement in the tensile modulus and yield strength with increases in the crystallinity of PEEK. Thus the crystallinity of the polymeric material should be optimized in order to get the desired combination of mechanical properties suitable for the targeted application of the polymer.

4.4.3 Temperature It is a well-known fact that polymeric materials display a significant change in their mechanical properties when the temperature increases from glassy state to rubbery state [74]. Therefore it is important to perceive the mechanical behavior of polymeric materials above and below Tg. Upon heating under a certain temperature, the segmental chain mobility of polymers increases dramatically resulting in a dramatic decrease in their mechanical stiffness [75]. For amorphous polymers, the increase in the mobility of the chain segments occurs within the volume of the polymer, and for semicrystalline polymers, the increase in mobility of the chain segments occurs after the melting process [75]. Thus the range of working temperature to a large extent depends on the intermolecular structure of the polymer. Marciano and Reis [76] investigated the effect of temperature on the mechanical properties of epoxy and polyester-based polymer mortars in the temperature range of room temperature to 90 C. It was reported that the flexural and compressive strength of polymer mortars decreases significantly as the temperature increases. The temperature dependency of the mechanical properties of polymer mortars was correlated with the heat distortion temperature of the epoxy resin used. Kumarasamy et al. [77] reported the effect of low and high temperatures on the mechanical behavior of glass fiber
reinforced epoxy composites. At low temperatures (room temperature to 220 C), a slight decrease in the tensile modulus was observed, but no significant effect was observed on the tensile strength. As the temperature increased, both modulus and tensile strength decreased. The decrease in the tensile strength was attributed to the softening of the resin used for the composite formation. In another study, Sahin and Yayla [78] demonstrated the effect of temperature on the mechanical properties of a PP random copolymer. A Charpy impact test revealed that the crack initiation and propagation resistance of the PP random copolymer were sensitive to temperature. At low temperature (0 C), a brittle behavior was observed and the PP copolymer became too ductile to break when the temperature was increased to 85 C. In addition, the Shore D hardness was found to be decreased with an increase in temperature. Thus all the

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mentioned studies reveal that the mechanical properties of polymeric materials can be altered significantly upon exposure to temperature.

4.4.4 Processing methods The processing methods formulate the fundamentals of the mechanical properties of polymers. The processing methods for polymers include compression, injection, rotation, and blow molding. The mechanical properties of a polymer vary on the basis of processing methods. Hence the optimization of processing techniques is essential to determine the key characteristics of polymers. The value of storage modulus at room temperature (25 C) for three different types of polymers, namely LDPE, PP, and NEXPRENE 1287A were studied. The specimens were prepared using different processing techniques such as injection and compression molding [79]. Table 4.1 demonstrates that the low density polyethylene (LDPE) samples processed via compression molding possess higher values of modulus than the Table 4.1 Storage modulus of different polymers under different processing conditions [79]. Polymers LDPE

Processing techniques Injection Compression

PP

Injection

Compression

NEXPRENE 1287A

Injection

Compression

Processing condition 300 F 340 F  300 F, 1500 psi 300 F, 2000 psi 340 F, 1500 psi 340 F, 2000 psi 380 F 430 F 470 F  380 F, 1500 psi 380 F, 2000 psi 430 F, 1500 psi 430 F, 2000 psi 470 F, 1500 psi 470 F, 2000 psi 350 F 390 F 430 F  350 F, 1500 psi 350 F, 2000 psi 390 F, 1500 psi 390 F, 2000 psi 430 F, 1500 psi 430 F, 2000 psi

Storage modulus (MPa) 2.33E 1 08 1.39E 1 08 4.10E 1 08 4.22E 1 08 2.36E 1 08 2.08E 1 08 8.91E 1 08 6.25E 1 08 1.08E 1 09 7.43E 1 08 9.28E 1 08 1.01E 1 09 1.16E 1 09 1.26E 1 09 1.02E 1 09 1.04E 1 08 9.67E 1 07 7.70E 1 07 7.84E 1 07 1.11E 1 08 1.00E 1 08 1.01E 1 08 1.03E 1 08 8.69E 1 07

4.4 Factors affecting the mechanical properties of polymers

those processed via injection molding. Among the series of PP samples tested, those processed via injection molding at 430 F and compression molding at 380 F, 1500 psi, exhibit lower values of storage modulus. For injection molded NEXPRENE 1287A, the storage modulus decreased with an increase in the processing temperature. However, for compression molded NEXPRENE 1287A, no significant change in storage modulus was observed. Table 4.2 demonstrates the elongation at break values for LDPE, PP, and NEXPRENE 1287A processed under different processing conditions. From the results, it can be concluded that different processing methods can have a significant influence on the storage modulus and brittleness of polymers. Table 4.3 summarizes the values of different mechanical parameters obtained for various polymeric materials such as PEEK, PC, PS, PP, polybutadiene (PBT), polyamide-imide (PAI), polyimide (PI), polyphenylene sulfide (PPS), polyvinylidene Table 4.2 Brittleness values for different polymers under different processing conditions [79]. Polymers

Processing technique

LDPE

Injection Compression

PP

Injection

Compression

NEXPRENE 1287A

Injection

Compression

Processing condition 300 F 340 F  300 F, 1500 psi 300 F, 2000 psi 340 F, 1500 psi 340 F, 2000 psi 380 F 430 F 470 F  380 F, 1500 psi 380 F, 2000 psi 430 F, 1500 psi 430 F, 2000 psi 470 F, 1500 psi 470 F, 2000 psi 350 F 390 F 430 F  350 F, 1500 psi 350 F, 2000 psi 390 F, 1500 psi 390 F, 2000 psi 430 F, 1500 psi 430 F, 2000 psi

Strain at break (%)

Brittleness (raw)

1.79 1.88 0.998 1.20 1.01 0.692 0.275 0.358 0.281 0.0781 0.0875 0.0934 0.0833 0.0922 0.0746 11.9 11.9 11.9 4.12 3.25 4.70 5.23 4.63 4.53

2.41E 2 09 3.84E 2 09 2.44E 2 09 1.98E 2 09 4.19E 2 09 6.95E 2 09 4.08E 2 09 4.48E 2 09 3.31E 2 09 1.72E 2 08 1.23E 2 08 1.06E 2 08 1.03E 2 08 8.61E 2 09 1.31E 2 08 8.06E 2 10 8.68E 2 10 1.09E 2 09 3.09E 2 09 2.76E 2 09 2.13E 2 09 1.89E 2 09 2.10E 2 09 2.54E 2 09

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CHAPTER 4 Mechanical analysis of polymers

Table 4.3 Mechanical properties of few selected polymers [33]. Mechanical properties

Polymers PEEK PC PBT PAI PI PPS PVDF CTFE PFA PES PS (high/ medium flow) PSU SAN ABS ABS high impact CA HDPE homopolymer Nylon 6%
50% RH Nylon 66%
50% RH PP homopolymer PP copolymer ASA SMA Epoxy silicon PPO (high Tg) PEI (low viscosity) PUR (unsaturated) Acrylic polymer

Izod Tensile Elongation Tensile Flexural impact strength at break modulus stress Compressive notch (MPa) (%) (MPa) (MPa) stress (MPa) (J/m) 100 62 51.7 151 86 48 57 36 27.5 84 35.85

150 125 200 7.60 8.00 1.00 600 80 300 40 1.20

3792 2344 2344 5032 2068 3309 550 1034 827 257 2275.26

110 96.5 83 189 131 96 94 51
128 68.94

124 86 90 221 207 110 110 32

82.73

85 694 53.4 144 80 , 27 427 133 No break 85 2.41

70.32 68.9 47 43

50 2.00



2482.11 3275 2413 2206

106.17 76 83 72

275.79 96



8.27 21 320 374

43 13.78

70 12.0

2757 1068.68

83



21.37

416 27.57

70.32

300

979.05







20.68

77.22

300

1206.58




33.78

14.47

21.02

100

1103.16







2.75

22.7

200

896







13.78

35.85 55.84 34 48.26 105

25 30 60 60 60

1516.84 2344.21
2447 3309

41.36 55.15 117 65 151



193 113 151

62.05 13.78 16 267 32

76

3

4205

131




21

65.5

5

2447

72

72

16

References

fluoride (PVDF), chlorotrifluoethylene (CTFE), perfluoroalkoxy alkanes (PFA), polyethersulfone (PES), polysulfone (PSU), styrene acrylonitrile (SAN), acrylonitrile butadiene styrene (ABS), cellulose acetate (CA), high density polyethylene (HDPE), acrylonitrile styrene acrylate (ASA), styrene maleic anhydride (SMA), polyphenylene oxide (PPO), polyetherimide (PEI), polyurethane (PUR), and so forth [33].

4.5 Conclusion Polymeric materials can exhibit a wide range of mechanical properties. The mechanical properties of polymers are a measure of the deformation resistance under an applied force. The deformation of polymeric materials is a complex phenomenon that is influenced by many factors including the type of stress, strain rate, type of polymer, characteristics of polymers such as MW, chemical composition, degree of crystallinity, microstructure, degree of crosslinking between adjacent chains, etc. These factors are responsible for the solidity, flexibility, strength, and stability of polymers. The mechanical properties of polymeric materials also depend on the processing method of the polymeric material, testing method, and test conditions. This chapter gives an overview of the basic principles involved in the mechanical behavior of polymers including stress
strain behavior, viscoelasticity, crazing and shear yielding, fracture mechanisms, time
temperature relationship, fatigue and fatigue crack propagation, coefficient of friction, and abrasion resistance. The mechanical properties of polymeric materials change drastically under the influence of temperature. Thus DMTA is an extremely useful technique to investigate the thermal transitions at various segmental scales by evaluating the relaxation times of macromolecules in relation to temperature. The DMTA technique determines the storage modulus, loss modulus, and damping coefficient (loss tangent) of polymeric materials with respect to temperature, time, and frequency. Despite a lot of success in the mechanical analysis of polymeric materials all these years, their long-term mechanical performance under dynamic loading conditions is still a prime concern in view of their industrial applications.

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[61] Badia JD, Santonja-Blasco L, Martinez- Filipe A, Ribes- Greus A. Dynamic mechanical thermal analysis of polymer blends. In: Thomas S, Grohens Y, Jyotishkumar P, editors. Characterization of polymer blends: miscibility, morphology and interfaces. Germany: Wiley-VCH Verlag GmBH & Co; 2014. [62] Jayanarayanan K, Rasana N, Mishra RK. Dynamic mechanical thermal analysis of polymer nanocomposites. In: Thomas S, Thomas R, Zachariah AK, Mishra RK, editors. Thermal and rheological measurement techniques for nanomaterial characterization. Amsterdam, The Netherlands: Elsevier; 2017. [63] Ray SS. Dynamic mechanical properties of environmentally friendly polymer nanocomposites using biodegradable polymer matrices and clay/carbon nanotube (CNT) reinforcements. Environmentally friendly polymer nanocompsites. Woodhead Publishing Ltd; 2013. [64] Rainde E, Diaz-Calleja R, Prolongo MG, Masegosa RM, Salom C. Polymer viscoelasticity. Stress and strain in practice. New York: Marcel Dekker, Inc; 2000. [65] Akay M. Aspect of dynamic mechanical analysis in polymeric composites. Comp Sci Technol 1993;47:419
23. [66] Effect of molecular weight on polymer properties. Polymer Properties Database; 2015, https://polymerdatabase.com/polymer%20physics/MW%20Properties.html. [67] Gotro J. Polymer innovation blog. Polymer composites, Part 2, Polymer resins, https://polymerinnovationblog.com/polymer-composites-part-2-introduction-polymerresins/. Copyright © 2019 Innocentrix, LLC. [68] Boesel LF, Meur SL, Thony-Meyer L, Ren Q. The effect of molecular weight on the material properties of biosynthesized poly (4-hydroxybutyrate). Int J Biol Macromol 2014;71:124
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42. [71] Katogi H, Takemura K. The effect of crystallinity on the mechanical properties of plain woven carbon reinforced composites using polypropylene. WIT Trans Built Environ 2014;137:301
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temperature superposition principle. Polymer 2018;139:177
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60. [76] Marciano J, Reis LD. Effect of temperature on the mechanical properties of polymer mortars. Mater Res 2012;15:645
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[77] Kumarasamy S, Zainol Abidin MS, Abu Baker MN, Nazida MS, Mustafa Z, Anjang A. Effect of high and low temperature on the tensile strength of glass fibre reinforced polymer composites. IOP Conf Ser Mater Sci Eng 2018;370 012021. [78] Sahin S, Yayla P. Effect of testing parameter on the mechanical properties of polypropylene random copolymer. Polym Test 2005;24:613
19. [79] Dong Y. Effects of processing techniques on mechanical properties of selected polymers. Master of Science Thesis, University of North Texas; 2013, p. 124.

CHAPTER

Physical and thermal analysis of polymer

5 Pallabi Saikia

Department of Chemistry, School of Basic Science, Assam Kaziranga University, Jorhat, India

5.1 Introduction The main focus of this chapter is the analysis of polymers on the basis of their chemical properties. For the analysis of polymeric substances, the spectroscopic techniques most commonly employed are UV vis spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, Raman spectroscopy. In addition, polymers existing in the solid state are characterized using methods like X-ray diffraction, optical and electron microscopy, thermal analysis, and so forth. Other than these, to determine the colloidal scale structure of a polymeric substance, small-angle scattering, gel permeation chromatography, and so forth, are the methods to be used. Further, polymers are analyzed on the basis of their theoretical calculations such as quantum chemical calculations (QCCs), density functional theory (DFT), and so forth. The basis of the analytic techniques that are used in polymer characterization are specific physical principles since the behavior of polymeric materials is strongly dependent on the size scale on which an observation is made [1]. For example, the spectroscopic absorption patterns of a semicrystalline polymer and a crystalline polymer are indistinguishable from their lower molecular weight analogs. For the analysis of the topological arrangement of monomers in a chain (i.e., tacticity), a suitable technique is NMR. Similarly, nanoscale crystallites formed as a result of low transport coefficients and chain folding are studied efficiently by small-angle X-ray scattering, transmission electron microscopy (TEM), and to some extent by Raman spectroscopy. Colloidal to optical scale structures in polymers result from fibrillar crystallites and the best ways to observed micron-scale structures of fibrillar crystallites are through optical microscopy, scanning electron microscopy (SEM), and small-angle light scattering. The absorption of electromagnetic (EM) radiation by polymer samples is the basis of the major techniques used for the determination of the chemical composition and molecular topology of polymers. Absorption is a quantized inelastic phenomenon involving the transfer of energy from EM radiation to a material. In addition to inelastic absorption phenomena, elastic interactions between EM radiation and materials are possible and this gives rise to diffraction and scattering phenomena. Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00005-6 © 2020 Elsevier Inc. All rights reserved.

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This chapter attempts to briefly highlight the chemical behavior of polymeric substances such as their structure, morphology, biodegradability, surface topology, and so forth, in light of analytical techniques; the basic backbone of which include the absorption, diffraction, and scattering of EM radiations by a given material.

Techniques used for physical and thermal analysis of polymers 5.1.1 Infrared and Raman spectroscopy 5.1.1.1 Basic principle Using a Fourier transform infrared (FT-IR) spectrometer, the IR spectra of a polymer can be obtained, and by comparing the peaks with the spectral database, the composition of the polymer is readily determined. For example, in Fig. 5.1, the transmission spectra of the isocyante oligomer and the hydroxyl oligomer show numerous totally absorbing IR bands.

FIGURE 5.1 Mid-IR spectra of isocyante oligomer and hydroxyl oligomer [2]. Reproduced with permission from Kro´l P, Pilch-Pitera B. Urethane oligomers as raw materials and intermediates for polyurethane elastomers. Methods for synthesis, structural studies and analysis of chemical composition. Polymer 2003;44:5075 101. Copyright 2003.

Techniques used for physical and thermal analysis of polymers

IR spectroscopy is a suitable analysis technique for the determination of components in polymer mixtures. This technique is also applicable to study the progress of a reaction from the starting materials to the products as well as to identify the in-process intermediates [3]. It is a cost-effective, fast, and reliable technique that is based on the vibrations of the atoms of a molecule. The polymer sample under study is allowed to expose in IR radiation and the fraction of incident radiation absorbed at a particular energy is determined. The positions of the absorption bands in the spectrum give information about the presence or absence of specific functional groups in a molecule. A difference between two spectra indicates that the two samples are made up of different components. For example, the IR spectra of vinyl chloridehinyl acetate copolymers showed a peak at 1740 cm21 corresponding to the acetate mode and the spectra of vinyl chloride showed a peak at 1430 cm21 due to the methylene bending mode. The ratio between the two can be used for quantitative analysis. Most IR spectroscopy is carried out using FT-IR spectrometers [4]. The composition of polymer materials may be readily determined by measuring their IR spectra using a FT-IR spectrometer. The method is based on the principle that the two domains of distance and frequency are interconvertible using the mathematical method of Fourier transformations. Here, the data are converted to digital form using an analog-to-digital converter and then transferred to a computer for Fourier transformation to take place. The output of an IR analysis is expressed in a spectrum where, on the x-axis, inverse wavelength units (cm21) are used, which is known as the wavenumber scale; while the y-axis is represented by the percentage of transmittance, with 100% at the top of the spectrum. Although band intensity is measured in terms of both absorbance and transmittance, the transmittance is traditionally used for spectral interpretation and the absorbance is used rarely for quantitative work. The IR spectrum can be divided into three regions, namely the far-IR (,400 cm21), the mid-IR (400 4000 cm21), and the near-IR (4000 13,000 cm21). The mid-IR region is applied mostly in IR, although the near- and far-IR regions can also provide specific information about materials. The near-IR region consists largely of overtones or combination bands of fundamental modes appearing in the mid-IR region. The far-IR region can provide information regarding lattice vibrations. Spectrum interpretation is simplified by the fact that the bands that appear can be assigned to particular parts of the molecule, thus, producing group frequencies. In Raman spectrum, the scattered intensity is plotted against the energy and each peak shows the Raman shift from the incident light energy [5a]. Raman absorption depends on molecular vibration and the Raman shift has the same energy range as FT-IR absorption. For example, the IR and Raman spectra of monomer and polymer tetrachloroethyl acrylate (TeCEA) [5b] are shown in Fig. 5.2. The variation of the configurational and conformational structure of polymer molecules is reflected in specific frequencies of the IR and Raman spectra, hence, the physical state of the polymer, whether it is crystalline, liquid crystalline, or

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FIGURE 5.2 Raman scattering and IR transmission spectra of monomer and polymer TeCEA [5b]. Reproduced with permission from Jo¨hncka M, Mu¨llera L, Neyera A, Hofstraatb JW. Quantitative determination of unsaturation in photocured halogenated acrylates and methacrylates by FT-IR and Raman-spectroscopy and by thermal analysis. Polymer 1999;40:3631 40. Copyright 1999.

disordered (amorphous), can be identified. Also from the spectra, the amounts of the conformational isomers can be detected [6].

5.1.1.2 Applications If polydimethylsiloxane (PDMS) surfaces are treated with UV-ozone (UVO), they form a stiff silica-like layer and show surface degradation. The chemical effect of ozone on PDMS can be studied using FT-IR [7]. As shown in Fig. 5.3, mainly four bands appear in the FT-IR spectrum. The band at 785 815 cm21 is formed with decreased intensity due to CH3 rocking and asymmetric Si C stretching in Si CH3. The intensity is decreased at 1245 1270 cm21 due to symmetric Si C deformation in the Si CH3 group. The band appearing with increased

Techniques used for physical and thermal analysis of polymers

FIGURE 5.3 (A) Variation of the FT-IR spectra upon exposure to UVO at different irradiation times ranging from 0 up to 120 min. (B) Relative intensity of the bands at 688 and 709 cm21 in Raman spectra [7]. Reproduced with permission from Campo AD, Nogales A, Ezquerra TA, Rodriguez-Herna´ndez J. Modification of poly(dimethylsiloxane) as a basis for surface wrinkle formation: chemical and mechanical characterization. Polymer 2016;98:327 35. Copyright 2016.

intensity at 1075 cm21 is due to asymmetric Si O Si stretching as they form a rigid silica-like top layer. The further increase of relative intensity between 688 and 709 cm21, which are characteristic bands for Si CH3 symmetric stretching, evidenced that the surface chemical group is completely converted into a silicalike layer. The functional groups of different polyimide fibers are studied by FT-IR using typical PI fibers pyromelliticdianhydride (PMDA)/p-phenylenediamine (PDA), PMDA/4,4’-oxidianiline (ODA), and 3,3’,4,4’-biphenyldianhydride (BPDA)/PDA [8]. In the FT-IR spectra of poly(amic acid) (PAAs) (Fig. 5.4), the two absorption bands at 1655 and 1548 cm21 that correspond to amide-I and amide-II have disappeared and two new bands are aroused at around 1775 and 1710 cm21 due to the C 5 O asymmetric and symmetric stretching vibration respectively. In the spectrum, the C N stretching vibration band is observed at 1373 cm21 and the out-of-plane bending vibration band of the imide ring is observed at 722 cm21. These data reveal that imidization occurred completely in the PAA. Poly(ε-caprolactone) (PCL) has a long-range structure through three types of weak intermolecular hydrogen bonds between the CH2 and C 5 O groups [9]. In the IR spectra (Fig. 5.5), there are significant bands due to the influence of hydrogen bonding on the CH2 and C 5 O stretching vibration regions. In the IR spectra

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FIGURE 5.4 FT-IR spectra of different PI fibers [8]. Reproduced with permission from Lei H, Zhang M, Niu H, Qi S, Tian G, Wu D. Multilevel structure analysis of polyimide fibers with different chemical constitutions. Polymer 2018;149:96 105. Copyright 2018.

of PGA, polyhydroxybutyrate (PHB), and PCL, there is a peak at 1740 cm21 due to the free C 5 O group of PGA. On the other hand, the C 5 O group of PHB shows a peak at 1723 cm21 as there is C 5 O/CH3 hydrogen bonding. The peak position and the shape of the C 5 O stretching band of PCL appears as similar to that of PHB at 1724 cm21, supporting the existence of the C H/O 5 C hydrogen bonding. The isothermal crystallization of PCL is studied by IR spectra in the CH2 stretching vibration region [9]. As in Fig. 5.6, the CH2 antisymmetric and symmetric stretching modes of the crystalline parts of PCL resulted in the peaks at 2944 and 2864 cm21 respectively. The bands that appear at 2958, 2944, 2935, 2924, 2909, and 2895 cm21 are due to antisymmetric CH2 stretching vibration during the crystallization process of PCL. In the IR spectrum of PGA, the bands are few and involved in intermolecular interaction between the ether and methylene H C bonds. The PCL chain contains five CH2 groups per unit and the PGA chain has only one CH2 group per unit, and the bands observed are associated with hydrogen bonding caused by the crystallization of PCL. Polyethers and polyesters of various molecular sizes and natures are formed by nonstoichiometric step-by-step polymerization of 2,4- and 2,6-tolylene toluen

Techniques used for physical and thermal analysis of polymers

FIGURE 5.5 IR spectra of PGA, PHB, and PCL cast film in the C 5 O stretching vibration region at room temperature [9]. Reproduced with permission from Funaki C, Yamamoto S, Hoshina H, Ozaki Y, Sato H. Three different kinds of weak C-H/O1/ 4C inter- and intramolecular interactions in poly(ε-caprolactone) studied by using terahertz spectroscopy, infrared spectroscopy and quantum chemical calculations. Polymer 2018;137:245 54. Copyright 2018.

diisocyanate (TDI) with diols [2]. In this polymerization process, urethane oligomers are produced and decayed simultaneously and can be identified by IR spectrometry. The general consistency of the expected chemical compositions of oligomers can be confirmed by the findings for NCO group contents (Fig. 5.7). The composition and structure of two types of high-impact polypropylene (HIPP), namely HIPP-1 and HIPP-2, have been studied by atomic force microscopy-infrared spectroscopy (AFM-IR) [10]. As shown in Fig. 5.8, the FTIR spectra of the two HIPP types show identical spectra with the symmetric C H bending of the methylene group at 1456 cm21 and symmetric methyl deformation at 1378 cm21. The presence of the CH2 rocking band at 720 cm21 indicted the existence of a long polyethylene (PE) sequence. The in situ chemical oxidative polymerization of pyrrole using ferric chloride results in nanocomposites of boron nitride (BN), Ag, and polypyrrole (PPy). In the IR spectra (Fig. 5.9), a peak at 3424 cm21 appeared due to the N H stretching vibrations of PPy. The frequencies correspond to the C 5 C, C 5 N, and C N stretching of pyrrole ring appearing at 1542, 1466, and 1299 cm21 respectively. The band at 915 cm21 due to C 5 N1 C stretching reveals that the PPy support is in its oxidized state and contains positively charged entities (N1). An absorption band due to the BN stretching of BN/Ag appears at 1391 cm21. In the case of the PPy/Ag/BN nanocomposite, the N H, C 5 C, C 5 N, and C N stretching

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FIGURE 5.6 IR spectra of PCL in the CH2 stretching vibration region measured during isothermal crystallization at 48 C (bottom) and their second derivatives (top) [9]. Reproduced with permission from Funaki C, Yamamoto S, Hoshina H, Ozaki Y, Sato H. Three different kinds of weak C-H/O1/ 4C inter- and intramolecular interactions in poly(ε-caprolactone) studied by using terahertz spectroscopy, infrared spectroscopy and quantum chemical calculations. Polymer 2018;137:245 54. Copyright 2018.

frequencies are slightly shifted. The characteristic peak at 1371 cm 1 is due to the BN stretching frequency of PPy/Ag/BN. The slight decrease in the N H, C 5 C, C 5 N, and C N stretching frequencies can probably be attributed to the interaction of PPy with the BN/Ag nanoparticles [11]. The three components in a polyethylene/polypropylene/ethylene propylene (E/P/EP) copolymer are imaged chemically in nanoscale with the help of combined AFM-IR and deuterium labeling [12]. It can be seen from the AFM height images of the E/P/EP blend and the E/P/dEP blend that the blends contain both small and large phase separated domains of P in an E matrix. In bulk FT-IR studies (Fig. 5.10), the combinations of the bands appear due to different CH3 and CH2 stretching and bending modes (CH2 stretches at 2918 and 2850 cm21, CH2 bends at 1465 cm21, and CH3 bends at 1377 cm21), and the C D stretching of the dEP copolymer (Fig. 5.11) at 2192, 2139, and 2089 cm21 are used to identify

FIGURE 5.7 IR spectra of isocyanate oligomer and hydroxyl oligomer obtained from the first and second stages of the reaction of diol and TDI [2]. Reproduced with permission from Kro´l P, Pilch-Pitera B. Urethane oligomers as raw materials and intermediates for polyurethane elastomers. Methods for synthesis, structural studies and analysis of chemical composition. Polymer 2003;44:5075 101. Copyright 2003.

FIGURE 5.8 FT-IR spectra of the HIPPs (compared with homo PP and PE) [10]. Reproduced with permission from Tang F, Bao P, Roy A, Wang Y, Su Z. In-situ spectroscopic and thermal analyses of phase domains in high-impact polypropylene. Polymer 2018;142:155 63. Copyright 2018.

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FIGURE 5.9 FT-IR spectra of prepared FeCl3-doped PPy (black), BN/Ag (green), and PPy/Ag/BN (purple) [11]. Reproduced with permission from Sultan A, Mohammad F. Chemical sensing, thermal stability, electrochemistry and electrical conductivity of silver nanoparticles decorated and polypyrrole enwrapped boron nitride nanocomposite. Polymer 2017;113:221 32. Copyright 2017.

E, P, and dEP in the ternary blend. Hence it is established that the dEP copolymer is dispersed in the E matrix. The polymer which can undergo rearrangement under thermal condition have excellent gas separation properties for separations such as CO2/CH4, CO2/N2, and H2/CH4, and it is formed by a reaction of orthofunctional poly(hydroxyimide)s via a high-temperature (i.e., 350 450 ) solid-state reaction [13]. Polybenzoxazole bands for both Poly(hydroxyamide)-thermally rearranged (PA-TR) and poly(hydroxyimide)-thermally rearranged (PI-TR) polymers are observed through FT-IR at 1058 and 1480 cm21. Further, a weak band near 1734 cm21 in PI-TR is due to imide functionality in the sample. The chemical structure of N-aryl-substituted polyaniline-derived poly(diphenylamine) (PDPA) particles is examined by FT-IR spectroscopy, which is performed in transmission mode for both diphenylamine monomer and PDPA in the mid-IR range (from 4000 to 400 cm21) [14] (Fig. 5.12). The absorption peaks at 3388 and 3053 cm21 correspond to N H stretching and C H stretching of the aromatic rings of PDPA respectively. The strong peak at 1595 cm21 is due to the bending mode of C 5 C stretching in the quinoid ring and the one at 1504 cm21 is due to the C 5 C stretching of the benzene ring. Again, the bands at 1317 cm21 and at 1172 cm21 are due to the C N stretching of the secondary aromatic amine

FIGURE 5.10 Bulk FT-IR spectra of (A) E, (B) P, (C) dEP, (D) E/P/dEP copolymer blend, and (E) E/P/EP copolymer blend. The spectra are shown in absorbance [12]. Reproduced with permission from Rickard MA, Meyers GF, Habersberger BM, Reinhardt CW, Stanley JJ. Nanoscale chemical imaging of a deuterium-labeled polyolefin copolymer in a polyolefin blend by atomic force microscopy-infrared spectroscopy. Polymer 2017;129:247 51. Copyright 2017.

FIGURE 5.11 (A) Bulk FT-IR spectrum of the E/P/dEP copolymer blend compared to AFM-IR spectra collected in the (B) E phase and (C) P phase. (D) The AFM-IR image of the C D stretch (2191 cm21) confirms that the dEP is dispersed in the E phase [12]. Reproduced with permission from Rickard MA, Meyers GF, Habersberger BM, Reinhardt CW, Stanley JJ. Nanoscale chemical imaging of a deuterium-labeled polyolefin copolymer in a polyolefin blend by atomic force microscopy-infrared spectroscopy. Polymer 2017;129:247 51. Copyright 2017.

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FIGURE 5.12 FT-IR absorption spectra of (A) DPA and (B) PDPA [14]. Reproduced with permission from Kim MH, Bae DH, Choi HJ, Seo Y. Synthesis of semiconducting poly (diphenylamine) particles and analysis of their electrorheological properties. Polymer 2017;119:40 9. Copyright 2017.

and the vibration band of C H (in-plane) in the quinoid ring respectively. The out-of-plane bending of aromatic C H and parasubstitution in the aromatic rings result in the peaks at 821 and 748 cm21. Peaks due to terminal phenyl groups are observed at 748 694 cm21. Telechelic is a class of polymers containing two reactive functional groups at their chain-ends, and these are used for the synthesis of block copolymers and polymeric networks. Telechelic polycarbonates (PCs) with alkyne, alkene, and phenol end-groups are prepared by a two-step organocatalytic process [15]. FT-IR analysis of the polymer shows mainly the presence of peaks at 1736 and at 1445 cm21, which correspond to the carbonyl band of the carbonate function and the band of the methyl carbonate end-groups respectively. Phenol-terminated polycarbonate (PC4-phenol) is reacted with p-phenylene diisothiocyanate in the presence of DBTL as a catalyst to obtain poly(thiourethane). The FT-IR analysis shows the total disappearance of the isothiocyanate band at 2090 cm21, and the appearance of aromatic bands at 1550 and 852 cm21 that are ascribed to the

Techniques used for physical and thermal analysis of polymers

reacted p-phenylene diisothiocyanate. The valence vibration band at 3447 cm21 is due to the N H bond of the thiourethane, whereas the band at 1736 cm21 is due to the overlapped carbonyl band of the thiourethane and carbonate group. Further, toward the CuI-catalyzed huisgen 1,3-dipolar cycloaddition (HDC) with 1,12-diazidododecane, the alkyne end-groups of PC-alkyne reacted well. This can be evidenced from the FT-IR spectrum of the HDC polymer as shown in figure 5.13, which shows the disappearance of the band of the azide group at 2100 cm21. Moreover, it was observed that the thermal properties of the HDC polymer were quite close to those of the polycarbonate chains (PC4-alkyne). The chemical structure of a series of acylated chitosans is studied by IR [16]. In the IR spectra of chitosan and its acylated derivatives (Fig. 5.14), absorption at 3000 4000 cm21 due to O H and NH2 stretching is absent in the acylated chitosans, but new peaks have appeared at 1716 cm21 (C 5 O of N(COR)2), 1747 cm21 (C 5 O of ester 3JCH 3 carbon hydrogen coupling constant), 2924 cm21 (asymmetric CH2 stretching), 2854 cm21 (symmetric CH2 stretching), 1464 cm21 (CH2 bending), and 1182 cm21 (CH2 twisting). These results suggest that on the monosaccharide structure of the chitosan, the amino group is converted to an imide group and all four hydroxy and amino groups are fully acylated.

FIGURE 5.13 FT-IR spectra of acyclic diene metathesispolymer, poly(thiourethane), and HDC polymer [15]. Reproduced with permission from Bigot S, Kebir N, Plasseraud L, Burel F. Organocatalytic synthesis of new telechelic polycarbonates and study of their chemical reactivity. Polymer 2015;66:127 34. Copyright 2015.

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FIGURE 5.14 Infrared spectra of chitosan and acylated chitosans [16]. Reproduced with permission from Zong Z, Kimura Y, Takahashi M, Yamane H. Characterization of chemical and solid state structures of acylated chitosans. Polymer 2000;41:899 906. Copyright 1999.

5.1.2 Nuclear magnetic resonance spectroscopy 5.1.2.1 Basic principle NMR spectroscopy is a well-known and popular technique to characterize both natural and synthetic polymers [17]. Liquid-state NMR has been applied to a wide range of polymers in the domain of natural as well as synthetic polymers such as addition polymers, condensation polymers, ring opening polymers, and so forth. Lots of information can be obtained through NMR analysis of a polymer such as its stereochemistry, regiochemistry, tacticity, chain-end structures, geometrical isomerism, and so forth. However, the secondary and tertiary structure of the polymer cannot be analyzed in solution NMR because of dissolution, which

Techniques used for physical and thermal analysis of polymers

destroys the structure. Further, the morphology of polymers can be analysed by solid-state NMR study. Nanoscale hydrophobic and hydrophilic domain morphology formation phase separation is possible by disulfonated poly(arylene ether) and PI copolymers [18]. Also, the three different states of water in this domain can be characterized. Freezable water is bound loosely or exists as free water content. To analyze the binding of freezable water, T1 and T2 NMR relaxation experiments are conducted between water molecules and exchangeable protons on the sulfonic acid group. However, loosely bound and tightly bound water exist in the hydrophilic domain where fast exchange can be possible.

5.1.2.2 Applications Polymer aging-related changes and the condition for the oxidation level of a material was investigated by Assink et al. [19]. The derivatization reaction of trifluoroacetic anhydride (TFAA) with aged hydroxyl terminated polybutadiene (HTPB) elastomeric binder samples followed by 19F NMR studies confirmed the primary and secondary hydroxyl groups as the major oxidation products of this material (Fig. 5.15). During thermooxidative aging, a derivative of TFAA and hydroxyl functionalities, that is, trifluoroester is formed.

FIGURE 5.15 Liquid-state 19F NMR spectra of HTPB aged at 80oC for (A) 0, (B) 14, (C) 53, (D) 147, and (E) 221 days, after reaction with TFAA [19]. Reproduced with permission from Skutnik JM, Assink RA, Celina M. High-sensitivity chemical derivatization NMR analysis for condition monitoring of aged elastomers. Polymer 2004;45:7463 9. Copyright 2004.

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The NMR spectra of a thermally rearranged (TR) polymer [13] suggest that poly benzoxazole (PBO) functionality is formed. The TR polymers were analyzed through 1H and 13C correlation spectroscopy (COSY), heteronuclear single quantum correlation (HSQC) spectroscopy, and heteronuclear multiple bond correlation (HMBC) spectroscopy. From Fig. 5.16 it is observed that five hydrogens are clearly present in the PA-TR, which is consistent with the proposed PBO chemical structure. There is a stronger correlation in peak intensity in each chemical

FIGURE 5.16 (A) 1H and 13C assignment for PA-TR, and 2D correlations including (B) COSY, (C) HSQC, and (D) HMBC NMR [13]. Reproduced with permission from Smith ZP, Czenkusch K, Wi S, Kristofer L, Gleason KL, Hernandez G, et al. Investigation of the chemical and morphological structure of thermally rearranged polymers. Polymer 2014;55:6649 57. Copyright 2014.

Techniques used for physical and thermal analysis of polymers

repeat unit of the proton peaks with 14 hydrogens and protons separated by 4 bonds. The HSQC study showed that all of the protons are directly coupled with carbons. For the 2,2-bis (3-amino-4-hydroxyphenyl)-hexafluoropropane portion of the PA-TR, carbons have a strong 3JCH correlation with the protons and weak 2JCH correlations with the protons. Carbon has a strong 3JCH correlation with the protons and a weak correlation with the protons. The quaternary carbon is connected to protons and by a 3JCH correlation. In 13C NMR, the four peaks are characteristic of the oxazole moiety in the PA-TR, and the two additional peaks are characteristic of the hexafluoroisopropylidene groups between the oxazole and aromatic moieties. The 1H NMR and 13C NMR of merhylcarbonate terminated chemicallymodified telechelic PCs reveal the total conversion of methyl carbonate [15]. The peaks corresponding to the methyl carbonate groups at 3.77 ppm in the 1 H NMR spectrum and at 155 ppm in the 13C NMR spectrum have totally disappeared. While comparing the 1H NMR spectra of the original chitosan in F3CCOOD/ D2O and the H-chitosan in CDCl3 [16] in Fig. 5.17, chitosan shows a singlet at 3.0 ppm (H2) and multiplets at 3.5 and 3.8 ppm (H3, H4, H5, 2H6) corresponding to the ring of methine protons together with a singlet at 1.95 ppm (H7). The signal at 4.4 ppm (H1) is due to the N-acetyl glucosamine units. H-chitosan shows signals at 5.6 (H1), 5.3 (H3), 4.25 (H4), 3.3 3.8 (H6, H5), and 2.75 (H2) (ppm) due to the protons of the polysaccharide ring together with the signals at 2.45 ( CO CH2 ), 1.35, 1.65( CH2 ), and 0.9 ( CH3) (ppm), which are assigned to the hexanoyl chains. The chemical shifts of the ring protons of the H-chitosan are shifted downfield due to the presence of electron-withdrawing acyl substituents. Furthermore, from the integration of the hexanoyl groups, it can be concluded that the degree of substitution with acyl groups is four per monosaccharide ring. Similarly comparing the 13C-NMR spectra of H-chitosan with the original chitosan in F3CCOOD/D2O, Fig. 5.18 shows that in the original chitosan, the signals at 95.2 and 98.2 ppm are due to C1 and C10 carbons. The signals at 54 (C2 1 C20), 73 (C3 1 C30), 67.6 (C4 1 C40), 70.5 (C50), 75.8 (C5), and 58 (C6 1 C60) (ppm) appear due to two different units in the structure of chitosan. The peaks at 19 and 174 ppm are assigned to the methyl and carbonyl carbons of the acetyl groups of the N-acetylglucosamine units respectively. For H-chitosan, the signals shown at 58 (C2), 62 (C6), 69 (C4), 73 (C3), and 99 (C1) (ppm) are attributed to the carbons in the polysaccharide structures. The signals shown at 179, 177, 173, 171, and 169 ppm are because of the carbonyl carbons. From these data, it can be concluded that the amino and amide groups of the original chitosan are converted into imides. The elemental analysis of chitosan and its acylated products also shows reasonable agreement with the result that the degree of substitution is four for all of the derivatives, and the acylation does not alter the chain length of the original chitosan.

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FIGURE 5.17 1

H-NMR spectra of (A) chitosan in F3CCOOD/D2O and (B) H-chitosan in CDCl3 [16]. Reproduced with permission from Zong Z, Kimura Y, Takahashi M, Yamane H. Characterization of chemical and solid state structures of acylated chitosans. Polymer 2000;41:899 906. Copyright 1999.

Techniques used for physical and thermal analysis of polymers

FIGURE 5.18 13

C-NMR spectra of (A) chitosan in deuterated trifluoro acetic acid (F3CCOOD)/D2O and (B) H-chitosan in deuterated chloroform [16]. Reproduced with permission from Zong Z, Kimura Y, Takahashi M, Yamane H. Characterization of chemical and solid state structures of acylated chitosans. Polymer 2000;41:899 906. Copyright 1999.

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5.1.3 X-ray analysis 5.1.3.1 Basic principle

˚; X-rays are electromagnetic radiations in the wavelength range of 1022 102 A however, the structure of materials are studied in the wavelength range of ˚ . In general, most polymers are studied in radiation from a copper tar0.5 2.5 A ˚ . X-rays play an important role in studyget tube having a wavelength of 1.5418 A ing the arrangement of an atom in matter because the most interatomic distances in condensed matter have the same order of magnitude as the X-ray wavelength. In experiments, X-rays are generated through a filament tube. Electrons released from a hot tungsten filament are accelerated toward a metal target in an evacuated tube and X-rays are generated on the impact of the electrons. Under the influence of the anode potential, the filament electrons hit the target metal at high speed and induce the emission of X-rays in all directions. The ranges of wavelengths are calculated using the formula: ν 5 λc . X-ray photons can excite the inner shell electrons from the absorbing atom. As the X-ray energy is increased through an inner-shell ionization potential, it results in sharp steps of absorption cross section. The spectrum is analyzed in two regions, namely (1) the threshold region in the near edge (XANES) and (2) the extended X-ray absorption fine structure (EXAFS). In the edge region, the energy of the photon is sufficient to excite a core electron into the unoccupied atomic or molecular orbitals. At slightly higher energies, the spectrum shows a series of gentle oscillations in the absorption cross section because of the scattering of excited photoelectrons by the neighboring atoms (Fig. 5.19) [20]. In X-ray wavelength-dispersive spectroscopy, characteristic X-rays are distinguished on the basis of their wavelength, provided a crystal behaves as a threedimensional diffraction grating and reflects X-ray photons with a wavelength from the Bragg equation: nλ 5 2d sin θ

where n is the order of the reflection (usually the first order is used), and θ is the angle between the incident X-ray beam and atomic planes (with a particular set of Miller indices) of spacing d in the crystal. By continuously changing θ, the X-ray intensity can be measured as a function of wavelength.

5.1.3.2 Applications Koros and coworkers [21] have studied the correlation between the chemical structure of 4,40-hexafluoroisopropylidene) diphthalic anhydride (6FDA)-based PIs with gas transport properties with a special focus on CO2 and CH4 transport. The amorphous behavior of the PIs is observed with broad peaks in wide-angle X-ray diffraction (WAXD) analysis. The changes of d-spacing reveal the changes in the packing density of the polymer chain affecting the diffusion of small molecules through a glassy matrix. It can be seen from the Fig. 5.20, the PIs membranes 6FDA-2,4,6-trimethyl-1,3-diaminobenzene (DAM) has highest d-spacings

Techniques used for physical and thermal analysis of polymers

FIGURE 5.19 (A) (a) XANES spectra of unpoled and poled MneZnO/polyvinylidene fluoride (PVDF) with 3 1/4 0.01, 0.03, and 0.05 Mn doping measured at Zn K-edge along with the spectra for reference of Zn metal and ZnO/PVDF film. (b) normalized XANES spectra for unpoled and poled MneZnO/PVDF with 3 1/4 0.01, 0.03, and 0.05 Mn doping measured at Mn K-edge along with the reference Mn metal, MnCl2, and Mn2O3 samples [20]. (B) (a) Normalized EXAFS spectra of poled MneZnO/PVDF samples with 3 1/4 0.01, 0.03, and 0.05 Mn doping measured at Zn K-edge along with the spectra for poled ZnO/PVDF film and reference Zn metal. (b) Normalized EXAFS spectra for poled MneZnO/PVDF samples with 3 1/4 0.01, 0.03, and 0.05 Mn doping measured at Mn K-edge along with the reference Mn metal and Mn2O3 sample [20]. Reproduced with permission from Bhunia R, Yadav AK, Jha SN, Bhattacharyya D, Hussain S, Bhar R, et al. Probing local environment of Mn-doped nanocrystalline-ZnO/PVDF composite thin films by XPS and EXAFS studies. Polymer 2015;78:1 12. Copyright 2015.

with 6FDA-DAM: meta phenylenediamine (mPDA), 6FDA-DAM: 3,5-d-iaminobenzoic acid (DABA) and 6FDA-mPDA: DABA and 6FDA-DABA have lowest d-spacings which is a result of increase chain packing. The molecular packing and crystal orientation of PI fibers (PMDA/PDA, PMDA/ODA, and BPDA/PDA) are characterized by 2D WAXD measurements [8]. Co-PI fibers exhibit highly ordered structures with a low degree of lateral packing orders along the perpendicular direction to the fiber longitudinal axis.

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FIGURE 5.20 WAXD scans of polyimide membranes [21]. Reproduced with permission from Qiu W, Xu L, Chen CC, Paul DR, Koros WJ. Gas separation performance of 6FDA-based polyimides with different chemical structures. Polymer 2013;54:6226 35. Copyright 2013.

The polymer chains of PMDA/PDA, PMDA/ODA, and BPDA/PDA have straight, helical, and folded structures respectively (Fig. 5.21). X-ray photoelectron spectroscopy (XPS) shows a higher degree of reduction of graphene oxide (GO) during the preparation of a graphene/polybenzimidazobenzophenanthroline (BBL) nanocomposite from GO and reduced graphene oxide (rGO) nanosheets [22]. XPS analysis of GO and rGO shows two main peaks, one is C1s (284.7 eV) and intense O1s (531.0 eV) for both GO and rGOs (Fig. 5.22). The greater intensity of the O1s peak in the XPS spectrum of GO indicates the predominating effect of oxygen-containing groups. However, after reduction, the intensity of the O1s peak is largely suppressed, while the C1s peak is preponderated. Furthermore, a two-dimensional X-ray pattern study of a BBL/GO (5 wt.%) nanocomposite film revealed that GO did not show any scattering maximum in the X-ray pattern. Thus both GO and rGO fillers in the polymer matrix undergo good exfoliation and dispersion in the BBL matrix. Raman and XPS studies show that the reduction of GO is more effective in the combined process of NaBH4 treatment and thermal annealing at 1100 compared to chemical reduction alone. The nanocomposite structure of BN, Ag, and PPy obtained from FeCl3-doped PPy were studied by XRD analysis [11] (Fig. 5.23). A peak observed at 24.50θ is assigned to the repeating units of pyrrole with an ordered arrangement of PPy

FIGURE 5.21 Optimized images of PI amorphous cell and chains configuration of (A) PMDA/PDA, (B) PMDA/ODA, and (C) BPDA/PDA [8]. Reproduced with permission from Lei H, Zhang M, Niu H, Qi S, Tian G, Wu D. Multilevel structure analysis of polyimide fibers with different chemical constitutions. Polymer 2018;149:96 105. Copyright 2018.

FIGURE 5.22 The C1S XPS spectra of (A) GO and (B) reduced GO (rGOCT) [22]. Reproduced with permission from Park JH, Choudhury A, Farmer BL, Dang TD, Park S-Y. Chemically modified graphene oxide/polybenzimidazobenzophenanthroline nanocomposites with improved electrical conductivity. Polymer 2012;53:3937 45. Copyright 2012.

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FIGURE 5.23 XRD patterns of (A) PPy, (B) BN, (C) BN/Ag, and (D) PPy/Ag/BN [11]. Reproduced with permission from Sultan A, Mohammad F. Chemical sensing, thermal stability, electrochemistry and electrical conductivity of silver nanoparticles decorated and polypyrrole enwrapped boron nitride nanocomposite. Polymer 2017;113:221 32. Copyright 2017.

units. The peaks at 2q values of 25.66, 37.10, 40.83, 43.55, 49.24, 54.23, and 76.81 correspond to BN. The peaks suggest that BN is highly crystalline and in a pure state. In the case of BN/Ag, some BN and Ag peaks merged. The presence of a new peak at a higher 2q B63.67 due to (220) the anatase phase of Ag indicate the formation of BN/Ag. On further polymerization of pyrrole with BN/Ag, the disappearance of low-intensity peaks of BN is probably due to the fact that amorphous PPy might have shadowed the low-intensity peaks of BN. Therefore XRD analysis indicates the presence of BN, Ag, and PPy in the PPy/Ag/BN nanocomposite. The chemical structure of polyethersulfone (PESU) has a considerable gas separation performance compared to polysulfone. Among PESU there are four different backbone structures, namely polyphenylsulfone (PPSU), poly trimethyl benzene ethersulfone (TPESU), PESU, and hydrophilic polyethersulfone (HPESU); the permeability and selectivity of various gas pairs were studied [23]. The XRD patterns in Fig. 5.24 show broad peaks that are typical for densely packed amorphous polymers. However, it is observed that PPSU and TPESU ˚ and HPESU and PESU have smaller dhave d-spacing values of about 5.1 A ˚ . The sequence of fractional free volume follows the spacing values of 3.5 4 A

Techniques used for physical and thermal analysis of polymers

FIGURE 5.24 XRD spectra of PES membranes [23]. Reproduced with permission from Naderi A, Yong WF, Xiao Y, Chung T-S, Weber M, Maletzko C. Effects of chemical structure on gas transport properties of polyethersulfone polymers. Polymer 2018;135:76 84. Copyright 2018.

order of PESU , HPESU , TPESU , PPSU. The O2 and CO2 permeabilities of PPSU are highest with the values 1.61 and 9.13 Barrer at 35 C, which is probably due to the presence of two additional aryl groups contributing high segmental motion. PPSU has the lowest O2/N2 and CO2/CH4 selectivity because of the enlarged pore size and d-space. The crystal structure model of a poly(vinyl alcohol) (PVA) iodine complex including counter ions (K1) was studied referring to the X-ray analyzed structure (Fig. 5.25) [24]. The positions of the K1 ions were found to be in the highly negatively-charged space surrounded by OH groups and iodine atoms with minimum electrostatic potential created by the PVA chains and iodine ions.

5.1.4 Scanning electron microscopy and transmission electron microscopy 5.1.4.1 Basic principle If an object is too small to be examined then microscopy is the technique to analyze the object in the micrometer and nanometer ranges. The basic principle of the transmission electron microscope is that inside a material, atomic planes diffract electrons if a solid is crystalline. Through a thin specimen, a transmission electron diffraction pattern is formed, which is imaged with a special resolution by their short wavelength. However, a limitation of TEM is that if a specimen is not made thin, then rather than transmitted, the electrons are strongly absorbed or

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FIGURE 5.25 Crystal structure (complex II) of PVA I2 3 complex proposed by X-ray analysis. The brown circles are iodine atoms. The large purple circles are potassium atoms [24]. Reproduced with permission from Takahama T, Saharin SM, Tashiro K. Details of the intermolecular interactions in poly(vinyl alcohol)-iodine complexes as studied by quantum chemical calculations. Polymer 2016;99:566 79. Copyright 2016.

scattered within the specimen. However, in the scanning electron microscope, electrons are reflected from a bulk specimen as secondary electrons. These electrons are emitted with a range of energies. Thus primary electrons are focused into an electron probe and scanned across the specimen. Thus SEM is based upon the secondary emission of electrons [25].

5.1.4.2 Applications SEM studies of FeCl3-doped PPy, BN/Ag, and PPy/Ag@BN shows that PPy exhibits a globular morphology [11]. In the case of BN/Ag it is observed that

Techniques used for physical and thermal analysis of polymers

uniformly dispersed clusters are formed by the silver nanoparticles on the surface of the nanosheets of boron nitride (Fig. 5.26). TEM micrographs of the BN/Ag and PPy/Ag@BN nanocomposites show black spots of silver nanoparticles on the grey background of boron nitride nanosheets as well as on the polypyrrole matrix (Fig. 5.27). Using typical PI fibers, PMDA/PDA, PMDA/ODA, and BPDA/PDA, the internal structures of PI fiber were investigated using SEM analysis [8]. An ellipseshaped cross-sectional area of the PMDA/PDA fiber with loose internal structure was observed (Fig. 5.28). The cross-sectional shapes were circles for PMDA/ ODA and BPDA/PDA fibers with relatively dense internal structures. During coagulation of fibers, different chemical constitutions of the inner force and external force cannot reach equilibrium for the solvents and coagulants, which result in different fractured cross-sectional morphologies and performances of the fibers. The cross-sectional morphology indicates regular shapes and compact inner structures of BPDA/PDA and PMDA/ODA as a result of a more moderate coagulation process of BPDA/PDA and PMDA/ODA fibers compared to PMDA/PDA fibers. The TEM images of two types of HIPP, namely HIPP-1 and HIPP-2, show that core shell structured multilayered rubber particles are dispersed in the PP matrix [10]. The AFM phase images of HIPP-1 and HIPP-2 are similar (Fig. 5.29), showing soft rubber particles with rigid cores dispersed in PP matrix, but as observed in the AFM-IR images (Fig. 5.30), the PP distribution is not the same in the two samples. The band at 1378 cm21 corresponds to different ethylene contents in the two HIPPs. In HIPP-1, the PP content is high in the matrix and low in the rubber domains, but the cores in HIPP-2 are rich in PE. The basic requirement for an electron energy-loss spectroscopy (EELS) is small differences in kinetic energy. Primary electrons that enter in TEM specimen lose kinetic energy through inelastic scattering. In EELS, the number of electrons that lose energy in the specimen is plotted against their energy loss. In the analysis of poly(methyl metacrylate) (PMMA), EELS in TEM is an important tool in order to obtain chemical information on electron sensitive polymers [26]. Molecular orbital calculations and XANES experiments are compared to the EELS data. The EELS spectra of PMMA during main chain scission is slightly dependent on the molecular weight of the polymer. The peak due to 1s-πᴨ (C C) transition, is characteristic of irradiation damage since no C C bond exists in nondegraded PMMA. The second peak at 287.3 eV, is due to a mixed 1s-Rydberg/σ (C H). In the near edge fine structure (Fig. 5.31), the peak at 288.4 eV is due to σ orbital transition, which is delocalized on the carbonyl functional group. Two peaks appeared at 290.8 and 292.8 eV, representing σ (C O C) and σ (C C) orbitals. Also the peak at 295.6 eV in XANES is due to σ (C O C) orbital. Because of the thickness effect, the second peak of σ (C O C) orbital does not appear in the EELS. Moreover, the quantification of the EELS spectrum gives an O/C atomic ratio equal to 0.4 6 2 0.02 indicating that the ester group has not yet broken, but scissions have occurred at the main chain. At the oxygen K-edge, the

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FIGURE 5.26 Fe-SEM images of (A) PPy, (B) BN/Ag, and (C and D) PPy/Ag@BN [11]. Reproduced with permission from Sultan A, Mohammad F. Chemical sensing, thermal stability, electrochemistry and electrical conductivity of silver nanoparticles decorated and polypyrrole enwrapped boron nitride nanocomposite. Polymer 2017;113:221 32. Copyright 2017.

Techniques used for physical and thermal analysis of polymers

FIGURE 5.27 Energy-dispersive X-ray spectra of (A) BN/Ag and (B) PPy/Ag@BN [11]. Reproduced with permission from Sultan A, Mohammad F. Chemical sensing, thermal stability, electrochemistry and electrical conductivity of silver nanoparticles decorated and polypyrrole enwrapped boron nitride nanocomposite. Polymer 2017;113:221 32. Copyright 2017.

EELS spectrum shows three peaks, namely π (COO) and σ (C O C) orbitals and O1s-σ (COO) transition (Fig. 5.32).

5.1.5 Thermogravimetry and differential scanning calorimetry 5.1.5.1 Basic principle Thermal analysis techniques are analytical experimental techniques where the properties of a sample are examined against the temperature of the sample [27].

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FIGURE 5.28 SEM images of different PI fibers, namely (A) PMDA/PDA, (B) PMDA/ODA, and (C) BPDA/PDA [8]. Reproduced with permission from Lei H, Zhang M, Niu H, Qi S, Tian G, Wu D. Multilevel structure analysis of polyimide fibers with different chemical constitutions. Polymer 2018;149:96 105. Copyright 2018.

Techniques used for physical and thermal analysis of polymers

FIGURE 5.29 TEM images of (A) HIPP-1 and (B) HIPP-2 [10]. Reproduced with permission from Tang F, Bao P, Roy A, Wang Y, Su Z. In-situ spectroscopic and thermal analyses of phase domains in high-impact polypropylene. Polymer 2018;142:155 63. Copyright 2018.

The operation involves keeping the temperature constant, heating, or cooling at a fixed rate of temperature change. The most basic thermal analysis techniques for the determination of the variables of a state are (1) thermogravimetry (TGA), (2) differential thermal analysis (DTA), and (3) differential scanning calorimetry (DSC). In DTA, the sample is heated at a programmed rate with an inert reference material. On heating, the temperature of both the sample and the reference material increase uniformly. If there is a phase change in the sample, energy is absorbed or emitted and the temperature difference between the sample and the reference (ΔT) is measured. A DTA curve is the plot of the temperature difference as a function of the temperature or time at a constant temperature. In DSC, the difference in temperature is plotted against the change of heat flux. During the phase change, heat is absorbed or emitted by the sample and there is a difference in heat flux. In TGA, the mass change of a sample is examined as a function of temperature. TGA is used to characterize the thermal stability as well as decomposition of materials under a variety of conditions. In TGA, a curve mass change, Δm, expressed in percent is plotted against temperature (T) or time (t). Thermal analysis data are indirect and are collated with results from spectroscopic measurements such as NMR, FT-IR, X-ray diffractometry, and so forth.

5.1.5.2 Applications 5.1.5.2.1 Thermogravimetry applications 6FDA-based PIs were synthesized and the chemical structure and thermal behavior of 6FDA-DAM, 6FDA-DABA, 6FDA-mPDA, and 6FDA-DAM:mPDA were studied in the evaluation of 6FDA-based PI membranes for application in gas

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FIGURE 5.30 AFM-IR spectra for HIPP-1 (B) and HIPP-2 (D) taken at the locations marked with corresponding colors in (A) and (C), normalized to the band at 1378 cm1, and indicative of different ethylene contents as shown by the intensity of the 1456 cm21 band [10]. Reproduced with permission from Tang F, Bao P, Roy A, Wang Y, Su Z. In-situ spectroscopic and thermal analyses of phase domains in high-impact polypropylene. Polymer 2018;142:155 63. Copyright 2018.

separation [28]. Comparative study through TGA during the imidization of polyamic acid to form PI reveals the partial formation of PI when the acid is dried at room temperature as well as at different temperatures ranging from 110 to 300 . However, as chemically imidized PI undergoes polymer backbone degradation, it shows complete imidization with major weight loss. The thermal decomposition behavior of PIs with different chemical structures was investigated using TGA in an N2 atmosphere. The TGA profiles show different thermal decomposition behaviors of PIs with the introduction of carboxylic acid side groups. The first minor weight loss is caused by the removal of the carboxylic acid groups in a decarboxylation process and the second weight loss is due to the decomposition of the

Techniques used for physical and thermal analysis of polymers

FIGURE 5.31 (A) EELS spectra of PMMA at the carbon K-edge with (B) the decomposition of the ELNES signal after subtraction of the ionization continuum [26]. Reproduced with permission from Varlot K, Martina JM, Gonbeau D, Quet C. Chemical bonding analysis of electron-sensitive polymers by EELS. Polymer 1999;40:5691 7. Copyright 1999.

backbone of PI. The decarboxylation-induced thermal crosslinking below the glass transition temperature as well as the ester crosslinking of 6FDA-DAM: DABA membranes were studied by TGA and derivative weight analysis. It was found that the acid group was decarboxylated to create a phenyl radical capable of attacking other portions of the PI for crosslinking. The decarboxylationinduced crosslinking of the 6FDA-DAM:DABA (3:2) membrane could be performed below the glass transition temperature of the polymer. From the TGA

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FIGURE 5.32 (A) EELS spectra of PMMA at the carbon K-edge with (B) the decomposition of the ELNES signal after subtraction of the ionization continuum [26]. Reproduced with permission from Varlot K, Martina JM, Gonbeau D, Quet C. Chemical bonding analysis of electron-sensitive polymers by EELS. Polymer 1999;40:5691 7. Copyright 1999.

curve, in chemically crosslinkable PIs, propane-diol Monoesterified Crosslinkable polyimide, the monoesterfication reaction is not completed and there are still carboxylic acid groups in the polymer. In the case of Ag1-substituted polymers, when the membrane is soaked at 80 in AgNO3 for 90 h, all H1 in the carboxylic acid groups are exchanged to Ag1. Moreover, compared with the pristine membrane, the Ag1-containing 6FDA-DABA membrane yields a markedly decreased decarboxylation temperature, which indicates the thermal crosslinking of the polymer membrane at a much lower temperature. HTPB having Fe was developed as a catalyst without altering the properties of HTPB. The TGA of HTPB and Fe-HTPB showed that there is no difference in the thermal stability as well as the glass transition temperature of HTPB and FeHTPB (Fig. 5.33). It evidences that there is no change in microstructure in the Fe-containing HTPB compared to HTPB [29]. Polythiophene-based cationic conjugated polyelectrolytes with phosphorous and nitrogen pendants were prepared by Hladysh et al. [30]. The thermal

Techniques used for physical and thermal analysis of polymers

FIGURE 5.33 Thermal studies of HTPB and Fe-HTPB; (A) TGA and (B) DSC plots [29]. Reproduced with permission from Rao BN, Malkappa K, Kumar N, Jana T. Ferrocene grafted hydroxyl terminated polybutadiene: a binder for propellant with improved burn rate. Polymer 2019;163:162 70. Copyright 2019.

properties of cationic conjugated polyelectrolytes reveal that they have good thermal stabilities with different degradation temperatures. However, phosphorousbased polyelectrolyte PHT-buP1 (degradation started at 320 ) shows better thermal stability compared to nitrogen-based polyelectrolyte PHT-buN1 (degradation started at 210 ). Other than this, in the TGA curve of PHT-buN1, a negligible water loss was observed at 100 , which might be due to the release of residual water indicating a strong hydrophilicity of PHT-buP1. The DTA thermograms of mixed polyelectrolyte showed different endothermic melting transitions with the highest melting peak at 460 for PHT-buP1 and the lowest at 375 for PHTbuN1 polyelectrolytes confirming the high thermal stability of these mixed polyelectrolytes (Fig. 5.34). TGA, DTG, and DTA curves of FeCl3-doped PPy and PPy/Ag/BN have evidenced [11] the oxidative degradation of the PPy chains under thermal conditions and volatilization of FeCl3 (Fig. 5.35). The glass transition temperature of the polymer is reflected from a broad exothermic peak at B285 for PPy in the DTA curve. In the DTG curve, peaks at 64, 283, and 604 with 42, 78, and 131 mg/min weight loss respectively are due to the decomposition of PPy. The thermal stability of the composite is higher than that of PPy observed in the TGA curve. As a result of the interaction between PPy and BN/Ag, the decomposition temperature is shifted. The evaporation of the water molecules from the polymer framework results in a weight loss of B3% at 50 150 . A second weight loss of B8% at 151 400 corresponded to the loss of the dopant and organic molecules. The weight loss observed between 401 and 650 was

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FIGURE 5.34 Thermal studies of HTPB and Fe-HTPB; (A) TGA and (B) DSC plots [30]. Reproduced with permission from Hladysh S, Murmiliuk A, Vohlidal JI, Havlicek D, Sedlarik V, Stepanek M, et al. Combination of phosphonium and ammonium pendant groups in cationic conjugated polyelectrolytes based on regioregular poly(3-hexylthiophene)polymer chains. J Eur Polym J 2018;100:200 8. Copyright 2018.

due to polypyrrole degradation. The amount of BN/Ag in the nanocomposite is calculated by the remaining weight of B40%. The TGA graph of polydiphenylamine (PDPA) particles show (Fig. 5.36) an unexpected initial weight loss probably because of the loss of the residual water molecules and dopants incorporated during washing and drying [14]. After 250 , drastic weight loss occurs due to the thermal degradation of the PDPA backbone. The degradation occurs in two steps, at temperatures above 600 and at 600 650 . The degradation at 600 is due to the varied content of lowmolecular-mass oligomers of the PDPA polymer.

5.1.5.2.2 Differential thermal analysis and differential scanning calorimetry applications Cellulose linear acyl esters have isobutyric substituents with a terminal branched structure closely packed in helical molecular chain conformation. They are used as thermoplasticized materials and their properties can be varied by changing the carbon number (C) of acyl substituents [31]. The thermal decomposition behavior of cellulose ester was studied in TGA, which indicated that the acyl esters are quite stable thermally. Again, DSC 1st and 2nd heating curves of the cellulose ester (Fig. 5.37) show endothermic peaks with increasing carbon number of the acyl group indicating that both the Tg and Tm values decrease. It is, however, observed that a shorter branched acyl side chain increases the thermal phase transition temperature. By keeping the molecular weights constant, but varying the comonomer contents, a series of ethylene/1-hexene copolymers was synthesized under the catalytic condition of a metallocene [32]. The copolymer series were analyzed with solid-state DSC, solution DSC, and crystallization analysis fractionation (Crystaf).

Techniques used for physical and thermal analysis of polymers

FIGURE 5.35 Thermal gravimetric analysis of (A) PPy and (B) PPy/Ag/BN [11]. Reproduced with permission from Sultan A, Mohammad F. Chemical sensing, thermal stability, electrochemistry and electrical conductivity of silver nanoparticles decorated and polypyrrole enwrapped boron nitride nanocomposite. Polymer 2017;113:221 32. Copyright 2017.

Comparing the experimental solution DSC exotherms with the Crystaf profiles under similar crystallization conditions, it is seen (Fig. 5.38) that at the same cooling rate (0.18 C/min), samples with low levels of short chain branching show differences, but with increasing branching the discrepancy becomes less significant. A good agreement is also observed between the Crystaf profiles and the solution DSC exotherms of the metallocene copolymers at cooling rates of 2.5 and 5 /min and heating rates of 10 /min. Similarly, the Crystaf and solution DSC profiles of a Ziegler Natta linear low-density polyethylene (LLDPE) sample

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FIGURE 5.36 Temperature dependence of the weight loss of polydiphenylamine form in the course of heating to 800 at a rate of 5 /min [14]. Reproduced with permission from Kim MH, Bae DH, Choi HJ, Seo Y. Synthesis of semiconducting poly (diphenylamine) particles and analysis of their electrorheological properties. Polymer 2017;119:40 9. Copyright 2017.

show good agreement when crystallized at the same cooling rate of 0.28 oC/min. However, to investigate the fractionation mechanism in Crystaf, solution DSC is used. In DSC thermograms of chitosan and acylated chitosans, the chitosan shows (Fig. 5.39) a broad endothermic peak around 84 due to the vaporization of the water and a decomposition peak at 298 [16]. In the thermogram, a particular glass transition is not observed for H-chitosan. The thermal decomposition of H-,    D-, and L-chitosans result in exothermic peaks at 225 , 246 , and 255 respec tively. The acylated chitosans are stable below 225 . In disulfonated poly(arylene ether) and PI copolymers [18], DSC thermograms measure the tightly bound water contents. The water content depends on the chemical backbone as well as the strength of the conjugate base. The water content is also dependent on the nature of the sulfonic acid groups present. During the study, Nafion was taken as a standard example. The tightly bound water present in Nafion is in the least amount per sulfonic acid group present in the Nafion structure. The chemical structure of Nafion consists of hydrophilic sulfonic acid groups and a highly hydrophobic, fluorinated, relatively flexible backbone. The conjugate base of perfluorosulfonic acid is weak with a reduced effective charge

FIGURE 5.37 DSC thermograms of cellulose esters; first heating scan (left) and second heating scan (right) [26]. Reproduced with permission from Danjo et al. Copyright 2018.

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FIGURE 5.38 Solution DSC exotherms of poly(ethylene-co-1-hexene) samples obtained in thermally conductive board at a cooling rate of 0.1 8 /min [32]. Reproduced with permission from Sarzottia DM, Soaresa JBP, Simona LC, Britto LJD. Analysis of the chemical composition distribution of ethylene/a-olefin copolymers by solution differential scanning calorimetry: an alternative technique to Crystaf. Polymer 2004;45:4787 99. Copyright 2004.

density. The tight binding of water in Nafion results from a weak conjugate base, the highly hydrophobic backbone, and the flexible distance between the backbone and the sulfonic acid group. In the multiblock copolymers, the sulfonic acid groups are arranged in a concerted array. The local effective negative charge density is higher in the multiblocks as compared to the random copolymers, which resulted in an increased tightly bound water content in the multiblocks. Under fully hydrated condition, the influence of the degree of sulfonation on the melting endotherm of the freezable water was investigated through DSC study, which suggested the presence of an increased amount of freezable water with increases in the ion content. The freezable water is bound loosely or exists as free water content. The thermal properties of poly(ethylene 2,6-naphthalate) (PEN)/PC 50/50 (wt.) blends were studied by Wozniak-Braszak et al. [33]. The DSC thermograms (Fig. 5.40) show two glass transitions during two times heating indicating the immiscibility of the homopolymer component of the blend. The lower glass

Techniques used for physical and thermal analysis of polymers

FIGURE 5.39 DSC thermograms of chitosan and acylated chitosans [16]. Reproduced with permission from Zong Z, Kimura Y, Takahashi M, Yamane H. Characterization of chemical and solid state structures of acylated chitosans. Polymer 2000;41:899 906. Copyright 2000.

transition temperature, Tg1 5 387 K, is associated with the transition temperature of the two phases of the amorphous PEN, namely glassy to rubbery phase. The second higher glass transition temperature, Tg2 5 417 K, is related to the glass transition of pure PC. Since no crystalline exothermal or melting endothermal peaks are observed during two times heating, it can be concluded that the blend was completely amorphous. Golitsyn et al. [34] studied the crystallization and melting behavior of PEG networks using DSC thermograms. Compared to the linear uncrosslinked precursors, PEG networks have a wider distribution of crystal sizes and a smaller lamella thickness as the melting endotherms of the networks showed broad peaks with a shift to lower temperatures (Fig. 5.41). Also, the degree of crystallinity in the PEG network increased with decreases in crosslinking density.

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FIGURE 5.40 DSC thermograms obtained for the PEN/PC 50/50 (wt.) blends (A) without and (B) with a compatibilizer on heating and cooling at the first and the second run [33]. Reproduced with permission from Wozniak-Braszak A, Jurga K, Nowaczyk G, Dobies M, Szostak M, Jurga J, et al. Characterization of poly(ethylene 2,6-naphthalate)/ polycarbonate blends by DSC, NMR off-resonance and DMTA methods. Eur Polym J 2015;64:62 9. Copyright 2015.

Techniques used for physical and thermal analysis of polymers

FIGURE 5.41 DSC thermograms of different PEG networks together with their respective melting temperatures Tm and crystallinities XC, DSC [34]. Reproduced with permission from Golitsyn Y, Pulst M, Samiullah MH, Busse K, Kressler J, Reichert D. Chracterization in PEG networks: the importance of network topology and chain tilt in crystals. Polymer 2019;165;72 82. Copyright 2019.

DSC spectra in isomorphous poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with different HV contents, namely P(HB-co-4.9% HV) and P(HB-co-9.4% HV), show a double melting peak (Fig. 5.42) [35]. The similarities in melting points of the polymers (P(HB-co-4.9% HV) at 161.9 and 171.6 and P(HB-co-9.4% HV) at 162.4 and 172.6 ) indicate the formation of similar structures in the polymers. However the differences in fusion enthalpies of the polymers (fusion enthalpies for P(HB-co-4.9% HV) are 33.3 and 42.5 J/g and for P(HB-co-9.4% HV) are 26.8 and 55.9 J/g) indicate that the crystallization process is different in the two kinds of polymers. The synthesis of poly(ester amide)s by the condensation of citric acid (CA), dglucono-δ-lactone (GL), and various amino acids (AA) under melt condition was studied by DSC, employing a reaction time # 20 min at 160 [36]. It was observed that the glass transition temperature (Tg) increased in the copolymerization of AA with CA and GL compared to the polymerization of CA/GL only and the increase in the Tg could be correlated to the monomer structure of AA having both hydrophobic, nonreactive side chains and hydrophilic hydroxyl or thiol side

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FIGURE 5.42 DSC curves of P(HB-co-4.9%HV) and P(HB-co-9.4%HV) at a heating rate of 5.0 /min [35]. Reproduced with permission from Zhu H, Lv Y, Duan T, Zhu M, Li Y, Miao W, et al. In situ investigation of multiple endothermic peaks in isomorphous poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with low HV content by synchrotron radiation. Polymer 2019;169:1 10. Copyright 2019.

chains. The hydroxyl and thiol side chains are capable of the formation of additional ester linkages, thereby increasing the crosslinking density, which resulted in an increase in the Tg (Fig. 5.43). While preparing thermoplastic starch-based composite films incorporating microcrystalline cellulose (MCC) into a thermoplastic hydroxypropyl starch (TPS) matrix, the thermostability of the composite film was studied with the DSC behavior of the films. The Tg for the pure TPS film is higher compared to that of the MCC/TPS [37]. The semicrystalline behavior of the polymer under fused deposition condition is monitored by DSC using a three-phase model including a mobile amorphous fraction, a rigid amorphous fraction, and a crystalline fraction [38]. Polyphenylene sulfate (PPS) in the form of a monofilament was chosen for the study as it is a chemical- and temperature-resistant thermoplastic polymer. During the heating curve, an exothermic peak was observed in between the Tg (90 ) and the melting temperature (279 ) indicating cold crystallization during heating (Fig. 5.44). The decreasing cold crystallization peak with increasing temperature reveals the higher level of crystallinity in the polymer.

Techniques used for physical and thermal analysis of polymers

FIGURE 5.43 Glass transition temperatures for the melt polycondensation between CA, GL, and amino acids with hydroxyl and thiol side chains of Ser, Tyr, and Cys. The reaction time refers to the ITS [36]. Reproduced with the permission from Jongh PAJMD, Paul PKC, Khoshdel E, Wilson P, Kempe K, Haddleton DM. High Tg poly(ester amide)s by melt polycondensation of monomers from renewable resources; citric acid, D-glucono-δ-lactone and amino acids: A DSC study. European Polymer Journal 2017; 94: 11-19. Copyright 2017.

5.1.6 Quantum chemical calculations 5.1.6.1 Basic principle The interaction of nuclei and electrons in a molecular geometrical arrangement with minimum energy is assigned by quantum chemistry [39]. The basis of all quantum mechanical methods is the Schrodinger equation, which relates the potential energy and kinetic energy of electrons and the probability of finding an electron in a given volume. The motion of the electrons depends on instantaneous interactions between the electrons and the field created by all other electrons. The approximate correlation between the electrons is introduced by DFT, according to which, the exchange and correlation energies of a uniform electron gas are dependent on its density. However, DFT is applicable to polymers of moderate size.

5.1.6.2 Applications Natural bond orbital analysis and the calculation of the interatomic distances between the C H and O 5 C groups in PCL crystalline indicate that there are

197

FIGURE 5.44 (A) Heat flow data for a printed PPS part performed at a heating and cooling rate of 10 / min. (B) Cold crystallization peaks for as-printed PPS compared to samples annealed at 95 , 110 , and 120 [38]. Reproduced with permission from Emily et al. Copyright 2019.

Techniques used for physical and thermal analysis of polymers

three kinds of weak intermolecular interactions between the CH2 and C 5 O groups in the PCL chain [9]. The results of QCCs using the cartesian-coordinate tensor transfer method to assign the THz spectra of PCL suggest that the peaks at 47 and 67 cm21 correspond to the atomic motions of the C 5 O 1 CH2 moiety derived from the weak CH/O 5 C hydrogen bonding. QCC and the assignment of the THz spectra of PCL shows that the crystal structure must be taken into account for the calculation of the THz spectrum. The polarization spectra are measured by rotating the stretched PCL samples in the direction of the linearly polarized THz waves. The peaks at 47, 67, 111, 153, 197, and 216 cm21 arise due to the perpendicular polarization to the c axis (fiber axis) and the peak at 67 cm21 appears because of the parallel polarization. The main atomic motions of the vibrational modes of the crystalline PCL in the THz results in bands at 44 and 64 cm21. The peak at 64 cm21 reflects the CH2 group of one pattern (Fig. 5.45), which is assigned to mainly the bending mode of the CH2 (adjacent to the ester groups) and COO groups. The peak at 44 cm21 (\) is because of the CH2 group of another pattern assigned to the outof-plane bending mode of C 5 O and CH2 groups (adjacent to the methylene groups). The perpendicular and parallel modes of THz spectra in PCL indicate the intermolecular and intramolecular vibrations of different groups in PCL respectively. PCL has C H/O 5 C inter- and intramolecular hydrogen bondings with different strengths between the CH2 and C 5 O groups. Moreover, the C H/ O 5 C hydrogen bonding stabilizes the lamellar structure in the cases of PHB and its copolymers with propyl side chains of hydroxyhexanoate (HHx) units. The difference in melting points between the two polymers, PCL and PGA, is due to the difference in the number of intermolecular interactions within the lamellar thickness. Thus the strength of hydrogen bonding has a significant contribution to the evaluation of crystallinity. The intermolecular interactions in PVA iodine complexes were studied by QCC [24]. The driving force to stabilize this complex is the OH...I hydrogen bonding interaction between the OH groups of the PVA and iodine. The orbital interactions were calculated and revealed that the highest occupied molecular orbital (HOMO) of I2 3 interacts with the LUMO of PVA to transfer the charge from I2 3 to PVA. However, the interaction strength is strongest in the syndiotactic PVA iodine pair (Fig. 5.46). The hydrophobic nature of a SiO2-PMMA composite surface was studied using QCC [40]. The AFM image showed that the surface contact inhibits the penetration of water as the area accessible to water is low. QCC using DFT was performed to study the interaction between the SiO2-PMMA composite and stainless steel. The electron density from HOMO is localized on the SiO2 and the C 5 O group of the MMA of the composite. These two are the active sites for bonding with the steel surface. As the LUMO is localized on the entire molecule, the centers are available for accepting electrons from stainless steel. During the chemical interaction, the excess negative charges on the oxygen atoms of SiO2 and MMA are responsible for donating electrons to the stainless-steel surface

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FIGURE 5.45 (A) Temperature-dependent THz spectra of PCL in the temperature range of 35 65 (top) and 20 100 (bottom). (B) Plots of the wavenumber of the peak at 47 cm21\ (top) and the peak at 67 cm21\, // (bottom) versus temperature. (C) Temperature-dependent polarization (\) THz spectra of PCL (bottom) and their second derivatives (top) in the temperature range of 20 90 . (D) Temperaturedependent polarization (//) of the THz spectra of PCL (bottom) and their second derivatives (top) in the temperature range of 20 90 [9]. Reproduced with permission from Funaki C, Yamamoto S, Hoshina H, Ozaki Y, Sato H. Three different kinds of weak C-H/O1/ 4C inter- and intramolecular interactions in poly(ε-caprolactone) studied by using terahertz spectroscopy, infrared spectroscopy and quantum chemical calculations. Polymer 2018;137:245 54. Copyright 2018.

Techniques used for physical and thermal analysis of polymers

FIGURE 5.46 The structural change of -PVA I2 3 model during geometry optimization [24]. Reproduced with permission from Takahama T, Saharin SM, Tashiro K. Details of the intermolecular interactions in poly(vinyl alcohol)-iodine complexes as studied by quantum chemical calculations. Polymer 2016;99:566 79. Copyright 2016.

resulting in an excellent coating ability. The binding properties between the two reacting species are determined by the orbital energy gap between the HOMO and LUMO of the coating and the stainless steel. The interaction between the HOMO of the SiO2-MMA pair and the LUMO of the metal-atom cluster is weaker than that between the HOMO of the metal-atom cluster and the LUMO of SiO2-MMA pair. The lower the value of orbital energy, the higher will be the tendency for electron back donation from the metal-atom cluster to SiO2-MMA compared to electron donation from SiO2-MMA to metal-atom cluster. Thus the strength of the bonding is governed by the ability of SiO2-MMA to accept electrons from the d orbital of the metal-atom cluster. The enhanced physical adsorption between steel is also affected by the dipole moment. The larger the dipole moment of a molecule, the stronger will be the intermolecular attraction. The dipole moment of the SiO2-MMA pair is higher (about 5.63 debye) than that of H2O (1.85 debye), resulting in the relocation of water from the steel surface, thus, increasing the adsorption between SiO2-MMA and steel. The chemical structure of a series of acylated chitosans was studied by techniques like elemental analysis, IR, 1H-NMR, 13C-NMR, and gel permeation chromatography. Dynamic mechanical analyses showed that all acylated chitosans have two phase transitions in the solid state. The first transition at 210 , 242 , and 240 are assigned due to the Tg of H-, D-, and L-chitosans respectively. The second one around 888 may be because of the transition related to the structure formed by the side chains. Wide angle X ray scattering analyses indicate that these polymers form a layered structure in the solid state and the layer d spacing increases linearly with increases in the length of the side chains. Further, the existence of an H-bonding interaction between the hydrogen atom of the OH group and the iodine ion was revealed by local molecular orbitals calculation for optimized crystal structure.

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5.1.7 Gas permeation behavior The gas permeation behaviour of 6FDA-based PIs shows that DAM-Based polymers have higher CO2 permeabilities, whereas MPDA and DABA groups decrease the CO2 permeability 100 psia. The introduction of CH3 groups on to 6FDA-mPDA increases the CO2 permeability from 6.6-fold (He) to 117-fold (CH4), but decreases the selectivity from 1.8-fold (O2/N2) to 19.7-fold (He/CH4). The introduction of a COOH group in the diamine moiety of 6FDA-mPDA to form 6FDA-DABA resulted in the decrease of permeability with no change in the selectivity with different gas pairs [28]. The gas permeability is lowest for hydrophilic HPESU, but it shows the highest rubbery portion ( poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) ) may move between the benzene rings and prevents the formation of charge transfer complexes or π π stacking, but it shows the highest CO2/N2 and CO2/CH4 selectivity of 34.9 and 34.6 respectively may be because of its affinity toward CO2 through its PE oxide units. TPESU has a high gas permeability as well as good selectivity of some gas pairs (highest O2/N2 selectivity of 6.0 and the second highest CO2/CH4 selectivity of 33.8) as it has trimethyl benzene groups in its structure. TPESU has O2 and CO2 permeabilities of 1.33 and 5.74 Barrer at 35 respectively. Further, PESU has a low gas permeability and average selectivity because of its linear ethersulfone chain structure [23].

5.2 Conclusion In this chapter, the current state of chemical analyses for polymeric substances is briefly outlined. The chemical analysis of a polymer is an essential approach to predict the behavior of the polymer as well as to control its transformation. This chapter covers analytical techniques that include spectroscopic techniques such as IR, NMR, and X-ray diffraction. Further, brief discussions about microscopic analysis (SEM and TEM) and thermal analysis (TGA and DSC) are presented herein. We also try to briefly outline QCC, which is the theoretical basis of the chemical analysis of a polymer. During thermochemical studies, polymeric materials suffer some disadvantages like the limitation of maximum temperature, poor sustainability, handling difficulties, and so forth. These difficulties can be overcome by modifying the substance by adding some additives in a low percentage into the polymer matrix. In the past few years, polymer nanocomposites have received an enormous amount of interest due to their significant physical and chemical properties. The basic difference between polymer nanocomposites and traditional polymer composites is that there is a large amount of interfacial area between the matrices and the fillers in nanocomposites. Thus lots of research interests have been focused [41 45] on studying the physical, chemical, and mechanical properties of polymeric nanocomposites. The study of electromagnetic microwave absorption properties of poly(vinylidene fluoride)/magnetite/carbon

References

nanotubes (PVDF/Fe3O4/CNT) and poly(vinylidene fluoride)/magnetite/graphene (PVDF/Fe3O4/GN) [42], energy harvesting study of PVDF-HFP nanocomposites containing ceramic BaTiO3:h-BN nanolayers [43], study of the sensing performance of PVDF nanocomposites in presence of solvent vapors [44], electromagnetic shielding study of conducting polymer nanocomposites [45] are such examples where the thermal, mechanical, physical properties of polymeric nanocomposites are elaborately demonstrated. Although adequate works on polymeric nanocomposites have been done in the past decades, there are still lots of studies that need to be done. Thus it is expected that in the near future more fruitful results will come up in terms of morphology, strength, physical as well as chemical properties of polymeric materials.

Acknowledgment The author is highly thankful to Assam Kaziranga University for providing all the supports while writing this article.

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6

Theoretical simulation approaches to polymer research

Tao Wei1 and Chunlai Ren2 1

Department of Chemical Engineering, Howard University, Washington, DC, United States National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, P.R. China

2

6.1 Introduction Computer simulations have become an important tool to complement experimental research on polymers. In general, simulations can be categorized into particlebased and field-based approaches. Particle-based approaches such as molecular dynamics (MD) [1,2], Monte Carlo (MC) [1,3
5], and dissipative particle dynamics (DPD) [6,7], utilize calibrated force field parameters for pairwise interactions of atoms or particles to elucidate their trajectories (in MD and DPD), structural changes (in MD, DPD, and MC), or ensemble averaged properties (in MC) according to specific conditions such as target temperature, pressure, volume, or chemical potential. These techniques have unique advantages in terms of quantitative analysis, and are capable of taking into account the complicated polymer chemistry and molecular structure of polymers as well as the effects of the external environment in which they are immersed, for example, the chemical nature of solvents and/or the presence of functionalized nanoparticles. On the other hand, field-based approaches, founded on statistical mechanics, integrate multiple-body interactions into one-body interactions in the presence of an effective field. Typical examples include density functional theory [8,9], self-consistent field (SCF) theory [10
16], field theory [17,18], and molecular theory (MT) [19
21]. In addition to the remarkable efficiency in large-scale simulations, field-based methodologies are capable of elucidating thermodynamics more explicitly. Polymeric materials display distinguished characteristics at various length and time scales. According to the specific system and properties of interest, different simulation methodologies can be applied. Some specific properties that can be studied include the strength of individual chemical bonds, the size of a subnanopore at an angstrom level, the length of the chain gyration radius in nanometers, phase morphologies in polymer melts, blends, solutions, and polymer nanocomposites (PNC) from micrometers to millimeters. Simulations are also used to probe the dynamics of various time scales relevant to the properties of different polymers, ranging from Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00006-8 © 2020 Elsevier Inc. All rights reserved.

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femtoseconds to seconds, or even hours for large-scale ordering processes such as phase separation in blends, crosslinking in polymer chains or between monomers in polymer membranes, and nanoparticle aggregation in PNC. This chapter focuses on the description of particle-based approaches (atomistic MD and coarse-grained (CG) DPD) and a field-based approach (MT), along with typical applications including their use in subnanoporous cross-linked polymer membranes, semiconducting polymers, and polymer brushes.

6.2 Methodologies and applications 6.2.1 Molecular dynamics simulations Atomistic MD is an important simulation method in polymer research due to its ability to provide insights on spatial details at the atomic level and at a time resolution spanning from subnanoseconds to microseconds or even milliseconds [22
31]. MD has some unique advantages over another kind of atomic-scale simulation known as MC, since MD can estimate dynamic properties, for example, transport coefficients [22,23,32], rheological properties [33,34], and spectra [35]. Extensive research works [22,31,36
41] using atomistic MD simulations have been carried out to guide the experimental design of polymer membranes with improved performance. MD simulations numerically solve Newton’s equations of motion for a system of N interacting atoms to compute their trajectories and positions; Fi ; i 5 1; . . . ; N mi 
  Fi 5 2 rri V rj

r_i 5 vi ; v_i 5

(6.1) (6.2)

where mi is the mass of atom i; ri represents its position vectors, and Fi stands for the force acting on atom i, derived from the interatomic potential, that is, force field potential (V) related with 3N atomic coordinates. The force field potential is described in terms of molecular mechanics, which defines the bonded and nonbonded potentials. According to their various physical modes of motion, the bonded potentials consist of bond (two-body interaction), angle (three-body interactions), torsion (four-body interactions), and improper dihedral (four-body interactions) (see Fig. 6.1). The nonbonded potentials are typically composed of Lennard
Jones and Coulombic interactions. Several classical nonreactive force field parameters are applied in full-atom simulations (MD or MC) of soft matters, the example, Chemistry at Harvard Macromolecular Mechanics (CHARMM) [42
44], Assisted Model Building with Energy Refinement (AMBER) [45
47], optimized potentials for liquid simulations-all atom (OPLS-AA) [48,49], consistent valence force field (CVFF) [50], polymer consistent force field (PCFF) [51], and DREIDING [52]. CHARMM [42
44] is herein used as an example to

6.2 Methodologies and applications

FIGURE 6.1 Molecular mechanics, namely, bond, angle, dihedral angle, and improper dihedral angle.

illustrate the formalism of the total potential energy of a molecular system with covalent interactions: X Kb ðr2r0 Þ2 1 Kθ ðθ2θ0 Þ2 bondsX angles X 1 K[ ð1 1 cosðn[ 2 δÞ 1 Kψ ðψ2ψ0 Þ2 dihedrals impropers 2 ! !6 3 12 X qi qj X 1 ε4 rrm 2 2 rrm 5 1 4πε0 εr i.j i.j

V5

X

(6.3)

In Eq. (6.3), the first term represents the bond stretching in terms of a harmonic function; the main parameters of which are a harmonic bond constant Kb and an equilibrium bond distance r0 . The second term is a harmonic angle potential, which is defined in terms of a harmonic angle constant Kθ and an equilibrium angle θ0 . The third term represents the torsion (i.e., proper dihedral) interaction in terms of the Ryckaert
Bellemans function with parameters of a potential constant K[ and a phase shift δ. The fourth term is the improper dihedral, which takes into account the out-of-plane bending of the planar groups via a harmonic potential constant Kψ and an out-of-plane angle ψ0 . The fifth term is associated with Lennard
Jones interactions, which are given in terms of a distance rm where the potential reaches its minimum energy ε, due to the London dispersion and the short-range repulsion originating from the Pauli exclusion principle. The last term of Eq. (6.3) is the long-range electrostatic interaction from partial atomic charges (qi and qj of atom i and j), with ε0 standing for the permittivity of vacuum and ε for the dielectric constant. The potential parameters and partial charges can be estimated from quantum simulations or experiments, for example, enthalpies, free energy, and vibration spectra. In nonreactive potentials, the atom types are used to distinguish the potential parameters according to different bond conditions.

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MD simulations are performed to generate configurations in an ensemble, for example, microcanonical ensemble (NVE), canonical ensemble (NVT), isothermal
isobaric ensemble (NPT) and grand canonical ensemble (μVT) [35,53
55]. An appropriate thermostat and/or barostat need to be applied in simulations according to the type of ensemble used [35,53
55]. A number of algorithms have been proposed for the numerical integration of the equations of motions [35,53,54]. The velocity-Verlet algorithm is a widely-adopted method [35,53,54]. Several powerful software packages are available for MD simulations such as Groningen Machine for Chemical Simulations (GROMACS) [56], Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [57], CHARMM [42
44], Nanoscale Molecular Dynamics (NAMD) [58], AMBER [45
47], and Highly Optimized Object-oriented Many-particle Dynamics—Blue Edition (HOOMD-blue) [59]. These simulation codes are supported by parallel computations to achieve large-scale computing. Statistical mechanical methods such as MC utilize the same force field potentials to simulate microscopic processes and predict polymer morphologies by allowing atoms to move within a certain thermodynamic ensemble. Atomistic MD simulations have played a critical role in the design of polymer membranes. In the past decade, membrane technologies such as reverse osmosis (RO) [60], forward osmosis [61], and nanofiltration (NF) [60] have demonstrated advantageous flexibility and efficiency in water treatment. Aromatic polyamide (PA) thin-film membranes have been widely used with commercial success in water desalination and purification. In water desalination, the majority of pores in PA membranes have a pore radius of B0.20 nm, which results in water
salt separation and selectivity in polymer membranes [62]. Atomistic MD simulations [22,41], which can reach an atomistic scale, have proved indispensable in facilitating the experimental investigations of these polymer membranes. Atomistic MD can illustrate molecular transfer phenomena [22] through polymer membrane subnanopores, salt
water separation on the membrane surfaces, the microscopic structure of membranes [22], and the surface fouling mechanism [41]. For PA membrane experimental synthesis, the crosslinking process of monomers takes place at the solution interface, involving coupled chemical reactions and diffusion. Phase-transfer catalysts are usually added to aid monomer diffusion [63]. The interfacial polymerization can take from a few seconds to a minute, which goes beyond the timescale of current atomistic MD simulations. A reliable atomistic model for a cross-linked polymer membrane with properties (density, monomer ratio, crosslinking degree, water content, pore size distribution, water diffusion, and salt rejection) that match experimental measurements is key to all subsequent investigations in simulations. To establish a reasonable polymer membrane structure using atomistic MD simulations, the most common strategies [22,36
40] consist of crosslinking monomers or the hybrids of monomers and linear PA chains in different initial configurations with the heating/annealing MD protocol or through a hybrid MC/MD approach without explicit solvents. These strategies can produce a reasonable PA structure in terms of the morphologies and other structural properties (e.g., the pore size distribution) of membranes as

6.2 Methodologies and applications

well as membrane performance (water flux and salt rejection capability). For example, Wei et al. performed atomistic MD simulations to investigate a PA RO membrane in desalination [22]. In their simulations, the cross-linked polymer membrane structure was constructed through an efficient protocol of hierarchical crosslinking. The crosslinking of monomers (m-phenylenediamine and hydrolyzed trimesoyl chloride) was performed in a small simulation cell, which was then expanded to a large cell by duplicating itself along each axis for further crosslinking (Fig. 6.2) [22]. The constructed cross-linked PA membrane exhibits different characteristics (i.e., pore size distributions, water diffusion, and salt exclusion) [22] that agree with the results of the experimentally synthesized membranes [62,64]. The results of the MD simulations [22] showed three main structural components inside the cross-linked polymer membrane including the primary two-layer thin slip structure with the length of not more than two linked benzene rings due to the short-range anisotropic π
π interactions among the aromatic benzene rings (Fig. 6.3A). The water diffusion coefficient (Dinter) on the PA polymer membrane surface is about 50% of its value in bulk water; however, inside the inhomogeneous polymeric structure of the membrane, the water molecules exhibit heterogeneous diffusivities (Fig. 6.3B). The less cross-linked local area inside the membrane offers fast pathways for the more coordinated water molecules with

FIGURE 6.2 (A) Crosslinking reactions and the definition of polymer degree, n (0 # n # 1; n 5 1 for fully cross-linked polyamide and n 5 0 for the fully linear one) and (B) hierarchical crosslinking process [22]. Copyright 2018. Reproduced with permission from ACS.

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FIGURE 6.3 (A) PA membrane in static water [water in two reservoirs (red), polymers (blue), water inside membrane (green), and water in the fast pathway (red)], and the main structural components; (B) water diffusion coefficients in Z-direction in different regions (the bulk; polymer
water interface; internalmembrane); (C) zoom-in water fast pathway colored red; (D) pore-size distribution; and (E) density profiles of water, polymer atoms, and ions (Na1 and Cl2) across the PA membrane. Z-axis is normal to the polymer surfaces [22]. PA, Polyamide. Copyright 2018. Reproduced with permission from ACS.

increased diffusion (Fig. 6.3C) [22]. In agreement with experiments [62], the simulated PA membrane [22] shows pore size distribution with the majority of the pore radiuses at B0.20 nm, giving rise to the observed membrane selectivity (Fig. 6.3D). The simulation results [22,65] also revealed that the feasibility of water
ion separation through such subnanopores is indeed controlled by the strength of the dehydration free energy of the ions in addition to the size of hydration cluster. The water molecules were found to associate with other water relatively flexibly through hydrogen bonding with smaller hydration free energy compared to the hydrated ions. Atomistic MD simulations [22,36] revealed that inside the PA polymer membrane, the water coordination number, that is, the number of water molecules inside a hydration shell, is 2.4
2.7, compared to the value of 4.4
4.5 for water in the bulk and 4.0 for water molecules along the fast pathway. Therefore a pore with a 0.2 nm radius allows for limited water diffusion, because the water hydration cluster is larger than the majority of the pore size. The results also indicated that hydrogen bonding among water molecules facilitates their transfer through the PA membrane subnanopores [22]. In the meantime, the 0.2-nm pores block the passage of the ions, since the pore is actually smaller than the radius of a hydrated Na1 or Cl2 ion. The simulations showed that the membrane
water interface is effective in rejecting ions from the bulk saline solution (Fig. 6.3E). Moreover, the presence of the fast pathways for water permeation does not affect the membrane’s salt rejections [22]. The irreversible adsorption of natural organic matter and microorganisms onto membrane surfaces presents a critical issue limiting polymer membrane performance,

6.2 Methodologies and applications

for example, leading to the loss of water flux in desalination and water purification [66]. Fundamental study of interfacial phenomena between biomolecules and polymer membranes is urgently needed in the development of efficient antibiofouling membrane materials. Leng et al. performed MD simulations [41] to study the fouling mechanism of alginate on a PA membrane. Their results [41] showed that the ions (Na1 and Ca21) can exhibit strong binding with the carboxylate groups on the PA surface and that the formation of an ionic binding bridge leads to PA-alginate fouling. Takizawa et al. [31] used a combination of experiments and MD simulations to investigate the antibiofouling mechanism of a multiwalled carbon nanotube-PA nanocomposite (MWCNT-PA) RO membrane. On the composite membrane, the reduction of bovine serum albumin (BAS) protein adsorption was caused by the addition of the MWCNT in the PA membrane, which led to a stiffer PA structure, a smoother surface morphology, and an increased hydrophilicity with the formation of an interfacial water layer [31].

6.2.2 Dissipative particle dynamics simulations DPD simulations [67] play an important role as an intermediate simulation technique to bridge the gap between atomistic simulations and macroscopic simulations. The DPD model maps a cluster of atoms into a single bead, thereby substantially reducing the computational load to achieve larger time and length scales than in conventional MD simulations, while keeping enough precise resolution compared to macroscale simulations [6]. Compared to the hard sphere models in CG MD simulations, for example, MARTINI [68] or Lennard
Jones potential in atomistic MD simulations, CG particles of DPD simulations represent segments or monomers of soft interaction potentials. DPD particles are subject to pairwise conservative, dissipative, and random forces [69,70]. The employment of the CG representation and the soft-core potentials enable simulations of large temporal and spatial scales. Moreover, the forces among beads are in the short-range with simple analytical forms, resulting in fast computation per time step and, hence, providing an opportunity to expand the simulation time scale [6]. In general DPD, CG particles represent segments, monomers, or solvent molecules rather than single atoms with simplified pairwise soft conservative (FCij ), R dissipative (FD ij ), and random (stochastic) forces (Fij ) [7]. The time evolution of each DPD particle is calculated by Newton’s second law, similarly to MD simulations. The force acting on particle i is mi

X d 2 ri R 5 FCij 1 FD ij 1 Fij dt2 j6¼i

(6.4)

where the sum is over all other particles j located within the cutoff distance rc ; mi and ri are the mass and the position vector of particle i, respectively; and t is time. In the standard implementation, the pairwise conservative interaction force

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FCij applies a soft pure repulsive (excluded volume) force through a distance rij between beads i and j;   Fcij 5 aij ωC rij eij 80 1 > > r < ij   @1 2 A rij , rc ωC rij 5 rc > > : 0 rij . rc

(6.5)

(6.6)

  where aij is the maximum repulsion force between beads i and j; ωC rij stands for the weight function of the conservative force, ranging from 0 to 1, which represents a simple decaying function of the distance related with the cutoff disðr 2 r Þ tance rc ; eij 5 i rij j is the unit vector pointing from particle j to particle i. The pairwise dissipative or frictional force FD ij takes into account the effects of viscosity, which slows down the particles’ motion with respect to each other;    D FD ij 5 2 γ ij ω rij eij vij eij

(6.7)

  where γ ij represents the friction  coefficient; ω rij is the weight function of dissipative force; and vij 5 vi 2 vj stands for the particles’ relative velocity. The pairwise stochastic force FRij is the thermal or vibrational energy of the system; D

  FRij 5 σij ωR rij ζ ij Δt20:5 eij

(6.8)

  where σij represents the amplitude of the noise; ωR rij stands for the weight function of the stochastic force; Δt is the integral time step; and ζ ij ð 5 ζ ji ) is a symmetrically and uniformly distributed random number with zero mean and unit variance to ensure the total conservation of momentum. In accordance with the fluctuation
dissipation theorem, forces FRij and FD ij are coupled to maintain the Langevin thermostat [7], which yields the relationship:     2 ωD rij 5 ωR rij qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi σij 5 2γ ij kB T

(6.9) (6.10)

where temperature.  kB stands for the Boltzmann constant and T is the  equilibrium  C ωR rij  can be r assigned to the same function of ω in the simple form ij   (ωR rij 5 ωC rij ). In DPD, the solvent effect can be presented with explicit solvent CG particle models [71,72]. It is worth noting that those pairwise DPD forces (FCij , FD ij , and FRij ) conserve momentum and ensure correct hydrodynamic behavior [69,70], which is different from the implicit-solvent method of Langevin dynamics [73] as the latter applies dissipative random (Brownian), drag, and gravity forces to individual particles, and violates momentum conservation. Moreover, to present a polymer structure, additional mechanical forces for bonded-interactions can be included in Eq. (6.4). For example, adjacent polymer CG beads are joined by a

6.2 Methodologies and applications

spring force FðSi;i11Þ , which is defined in terms of the spring constant ks and the equilibrium bond l0 [70,72,74]: FSði;i11Þ 5 2 rUðSi;i11Þ S Uði;i11Þ 5

X1 i

2

ks ðlði;i11Þ 2l0 Þ2

(6.11) (6.12)

where lði;i11Þ represents the bond length between the two adjacent and connected particles (i and i 1 1). In addition, to describe the bond rigidity [74,75] in addition to the spring force FðSi;i11Þ , the angle force FðAi21;i11Þ is applied to the connected polymer CG beads: FAði21;i11Þ 5 2 rUðAi21;i;i11Þ X   kA ½1 2 cos [ði21;i;i11Þ 2 [0  UðAi21;i;i11Þ 5

(6.13) (6.14)

i

where [ði21;i;i11Þ is the angle of the three adjacent particles (i 2 1, i, and i 1 1); [0 is the equilibrium angle; and kA is the harmonic potential constant. DPD simulation parameters can be estimated using experiments such as differential scanning calorimetry [76] or with atomistic MD simulations [74,76
80]. For example, Groot and Warren [6] showed that the key parameter of the interactions (aij) between DPD beads (i and j) will be obtained with Flory
Huggins parameter χij, which, being a function of the solubility, can be determined by experimental measurements. Atomistic MD can also be performed to estimate the solubility of polar and apolar polymer segments using the standard protocol, which has been widely published in the literature [74,76
81]. The equations for the motion of DPD beads can be numerically solved using a modified velocity-Verlet algorithm [75,82]. DPD simulations are implemented in open-source packages such as LAMMPPS [57], GROMACS [56], and HOOMD-blue [59]. Due to their outstanding computational efficiency, DPD simulations have been widely applied to the study of polymers, for example, microphase separation of diblock copolymers [83], polymer
surfactant aggregation [84], hydration of polymer membrane [72], and interactions of polymer brushes with functionalized nanoparticles [71]. An illustration of DPD simulations of semiconducting polymers will be presented here. Organic π-conjugated materials, particularly semiconducting polymers, have attracted considerable attention due to their unique electronic properties, intrinsic mechanical flexibility, and their wide range of electronic applications [85,86] such as in sensors [87], field effect transistors [88], photodetectors [89], photovoltaics [90], and in biological and medical applications [91
93]. Simulations of semiconducting polymers represent one of the major challenges in computational chemistry due to the slow dynamics, large system size, and chain rigidity of polymers, especially for aromatic groups or π-conjugation. DPD simulations have been successfully used to predict the morphologies of semiconducting polymers as a function of the sidechains, backbone rigidity, and

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chain length at the mesoscopic level [69,74,75,94,95]. Some studies [75,94,96] used the DPD method to simulate the phase morphologies of semiconducting/ferroelectric blend polymer films and achieved results in agreement with experimental measurements. Both DPD simulation and experimental results showed that the phase separation formation process is dependent on the ratio of both components (Fig. 6.4) [94]. Cheung and Troisi used different types of DPD parameters in modeling the backbone and the sidechains of semiconducting polymers to investigate the relationship between the molecular architecture and the phase behavior in hairy-rod polymers. Their simulation results showed that the phase behavior is controlled by changes in the molecular structure, particularly the sidechain length and density, and the molecular interactions. Fig. 6.5 shows the effect of the polymer backbone and sidechain structure on phase diagram formation [75]. The growth of the sidechain length stabilizes the laminar phase of hairy-rod polymers (Fig. 6.5A and B), whereas the increase in backbone bead number in each monomer leads to the formation of inverted cylindrical (or honeycomb) phases (Fig. 6.5C and D). Gong et al. applied DPD simulations to investigate the effect of molecular weight and structure factors (backbone and sidechains consisting of flexible spacer, discotic mesogenic core, and peripheral substituents of aliphatic tails) on the phase morphologies of triphenylene-based sidechain discotic liquid crystalline polymers [74]. Their simulations revealed that peripheral aliphatic tails and the incompatibility between the mesogenic cores and substituents controlled the intracolumnar self-assembling patterns of the polymers [74]. AlSunaidi et al. used DPD simulations to predict the liquid crystalline phases (isotropic, smecticA, and crystalline) of both pure-rod fluids and rod
coil copolymers [95]. Their simulation demonstrated that the interplay between microphase separation and

FIGURE 6.4 Phase separation formation at weight ration 10/90. (F8T2)/P(VDT-TrFE): DPD simulation (right); SEM image (left) [94]. DPD, Dissipative particle dynamics. Copyright 2018. Reproduced with permission from Elsevier.

6.2 Methodologies and applications

FIGURE 6.5 Simulation snapshots of (A) A1B2, (B) A1B3, (C) A2B1, and (D) A2B2 polymers. Left hand column shows backbone beads (A) and right hand column sidechain beads (B) [75]. Copyright 2018. Reproduced with permission from RSC.

liquid crystalline ordering is affected by the repulsion of different compositions (i.e., DPD parameter aij or Flory
Huggins χ), which is sensitive to temperature [95].

6.2.3 Molecular theory In the study of polymers, the main constraint of computer simulations is that they demand a large amount of computational time due to the inherent complexity of polymer chains. Therefore theoretical approaches have been applied to the study of polymers such as scaling theories [97,98] and SCF methods [10
16]. The scaling approaches present, in an elegant and simple way, the correct trends under a wide variety of conditions. The SCF methods are more rigorous owing to the incorporation of polymer conformations through adopting the Gaussian chain model, and can provide more microscopic information that the scaling approaches cannot. However, such theoretical approaches are inappropriate for polymers when complex intra- and intermolecular interactions need to be considered. Based on the mean field theory, Ben-Shaul et al. [20] first developed MT, which focuses more on details at the molecular level. In MT, all the inter- and intramolecular interactions are included and the free energy is described as a function of the probability distribution function (PDF) of a large number of polymeric conformations. Taking an end-grafted polymer in water [19] as an example, it is assumed that the only inhomogeneous direction is the one perpendicular to the surface, that is,

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the z-direction. The Helmholtz free energy per unit area (A) of a polymer layer in an aqueous solution is given by: βF Sp Sw βFinter βUrep 2 1 1 52 kB A kB A A A A

(6.15)

The first term in Eq. (6.15) denotes the conformational entropy of polymer chains (sp ), which is given by: X 2 Sp 5σ PðαÞlnPðαÞ kB A α

(6.16)

where σ is the surface coverage of the layer, and PðαÞ is the PDF of finding a chain in conformation α. Given the PDF, the thermodynamic and average structural properties of the polymers can be calculated. For example, the polymer volume fraction profile is given by: X

φp ðzÞ dz 5 σ PðαÞvp ðz;αÞdz

(6.17)

α

where vp ðz;αÞdz denotes the volume that a polymer chain in conformation α contributes to the layer between z and z 1 dz. The second term in Eq. (6.15) is the z-dependent translational (mixing) entropy of the water molecules, which is given by: 2 Sw 5 kB A

ð

dzρw ðzÞ½lnρw ðzÞvw 2 1

(6.18)

where ρw ðzÞ corresponds to the number density of water molecules at z with φw ðzÞ 5 ρw ðzÞvw . The third term in Eq. (6.15) describes the effective intermolecular interactions of the system, which could be specific or nonspecific interactions. The last term in the free energy, Eq. (6.15), represents the repulsive interaction of the system. These are modeled as hard-core repulsions and can be written in the form: ð   Urep 5 β dzπðzÞ φp ðzÞ 1 φw ðzÞ 2 1 A

(6.19)

where πðzÞ represents the position-dependent repulsive interaction field. This field is determined by the requirement that the total volume is filled with either polymer or solvent, that is, the packing constraints are associated with the excluded volume interactions, which are position-dependent:

φp ðzÞ 1 φw ðzÞ 5 1

(6.20)

After minimizing the free energy, the quantities of PðαÞ and φw ðzÞ can be obtained. Together with the packing constraints in Eq. (6.20), a set of coupled equations is solved. In practice, the space is discretized and the integral equations are thereby converted into nonlinear equations, which are solved numerically. It should be mentioned that the input necessary to solve those equations includes the set of conformations of polymer chains, which basically originate in

6.2 Methodologies and applications

the chain architecture and flexibility. For linear and flexible chains, a rotational isomeric chain model [99] is used, where each bond can have three isoenergetic states. By adjusting the different energies of the three states, different flexibilities of the chain can be obtained. In principle, the number of possible conformations of a polymer chain is 3N21. However, from a practical point of view, this is an unrealistic number of conformations. Therefore taking a chain with 50 monomers for example, theoretical results can be obtained using around 106 independent chain conformations, which are generated by a simple sampling method. The chain model provides quantitative agreement between the calculated behavior and the experimental observations of PEG-tethered layers [100]. MT has been generalized for polymers with complicated architectures such as networks [101,102] as well as polymers with complex interactions, for example, ligand
receptor binding [103], polyelectrolytes [104,105], and responsive polymers [106,107]. Particularly, the theory has provided accurate information as confirmed by experiments concerning the amount of protein adsorption on surfacepassivating PEG [108,109] and peptoid layers [110] as well as the passivation of DNA oligonucleotides on PEG layers [111]. Such accuracy fully illustrates the advantages of MT in polymer studies. For example, Fig. 6.6 shows good agreement between the MT simulation and experiments on protein adsorption as a function of surface coverage of polymer brush for different chain lengths [110]. In addition, the theory not only provides a quantitative comparison with experiments, but it also reveals more detailed information that is not directly available in experiments. For instance, the distribution of bound proteins is found to undergo a transition from one layer to two layers mediated by the ligand
receptor binding in Fig. 6.7, where the microstructure of the layer is well represented by MT [103]. Moreover, MT helps identify important factors in a complex system

FIGURE 6.6 (A) An experimental-theoretical analysis of a peptidomimetic polymer brush is adopted forpreventing protein adsorption. Excellent agreement in terms of both the polymer brush structure and the critical chain density was observed. (B) Comparison between the adsorbed fibrinogen mass densities, which are obtained experimentally by ellipsometry (symbols), and data from MT (line trances) for three different polymer brushes. The error bars indicate 6 1 SD [110]. MT, Molecular theory. Copyright 2018. Reproduced with permission from ACS.

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FIGURE 6.7 (A) Schematic representation of streptavidin
biotin binding in the presence of a polymer spacer. Biotin (green) is chemically attached to the free ends of polyethylene glycol (PEO) chains (purple) tethered to a planar interface at the other end-group. Free tethered polymers (thin purple lines) are those not bound to streptavidin. Bound streptavidin proteins (light blue) are linked to two PEO chains (thick purple lines) through their biotins. (B) Volume fraction of the bound proteins as a function of the distance from the surface for three different surface coverages of bound proteins, namely, σb 5 0:006 (full line), 0.009 (dotted line), and 0.012 nm22 (dashed line) [103]. The theoretical results imply that the distribution of bound proteins transitions from one layer to two layers. Copyright 2018. Reproduced with permission from ACS.

FIGURE 6.8 (A) Schematic representation for the passivation properties of PEG-coated substrate surfaces against Cy3-labeled DNA oligonucleotides. Red lines are DNA chains. Blue dots are Cy3. Black lines are PEG brush. (B) Different contributions to the potential of mean force for PEG5000 as a function of the distance to the surface to explore the mechanism of surface passivation at the molecular level [111]. Copyright 2018. Reproduced with permission from ACS.

with complicated interactions. For example, in Fig. 6.8, by including all the intermolecular interactions (i.e., steric, electrostatic, van der Waals, and entropic interactions) in the system, MT demonstrated the effects of each interaction from an

6.3 Conclusion

energy perspective, which definitively enables a deeper understanding of the underlying physics [111].

6.3 Conclusion Polymeric materials display characteristics at various spatial and time scales. A comprehensive understanding of the fundamentals of the hierarchical structure and multiscale behavior of polymers is critical for further development of polymer systems. However, it still remains a challenging issue for current simulation methodologies to handle fine spatial resolutions and long time scales simultaneously due to the heavy computational load involved. To facilitate experimental investigation, different simulation approaches have been developed and applied to specific systems to explore relevant properties of interest. The unique advantage of atomistic MD simulations is its ability to achieve atomistic resolution. A typical example is its application in the investigation of cross-linked subnanoporous polymer membranes [22,36
40]. MD simulations can offer insights into the local microscopic structures, molecular transfer along subnanopores, water
salt separation mechanism, and membrane surface biofouling of polymer membranes. The on-going development of polymer membranes necessitates further investigation of polymer crosslinking using simulation. Polymer crosslinking, in principle, is a kinetics-controlled process, involving reactions and diffusion in a solvent environment, which represents a complex issue in simulations. For example, experiments [112] showed a unique Turing structure that can be fabricated on a NF membrane using a facile route based on interfacial polymerization to control the reaction and diffusion in solvents. Such unusual nanostructures, as a result of diffusion-driven instability, display outstanding transport properties in terms of both water permeability and water
salt selectivity. The newly developed reactive force-field (ReaxFF) MD simulations [113], which are based on a general bond-order-dependent potential, can provide accurate descriptions of bond breaking and bond formation in chemical reactions. ReaxFF MD enhances the capability of conventional MD simulations to model chemical reactions and also to overcome the scale problem of quantum simulations. A more efficient simulation software package, named RXMD (Reactive MD) [114], has been newly developed for inorganic materials design [114
116]. A similar RXMD simulation approach [117], which takes into account the charge evolution during polymerization, has been applied to the study of the polymer crosslinking process. Future development of ReaxFF simulation methods are highly desirable toward understanding the polymer crosslinking process, membrane structures, and interfacial interactions, particularly in a solvent environment. Similar to atomistic MD simulations, DPD simulations also provide efficient quantitative predictions of polymer morphologies at the mesoscopic scale, for example, block polymers [82,83], semiconducting polymers with crystalline structures [69,74,75,94,96], and polymer
nanoparticle composites [118]. However,

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the soft-core potentials such as those derived from Flory
Huggins free energy approximation, limit the prediction accuracy of the mechanical properties of polymers. It typically requires a combination of CG DPD simulations and atomistic MD simulations through certain mapping and reverse mapping procedures [119]. MT, based on the mean field theory, is a unique efficient approach to predict polymer phase morphology with CG resolution that offers explicit representation of the underlying thermodynamics. Such statistical mechanics approaches have been widely and successfully applied to the study of polymer brushes [100], ligand
receptor binding [103], polyelectrolytes [104,105], responsive polymers [106,107], and peptide aggregation [110]. Further exploration will be needed with regard to more complicated and specific interactions at the molecular level and their correlation in long-range interactions.

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CHAPTER

7

An example of theoretical approaches in polymer hydrogels: insights into the behavior of pH-responsive nanofilms

Gabriel S. Longo Instituto de Investigaciones Fisicoqu´ımicas Teo´ricas y Aplicadas (INIFTA), UNLP-CONICET, La Plata, Argentina

7.1 Introduction Hydrogels are made of chemically or physically cross-linked polymer chains forming a highly hydrated, generally biocompatible network. The main feature that makes these materials exceptional is that through the appropriate selection of the chemical composition of the polymer used, hydrogels can be designed to respond to external stimuli. For example, the thermosensitive polymer, poly(N-isopropyl-acrylamide) (PNIPAm), exhibits a low critical solution temperature (LCST) in the 30
C34
C range. Micrometer-sized hydrogels (microgels) of chemically crosslinked PNIPAm display a dramatic size change in aqueous solution when heated above this LCST [1]. Similarly, microgels including poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) show a many-fold size increase when the solution pH is raised above the intrinsic pKa of the particular acid [2,3]; this behavior is triggered by an increase in the electric charge of the polymer. Microgels of copolymers of PAA (or PMAA) and PNIPAm display both temperature and pH response [4,5]. Resulting from this external control of the physicochemical properties of these materials, polymer hydrogels are currently one of the first choices to consider when designing smart, responsive biomaterials with applications ranging from biosensing [6,7] to tissue engineering [8,9], bone regeneration [10], biomimetic materials [11,12], drug delivery [13,14], wound healing [15], and oral drug delivery [1618], where pH-responsive hydrogels are particularly suitable due to the pH variations that occur along the digestive tract. Crosslinked weak polyacid hydrogels are unswollen within the acidic stomach medium (pH 1.22), which slows down the diffusion of the encapsulated agent to escape the hydrogel; on the

Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00007-X © 2020 Elsevier Inc. All rights reserved.

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other hand, these networks swell at the environmental conditions of the intestines (pH 78), facilitating drug diffusion and delivery. Within the tissue-like aqueous environment inside polymer hydrogels, many proteins find a suitable host that can help prevent aggregation and denaturation [1921]. Hydrogels of pH-sensitive polymers are the subject of intensive research in oral drug delivery because they can enclose and release active and structured proteins [22]. As an administration vehicle, these hydrogels can protect a protein from rapid degradation during its transit through the gastrointestinal tract [17,18,23,24]. However, applications that require pH-responsive hydrogels to interact with proteins face several design challenges. The behavior of these materials in protein solutions is generally difficult to predict, and oftentimes unexpected phenomena emerge that can only be rationalized with a posteriori considerations. For example, there is considerable interest in the oral delivery of calcitonin for the treatment of osteoporosis, but the high isoelectric point (pI) of this protein hinders its administration using pH-responsive hydrogels [25]. In the acidic gastric environment, the protein is strongly and positively charged; electrostatic attractions with the negatively charged hydrogel network can prevent or retard protein release [25]. Therefore understanding the physical chemistry involved in the interaction between proteins and these hydrogels is essential on the path to smart biomaterials with targeted performance. If these polymer hydrogels are to be used as smart, environment-sensitive components in biomaterials, then how they interact with proteins must be understood. Contact lenses are sometimes based on pH-responsive PMAA. Although tear fluid contains many proteins, adsorption to the hydrogel must be generally prevented because it can affect wear comfort and lead to inflammation [26,27]. However, the adsorption of some specific proteins might be beneficial; tears contain lysozyme, for example, which is well known to have antibacterial and antiinflammatory properties [27]. The interplay between different degrees of freedom and molecular forces determines the interaction of proteins with polymermodified surfaces [28,29]. The protonation/deprotonation of the hydrogel units changes the nearby environment, which allows proteins to regulate their electric charge. The purpose of this chapter is to describe the physical chemistry that originate the mentioned effects. To achieve this goal, theoretical studies by the author and collaborators will be reviewed addressing the role of environment composition on peptide and protein adsorption to pH-responsive polymer hydrogel films. The adsorption of proteins to polymer hydrogels has been investigated using a wide range of experimental methods [28,3033]. Theory and molecular simulations can provide insights into the physical chemistry behind the different phenomena associated with protein adsorption to different materials. However, only a few theoretical and simulation studies of protein adsorption to hydrogels can be found in the literature [3439]. Theoretical studies have unveiled many features of the physical chemistry that governs protein adsorption to pH-responsive polymeric materials. In these studies, however, the protein is generally modeled as a geometrical object

7.2 Acidbase equilibrium in dilute solutions: ideal behavior

without detailed information of its three-dimensional structure. For this reason, these works cannot account for the distribution of titratable amino acids in the protein structure. In most molecular simulation studies, on the other hand, the protein is assigned a fixed charge on the basis of the solution pH and the intrinsic pKa of its residues. As a result, the effect of amino acid protonation is ignored. In the past few years, new molecular simulation methods have been developed that are capable of describing amino acid protonation. So far, these studies have focused on the adsorption of charge-regulating proteins to charged surfaces and strong polyelectrolytes [4044], and the possibility of a charge-regulating adsorbent material has not been explored, to the best of the author’s knowledge. We have developed a molecular theory to study the thermodynamics of hydrogels of cross-linked weak polyacid chains including bulk gels [4547], freestanding surface-deposited films [48], and surface-grafted films [49,50]. The same method has been extended to investigate peptide and protein adsorption to these pH-responsive hydrogel films [5155]. The most general version of this molecular theory is presented in Ref. [54], which is based on previous work by Szleifer et al. considering protein adsorption to polymer brushes [5658], and the behavior of grafted weak polyelectrolyte layers [59,60]. In this theory, a thermodynamic potential is derived that considers all the physicochemical contributions, namely the acidbase equilibrium of all titratable units, the entropic loss of molecular confinement, the conformational degrees of freedom of the network and the proteins, and the electrostatic, van der Waals, and steric interactions. The approach also incorporates a molecular model that accounts for size, shape, charge distribution, protonation state, and conformation of each species. The goal of this chapter is to introduce the reader to the interesting phenomena that emerge from the effect of protonation in mediating the interactions of peptides and proteins with pH-responsive hydrogels. Hence results newly calculated with this molecular theory are included that correspond to the equilibrium adsorption in NaCl solutions of hexahistidine (His-tag), myoglobin, lysozyme, and cytochrome c to surfacegrafted hydrogel films of cross-linked PMAA chains.

7.2 Acidbase equilibrium in dilute solutions: ideal behavior First consider protonation under ideal conditions where an isolated molecule bearing a titratable group is immersed in an aqueous solution. This molecule can exist in two different chemical states, either protonated (AH) or deprotonated (A). Chemical equilibrium between these species is described through the proton dissociation constant Ka, which in a dilute or ideal solution satisfies: Ka 5

½A½H 1  ½AH

(7.1)

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where square brackets indicate molar concentration. In this solution, the fraction of molecules in the deprotonated state or degree of proton dissociation is: ½A ½A 1 ½AH

(7.2)

fd 5

1
 1 1 ½AH=½A

(7.3)

fd 5

1
 1 1 ½H 1 =Ka

(7.4)

1 1 1 10pKa 2pH

(7.5)

fd 5

fd 5

where pKa 5 2 log10 Ka is the more familiar logarithmic equilibrium constant, and pH 5 2 log10 ½H 1  is defined with ½H1  being the hydronium concentration. For an acidic molecule, methacrylic acid (MAA), for example, the deprotonated state is negatively charged ðA2 Þ, while the protonated species is charge neutral. Then, fd also gives the fraction of charged molecules in the solution or degree of charge fc . For basic molecules, the
 amino acid His, for example, the protonated state is positively charged AH 1 , while the deprotonated species is charge neutral. In this case, the degree of charge is fc 5 1 2 fd . Given the intrinsic pKa of a particular acid/base, Eq. (7.5) shows that the degree of dissociation (and that of the charge) is determined by the pH of the solution. If pH 5 pKa , molecules occupy the protonated and deprotonated state with equal
 probability ðfd 5 0:5Þ, half of them being electrically charged fc 5 1=2 . If pH 5 pKa 2 1, less than 10% of the molecules in the solution are deprotonated ðfd , 0:1Þ, while when pH 5 pKa 1 1, more than 90% of these molecules populate this chemical state ðfd . 0:9Þ. In other words, when the pH of an ideal solution increases to around the pKa , the deprotonation transition occurs in 2 unit of pH, where the degree of dissociation increases from 10% to 90%. Fig. 7.1 illustrates the degree of charge of MAA ðpKa 5 4:65Þ and the side chain of His ðpKa 5 6:6Þ under dilute conditions as a function of the solution pH. The inset shows the degree of dissociation of each species under the same conditions. Both species deprotonate or dissociate as the pH increases (see Fig. 7.1 inset). At low pH, MAA is charge neutral and becomes negatively charged as the pH increases. On the other hand, His is positively charged at low pH and becomes charge neutral as the pH increases. The titration curves displayed in Fig. 7.1 are often considered to predict behavior under conditions other than dilute solution. Consider, for example, the adsorption of a His-rich peptide to a PMAA hydrogel. Eq. (7.5) tells us to expect adsorption between pH 5 and 7, where the titratable units of the adsorbate and that of the adsorbent species are strongly and oppositely charged (see Fig. 7.1). The objective of this chapter is to show that, in general, such considerations lead

7.3 Protonation of weak polyacid hydrogel films

FIGURE 7.1 Plot of the degree of charge of MAA and the side chain of His under dilute conditions as a
 function of the solution pH. The intersection between the dotted fc 5 1=2 and solid lines gives the intrinsic pKa of each species. The inset shows the degree of proton dissociation for both solutes. His, Histidine; MAA, methacrylic acid.

to qualitatively wrong conclusions. The confinement of MAA units to the polymer network makes the hydrogel backbone significantly deviate its protonation state from ideal behavior. As a result, a lower pH occurs in the interior of the hydrogel, which causes proteins to protonate after adsorbing and increasing their positive charge.

7.3 Protonation of weak polyacid hydrogel films In this section, the protonation of pH-sensitive polymer hydrogels is addressed along with how this behavior modifies the environment inside the material. For simplicity, but without loss of generality, this discussion is limited to hydrogels of cross-linked weak polyacid chains, MAA in particular. Moreover, surfacegrafted thin hydrogel films (see Fig. 7.2) receive special focus because they present faster response to variations of pH than bulk hydrogels. The response time of stimuli-sensitive hydrogels is one of the most critical features to evaluate when devising applications [61]. As Tanaka and Fillmore [62] have shown, the response following an external perturbation occurs in a time interval proportional to the square of the smallest dimension of the hydrogel. Thin hydrogel films are excellent candidates as smart components in applications requiring a fast response. Indeed, micro- or nanosized hydrogel films can be exploited in photonic materials [63], continuous glucose sensors [6], and mechanical microactuators [64].

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FIGURE 7.2 Top: A graph illustrating how the local pH drops in the interior of a hydrogel film of crosslinked PMAA. Bottom: Schematic representation of surface-grafted PMAA network; the surface sits at z 5 0 and the polymer extends up to roughly 130 nm. In calculating this result, each methacrylic acid unit is modeled using a coarse-grained particle. PMAA, Poly (methacrylic acid). Data partially published in Hagemann A, Giussi JM, Longo GS. Use of pH gradients in responsive polymer hydrogels for the separation and localization of proteins from binary mixtures. Macromolecules 2018;51 (20):820516. https://doi.org/10.1021/acs.macromol.8b01876.

7.3.1 Local pH Fig. 7.2 shows how the pH changes in proximity to and inside a thin hydrogel film of cross-linked PMAA. These and all the results have been obtained by evaluating the molecular theory presented in Ref. [54]. The local pH is defined as:
 pHðrÞ 5 2 log10 ½H1 ðrÞ

(7.6)

because it is assumed that the system is homogeneous in the plane of the surface, the only relevant coordinate is z, which measures the distance to the grafting surface that supports the film. Three clearly distinct spatial regions are observed in Fig. 7.2. Sufficiently far from the film (large z), the local pH is equal to the bulk pH. Inside the film, a lower pH establishes, which depends on the salt concentration of the solution. Fig. 7.2 shows that pHgel, the average of the local pH within the gel, is a welldefined quantity [48,49]. Finally, there is an interfacial region where the pH drops

7.3 Protonation of weak polyacid hydrogel films

from the bulk value to pHgel as z decreases and enters the film. The width of this interface extends from a few nanometers at high salt concentrations to tens of nanometers at lower salinity conditions [48]. The pH drop inside the hydrogel results in a lower dissociation of the MAA segments that compose the network. Fig. 7.2 displays the average degree of charge of MAA units at different conditions; hi denotes the ensemble average over the different molecular conformations of the crosslinked polymeric structure. The evaluation of Eq. (7.5) using pHgel instead of the bulk pH provides a good estimation of the degree of charge of the MAA units in the network [53], which appears to make easier the task of determining the state of charge of the different molecular components (network or a potential adsorbed protein) inside the material. However, when calculating the local pH, an identical difficulty is encountered as in the original problem. Inside the material and in the polymersolution interface, the local pH results from an intricate balance between contributions to the free energy resulting in the thermodynamic equilibrium at the experimental conditions (pH, salt concentration). These contributions include physical interactions, chemical equilibria, and molecular organization. Nevertheless, the importance of pHgel and pH(r) should be stressed because, if known, they supply complete information on the local state of charge and protonation of all titratable units.

7.3.2 Displacement of chemical equilibrium: the role of salt concentration As opposed to dilute solutions, the acidic units that make the polymeric backbone of a pH-responsive hydrogel are relatively close to each other. If charged, these segments repel each other. To lessen the magnitude of these intra-network repulsions, titratable segments deprotonate less than ideal dissociation. Fig. 7.3A displays this behavior for a wide range of solution compositions, presenting the average charge degree of the MAA units of the hydrogel. The dissociation curves of the PMAA network displace to higher pH values as compared to dilute solutions. In other words, at any given pH, a network segment is less likely to be charged than what would be anticipated from dilute solution considerations. For example, the apparent pKa, which gives the pH at which on average half of the acidic groups are charged, displaces almost 3 unit for a 1 mM NaCl solution from its intrinsic value of 4.65 to pKapp 5 7.4 in the polymer network. In addition to the displacement to higher pH values, characterized by an increasing apparent pKa, the dissociation curves of the PMAA network deform as well; the pH width of the transition from 10% to 90% dissociation ðΔpH Þ changes from 2 in the ideal solution to almost 3 unit for the hydrogel in contact with a 1 mM NaCl solution. Clearly, the salt concentration is the variable that regulates the behavior described in Fig. 7.3A. Relatively high salt solutions result in a significant density of counter- as well as co-ions within films. These ions screen the intra-network

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FIGURE 7.3 Protonation of a cross-linked poly(methacrylic acid) hydrogel immersed in NaCl aqueous solutions of different ionic strengths. (A) Graph of the degree of dissociation/charge of the polymer network as a function of pH, where the dashed-line curve represents the dissociation of methacrylic acid in a dilute solution. (B) Plot of the average local pH established in the interior of the film, where the dashed-line curve corresponds to situation where bulk and film pH are identical. Adapted from Longo GS, Pe´rez-Cha´vez NA, Szleifer I. How protonation modulates the interaction between proteins and pH-responsive hydrogel films. Curr Opin Colloid Interface Sci 2019;41:2739. ,https:// linkinghub.elsevier.com/retrieve/pii/S1359029418301183..

electrostatic repulsions, which effectively become short range, allowing for the network to charge more and reduce the chemical free energy cost of displacing the acidbase equilibrium. Eventually, when the salt concentration of a solution is sufficiently high, the dissociation behavior becomes ideal. On the contrary, low salt concentration solutions increase the entropic cost of confining ions inside gels. Only a sufficient number of counterions adsorb to neutralize the electric charge of the polymer network. The shielding effect of mobile ions diminishes,

7.4 Histidine-tag adsorption to pH-responsive hydrogels

and the intra-network electrostatic repulsions can effectively reach further. In consequence, the polymer charges less to decrease such repulsive interactions, paying the chemical free energy price of displacing the acidbase equilibrium. Fig. 7.3B shows how the average pH drops inside the hydrogel film under different conditions. Depending on the composition of the solution, the interior of the hydrogel can present a local pH that is lower than that of the bulk solution. The behavior presented in panels (A) and (B) of Fig. 7.3 are two sides of the same coin; a lower pH establishes inside the hydrogel as a result of a higher degree of protonation or vice versa. Consequently, the variable that modulates the pH drop is also the salt concentration of the solution. At any given solution pH, a lower salt concentration leads to a lower local pH in the interior the gel. Finally, it should be emphasized again the significance of the pH drop that occurs inside these hydrogel films. Because the net charge of proteins depends on the pH, this lower gel pH allows for proteins to modify their protonation state and regulate their electric charge, thus, enhancing the electrostatic interactions with the polyacid network; these interactions control the adsorption. In the next sections, peptide and protein adsorption to pH-responsive hydrogels are discussed with particular emphasis on the role of amino acid protonation.

7.4 Histidine-tag adsorption to pH-responsive hydrogels 7.4.1 Adsorption is a nonmonotonic function of pH Let us consider how a short homopeptide composed of six His residues adsorbs to a pH-responsive hydrogel film. This problem represents an initial approach to the more complex behavior of larger proteins having several different amino acid residues. At the same time, the adsorption of these peptides to PMAA films displays most of the physicochemical behavior that the authors want to describe in this chapter. Short His sequences, known as His-tag, and particularly His6 are commonly applied to label proteins in chromatography [65]. This amino acid has a pKa around 67, bearing a positive charge under acidic conditions (see Fig. 7.1). For the sake of simplicity, in this discussion, the effects of the titratable peptide’s C- and N-terminus groups are not included. The partition of His-tag to different regions of the hydrogel film can be quantitatively described using the (excess) adsorption, which is defined as: ð


 dr ρðrÞ 2 ρbulk

Γ5

(7.7)

V

where ρ(r) is the adsorbate density at position r and ρbulk is its density in the bulk solution. As expressed in Eq. (7.7), Γ measures the total peptide mass within the volume V minus the amount of peptide contained within the same volume of the bulk solution. In the case of films, this quantity gives the excess amount of

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FIGURE 7.4 (A) Adsorption of hexahistidine to a poly(methacrylic acid) hydrogel film from aqueous peptide solutions. (B) Plot showing the degree of protonation (charge) of peptide residues belonging to adsorbed (solid-line curves) and solution (dashed-line curve) hexahistidine. In both panels, the peptide concentration is ½His6  5 10 μM. Reprinted from Longo GS, Pe´rez-Cha´vez NA, Szleifer I. How protonation modulates the interaction between proteins and pH-responsive hydrogel films. Curr Opin Colloid Interface Sci 2019;41:2739. ,https:// linkinghub.elsevier.com/retrieve/pii/S1359029418301183..

adsorbate inside the hydrogel, but it also accounts for the adsorption at the polymersolution interface described in Section 7.3.1. Fig. 7.4A shows the adsorption of His-tag from solutions of different compositions to a grafted film of cross-linked PMAA. The magnitude of the adsorption depends most sensibly on the salt concentration; decreasing NaCl enhances adsorption, which can be rationalized as a high density of salt ions inside the film leads to an effective screening of the MAAHis attractions, thus, weakening the driving force for adsorption. Additionally, under these conditions, there is no significant entropic gain in releasing counterions to the bulk solution where their

7.4 Histidine-tag adsorption to pH-responsive hydrogels

concentration is high. On the other hand, at low salt concentrations there are only enough counterions confined within the hydrogel to balance the polymer charge and yield electroneutrality. The shielding of MAAHis electrostatic interactions is less relevant under such conditions, and these attractions drive substantial quantities of peptide to the film interior. Moreover, upon adsorption, there is an important entropic gain of releasing salt counterions to the bulk solution where their concentration is low. To understand why Γ is a nonmonotonic function of the solution pH, we need to apply ideal solution concepts and consider that the electrostatic attractions between charged MAA and His units drive the adsorption. At low pH, His residues are mostly charged in the solution, but the MAA units that make the hydrogel network are uncharged (see Fig. 7.1). At alkaline pH values, MAA is expected to be charged; while His residues, however, are not. In both situations, the electrostatic attractions are weak and insufficient to result in a significant degree of adsorption. Adsorption takes place when both His residues and MAA segments are strongly and oppositely charged within intermediate values of pH, which yields the nonmonotonic behavior seen in Fig. 7.4A. Ideal solution considerations cannot give the pH of maximal adsorption, which is mainly a function of the salt concentration. As seen in Fig. 7.3, knowing the intrinsic pKa and the bulk pH is not enough to determine the state of charge of the hydrogel network. Moreover, His residues protonate more in the lower pH that occurs within the film (see Fig. 7.4B), which increases the net positive charge of the peptide after adsorption. The apparent pKa of His residues, which is defined as the pH at which half of these units are protonated (charged), can be considerably larger than its intrinsic value for adsorbed molecules. For example, the apparent pKa of His residues displaces from its 6.6 intrinsic value to 9.6 for 1 mM NaCl solutions (see Fig. 7.4B). The width of the dissociation transition increases as well (ΔpH 5 2.9). However, both the apparent pKa and ΔpH are not sufficient to completely characterize the shape of the titration curves of the adsorbed molecules. For example, a plateau region is observed when NaCl 5 1 mM, where the degree of charge of His residues remains approximately constant between pH 7 and 9 (see in Fig. 7.4B).

7.4.2 Adsorption can modify the pH inside the hydrogel The equilibrium protonation state of His residues displaces to result in a more positively charged peptide, which enhances the electrostatic attractions with the hydrogel network. As has been seen, the solution salt concentration is the key element that modulates such behavior; lowering the NaCl concentration leads to more protonation of adsorbed His-tag molecules and increases adsorption. In this context, if the results of Figs. 7.3A and 7.4A are compared, a counterintuitive behavior emerges. In the absence of peptide, lowering the salt concentration leads to less dissociation and charge in the polymer network (see Fig. 7.3A). However, His-tag adsorption is optimal under these conditions (Fig. 7.4A). Namely, the

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FIGURE 7.5 (A) Plot of the average degree of charge of network segments as a function of pH for a PMAA hydrogel film. Three different situations are considered, namely the isolated MAA in a dilute solution (dashed line), and the network in contact with a salt solution with (solid line) and without (dotted line) His6. Panel (B) presents the average gel pH for the same conditions as panel (A). The salt and peptide concentrations are 1 mM and 10 μM, respectively, when it applies. The solid-line curves represent identical conditions to those shown in Fig. 7.4.

weakest charged network displays the strongest adsorption. It should be emphasized that the results of Fig. 7.3 correspond to salt solutions without peptide, because even at this low His-tag concentration (10 μM), peptide adsorption modifies the charge on the polymeric backbone of the gel [51]. Fig. 7.5A exemplifies such behavior through showing the degree of charge of the network after His-tag adsorption for the lowest salt concentration considered. This phenomenon can also be described considering that after the His-tag adsorbs, the pH that establishes within the gel changes. Fig. 7.5B shows that the pHgel increases significantly with respect to solutions without peptide. As a

7.5 Adsorption of proteins to pH-sensitive hydrogels

consequence, the polymer network is more negatively charged than in the absence of peptide (see Fig. 7.5A). At the same time, the pHgel still drops with respect to the solution, which allows for the peptide to be more positively charged than in the solution. Here, it can be seen again that the gel pH is the result of a complex interplay between the different terms that contribute to the total thermodynamic potential. The chemical free energy that describes the acidbase equilibrium is optimized when the bulk and hydrogel pH are identical. At the same time, the electrostatic energy drives variations in pH to modify the charge of the different titratable species, and to reduce intra-network repulsions or enhance adsorbatenetwork attractions. In this context, counterion confinement and salt concentration play decisive roles in modulating the electrostatic interactions. A high concentration of salt ions can screen such interactions and weaken their influence. At low salt concentrations, on the other hand, counterion confinement is entropically costly; in the absence of adsorbate, the network charges less to prevent counterion absorption as imposed by electroneutrality, while for adsorbate solutions, similarly, counterion release favors protein/peptide adsorption.

7.5 Adsorption of proteins to pH-sensitive hydrogels 7.5.1 Protein model and solution titration curves Over the past few years the equilibrium adsorption of proteins to pH-sensitive hydrogel films have been studied [5255]. To this goal, a molecular model has been developed where proteins are represented through applying a coarse-grained scheme on the structure obtained from crystallographic data; each residue is a single particle centered at the original position of the alpha carbon. A similar molecular model has been applied to describe His-tag, but the configurations of the peptide chain are generated using a rotational isomeric state model [66], where each coarse-grained segment can assume three different isoenergetic relative orientations [51]. Each of the coarse-grained units that make the protein can be either titratable (acidic or basic) or charge-neutral. Myoglobin (16.7 kDa), lysozyme (14.4 kDa), and cytochrome c (12.4 kDa) have been considered (protein data bank entries 3RGK, 193L, and 2B4Z, respectively), which are similar-sized globular proteins having 1.5 nm hydrodynamic radii approximately. Fig. 7.6 presents the coarse-grained scheme as well as the pKa values assigned to the different titratable units. Fig. 7.7 plots the net charge of the proteins under dilute conditions as a function of the solution pH using the model previously described. For acidic pH values, the proteins studied are highly and positively charged because all of their residues are protonated; acidic units are charge neutral and basic units are positively charged. As the pH increases, the degree of dissociation of the

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FIGURE 7.6 Top: Table showing the pKa of most of the titratable residues that compose proteins [54]; these values represent averages obtained from several experimental results considering different proteins [67]. The table also presents the number of these coarse-grained units in each of the proteins of interest. Bottom: Scheme of the molecular model applied to represent proteins; an amino acid residue is described using a single coarse-grained particle, which can either be charge neutral (gray spheres), acidic (red spheres), or basic (blue spheres). Adapted from Hagemann A, Giussi JM, Longo GS. Use of pH gradients in responsive polymer hydrogels for the separation and localization of proteins from binary mixtures. Macromolecules 2018;51(20):820516. https://doi.org/10.1021/acs.macromol.8b01876 and Longo GS, Pe´rez-Cha´vez NA, Szleifer I. How protonation modulates the interaction between proteins and pH-responsive hydrogel films. Curr Opin Colloid Interface Sci 2019;41:2739. ,https://linkinghub.elsevier.com/retrieve/pii/S1359029418301183..

titratable units increases; acidic units become negatively charged and basic units are increasingly found in the charge neutral state. This behavior is illustrated for myoglobin in Fig. 7.7. The bulk pH where the protein is overall charge-neutral is known as the isoelectric point, pI. The net charge of the protein is positive if pH , pI, and negative if pH . pI. Within the model used here, the pI of myoglobin, lysozyme, and cytochrome c are 7.1, 10.8, and 9.8, respectively, in agreement with the experimental values. Throughout the rest of this chapter, the term intrinsic or solution pI will be used because the deprotonation curves of proteins when adsorbed to the hydrogel are different from those shown in Fig. 7.7, resulting in an apparent displacement of the pI.

7.5.2 The role of pH and salt concentration in the magnitude of adsorption Next, the adsorption of myoglobin, lysozyme, and cytochrome c to a film of cross-linked PMAA are discussed. With this aim, the protein adsorption results calculated using the molecular-level theory developed in Ref. [54] are included,

7.5 Adsorption of proteins to pH-sensitive hydrogels

FIGURE 7.7 Top: Net charge number of the proteins as a function of pH. Bottom: The scheme illustrates how myoglobin deprotonates as the solution pH increases; positively charged (basic) coarse-grained units are shown in cyan, negatively charged (acidic) residues in orange, and charge neutral units are represented by gray spheres.

considering different NaCl concentrations. It should be emphasized again that the present method provides as a result of the local protonation state of protein residues and network segments; that is, the degree of charge of titratable units is not imposed a priori on the basis of its pKa and the bulk pH, but it is predicted instead from the spatial location of the group and the neighboring microenvironment. The independent variable in these calculations is the composition of the bulk solution (i.e., pH and salt and protein concentration). Fig. 7.8 shows that the adsorption of myoglobin, lysozyme, and cytochrome c to the PMAA hydrogel film depends nonmonotonically on the bulk pH. At acidic pH values, the protein bears several positive unit charges, but the polymer network is not strongly charged (see Figs. 7.3A and 7.7). When the pH is higher, the hydrogel backbone can be strongly and negatively charged, but the net charge of proteins in the solution is either weakly positive or even negative. Then, in both sides of the pH scale, the polymerprotein electrostatic interactions are only weakly attractive, if not repulsive, and insufficient to drive any significant degree of adsorption. However, adsorption can occur in the intermediate range of pH where both species are strongly and oppositely charged; necessarily, adsorption profiles display a maximum under such conditions.

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FIGURE 7.8 Adsorption to a poly(methacrylic acid) hydrogel from solutions having 10 μM of a single type of protein. Each panel presents a particular salt concentration. Reprinted from Longo GS, Pe´rez-Cha´vez NA, Szleifer I. How protonation modulates the interaction between proteins and pH-responsive hydrogel films. Curr Opin Colloid Interface Sci 2019;41:2739. ,https:// linkinghub.elsevier.com/retrieve/pii/S1359029418301183.; data originally published in Hagemann A, Giussi JM, Longo GS. Use of pH gradients in responsive polymer hydrogels for the separation and localization of proteins from binary mixtures. Macromolecules 2018;51(20):820516. https://doi.org/10.1021/acs. macromol.8b01876.

Myoglobin, lysozyme, and cytochrome c adsorption to PMAA hydrogel films depends sensibly on the salt concentration of the solution (see Fig. 7.8). Lowering the salt concentration enhances the magnitude and widens the pH range of adsorption. Comparing both panels of Fig. 7.8 yields an approximately one order of magnitude decrease in adsorption when the NaCl concentration is raised from 1 to 10 mM. The pH at which adsorption is maximal depends on the salt concentration as well.

7.5 Adsorption of proteins to pH-sensitive hydrogels

Similarly to His-tag adsorption, here it can be seen again that the weakest charged network incorporates the most protein, which occurs at low NaCl (compare Figs. 7.3 and 7.8). In other words, as the salt concentration decreases, the polymer network protonates more and charges less, but it adsorbs more protein. In this case, as opposed to His-tag solutions, adsorption only marginally affects the network state of protonation, which is true at the relatively low protein concentration seen in Fig. 7.8 (10 μM). At the risk of being repetitive, but without going into much detail, there are three main reasons for protein adsorption to depend so critically on the salt content of the bulk solution. First, lowering the NaCl concentration weakens the screening effect of salt ions, thus, enhancing proteinattractions, and, therefore, increasing the adsorption. Second, when the protein adsorbs, the release of counterions from the hydrogel increases the entropy; this entropic gain favoring protein adsorption strengthens as the bulk concentration of salt diminishes. Third, the lower the salt concentration (at a given bulk pH), the more positively charged adsorbed proteins are, which is because the gel pH decreases as well. This last phenomenon is described next.

7.5.3 Protein charge regulation As it adsorbs to the hydrogel film, the protein enters an environment where the local pH is lower than that in the solution. How does this pH drop modify the electric charge of the protein? To answer this question with an example, Fig. 7.9 shows how the net charge of lysozyme increases suddenly as the pH drops in the few nanometers that compose the polymersolution interface. In this case, lysozyme gains 2.5 protons on average in moving approximately 25 nm from z 5 150 nm toward the interior of the film. Within this spatial range, the local pH falls from neutral to 4.5 inside the film. Fig. 7.9 also shows that, except in this interface, the pH inside the film is not significantly modified by protein adsorption (at 10 μM concentration). To further illustrate the relation between charge regulation and protein adsorption, Fig. 7.10A shows how the net charge number of adsorbed lysozyme depends on the pH and salt concentration. Depending on the solution composition, the protein can increase its positive charge by several units as it enters the film. Fig. 7.10B displays the number of protons that lysozyme captures once it enters the film under different conditions. As has been seen, proteins are more positively charged inside the hydrogel as a result of the drop in local pH; this enhances the electrostatic interactions with the hydrogel network, which is negatively charged, and consequently favors protein adsorption. By lowering the salt concentration, the protein becomes more positively charged inside the hydrogel. In other words, decreasing the NaCl concentration results in a lower film pH, which leads to a more positively charged adsorbed protein.

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FIGURE 7.9 The plot illustrates how the protein charge (top) increases and the local pH drops (bottom) in the interior of a poly(methacrylic acid) hydrogel film in contact with a 10 μM lysozyme solution; the dashed-line curve corresponds to a bulk solution with the same pH and NaCl concentration, but without proteins. The supporting surface sits at z 5 0 and the polymer extends up to roughly 130 nm. Data partially published in Hagemann A, Giussi JM, Longo GS. Use of pH gradients in responsive polymer hydrogels for the separation and localization of proteins from binary mixtures. Macromolecules 2018;51 (20):820516. https://doi.org/10.1021/acs.macromol.8b01876.

Solution proteins are negatively charged if the pH is above the pI. However, a lower pH occurs in the interior of the film and the apparent pI of adsorbed proteins increases. This result implies that there are conditions where the protein reverses the sign of its electric charge upon adsorption, from negatively charged in the solution to positively charged inside the film [52]. The charge regulation behavior illustrated in Fig. 7.10, which can even lead to charge reversal, takes place within the few nanometers that compose the interface between the hydrogel film and the solution [52] as seen in Fig. 7.9.

7.5.4 Protonation of amino acids after adsorption Clearly, the charge regulation behavior described in Section 7.5.3 occurs because the different amino acid residues are more likely to be protonated when the protein is immersed in the lower pH medium within the hydrogel. Namely, these residues modify their chemical state and displace equilibrium toward the protonated species. A higher degree of protonation means that when the protein adsorbs, acidic residues are less likely to be negatively charged, while basic residues are more likely to be positively charged. Within the molecular theory discussed in this chapter, it is possible to individualize the contribution of each amino acid to this charge regulation phenomenon. To provide more insight into the charge

7.5 Adsorption of proteins to pH-sensitive hydrogels

FIGURE 7.10 These graphs illustrate how lysozyme protonates and modifies its electric charge after adsorption to a poly(methacrylic acid) hydrogel film. Panel (A) displays the net charge of adsorbed (solid-line curves) and solution (dashed-line curve) lysozyme. Panel (B) presents the number of protons gained by the adsorbed protein. These results are presented as a function of pH for solutions having different salt concentrations and 10 μM lysozyme. Data partially published in Hagemann A, Giussi JM, Longo GS. Use of pH gradients in responsive polymer hydrogels for the separation and localization of proteins from binary mixtures. Macromolecules 2018;51 (20):820516. https://doi.org/10.1021/acs.macromol.8b01876.

regulation behavior, Fig. 7.11 displays the state of charge of glutamic acid (acidic) and His (basic) residues in lysozyme after the protein has adsorbed to the hydrogel film under different conditions. Although the degrees of protonation of all residues increase when the protein adsorbs, this behavior depends not only on the experimental conditions, but also on the particular amino acid in question. Both quantities that characterize the titration curves, the width in pH units of the deprotonation transition (ΔpH) and the displacement of apparent pKa, change with the amino acid type. More generally, the

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FIGURE 7.11 Using the examples of glutamic acid (top) and histidine (bottom) these graphs illustrate how the different residues modify their protonation state when lysozyme adsorbs to a poly (methacrylic acid) hydrogel film. Both panels present the average degree of charge of the residue as a function of pH for solution proteins (dashed-line curves) and proteins adsorbed under different salinities (solid-line curves). These results can be directly compared to those shown in Fig. 7.10. Data partially published in Longo GS, Szleifer I. Adsorption and protonation of peptides and proteins in pH responsive gels. J Phys D Appl Phys 2016;49(32):323001. ,http://stacks.iop.org/0022-3727/49/i 5 32/ a 5 323001..

deviation from ideal behavior of the titration curves is different for each of the various types of residues. Thus the distinctive intrinsic pKa values of the residues endow the protein with degrees of freedom to adjust their net charge under several environmental conditions. In the interaction with a PMAA hydrogel, this freedom translates into the protein’s ability to modulate the electrostatic interactions with the hydrogel and enhance its adsorption [53].

7.5 Adsorption of proteins to pH-sensitive hydrogels

FIGURE 7.12 Plot showing the adsorption from binary protein solutions to a poly(methacrylic acid) hydrogel; the results are presented as a function of pH, the concentration of each protein is 10 μM and NaCl 5 1 mM. The adsorption from solutions having a single type of protein (Continued)

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7.5.5 Adsorption from binary protein mixtures So far, the behavior of single peptide/protein solutions has been described. In the context of biomaterials applications, however, pH-sensitive hydrogels will almost exclusively be exposed to fluids containing many different proteins. Considerations based on the adsorption from single protein solutions are not useful when estimating the behavior of mixtures [68]. Because even binary protein solutions display a rich pH-dependent behavior [54], one can only speculate about the complexity of biological fluids that contain many proteins. One example of the phenomena that originate from protein mixtures is selective adsorption. While one protein would strongly adsorb from a single protein solution, adding another protein to the same solution might completely exclude the first one from the biomaterial. This behavior is shown in Fig. 7.12, which presents the adsorption to a PMAA hydrogel film from binary solutions of myoglobin, lysozyme, and cytochrome c in contrast to single protein solutions. The figure also shows that changing the solution pH allows for the selection or control ever which protein adsorbs to the hydrogel. Albeit the complexity of the behavior, hydrogel-based biomaterials can take advantage of the adsorption from protein mixtures. Selective adsorption of some specific proteins while preventing the nonspecific adsorption of others can enhance biomaterial functionality and durability. At the same time, confinement of some proteins within particular regions of the biomaterial (or the exclusion of other proteins from those regions) can significantly enhance biomaterial functionality. New studies have suggested that the incorporation of comonomers into a PMAA polymer network can be used to separate and localize specific proteins to nanometer-sized regions of the material [54]. Copolymer hydrogel films generate different pH gradients that can trigger the selective adsorption of proteins; behavior that can be externally manipulated using the composition of the bulk solution. In the context of designing functional biomaterials based on pH-responsive hydrogels, the emergent phenomena associated with protein localization show how critical it is to understand the physical chemistry controlling the adsorption from multicomponent protein mixtures.

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is represented using dotted-line curves. Each panel corresponds to a different protein mixture, as indicated. Reprinted from Longo GS, Pe´rez-Cha´vez NA, Szleifer I. How protonation modulates the interaction between proteins and pH-responsive hydrogel films. Curr Opin Colloid Interface Sci 2019;41:2739. ,https:// linkinghub.elsevier.com/retrieve/pii/S1359029418301183.; data originally published in Hagemann A, Giussi JM, Longo GS. Use of pH gradients in responsive polymer hydrogels for the separation and localization of proteins from binary mixtures. Macromolecules 2018;51(20):820516. https://doi.org/10.1021/acs. macromol.8b01876.

References

7.6 Conclusion Hydrogel of pH-responsive polymers show potential as smart, function-specific components in different biomedical applications. A rational bottom-up approach to devising these biomaterials requires a deep understanding of how hydrogels interact with protein solutions. Molecular simulations can reveal new features of this interaction as well as provide information that is challenging if not impossible to access from experiments. On the basis of recent work using molecular theory, the purpose of this chapter was to present the reader with interesting behavior that emerges from the ability of both proteins and hydrogels to regulate electric charge. Inside a pH-responsive hydrogel film, the protonation equilibrium of the acidic units of the polymer displaces, resulting in a lower density of charge along the network. In other words, the local pH drops inside the film as compared to the bulk solution. Amino acid residues protonate in the lower pH environment found inside the hydrogel, which allows the protein to increase its positive charge, giving rise to new and interesting phenomena. For example, having several amino acid types that protonate/deprotonate in different regions of the pH scale supplies the protein with flexibility to regulate electric charge and increase adsorption within a wide range of environmental conditions. Such rich behavior can be exploited to separate proteins or localize them within precise regions inside hydrogels. A few methods of molecular simulation are currently capable of describing acidbase equilibrium. To date, these methods have only been employed to study the interaction of proteins with either charged surfaces or strong polyelectrolytes. In the near future, these molecular simulation methods will be used to model how proteins behave when placed in contact with pH-sensitive materials, which will uncover new phenomena emerging from the ability of these molecules to modify the protonation state of their titratable units. It is likely that these discoveries will pave the way to new developments in functional biomaterials. From the point of view of physical chemistry, it is completely clear that exciting research awaits.

Acknowledgment This work has been supported by ANPCyT (PICT-2017-3513) and CONICET, Argentina.

References [1] Pelton R. Temperature-sensitive aqueous microgels. Adv Colloid Interface Sci 2000;85(1):133. Available from: https://doi.org/10.1016/S0001-8686(99)00023-8.

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CHAPTER

Pectin as oral colon-specific nano- and microparticulate drug carriers

8

Badrul Hisyam Zainudin1,2,3,4, Tin Wui Wong1,2 and Halimaton Hamdan4 1

Non-Destructive Biomedical and Pharmaceutical Research Centre, iPROMISE, Universiti Teknologi MARA, Puncak Alam, Malaysia 2 Particle Design Research Group, Faculty of Pharmacy, Universiti Teknologi MARA, Puncak Alam, Malaysia 3 Malaysian Cocoa Board, Cocoa Innovative and Technology Centre, Nilai, Malaysia 4 Razak School of Engineering and Advanced Technology, Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia

8.1 Introduction 8.1.1 Synthetic polymers Synthetic polymers are manmade polymers and are usually derived from petroleum oil. They can be classified as thermoplastics and thermosets. Thermoplastic polymers turn soft upon heating and can be reversibly melted, while thermoset polymers undergo chemical reaction under heat or with chemicals forming insoluble materials that cannot be melted [1]. Most synthetic polymers are organic in nature with backbones made up of carbon carbon bonds. Elements such as oxygen, sulfur, or nitrogen may be inserted alongside the backbone to produce heteropolymers. Synthetic polymers can exist in the absence of carbon carbon bonds such as in polysiloxane (silicone oxygen backbone). They are known as inorganic polymers. Examples of synthetic polymers that have been employed in pharmaceutical and biomedical applications are poly(ethylene glycol), poly(propylene glycol), poly(oxalate), poly(dimethylsiloxane), poly(vinyl acetate), poly (acrylic acid), polystyrene, polyurethane, and poly(maleic anhydride). One of the major advantages of synthetic polymers are their high chemical purity [2]. Medical devices such as suture materials, medical implants, fracture fixation devices, and dialysis tubing make use of the biological inertness of synthetic polymers and reduce the host response to the material to a minimum level [3]. Degradable synthetic polymers are beneficial in the design of regenerative medicine and drug delivery systems [4 7]. Polymers can be tailor-made to be deliberately degraded to interact with the host for specific applications.

Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00008-1 © 2020 Elsevier Inc. All rights reserved.

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8.1.2 Natural polymer Natural polymers are polymers that can be extracted chemically from nature. They can be isolated from plant and animal sources or be synthesized by microorganisms [8]. Natural polymers are broadly classified based on their sources in nature such as polysaccharides, proteins, polynucleotides, polyisoprenes, and polyesters [9] (Fig. 8.1). Polysaccharides consist of polymeric carbohydrate molecules composed of chains of monosaccharides bound together by glycosidic linkages. They are the most abundant natural polymers in nature and an important class of biomaterials with significant research interest in medical and pharmaceutical applications owing to their safety, biocompatibility, and bioactivity. Polysaccharides possess structural similarities, chemical versatilities, and biological performances similar to those of the extracellular matrix of a biological host, thus, making them nontoxic with minimum host immune response [10]. Examples of polysaccharidic polymers include pectin, chitosan, alginate, cellulose, hyaluronan, lignin, and dextran. Proteins are macromolecules that consist of one or more long chains of amino acid residues linked by amide bonds to form threedimensional macrocomplexes. They are the principal compounds of all living cells. Bioactive proteins known as enzymes are responsible for the cell lifecycle and the metabolism and synthesis of compounds such as lipids and carbohydrates. Examples of such are proteinase, alcohol dehydrogenase, and deoxyribonuclease. Nonbioactive proteins known as storage proteins are stable and stay dormant until seed germination. Examples of storage proteins include casein and ovalbumin [11]. Polynucleotides are natural polymers consisting of nucleotide monomers

FIGURE 8.1 Types of natural polymers.

8.2 Pectin as bioactive dietary fiber

bound covalently in a chain. Two of the most important polynucleotides with distinct biological functions are deoxyribonucleic acid and ribonucleic acid. Polynucleotides have promising functions as vaccines [12] and gene therapeutics [13,14]. Polyisoprenes are natural polymers consisting of isoprene monomers and are categorized into cis and trans forms. Cis-1,4-polyisoprene, which has a highly specific associative structure, is a major ingredient for natural rubber [15]. Its distinct properties such as elasticity, abrasive performance, good thermal dispersion, and impact resistance make it an important material in tire industries. On the other hand, trans-1,4-polyisoprene, which has the same flexibility and plasticity as a natural rubber [16], can be applied as an additive [17] or a root canal filling [18,19]. Natural polyester polymers are characterized by the ester functional group in the main chain. Examples are cutin, suberin, and polyhydroxyalkanoates [9]. Cutin and suberin are two complex lipid-based polyesters that are unique to the plant kingdom [20]. Polyhydroxyalkanoates is produced in transgenic plants and used as a source of renewable and environment-friendly plastics [20].

8.2 Pectin as bioactive dietary fiber Pectin can be found in the cell walls of most plants. It is a naturally occurring heterogeneous anionic polysaccharide that consists mainly of linearly connected α-(1,4)-D-galacturonic acid monosaccharide units. In addition, rhamnose, arabinose, and xylose are usually found and introduced kinks in the normally linear molecule [21]. Three main pectic polysaccharides have been isolated from the cell walls of plants [22]. The most abundant pectic polysaccharide, which represents about 65% of the pectin molecule, is homogalacturonan (HG) [23]. HG is a linear homopolymer of D-galacturonic acid linked together by α-(1,4) glycoside bonds, and is considered as the backbone and smooth region of pectin. The second most abundant component is the hairy region of pectin comprising of rhamnogalacturonan I (RG I), which represents about 20% 35% of the pectin molecule [23]. The rest of pectin comprises of substituted galacturonans (GS). Among these GS, rhamnogalacturonan II (RG II), occurring with much less frequency than RG I, makes the most highly complex branched structure in pectin and represents about 10% of the pectin molecule [24]. Extraction of pectin from plants results in methoxylation of the acidic groups of the monosaccharide [21] (Fig. 8.2). The most important molecular parameter of pectin is the degree of esterification (DE), which denotes the percentage of methoxylated C6 atoms in the galacturonic acid backbone [25]. Pectins are classified as high methoxyl pectin (HMP) or low methoxyl pectin (LMP), with DE .50% and ,50% respectively [26]. HMP forms gels with sugar and acid as a result of the partial dehydration of the molecule to a degree where it is in a state between fully dissolved and precipitated [27]. LMP requires the formation of ionic linkages such as calcium bridges with carboxyl groups to form a gel, popularly known as the egg-box model [28,29] (Fig. 8.3). Pectins are commercially

259

FIGURE 8.2 Structure for (1) amidated, (2) methyl esterified, and (3) carboxylated pectin.

FIGURE 8.3 Egg-box model for calcium pectinate.

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extracted from citrus peel and apple pomace [30]. Other sources of pectin include passion fruit peel, hawthorn wine pomace, grapefruit peel, creeping fig seeds, pumpkin biomass, dragon fruit peel, cocoa husks, cocoa pulp, and jackfruit peel [26,31 38]. These pectins are different in their molecular weights, charges, neutral sugar contents, DE, and the presence of ferulic acid and protein [39]. The plethora of pectin sources makes it a unique polysaccharide with highly variable physical, chemical, and biological characteristics. As a dietary fiber, pectin inherits bioactive properties and can act as an antibacterial, antioxidant, antifungal, and antitumor agent. The main advantage of pectin in pharmaceutical, nutraceutical, and medical applications is its noncytotoxic characteristic to living cells. Further, pectin is easily degraded in the colon through enzymatic digestion and is regarded as generally recognized as safe (GRAS) by the United States Food and Drug Administration [40].

8.2.1 Prebiotic A prebiotic is defined as a nondigestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of colonic bacteria and improving host health [41]. Research studies indicate that pectin exhibits a prebiotic potential. As a dietary fiber, pectin is poorly digested in the small intestine, but it is readily digested in the colon by pectinolytic enzymes and forms short chain fatty acids such as acetate, propionate, and butyrate [42,43]. Pectin can then activate the growth of beneficial bacterial species such as Lactobacillus bravis, Bifidobacterium bifidum, and Bifidobacterium longum. It inhibits the growth of harmful bacteria such as Escherichia coli and Clostridium perfringens [44]. Pectic oligosaccharides obtained from sugar beet pulp and lime peel waste demonstrate better results compared to commercial oligosaccharides such as fructooligosaccharides in promoting joint population of Bifidobacterium and Lactobacillus growth, thus confirming their potential as prebiotic [45]. Citrus pectin oligosaccharides show a significantly high prebiotic score for B. bifidum, Lactobacillus acidophilus, and Lactobacillus paracasei when compared to commercial prebiotics [46 48]. Prebiotic effects on colonic bacteria are related to galactan chain length in pectin where shorter chain galactan oligosaccharide leads to a higher level of bacterial growth [49 52]. The degree of methoxylation does not have any impact on the fermentability properties or short chain fatty acid production, regardless of the origin of pectin [50]. It was found that pectin can be superior to inulin, a gold standard prebiotic in increasing the adhesion of Lactobacillus rhamnosus and at the same time lowering the chance for Salmonella typhimurium adhesion to Caco-2 cells [53]. A synergistic effect is observable when pectin is used in combination with probiotic strain B. longum BB-46 [54]. This combination is relatively efficient in decreasing the intestinal NH41 levels and increasing the butyric acid-producing bacteria, hence, providing beneficial effects on human health.

8.2 Pectin as bioactive dietary fiber

8.2.2 Antibacterial An antibacterial is an agent that kills microorganisms and suppresses their growth. Pectin isolated from fruit has a small antibacterial effect at physiologically relevant concentrations [53]. Pectin does not show any significant antibacterial activity against E. coli and Klebsiella pneumoniae. It, however, has been found to produce good antibacterial effects on all the 16 clinical isolates and 2 reference strains of Helicobacter pylori with the highest antibacterial effect at a low pH (5.0) compared to higher pH environments [55]. Another study has found that citrus oligogalacturonide exhibits a bactericidal effect with a minimal inhibitory concentration of 37.5 μg/mL for Pseudomonas aeruginosa, Listeria monocytogenes, and S. typhimurium, and at 150 μg/mL for Staphylococcus aureus [56]. Smaller molecular weight pectin oligosaccharides give rise to higher antibacterial activities compared to larger pectin oligosaccharides [51,52].

8.2.3 Antioxidant Antioxidants are compounds that may prevent or delay cellular damage by free radicals. They act by scavenging reactive oxygen species to stop radical chain reactions or by inhibiting reactive oxidants from being formed [57]. Pectic polysaccharides from oil pumpkin display moderate antioxidant and radical scavenging activities, which correlate with their content of phenolic compounds [58]. This synergistic effect of pectin and other antioxidants such as polyphenol is reflected in the study of two different species of horsetail plant, namely Equisetum arvense and Equisetum sylvaticum [59]. The phenolic compounds associated with polysaccharidic moieties are apparently responsible for the differences in the anti-α,α-diphenyl-β-picrylhydrazyl (DPPH) scavenging activity of the pectins of different horsetail species. In another study, the galactose residues in the side chains of plum pectin macromolecules appear to play an important role in superoxide anion radical scavenging activity. A higher content of galactose residues in the pectin fraction results in a higher antioxidant effect compared to pectin fractions with lower galactose residues and parent pectin [60]. These observations are supported by pectic polysaccharides isolated from turmeric, where parent pectin and modified pectin differ in their molecular weights, sugar compositions, and side chain characteristics [61], with galacturonic acid and galactose-rich modified pectin giving higher antioxidant properties than parent pectin. The antioxidant effect of pectin is dose-dependent, where a higher dose of citrus pectin is more potent [62]. Hawthorn pectin penta-oligogalacturonides similarly display concentration-dependent scavenging activities against superoxide anion, hydroxyl, and DPPH radicals [63]. The pectic polysaccharide fraction of okra shows a remarkable antioxidant activity in vitro, and by inhibiting the peroxidation chain reaction, it also shows significant antihyperglycemic activity in diabetic mice [46,47].

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8.2.4 Antidiabetic Diabetes is a chronic disease in which the body no longer produces adequate insulin and is translated to hyperglycemia [64]. In vivo animal experiments show that ginseng pectin exhibits significant antihyperglycemic activities in alloxan-induced diabetic mice [65]. Heat processing could change the physicochemical properties of ginseng pectins and influence their physiological properties. The antihyperglycemic activities of pectin increase with the use of a higher processing temperature. The pectic polysaccharide of okra exhibits a significant antihyperglycemic activity in streptozotocin-induced diabetic mice by inhibiting the lipid peroxidation chain reaction [46,47]. The bioactive pectin fragment is speculated to constitute of an RG I backbone with type II arabinogalactan side chains. The pectic polysaccharide from the fruit of Ficus pumila Linn. shows antihyperglycemic activity through the activation of the IRS-1/PI3K/Akt/GSK3β/GS insulin signaling pathway and the AMPK/GSK3β/GS signaling pathway, and the regulation of glucokinase, phosphoenolpyruvate carboxykinase, and glucose-6-phosphatase expression involved in hepatic glycogenesis and glycogenolysis [66]. Citrus pectin significantly reduces insulin resistance [67]. Using pectin, phosphorylated Akt expression is upregulated and GSK3β expression is downregulated, indicating that the potential antidiabetic mechanism of citrus pectin may be mediated through the regulation of the PI3K/Akt signaling pathway.

8.2.5 Antitumor Lung, stomach, colon, and breast cancers are the main causes of death globally [68]. As a dietary fiber, pectin has been shown to decrease the risk of several cancers even though the mechanisms are still not clear [69]. The elucidation of the mechanism of action of pectin is complicated by its structural complexity with modifications in the structure of pectin resulting from the extraction and diverse fragmentation processes [70]. There is growing evidence linking modified forms of pectic polysaccharides with anticancer activity, where modified polysaccharides contain structural elements that can bind to and inhibit galectin-3 [24]. Galectin-3 is a multifaceted and prometastatic protein whose expression is upregulated in many cancers. The spread of tumors involves the exit from the blood vessels and the invasion of the organs. This action can only be accomplished when there is an interaction between galectin-3 and the extracellular matrix. Modified pectins with lower molecular weights can bind to and inhibit the role of the prometastatic regulatory protein, galectin-3, which is responsible for their anticancer action [42,43]. Pectin obtained from apple pomace or citrus peel has been widely evaluated for its anticancer activity [69,71 81]. Other sources of pectin that show promising anticancer activity include flowering plants [82], sugar beet [83], herbs [84,85], okra [86], potato [87], and rose apple [88]. Pectin, either in native or modified form, shows anticancer properties toward breast [75,76,79],

8.2 Pectin as bioactive dietary fiber

leukemia [72], liver [69,77,78], lung [82,84], skin [86], and colon cancers [71,73,74,80,81,83,85,87]. A study suggests that the biological properties of LMP toward cancerous cells are influenced by structural characteristics including molecular weight and monosaccharide composition [79]. In comparison with native pectin polysaccharide with moderate antitumor activity, pectic oligosaccharide fractions possess significant antitumor activity against MCF-7 cells (breast cancer). The high temperature treatment of ginseng pectin translates into the formation of HG-rich pectin, which induces apoptosis in addition to cell cycle arrest, giving rise to better antitumor activity than that of original pectin [85]. Research on modified sugar beet pectin and high-methoxyl HG pectin from Hippophae rhamnoides berry reveals that the DE does not play a significant role in the anticancer activity [82,83]. Neutral sugar-containing RG I regions are essential for bioactivity. The removal of galactose and arabinose through an enzymatic process decreases the anticancer effect of pectin on HT-29 cancer cells (colon cancer) [83]. Nevertheless, the study reveals that the anticancer effect on cells is not depleted entirely when neutral sugar side chains are removed from the parent pectin. This suggests that the backbone of RG I/HG may probably be bioactive on its own. Investigation on apple pectin in the form of pectic acid without any modification found that it is able to induce death in 4T-1 cancer cells (breast cancer) and inhibit tumor growth in vivo [75]. Pectin fragments display concentration-dependent inhibition effects on Caco-2 cells (colon cancer) [74]. Pectin fragments that originate from high methoxy citrus pectin rich with RG II exhibit potentially high antiproliferation activity because of their irregular branched structure and low molecular size. 4,5-Dihydroxy-2-cyclopenten-1-one is identified as a cytotoxic molecule in modified pectin that acts against HepG2 cancer cells (liver cancer) [78] (Fig. 8.4). This molecule possesses an α, β unsaturated carbonyl group that may form covalent adducts with thiols through a Michael conjugate addition, which leads to the denaturation of proteins. This molecule may not be the only active molecule present in modified pectin. Pectic polysaccharide isolated from Codonopsis pilosula composed of rhamnose, arabinose, galactose, and galacturonic acid is cytotoxic to human lung adenocarcinoma A549 cells in a dose- and time-dependent manner [84]. A comparison between different domains of pectin in the form of HG, RG I,

FIGURE 8.4 Cytotoxic molecule in modified pectin that contribute to cytotoxic effects; 4,5-dihydroxy-2cyclopenten-1-one.

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RG II, and arabinogalacturonan (AG) indicates that the RG I domain of pectin from potato possesses antiproliferative activity on HT-29 cancer cells (colon cancer) [87]. Modified pectin shows selective growth inhibition in human cancer cells and normal cells. Enzyme-modified pectin causes an increase in the membrane permeability and damage of human hepatoma HepG2, human lung carcinoma A549, and human colon carcinoma Colo 205, while the growth inhibition of human normal embryonic kidney HEK293 cells is almost not affected [69]. Cancer cells are characterized by a slight increase in galectin-3 expression with reference to normal cells [89]. Through inhibiting the binding of galectin-3 to its ligand, modified pectin can selectively enhance apoptosis and prevent tumorigenesis in cancer cells [81,86].

8.3 Pectin-based oral drug delivery system When a drug is taken by a patient, the resulting biological effects are primarily determined by the pharmacological properties of the drug. These are usually produced by an interaction of the drug with specific receptors at the site of action at a cellular level. Unless a drug can be delivered to its site of action at a rate and concentration that both minimize side effects and maximize therapeutic effects, the efficiency of the therapy is compromised. A delivery system is imperative to enhance or facilitate the action of therapeutics. Pectin has a long standing reputation of GRAS status, relatively low production costs, and high availability [42,43]. There is a growing interest in using pectin as a primary or complimentary oral drug delivery excipient, specifically as a drug delivery system to target the therapeutics at cancerous colon cells. Pectin has great potential for use as an oral colonic drug delivery carrier due to its poor degradation characteristics in the upper gastrointestinal tract and nearly complete degradation by the bacteria living in the colon. The drug release is triggered by the colonic microflora and this introduces a high degree of site selectivity in drug delivery [90]. The design of an oral colon-specific drug delivery system is met with several challenges [91]. The drug delivery system needs to remain in its original dosage form when traveling through the upper gastrointestinal tract and be able to release the incorporated drugs immediately upon reaching the colon. The released drug then needs to be effectively absorbed at a proper rate in order to be therapeutically effective. The application of an oral pectin drug delivery system is hindered by a low colon-specific delivery efficiency, which prevents the accumulation and retention of the drug in the tumor because of its proneness to swelling, leading to premature drug release in the upper gastrointestinal tract. To overcome the challenge of pectin unfavorably dissolving in the upper gastrointestinal tract, site-specific and cell-specific active targeting modalities are rapidly advancing with several kinds of formulations such as tablets, beads, microparticles, pellets, and nanoparticles.

8.3 Pectin-based oral drug delivery system

8.3.1 Tablet Tablets can be produced via a direct compression technique or by wet granulation followed by a compression process [51,52,92 100]. A matrix tablet consisting of zinc pectinate microparticles bearing ketoprofen and mixtures of pectin/dextran has been prepared by direct compression using a hydraulic press [99]. The formulations prevent premature drug release in the upper gastrointestinal tract with 50% of the drug being released after 7 hours of dissolution. Through calcium ionic cross-linkage, a matrix made of low methoxylated pectin requires a longer period to release its encapsulated drug [95]. Increasing the fraction of calcium ionic cross-linkage leads to a higher level of drug release retardation as in the case of hydroxypropylmethylcellulose/pectin/calcium tablet formulation [96]. A pectin and starch paste tablet with a Eudragit S 100 coating is capable of preventing 5fluorouracil release in the gastric environment [98]. An optimized formulation of pectin (66.67%, w/w) and starch paste (15%, w/w) releases a negligible amount of drug at pH 1.2 and 7.4, whereas significant drug fractions are released at pH 6.5 in the presence of rat cecal contents (4%, w/v). In another study, a higher pectin weight ratio in a tablet formulation retards the drug release rate to a greater extent [97,100]. The tablet tensile strength can be increased using higher molecular weight pectin with a lower DE [97]. However, the hardness of a tablet formulation has no effect on drug release. The use of amidated pectin as a tablet excipient gives rise to a dramatic decrease in water solubility as well as favorable controlled-release properties where the tablet hardly releases the drug at pH ,6.8 [51,52].

8.3.2 Beads Beads can be prepared via ionotropic gelation between an ionic polymer and oppositely charge ions (Fig. 8.5). The ionic interaction between the negatively charged carboxylic acid groups of the pectin chains and the positively charged divalent calcium or zinc ions leads to intermolecular crosslinking known as the “egg-box” conformation and forms gelled spherical beads [101]. The insoluble gels function to entrap a given drug and modulate its drug release as a function of pectin DE, molecular weight, counter ion concentration, and solution pH [102]. The release of a poorly-soluble drug such as indomethacin in calcium pectinate gel beads is dependent on the pectin, crosslinking agent, and bead properties [103]. The drug is released from the beads via a diffusion-controlled model of an inert porous matrix. The release of ketoprofen in simulated intestinal fluid is, on the other hand, strongly affected by crosslinking agent concentration and the initial drug load, but not affected by the amount of pectin in the beads [99]. Increasing the crosslinking agent concentration results in a slower drug release pattern, while increasing the drug load increases its release rate due to a reduction in the weight ratio of polymer to drug. Different types of crosslinking agents provide different drug release profiles. Calcium pectinate beads are hydrated and

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FIGURE 8.5 Ionotropic gelation technique of pectin and oppositely charge ion.

8.3 Pectin-based oral drug delivery system

swelled rapidly on contact with simulated intestinal fluid, leading to subsequent drug release due to the formation of a matrix with a high molecular porosity compared to zinc pectinate beads [101]. Zinc ions pose a higher binding ability with a higher affinity than those of calcium ions for pectin. Zinc ions form pectinate beads with a higher gel strength. It is suggested that calcium ions only interact with the carboxylate groups of pectin, while zinc ions interact with both the carboxylate and hydroxyl groups of galacturonate units of pectin [104]. A cationic polymer such as chitosan can be introduced to induce polyelectrolyte complexation with anionic pectin to sustain drug release. Chitosan aids in the encapsulation of an anionic drug such as bovine serum albumin in its coacervation process with pectin to form beads [105]. These beads exhibit an improved pH-sensitive drug release property with respect to colon delivery. An in vivo rat study demonstrated that 72.6% of biologically active anti-A/B toxin immunoglobin of egg yolk (IgY) is released specifically in the colon when chitosan calcium pectinate beads are employed as the carrier [106]. The platform of chitosan calcium pectinate beads can be used as an effective oral delivery system for the biological treatment of a Clostridium difficile infection. A blend of pectin, starch, and chitosan cross-linked by calcium chloride is able to sustain the release of doxorubicin in simulated media of the stomach and small intestine with most of the loaded drug reaching the colon [107]. Lately, pectin silica beads have been designed [108]. The addition of silica in the formulations provides a slower release of mesalazine in vitro within the simulated gastrointestinal conditions in comparison to calcium pectinate beads. A dual crosslinking strategy using calcium ions and epichlorhydrin as the crosslinking agent of pectin alginate beads has been adopted to further reduce the release rate of repaglinide in simulated gastric and intestinal fluids, instead of a single crosslinking approach [109]. The pectin alginate mixture is first cross-linked with a calcium chloride solution by ionotropic gelation method. The beads are then immediately transferred into an epichlorhydrin solution for secondary crosslinking to occur. Through increasing the degree of matrix crosslinking using a dual crosslinking strategy, the beads exhibit a more sustained drug release behavior. Coating pectin beads using Eudragit can assist to sustain the release of the drug by protecting the beads from dissolving in the gastric environment and prolonging the intestinal residence time of the beads and drugs [110]. Enzymes such as pectin methylesterase can act on pectin and produce pectin with different structural and functional properties [111]. Pectin methylesterases show a blockwise deesterification pattern in citrus and sugar beet pectin, which increases the sequential structure of the deesterified groups and decreases the sequential structure of the ethyl-esterified groups [112]. Such modification of HMP gives rise to a significantly higher drug entrapment efficiency and lower drug release rate than those of the commercial low methoxyl citrus pectins [102]. Beads prepared from modified HMP have a smoother and denser surface morphology than those of commercial LMP. Modified HMP is more sensitive to calcium ions during bead formation and enables effective matrix crosslinking.

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8.3.3 Pellets Pellets are defined as small, discrete, free flowing spherical units prepared from fine powders with narrow size distributions, and they are characterized by mean particle sizes ranging from 500 to 1500 μm [113]. With reference to oral colonspecific drug delivery, pellets have been designed as enzymatically-degradable, pH-sensitive, and time-controlled systems for the purpose of site-specific drug targeting [114]. Pectin has been employed in the development of enzymaticallydegradable systems since it can be selectively degraded by colonic pectinolytic enzymes and its use as a drug vehicle can have drug release triggered in the colon [90]. Pectin can act as either a pellet core [115 118] or as a coating polymer [119 121]. The pellet core used in the development of oral colon-specific drug delivery systems usually consists of a drug and microcrystalline cellulose, and is prepared using the extrusion-spheronization technique. As a coating material, pectin can be blended with hydrophobic ethylcellulose to provide sufficient time-delay before the drug is released in the colon [119 121]. A comparison between uncoated pellets and coated pellets with pectin/ethylcellulose shows that 5-fluorouracil, a cancer therapeutic, released from uncoated pellets mainly distributes in the upper gastrointestinal tract and 5-fluorouracil released from the coated pellets mainly distributes in the cecum and colon [121]. Most coated pellets are eliminated from the stomach in 2 hours, move into the small intestine after 2 4 hours, and reach the large intestine after 4 hours of transit [119]. The peak plasma concentration in vivo is 3.21 μg/mL at 16 hours for coated pellets and 22.2 μg/mL at 0.75 hours for uncoated pellets. The coating of pellets by pectin helps to decrease the release of drug in the upper gastrointestinal tract and mitigate oral drug toxicity. The coating of core pellets using pectin/ethylcellulose can be initiated in vivo instead of by in vitro fluid-bed technique [116,117]. In situ intracapsular pellet coating with pectin/ethylcellulose is initiated via wetting of the pectin coat by dissolution medium, followed by the formation of ethylcellulose plug interconnecting with pellets through the binding action of soluble pectin. This in vivo intracapsular coating of pellets is translated to reduced systemic drug bioavailability and enhanced drug accumulation in colon. The majority of 5-fluorouracil is released upon prolonged dissolution and in response to colonic enzyme pectinase, which digests the coat as well as core pectin to facilitate site-specific drug release. The collective action of reduced premature drug release and eventual drug release via colonic enzymatic digestion of pellets as well as coat reduce the tumor number and size through reforming tubular epithelium with basement membrane and restricting expression of cancer from adenoma to adenocarcinoma. Thiolated pectin as the pellet core is able to sustain the release of ketoprofen without additional coating [118]. The pellet core, consisting of thiolated pectin, microcrystalline cellulose, and ketoprofen, is prepared and subjected to mucoadhesion examination through a wash-off test using porcine intestinal mucosa. The inclusion of thiol groups in the pectin structure changes the nature of the

8.3 Pectin-based oral drug delivery system

interaction between the polymer and the mucin. The thiolated pectin core pellet is able to adhere to the intestinal mucosa over 480 minutes of contact and shows a gradual release of ketoprofen. Microcrystalline cellulose appears to be essential in sustaining drug release. The substitution of microcrystalline cellulose with other polymers such as pectin and chitosan results in a faster release of theophylline, dimenhydrinate, and ibuprofen [115]. Replacing microcrystalline cellulose with polysaccharides, especially pectin, leads to increased pellet porosity and faster release of drug from the pellet core, which are unfavorable where oral colonspecific drug delivery is concerned.

8.3.4 Nanoparticles The development of nanoparticles has attracted the interest many scientists in the field of drug delivery systems as well as in tissue engineering, gene delivery, and imaging studies since the 1980s [122]. The widespread use of polymers as biomaterials in the past decade, owing to their unique properties such as good biocompatibility and easy design and preparation, has led to a variety of structures and interesting biomimetic characters being demonstrated from the perspective of nanomedicine [123]. Pharmaceutically, nanomedicine refers to the application of nanotechnology in the design of particles of sizes between 1 and 1000 nm [124]. In the arena of drug delivery, nanoparticles are usually tiny particles of between 20 and 500 nm in dimension. They can be formed from either natural or synthetic polymers, which are used to entrap drugs for improved drug targeting and sustain release [125]. Nanoparticles have outstanding advantages, for example, they can pass through the smallest capillary vessels because of their ultra-tiny volume and avoid rapid clearance by phagocytes so that their duration in the blood stream is greatly prolonged. They can also penetrate cells and tissue gaps to arrive at target organs such as the liver, spleen, lungs, spinal cord, lymph, and colon. They show controlled-release properties due to the biodegradability, pH, ion and/or temperature sensibility of the materials. Last but not least, they can improve the utility of drugs and reduce the toxic side effects [126]. Ionotropic gelation [127 131], polyelectrolyte complexation [132 142], and self-assembly complexation [5,6,143,144] have been widely applied to decorate pectin nanoparticles for oral delivery. Typically, oppositely charged ions such as calcium and magnesium ions or polysaccharides such as chitosan are employed to crosslink/coacervate with anionic pectin [126,131 134,137 142]. Polyelectrolyte complexation can be initiated with or without the addition of bivalent cations. Magnesium ions are divalent in nature [130]. It is reported that the size of the nanoparticles formed through ionotropic gelation with magnesium ions are smaller than those cross-linked by calcium ions [145]. Pectin methotrexate nanoparticles formed via magnesium ionic crosslinking exhibit a higher level of cytotoxicity than that of free methotrexate. 5-Fluorouracil-loaded pectin nanospheres and vesicles have been developed in aqueous media containing calcium ions and carbonate ions under mild conditions

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through the ionotropic gelation method [129]. The uniqueness of this formulation is that no surfactant is introduced. Using calcium ions and carbonate ions in the development of pectin-based nanoparticles, the permeability of the encapsulated drug is reduced, resulting in sustained drug release behavior. These nanoparticles display a greater potency in killing cancer cells compared to that of the free drug [128]. The results are corroborated with a pharmacokinetics study using Sprague Dawley rats, where the nanoparticles are characterized by a much longer half-life in the systemic circulation than the free drug, and a longer drug exposure in vivo is possible. The encapsulation of pectin-based nanoparticles with Eudragit S100 is an alternative approach to prevent premature drug release and allow the nanoparticles to reach the colonic region, wherein they are taken up by the mucosa and have the drug released over an extended period of time [127]. Superparamagnetic iron oxide has been added to pectin nanoparticles loaded with oxaliplatin [131]. Through external magnetic stimuli, the nanoparticles can be positioned at the colon via magnet superparamagnetic iron oxide interaction. Luo et al. [142] fabricated nanocomplex particles of sodium caseinate and pectin as potential oral delivery vehicles. The thermal treatment of the matrix at 85 C for 30 minutes facilitates the formation of stable, compact, and spherical nanocomplexes. The heating significantly improves the encapsulation and controlled-release properties of the model drug, rutin, in simulated intestinal conditions. Rutin in nanocomplexes exhibits a much slower release rate, where only 20% of the drug is released within the first 2 hours under gastric conditions followed by a rapid release under intestinal conditions, where 80% of rutin is released after 4 hours of dissolution. In another study, β-lactoglobulin undergoes complexation with pectin to form stable nanocomplexes [140]. Excess pectin is added during complexation in order to form soluble and negatively charged nanocomplexes that remain stable due to their mutual electrostatic repulsion. The β-lactoglobulin pectin nanoparticles are designed as an oral colon-specific carrier for a newly synthesized anticancer platinum complex (bipyridine ethyl dithiocarbamate platinum(II) nitrate). The nanoparticles only release the drug at pH 7 in simulated colonic fluid. The self-assembly method has been used to prepare pectin nanoparticles with liposome and protein [143,144]. Self-assembly can be defined as the process through which a system’s components, be it molecules, polymers, colloids, or macroscopic particles, are organized into ordered and/or functional structures or patterns as a consequence of specific, local interactions among the components themselves, without external direction [146]. The self-assembly method may engage a layer-by-layer approach using an insoluble matrix as a template (silica) and oppositely charged coating materials (pectin and chitosan) as layers [147]. In the latter, the shell of the nanocapsule is constructed via the electrostatic interactions between pectin and chitosan. The nanocapsules exhibit a high drug loading and pH-sensitive release behavior. A study on a doxorubicin model suggests that nanoparticles can be easily taken up by HepG2 cells and the doxorubicin is rapidly released in response to a low intracellular pH stimulus. Pectin can undergo

8.4 Oral colon-specific drug delivery mechanism

polyelectrolyte complexation with a drug itself such as in the case of cisplatin [135]. The formation of a coordination bond between the platinum(II) atom and the carboxylic group of pectin causes the spontaneous folding of the molecules into a nanoconjugate. A tissue biodistribution study indicated that such a nanoconjugate reduces kidney drug accumulation and improves nephrotoxicity. The nanospray drying technique has been employed to transform pectin molecules into nanoparticles [134,136,148]. Spray drying is a single continuous process. It is simple to scale up and shows minor variations in terms of feed flow as well as the concentration of polymer and the temperature needed to fabricate particles of the desired particle size range [149]. In a study of freeze-dried and spray-dried caseinate zein pectin preformed nanoparticles, the sizes of the original nanoparticles differed following the freeze drying process [137]. The redispersed nanospray-dried caseinate zein pectin nanoparticles nonetheless display small particle sizes with narrow distribution, similar to that of the original nanoparticulate complexes. Despite all the advantages, the nanospray drying process is not suitable for use when processing materials sensitive to the mechanical shear of atomization [150].

8.4 Oral colon-specific drug delivery mechanism According to the World Health Organization, cancer is a leading cause of death worldwide, accounting for an estimated 9.6 million deaths in 2018 [151]. Colorectal cancer is the third most common cancer with 1.8 million new cases reported in 2018 and 862,000 deaths recorded. In principal, cancer is a generic term for a large group of diseases that can affect any part of the body. It arises from the transformation of normal cells into tumor cells in a multistage process that generally progresses from a precancerous lesion to a malignant tumor. Fortunately, when identified early, cancer is more likely to respond to effective treatment and can result in a greater probability of surviving, less morbidity, and less expensive treatment. Surgery remains the primary mode of cancer treatment with chemotherapy and/or radiotherapy recommended upon the nature and extent of the severity of the disease [90]. With reference to local colon cancer treatment, oral colon-specific drug delivery is deemed advantageous from the perspectives that it is noninvasive and carries lower risk of infection and systemic adverse effects. Additionally, complications and costs derived from intravenous chemotherapy can be avoided while maintaining patients’ quality of life since the treatment can be given at home [152]. However, oral chemotherapy is not without limitations with one of the main drawbacks of its intended use being bioavailability [152]. Bioavailability is the extent and rate to which a drug is absorbed or becomes available at the site of drug action [153]. When compared to intravenous administration, the unpredictable absorption pattern of drugs through the oral route may result in either toxicity or subtherapeutic drug dosing. Administering a

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drug through the oral route is not always as simple as increasing or decreasing the drug amount taken. Careful and thorough study of bioavailability through pharmacokinetic and pharmacodynamic methods of a given drug is an important step in oral colon-specific drug delivery system design. Oral colon-specific drug delivery requires the drug to be transported through different environmental conditions beginning with low pH and constant mechanical pressure in the stomach, then protease digestion in the stomach and small intestine, and finally microflora digestion in the colon [154]. Matrix excipients such as pectin show good properties in the form of physicochemical stability, low toxicity, hydrophilicity, gelling ability, and biodegradability. Nonetheless, they are commonly water-soluble [155]. Being soluble in water, pectin is not able to sustain and defend its drug loading capability during its transit through the stomach and small intestine [156]. Designing an oral colon-specific drug delivery system using pectin as the main matrix substance requires strategy to reduce the solubility of pectin in the upper gastrointestinal tract while promoting its degradability in the colonic microflora. Different strategies have been used to deliver drugs to the colon, namely prodrug, pH-dependent system, time-dependent system, pressure-dependent system, and microbial-triggered system (Fig. 8.6). Prodrugs are molecules with little or no pharmacological activity that are converted to the active parent drug in vivo by enzymatic or chemical reactions or by a combination of both [157]. They are designed to overcome pharmaceutical and/ or pharmacokinetic issues associated with the parent drug molecule that would otherwise limit the clinical usefulness of the drug [158]. Since many therapeutic agents nowadays are manufactured and administered in prodrug forms, a new

FIGURE 8.6 Oral colon specific drug delivery approach.

8.4 Oral colon-specific drug delivery mechanism

classification system has been proposed to provide useful information about where in the body a prodrug is converted to the active molecule [159]. Type I prodrugs are bioactivated intracellularly, whereas type II prodrugs are bioactivated extracellularly. Prodrugs can be classified according to their sources, either from natural sources or synthetically made during drug development. Pectin has been used to prepare prodrugs in the form of pectin drug conjugates either through amide, ester, imine, or disulfide bonds [104,160 165]. Ideally, a drug conjugated to the pectin backbone will be in an inactive state and travel through the upper gastrointestinal tract unaffected by the surrounding environment. When the prodrug reaches the lower gastrointestinal tract, the combination effects of pH and microbes trigger glycosidic cleavage and degrade the pectin to release the active drug in the colon. Additionally, a low molecular weight ligand such as folate can be conjugated to the pectin backbone and utilized for targeting cancerous cells in the colon via the folate receptor, which has been known to be overexpressed in several human cancers. Contrary to the azo-polymer technique, the prodrug approach via pectin drug conjugate does not result in the formation of toxic byproducts. However, the synthesis of covalent bonds between pectin and a given drug involves toxic reagents and catalysts that if not cleaned up and purified properly, will eventually leave chemical impurities that will harm the whole drug delivery system. In a normal healthy subject, there is a progressive increase in pH from the stomach (pH 1 2) to the duodenum (pH 4 5.5), jejunum (pH 5.5 7), ileum (pH 7 7.5), and finally the colon and rectum (pH 7 7.5) [166]. The acidic nature of the pectin molecule with a carboxylic acid functional group makes it insoluble in the acidic medium in the stomach and highly soluble at the neutral or slightly alkaline pH in the colon. There is a likelihood that during the long passage of the drug through the harsh environment of the upper gastrointestinal tract to the colon, body fluid may enter the matrix and come into contact with the pectin to trigger a premature release of the drug [167]. The hydrophilicity of pectin renders it susceptible to drug loss in the upper gastrointestinal tract. A thick coat of a hydrophobic polymer can be used to decrease the pH-sensitive solubility attribute of pectin in aqueous media and protect the drug from premature release [116,117,119 121,168]. However, the high weight ratios of ethylcellulose to pectin coating may render the formulation insusceptible to the anaerobic microflora of the colon, which may prevent drug release in the colon [119]. On the other hand, the effects of diet, disease, and the presence of fatty acids, carbon dioxide, and other fermentation products, make the intestinal pH unstable and result in irreproducible colon drug delivery [169]. The pH of the small intestine at the jejunum and ileum (5 7.5) resembles that of the earlier part of the large intestine. This small variation window of pH can further affect the reproducibility of colonic drug release from dosage forms with a pH-dependent approach. The colon is inhabited by a large number and variety of bacteria (1012 CFU/mL), which inevitably secrete many enzymes [169,170]. The unique presence of biodegradable enzymes in the colon from various bacteria such as Enterobacteria,

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Enterococci, Lactobacilli, Clostridia, and Bacteroides makes bacterial degradable polymers such as pectin site specific for colonic drug delivery [156,171]. The combination of a pH-dependent system and a microbial-degraded system has been used by many researchers because of its synergistic effects toward pectin colon-specific drug delivery [116,117,172 174]. The lower pH value and microflora count in the upper gastrointestinal tract help in sustaining the dosage forms and prevent the drug from undergoing premature release. However, in its unmodified form, pectin does not have the characteristics and properties of an oral colonspecific drug carrier. It needs additional excipients as core and/or coating materials to have the drug release modulated. The solid dosage form of pectin can be coated with pH-resistant and hydrophobic materials to withstand the low pH and highly aqueous environment of the stomach and small intestine. Drug release studies of Eudragit S100-coated citrus pectin nanoparticles show that around 60% 5-fluorouracil is released in the presence of rat cecal content [127]. The omission of Eudragit results in the release of around 90% of the drug under the same dissolution circumstances. In situ intracapsular pellet coating similarly utilizes a combination of microflora and pH-sensitive mechanisms [116,117]. The in situ intracapsular wetting of pectin coat provides soluble binder to aggregate the ethylcellulose with pellets forming a plug with less than 25% 5-fluorouracil release in the upper gastrointestinal tract. The majority of the drug is released in the colonic medium where the core pellets and pectin coat are digested by the colonic enzyme pectinase. The potential application of multi-walled carbon nanotubes and a pectin complex for colon-specific delivery based on the enzyme responsive mechanism has been explored [175]. The tablet is selected as the dosage form of interest as it can withstand the various pH conditions of the gastrointestinal tract and effectively prevent the leakage of multi-walled carbon nanotubes. The microbial-triggered system has its own weakness with reference to a high variability in the composition of the gut microflora [176]. Most of the reported methods of drug delivery to the colon are more or less susceptible to changes in diet, diseases, and environmental variables. These factors bring about variations in the bacterial population of the colon from one animal species to another and even from one individual to another [177], affecting the performance reproducibility of the microbial-triggered drug delivery system.

8.5 Conclusion The application of pectin as an oral colon-specific drug carrier has gained increasing interest lately, either as a primary excipient or as a complimentary polysaccharide. The numerous advantages of pectin together with its GRAS status has made pectin the polysaccharide of choice for prodrug, pH-dependent, and microbial-triggered systems. Although its hydrophilicity could prevent it from being an optimum carrier for colonic delivery, the numerous carboxylic acid

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groups in its polymer chains make it modifiable chemically by covalent conjugation, capable of gel forming, and transformable into an insoluble matrix in a low pH gastric medium and small intestinal fluid. As a dietary fiber, pectin inherits the rich bioactive properties of its sources and can act as antibacterial, antioxidant, antifungal, and antitumor agents. Despite the abundance of polysaccharides used as drug carriers, especially for oral colon-specific drug delivery, pectin appears to be one of the most exciting and promising carriers to the colon. Pectins represent an important class of biomaterial with significant research interest in medical and pharmaceutical applications owing to their safety, biocompatibility, biodegradability, ease of physicochemical modification and therapeutic bioactivity.

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[173] Lee CM, et al. Pectin microspheres for oral colon delivery: preparation using spray drying method and in vitro release of indomethacin. Biotechnol Bioprocess Eng 2004;9(3):191 5. [174] Liu J, et al. Preparation and evaluation of pectin-based colon-specific pulsatile capsule in vitro and in vivo. Arch Pharmacol Res 2012;35(11):1927 34. [175] Zhu W, et al. Enzyme-responsive mechanism based on multi-walled carbon nanotubes and pectin complex tablets for oral colon-specific drug delivery system. J Radioanalyt Nucl Chem 2019;320(2):503 12. [176] Gazzaniga A, et al. Time-controlled oral delivery systems for colon targeting. Expert Opin Drug Deliv 2006;3(5):583 97. [177] Sinha VR, Kumria R. Microbially triggered drug delivery to the colon. Eur J Pharm Sci 2003;18(1):3 18.

CHAPTER

Starch as oral colon-specific nano- and microparticulate drug carriers

9

NorulNazilah Ab’lah1,2,3 and Tin Wui Wong1,2 1

Non-Destructive Biomedical and Pharmaceutical Research Centre, iPROMISE, Universiti Teknologi MARA, Puncak Alam, Malaysia 2 Particle Design Research Group, Faculty of Pharmacy, Universiti Teknologi MARA, Puncak Alam, Malaysia 3 Centre of Foundation Studies, Universiti Teknologi MARA, Dengkil, Malaysia

9.1 Introduction Polysaccharides are polymers with a high molecular weight attribute and they are formed through glycosidic bonding of repeating monosaccharides units [1]. Fig. 9.1 shows sources of polysaccharides from animal, vegetal (land and marine), and microbial/fungi origins. Chitin, chitosan, and glycosaminoglycan are derived from animal polysaccharides. Land vegetal polysaccharides include cellulose, pectin, galactomannas, acacia gum, and starch, whilst marine vegetal polysaccharides include agar and carrageenan. Alginate, dextran, gellan, pullulan, sceroglycan, and xanthan are examples of microbial/fungi polysaccharides [2]. Polysaccharides are available in abundance in nature and at low costs [3]. They can be easily manipulated chemically and biochemically to produce new derivatives due to the accessibility of reactive functional groups present in their molecular backbone such as hydroxyl, carboxyl, and amino moieties [4]. With reference to pharmaceutical applications, these chemical moieties can be utilized to conjugate with drugs directly or via linkers. Polysaccharides are biocompatible with mammalian or living tissue, biodegradable, stable, safe, and nontoxic with hydrophilic and gel-forming properties. Combined with their structural versatility, polysaccharides have received widespread commercial interest in pharmaceutical applications as drug carriers and targeted drug delivery systems [1,5,6].

9.2 Polysaccharides as anticancer drug carriers Cancer chemotherapeutics are characterized by their narrow therapeutic index and poor aqueous solubility. The conjugation or complexation of these drugs to polysaccharides provides a safe, biodegradable, and soluble option that is essential to Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00009-3 © 2020 Elsevier Inc. All rights reserved.

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Animals

Microbial/fungi

Vegetal (land/marine)

Alginate

Acacia gum

Chitin Dextran

Sources of polyaccharides

Gellan Chitosan

Agar Carrageenan Cellulose

Pullulan Sceroglycan Glycosaminoglycan Xanthan

Galactomanna Pectin Starch

FIGURE 9.1 Sources of polysaccharides.

promote drug bioavailability [4]. Table 9.1 summarizes examples of polysaccharides to be used as anticancer drug carriers with in vitro and in vivo evaluation of biological performances. These carriers are suitably designed as pH-dependent, slow-release, sustained-release, targeted-release, and controlled-release nanoparticulate or microparticulate systems. In vitro and in vivo evaluations show that they are able to suppress tumor growth through enhancing the cytotoxicity of drugs, and with reduced adversity to normal cells when the targeting attributes are installed in drug delivery systems.

9.3 Colon anatomy and physiology The large intestine (colon) is segmented into four parts, that is, the cecum, colon, rectum, and anus, as illustrated in Fig. 9.2. The average length of the large intestine is approximately 1.5 m (5 ft.) to 1.8 m (6 ft.) with a diameter greater than the small intestine that varies between 2.5 and 7.5 cm (1
3 in.) [23]. Only the last 0.3 m of colon is accessible from the anus since the folding of the splenic flexure resists materials entering the transverse colon [24]. Anatomically, the colon is organized in an inverted “U” shape and separated into four sections, namely the ascending (proximal) colon, transverse colon, descending (distal) colon, and sigmoid colon [23,25]. The colon receives only 5%
10% of all food consumed, is shorter in comparison to the small intestine, and consists of a series of concentric layers that may be detected by means of a light microscope [23,26]. Starting in the lumen, these layers are the columnar mucosa, basement membrane, lamina propria, muscularis mucosae, submucosa, muscularis

Table 9.1 Examples of polysaccharides for use as anticancer drug carriers. Polysaccharide type

Dosage form

Drug

Administration route

Drug release mode

Alginate

Composite

5-Fluorouracil

Not specified

Targeted-release

Amylose

Nanoparticles

Curcumin, doxorubicin

Not specified

pH-dependent

Remark

Reference

Sodium alginate grafted on graphene oxide loaded with 5fluorouracil is designed to treat colon cancer characterized by liver metastasis. In vitro and in vivo studies show that the delivery system is characterized by colontargeted drug release behavior, biocompatibility, and reduced drug cytotoxicity. In vivo experiments with mice indicate that the tumor growth is highly suppressed and the animal survival time is prolonged. Amylose is produced as a biopolymeric nanocarrier for codelivery of curcumin and doxorubicin anticancer drugs. The drug release study shows that the doxorubicin that is adsorbed on the surfaces of the nanocarrier is released earlier and faster than curcumin that is entrapped in the core of the delivery system. The nanocarrier made of amylose has a great potential application in drug delivery and cancer therapy. No in vivo biological performance tests were conducted.

[7]

[8]

(Continued)

Table 9.1 Examples of polysaccharides for use as anticancer drug carriers. Continued Polysaccharide type

Dosage form

Drug

Administration route

Drug release mode

Chitosan

Nanocomposite

Bis-demethoxycurcumin analogue

Not specified

Sustained-release

Nanogel

Doxorubicin

Not specified

Not specified

Nanocomposite

Cisplatin

Not specified

Controlled- and targeted-release

Remark

Reference

Chitosan, blended with starch via ionic gelation method is loaded with drug for breast cancer treatment. The delivery system shows anticancer activity against the breast cancer cell lines (MCF7). Chitosan nanogel is prepared through electrostatic interaction between chitosan and carboxymethyl-chitosan. Cell culture testing using colorectal cancer cells (Caco-2 cells) shows greater reduction in the percentage of cell viability, improved mucoadhesion, and limited cellular permeability, which enable prolonged contact of drug with intestinal mucosa and improved local drug action. Cisplatin is encapsulated in a chitosan-poly oxalates nanocomposite. The cytotoxicity of cisplatin-loaded nanocomposite is similar to that of free cisplatin toward MCF-7 cells where it is able to inhibit the growth of the cells.

[9]

[10]

[11]

Cyclodextrin

Nanoparticles

Doxorubicin

Not specified

Sustained-release

Nanoparticles

5-Fluorouracil

Not specified

Slow-release

Cyclodextrin is blended with poly (2-dimethylamino) ethyl methacrylate and poly(ethylene glycol) where the drug is loaded into the polymer via host
guest interaction. The cellular uptake of doxorubicin-loaded nanoparticles is promoted in the case of human cervical cancer cell line (HeLa) and hepatic carcinoma cell line (HepG2). The in vivo antitumor experiment on BALB/c mice bearing cervical tumors shows that the doxorubicin-loaded nanoparticles can effectively suppress the growth of tumors without any significant side effects. Cyclodextrin is conjugated with serum bovine albumin and then loaded with drug prior to final conjugation with folic acid. The biological performance of the conjugate is assessed by means of a cell culture approach using human hepatocellular carcinoma cell line (SMMC-7721) and HeLa cells. The folic acid-labeled conjugate enhances folate receptor-mediated endocytosis¸ which leads to an increase in the intracellular uptake propensity of the drug and induces the apoptosis of cells by downregulation of adenosine triphosphate level and overexpression of caspase-3.

[12]

[13]

(Continued)

Table 9.1 Examples of polysaccharides for use as anticancer drug carriers. Continued Polysaccharide type

Dosage form

Drug

Administration route

Drug release mode

Dextran

Nanoparticles

Doxorubicin

Intravenous injection

pH-dependent

Guar gum

Magnetic nanoparticles

Doxorubicin

Intravenous injection

Targeted-release

Hydrogel

5-Fluorouracil

Oral

Controlled-release

Remark

Reference

Dextran coated with super paramagnetic iron oxide is designed as a magnetic carrier for doxorubicin with its targeted attribute induced by external magnetism. This system demonstrates less toxicity with better antitumor effectiveness at in vitro (MTT assay) and in vivo (rabbit VX2 liver tumor model) levels. Guar gum is coated on cobalt ferrite nanoparticles and conjugated with doxorubicin to magnetically control the drug targeting using an external magnetic field gradient. The delivery system efficiently reduces the growth of Huh-7 cells (liver parenchymal cancerous cells) and shows negligible effect on CHO cells (normal cells). Guar gum, grafted with lysineβ-cyclodextrin and acting as a drug carrier for 5-fluorouracil, is suggested to offer an operative therapy for colorectal cancer with reduced drug dose and duration of treatment. The formulation displays a great cytotoxicity level against subline of the ubiquitous keratin-forming (KB) tumor cells through generating reactive oxygen species.

[14]

[15]

[16]

Inulin

Nanocomposite

5-Fluorouracil

Intravenous injections

Targeted-release

Pectin

Nanoparticles

Epirubicin

Intravenous injections

Targeted-release

Starch

Nanoparticles

Doxorubicin

Not specified

Sustained-release

Inulin is conjugated with silvergraphene quantum dots and hyaluronic acid as a targeted drug delivery vehicle specifically for pancreatic cancer. Inulin acts as a surface modifier for metal nanoparticles to mitigate their toxicity. In vivo and in vitro studies show that the delivery system is able to inhibit the growth of pancreatic cancer cells. Inulin-ibuprofen is conjugated with arginylglycylaspartic acid-peptide (RGD-peptide), tumor-specific targeting molecules, and loaded with epirubicin. The delivery system increases cellular uptake and the cytotoxicity of the drug at in vitro level with BCG 823 cells (gastric cancer cells). It shows superior tumor growth inhibition and reduced systemic toxicity when compared with free epirubicin and non-RGD peptideconjugated nanoparticles at in vivo level. Acetylated starch nanoparticles are synthesized from broken rice and loaded with doxorubicin. Toxicity analysis using rat hepatocyte model suggests that the modified starch is biocompatible and can be used in drug delivery system design. In vitro analysis shows that the nanoparticulate system significantly enhances the cytotoxicity of doxorubicin against HeLa cells.

[17]

[18]

[19]

(Continued)

Table 9.1 Examples of polysaccharides for use as anticancer drug carriers. Continued Polysaccharide type

Dosage form

Drug Curcumin

Xylan

Nanoparticles

5-Fluorouracil

Administration route Oral

Oral

Drug release mode Sustainedrelease

Targeted-release

Remark Cassava starch is blended with poly(vinyl alcohol) and loaded with curcumin for use in cancer treatment. The system is nontoxic to normal cells and demonstrates anticancer potential on skin cancer cells against the pure curcumin. Xylan is conjugated with modified 5-fluorouracil to act as colonspecific prodrugs. The cytotoxicity study with human colorectal cancer cell line (HTC-15 and HT29) shows that the conjugates are more active against the cell lines as compared to the free drug.

Reference [20]

[21]

9.3 Colon anatomy and physiology

Right colic (hepatic) flexure

TRANSVERSE COLON

Left colic (splenic) flexure

Taenia coli

Haustra

ASCENDING COLON

DESCENDING COLON

Ileum Sigmoid flexure

Ileocecal valve Cecum

SIGMOID COLON

Appendix

Rectum Anus

FIGURE 9.2 Colon anatomy [22].

propria, inner circular layer, outer incomplete longitudinal layer (taenia coli), and serosa [27]. Mucosa in the large intestine is characterized by the absence of villi and fewer microvilli of epithelial cells in comparison to the small intestine [28]. The mucosal surface quality dictates the chances of microbial pathogens entering into the host and the formation of prominent sites of microbially induced diseases that lead to organ dysfunction. Previous studies indicate the role of epithelial cells as an integral component of a communications network that involves interactions between epithelial cells, luminal microbes, and host immune and inflammatory cells [29]. The transverse colon can be separated into two parts, which are the right and the left colon. The cecum and ascending colons are known as the right colon and play a major role in the fermentation of undigested sugars, and in water and electrolytes absorption with a maximal absorptive capacity of up to about 4.5 L/day [25]. The absorption process is assisted by segmental contraction, which circulates chyme across the colonic mucosa [30,31]. The left colon, which consists of the descending colon, sigmoid colon, and rectum is principally involved in the storage and evacuation of stool. The aldosterone hormone, which is absent in the small intestine, is secreted in response to total body sodium depletion or potassium loading to stimulate sodium absorption and potassium secretion in the colon

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Table 9.2 Colonic pH profiles. Region

pH

Cecum Ascending colon Transverse colon Descending colon Rectum

5.5
7 5.7
6.9 5.8
7.4 6.3
7.7 B7

Adapted from Amidon S, Brown JE, Dave VS. Colon-targeted oral drug delivery systems: design trends and approaches. AAPS PharmSciTech 2015;16:731
41.

[25]. Table 9.2 displays the colonic pH values for the various regions of the colon. The presence of short chain fatty acids arising from the bacterial fermentation of polysaccharides changes the pH in every part of the colon [32].

9.4 Colon cancer Colon cancer is cancer of the colon or sometimes known as the large intestine or large bowel located in the lower part of the digestive system, whilst rectal cancer is cancer found in the end of the colon. These two cancers are often referred to as colorectal cancer [33]. Tumors that grow inside the bowels are often called polyps or neoplasms and are sometimes known as neoplastic polyps, colonic polyps, or lesions. Polyps can be divided into two types, which are small hyperplastic polyps and larger adenomatous polyps [23]. Adenomatous polyps are common in adults over the age of 50; however, the majority of polyps will not develop into adenocarcinoma, but it is still the most common and clinically important form of polyps since it represents approximately one half to two thirds of all colorectal polyps and is associated with a higher risk of colorectal cancer [34]. Cancer can metastasize through the tissue, lymph system, and blood in the body. Under such circumstances, the cancerous cells break away from where they began (primary tumor) and travel through the lymph system or blood to other parts of the body [35]. Early colorectal cancer often has no prominent symptoms, but once the polyps grow, it can bleed or obstruct the intestine. The warning signs that can be seen are rectum bleeding, blood in the stool or in the toilet after having a bowel movement, dark- or black-colored stools, cramping pain in the lower stomach, a feeling of discomfort or an urge to have a bowel movement when there is no need to have one, new onset of constipation or diarrhea that lasts for more than a few days, and unintentional weight loss [36]. Age, sex, a sedentary lifestyle, a previous history of colorectal cancer or certain kinds of polyps in patients, having a history of ulcerative colitis or Crohn’s disease, positive family history of colorectal cancer, race or ethnic background, type II diabetes, certain family syndromes like familial adenomatous polyposis or hereditary nonpolyposis colon cancer (also known as Lynch syndrome), meat

9.4 Colon cancer

consumption, smoking, alcohol consumption, metabolic syndrome, and obesity are some risk factors that may increase a person’s chance of getting polyps or colorectal cancer [37
40].

9.4.1 Colon cancer statistics Cancers have exhibited an increasing incidence worldwide over the years. According to the National Cancer Society of Malaysia, 37,000 new cases of cancer and 22,000 cancer-related deaths are reported annually nationwide. Developing countries are expected to receive a greater surge in statistics, where it is estimated that 21.4 million new cases will be reached by 2030. Amongst all the cancer cases, colorectal cancer is one of the main contributors to the figures stated [41]. According to the National Cancer Institute, the numbers of estimated new cases of colon and rectal cancers in the United States for the year of 2014 were 96,830 and 40,000 cases respectively where the death rate for colorectal cancer patients was 50,310 cases [33]. Based on the data provided by the International Cancer Screening Network for Colorectal Cancer Incidence and Mortality 2008 on 100,000 males and 100,000 females in the United States, the annual incidence rates and annual colorectal deaths for males are 34.1% and 9.9% respectively, whilst for females are 25% and 7.7% respectively. In Malaysia, the annual incidence rates and annual colorectal deaths for males are 19.6% and 13%, whereas for females are 15.5% and 10.2% respectively [42]. The colorectal cancer death rate in 2015 (14 per 100,000) was half of what it was in 1975 (28 per 100,000) because of increased screening, a decline in incidence, and improvements in treatment. Between 2006 and 2015, the death rate declined by 2.9% per year among individuals aged 55 and older, but increased by 1% per year among adults younger than 55 [43]. In 2018, the number of new cases of colon cancer was estimated to be 1,096,601 (6.1%) with 551,269 (5.8%) deaths [44]. The American Cancer Society recommends that adults aged 45 years and older with an average risk of colorectal cancer should undergo regular screening tests either by noninvasive (fecal tests) or invasive structural examination for the detection of the possibility or extent of cancer growth (Fig. 9.3), depending on patients’ preference and test availability [45]. Generally, fecal tests are a diagnostic tool to primarily identify cancer [34,46]. These inexpensive tests are the guaiac fecal occult blood test and fecal immunochemical tests for hemoglobin. Deoxyribonucleic acid, ribonucleic acid (RNA), protein biomarker stool and blood tests are newer noninvasive tests [47,48]. Structural examination, on the other hand, includes flexible sigmoidoscopy, colonoscopy, double-contract barium enema, computed tomographic colonography [34], colon capsule endoscopy, and magnetic resonance colonoscopy [47]. Both of the mentioned screening tests may be used alone or in combination to improve sensitivity or in some instances to ensure a complete examination of the colon [34]. Colon cancer is characterized by its stages of occurrence. The clinical stage is derived from the outcomes of physical examination, biopsy, and imaging tests, while the pathologic stage combines the outcomes of surgery and

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Invasive test

Non-invasive test

• DNA, RBA and protein biomaker stool/blood tests • Fecal immunochemical tests • Guaic fecal occult blood test

• • • • • •

Colonoscopy Colon capsule endoscopy Computed tomographic colonography Double-contract barium enema Flexible sigmoidoscopy Magnetic resonance colonocopy

FIGURE 9.3 Colon cancer screening tests. Dukes Staging System (Introduced by Cuthbert E.Dukes, 1932) Type A: carcinoma only limits to the submucosa (rectal wall) Type B: Tumours or lesions spread through the muscular layer of bowel wall excluding lymph nodes Type C: Metastasis in regional lymph nodes

TNM Staging System Stage 0: Cancer cells are found in the mucosa (the inner most layer) of colon wall Stage I: Caner cells grow through the superficial lining (mucosa) of colon or rectum but do not spread beyond the colon wall or rectum Stage II: Cancer cells grow into or through the wall of colon or rectum but do not spread to nearby lymph nodes Stage III: Cancer has invaded nearby lymph nodes Stage IV: Cancer spreads to distant organs

Kirklin’s Classification (Modified by Kirklin Dockery and Waugh, 1949 on Type B Dukes staging system) Type B1: Lesions extend into muscularis propria Type B2: Lesions penetrate muscularis propria with negative nodes

Astler-Coller System (Modified by Astler and Coller, 1954 on Type C Dukes staging system) Type C1: Lesions limit to wall with positive nodes Type C2: Lesions is found through all layers with positive nodes

SEER Summary Staging System In situ: Cancers confine in the wall of colon or rectum Local: Cancers penetrate into the wall of colon and rectum but still do not invade nearby tissues Regional: Cancers spread through the wall and invade nearby tissues or lymph nodes Distant: Metastasis of cancer normally confined at liver or lung

FIGURE 9.4 Colon cancer grading systems.

observations made under the clinical stage [49]. Dukes [39,50], tumor, node, and metastasis [35,51,52], and Surveillance, Epidemiology and End Results summary [36] staging systems are devised for use in the characterization of colon cancer development as summarized in Fig. 9.4.

9.4.2 Treatment modes, their disadvantages, and limitations There are four main therapeutic cures that will be endorsed depending on the cancer stage, which are surgery, radiation therapies, chemotherapies, and targeted

9.4 Colon cancer

therapies. Surgery is the regular core treatment for rectal cancer and for the earlier-stages of colon cancer. With reference to colon cancer, surgery can be segmented into three categories, which are polypectomy and local excision, colectomy (either open colectomy or laparoscopic-assisted colectomy), and diverting colostomy. In the case of rectal cancer, the modes of surgery are extended further to polypectomy and local excision, local transanal resection, transanal endoscopic microsurgery, low anterior resection, protectomy with coloanal anastomosis, abdominoperineal resection, pelvic exenteration, and diverting colostomy [36]. According to Hohenberger et al. [53], colon cancer with a low risk of lymph node metastases and local recurrence can be treated by endoscopic polypectomy or segmental resection. Complete mesocolic excision with central vascular ligation was introduced by Hohenberger, applying the concept of the total mesorectal excision, which is a standard surgery for rectal cancer adopted in many western countries to treat stage three colon cancer with an excellent resection outcome [54]. The hitches of complete mesocolic excision, according to Willaert and Wouter [55], are routine implementation that may harm the patients in association with longer operating times, major vascular damage, and autonomic nerve injury [55]. Randomized trials reporting relevant endpoints are required before the mentioned technique can be recommended as a standard approach in colon cancer surgery [55]. Laparoscopic resection has many successful short- and long-term surgery results and its suitability has been compared to the conventional open surgery technique. Laparoscopic-assisted colectomy offers many advantages such as an increased probability of cancer survival in terms of faster postoperative recovery, less pain for patients leading to less use of analgesic, shorter hospital recovery time, and less blood loss. However, the time taken for the excision is longer than in open surgery [56
59]. Overall, the laparoscopic approach is regarded to be a safe alternative treatment with better short-term outcomes and similar long-term oncological control compared with open resection [58]. With reference to radiation therapy, recurrent treatment brings tumor shrinkage and down-staging and improves resectability and sphincter preservation [60]. The treatment can be applied in three different ways, that is, it can be used as an adjuvant treatment either postoperatively after complete adenocarcinoma resection or preoperatively to transform a nonresectable tumor to a resectable one and lastly the therapy can also be used alone in patients who are medically inoperable or refuse radical surgery [61]. Radiation therapy can be categorized into external beam radiation therapy, endocavitary therapy, and brachytherapy. The side effects such as skin changes in the area where the radiation passes, nausea and vomiting, diarrhea, rectal irritation, bladder irritation, and tiredness can be witnessed for patients undergoing the treatment [38]. In the case of chemotherapy, neoadjuvant treatment before surgery is used to shrink the tumor and to control its local spread, whilst adjuvant and palliative treatment is implemented after surgery to destroy any microscopic cancer cells that may remain after surgery and to reduce the risk of cancer recurrence

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and increase the disease-free survival and overall survival rates [62]. Examples of anticancer drugs that are used in the treatment of colon cancer are doxorubicin, bevacizumab (Avastin), capecitabine (Xeloda), oxaliplatin (Eloxatin), 5fluorouracil, irinotecan (Camptosar), levoleucovorin (Fusilev), irinotecan hydrochloride, leucovorin calcium, nivolumab (Opdivo), panitumumab (Vectibix), regorafenib (Stivarga), and panitumumab (Vectibix). These drugs can be used in combination such as CAPOX (capecitabine and oxaliplatin), FOLFIRI (leucovorin calcium, 5-fluorouracil, and irinotecan hydrochloride), FOLFIRI-BEVACIZUMAB (leucovorin calcium, 5-fluorouracil, irinotecan hydrochloride, and bevacizumab), FOLFIRI-CETUXIMAB (leucovorin calcium, 5-fluorouracil, irinotecan hydrochloride, and cetuximab), FU-LV (5-fluorouracil and leucovorin calcium), XELIRI (Xeloda and irinotecan hydrochloride), and XELOX (Xeloda and oxaliplatin) [63]. The drugs are given in cycles (either in combination or alone) of treatment days followed by days of rest with varied durations as a function of drug type [64]. The latest colorectal cancer treatment mode involves targeted therapies that use the drugs of standard chemotherapy to treat advanced cancers in a targeted manner [38]. Generally, the targeted therapy works via (1) blocking or turning off the chemical signals that tell the cancer cells to grow and divide (signal transduction inhibitors); (2) modifying the protein function within the cancer cells that plays a role in controlling gene expression so that the cells will die (gene expression modulators); (3) blocking the growth of new blood vessels supplying tumors (angiogenesis inhibitors); (4) triggering the immune system to destroy the cancer cells (immunotherapies); and (5) carrying a drug to the cancer cells instead of normal cells (monoclonal antibodies as targeting ligand) [65,66].

9.5 Colon-specific drug delivery Table 9.3 summarizes the experimental chemotherapy used for the treatment of colon cancer. The mode of drug administration is commonly mediated by means of injection route. The intravenous administration of cancer chemotherapeutics is met with a major limitation in relation to their nonspecific action against healthy cells apart from colon cancer cells [73]. This brings about bone marrow, liver, and gastrointestinal tract toxicity [74]. Tenner and O’Neil highlight that such a situation can be improved through optimizing the use of traditional chemotherapeutic agents or even new cytotoxic agents with targeting ligands [75]. The therapeutic index of drugs can be improved by enhancing their targeted delivery attribute and reducing their associated adverse effects over normal cell populations [76,77]. With reference to early stage colon cancer, oral colon-specific delivery systems are envisaged to provide an effective and safe chemotherapy with reduced systemic adverse effects, reduced drug dose, reduced treatment duration, and

9.5 Colon-specific drug delivery

Table 9.3 Experimental chemotherapy for treatment of colon cancer. Drug and dosage form Avastin and siRNA (small interfering RNA) in the form of hydrogel patch

Cisplatin

Material Dendrimer and dextran

Shikonin

Remark Implantable dendrimer
dextran hydrogel patch doped with both drug
gold nanorods and siRNA
gold nanospheres for local colorectal cancer using triple-combination therapy, namely gene, drug, and phototherapy is developed. Spherical gold nanoparticles are used as a first wave of treatment to deliver siRNAs against a key oncogene driver (Kras) and rod-shaped nanoparticles mediate the conversion of near-infrared radiation into heat causing the release of a chemotherapeutic as well as thermally induced cell damage. Subcutaneous tumors are induced in male severe combined immunodeficient hairless congenic mice by injection with LoVo-6-Luc-1 colorectal cancer cells. The patch is implanted adjacent to the colorectal tumor when the tumor volume reaches about 100 mm3. Its application to nonresected tumors is characterized by complete tumor remission. The same application to the resected tumor site eliminates tumor recurrence. Five-week-old athymic BALB/ cA nu/nu female mice are subcutaneously injected with human colorectal carcinoma cell line (HCT)-116 cells into the right flank. Shikonin is used as a reactive oxygen species inducer to enhance cisplatin-induced colon cancer cells apoptosis by triggering reactive oxygen species-mediated mitochondrial dysfunction pathway. The growth of the tumor is effectively inhibited. Combining

Reference [67]

[68]

(Continued)

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Table 9.3 Experimental chemotherapy for treatment of colon cancer. Continued Drug and dosage form

Material

NS398 and thiostrepton




Doxorubicin and TRAIL (tumor necrosis factorrelated apoptosisinducing ligand) nanoparticles

Human serum albumin

Curcumin-loaded micellar nanoparticles

Monomethyl poly (ethylene glycol)
poly (ε-caprolactone)
poly (trimethylene carbonate) (MPEG-P (CL-coTMC))

Remark a low dose of shikonin with cisplatin is a potential approach for the treatment of human colorectal cancer. Cyclooxygenase-2 (Cox-2) has been found to be overexpressed in various cancer cells, whilst forkhead box protein M1 (FoxM1) signaling has been implicated to be associated with the carcinogenesis of tumor development in colorectal cancer as well as other solid tumors. The cotreatment with Cox-2 inhibitor, NS398, and FoxM1 inhibitor, Thiostrepton, in nude mice bearing subcutaneous HT-29 cell xenografts significantly downregulates Cox-2 and FoxM1 expression. BALB/c nu/nu 6
7-week-old male mice are used as the tumor xenograft model where the tumors are established by inoculating human colorectal cancer cell line (HCT-116 cells) subcutaneously into the right dorsal flank of each mouse. Albumin based nanoparticles codeliver doxorubicin and TRAIL display marked tumor targeting and excellent antitumor efficacy in HCT-116 colon cancer-bearing mice compared to single administration of doxorubicin
albumin nanoparticles. Curcumin is encapsulated in MPEG-P (CL-co-TMC) in the form of micelles through a single-step solid dispersion processing method. BALB/c female mice are subcutaneously injected with CT26 colon tumor model cells. The tumor-bearing

Reference

[69]

[70]

[71]

(Continued)

9.5 Colon-specific drug delivery

Table 9.3 Experimental chemotherapy for treatment of colon cancer. Continued Drug and dosage form

Doxorubin and cisplatin nanoparticles

Material

Dextran

Remark mice are treated with curcumin micelles (50 mg/kg curcumin equivalent) daily for 3 weeks. In vivo anticancer studies confirm that apoptosis induction and cellular uptake of drug by CT26 cells increase with curcumin micelles compared with free curcumin. The curcumin micelles are more effective in suppressing the tumor growth of subcutaneous CT26 colon tumor. Doxorubin-loaded cisplatin cross-linked polysaccharidebased nanoparticles are developed. The therapeutic efficacy of the nanoparticulate formulation is evaluated in three solid tumor animal models (male or female BALB/c mice at 5
6 weeks of age), which are characterized by subcutaneous colorectal carcinoma (established by subcutaneous injection of CT26 cells in the right flank of mice), primary colorectal carcinoma (established by intraperitoneal injection of dimethylhydrazine dissolved in ethylene diamine tetraacetic acid disodium, EDTA-Na2 with a pH of 6.5), and metastatic mammary carcinoma (established by subcutaneous injection of breast cancer cell line, 4T1 cells into the mammary fat pad of female mice) are developed. The application of nanoparticulate formulation carrying a combination of anticancer agents efficiently inhibits cancer tissue growth in vivo in all solid tumor animal models.

Reference

[72]

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improved patient compliance due to convenience in drug administration compared to that of injection routes [32,40,78]. A vast range of review studies has been published on the use of polysaccharide/polymeric matrices and reservoir drug delivery systems to convey colon-specific drug administration via the oral route [31,39,74,79,80]. Among all polysaccharides, pectin has been the primary interest of many researchers in the development of oral colon-specific nanoparticulate and microparticulate delivery systems due to its biodegradable, biocompatible, anticancer, cancer preventive, and ease of processing attributes, and its digestibility by colonic microbial enzymes to elicit colon-targeted drug release or delivery.

9.6 Starch as a drug carrier Carbohydrate polymers such as polysaccharides have drawn a great deal of attention in pharmaceutical industries with respect to oral colon-specific drug delivery system development. While the sources are abundant, easily available, and inexpensive, these polymers can also be modified chemically to meet site- and cellspecific delivery action. Starch is an example of a polysaccharide stored in plants as a source of energy like glycogen in the human body. Starch is also known as a renewable raw material since it is an indirect product of photosynthesis that is synthesized from glucose [81]. Normally, unmodified starch is not directly applied in drug delivery system design due to its low shear stress resistance, tendency of thermal decomposition, high retrogradation and syneresis, and poor processability and solubility in common organic solvents [82]. The use of starch derivatives, either physically, enzymatically, or chemically prepared, is met with growing interest in the medical and pharmaceutical industries. In the medical industry, starch and its derivatives are effective in treating dermatitis, iodine poisoning, and acute diarrhea, and act as an emollient and major base in enemas. In the pharmaceutical industry, starch derivatives have been used as encapsulating agents, matrices for drug carriers, and excipients of specific functionalities [83].

9.6.1 Physicochemical properties of starch The main elements in starch are amylose and amylopectin. The percentage of these two elements differs based on the botanical origin of the plant itself. In native starch, the percentage of amylose is typically smaller, between 10% and 35% when compared to amylopectin. In amylose-rich starch, the amylose fraction is higher and can reach up to 70%. Waxy starch, known as high-amylopectin starch, has in a trace amount, 0%
4%, of amylose available [81]. Amylose and amylopectin are the major components found in starch with about 98%
99% of the dry weight of native granules with the remainder being lipids, minerals, and phosphorus in the form of phosphates esterified to an

9.6 Starch as a drug carrier

6

CH2OH CH2OH 5 O 4 OH O 1 OH O O O 2 3 OH OH -1, 4-glycosidic bonds Amylose CH2OH OH

OH

O

O OH

OH

O

OH

O OH

O O OH

n

CH2OH OH

O

OH

O -1, 6-glycosidic bonds

O OH

n

6

CH2OH

O

O

OH

O

OH

CH2OH

CH2OH

O

6

CH2OH

O

CH2OH

O O 4

OH

OH -1, 4-glycosidic bonds Amylopectin

CH2

5 O OH 3

2 OH

CH2OH OH

O

O O

n

OH

FIGURE 9.5 Chemical structures of amylose and amylopectin.

hydroxyl moiety of glucose [81]. Fig. 9.5 shows the chemical structures of amylose and amylopectin. Amylose is largely linear, that is, a flexible polymer of glucosyl residues linked via α-1,4-glucosidic linkages [84]. The amylopectin exists as a highly branched polymer containing on average one branch point of α-1,4- to α-1,6-D-glucan polymer linked for every 20
25 straight chain residues [85]. The molecular weight of amylose is between 105 and 106 g/mol with a degree of polymerization of 1000
10,000 glucose units. Whereas amylopectin has a molecular weight of about 107
108 g/mol and a degree of polymerization that may exceed one million [86,87]. Starch granule crystallinity can be divided into four types, namely A-type (obtained from cereal starches unless amylose content exceeds 40%), B-type (obtained from root and tuber starches, high amylose varieties, and retrograded starch) [88], C-type (legume, root, some fruits and stem starches [86], and beans and peas), and V-type (some high-amylose starches and gelatinized lipid-containing starches). The highest level of crystallinity is observed at intermediate water contents of granules [88]. It is assumed that amylose helices may contribute to granule crystallinity in high-amylose starch [86].

9.6.2 Resistant starch Rapidly digestible starch, slowly digestible starch, and resistant starch are three main categories of starch derivatives [89]. Rapidly digestible starch refers to

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starch that is hydrolyzed in vitro within 20 minutes with α-amylase and amyloglucosidase [90]. Slowly digestible starch falls in the categories of enzyme inaccessible starch and raw starch, which is fully hydrolyzed in vitro with a prolonged incubation of 20
120 minutes, whilst resistant starch is starch that is not hydrolyzed after 120 minutes of incubation with pancreatin and amyloglucosidase [89]. Thus it is envisaged that resistant starch is an ideal candidate for the development of oral colon-specific drug delivery systems. Resistant starch is broadly categorized into four denominations, namely RS1 (physically entrapped starch or inaccessible to digestion by entrapment in a nondigestible matrix; or physically protected), RS2 (ungelatinized starch; raw starch granules such as potato, high amylose corn, and green banana; or ungelatinized resistant granules with type-B crystallinity), RS3 (retrograded starch), and RS4 (chemically modified starch due to crosslinking with chemical reagents) [81,91
94]. The latest research studies have led to the exploration of another resistant starch denomination known as RS5 (amylose
lipid complex and considered as thermally stable) [95,96]. Resistant starch has been evidenced to be propitious to health based on its physiological effects. Resistant starch provides the dietary fiber fraction of food and is believed to function as fiber in the human digestive tract. It reduces postprandial glycemic and insulin response, thus, reducing the risk of developing type II diabetes, obesity, and cardiovascular diseases [97]. Resistant starch reveals a positive impact on colonic health by increasing cell production rate or by decreasing the colonic epithelial atrophy in comparison to no-fiber diets [93,94,98]. The fermentation of resistant starch to hydrogen, methane, carbon dioxide, lactic acid (transient), and short chain fatty acids (acetate, propionate, and butyrate) after reaching the colon gives beneficial effects in colon disease prevention (protection against colonic carcinogenesis) [99,100]. Butyrate is one of the main energy substrates for large intestinal epithelial cells and has shown to have inhibitory effect on the growth and proliferation of tumor cells in vitro by cell cycle arrest [101]. The administration of resistant starch may impede the growth and/or development of neoplastic lesions in the colon suggesting that colon tumorigenesis may be highly sensitive to dietary intervention. Adults who may have preneoplastic lesions in their colon may benefit from dietary resistant starch. Resistant starch is suggested to be beneficial as a preventive agent for individuals that are diagnosed as high risk for colon cancer development [102]. It is also claimed that resistant starch can act as a substrate for the growth of probiotic microorganisms that keep the colon healthy [103]. Other beneficial physiological effects of resistant starch are the inhibition of fat accumulation and the reduction of gall stone formation [104].

9.6.3 Preparations of resistant starch Resistant starch can be prepared via several approaches such as acetylation, acid hydrolysis, amylose
lipid complexation, crosslinking, enzymatic debranching,

9.6 Starch as a drug carrier

Reduce molecular weight Heat-moisture treatment and annealing methods Hydrothermal treatment

Convert amylopectin into amylose

Substitute hydroxyl groups (hydrophilic) with acetyl groups (hydrophobic)

Acid hydrolysis

Acetylation

Enzymatic debranching

Amylose-lipid complexation Crosslinking

Decrease enzyme susceptibility

Strengthen the hydrogen bonds

FIGURE 9.6 Preparative approaches of resistant starch.

and hydrothermal treatment (Fig. 9.6). The inherent properties of starch (starch crystallinity, granular structure, amylose:amylopectin ratio, amylose retrogradation, amylose chain length, amylopectin linearization), interaction profiles of starch with other components (protein, dietary fiber, enzyme inhibitor, ion, sugar, lipid, and/or emulsifier), processing conditions such as heat and moisture, processing approaches (steam cooking, autoclaving, parboiling, baking, extrusion cooking, pyroconversion, microwave irradiation), mechanical and/or biological treatments (milling, germination, fermentation), and storage conditions have a strong bearing on the quality attributes of the resistant starch produced [94].

9.6.3.1 Acetylation Indica rice (RS4-type) is used as a starting material to prepare highly resistant starch through acetylation (esterification) [100]. The resistance attribute is introduced via substituting the hydrophilic hydroxyl groups of starch with hydrophobic acetyl groups, leading to the starch being more hydrophobic and having the formation of hydrogen bonding between the hydroxyl groups and water molecules negated [82].

9.6.3.2 Acid hydrolysis The acid hydrolysis of amylotype corn starch samples (Hylon V and Hylon VII) discloses that the molecular weights of the samples are reduced as a function of hydrolysis duration. The resistant starch content nonetheless does not seem to differ from those of native starches. On the other hand, starches that have undergone oven drying following acid hydrolysis are found to have a higher resistant starch

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content than those subjected to a freeze-drying process. Using the freeze-drying process, the molecular chains of starch are less flexible to interact with each other and this translates into the reduced formation of tightly packed areas (resistant starch regions). The oven drying introduces heat. The combination of acid hydrolysis with heat can increase the resistant starch content. An example of such is the increased RS3 contents of amylotype corn starches [105].

9.6.3.3 Amylose
lipid complexation Amylose
lipid complex formation distresses the enzyme susceptibility of sago starch by decreased starch granule swelling, building less opportunity for enzymes to gain access to granule interior, and less amylose leaches from the granules. This complex is more resistant to digestive enzymes than amylose [106]. The susceptibility of amylose
lipid complexes prepared with potato amylose with fatty acids and lysophosphatidylcholine to hydrolysis by glucoamylase from rhizopus niveus is reduced through decreasing the ability of starch to adsorb enzymes such as glucoamylase [107].

9.6.3.4 Crosslinking Crosslinking strengthens the hydrogen bonds in the starch granule through bringing the starch molecules closer for hydrogen bonding to take place. The degree of crosslinking is dependent on the chemical composition of the reagent, reagent concentration, pH, reaction time, and temperature [82]. Crosslinking agents such as sodium trimetaphosphate, sodium tripolyphosphate, epichlorohydrin, and phosphoryl chloride are commonly employed in the development of resistant starch via the crosslinking approach [108].

9.6.3.5 Enzymatic debranching The retrogradation process is an approach that can possibly be used to produce resistant starch. Starch contains a large fraction of amylopectin. The retrogradation process proceeds by first enzymatically converting the amylopectin into amylose using debranching enzymes such as pullulanase, isoamylase, or by partial hydrolysis with α-amylase. The retrograded starch is then subjected to enzymatic or acid hydrolysis to eradicate the amorphous regions and generate starch that is essentially free of amorphous regions and contains at least 90% crystalline material [109]. A research study based on the influence of amylose content on the production of resistant starch by means of enzymatic treatment indicates that the amount of resistant starch (RS3) developed in raw flours increases with the use of flours of a higher amylose content [110].

9.6.3.6 Hydrothermal treatment Heat
moisture treatment and annealing are both hydrothermal treatments that are usually used in increasing or modifying the resistant starch level in RS2 [111]. These physical modifications change the physicochemical properties of starch without destroying its granular structure [112]. Heat
moisture treatment and

9.6 Starch as a drug carrier

annealing can either be used alone, in combination, or in addition to other treatment modes in order to attain the desired resistant starch level in starch. Starch is treated under limited water content (,35%) at high temperatures for a specific time in the heat
moisture treatment process, whereas in annealing, the treatment involves starch incubation in excess water (,65%) or intermediate water levels (40%
55%) for a specific duration at a temperature below the onset temperature of gelatinization but above the glass transition temperature. The modification of resistant starch level using these two methods results in structural changes within the amorphous and crystalline regions of starch to different extents. Both approaches create higher ordered structures as a function of the initial structural order, and either type of hydrothermal treatment is potentially viable to improve the resistant starch level without destroying the granular structure. Both heat
moisture treatment and annealing methods promote an increase in the degree of interaction between starch components at the amorphous regions and closer packing of the double helices within starch crystallites. The degree of resistant starch development requires a judicious choice of temperature and moisture conditions [111].

9.6.4 Pharmaceutical applications of starch Both starch and resistant starch can be further modified and applied in the design of drug delivery systems for pharmaceutical applications. Table 9.4 summarizes the selected examples of modified starch that have been developed into various dosage forms for drug delivery through skin, gastrointestinal, vaginal, pulmonary, ocular, and parenteral routes. High-amylose maize starch, high-amylose corn starch, resistant starch type-3, resistant starch acetate, cross-linked pregelatinized maize starch, and other variants have been used in the design of colon-specific drug delivery systems.

9.6.5 Starch as oral colon-specific drug carrier The parenteral and nasal routes of administration have been used to deliver drugs to specific sites of the gastrointestinal tract, but both modes of drug administration are regarded to be less convenient than the oral route [141]. Oral drug administration is the preferred choice of drug administration route primarily due to the high drug absorptive capacity of the gastrointestinal tract [142]. The colonic drug delivery system is a focal point amongst researchers for the treatment of colonic diseases such as colorectal cancer, amebiasis, ulcerative colitis, and Crohn’s diseases since drug dosage can be minimized while reducing the adverse systemic effects [143
145]. Further, drugs that are polar and/or susceptible to chemical and enzymatic degradation in the upper gastrointestinal tract are suitable candidates for use in colonic delivery [146]. The colon-specific drug delivery system is available in single-unit and multiunit systems. Single-unit formulations contain the active ingredient within a single

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Table 9.4 Pharmaceutical applications of starch as drug carrier. Scope

Remark

Reference

A systematic evaluation of hydroxyethyl starch as a potential nanocarrier for parenteral drug delivery

The drug encapsulation and release behavior of indomethacin and ibuprofen sodium indicates the potential of hydroxyethyl starch for use as a drug carrier. The starch nanoparticles give excellent biocompatibility for parenteral use as they lack in vivo inflammatory potential and organ toxicity. The potential of hydroxyethyl starch in the form of macromolecular drugcarrier conjugates to treat leukemia through intravenous administration is evaluated. The hydroxyethyl starch does not exhibit any cytotoxicity in vitro. The modified starch is conjugated with methotrexate. The survival time of murine leukemia P388 and MV4
11 human leukemia-bearing mice treated with a hydroxyethyl starch
methotrexate conjugate is longer than that of untreated animals and of mice treated with free methotrexate. In vivo study indicates that the hydroxyethyl starch
methotrexate conjugate displays a significant inhibitory activity against MV-4
11 tumor growth in relation to free methotrexate. The conjugate is designed to reduce the side effects caused by nonspecific drug delivery and improve the antitumor efficacy of doxorubicin and glutathionemediated intracellular drug release. The conjugate is proven to be a promising prodrug of doxorubicin with clinical potentials to achieve tumor-targeted drug delivery and timely intracellular drug release for effective and safe cancer chemotherapy. Aldehyde starch is conjugated with 5-fluorouracil and the conjugate is in a nanoparticulate form as a drug carrier to treat breast cancer. The

[113]

Hydroxyethyl starch as an effective methotrexate carrier in parenteral anticancer therapy

Redox-sensitive hydroxyethyl starch
doxorubicin conjugate for tumor-targeted parenteral drug delivery

Dialdehyde starch nanoparticles as antitumor drug delivery system: an in vitro, in vivo, and immunohistological evaluation

[114]

[115]

[116]

(Continued)

9.6 Starch as a drug carrier

Table 9.4 Pharmaceutical applications of starch as drug carrier. Continued Scope

Characterization of polyacryl starch microparticles as carriers for protein and drugs

Characterization and evaluation of dialdehyde starch as an erodible medical polymer and a drug carrier

Porous starch-based injection/ implantation drug delivery systems processed by a microwave route

Remark modified starch is capable of carrying the drug to the specific target by minimizing its premature release. In vitro cell culture study indicates that the aldehyde starch
5-fluorouracil conjugate nanoparticles are able to significantly inhibit MCF-7 growth when compared to free 5fluorouracil. In vivo antitumor activity is assessed through a single subcutaneous injection of nanoparticles into the animal model. The tumor tissue is induced with necrosis by approximately 61% 6 6% as compared to free 5fluorouracil which is about 42% 6 4%. Hydrolytic enzymes and an isolated lysosome-enriched rat liver are used to study the biodegradability of polyacryl starch microparticles that are prepared from either acryloylated maltodextrin or acryloylated hydroxyethyl starch. The derivatization of starch with acrylic acid glycidyl ester does not alter the biodegradability of polyacryl starch microparticles. These microparticles are suitable as a drug carrier since their metabolism can be controlled and anticipated. Dialdehyde starch is conjugated with isoniazid drug through hydrazine-bonding. The efficiency of the dialdehyde starch to act as a drug carrier is evaluated in vitro. The dissolution profile of this conjugate shows that it is dissolved gradually and almost linearly over the period of a month. This indicates that the dialdehyde starch is suitable for use as a sustained-release drug delivery system. Starch-based porous material is designed based on the utilization of a baking powder as an additional

Reference

[117]

[118]

[119]

(Continued)

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Table 9.4 Pharmaceutical applications of starch as drug carrier. Continued Scope

Effective insulin delivery using starch nanoparticles as a potential transnasal mucoadhesive carrier

Starch acetate microparticles for drug delivery into retinal pigment epithelium: an in vitro study

A novel antigen-carrier system: raw starch microparticles as the carrier of Mycobacterium tuberculosis alpha crystalline (Acr) protein

Remark blowing agent. Meclofenamic sodium salt drug is loaded during the preparation of the porous starch to ensure homogeneous dispersion of the drug. The mixture is then processed in a microwave oven. The release profile in isotonic saline solutions is characterized by an initial burst effect followed by a slow controlled-release of drug for up to 10 days. Starch nanoparticles are prepared using two different methods, namely crosslinking by gel and crosslinking by emulsion using epichlorohydrin and phosphoryl chloride as crosslinkers. The insulin is loaded into nanoparticles using a postloading method. Crosslinking using epichlorohydrin emulsion method gives smaller nanoparticle sizes compared to other formulation methods and faster drug release in vitro. The higher drug release rate from the nanoparticles is translated into a higher nasal surface drug content for absorption. This study suggests that the natural enzyme-sensitive starch acetate may well be suitable for drug delivery to the retinal pigment epithelium without any significant toxicity. The retinal epithelial cells are able to degrade starch acetate. Raw starch microparticles and starch binding domain tag fusion protein are proposed as a delivery system and an immobilization platform respectively using Acr protein as a model active. The immunogenicity of the system is inspected in BALB/c mice inoculated with purified Acr-starch binding domain tag fusion protein and starch immobilized Acr-starch binding domain tag fusion protein by oral and intranasal routes. The system exhibits the necessary

Reference

[120]

[121]

[122]

(Continued)

9.6 Starch as a drug carrier

Table 9.4 Pharmaceutical applications of starch as drug carrier. Continued Scope

Starch-coated magnetic liposomes as an inhalable carrier for the accumulation of fasudil in the pulmonary vasculature

“A quality by design” approach on starch-based nanocapsules: a promising platform for topical drug delivery

Development of starch-based mucoadhesive vaginal drug delivery systems for application in veterinary medicine

Synthesis of starch-based drug carrier for the controlled release of estrone hormone

Remark characteristics to improve antigen release and presentation to antigen presenting cells in the mucosae. The concept of work may be suitable for vaccine development. The viability of magnetic liposomes as carriers for localized delivery of fasudil, an investigational pulmonary arterial antihypertensive drug, into the lungs is evaluated. Starchcoated magnetite, which is biocompatible and biodegradable, is used to reduce iron particulate toxicity. The design of starch-based nanoparticulate carrier systems (nanocapsules) is optimized using a quality by design approach and it is ascertained to be a promising formulation strategy and a potential nanocarrier for topical lipophilic bioactive molecules with a good acceptance by human volunteers, boosting patient comfort for maximum compliance and treatment results. Mucoadhesive vaginal tablet formulation (75 mg progesterone) is prepared with nontoxic, biocompatible wheat starch-based graft-poly (acrylic acid) copolymers. The formulation demonstrates a promising therapeutic result and may be suitable as a substitute product to the commercial brands in veterinary medicine. Bromoacetylated starch is fabricated to provide more reactive sites for coupling of bioactive estrone and a suitable spacer between the drug carrier and the hormone. Estrone is coupled to the bromoacetylated starch using estrone sodium salt via nucleophilic substitution. The solubility data displays that bromoacetylated starch and estrone conjugate starch are soluble at room temperature and polar aprotic solvents, unlike raw starch.

Reference

[123]

[124]

[125]

[126]

(Continued)

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Table 9.4 Pharmaceutical applications of starch as drug carrier. Continued Scope Starch cellulose acetate coacrylate polymer as a drug carrier

Direct compressible polymethacrylic acid
starch compositions for sitespecific drug delivery

Preparation and use of acrylamide grafted starch as polymeric oral drug carrier

Starch-based coatings for colonspecific delivery focusing on physicochemical properties and in vitro drug release profiles of high amylose maize starch film

Remark Starch cellulose acetate coacrylate is produced from the reaction between acrylic acid with starch/ cellulose acetate blends (weight ratio of 90:10). The anticancer model drug is 8-(2-methoxyphenyl)3 and 4-dioxo-6-thioxo-3,4,6,7tetrahydro-2h-pyrimido[6,1-c][1,2,4]triazine-9-carbonitrile. Starch cellulose acetate coacrylate loaded with drug is completely degradable in alkaline solution of pH 12 and above after an hour of contact. The drug release in acid and moderately basic solutions is low. The carrier can sustain drug release for up to 240 or 480 h. Polymethacrylic acid-starch, at a weight ratio of 1:1.38, is used as an oral carrier composition. A model peptide peroxidase and model drugs, amoxicillin and rifampicin, which all are unstable and rapidly degradable in the stomach, are used. The formulations are compressed into the form of a tablet. The dissolution studies in simulated gastric fluid (pH 1.2) show no drug release within 120 min for either peptide peroxidase or the model drugs. The polymethacrylic acid
starch is a suitable carrier system for intestinal drug delivery. The gelation and ceftriaxone sodium release profiles of raw starch and acrylamide grafted starch hydrogels are evaluated in distilled water, normal saline, and buffer solution of pH 2. The acrylamide grafted starch demonstrates greater swelling and drug release propensities. 5-Aminosalicylic acid release from pellets coated with a blend of high amylose maize starch/Surelease is found to be marginal in simulated gastric and intestinal fluids. This suggests that the mixed coats provide starch domains that are

Reference [127]

[128]

[129]

[130]

(Continued)

9.6 Starch as a drug carrier

Table 9.4 Pharmaceutical applications of starch as drug carrier. Continued Scope

Development and characterization of nanoscale retrograded starch as an encapsulating agent for colonspecific drug delivery

Preparation and characterization of high-amylose corn starch/pectin blend microparticles

Preparation and characterization of glycoprotein-resistant starch complex as a coating material for oral bioadhesive microparticles for colon-targeted polypeptide delivery

Resistant starch as a drug carrier for oral colon-targeting

Remark resistant to the enzymes present in the upper gastrointestinal tract and, thus, can potentially be used in the preparation of colon-specific delivery devices. This study shows that high amylose maize starchbased films can be used potentially in the development of colon-specific delivery devices. RS3 nanoparticles are developed for colon-specific drug delivery using high-speed shearing emulsification technique. The nanoparticles are stable in both neutral and alkaline pH or against low sodium chloride concentration. They have a desirable drug loading capacity and in vitro drug release property. High-amylose corn starch/pectin blend microparticles are prepared by spray drying technique to obtain effective targeted drug release to the colon. The blending improves drug encapsulation efficiency and decreases the drug dissolution propensity in the gastric condition compared to the pectin-based microparticles. This suggests that colonic-controlled drug delivery may be obtainable with the use of highamylose corn starch/pectin blend. The use of resistant starch acetate conjugated with concanavalin A (a glycoprotein) as a film coating material for insulin-loaded microparticles is translated to a good hypoglycemic response where the plasma glucose level is kept within the normal range for 44
52 h. This film-coated microparticles system has been demonstrated to be capable of improving the oral bioavailability of bioactive proteins and peptides. Cross-linked pregelatinized maize starch has been used to develop oral tablet dosage form for targeting

Reference

[131]

[132]

[133]

[134]

(Continued)

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Table 9.4 Pharmaceutical applications of starch as drug carrier. Continued Scope

In vitro and in vivo evaluation of Assam Bora rice starch-based bioadhesive microsphere as a drug carrier for colon targeting

Acetylated starch-based biodegradable materials with potential biomedical applications as drug delivery systems

Remark peptide and protein drugs that are deemed susceptible to enzymatic digestion by proteolytic enzymes. Bovine serum albumin is used as a model drug where it is compressed and dry-coated with the modified starch. In vitro drug release analysis using simulated gastric, intestinal, and colonic fluids shows that only 10% of bovine serum albumin are released after 8 h of incubation under upper gastrointestinal conditions, and the drug is released to above 90% over 36 h in the simulated colonic region. Colon-targeted bioadhesive microspheres have been produced using Assam Bora rice starch with metronidazole as the model drug. In vitro drug release study in different physiological environments of the gastrointestinal tract indicates that the release profiles of the drug are unaffected by the hostile environment of the gastrointestinal tract. The microspheres are able to exert in vitro growth inhibition against the metronidazole-sensitive Bacteroides fragilis. In vivo organ distribution study using male albino rats shows the drug targeting potential of the microspheres where the drug is only released after reaching the colon following the microbial degradation of the starch. Maize starch acetate prepared by acetyl esterification approach is used to deliver bovine serum albumin to the colon in the form of a tablet. The in vitro drug release study in simulated gastric fluid (2 h), simulated intestinal fluid (6 h), followed by simulated colonic fluid (16 h) indicates that less than 10% of the drug is released from the tablet coated with maize starch acetate after 6 h of incubation. The uncoated tablet releases almost all the drug after 4 h of incubation.

Reference

[135]

[136]

(Continued)

9.6 Starch as a drug carrier

Table 9.4 Pharmaceutical applications of starch as drug carrier. Continued Scope Synthesis and characterization of a starch-modified hydrogel as a potential drug carrier

Characterization and in vitro evaluation of starch-based hydrogels as carriers for colonspecific drug delivery

Starch-based microspheres for sustained-release of curcumin: preparation and cytotoxic effect on tumor cells

Remark Starch is chemically modified using glycidil methacrylate followed by crosslinking using sodium persulfate to transform it into hydrogels. The swelling characteristics of the starch hydrogel are not affected by the temperature or pH of the surrounding liquid. The starch hydrogel has the ability to transport and preserve acid-responsive drugs such as corticoids for the treatment of colon diseases in a colontargeted manner. Starch/methacrylic acid hydrogels of different compositions are synthesized using γ-ray induced polymerization and crosslinking techniques and loaded with ketoprofen as a model drug. The hydrogel possesses good pH sensitivity and shows Fickian diffusion behavior at pH 1 and nonFickian behavior at pH 7. The drug release from the hydrogel only occurs in a buffer solution of pH 7. The factors that control the release rate are the degree of crosslinking and the methacrylic acid concentration. The increment in the crosslinking density and decreased content of methacrylic acid decrease the drug release rate. The starch
methacrylic composition can be modulated for colon-specific drug delivery. Chemically modified starch crosslinked with N,N0 methylenebisacarylamide is prepared in the form of a water-inoil emulsion system and loaded with curcumin. In vitro drug release study in simulated gastric fluid (pH 1.2) and simulated intestinal fluid (pH 6.8) shows that the curcumin release is governed by anomalous transport and that it is a pHdependent process. The release is more pronounced in an acid condition. Cytotoxicity assays show

Reference [137]

[138]

[139]

(Continued)

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Table 9.4 Pharmaceutical applications of starch as drug carrier. Continued Scope

Development of resistant corn starch for use as an oral colonspecific nanoparticulate drug carrier

Remark that the starch microspheres can improve the cytotoxicity of curcumin toward Caco-2 and HCT-116 tumor cell lines compared to that found against pure curcumin due to the slow and sustained release of curcumin from the microspheres in a near neutral medium. Resistant starch is formed by amylopectin debranching via heat
moisture or acid treatment of native starch. Heat
moisture treatment of native corn starch enables the formation of resistant starch through amylopectin debranching and molecular weight reduction, thereby enhancing hydrogen bonding between the starch molecules at the amorphous phase and gelatinization capacity. The nanoparticles prepared from resistant starch demonstrate similar drug release as those of native starch in spite of the resistant starch having a lower molecular weight. The resistant starch is envisaged to be resistant to the digestive action of amylase in the intestinal tract without the nanoparticles exhibiting excessively fast drug release in comparison to native starch. With reduced branching, the resistant starch represents an ideal precursor for targeting ligand conjugation in the design of oral colon-specific nanoparticulate drug carrier.

Reference

[140]

tablet or capsule, whereas multiple-unit dosage forms comprise of a number of discrete particles that are combined into one dosage unit [147]. Single-unit systems may encounter unintentional disintegration due to manufacturing deficiency or unusual gastric physiology. This can lead to drastically compromised systemic drug bioavailability or loss of local therapeutic action in the colon [148]. Nowadays, multiparticulate dosage forms are preferred over single-unit systems since the former enables the drug to reach the colon quickly and is retained in the ascending colon for a longer duration [148]. The total drug is divided into many units. The failure of a few units may not be as substantial as the failure of a

9.6 Starch as a drug carrier

single-unit system [147]. Other potential benefits offered through multiparticulate systems are increased drug bioavailability, reduced risk of systemic toxicity due to dose dumping, reduced risk of local irritation due to its widespread locality in the gastrointestinal tract, and predictable gastrointestinal transit in addition to good patient compliance due to the ease of swallowing and flexible dose adjustment [149,150]. Various multiparticulate systems such as granules, pellets, beads, microparticles, and nanoparticles have been adopted in oral colon-specific drug delivery systems [148].

9.6.5.1 Beads Microparticulate beads are prepared using the ionic gelation method. Pectin/chitosan-coated beads containing doxorubicin-loaded porous maize starch exhibit lower drug release in the upper gastrointestinal tract and a higher drug fraction is deemed to release in the colon [151].

9.6.5.2 Hydrogels Hydrogels are defined as two- or multicomponent systems consisting of a threedimensional network of polymer chains and water that fills the space between the macromolecules. The classification of hydrogels can be based on source (natural or synthetic origins), polymeric composition (homopolymeric, copolymeric, or multipolymeric interpenetrating polymeric hydrogels), configuration (amorphous, semicrystalline, or crystalline), type of crosslinking, physical appearance (matrix, film, or microsphere), and network electrical charge (nonionic, ionic, amphoteric, or zwitterionic) [152]. Starch-based hydrogels have many favorable properties such as biodegradability, hydrophilicity, biocompatibility, low cost, and nontoxicity [153]. The porous structure in hydrogels can be adjusted to where it helps to load drugs into the gel matrix and release them based on the diffusion coefficient rate through the hydrogel network [154]. There are several studies on starch hydrogel as a drug carrier (Table 9.4), but the distinct use of starch hydrogel in oral colon-specific drug delivery has yet to be clarified.

9.6.5.3 Microparticles Microparticles as a drug carrier can be primarily prepared by single emulsification, double emulsification, polymerization, spray drying, spray congealing, and solvent evaporation techniques [155]. With reference to the single emulsification approach, the drug is commonly dissolved in an aqueous medium that acts as the dispersed phase. The aqueous medium is introduced into an oil phase that acts as a continuous phase. A single emulsion is formed through agitation of both phases in the presence of a surface active agent with the addition of a crosslinking agent or heating to stabilize the emulsion system [155]. The single emulsification technique has been employed to prepare starch-polyvinyl alcohol microparticles with glutaraldehyde as the crosslinking agent and ornidazole as the drug of choice [156]. The weight ratio of starch to polyvinyl alcohol dictates the drug release pattern. Within 48 hours of drug release, high and low amounts of starch in

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relation to polyvinyl alcohol give low levels of drug release, whilst an intermediate amount of starch brings about a high level of drug release from the microparticles. Another method of forming microparticles is extrusion
spheronization. The formed microparticles are subsequently coated with starch in the form of an aqueous dispersion by a bottom-spray fluid bed coating technique. The insulinloaded microparticulate core is coated with resistant starch type-3 [157] and with concanavalin A
resistant starch acetate [133], which demonstrate an excellent oral colon-specific controlled drug release behavior. Their application in type II diabetic rats is capable of improving the oral bioavailability of bioactive proteins and peptides. 5-Aminosalicylic acid coated with resistant starch type-3 likewise exhibits high acidic and enzymatic resistibility in vitro and is potentially viable to accurately target the bioactive compound to the colonic region [158]. Other examples of oral colon-specific drug delivery systems that have been designed in the form of a microparticulate system are Assam Bora rice starch, high-amylose corn starch/pectin blend, and N,N-methylenebisacrylamide-cross-linked starch microparticles (Table 9.4).

9.6.5.4 Nanoparticles Nanoparticulate systems appear to be the delivery system of interest for cancer therapeutics [159]. According to the National Nanotechnology Initiative of the United States, nanotechnology refers to structures of between 1 and 100 nm in size in at least one dimension. In pharmaceutical fields, the use of nanotechnologies such as nanocarriers with definite shape, size, and surface properties offers many attractive features such as improving the dissolution and bioavailability of poorly water-soluble drugs; improving drug stability through matrix protection from harsh environments; improving drug biodistribution profiles; targeting the delivery of drugs in a cell- or tissue-specific manner, thereby reducing adverse effects; lowering immunotoxicity; enhancing the transcytosis of drugs across tight epithelial and endothelial barriers; enabling the delivery of large macromolecular drugs to intracellular sites of action; codelivering multiple types of drugs and/or therapeutic modalities for combination therapy; enabling the visualization of the sites of drug delivery by combining therapeutic agents with imaging modalities; and facilitating real-time readout on the treatment efficacy of therapeutic agents [160
162]. Nanoparticles are stable in the gastrointestinal tract and can protect the encapsulated therapeutics from the degradative actions of pH and enzyme degradation as well as the efflux action of drug pumps with drug release modulation in a temporally or spatially controlled manner [163]. Cancer therapeutics formulated in the form of nanoparticles or more explicitly known as cancer nanotherapeutics can be designed to overcome nonspecific biodistribution and targeting, a lack aqueous solubility, poor oral bioavailability, and low therapeutic indices of drugs [164]. Nanoparticles have the ability to accumulate in cells without being recognized by P-glycoprotein, a main mediator of multidrug resistance [164]. Thus encapsulating chemotherapeutic agents in nanoparticles can enable the oral administration of drugs currently limited to intravenous administration [163]. Oral colon-specific drug delivery has attracted a great deal of attention in

Acknowledgment

pharmaceutical applications. However, there are limited studies on starch-based nanoparticles as oral colon-specific drug carriers (Table 9.4). Resistant starch isolated from soybean meal is a stable coating material and has been applied in nanoparticle design suitable for colon-targeted applications since it is stable under simulated human physiological conditions [165].

9.6.5.5 Pellets Pellets are microparticles with smooth surface morphology, narrow size distribution (typically between 500 and 1500 μm), spherical shape, and low friability [148,166]. They can be easily coated to enable drug release kinetics modulated to the intended profile. The drug release of pellets coated by a resistant starch film decreases in simulated digestive fluid due to the increased resistibility to enzymatic digestion and hydrophobicity of the dosage form. Pellets coated by a resistant starch film can easily pass through the upper gastrointestinal tract and release the drugs in the colon [167]. Several active components, incompatible drugs, or drugs with different release profiles may possibly be combined through mixing different pellet batches in the same unit dosage form [166]. Tablets from pellets can be prepared at lower costs and with higher production rates than pellet-filled capsules [147].

9.7 Conclusion Starch is biocompatible, nontoxic, biodegradable, environment-friendly, and inexpensive. It is relatively pure and does not require intensive purification procedures, unlike other naturally occurring biopolymers such as cellulose and gum [168]. Modified starch has the ability and potential to be used as a drug carrier due to its sustained-release attribute. Resistant starch is believed to benefit colonic health as butyrate, one of the fermentation products, can have inhibitory effects on the growth and proliferation of tumor cells. Further, resistant starch is claimed to act as a substrate for the growth of probiotic microorganisms, which are essential to keep the colon healthy. Starch and resistant starch are ideal candidates for use in the development of oral colon-specific drug delivery systems for their drug release modulation and colon health properties. To date, there are limited studies related to starch for oral colon-specific drug delivery. With reference to colon cancer and related ailments, the use of starch as the core and coat materials of oral colon-specific drug delivery systems has not been explored in great detail.

Acknowledgment The authors wish to express their heart-felt gratitude to UiTM for fund and facility support (0141903) and the Centre of Foundation Studies and the Ministry of Higher Education Malaysia for scholarship support.

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94.

CHAPTER

Polymers in textiles

10 Mabrouk Ouederni

Qatar Petrochemical Company (QAPCO), Doha, Qatar

10.1 Introduction Polymeric fibers, whether natural or synthetic, constitute the backbone of textile fabrics. In fact, it is the versatility of polymeric materials and their ability to be shaped into fibers of different geometries and sizes that confer on textiles their unique properties. The final properties of a textile fabric, however, are determined by the method of constructing these fibers into a fabric as well as the finishing process [1 4]. Natural fibers such as cotton have been used by various civilizations for thousands of years in applications such as clothing, home furnishings, and building materials [5]. The use of cotton, silk, and wool for clothing and other textile applications is well established in various cultures around the world [6]. Agricultural fibers such as straw were mixed with clay by the Egyptians to make walls and for building structures [7]. The early Egyptians also used linen, a natural fiber, to make shrouds for the burial of Pharaohs [8]. The Chinese cultivated hemp, a natural fiber, as early as 2800 BCE [9]. Along with the Greek, the Chinese also used wool to make nonwoven felted materials as evidenced by their writing from thousands of years ago [10]. Natural fibers, however, had several shortcomings that limited their use and made them inadequate for certain textile applications [11]. It was well known, for example, that cotton and linen had a wrinkling problem upon wearing and washing, while wool had a shrinking problem and could be irritating to the touch [12]. The focus of this chapter is on manmade fibers, also known as manufactured fibers. More specifically, this chapter focuses on the use of polymeric materials to manufacture synthetic fibers for textile applications. It is also important to note that greater emphasis will be put on technical textiles as opposed to the more traditional, commodity garment and clothing applications. Technical textiles cover applications such as medical, military, transportation, building, modern apparel, and consumer goods. The most common polymers used for textile applications will be covered including a discussion on their inherent properties and the characteristics that make them unique for those applications. Technical textiles and nonwovens will be discussed including their requirements and polymer suitability to Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00010-X © 2020 Elsevier Inc. All rights reserved.

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meet those requirements. Finally, the chapter will venture into modern applications, and the potential to develop polymeric materials to meet demanding and challenging textiles for the future.

10.2 Brief history of manmade fibers The story of polymer-based manmade fibers started to take shape in the early 1800s [13,14]. Count de Chardonnet, a French chemist, started a series of trials attempting to produce artificial silk from dissolved nitrocellulose. Chardonnet introduced his artificial silk fabrics at the Paris exhibition in 1889 and later succeeded in establishing the first commercial scale production plant of rayon in France [12]. In the United States, the first commercial production of artificial silk did not begin until the American Viscose Company opened a rayon plant in 1910 [15]. Before conquering the textile fiber market, cellulose acetate (CA) was made as dope for airplane wings in England and later in the United States by Swiss brothers Camille and Henry Dreyfus by invitation from the government to fulfill the needs of US war planes. It was not until the early 1920s that Celanese Corporation turned CA into an important textile fiber [12]. Seventy years later (mid 1990’s) I joined Hoechst Celanese Corporation, as an R&D Engineer at the Dreyfus Research Park (DRP) in Charlotte, North Carolina. DRP, as well as various other research & innovation labs, were a testament to the contributions made by the Dreyfus brothers and other early pioneers, to the polymer and synthetic fiber industries in the 20th century. The 1920s also witnessed the recognition, for the first time, by Hermann Staudinger, of the macromolecular nature of the polymer structure [16]. This discovery, rewarded by the Nobel Prize in Chemistry in 1952, paved the way for the eventual establishment and fast growth of the polymer and synthetic fiber industries. This impact was no more evident than in the work of US scientist Wallace Carothers in the research labs of DuPont [17]. Carothers synthesized polyamide 66, known as nylon 66, which revolutionized the synthetic fiber industry [18]. Unlike acetate, which is derived from plant cellulose, nylon is derived from petroleum-based products. Nylon stockings were a fashion hit with US woman and the fabric became extremely popular in the fashion industry until World War II started and nylon fiber production became totally devoted to military use [19]. Nylon fiber, due to its importance in textile applications, will be discussed in more detail in the subsequent sections of this chapter.

10.3 Terminology and definitions Fibers, whether natural such as cotton or wool, or synthetic such as polyester or polypropylene, are the fundamental building blocks of any textile product. Fibers are characterized by a large length-to-diameter ratio. Before being knitted or

10.4 Fiber manufacturing

woven into a fabric, textile fibers can exist in various forms [20]. It is, therefore, important to make the distinction between certain textile terms, namely:

• Filament: A single fiber of continuous length. It is sometimes called a

• • • • • •

monofilament, but it is not to be confused with monofilaments of larger diameters, which are extruded, wound on bobbins, and used in applications such as fishing line and paper machine clothing [21]. Filament yarn: A number of continuous filaments held together by twisting or other methods [22]. Filament tow: Large bundles of filament held together without twisting [23]. Staple fiber: Short cut fibers with specific length made from filament tow, packaged in bales of staple fiber before textile processing [24]. Spun yarn: Staple fiber is sometimes spun for easy processing on cards and other textile machinery [25]. Textured filament yarn: Refers to filament yarn when it is twisted and manipulated in order to impart texture and bulk to it before being used in a textile fabric. This step can also impart a soft hand feel to the yarn [26]. Nonwovens: Nonwovens are clearly distinguished from traditional textile fabrics, paper sheets, and plastic films. Unlike traditional fabrics knitted or woven from yarn, nonwovens are engineered fabric structures made directly from fibers or from a web that is itself made up of fibers [27,28]. The Textile Institute defines a nonwoven as “a textile structure made directly from fiber rather than yarn. Fabrics are normally made from continuous filaments or from fiber webs or batts strengthened by bonding using various techniques: These include adhesive bonding, mechanical interlocking by needling or fluid jet entanglement, thermal bonding and stitch bonding” [10].

The Association of the Nonwoven Fabrics Industry (INDA) defines a nonwoven as “a sheet, batt or web of natural and/or manmade fibers or filaments, excluding paper, that have not been converted into yarns, and that are bonded to each other by any of several means” [29]. Disposable nonwovens include baby diapers, face masks, wet wipes, and several hygiene and medical fabrics. Durable nonwovens include filtration media, automotive fabrics, roofing fabrics, geomembranes, interlinings for clothing and shoes, and military protective garments. Nonwovens are widely used in consumer, industrial, and healthcare products due to their unique combination of lightweight, open structure, filtration capability, and fluid management properties, depending on the specific application. These fabrics are versatile in structure and appearance due to the various processes and bonding techniques used to make them [30].

10.4 Fiber manufacturing The conversion process of polymers into fibers depends on the polymer type and chemical structure. There are four fiber spinning processes commonly used in the

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fiber industry, namely melt spinning, dry spinning, wet spinning, and gel spinning [31]. More recently, electrospinning has gained attention as a way to make fibers in the nanoscale [32,33]. A brief description of these processes, which will be helpful in understanding various fiber properties and uses is given here.

10.4.1 Melt spinning Melt spinning is the most common process used to make textile-type polymer fibers. Almost any thermoplastic polymer (thermoplastics are polymers that are able to flow upon heating and are readily shaped into various forms as the polymer melt is cooled down) is made into fiber form via melt spinning, with varying degrees of difficulty. The melt spinning of polymers into continuous filament yarn consists of these steps [34]:

• Melting the polymer pellets in an extruder and pushing it through a spinneret •

that consists of tiny holes evenly distributed through the surface. A spinneret pack with polymeric filaments is depicted in Fig. 10.1 [12]. Solidifying the filaments exiting the spinneret using cool air and collecting them in a bundle on a rotating roll.

FIGURE 10.1 Filaments exiting through the spinneret holes of a spinneret pack. Fiber manufacturing, ,http://www.fibersource.com/f-tutor/techpag.htm., Fibersource.com; 2013.

10.4 Fiber manufacturing

Highly oriented yarn Moderatelly oriented yarn Tenacity

Low oriented yarn

0

20

40 60 Elongation%

80

100

FIGURE 10.2 Effect of orientation on polypropylene fiber tenacity [36].

• Drawing the filaments between two rolls of different speeds in order to orient the molecules and increase the filament strength.

• Heat setting the filaments and collecting them on bobbins on a wind-up roll for further textile processing. The drawing of extruded fibers is important in industrial fiber manufacturing in order to impart the required tenacity and strength to the fiber. In the case of semicrystalline thermoplastics such as polyester and polypropylene, drawing orients the polymer chains and extends them in the drawing direction, which results in a higher percentage of crystallinity and a stronger fiber [35]. Fig. 10.2 shows the significant effect of orientation on the tenacity and elongation of polypropylene filament yarn [36]. Highly oriented yarn becomes extremely strong (high tenacity) while having a high modulus and a low elongation to break (Eb). Staple fiber is made from spun bundles of fibers that are collected in large cans from several parallel extruders in the form of a large tow. The tow is then taken through several steps that include surface chemical treatment, drawing to orient fibers, crimping to increase bulk, finishing, and finally cutting into several millimeter long staple fibers (fiber length depends on subsequent process and application), and packing into bales [37]. It is important to note that “fiber spinning” as it relates to fiber production should not be confused with “textile spinning,” which is a process by which staple fiber is twisted and bundled into “spun yarn” that is further processed to make textile fabrics [37].

10.4.2 Dry spinning When polymers cannot be melted in a melt spinning process, an adequate solvent is chosen to dissolve the polymer. The solution is then extruded through a spinneret; as the fibers exit the spinneret, the solvent is evaporated by hot air and the

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dry filaments collected on a bobbin. Dry spinning has relatively lower spinning speeds than those of melt spinning. Dry spinning is used to make fibers such as acetate and acrylics [38].

10.4.3 Wet spinning In wet spinning, the polymer is dissolved in a solvent, but instead of being extruded into hot air, it is extruded into a liquid bath. The solvent is then removed chemically in the bath as opposed to being evaporated as in dry spinning. The speeds used are low in wet spinning and the filaments are oriented after the bath in order to impart strength on them [38]. Aramid, spandex, and modacrylic fibers are made by wet spinning.

10.4.4 Gel spinning The gel spinning process consists of extruding a heated gel such as ultrahigh molecular weight polyethylene (UHMWPE) through a spinneret, drawing it through air, and cooling it down in a water bath. The key in gel spinning is to separate the molecular chains in the solvent to minimize entanglement and give the chains a chance to achieve a high degree of orientation.

10.4.5 Nonwovens processing The processes used to make nonwovens are either textile based, paper based, or polymer based [39]. Textile-based processes such as carding and air laid, convert fibers into nonwovens. Paper-based processes such as wet laid, also convert fibers into nonwovens in a hydro-process similar to paper making. The third type of process, polymer based, is the one that concerns us in this chapter. In the so-called “polymer-to-fabric” conversion processes, polymers are extruded into fibers that are directly converted into a web in a continuous process. These processes include spun-bonding, melt-blowing, and porous film. Nonwovens made by these processes are known as spunbond (SB) nonwovens, melt blown (MB) nonwovens, and apertured-film nonwovens [37]. SB nonwovens: Freudenberg and DuPont were among the early leaders in this technology, while Lurgi of Germany commercialized the process and licensed it to manufacturers around the world in the 1970s [40]. In this process, the polymer is extruded and pushed through a spinneret, similar to a regular fiber spinning process. Bundles of fibers from multiple spinnerets are then cooled down and stretched pneumatically or mechanically in order to orient the fibers to make them stronger. An attenuation step reduces fiber diameter and improves fiber morphology through orientation. The fibers are then accelerated by high velocity air stream and laid down randomly on a moving belt to form a web. The web is subsequently made even stronger by either chemical

10.5 Characterization and testing of textile fibers

bonding with latex or another agent or through thermal bonding using low melt fibers and passing the web between hot rolls [37]. One process widely used in the industry is the Kasen process shown in Fig. 10.3 [41]. The SB process can deliver good strength-to-weight ratios, which is important for many consumer and industrial applications. MB nonwovens: MB nonwovens are also made directly from the polymer into a fabric. It was developed first at the United States Naval Research Laboratories and was later improved by researchers at Exxon and 3M and commercialized in the 1960s [29,40]. In the MB process, the polymer is processed through an extruder before passing through an orifice. As it exits the orifice, the polymer is blown with high velocity air at a temperature ranging from 250 C to 500 C. The high velocity air shatters the polymer and stretches it into a fine fibrous structure made of extremely fine diameter fibers [37]. The randomly oriented web of fibers is then separated from the air stream and passed through heated rolls for pressing and bonding. The web is then slit and wound in a roll as seen in Fig. 10.4. MB fabrics are characterized by a large surface area and extremely high number of pores per unit area due to the fine diameter fibers, the random entanglement, and the close packing of fibers [42].

10.5 Characterization and testing of textile fibers Polymeric fibers are characterized for purposes of identification, quality control, and fabric design using standard test methods [43]. Most tests are common to all textile fibers while others are specific to certain polymer types. Some common tests and fiber properties are provided here that are important to understand before any detailed discussion of specific types of fibers and their characteristics.

10.5.1 Density The density of a fiber is usually measured in terms of a quantity called denier. Denier is a measure of the linear density of the fiber and it is defined as the weight in grams of 9000 m of fiber [44]. For the same type of fiber, the larger the diameter is, the bigger the denier. Because fibers are usually bundled into a yarn as mentioned earlier, the term total denier is used and it refers to the denier of the yarn. The term denier per filament is a common term in the fiber industry and it refers to the denier of a single filament (it is equal to the total denier divided by the number of filaments in the yarn) [45]. The denier is commonly used in North America, whereas the European fiber industry prefers to use the term Tex, which is the weight in grams of 1000 m of fiber. The common unit of fiber linear density in Europe is actually the dtex (decitex), which is the weight in grams of 10,000 m of fiber.

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FIGURE 10.3 Schematic of the spunbond nonwoven process. Reproduced with permission from Spunbond and melt blown process charts, KASEN Corporation Japan. ,http://www.kasen.co.jp/English/product/line/work.html..

FIGURE 10.4 Schematic of the melt blown nonwoven process. Reproduced with permission from Spunbond and melt blown process charts, KASEN Corporation Japan. ,http://www.kasen.co.jp/English/product/line/work.html..

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10.5.2 Mechanical properties 10.5.2.1 Tenacity Tenacity is the stress at which the fiber breaks, expressed in grams per denier [46].

10.5.2.2 Elongation to break The Eb is a measure of the extensibility of the fiber and its ability to stretch before breaking. Eb is derived from a stress strain curve and it is measured according to standardized testing such as american society for testing and materials. The Eb of textile fibers depends on the stretching and drawing of the fiber and on the heat setting at the end of the fiber making process [47]. It is a measure of the flexibility of the fiber and producers usually try to strike a balance between tenacity and elongation as required by the specific end use application. Extensibility is also important in order to allow the fiber to go through subsequent textile machinery and processes without breakage [47].

10.5.3 Fiber structure and morphology In addition to the chemical structure of the fiber forming polymer, fiber morphology plays a significant role in determining the ultimate strength and extensibility (or elasticity) of a fiber [48]. Morphology is also important in determining the ability of the fiber to be dyed (penetrated by pigments). In the case of the melt spinning of semicrystalline thermoplastics (such as polyester or nylon), the final morphology of the fiber is dictated by:

• Polymer chemical structure • Melt spinning conditions (spinning speed, cooling type, cooling rate, etc.) • Drawing conditions (draw ratio, heat setting temperature, relaxation, etc.) The relative amounts of amorphous and crystalline regions as well as the type of crystals and their arrangement along the fiber length will determine its acceptance of chemical dyes and its mechanical behavior. High draw ratios are necessary to obtain highly crystalline, strong fibers, but this may be to the expense of dyeability. Fig. 10.5 illustrates the morphology of polyester fiber and a typical distribution of crystalline and amorphous regions and the orientation of crystals along the fiber length [49].

10.5.4 Fiber identification The fiber industry uses several techniques to identify fibers and distinguish them from one another. Some of the most common methods include microscopic examination, solubility test, heating and flammability tests, and density and staining techniques [50].

10.5 Characterization and testing of textile fibers

FIGURE 10.5 Morphology of polyethylene terephthalate (PET) fiber [49]. Reprinted from Mark HF. Polyester fibers. In: Encyclopedia of polymer science and technology. John Wiley & Sons, Inc. © 2003 John Wiley & Sons.

10.5.4.1 Microscopy test The optical microscope is a useful tool in any lab to examine textile fibers and identify them. It is easier to identify natural fibers than synthetic ones because of the similarity in synthetic fiber appearance. Shape and cross section are common characteristics to examine under the microscope.

10.5.4.2 Chemical test Solubility is a highly effective test to identify fibers. The chemical structure of a polymer determines its resistance to solvents and other chemicals. Polyolefins, for example, have excellent chemical resistance to most common solvents and they can be easily distinguished from a polyester or nylon fiber by a simple solubility test. Functional groups can also be identified using techniques such as FT-IR, which will help identify the fiber type.

10.5.4.3 Burn test When in contact with an open flame, different fibers will generate different types of flame, ash, and smell. What is important in this test is to ask: Does the fiber melt or burn? Does the fiber shrink from the flame? What is the odor of the

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fumes? What kind of residue does the flame leave? The answers to these questions are used as a guide to identify the fiber based on known behavior of each type of fiber when exposed to a flame.

10.5.4.4 Density test Fiber density is an indication of the type of fiber being identified. Polyolefin fibers, for example, will float in water, which is a unique characteristic.

10.5.4.5 Stain test This test is based on the affinity of various fibers toward different dyes. A series of dyes that have known affinities for specific fibers are prepared and the fiber in question is immersed to check its dye pick-up. The morphology of the fiber and its chemical structure determine its dyeability and hydrophilicity.

10.5.5 Other characterization and identification techniques Textile fibers are characterized by numerous analytical techniques such as thermal analysis, spectroscopy techniques, electron microscopy, and nuclear magnetic resonance. Differential scanning calorimetry is particularly important in determining the melting point of polymeric fibers and quickly distinguishing between polymers [51]. Basis weight is important to characterize nonwovens, which is the number of grams per square meter of fabric, and can be as low as 10 g/m2 or as high as a few thousand grams per square meter depending on the application [52].

10.6 Polymers in textiles: major manmade fibers This chapter is concerned with the technology of synthetic fibers, rather than the economics. It is, however, important to remind the reader of the size and economic weight of the synthetic fiber industry. According to a report by Grand View Research Inc., the global synthetic fiber market is expected to reach USD88.5 billion by the year 2025 [53]. The industry is expected to grow at an annual rate of 6.3%, largely driven by the growth in polyester [54]. In the subsequent sections, the major manmade polymer fibers will be discussed in detail. For each fiber type, the base polymer (s) will be introduced, the unique characteristics described, and the main uses and applications detailed. As mentioned earlier, the focus will be on the technical and modern textile applications.

10.6 Polymers in textiles: major manmade fibers

10.6.1 Polyester 10.6.1.1 Chemistry Polyester fiber is defined, according to the Federal Trade Commission (USA), as a “manufactured fiber in which the fiber forming substance is any long chain synthetic polymer composed of at least 85% by weight of an ester of a di-alcohol and terephthalic acid” [34]. The most common polyester fiber is made from polyethylene terephthalate (PET). PET is a polymer made through the condensation polymerization of ethylene glycol and terephthalic acid. The chemical structure of PET is shown in Fig. 10.6.

10.6.1.2 Properties Polyester fibers have a smooth surface and are usually round and uniform. Polyester fibers are sometimes blended with cotton to balance the fabric properties, often to the expense of the qualities of cotton such as breathability and heat resistance. Some of the characteristics that PET fibers are known for include:

• • • • • • • •

Good strength Good resistance to stretching and shrinking Good abrasion resistance Good chemical resistance Dyeability Excellent wrinkle resistance Quick drying Moderate resiliency both dry and wet

One important parameter to consider in choosing the polymer grade for fiber manufacturing is the intrinsic viscosity (IV). It is determined by extrapolating to zero concentration the relative viscosity concentration curve. The IV of polyester resin is extremely important for fiber grade polymers as it is critical for both fiber spinning and the final fiber properties [55]. Textile fiber grade polymers should have an IV ranging from 0.40 to 0.70, whereas a range of 0.72 0.98 is required for more demanding applications such as technical textiles. Another important point to highlight, which is true for all other synthetic fibers, is the effect of manufacturing process on fiber properties. Polyester fiber

FIGURE 10.6 Polyethylene terephthalate chemical structure.

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can be made into high tenacity (Ten), low elongation yarn (Ten 85 cN/Tex and Eb 7%) or low tenacity, high elongation yarn (Ten 26 Cn/Tex and Eb 40%) depending on the melt spinning and drawing conditions [56].

10.6.1.3 Uses Polyester fiber was first manufactured by DuPont in 1953 [57]. Since then, the demand for polyester fiber has increased steadily; it has already surpassed cotton as the most widely used manufactured fiber. Due to its characteristics, polyester is widely used in apparel and home furnishings. Polyester fibers are found in shirts, suits, children’s wear, dresses, and blouses. Polyester is also popular in home furnishings such as blankets, bedroom sheets, and cushions [58]. One of its large volume applications is the so-called polyester fiberfill. For this application, polyester fibers are made to be bulky through mechanical crimping and coated with silicone-based finishes to impart springiness and an ability to recover from an applied load [59]. This makes fiberfill an excellent choice for pillow stuffing and sleeping bags. Polyester is widely used in technical textiles, especially fully oriented fiber with high tenacity. In transportation, polyester filaments are well established as reinforcement for rubber in tires. Polyester tire cords are durable and heat resistant. Polybutylene terephthalate, another type of polyester fiber, is used in V belts and other types of belts in automobiles [55]. Polyester fabric is an excellent choice for sail cloth due to its durability, high modulus, and low cost. Fig. 10.7

FIGURE 10.7 Dacron polyester sail cloth.

10.6 Polymers in textiles: major manmade fibers

FIGURE 10.8 Polyester swimming pool filter cartridge. Courtesy Walmart.

shows an example of a sail cloth made from high tenacity PET. In building and construction, PET SB nonwovens represent a material of choice for roofing and geotextile applications. PET nonwovens are also used as liquid filtration media for pool and spa filter cartridges such as the filter shown in Fig. 10.8.

10.6.2 Nylon Nylon is an important manmade fiber due to its excellent combination of properties [60]. It is also important from a historical perspective as discussed in Section 10.2.

10.6.2.1 Chemistry Nylon, according to the federal trade commission, is a “manufactured fiber in which the fiber forming substance is any long chain synthetic polyamide having

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FIGURE 10.9 Nylon 66 chemical structure.

recurring amide groups as an integral part of the polymer chain” [34]. Nylon 66 and nylon 6 are the most common types of nylon fibers [61]. Nylon 66 is made via the polycondensation of hexamethylene diamine and adipic acid (Fig. 10.9). Nylon 6, on the other hand, is made via the ring opening polymerization of caprolactam.

10.6.2.2 Properties Some unique properties of nylon fibers are:

• • • • •

Excellent combination of strength and elasticity Excellent resiliency Good abrasion resistance Excellent wrinkle resistance Good flame resistance

Nylon fibers are also known to have poor resistance to sunlight and are prone to UV and thermal degradation [47]. Nylon 66 owes its high tenacity, in part, to hydrogen bonding, which increases the percentage of crystallinity in the polymer chains [62].

10.6.2.3 Uses Nylon fiber is widely used in carpets, upholstery, hosiery, raincoats, and sewing thread. It is more expensive than polyester so several textile applications have switched to polyester fiber use. Nylon, however, remains a fiber of choice for various technical textile applications. In automobiles, nylon fiber is used as fabric for airbags and seat belts and it is also used as tire cord for rubber reinforcement (Fig. 10.10). In civil and military applications, nylon fiber is widely used in tents, ropes, parachute canopies, and harnesses [63].

10.6.3 Acetate fiber 10.6.3.1 Chemistry Acetate is an important member of the manufactured fiber family, it was first manufactured by Celanese Corporation in 1924 [34]. It refers to fibers made from CA where not less than 92% of the hydroxyl groups are acetylated. Acetate fibers are made using the dry spinning process.

10.6 Polymers in textiles: major manmade fibers

FIGURE 10.10 Automobile airbag made from nylon fabric.

10.6.3.2 Properties Acetate is sometimes called acetate silk fiber as it is extremely soft to the touch, but its tensile strength is low. Some of the characteristics of acetate fiber are:

• • • • • • • • •

Excellent softness Pleasant feel Low strength Low thermal stability Poor elasticity Poor abrasion resistance Fast drying Good drape Good resistance to shrinkage

10.6.3.3 Uses Acetate fibers are popular in apparel applications such as dresses and lingerie. They are also used in draperies, blouses, sportswear, and satin fabrics. Acetate fibers are also used in umbrellas and as filters in cigarettes. Triacetate fibers are used in sportswear and garments where pleat retention is important due to their shrink resistance [64]. The low tenacity of acetate fibers makes their use in technical textiles somehow limited.

10.6.4 Acrylic fiber The first acrylic fiber was manufactured by DuPont in the United States in 1941 [65].

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FIGURE 10.11 Chemical structure of polyacrylonitrile.

10.6.4.1 Chemistry Acrylic fibers refer to fibers made from polyacrylonitrile (PAN). According to the United States Federal Trade Commission, it refers to manufactured fibers where the fiber forming substance is any long chain synthetic polymer composed of at least 85% by weight of acrylonitrile [34] (Fig. 10.11).

10.6.4.2 Properties • Excellent warm feel • Lightweight • Good resistance to sunlight • Good chemical and oil resistance • Excellent wickability and fast drying 10.6.4.3 Uses Acrylic fiber is ideal for use in sweaters, blankets, and socks. Acrylic fiber is also popular in ski and snow suits because of its keep-warm property. Other applications include baby garments and sportswear. Acrylic fiber can be easily used as a substitute for cotton and wool and it can also be blended with these natural fibers for various applications [66]. A drawback of acrylic fiber is its poor flame resistance, which is a point to consider when blending it with cotton or wool [67]. The colorfast characteristic of acrylic fibers makes them highly attractive in applications where the textile article requires frequent washing.

10.6.5 Modacrylic fiber 10.6.5.1 Chemistry When acrylonitrile is mixed with a comonomer such as vinyl chloride or vinyledene chloride in a dry spinning process, the resulting fiber is referred to as modacrylic. A typical PAN content of between 35% and 85% is necessary for modified acrylic to be classified as modacrylic [34].

10.6.5.2 Properties The main advantage of modification is to impart flame retardant characteristics to acrylic fiber. Modacrylic fiber has the ability to be softened at low temperatures, which allows the fibers to be stretched and curled due to varying shrinkage levels.

10.6 Polymers in textiles: major manmade fibers

Some of the main characteristics of modacrylic fiber are:

• • • • • •

Excellent flame resistance, that is, it does not burn Good abrasion resistance Good resiliency Excellent chemical resistance Excellent resistance to sunlight Moderate strength

10.6.5.3 Uses Their unique properties make them useful in applications such as simulated fur, hair extensions and wigs, paint rollers, rugs and carpets, and protective clothing [68].

10.6.6 Spandex fiber Spandex is an elastic, rubber-like fiber made from polyurethane. It is unique due to its excellent stretchability and ability to recover to its original length and shape. It is also an extremely durable fiber, but has several drawbacks [47].

10.6.6.1 Chemistry Polyurethane fiber is made by reacting diisocyanate with a dialcohol resulting in a hard segment and an elastic, soft segment in the polymer chain as shown in Fig. 10.12.

10.6.6.2 Properties • Poor tenacity • Superior elasticity • Good resilience • Excellent abrasion resistance • Excellent wrinkle resistance • Good resistance to body oils • Poor heat resistance • Slow burning characteristics

FIGURE 10.12 Polymerization reaction of polyurethane.

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10.6.6.3 Uses Spandex, and other elastic fibers, are widely used in athletic wear, swimsuits, bras, and a number of other elastic textiles. In technical textiles, spandex is used as a support and surgical hose in operation rooms [69].

10.6.7 High-performance fibers High-performance fibers are a class of polymeric fibers with high temperature or superior mechanical performance characteristics that are not achievable with some of the more conventional fibers mentioned in the previous sections. Their development in the early 1980s “provided some of the most significant and dramatic impulses to the evolution of technical textiles” [70]. The most common ones are aramids (Nomex and Kevlar), but this class also includes fibers such as polyphenylene sulfide (PPS), poly ether ether ketone (PEEK), and UHMWPE [71]. The availability of various high-performance fibers, along with the diversity of fabric forming techniques, is critical to the optimization of military clothing in terms of balancing performance with lightweight and the comfort of soldiers, especially in harsh environments. High-performance fibers are the materials of choice for demanding military and security applications, both in marine and terrestrial environments.

10.6.7.1 Aramids (Nomex and Kevlar) 10.6.7.1.1 Chemistry Aramids are simply aromatic polyamides (nylons). The fiber forming substance is a polyamide where the amide groups on the polymer chain have two aromatic rings adjacently attached on both sides (Fig. 10.13). The meta form of aramid, Nomex, is known for its high temperature resistance and is widely used in protective clothing. The para form of aramid, Kevlar, is known for its high strength and modulus and is widely used in bullet proof vests.

10.6.7.1.2 Properties Some of the major properties of aramid fibers are:

• High temperature resistance • Excellent strength and stiffness • Excellent dimensional stability

FIGURE 10.13 Aramid chemical structure.

10.6 Polymers in textiles: major manmade fibers

• • • • •

Low flammability Excellent heat resistance Good chemical resistance Bullet proof Cut resistance

10.6.7.1.3 Uses Aramid fibers such as Kevlar and Nomex are used in high temperature and mechanically demanding applications. Nomex is the fiber of choice for fire fighter suits due to its excellent heat resistance. It is also used in space suits, heat resistant gloves, and filters (Fig. 10.14). Kevlar is used in bullet proof vests, cutresistant gloves, reinforcement for hoses, tires, belts, ropes, and advanced composites (Fig. 10.15) [70].

FIGURE 10.14 Fire fighter suit made from Nomex fiber.

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FIGURE 10.15 Police body armor made from Kevlar fiber.

10.6.7.2 Ultrahigh molecular weight polyethylene UHMWPE is considered to be a polymer of choice for high strength and low weight fiber applications. This fiber is currently made by DSM, LLC, in Europe and Honeywell Advanced Fibers & Composites, USA. The fibers are known respectively as Dyneema and Spectra [37].

10.6.7.2.1 Chemistry UHMWPE is simply polyethylene, but its molecular weight is extremely high, which makes it unique and impossible to process using common melt processing techniques. The fiber is made by the gel spinning method, which was first invented in Europe and commercialized by DSM in the 1990s. UHMWPE usually has a molecular weight in the range of 2 6 million g/mol.

10.6.7.2.2 Properties The outstanding strength of UHMWPE is attributed to its fully oriented, polyethylene chains [72]. In fact, it is one of the world’s strongest and lightest fibers. UHMWPE fiber is 10-times stronger than steel and up to 40% stronger than aramids, compared on a similar weight basis.

10.6 Polymers in textiles: major manmade fibers

UHMWPE has the properties:

• • • • •

Outstanding strength Excellent chemical resistance Good water resistance Floats on water Excellent fiber-to-fiber abrasion.

10.6.7.2.3 Uses UHMWPE fibers are used in applications such as marine cordage and lifting slings. The fact that it floats on water gives it an advantage in many marine applications. Other uses include police and military ballistic vests, armored vehicles, cut-resistant gloves, fishing lines, and safety clothing. The physical properties of a spectra grade made by Honeywell are summarized in Table 10.1 showing the exceptional mechanical properties of this fiber [73].

10.6.7.3 Carbon fiber Carbon fibers are among the strongest manmade fibers available. Their performance is unmatched in terms of strength-to-weight ratio while they have excellent rigidity [74].

10.6.7.3.1 Chemistry Carbon fibers are made from what is known as a precursor, which is either PAN or rayon. PAN is spun into filaments, which are collected in a flat tow that is heated in the absence of oxygen in a pyrolysis step that removes all noncarbon atoms. Silicone-based finishes are usually used to lubricate the filaments and prevent them from sticking to each other during processing [75] (Fig. 10.16).

10.6.7.3.2 Properties Carbon fibers are uniquely known for:

• • • • •

Superior strength and stiffness at low weight Lightweight Excellent fatigue resistance Corrosion resistance Chemical inertness

10.6.7.3.3 Uses Carbon fibers are woven into bidirectional sheets that are later impregnated with epoxy resin and cured to make carbon fiber reinforced plastic (CFRP) molded parts. CFRPs took off as important commercial materials with the increasing demand of the military and aerospace industries. The need for lightweight materials in the design of automobiles and airplanes in order to reduce total weigh and save on the cost of fuel made CFRPs an excellent choice due to their outstanding mechanical properties. In renewable energy, carbon fiber makes it possible to design huge wind blades with excellent balance between strength and lightweight [76].

353

Table 10.1 Partial product data sheet showing physical properties of the HT spectra grade.

Product family

Spectra fiber

Weight/unit length (Denier/ Decitex)

HT

240 300 375

267 333 417

Spectra datasheet, partial, Honeywell USA.

Ultimate tensile strength (g/ den)/(GPa)

Modulus (g/den)/ (GPa)

Elongation (%)

Breaking strength (kg)

Density (g/cc)

Filament tow

Filament (dpf)

41 39.5 45

1350 1650 1200 1500 1300 1700

3.1 3.5 2.9 3.7 2.9 3.6

10.0 11.8 17.2

.97 .97 .97

60 60 120

4 5 3.1

10.6 Polymers in textiles: major manmade fibers

FIGURE 10.16 Carbon fiber on a bobbin. Courtesy Cytec Corp.

Carbon fiber composites are used as frames and panels in wide body airplanes and space rockets. Other uses include various parts in military and racing vehicles (Fig. 10.17). CFRPs are also used in sporting equipment such as bicycles, fishing rods, tennis rackets, and hockey sticks (Fig. 10.18). In fact, the use of carbon fibers in sporting equipment grew as military spending decreased towards the end of the 20th century and the beginning of the 21st century, which forced manufacturers to develop new applications. It is important to mention, however, that the cost associated with making carbon fibers is rather high, which makes it challenging to justify the economics associated with their use in some applications.

10.6.8 Polyolefins Polyolefin fibers have a clear cost advantage compared to other synthetic fibers; they also possess unique properties. Polypropylene is more popular than polyethylene in textile applications, but both have relatively low melt points (compared to other synthetic fibers), which is a drawback [77].

10.6.8.1 Chemistry According to the United States Federal Trade Commission’s official definition, an olefin fiber is “A manufactured fiber in which the fiber forming substance is any long chain synthetic polymer composed of at least 85% by weight of ethylene, propylene or other olefin units” [34] (Figs. 10.19 and 10.20).

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FIGURE 10.17 Tail of radio controlled helicopter (carbon fiber reinforced polymer).

FIGURE 10.18 High modulus carbon fiber tennis racket.

10.6 Polymers in textiles: major manmade fibers

FIGURE 10.19 Polyethylene chemical structure.

FIGURE 10.20 Polypropylene chemical structure.

10.6.8.2 Properties Polyolefin fibers (polyethylene and polypropylene) are known to have good tensile strength and toughness and good abrasion resistance, they also have good chemical resistance, but this means that they are difficult to dye, which is a drawback of polyolefin fibers. These fibers are characterized by a smooth and round cross section. General textile use polypropylene fibers can have a tenacity of 40.5 50 cN/dtex, whereas high tenacity yarns used in ropes and nets can have tenacities up to 81 cN/dtex [78]. Polyethylene (high density) softens at 130 C and melts around 140 C, whereas polypropylene softens at 150 C and melts around 160 C. Polyethylene fiber burns quickly, giving a blue flame with a yellow tip, and it also drips while it burns. Polyethylene gives off a paraffin odor similar to a burning candle. Polypropylene melts and burns with a steady flame with almost no smoke and a clear melting portion. Polypropylene gives off a slightly celery odor or no odor at all. Polypropylene is characterized by a hard tan-colored residue [79]. A summary of some of the main properties and characteristics of polyolefin fibers is given here:

• Lightweight: polyolefin fibers have the lowest specific gravity of any • • • • • • • •

synthetic or natural fiber. Ability to give good bulk, which translates into good cover properties. Good strength (both wet and dry) and resiliency. Good abrasion resistance. Low moisture regain (close to zero). Quick drying ability since the fiber does not absorb moisture. Good chemical and stain resistance. Excellent thermal bonding properties, which is important for nonwovens. Good wicking characteristics and a comfortable feel.

10.6.8.3 Uses Polyolefins have long been used in applications such as carpets, ropes, geotextiles, hygiene nonwovens, construction, and technical textiles. They offer good cost benefits for many applications. Most polyolefin fibers are used in consumer

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products with carpets and rugs being the largest market. This is due mainly to the ability of polypropylene to replace nylon and jute in carpets. Polyolefin fibers, especially polypropylene, are widely used in carpet backing, carpet face yarn, laundry bags, sportswear and sweaters, rope and cordage, hosiery, undergarments, sewing thread, and knitwear. Polypropylene slit films and monofilaments are used in ropes, agricultural nets, and flexible intermediate bulk containers. Polyolefin fibers are also used in furniture and equipment covering, outside furniture, and more recently, as artificial turf [37]. Polypropylene in nonwovens include baby diapers, hygiene fabrics, wet wipes, and adult incontinence fabrics, which require a soft hand. Automotive fabrics, geotextiles, disposable hospital clothing, and industrial wipes are also important polypropylene applications. Some of the main uses of polypropylene in MB nonwovens are in filtration media, face masks to protect from viruses and air pollutants (Fig. 10.21), and battery separators. Metallocene catalyzed polypropylene was used to develop submicron diameter MB fibers [53]. The polyolefin used in this work had a narrow molecular weight distribution and an melt flow rate (MFR) greater than 1000 g/10 min, a typical range for MB nonwovens polymers. Product innovation through catalysts such as metallocenes makes it possible to make SB and MB fibers with finer diameters, which translates into softer fabrics and textile-type SB and MB nonwovens [80,81]. The world consumes in excess of 2.5 million tons of polypropylene in nonwovens today. According to a report by Chemicals Market Resources Inc. “Diapers, adult incontinence products, geotextile and carpets, are the end use applications expected to see rapid increase in demand in developing regions” [82].

FIGURE 10.21 Face mask made from melt blown polypropylene nonwoven fabric.

References

10.7 Conclusion The advances made in the synthetic fiber industry to shape polymers into fibers of various shapes and sizes have led to a revolution in textile manufacturing and design. Synthetic polymers made it possible to overcome some of the deficiencies of natural fibers. Synthetic polymers also made it possible to make technical textiles with superior properties and outstanding performances. Even though melt spinning remains the most commonly used fiber manufacturing technique, other techniques such as gel spinning offer alternatives when a polymer cannot be easily melt processed. The choice of polymer is critical in order to meet the specific requirements of textile fabrics. Polyester remains the most popular synthetic fiber due to its unique properties such as strength, abrasion resistance, and reasonable economics. It is widely used as sail cloth and SB nonwovens roofing. Nylon is also popular with an excellent combination of strength and elasticity and dominates in applications such as airbags, military tents, and parachute canopies. When softness is required, acetate fiber offers a good choice, whereas spandex (polyurethane) offers unmatched stretch and elasticity. Acrylic is the fiber of choice when warmth and flame retardancy are required. As for polyolefin fibers, they are popular in certain applications such as carpet and disposable non-wovens due to a clear cost advantage, but they remain limited by their low melt point and moderate mechanical properties. In highly demanding applications such as military and aerospace, however, the challenges can only be met by high-performance fibers. Aramids, carbon fiber, and UHMWPE are some of the most popular fibers with outstanding mechanical properties. These fibers are expensive, but they offer unique toughness, high temperature resistance, and, in the case of aramids, bullet proof performance. Carbon fibers are extremely successful in aerospace due to a combination of their mechanical performance and lightweight. Polymer innovation will continue to shape the growth of the textile industry.

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11

Kishor Kumar Sadasivuni1, Sara Mohamed Hegazy2, AAliah Aboubakr Moustafa Abdullah Aly2, Sadiya Waseem3 and K. Karthik4 1

Center for Advanced Materials, Qatar University, Doha, Qatar 2 College of Arts and Sciences, Qatar University, Doha, Qatar 3 Advance Carbon Products, CSIR-NPL, New Delhi, India 4 School of Physics, Bharathidasan University, Tiruchirappalli, India

11.1 Introduction Polymer electronics represent unique microdevice technology that uses plastic diodes, transistors, sensors, light-emitting devices, and photovoltaics. The availability of materials is considered to be one of the advantages of organic or polymer semiconductors over more traditional materials. There are innumerable side groups that can be added to the backbones of polymers to fine-tune specific properties. Chemical, electrochemical, and plasma synthesis of polymer materials are possible. These can vary from purely amorphous to highly crystalline in structural order. The shape of substrates can be varied from fibers to films as well. Thus polymeric materials have more flexibility to work with than inorganic semiconductors and can be tailored to specific physical, electrical, and optical properties; an extensive range of devices can, therefore, be integrated potentially with this technology. Facility and manufacturing costs other benefits of polymer electronics as well as the fact that a variety of solvents can be used to process polymers at low temperatures. Mask-less manufacturing can make a rapid turn-around and custom circuits are constructed layer by layer using common “printing” techniques. Furthermore, combining an active polymer electronic backplane with more traditional high-performance silicon and compound semiconductor technologies can open up an entirely new and powerful multitechnology integration approach. Integration with on-board, polymer-based power supplies and full-color, flexible organic light-emitting diode (OLED) displays on flexible substrates are expected to have an imaginable cost-effective plastic microsystem in a credit card size. This technology can also be used to print on large areas (many square meters) and at (or near) room temperature [1
3]. Naturally, all the potential benefits of the mentioned organic electronics are accompanied by major challenges. In order to develop a solvent, viable technology, material interactions, material lifetime, microstructure
property relationships, and durability need to be Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00011-1 © 2020 Elsevier Inc. All rights reserved.

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understood. All of these depend on the consistency and purity of the starting materials. The variability of the results reported in the literature for nominally identical materials indicates that for reliable electronics applications, a supplier level of 90% purity is probably not enough. The lack of consistency in the properties of polymer-based electronic devices in such materials causes a lack of basic understanding of the mechanisms of electrons and the whole transport. A typical electrical system will consist of a support structure that allows circuitry construction while providing isolation between circuit components. A print circuit board containing materials such as epoxy resin, phenol-formaldehyde, or polyester resin could be an example. Switches can be constructed as the main switch body from thermosetting polymers, molded with a specific structure to allow certain mechanical operations to occur. Moreover, other components such as push buttons are produced using thermoplastic materials through injection molding. This circuitry can be environmentprotected in a molded housing. Because of the insulative properties of plastics, they have a long history of use in electrical devices. Thermoplastics such as PVC are used as cable or wire insulation and have a low flammability as an added benefit. Some electrical systems are produced as components of “box-out, box-in,” in which case the component is either disposable or recycled for repair by the manufacturer. Other systems may be designed to be maintained in the field; in which case the housing must be designed to be accessible to the electrical components. This type of design may require the use of elastomeric materials to provide a seal to the environment to prevent the entry of substances that may affect electrical function. Therefore in the construction of an electrical device, a wide range of elastomers, thermoplastics, and thermosets can be used. Some of these materials may contain volatile substances that may cause the contacts and circuits inside the device to corrode. For example, it is known that residual sulfur or amine compounds react with copper resulting in a conductivity breakdown. If it is found that a sulfur vulcanized elastomer provides adequate physical properties, it may be necessary to keep the levels of vulcanization chemicals to a minimum. Or in other words, polymer compounds should be carefully selected to ensure compatibility with a device’s electrical components [4,5]. Another factor that should be considered is a polymer’s permeability. Unlike materials like glass and metals, polymers can be permeated to varying degrees depending on the type of chemical substance that comes into contact with the electrical device. Elastomers are generally more permeable than plastics. It is, therefore, necessary to consider their inclusion in the design of a device depending on the degree of exposure and the location of the seal within the device. Therefore the challenge is to identify materials with adequate physical properties that also provide chemical compatibility and chemical protection. Smithers Rapra’s consultancy team has extensive knowledge of polymeric materials and they assist with material specification for electrical devices to ensure the correct polymers are identified for a particular application [6
9].

11.2 Type of polymers

In electronics and polymer science, there are some outstanding polymers that can conduct electricity. They further explain the prospect of materials by combining novel electronic properties with the ease of polymer processing, and whose properties can be adjusted to give desired features through chemical modification. This novel typology of electronic material means that it will be projected to have giant flexibility, especially for printing displays and electronic circuits. In this chapter, the outstanding developments in this area are examined, giving significance to light-emitting diodes (LEDs) and plastic transistors, and then ingredients that may be combined to give upcoming innovations are discussed. Various polymeric types such as conjugated polymers [10
12], conducting polymers, ionic polymers, and so forth, are discussed. Moreover, the challenges that these materialists often face in fabricating polymeric products and in designing specific applications out of them will be investigated throughout.

11.2 Type of polymers 11.2.1 Conducting polymers 11.2.1.1 Traditional sequences of conducting polymer Polymers were only believed to be insulators until 1977, and then the “conductivity of polymers” concept came out. Polyacetylene (PA) after doping with electron withdrawing AsF5 was the first reported conductive polymer with a ninefold increase in conductivity. Thereafter various conducting polymers were identified such as polypyrrole (PPy), polyaniline (PAn), and polythiophene (PTh) during 1971
81. This imparted great impact in advancing the path of research on conducting polymers. The discovery of conjugated conductive polymers opened the applicability of those in electroluminescence (poly(p-phenylene vinylene) (PPV)), polymer light-emitting diodes (PLEDs) (semiconducting intrinsic conjugated polymers), photovoltaics, and so forth. In organic photovoltaics, bulkheterojunction polymer solar cells (PSCs) are developed with a conjugated polymer poly[2-methoxy-5-(20 -ethylhexyloxy)-1, 4-phenylene vinylene] (MEH
PPV) as a donor and a fullerene derivative, phenyl-C61-butyric acid methyl ester, as an acceptor. Subsequently optoelectronic materials and devices including PLEDs and PSCs were also established. In 2000, Heeger, MacDiarmid, and Shirakawa were presented with a Noble Prize in Chemistry for their achievements in the area of conducting polymers [13
15]. Fig. 11.1 shows an electrical conductivity comparison between a conductive polymer and a typical metal conductor.

11.2.1.2 Features of conducting polymers 1. Conducting polymers include both electronic conducting polymers and ionic conducting polymers. 2. Ionic conducting polymers are frequently called polymer electrolytes.

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FIGURE 11.1 Schematic representation of electrical conductivity comparison between a conductive polymer and a metal conductor.

3. Electronic conducting polymers can also incorporate conjugated conducting polymers and insulating polymers mixed with conducting materials.

11.2.1.3 Structure of conducting polymers Conducting polymers have conjugated molecular arrangements in the core chain where π-electrons delocalize over the entire molecular chain. In conjugated polymers, PA displays the simplest key chain structure composed of an alternate single bond and double bond carbon chain. According to the positions of the hydrogen atoms on the double bonded carbons, there are two types of structures: 1. Trans-polyacetylene In trans-polyacetylene, two hydrogen atoms are present on the opposite sides of double bond carbon. trans-Polyacetylene is a degenerate conjugated polymer that possesses an identical structure after swapping its double bond and single bond. 2. cis-Polyacetylene In cis-polyacetylene, two hydrogen atoms are located on the same side of the double bond. cis-Polyacetylene and other conjugated polymers are nondegenerate conjugated polymers that have nonequivalent structures after exchanging their double and single bonds.

11.2 Type of polymers

The key structures of representative conjugated polymers include PA, PPy, polyaniline (PAn or PANi), polythiophene (PT or PTh), PPV, poly(p-phenylene) (PPP), and polyfluorene.

11.2.1.4 Advantages of conducting polymers 1. They possess both the optical and electronic properties of metals and inorganic semiconductors. 2. They have adaptable mechanics and processability. 3. There is special electrochemical redox activity for conducting polymers. In future, conducting polymers including doped conducting polymers and intrinsic semiconducting conjugated polymers, will show a key enhancement in the development of organic optoelectronic and electrochemical devices [16].

11.2.2 Semiconducting polymers Due to the wide applications in plastic electronics, abundant progress has been made in the growth of electroplastics. There is still a serious need for new materials with enhanced properties and functionality to strictly fulfill the quest of having all-plastic electronic devices. In particular, we need printable electroplastics with (1) a wide range of controlled conductivity, (2) with high electron mobility (ntype materials), and (3) blue light emission. The basic requirements for such modern electroplastics are availability, processability, and reliability. Electroplastics indicate electroactive polymers with charge transporting properties. The majority of electroplastics are semiconducting with electrical conductivity in the range of somewhere from 101 to 102 ohm cm. Semiconducting polymers are categorized as per the schematic representation provided in Fig. 11.2.

11.2.2.1 Filled polymers Polymers that are loaded with conductive fillers such as carbon black, graphite fiber, metal particles, or metal oxide particles are referred to as filled polymers. Filled polymers have the greatest history and the widest application in electronic

FIGURE 11.2 Schematic representation for the classification of semiconducting polymers.

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devices. Conducting filled polymers were invented in 1930 for avoiding corona discharge and have been applied in advanced printed circuits. The main usage of filled polymers as semiconducting materials can be credited to their ease of processing and extensive range of electrical properties. Filled polymers are inhomogeneous in nature, and their heterogeneous nature is considered as the main weakness of such systems. Filled polymers have three major phases, namely: 1. The polymer 2. The filler 3. The interface Such heterogeneity tends to result in problems such as the absence of reproducibility, heavy process dependency, sudden percolation threshold in conductivity, or weak dielectric strength. Thus monitoring the quality of filler dispersion in a polymer matrix is the most critical and challenging technical issue in filled polymers.

11.2.2.2 Ionic polymers or ionomers Ionic polymers or ionomers have been notorious for more than 30 years; their ionic conductivity was not researched until 1975. Since then, ionically conducting polymers or polymer electrolytes have had an extensive range of commercial electronic applications, comprising of rechargeable batteries, fuel cells, and PLEDs. The drive concerning these important commercial applications has forced our understanding of ionically conducting polymers significantly. Ionic polymers also have the advantages of being extremely processable and richly available. However, ionic conductivity is highly sensitive to humidity. This is linked to the ionic conduction mechanism, which needs the dissociation of the opposite ionic charges followed by solvation. Solvation is usually disturbed by polar polymer matrices and by the presence of polar solvents, specifically water. As a result, most ionic conductors become extremely insulating upon drying. In several practical ionic polymer systems, one part of the ionic species is attached to the polymer chains, whereas the counter ions are mobile. This can avoid the rapid depletion of ionic species in these systems. Materials with both ionic and electronic conductivity are well known and may have new electronic applications. It should be noted that certain ionic conducting polymers can also be observed as filled polymers when the ionic additives are not entirely soluble in a polymer matrix.

11.2.2.3 Charge transfer polymers The field of charge transfer polymers originated from the discovery of electrical conductivity in molecular charge transfer complexes in the 1950s. A parallel observation was made in iodine-doped poly(vinyl carbazole) (PVK) in the mid1960s. The discovery of metallic conductivity in doped-PA in 1977 created incredible excitement and launched the field of synthetic metals. For the next

11.2 Type of polymers

decade, the field focused on searching for novel materials with metallic conductivity, without truly addressing the critical application parameters such as availability, processability, and reliability. After the discovery of superconducting molecular charge transfer complexes in 1980 for tetra methyl selena fulvalene and in 1986 for fullerene, the field of semiconducting organics finally evolved from a phenomenological chase to motivated research regarding applications. Currently, the exploration of semiconducting polymers as active constituents in electronic devices has become the main focal point. The ultimate challenge of the field is to develop intrinsically conducting polymers with electronic properties like molecular systems as well as processability and mechanical properties like conventional polymers [17
19].

11.2.2.4 Conjugated conducting polymers In the conjugated polymer front, polyaniline and modified PANi are being used as conductive fillers to provide conducting filled polymers. Water-soluble stable conducting PThs (Baytron) have been commercialized by Bayer Corporation. This revolution has inspired abundant work in the area of watersoluble conducting polymers. Ionic conductivity may also be present in such water-soluble conducting polymers. It has also been revealed that condensation polymers having conjugated oligomeric segments of well-defined structures such as an aniline trimer, exhibited desired electroactivity as well as processability and mechanical properties. Because of their commercial application in xerographic photoreceptors, charge transport polymers have become the most reliable semiconducting organic systems. The first commercial organic photoreceptor, presented by IBM in 1972, was a single layer device based on a charge transfer polymer, namely trinitrotoluene-doped poly(vinyl carbozole). Many aromatic organic materials, with conjugated polymers such as PPV, have since been evaluated for photoreceptor applications [20
22].

11.2.2.4.1 Charge transport polymer Instead of charge transfer polymers, charge transport polymers are being used in the latest multilayer photoreceptors. The majority of charge transport polymers such as PVK, triarylamine-doped polycarbonate (PC), and polysilanes are hole transporting or p-type materials. Though several electron transport materials have been identified, their charge mobility of 10v6 cm2 Vv 1 sv1 is about three orders of magnitude lower than that for hole transport materials. The absence of high mobility electron transport materials is a critical weakness related to semiconducting organics. The group of semiconducting organics will possibly remain p-type if high mobility electron transport materials are not realized. The theory of adding an oxidant (such as SbCls and tri-(p-bromophenyl)aluminum hexachloroantimonate) to a charge transport polymer (such as PVK) to provide a semiconducting charge transfer polymer was first established in the 1970s. Various oxidant and charge transport polymer groupings have been developed to give different semiconducting polymers and have been used as contact modification layers in organic

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LEDs. For example, cation radical salts of N,N,N0 ,N0 -tetra-p-tolyl-4,40 -biphenyldiamine were initiated and used in groupings with triarylamine-doped PC charge transport polymers to provide semiconducting polymers. The conductivity of these polymers could be altered readily by regulating the concentrations of the charge transport group and the oxidant. These semiconducting polymers are near to ideal because they have several advantages including a broad range of highly stable, tunable, homogeneous conductivities [10vl 2
10v 5 (Ωcm)v 1], tremendous wear resistances, and extreme dielectric strengths.

11.3 Applications of semiconducting polymers Semiconducting polymers are widely used in plastic electronics in the form of plastic transistors, plastic displays, plastic photodiodes, plastic lasers, and plastic fuel cells. The only challenge for their wider applicability is that maximum conductivity is not yet achievable. Additional researches in this direction may result in more highly conducting polymers in order to enlarge the range of applications [23
26]. Some of the notable applications of semiconducting polymers are discussed here.

11.3.1 Fuel cells Polymer electrolyte fuel cells (PEFCs) have been the center of attention as energy harvesting devices. There is an immediate requirement of energy that is highly efficient, nonharmful toward the environment, and that can act as a power source for vehicles and in transportation. Since governments all over the world are serious about such power sources, huge research works are being done related to fuel-cell-based vehicles of great efficiency and low tail pipe emissions. Hydrogen (H2) is easily available and quite a suitable fuel for fuel-cell-powered vehicles, offering extreme conversion efficiency and a clean by product (water). Hydrogen fuel is easy to handle as it can be carried on board the vehicle in the form of neat hydrogen, as cryogenically stored liquid, pressurized gas, or in the form of a more ordinary liquid fuel such as methanol or liquid hydrocarbon, which would require further processing on board the vehicle at the time of use. Vehicles using pure H2 fulfill the criteria of a zero-emission-vehicle, but with conversion to H2 ones are called ultralow emission vehicle. Fuel cells have numerous types and varieties and among them; the PEFC technology seems to be highly appropriate for terrestrial transportation applications. The key factors that lead to the preference of such fuel cells over others are their low temperature of operation (hence fast cold start), flawless carbon dioxide (CO2) tolerance by the electrolyte, and a blend of both high power density and high energy conversion efficiency. Present hurdles associated with the development of this fuel cell technology for terrestrial applications have been successfully

11.3 Applications of semiconducting polymers

overcome and the technology has prospered a lot. Therefore automotive and fuel cell manufacturing industries have started significant work for technology authentication programs and demonstrations, which include fuel-cell-powered vehicles, stationary power generation systems, and battery replacement devices. The market entry of PEFCs through these applications may actually lead to the implementation of such fuel cells in vehicular power systems, majorly because of the less stringent demands on systems budgets [27
29].

11.3.2 Piezoelectric materials In the investigation of piezoelectric materials, polyvinylidene fluoride (PVDF), and its copolymers are of utmost significance [30,31]. The piezoelectric effect of these polymers is mainly due to the strong molecular dipoles existing inside the polymer chain. The shift in the dipole density when a mechanical stimulus is applied is yet another factor affecting it. Ferroelectric polymers reveal moderate piezoelectric coefficients (d33 around 20
30 pC/N) in comparison to ceramic piezoelectrics with an acoustic impedance analogous to that of water and other liquids [32
34]. Multiple applications of ferroelectric polymers immersed in many niches like hydrophones and clamp-on transducers consumed as pressure sensor for diesel injection lines, with a demand of sale of over 50 million pieces per year, in addition to piezoelectric ignition systems for evaluating the mechanical and physical state of matter under examination. Current research on piezoelectric materials extend to many ferroelectric systems such as electron-irradiated PVDF
trifluoroethylene copolymers [8] or terpolymers of vinylidene fluoride, trifluoroethylene, and chlorofluoroethylene [9]. Other than ferroelectric copolymers, charged cellular polymers also exhibit large piezoelectric d33 coefficients, and are collectively called ferroelectrets [35,36]. Ferroelectrets show great intrinsic piezoelectric d33 coefficients at temperatures higher than 100 C, relatively small d31 and d32 coefficients and an acoustic impedance flawlessly welladjusted for coupling with air as well as other gases. The premium ferroelectret, reliant on cellular polypropylene films, shows a restricted thermal stability of up to 50 C. The materials have been made in Finland and are available on the market for commercial use, likely for use in flexible keyboards and musical pick-ups. They have potential for use in large scale applications like surveillance and intruder systems. Cellular polytetrafluoroethylene and its copolymers report a thermal stability of up to 100 C as well. Smart piezoelectric materials also demand wide bandwidth, fast electromechanical reactions, relatively low power necessities, and high generative forces. The word piezoelectricity was coined from “pressure electricity,” which means the generation of electrical polarization in a material in a reaction to a mechanical stress. This phenomenon is labeled as the direct effect. Piezoelectric materials also display the converse effect, which means that mechanical deformation takes place upon the usage of an electrical charge or signal. Piezoelectricity is an intrinsic asset of many noncentrosymmetric ceramics, polymers, and other such

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biological systems. Ferroelectricity is a characteristic of certain particular dielectrics that show an impulsive electric polarization (separation of the center of positive and negative electric charge, making one side of the crystal completely positive and the opposite side entirely negative), which can even be inverted using a certain electric field. Ferroelectricity is entitled by analogy with ferromagnetism, which arises in some materials such as iron. Conventionally, ferroelectricity is known in crystalline materials and in the region of crystallinity in semicrystalline materials. A number of researchers have opened up the possibility of the presence of ferroelectricity in amorphous polymers too, that is, ferroelectricity can exist even with a lack of crystal lattice structure [37]. Though the piezoelectric strain constant (d31) for polymers is lower than that of ceramics, piezoelectric polymers have larger piezoelectric stress constants (g3), which suggests their better sensitivity in comparison to ceramics. Piezoelectric polymeric sensors and actuators have the benefits and advantages of processing flexibility due to being lightweight, tough, easily made into large areas, and the ability to be cut and assembled into complex shapes as well. These polymers also show high strength and high impact resistance. Other distinguished aspects of these polymers are their low dielectric constants, low elastic stiffnesses, and low densities, which result in high voltage sensitivity (excellent sensor characteristic) and low acoustic and mechanical impedances (crucial for medical and underwater applications). Polymers usually have a high dielectric breakdown and have high operating field strengths, which suggest that they can hold extremely large driving fields compared to ceramics [38,39].

11.3.3 Optoelectronics Optoelectronic devices such as OLEDs and organic photovoltaic cells (OPVs) have attracted great attention for replacing their inorganic counterparts in numerous applications. Such devices use organic materials as their active elements. However, conventional optoelectronic components use semiconductors (silicon, gallium arsenide), which require complex dispensation leading to high production costs, especially for large-area applications. Table 11.1 compares the inorganic and organic materials that are used in manufacturing optoelectronic devices. Table 11.1 Comparison of inorganic and organic materials used in optoelectronics. Inorganic materials

Organic materials

Complex processing Heavy weight in comparison to organic materials Small area Low flexibility High cost

Simple processing Lightweight in comparison to organic materials Large area High flexibility Low cost

11.3 Applications of semiconducting polymers

It is clear that organic materials are better suited for fabricating optoelectronic materials. Numerous applications including small-sized organic display screens and solar cells have already entered the consumer market. These applications prove that organic materials are flawlessly adapted to industrial production regardless of their low efficiency compared to that of inorganic semiconductors. The main shortcomings of using organic materials for devices are their weak stability, which can lead to device failure even at ambient temperature. Such instability is found to be associated to the structure of organic materials, which depends on their glass transition temperatures and photooxidation. Additional dilapidation can be caused by changes in morphology, diffusion of components, and the modification of interfaces throughout device operation. Because of these shortcomings, the lifetime of organic devices is still shorter (industrially applicable levels B10,000 hours) as compared to that of conventional semiconductors. The short lifetime of such devices is caused by deprivations occurring in different parts of the diode during operation. Several degradation processes have been acknowledged such as thermal instabilities, chemical and photooxidation of the active layer, and the diffusion of metal from electrodes. In order to improve and enhance the stability of organic films, inorganic nanostructures (oxides, semiconductors) are added into the host materials to form composites. Not only is the stability of a film expected to be heightened due to the presence of an inorganic component, but its optical possessions can also be modulated by regulating the fillers (concentration and shape) for specific optoelectronic applications. A supplementary advantage of the use of composites in devices is the formation of pathways for carrier transport, which expands the electrical conductivity of thin films. Hence the use of nanocomposites as an active material in devices is predicted to substantially improve their performance and their lifetime as well. Semiconducting polymers are a smart choice as active materials for a variety of optoelectronic applications. These materials have a unique and extraordinary combination of optoelectronic and mechanical properties such as large visible light absorbing strength, which results in a sizable density of the photogenerated carrier, and sand solution processability. The sizeable optical captivation of these materials simplifies the requirement in form of thin (submicron) films rather than the thick layers as in the case of silicon-type materials. These structures offer the possibility of a seamless integration of device structures grounded on active materials with biological systems. For instance, the optoelectronic properties and features of materials have been utilized as active triggers for neuronal stimulation. Polymers are modified for their charge-carrier nobilities using regular and normal molecular structures, stretch orientations, or liquid-crystal phases [40,41]. There have been some momentous successes in the rational design of materials with the required properties such as cyano-substitution on PPV to enhance its electron affinity. Another way is by using an ethyl-hexyloxy side group to make the soluble polymer MEH/PPV. The asymmetrical substitution hinders the crystallization of the polymer making it extra soluble in nature.

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Yet another example is color tuning by making copolymers of two different materials to tune the color of one. This trend is inclined to grow and helps future developments in linking the properties and features of conjugated polymers to their structure and morphology, which will mean that larger materials can be designed and industrialized with the desired properties. However, it is not sufficient to know which structures would make flawless materials, but it is compulsory to be able to synthesize them. Developments and advancements in chemistry will be another critical factor in the development of polymer electronics. Current augmentations in synthesis provide improvements in purity, control of end groups, and a better control of molecular weight. By altering the side groups, morphology and solubility features can be organized and enhanced, allowing for the control of charge transportation and processing. An impurity that obstructs luminescence at a concentration of one part per million may lead to a significant decrease in the efficiency of an LED. Currently, many organic LEDs have multiple layers, each individually deposited and adapted for a particular purpose (e.g., light emission, electron transport, hole transport). An anticipated architecture would be to have single molecule dispersion from contact to contact, and molecules encompassing different sections effectively adapted to different functions. The part of the molecule near the holeinjecting interaction would have to have a chemical composition that is perfect for transporting holes to a central section, which is modified for light emission, while the other end of the molecule transports electrons to the light-emitting region. Generally, the molecules would be exceedingly ordered to allow for efficient charge transport, and provide with a higher physical robustness. A path breaking development of this knowledge would be to include segments of conducting polymers on each end of the molecule instead of conducting the contacts. If each section of the molecule was designed to be inclusive to similar sections in other molecules, then that would be the best-case scenario. This means that if the molecules were mixed together in a solution, then they would amalgamate to form a light-emitting polymer layer. Such a stimulating architecture would open endless possibilities and opportunities of tuning each and every important property; by changing the repeat units, mixing different repeat units, changing the length of segments, or even having a large gradation of assets rather than discrete segments. Researches have been carried out in order to make molecular wires and even some molecular diodes. An encouraging method is the utilization of “self-assembly” techniques, in which a structure is developed by depositing many layers of molecules in an ordered and organized way so that there exists a chemical bond between each consecutive layer. LEDs are advanced in this fashion. A probable future technique is to prepare a template such as a system of nanotubes created in a liquid-crystal mesophase of silica. These can be used to make an array of straight conjugated polymer molecules. Otherwise, a potential rigid conjugated polymer can also be made that could form such a mesophase on its own without using a silica template. The influence of a particular control over structures is

11.3 Applications of semiconducting polymers

obvious in the era of inorganic semiconductors, where molecular beam epitaxy and chemical vapor deposition have delivered exquisite control over material grasp and possessions [42].

11.3.4 Flexible electronics An interesting aspect of polymer electronics is the opportunity to build flexible electronic circuits by depositing the polymer layers on flexible substrates. Perhaps the rest of the question to reflect in this context is whether anybody would need flexible electronics, and in some behaviors this is a test of thoughts. In the respect of demonstrations, one noteworthy practice of flexibility would be to permit a desired shape of the display to be understood easily. It can be seen in some displays (e.g., car dashboards) that are curved, allowing optimal use of space and assisting to keep on the stray light. In some of these solicitations, there is no requirement to alter the shape of the display once it has been tailored, but flexibility would deliver an appropriate way of comprehending a specific shape. Nevertheless, there could also be habits for flexible displays; for example, they could be readily stored by folding (or rolling) them up when they are not in utilization. In the context of electronics, smart cards that comprehend a simple chip and electronic barcodes are expanding. It is necessary for such devices to be flexible, and polymer FETs promise a likely way of attaining this. Nevertheless, there are two additional inferences of the flexibility that go beyond the customs explained previously. The gist is that, flexibility convenes robustness. A propensity to bend means that brittle fracture can be sidestepped. This is significant for all applications, but predominantly to those regarding large-area devices. For example, a display panel of the dimensions of a few meters may be tremendously vulnerable to rupture unless it was flexible or had a cumbersome support. The other substantial implication is that flexibility dramatically improves the manufacturability of the process. It opens the avenues to having a procedure that starts with a drum of substrate that is then coated with suitable polymer layers, and then naturally cut into polymer electronic devices in a single procedure. Flexible PLEDs are established on a poly(ethylene terephthalate) substrate (as used for transparencies for overhead projectors) painted with conducting PANi as a hole-injecting contact. A semiconducting polymer layer is coated on top, and the device is concluded with a top layer of calcium and it can be perfectly fixed. Flexibility can be helpful in the future for lighting solicitations. A large-area light-emitting polymer device may be used to give a dissimilar and possibly more restful lighting in which low brightness over a proper area can be developed. Flexibility can also be used to make light-emitting clothing for safety or fashion purposes. A major drawback of using polymer substrates is that they are exaggeratedly permeable to air and water in comparison to glass, commonly known as a substrate. This is a tricky situation since oxygen and water lead to the degradation of polymer devices. There are two significant methodologies to solve this

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problem. The first is to design substrate materials with enhanced barrier properties; while the second is to make conjugated polymers that are more resilient to air and water [43
45].

11.3.5 Printable electronics Developments in polymer dispensation produce remarkable applications in polymer electronics. The utmost advantage of printing is its probability for mass production and demonstrating. It can be in contacts for pixelated exhibitions or in simpler terms in the shape of a light-emitting company logo. Moreover, it is an additive process and, hence, reduces material wastage. Some forms of printing can also be well-matched to instant prototyping or small-scale personalized synthesis, while other forms could permit mass production. Yang et al. have established the ink-jet printing of polymer LEDs. It has been discovered that FETs can be printed with trustworthy charge-carrier mobilities. Even more remarkable is the development of a prototype miniature television screen 50 mm2 and 2 mm thick. It led to the connotation between Cambridge Display Technology and Seiko Epson, and was developed by ink-jet printing of polymers onto polysilicon thin-lm transistors. The prototype is yellow and green and companies are now enthusiastic for a full-color version of the display. Fundamental lithographic printing presses can achieve 10,000 impressions per hour. A substantial development by Harrison et al. has been a principal ink compatible with lithographic printing. The ink comprises of conducting metal particles in a polymer matrix. It permits conductive circuits to be printed promptly. The predictable process for making “printed” circuits includes a photographic exposure monitored by etching, which harvests acid waste. The lithographic printing method is dramatically faster and comparatively clean. If it can be exploited to print polymer electronic devices, it would facilitate electronics fabrication at unprecedented speeds and lower costs. This can transform the range of applications in areas of electronics. The present existing electronics printing techniques are:

• Flexo printing: a high-pressure method that is especially well applicable to print on plastic substrates.

• Offset printing: a flat printing technique that helps make a high resolution conceivable.

• Gravure printing: a low-pressure printing technique that allows high volumes and the use of organic dissolvent possible.

• Rotary screen printing: a method that allows printing in thick layers easily. • Coating methods: diverse methods to join homogeneous and thin layers together. Polymers in printable electronics represent a widening area for applied research and have many advantages and disadvantages. The major advantages can be summarized as:

11.3 Applications of semiconducting polymers

• • • • •

Manufacturing is relatively simple and cost effective Polymers are lightweight, flexible, and durable under immense stress and flex Can be easily used over a large surface area Freedom of choice of the chemical composition used Adaptable in various situations because of printing techniques that can be synced to the current requirements quickly (printed electronics)

In the same way, this emerging field is facing certain disadvantages also, some of which include:

• Due to their intrinsic physical assets (i.e., restricted mobility of charge • •

carriers), the presentation of polymer electronic products lacks the speed of its silicon counterpart. Research is still on going to enhance performance for more complex functionality. To be able to enhance performance, one should be able to differentiate between problems familiarized during preparation, intrinsic material properties, and device characteristics [46,47].

11.3.6 Dielectrics In the early 1800s, Michael Faraday first used the word dielectric to define a phenomenon that he observed when an insulator material was positioned between two capacitor parallel plates. The dielectric material between them becomes polarized to store electrical charge as an electrical field is applied to the metal plates. Several books on solid-state physics and dielectric theory cover a thorough mathematical treatment of this subject. Due to their lightweight, low price, graceful failure mode, and ease of processing, polymers have achieved prominence as dielectric materials. In dielectric applications, several distinct polymers are used, most particularly polyethylene (PE) in extruded wires and polypropylene with biaxial orientation (BOPP) for thin film capacitor applications [48
50]. In addition to polymers, inorganic materials such as ceramics, owing to their high dielectric constants, often .1000 times, have been used for capacitor applications. However, inorganic capacitors suffer from low breakdown resistance and nongraceful failure mode despite their elevated dielectric constants. This leads ceramic capacitors to fail in medium to high areas, resulting in a low general density of energy. Consequently, even capacitors produced from specific ceramics such as barium titanate (BaTiO3) with a dielectric constant of approximately 1700 or strontium titanate with a dielectric constant of approximately 2000 have lower energy densities than BOPP, which has a dielectric constant of only 2.2 for any implementation requiring a high energy storage ability. Enhanced focus has appeared to overcome these limitations by producing polymers with metal atoms attached to their backbone that could deliver the elevated dielectric constant observed in ceramics with the graceful failure and ease of processing observed in polymer films.

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Among the polymers, PP is a notable dielectric material with wider applicability. Because of its steric regularity, isotactic PP can be extremely crystalline, although it includes an amorphous phase as a few chain sections do not crystallize and may coexist with a few atactic chains. Therefore PP displays the features of both stages in general. In the crystalline region, by ordering regularity and stability, the polymer chain forms crystalline lamellae. The amorphous parts have loose loops, tie chains, cilia, and arranged randomly as segregation, leading to branching at times. The interface domain between the crystalline and amorphous stages plays a significant role in the general morphological characteristics of polymers and, as a consequence, their electrical characteristics. Treatment with acetone may decrease the amorphous stage and remove antioxidants from PP, reducing dielectric loss. At greater temperatures, the existence of antioxidants can also produce high dielectric loss. Other impurities may result in a loss of conductivity. The spherulites structures in the PP are deformed, elongated, and lose their crystalline identity to stack in microfibrils during the process of biaxially orienting the polymer film. This method of biaxial orientation can induce, as tempter or tubular forms, two distinct kinds of morphologies within BOPP. A high degree of crystallinity, a few orientation imbalances, and a standardized film thickness are needed for high dielectric strength. Polyphenylene sulfide (PPS) is another dielectric material, and it is an aromatic benzene ring polymer with a sulfur atom in the primary polymer chain. PPS is a thermoplastic with a recorded dielectric constant of 3.5 in high temperature engineering. PPS exists in semicrystalline and mostly amorphous form, or it can be crystallized above its glass transition temperature (Tg). In the amorphous material, the amorphous phase relaxation peak at 100 C occurs at 105 Hz and a second relaxation peak (α relaxation) occurs at 140 C owing to the semicrystalline content. Therefore the proportion of relaxing dipoles rises with temperature and the distribution of relaxation time changes from optimal behavior to nonideal behavior as the temperature rises. PPS is a good candidate for replacing PC dielectric films due to its exceptional chemical resistance, low water absorption (approximately 0.02%), excellent thermal stability (heat deflection temperature above 260 C), elevated elastic modulus and flame retardance. PPS displays reduced dielectric losses and dielectric equivalents compared to PC [51,52]. Polyethylene terephthalate (PET) is also a commonly used capacitor dielectric polyester. It has good mechanical strength, resistivity to chemicals, and excellent thermal stability (m.p. 260 C). This mixture of properties makes it an excellent insulating material for electrical use. Relaxation procedures caused either by a particular molecule’s local motion or by chain groups are called β, whereas relaxation owing to the entire chain’s motion is α. For PET, it is necessary to consider three distinct kinds of dielectric relaxation techniques. Due to the relaxation of dipoles in the main polymer chain, the first relaxation process correlates with the mechanical and thermal properties of the polymer. The second is due to the presence of OH bands, and the third relaxation at low frequencies and elevated temperatures is connected with the conduction of charges through the material. In

11.3 Applications of semiconducting polymers

general, PET film shows β-relaxation in the range between 50 C and 40 C due to the motion of ester groups with partial support of the motion of phenyl group. The α-relaxation is located at a temperature of 120 C in the range of 40 C
140 C. PET β-strengthening does not involve the ends of the hydroxyl chains (OH) and methylene units (CH3). The α-relaxation results in much greater losses, resulting in a new chain setup, which results in modifications in dielectric and mechanical characteristics. The α-relaxation peak level for PET is 120 C, which is the temperature of the crystal change. Over a wide spectrum of frequencies, PET has a dielectric constant B3.3. PET has a considerably greater dielectric loss that improves with temperature and frequency relative to PP. Although the energy density is 50% greater than that of PP, the difference in dielectric characteristics does not make PET an appropriate choice for applications with elevated pulsed power. With PET films, polyethylene oxide, polycaprolactone, and silica (SiO2) were used as coatings to improve the electrical resistance by 10%
15% and dielectric constant by up to 5 μm film thickness. Another polymer, PC was first introduced in Germany in 1953 by Bayer and in the United States by General Electric. Due to its capacity to be recycled, PC is regarded as a high-performance engineering thermoplastic and an environmentfriendly plastic. In the 1960s, PC films were first used in capacitors and it has been effective for decades since then. PC has a dielectric constant of 55 C
125 C, which makes it a more appealing product for greater heat applications than BOPP. Most providers stopped making PC films in 2000. As of 2017, one company, Electronic Concepts, is still producing in-house PC films for its capacitors. Because of the β-relaxation molecular movements of the primary chain at low temperatures, PC has a strong effect force below its Tg. The wide β-strengthening ranges in PC are connected with more than one relaxation method, one for carbonate group relaxation (the peak at a reduced temperature) and the other (the peak at a greater temperature) where phenyl group movements are limited. Limitations such as high shrinkage, small Young’s modulus, and simple defect formation decreased capacitor use, and PC was gradually substituted with other polymeric dielectric films such as polyimide or PPS. In the case of polyether ether ketone (PEEK), ether connections are included between the aromatic units in the core of the polymer. PEEK is synthesized with step-growth polymerization with hydroquinone disodium salt and 4,40 -difluorobenzophenone in a high temperature polar protic solvent (300 C). PEEK usually provides polymers of elevated molecular weights and acts with excellent thermal stability as a thermoplastic. The crystalline/amorphous proportion in PEEK differs in an inert atmosphere by selectively annealing the film. The high heat capacity, chemical strength, excellent mechanical characteristics, and elevated crystal transition temperature (Tg) make it a great option for capacitors. PEEK performance investigations at varying frequencies from 1 to 100 Hz and at different temperatures found that dipolar orientation occurs at temperature ranges of 100 C
180 C and ionic conductions occur at 180 C
300 C. Two phenomena are usually found in the first temperature area. The first is due to the dipolar relaxation of the

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FIGURE 11.3 Variation of dielectric constant of cellulose/reduced graphene oxide composites and pure cellulose with frequency variation at 1 V (RH 5 25% and T 5 25 C) [55].

amorphous PEEK at 150 C connected with glass transition. Second, the dielectric loss increases from 165 C to 180 C above the Tg [53,54] (Fig. 11.3).

11.3.7 Sensors Smart sensors are quite problematic to define in an explicit way. They may be described as affluences that respond to a need arising from external surroundings, in the shape of a chemical or physical incentive that leads to some change in material properties. It can also be concluded that each material reacts in a certain way to fluctuations occurring in its environment. A vivid case of this kind of response could be the effect of thermal expansion occurring due to the escalation of temperature of the surroundings or a material’s temperature itself. Yet, this is not referred to as “smart.” Hence it is much more accurate to state that so-called “smart materials” are materials that react to fluctuations in their atmosphere in an explicit and practical way, and that this outcome is reproducible. Currently, a variety of compounds have been synthesized as polymers owing to the diversity of their chemical and physical properties, and their adaptations to numerous applications. In the early decades or two, outstanding interest has been shown in polymeric materials that could reversibly or irreversibly engineer their physical and chemical properties for an external effect such as pH, temperature, and presence of specific ions, light radiation, mechanical forces, magnetic fields, electric fields, and bioactive molecules. Smart polymers are obtainable in the form of solutions, gels, self-assembled nanoparticles, films, or solids. Researchers have been trying hard to include the

11.3 Applications of semiconducting polymers

already renowned features of these materials in extra complex conditions such as organized distribution of drugs and genes, catalysis, detection and imaging, adaptive coatings, or self-healing materials. Smart materials originated from polymers, and in contrast to their low molecular weight counterparts, they show a variety of profits when it comes to structural stability, dispersion in aqueous solutions, biocompatibility, affluence of handling, and ensuing amalgamation with detecting devices. It a popular notion that a sensor’s function is to provide information regarding the environments in which a material is placed. The overview of further statutory acts has strengthened the requirement for the development of materials/ equipment for observing human factors that can impend human life such as the existence of toxic vapors and gases at the work place, water pollution made by industrial wastes, or pesticides used in crop fields. Medicine is yet another field that needs the inclusion of such materials. Polymers, as materials that are efficient in tending to precise tasks through their appropriate alteration or synthesis, have found great use in the development of sensory devices. A comprehensive literature study by Adhikari and Majumdar states that polymers are the first choice of materials for use in the building of sensors for varied applications [56
59] (Fig. 11.4).

11.3.7.1 Temperature sensors The introductory reports about the initiation of fluorescent thermometers produced with the help of polymers date back to 2003. Further constituents of these types of mixtures are polarizable fluorescent dyes and polymers that can be combined. The suitable choice of such materials is highly crucial for the process of sensor design, mainly due to slight variations in the surroundings for which the final product is projected to work. As a polymer matrix, required for these products, aqueous solutions of high molecular weight compounds are taken into consideration. The growth of combinations with less polarity above their lower critical solubility temperature (LCST) is recorded. Several homopolymers or copolymers of different compounds like N-isopropylacrylamide (NIPAM), N-isopropylmethacrylamide, N-propylacrylamide, and N-tert-butylacrylamide have been used in the course of the expansion of polymer thermometers. Benzoxadiazole-including compounds, such as 4-N-(2-acryloyloxyethyl)-Nmethylamino-7-(N,N-dimethylamino) sulfonyl-2,1,3-benzoxadiazole, satisfy the role of a necessary pigment. Dyes required to develop fluorescent thermometers display two important characteristics, namely poor fluorescence in a hydrophilic environment and an extreme extent of fluorescence in hydrophobic domains. The dye is copolymerized with a polymeric matrix, producing a material in which the intensity of fluorescent release at temperatures beyond the LCST rigorously rises. The thermal phase transition tends toward the disintegration and continuous accumulation of the polymer chains, which causes the modification of the microenvironment of the dyes from hydrophilic to hydrophobic and also leads to an addition in the quantum yield capacity of covalently deposited dyes. There are also famous examples of temperature sensors [61
64].

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FIGURE 11.4 Relative resistances (ΔR/R0) versus time (min) for styrene-isoprene block copolymers (SIS)/PANI composite films in different media including (A) oil, (B) water, and (C) oil in water. (D) The experimental set up for oil in water sensing by SIS/PANI composite film [60].

11.3.7.2 pH sensors The surroundings in which chemical reactions are progressed often play a substantial part in, and control, the final structure of a substance. Hence both pH regulation and measurement are extremely vibrant for all kinds of sciences. In this scenario, pH sensors have the utmost significance. Photochemical polymerizable copolymers as a blend of acrylamide and methylene-bis-acrylamide, having aminofluorescein, which has been covalently related to the surface of the fiber and makes a material that acts as a pH sensor. However, several research papers state that PANi should be measured as a suitable material, which can be used to form a pH sensor that is operational in aqueous media. Currently, there are commercially used blood pH sensors that are available on the market. A thin and sensitive layer was applied for altering the surrounding after responding to aminoethylcellulose

11.3 Applications of semiconducting polymers

fibers with 1-hydroxypyrrole-3,6,8-trisulphochlorane. The achieved surface was combined with a polyester film and later placed on an ion-permeable hydrogel based on polyurethane (PU). The produced material has variable conductivity to a measurable extent when it touches the surface of hydrogen ions, enabling pH measurements. Similar electrodes were also formed using poly(3-cyclohexyl)thiophene, which appear to be beneficial for both aqueous and nonaqueous media.

11.3.7.3 Gas sensors In a similar way, the benefit of measurements of potential electrodes vary and was used to signify the existence of gas in the surroundings. Conductive polymers and their connected composites made with other polymers such as PVC and PMMA have many uses in such devices. The only necessary condition is that they must have active functional groups in their ending structure. Gas sensors with few types of materials are efficient for use as moisture sensors. They are used to find the relative humidity value in the atmosphere. Monitoring air moisture in many areas such as industry, medicine, or in a household is important, hence, they are the most commonly used devices [65
67] (Fig. 11.5).

11.3.7.4 Ion-selective sensors In ion-selective sensors, the polymer acts as a conductive component or performs as a matrix of an electrically conductive system. When such a setup comes into contact with the analyte, a reaction or ion exchange with the sensor system occurs. This phenomenon leads to the production of an electrical signal, which is recorded. Wellknown examples of these sensors are ion-selective electrodes. They permit the detection of the presence of specific ions in solution, in spite of the incidence of other dissolved ions. The results of incorporating silicone rubber and PU/PVC copolymers as ion-selective membranes have also been cited in the literature. Silicone rubber

FIGURE 11.5 (A) Temperature and (B) flexibility effect on the relative resistance change of cellulose nanocrystal/iron oxide nanocomposite in the presence of NO2 gas at 500 ppm [68].

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membranes have been used to detect Na 1 ions in body fluids. Moreover, PANibased electrodes that sense calcium ions have been synthesized.

11.3.7.5 Stress sensors Stress sensors indicate a relatively advanced collection of intelligent materials. They are established on the photoluminescence phenomenon. Colors ascending at the time of deformation are reliant on various types of forces applied on the material such as shearing or stretching. Previous readings have stated many methods in which a material may reply to stress. Apart from the photoluminescence effect, the occurrence of color changes or fluorescence emissions can also be used. Also, several mechanisms and substances could be used. An inspiring example is that of a sensor in which oligo(p-phenylene vinylene) was used as a dye. It is considered by a diversity of chemical structures as well as their exclusive aptitude to form intense fluorescence in solution and a crystalline state. This dye was offered into low density polyethylene in a plastic state. The ultimate produced material is branded by luminescence at the time of stretching. Before deformation, the presented dye submerges within the polymer matrix, which leads to excimer emissions. Afterwards, the dye collections are detached owing to the effect of stretching, and the ionized form of this compound exhibits luminescence with different characteristics [69].

11.3.7.6 Biosensors Biosensors involve the arrangement of a biological element, a bioreceptor, and a polymer sensor. The job of a typical biosensor is to convert biological reactions into an electrical signal that can be sensed and recognized. Biosensors have several applications in medical diagnostics and environmental pollution control in the domains of both water and air. The ideal design and functioning of a biosensor is depicted in Fig. 11.6. The enzymatic reaction and the product made, partly diffuse into the sensor, which leads to fluctuations in its electrical characteristics and yields an impulse that is delivered by an appropriate detector.

11.3.7.7 Multisensors Currently, many technologies for producing sensors sensitive to numerous stimuli have been established. However, producing materials that react differently and instantaneously toward multiple stimuli is still a challenge. Such materials may have unquestionably numerous applications for analyzing biological methods that are customarily complex and arise from atmospheric situations made by various factors. It is possible to form sensors sensitive toward a reduction or an upsurge in temperature and pH. These materials can be made as a pseudorotaxane structure performed with the use of 1,5-diaminonaftalene functionalized with poly(Niso-propyl)acrylamide (Napht-NPNIPAM) and cyclobis(paraquat-p-phenylene) (CBPTQT4 1 , 4Cl
). In this scenario, through NIPAM reversible addition
fragmentation chain transfer-type polymerization, Napht-N-CTA having 1,5-diaminophthalate moieties was utilized to present and comprise materials into the polymer chain functional groups, which are sensitive to pH changes [71
73].

Acknowledgment

FIGURE 11.6 (A) Steady state amperometric i
t curve (inset: 1
10 μM of glucose). (B) Calibration plot of steady state current versus concentration of glucose for synthesized CeO2
CuO core
shell nanostructure [70].

11.4 Conclusion Polymer electronics include similar systems to those of conventional electronics, namely transistors (OFETs), diodes, capacitors, inverters, and polymer ring oscillators. Mainly conducting and semiconducting polymers are utilized to achieve such electronic materials. The field offers light, flexible, and cost-effective products on a larger scale than in conventional electronics. Major electronic applications that involve polymers include OPVs, OLEDs, polymer transistors, printed electronics, and so forth. The advancement of polymer LEDs from its original innovation to the point of commercial synthesis in the space of a decade is considered to be quick in comparison to other areas of semiconductor electronics. Simple polymers show outstanding responses such as liquid-crystal displays in mobile phones or backlights, and colorful displays are more investigated. It is likely to combine numerous devices onto a single sheet of plastic so that display, logic, light detection, and even power generation are all combined. There is also the probability for hybrid devices combining organic and inorganic materials; the compatibility of semiconducting polymers with silicon is an eye-catching feature.

Acknowledgment This publication was partially made possible by UREP Grant 23-116-2-041 from Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors.

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6. [73] An KH, Jeong SY, Hwang HR, Lee YH. Enhanced sensitivity of a gas sensor incorporating single-walled carbon nanotube 2 polypyrrole nanocomposites. Adv Mater 2004;16:1005
9.

CHAPTER

Polymers in robotics

12

Arunima Reghunadhan1, Athira Krishna2 and Ajith James Jose2 1

International and Interuniversity Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India 2 Department of Chemistry, St. Berchmann’s College, Kottayam, India

12.1 Introduction Robotics is a branch of expertise that is a compilation of different engineering and technological branches like electrical, electronics, computer science, material science, mechanical engineering, bioinformatics, etc. The manufacturing and operation require coordination among all the engineering branches and their technologies. Robots are used in a variety of applications from simple manufacturing to defense and military. Different applications demand different operative techniques and different types of materials for components. In a general point of view, all robots require flexibility, durability, and strength. For certain applications like in biological fields, they are required to be lightweight as well. Polymers, a broad class of materials, which includes thermoplastics, elastomers, thermosets, nanomaterials, etc., comes into action because of these mentioned qualities. One of the advantages of polymers is that they can be tuned to obtain desired properties. This chapter provides a brief discussion on the role of different polymers in the robotics field.

12.1.1 Robotics: the term, the idea Robotics is a division of science that is concerned with the design, making, working, and applications of robots. The design and operations of the computer systems for their control also come under the scope of robotics. The term “robot” was first used by a Czech novelist, Karel Capek, in his play entitled, Rassum’s Universal Robots (RUR) in 1920. The word robot originated from the Czech language, meaning worker or servant. A robot can be described as “a machine capable of carrying out a complex series of actions automatically, especially one programmable by a computer” according to the Oxford dictionary.

Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00012-3 © 2020 Elsevier Inc. All rights reserved.

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12.1.2 History of robots The first written record on automation come from Aristotle, where he said “If every tool, when ordered, or even of its own concurrence, could do the work that befits it. . . then there would be no need either of apprentices for the master workers or of slaves for the lords.” These words are from 320 BCE; any breakthrough in this direction came after 2000 years, in 1495, in the form of Leonardo Da Vinci’s Mechanical Knight. His years of research on anatomy helped him to develop a sketch for a self-working model resembling a medieval knight (Fig. 12.1). In 1920, the play, RUR, by Karel Capek was performed, in which the word robot was used for the first time [2,3]. In 1932, Japan introduced a toy robot named Lilliput, with a canonical, rigid, stocky appearance. The Lilliput was a 15 cm tall, thin figure with the ability to walk. In 1942, Isaac Asimov’s Runaround presented three laws for the working of robots: 1. A robot may not harm a human being or, through inaction, allow a human being to come to harm. 2. A robot must follow any orders given to it by human beings, except where such orders would clash with the first. 3. A robot must look after its own subsistence as long as such protection does not conflict with the first or second law. In 1950, Alan Turning introduced a test for measuring the artificial intelligence of computers. One of the chief signposts in the history of robotics took place in 1954 when Joe Engelberger and George Devol developed the first programmable robot, the Unimate. It was later used in General Motors’ assembly

FIGURE 12.1 (A) Schematic representation of the structure of an electro-strictive graft copolymer and (B) the mechanism of deformation in an electro-strictive polymer [1].

12.1 Introduction

line. This machine consists of an arm that can perform dangerous tasks. In 1956, the team formed the first company for developing robotics named Unimation. To the date, these robots are used mainly in automobiles and any use of them as a substitute for humans remains only in science fiction. In 1966, the world’s first mobile robot, Shakey was created at the artificial intelligence center at Stanford. It could be controlled using a computer and was able to sense its environment. The journey of robotics beyond automobiles or science fiction started with the innovation of The Stanford Cart, successor of Shakey, with a television for the vision. The Stanford Cart can navigate by itself around chairs and can also create its own program. This became a cherished moment for robotics and also for artificial intelligence. The next memorable event in the robotics developmental chronology happened in 1993 when an eight-legged robot, Dante, developed by Carnegie University, was employed in volcanic environments for research and exploration. This paved the way for the use of robots in potentially harmful environments, thereby escalating researchers interest in this direction. Six years later, Sony introduced an animal robot named Sony Aibo. Aibo was an artificial pet dog with learning and communication potential. In 2000, Honda introduced ASIMO, a 4 ft. 3 in. walking and interacting robot with the ability to recognize objects, body language, faces, and sounds and with autonomous navigation. In 2002, Roomba, the robotic vacuum cleaner, was introduced by iRobot as a domestic help. By 2008, the Roomba had become the most commercially flourishing domestic robot in history and the idea of a robot in the home became quickly regularized. In 2012, NASA sent a humanoid robot, R2 Robotnaut, to the International Space Station on the ultimate task of space shuttle discovery. With a near-human range of movement unparalleled in humanoid development so far, NASA will depend on its R2 machines to complete tasks unsuitable for human astronauts as we delve further into space. Even though robotics has seen enormous growth, there are still miles to go. This chapter mainly focuses on the role of different types of polymers in robotics.

12.1.3 Classification of robots A robot can be classified based on different measures such as its degree of freedom, cinematic structure, drive technology, workshop geometry, movement characteristics, and applications (Chart 12.1) [4,5].

12.1.3.1 Degrees of freedom It is a well-known fact that a manipulator needs a minimum of 6 degrees of freedom to run an object freely in a 3D space. Grounded on this, robots can be classed as: 1. General purpose robot: It has 6 degrees of freedom. 2. Redundant robot: It has more than 6 degrees of freedom. 3. Deficient robot: It has less than 6 degrees of freedom.

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Classification of robots

Degrees of freedom

General

Redundant

Deficient

Drive technology

Electric

Parallel

Hybrid

Hydraulic

Nuclear powered

Green

Planar

Spherical

Mobile

Data acquisition and control

Spherical

Cylindrical

Manipulation Workspace geometry

Motion characteristics

Kinematic structure

Serial robot

Pneumatic

Applications

Spatial

Articulated

SCARA

Cartesian

CHART 12.1 Chart showing the classification of robots.

12.1.3.2 Kinematic structure Based on kinematic structure, robots can be classed as: 1. Serial robot or open-loop manipulator: In these kinds of robots, the kinematic structure takes the shape of an open-loop string. For example, the AdeptOne Robot. 2. Parallel manipulator: In these kinds of robots, the kinematic structure is made up of a closed-loop chain. It has the advantages of higher stiffness, higher payload capacity, and lower inertia than a comparable serial robot. 3. Hybrid manipulator: In these kinds of robots, both open-loop as well as parallel kinematic structures coexists.

12.1.3.3 Drive technology Established on the source used for the power supply, robots can be classed as: 1. Electric robots: These kinds of robots use DC electric current as their source for power provision. 2. Hydraulic robots: Hydraulic power supplies are not a clean form of power supply since they are associated with the leakage of oil. A hydraulic drive is naturally flexible because of the high bulk modulus of oil. This type of power provision is principally applied for those sorts of robots that are utilized for high power applications. 3. Pneumatic robots: A pneumatic drive is clean and fast, but it is difficult to control because air acts is a compressible fluid. 4. Nuclear powered robots: These carry their own nuclear reactor. These kinds of robots were used by NASA for space exploration. They can operate endlessly for decades without any human contact. 5. Green robots: These types of robots use those power sources that can be replaced without much impact on the environment like solar, wind, organic sources, and natural heat sources.

12.1 Introduction

12.1.3.4 Workspace geometry The workspace of a robot can be defined as the space in which a robot can go about. Grounded on this, a robot can be classed as: 1. Cartesian robot: These kinds of robots can move in x, y, and z directions. 2. Cylindrical robot: These kinds of robots have cylindrical workspace geometry. 3. Spherical robot: These kinds of robots have spherical workspace geometry. 4. Articulated robot: Articulated robots have a spherical-type workspace geometry that is inhibited by the assembly of the robot. 5. The SCARA (selective compliance assembly robot arm) robot: This is a special type of robot that combines the cartesian motion with the rotational motion of articulated robots. SCARA has a cylindrical geometry with one axis moving in a linear fashion and the other two axes moving in a rotational manner.

12.1.3.5 Motion characteristics Robot manipulators can also be classified according to their motion as: 1. Planar: A manipulator is called a planar manipulator if its mechanism is a planar mechanism. Planar manipulators are useful for manipulating an object on a plane. 2. Spherical: A manipulator is called a spherical manipulator if it is made up of a spherical mechanism. A spherical mechanism is a motion in which all the points in a moving body perform spherical motions with respect to a common stationary point. 3. Spatial manipulator: If a particular robot’s motion cannot be identified as planar or spherical then it can be classified as a spatial robot.

12.1.3.6 Applications 1. Manipulation robotic system: This is the most extensively used robotic system, mainly found in manufacturing industries. 2. Mobile robotic system: Robots used for transporting purposes come under this classification. 3. Data acquisition and control robotic system: These are used for acquiring, processing, and transforming data.

12.1.4 Components of robots The basic components of a robot include (Chart 12.2) [6,7]: 1. 2. 3. 4.

A mechanical platform Sensors Motors Power supplies

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CHART 12.2 Components of a robot.

5. 6. 7. 8. 9.

Electronic controls Microcontroller systems Languages Pneumatics Driving high-current loads from logic controllers

12.1.4.1 Mechanical platform A mechanical platform is the base on which a robot is mounted. This part will interact with the environment.

12.1.4.2 Sensors As the name suggests, these are the parts of a robot system that can sense. They can detect objects or can sense environmental stimuli like heat or light and then convert that signal to analog or digital form for the understanding of the robot,

12.1 Introduction

then the robot can act according to the information provided by the sensor. There are different types of sensors: Vision sensors: These help a robot to see things. For example, a camera, frame grabber, or image processing unit. Proximity sensors: This type of sensor will help a robot to calculate the distance to any objects around it. Proprioceptive sensors: This type of sensor help a robot to understand itself and help it to maintain its internal status, like battery charge, current variations, heat variations, etc.

12.1.4.3 Motors Motors provide power to a robot for different programmed movements. AC or DC motors may be used.

12.1.4.4 Power supply In a robot, power supply can be offered by two means, namely (1) nonrechargeable batteries, which can be discarded after use and (2) rechargeable batteries, which can be recharged multiple times.

12.1.4.5 Electronic controls These are used to control the mechanical system present in a robot.

12.1.4.6 Microcontroller systems The microcontroller system is the brain of a robot. It helps a robot to complete assigned tasks. A robot’s efficiency can be measured in terms of certain characteristics of its microcontroller system including: 1. Speed: Designated in clock cycles, and expressed in millions of cycles per second (megahertz, MHz). 2. Size: A measure of the number of bits of data a microcontroller can process in one step. 3. Memory: There are both read only memory and random-access memory, usually given in bytes.

12.1.4.7 Languages These refer to the programming language used in a robot, like variable assembly language (VAL), which was the first language used in a robotic system, robotic markup language (Robo ML), extensible robot control language (XRCL), etc.

12.1.4.8 Pneumatics These systems are used mainly for linear motions.

12.1.4.9 Driving high-current loads from logic controllers These are used to edge between logic circuitry and high-current loads.

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12.2 Role of polymers in robotics Polymers and plastics play a major role in the manufacturing of robots, and robots are being utilized in the production of polymers. Commercial and industrial robots are being used widely in polymer manufacturing, packaging, etc. Polymers are cheap, lightweight, flexible, pliable, and easily manufactured in most cases. These qualities make them suitable for widespread applications like in automobiles, aerospace, household goods, and electronics. Polymers can be cast into any shape and their properties can be customized according the field in which they are used. Many polymers are being used in the robotics field; mainly conducting polymers, their composites, electroactive polymers (EAP), nanomaterials, etc., are considered in major portions. Soft robotics is preferred nowadays because of the ease of interaction with human beings when compared to conventional robots. Materials that are apt for use in soft robotics are always in demand, and this is where the role of polymers comes in.

12.2.1 Types of polymers used in robotics The types of polymers utilized in robotic applications are mainly thermoplastics, epoxy- and acrylic-based materials, elastomers, polymeric gels, etc. These are discussed in detail in the coming sections.

12.2.1.1 Electroactive materials Polymers that respond to any external stimuli such as electrical and magnetic fields, temperature, pH, light, etc., by changing shape or size are collectively termed active polymers. As previously mentioned, electroactive materials are mostly preferred in soft robotics. Soft robotics is a subfield of robotics that deals with constructing robots from highly acquiescent resources, similar to those found in living organisms. They typically consist of soft materials, rather than metals or hard plastics. The elastic modulus of these materials is often close to the that found in living organisms. The major benefits of using polymers are their ease of processing, low cost, and lightweight nature. Active polymers can also be biocompatible and they can mimic bioactivities or functions. These polymers have characteristics comparable to biological components like flexibility, large actuation, and damage tolerance. They are more supple than conventional motor parts and can function as vibration and shock hindrances. The use of polymers reduces the use of complex mechanisms and operational procedures. There are a variety of active polymers with tunable properties. Such active polymers are able to produce permanent or reversible responses, and can be passive or active creating smart structures. Active polymers are mainly shape memory polymers, photoactivated materials, magnetically activated materials, etc. EAPs are those which respond to electric stimuli such as changes in voltage or current. EAP materials are particularly apt in actuators, which are components

12.2 Role of polymers in robotics

used to move or control mechanisms. EAPs were discovered in the 1880s by Wilhelm Rontgen. EAPs are lighter and their striction capability can be as high as two orders of magnitude more than previously used materials [8]. Their actuation and shape recovery are faster than normal shape memory alloys. The use of EAP actuators that can twist or extend and contract is capable of producing unique robotic devices that emulate human hands [9]. Important properties of EAPs include:

• • • • • • • • •

Stress (MPa) Strain (%) Drive voltage (V) Bandwidth (Hz) or response rate (s) Power density (W/cm3) Efficiency (%) Lifetime (cycles) Density (g/cm3) Operating environment (temperature, pressure, humidity, etc.)

EAPs and their usage in robotics have led to a great revolution in the area of biomimetics. Actuators developed from EAPs match the performance and appearance of actual muscles. These constituents have functional likenesses to biological muscles, together with resilience, impairment tolerance, and great actuation responses like elongating, shrinking, or bending. They can possibly deliver more convincing aesthetics, shaking and shock inhibition, and more supple actuator configurations. Once suitable EAP materials are selected, they will be displayed into the control system by undergoing surface shape adjustments and control instructions for the creation of the anticipated facial expressions [10].

12.2.1.1.1 Mechanism of electroactive polymers Electronic EAPs are conductive in nature and this conductivity arises from the highly conjugated electronic structure and charge mobility of these materials. Their properties can be changed by changing or modifying the chemical backbone of the materials (Fig. 12.2). Conductivity or stimuli response is mainly created due to changes in the electronic arrangement of polymers. In the structure of conjugated polymers, the π bonds in these polymers will be weaker than the sigma bonds and the electrons in the π bonds can be easily migrated. Conjugated polymers have an arrangement suppleness that lets them locally distort and, thus, neutralize, the generated charge. Introduced local charge imperfections in polymers are identified as polarons, bipolarons, and solitons. When an electron is removed (added), a radical cation (anion) called a polaron is created, and it can be converted into a bipolaron if a second electron is removed (added). When the polymer is oxidized (forming a positive charge) it combines with anions, and when reduced (forming a negative charge) it combines with cations or ejects anions [11].

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FIGURE 12.2 EAP properties including (A) the conductivity range of conducting polymers and polymerbased composites and (B) polaron and bipolaron formation upon oxidation (p-doping) of polypyrrole.

They are mainly classified depending on the mechanism responsible for their actuation into electronic or ionic.

• Electronic electroactive polymers: These are driven by electric fields or

•

coulomb forces. They include piezoelectric polymers, electro-strictive polymers, dielectric elastomers, liquid crystal elastomers (LCEs), ferroelectric polymers, etc. Ionic electroactive polymers: These change shape through the mobility or diffusion of ions and their conjugated substances. These include ionic polymer metal composites (IPMC), carbon nanotubes (CNTs), ionic polymer gels (IPGs), conductive polymers (CP), electrorheological fluids, etc.

12.2 Role of polymers in robotics

12.2.1.2 Electronic electroactive polymers Electronic EAP materials fluctuate in shape or properties with respect to changes in the applied electric current or voltage. EAPs are utilized in robotics mainly as actuators, which enables new capabilities.

12.2.1.2.1 Piezoelectric polymers Piezoelectric behavior is a property of certain materials that expand or contract in an electrical field or a property of certain materials that generates an electrical charge when pressure is applied. One of the most important piezoelectric materials or polymers is poly(vinylidene fluoride) (PVDF). When an electric field (E) is spread over these sheets they either contract in thickness and enlarge along the stretch direction or expand in thickness and contract along the stretch direction depending on which manner the field is applied. This is due to the physical environment of the positive hydrogen atoms attracting the negative side of the electric field and resisting the positive side of the electric field. The negative fluorine atoms attract the positive side of the electric field and repel the negative side of the electric field. Piezoelectric polymeric materials have been utilized mainly as transducers including ultrasonic, audio, and medical transducers [12]. Piezoelectric actuators have been used for the manufacture of robotic arms and fingers, inchworm robots, legged robots, underwater robots, flapping robots, etc. [13 15]. In piezoelectric actuators, bimorph-type actuators are most widely used, where two layers of piezoelectric elements are stacked with a thin structure between them. When an opposite polarity is put onto two sheets, a bending stroke can be produced. Bimorphs result in a larger movement and smaller force as related to single piezoelectric components. The distinctive advantages of a piezoelectric actuator include a linear output to input voltage, fast response, wide working frequency range, high actuating force per unit volume, and low power consumption [16].

12.2.1.2.2 Electro-strictive polymers These materials have the dual functionality of actuation and sensing. Electrostrictive materials are polymer blends or composites in which one component is conductive. They offer noteworthy field-induced strain, high mechanical output force, and exceptional strain energy density. Electro-strictive polymers are conformable, lightweight, and tough. An electro-strictive graft elastomer has a mainstay molecule, which is a noncrystallizable, elastic macromolecular chain, and a grafted polymer, creating polar graft moieties with backbone molecules. The polar graft moieties are rotated by an applied electric field into substantial polar alignment. The rotation is sustained until the electric field is removed. They are used as actuators and have the ability to harvest energy from environmental sources like human movement [17]. A large number of electro-strictive polymer actuated robots are likely to be developed in the near future. These robots could perform nondestructive

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FIGURE 12.3 Change in strictive strain as a function of applied field in the case of PVDF TrFE films.

evaluations and other complex procedures. Electro-strictive polymers having a low modulus of elasticity and great compliance electrodes are used in a class of actuators that offer an overall performance similar in some respects to that of biological muscle. One among the best examples of energy harvesting electrostrictive polymers is PVDF trifluoroethylene (TrFE) copolymer films. PVDF TrFE shows an ferroelectric β crystalline segment. The fluorine atom from TrFE steadies the β-crystalline phase and depresses α-crystalline phase formation. This property allows the PVDF TrFE copolymer to be molded in the form of thin-films by spin coating, and permits an appropriate control of sample thickness, which is perfect for the production of energy harvesting microstructures [18] (Fig. 12.3). In the case of these films, applied fields on the order of 10 MV/m are desirable in order to induce strains reaching 2.5%, and offer an elastic energy density of B0.5 MJ/m3.

12.2.1.2.3 Dielectric elastomeric actuators Dielectric elastomer actuators (DEAs) are composed of incompressible soft dielectric elastomer membranes sandwiched between acquiescent electrode layers to form dynamic capacitors. When an electric field is applied through the electrodes, the columbic force produces a stress called Maxwell stress, which attracts other electrodes together and squeezes the sandwiched dielectric elastomer layer. As a result, the in-plane extension of DEA can be observed due to elastomer incompressibility. The stress is proportionate to the square of the applied field and to the dielectric constant. Low modulus (B1 MPa) and high dielectric strength ( . 100 MV/m) can result in strains of up to 380% at high applied fields.

12.2 Role of polymers in robotics

FIGURE 12.4 Schematic representation of dielectric elastomer actuator.

Usually strains are 10% 100%, which associates favorably with the 20% observed in skeletal muscle (Fig. 12.4). DEAs are capable of generating large strains and strain rates. They are used in insect-like robots. The elastomers most engaged are silicone and acrylic elastomers.

12.2.1.2.4 Liquid crystal elastomers LCEs are made of liquid crystals and elastomers as the name implies. So they have the properties of both materials. They have elastomer elasticity and liquid crystal anisotropy. LCEs are made of an elastomeric silicon or carbon net that dictates their macroscopic properties such as deformation and shape. In various ways, mesogenic groups with microscopic properties of liquid crystals are attached to the net, thus, introducing the order of liquid crystals. There are two types of elastomers of liquid crystal, namely main and side chain. LCEs are good artificial muscle materials because they resemble many properties. LCEs can be made to be active by applying electrical energy through joule heating. The conducting substance’s electrical activation induces a rapid joule heating in the sample, likely to result in a nematic to isotropic phase transition where the dimensional elastomer contracts in less than a second. The process of cooling, from isotropic to nematic, is where the elastomer expands back to its original length. The mesogenic units are preferentially aligned in one specific direction in the nematic phase, but have no positional order and no crystalline regularity. When these units are topologically fixed through incorporation into a cross-connected polymer network, an overall distortion of the polymer network dimensions could occur through liquid crystalline phase transition. To translate the distortion into a change in bulk material dimensions, all of its mesogenic units must be preferentially aligned in the same direction. Researchers have developed a bioinspired microrobot capable of using LCEs to imitate the gait of a caterpillar on a natural scale. The 15 mm long soft robot harvests green light energy and is controlled by a spatially modulated laser beam. Apart from traveling on flat surfaces, it can also climb slopes, squeeze through narrow slits and transport loads. Light can also be used as stimuli for the LCE. Light-induced deformation allows a monolithic LCE structure to perform complex actions without numerous discrete actuators [19]. The mesogenic groups are

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important in the determination of the effectiveness of actuation. Azo dyes can be used as mesogenic groups in LCEs. When these dyes are included in the polymeric backbone, then cis trans isomerization governs the actuation (Fig. 12.5). Propulsion systems from photoactuated cross-linked liquid crystalline polymeric (CLCP) materials have been developed. A study of an azo dye-doped LCE film floating on a water surface was performed. The study showed that the mechanical deformation of the LCE sample in which azobenzene colors were dissolved by visible light in response to nonuniform illumination became extremely large (the sample bent over 60 degree). When laser light from above was shone on a dye-doped LCE sample floating on water, the LCE “swam” away from the laser beam—the action resembled that of a flatfish [20]. Taking advantage of the alternating bending and stretching of azo-CLCP materials on UV films and subsequent visible light irradiation, CLCP-laminated polyethylene (PE) films showing photomobility-sophisticated 3D movement have been developed. These CLCPlaminated films were also used to fabricate robotic arms. Lamination is done by rolling the films up and also with azo-CLCP layers at two places. Azobenzene mesogens were aligned along the long axis of the film. At room temperature, the robotic arm underwent a sequential, flexible movement under light. Upon exposure to UV light, the laminated parts of the CLCP extended from a curved shape to a flat one and returned to their initial state after irradiation with visible light, functioning as a “hinge joint,” resulting in a wide and flexible movement of the entire film. By controlling the position and intensity of the irradiation, the film could be driven to manipulate objects in a chosen manner (Fig. 12.6). A prototype of a fully plastic robot was developed with these CLCP-laminated films. The microrobot’s mobile parts were assembled with various shaped CLCPlaminated films, which also consisted of an azo-containing CLCP layer and a PE layer. The microrobot composed of a “hand,” a “wrist,” and an “arm” was realized by the combination of CLCP/PE bilayer films with different initial shapes and photodeformation modes [21].

FIGURE 12.5 (A) Mesogenic units (blue) and azo-cross-linker (yellow) in the trans-state aligned in parallel. (B) On irradiation with light, the crosslinker undergoes trans cis isomerization, contracting the network in the horizontal direction on top, and dilating it on the bottom, causing bend.

12.2 Role of polymers in robotics

FIGURE 12.6 A series of photographs showing time profiles of the motion of a flexible robotic arm of a CLCP-laminated film induced by irradiation with UV (366 nm, 240 mW/cm2) and visible light ( . 540 nm, 120 mW/cm2) at room temperature. Arrows indicate the direction of light irradiation [21].

12.2.1.2.5 Ferroelectric polymers Ferroelectric polymers are easy to process, cheap, lightweight, and compatible with complicated shapes and surfaces, but these polymers practical applications are limited by their low strain and low strain energy. They are similar to ferromagnets where applying an electrical field aligns the material’s polarized domains. Even after the removal of the field, permanent polarization exists, and the Curie temperature in ferroelectric materials, similar to ferromagnetic materials, interferes with the permanent polarization by thermal energy. Intrinsic dipole moments can reverse their direction due to the electric field in ferroelectric materials. By converting mechanical and thermal inputs into electrical signals, some of the sensing organs in human skin function. A multimodal system would be needed to imitate the complex behavior of human skin in a humanoid robot. Certain polymers like PVDF can act as sensors and actuators since they have a ferroelectric character and, hence, can be used in mimicking robots [22]. The polymer poly(vinylidene fluoride) (PVF2) was found to be ferroelectric during the 1970s. It has a simple chemical structure with a repeat unit of 42H2BCF2 , and in a flexible thin-film form, PVF2 is readily produced. The combination of ferroelectricity and these desirable mechanical properties has led to investigations of many possible applications. PVDF and its most important copolymer,

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PVDF TrFE, have a Young’s modulus of nearly 1 10 GPa, which allows a high mechanical energy density to be obtained. Up to 2% electrostatic strains were obtained with the application of a large electric field (B200 MV/m), which is nearly equal to the dielectric breakdown field of the material. PVDF TrFE contracts in the direction of the field and expands in the direction perpendicular to the field. The strain can be enlarged by prestraining, and moderate strains (up to 7%) with high stresses (reaching 45 MPa) have been achieved. A high stiffness (70.4 GPa) was achieved, but was dependent on the density of imperfections and large work per cycle (approaching 1 MJ/m3) [23]. Researchers have developed a tactile sensing composite transducer with a skin-like structure that makes it potentially useful in both in prosthetics and robotics. Such a transducer consists of an “epidermal,” thin film, PVF2 sensor, and two inner layers consisting of a conductive rubber sheet and a range of 128 PVF2 sensors that are indented to partially reproduce some of the mechanical characteristics and sensing capabilities of the human dermis. Odd numbered polyamides (nylons) are another class of ferroelectric and piezoelectric polymers. The NH and CQO hydrogen bonds provide essential polar elements in such polymers for piezoelectricity. The significant piezoelectric properties of nylon 11 were studied and reported in the early 1980s. It was predicted that its ferroelectricity was based on its chemical structure and crystal phases. The investigation also found that cold stretching after melt-quenching is a critical step in obtaining the polyamide chains in parallel form as required for ferroelectric polarization and piezoelectricity switching.

12.2.1.3 Ionic electroactive polymers Ionic electroactive polymers (IEAPs) are highly attractive for use in sensors and actuators due to specific characteristics such as their lightweight, noiseless operation, and the ability to generate large strains at low operating voltages [24].

12.2.1.3.1 Ionic polymer metal composites IPMCs contain a polyelectrolyte membrane sandwiched between two highsurface-area compliant electrodes. Polyelectrolyte membranes are stereotypically perfluorinated or sulfonated polymer membranes that are permeable to cations, but impervious to anions. The negatively charged polymer backbone is neutralized with mobile positive ions, which are dissolved in water or other solvents. When a low voltage (B1 4 V) is applied between the two electrodes, the solvated mobile cations drift toward the negatively charged electrode, resulting in a swelling of the electrolyte on the cathode side and a shrinking on the anode side. This electrically controlled transport of ion/liquid causes the actuator to bend quickly (up to 100 Hz). Simply put, these composites bend in response to electric current and their working principle is based on ion conduction. Nowadays, ionic liquids and porous electrodes are being employed in the IPMCs to increase their performance. The use of ionic liquids as electrolytes provides a large electrochemical stability window that allows actuators to be operated in open-air

12.2 Role of polymers in robotics

FIGURE 12.7 Structure of an IPMC.

environments at a high electric potential without causing electrolysis and evaporation [25]. They have been widely used in fins for robotic fish, artificial flies, aquatic insectile robots, biomimetic robots, terrestrial walking robots, etc. [26 28] (Fig. 12.7). Fourth generation bionic robots were designed to imitate the external shape, motion, and biological behavior of living creatures and these types of robots mainly consist of IPMC actuators. IPMC can be easily manipulated, is flexible, has a small elastic modulus, emits no noise, and can undergo large deformation. Researchers designed a bioinspired robotic fish driven by gold-plated electrode IPMCs, each of which is controlled separately. Different excitation signals are sent to achieve different modes of motion. The caudal fin swings, twists, and tilts, and the two pectoral fins bend upward and twist in the same or opposite directions [29]. Based on the motion of a jelly fish, researchers designed a jellyfish-like robot mimicking the prototype of hydrozoan medusae. The propulsion force was provided by an shape memory alloy (SMA) coil spring, and an IPMC actuator was used to adjust the swimming direction [30]. The electrode architecture was adjusted and the internal solvent was changed to optimize the IPMC drive. PE, acrylic, and polydimethyl siloxane (PDMS) films were used as a support umbrella. An IPMC drive was used to design a robot that resembles this creature with an average speed of 0.77 mm/s and an energy consumption of 0.7 W [31].

12.2.1.3.2 Carbon nanotubes In 1952, Radushkevich and Lukyanovich first discovered CNTs. Single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs) consist of one and several layers respectively of rolled graphene sheets. The diameter of SWCNTs depends on the direction in which the sheet of graphene is rolled up and typically is a few nanometers. Baughman found, in 1999, that nanotube sheets act as electrodes when used in electrochemical cells [32]. Several actuation mechanisms including CNT nematic LCEs, electrostatic, light-driven, and pneumatic actuation have been reported for CNTs since then. CNT actuator technology is at an early stage of development and several problems need to be addressed such as creep, low

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electromechanical coupling of a currently ,1%, low stress rates due to relatively high internal electrolyte resistance, and the poor mechanical properties of macroscopic structures (such as yarns and sheets) compared with single CNTs. CNTs are used in nanorobotics, a nanometer-scale emerging field of robotics. For nanorobotics, some of the most significant features of nanotubes comprise their nanometer diameter, large aspect ratio (10 1000), terapascal scale Young’s modulus, outstanding elasticity, ultrasmall interlayer friction, excellent fieldemission properties, various electric conductivities, high thermal conductivity, high current carrying capability with essentially no heating, sensitivity of conductance to various physical or chemical changes, and charge-induced bond length changes. CNTs can serve in nanorobotic systems as structural elements, tools, sensors, and actuators [33]. The properties of nanotubes suitable for robotic applications are given in Table 12.1. Table 12.1 The properties of nanotubes suitable for robotic applications. Property

Item

Data

Geometrical

Layers Aspect ratio Diameter

Single/multiple 10 1000 B0.4 nm to .3 nm (SWNTs) B1.4 nm to .100 nm (MWNTs) Several micrometers (up to centimeters) B1 TPa

Length Mechanical

Electronic

Young’s modulus Tensile strength Density Interlayer friction Conductivity Current carrying capacity Field emission

Electromechanical

Piezoresistivity

Thermal

Heat transmission

Potential application in nanorobotics Structures, probes, grippers/tweezers, scissors

45 GPa B1.33 1.4 g/cm3 Ultrasmall

Metallic/ semiconducting B1 TS/cm3

Active phosphorous at B1 3 V Positive/negative .3 kW/mK

Actuators, bearings, syringes, switches, memories Diodes, transistors, switches, logic gates Wires/cables

Proximity/position sensors Deformation/displacement sensors Circuits, sensors, thermal actuators

12.2 Role of polymers in robotics

A skin-like polymeric material used CNTs in research conducted by Benjamin et al. to bring a sense of touch to robotic and prosthetic devices. The sensors were made of a composite of CNTs scattered in a flexible plastic of polyurethane and molded into pyramidal structures. The pyramidal shape was crucial because it allowed the pressure range of the sensor to be tuned to that of skin [34].

12.2.1.3.3 Ionic polymer gels A polymer gel is a type of gel that consists of a cross-linked polymer network of three dimensions that can be significantly deformed. This is possible because these gels are soft and filled with a solvent like water or some other liquid solvent that imparts properties of deformation in different shapes and sizes. These gels can grow or shrink in volume up to 1000 times. External stimuli can easily deform polymer gels, generating a force or performing work on the external environment. In polymeric gels, phase transformation can be induced by stimuli such as temperature, pH, ionic strength, etc. These gel materials are, hence, utilized in actuators, sensors, controllable membranes for separations, and modulators for the delivery of drugs, etc. The most common types of polymer gels used in industry are poly(vinyl alcohol) (PVA), poly(acrylic acid), poly(acrylonitrile) (PAN), polyvinyl chloride (PVC) gels, etc. An IPG can be treated as a three-dimensional charged system of a crosslinked macromolecular polyelectrolyte capable of collapsing or swelling in an acidic or alkaline environment respectively, purely due to pH variations. In such macromolecular networks, fixed electrical charges exist in all crosslinks in the presence of wandering mobile charges that tend to change their spatial distribution within the gel network. In the presence of an electric field, the mobile ions redistribute themselves in the gel network and cause the network to deform accordingly. Smart polymeric and gel actuators have numerous advantages compared to traditional actuators, for instance, they can be safely used. The first prototype of a gel robot was made using an EAP gel, namely poly(2-acrylamido-2methylpropane sulfonic acid) (PAMPS) gel. PAMPS gel swings reversibly in a surfactant solution with the application of an electric field. PAMPS gel is a kind of chemomechanical polymer and can be used as mechanochemical actuators. Examining the mechanism of gel materials, if an electric field is applied to a sheet of PAMPS gel in a surfactant solution containing sodium sulfate, the gel shows significant and quick bending toward the anode. If the polarity of an electric field is altered repeatedly, the gel sheet swings repeatedly. This is based on an electrokinetic molecular assembly reaction of surfactant molecules on the hydro gel. If the gel network is anionic, then the positively charged surfactant molecules can bind to its surface. This causes anisotropic contraction or the bending of the gel toward the anode. Like other gels, polyaniline-based gels are important and they resemble polyacrylic gels in action and properties. Among the available polymer-based actuator materials, PAN fiber is already produced commercially in large volumes and used in the production of textiles. When polyaniline fibers transform into gels, they

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have stronger mechanical properties and larger volume changes, more closely resembling biological muscle than any other polymer gel actuators. The use of polymer gels as actuators creates a quick and reliable control system, and the use of electric or magnetic stimuli facilitates the development of these control systems. By controlling the magnetic or electric field, PVA-based gel systems can be utilized in artificial muscles. Encapsulated polypyrrole actuators have been developed using a gel, doped with salt, as the electrolyte. The gel electrolyte was made of agar or polymethylmethacrylate (PMMA) and gave good actuation responses. There are so many other gel forms that respond to external stimuli. But all of these are not used in robotics because of certain drawbacks.

12.2.1.3.4 Conductive polymers CPs are organic semiconductors with conjugating double bonds. So they are also called conjugated polymers. CPs have the benefits of actuation from low operation voltages, lightweight, good flexibility, biocompatibility, negligible selfdischarge, high force generation capabilities, high energy density per cycle, and that they can be manufactured on the nano- and microscale. CPs have several properties including high tensile strength, large stresses, stiffness, and low actuation voltage that make them striking actuator materials. Conductive materials or polymers give deformation by the application of an electric field and this deformation can be utilized in robots. The most common CPs are polypyrrole, polyaniline, polythiophene, etc. These polymers alone or their suitable composite materials can be used in robotic actuators.

12.2.1.3.5 Electrorheological fluids Rheological fluids are those that can change their flow behavior by the action of electric or magnetic fields. They are normally used in automotive fields. These fluids can be incorporated into soft matter and the resulting material is used to increase the stiffness of soft structures. These were used in soft robotics, but are now limited because of the drawbacks of the materials such as environmental issues, particle settling, sealing problems, etc. Electrorheological fluid uses ferroelectric materials and polymers.

12.2.1.4 Thermoplastics in robotics The robotic branch of technology utilizes soft matter in various forms. Soft matter includes fluids, polymers, colloids, granular materials, and biological materials. A common characteristic of soft matter is that it comprises of large molecules or assemblies of molecules that move collectively, and, as a result, it gives large, slow, and nonlinear response to small forces. Thermoplastic polymers have been used as supporting structures or kinematic linkages. They are considered to be cheap alternatives to metals. Rubber or sponge has been used to cover surfaces to absorb impact or maximize the contact surface. Stretching/elongation functions require elastomeric materials. Common elastomers or dielectric elastomeric actuators are considered for such functions. For example, in the artificial muscles in

12.2 Role of polymers in robotics

robots, dielectric elastomers are used. Pneumatic artificial muscles are directly used as body segments. The bodies are made from meshes of thermoplastic polymers. They are actuated by shape memory alloy or cables with a motor respectively to achieve the contraction and extension of different segments. Robotic systems with flexure hinges are made from thermoplastic materials. A variety of thermoplastics or elastomers are used in fingered gripping, highfrequency flapping flying, and legged locomotion. The use of polymers to replace metals for flexure hinges not only reduces cost and weight, but also allows larger bending angles due to the larger yield strain of the polymers. In addition, polymers such as thermoplastics have a viscoelastic property, which is thought to be Table 12.2 Types of deformation, soft matter, and robotic functions. Deformation

Soft matter

Elongation/ shortening

Thermosetting polymer Thermoplastic polymer EAPs Thermosetting polymer

Bending

Thermoplastic polymer

Robotic application Silicon-based elastomer polyether ether ketone (PEEK) mesh Silicon-based elastomer

polyurethane (PU) elastomer Polyimide plastic EVA, polyester, polyethylene, polytetrafluoroethylene (PTFE)

EAPs

Flowing

Transformation/ reconfiguration

Polymer gel (not EAP gel) Granular materials Smart fluids

Thermoplastic polymer colloid

Iron powder, plastic powder, coffee beans Electrorheological fluid Magnetorheological fluid EVA foam

Reaching, peristaltic locomotion Peristaltic locomotion Artificial muscle actuators Gripping, crawling, legged locomotion, rolling, tailed swimming, jellyfish-like, octopus-like locomotion Hinged flexure, rolling locomotion Hinged flexure Hinged flexure

Gripping, crawling locomotion, tailed swimming, octopus-like locomotion Gripping, tailed swimming locomotion Gripping Gripping Gripping Dragline forming locomotion, gripping, scooping, legged locomotion

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important in passive damping for gripping and legged locomotion. Ethylene vinyl acetate (EVA)-based thermoplastic adhesive has been utilized in robotic arms. Silicone-based elastomers like polydimethylsiloxanes, block copolymers like acrylonitrile butadiene styrene (ABS), etc., are used as actuators. Bending deformations are one of the most important mechanisms required in robots. These deformations are brought about by thermoplastic elastomers like polyurethane thermoplastics such as EVA, polyester, PE, and polytetrafluoroethylene. Table 12.2 gives an idea of the mechanisms required in robots and the type of materials preferred for such applications [35]. Like thermoplastics, thermoplastic-based adhesive materials are also used in robots. The robotic active sensing system is able to adjust its sensor morphology in order to sense different physical quantities with desirable sensing characteristics. For this purpose, thermoplastic hot melt adhesives have been utilized. These adhesives regulate the plasticity of the material to autonomously fabricate sensors and adjust their morphology. When heated, these materials become viscous and adhesive, and are placed in robotic arms to fabricate the structures. By knowing the adhesive material’s Young’s modulus and the geometry of the softness sensor, the bending angle and object softness can be found out. Thermoplastics have been widely used as molding materials in the manufacturing of robots. Their flexibility and high mechanical properties make them suitable for this.

12.2.1.5 Epoxy-based materials in robotics Epoxies are a well-known class of adhesives. Epoxies are a large group of resins that set via a polymerization reaction involving an epoxide group (a threemembered ring of one oxygen and two carbon atoms). They are generally formulated as two components that must be thoroughly blended together before use. Cured epoxy is a highly durable material, that is, strong, rigid, waterproof, heat resistant, and unaffected by most chemicals. Epoxies are nearly unique among high-performance adhesives in having good gap-filling abilities. In 3D printed robots, the gaps are normally filled with epoxy-based adhesives. The internal voids in these 3D printed structures act as molds for the thermoset. The hardened epoxy provides structural reinforcement to the component in these robots. These epoxy-filled parts will have high tensile strength and flexural modulus. Epoxies have been widely used in printed circuit boards, electronic encapsulating materials, dielectric materials, etc. Epoxy-filled composites are also used widely. Assembling robots have been manufactured from graphite-filled epoxy resins. The high specific stiffness and high damping characteristics of composite materials are promising material properties for the direct drive robot arm structure. These composite materials are cost effective [36,37]. Similar to graphite-filled composites, carbon fiber-filled epoxy composites are also used in the manufacture of robotic arms. With epoxy-filled composite materials, a high degree of freedom and modulus can be achieved. One of the attractive applications of epoxies in the robotics industry is in the manufacturing of climbing robots. In this type of robot, the legs of the robot contain fine hairs that are connected to a pumping system of

12.3 Applications of robotics

adhesives. A regular flow of adhesive is required to stick to the walls or ceiling. A thin layer of epoxy resin or its composite adhesives can be used successfully for in climbing gecko-like robots.

12.2.2 Composites in robotics Polymeric composites have numerous applications. EAP-based composites have been key materials in robots. These materials have been discussed in detail in the previous sections. All EAP composites like IPMCs, electronic and magnetic polymeric gel composites, conductive material-filled polymer composites, metal-filled polymer composites, etc., have their own roles in the manufacturing of robotic body parts, robotic sensing parts, and artificial organs like artificial muscles, biomimetics, actuator systems, etc. PVDF, PVDF TrFE, PMMA, polyvinyl alcohol (PVA), PPy, polyaniline (PANi), CNT-filled polymeric composites, thermoplastics, dielectric elastomers, etc., have been utilized widely.

12.2.3 Polymeric sensors Sensing is the most important mechanism in robotics. Sensing and actuation are the main operating mechanisms in robots. Robotic sensors are used to estimate a robot’s condition and environment. Sensors in robots are just like the sensory systems in living beings. Robotic sensors can be of different types like light sensing, tough sensing, vision sensing, sound sensing, temperature sensing, distance sensing, pressure sensing, etc. Ionic polymer composite materials can be used as micro- or nanosensors in biomimetic systems. Tactile sensing systems can be made of polymer materials and metal thin-film sensors, which can detect the hardness, thermal conductivity, temperature, and surface contour of a contact object for comprehensive evaluation of contact objects and events. A tactile sensor is a device that measures information arising from physical interactions with its environment. Tactile sensors are generally modeled after the biological sense of cutaneous touch, which is capable of detecting stimuli resulting from mechanical stimulation, temperature, and pain. Polymer materials reduce the cost and the fabrication complexity of sensor skin, while increasing mechanical flexibility and robustness [38]. Polyimide- and polydimethylsiloxane-based composite materials are also used in tactile sensors for sensing loads. Ionic metal polymer composites are used in sensors for prosthetic.

12.3 Applications of robotics Robots have a wide variety of applications; this particular section deals with the purposes of robots in different areas. Different uses are classified into three main groups, namely terrestrial, space, and underwater [39].

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12.3.1 Terrestrial applications The terrestrial applications of robots can again be categorized into medical, industrial, and other miscellaneous sectors.

12.3.2 Medical sector In the medical field, robots are used for rehabilitation, motion assistance, telesurgery, etc. [40]. Iqbal et al. [41] proposed a novel exoskeleton system for hand rehabilitation. The newly developed robotic hand has better compatibility with the human hand and it can apply force beyond 45 N, which is the best ever reported [42]. A multiobjective optimization procedure [43] and a series of experiments on human hands of various sizes and age groups [44] helped the device design.

12.3.3 Industrial sector The importance of robots in industries is mainly due to their role in improving accuracy, repeatability, reliability, precision, and efficiency. Industrial robots have replaced a wide range of human tasks since they lack a lot of problems like fear, tiredness, lack of interest, or any other such factor that could be a restriction in realizing targets. The major industrial processes in which robots find applications are assembling, painting, polishing, welding, drilling, picking and placing, sorting, etc. Serial articulated robots comprising of all revolute joints are common in industrial automation. An example is the 6 degrees of freedom (DOF) pseudoindustrial manipulator, Autonomous Articulated Robotic Educational Platform [45]. It is a servoactuated robot that can be used to test trajectories before their execution on a real industrial arm [46]. To facilitate application development, kinematic and dynamic models of the arm have been derived [47]. The manipulator also has the potential to validate advanced control strategies [48,49] for multiDOF manipulators.

12.3.4 Miscellaneous applications Other terrestrial applications of robots are in the areas of defense, agriculture, entertainment, rescue and safety, etc. [50]. Tethers support a robot for managing in narrow spaces and decreases the size significantly. Tethers can also be used for power supply and communication purposes. Other applications of robots in scenarios where human involvement is either risky or impossible include nuclear power plants [51] and petroleum tank inspection.

12.3.5 Space applications Space research consists of extreme conditions, severe environmental constraints, and long traversal times on many planetary surfaces like Mars where robotic

12.3 Applications of robotics

rovers are major sources of information. Researchers send a lander during the voyage; a rover is fixed inside one of the petals of the lander. After landing, these petals are opened and sample collection starts. The rover is teleoperated and it operates in close proximity to the lander. Navigating using the lander camera, the rover moves to a selected target and performs drilling in an automatic way. After sampling, the rover follows a tether to return back to the lander. Finally, the robot delivers the collected samples to the lander, where the samples are analyzed [27,52]. Considering a planetary robotic rover based on the extended Kalman filter, Iqbal et al. [53] proposed a state estimation approach by fusing attitude data measured by an inertial measurement unit and position data from odometry sensors.

12.3.6 Underwater applications Underwater robots are more commonly known as remotely operated vehicles (ROVs), which are reconfigurable, reliable, productive, and flexible vehicles that can operate in deep subsea depths. The basic equipment on a typical ROV includes light sources, a video camera, and water samplers, while advanced accessories can be sound navigation ranging (SONAR), a manipulator arm, a magnetometer, and other sensors, for example, to measure light, temperature, or water characteristics. An example of an ROV is French Victor 6000, which is a remotecontrolled ROV to explore deep ocean floors [54]. ROV can go up to 6000 m in depth with a speed of 0.77 m/s. It is a four-ton robot with six thrusters. Based on a highly modular and flexible design, scientific instruments can be changed as per the requirements of a specific assignment. The sensory system consists of three CCD cameras, two pilot cameras, SONAR, and sensors to measure altitude and pressure depth. The vehicle is equipped with two robotic arms; a five-function arm for grasping and a seven-function arm for manipulation. Two virtual environment manager (VEM) computers implement a real-time system to control the vehicle. Major operations of Victor include geological, optical, and mid-ocean rift survey, German icebreaking research vessel polar stern, West African continental margin ecosystems, hydrothermal vent inspection and local area investigation, and investigating significant reasons for functional benthic biodiversity in polar deep sea.

12.3.7 Military applications Robots on the warfront seem merely to be fiction. But in real practice, robotic technology has been widely used by armies across the world. Robots and the familiar term, drones, became popular in the field of war many years ago. Substituting humans with robots and drones is meant to increase economic savings. The United States is considered to be the largest producer of military robots. Technologically advanced European and Asian countries are also making use of artificial devices instead of manned force in military operations [55,56]. Some

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examples of military robots in the current era are Defense research and development organization (DRDO) Daksh of India, Elbit Hermes 450 and D9T Panda of Israel, Iran aviation industries organization (IAIO) Fotros of Iran, Goalkeeper close-in weapon system (CIWS), Ripsaw MS1, and Army mules of the United States, etc.

12.3.8 In mining Mining is considered to be among the most dangerous and risky jobs across the globe. Unexpected explosions, falling, machinery faults, temperature, decreases in oxygen supply, etc., are considered to increase the risk. So the mining has sector searches for an alternative to humans or a helping aid to avoid or overcome the risks. Robots can make mining safer by mapping the risk factors and entering the mining area prior to men. They are more efficient. Coal mines and ore mines across the world are utilizing this kind of machine. In order to work in hazardous jobs like mining, robots require logic and to be able to interpret circumstances, design strategies, and implement tasks with absolute consistency. They come under the category of field robots. This type of automated machine includes selfdriving carriers, automated drillers, position markers, and assistants [57,58].

12.4 Conclusion Robotics is an advanced technology that incorporates almost all engineering and technology branches. The use of robots is increasing in many fields including industry, research, and biomedical fields. Biomimetics is the most common and demanding area of robotics. These robots can act as exact substitutes for the muscles of the body. Different materials are being used in the robotics field. Polymers and macromolecules having a wide range of applications also play an important part in robotics. Thermoplastics, thermoset adhesives, elastomers, etc., are used in the manufacturing of robotic components. Actuators and sensors are major components of robots. Electroactive materials like ferroelectrics, conducting polymers, CNTs, etc., are utilized for the manufacturing of sensors and actuators. Polymers provide flexibility, durability, and a lightweight nature to components. The ability of polymers to be molded into different shapes makes them a favorable candidate in robotics.

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[41] Iqbal J, Khan H, Tsagarakis NG, Caldwell DG. A novel exoskeleton robotic system for hand rehabilitation conceptualization to prototyping. J Biocybern Biomed Eng 2014;. [42] Iqbal J, Tsagarakis NG, Fiorilla AE, Caldwell DG. A portable rehabilitation device for the hand. In: IEEE international conference on engineering in medicine and biology society (EMBC); 2010. p. 3694 7. [43] Iqbal J, Tsagarakis N, Caldwell D. Design optimization of a hand exoskeleton rehabilitation device. In: Proceedings of RSS workshop on understanding the human hand for advancing robotic manipulation, Seattle, US; 2009. p. 44 5. [44] Iqbal J, Tsagarakis N, Fiorilla AE, Caldwell D. Design requirements of a hand exoskeleton robotic device. In: 14th IASTED international conference on robotics and applications (RA), Massachusetts, USA; 2009. p. 44 51. [45] Iqbal U, Samad A, Nissa Z, Iqbal J. Embedded control system for AUTAREP a novel AUTonomous Articulated Robotic Educational Platform. Tehnicki VjesnikTech Gazette 2014;21:1255 61. [46] Islam RU, Iqbal J, Manzoor S, Khalid A, Khan S. An autonomous image-guided robotic system simulating industrial applications. In: 7th IEEE international conference on system of systems engineering (SoSE), Italy; 2012. p. 344 9. [47] Islam RU, Iqbal J, Khan Q. Design and comparison of two control strategies for multiDOF articulated robotic arm manipulator. J Control Eng Appl Inform 2014;16:28 39. [48] Ajwad S, Ullah M, Baizid K, Iqbal J. A comprehensive state-of-the-art on control of industrial articulated robots. J Balkan Tribol Assoc 2014;20:499 521. [49] Khan MF, Iqbal J, Islam RU. Control strategies for robotic manipulators. In: International conference on robotics and artificial intelligence (ICRAI); 2012. p. 26 33. [50] Ajwad SA, Iqbal J. Recent advances and applications of tethered robotic systems. Sci Int 2014;26:2045 51. [51] Iqbal J, Tahir A, Islam RU, Nabi SRU. Robotics for nuclear power plants—challenges and future perspectives. In: Applied robotics for the power industry (CARPI), 2012 2nd international conference on; 2012. p. 151 6. [52] Iqbal J, Heikkila S, Halme A. Tether tracking and control of ROSA robotic rover. In: 10th IEEE international conference on control, automation, robotics and vision (ICARCV); 2008. p. 689 93. [53] Iqbal J, Saad MR, Tahir AM, Malik A. State estimation technique for a planetary robotic rover. Rev Facul Ingen 2014;58 68. [54] Michel J-L, Klages M, Barriga F, Fouquet Y, Sibuet M, Sarradin P-M et al. Victor 6000: design, utilization and first improvements. In: 13th int. offshore polar eng. conference; 2003. p. 25 30. [55] Springer PJ. Military robots and drones: a reference handbook, ABC-CLIO, ISBN: 9781598847321. [56] Borges JV. Robots and the military: a strategic view. In: Aldinhas Ferreira M, Silva Sequeira J, Tokhi M, Kadar EE, Virk G, editors. A world with robots. Intelligent systems, control and automation: science and engineering, vol. 84. Cham: Springer; 2017. ISBN: 978-3-319-46665-1. [57] Pilania VK, Chakravarty D. Application of wireless visual sensor for semiautonomous mine navigation system. World Acad Sci Eng Technol 2008;21. [58] Nanda SK, Dash AK, Moharana SAA. Application of robotics in mining industry: a critical review. Indian Min Eng J 2010;108 12.

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Polymers in optics

13

Sneha Bhagyaraj1,2,3, Oluwatobi Samuel Oluwafemi2,3 and Igor Krupa1 1

Center for Advanced Materials, Qatar University, Doha, Qatar Centre for Nanomaterials Science Research, University of Johannesburg, Johannesburg, South Africa 3 Department of Chemical Sciences, University of Johannesburg, Johannesburg, South Africa 2

13.1 Introduction The increasing demand for more advanced functional materials with improved properties during the past few decades have encouraged researchers to come up with new composite materials [1]. Among these composite materials, polymers serve as one of the most commonly used matrix elements. The unending possibilities of tuning and expanding polymer functionality have made these materials suitable for introducing them in a myriad of fields [2,3]. With no exception, in the past few years, photonics has also exploited polymers creatively in diverse optical applications. Their ease of processing and tunability has enabled polymer photonics to witness an immense boost in research level and practical applications. The ease of modifying polymer materials to exhibit unique optical and electrical properties attracts the interest of researchers to exploit the multifunctionalities of this system and utilize them in high-end applications. Extremely transparent and reliable optical polymeric materials are abundantly available to be used for the fabrication of various waveguide components. Developments in the synthesis of active polymers have empowered researchers to develop advanced devices such as photonic molecular lasers [4], deformable lenses with tunable abrasions [5], ultrafast electro-optic modulators, efficient W-light emitting diodes (LEDs), solar cells, flexible displays, and so forth. The main attractions of these polymeric systems are the ease in their fabrication technology, their flexibility, and the broad compatibility of these materials with other semiconductor processing technologies. Applications of polymer optics include, but are not limited to, ferroelectrics, conductors or semiconductors, sensors, optical waveguides, and so forth. Fig. 13.1 shows a schematic representation of various applications of polymers based on their optical properties. To pave the way for polymers to be applied in various optics and electro-optics applications, they must be defined by suitable chemical and physical properties. Properties like light transmission, Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00013-5 © 2020 Elsevier Inc. All rights reserved.

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FIGURE 13.1 Applications of polymers based on their optical properties.

refractive index, abbe value, melt flow rate, specific gravity, chemical resistance, and so forth, also need to be monitored while fabricating optical devices. They also possess properties like photoconductivity, piezo or pyro electricity, or nonlinear optical (NLO) characteristics. Transparent optical polymers (TOPs) show good light transmission in the UV
visible and near infrared (IR) regions of the electromagnetic spectrum [6]. By substituting TOPs for glass in various aspheric and miniature elements, huge reductions in manufacturing costs can be achieved. The structural peculiarities of polymers include different types of bonding, that is, one-dimensional covalent bonds and the existence of secondary networks between the primary covalent bond long chains. These distinctive structures contribute to the many interesting properties of these polymers like thermal dependence, flexibility, and low thermal conductivity. For example, polymer dispersed nematic liquid crystal films have successfully being used to develop electrically controllable polarizers [7]. Polymers with special features like polyvinylidene difluoride (PVDF) and its

13.1 Introduction

modifications have enabled the development of various technological devices such as thermo-optic (TO) devices, single-mode waveguides, flexible waveguide devices, and polarization-controlling devices [8
10]. In spite of their many advantages, polymers are prone to disadvantages like weak thermal stability and photooxidation. In order to rectify these defects and improve the properties and applicability of polymers, various techniques like introducing fillers, plasma modifications, preparing blends, and so forth have been studied [11,12]. To overcome the problem of optical loss due to vibrational overtone absorption, fluorinated polymers have been introduced [13]. This has helped to reduce propagation loss below 0.1 dB/cm, which is quite useful. The C
F bond is stable compared to the C
H bond due to decreased electron energy level and has improved thermal and chemical stability. At higher temperatures, fluorinated polymers are found to be stable and no photooxidation is observed. An example of a fluorinated polymer used for integrated optic waveguide devices is ZPU polymers developed by ChemOptics. They possess excellent processability and are suitable for producing layered polymer coatings using simple techniques like UV curing processes and spin coating. Many of the unique properties of fluorinated polymers make them useful as a reliable material in the manufacture of different optical waveguide devices. In the case of polymers, packing density has an important role in influencing the refractive index of a material. The expansion of polymers in the presence of heat results in a drop in molecular density, which leads to a decreased refractive index and negative TO coefficient. Compact tunable filters are used as channel selectors to simplify the receiver configuration in an optical fiber network with multiplex circuits. Various research groups have intensely worked on fluorinated polymers to develop high-end devices including polarization controllers, integrated optics, and flexible waveguide tunable lasers with good performance and reliability [14,15]. Another significant aspect of polymer optics is their use as adaptive optics (AO) systems. It is a technique that is used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions [16]. The mechanism is that the polymer acts as a deforming mirror to rectify the effect of distortion. AO systems have already confirmed their importance in various applications including in telescopes [17], microscopy [18], optical communications [19], high energy lasers [20], and so forth. AO is a considerable scheme to solve the challenges involved in establishing high-speed laser communications between satellites and ground stations. In early versions, polysilicon membranes were used as deformable mirrors. One of the disadvantages of the silicon material was the high driving voltage. In order to overcome this, flexible polymer organic materials have been employed. An epoxy-based SU-8 membrane was able to achieve a stroke of 12 m at 220 V [21]. Polymer mirrors based on polyimide offer a large 15 m stroke at below 1 V with 2.5 A current [22,23]. This chapter is dedicated to giving insight into the optical properties of polymers and their importance. Initially the chapter outlines the significance of the optical properties of polymers followed by a classification of various optical parameters.

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A brief description of various techniques used to characterize the optical properties of polymers is given. More importance is given to the application aspects of optical polymers in various fields. Finally, the chapter concludes by discussing the future perspective of polymer optics in various day to day applications.

13.2 Properties of optical polymers Optical polymer materials possess many unique properties like refractometric and dispersive attributes that are missing in several inorganic optical moieties. Properties like flexibility, TO effect, index tenability by preparation protocol, low thermal conductivity, structural diversity, and controllable birefringence are some worth mentioning. Compared to glass, polymeric materials possess some advantages like low weight, high impact resistance, and the ability for the optical and mechanical features to be tailored cost effectively. The optical properties of a polymer depend on many factors like its structure, composition, preparation methods, and processing conditions. Any negative influence on the optical properties of a polymer results in the aging of the polymer, which in turn affects the lifetime of the product. It has been found that scattering phenomena play a major role in determining the optical properties of a polymer. Fig. 13.2 illustrates some important optical properties of polymers.

FIGURE 13.2 Various optical properties of polymers.

13.2 Properties of optical polymers

13.2.1 Refractive index An important property of transparent polymers that determines its optical applications is refractive index. This property of a polymer can be used to determine many other parameters of the respective polymer. Various parameters like the glass transition temperature and molecular weight of a polymer can be determined if the refractive index of the material is known. The refractive index is the relative velocity of light when compared with the air and polymer medium. The polarizability and wavelength of a polymer plays a crucial role in determining the refractive index of the polymer.

13.2.2 Abbe number (V number) Abbe value is significant when it comes to the use of polymers in various lenses. It classifies materials based on their chromaticity. The Abbe number, VD, of a material is defined as: VD 5 nD 2 1=nF 2 nC

(13.1)

where nD, nF, and nC are the refractive indices of the material at the wavelengths of the Fraunhofer D-, F-, and C-spectral lines (589.3, 486.1, and 656.3 nm, respectively). The value varies for different polymers based on the composition and structure of the polymer. Polymers like poly methyl methacrylate (PMMA), with good Abbe values are used for preparing optical lenses.

13.2.3 Birefringence Birefringence is an optical property of a material having a refractive index. It depends on the propagation and direction of light while passing through the material. This property is typical for materials possessing anisotropy. Polymer plastics are isotropic by nature and induced stress during manufacturing procedures gives rise to anisotropy, resulting in birefringence in the finished material.

13.2.4 Transparency In transparent polymers, light passes through these materials without any reflection. But this is an ideal condition and a small percentage of scattering happens inside a polymer matrix due to the presence of structural molecules inside the matrix. This scattering distorts the transmitted image.

13.2.5 Color Most polymers are colorless since they absorb in the near or far IR range. Few exceptions like some elastomers (e.g., natural rubber) and thermosets (e.g., epoxy) contain double bonds, which act as chromophore and induce color in polymeric matrices.

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13.2.6 Gloss Gloss is a surface optical characteristic property of transparent or opaque polymers. It is a mirror-like reflecting surface with a high scattering property that reflects light equally in all directions at all angles of incidence. The light from one or more incident angle reflected with particular angular distributions results in this phenomenon.

13.3 Characterization of optical properties of polymers Some of the most common instrumental techniques used to measure the various optical properties of polymers are briefly discussed here.

13.3.1 Abbe refractometer A refractometer is a simple instrument used for measuring the refractive index of liquids, gases, and translucent solids. It is associated with the reduction in speed of light when traveling through a medium that is under monitoring. The calculation of the index of refraction is done using Snell’s law and in some cases using rules such as the Gladstone
Dale relation and the Lorentz
Lorenz equation [24]. To measure the parameters, a small amount of the sample is required and this technique is extensively used in many industries like food, agricultural, chemical, and manufacturing units.

13.3.2 UV
visible absorption spectroscopy UV
visible spectroscopy is useful in evaluating the light absorption properties of polymers. The principle of this spectroscopy is associated with the excitation of electrons, in both atoms and molecules, from lower to higher energy levels. Some forms of advanced study like UV
visible derivative spectroscopy are also useful to carry out detailed analyses of optical properties and the nature of chromophore responsible for many of the absorptions [25].

13.3.3 Photoluminescence spectroscopy Photoluminescence (fluorescence) spectroscopy is a contactless and nondestructive method to probe the electronic structure of materials. When light falls on the sample, it is absorbed and imparts excess energy into the material in a process called photoexcitation. This excess energy can be dissipated through the emission of light, which is known as photoluminescence. This technique can be used for the analysis of various structural, morphological, and dynamical phenomena in natural and synthetic polymers [26]. Time resolved fluorescence spectroscopy is an extended characterization technique that involves fluorescence measurement. This technique is used to study the fluorescence of a sample and is represented as a function of time after excitation by a laser beam.

13.4 Polymer optics: the manufacturing technology

13.3.4 Raman spectroscopy Raman spectroscopy is an example of a vibrational spectroscopic technique that is used to provide information on the molecular vibrations and crystal structures in a sample. In this technique, the sample is initially irradiated using a laser source, which results in the generation of Raman scattered light. This signature light is recorded as the Raman spectrum by a CCD camera. By analyzing the characteristic pattern in a Raman spectrum, it is possible to identify an unknown substance, and evaluate its crystallinity, orientation, and stress [27]. An advantage of this technique is that it is nondestructive and all form of samples can be analyzed using this technique.

13.3.5 Brillouin spectroscopy Brillouin spectroscopy is useful to study the mechanical properties of polymer nanocomposites. In Brillouin spectroscopy, Brillouin scattering (BLS), that is, the inelastic scattering of light in a physical medium by thermally excited acoustical phonons is measured [28]. From the BLS measurement, various parameters including the elastic constants, acoustic velocities, the refractive index, and the glass transition temperature of polymers and nanocomposites can be analyzed. BLS and friction experiments can be used to correlate the elastic modulus and tribological properties of nanocomposites.

13.4 Polymer optics: the manufacturing technology To achieve the complicated and unique geometries necessary for optical devices as well as to reduce costs and achieve targeted volume production, manufactures are keen to invest in plastics as a good alternative to glass. The most important production technique suitable for translucent polymers is injection molding. Injection molded products tend to have better resilience to breaking and are extremely repeatable. Depending on the end use, either plastics or a composite of glass and plastic can be used. Adding glass into plastic during processing improves the strength of the polymer and the final property of the material can be varied by changing the ratio of both. The manufacturing technology in polymer optics includes different stages.

• Designing • Material upgrade • Manufacturing process Various stages of plastic optics manufacturing have highly benefitted from advanced designing techniques like computer-aided design and computer-aided machining. Advanced software that are able to design complicated 3D structure models of the proposed product, sophisticated instruments for manufacturing, and improved quality and quantity of raw materials are also significant during the production stage [29].

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Manufacturing technology for optical polymers can be easily explained by considering the example of optical lenses. In the first stage of a project, to meet the quality standards and development schedule, single-point diamond turning (SPDT) manufacturing processes are commonly used [30]. This is a prototyping stage and can be done in parallel with the design and building of the plastic injection mold and preparing the initial samples in lab. During the prototyping stage itself, an idea regarding the choice of polymers to be used in the manufacturing process will arise. The SPDT technique can be used to prepare various optics including free foams, spheres, parabolas, mirrors, and so forth. Many simple polymers used in the production molding can also be prototyped in the SPDT stage. Simple polymers like cyclic olefin polymer, PMMA, polystyrene (PS), and so forth can be prototyped using SPDT. When it comes to materials like polyetherimide or polyether sulfone, initially another technique called high refraction diamond turning (HRDT) is used [31]. In the HRDT method, lenses made from high index and high thermal property plastics are fabricated. The first step is to anneal the surface to give it an optical quality followed by the SDPT process for lens fabrication. The HRDT method is mainly used to prepare lenses with multiple diffractive structures. Together with advance prototyping, material grade improving also need to be continuouswhen it comes in the manufacturing sector. According to the need of the end user, the material should be able to withstand harsh environments in diverse applications. An example is an optical device used for military applications. When it is dropped from a high altitude to a lower one, it should withstand the drastic change in temperature over a short period of time.

13.5 Applications of polymers in optics Based on the optical properties of polymers and their composites, they are applicable for ferroelectrics, conductors or semiconductors, sensors, optical waveguides, and so forth. They also possess properties like photoconductivity, piezo or pyro electricity, or NLO properties. Properties like light transmission, refractive index, Abbe value, melt flow rate, specific gravity, chemical resistance, and so forth also need to be monitored while fabricating optical devices. Some of the important applications of polymer optics are discussed here.

13.5.1 Polymers in fiber optics Fiber optics, which is frequently used in applications like networking, broadcasting, and electronics, represents a technology or a medium in which information is transmitted as light pulses from one end to the other without a great loss in intensity [32]. Optical fibers are basically made from extremely pure glass or polymers and are lightweight and small in size. They are widely used in optoelectronic

13.5 Applications of polymers in optics

applications due to their strength of signal and compatibility. An optical fiber cable comprises mainly three parts, namely core, cladding, and buffer. The center of the fiber is called the core and it is through this that the light is “guided” during transmission. An optical material called “cladding” surrounds the core, which traps the light in the core using total internal reflection. To protect the fiber from moisture and physical damage, mechanical isolation, and so forth, it is covered by another layer called the “buffer.” This layer helps to identify the fiber and also helps to improve scattering loss due to microbends. Fig. 13.3 represents a schematic representation of an optical fiber. Over the years, silica fibers have played an important role in optical communications due to their lowest attenuation and large availability [33]. Silicon-based optical fibers have been immensely used in various strain, temperature, and angle related measurement technologies due to their characteristic properties like their lightweight, magnetic field immunity, and multiplexing capabilities. In spite of their large number of advantages, silica-based optical fibers possess some disadvantages like fragility, low power, limited applications, and high production cost. In search of an alternative to silica fibers, researchers noticed that polymer optical fiber (POF) is generally cost effective. In the case of POF, both the core and the cladding are manufactured from polymer. The manufacturing techniques used for the production of POFs depend on the requirements of the final product [34]. There are mainly two techniques, namely the continuous manufacturing technique and the discontinuous manufacturing technique. The continuous manufacturing process can produce optical fiber with unlimited length, while in the discontinuous process, the fiber length is limited. POF has some advantages over silica fibers such as high sensitivity in bending, ease of fabrication, biocompatibility, mechanical robustness, high fracture toughness, and high sensitivity in strain, which make it suitable for sensing applications [35,36]. One of the disadvantages of POFs is their higher transmission

FIGURE 13.3 Structure of an optical fiber.

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FIGURE 13.4 (A) Optical microscope image of the Rh6G-doped POF. (B) Fiber length-dependent photoluminescence spectra of Rh6G-doped POF (404 nm excitation) [39]. Copyright 2015. Reproduced with permission from Elsevier.

loss, which makes them unfit to be used in short distance communications. Similar to silica glass fibers, POFs also transmit data through the core of the fiber. But compared to the core size of glass fiber, POFs possess a higher core size sometimes hundreds of times greater than that of glass fiber. This higher core size allows the core to transmit a signal from point to point, which makes it suitable to be used in high bandwidth transmission over short distances. Common polymers that are used to make high quality optical fibers include polycarbonate (PC), PMMA, PS, fluoropolymers like CYTOP, and so forth [37]. Nowadays, POFs are used in many applications like curvature sensors [38], fiber light systems [39], temperature sensors, [40] and accelerometers [41]. Polymer optical fibers can be used to develop fiber lighting systems. An example is a tracer dye (Rhodamine 6G; Rh6G)-doped PMMA-based polymer optical fiber. In conventional fiber light systems, the lateral emission comes from pump radiation. But in the case of the dye-doped polymer optical fiber system, a combination of both pump radiation and photoluminescence from the dye molecule produces a multicolor lateral light emission. Fig. 13.4 shows an optical microscope image of dye-doped POF and its fiber length-dependent fluorescence properties. A detailed study on the axial photoluminescence spectra (shape, intensity, etc.) was done with respect to the length of the polymer optical fiber.

13.5.2 Polymers in optical lenses Optical components that are designed to focus or diverge light are collectively called optical lenses. They can be made of a single component or multiple

13.5 Applications of polymers in optics

components depending on the level of application varying from microscopy to laser processing. Nowadays, in order to reduce the production cost and improve quality and applicability, optical grade polymers have successfully replaced traditional optical glass in many applications. The main reason for this replacement is there low cost. Many fabrication techniques have also evolved that offer repeatable and high precision products with high production volumes. Another advantage of polymer lenses, is their high impact resistance, which makes them durable for long-term applications. Optical grade polymers with low birefringence and stable refractive indices are now available offering better design flexibility than glass optics. Chalcogenide materials that are commonly used to make lenses inherit many disadvantages like fragility, toxicity, and high processing costs. These materials are not suitable for the fabrication of integrated optic elements. Piezo electric polymers like PVDF have emerged as a good alternative for this cause. It is possible to use PVDF microlenses in chip to chip interconnects in digital systems [42]. Flexible elastomers are used to fabricate microlenses by applying controlled strain [5]. The lens abrasions can be controlled by varying the strain. Compared to pressure-actuated membranes, elastomeric membranes are simple and they possess well defined optical surfaces even in the unstrained state. These properties make them good candidates for application in camera lenses. To be used in the preparation of microlenses, features like refractive index, dispersion properties, and transmittance play a crucial role. After device fabrication, properties like birefringence, stability, haze, and so forth influence the quality of the device. Various thermoplastic polymers like PMMA, PS, PC, and copolymers like styrene-acrylonitrile (SAN) polymers [43] are commonly used for the fabrication of microlenses. Contact lenses, which are alternative medical devices for the eyes, play an important role in rectifying human sight disorders. They works like eyeglasses and can be manufactured from high-end polymers. For a polymer to be used as a raw material in contact lenses, they should possess certain features. The top priorities are transparency and flexibility. Other properties like low density, being unreactive to chemicals on the eye surface, hydrophilicity, and ease of manufacture are also considered. The lenses should allow oxygen to pass through the eye surface. The first commercial contact lenses were based on PMMA and were popularized during the 1960s [44]. But PMMA lenses are hard and not very comfortable. Sometimes it may take weeks for users to get adjusted to them, and above all, they don’t allow oxygen to pass through the cornea. The introduction of soft lenses in 1971 made from polyacrylamide (PAM) was another breakthrough in this field. Due to the presence of nitrogen in PAM, after crosslinking, the material absorbs water. Since around 40%
80% of soft lenses are water, these lenses remain soft and flexible. Rigid gas permeable lenses made of PMMA, silicones, and fluoropolymers were introduced in 1979. These types of lenses are mainly applicable to correct astigmatism and bifocal errors. But the high manufacturing cost and inflexibility in these lenses limit their applications.

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Lens implantation using intraocular lenses (IOLs) is an effective way to treat cataract patients partly to recover from visual problems. Since cataract is common among the elderly, IOLs with good adjustability and biocompatibility are extremely important. Polydimethylsiloxane (PDMS), which is an organosilicon polymer with excellent rheological properties, is commonly used in contact lenses. A porous-structured IOL with a meniscus polymer membrane using PDMS fabricated through thermal and gravity assisted methods has recently been reported [45]. The accommodating range of the lens was 13.5D, while its focal length varied between 24 and 18 mm. During the imaging trial, the system showed larger resolution in the middle stage of the lens deformation process. It is claimed that this wide accommodating range of the IOL and feasible fabrication procedure will make this useful in various fields including the biomedical field such as in visual disturbance therapy. It can also be applied in various optical zooming systems like microscopes and smart cameras. Contact lenses with an antibacterial property made of fluorinated ethylene propylene copolymer and TiO2 have also been reported [46]. Fluidic tunable lenses with a polymer membrane and multiflow structure can be used for various low compact optical systems. The lens design includes five layers including one polymer layer, two solid layers, and two liquid layers [47]. Zoom optical systems based on tunable polymer lenses have also been reported [48]. An inexpensive polymer lens that can be used in high power dye sensitized solar cells as a solar concentrating device is also in the developing stage [49]. Fig. 13.5 shows a prototype of a polymer-based lens together with its power conversion efficiency. A tunable focal lens based on flexible bionanocomposites with an electric field
induced actuation has also been reported. A deformable smart composite lens made from a poly(diethylene glycol adipate) and cellulose nanocrystals composite showed an excellent optical transmittance of 93% [50]. Intensive research to improve the properties of polymer-based lenses in order to use them as drug delivery systems to the ocular tissues in addition to their basic purposes is under progress. The addition of various nanofillers like metal nanoparticles, nanoproteins, and so forth to polymer matrices, improves their antibacterial properties and drug loading efficiency together with eye rectification [51
53]. A hybrid, soft contact lens with a laser shielding effect has also been newly developed [54]. In order to introduce the shielding effect, gold-based plasmonic silica-shelled nanocapsules were incorporated into a methacrylate-based polymer matrix, which tremendously improved the optical density of the hydrogel.

13.5.3 Polymers in lasers Semiconducting polymers, which combine the characteristics of both a polymer and a semiconductor, show potential for various applications in electronics and optoelectronics [55]. The semiconducting properties of long chain polymer molecules arise due to the presence of conjugated single and double bonds.

13.5 Applications of polymers in optics

FIGURE 13.5 (A) Polymer lens diode (PLD) based solar system, (B) measurement setup, and (C) graph representing increase in output power density against PDMS height of the PLD solar system [49]. Copyright 2016. Reproduced with permission from Elsevier.

This conjugation enables polymers to be capable of fluorescence emission. This emission from polymers can be achieved in two ways; either by shining light on the surface to give fluorescence or by applying voltage to a polymer-based lightemitting diode. The main reason for the interest toward semiconducting polymers is that they have a broad spectrum, which highlight the scope of making tunable lasers. The strong absorption coefficient of polymers results instrong amplification of light. Above all, the flexible nature of polymers enables them to be processed according to the nature of their application. The first semiconducting polymer laser used a resonator design and was made of a low concentration solution of poly(2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) as reported by Moses in 1992 [56]. In another example, lasing action was observed at a wavelength of 545 nm when PPV was sandwiched between two mirrors. Polymeric random lasers have always been attractive due to their high stability and feasibility of processing [57]. Dye-doped electrospun polymer nanofibers with random orientations and alignments, which showed optical amplification and random lasing action, were reported; which increases the possibility of the application of organic microfibers. For example, nanofibers prepared from polyvinylpyrrolidone (PVP) and 4-(dicyanomethylene)2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) showed good random lasing properties [58]. Lasers that are free of resonance cavities with reflection mirrors are called random lasers. Dual-wavelength polymer

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FIGURE 13.6 A scheme representing the fabrication process for a dual-wavelength random laser (A)
(G) exhibits the fabrication procedure of the step-cavity random laser of polymer film and (H) demonstrate the diagram of the dual-cavity polymer random laser excited by a short pump [59]. Copyright 2018. Reproduced with permission from Elsevier.

random lasers are also reported, which can be fabricated using the spin coating method [59]. Fig. 13.6 represents a scheme for the fabrication of a dualwavelength random laser. To adjust the wavelengths and intensities of the dualwavelength polymer random laser, the pump location and cavity illumination area can be varied. Poly[9,9-dioctylfluorenyl-2,7-diyl], which is a light-emitting polymer, end-capped with dimethyl phenyl is the active material. Electrospun, dye-doped polymeric nanofibers also showed improved optical properties and lasing action. The lasing properties of the nanofibers were independent of the alignment of the fiber. Electrospun composite fiber prepared from PVP and DCJTB dye showed excellent optical lasing properties. In spite of their orientation, both aligned and random nanofibers highlighted excellent optical amplification. The aligned fibers, due to their arrangement, showed a better threshold and polarization process. Nanoporous polymer films prepared through spin coating also showed random lasing action [60]. A PMMA/PS blend in a 1:1 ratio was spin coated into a silicon substrate followed by doping Rh6G into the nanoporous thin film. The as prepared polymer film showed laser emission with a low threshold due to its hole size and can be extended possibly into developing an organic coherent light source. Fluorescent polymer nanocomposites, which can be used in fabricating LEDs, can be prepared by incorporating various quantum dots (QDs) with core, core
shell and core
multishell architectures by introducing fluorescent QDs into the

13.5 Applications of polymers in optics

polymer matrix. QDs are fluorescent semiconductor nanocrystals that basically have sizes of up to 10 nm [61
63]. A highly transparent and luminescent epoxy nanocomposite was reported that was prepared using fluorescent amido group
functionalized CdSe QDs for probable application in white LEDs. The amidofunctionalized QDs showed better dispersion and improved optical properties inside the polymer matrix [64]. A yellow light
emitting QDs
NH2/epoxy nanocomposite was prepared, and in order to stimulate the working condition, it was exposed to a blue light source under fluorescence microscopy. The clear white light emission was obtained by mixing the blue light and the reemitted yellow light. A fluorescent epoxy nanocomposite prepared from carboxyl-functionalized CdSe QDs that emits a yellow and red color was also reported [65].

13.5.4 Polymers in optical sensors Detectors that can record the optical properties in a material and convert the changes into an electronic signal can be called optical sensors. Polymer-based optical sensors are common these days. In order to use a polymer material as an optical sensor, the key parameter that needs to be controlled is its modify refractive index (RI). To modify the refractive index of a polymer to suit an application, approaches like preparing multicomponent hybrid materials with inorganic
organic components, adding ceramic nanoparticles to the polymer matrix, and so forth have been considered. Due to the possibility of forming agglomerates, nanoparticle incorporation is not that favorable under certain conditions. An effective and scalable way to improve and tune the refractive index of a polymer is to add guest molecules like phenanthrene [66]. When it comes to the production of polymer devices, together with refractive index, polymer viscosity also plays an important role. For example, in optic waveguide fabrication processes like nanoimprint lithography and inkjet printing, initially the monomer is in a flow state, which, after UV-induced curing, undergo polymerization. Organic field-effect transistors (OFETs) have great potential in the field of biomedicine and can be used as highly sensitive and selective drug sensors. An OFET fabricated using a polyethylene naphthalate flexible foil as a substrate and a transparent aluminum oxide transparent gate dielectric has been tested successfully [67]. Organic conductive polymers show an electrochromic (EC) effect. This effect basically occurs when the absorption spectrum changes in response to a voltage driven redox reaction. Polymers like poly(3,4-ethylene-dioxythiophene) doped with poly(styrene-sulfonate) (PEDOT:PSS) show an EC effect [68]. Compared to liquid crystals, EC materials possess good compatibility with organic and aqueous electrolytes, ease of processing, low voltage DC actuation, and so forth that enable them to be used as gratings, lenses, and so forth. Another category of polymer modification for sensing applications is the molecular imprinting process. This process introduces highly sensitive and specific recognition sites in a polymer through a template-assisted imprinting technique. When compared to natural receptors, the imprinted synthetic receptors

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show high specificity, robustness, high affinity, and low-cost production. Nanotechnology has contributed much to the development of high-performance molecular imprinted polymer (MIP) sensors. An example of a mechanism of MIP sensing is the change in RI (Refractive index) of a polymer as a result of binding of protein or other large biomolecules of the MIPs immobilized on the surface plasmon sensor [69]. Fluorescence sensors are gaining attention due to their low detection limit and simple protocol. Many sensors are commercially available for checking blood sugar level. MIP-based sensors are also in the advanced stages of development and commercialization. To highlight this, a schematic representation of a glucose sensor based on a hybrid MIP microgel is shown in Fig. 13.7. The fluorescent property of the polymer microgel is due to the incorporation of nontoxic fluorescent carbon dots into the hydrogel matrix during the imprinting process [70]. Carbon dots have excellent optical properties, which is significant for the fabrication of sensors. They are highly inert, photostable, and possess superior electron/hole transfer,

FIGURE 13.7 Representation of protocol for glucose sensor fabrication using MIP technique [70]. Copyright 2015. Reproduced with permission from ACS.

13.5 Applications of polymers in optics

which make them suitable for various biosensing applications. The as prepared MIP was highly sensitive to the presence of glucose and also highly selective, even in the presence of other body fluids. In another example, microsphere-tipped micropillars of polydimethylsiloxane (MSMPs-PDMS) with outstanding flexibility have been studied as an optical airflow sensor. In the presence of airflow, the micropillar bends, resulting in the decrease of reflection level and by simulations and calculations we can arrive at the solution [71]. New developments include ultrasonic wideband sensors for biomedical applications fabricated using single-mode polymer optical fibers [72]. The sensitivity of the as designed sensors was more than that of traditional singlemode silica optical fiber. Polymer optical fibers, due to their transparency, flexibility, and compatibility, have great potential to be used in optical sensing, power delivery, and optical networking. Due to their nanoscale structure, they possess excellent optical confinement. One of the unique properties of polymer nanofiber is its flexibility toward modification with various dopants, fillers, or even the formation of blends. Its high surface-to-volume ratio offer more possibilities for optical applications like sensors, photodetectors, in-coherent light-emitting devices, and so forth. [73].

13.5.5 Polymers in waveguide fabrication The growing population and their increasing demand for more bandwidths in future interconnects have forced researchers to think solutions to come out with technologies which combine high performance and low complexity with budget friendly approach. In this path, integrated optical connections can offer an increase in performance for future electronic systems [74]. Compared to electrical copper-based interconnects, improved energy efficiency and bandwidth density are needed [75]. Applying optical integrated planar waveguides instead of fibers is a promising approach to avoid excessive use of fibers. The most applied waveguide materials for efficient, low-cost electro-optical devices are glass and polymer [76]. In order to use a polymer as an optical waveguide material, it should possess good TO conductivity and structural diversity. An example of a pressure sensor fabricated using flexible polymer waveguide material is given in Fig. 13.8. Due to the small Young’s modulus of polymer material, a large strain is induced, which can produce significant changes in the device length and the Bragg grating period. This polymeric Bragg reflector can be used to measure strain with higher sensitivity than that of silica fiber gratings. Various waveguide structures including planar single-mode, multimode, and microoptical waveguide structures can be readily fabricated from optical polymers. They range from one micrometer to several hundred micrometers. An optical mode filter, which is a device to filter a single mode from different modes, is an integrated optical waveguide formed by a higher-refracting or an absorbent material disposed near a flat waveguide core. Various optical polymers have been

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FIGURE 13.8 Prototype of a flexible strain sensor [77]. Copyright 2016. Reproduced with permission from Elsevier.

used to fabricate different mode filters. An example is SAN (styrene- acrylonitrile resin) and K-resin based four layer polymeric waveguide structure which showed relatively low propagation losses compared to a monolayer configuration [78]. The isolation of the required wavelengths from multiplex wavelengths is an important requirement when it comes to the applications of waveguide optics. For that purpose, Bragg reflectors serve. Compared to inorganic waveguide materials, polymers possess a high thermal insulation property and TO coefficient. Hence simple microheaters can be used to tune the reflection spectrum of the reflector. Compared to other devices, polymeric wavelength filters have many advantages. Temperature plays a huge role in tuning the refractive index of polymers. Due to this TO effect of polymers, only a single heating electrode is needed to tune the direct wavelength [79
82]. In addition, their ease of processability allow these polymer tunable filters to be coupled with various other devices such as optical switches, phase controllers, and polarization control devices [77,83]. Fluorinated acrylate polymers have also been used to fabricate Bragg reflectors. By increasing the fluoroorganic compound, the optical absorption loss can be reduced together with achieving additional properties like thermal stability and strong chemical resistance [84]. Polyamide-imide polymers, due to their high thermal stability and good chemical resistance, have been investigated for various applications including sensors, separation membranes, and so on. They can also be used in certain microdevices due to their inherent stability and mechanical strength [85]. During optical transmission in polymer integrated waveguides, transmission loss plays an important role as a limiting factor for the use of these materials in practical applications. Polymers like poly methacrylate and polyvinyl derivatives have interested researchers due to their ease of processability and film forming including waveguide patterning. But fabricated polymer waveguides show relatively high optical loss. This phenomenon is basically due to the intrinsic absorption of the material and to the light scattering in the bulk [86].

13.5 Applications of polymers in optics

13.5.6 Polymers in nonlinear optics NLO properties mainly show the behavior of high intensity laser light when it passes through a nonlinear media. During the interaction of laser light with matter, the dielectric polarization (P) of these matters behaves nonlinearly with the electric field of the light. This phenomenon can be observed only when the light is given from a high intensity laser source. The incident laser beam can either generate new light frequencies or change the optical properties of the materials depending on the nonlinearity of the medium. Hence nonlinearity is a property of the medium, and various polymers and their composite systems possess nonlinear properties under various conditions. It has been shown that attaching various NLO chromophores to polymer side chains can effectively increase the stability, reaction efficiency, and effect of the byproduct. A pyronin Y/flexible organic substrate composite showed excellent linear and NLO properties. It can be used as a nonlinear absorption media for saturation applications due to its high third order NLO value χ(3) and refractive index n(3) values [87]. For optical communications, which need fast modulation and switching of optical signals, organic NLO polymer-based integrated devices play an important role. They are able to modulate and switch optical signals at a high speed of 100 GB/s or above. The NLO properties of polymers enable them to be used in a vast number of applications in optoelectronics. To induce NLO properties in a non-NLO polymer, various fillers can be added into the polymer matrix to prepare its respective nanocomposites with NLO activity. Another way is to modify the polymer structure by introducing functional side chains. This can be done in two ways, namely though doped polymers and grafted polymers. In doped polymers, a host
guest system is evolved by dispersing chromophores inside the polymer matrix, making this a simple system. The main disadvantage of this system is the instability of the orientation of the chromophore in the composite. In the case of grafted polymers, the orientation of the chromophore is stable. They show higher nonlinear response compared to doped polymers due to the presence of a large number of grafted chromophores. Since it is grafted to the polymer skeleton, it is not easy to move maintaining the orientation. Novel polymer materials like cyclic olefin copolymers exhibit good chemical resistance, flexibility, and transparency. Hence they are commercially used to fabricate fundamental parts like optical and tetrahertz waveguiding, nanofluidic devices for tetrahertz spectroscopy, and so forth [88]. Photoresponsive polymers have emerged as important candidates for widespread applications in NLOs and solubility switching. They are promising materials due to their ease of processability and being versatile together with their light-triggered spatial response. They have found established applications in aerospace and communications and are now being used in biomedical applications due to their photoswitchable solubility [89]. The light-induced reversible isomerization of the azo bond between the cis and trans configuration offers a reversible control over a wide variety of their properties [90]. Azobenzene-incorporated liquid crystal networks, which

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show photoresponse like shrinking and oscillation, demonstrate a promising future for developing these materials into various applications like self-cleaning surfaces.

13.5.7 Polymers in solar cells As the need for clean energy is increasing, there is an acceleration in the research and development of solar cells [91]. Photovoltaics, which is the science behind the harvesting of solar energy, is the conversion of light into electricity with the aid of certain semiconducting materials. Due to their lightweight properties, environment-friendliness, and solution processability, polymer photovoltaics is currently a hot research area. Polymer solar cells (PSCs) or “plastic solar cells” are in fact semiconducting materials made from organic molecules. In function and working principle, they are similar to silicon solar cells, but different in material. The concept of PSCs is still at the research level and is not yet commercialized. The layered PSC structure consists of mainly three layers, namely a transparent front electrode, an active semiconducting polymer material layer, and a back electrode printed onto a plastic substrate [92]. The thickness of the active layer varies between 150 and 200 nm, which is low compared to traditional silicon solar cells. The use of low-cost materials like plastics and aluminum highly affect the production cost of polymer-based solar cells. Another advantage is that they can be fabricated using existing low-cost technologies, which significantly reduce their product price. The energy efficient production protocols and low use of toxic metals and chemicals makes this an environment-friendly system. The working of PSCs is illustrated in Fig. 13.9. As in a traditional solar cell, there exist three operational stages for the working of PSCs.

FIGURE 13.9 Schematic illustration of three-stage mechanism including (A) light absorption, (B) charge separation, and (C) charge collection, in PSCs [92]. Copyright 2010. Reproduced with permission from Elsevier.

13.5 Applications of polymers in optics

1. Electron
hole pair creation by the absorption of photons 2. Charge separation by exciton diffusion at the interface 3. Charge transport to the corresponding electrodes The efficiency of PSCs can be calculated using the equation: PCE 5

ðJSC VOC FFÞ Pin

(13.2)

where JSC is the short circuit current, VOC is the open circuit voltage, FF is the fill factor and Pin is the incident light power, which is standardized as 100 mW/cm2. PSCs, which can absorb light and convert it into electricity, cannot match the durability of inorganic solar cells currently, but have the potential to produce mass, nontoxic solar panels with low production costs [93]. This can be seen as a complimentary source that can provide energy for point-to-use devices instead of toxic batteries. In PSCs, polymers can be dissolved in a solvent and printed into any flexible surface even in a single coating, which makes this material cost effective. Long rolls of solar cells, which can be used to cover buildings, motor vehicles, and so forth, could improve the energy efficiency of entire nations. An effective PSC needs two well-matched organic molecules to be more efficient. One polymer absorbs light and produces excited states, which release charges, and the second one pulls electrons from the first so the cell can generate a current. An efficient PSC can be made only if the two molecules go hand in hand and interact closely to enable the current transfer efficiently. The current record of conversion efficiency for PSC is 11.5% for an organic solar cell made by Henry Yan at the Hong Kong University of Science and Technology. To fabricate PSCs, organic materials with delocalized π electrons that can absorb sunlight to create photogenerated charge carriers and have the ability to transport these carriers, can be utilized. So they come under the classification of electron donors and electron acceptors. Some commonly used electron donor polymers are P3HT [poly(3-hexylthiophene)], [poly[2,7-(9,9-dioctyl-fluorene)-alt5,5-(4,70 -di-2-thienyl-20 ,10 ,30 -benzothia diazole)]] [94], and [poly[N-90 -heptadecanyl-2,7-carbazole-alt-5,5-(40 ,70 -di-thienyl-20 ,10 ,30 -b3nzothiadizaole)]] [95], and some electron acceptor polymers are F8TB [poly(9,90 -dioctylfluorene-co-bisN,N0 -(4-butylphenyl)-bis-N,N0 -phenyl-1,4-phenylenediamine)], CN MEH-PPV poly-[2-methoxy-5,20 -ethylhexyloxy]-1,4-(1-cyanovinylene)-phenylene and so forth. The best, known electron acceptors are fullerenes. Another class of PSC is polymer nanocrystal solar cells (HSCs) [96]. They possess unique properties of both polymers and nanocrystals; the lightweight and mechanical flexibility of polymers and the absorption, high carrier mobility, and structure stability of nanocrystals. In spite of the advantages, the development in the field of HSCs when compared with other PSC is relatively sluggish. This is due to the lack of understanding in the mechanism of operation in HSCs. The existence of defects and ligands impacts the carrier transport in HSCs, which further complicates

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the optimization [97]. A number of polymer
nanocrystal systems have been studied for the purpose of HSCs cells. Systems like P3HT
CdS [98], P3HT
ZnO [99], and methoxy substituted poly(phenylene vinylene)
CdTe [100], for example. Transparent organic photovoltaics have been receiving immense attention due to their unique potential in future applications other than solar energy harvesting [101]. Polymer photovoltaics are significant in visible spectrum applications because of the noncontinuous nature of their absorption spectra. Highperformance polymer photovoltaics with superior transparencies and power conversion efficiencies have been developed. By introducing new low bandgap donors and non-fullerene acceptors, the active-layer absorption spectrum can be shifted to the IR region, thereby increasing the efficiency of the photovoltaic. Expect solar cells, polymer photovoltaics can be successfully introduced in applications like small electronic gadgets, off-grid community power generation, or power plant energy production, and so forth. Another interesting area of application is the integration of photovoltaics into buildings. Since they are cheap, environment-friendly, thin, light, and highly flexible, they can easily be integrated into building surfaces such as roofs, windows, facades, and so forth. In organic photovoltaic cells, the polymer-based active material used will be lightweight, flexible, and highly sensitive. This makes them suitable to develop technologies for electricity development integrated into everyday life [102]. Reportedly, the single-junction opaque organic photovoltaic cells that currently exhibit the highest efficiency are cells made with bulk heterojunctions of PTB7: PC71BM [103,104].

13.5.8 Polymers in photocatalysis The sustainable development of human society depends on the development of technologies that are pollution free and can supply clean energy. Photocatalysis is a promising approach among various green technologies to realize solar energy conversion [105]. It was in 1972, in a ground breaking discovery, that Fujishima and Honda demonstrated the feasibility of photoelectrochemical water splitting using an n-type semiconductor TiO2 anode coupled with a platinum (Pt) cathode [106]. Photonic energy conversion through a photocatalytic method was recognized widely after this discovery. It has been shown that semiconductors have great potential in the photolysis of water to yield hydrogen fuel, which is a form of clean energy. But the main disadvantage is that they tend to agglomerate easily and the conversion efficiency is low, which limit their application at large scales. This urged the development of new photocatalysts that possess high efficiency and stability [107,108]. An attractive strategy for clean energy production is direct photocatalytic water splitting. To meet future energy demands, the direct production of hydrogen from water using solar energy is significant. In order to harvest solar energy, semiconductors with appropriate band gaps are required. Compared to inorganic photocatalysts, organic photocatalysts have advantages like tunable band gaps,

13.5 Applications of polymers in optics

controlled structures, good processabilities, and abundant sources. A few organic polymeric compounds have been studied for photocatalytic hydrogen evolution. Some examples include poly(azomethine) networks [109], triazine-based frameworks with covalent bonds [110], conjugated microporous polymers [111], linear conjugated polymers [112], and so forth. Two-dimensional polymer nanosheets have successfully tested as a photocatalyst for water splitting. Almost all reported polymer photocatalysts for water splitting are based on 2D polymers, indicating their importance in solar water splitting. Because of the 2D surface, these catalysts possess high surface areas, abundant active sites, and efficient charge separation and migration [113]. An example is 1,3-diyne-linked conjugated polymer nanosheets prepared from terminal alkynes such as 1,3,5-tris-(4-ethynylphenyl)benzene and 1,3,5-triethynylbenzene. Under visible-light irradiation, they exhibited excellent photocatalytic activity in overall water splitting [114]. Conjugated polymer nanostructures can also be effectively used in photocatalysis. Polypyrrole films with TiO2 nanoparticles, TiO2-modified conjugated derivatives of polyisoprene, and so forth, show excellent photocatalytic activity in the degradation of dye under visible light when compared to TiO2 alone [115]. Large structural diversity and controllable synthetic procedures make coordination polymers (CPs) promising candidates for use in light-harvesting antennae and catalytic centers to achieve solar energy harvesting [116]. Furthermore, ease of introducing new functional materials into the CPs structure resulting in new multifunctional/hybrid composite with superior photocatalytic performance is also highlighting its significance [117].

13.5.9 Polymer optics in the biomedical field Biophotonics, which is an emerging science mainly in the medical field comprises the principles of both biology and medicine to design new therapeutic strategies. To improve the efficiency and quality of advanced medical treatments available, there is need to introduce sophisticated protocols and material platforms to enhance the existing technology. In this aspect, biophotonics plays an important role by introducing new imaging and sensing strategies to improve the currently existing treatments [118]. The most important challenge in the field of biophotonics is to design a material that is both biocompatible and biodegradable and that possess all the necessary properties for the particular application. To introduce it into in vivo use, it is necessary that the developed material is biocompatible. Hence researchers are attracted to various natural and synthetic polymers to replace the existing brittle and nondegradable silica-based optical materials. Terminal diseases such as cancer can be diagnosed in the early stages using biophotonic technology imaging, which is a noninvasive and high sensitive technique [119]. The main factor that makes a material suitable for use in bioimaging applications is that it should be able to deliver light with high efficiency and low loss. With silica being the most used material, some polymers like PAM, cellulose, polyethylene glycol (PEG), chitosan, PDMS, and so forth, also show excellent optical properties for applications in various devices [120
123].

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Photodynamic therapy is an available efficient treatment for cancer that uses special photosensitizing substances to initiate cell death. It offers localized cell death, which is effective to treat region-specific tumors. Porous polymer optical fibers can overcome some of the disadvantages existing with the currently available photosensitizing materials. Polymer optical fibers with high porosity made of PC and PMMA were prepared by thermal drawing under controlled water [124]. Natural materials like silk, DNA, cellulose, chitosan, and so forth, with high transparency and light guiding efficiency, can be used for biophotonic applications. DNA and its complexes like DNS
PDMS, DNA
cetyltrimethylammonium, and so forth, were successfully used to prepare light-emitting and waveguide materials for various biodevices [125]. Silk, which is a natural protein fiber has also been studied to develop lenses, gratings, and waveguides for in vivo implanting applications [126]. Synthetic polymers are considered to be better candidates for biodevices due to their more controllable physical and chemical properties. Synthetic polymers like PEG, PAM, PDMS, and so forth, show promising applications in various biomedical devices. Hydrogels prepared from PEG and PAM possess good features suitable for tissue engineering and drug delivery. Techniques like soft lithography can be used to fabricate a wide range of structures from silicone-based PDMS, which in turn can be used for fiber resonators [127]. Light-guiding polymer hydrogels exhibit potential photonic functionalities. The main challenge while working with light in biomedical applications is the limited penetration of light into biological tissues. Cell-based, light-mediated therapy being a sensitive technique, possesses many challenges including high optical loss in biological tissue due to scattering and absorption. Depending on the specific applications, there is need to develop desired optical and mechanical functionalized biomaterials. A single polymer alone may not be able to provide all the necessary conditions needed and, hence, unique blends or composites need to be developed to suit the application. A hydrogel optical fiber prepared based on PEG hydrogel as the core and alginate hydrogel as the cladding material demonstrated high light guiding efficiency with a propagation loss of 0.32 6 0.02 dB/cm in air and 0.42 6 0.01 dB/cm in tissue at 492 nm [128]. Another significant factor of polymer hydrogel systems is the ability to incorporate functional materials like organic dyes, nanoparticles, and so forth, into it to modify the optical properties, fluorescence, and photothermal heating nature of the parent system. Highly stretchable and tough step-index optical fibers with a low propagation loss of 0.45 dB/cm in air were made from alginate
PAM hydrogels. The hydrogel fibers can potentially be applied in many applications including daily wearable sensors and in vivo implantable therapy devices [129]. Fluorescent polymeric nanoparticles (FPNs) have emerged as potential candidates for biological imaging theranaustic applications due to their high sensitivity and ease of operation. Benzophenone hydrazone-poly(3-formyl-4-hydroxybenzyl methacrylateco-2-Methacryloyloxyethyl phosphorylcholine polymer) FPNs, which were prepared using a novel combination of reversible addition fragmentation chain transfer polymerization and postpolymerization showed various properties

Acknowledgments

like good water dispersibility, fluorescence, large stokes shift, biocompatibility, and photostability. All these properties, which are significant when it comes to biological imaging, make them highly promising for biophotonic applications [130].

13.6 Future perspective and challenges in polymer optics In spite of their many advantages compared to glass in various applications, polymer optics still need immense research and improved modifications to overcome various disadvantages. One of the main areas that needs to improve is the temperature resistance of polymers. If polymers need to be used as a replacement for silica in an industrial range, then the thermal properties of polymers need to be improved according to the processing conditions and applications. Various methods including preparing blends, composites, introducing heat resistant materials, hybrid fillers, and so forth, are already in use to improve the thermal properties of polymers. The concept of multifunctional materials is highly significant at this point. It has already been shown that the addition of nanofillers like graphene or its derivatives, metal nanoparticles, and so forth, into various polymers at certain filler loadings improve their electrical properties, which in turn makes them useful to fabricate various sensors [131]. Developing new and advanced 3D holograms that can enable compact 3D displays, freeform optics, biophotonic materials for medicine, and so forth, are promising areas that need to be focused on.

13.7 Conclusion Polymers have been in the limelight since the day they were discovered for their amazing physical and chemical properties that can be tuned effectively to suit various high-end applications. Selecting polymers for optical applications mainly depends on the optical and material properties. Polymer plastics are organic systems that possess good light transmission properties in various regions of the electromagnetic spectrum. In sophisticated electronic and semiconducting devices, integrated device parts made of optical polymers play an important role. There is still immense scope for optically active polymers and their various modifications to be exploited for optics and electrooptics related applications. It is worth noting that optoelectronic components developed from polymers will be an important part of future smart optics systems.

Acknowledgments This publication was supported by Qatar University Collaborative Grant QUCG-CAM-19/ 20-2. The findings achieved herein are solely the responsibility of the authors.

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Polymers in space exploration and commercialization

14 M.Z. Naser1 and A.I. Chehab2

1

Glenn Department of Civil Engineering, Clemson University, Clemson, SC, United States Department of Civil and Environmental Engineering, Wayne State University, Detroit, MI, United States

2

14.1 Introduction Apollo 11 marked the first tangible effort toward realizing manned deep space exploration, and since 1961, humans have managed to explore a number of moons, planets, and galaxies. The primary objective of such exploration attempts is to find a suitable body (i.e., Earth analog/exoplanet) such that humans can securely migrate and prospect. Due to their proximity to Earth, a natural inclination to visit and explore the Moon and Mars has emerged (see Fig. 14.1). This vision started an inertia toward developing emerging technologies and solutions that would enable successful exploration missions [1]. Such solutions aim to solve fundamental complications such as developing resilient materials and affordable technologies to enable explorers to safely inhabit unique space environments. The fact of the matter is that in order to enable safe manned missions aimed at facilitating research and site exploration, astronauts need to be provided with tools, vehicles, and equipment that can withstand the extreme environments of space. This gear should also be fabricated of durable and light materials that are ideal to be fabricated and maintained using in situ resources as available. The emphasis of using in situ materials is prompted by the estimation that it can cost between US$5000 and US$20,000 to deliver one pound (B0.45 kg) of materials from Earth to the Moon or Mars [2]. Hence a vast number of research programs has been commissioned over the past five decades with the prime goal of enhancing our knowledge in order to realize suitable materials for space applications. Of the most investigated materials, there is a convergence on the notion that polymers and composites are best suited for space environments due to their superior performance against vacuum, radiation, elevated temperatures, and so forth [3,4]. As a result of their good ratio of mechanical strength (stiffness) to weight along with their good thermal, physical, electrical, and optical properties, polymers have been being extensively used in exterior and interior-based space Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00014-7 © 2020 Elsevier Inc. All rights reserved.

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FIGURE 14.1 Landing sites on (A) the Moon and (B) Mars. Courtesy NASA.

applications [5,6]. Typical polymers used for space applications typically include silicones, epoxies, polyurethanes, acrylics, fluorocarbons, polyimides, and so forth. According to studies, with slow improvements, polymers could potentially outperform traditional materials (e.g., metals); especially when it comes to offering solutions to reduce the overall weight-to-size ratio of transport, and, hence, the fuel needed in spacecrafts [7]. This chapter is inspired by the 2018 successes of the National Aeronautics and Space Administration’s (NASA) landing of InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport mission) robotic lander on Mars as demonstrated in Fig. 14.2, in addition to the Japan Aerospace Exploration Agency’s (JAXA) Hayabusa2 landing on asteroid Ryugu. The displayed inertia toward restarting crewed missions to the Moon and Mars as announced by NASA, the European Space Agency (ESA), as well as the China National Space Administration (CNSA), among others, motivates promising research in this field. As a major outcome of these announcements, a plan for manned missions to land on Mars in the early 2030s is set, with the premise of building human habitats in the near future. In favor of the mentioned efforts, this chapter reports previous and current findings, and also identifies challenges related to the use of polymers in space colonization and commercialization. First, this chapter reviews the adverse effects of space environments on the properties of polymers, followed by a discussion on the feasibility of developing and using inorganic polymers and space resources as construction materials. Then, concepts for “space-resilient” habitats are highlighted. Finally, a number of issues and challenges facing the use of polymers in space exploration and venues that warrant research are emphasized.

14.2 Space environments, actions, and conditions

FIGURE 14.2 Demonstration of InSight collecting data on Mars. Courtesy NASA.

14.2 Space environments, actions, and conditions Before discussing the role of polymers in space exploration and commercialization, a concise review of the alien environments in space is essential. For brevity, this section primarily discusses the environment of low Earth orbit (LEO), as well as those of the Moon and Mars. Geometrically, the LEO extends to about 700 km from the Earth’s atmosphere. At such an altitude, the damaging effects of vacuum, atomic oxygen (ATOX), ultraviolet (UV), radiation, micrometeorites (i.e., space debris), and thermal cycling become widely apparent, especially on polymers [6]. The main key differences that seem to stand out when analyzing the environments of the Earth, Moon, and Mars can be grouped under four main categories, namely (1) low gravity, (2) absence/lack of atmospheric pressure (vacuum), (3) micrometeorites/meteorites impacts, as well as (4) extreme radiation. These differences stem from the natural variations between the Earth, Moon, and Mars. These variations are further highlighted in Table 14.1. A closer look at Table 14.1 shows that both the Moon and Mars have smaller sizes, in terms of mass and radius, than that of the Earth. As a result, the gravity of these two planets is also smaller than the Earth’s and are often estimated at 1.62 and 3.71 m/s2 for the Moon and Mars respectively, compared to the gravity on Earth, which is approximately 9.81 m/s2. This lower gravity influences material formation such as pore development, and so forth [13]. Similarly, low gravity conditions can also affect other material-based processes such as mixing and the transport of mass or heat. In lieu of the magnitude of gravity, another difference between the Moon, Mars, and Earth is the fact that the atmosphere of the latter is thick and primarily consists of oxygen (21%) and nitrogen (78%). The atmosphere

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on Earth eliminates much of space radiation, and, thus, radiation levels become insignificant. On the other hand, the Moon lacks atmosphere, and, hence, it experiences hard vacuum (estimated at 3 3 10213 kPa), combined with the unceasing impact of meteorites and micrometeorites, diurnal temperature fluctuations (ranging from 2173 C to 127 C C), and radiation of up to 380 mSv [14]. It should be noted that vacuum conditions not only can cause materials to outgas, but can also accelerate material deterioration and fluid loss [15]. Unlike the Earth and Moon, the atmosphere on Mars mainly consists of carbon dioxide and is 100-times thinner than that on Earth [16]. Despite its thin atmosphere, a low atmospheric pressure of about 0.7 kPa exists on Mars too, which serves as a weak shield against radiation (B100 mSv).

14.3 Effect of space environment on polymers Unlike other loading conditions, space demands stringent requirements so as to mitigate the extreme nature of the environments affecting these materials. Thus the influence of space environments, together with their inherent synergy, need to be well-understood for the proper selection of polymers in space applications. Despite their remarkable characteristics, and similarly to other materials, polymers experience deterioration under harsh space environments. For example, polymers used in the coating of the exterior surface of space systems such as rockets, vehicles, and habitats can be severely affected by harsh space conditions. This has been observed in tests carried out in space shuttles and/or simulated environments [6]. In fact, polymer degradation is a major concern as this phenomenon can accelerate under a combination of vacuum, radiation, and thermal cycling. Initially, both a high flux of electromagnetic radiation as well as spaceborne particles can damage the stability of polymers and lead to bond scission and embrittlement. Vacuum can cause volatilization, which may contaminate sensitive equipment or components, while thermal cycling can result in polymer instability and a reduction in load-bearing capabilities [17]. Overall, outer space Table 14.1 Main differences between the environment of the Earth, Moon, and Mars [8 12]. Parameter Mass compared to Earth (%) Distance from Earth (km) Average surface temperature ( C) Mean radius (km) Surface gravity (m/s2) Radiation level (mSv) Atmospheric pressure (kPa)

Earth

Moon

Mars

13 6371 9.81 2.4 101.3

1.2 3.84 3 105 230 1737 1.62 380 Negligible

10.7 2.25 3 108 257 3389.5 3.71 100 0.7

14.3 Effect of space environment on polymers

Table 14.2 A list of adverse effects of space environment to polymers [6]. Environment

Effect

Ultraviolet radiation

• • • • • • •

Charged particle radiation Vacuum Thermal cycling Micrometeoroid Atomic oxygen Ionizing radiation Solar ultraviolet radiation

Develops defects in lattice Develops chain scission and crosslinking in organic materials Same as above Develops secondary radiation damages Causes volatilization of low vapor pressure Diffusion/vacuum welding Leads to mechanical and chemical degradation and embrittlement • Mechanical failure • Fracture of materials and structural systems • Causes oxidation and surface erosion • Cracking/crazing of materials • Property degradation • Property degradation

can be particularly severe to polymers with environmental threats including vacuum, UV radiation, ATOX, debris impact, thermal cycles, and electrostatic charging [6,17]. Due to the effects of the nature of space, which can significantly damage polymers, a thorough discussion on such damages is presented in the following sections and summarized in Table 14.2.

14.3.1 Vacuum As the atmospheric pressure tends to decrease with continuous increases in altitude, this decrease in pressure facilitates the migration of volatiles. In the case of polymers, the released impurities comprise of catalysts, unreacted monomers, plasticizers, and so forth. Thus once polymers are exposed to hard vacuum, polymeric volatiles evaporate, and then migrate to eventually fixate on a cold surface (i.e., solar panels, sensors, etc.). Vacuum conditions also accelerate the volatilization and diffusion of low molecular weights (as well as additives). Vacuum can also induce the evaporation and sublimation of low-molecular components from polymeric materials. Hence the corresponding properties of polymers change as they release low-molecular compounds that are then deposited on nearby surfaces. In practice, the volatilization of polymers is often assessed as per ASTM Standard E595-7. In this procedure, the tested polymer material is first heated for 24 hours at 125 C while being pressured under 1026 torr, after which volatiles are collected at ambient temperature (i.e., 25 C). In this procedure, the total mass loss is collected, and the tendency of a given polymer to volatile is estimated. It is worth noting that the requirements needing to be satisfied for space applications include

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Table 14.3 Outgassing levels of common polymeric materials [6]. Material

Application

TML (%)

CVCM (%)

Kapton PEP film Buna-N Fiber optic cable CE-100 Acrylic sheet

Insulation O-ring Optics Structural

0.31 14.74 0.27 0.51

0.18 6.25 0.07 0.05

a value of 0.1% for collected volatile condensable material and 1% for total mass loss. The outgassing of polymeric constituents is another action that can occur due to vacuum and this can degrade physical, mechanical, and/or electrical properties, even at small levels, when this degradation slightly exceeds an amount of 2%. A major concern regarding this phenomenon is the toxicology of volatile byproducts on astronauts in manned missions. It is worth noting that the addition of stabilizers can improve the required time to undergo damage under exposure to vacuum by a factor ranging from 3 to 10. Table 14.3 lists the outgassing levels of some of the most commonly used polymers in space applications.

14.3.2 Thermal cycling The thermal balance of a space-based system (i.e., space station) can be simply evaluated through checking how much heat the exterior surface can absorb. In general, large cycles of temperature may trigger the cracking and delamination of structural components made of polymers. This can produce thermal-induced stresses leading to mechanical defects or failures. Further, thermal cycling deteriorates mechanical properties (i.e., strength and stiffness), as well as chemical composition. Thermal cycling at temperatures between 2100 C and 100 C can stimulate the exfoliation of composites, especially in cases where components are of different coefficients of thermal expansion. As a rule of thumb, thermoplastic-based composites can outperform thermosets due to the better synergy between the fiber and matrix. In terms of thermal cycling effects on binding materials (i.e., adhesive), high temperatures can cause decay, while low temperatures can cause concrete solidifying and weakness, both effects can hinder the use of glues in space. As natural polymers, glues are primarily made out of adaptable alkyl chains and, hence, their thermal stability is sensitive to the vitality of chemical bonds. With the aim of reducing the oxidation of chemical bonding, the addition of alicyclic or aromatic rings or heterocyclic groups to the primary polymer networks is an effective technique to improve the resistance of adhesives to heat.

14.3.3 Atomic oxygen ATOX can oxidize polymers and alter their surface properties. ATOX may also cause the oxidative erosion of many polymers. The collective interaction of all of

14.3 Effect of space environment on polymers

these environments can cause a much more severe damage to polymers than each of the individual effects (i.e., independently). ATOX erodes polymer surfaces, thereby changing their morphology, optical, thermal, and mechanical properties and may also cause mass loss. However, not all polymers are sensitive to ATOX (see Table 14.4); for example, Teflon polymers (i.e., polytetrafluoroethylene) have slow reactivity with ATOX. This is credited to the higher bond strength between carbon fluorine as well as the endothermity of the oxidation reaction [18]. In most cases, however, polymers are coated (either with inorganic or organic layers). While inorganic oxide coatings are brittle and difficult to apply, organic coatings are much more flexible and easier to apply. Based on research conducted on the effect of ATOX on polymers [19], ATOX was found to cause changes in the optical properties of polymers including cracking and quality loss of the surface and volatile fragments (short-chain oxidation products) leaving the surface of the material. For example, in the case of silver, ATOX may also diffuse into the material form as demonstrated in Fig. 14.3. Such effects can occur during a flight mission, where ATOX and coating materials undergo compound chemical and physical reactions that yield damage. Other significant changes include severe stripping on windward surfaces and loss of shape Table 14.4 Reactivity of commonly used polymers with atomic oxygen [18]. Material

Reactive efficiency (cm3/atoms 3 1024)

Polyester (Mylar) Polystyrene Teflon Silicone

3.40 1.70 ,0.05 ,0.05

FIGURE 14.3 Reaction between atomic oxygen and organic materials [19,20]. Courtesy NASA.

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Table 14.5 Dissociation energies for chemical bonds exposed to space environment [7]. Chemical bond

Material type

Dissociation energy (eV)

C6H4C(5O) C N CF3 CF3 Si O Zr O Al O

Kapton Kapton FEP Teflon Nanocomposite Nanocomposite Nanocomposite

3.9 3.2 4.3 8.3 8.1 5.3

integrity (especially for high-polymer materials), causing contamination to the optical system of a spacecraft or space base [21]. Experimentally verified, dissociation energies of bonds in polymers commonly employed in outer space, for example, Kapton and FEP Teflon are extremely low (,4.5 eV) as shown in Table 14.5. Since traveling in LEO encounters ATOX collision energies of about 4.5 eV (to the surface in the direction of travel), this can result in rapid and localized degradation of polymer materials. Thus protecting polymers from such harsh conditions is essential, and can be achieved by depositing an inorganic coating with a much higher bond dissociation energy (see Table 14.5).

14.3.4 Ionizing radiation The resulting effects of energy transfer occurring from incident radiation to polymers include ionization and atomic displacement. Both of these effects may lead to significant degradation in polymer properties depending on the nature of the radiation as well as the radiation energy and dosage. Energetic particles can cause radiation damage to electronic components comprising of polymers, and similarly to solar cells [5].

14.3.5 Solar ultraviolet radiation In general, radiation (i.e., UV- and charged particle-based) can lead to chain scission and crosslinking in polymers [6]. UV radiation is about the same level at LEO as at geostationary Earth orbit (GEO) and tends to decrease toward the ground surface. Specifically, polymers receive radiation doses of 1012 1015 erg/ g/year primarily from short wavelength sunlight, which is well above the established damage threshold for common polymers (i.e., 108 1010 erg/g/year). This radiation also affects the optical properties of polymers (by increasing solar absorptivity) as damage from radiation is concentrated at the surface. In addition, UV radiation yields discoloration and degradation in the mechanical, physical, and electrical properties of polymers, either owing to crosslinking or chain scission. Solar radiation, which comprises of electromagnetic and corpuscular, stimulates various phase-change transformations within polymers [22]. In one study,

14.4 Use of inorganic polymers as building materials

Krishnamurthy [6] showed how some types of polymers such as phenyl-methylsilicone, polyethylene-terephthalate, polyvinyl-chloride, and poly-methylmethacrylate experienced rapid crosslinking upon exposure to space sunlight. Solar UV radiation holds an energy large enough to break polymeric-based bonds and functional groups [5] given that the total energy provided by solar UV radiation can be estimated by 8% of the solar constant 1366 W/m2, over wavelength range of 100 400 nm. This type of radiation may induce material degradation in thermo-optical properties, leading to a decreased efficiency of the thermal control and mechanical strength. Furthermore, solar UV radiation can often cause crosslinking of polymer surfaces, resulting in embrittlement and cracking [23].

14.4 Use of inorganic polymers as building materials The exceptional achievement of polymer scientists in creating natural polymers has been a motivation to create inorganic polymers. In any case, research has not yet uncovered a completely new inorganic polymer framework other than silicones. In addition, silicone chains are hindered by natural substituents and can simply connect with one another by powerless Van der Waals forces. Normally, polymers are inflexible solids at low temperatures with their subatomic units bonded in threedimensional (3D) structures through Van der Waals forces, hydrogen bonds (viz., long-chain polymers), covalent linkages, or ionic powers (e.g., system polymers). Numerous basic inorganic substances are polymers made out of huge quantities of indistinguishable auxiliary units connected by covalent bonds. It is worth noting that the units in such polymers may not participate in building long chains, but rather in framing 3D systems. Inorganic polymers do not contain carbon, yet may have carbon in the pendant gatherings or in the side chain(s). Such polymer compounds could be integrated from accessible lunar substance components, silicon, aluminum, and oxygen using inorganic system polymers that contrast from natural polymers in that the network atoms are not joined explicitly, but rather are joined through halfway associating molecules that might be oxygen (as in the straightforward mixes of boric corrosive or silica). The bonds inside these polymers are covalent among oxygen, boron, or silicon. Such materials are ordinarily not adaptable, elastomeric, or safe. Examples of 3D inorganic polymers are shown in Fig. 14.4 [24]. Organic materials have weak protection from heat, while in comparison, inorganics cannot experience strain. To deliver a material between these limits, the ideal structure is required to comprise of a straightly organized polymer with nonionic substituent gatherings to support adaptability. This is the fundamental idea driving the commercial accomplishment of inorganic polymers, known as silicone polymers. One of the most generally utilized silicon polymers is siliconepolydimethylsiloxane, which comprises of a chain of substituting silicon and oxygen atoms with two methyl groups connected to every silicon heteroatom as shown in Fig. 14.5 [24]. These materials show comparative nonstructural nuclear

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FIGURE 14.4 Examples of inorganic polymers [24]. Courtesy NASA.

FIGURE 14.5 Atomic formulation of polydimethylsiloxane [24]. Courtesy NASA.

development of inorganic materials that are present in nature such as quartz, feldspars, and zeolites. The search for inorganic polymers that can be attainably processed need not be built based on chainlike particles, as the utilization of direct polymers necessitates that at least one unreactive substituent be joined to every atom in the chain. These univalent assemblies are either volatile to oxidation/ hydrolysis or are lost under elevated temperatures. Silicones are uncommon in

14.5 Space resources

light of the fact that substituents like methyl, for example, when connected to silicon, show conceivable formation of a straight polydimethylsiloxane chain. A potential generation method to construct space structures could be to adjust lunar silicates by combining them with a synthesized inorganic polymeric binder to shape cold-form building. For a system polymer, a higher temperature is required to accomplish viscous stream than for a comparable chain polymer. Higher temperatures support progressively complex responses and conceivable debasement. Mainly, inorganic polymer science has been approached by exploring two fundamental types, namely (1) polymers comprising of chains of homoparticles (a solitary element chain) as in plastic sulfur or black phosphorus; and (2) those having heteroatoms (at least two disparate exchanging component chains) that use metals such as aluminoxanes that have been found to hydrolyze effortlessly, which may not be a lunar issue, but might be a concern on Mars. Accessible lunar silicate minerals could possibly be valuable for use as coldmold composites, where fiber wound structures could be created from glassy metals as added substances to lunar composites. Another potential result of lunar inorganic polymeric silicates can be in the form of a lightweight building material for sandwich development (or protection). This material could be created by oppressing lunar inorganic polymeric silicates and a synthesized inorganic binder blend, together to a sintering and frothing procedure. The utilization of binding materials, for example, cements and sealants, is a basic procedure in the construction of habitats in space. Such terrestrial materials can be produced from scare elements available in the soil found on the Moon such as carbon, hydrogen, and nitrogen. Regularly, polymer glues can be connected to optoelectronic and bearing components such as couplers, and waveguides. For example, rocket glues can be grouped into certain classes, namely polysiloxane, epoxyphenolic tar, and polyimide. The effect of vacuum on polymer adhesives is commonly insignificant, given that the emanation segment is condensable, however, outgassing may contaminate electronics and gadgets [25]. Charged radiation also does not significantly impact polymer adhesives with the exception that bright radiation can harm gleam adhesives. Changes in the mechanical properties of a choice of adhesives under vacuum aging are outlined in Table 14.6 [27]. The development of new and increasingly achievable subatomic building methods is fundamental to create feasible manufactured methodologies. However, several difficulties still persist in the synthesis of lunar inorganic polymers to be utilized as binders and cements as noted in a recent work [28].

14.5 Space resources 14.5.1 Materials from space resources The retrieval of asteroidal materials has been explored. A number of concepts and designs have been established with the aim of developing space-based mining

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Table 14.6 Mechanical properties of adhesive materials under vacuum aging test [26].

Adhesive

Stage

Maximum load (N)

Silicon rubber (condensed)

Before aging After aging Before aging After aging Before aging After aging

5.1 3.6 1744 1365 566.4 360

Epoxy resin Acrylic resin

Tensile strength (MPa)

Elongation (%)

Elastic modulus (MPa)

0.49 0.34 30.6 29 44.7 36.6

271.1 102.5 4.52 3.52 7.17 6.84

0.2 0.25 854 1179 1024 1110

equipment and approaches for processing volatiles and free substances at the asteroid surface or at nearby mining stations. In 2018, two small hopping robots successfully landed on an asteroid called Ryugu, sent from JAXA’s Hayabusa2 spacecraft. In the same year, NASA launched its first asteroid-sampling mission, Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer, with the main goal of catching a considerable sample of asteroid material and returning it to Earth in 2023. Regardless of the challenges associated with mining lunar/Martian resources for space construction, this approach can still provide a distinctive advantage over relying on terrestrial materials for building habitats in space. The chemical and physical natures of such bodies can be gathered from the analysis of remote-sensing data, the analysis of meteorites, and by studying their source regions. A metal-rich carbonaceous substance is viewed as the most alluring to recuperate and develop polymers due to the presence of carbon and oxygen, and so forth. Three possible sorts of asteroidal materials containing abundant unstable and free metals are outlined in Table 14.7.

14.6 Use of polymers in space 14.6.1 Inflatable bases Inflatables utilize high-quality polymer fibers as the main load-bearing component to carry applied external loads (for example, regolith) as well as support internal loads (i.e., live load, pressure, etc.). Inflatables were at first used as cushions in the Martian Pathfinder lander on July 4, 1997. After that, inflatables paved the way for new concepts and designs for bases. Overall, inflatable structures can be completely preassembled, arranged to fit into small compartments, and transported to the Moon or Mars. Upon arrival, the inflatable structure is unfurled and swelled by means of a mechanical or computerized process (see Fig. 14.6). Some of the primary benefits of inflatable bases is their easy deployment and ability to house large volumes.

14.6 Use of polymers in space

Table 14.7 Possible asteroidal materials by weight percentage [24]. Type

Metal-rich carbonaceousa

Matrix-rich carbonaceousb

Type 3 4, L H chondrite

Fec (metal) Ni (metal) Co (metal) C H 2O S FeO SiO2 MgO Al2O3 Na2O K 2O P2O5

10.7 1.4 0.11 1.4 5.7 1.3 15.4 33.8 23.8 2.4 0.55 0.04 0.28

B0.1

6.19 1.2 B0.1 B0.3 B0.15 B1.5 B10 38 24 2.1 0.9 0.1 0.28

1.9 3.0 B12 B2 22 28 20 2.1 B0.3 0.04 0.23

a

Data from metal-rich C2 meteorite Renazzo. Data from C2 meteorite Murchison and average C1 C2 types. c Chemical analysis in weight percent. b

FIGURE 14.6 Concept of inflatable lunar bases. Courtesy NASA.

With regards to structure transportation, manufacturing and development must be done cautiously to avoid harm to the membrane and texture. Perhaps one of the primary burdens is the fact that composite layers can’t be created from nearby

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materials on the Moon. While calls continue to advocate for the utilization of multilayered polymeric or composite membranes as they provide high resistance to abrasion and radiation, Earth-based membranes may not potentially meet space requirements (i.e., resistance to radiation, etc.) [10]. It ought to be likewise noted that the utilization of lunar fiberglass is being examined, which, if effective, could be a suitable answer for this issue [29].

14.6.2 Construction materials This section provides insights into the use of polymers in building materials suitable for extraterrestrial construction applications. Such building materials include, but are not limited to, both polymer concrete and geopolymer concrete.

14.6.2.1 Polymer concrete Concrete is a composite material that is primarily made of aggregates, cement, fines, and water. Once water interacts with cement, an adhesive develops that bonds the raw materials together. From the point of view of this chapter, polymers can also be used to develop concrete. In order to avoid using water to cast and cure concrete, polymers can be used instead. Polymer concretes are materials that mainly comprise of aggregates and polymeric binders [4,30]. Polymer concretes are sensitive to the nature and magnitude of the epoxy/resin binder used, type of aggregates, technology of the preparation of concrete, and also to the degree of adhesion that develops between both polymer binder and aggregates. Overall, the compressive strength of polymer concrete fits into the range of 17 129 MPa. A typical sample of a polymer concrete with 13% polymer as a binder material is shown in Fig. 14.7, and the properties of various types of polymer concrete are listed in Table 14.8.

FIGURE 14.7 SEM observations of two polymer concretes with varying polymer contents [31]. Reproduced with permission from Elsevier. Copyright 2011.

14.6 Use of polymers in space

Table 14.8 Properties of polymer concrete.

Property Compressive strength (MPa) Modulus of elasticity (GPa) Tensile strength (MPa) Flexural strength (MPa) Strain at failure (%)

Polyester concrete [4]

Carbon fiberreinforced polymer concrete [32,33]

Glass fiber-reinforced polymer concrete [31]

Epoxy concrete [4]

54

30 69.2

64.8

17 129

11

11.5

10.8

15

11.6 15.1

16.3 42.6

24 37.6

21.3

0.1 0.2

0.17

1 11

The behavior and effectiveness of polymer concrete was studied by Mani et al. [4]. In this study, the researchers examined two types of polymer concretes, wherein the first is made of unsaturated polyester binder and the second incorporated an epoxy binder. The outcome of this study showed that both polyester concrete and epoxy concrete could achieve higher strengths (i.e., compressive and tensile) than those of common “traditional” concrete. The measured compressive strengths in this study were 54 and 84 MPa, while the tensile strengths were 11 and 15 MPa, for polyester concrete and epoxy concrete respectively (as compared to 24 and 3.2 MPa for traditional concrete). In another work, Sik Lee et al. [34] designed a polymer concrete with proportions of one part polymer to nine parts lunar simulant soil. The procedure to achieve this polymer-based concrete followed certain steps; first, this concrete was exposed to a relatively high temperature reaching 230 C in order to melt the polymer and reach a hardened concrete. Upon a close examination of the hardened concrete, these researchers reported that the porosity of the polymer concrete is comparable to ordinary concrete. The researchers reported that the average compressive strength reached 12.75 MPa after 5 hours of casting. This shows the merit of polymeric concrete in rapid construction. In order to develop polymer concrete with load-bearing capabilities as well as radiation shielding properties, the research group of Mart´ınez-Barrera et al. [35,36] tested a number of specimens made of polyester-based polymer concrete while being exposed to gamma radiation (at three doses of 0.2, 0.25, and 0.3 MGy). In those tests, the tested “irradiated” specimens achieved an increase in compressive strength estimated at 19% as compared to nonirradiated specimens. The same researchers also reported that while the addition of polyester fibers reduced the stiffness of concrete, these fibers still managed to increase the strain at which the concrete fails. It is worth noting that similar studies noted that the properties of polyester polymer concrete

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could be further improved by embedding steel and fiber-reinforced polymer (FRP) rebars as well as powder made from glass waste [37,38]. Garnock and Bernold [39] attempted to develop water-free polymer concrete. In their study, these researchers developed waterless concrete made of 90% 95% lunar simulant (similar to samples collected in Apollo 11, 12, and 14) and 5% 10% polypropylene powder. Garnock and Bernold [39] tested samples of this concrete in compression and tension. These researchers reported that the concrete utilizing 5% polymer crumbled, however, the concrete made of 10% polymer achieved poor mechanical strength (compressive and tensile of 4 and 1.4 MPa, respectively). Both Ribeiro et al. [40] and Tavares et al. [41] documented that a key challenge of developing polymer concrete is the nature of the viscoelastic properties of polymers as they have high sensitivity to temperature (within the glass transition temperature i.e., ,80 C) as well as creep effects, thermo-oxidative degradation, and associated debonding between aggregates and binders. More specifically, these tests also showed that epoxy concretes are more sensitive to temperature fluctuation than polyester concretes.

14.6.2.2 Geopolymer concrete Another type of polymeric concrete can be prepared using geopolymers. Such materials can be made from stable aluminosilicates, that is, fly ash or metakaolin, mixed with a high-pH liquid such as alkali hydroxide or alkali silicate solution [42,43]. The procedure of geopolymerization begins with joining silalate ( Si O Al ) or silalate-siloxi ( Si O Al O Si ) monomers in a homogeneous mixture to form inorganic polymers that possess properties similar to Portland cement [44]. The nature and performance of geopolymer concretes depend extensively on the silicon to aluminum (Si:Al) ratio (see Fig. 14.8). In general, a higher Si:Al ratio (of about 2:1) produces a smoother microstructure, and a correspondingly higher compressive strength [45]. Geopolymers have unique features that naturally match the needs of lunar and Martian environments such as near-zero water consumption, vacuum stability, and resistance to thermal fatigue [46]. Geopolymer concrete can be prepared with approximately 70% 80% aggregates and 20% 30% geopolymer binder [47]. In fact, if lunar (or Martian) regolith is found to exhibit geopolymerization, then the raw materials needed (up to 90% of the total mass) to produce geopolymer concrete could be collected from in situ materials. Wang et al. [46] developed geopolymer concrete made of volcanic ash and sodium hydroxide, a mixture that only consumed 1.39% of water. The performance of this geopolymer concrete was examined under a variety of conditions such as vacuum and freeze thaw cycling. This concrete achieved a compressive strength ranging between 26 and 45 MPa. In another study, Saavedra and de Gutie´rrez [48] examined the performance of geopolymer concrete at elevated temperatures varying between 300 C and 500 C. At such high temperatures, the geopolymer concrete mixtures lost 13% 45% compressive strength and gained minor increases in porosity when compared to traditional concrete under the same

14.6 Use of polymers in space

FIGURE 14.8 SEM observations on the impact of the Si:Al ratio on the microstructure of geopolymer concrete: (A) 1.45, (B) 1.50, (C) 1.55, (D) 1.60 [45]. Reproduced with permission from Elsevier. Copyright 2017.

temperatures. A much in depth discussion on the use of polymer concrete, as well as other materials, is provided in a recent work by the author [28]. (see Fig. 14.9) The possibility of geopolymerizing lunar and Martian regolith to enable the production of in situ geopolymer concrete has been examined by Alexiadis et al. [44]. As part of this investigation, the outcome of tests showed that the lunar geopolymer managed to outperform cement in both compressive and flexural strength. An interesting finding was that lunar and Martian geopolymers, unlike cement, did not seem to undergo large reduction in flexural strength with respect of compressive strength. In this particular study, the Martian geopolymer was found to have a compressive strength that is lower than that of the lunar and traditional concretes and this was credited to the low reactivity of JSC MARS-1A Martian simulants used in developing this Martian geopolymer concrete. Some properties of the lunar and Martian geopolymer concretes are listed in Table 14.9, and samples are shown in Fig. 14.10.

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(A)

Cured at ambient conditions Cured in average daytime lunar heat Cured under vacuum

25

Compressive strength (MPa)

474

20 15 10 5 0 0

5

10

15 Age (days)

20

25

30

(B)

(C)

FIGURE 14.9 SEM microscopy of geopolymer binders [45]. (A) Compressive strength of concrete under various curing conditions. (B) Cured in ambient conditions. (C) Cured in both heat and vacuum. Reproduced with permission from Elsevier. Copyright 2017.

14.6.2.3 Advanced polymer-based materials Langley Research Center (LaRC), VA, has developed a number of advanced materials; for example, LaRC SI, LaRC TEEK, LaRC RP-50 polyimides, and LaRC PETI-5, among others [50]. Such materials are specifically engineered to

14.6 Use of polymers in space

Table 14.9 Properties of geopolymer concrete. Property Compressive strength (MPa) Young’s modulus (GPa) Tensile strength (MPa) Flexural strength (MPa) Thermal expansion (1/ C) Strain at failure (%) Fracture energy (J/m2)

Geopolymer lunar concrete [44,49]

Geopolymer Martian concrete [44]

2 37.6

0.7 2.5

13

3.6

FIGURE 14.10 Geopolymers in loose and compacted conditions: JSC LUNAR-1A (left) and JSC MARS-1A (right). Reproduced with permission from Alexiadis A, Alberini F, Meyer ME. Geopolymers from lunar and Martian soil simulants. Adv Space Res 59;2017:490 5. doi:10.1016/j.asr.2016.10.003. Elsevier. Copyright 2017.

Table 14.10 Properties of high-performance polymer-based materials. Material property

LaRC-SIa

LaRC-PETI-5a

Shape memory polymerb,c

Density (g cm3) Ply thickness (mm) Tensile strength (MPa) Tensile strain Young’s modulus (GPa) Toughness (kJ/m3) Extent of deformation (%)

1.37 141

59.4

0.9 1.2 3 6 20

4.00

3.82 106

1.0 Up to 800

a

Armanios and Reeder [51]. Gross [45]. c Liu et al. [52]. b

comprise of high temperature/performance polyimides. LaRC materials have preeminent thermal stability at inert atmosphere and ambient environment conditions as well as high glass transition temperatures. The mechanical properties of samples of LaRC materials are listed in Table 14.10.

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In addition to LaRCs, another type of advanced materials is known as shape memory materials. For example, shape memory alloys (SMAs) have been developed by NASA as a means to join composite structural members [53]. The feasibility of integrating SMAs into intelligent and adaptive space structural framing for communication facilities has been investigated by Kalra et al. [54]. Similar to SMAs, shape memory polymers (SMPs) can dynamically respond to external stimulus and can recover from mechanically induced strains [47]. These materials can return to their original shape when heated above their transformation temperature. This unique feature of SMPs allows for self-deployment mechanisms that can be useful for construction practices in space [55]. It is worth noting that SMPs have a low tensile strength, ranging between 2% and 5%, compared to SMAs, but can still be used as low mass and cheap self-deployable structures for space construction. Some mechanical properties of SMPs are listed in Table 14.10. In a new work, Liang et al. [56] effectively managed to add reinforcing fibers to enhance the mechanical properties (primarily strength and stiffness properties) of SMPs, allowing their use as load-bearing structural components. Other studies were also performed on shape memory ceramics as well as on intermetallics by Lai et al. [57] and Reyes-Morel and Chen [58], and it was noted that these materials can fail through excessive cracking and low levels of strain.

14.7 Research needs and future directions 14.7.1 Utilizing robotics The utilization of robots for exploration and construction missions on the Moon and Mars is of extraordinary significance in terms of decreasing the dangers of severe space conditions on humans. Such missions require that robots and self-ruling vehicles be made of versatile materials as well as to have comprehension and knowledge so as to make quick decisions without depending on human administrators. In terms of suitable materials for robotic applications, electroactive polymers (EAPs) are profoundly attractive for their low-thickness and vast deformability capacity, which may be multiple-times more stretchable than inflexible and delicate electroactive earthenware products. The most appealing advantage of EAPs is their capacity to imitate organic muscles with high strength and large actuation strain and vibration damping. These “artificial muscles” offer the capability of developing biologically inspired robots for mining and construction applications. A presentation of an artificial muscle tissue concept is presented in Fig. 14.11. However, these materials reach their flexible limit at low stress levels with actuation stress much lower than that electroactive ceramics (EACs) and SMAs actuators. A comparison between EAP, EAC, and SMA properties is shown in Table 14.11, where the actuation

14.7 Research needs and future directions

FIGURE 14.11 Artificial muscle and sensor array concept for space robots [59]. Courtesy NASA.

Table 14.11 Comparison between the properties of EAP, EAC, and SMA [60]. Property

EAP

EAC

SMA

Actuation strain Force Reaction speed Density Drive voltage (V)

.300% 0.1 40 µs to min 1 2.5 g/cm3 1 7 (ionic EAP)

0.1% 0.3% 30 40 µs to 1 s 6 8 g/cm3 50 800

,8% 200 ms to min 5 6 g/cm3 5

strain of EAPs is clearly the highest ( . 300%) in this group. Improvements in this field are fundamental to the accessibility of resilient actuation materials for practical applications in space and robotic engineering [60].

14.7.2 Processing and printing of polymers in space Additive printing is a modern concept based on accumulating materials such as concrete and polymer composites to realize quick and precise construction (see Fig. 14.12). This concept is successfully used in the automation industry, and appears to satisfy the requirements for space habitat construction [61]. Regrettably, limited works have explored the application of additive printing of polymers in environments simulating space conditions (i.e., low gravity, radiation, etc.).

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FIGURE 14.12 Representation of a 3D robotic printer on the Moon. Courtesy NASA.

14.7.3 Flexible and energy harvesting polymers With the advent rise in technological advancements, the integration of efficient energy harvesting materials into flexible and soft substrates such as polymers could lead to substantial innovations in space applications. The work of Ponnamma et al. [62] has explored the use of a triphasic filler combination of 1D titanium dioxide (TiO2) nanotubes, 2D reduced graphene oxide, and 3D strontium titanate (SrTiO3) in a semicrystalline polymer, poly(vinylidene fluoride-co-hexafluoropropylene). In this work, an affordable and simple mixing method was developed to enable the fabrication of the proposed composite (see Fig. 14.13). The outcome of this research reported the increase of the piezoelectric constant to a value of 7.52 pC/N at a 1:2 filler combination with an output of 10.5-times that of the voltage generated by a neat polymer. This work showcases the merit of utilizing flexible and energetic future polymers that could be used in space applications.

14.8 Novel polymers Novel polymers such as those with thermoelectric capabilities have high potential in applications relating to power generation. This thermoelectric-based energy generation could be used either as a power source or as an energy harvesting system. Both of these items seem to fit the requirements of space applications, and, hence, novel polymers have newly been explored. For instance, Yao et al. [63] prepared hybrid nanocomposites containing carbon nanotubes and ordered polyaniline (PANI) through an in situ polymerization reaction (see Fig. 14.14). In this

14.8 Novel polymers

FIGURE 14.13 Schematic representation of the preparation of composites [62]. Reproduced with permission from Springer. Copyright 2018.

FIGURE 14.14 TEM images for single-walled carbon nanotube (SWNT)/PANI composites with 25 wt.% SWNT. Inset of (A) is the SEM top view of the nanocable [63]. Reproduced with permission from Yao Q, Chen L, Zhang W, Liufu S, Chen X. Enhanced thermoelectric performance of single-walled carbon nanotubes/polyaniline hybrid nanocomposites. ACS Nano 4;2010:2445 51. doi:10.1021/nn1002562, American Chemical Society. Copyright 2010.

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study, the nanocomposites showed improved (higher) electrical conductivity (1.25 3 104 S/m) and Seebeck coefficient (40 µV/K), as compared to pure PANI. The developed material also achieved a power factor of 2 orders of magnitude higher than that of pure PANI. Overall, Yao et al. [63] noted that the developed procedure is effective in enhancing the thermoelectric properties of polymers.

14.9 Conclusion This chapter presents a comprehensive overview on key challenges associated with space exploration with especial emphasis toward the role of polymers as a promising material that could best suit exploration efforts. Space environments and their harsh effects on the composition and performance of polymers are discussed with reported data from the literature. This discussion is then followed by concepts for polymeric-based materials suitable for extraterrestrial construction and developing temporary and permanent habitats that could facilitate various space exploration missions. Key needs facing the use of polymers in space exploration programs, together with venues warranting research, are also identified and examined. The outcome of this chapter review can be concluded in the following points:

• The extreme environment of space poses unique conditions to polymeric

• •

materials. These effects (e.g., radiation, chain scission, and crosslinking) are to be taken into account when developing polymers for various space applications. Polymers could be used to develop superior construction materials that could be fabricated from space in situ resources. In order to realize large-scale deep exploration programs, current technological challenges such as those associated with polymer-based characterization of in situ materials, processing methods suitable for vacuum, low gravity conditions, and so forth, need to be further investigated.

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CHAPTER

Polymers in sports

15 Meena Sadashiv Laad

Professor (Physics) Department of Applied Sciences, Symbiosis Institute of Technology, Symbiosis International University, Lavale, Pune, Maharashtra, India

15.1 Introduction Sports have been an essential part of healthy living and the lifestyle of people. With the spread of education, there has been increasingly more awareness about hygiene and physical fitness among people. Physical activities protect people from several lifestyle induced diseases such as diabetes, hypertension, obesity, high/low blood pressure, and so forth. It also helps children grow physically healthy and fit. Their bones grow stronger and healthier and they develop powerful lungs and stronger cardiovascular systems. People playing any sport also bond with teammates and develop a sense of belongingness. Major global sports events are organized worldwide every year such as the Olympics and the Cricket World Cup or Men’s and Women’s Hockey World Cup, which manifest worldwide participation and increasing interest in sports activities. Sports persons keep participating in national and international sports events and compete for recognition and acknowledgment of their talent and to make their countries proud. The healthy competition in sports pushes athletes to work hard and improve upon their performances. Sports is a window to the advancements in science and technology, manifesting the achievements of a nation in the area of science and technology. Advancements in material technology play a significant role in the improvement of the performance of athletes. Modern sports starting from natural, simple equipment and activities and gradually developed into high-tech advanced materials, equipment, and venues. Traditionally, the majority of sports equipment was made of wood and metals, which were heavy, inconvenient, and not very functional. Now, there exist lightweight, strong, and durable sports equipment made with novel polymer materials. Not only sports equipment, but even sportswear made of breathable fabric for easy respiration and adjusting to body temperature has helped in improving the performance of players. In sports clothes, a nylon taffeta coated amino acid polymer is used, which is also waterproof [1]. People’s living standards are improving with the economic development currently seen. Sports experts focus on scientific training and attach a lot of value to Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00015-9 © 2020 Elsevier Inc. All rights reserved.

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the improvement and development of sports equipment. The performance of athletes depends on the physiological and psychological state of their competitors, which arises due to human aspects and the design, materials, and efficiency of the sports equipment used by the athlete. The demands of the sporting goods industry require highly functional and advanced materials that can meet the specific requirements of sports and are also cost effective at the same time. Physical activities such as walking and running do not require any special equipment, but for other sports activities, it is essential to have the correct equipment to prevent injury. Equipment for any sports activity may range from special clothing to special protective suits or apparatus. For good performance in sports, it is essential to use the right kind of equipment. All sports equipment need suitable materials to construct them, which will help in improving the performance of the users. In order to get appropriate sports equipment that fulfills all the requirements, the selection of materials is important and often the materials of choice are a good combination of metals, ceramics, polymers, and composites. These materials are used to make the required equipment by taking into consideration the specific design requirements, functionality, convenience, strength, and biomechanical necessities. For example, if the requirement for making a specific sports equipment is to have a material that shows the highest attainable stiffness with the least possible weight, then a material should be selected that has the highest specific stiffness. The design as per the requirement of that specific sport needs to be essentially defined first for making the best possible selection of the appropriate material [2]. In the past few years, there has been significant improvements in sporting performance due to advancements in material technology. More efficient sports equipment with better specifications and designs are being developed with advanced materials and novel processing methods. There is interdependence between material technology and design, and it has significantly affected the overall performance in some sports. Advancements in material technology have significantly influenced the goods and equipment used in sports over the centuries, thereby improving the performance of players significantly. With the continuous research, new, strong, and lightweight materials are synthesized and new sports and related sports activities are developed. Due to the advancement in materials, the performance of professional sports men and women as well as amateur players has tremendously improved. For example, in golf, it is now possible for amateur players to achieve distances equal to those achieved by professionals. It has become possible because the oversized driver clubs made from lightweight materials (for example, titanium), have a large “sweet spot,” which gives an amateur player a greater chance of hitting the ball accurately. Advancements in materials have also had a huge impact on the venues where sports activities are held. The application of steel and aluminum in constructing sports stadiums have allowed for larger seating capacities with better visibility at the same time. With the development in materials technology, sporting performance has increased significantly in the past two decades. Efficient, lightweight, and more

15.2 Materials used in sports

functional sports equipment with improved properties and better design are being made of new materials, thus, enhancing sporting performance [3]. Sports equipment has been evolving over time with the development of new and advance materials. Intelligent designs of sports equipment enhance the performance of athletes significantly [4].

15.2 Materials used in sports Equipment design is one of the most important aspects of material science and engineering. The continuous development of new materials has significantly affected the development of highly efficient sports equipment. Developments in material technology led to developments in sports equipment with innovative design. Significant improvements are observed in sports such as in tennis in which graphite fiber reinforced polymers are used, in golf where golf clubs are weighed with tungsten, and in vaulting poles in which glassy metal inserts are introduced. These are examples that show that the use of advanced materials help in improving the design, comfort, and functionality of sports equipment [5 7]. New materials have several advantages over traditional materials in terms of their lightweight, weight-to-strength ratio, corrosion resistance, and so forth. These new materials are stronger and stiffer than traditional materials [8]. The new materials are also found to have better torsion strain resistance and improved toughness. These properties are used in a variety of sporting equipment. A lot of research is going on globally to develop new materials to make sports equipment. A variety of materials are used in making sports equipment ranging from wood, twine, gut, and rubber to advanced materials such as advanced metal-matrix composites, ceramics, polymers, and synthetic hybrid materials [9]. Materials are everywhere within sports. Different sports require a variety of materials for different purposes, some of them occurring naturally, while others are manmade or synthetic. Natural materials are derived from plants and animals. Fabric materials such as silk and cotton are obtained from trees and plants. Different types of polymers, glass, plastics, and so forth, are manmade or synthetic materials. Plastic is the most common synthetic material, and it is produced as a byproduct of the distillation process of crude oil [10]. Fabric materials such as silk and cotton are obtained from plants and trees. Plastics, glass, and materials such as neoprene, synthetic leather, and so forth, are manmade or synthetic materials. The properties of these materials are improved by making use of appropriate reinforcing materials or fillers, for example, adding organo-modified clays into clay polymer nanocomposites produces significant increases in physical properties at a lower filler loading [11]. Many important polymeric products are being fabricated by reinforcing materials with polymers, thereby improving the mechanical and structural properties [12]. Plastic is the most common synthetic material that is derived from crude oil [13].

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Rubber is another material that frequently finds applications in sports equipment. It is highly elastic in nature, hard to tear, and is highly durable. Neoprene is another popular material used for making wet suits and can withstand different weather conditions. In 1998, polyurethanes (PUs) and engineering plastics were the most popular composite materials used for making sports goods. After that, it was fiber-reinforced materials, which together with metals were later found to be more useful materials for making sports equipment [14]. For molded sports products, the preferred polymeric material is polypropylene (PP). Another popular polymer used for making sports equipment is acrylonitrile butadiene styrene [15]. There have been significant improvements in the performance in many sports due to the applications of advanced materials in making well designed and more functional sports equipment and clothing. Sports such as vaulting, ice hockey, tennis, and golf have been considerably improved. The introduction of polymers into sports has brought overwhelming response from sportsmen due to their convenience of use, comfort, and improved consistency in comparison with traditional materials. The use of polymers in artificial surfaces that can withstand all weather conditions is worth appreciation. Polymeric materials are used in developing materials for various sports related goods such as head gear, protective jackets, gymnastics mats, running tracks, and sports surfaces, and so forth. Modern vaulting poles made of polymers have helped athletes to improve their performance due to the inherent flexibility and strength of the polymeric materials used. Artificial sports surfaces made of polymers offer various advantages over traditional surfaces made of wood or concrete including improved performance, consistency, safety, and cost. Different types of polymer materials are used in sports. Tire crumb is one of the major constituents of artificial surfaces. Advancements in material technology have tremendously affected the equipment used in sport over the centuries and a noticeable impact is observed on sporting performance. The developments in material technology and processing techniques have enabled the manufacturing of sport equipment with improved design and better functionality. To meet the requirements of sports equipment with improved efficiency and functionality, the selected materials are often a combination of metals, ceramics, polymers, and composites. Further improvements in sports performance can be made by developing the equipment used. The applications of strong, lightweight materials have also allowed new sports and activities to be developed. Many manmade materials are also much cheaper than their natural alternatives, and it is one of the reasons that they could easily replace traditional materials. Synthetic leather finds applications in sports goods like football boots, shoes, horse saddles, and so forth. Another polymer material, latex, is used in sports clothing and is also used as a sealant on waterproof garments. The goalkeepers in football, wear gloves made of rubber, neoprene, cloth, and leather or a combination of these materials. These materials provide strength, durability, and resistance to abrasion. Materials find applications everywhere in every sport, and constant research is needed in the areas of advance materials and material technology in order to

15.3 Evolution of materials used in sports from traditional

improve the performance of players. In football, different materials are used for making the uniform of the players, the ball, goals, pitch, and so forth. Each of these materials are carefully selected for their specific properties to enhance the performance, comfort, and convenience of the players. Materials have not only played an important role in sports equipment; they have also had a huge impact on sports arenas. The introduction of steel and aluminum to sports stadium construction has allowed larger capacities to be accommodated at the same time maintaining good visibility in sports arenas. In order to get the best design of sports equipment for enhanced performance and as user-friendly as possible, one has to have knowledge of a number of disciplines ranging from human anatomy to material technology. In order to make sports equipment of good design, the properties such as strength, density, toughness, ductility, fatigue resistance, modulus (damping), and cost of materials are of prime importance.

15.3 Evolution of materials used in sports from traditional to composites 15.3.1 Wood Wood is an organic material containing porous and fibrous tissues in its stem and roots. Wood is a naturally occurring composite made of cellulose fibers. These fibrous tissues show strong tension and are embedded in a lignin matrix. These tissues are also resistant to compression and are known as the secondary xylem in the stems of trees [16]. Wood was the primary material used for making sports equipment in the past. Even today, it is used in many sports goods, for example, white willow wood is used to make cricket bats, while baseball bats are made of ash wood or hickory. There are various other forms of equipment for sports and recreation such as golf clubs, skies, hockey sticks, and archery bows that were produced out of wood traditionally. With the advancement in material technology, wood has been replaced with new lightweight materials such as aluminum, titanium, or composites. Wood has several advantages such as being an environment-friendly, strong, and low-density material; it is a good insulator and, thus, an energy saver and is also low cost compared to other advanced composite materials. But there are disadvantages to using equipment made of wood too. Wood is susceptible to termites, woodworms, and infestations and it can rot. It is highly combustible and can’t be used at high temperatures.

15.3.2 Metals Metals are strong, durable, rigid, shiny, and good conductors of heat and electricity. Metals are often used for making sports equipment. One of the most

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commonly used metals is steel, which is known for its strength, durability, and flexibility. One of the major problems with metals is that they easily get corroded when they come into contact with water or other liquids. To protect them from corrosion, they are coated with layers of varnish or paint. Steel easily gets rusted if it is not coated with paint. The rusting of steel reduces its strength. Steel is used widely in sports in constructing stadiums, bike frames, hockey pucks, making tables, sinks, and so forth. Monolithic metals such as aluminum, magnesium, steel, titanium, and metalmatrix composites are used to make modern tennis rackets. Composite materials reinforced with carbon fiber are considered to be better than metals due to their high stiffness as the force imparted to the ball is high.

15.3.3 Composite materials A composite is made by the combination of two or more materials with different properties. In a composite material, one of the components is either a particulate or a fiber, while the other component is the binder of the matrix material. By proper combination of matrix and filler material, a new composite material can be synthesized whose properties can be customized as per the requirements of specific applications. Composite materials provide a lot of design flexibility and can be molded into complex shapes. They are lightweight and have good strength. Composite materials have comparatively greater specific strength, improved stiffness, better fatigue resistance, and also high corrosion resistance [17]. The first modern composite material was fiberglass, which was widely used in making a variety of sports equipment and sports goods. A fiberglass composite material consists of plastic as the matrix material and glass as the reinforcement. Highly functional composite materials are now synthesized using carbon fibers instead of glass. These materials are lighter and stronger in comparison with fiberglass, but are more expensive. Fiber-reinforced composites are a mix of two materials in which fiber is reinforced with another matrix material. In fiberglass composite materials, fibers are embedded in the matrix phase [18]. Composite materials are used in making aircraft structures and other expensive sports equipment such as golf clubs. Most sporting goods are made of complex and advanced fiber-reinforced composite materials nowadays. Composite materials offer a wider combination of properties than is available in single-component materials. The importance of fiber-reinforced composites and other advanced composite materials that are lightweight, strong, and provide a wide range of properties are well recognized and these are used in making highquality sports equipment. Most sports equipment need to make the movements of players lighter, easy, and more comfortable. Sports equipment should essentially have good usable performance and excellent mechanical properties. Fiber-reinforced composites, due to their good strength, lightweight, and durability, find applications in many forms of sports equipment. Golf clubs made of carbon fiber reinforced materials

15.4 Common polymers in sports

exhibit much superior mechanical properties than those of metal rods. Developments in composite material technologies have greatly enhanced their design degrees of freedom compared to traditional materials. Fiber-reinforced composites have various advantages in terms of their characteristic properties such as low hygroscopicity, stability in size, resistance to heat and aging, and good chemical resistance. These composites also show improved elastic modulus, tensile strength, and lesser elongation. Composite materials are used to make poles for use in pole vaulting to support athletes in jumping higher. Nowadays, carbon fibers are reinforced with E-glass and S-glass materials. Glass fiber reinforced composite materials find wide applications in sport equipment due to their tensile strength, bending capacity, impact strength, and stiffness. Thermoplastic composites can be recycled and the cost of composite materials is also low.

15.4 Common polymers in sports Materials have different mechanical, structural, thermal, and chemical properties. The different properties of materials are taken into consideration during the design of sports equipment such as mechanical strength, ductile behavior, fatigue resistance, density, hardness, modulus (damping), cost, and so forth. The design of sports equipment involves the knowledge and applications of a number of subjects in order to make sports equipment that is more efficient and user-friendly. Because of the availability of sports equipment with improved quality and safety features, many of sports have gained popularity. For example, skiing was not a popular sport, but with increased leisure, cheaper flights, and improved safety gear, it has developed into a mass sport today. For the same reasons, winter sports are attracting more and more people, and as a result, there is a growing demand and requirement for the related sports equipment. With the huge demand for sports equipment, there is a lot of research and developments going on in this area. Newer materials are being synthesized for making these forms of equipment more efficient, stronger, and more durable. The obvious choice for making these materials is polymers with their inherent properties. Plastics are used largely in the manufacturing of sports goods for winter sports. The materials used for making winter sports goods started with the use of glass fiber hulls and today a wide range of polymers are used, which include materials like rigid foam and composites reinforced with carbon and aramid fibers. Polymeric materials reinforced with glass are used for making small pleasure boats, personal jet skis as well as large vessels. Developments in polymeric materials with enhanced properties have brought a radical change in the design and construction of many sports equipment and goods. The application of polymers in sports goods and equipment has contributed immensely in making these sports popular among the masses as not only the cost

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of sports equipment is reduced greatly, but also there is improved safety, strength, and durability in these sports goods. Polymeric composites have high strength-toweight ratios, better fatigue resistance, and are lighter in weight. Polymers commonly used in sports equipment are described here.

15.4.1 Cyanoacrylate

Cyanoacrylate represents a family of strong adhesives that are used for industrial, medical, and household applications [19]. Cyanoacrylate finds applications in sports such as in archery to join fletching to arrow shafts. In archery, arrows are propelled using a bow. Archery equipment consists of bows, bow string, arrows, fletching, protective gear, release aids, and stabilizers. Cyanoacrylate is used as a strong glue for making many other equipment required for archery.

15.4.2 Vectran

Vectran is a synthetic fiber made up of a liquid crystal polymer called vectra. Vectran is an aromatic polyester. It is obtained through the polycondensation of 4-hydroxybenzoic acid and 6-hydroxynaphthalene-2-carboxylic acid. The liquid crystal polymer vectran exhibits excellent mechanical strength and is also capable of withstanding a wide range of temperatures. It also shows high chemical resistance and low moisture absorption. It exhibits high abrasion resistance [20]. Even when this fiber is loaded in up to 50% of the breaking load, there is no measurable creep shown. Vectran is a polyester consisting of aromatic rings on both monomer constituents. Its melting point is relatively high,

15.4 Common polymers in sports

276 330 . It was first used in sporting equipment as synthetic strings for badminton racquets. The use of vectran in racquets resulted in notable impact resistance and high tensile strength. Moreover, the racquet became lighter in weight with a strong head, which allowed players to give more powerful shots.

15.4.3 Gutta-percha The chemical name for gutta-percha is polyterpene. It is a trans-isomer of polyisoprene. Chemically, gutta-percha has a trans-1,4-polyisoprene structure [21]. It has a similar molecular structure to that of natural rubber. Since rubber is a cisisomer of polyisoprene, there are a lot of similarities between gutta-percha and rubber. Gutta-percha behaves more like a crystalline polymer. Its chemical composition includes gutta-percha (75% 82%), alban (14% 16%), and fluavil (4% 6%) [22]. It does have small amounts of tannin, salts, and saccharine too [19]. The trans-1,4 polymer is a highly crystalline, tough, hard, and leathery material. It is used as shielding for underwater cables and golf balls.

15.4.4 trans-1,4-Polyisoprene

trans-1,4-Polyisoprene finds applications in different types of sports equipment, for example, it is used in making golf balls and as adhesives in making various other sports goods.

15.4.5 Surlyn copolymer

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The trademark name of this ionomer is Surlyn, which is a popular polymer because of its self-healing properties. Surlyn, a copolymer, contains poly(ethylene-co-methacrylic acid). Chemically, Surlyn consists of 5.4 mol.% methacrylic acid, which is neutralized with alkali metals or zinc hydroxides [23]. Rubber balls were one of the earliest applications of polymers in sports. Copolymers from the family of high-performance ethylene consist of acid groups that are partially neutralized by adding metal salts such as zinc and sodium, while others are used in the construction of golf balls [24].

15.4.6 Polycarbonate

Thermoplastic polymers known as polycarbonates essentially contain a carbonate functional group (O (C 5 O) O) in their chemical structure and are commonly formed from acid chloride (RCOCl) precursors and bisphenol A. There is a linking between the monomers by multiple nucleophilic acyl substitutions where HCl is lost as a byproduct. The resultant polymer layers are flexible and nonbrittle; properties that allow them to be molded into shapes as a solid sheet without cracking or breaking such as canoes and kayak hulls. Polycarbonates are strong, shatter-resistant, hard materials. Some grades of polycarbonates are also found to be optically transparent. These polycarbonates are easy to work, mold, and thermoform. Because of these properties, polycarbonates are used for making protective sports equipment. They are often used in helmets for protecting bikers and riders competing in equestrian and cycling competitions. In some sports, there is a requirement for goggles, visors, or face masks to be worn by players. Polycarbonate is used in making sunglasses and protective visors that not only provide clear visibility, but also offer shatter resistance. Swimmers use goggles made of polycarbonate materials. Polycarbonates are also used in making lenses [25]. These lenses consist of two layers, one 3 mm thick layer and another layer of 0.5 mm on top of the first. The outer layer is disposable and is

15.4 Common polymers in sports

used to protect the base layer so that it does not crack under environmental stress or from stress concentrations. In safety equipment such as crash helmets used in both cycling and motorcycling, the outermost polycarbonate “shell” is able to absorb some of the impact energy by deforming and flexing slightly. If the shell is too rigid it would either be too brittle and shatter or transfer much of the impact to the skull of the wearer. A slightly malleable (pliable) polycarbonate shell absorbs much of the initial impact because of its soft, foam, expanded polystyrene (EPS) interior and it reduces the impact of collision.

15.4.7 Epoxy resin Epoxy resins are extremely strong polymers that can withstand high temperatures. They are formed by the nucleophilic attack of phenols (or deprotonated phenols) on epoxides, leading to the opening of their strained three-membered ring. This forms a prepolymer that can be reacted with nucleophiles such as triamines (e.g., H2NCH2CH2NHCH2CH2NH2); each of the three amine groups can react with an epoxide on a prepolymer. As the three amine groups can each react with different molecules of the prepolymer, this forms a densely cross-linked structure that is extremely strong. When a carbon fiber is mixed with an epoxy resin it adds additional strength.

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15.4.8 Polyurethane

PUs are polymers having a molecular backbone consisting of carbamate groups ( NHCO2). PUs are obtained from the reaction between diisocyanate and polyol groups; its mechanical properties can be altered or modified by making changes to the synthesizing conditions [26]. PUs find wide applications in insulation in buildings, surface coatings, adhesives, solid plastics, and athletic apparel due to their versatility. In sports, PUs are frequently used in shoes for running or other athletic activities owing to their flexibility [27]. PUs also find applications in a range of common sports equipment. For example, soccer balls, judo mats, and binders on running tracks are made of PU. These are also used in producing a range of sports flooring and pour-in-place track surfaces. There are a variety of shock pads made of PU available today that are capable of maintaining a flat surface with high dimensional stability [28]. Crumb rubber shock pads have been in use for protection. These contain a crumb rubber sheet joint together with PU. Rubber is also used in asphalt layers for enhancing their impact absorbing capacity. Skiing has drastically been improved with the use of polymers, which made the take-off during skiing smooth with enhanced comfort, safety, and reduced cost. The first change in making equipment for skiing was to encase a wood core in a polyester reinforced with glass fiber. Later PU foam cores were developed. Since then, there have been rapid changes in the materials used for the construction of skis including honeycomb cores, highly advanced and functional composite materials, a movable layer of graphite impregnated polyethylene (PE), and so forth. The sports equipment used in skiing needs to be extremely hard and strong and should have the ability to maintain their properties even at low temperatures. PU matches well with these requirements and is used for constructing different parts of boots used for skiing. Therefore PU is a preferred material for making skiing boots. Similarly, surfboards are produced with a PU foam core on which there is a polyester layer reinforced with glass. The core blanks are carved into the appropriate shapes and then laminated.

15.4 Common polymers in sports

PU is also used for constructing sports surfaces and athletic running tracks. In a bound rubber crumb system, rubber shred or granules are mixed with a PU binder. The granules of rubber are held together by the PU, which acts as a binding agent coated with a high traction coating. PU is also used in constructing polymeric tracks. The most common materials used for making these tracks are cast rubber, natural rubber, or polychloroprene latex with a dressing of rubber chips or granules. They show excellent wear resistance and are impermeable. PU foams are widely used for thermal insulation purposes in buildings to save energy due to their excellent thermal insulation properties and low flammability [29].

15.4.9 Acrylonitrile butadiene styrene

The chemical formula for acrylonitrile butadiene styrene (ABS) is ((C8H8)x
(C4H6)y
(C3H3N)z). ABS, a thermoplastic polymer, exists in a variety of forms and has a wide range of applications. It has a glass transition temperature of approximately 105 [30]. ABS finds a lot of applications in sports equipment. For making a lighter and stiffer aircraft, ABS is the preferred choice. One of its most important marine applications is in making the fabric for sails. Injection molded ABS is also used in making helmets for baseball that are crack resistant. In the case of tennis balls, they require specialized production techniques. The bouncing characteristics of pressurized tennis balls directly depend on the pressure inside the ball, which is approximately 100 kPa above atmospheric pressure. As air permeates out, there is a drop in pressure and the playing characteristics of the tennis balls change. To reduce this loss, it is filled with a gas that permeates at a much slower rate than air. An elastomer having low permeability is also used for this purpose. Specifically designed containers are manufactured from ABS to maintain the pressure inside the gas-filled balls.

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15.4.10 Polyvinyl chloride

Polyvinyl chloride (PVC) is a synthetic resin made from the polymerization of vinyl chloride. PVC is a commonly used polymer that finds applications in a variety of products due to its excellent properties and low cost. It is used in several products in day to day use such as pipes, cable insulation, roofing sheets, packaging foils, floor coverings, bottles, medical products, and so forth. Its degradation starts at comparatively low temperatures (B300 C) in the presence of light to release hydrogen chloride [31]. Flexible PVC is used in mouth guards for basketball players due to its high durability. In the design of advanced racing boats, epoxy skins reinforced with carbon fiber and a core made of PVC are used. Traditional leather footballs become heavier when wet and are difficult to maintain. Footballs made from PVC, ethylene vinyl acetate, or PU show much better playing characteristics, are more consistent, and are of better quality than a traditional leather football. To prevent the leather used in football from absorbing water, it is coated with a layer of an elastomer.

15.4.11 Poly(ethylene-vinyl acetate) Poly(ethylene-vinyl acetate) (PEVA) is a copolymer of ethylene and vinyl acetate. Usually the weight percent of vinyl acetate varies from 10% to 40%, with the remaining consisting of ethylene. The chemical formula of PEVA is (C2H4)n(C4H6O2)m.

15.4 Common polymers in sports

In products made of PVC, volatile organic compounds readily evaporate and gases enter the surroundings and cause serious health hazards for humans and animals. PEVA plastic is considered as a popular substitute for PVC since it does not contain chlorine [28]. The main constituent of many different forms of protection equipment is PEVA, which is prepared by the polymerization of ethene with vinyl acetate.

PEVA is also used for making shock-absorbers in the shoe soles and hockey pads. Ethylene vinyl acetate produces lighter midsoles of low density of up to 0.25 g/cm3.

15.4.12 Carbon fiber reinforced polymer In both high performance and noncompetitive canoeing, carbon fiber reinforced polymer (CFRP) is the optimal construction material for canoe paddles and oars. Poly(acrylonitrile) (PAN) is the main polymeric component of carbon fiber. PAN fibers are spun before being heated in the air, fusing the polymer into a series of pyridine rings (fully unsaturated six-membered rings with a nitrogen atom) at approximately 700 .

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The slow heating of the polymer to 600 expels hydrogen gas and fuses adjacent polymer chains together. At conditions up to 1300 , multiple chains are fused together, eliminating more hydrogen as well as nitrogen, leaving a pure sheet of interlinked carbon atoms. The high temperatures and pressures involved in this process align the microscopic crystals, which the carbon sheets form all along the same axis as the fiber. As such, the overall structure is highly resistant to bending or stretching pressures.

15.4.13 Soft and hard polyethene PE or polythene is the most common plastic. High density as well as low-density polymers are the most important thermoplastic polymers used in our daily lives for various purposes. Polythene is highly chemically stable, highly flexible, easy to process, shows good thermal and electrical insulating properties, and has low

15.4 Common polymers in sports

toxicity [29]. Its primary use is in packaging. Its chemical formula is (C2H4). PE is a thermoplastic polymer with variable crystalline structure that has a wide range of applications and is the most widely produced plastic in the world. High density polyethylene (HDPE) is much more crystalline and has a much higher density. Low-density polyethene (LDPE) is widely used in plastic packaging materials. In sports, it is also used in making protection equipment such as shoulder pads. In HDPE, the polymer chains do not have any branching; HDPE has a high density. Since the polymer chains are arranged close together, intermolecular forces are formed between the chains, which make the material stronger. When a plastic bag is stretched, it increases in length and decreases in width. If the bag is stretched beyond a certain limit, it leads to the breakage of the intermolecular forces and the straightening out of the polymeric chains. However, the covalent bonds in the polymer do not break till the plastic rips. In softer foam, there is branching in the polymer chains, which makes the material more flexible. LDPE has maximal branching in the polymer chains, making it highly flexible. Though it is the weakest polymer, LDPE, when used in shoulder pads, makes the pads lighter in weight, which makes the movements of players easier.

In synthetic turf, the fibers used are a combination of PE and PP, which after treatment are arranged in a specific pattern, very similar to real grass. There are many advantages of synthetic turf such as all-weather capability, easy maintenance, the absence of bumps, and so forth.

15.4.14 Polymeric foams Polymeric foams find applications in mats used for high jump, martial arts, gymnastics, and pole vault. The thickness and density of the foams used vary to suit the specific requirements of the different sports. For example, a foam of around 750 mm in thickness is used for a pole vault landing area, while it is 20 mm for a basic exercise mat. The composition of foam is generally polyether or reconstructed PU, but elementary thin mats are sometimes made of PE or ethylene

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vinyl acetate. The main use of mats is to absorb impact and protect athletes from injury, but their resilience and stiffness play equally important roles. In sports like judo, a big deflection hampering movement could prove to be more dangerous for an athlete than the effect caused due to falling. To prevent from injuries, a composite construction of mats that uses a number of layers is used to offer more safety and the desired characteristics. These mats are properly covered with PVC-coated nylon and have a nonslipping rubber base. These mats are potentially a significant fire hazard and are subjected to rigorous flammability testing conditions in some countries.

15.4.15 Neoprene Neoprene (also polychloroprene or pc-rubber) represents a group of synthetic rubbers that are derived from the polymerization of chloroprene. Neoprene is highly chemically stable and is flexible over a wide range of temperatures. Neoprene is available either in solid rubber form or in latex form. Both forms find wide range applications in various products [30]. Neoprene is synthesized by the free-radical polymerization of chloroprene. For commercial production, a free-radical emulsion polymerization method is used. Polymerization is initiated using potassium persulfate. Bifunctional nucleophiles, metal oxides (e.g., zinc oxide), and thioureas are used for crosslinking individual polymer strands [31].

Hand protection is extremely important in sports like rock climbing, sailing, white-water rafting, and so forth, to protect the skin from cuts, bruises, or burns. Hand gloves are important to protect the skin, to retain flexibility and motor control, and to provide the necessary grasp in wet or icy conditions. Protective gloves for hand safety are made by combining neoprene for chemical resistance, reduced weight, enhanced flexibility, and breathability, and silicones for water and heat resistances. Footballs are made from synthetic leather, rubber, and neoprene.

15.4.16 Polydimethylsiloxane Polydimethylsiloxane (PDMS) is from a family of polymeric organ silicon compounds, commonly known as silicones [32]. PDMS is a popularly used siliconbased organic polymer, particularly known for its properties related to rheology (or flow). PDMS is an optically transparent, inert, nontoxic, and inflammable material. PDMS is obtained from the hydrolysis of dimethyldichlorosilane.

15.4 Common polymers in sports

These materials significantly reduce the number of impacts and injuries that players might receive.

15.4.17 Nylon Nylon is a general name for a group of synthetic polymers that are derived from aliphatic or semiaromatic polyamides. It is a thermoplastic polymer that is silky in nature and is melt-processed into fibers, films, or other shapes. The most common polymers used in synthetic grass for sports grounds are nylon, polyolefins, or polyester extruded as monofilaments.

15.4.18 Polyamides

A macromolecule of polyamide consists of repeating units linked by amide bonds [33]. Polyamides can occur naturally as well as artificially. For example, proteins in wool and silk are naturally occurring polyamides. Artificial polyamides are synthesized by a step growth polymerization method or a solid phase synthesis method. Materials like nylons, aramids, and sodium poly(aspirate) are obtained by this method. Artificial polyamides are highly durable and extremely strong materials and are frequently used in textiles, automobiles, carpets, and sportswear applications.

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15.4.19 Polyolefins Macromolecules of polyolefins can be produced by the polymerization of olefin monomer units. PP and PE are the two most popular polyolefins used for constructing modern day stadium turfs. Traditionally, the floors used in sports halls were made of wood or concrete. With rising awareness and interest in sports activities, there has been a sharp increase in the requirement of sports halls in educational institutes, local community centers, and so forth. People are more concerned about the quality of sports equipment and facilities. Traditional floors have some disadvantages such as difficult installation, high cost, noise generation and low friction, and unwanted vibrations that affect nonparticipants. Polymers are used with wooden floors as foam pads to offer a certain extent of spring at low cost. Flooring that is normally laid on battens is supported by foaming pads. These types of structures are prefabricated to make installation less complicated. Polymers in the form of sheets or carpets are used as an extra top layer on a wooden floor for further improving several properties of the floor though this increases the cost. There are many polymers such as rubber, PVC, and nylon or PP that are available, which, in the form of sheets or carpets, can be laid on any basic floor to reduce the impact and provide specific properties and good durability.

15.5 Polymers in winter sports Snowboarding, skiing, hockey, and so forth, are exciting winter sports and any of these sports could not have existed in their present forms without the application of polymer materials. PU foam plastic cores are used to make strong and lightweight modern snowboards. These cores are then covered with highly advanced composite plastic sheets such as glass-reinforced plastics or strong plastic fibers that are tougher and more durable. The base of these boards is made with slightly porous PE plastic so as to absorb wax for a smooth glide on the snow. Boots worn by snowboarders are made of plastic such as nylon because of its strength, durability, and lighter weight. These boots provide thermal insulation, are sturdy, and keep the feet warm, and also reduce the energy needed to hike to the lift. These boots are tightly held to the board through plastic bindings. The safety and comfort of snowboarders depend on the materials used for making the boots, gloves, and other safety gear required. The protection gear used such as knee pads, wrist guards, goggles, and so forth, are made of different types of plastics that provide more safety, comfort, and convenience. Specifically designed goggles are made of polycarbonate plastic lenses that are impact resistant and offer protection to the eyes from wind, dust, snow, UV rays, and debris.

15.5 Polymers in winter sports

15.5.1 Skiing There has been a lot of changes in the materials used for making skis. Traditionally skis were made using bones, then wood and metals were used. Modern skis are made from highly advanced composite materials, mainly polymer composites. The cores of skis are made from a lightweight yet extremely strong polymer PU plastic. This core is then covered with glass-reinforced plastic to offer additional strength. There is a coating of slick PE plastic on the bottom of skis. Till the end of the 19th century, skis made of wood were used. Later, laminated wooden skis came into production that contained a hickory bottom layer and a top layer of pine or ash. Additionally, the edges of skis were made out of steel to make them even stronger. In 1950, the first ski was made with aluminum and wood. It also had a phenolic running surface and steel edges. Around 1955, Kofler in Austria, used PE as a ski base, soon it was followed by Inter Montana in Switzerland. In 1960, yet another new significant change took place in ski manufacturing when Franz Kneissel used epoxy resin and glass fiber. These glass fiber based skis soon became popular and replaced wooden skis. By 1976, at the Nordic Ski World Championships in Falun, Sweden, the majority of the competitors used skis made out of fiberglass. Ultrahigh molecular weight polyethylene (UHMW PE) is the most commonly used polymer as a ski base material owing to its high wear resistance, high hydrophobicity, and affordable cost [34]. Skiing and snowboarding are directly related to the safety and performance of athletes and the equipment used in these sports are complex. Skis made of wood are comparatively lighter and cheaper, but are prone to be affected by dampness. Skis made of fiber composites are suitable for any type of weather condition and also require much less maintenance. The core material of skis is made of wood, PU, PVC, and so forth. The upper core layer of skis is made of carbon fiber, which strengthens the flexion degree of skis; glass fiber in the upper layer joins the panel and the core layer, and improves the hardness and durability of skis. Due to the development and availability of suitable materials, there have been improvements in designs as well. The new binding configuration includes the toes and the heel of the boots, both fixed onto the ski. Due to developments in material technology, skiing today is a popular winter sport. The first plastic ski boots were made in the 1960s, which were more waterresistant, comfortable, and stronger; these contained plastic fabric liners and foam providing cushion and insulation inside the hard-plastic shell. With the improved design and highly durable, stronger, and water-resistant materials, athletes today are able to dramatically increase their speed and control. There are safety gears used by skiers that are also made from polymeric materials. Ski bindings are made from strong plastic reinforced with carbon fiber. These are designed to release from a skier’s foot laterally during a fall, thereby reducing the possibility of painful ligament injuries. Proper care needs to be taken so as to avoid ligament injuries, which could be so serious that they might require immediate surgery.

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The binding used to attach the boot to the ski is an important part of skis. It is difficult to maintain the boot safely bearing all the stresses of skiing throughout and release when the stress reaches a danger point. Several materials have been used for making the bindings such as aluminum, fiber-reinforced nylon, and so forth. But now, plastic has replaced aluminum and other metals. Hybrid fiber reinforced materials are used for making top-range ski pole tubes. Polymers are also used in various sledges and other snow vehicles. With advancements in material technology, new polymer materials are available and are being used in stronger, more durable, tougher sports equipment, protection gear, and sports clothing.

15.5.2 Ice hockey Modern hockey sticks made of glass- and carbon-reinforced composite plastics have replaced traditional wooden hockey sticks. In earlier times, leather was used to make hockey skates, but today a wide range of materials are available such as plastics that are more functional and durable. Skates made of plastics not only provide cushion and comfort to the feet, but also provide strong support to the ankle for enhanced steadiness and swiftness. In a hockey rink, safety glasses are used that are made of transparent acrylic plastic and are more impact resistive than glass. Protection gear such as helmets, knee guards, and so forth, are essential for players to protect themselves from injuries. The outer plastic shell of hockey helmets is made of vinyl nitrile, which is crack resistant, and there are various plastic foam liners inside it to absorb impact and minimize the effect of force. Usually, these helmets are fitted with a visor made of polycarbonate plastic, which is transparent and offers resistance to impact. Other protection gear such as mouth guards, neck guards, gloves, elbow pads, shoulder pads, athletic protectors, padded shorts, and shin guards are all made of polymer materials. In hockey skates and other sports shoes, comfortable yet sturdy foot beds are required, which are inserted into the skates or in shoes. These are made of a combination of soft foam and rigid plastics. These soft foam insertions protect a player’s knees from bending inward while skating, thereby reducing knee pain and foot cramps. They also provide extra power and control on the ice. Polymeric materials normally used in ski and snowboards include rubber, glass-reinforced epoxy structural components, aramid, carbon and graphite fibers, PU or thermoset cellular core materials, and so forth.

15.6 Polymeric sports surfaces Various materials can be used to modify the properties of athletic tracks. Tracks can be constructed to be porous so as to avoid the necessity for a slope for drainage. A polymeric sport surface consists of rubber granules held together by a

15.7 Polymers in sports protection equipment

binding agent of PU and with a thin spray of a high traction, colored top-coat. This polymer composite is laid onto a solid base, mainly tarmac and retained by kerbing. Most athletic running tracks are made of polymeric materials. Many sports such as badminton, tennis, netball, and basketball, make use of polymer composites to make the playing areas and courts. These polymeric surfaces come with different specifications to meet different performance standards. Runners and athletes like synthetic surfaces as these surfaces provide good shock absorption, which allows them to maintain a good speed unlike the loss of speed experienced on soft natural surfaces. This characteristic property of synthetic tracks that allows athletes to maintain good speed has earned synthetic tracks the reputation for being fast tracks. Synthetic surfaces also provide good traction, even in inclement weather, unlike natural surfaces, which change depending on the weather. It helps in maintaining training and competitions more consistent, allowing athletes to continue uninterrupted training for longer durations of time irrespective of the change in seasons. Synthetic tracks offer a uniform and even surface. These tracks can be used for indoor as well as outdoor sports. These tracks offer an even surface for athletes, players, stadium designers, and event organizers to make use of. Polymeric materials are also preferred for their characteristic properties such as durability, resistance to weathering, and relatively low maintenance requirements. Athletics tracks essentially need a higher proportion of bound crumb to make the tracks more durable and more resistive to spikes. The selection of granule plays a significant role because even a small change in size, surface texture, or shape can significantly affect the performance, strength, and durability of these running tracks. Natural tracks require a lot of maintenance to preserve good running surfaces. Also, their properties change with changing weather conditions. A variety of polymers are used for constructing tracks such as styrene butadiene rubber, polychloroprene, PVC, and so forth. Prefabricated sheets of different types of polymers are used for indoor installations where an uneven floor may cause lesser problems.

15.7 Polymers in sports protection equipment Athletes, runners, and players frequently suffer from injuries. The competitive nature of athletes, their desire to perform better, and the fast pace of certain sports sometimes lead to rash challenges, fractures, and muscle strains. Thus it becomes necessary to use protective gear in sports for safety reasons. There has been a significant advancement in the protective equipment used in sports. The protective gear used against balls, bats, sticks, and opposing players have gone through major changes. Traditional wooden and cotton plates have been replaced by

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modern advanced foams and high-quality polymers that provide more comfort, convenience, and safety.

15.7.1 Protection for the mouth The neck, face, and mouth are some of the most delicate and vital parts of the human body. Gum and mouth guards prevent against concussions and tongue injuries, which might occur from powerful impacts to the jaw. “Boil and bite” type mouth guards are the most commonly available mouth guards. These are molded around the teeth of the user by heating and then form a strongly resistant solid when cooled. The main chemical constituent in these mouth guards is PEVA. PEVA is also used in various other sports applications such as in making shock-absorbers, hockey pads, the soles of shoes, and so forth. Other mouth guards available today are made of alginic acid, which is a gum-like polymer. It is used to make a tooth mold and then forms a gum shield that properly fits the user. These mouth guards are more expensive; though they offer more protection than the “boil and bite” type of mouth guards.

15.7.2 Protection for the head The human brain is a highly delicate and complex organ. Even a small movement of the brain can cause severe damage by straining the nerve cells and other parts within the brain. It can also permanently damage the nerve endings, which might result in memory loss or reduced control over motor activities. Helmets are used as protective gear to protect from a linear impact and skull fracture. The construction of helmets includes an inner foam pad, normally made of PU, and a hardplastic outer layer, which is normally made of a composite or polycarbonate, which together reduce the effect of force from a direct impact considerably. On an impact, the polycarbonate flexes to absorb a major part of the force, and as a result, much less impact is imparted to the head.

15.7 Polymers in sports protection equipment

However, a few impacts are direct and cause serious damage to the skull and the brain. The major cause of head injuries is due to rotational impacts that hit the skull and also twists it, straining the brain considerably. In order to minimize the effect of these additional forces, a multidirectional impact system is being used for testing. A plastic layer is placed below the helmet pad and the helmet is kept afloat above the plastic layer. It helps in absorbing some of the impact from the force of a nonlinear rotation. This system reduces an impact to the head by 15% 55%.

15.7.3 Protection for the shoulders Shoulder pads are used as protective gear to safe guard the shoulders. The material used for making shoulder pads is a combination of soft and hard shells of PE foam. The hard shell is made of HDPE. As it does not have branching, it is very dense. The foam is comparatively softer as there is lot of branching in its polymer chains. The branching makes the material more flexible. LDPE shows the maximum amount of flexibility as the polymer chains have maximum branching. Due to its low density, the shoulder pads are lightweight and comfortable for the players for shoulder movements.

15.7.4 Protection for the hands Hand gloves are used for protecting the hands from cuts, blisters, or burning sensation while participating in adventurous sports like rock climbing, sailing, whitewater rafting, and so forth. Gloves used for hand protection should retain the flexibility and motor control required to provide a strong grip in wet or icy conditions at the same time. Gloves used for hand protection are made of neoprene and silicones combinations. Neoprene offers chemical resistance, breathability, and flexibility, whereas silicones offer resistance from moisture and heat.

PDMS is chemically synthesized by the hydrolysis of dimethyldichlorosilane. It includes a chain of nucleophilic substitution reactions in which strong Si O bonds are formed. By using different chlorosilanes, the methyl groups on the silicon atoms of silicone can be replaced. It gives a wider range of structures, and crosslinking to form a sturdy, overlapping silicone polymer chains.

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15.8 Polymers in tennis A variety of polymers are used in tennis. All professional tennis rackets were made of wood until 1965. A French player, Rene Lacoste, patented a steel tennis racket in 1965, and in 1968, the first aluminum rackets came onto the market. Slowly metal tennis rackets started gaining popularity. The use of metals helped in making changes in the designs of rackets such as making rackets with a broader head. When the head in wooden rackets was made too broadly, then it resulted in increased tension and such a racket would not play well. But as the metal frames had greater strength than the wooden frames, metal frames could accommodate greater string tension. Polymeric materials are lighter in weight and often stronger per gram in comparison with steel, and can be relatively cheap to process. Some polymers also show a shape-memory property. One of the simplest requirements for tennis string is a minimum tensile strength. The materials used in making tennis strings are generally polymers, although there are some types of string with titanium in them. Primarily, there are four polymeric materials used for making the strings of tennis rackets, namely nylon, polyester, Kevlar, and natural gut.

15.8.1 Nylon string Nylon is the most commonly used string material. It is also called “synthetic gut.” It comes in a variety of constructions and shapes and is stiffer than natural gut, but softer in comparison with polyester or Kevlar.

15.8.2 Polyester string Polyester string offers improved spin for players with long and fast swing speeds. It is often used in hybrids as a stiffer, more durable alternative to nylon or natural gut.

15.9 Polymers in athletics and gymnastics

15.8.3 Kevlar string Kevlar string is a stiff and durable string. It is generally used in hybrid string. Hybrid strings are mixtures of two types of string; one for the mains and the another for the crosses.

15.8.4 Natural gut string Natural gut string is the softest, but most expensive string material available on the market. Natural gut offers good tension maintenance. Modern tennis rackets available today are made of polymer composite materials that are stronger and more durable. Carbon fiber composite materials are effective shock absorbers and also provide design flexibility. In comparison with other materials, carbon fiber used in tennis rackets has many advantages. Tennis rackets of bigger size can be made in comparison to those made of wood of the same weight. The area of the racket can be increased significantly and also the tension of the cable can be increased by 20% 45%. The vibration damping performance of carbon fiber composites is outstanding. Tennis balls are an equally important part of the game. In 1870, tennis was first played with balls made of leather or cloth stuffed with rags or horse hair. Tennis balls made from rubber were first made in India and soon became the standard for lawn tennis games. Later, flannel coverings were added to rubber tennis balls to enhance their durability. To make tennis balls bouncier, pressurized air was filled in the center of the rubber core. To achieve a proper bouncing height, a very specific amount of air is filled in the core of tennis balls. Modern tennis balls consist of a hollow rubber core covered in a wool or nylon shell, called the nap. These balls bounce to the required height due to the pressurized air filled inside the rubber core. If the air filled inside the core does not have the appropriate pressure then the balls make a thud when they hit the floor. Such balls are called dead balls.

15.9 Polymers in athletics and gymnastics The most commonly used materials for making mats for gymnastics, high jump, martial arts, and pole vaulting are polymeric foams. Their thickness and foam density can be varied as per the requirements of its use. When carbon fibers are added into polymeric foams, the mechanical properties of vaulting poles are enhanced, but their weight is reduced. The bending stiffness and other mechanical properties of vaulting poles depend on the number and arrangement of the fibers. While designing the vaulting pole, the ability and physical characteristics of individual athletes are of prime consideration. Poles are evaluated by “weight” and athletes select a suitable pole according to their ability [35].

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The platforms used for pole vaulting are also made of rubbers that are corrosion resistant. These platforms are used to keep the pole vaulting and high jump landing systems at a certain height above the ground. It also helps in reducing the possibility of vinyl deterioration due to contact with water and debris. By nature, polymers resist the tendency of pit migration over the course of numerous landings. Platforms that are made up of sections are not only easy to carry, but also convenient to store. These can also be quickly assembled and disassembled as and when required. With the advancements in material technology, a new category of polymers called shape memory polymers (SMPs) has been developed. These shape memory alloys have the ability to change shape at different temperatures. With shape memory alloys, many new developments are possible such as bendable mobile phones, airplanes with wings that can change shape during flight, rockets that can open up into space stations, and so forth. Mobile phones are popularly used by athletes and players for making use of various fitness apps and also for time keeping. One of the most important applications of smart polymeric materials is in making flexible smart phones. Smart phones made of SMPs would change shape as per the requirement of the user. For example, while running, a mobile phone could be wrapped into a wrist band and later it can attain its normal shape according to changes in temperature. The right mix of carbon fibers, foams, membranes, films, or composites need to be used for various applications of SMPs to get the desired material stiffness, tension retention, hardness, and other mechanical properties. Smart mobile phones could be made limp and then change to floppy and then stiff with changes in temperature.

15.10 Polymers in golf Most golf balls available on the market today can be classified into two general categories, namely solid and wound. Solid golf balls consist of one-piece, twopiece, and multilayer golf balls. One-piece golf balls can be constructed easily. They are not expensive either. But they offer limited playing characteristics. Two-piece golf balls consist of a solid core made of a polybutadiene and a cover. These are highly durable and offer good distance. Multilayer golf balls comprise of a solid core and a cover, either of which may be made of one single layer or more layers. Multilayered balls offer a large range of playing characteristics, though they are more expensive. In wound-type golf balls, the center is filled with a fluid, which is surrounded by an elastomeric material and a cover. These balls offer good spin and playing characteristics and, therefore, are preferred over other balls by golf players. The construction of wound golf balls is difficult and they are expensive. The properties of these balls can be altered by changing the polymeric compositions or the physical construction of their various parts such as centers, cores,

15.11 Polymers in pole vaulting

intermediate layers, and covers. It is not easy to find an appropriate combination of materials used for the core and the layers and construct an ideal golf ball suitable for predetermined performance criteria. The materials used for making golf balls consist of at least one polymeric material and one healing agent that could improve impact durability. Polymers are macromolecules obtained by linking a large number of monomers together. With prolonged use and repetitive impact, golf balls made of different polymers start developing microcracks. One component of polymeric golf balls can be repaired by heating and cooling of the polymer backbone. The repaired plastic regains the same strength as that of the undamaged polymeric material. Golf balls can be constructed in different ways. For example, the core of a golf ball can consist of a solid core surrounded by a cover layer. The core may contain a single layer or it could be multilayered. The center of the core can be a solid- or a liquid-filled sphere surrounded by an outer core layer. Similarly, the cover layer may also have multiple layers. The core can also have a surrounding wound layer made of a polymer material, which can be a self-healing polymer as well. The base material of a golf ball can be made of a thermoplastic, a thermoset such as PU, PU-urea, polyurea-urethane, polyurea, or a cross-linked polybutadiene. Microencapsulated healing agents behave as adhesives that fix the microcracks formed in a composite material. When a microcrack is formed in the base material, it spreads throughout the material. As a result, the crack ruptures the microcapsules and releases the healing agent. When this healing agent flows through the crack and comes into contact with the catalyst, then the polymerization process begins [36].

15.11 Polymers in pole vaulting The use of polymers in pole vaulting has significantly improved the performance in the sport. Carbon-reinforced polymers are used in making vaulting poles, which gives more strength and reduces the weight of the pole. The rules covering this sport allow the vaulting pole to be constructed of any material and of any desired length and diameter. Vaulting poles are basically hollow and have varying thicknesses that change along the length. The performance of an athlete in this sport depends upon their ability in terms of physical fitness, skills, and techniques used, and also on the quality of the pole, whose characteristics depend on the type of materials it is made of. In 1960, the materials used for making vaulting poles were bamboo or wood. Soon these materials were replaced by glass-reinforced polymer (GFRP) composites. These lightweight GFRP poles supported athletes to have a faster run-up and greater take-off speed, thereby providing more kinetic energy to convert into potential energy and, hence, greater attainable height. It helped in improving the performance of athletes. The failure stress of vaulting poles made

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of GFRP is also quite high in comparison with that of bamboo. It provides more bending to the poles under the load of an athlete, thereby storing more elastic strain energy that can be released when the pole straightens, which results in greater energy efficiency. The flexibility of GFRP materials also allowed athletes to change their technique from a style in which the body remained almost upright while vaulting to another where an athlete could go over the bar keeping their feet upward. Modern vaulting poles are synthesized with GFRPs, with or without CFRP composite reinforcement. Reinforced carbon fiber helps in maintaining the mechanical properties of poles, and also reduces the weight. The mechanical properties of vaulting poles can be altered by changing the number and arrangement of the carbon fibers. Vaulting poles can also be specifically designed for individual athletes taking into consideration their ability and physical attributes. The rating of poles is by weight, which helps athletes to choose an appropriate pole [35]. The platforms used for pole vaulting are also made of rubber, which is corrosion resistant.

15.12 Polymers in water sports With the advent of new efficient polymer composite materials and advanced processing techniques, there has been a significant change in the construction and designing of boats and other equipment used in water sports. This has resulted in an increased strength-to-weight ratio and a reduction in cost of sports equipment and sports goods used in water sports. The material used initially for making equipment for water sports was glass fiber and today a wide variety of polymers are used such as materials reinforced with polycarbon, rigid foams, and so forth. Glass-reinforced materials are used mostly for making boats and large vessels. GFRPs are also used in the construction of small leisure boats and in private jet skis. Advanced composite materials exhibit good fatigue resistance, which can be further improved with sandwich constructions. Since polymer composites show good strength-to-weight ratio, it helps in making equipment lighter in weight as well. Advanced racing boats are constructed using epoxy skins reinforced with carbon and a PVC foam core. This kind of sandwich construction gives a high strength to boats, and can be made by using a foam core between two glass-reinforced panels. In one such design, a cellular core, a thermoformed skin, and a glass fiber reinforced layer were used [37]. Boats can be rotomolded and made at a fast rate using more conventional processes [38]. According to a research study report, a pedal boat made of plastic reinforced with glass was not found commercially viable and, thus, the material was replaced by rotomolded PE to make it more economically and commercially viable [39].

15.13 Polymers in motor sports

Canoes are an important part of water sports goods. GFRP canoes were common and easy to make. Many changes were made using carbon and aramid as reinforcement materials to make them even stronger and water resistant [40]. Rotational molding from HDPE is faster, more economic, and is used for making recreational canoes. Kayaks are constructed using recycled HDPE by thermoforming and rotomolding [41]. Elastomer-coated polyester or nylon fabric is used in inflatable boats. The best rubber for the outer coating for durability is generally taken as Hypalon, while polychloroprene is the best suited material for making the inner layer. PU, butyl rubber, and PVC are also used for making the inner layer. Fabric materials used for sails are also an important marine application of polymers. Nylon and polyester replaced cotton almost 50 years ago. Kevlar came into existence nearly 25 years ago. The characteristic properties of polyester such as stiffness and strength, which are much higher than the values for cotton, made it popular in the construction of sails. For high-performance applications, Kevlar exhibits 1000% increase in modulus and a 300% increase in strength. The new composite materials that have been developed, namely carbon fiber and polybenzoxazole, which have shown considerable strength and high stiffness, are fast replacing other materials. But the cost of these new materials is high. Another promising material is PE naphthalate, which is as durable as a woven material and also shows good performance, matching with that of a laminate [42]. Rubbers and plastics are extremely popular polymers that find applications in many marine components such as the construction of seats and making attractively designed trim on oars and rowlocks. In boat engines, nylon reinforced with glass is used. Surfboards are manufactured in large volumes every year using PU foam cores that are sheathed with GFRP. The core blanks are carved in appropriate shapes and then laminated. Best quality blanks are produced using methylene diphenyl isocyanate instead of toluene diisocyanate [43]. Polymers are also used in sportswear. The latest development in sportswear is based on wind surfing technology, which favors composite skins over EPS [44]. Elastomers are also used in making swimming caps, fins, and masks. Thermoplastic elastomers are gaining popularity for use in twin bladed fins [45]. Polymers also find applications in wet suits and diving gear. To stay afloat in water, swimmers make use of buoyancy aids, which are also made from polymeric materials.

15.13 Polymers in motor sports The majority of motor vehicles are used for recreational purposes and also for motor sports. People participate in different forms of motor sports, which vary from trials (“mud plugging”) to Formula 1 Grand Prix. The developments in the

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design of racing cars in turn influence the design of everyday cars that normal people use. There are a lot of applications of polymers in different parts of a vehicle such as in tires, seat covers, coatings, and so forth. The tires used in motor sports are almost like those used in passenger cars, but their constructions and the materials used are highly specialized to give better performance. Rubbers are used for constructing a wide range of motor vehicle components, which include seals, bushes, mountings, and so forth. Glass fiber reinforced body shells also find applications in small-volume specialist sports cars. But nowadays, a significant amount of plastics are used in mainstream car production. Plastics not only provide strength and durability, but also reduce the weight of cars. Glass fiber is widely used in the components of kit cars. The use of carbon fibers is also steadily increasing in items like wings and dashboards. Advanced composite materials find applications in many important parts of racing cars where the stiffness-to-weight ratio plays a significant role. There is a huge demand for vanity components made of plastic reinforced with glass with a thin veneer plastic reinforced with carbon similar to the components used in Formula 1 cars. Plasticized PVC, PUs, and foams, are used for making seats and interior trims, though they are not used much in racing cars.

15.14 Polymers in cycling A few years ago, the use of polymers in bicycles was limited to a few applications. They were used in making rubber tires, handle bar grips, and brake blocks only. A small amount of plastics was also used in saddles, cable covers, mud guards, and lights. The main components of a bicycle were made of only metals. In 1966, the first bicycle using composite polymer materials was designed. In this bicycle, some of the parts such as the rear arm frame and seat pillar were made of polyester reinforced with glass fiber. Another bicycle was also launched almost at the same time in which the material used for its construction was thermoplastic PU that was reinforced with carbon and glass fibers. Today, the major components of bicycles are made of composite materials, which are fast replacing metal. The use of composite polymers improves the strength and comfort and reduces the weight of bicycles. There are many other sports and recreational activities that make use of polymers, for example, plastic flights used for darts, synthetic fiber boards, and plastic dominoes are seen in many sports clubs. Plastic glasses, plates, and spoons are frequently used for serving food and soft drinks at sporting events. Polymers are used in molded chess pieces, chips and counters, dice, and a wide variety of toys for children. The use of polymers drastically reduces the weight and, hence, are convenient to carry or use. In skating, the chassis of inline skates is made of nylon. Hard PU is used in wheels. Fishing rods are also made of polymer composites. Polymers are also important for making the components of the reels and line. Polymers are not only

15.15 Polymers in sportswear

used in sports equipment or sportswear, but have also found applications in almost every activity that we do in our daily lives. Sport is for everyone, ablebodied as well as differentially-abled people. The world record for the 100 m sprint by an amputee was made possible because of prosthetic limb technology, which includes carbon fiber composites. There are also other facilities required along with sports, for example, flood lighting becomes essential for organizing certain sports events, which includes polymer applications such as in lenses, cases, seals, and cables. Sports stadiums also use a huge volume of plastics in making seats or in the fabric of the building. The security passes used at sporting events are also made of polymers. Polymer materials containing carbon fiber and polybenzoxazole have shown considerable applications. Though these advanced materials are highly effective and convenient, their use is limited due to their high cost.

15.15 Polymers in sportswear Every sport requires special clothing and a large amount of synthetic fibers are consumed for making specially designed sportswear. Though sports clothing is not much different from normal clothing, some characteristic properties, depending on the nature of the sport, are essentially required or desired from the sportswear. For example, a breathable fabric is required for activities like walking and climbing. Polymers are frequently used for making sports clothing and have significant impact on sports in making smart, comfortable, fashionable, and functional sports clothing. In some sports, there is a particular requirement for protective clothing, where foams are used abundantly. For example, the use of protective jackets is now compulsory in all types of riding activities. Polymers are also used in shin guards and other similar sports equipment. An effective design consists of a foam interior surrounded by a rigid molded plastic shell. Gloves are made from a variety of materials for a wide range of applications; for example, padding or the addition of a latex nonslip surface. Commonly used sports clothing includes tracksuits, shorts, t-shirts, and polo shirts. For water sports and swimming, specialized garments are worn, which include swimsuits, wet suits, ski suits, and leotards. Some sports make use of fire-resistant fabric such as Nomex in underwear, boots, gloves, and race suits to be used in motor sports. A mix of silicone gel, Kevlar, and closed-cell foam is used in knee pads and Kevlar heat sleeves for protection from heat. The head gears used for the protection of the head are made of thermoplastics or reinforced plastics and are used in cycling, motor sports, horseriding, and so forth. There are several other applications of polymers in sports. For example, plastic shuttlecocks are manufactured by a high-tech injection molding process in which a

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polyamide skirt is completely formed in the mold, yielding a product similar to the traditional hand-built feather design. There are other sports item made of polymers such as bats, sticks, goal posts, racquets, nets, and so forth. With the characteristic properties of polymers such as high strength-to-weight ratio, durability, flexibility, and strength, there is immense potential for innovation and improvement in designs of sports equipment, which will provide more comfort and convenience to athletes. Sportswear not only includes clothing, but also footwear. Specific sportswear is worn for most sports and physical exercises for comfort, convenience, functionality, and safety reasons. Sports footwear used in different types of sport activities, normally includes trainers, football boots, riding boots, and ice skates. Depending upon the specific requirements of the sport, athletes make use of different types of sportswear such as sport shoes, pants, and shirts. For some sports, it becomes essential to wear protective gear for safety reasons such as helmets, hand gloves, and so forth. Improvements were made using carbon and aramid as reinforcement materials. Sports fabrics are specially designed technical materials that are required to provide comfort and safety to its wearers during sports activities. The type of material required for sports clothing will depend upon the intensity and nature of the exercise and the type of activity. For example, the fabric used in sports garments for yoga should have good stretchability for easy movement. The sports attire needed for long distance running should provide athletes with a lot of comfort and convenience. Therefore these should have good moisture wicking properties to enable sweat to transfer from the interior of the clothes to the outside. Sports clothing for outdoor sports and activities for winter sports or snow sports normally make use of breathable fabrics that also have good insulating properties. In the case of football, opponent teams wear different color outfits, while the referee wears a specially designed sportswear that is lightweight and provides maximum comfort to the wearer. Good quality sportswear should be functional, lightweight, breathable, and should not create drag. Sportswear should be sufficiently loose so as to make movement easy while exercising. Some sports have characteristic, specific style requirements, for example, the keikogi used in karate. Sports uniforms are also specially designed to distinguish between different teams such as in hockey, cricket, and so forth. Usually the opposing sides in team sports are identified by the colors of their sports uniform, while individual team members are recognized by a number on their shirts. In some sports, in order to differentiate the roles of people in a team, specific garments are worn by specific team members. For example, in volleyball, the libero (a specialist in defensive play) wears a different color to what the rest of the team wears. In soccer, the uniform of the goalkeeper is of a contrasting color or pattern. In some sports, sportswear may also give information about the current status or past achievements of a participating player. In cycling, if a cyclist wears a rainbow jersey, it indicates that they are the current world champion. In major road cycling races, the race leader wears a jersey of a particular color.

15.15 Polymers in sportswear

The most popular material for making sportswear is spandex, which is used in wrestling, track and field, gymnastics, speed skating, and swimming. Different companies/sponsors of sports events also provide sportswear for the participants for the promotion of their products. There are rules and regulations in some sports on the design of sponsorship, size, brand names, and logos on items of clothing.

15.15.1 Thermal properties of sportswear Sportswear designs need to take into account the thermal insulation and breathability properties of the sports clothing. In hot weather conditions, sportswear should allow players to stay cool; while during cold weather, sportswear should keep players warm. Sportswear should also have the ability to transfer perspiration away from the skin. For this purpose, sportswear is made of moisture transferring fabric. Spandex is generally used for making the base layer to absorb sweat. In skiing and mountain climbing, sportswear that helps in moisture transfer (wicking) is worn next to the skin. Then there is an insulating layer followed by wind and water-resistant shell garments. Moisture wicking fabrics belong to a class of highly advanced fabrics that gives moisture control for the skin and body. These fabrics take away sweat from the body of athletes and transfer it to the outer surface of the fabric where it gets evaporated. These fabrics are soft, stretchy, and light in weight. They are highly functional and comfortable for any sports activity. These fabrics are moisture absorbent and keep athletes dry by removing sweat. The latest variation in moisture wicking is “dry wicking.” It consists of a smart two-tier fabric that breaks the surface tension of sweat and pushes it through a hydrophobic layer toward a natural wicking outer layer like cotton where it is supported by evaporative cooling, leaving the skin completely dry. This not only helps the wearer to perform better, but also keeps their body odor free and their skin dry. This fabric is used to make different types of sports garments such as t-shirts, sports bras, running and cycling jerseys, socks, tracksuits, and polo-style shirts.

15.15.2 Golf attire Golf has a long-standing tradition of characteristic attire. Golf attire reflects the tradition of Scottish aristocrats breathing in fresh air while walking around the golf course and swinging their golf clubs. Modern golf attire is influenced by comfortable fabrics and trends that are more functional and durable. Athletes in all sports nowadays prefer moisture wicking fabric materials with novel designs and fashionable colors.

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15.16 Polymers in sports footwear In sports footwear, polymers are used frequently to provide strength and comfort. Athletic shoes are designed according to the requirements of specific sports and can offer greater flexibility, support, functionality, and stability. The selection of the correct footwear depending on the requirements of the sport is extremely important. The wrong choice of footwear may cause serious injury. Different types of sports footwear for men, women, and kids are available on the market. In these types of footwear, rubber soles are used to provide flexibility and comfort. While jumping, running, or playing, our feet bear the pressure of up to seven-times the normal body weight. Modern shoes normally made of a variety of polymers have the ability to absorb shock and at the same time offer more support, flexibility, and traction. Polymers are the preferred materials to make sports footwear as it is easy to achieve the required material properties. The most common polymers used in sports shoes are nylon, PP, polystyrene, PE, PVC, Bakelite, epoxy resins, organic glass, PU, synthetic rubber, Ethylene-vinyl acetate (EVA), and so forth. Polymers are mainly used for making specific parts of sports footwear such as the midsole, upper part, insole, laces, and so forth. Midsole: In this part of shoes, the maximum absorption of shock takes place. The most commonly used material for making midsoles is ethylene vinyl acetate also known as foam polymer. Some sports shoes use a denser foam polymer that is synthesized from PU. It is also used for making skateboard wheels with air bubbles in it. Advanced plastic materials are also used for making midsoles. Upper part: The upper portion of sports shoes includes shoe laces, the outer coloring, and the design. The upper part is usually made from leather or a synthetic material specific to that sports activity. For instance, shoes used for running are made from polyester. It is lightweight and provides good support, comfort, and breathability to the feet. Insole: The insoles also absorb shock and provide support to the muscles. Plastic foam and silicone gel are commonly used for making insoles. Laces: Shoe laces are usually made of leather, cotton, or a mix of natural and synthetic polymeric materials. Toe box: Depending upon the type of sport, the toe box is designed to provide maximum support and strength to the feet. In the case of sports such as soccer or baseball, hard rubber is used for making the toe box in order to protect the toes. A softer type of rubber is used in jazz shoes for dancing on tiptoes. Outer sole: Different forms of rubber are used for making the outer sole, which provides good grip to a playground or gym floor to the wearer.

15.17 Conclusion Athletes strive hard to improve their performance and constantly look for good quality sports equipment with comfortable and functional designs. Their good

15.17 Conclusion

performance can be attributed to polymers allowing new and superior equipment to be produced. Polymers are used over a wide range of sports applications. With the advancements in material technology, new, efficient materials are available today to make more functional, flexible, strong, and comfortable sports equipment. Sports equipment such as tennis rackets are developed with the use of glass fiber, carbon fiber, and aramid fiber reinforced composite materials. Laminated composite materials are used for making ski, paddle, golf, and hockey sticks, and so forth. Fiber-reinforced composites have a huge demand in sports equipment. Polymers are fast replacing traditional materials and more and more new materials are being synthesized with improved properties. This clearly proves that continuous changes in sports equipment are taking place and athletes keep looking for high quality, comfortable, and more functional sports equipment with reduced cost to improve their performance. Though polymers have been widely used in sports equipment, there is still a lot of scope for innovation and improvement including the cross-fertilization of practices between different types of sports. With the increasing awareness of health and fitness, the demand for sport and leisure facilities and equipment will grow and there will be constant research and development in material technology and novel materials with improved properties. New polymer materials are constantly emerging and will continue to be used in sports equipment manufacturing. With the improvement of manufacturing technology and processes, there will also be a reduction in the cost of sports equipment. There have been significant improvements in some sports, for example, golf clubs, which were made of wood traditionally, used to be very heavy and prone to rot. Nowadays, golf clubs are made of carbon fiber mixed with graphite and composite materials. They are simple, more flexible, stronger, and lighter, and also have improved the swing speeds of players. Similarly, the poles used in pole vaulting used to be made of wood. But now they are made of polymeric materials. These new polymeric materials are stronger, tougher, and provide more comfort and safety to the users. Ski boots used to be made of PU, which used to cause ankle injuries for athletes, but modern ski boots are made of nylon, which are lighter, strong, and safe in use. There is a lot of improvement in the quality and comfort of sportswear too with the use of new polymeric materials. The sportswear available today is made of polymers that are made of breathable materials, are comfortable, fashionable, and convenient to use. There is a coating of nylon taffeta coated amino acid polymer, which makes sports materials waterproof, windproof, and skin-friendly. The quality of sports shoes has also improved with the application of new polymer materials. These shoes are lighter and more comfortable, which help in improving the performance of players.

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[25] Fehim Findik A. Case study on the selection of materials for eye lenses. ISRN Mechanical Engineering; 2011. Article ID: 160671, 4 pages. [26] Calvetea DP, Restrepo Holgu´ına DF, Lunaa MP. Grafting polymer based in active polyurethane matrixes via free radical. Procedia Mater Sci 2015;9:491 5. [27] Janssen RPM, de Kanter D, Govaert LE, Meijer HEH. Fatigue life predictions for glassy polymers: a constitutive approach. Macromolecules 2008;41(7):2520 30. [28] Meng TT. Volatile organic compounds of polyethylene vinyl acetate plastic are toxic to living organisms. J. Toxicol. Sci. 2014;39(5):795 802. [29] M. Kaseem, K. Hamad, Polymer Science Series A 57(6), July 2015, Material Properties of Polyethylene/Wood Composites: A Review of Recent Works. [30] Myhre M, MacKillop DA. Rubber recycling. Rubber Chem.Technol. 2002;75 (3):429 74. [31] Howe M. 2018, Management of sports and physical education, Waltham Abbey Essex, UK: ED Tech Press, p. 82-83. [32] Mojsiewicz-Pie´nkowska K, Jamro´giewicz M, Szymkowska K, Krenczkowska D. Direct human contact with siloxanes (silicones) - safety or risk part 1. Characteristics of siloxanes (silicones). Front. Pharmacol. 2016;7:132. [33] Lee J, Seo WG, Kim J, et al. Amide-based oligomers for low-viscosity composites of polyamide 66. Macromol. Res. 2017;25:1000 6. [34] Fischer J, Wallner GM, Pieber A. Morphology of polyethylene ski base materials. J. Sports Sci. 2010;28:555 62. [35] Davis CL, Kukureka SN. Effect of materials and manufacturing on the bending stiffness of vaulting poles. Phys. Educ. 2012;47:5. [36] A€ıssa B, Therriault D, Haddad E, Jamroz W. Self-healing materials systems: overview of major approaches and recent developed technologies. Adv Mater Sci Eng. 2012;2012(854203):1 17. [37] Vogler H. Wettstreit um die Polyamidfasern. Chem unsererZeit 2013;47:62 3. Available from: https://doi.org/10.1002/ciuz.201390006. [38] Palmer RJ. Polyamides, plastics. In: Encyclopedia of polymer science and technology; 2001. doi:10.1002/0471440264.pst251. [39] Giesbrecht JL. Polymers on snow towards skiing faster [Dissertation submitted for the degree of Doctor in Science]. [40] Davis CL, Kukureka SN. Phys Educ 2012;47:524. [41] Harris KM, Rajagopalan M. 2004-09-21US6794472B2Grant. [42] High Performance Textiles. Alliedsigolythylene naphthalate fibre makes tougher sailcloths. Allied Signal Inc, July 1997, p. 2. [43] Boatshed Ltd. Homeblown surf board foam the new way. European Community, European Union, UK, Western Europe, Plastics and Rubber Weekly, No. 1850, August 18, 2000, p. 7. [44] Forest JP. Revue Generale des Caoutchoucs et Plastiques 77, No. 783, February 2000, p. 32. French Boating: FBOATINine Weather for Plastics. [45] Philips, SP Technologies, Goss Challenge Ltd, European Community, European Union, UK, Western Europe. Material not design snag for team, No. 5, May 2000, p. 10.

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Polymers and food packaging

16

Behjat Tajeddin1 and Mina Arabkhedri2 1

Agricultural Engineering Research Institute (AERI), Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran 2 Applied Science in Mechanical Engineering, University of British Columbia, Vancouver, Canada

16.1 Introduction The packaging of an item for consumption is progressively becoming recognized in terms of its marketing value as products sit on shelves next to similar products and compete for attention from potential buyers. Packaging can give a positive customer experience if it utilizes a good design, graphics, and information label about its contents. It is apparent that some products are not the healthiest, but the design of the packaging of an unhealthy food can make it more attractive to potential consumers than a healthy product contained in a poorly designed package. In fact, nowadays industries use packaging not only to protect and cover products, but also as a tool to advertise and convey their brand to their customers. One of the most popular materials used in the food industry for packaging is petroleum plastics (synthetic polymers). Such polymers include polyethylene terephthalate (PET), low and high density polyethylene (LDPE and HDPE respectively), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS) [1]. Another topic of course, is sustainability, which has gained the interest of many scientists, researchers, and environmentalists. In the polymer world, biopolymer materials are an environment-friendly substitute for synthetic polymers. This is due to their biodegradability, agro-industrial waste (biomass) usage, and renewable raw materials. In addition, they are desirable in terms of their availability and cost effectiveness. Furthermore, biopolymer materials can be prepared in such a way that they are edible, and possibly with active antioxidants and/or antimicrobial agents. These biopolymer materials can also be formed as composites and laminated so as to enhance their properties [2]. While remembering the main purpose of food packaging (which is to preserve the quality of food products), it is critical to recognize the importance of health and safety in the different materials used in the making of food packaging. This is an important issue due to the potential for the migration of components from packaging materials into food, thus, causing contamination. Examples of possible Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00016-0 © 2020 Elsevier Inc. All rights reserved.

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contaminants are additives (such as antioxidants, stabilizers, and plasticizers) and monomers; while there is also a possibility of unknown substances that may be transferred unintentionally. Gra´ın˜o et al. [3] identified numerous migrant compounds that contained acetyl tributyl citrate, N-alkanes, tributyl aconitate, phthalates, butylated hydroxytoluene, bis(2-ethylhexyl)adipate, etc., in paper-based candy wrappers and plastics [3]. Many of the substances detected in samples of plastics are not authorized for consumption by health and safety organizations. Since biopolymers are progressively being substituted for common synthetic polymers (obtained from oil derivatives) [4], the migration risk from bioplastics into foods is also inevitable and a possible source of contamination as well. However, as these bioplastics are made using natural ingredients, the risk to food safety caused by contamination is excessively reduced as natural ingredients do not pose a severe threat to food and consumer safety.

16.2 Food packaging Packaging is used in all sorts of industries with applications in the medical field, pharmaceuticals, food, electronic devices, etc. A variety of materials are used in order to prepare and manufacture packaging. Food packaging and its related technologies are an essential part of the food industry as they involve the protection and preservation of foods and products [5]. In fact, food packaging is designed to protect and conserve the quality of foods without deteriorating their appearance, taste, smell, or nutritional content; and also to inform the consumer about the ingredients and nutritional information enclosed inside. Consumers have a right to know what the contents of each package are, and, therefore, product information is always to be printed on the packaging. As products compete for attention from potential buyers, there is an ever-increasing interest in creating attractive marketing solutions and improving packaging designs to have more ergonomic and effective products. As consumer demand rises, producers are going to be challenged as they encounter more concerns in terms of creating potential threats to product quality. According to a review by Jacob [6], protection, preservation, and presentation are the three fundamental functions of a package. Although determining these package functions is rather simple and logical, it leaves aside other aspects for consideration [6]. Robertson identifies four functions of packaging, namely containment, convenience, protection, and communication. In view of the convenience factor (which is defined as consumer usability), a package should be user-friendly, meaning it should be easy to open (and possibly reseal), handle, carry, and recycle (and/ or dispose of as waste) [7]. Many other authors have different opinions about packaging functions based on their expertise, which can be found in their extensive research and literature works. For example, Burke [8] describes three major classifications for a successful package design. These three classifications are authenticity, meaning, and the package’s ability to convey the brand image to the consumer [8].

16.3 Packaging materials

16.3 Packaging materials The packaging materials used in food and allied industries are highly varied. The main responsibility of these materials is to keep foods fresh and safe from the production stage all the way to consumption including the storage and distribution chains as well. Materials that have traditionally been utilized in food packaging and have worked well throughout the years include glass, metals (aluminum, foils and laminates, tin-free steel, tin plate, etc.), plastics, paper, and paperboards. In general, flexible and rigid synthetic packaging materials are used in food products [9]. Throughout history, the most common materials used for protecting and covering goods have been glass and paper. Similar to many other matters in day to day life, however, each of these materials has its own advantages and disadvantages. For example, glass containers used in different applications and shapes such as glass bottles are the oldest packaging group and are still used to this day. Glass packages are most suitable for containing liquids as they are made of natural components with a nonpermeable barrier (one of their advantages). However, their biggest disadvantage is their fragility. The paper packaging industry also offers the best quality kraft and sack paper. Kraft paper is a strong paperboard or cardboard (usually brown paper) that is produced by processing wood chemical pulp. It is mainly used for bags and or as wrapping paper. Sack paper is a kraft paper that is permeable and sponge-like with high tear resistance and elasticity. It is widely used for packaging products that demand strength and durability. An advantage point for the paper packaging industry is that innovative solutions marketing experts can design on it for clientele in the industrial, medical, and consumer sections. The greatest disadvantage for paper packaging, however, is the absorption of water and moisture. Over the course of history, especially after the industrial revolution in the eighteenth century, manufacturing technology revolutionized the entire concept of packaging when manufactures were pushed to develop more durable and resistant types of protection (packaging) to make the transportation of products from factory to shop and later to customers’ homes possible. Soon after, the development and manufacturing of plastic materials for packing applications began in the 1860s [7]. This was done by altering hard rubber. As science progressed, synthetic plastics and multiple compounds such as PVC and PET were gradually invented [10]. For a long time, petrochemical polymers (plastics) have served as the go-to packaging materials due to their economical abundance, desirable presentation, excellent barrier properties toward oxygen, aroma compounds, softness, tensile and tear strength, lightness, and transparency [5]. An example of petrochemical polymers is polyethylene (PE) foam, which is a durable, lightweight, closed-cell material. Its main applications are in industrial and agricultural packaging. PE is an excellent material choice in these applications due to its insulation and

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vibration control properties. It also offers high resistance to chemicals and moisture and comes in various shapes such as pouches, sheets, bundles, die cuts, and tubing. The advantages of PE foam in pouch shape, for example, are its flexibility, lightness, surface sensitivity, low cost, shock absorbability, and resistance to mildew or mold [11]. Nowadays, with the arrival of new technologies such as three dimensional (3D) printing, it is possible to have a diverse, good-quality packaging material. The application of 3D printing technology in packaging industries has shifted the dynamic of packaging in many different aspects such as in the potential to have full color graphics and text for labels. 3D printing is a process that uses computer control to form layers and coatings for materials. It is convenient in terms of package design. People are able to provide color coatings and layers in almost any material regardless of shape or geometry [12,13]. Although the current advantage of 3D printing technology in packaging is not clear, it requires a lot of future exploration because of its huge potential. It should be noted that the packaging industry and researchers work on the recycling and reusing of synthetic packaging materials for waste management and control. For example, PET can be recycled repeatedly, and about 681,000 metric tons of recycled PET containers and bottles are recovered each year in the United States. One method of recycling is to wash and remelt PET, and use it in new products that require PET as a component. Another method of recycling PET is to break it down with chemical processes into its raw materials. Due to this reusability, PET is highly sustainable and its sustainability is an ever-increasing pursuit for scientists and a lot of research is going into the development of facilities that are capable of transforming used PET packages and bottles into new foodgrade PET containers and bottles. The only drawback to the reuse of PET is the amount of material collected. One of the most interesting uses of recycled PET resin is in the production of filament for use in 3D printers [14]. Nevertheless, the use of petrochemical polymers (plastics) has many disadvantages such as a poor water vapor transmission rate (depending on the goal of packaging, it is sometimes an advantage) and a significantly negative effect on the environment. In fact, the worst disadvantage of these polymers is their nonbiodegradability and noncompostability, which has a great impact on environment pollution and global warming. The waste disposal problem of petroleum materials and their nonrenewable nature have caused a spike in the level of interest in the sustainable development of biodegradable polymers, recycling, and/or environment protection. The degradation of materials causes structural and morphological transformations that can result in major changes in the properties of polymer materials. Generally, biodegradable polymers get hydrolyzed and turn into methane (CH4), carbon dioxide (CO2), mineral mixtures or compounds, or biomass. To create biodegradable polymers, bio-origin materials obtained from cellulose, starch, and microbial fermentations are used. This has led to their incredible success in the food packaging industry in the past few years [5,15,16].

16.3 Packaging materials

16.3.1 Polymers A polymer is a material that has a large molecular structure composed entirely of rings or chains of many distinctive repeating units (monomers). To simplify the meaning of polymers, one can refer to the origin of the word in Greek: words “poly” and “mer” which mean “many” and “parts”, respectively. Both synthetic and natural polymers are widely used in different industries due to the broad range of their properties. Many synthetic substances are used as plastics and resins [1]. In fact, plastics are polymers with additives and are based on long-chain molecules developed from alkenes. To prepare different polymers, usually one or more hydrogen atoms is replaced by a different atom [9]. Plastics are commonly used in the packaging industry due to their easy molding and transformation. Various plastics and their different properties allow for many packaging options such as shape, color, size, weight, function, printing, etc. [1]. Today, polymers are an integral part of modern life owing to their desirable properties including stability, resilience, and ease of production [17]. In other words, plastics are favorable for producers for the reasons that they are light in weight and malleable and/or flexible, thus, can be formed into any shape by different means such as blowing, extrusion or coextrusion, casting, lamination, etc. This makes it possible to package unique objects that are difficult to fit into normal/basic containers. Polymer plastics are useful for different parts of food packaging due to their barrier properties that help to keep products fresh, prevent contamination, and increase shelf-life. Polymer packaging can be seen as beneficial to the environment because food industry manufacturers and businesses are able to reduce waste by preserving foods for longer than in a case without polymer packaging [1]. For example, Tajeddin et al. [9] reviewed polymers for modified atmosphere packaging (MAP) applications. These compounds are the main materials for flexible package structures used for MAP, but they can also be used in a rigid or semirigid packaging solution such as a lidding on a tray. PEs including LDPE, linear LLDPE, HDPE; PP; PS; polyesters including PET or PETE, polycarbonate (PC), and polyethylene naphthalate; PVC; polyvinyl dichloride; polyamide (PA) or nylon; and ethylene vinyl alcohol are the main polymeric materials (plastic films) used for MAP applications [9]. On the other hand, researchers are still working to correct the structure of some of these polymers for different purposes. For instance, Kobayashi and Saito [18] worked on the structural evolution of blends of PC and poly(methyl methacrylate) by simultaneous biaxial stretching. Since the mechanical and gas barrier properties of polymer films can be improved using the biaxial stretching process, these biaxially stretched films are greatly utilized as packaging materials for industrial and food products [18]. In another example, Han et al. [19] synthesized polyvinyl formal (PVF) through the reaction of polyvinyl alcohol (PVA) and formaldehyde. The synthesized PVF showed a higher decomposition and glass transition temperature (Tg), and a lower melting point compared to PVA. The synthesized PVF could be melted and processed at much lower temperatures than PVA. Polypropylene carbonates (PPC) and the synthesized PVF were melted and blended in a Haake mixer machine.

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The PPC/PVF blends presented a higher Vicat softening temperature and tensile strength compared to those of pure PPC. Thermogravimetric analysis findings showed that the thermal stabilities of the PPC/PVF blends were reduced as the PVF content was increased. Observations obtained from scanning electron microscopy revealed that the interfacial compatibility of the PPC and PVA matrix was worse than that of the PPC and PVF matrix. The PPC/PVF blends indicated superior broad properties compared to those of pure PPC, which presents as a feasible method to expand the application of PPC copolymers [19]. As previously mentioned, polymers (plastic materials) are the most used materials in the packaging industry. For example, in an analyzed packaging categories, it is estimated that 95% 99% of plastics use in North American packaging. Comparative to other packaging substances such as aluminum, glass, paper, steel, etc., plastic-based packaging materials make up 39% 100% of total North American market request for the packaging categories analyzed in this study. However, the most important problem with plastics is that the production and processing of plastics are energy used processes, leading to the production of greenhouse gases and contributing enormously to global warming [20]. Furthermore, Smith [21], as cited by Yadav et al. [17], stated that on burning, plastics release noxious emissions such as CO2, chlorine, hydrochloric acid, dioxin, furans, amines, benzene, 1,3-butadiene, and acetaldehyde, which threaten the environment and public health [21]. Created wastes such as of plastics are also a big challenge for many years due to their degradation resistance. They produce substantial environment pollutions, thus, hurt natural resources and wildlife when they are spread in nature. For example, the disposal of nondegradable plastic bags, to-go food containers, plastic straws, and plastic bottles for bottled water has a major and direct negative impact on sea life [22] and can ultimately disrupt the natural food chain. It is estimated that by 2050, there will be more plastic waste in our oceans than fish! Plastic bags are choking our planet and, in particular, the oceans. They expose children to lung problems and can increase the risk of prostate cancer in men. Every year, more than 500 billion plastic bags are disposed of in landfills, and less than 3% of these bags are being recycled. These plastics are mainly made of PE and can take hundreds of years to degrade without proper recycling technology and also release damaging greenhouse gases that contribute to global warming [23]. Therefore it is better to reduce the distribution of these materials and to use other packaging materials instead. Fig. 16.1 shows an example of packaging materials composition for replacing plastic materials. In fact, Brandt and Pilz [24] developed a model for a theoretical substitution of plastic packaging. The results of their study are summarized in Fig. 16.1, if plastic packaging (LDPE, LLDPE, HDPE, PP, PVC, PS, expanded polystyrene, and PET in this study) were replaced by other materials (tin plate and steel packaging, aluminum, glass, corrugated board, cardboard, paper and fiber cast, paper-based composites, and wood in this study) [24]. As shown in Fig. 16.1, in addition to the possibility of replacing plastic materials with other packaging materials (which is a widespread debate and needs to

16.3 Packaging materials

FIGURE 16.1 A case study of the replacement of plastic packaging with other packaging materials [24].

be addressed elsewhere), through the use of renewable natural materials and new technologies, there may be other alternatives to substitute for polymeric materials.

16.3.2 Biodegradable polymers Every day people are coming up with new ideas on how to better package products. First, it was being able to recycle packaging materials, and now people are continually coming up with newer and better biodegradable and environment-friendly materials. Companies have already begun the transition into better materials [25]. The fact is that most synthetic polymers like PE, PET, PA, polyethylene furanoate (PEF), polyurethanes, etc., are biobased materials due to the fact that they come from the Earth originally; however, they are nonbiodegradable and unsustainable. The word biodegradable is used to explain materials that decompose by way of the enzymatic action of living organisms (for example, yeasts, fungi, and bacteria). The ultimate products of the decomposition process are water (H2O), CO2, and biomass under hydrocarbon and aerobic conditions, and biomass and CH4 under anaerobic conditions [26]. Thus the main challenge is replacing conventional packaging with sustainable materials that are biodegradable such as polybutylene adipate-co-terephthalate, PBS, polylactic acid (PLA), polyhydroxyalkanoates (PHAs), starch blends, etc. It is worth noting that according to a statement of the European Bioplastics Organization, a plastic material is defined as a bioplastic if it is either biobased, biodegradable, or features both properties. The necessity of the replacement of petroleumbased plastic with natural-based polymers is logical because producing unsustainable (due to environmental problems) synthesized plastics consumes 65% more energy and emits 30% 80% more greenhouse gases than bioplastics [27,28 as cited in 17].

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At this time, bioplastics represent about 1% of the approximately 320 million tons of plastic produced annually. But as demand is rising and with more high-level biopolymers, applications, and products emerging, the market is continuously growing. Along with the newest market data collection of the European Bioplastics Organization, and the research Nova-Institute, global bioplastics production volume is growing from around 2.05 million tons in 2017 to approximately 2.44 million tons in 2022. PLA and PHAs are the main substances contributing to this growth in the field of biobased, biodegradable plastics [29]. There is a bioplastic substitute for almost every typical plastic material and corresponding application. What is certain is that the properties of these new bioplastics should be similar to those of the traditional ones because their good properties have been proven. Beyond this, bioplastics, in addition to having the same properties as common plastics, should be offered additional advantages such as a reduced carbon track or further waste management options such as industrial composting, depending on the material. For example, both scientific researches and industries are presently dedicating many attempting to improve high gas barrier bioplastics as replace with conventional fossil-based polymers [30]. In general, biopolymer materials are derived from polysaccharides, proteins, or lipids, based on the compositional units. In order to get better biopolymer material properties, they can be treated, that is, laminated or formed as composites. In addition, biopolymer materials can be made to be edible and/or active with strong antioxidant and/or antimicrobial properties [2]. Sources of these biodegradable polymers are lignocellulose products, wood, straw, pectin, chitosan/ chitin, gums, wheat, starches, cassava, potatoes, maize, etc. [22]. Chitin and chitosan are the main and most abundant sources of natural polymers subsequent to cellulose. Chitosan has proven useful for the development of bioactive materials due to incomparable properties like nontoxicity, biodegradability, chelating, anticoagulant, antioxidant, and antimicrobial characteristics, and biocompatibility [31]. Biopolymers can also be produced by microorganisms through the fermentative processes of altered bioresources such as PHAs and biomass may be produced directly from various plants [4]. In addition, there are some plastic and paper products made from raw materials such as bamboo, wood, recycled paper, bagasse (sugar cane), etc. Bioplastics are made from components found in plants like hemp oil, soy bean oil, and corn starch. Bagasse has the least effect on the environment. Because it is strong and does not deform easily, it is suitable for use in take-away boxes, plates, bowls, and ice cream cups. It is heat and water resistant and will not suffocate food stuffs [25]. However, different types of wood are used for different packaging materials. Furthermore, plants like bamboo are consumed as packaging materials. They come from renewable resources that are compostable and biodegradable. For instance, prepared cellophane is used for bags and sheets because it can block out or keep in air, grease, and bacteria really well and can be sealed by heat [32]. The environmental impact of paper products was compared with that of plastic products in terms of four factors. On the whole, the results showed considerably

16.3 Packaging materials

less of an environmental impact for paper products including 50% 70% lower emissions of greenhouse gases [25]. From a collection of the mentioned studies and other valuable researches, there is a comprehensive classification of natural polymers currently used in food packaging, which is summarized in Table 16.1. As shown in this table, excluding the fourth family (which is of fossil origin) and the fifth family, most polymers (families 1 3) are obtained from renewable resources or biomass [33]. If families 1 3 (which are fully biodegradable) are not available for use however, the use of families 4 5 (which are semibiodegradable as they are petroleum-based) is still far better than the use polymers made solely of petroleum products. As an example of the fourth group, Guidotti et al. [30] compared a biobased aromatic polyester, polypropylene 2,5-thiophenedicarboxylate (PPTF) with a furanbased counterpart (PPF). The results showed that with rising RH, the permeability performance of PPTF was better compared to that of the PPF as a result of the polar nature of the furan ring. Both PPTF and PPF are distinguished in terms of their gas barrier properties in comparison to PET and PEF. This simple synthetic approach results in excellent barrier performances in these biobased polyesters and makes them an attractive substitute for conventional materials, and suitable for a greener and more sustainable strategy in the packaging industry. It was shown that the stability and different polarity of heterocyclic rings is an effective means for directing the ability of crystallization. Crystallization then influences the barrier and mechanical properties [30]. In addition, Guidotti et al. [35] presented polybutylene PPTF, which is a biobased aromatic polyester, and studied its characteristics. They suggested that PBTF has the potential to create alternative solutions for a greener world of sustainable and environment-friendly packaging materials [35]. Nowadays, there are many companies in different countries (such as Canada, the United States, Japan, the United Kingdom, etc.) that focus on bioplastics and their applications for food packaging. As always, similar to many other scientific subjects, bioplastics also have advantages and disadvantages. The advantages and disadvantages of a number of biodegradable polymers are summarized in Table 16.2. There are valuable studies about using natural biodegradable films for some foods. For example, Rhim et al. [44] coated SPI-based films with PLA to improve its mechanical and barrier properties as well as to determine some of its other properties. The results showed that the water barrier and mechanical properties of the PLA-coated SPI films were significantly increased. They reported that these films are well suited for food packaging applications [44].

16.3.3 Synthetic polymers and biopolymers hybrids As shown in Table 16.1, there is also the possibility of using a mixture of synthetic and natural polymers. Studies by many researchers such as Vasilevskaya and Yoshikawa [45] and the author of this chapter [46,47] show that the preparation of synthetic polymer and biopolymer hybrids, while reducing the consumption of synthetic polymer materials in nature, boosts some of the properties of

533

Table 16.1 List of common biodegradable polymers used in food packaging [33].

Family/group 1. Biomass products [from agro-resources (agropolymers)]

Natural polymers/ renewable materials

Source

Examples

Polysaccharides

Plant/algal

Starch (wheat, potatoes, maize, etc.), cellulose, pectin, konjac, alginate, carrageenan, gums Hyaluronic acid, chitosan/chitin Pullulan, elsinan, scleroglucan Chitin, chitosan, levan, xanthan, polygalactosamine, curdlan, gellan, dextran Soya, zein, wheat gluten, resilin, polylysine, polyamino acids, poly(g-glutamic acid), elastin, fibrin gel, polyarginyl polyaspartic acid Collagen/gelatin, casein, serum, albumin, silks, chitin [34], whey Acetoglycerides, waxes, surfactants, emulsan Polyhydroxyalkanoates, poly(hydroxyl butyrate) (PHB), poly (hydroxybutyrate co-valerate) (PHBV) Poly(lactic acid) (PLA)

Animal Fungal Bacterial Proteins

Plant Animal

2. From microorganisms (obtained by extraction) 3. From biotechnology (conventional synthesis from bioderived monomers) 4. From petrochemical products (conventional synthesis from synthetic monomers) 5. Synthetic or natural polymers reinforced with any other renewable resource-based materials

Lipids/surfactants Polyesters Polylactides

Polycaprolactones, polyesteramides, aliphatic copolyesters (e.g., PBSA), aromatic copolyesters (e.g., polybutylene adipate-coterephthalate (PBAT)), polyurethane (PU) Variety of biofibers (as reinforcement)

Green polymers and resins Biorenewable reinforcement

Lignocellulosic natural fibers, lignin, shellac, natural rubbers, vegetable (herbal) oils Ranging from natural clays to natural fillers including wood, straws, rice husk

16.3 Packaging materials

Table 16.2 Some advantages and disadvantages of a number of natural biodegradable polymers [26,36 43]. Raw material

Advantages

Disadvantages

Starch

Availability, relatively cheap cost

Chitosan

Antimicrobial and antifungal activity, good mechanical properties Good film forming, good tensile and moisture barrier properties, low water solubility (depending on the aim of packaging), resistant to microbial attack Desirable film formation, good oxygen barrier

Hydrophilic character, poor mechanical properties Low oxygen and carbon dioxide permeability, brittleness, high water vapor permeability Glossy appearance, tough, greaseproof, hydrophobic, brittleness, low water solubility (depending on the aim)

Zein

Whey protein isolate (WPI) Gluten Soy protein isolate (SPI)

Low cost, good oxygen barrier properties, good film forming Abundant, inexpensive, nutritional raw material, excellent film, high water absorption (this property may be good or bad depending on the aim of SPI application)

Low tensile and strength properties, high water vapor permeability High sensitivity to moisture, brittleness High water absorption, sensitivity against oxygen permeation, poor barrier properties, weak mechanical properties

biopolymers such as their mechanical properties. For example, Cannarsia et al. [48] studied the effects of switching PVC films with biodegradable polymers with the aim of preserving the characteristics of meat such as its color and to restrict its microbial contamination. Ten organically farmed meats from slaughtered animals at 16 18 months of age were each put on PS plates and packaged hermetically with PVC films and biodegradable polyesters. Their results showed that the mentioned biodegradable polyester films could be profitably used to replace PVC films for packaging fresh processed meat [48]. Xu et al. [43] also prepared blown film using modified starch and polyethylene with better characteristics than common polymers. However, it should be noted that in the use of a mixture of synthetic and natural polymers, despite all its benefits, it is not possible to achieve complete biodegradability. Depending on the purpose and the application, more studies are needed.

16.3.4 Nanomaterials As biodegradable films (natural polymers) have weak barrier and mechanical performances, their applications for food packaging have been extremely limited. The use of nanocomposites has great potential to extend the utilization of

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biodegradable and edible films. These will in turn reduce the waste from the packaging of processed foods by means of preserving fresh foods and extending their shelf-life [49]. The definition of nanotechnology is to create and utilize structures that have a minimum of one dimension in the nanometer (1029 m) length scale. Structures created by nanotechnology are called nanocomposites, which may present modifications in material properties. In order to obtain modifications, an interaction is desired between the nanofiller and the polymer matrix. An excellent method for enhancing the properties of biobased films is the incorporation of nanoparticles [40]. Nanotechnology can be utilized for food packaging applications to construct better performing, stronger, or lighter polymer structures. One method for food spoilage avoidance is the use of nanoparticles of titanium or silver dioxide as antimicrobials. In fact, incorporating nanoparticles into food packaging is an effective way to block moisture, CO2, and O2, from entering the food contents and, therefore, help in preventing food spoilage [50]. However, there are many valuable studies on different obtained nanocomposites from various materials. For example, Ozilgen and Bucak [51] reviewed the performing properties and the relation between the functional characteristics and the nanostructures of bacterial polysaccharides (e.g., xanthan, cellulose), plant/ algal polysaccharides (e.g., starch, agar, alginate, pectin), and animal polysaccharides (e.g., chitosan), and their main applications in the food industry [51]. Kadam et al. [39] developed a film implanted with titania (TiO2) nanoparticles using sonic technology to gain a consistent distribution of nanoparticles in WPI and characterized the results using different sonification levels. Their results showed that the incorporation of nanoparticles helps to recover the mechanical properties and to increase the thickness of films. They stated that WPI films implanted with nanoparticles can present a high promise for extending shelf-life and quality, and increasing the food safety in food packaging applications [39]. In addition, Malathi et al. [42] prepared SPI films implanted with titanium dioxide nanoparticles. The implanted nanoparticles successfully enhanced the opacity and thickness of the films [42]. Chi et al. [52] studied the effects of high pressure treatment on some properties of PLA/silver (Ag) nanocomposite films. They also studied the migration behavior of nano-Ag from nanocomposite films with 50% (v/v) ethanol present as a food simulant. The high pressure treatment on film-forming solutions generally presented improvements in the functional performance of the nanocomposite films. This was especially revealed for water vapor barrier performances compared with the untreated samples. It is worth noting that high pressure treatment at pressures between 200 and 400 MPa drastically decreased the migration of nano-Ag (P , .05) from the films [52]. Manigandan et al. [31] also studied the main analytical techniques used for the development of chitosan nanofilms as well as the major applications of chitosan-based active food packaging systems. The nanoencapsulated chitosan films showed a noticeably decreased water vapor permeability. Furthermore, the

16.5 Biopolymers and active packaging

dispersed chitosan presentation in protein significantly verified the improvement of the mechanical strength of the nanofilms, that is, make practicable the use of these films in the food industry [31].

16.4 Some methods for biopolymers production There are different methods for producing biopolymers like solution casting, melt mix, electrospinning, thermo pressing and casting, and extrusion blowing [17], which can be used to change the properties of these materials. For instance, Lopusiewicz et al. [53] used various fungal melanin concentrations as a modifier to prepare PLA-based composites using an extrusion method. The results showed that the mixing of fungal melanin with the PLA has the possibility to be developed as a value-added modifier due to improving of the overall properties of PLA. The mixture of PLA/melanin films exhibited valuable antioxidant activity and were active against Pseudomonas aeruginosa, Enterococcus faecalis, and Pseudomonas putida [53]. In another study, Thanakkasaranee et al. [54] prepared a series of poly(etherblock-amide) (PEBAX)/polyethylene glycol (PEG) composite films (PBXPG) using the solution casting procedure to investigate the effects of the integration of different molecular weight PEGs into PEBAX, and whether and how it would improve the composite films’ performance in gas permeability as a function of temperature. Based on the results, it was concluded that the high (H)-PBXPG composite films are suitable for safe microwave cooking and other applications as self-ventilating products [54]. In addition, Guidotti et al. [55] synthesized a biobased polyester (butylene 1,4-cyclohexane dicarboxylate) consisting of random copolymers as a material for flexible and sustainable packaging solutions using the lamination method. On the one hand, the linear butylene moiety was replaced by glycol subunits with alkyl pendant groups of diverse lengths. On the other hand, copolymers with different cis/trans isomer ratios of cyclohexane rings were produced. The presence of side alkyl groups considerably affected the formation of ordered phases that influence the functional properties, primarily the mechanical performance and barrier response. Specifically, the end products showed extensively improved barrier properties and a better flexibility compared to the homopolymer and all other polymers that are generally utilized for flexible packaging [55].

16.5 Biopolymers and active packaging A trending topic in improved packaging designs is the incorporation of additives into packaging materials. Antimicrobial packaging contains films that have antimicrobial additives incorporated into a polymer film used for barrier protection. These films maintain good physical barrier properties as well as the quality of the food. Accordingly, in the past few years, antimicrobial films for packaging

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applications have received growing attention from the food industry [31]. Kuorwel et al. [56] investigated biodegradable polymers derived from polysaccharides and protein-based resources for their potential usage to design packaging systems for the protection of food products from microbial contamination [56]. Food-grade biopolymers as well as their inherent nutritional properties, can be adapted and designed for improving food quality and safety with imparting functions like active antibacterial and antiviral properties [57]. For example, food hydrocolloids are high molecular weight long-chain biopolymers that are made of high molecular weight polysaccharides and proteins. Hydrocolloids are commonly used as functional food additives in many food products to save or improve the sensory characteristics of the foods and drinks, to improve the shelf-life of the food products, to formulate the production processes so as to be simpler and more efficient, and to produce functional food products. They can form films with applications as edible films and active packaging [51]. Razavi and Behrouzian [58] considered biopolymers from food hydrocolloids as fat replacers within foods such as cheese, ice cream, sauce, and yogurt due to consumer demand for low-fat or fat-free food products. Hydrocolloids, the largest number of biopolymers have functional properties such as textural, viscosity, and feel that let them represent the sensory and flow characteristics like fat behavior [58]. The academic research and constant pursuit of improved technology designs in the packing industry has led to innovative solutions for the enhancement of food safety and its quality along with extending shelf-life. One of such researches was done with the aim of comparing two barrier materials, namely biodegradable natural PHB and petrochemical PU, mainly focusing on their antibacterial agent realization performance. The study indicated that the kinetic energy released by chlorhexidine digluconate (which was the active agent in both polymers), was considerably different because of the surface degradation and superposition of diffusion in PHB. This brought to light the effect of active biodegradable packaging on the base of PHB [59]. Liang and Wang [60] prepared an active film by incorporating different concentrations of cortex philodendron extract (CPE) as an active agent into an SPI. The results showed that the crystallinity of the films and new hydrogen bonds formed between molecules in the films were reduced. With the incorporation of CPE, the barrier performances against light, oxygen, and water vapor as well as the antioxidant activity of the SPI films were increased. The SPI/CPE films were successful against Staphylococcus aureus (Gram-positive bacteria). Thus they suggested that the shelf-life of foods may be extended with the use of SPI/CPE films [60]. In another study, Liang and Wang [61] developed a pH-sensing film using a natural dye extracted from litmus lichen (LLE) and tamarind seed polysaccharide (TSP). The characterization outcomes exhibited that the interaction between LLE and TSP was via hydrogen bonding. The film color differed from orange (pH 4.0) to blue/violet (pH 10.0). A full cream milk spoilage test indicated that this film is a perfect solution for detecting this spoilage problem. Therefore the developed pH-sensing film can be utilized as an assuring food spoilage indicator [61].

References

Go´mez-Mascaraque et al. [57] also studied different food-grade biopolymers (mostly carbohydrates, proteins, and some biopolyesters) as potential solutions to encapsulating matrices for the protection of sensitive bioactives or as nanostructured packaging layers to regulate the growth of pathogenic bacteria and viruses [57].

16.6 Conclusion In this day and age, synthetic polymers are generally manufactured from petrochemicals, which are nondegradable. The use of biodegradable polymers decreases the toxic effects that nonrenewable plastics have on the environment. Even though the complete replacement of all conventional polymers with environment-friendly materials is an almost impossible task to achieve, it is possible to at least try to do that in some applications such as in food packaging. Common materials for producing green and environment-friendly packaging materials typically come from natural resources such as (but not limited to) plants or animal polysaccharides, plants or animal proteins, wood pulps, etc. These materials (bioplastics) can have applications in edible coatings, paperboards, carry bags, wrapping films, containers, etc. To make up for the shortcomings of bioplastics, however, the incorporation of nanoparticles is a great method to enhance the performance of biobased films. It also reduces the waste associated with the packaging of processed foods that extend the shelf-life of fresh foods. Even though, the use of bioplastics in replacing traditional plastics seems promising, it is, as the saying goes, “too good to be true!” Even if the use of nanotechnology helps in reducing the defects of bioplastics, the food industry applications of all bioplastics still require further investigations and research. For example, as the migration of monomers to food is a highly concerning issue in conventional packaging materials, the same issue should be considered for new bioplastic technology as well. The presence of nanoparticles, especially metallic nanoparticles, in bioplastics and their migration to foods can cause contamination and other health and safety problems that science may not yet be aware of. Therefore it is essential to study, assess, and understand the properties, structures, and behaviors of advanced materials for future food packaging applications. The best practice for a greener tomorrow, however, is to have the packaging industry and consumers reduce their production and usage of packaging materials respectively, and to create less waste in general.

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[39] Kadam MD, Thunga M, Wang S, Kessler RM, Grewell D, Lamsal B, et al. Preparation and characterization of whey protein isolate films reinforced with porous silica coated titania nanoparticles. J Food Eng 2013;117:133 40. [40] Peelman N, Ragaert P, Meulenaer DB, Adons D, Peeters R, Cardon L, et al. Application of bioplastic for food packaging. Trends Food Sci Technol 2013;1 14. [41] Serna CP, Filho JFL. Biodegradable zein-based blend films: structural, mechanical and barrier properties. Food Technol Biotechnol 2015;53(3):348 53. [42] Malathi AN, Kumar N, Nidoni U, Hiregoudar S. Development of soy protein isolate films reinforced with titanium dioxide nanoparticles. Int J Agric Environ Biotechnol 2017;10(1):141 8. [43] Xu L, Jiang X, Zhao Y, Xia L. Preparation of starch-based biodegradable film and the application in agriculture. J Agric Sci 2017;9(3):1 6. [44] Rhim WJ, Lee HJ, Perry KW. Mechanical and barrier properties of biodegradable soy protein isolate-based film coated with polylactic acid. LWT 2007;40:232 8. [45] Vasilevskaya V, Yoshikawa K. Hybrids of synthetic polymers and biopolymers. In: Kobayashi S, Mu¨llen K, editors. Encyclopedia of polymeric nanomaterials. Berlin, Heidelberg: Springer; 2014. [46] Tajeddin B. Cellulose-based polymers for packaging applications. In: Thakur VK, editor. Lignocellulosic polymer composites. Willey Scrivener Publishing LLC; 2014. p. 477 98. [47] Tajeddin B. Natural nano-based polymers for packaging applications (chapter 8). In: Thakur VK, Thakur MK, editors. Eco-friendly polymer nanocomposites: chemistry and applications. India: Springer; 2015. p. 239 77. [48] Cannarsia M, Baiano A, Marino R, Sinigaglia M, Del Nobile MA. Use of biodegradable film for cut beef steaks packaging. Meat Sci 2005;70:259 65. [49] Sorrentino A, Gorrasi G, Vittoria V. Potential perspectives of bio-nanocomposites for food packaging applications. Trends Food Sci Technol 2007;18(2):8495. [50] Lam D. Packaging application using nannotechnology. San Jose State University; 2010. [51] Ozilgen S, Bucak S. Functional biopolymers in food manufacturing (chapter 6). In: Grumezescu AM, Holban AM, editors. Biopolymers for food design, a volume in handbook of food bioengineering. Academic Press; 2018. p. 157 89. [52] Chi H, Xue J, Zhang C, Chen H, Li L, Qin Y. High pressure treatment for improving water vapour barrier properties of poly(lactic acid)/Ag nanocomposite films. Polymers 2018;10(9):1011. [53] Lopusiewicz L, Jedra F, Mizielinska M. New poly (lactic acid) active packaging composite films incorporated with fungal melanin. Polymers 2018;10(4):386. [54] Thanakkasaranee S, Kim D, Seo J. Preparation and characterization of poly (etherblock-amide)/polyethylene glycol composite films with temperature-dependent permeation. Polymers 2018;10(2):225. [55] Guidotti G, Soccio M, Siracusa V, Gazzano M, Munari A, Lotti N. Novel random copolymers of poly (butylene 1,4-cyclohexane dicarboxylate) with outstanding barrier properties for green and sustainable packaging: content and length of aliphatic side chains as efficient tools to tailor the material’s final performance. Polymer 2018;10(8):866. [56] Kuorwel KK, Cran JM, Sonneveld K, MiltZ J, Bigger WS. Antimicrobial activity of biodegrdable polysaccharide and protein-based films containing active agents. J Food Sci 2011;76:90 106.

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[57] Go´mez-Mascaraque LG, Fabra MJ, Castro-Mayorga JL, Sa´nchez G, Mart´ınez-Sanz M, Lo´pez-Rubio A. Nanostructuring biopolymers for improved food quality and safety. In: Grumezescu AM, Holban AM, editors. Biopolymers for food design, a volume in handbook of food bioengineering. Academic Press; 2018. p. 33 64. [58] Razavi SMA, Behrouzian F. Biopolymers for fat-replaced food design. In: Grumezescu AM, Holban AM, editors. Biopolymers for food design, a volume in handbook of food bioengineering. Academic Press; 2018. p. 65 94. [59] Iordanskii A, Zhulkina A, Olkhov A, Fomin S, Burkov A, Stilman M. Characterization and evaluation of controlled antimicrobial release from petrochemical (PU) and biodegradable (PHB) packaging. Polymers 2018;10(8):817. [60] Liang S, Wang L. A natural antibacterial-antioxidant film from soy protein isolate incorporated with cortex Phellodendron extract. Polymer 2018;10(1):71. [61] Liang T, Wang L. A pH-sensing film from tamarind seed polysaccharide with litmus lichen extract as an indicator. Polymers 2018;10(1):13.

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Polymers in cosmetics

17

Rohini P. Gawade1, Shamal L. Chinke1,2 and Prashant S. Alegaonkar3 1

Department of Applied Physics, Defence Institute of Advanced Technology, Pune, India 2 Department of Electronic Science, Savitribai Phule Pune University, Pune, India 3 Department of Physics, School of Basic and Applied Sciences, Central University of Punjab, Bathinda, Punjab, India

17.1 Introduction Cosmetics can be defined as the science that deals with articles that are intended to be applied on the human body or any of its parts for beautifying, cleansing, making it to look attractive, promoting, and/or modifying the appearances [1
4]. Polymers plays a crucial role in the formulation of cosmetics [5], hence, cosmetic products contain different types of polymers in their formulations according to their intended functions [6]. Polymers are used in cosmetic formulations as rheological modifiers, emulsifiers, stimuli-responsive reagents, conditioners, film formers, fixations, foam stabilizers, skin-feel beneficial agents, antimicrobials, and so forth [5]. Broadly, in cosmetics, polymers can be classified into four main types, namely (a) synthetic polymers, (b) polysaccharide-based polymers, (c) proteins, and (d) silicones [7]. Cosmetic products with polymer-type ingredients are shown in Fig. 17.1. In the 21st century, cosmetic and personal care industries are increasing rapidly with the development and use of new synthetic polymer-based products [8]. Monomers are the building blocks of synthetic polymers. Polysaccharides have been in cosmetic use for centuries and they play a vital role in the development of personal care formulations. When other ingredients are added to polysaccharides, their properties are modified, which makes them more suited for cosmetic applications [9]. Polysaccharide-based polymers are safer for use than syntheticbased polymers [10]. Polysaccharide-based polymers in liquid form may exist as loose rigid helices or randomized coils [11,12]. They can be cationic, nonionic, anionic, or amphoteric depending on the chemical identity in their native group, and their properties can be modified by the temperature, concentration, salts present, and so forth. They are best suited in rheology modifiers, conditioners, and healing and suspending agents [13,14]. They are used in the formulation of moisturizers, hydrators, and emulsifiers [15]. It is difficult to understand their functionality, however, a lot of research has been going on in this field in the past decade. Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00017-2 © 2020 Elsevier Inc. All rights reserved.

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FIGURE 17.1 Typical classification of polymers used in cosmetics and cosmetics products.

The types of polysaccharides and their functions in cosmetic and personal care industries are discussed briefly in further sections. The use of proteins derived from milk, egg, and so forth, for hair/skin care has been well known since ancient times [16,17]. Proteins extracted from animal fats, plant oils, and mineral pigments have promising applications in this field and, therefore, attract the attention of many researchers. In the past few decades, there have been major developments in the extraction methods/techniques of these proteins. These ingredients are environment-friendly and their properties and use for cosmetic formulations are verified scientifically. Studies revealing the relationship between the molecular characteristics and cosmetic applications of proteins have been reported [18,19]. By making use of the amphoteric and buffering properties of proteins, a number of products providing glow, softness, and conditioning to hair have been developed [20]. In this chapter, a brief discussion on the applications of proteins in hair/skin care and cleansing formulations is presented. The use of silicone-based materials in hair/skin care products has been well known since the mid-20th century. Polydimethylsiloxanes/dimethicone was the first silicone family that was commercially used in the cosmetics industry [21]. Though dimethicone is used in skin and hair care products, skin care products have attained more popularity among customers. In the late 1940s, Revlon launched Silicare skin lotion, which became extremely popular and attracted the interest of customers and manufacturers. This triggered scientists and engineers to work in this field and soon afterward, they had come up with a wide range of products. They observed that when dimethicone is used as an ingredient, it provides protection to the skin and develops a breathable barrier on the skin [22]. In 1950, Sudden Date, a silicone-based lotion spray, entered the hair care market, which gave birth to a new area of research, that is, the application of silicones in hair care products. In the past few decades, the use of silicone ingredients in cosmetic

17.3 Use of polymers in cosmetics

products modifying properties of the formulations and made them suitable for skin/hair care products [23,24] is discussed in further sections.

17.2 Understanding polymer/surfactant interactions The use of polymers in cosmetics is mainly governed by polymer/surfactant interactions, hence, polymer/surfactant interactions are discussed here. Surfactants are commonly used in skin cleansing products, spreading agents, emulsifiers, and so forth, as they break up the oily component and make it easy to wash [25]. Surfactants are classified based on the charge present in its hydrophilic group. Surfactants interactions with polymers are controlled by Van der Waals forces, dispersive forces, hydrophilic effect, and dipolar/acid base and electrostatic interactions [26]. Several factors play a mutual role in affecting surfactant and polymer interactions. (1) The chain length of the surfactant; where, if the polymer is uncharged, then the binding concentration of the ionic surfactant in homogeneous series decreases with increases in the chain length of the surfactant [27]. (2) The structure of the surfactant; in which the nature of the head group in the surfactant governs the interactions of the surfactant with uncharged/water-soluble polymers [28]. Nonionic surfactants are unreactive toward simple uncharged polymers, while anionic surfactants are strongly reactive toward cationic polymers, but weakly/nonreactive to anionic polymers. (3) Polymer characteristics including (a) the weight of the polymer used—a minimum range of weights of polymer is required for the interaction to occur. Particularly, hydrophobically modified polymer required weight 1000 times larger and readily interacts with caffeine leave but also nonionic surfactant. (b) The amount of polymer, where the amount of polymer and surfactant must be the same for effective interactions between the two. (c) Polymer structure as there is a definite difference in the reaction affinity of polymers and surfactants. For effective interactions between ionic surfactants and uncharged polymers, there must be definitive differences in the reaction affinity between the polymers and surfactants. (d) Added salt, where the addition of salts affect the interactions between ionic surfactants and polymers [29].

17.3 Use of polymers in cosmetics In this chapter, the use of polymers in cosmetics is discussed based on their classifications, namely (a) synthetic polymers, (b) polysaccharide-based polymers, (c) proteins, and (d) silicones, used in cosmetic formulations.

17.3.1 Synthetic polymers In the 21st century, the use of synthetic polymers is tremendously increasing in the cosmetics industry and in personal care products. Synthetic polymers are classified

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FIGURE 17.2 Structures of some commonly used synthetic polymers in cosmetics.

into two major types as condensation polymers or additive polymers. Water-soluble synthetic polymers like polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyethylene oxide (PEO), and so forth, are blended with natural polymers like collagen, elastin, keratin, silk, and gelatins to make thin films, hydrogels, or other formulations used in cosmetics [30
32]. The structure of some synthetic polymers is shown in Fig. 17.2. Mostly, synthetic polymers are used as thickening agents by different mechanisms such as chain entanglement, covalent cross-linking, and by an associative mechanism.

17.3.1.1 Thickening by chain entanglement Chain entanglement is a simple and straight forward mechanism to achieve thickening in polymers. For cosmetics, the polymer chains are dissolved into the solvents like water, alcohols with low molecular weight, or combination of both, that provides soft entanglement. Then the viscosity of the solution increases with increases in the concentration of polymer because more chains are occupied in less space [33]. Further, with increases in the polymer concentration, the separate individual polymers become difficult because of the shear force generated over the given area. Additionally, increases in molecular weight play a crucial role in chain entanglement. The chemical identity does affect the behavior of polymers, copolymers of cross-linked polymers, and hydrophobically modified polymers. Polymers with different viscosities behave differently in formulations as simple linear polymers such as poly(methacrylic acid), PEO, PVA, PVP influence solution viscosity through random chain entanglement [34
36].

17.3.1.2 Thickening by covalent cross-linking Thickening by covalent cross-linking can be realized when two polymer chains are attached to each other by the interactions of a bifunctional monomer, which reacts with both the chains, radically modifying their properties [37,38]. Such cross-linking in an aqueous solution of polymers increases the usefulness of those

17.3 Use of polymers in cosmetics

polymers to some extent. The vulcanization of rubber is the most familiar example of functional cross-linking. Rubber is a form of natural gum. It is a tacky flowing resin when harvested from the rubber tree. When the this is heated in the presence of sulfur, it gets covalently cross-linked and makes the rubber used for automobile tires. Covalent cross-linking is one of the most important ways to achieve thickening; and acrylate copolymers, 2,5-Furandione, polymer with methoxyethene; copolymer of methyl vinyl ether and maleic anhydride decadiene cross polymer [39,40], carbomer [41,42], and acrylate/VA cross polymer [39] are the most popular polymers used in cosmetics.

17.3.1.3 Thickening by an associative mechanism Thickening can be achieved by an associative mechanism; hydrophobically modified polymers that provide surfactant-like behavior are used [43]. This associative aggregation affects the viscosity of the solution, spreading behavior, film thickening, and the feel of cosmetics [44]. An associative thickening is observed when the behavior of amphiphiles and surfactants in aqueous solutions is studied. A surfactant can perform two functionalities as a head group that is hydrophilic attached to a hydrophobic tail group and their behavior is governed by the interactions of surfactants in aqueous solutions [45]. Hydrophobes in liquid phase interfere with the hydrogen-bonding network in water molecules. These water molecules may create molecular cavities, allowing for a minimization in solvation energy. This process is referred to as the hydrophobic effect and is mainly a function of entropy. Hydrophobically modified acrylate associative thickeners, hydrophobically modified cationic acrylate associative thickeners, and hydrophobically modified polyether associative thickeners are examples of polymers in which thickening is achieved by an associative mechanism.

17.3.2 Polysaccharide-based polymers Polysaccharides are complex carbohydrates with hydroxyl groups that exhibit strong interactions with water, and their good mechanical properties make them suitable for use in adhesives, fibers, hydrogels, or as drug delivery agents in cosmetics [46,47]. Therefore polysaccharide-based polymers play a major role in cosmetic formulations along with surfactants, salts, and other polymers. These multipurpose polymers can be used in personal care products as thickeners, stabilizing agents, skin cleansers, sunscreens, and moisturizing ingredients [48]. The use of polysaccharides is increasing rapidly in cosmetics and in turn drawing the attention of manufacturers toward these long lifespan, green, raw materials having superior skin feel. The properties of polysaccharides are influenced by substituent groups bonded to monosaccharides. These substituents can be derived from nature or synthesized in a laboratory. Broadly, polysaccharides are classified into five main categories, mainly anionic, cationic, nonionic, amphoteric, and hydrophobic. Nowadays, these products are gaining popularity in skin care resins against inflammatory disorders and skin aging.

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17.3.2.1 Anionic polysaccharides Anionic polysaccharides are mainly comprised of materials found in nature. Polysaccharides are made anionic by the intervention of humans [49]. The most commonly and commercially available polysaccharides are carboxymethylcellulose (cellulose gum), carboxymethyl chitin, and carboxymethyl glucan [50]. Examples of naturally occurring anionic polysaccharides include alginic acid, pectin, carrageenans, xanthan gum, hyaluronic acid, chondroitin sulfate, and arabic, karaya, and tragacanthin gum. Cellulose gum (sodium carboxymethylcellulose) and carboxymethyl chitin are examples of seminatural anionic polysaccharides. Alzamic acid is a linear polysaccharide made up of uremic and mannuronic acids for use as a thickening agent in many formulations such as toothpaste, soap, sharing creams, and hair gels. Also, several natural polysaccharide polymers produced from plants are used as thickening agents, creams, suspensers, and so forth.

17.3.2.2 Cationic polysaccharides Cationic polysaccharides that are used in cosmetic industries mainly consist of a group of synthetically modified polyglycans because nature does not provide any polysaccharides that can carry cationic charge and contain nitrogen. For a polysaccharide to conduct a charge it requires synthetic intervention. The structure of cationic polysaccharide can be represented as shown in Fig. 17.3. Chitosan is a cationic polyglucan found in nature. It can be cationically charged up to pH , 7.0 [51
54]. Most of these polysaccharides are tightly bound to anionic surfaces. The isoelectric points of proteins for human skin and hair are B3.2 and 5.0 respectively because of which amino acids containing proteins of the hair and skin get negative charge over a useful pH range. This can be verified by treating the hair/skin by surfactant and subsequent dyeing that will allow to increase in the anionic charge of the surfaces of hair/skin. Because of the reverse charge exhibited by cationic polysaccharides, they are extensively used in damage control agents such as conditioners. Cationic polysaccharides having film-forming properties are mostly used in hair fixtures. It has been reported that most polysaccharides can be made cationic by human manipulations. Cationic hydroxyethyl +· CH2OCH2(CH2)nNHR1R2 Cl

CH2OH O

H

H OH H O

O

H

OH H

O H

OH

H

H

O H

OH q

FIGURE 17.3 Structure of cationic polysaccharide.

17.3 Use of polymers in cosmetics

cellulose, cationic guar, and cationic hydroxyl propyl guar are examples of cationic polyglycans that are of commercial significance, whereas other cationic polysaccharides are used in paper making and personal care products. Cationic polysaccharides are further classified into naturally occurring and seminatural cationic polysaccharides. Examples of naturally and seminatural cationic polysaccharides include chitosan, cationic hydroxyethyl cellulose, cationic guar/cationic hydroxy-polyguar. Cationic polysaccharides are widely used in hair conditioner to control hair damage. Polysaccharides can be neutralized with cosmetically functional carboxylic acid such as lactic acid or glycolic acid, a component of the natural moisturizing factor in skin.

17.3.2.3 Nonionic polysaccharides Nonionic polysaccharides are those that do not carry any formal charges, however, nearby charges may affect their charge characteristics [55]. Rheology modifiers and thickeners are the two main uses of these polysaccharides. Viscosity is associated with the ratio of amylose to amylopectin used in cosmetic formulations. Viscosity is associated with the ratio of amount of amylose to amylopectin used in cosmetic formulations. The higher the ratio; higher is the thickening. Factors that affect the behavior of nonionic polysaccharides include the solvent used, surfactants, polymers, added salts, and so forth [56]. The structure of nonionic polysaccharides can be seen in Fig. 17.4. A potential use of nonionic polysaccharides is as rheology modifiers/thickeners in cosmetic formulations. Guar-based materials and ether-modified cellulose are popularly used in the synthesis of nonionic seminatural polysaccharides. Most natural nonionic polysaccharides are not commonly used in personal care products, but are used in expensive thickeners. Examples of these polysaccharides are starch, maltodextrins/cyclodextrins, guar/locust bean gum, sclerotium gum, cellulose ethers, nitrocellulose, hydroxypropyl guar.

17.3.2.4 Amphoteric polysaccharides Amphoteric polysaccharides carry cationic as well as anionic charges on the same chain. The name “amphoteric” is derived from the Greek word “amphoteros” meaning both. Most amphoteric polysaccharides are seminatural and derived from natural polysaccharides through modification. This type of polysaccharide is not CH2 O

H OH

H

H

OH

O

OH

FIGURE 17.4 Structure of nonionic polysaccharides.

n

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–O CH C 2 2

OH NH2+

HO

O O

O O OH

HO

NH+ HO2CH2C

CH2CO2–

FIGURE 17.5 Structure of amphoteric polysaccharide.

well known in the cosmetics industry, hence, their use is limited. But in personal care, the use of amphoteric polysaccharides as surfactants is well known and popular. The structure of amphoteric polysaccharides is shown in Fig. 17.5. The use of amphoteric polysaccharides is challenging due to the both factors i.e. the presence of charges and insoluble mass in zwitter ionic form. Carboxyl methyl cytogen and modified potato starch fall under this category. Hydrophobically modified polysaccharides and polysaccharides with topical physiological effects are two other types of polysaccharides that are usually used in the preparation of cosmetics. Amphoteric polysaccharides are chemically sophisticated and need formulators to explore their applications. The pH of a cosmetic formulation is used to classify whether the polysaccharide is cationic, anionic, or both. It is difficult to formulate amphoteric polysaccharides as they have higher solubility in cationic or anionic form, but have low solubility in zwitterionic form. Amphoteric polysaccharides show complex chemistries when dealing with salts and surfactants. The addition of salts may change the value of the pH. When an amphoteric polymer is cationic, it is not compatible with anionic surfactants, and when it is anionic, it shows compatibility with anionic surfactants. The overall pH of a system is responsible for these changes. Examples of these polysaccharides are carboxymethyl chitosan, N-[(20 -hydroxy-20 ,30 -dicarboxy) ethyl] chitosan, and potato starch.

17.3.3 Proteins Milk protein was the first used in the cosmetics industry. The hydrophobicity of peptide linkages affected the cosmetic properties of the protein like substantivity to human skin, binding capacity, emulsification performance and solubility [22,64,65]. The protein ingredients in cosmetics are classified into five main categories of hydrolyzed protein. There are perhaps non-hydrolyzed protein like abdomen, calcium calcinate cosier, protein siren, banchan protein [66]. In hydrolyzed catagory like ammonium hydrolyzed, gelatine, and sink sialoprotein, [67]. In quaternary protein such as coca aminopropyl, dimethyl amino, collagen

17.3 Use of polymers in cosmetics

to sulfate, hydrolyzed collagen, and hydrolyzed plant protein. The next type is condensate isochloride hydrolyzed collagen, iodized hydrolyzed sodium subsided gelatine, and few other enzymatic types like amylase cartage, glucose oxide paper, and parser are also under the class of enzymes. Lather enhancer cellulosic protein is a personal care type product. The multifunctional family of water-soluble polymers, mythical cellulose ethers, are an extensive family of methyl cellulose (MC) and hydroxypropyl methyl cellulose (HPMC) polymers that are extensively used in personal care products [61]. They are extensively used as thickeners in liquid formulations and as binders in solid and semisolid formulations. Moreover, neta sol products are used as feel formers, lubricant, lather enhancers, and gelling agents in personal care products. Alkyl ethylated sulfate, sodium and/or potassium salts of coco such as n-alkyl triethylene glycol ethoxylated sulfate, alkyl triethylene glycol, ethoxylated sulfate, and alkyl hexa-oxyethylene sulfate are important lather enhancers used in formulations of mild cleaning agents. The commercial preparation diarylethene hydroxypropyl MC is highly pure water-soluble nonionic cellulose ether and it is used in personal care products like shampoos, body washes, and shaving creams as a thickener.

17.3.3.1 Proteins in skin care Proteins are widely used in several skin care and makeup products. Soluble protein ingredients are added to products in all forms such as gels, powders, and lotions. Insoluble proteins are used for specific applications as (1) micronized powders of insoluble elastin and keratin for use in cosmetic powders, (2) insoluble fibrous collagen prepared by the lyophilization of aqueous dispersions for use in moisturizers to help to make skin smoother and shinier [62,63], and (3) fibroin protein obtained from powder made by fine grinding of pure silk for use in preparations of lipsticks and other decorative products; when used, they make the skin smooth and provide lubrication to these preparations. It also helps in enhancing the oil
water absorption capability of these preparations. (4) Native proteins with higher than average molecular weights are used as ingredients in skin care products because of the film-forming property exhibited by them, for example, soluble collagen and desamido collagen, and because of the structure and length of their molecules they are able to form a colloidal film on the surface of skin, creating a soft feel. A hydrating effect is observed due to the exposure to a large number of hydrogen binding sites available for water-linking. Therefore they are added just after the process of emulsification and cooling (B5 C below the melting temperature). (5) Wheat protein derivatives were studied in aqueous solution form and in an O/W emulsion, which showed their effective use in improving the viscoelasticity of skin and providing a moisturizing effect. The film-forming properties were also improved using this. With these properties, protein derivatives can be made [64]. (6) Protein hydrolysates are used as one of the main ingredients in formulations of conditioners and moisturizers with active concentrations (B0.1%
2%) [62].

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17.3.3.2 Proteins in hair care Proteins and their derivatives are used in hair hygiene and care formulations including shampoos, conditioners, coloring lotions, hair straightening products, and so forth. It has been proven that proteins help in increasing the strength, elasticity, and softness of hair. Furthermore, they help repair split ends and protect from the adverse effects of bleaching. Different protein derivatives have been observed to be useful for specific hair treatments and the dose range varies with the formulation. The hair conditioning observed with proteins may be due to the property to get absorbed/react with cuticle keratin. Some proteins get attached to hair keratins, which are also used in shampoo formulations with shorter times of contact for shampooing procedures. Hence the protection and conditioning of hair can be achieved in application conditions which are non-favorable. Protein hydrolysates and their derivatives are used in shampoos, and when tested show increase in tensile strength [65]. The protective effect was observed to be more prominent in quaternized derivatives rather than parent hydrolysates [66]. When tested, protein hydrolysate of wheat was more effective than collagen hydrolysate because a higher hydrophobicity was exhibited by the wheat protein. The use of sericin hydrolysate in shampoo was studied and compared with a placebo [67], in which sericin peptides showed superior performances in hair smoothness, combability, and tolerability to the skin and eyes. Such protein derivatives also exhibit foaming properties, which make them useful in shampoo formulations [68]. Examples of these polymers are protein hydrolysates, soluble collagen, desamido collagen, serum albumin, sodium caseinate, gelatin, hydrolyzed wheat protein, polysiloxane copolymer, aminomethyl propanolisostearoyl hydrolyzed protein, alkyldiammonium hydroxypropyl (or ethyl) hydrolyzed protein, soluble keratin, and wheat protein.

17.3.3.3 Proteins in cleansing products Protein additives can be used to improve tolerability to the skin and eyes in cleansing products. It also provides protection from adverse effects such as skin dehydration, roughness, and so forth. Quantitative data on their protective action have been reported [69]. Protein fatty acids with high molecular weights are reported to be effective in increasing the tolerability of the eyes and skin to various anionic tensides; the same can be verified by red blood cell tests [70]. The quaternized derivatives of proteins are reported to exhibit antiirritant properties when used in anionic-based cosmetic products [71]. Examples of these polymers are sodium laureth sulfate, cocamidopropylamine oxide, disodium cocoamphodiacetate, pearlescent 35% (sodium laureth sulfate, glycol distearate), cocamide diethanolamine, PEG-7 glyceryl cocoate glycerin, citric acid, preservatives, dyes, water.

17.3.4 Silicones Over the past 50 years, silicon materials have been used in hair care and skin care in the cosmetics industry [72
75]. The incorporation of silicon into skin care

17.3 Use of polymers in cosmetics

products [76] is quite new as compared to hair care formulations. Ravel on was the first to launch skin care lotions containing silicon [77]. The increased use of silicon in cosmetics can be attributed to technological developments in suspending agents and processes such as emulsification and associative thickening. Organofunctional silicone has the additional advantage of providing multifunctional benefits to formulations. Silicon and its composites are used in cosmetics such as deodorants, shampoos, antiperspirants, lotions, and so forth. Silicon derived from natural products is known as silica basic sand and it can be used in personal care products after appropriate chemical modifications. Biomethane and cyclomethycaine are examples of modified silicones that are used in personal care products such as

FIGURE 17.6 Structures of commonly used silicones in cosmetics.

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water-proofing silkiness of hair, in deodorants to provide den vacate feel in sunscreens, and in creams as water resistant, lubricating, and messaging agents, as well as in antiaging and antiwrinkle creams. Dimethylene and dimethicone are two types of silicon with the ability to spread uniformly into thin films over the surface of hair and skin to provide hydrophobicity to the surface of skin and hair. The added lubricity, humidity resistant into hair creams may provide a path to protect skin. To increase the durability of thin films used on the skin and hair, high molecular weight dimethicone gums or silicon resin can be added into formulations. Alkynemodified silicone, also popularly known as Arms, is used in skin care sunscreens and the cosmetic products to form an acrosine barrier onto the skin. This material is an excellent moisturizing agent and reduces tears and epidermal water loss to a level comparable to petroleum jelly. In lipstick formations it is widely used as a stick ingredient agent to impart occlusive barrier to the lips, provide moisturizing effect, it enhances pigment release, however, reduces pigment transfer from one ingredient to another type. The structures of the most common types of silicones used in cosmetics are shown in Fig. 17.6.

17.3.4.1 Cyclomethicones Cyclomethicones are colorless, nonpolar, low-viscosity fluids with characteristic odor and high volubility [78]. They are insoluble in water, but partly soluble in organic solvents. They are represented by ring structures containing silicones. Commercially used silicones have 3
6 Si3/4O groups per ring. Cyclomethicones are used as carriers/diluents in hair care products as they help in the uniform delivery of fluids with high molecular weights to hair fibers. They are also used in as plasticizers in fixative products and in delivery systems in glossing sprays. Cyclomethicones are useful in conditioners and shampoos [79] as they support wet combing, but it is recommended to add external thickeners and stabilizing agents. Some researchers have reported an impressive use of cyclomethicones as a shine enhancer to hair. In luster formulations, they are used to provide uniform deposition materials in the formulation. Cyclomethicones have a low heat vaporization property that makes them suitable to be applied in hair/skin care formulations for shorter times. D4 evaporates in 10 minutes at 37 C and in 25 minutes at 30 C because of which they are also used in deodorants. Cyclomethicones exhibit lower value of evaporation so they can also be employed in the systems where low friction is required such as skin care applications. Cyclomethicones are used as carriers for delivering uniform pigments over the skin, as thickeners, as sticking agents, and as lubricants.

17.3.4.2 Dimethicone Dimethicone is referred to as a capped silicone polymer because of the attachment of nonreactive trimethylsilyl as the end group. On the other hand, dimethiconol is an uncapped polymer that may undergo self-condensation in drying with hydroxyl/methoxyl end units. Dimethicone and dimethiconol uniformly and readily spread on hair/skin, resulting in the formation of thin hydrophobic films

17.3 Use of polymers in cosmetics

providing lubricity, increases in shine, protection of hair from humidity, and generating a breathable barrier on the skin. Dimethicone can be used as a defoamer to protect from soaping effects in formulations containing fatty alcohols. Dimethicone polymers have high diffusivities for gases, this allows respiration of the skin, and, therefore, several of these are used as skin protectors [80,81]. Dimethicone liquid formulations are used in skin care to lubricate and make the skin soft. Dimethicone high molecular weight gums are used as the main ingredient in color cosmetics and sunscreen lotions. Polydimethylsiloxane polymers are used in conditioner additives and shampoos. Dimethyl silicone fluids are used as one of the main ingredients in some bleaching and dyeing formulations to provide conditioning during the application of the same [82
85]. In temporary hair dyes, organofunctional silicones have shown reduced rub-off tendencies when exposed to hands/cloths [86]. Dimethyl silicone fluids are used in hair straightening formulations [87].

17.3.4.3 Amodimethicone Amodimethicone is a fluid that is polymerized by emulsion, prepared by linear processing technology, and its emulsification is performed by mechanical route. The most popularly used amodimethicone emulsions contain tallowtrimonium chloride and nonoxynol-10 or cetrimonium chloride and trideceth-10 or -12 as functional pairs. These functional aminosilicones may be linear or branched in structure. In both cases, they undergo a condensation reaction while drying in forming films on hair and provide wet as well as dry combing advantages. They lower the effects of charge and increase the softness of hair when used in hair formulations. They are found to extremely good in conditioners, lotions, and two-inone shampoos.

17.3.4.4 Alkyl-modified silicones Alkyl-modified silicones (AMS) are well known for their use as lubricants in textile and metallic parts manufacturing industries. In the past two decades, these materials have been introduced in personal care formulations and have now become popular in western countries. The main reason is: earlier grafting of alkyl groups onto silicone polymers generated linear long-chains that resulted into hard and brittle waxes. Introduction of alkyl moieties, broadly, modified the molecular symmetry that resulted into forming hard wax materials. Their melting points and softness can be programmed by the synthesis of linear/branched copolymers and long/short chain olefin moieties on the siloxane backbone. Alkyl-modified silicones have moisturizing behavior with compatibility to other ingredients, which make them suitable candidates in moisturizer formulations. These AMS polymers can give substantivity, rheology modification, improved stability, and lubrication in cosmetic and hair care products such as conditioners [88
92]. Because of the sticking properties exhibited by these AMS polymers, they are used in lipstick formulations as they enhance the sticking properties and create an occlusive barrier for moisturization. The series of alkyl-substituted siloxanes includes

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silicon hybrids. The synthesis of alkyl-substituted siloxanes requires a suitable solvent using a platinum catalyst and combining unsaturated α-olefins with silicon hybrid compounds. The alkylation of silicon enhances its character of moisturization and its compatibility with other cosmetic raw materials. The alkylation of silicones provides enhancements in the rheology and product stability of products. Silicon is compatible with terpolymers of dimethyl, methyl alkyl, methyl polyethers and adds functionality to them. Cetyl dimethicone copolyol can be used to improve the stability of several cosmetic formulations. They are used as rheology modifiers in cosmetic formulations from low-viscosity lotions to thick creams. Alkyl-modified silicones act as adhesive barriers on the skin in sunscreen creams. Silicones are also used in skin care and color cosmetic products because of their high degree/longer chain alkylation, which improves the water resistance property of formulations. Alkyl-modified silicones are available in different physical forms. In methicone, physical forms are given along with the silicon polymer, where there are physical forms ranging from liquid to hard wax. Decyl dimethicone is a liquid physical form, lauryl dimethicone is another liquid physical form, cetearyl methicone is a soft wax, stearyl dimethicone is a brittle wax, and stearoxy dimethicone is a hard wax. The major benefit these types of silicones is that they can act as rheology modifiers, emulsifiers, moisturizing agents, and provide resistance to water.

17.3.5 Examples and case studies 17.3.5.1 Lather enhancer cellulose in personal care Methocel cellulose ethers are polymers that are soluble in water and belong to the class of MC and HPMC polymers having a wide range of applications in cosmetics such as thickeners in liquid and binders in solid formulations. Methocel products also act as film formers, lubricants, lather enhancers, and gelling agents in personal care products. Alkyl ethoxylated sulfate and sodium salts of ethylene/ tallow alkyl triethylene sulfate are used as lather enhancers in cleansing agents. Commercially prepared hydroxyl propyl MCs are water soluble, nonionic, cellulose ethers that are used in personal care products like shampoos, body washers, shower gels, and shaving creams as thickeners, lather enhancers, water binders and film formers.

17.3.5.2 Polymers in hair care The main uses of polymers in hair care formulations are as fixatives and viscosity controllers. Nowadays, they are also used as conditioning agents. The most commonly used polymers in hair care products are guar gum, cellulose, proteins, polypeptides, chitosan, lanolin, starches, sugars, and aminosilicones. Polymers are used as a primary ingredient in hair care and styling products. The role of polymers in hair care products include conditioning to improve substantivity or other ingredients to hair improving combing, curl attention, thickening formulations, and

17.3 Use of polymers in cosmetics

enhanced emulsions. The most important polymers used in hair care products are cationic polymers. Many commercially available shampoos use cationic surfactants. To make use of anionic polymers in shampoo formulations, one needs to utilize charges; excess cationic charges or surfactant can also be utilized by using anionic surfactants. In cationic polymers, the large molecular weight polymers spread slowly onto hair in comparison to the small molecular weight polymers. Along with molecular weight, the charge also plays an important role in the absorption of formulations in hair. Charged polymers absorb faster than uncharged polymers. The maximum absorption of a polymer is at pH 7 and absorption decreases at pH 4 to pH 10.

17.3.5.3 Application of acetylene-derived polymers for personal care A German chemist, Walter Reppe, invented PVP from acetylene by a process known as the Reppe process. In this process, at high temperature and pressure conditions, acetylene is allowed to react with formaldehyde to give methandienone. Methandienone on oxidization gave gamma butyrolactone. It further reacting with ammonia produces 2-pyrilidone that finally yield to give PVP l or methyl vinyl ether series of polymers. These types of polymers have film-forming capacity and are proven to be important ingredients in hair styling products. Lepton-based polymers are functionally active and are used in various personal care products like hair sprays, hair styling gels, hair conditioning agents, teeth whitening agents, and water-proofing sunscreens. PVP has ability to form thin film due to less frictional properties and ability to thicken the medium compounded with properties like lubricant, skin protectant, adhesion promoter, and gelling agent in cosmetics used for personal care. PVP has been used in both the pharmaceutic and the cosmetic industries as well as in food the industry as an adhesion agent in the pen industry. In the pharmaceutical industry, PVP is used as a binder. It is also used as a disintegrating agent. In the case of tablets, it is used as a suspending agent in suspensions and also in solutions to increase viscosity. In the cosmetics industry, it is used as a film former for hair spray, high setting lotions, and in conditioning shampoos. In the food industry, it is used in the symbolization of beverages and in the pen industry. It is used as an adhesive and as adhesive sticks. In this module, a number of polymers, their applications in a number of fields of the cosmetics industry, and their prototypes with examples have been seen. The various types of polymers that are used in cosmetic applications and the use of these polymers in various types of cosmetic products have been discussed.

17.3.5.4 Cosmetic use of chitin and chitosan Chitin and chitosan are not found in the skin of humans, but when they are applied on the skin, they speed up the wound-healing process and minimize scarring. Chitin fibers are prepared by weight spilling in a 14% sodium hydroxide solution and used as nonallergic dualizing antibacterial and moisturizer controllers in cosmetics. Chitin is a highly effective hydrating agent; it supplies water and it avoids dehydration and provides long lasting hydrating effects. Chitin and its

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derivatives allow active principles in close contact with the skin. Chitosan is the only naturally occurring cationic polysaccharide. Chitosan is a random copolymer comprising of N-acetyl-β-D-(1,4)-glucosamine and β-D-(1,4)-glucosamine generally in proportions of 1:4. Chitosan salts have a strong affinity to the anionic structures of skin and hair. It is an important film-forming polysaccharide that is widely used in cosmetic formulations and hair shampoos. Chitosan also facilitates the formation of a thin film on the skin surface that keeps the active ingredient intact onto the skin. Enhanced amount of chitin and chitosan are widely used in skin creams, shampoos, skin lotions, warmish, and other uses.

17.4 Conclusion Polymers play a vital role in cosmetic formulations, and cosmetic products contain different types of polymers according to their intended functions. In this chapter, the use of synthetic polymers as thickening agents and their mechanisms such as chain entanglement, covalent cross-linking, and the associative mechanism were discussed. The five main types of polysaccharide-based polymers (anionic, cationic, nonionic, amphoteric, and hydrophobic) used in cosmetics were studied with their use as thickeners, stabilizing agents, and as an ingredient in skin cleansers, sunscreens, toothpastes, shaving creams, hair gels, moisturizers, and antiaging formulations. Proteins and their derivatives exhibit properties such as substantivity, binding capacity, solubility, film formation, and emulsifying performance that make them well suit for cosmetic formulations. Silicones and their derivatives are studied with their use in deodorants, shampoos, sunscreen lotions/creams, messaging agents, antiaging and antiwrinkle creams, and lipstick formations.

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8. [2] Jones A, Kramer R. Facial cosmetics and attractiveness: comparing the effect sizes of professionally-applied cosmetics and identity. PLoS One 2016;11(10):e0164218. [3] Aranaz I, Acosta N, Civera C, Elorza B, Mingo J, Castro C, et al. Cosmetics and cosmeceutical applications of chitin, chitosan and their derivatives. Polymers 2018;10 (2):213. [4] Sakamoto K, et al., editors. Cosmetic science and technology: theoretical principles and applications. Elsevier 2017. [5] Lochhead R. The role of polymers in cosmetics: recent trends. ACS Symp Ser 2007;3
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[30] Suberlyak ОV, Mel’nyk YY, Skorokhoda VI. Regularities of preparation and properties of hydrogel membranes. Mater Sci 2015;50(6):889
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18. new high molecular weight protein surfactant. SO [71] Henkel KGaA. Cospha protein hydrolysates and protein derivatives. Technical Bulletin; 1992. [72] Rochow EG. Silicon and silicones. New York: Springer-Verlag; 1987. [73] Noll W. Chemistry and technology of silicones. New York: Academic Press; 1968. [74] Kulkarni R, Goddard E, Kanner B. Mechanism of antifoaming action. J Colloid Interface Sci 1977;59(3):468
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[75] Kulkarni RD, Goddard ED, Rosen MR. Anitfoams. J Soc Cosmet Chem 1979;30 (2):105. [76] Pellicoro C, Marsella R, Ahrens K. Pilot study to evaluate the effect of topical dimethicone on clinical signs and skin barrier function in dogs with naturally occurring atopic dermatitis. Vet Med Int 2013;2013:1
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227S. [79] Rathi S, D0 Souza P. Shampoo and conditioners: what a dermatologist should know? Indian J Dermatol 2015;60(3):248. [80] Stewart David P, Sherman Laura B. Develoments at the Iran-United States claims tribunal: 1981-1983. Va. J. Int’l L 1983;24:1. [81] Mills MJ. Fishery data series NO. 90-44 Harvest and Participation in Alaska Sport FIsheries During 1989l. (1990). [82] Federal Register 1990;55(119):25203. [83] Brieva H, Jose N, Tietjen M, assignee. US 5,066,485, Revlon, Inc, November 19, 1991. [84] Fridd PF, Taylor RM, assignee. DE 3,706,053 Al, Dow Corning Ltd, February 26, 1986. [85] A, assignee, JP 4,059,721, Kao Corp, February 26, 1992. [86] Mandrange A, Canivet P, assignee. GB 2,198,949 A, L’Oreal, October 14, 1987. [87] Wolfram LJ, Cohen D, assignee. US 4,770,873, Clairol Inc, September 13, 1988. [88] Berthiaume MD, Miranda PM, assignee. US 5,684,112, General Electric Co, November 4, 1997. [89] Lance-Gomez TE, Husam AA, assignee. US 5,393,521, DEP Corp, February 28, 1995. [90] Berthiaume MD, Baum AD. Organofunctionlized silicone resins for personal care applications. J Soc Cosmet Chem 1997;48(1):1. [91] Marcus FK, unassigned. DE 3,030,119 A1, August 6, 1980 [92] Moses RE, Roberto FM, assignee. US 5,213,793, T Gillette Co, May 25, 1993.

Further reading Ahmed S, Ikram S. Chitosan based scaffolds and their applications in wound healing. Achiev Life Sci 2016;10(1):27
37. Andreu V, Mendoza G, Arruebo M, Irusta S. Smart dressings based on nanostructured fibers containing natural origin antimicrobial, anti-inflammatory, and regenerative compounds. Materials 2015;8(8):5154
93. Arca H, Mosquera-Giraldo L, Bi V, Xu D, Taylor L, Edgar K. Pharmaceutical applications of cellulose ethers and cellulose ether esters. Biomacromolecules 2018;19(7):2351
76. Azuma K, Izumi R, Osaki T, Ifuku S, Morimoto M, Saimoto H, et al. Chitin, chitosan, and its derivatives for wound healing: old and new materials. J Funct Biomater 2015;6 (1):104
42.

Further reading

Dai T, Tanaka M, Huang Y, Hamblin M. Chitosan preparations for wounds and burns: antimicrobial and wound-healing effects. Expert Rev Antiinfect Ther 2011;9(7):857
79. Golann S. On description of family therapy. Family Process 1987;26(3):331
40. Gutha Y, Pathak J, Zhang W, Zhang Y, Jiao X. Antibacterial and wound healing properties of chitosan/poly(vinyl alcohol)/zinc oxide beads (CS/PVA/ZnO). Int J Biol Macromol 2017;103:234
41. Ho¨ssel P, Dieing R, No¨renberg R, Pfau A, Sander R. Conditioning polymers in today’s shampoo formulations
efficacy, mechanism and test methods. Int J Cosmetic Sci 2000;22(1):1
10. Keary C. Characterization of METHOCEL cellulose ethers by aqueous SEC with multiple detectors. Carbohydr Polym 2001;45(3):293
303. Liu H, Wang C, Li C, Qin Y, Wang Z, Yang F, et al. A functional chitosan-based hydrogel as a wound dressing and drug delivery system in the treatment of wound healing. RSC Adv 2018;8(14):7533
49. Matras Z, Kopiczak B. The effect of surfactant and high molecular weight polymer addition on pressure drop reduction in pipe flow. Braz J Chem Eng 2016;33(4):933
43. Ohlinger C, Kraushaar-Czarnetzki B. Improved processing stability in the hydrogenation of dimethyl maleate to γ-butyrolactone, 1,4-butanediol and tetrahydrofuran. Chem Eng Sci 2003;58(8):1453
61. Cosmedia Triple C: for hair and skin care products. Focus on Surfactants. 2010;2010(3):4. St.Denis T, Dai T, Huang Y, Hamblin M. Wound-healing properties of chitosan and its use in wound dressing biopharmaceuticals. Chitosan-Based Syst Biopharmaceut 2012;429
50.

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Polymers in food

18 Pathik Shah

CIPET-Institute of Plastics Technology, Ahmedabad, India

18.1 Introduction Polymer particles in their complex structure are straight long chains, branched and cross-linked with covalently attached monomers. Single monomers can likewise have diverse traits, for example, charge, hydrophobicity, and adding complications. These complex structures are what frequently provide polymers with their ability to be used in foodstuff [1]. Most polymeric foodstuffs are useful segments of living beings and are devoured as either living beings or fixings from previous living beings. All foodstuff comprises macromolecules, and almost all of these macromolecules are polymers of some kind. The expanding trend of fast food items with a long time span of usability adds to the development of novel ingredients that guarantee that an item’s inherent characteristics and look are not significantly changed. Knowledge of food polymers in the commercial market is essential as it will offer appreciated guidance toward the improvement and largescale production of foodstuffs. New investigations of polymeric foods comprise “molecular design, synthesis, extraction, modification, structure and property, materials preparation, and applications,” which provide unique guidelines in food science [2]. Hence research in polymer science for foodstuff utility could offer a superior knowledge of food schemes, make healthier usage of polymeric food, and increase nutrient qualities. Developments in the research area of polymers, particularly polymeric food, experience rapid improvements with the aim of developing foodstuff systems. Food structures normally contain an unpredictable blend of polypeptides and polysaccharides, and their blending, combinations, and structures varying in chain length, substance and physical properties, and conglomeration [3]. Polymeric foods obtained from plants, animals, and microorganisms that are utilized in food systems contain peptides, polysaccharides, and proteins. Natural polymeric foods play a significant part in the structure, functional characteristics, and shelf life of foods [4]. Hence the analysis of polymeric food utility could offer a greater knowledge of food patterns, increase food standards, and provide improved usefulness to food polymers. Polymeric foods can be effectively consumed by humans entirely or in part, through the mouth and provide innocuous Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00018-4 © 2020 Elsevier Inc. All rights reserved.

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impact to the health. Polymeric foods might be connected specifically superficially as extra insurance to save item quality and soundness. There are several purposes for researching polymers in food. One of them is the demonstration of new food item classes, for example, protected, advantageous, and great items. They safeguard foods against the loss of supplements. Practically speaking, edible coatings that regulate the speed of transport of an item’s subatomic segments from within to the exterior of the packaging may delay unfavorable responses that are in charge of inconvenient changes in food items [5]. Technologists create and design embedded drug delivery systems prepared by natural polymers, which discharge accurate amounts of healing agents systematically after a certain period of time [6]. Polymeric drug systems using capsules of nanomedicine are utilized in chemotherapy treatment [7]. Polymeric foods are deliberated as an alternative to progressively conventional recycling processes and this motivates scientists to compose innovative polymers that can safely return to environmental cycles after use. In this way, the utilization of agrarian polymers that were effectively biodegradable would tackle these issues, yet would likewise give a proficient utility for surplus agricultural items. In the foodstuff packaging segment, starch-based substances have received excessive responsiveness due to its biodegradability, eatable, wide availability as agricultural surplus raw material, abundant, can be produced at low cost and at large scale, non-allergic, easy to use and thermo-processable [8]. Generally, foodstuff and chyme are phase-separated systems, but only the first steps have been taken toward understanding the phase behavior of polymeric foods in food processing and basically nothing in food digestion. The impact of thermodynamic compatibility of mucopolysaccharides, food fibers, and exopolysaccharides of different microbes species on the selection of microflora in the intestine and colon have not been studied till date [3]. The components of bacteriological formation in the digestive system and the peristaltic motions of nutrients have likewise not been studied yet. Our perception regarding generic immunity and humanoid nourishment including the systems of food processing and retention, is incredibly weak. Microbes, which can be overwhelmed with foodstuff, the characteristics of their surfaces, and the chemical kinetics of their polysaccharides and polypeptides including in materials with dietary strands, hydrolyzing chemicals, and mucopolysaccharides, are yet to be explored. Among natural polymers, polysaccharides and polysaccharide containing substances are largely responsible for ecological contacts of living entities mainly for nonspecific immunity and, together with proteins (enzymes), for digestion of food. Most polymeric foods are comprised of the three fundamental kinds of natural-origin polymers, namely polysaccharides (i.e., starches, celluloses), polyamides (proteins), and polynucleotides like DNA and RNA found in cell material [1]. Polymeric foods and the behavior of their blends are principally accountable for the structure properties relationship in foodstuffs and nutrients. Any foodstuff contains two main entities, first polymeric structures (proteins and polysaccharides) and water as the main medium, solvent, and plasticizer.

18.1 Introduction

Polymeric foods are mainly made up of multicomponent physical systems; therefore, the interfaces amongst segments are more extraordinary than the synthetic and physical features of each of the parts [9]. Generally, polymeric foods can be categorized into three groups as per their origin, namely (1) plant-based polymers such as cereal protein, dietary fiber, and starch, (2) animal-based polymers such as meat protein, and (3) synthetic polymers, which can be edible. The main benefit of edible food polymers over the earliest synthetic polymers is that they can be consumed with foodstuffs [10]. There is nothing left over as waste, and although these films are not consumable, food polymers could yet contribute toward a decrease of environmental contamination. Polymeric foods are manufactured entirely from renewable, palatable elements, and, hence, are expected to degrade rapidly compared to other polymer types. Polymeric foods can increase the organoleptic characteristic of wrapped foods, providing various constituents like flavors, colorings, and sweeteners [11]. The applications of natural polymers and food grade extracts have been constantly growing in the food manufacturing and clinical industries. Edible polymers may be manufactured with an expansion of natural crops including lipids, proteins, and polysaccharides with the mixing of plasticizers and surfactants. The proficiency of polymeric foods especially relies on their color, mechanical, and barrier properties, which sequentially depend on coating conformation and preparative method. Foodstuffs have typically been coated by way of dipping or spraying, thus, forming a thin film on the surface of foods that acts as a semipermeable membrane, which in turn manipulates moisture loss or/and suppresses gas transfer [12]. Polymers likewise work as bearers of antimicrobial and antioxidant agents. The generation of edible food polymers causes less waste and contamination, and their mechanical and barrier properties are commonly weaker compared to engineered polymers. Segments utilized for the arrangement of polymeric foods can be divided into four classes, namely lipids, hydrocolloids, polypeptides, and their composites. Hydrocolloid coatings have great permeable properties of gases and lipids, however, not of moisture. Most hydrocolloid polymers likewise have wonderful mechanical properties, which are helpful for delicate food items. Among them, protein-based polymeric foods are the most fascinating. Protein-based polymeric foods have amazing gas interference characteristics analogized with those of polysaccharides and fats. At the boiling point of water, the oxygen barrier property of a soy protein-based film was found to be inferior to that of a thin film of starch, methylcellulose, and gelatin respectively [13]. Polymeric foods having polypeptide components have higher mechanical properties than those of polysaccharide and lipid films, for example, rapeseed protein blended with gelatin [14]. Polypeptide foods can make chemical bonds at various places and provide the extraordinary potential for shaping various chemical links. Low-priced fish, for example, lizardfish, are typically dismissed from surimi fabricating due to poor surimi gel quality [15]. Thus they have until now, for the most part, been utilized as animal feed and additionally sold at low cost in the absence of techniques for

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utilizing them as foodstuffs. Currently, there is an environment-conscious tendency of reducing the use of synthetic polymer components. An alternative to this problem is to replace synthetic polymer packaging with innovative, biodegradable materials. Polymeric foods seem to be a respectable substitute for plastic foils [16]. As edible polymers were considered packaging as well as a food component, they should satisfy a number of requirements, such as high barrier and mechanical efficiencies, biochemical, physicochemical and microbial stability. Polymeric foods should also nonpoisonous, noncontaminating, and cheap. Food polymers have two kinds of configuration, namely (1) proteins (like myosin, collagen, keratin, and actin) and (2) polysaccharides (like long-chain cellulose and short-chain hemicelluloses). Similarly, there are numerous ingredients that are blends of these dual polymers, for instance, glycosaminoglycans, which have a variety of functional groups.

18.2 Classification of food polymers Food polymers can be classified by their basic chemical structure into four groups: (1) Polysaccharides (hydrocolloids), (2) polypeptides (proteins), (3) lipids (fat and wax), and (4) synthetic and composite food polymers.

18.2.1 Polysaccharides Hydrophilic polymers (also called hydrocolloids) are natural or synthetic polymers that are commonly comprised of multiple alcohol (hydroxyl) functional groups (like polysaccharides) and could be ionic polymers. Hydrocolloids like polysaccharides are typically used in food production and nonfood productions as crystallization inhibitors, encapsulating agents, gelling agents, solidifying agents, and stabilizers [17]. Several hydrocolloids are ionomers (ionic polymers), for instance, chitosan, pectin, xanthan gum, carrageenan, starch, alginate, and gum arabic. Polysaccharides are polymers having backbones containing heterocyclic ring structures of sugars and their derived constituents. They may be straightchain or branched structures, and some of them are ionic polymers. Polysaccharides have a higher degree of polymerization usually 10,000 100,000. The important biological roles of polysaccharides are as nutrients that store energy for digestion mainly glycogen in mammalians and starch in vegetables, and other roles are as the building blocks in plants (like cellulose) and chitin constituents of the cell walls of fungi [1]. In scientific terms, proteins and polysaccharides are frequently called hydrocolloids. They also called structural polysaccharides as they are available in various types and are frequently in used in blends with other in complicated structures.

18.2 Classification of food polymers

The building blocks of plants are made of cellulose, which is a polymeric compound of glucose and fructose. This tough stuff makes wood and the stems of plants. Cellulose also makes filaments; for example, in cotton and hemp, which can be wound into strings and mesh in apparel and several plants similarly make starch. Starch is abundantly available in corn, grains, potatoes, and rice. Starch is a polymeric compound made of glucose monomer. Although starch and cellulose have the same glucose monomer, their behavior is quite different. This is due to their glucose linkages being different. Starch is soluble in an aqueous solvent, however, cellulose is difficult to make soluble in water [18]. Therefore food can be made from starches and clothes from cellulose. Starch is actually a condensed system to store a lot of glucose in a compact structure. In the digestive system, starch decomposes into glucose, which can be utilized for energy. This energy is used for running, jumping, playing, and thinking. Cellulose gives strength to plants. Cellulose chains are altogether extended and like to remain tight alongside one another, similar to crude spaghetti that is altogether stuck together. That is the reason cellulose can hold up the tallest trees. Cotton is available mostly in the form of fiber; it typically consists of the cellulose-like structures found in vegetables and grains [19]. Humans find it difficult to digest, yet it is beneficial for us since it helps keep our inner parts clean. Polysaccharide includes many glucose units (sugar molecules), for example, cellulose and starches. The chemical structures of starch and cellulose are presented in Scheme 18.1. Polymeric compounds like protein and starch are an important component of food and provide nutrients to our bodies. These two naturally occurring polymeric foods can be categorized into two classes for use in different applications. (1) The formation and stabilization of food microstructures, for example, gelling, thickening, emulsification, and for use as cryoprotectants to increase freeze soften stability, as drying aids, and as encapsulating components. Further, (2) for physiological and

SCHEME 18.1 The chemical structure of starch (left) and cellulose (right). (Left) https://link.springer.com/chapter/10.1007/978-3-319-50766-8_3 and (right) https://link.springer.com/ chapter/10.1007/978-3-319-45340-8_5. Copyright 2018. Reproduced with permission from Springer.

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biotic jobs, for example, in food having specific functional health tenders like blood improving satiety, enhanced bioavailability, control of cholesterol levels, and as preservative agents by antioxidative and antimicrobial action [20]. Polysaccharides, for example, different food gums, have some promising features, for example, incredible structural versatility, extraordinary hydrophilicity, and simple procedures, which encourage the wide-ranging use of polysaccharide in the medical field [21]. Hence in tissue engineering and biomedical applications, polysaccharidebased hydrogels have been used extensively due to their biodegradability and biocompatibility. Natural-origin tannins and lignins contain phenolic polymers [22]. Mostly, consumable foodstuffs are abundant sources of phenolic polymers, particularly black foods conventionally consumed in East Asia. Agricultural waste materials that contain nonconsumable phenolic polymers, like seaweed, wood, grape pomace, and other byproducts of fruit and coffee processing have industrial interest. Pomegranates are chief sources of ellagitannins, which are also available in many fruits and nuts like almonds, blackberries, cloudberries, muscadine grapes, raspberries, strawberries, and walnuts [23]. The subunit structures of tannins from grape seed are presented in Scheme 18.2. Consumable polysaccharides, for instance, starch, are broken down in the mouth and small intestine in many phases that ultimately produce glucose, which is absorbed in our bodies and gives us energy [24]. They make available carbon atoms for the preparation of body fat, proteins, and other matters in the body. Nonconsumable polysaccharides or dietary fiber, for example, cellulose, provide the path for foodstuff through the gut and help support bowel regularity. Certain nondigestible polysaccharides, for instance, inulin, might similarly stimulate the

SCHEME 18.2 Subunit structures of tannins from grape seed. https://journals.plos.org/plosone/article?id 5 10.1371/journal.pone.0161095. Copyright 2016.Reproduced with permission from Ma et al. PLoS One.

18.2 Classification of food polymers

development of advantageous intestinal microorganisms [25]. Polysaccharides are less important nutrients as it is not essential to consume them with the purpose of being healthy. In commercial foods industries, natural and synthetic polysaccharides are used as thickeners and as fibers containing several kinds of dextrin, gums, inulin, polydextrose, and starches. Cellulose is also available in plants and animals, while glycogens are available in humans as storage polysaccharides. Structural polysaccharides provide structural support to plants and they consists of cellulose in vegetation and chitin in the shells of ocean animals. Table 18.1 lists polysaccharides with examples of their food sources, and Table 18.2 lists carbohydrates and modified carbohydrates.

Table 18.1 List of polysaccharides and foodstuff examples. Polysaccharide Digestible Starch

Dextrin (starch gum) Glycogen

Food source Cereal grains (wheat, oats, corn, rice) and their products (bread, pasta, pastries), potatoes, tapioca, yam, legumes An artificially produced food additive Shellfish, animal liver

Nondigestible (dietary fiber) Cellulose Hemicellulose Inulin Beta-glucan Pectin Psyllium husk mucilage Galactomannans or gums: betamannan, carob, fenugreek, guar and Tara gum Glucomannan or konjac gum Other natural gums: gum acacia (arabic), karaya, tragacanth Artificially produced gums: arabinoxylan (soluble) gellan, xanthan Seaweed polysaccharides: agar agar, alginate, carrageenan Chitin and chitosan

Whole grains, green leafy vegetables, beans, peas, lentils Cereals bran Wheat, onions, chicory root, leeks; a food additive Barley, whole oats, supplements Fruits, carrots, sweet potatoes; a food additive Psyllium seed husk A food additive derived from beans and seeds

A food additive extracted from konjac plant Food additives Food additives Food additives derived from marine algae Dietary supplements, derived from shells of crustaceans

www.nutrientsreview.com/carbs/polysaccharides.html.

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Table 18.2 List of carbohydrates and modified carbohydrates. Carbohydrates and modified No. carbohydrate

Sources

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Isolated from seaweed Isolated from brown algae A kind of starch, isolated from starch A kind of starch, isolated from starch Modified cellulose, isolated from wood, etc. Isolated from seaweed Isolated from seaweed Isolated from legume seed Exudate of Acacia tree from Middle East Exudate of tree from India Exudate of Astragalus, a Middle Eastern shrub Exudate of tree from India Isolated from fruit and other plants By chemical reactions in laboratory

Agar Algin or alginate Amylopectin Amylose Carboxymethylcellulose Carrageenans Furcellaran Guar (or guaran) gum Gum arabic Gum ghatti Gum tragacanth Karaya gum Locust bean gum pectin Starch and modified starches

www.scribd.com/document/7469327/Food-Polymers.

Plant polysaccharides and lignins that are difficult to hydrolyze for intestinal enzymes in the human body are called dietary fibers. Dietary fibers are comprised of cellulose, hemicellulose, pectin, and lignin as plant cell-wall constituents. The normal composition is about 42% of cellulose, 25% of hemicellulose, and 20% of lignin. Polysaccharides like the aromatic polymer lignin interact in the plant cell wall and give the strength and structural form of the plant cell [26]. Sugar contains three or more hydroxyl groups called polyols. These sugar polyols can be classified into (1) acyclic polyols, for example, alditols, xylitol, and glycitols and (2) cyclic polyols for example myoinositol. These functional groups are responsible for low caloric consumption by partial absorption, sweeteners in diabetic diets, and carry reducing agents [27]. Inulin is a polysaccharide of starchy material found abundantly in different kinds of fruit like bananas, and vegetables like wheat, onions, leeks, artichokes, and asparagus. The inulin used as a drug is extracted by soaking chicory roots in hot water [28]. Gamma inulin has been used extensively in contraceptive, flu, hepatitis B, and malaria vaccines with minute side effects. Delta inulin is more efficient than gamma inulin and has better temperature stability. AdvaxTM, an innovative micro-polysaccharide particle built from delta inulin, when formulated with recombinant or inactivated vaccine antigens increases immunity with a minor inflammatory profile and protects against viral pathogens. Polysaccharides can be classified into three types: 1. Food storage polysaccharides 2. Structural polysaccharides 3. Mucosubstances

18.2 Classification of food polymers

18.2.1.1 Food storage polysaccharides Food storage polysaccharides (starch and glycogen) work as reserve food and when required, they are hydrolyzed to sugar for the production of energy and biosynthetic activity. Human beings acquire starch from cereal grains, legumes, and potato, tapioca, banana, and so forth. Food storage polysaccharides are formed by photosynthesis in plants. Starch is available in the inner side of chloroplasts and leucoplasts known as amyloplasts. Starch is produced in small grains known as starch grains. Glycogen is the polysaccharide foodstuff reserve in animals, bacteria, and fungi and is also called animal starch. Glycogen is mainly stored inside the liver and muscles. Polysaccharides look like ellipsoid compressed grains that lie generously inside the cells. Polysaccharide provides a blue-black color with iodine. Inulin is difficult to digest in the human body and is freely filtered through the kidney, and consequently, inulin is used in the analysis of kidney function, exclusively glomerular filtration [29].

18.2.1.2 Structural polysaccharides Polysaccharides that contribute to the formation of the skeleton of plants and animals are known as structural polysaccharides, for example, chitin and cellulose. Heteropolysaccharide chitin is the most abundant organic substance, which originates as a structural constituent of the walls and skeleton of the velvet worm. Chitin is available in fungal walls known as fungus cellulose. As chitin is rubbery and soft, it provides flexibility as well as strength. Chitin becomes rigid when impregnated with certain proteins and CaCO3. Cellulose is a fiber-like structural polysaccharide having a high tensile strength that makes a fundamental structure of the cell wall in vegetation, certain yeasts, and protozoan. Tunic of tunicates is known as animal cellulose. Nutritional facts about carrots and strawberries are presented in Fig. 18.1. Cellulose creates the majority of human food. Amylase enzyme present in human digestive juices did not respond to the β-glucose linkage present in cellulose [30]. Cellulose is a significant component of food for mammals such as buffaloes and cows. The gastrointestinal tract of cows and buffaloes contains microbes capable of breaking down cellulose. Termites and snails similarly have microbes in their gut for the digestive purpose. In the food industry, microorganisms (enzymes) are used in the manufacturing of monosaccharaides from cellulose, which then undergo fermentation for the production of acetone, acetic acid, butanol, ethanol, methane, and so forth. High content cellulose timber is used in the manufacturing of furniture, articles, paper, tools, sports equipment, and so forth. Cellulose in the form of fibers is used in the textile industry, for example, in the production of cotton, linen, jute, hemp, China jute, and deccan hemp. From cellulose, xanthate, Rayon, and cellophane are formed. Cellulose acetate is obtained by the esterification of wood pulp. Cellulose acetate fibers are applied in the preparation of double knits, mothproof, tricot, and wrinkle-proof clothing. Cellulose acetates are used in the preparation of cigarette filters, plastics, and shatterproof glass. In propellant

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Nutrition facts Serving size: ½ cup carrots, sliced (61 g)

Nutrition facts Serving size: ½ cup strawberries, sliced (83 g)

Calories 25

Calories 27

Total fat 0 g Saturated fat 0 g Trans fat 0 g Cholesterol 0 mg Sodium 45 mg Total carbohydrate 6 g Dietary fiber 2 g Sugars 3 g Protein 1 g Vitamin A 204% Vitamin C 6%

Calories from fat 0 % daily value 0% 0% 0% 2% 2% 7%

Total fat 0 g Saturated fat 0 g Trans Fat 0 g Cholesterol 0 mg Sodium 1 mg Total carbohydrate 6 g Dietary fiber 2 g Sugars 4 g Protein 1g Vitamin A 0% Vitamin C 81%

Calcium 2% Iron 1%

Calories from Fat 0 % daily value 0% 0% 0% 0% 2% 7%

Calcium 1% Iron 2%

FIGURE 18.1 Nutrition facts of carrot and strawberries. http://harvestofthemonth.cdph.ca.gov/Pages/nutrition-labels.aspx.

explosives, cellulose nitrate is used. Carboxymethylcellulose is utilized in the production of smoothening additives and emulsifiers in cosmetics, medicines, and ice cream.

18.2.1.3 Mucosubstances Substances that are known to form mucilage, mucus, or slime in the human body are called mucosubstances. Two types of mucosubstances are available, namely (1) mucopolysaccharides and (2) mucoproteins (glycoproteins). Mucopolysaccharides are thick and gluey substances that have aminated or carboxylic acid polysaccharides derived from uronic acids, mannose, and galactose, and are somewhat common in both plants and animals [31]. Mucopolysaccharides are detected by slicing the immature fruit of gumbo like okra and Plantago ovata seeds. Mucopolysaccharides are available in plant cell walls and the bodies of aquatic plants, microbes, and algae. They form underlining layers between cells, the inner side of body fluids, cartilages, and connective flesh. Mucopolysaccharide helps to hold water in interstitial spaces. Mucopolysaccharides form a film of mucilage around algae along with other marine floras, defend algae from the decaying impact of water, and inhibit desiccation and the attack of pathogens. Mucopolysaccharides offer lubrication between ligaments and tendons. Keratan sulfate, a mucopolysaccharide, is produced inside the skin and cornea and gives both strength and elasticity [32]. Mucopolysaccharide, which contains chondroitin sulfate, produced surrounding connective tissue and cartilage provides strength and flexibility. Hyaluronic acid is a mucopolysaccharide found in the extracellular fluid of animal tissues, transparent gelatinous tissue filling the eyeball behind the lens, and synovial liquid, and so forth. It is similarly found in reinforcing

18.2 Classification of food polymers

substances between animal cells along with the inner side of the cell coat. P. ovata husk contains mucilage that is utilized pharmaceutically in the curing of stomach problems and reducing irritation [33]. Aloe barbadensis contains mucilage that reduces swelling. Mucopolysaccharide heparin helps to prevent blood clots [34]. Aquatic brown and red algae contain mucopolysaccharides that have commercial value. It used to produce carrageenan, agar, and alginic acid. Agar contains mucopolysaccharides of D-galactose, 3-6 anhydro L-galactose units. Agar agar has been utilized in a culture medium in a microbiology test center as a stabilizing, purgative, emulsifying, and solidifying agent [35]. Agar agar is extracted from the cytoplasm of certain marine algae, for example, Gelidiella, Gelidium, and Gracilaria. Pectin as a calcium pectate undergoes sol gel interchange. It is used in the production of gelatin and jams [36]. Polysaccharides of xylans, arabino-galactans, galactans, and glucomannans combine and produce hemicellulose. Hemicellulose produces bridges among pectic components and cellulose microfibers in the cell membrane. Lipopolysaccharide is a composite made of polysaccharide and lipid that produces the external cell wall of Gram-negative microbes. Lipopolysaccharide induces fever, shock, and other toxic effects [37]. Mucus (mucoproteins conjugated with monosaccharide) is created in nasal excretion, the intestine, stomach, and vagina. These compounds are antibacterial and protective in function. Starch comprises two kinds of fragments, namely amylose and amylopectin, which are primarily obtained from wheat, tapioca, maize, rice, and potato. Chiefly, the amylose part of starch, increases its film-forming capacity. High amylose content starch films are more flexible, oxygen impervious, oil resilient, and heat sealable. High-amylose containing edible polymers like cornstarch and potato starch was highly stable during aging. Polymeric foods, particularly those containing starch, are biologically absorbable, colorless, nontoxic, odorless, tasteless, and partially permeable to carbon dioxide and oxygen [38]. However, edible starch-based films are easily affected by microbial, chemical, and enzymatic activities. Edible chitosan, carrageenan, and starch films increase the service life of strawberries mainly for food processing industries [39]. Coatings prepared with modified (oxidized) starch, exhibit better mechanical properties compared to those without modified starch. Oils extracted from ginger family plant (Kaempferia rotunda and Curcuma xanthorrhiza essential oils) blended with cassava starch used as an edible coating for fish preservation [40]. Biopolymer alginates possess good film making properties, which make them beneficial in food applications, which comprise film forming, thickening, gel producing, stabilizing, suspending, and emulsion stabilizing properties. Alginate is used in the formulation of several medicines such as Asilone, Bisodol, and Gaviscon. Gaviscon is an over-the-counter medicine, which is taken orally to reduce esophagus and stomach reflux [41]. Esophagus and stomach reflux can be treated by mixing of sodium bicarbonate, sodium alginate, and calcium carbonate, which is also available in the form of chewable tablets. The blending of alginate beads and 5-fluorouracil is utilized for curing breast cancer. Alginate is

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added into phenobarbitone tablets for the curing of chronic seizures as a drug release agent. Alginate is exploited comprehensively in dentistry and prosthetics [42]. Alginate is similarly employed in food manufacturing to increase the viscosity of soups and jellies. Carrageenan (sulfated galactans) is obtained from the cell membrane of several red seaweeds. Carrageenan coatings are prepared using a heating cooling mechanism [43]. This jelly-type carrageenan is consumed in dairy products such as ice cream, milkshakes, soymilk, and sweetened condensed milks. Hydroxypropyl cellulose, hydroxypropyl methylcellulose, carboxymethylcellulose, and methylcellulose are synthetic celluloses used for edible food coatings and films. Synthetic cellulose derivatives exhibit thermogelation properties (heating cooling process with water) [44]. Synthetic cellulose derivatives have adequate strength and are impervious to lipids. They are tasteless, colorless, translucent, and flexible. Methylcellulose is resilient to water (least hydrophilic) and has poor mechanical properties. The hydrophobic characteristic improved with the addition of fatty acid in a synthetic cellulose derivative film. Another example is the bilayer films formed by blending lipids and methylcellulose [45]. Edible films prepared from hydroxypropyl cellulose; hydroxypropyl methylcellulose; carboxymethylcellulose; and methylcellulose are employed for certain vegetables and fruits to protect them through providing barrier layers. Microcrystalline and macrocellulose are consumed as sedentary additives in medicine and are used as jelling agents in food processing industries [46]. A large amount of cellulose powder is exploited in several types of cheese to avoid hardening inside the tube. Hydroxypropyl methylcellulose and methylcellulose are utilized for coating purposes to decrease oil absorption in dough discs and potato fries. The moisture barrier of edible films can be enhanced by blending fatty acids with hydroxy propyl methylcellulose. A biodegradable and uniform packaging coating (film) was synthesized using hydroxypropyl methylcellulose by chemical modification (cross-linking) permitting a decrease in hydrophilicity [47]. Pectin, a structural polysaccharide, is chiefly available in several vegetables and fruit, mainly in apple pomace and citrus peel [48]. Pectin is used in jelly, chocolates, jam, and in sweet juices. Pectin has the ability to increase the thickness and bulk of human feces. Due to this characteristic, pectin is used for the curing of constipation and diarrhea in medicine. The agars have the ability to form reversible gels simply by cooling a hot aqueous solution. Therefore it used as gelling additives in the processed food industry [35]. Conversely, even though it is biodegradable and has good gelling power, agar is not used generally due to poor aging. Due to photodegradation and variations in ambient temperature and humidity, agar forms a crystalline structure, leading to the formation of microfractures and brittleness. Phan [49] studied the effects of agar on the characteristics of emulsified edible films. Chitosan is an auspicious polysaccharide having characteristics of biodegradability, biocompatibility, chemical resistance, good tensile strength, high filmforming characteristics, and cost effectiveness [50]. Moreover, chitosan bears a

18.2 Classification of food polymers

high-volume modification in response to variations in solvent composition, pH, and temperature, so it is used for the fabrication of simulated muscles. A new analysis has proposed that chitin can be used to treat human allergies. Chitosan is a widely available natural and nontoxic polymer. Chitosan has the property that it does not require any additives to form films. It also shows high oxygen and carbon dioxide permeability, admirable mechanical properties, and antimicrobial activity against microbes [51]. Chitosan materials were highly viscous, just like natural gums. Translucent films of chitosan can enhance the quality and increase the storage life of foodstuffs. Virgin chitosan coatings are normally consistent, compact, and the coating surface has a suave appearance without any damage. Chitosan is employed in wastewater purification, membrane technology, food packaging, drug delivery systems, and biosensors [52]. Blends of chitosan and conducting polymers like polyaniline are used as flexible conductive polymers [53]. Guar gums have good texturizing capabilities. They are employed to create viscosity, as thickeners, and in stabilizers [54]. Gum arabic is soluble in water and produces less viscous hydrocolloid gums. Xanthan gum is also dissolved in water; therefore, extraordinary stability is obtained quickly in the water phase. A mixture of xanthan gum, guar gum, and gum arabic delivers a uniform film with decent adhesion in wet conditions. The blending of mesquite gum and small amounts of lipids gives films with exceptional water vapor barrier properties.

18.2.2 Polypeptides Polypeptides are nitrogenous organic polymers having amino acid groups and are an important part of all living organisms. For example, important constituents of the body like muscle, hair, enzymes, and antibodies are made of polypeptides (proteins). Polypeptides (proteins) are acute nutrients that are useful to generate new enzymes, muscle tissue, and hormones in the human body. They are available in foodstuff. Actually, proteins ensure that the body continues to function accurately. Every protein molecule consists of amino acids, which are attached in chains to form long-chain polymers. Subsequent to digesting dietary protein, the body adjusts these amino acids to make new protein structures expected to perform different physiological procedures. On the market, protein is available in the form of protein powder to make milkshakes. The most well-known proteins are soy protein and whey protein available on the market, which allows one to make the healthiest choice for their body. Polypeptide-based polymeric foods can be used for the packaging of small portions of food such as beans, cashew nuts, and nuts [55]. Additionally, polypeptide-based edible food polymers work as antimicrobial and antioxidant agents for coating applications. Polypeptide based edible food polymer can use for multilayer food packaging materials together with the non-edible polymer, for example, consumable polymers (protein) produces coatings that direct touching with foodstuff [56]. Natural proteins have poor mechanical and barrier properties

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and, therefore, in industries they are replaced by edible synthetic polymer films. Scheme 18.3 depicts the common structure of polypeptide. Collagens are structural proteins of connective tissue that join tissues like bone, hide ligaments, and tendon cartilage. The principle use of collagen films was a barrier membrane. These flexible films can easily sterilize and operate in the delivery of drugs to the eyes as well as be applied to injuries to quickly cure ophthalmology [57]. Polymeric collagen has characteristics of being biocompatible and nontoxic to most tissues. Collagen is easily isolated and purified in large amounts and has good immunological properties. Gelatin has the unique property to form reversible gel materials at 36 C and is essentially important in edible and medicinal uses. By controlling the hydrolysis of protein (collagen), gelatin is prepared. Gelatin contains a large amount of peptides like hydroxyproline, glycine, and proline. Gelatin is used in low oil and moist phase medicines and foodstuffs [58]. This encapsulation offers protection against oxygen and light. Gelatin films are used for packaging meats, microencapsulation and tablet coverings in drugs, and in processed food industries to reduce the transportation of oil, oxygen, and humidity. Low molecular weight gelatins are effective in the curing of gastrointestinal ulcers. Fig. 18.2 depicts nutritional facts about great-lakes collagen. Soy protein is industrially accessible in soy flour. The hydrolysis of soy proteins produces albumin, glutelin, and globulin fractions. Soy protein is used in different kinds of foodstuff like bread, breakfast cereals, salad dressings, beverage powders, cheeses, frozen desserts, infant formulas, meat analogs, nondairy creamer, pasta, soups, and whipped toppings [59]. Gluten protein is abundantly available in grains like wheat. The corn kernel comprises 10% zein protein. Zein protein is widely used in food processing and drugs [60]. Zein protein is used as a coating for toffee, dry and wet fruit, pills, and foods and medicines. In the United States, it is branded as “confectioner’s glaze” and used as an edible packaging film on bakery products. Zein is used to increase surface gloss and as a moisture barrier for nuts, chocolates, and confectionery products [61]. Zein protein is soluble in 75% ethanol, less hydrophilic and thermoplastic substance. Zein protein

SCHEME 18.3 Common structure of polypeptide. http://biolishl.blogspot.com/2013/09/7.html.

18.2 Classification of food polymers

Supplement facts Serving size12 g (around 2 rounded tbsp) Servings per container: 38 Amount per serving

Nutrition facts Serving size (30 g) Servings per container: 13 Amount per serving Calories 143 Caloriesfrom fat 63

Calories 43

% daily values* Total fat 7 g 11% Saturated fat 1 g 5% Trans fat 0 g Cholesterol 0 mg 0% Sodium 90 mg 4% Total carbohydrate 6 g 2% Dietary fiber 5g 20% Sugars 0g Protein 11 g 22% Vitamin A 2% Vitamin C 2% Calcium 2% Iron 6% *Percent daily values are based on a 2000 calorie diet. Your daily values may be higher or lower depending on your calorie needs. Calories 2000 2500 Total fat Less than 65 g 80 g Sat fat Less than 20 g 25 g Cholesterol Less than 300 mg 300 mg Sodium Less than 2400 mg 2400 mg Total carbohydrate 300 g 375 g Dietary fiber 25 g 30 g

% daily value*

Total fat Sodium Total carbohydrate

0g 12 mg 0g

0% 1% 0%

Protein 11 g Not a significant source of calories from fat, saturated fat, cholesterol, dietary fiber, sugars, vitamin A, vitamin C, calcium and iron.

*Percent daily values are based on a 2000 calorie diet. There are no additives or preservatives.

FIGURE 18.2 Nutritional facts about great-lakes collagen. https://glutenfreedietwithnutrition.com/great-lakes-gelatin-collagen-hydrolysate-beef-kosher/great-lakescollagen-nutrition-facts/.

coating has likewise demonstrated a capacity to reduce moisture and loss of hardness and defer color modification in fresh fruit. Wheat gluten is an insoluble globular protein of wheat flour. The cohesiveness and elasticity of wheat gluten offer consistency to wheat blends and accelerates their film-forming ability. It also produces hydrophilic films. To increase flexibility and hydrophobicity, a plasticizer, a cross-linking agent, and a heat curing agent can be added. The aerobic degradation of these films can take place in 1 month, while anaerobic degradation takes place over 2 months in soil without discharging any poisonous materials [62]. They can widely be use in milling, bakery foodstuffs, meats, pasta, bread, and more. Wheat protein is a chief constituent of bakery products like bread, crackers, cookies, biscuits, pastries, and so forth. In the production of wheat foodstuffs, gluten is an important component to increase the viscoelastic properties of dough, allowing for the manufacturing of various processed foods, for example, bread, pasta, and noodles. Wheat is an important food for human nourishment, predominantly in underdeveloped countries where wheat materials are chief foodstuffs. Whole grain wheat gives several nutrients and dietary fiber, which are

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recommended for children and adults. Soy protein (obtained from soybeans) consists of a large amount of nutrients and has outstanding functional characteristics. Subsequently, it is widely available in nature, cheap, biodegradable, and dietary; it is used in edible and biodegradable films [63]. Milk proteins are classified into whey and casein protein. The addition of alkali (neutralization) into acid casein produces soluble caseinates. Edible caseinate films can be prepared by dissolving various caseinates in water then casting and drying. These films protect vegetables and dried fruit from moisture and aerobic degradation. The blending of lipid and a caseinate film was effective in decreasing humidity loss from zucchini and peeled carrots. A blend of casein and rennet can be used as a organoleptically modified agent. It is utilized as a texturing and flavoring agent in the manufacturing of chocolate products [64]. Mung grams are useful as a prospective constituent of biodegradable coatings since it contains large amounts of polypeptides. Bourtoom developed and investigated a film made from mung bean protein [65]. They found that this film had good tensile properties and permeability compared to other natural proteins. Conversely, the mung bean protein coating still exhibited significantly poorer mechanical and moisture barrier characteristics compared to synthetic polymers. Chitin is a hydrophobic material consisting of polysaccharides, which are the main component in the exoskeleton of arthropods and the cell walls of fungi and mushrooms. Purified chitin (off-white powder) is used as a thickening agent by farmers, doctors, and in food processing industries.

18.2.3 Lipids Lipids produce a unique kind of polymer, recognized for being a basic constituent of cell membranes and hormones. Normally polymers are long chains of identical monomers and high molecular weight compounds, while lipid contains different molecules attached to each monomer chain. This molecule differs with the category of lipid; some contain carboxylic groups while others contain glycerol or phosphate groups. Certain lipids make macromolecular structures (polymers) with different types of fat molecules, for example, steroids (cholesterol), but they are not considered as perfect polymers. Lipids are not considered to be true polymers, but still have a long chain of carbon in the triglyceride unit and high molecular weights. Lipid has remarkable significance in food processing industries. Lipids are one kind of fats and oils, which are used as cooking oils, ghee, margarine, and edible fats. Fats are solids and oils are liquids depending on temperature. Lipid materials are well known as consumable macromolecules containing acetylated monoglycerides and natural wax [66]. Examples of lipids are beeswax and paraffin wax. Due to their relatively low polarity, lipids used to block the passage of moisture. The hydrophobic nature of lipids produces less flexible and bulky coatings. Therefore lipids must be blended with polysaccharides and polypeptides to get good thin films, which also increases mechanical strength. Normally, moisture permeability is reduced when the amount of hydrophobicity phase increases.

18.2 Classification of food polymers

SCHEME 18.4 Common structure of lipids. https://en.wikipedia.org/wiki/Lipid. Copyright 2018, Reproduced with permission from wikipedia.

During the fractional distillation of crude oil at 300 C 350 C, paraffin wax is obtained. Paraffin wax is used as a coating material for fruit, vegetables, and cheese. From palm tree leaves, carnauba wax is obtained. Beeswax is produced by honeybees. Candelilla wax is obtained from the candelilla plant. These waxes are used as humidity barriers, for example, skin on fresh fruit. They also improve the surface glossiness of several foodstuffs. Thick wax coatings are compulsorily removed before foods can be eaten (like many cheeses). When used in thin coatings, they are considered to be edible with foods. Stearodiacetin (acetylated monoglyceride) shows the special property of hardening from the melted state into a flexible state [67]. The elongation property of lipids is good, for example, acetylated glycerol monostearate can stretch up to eight-times its original length. Lipid films have a weak moisture barrier property. Lipid films (acetylated monoglyceride) are employed on meat and poultry to delay moisture loss at the time of packaging. Scheme 18.4 depicts the common structure of lipids. Shellac is a material excretion by the female lac bug (Laccifer lacca) on plants in the jungles of central India and Thailand. Shellac resins are made of a complex blending of aleuritic and shelloic acids. Shellac is recognized as a toxic material. Shellac resins are acceptable as bonding agents and secondary food coatings. Shellac resins are employed in coatings for drug production and have limited use in food coatings. Rosins oil obtained from oleoresins of pine trees, widely used in coating for citrus fruits [68]. Shellac and wood resins are utilized to coated citrus fruit to preserve the flavor of the fruit. Coatings of shellac and wood films increase the prevalence of postharvest pitting [69].

18.2.4 Synthetic and composite food polymers Food polymers may be different in characteristics, comprising a mixture of lipids, proteins, and polysaccharides and consequently have different film-forming

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properties. The intention of manufacturing composite films (coating) is to increase barrier and tensile strength as per the requirements of consumers. Composite polymeric coatings are applied by different methods like dispersion, emulsion, and suspension of the insoluble components or in successive films. The technique of application is important and directly affects the permeability features of the coatings achieved. The blending of methylcellulose and fatty acid coatings by emulsion increases the humidity barrier properties of cellulose films [70]. Synthetic cellulose nanocrystals encourage development in the cohesion of hydroxypropyl methylcellulose and starch molecules in mixtures and form homogeneous films, which increases coating characteristics [71]. Polyvinyl acetate is widely used for food coating and drug coating as it is a nontoxic polymer to humans [72]. In antimicrobial chemotherapy, some biopolymers (polysaccharides and proteins) and synthetic polymers (polymethyl methacrylate) are used that are biocompatible [73]. They are also used for water sterilization, foodstuff packaging, various antifouling applications, and food preservation. Coating techniques such as spraying, encapsulation, and dip coating are exploited in the medical field to combine edible polymers with bioactive agents. These edible polymers are mostly comprised of cellulose derivatives, poly(Nvinyl pyrrolidone) and polyethylene glycol. Grafts of polymethacrylic acid with polyethylene glycol are consumed as pH-responsive hydrogels for oral protein delivery [74]. Copolymers like polyethylene oxide and polypropylene oxide have been comprehensively studied in drug delivery [75]. Electrostatic interfaces among polyethyleneimine and DNA, called polyplexes, have been extensively studied for gene delivery. Polyethylene glycol is used in vaginal suppositories, drug and pill lubricants, cream bases, and so forth. N,N-Dimethyl amino ethyl methacrylate (polyacrylamide) is used as a barrier between the bloodstream and insulin [76]. In tablets, polymers like methylcellulose, hydroxyl ethyl cellulose, and hydroxyl ethyl methylcellulose are used as binders. Polymers like carboxyl methylcellulose sodium used as disintegrating agents as they encourage the breakup of the tablet into smaller parts in an aqueous solution and stimulating the release of the drug substance. Polymers like the entire cellulose derivative used as coating materials. Synthetic edible polymers like cellulose acetate phthalate, polyvinyl acetate phthalate, hydroxypropyl methylcellulose acetate succinate, and methacrylate and ethyl acrylate copolymers are used as coatings for oral medicines. Copolymers of methyl methacrylates and methacrylic acid are utilized for the covering of drugs that are resistant to gastrointestinal juice and increase protection against tropical conditions [77]. Scientists have made polymer pills with a sucrose layer on the outer side and a drug layer on the inner side. Polymer spheres made by blending glycosylated polybutadiene and polyethylene oxide form colloids called vesicles. These vesicles are used in drug delivery containers, living cell mimics, target drugs, and biomolecules to injure cancerous tissues [78]. Now a day, there is a demand for active packaging. This active packaging modifies environments surrounding the foodstuff to preserve product quality and

18.2 Classification of food polymers

freshness, upturn sensory properties, or enrich product safety and shelf life. To satisfy the demand for recyclable and natural packaging materials, novel food grade coating materials have been developed. Novel coating compounds comprised of biopolymers, bioplastics, and edible polymer packaging materials are made from agronomic or aquatic sources. The application of nanocomposites has the potential to enlarge the utility of biodegradable and consumable polymers [79]. Nanocomposite films support to the reduction of packing waste related to packed foods, contribute to the conservation of fresh foods, and improve the shelf life of foods. Polymer composites are combinations of polymers with fillers in certain proportions. Polyvinyl acetate is used as an efficient protective edible coating on green-stage tomato [80]. The consumable polymeric film did not considerably affect glossiness and weight loss. Some scientific results showed that PVA coating was given more protection of the fruit compared to uncoated fruits. The gelatin used for the capsulation of numerous medications and supplements. These capsules can make it easy to take certain drugs and supplements, specifically those in a powder or liquid form [81]. This pill was practically odorless, flavorless, did not interact with other medicines, and was easy to digest without any problems to humans. Mammalian (fish) gelatins have better film-forming capacities than synthetic polymers to preserve foods. A food grade polyvinyl acetate coating was found to be inexpensive to utilize and highly efficient compared to natural polymers in avoiding postharvest fruit deterioration without any alterations in the color of fruit. Polyvinyl acetate films easily formulate to vegetables and fruit by simple spraying and plunging processes. Polyvinyl acetate is also employed as a component in chewing gum [82]. In candy and baked goods, polyvinyl acetate is used as a coating material; it also includes other additives like plasticizers, optical brighteners, and surfactants, which increase coverage. This coating has the benefits of keeping fruit firm and reducing fruit breathing. Shellac whitens when it is exposed to moisture, for example, apple transferred from cold storage to a humid environment. An additional difficulty is that apple and citrus fruit slightly change flavors. Shellac coatings are chiefly of insect origin, which has made them disagreeable to many customers. Some snacks are orally digestible polymers. These snacks consist of one or more layers of polymer film that was orally soluble and the human tongue can break it rapidly without leaving substantial residue [83]. Flavored chocolate contains the orally soluble polymeric cocoa procyanidin. These snacks are orally soluble so that they break down comparatively slow when placed in the human mouth. Chewing gums contain flavorings, corn syrup, gum base, sweeteners, and softeners. The gum base is the indigestible part that remains in the mouth while chewing. Natural latexes (rubber) and synthetic polymers are employed to make chewing gum bases. Thomas Adams wanted to utilize synthetic styrene butadiene rubber and polyvinyl acetate polymer as the gum base in chewing gum [84]. Present bubble gums are prepared with artificial elastomers that provide flexibility and an obstinate and sticky quality. Chewing gum contains a gum base and

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insoluble hydrophilic polymers. These polymers are methacrylate or hydroxyalkyl acrylate having a releasable adsorbed flavor. Poly-(2-hydroxyethyl methacrylate) is also used in chewing gum [85]. Petros developed consumable gum compositions that rise in volume when chewed [86]. Superabsorbent hydrophilic polymer blends with a gum base are used in gum composition. Scientists have discovered techniques for increasing the healing ability of medicines by adjusting the formulation systems. Innovative edible polymers offer benefits; however, care should be taken in the precise selection of polymers for drug delivery formulations. The ultimate aim is to manufacture cheap, nontoxic, and biocompatible polymers so that delivery formulations meet the several stages of scientific criteria and are an advantage to humanity. Polymeric starch has extensive applications extending from its use as a filler or binder to a component in the formulation of tablets, coatings, and intravenous implants. Starch is available in pure form and soluble in water. So it is suitable for delivery by injection. Nanoprotein has the benefit of improved stability during storage. This is a good protein-based medicine utilized in targeted drugs for attaining an improved pharmacologic effect [87]. Hyaluronic acid is a polysaccharide that is dispersed in the joint liquid of animals. The Food and Drug Administration (FDA) [88] has sanctioned it for injections. Hyaluronic acid is a polyanionic mucoadhesive. It is nontoxic, biocompatible, biodegradable, and mostly dispersed in the connective muscle, lungs, intestines, and eyes. In drug formulations, guar gum is consumed as a preservative, suspending agent, emulsifier, and gelling agent. Guar gum has been extensively developed for colonic drug delivery systems. The swelling capability of guar gum utilized in the delay of drug release from the dosage forms. Chitosan derivatives like N-carboxymethyl chitosan are used broadly in drug industries as a drug releasing agent and connective tissue. Pectin hydrogels (spherical calcium pectinate gel) are used as a binder in drug formulations and in controlled release drug formulations [89]. Polysaccharides contain many hydroxyl groups that directly react with the carboxylic acid functions of drugs and create ester linkages. These functional groups support drug release in the human body. Vaccines (drug delivery systems) are categorized into proteins and nucleic acids. For both categories, they need polymeric configurations since vaccines are prone to degradation by peptidases or nucleases. Vaccines are condensed into polymeric carriers, thus, are protected from deterioration [90]. Researchers in the United States made an ingestible electronic device. This device is manufactured totally using edible polymers and produces its own electrical current. Specialists formulated and invented an ingestible current source containing polymer terminals and a sodium particle cell [91]. These elastic polymeric electrodes can fold into an edible pill, while the sodium particle cell works as an onboard energy source. Patients swallow a capsule that contains the device. Different kinds of polyesters are utilized for the encapsulation of various kinds of healing agents for tumors and microbial infections. Hydroxyl propyl methyl cellulose is used as an ophthalmic lubricant, as an excipient, and as a controlled

18.3 Conclusion

delivery component in oral pharmaceuticals [92]. Polymeric materials containing carboxybetaine ester with mucus and cellulose sulfate, demonstrate photoswitchable characteristics with bioactive substances used in several medicinal applications [93]. Polyoxalate-cross-linked chitosan nanocarriers are utilized in anticancer drug delivery systems [94]. Natural lignin grafted with methacrylate hollow-nanofiber composites is utilized as a biocompatible nanocarrier with improved anticancer ability [95]. The control of pH has increased as an amazing methodology in disease treatment since pH profoundly affects the interactions of a polymer containing a drug delivery system with cancer cells. Guar gum-grafted lysine-β-cyclodextrin as a drug carrier for the release of 5-flourouracil is an efficient drug delivery system for chemotherapy and is utilized for preventing tumor cell enlargement [96]. The uses of polymers in food also have some drawbacks. During manufacture, polymeric foods are completely exposed to the exterior environment and, therefore, there are probabilities of bacteriological adulteration. The synthetic production of polymeric films and coatings is a controlled technique with fixed amounts of components, while the manufacturing of natural polymers is reliant on the environment and several physical issues. Due to variances in the assortment of natural polymers at different periods, besides variances in the area, species, and environmental conditions, the percentage of chemical compositions present in a given material may differ. As the manufacturing rate depends upon the surroundings and several other issues, polymeric foods have a slow rate of fabrication. There are probabilities of heavy metal impurities often associated with polymeric foodstuffs.

18.3 Conclusion The development of innovative polymeric foods is limited by the huge investment charges related to attaining the required judicial and governing endorsement. Consequently, it is likely that innovations will be made concerning new combinations, application areas, or technological advances. The utilization and expansion of natural polymers in foodstuffs are probable to endure and increase in future; though, there may be moves from one source to another because of sourcing problems, fluctuating prices, and customer preference fluctuations. The introduction of innovative technologies, for instance, micro- and nanoencapsulation in the foodstuff area has permitted the utilization of numerous components such as modified starch, hydrocolloids, and polymeric emulsifiers to generate core shell constituents, which assist both as shell components and as adhesive agents, thus, increasing the textural characteristics of finishing materials and process stability. A novel type of food polymer is under development with the objective of approving the combination of controlled release drug systems using nanotechnology, for example, nanoencapsulation and multifaceted systems. Currently,

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nanotechnologies are utilized to improve the dietary features of food using nanoscale additives and nanosized delivery systems for bioactive polymeric mixtures. Nanocomposite phenomena exhibit encouraging directions for generating new and innovative ingredients, also in the field of polymeric foods. Micro- and nanoscale active ingredients within edible polymeric films or coatings, support the regulation of their release in particular circumstances, consequently defending them from heat or water vapor or other dangerous influences and improving their feasibility and stability. Nano-laminates associate with either dipping food into a series of solutions containing ingredients that would adsorb to a foodstuff’s surface or by spraying ingredients onto the food surface. These nano-laminate films derived exclusively from food-grade elements and contain several functional agents such as antimicrobials, antioxidants, enzymes, flavorings, and colorants. Nanocomposites achieved by blending natural, polymeric foods and layers of crystalline solid, offer incredible properties. Nanocomposites have low cost and are high in efficiency compared with traditional synthetic polymers. Micro- and nanoencapsulation are utilized to increase the stability or bioavailability of other precious components, for example, vitamins, volatile fragrances, and functional food constituents.

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Further reading Shit SC, Shah PM. Edible polymers: challenges and opportunities. J Polym 2014;2014:1 13.

CHAPTER

Future needs and trends: influence of polymers on the environment

19

Ammavasi Nagaraj and Mariappan Rajan Biomaterials in Medicinal Chemistry Laboratory, Department of Natural Products Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai, India

19.1 Introduction The easiest development of a polymer is a helpful substance made of many repeating units. A polymer can be made up of a three-dimensional system (think about the repeating units connected together left and right, front and back, all over) or a two-dimensional system (think about the repeating units connected together left, appropriate, up, and down in a sheet) or a one-dimensional system (think about the repeating units connected left and right in a chain) (Fig. 19.1). Each repeating unit is represented by the term “-mer” as fundamental units within a “polymer,” which means many repeating units [1]. Repeating units are frequently made of carbon and hydrogen and sometimes oxygen, nitrogen, sulfur, chlorine, fluorine, phosphorous, and silicon. To make the chain, abundant links or “-mers” are chemically entrapped or polymerized together [2]. As a visual example of polymers, imagine connecting infinite segments of development paper together to make a paper garland or entrapping together many paper garlands to shape chains or hanging beads. Polymers happen in nature and can be made to serve particular needs. Fabricated polymers can be three-dimensional systems that don’t liquefy once shaped. Such systems are called thermoset polymers. Epoxy gums utilized in two-section glues are thermoset polymers [3]. Manufactured polymers can likewise be one-dimensional chains that can be softened. These chains are thermoplastic polymers and are likewise called straight polymers. Plastic jugs, CDs, mugs, and filaments are thermoplastic polymers and represent the two-dimensional structure of the simplest polymers (Fig. 19.1).

19.1.1 The structure and properties of polymers Polymers abound in nature. Definitive natural polymers are the deoxyribonucleic acid and ribonucleic acid that describe life. Hair, spider silk, and horns are protein polymers. Starch can be a polymer as is cellulose in wood. Rubber tree latex and cellulose have been utilized as crude materials to make fabricated polymeric Polymer Science and Innovative Applications. DOI: https://doi.org/10.1016/B978-0-12-816808-0.00019-6 © 2020 Elsevier Inc. All rights reserved.

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FIGURE 19.1 Two-dimensional network of polymeric structure.

polymers and rubbers. The first synthetic plastic was Bakelite, made in 1909 for telephone casing and electrical parts [4]. The first fabricated polymeric fiber was Rayon made from cellulose in 1910. Nylon was developed in 1935 in an attempt to find a synthetic spider silk.

19.1.1.1 The structure of polymers Different major classes of polymers are made out of hydrocarbons and mixes of carbon and hydrogen [4]. These polymers are especially made of carbon atom reinforced together, one to the another with, into long chains that are known as the foundation of the polymer. In perspective of the possibilities of carbon, no less than one distinctive atom can be associated with each carbon molecule in the backbone. There are polymers that contain just carbon and hydrogen particles. Polypropylene (PP), polyethylene (PE), polystyrene (PS), polybutylene, and polymethylpentene are models of these. Polyvinyl chloride (PVC) has chlorine added to the all-carbon backbone. Teflon has fluorine joined to the all-carbon backbone [5]. Other ordinary distributed polymers have chains that join parts other than carbon. Nylons contain nitrogen atoms in the repeat unit chain. Polyesters and polycarbonates contain oxygen in the long chain. There are, in a similar manner, a number of polymers that, as opposed to having a carbon backbone, have a silicon or phosphorous backbone [6].

19.1 Introduction

19.1.1.2 Molecular arrangement of polymers Think of how spaghetti looks on a plate. This is how the arrangement of polymers can be composed on the off chance that they require explicit interest or are poorly characterized. Controlling the polymerization approach and extinguishing liquid polymers can result in an unclear alliance. An indistinct approach atoms has no long-run request or shape in which the polymer chains organize themselves [7]. Structureless polymers are viewed as clear. This is a fundamental trademark for a couple of uses, for example, plastic windows, sustenance wrap, front light central focuses, and contact central focuses. Essentially not all polymers are straight. The polymer chains in articles that are translucent and dim might be in a crystalline structure. By definition, a crystalline structure has particles, or for this situation, atoms dealt with in patent models. For the most part, crystalline struct ures are observed in table salt and gemstones; however, they can happen in polymers. Similarly, as extinguishing can deliver undefined plans, handling can control the level of crystallinity for those polymers that can take shape. A few polymers intended to never have the capacity to take shape. Other polymers are intended to have the capacity to be solidified. The higher the dimension of crystallinity, by and large, the less light can go through a polymer. Thus the dimension of haziness or translucence of a polymer can be especially affected by its crystallinity. Crystallinity imparts benefits in quality, hardness, substance obstruction, and quality. Investigators and specialists are diligently making all the more pleasing materials by controlling the atomic structure that impacts on previously manufactured polymers. Processors and creators present one of kind fillers, clips, and included substances into base polymers, creating inconceivable outcomes.

19.1.1.3 Characteristics of polymers The largest percentage of manufactured polymers consist of thermoplastics, recommending that once these polymers are formed they may be warmed and changed over and over again. This property thoroughly considers essential arranging and engages reusing. The other gathering, the thermosets, can’t be dispatched. Once these polymers are framed, warming will make the material at last corrupt, yet not dissolve. Each polymer has incredibly explicit qualities; in any case, most polymers have general properties. Polymers can be unbelievably similar to synthetics. Consider all the cleaning liquids in your home that are packaged in plastic. Perusing the notice names that portray what happens when the concoction interacts with skin or eyes or is ingested will underline the requirement for substance obstruction in the plastic bundling. While solvents effectively disintegrate a few plastics, some plastics give safe, non-flimsy bundles for forceful solvents [8]. Polymers can be both heat and electrical separators. A stroll around your home will fortify this idea as most of the machines, ropes, electrical outlets, and wirings observed will most likely be made or moored with polymeric materials. Heat resistant materials are clear in the kitchen in pot and holder handles made of polymers, espresso pot handles, the froth point of convergence of iceboxes and

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coolers, verified glasses, coolers, and microwave cookware. The warm clothing items that skiers wear are made of PP and the fiberfill in winter coats is made of acrylic and polyester. By and large, polymers are light in weight with significant worth. Think about their degree of uses from toys to the edge structure of room stations or from fragile nylon fiber in pantyhose to Kevlar or insusceptible vests. Two or three polymers float in water while the others sink. All things considered, compared with stone, solid steel, copper, or aluminum, all plastics are lightweight materials [9]. Polymers can be dealt with in different ways. Expulsion creates thin filaments or overwhelming channels or movies or sustenance bottles. Polymers can be molded into drums or be blended with solvents to produce glues or paints. Elastomers and two or three polymers expand and have incredible degrees of flexibility. Two or three polymers are similar in their ability to hold their shape, for example, soda pop compartments. Different polymers can be frothed like PS (Styrofoam), polyurethane (PU), and PE; several characteristics of polymers are diagrammatically represented in Fig. 19.2 [11]. Polymers are materials with an obviously boundless degree of attributes and traits. Polymers have different characteristic properties that can be improved by an expansive variety of added substances to widen their utilization and applications. Polymers can be made to imitate cotton, silk, and wool strands; porcelain and marble; and aluminum and zinc. Polymers can likewise make conceivable items that don’t promptly originate from the common world, for example, adaptable CD’s and DVD’s. Polymers are consistently made of oil, yet not all polymers are. Different polymers are made of rehashed units from vaporous oil, coal, or foul oil. In any case, assembling square recurrent units can a portion of the time be passed on utilizing boundless materials, for instance, polylactic ruinous from corn or cellulose from cotton linters [10]. A few plastics have dependably been produced using inexhaustible materials, for example, cellulose acetic acid derivation utilized for screwdriver handles and blessing strip. Polymer structure squares can be made more cost effectively from viable materials than from oil-based merchandise; either old plastics are used in grungy materials or new plastics are produces.

FIGURE 19.2 Diagrammatic representation of some characteristics of polymers [10].

19.1 Introduction

Polymers can be utilized to settle on things in coatings in applications where only specific materials are required. Polymers can be made into clear, waterproof films. PVC is utilized to make accommodating tubing and blood packs that broaden the lifespan of blood and things related to blood. PVC securely conveys combustible oxygen in non-consuming adaptable tubing. Moreover, thrombogenic materials, for example, heparin, can be converted into adaptable PVC catheters for open heart remedial frameworks, dialysis, and blood gathering. Different helpful gadgets depend upon polymers to allow their working. Polymer composites (PCs) will remain an area of interest. Based on the strength and lightweight (their thickness is lower than most of the ordinary composites) and environmental impact and biocompatible nature (especially on account of their direct reusing idea relax reprocessing) [12]. With regard to network/fortification mixes, shapeless/semicrystalline and semicrystalline/semicrystalline mixes [13] are particularly significant. Endeavors will be additionally made to adjust the support by nanofillers, particularly by those that have high viewpoint proportions. Their fuse should yield expanded solidness, quality, and heat resistance. The resistance to fire of PCs ought to be improved. This is an important need for further potential applications. The organized action toward this path yielded some particular outcomes. PCs of increasingly sample structures, for example, boards containing honeycomb or froth centers, may procure new application fields. A further exciting issue is the improvement of multifunctional PCs. Further works are expected to check the capability of electrospun strengthening in PCs. Liquid composite modeling (LCM) generation strategies are extremely encouraging, particularly when using conceivable trans responses between strengthening and polymerizing melts. This perspective was covered in an ongoing patent to deliver a unidirectional arrangement carbon fiber strengthened composite center through LCM as burden a bearing link of an electric transmission line. If there is need for the occurrence of hot squeezing of nonconsolidated preforms, preliminaries should be used to avoid issues of moderate heat conductivity [14]. The loading of appropriate (nano) particles in the lattice of the polymer going about as problem zones (heat sources) by outside enacting (electromagnetic fields, microwave, etc) is apparently a sound system [12]. The joining of PCs by means of different strategies is an extraordinary test. Portraying the invasion of liquid tar to strength structures and displaying the structure
property connections in PCs are further imperative undertakings [15].

19.1.1.4 Mechanical and thermal stabilities of polymers Trademark highlights of most polymers incorporate their low heat conductivity, high coefficient of direct warm augmentation, and their smoothness at high temperatures, and moderate low heat protections. Warm methodologies are the principal procedures for research of helper loosening up structures in polymers, especially the unfreezing of various degrees of chance by growing the intensity of warm development [16]. At the point when a material is heated, the recurrence of vibration of the molecules and the interatomic separations increase because of the

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FIGURE 19.3 Mechanical stability of polymers [19].

expanded energy supply [17]. Expanded vitality builds the normal separation among molecules and the strong extends. The estimation of warm augmentation dependent on the essentialness of the interatomic distance, so polymers with powerless interatomic joint effort unusual to higher spreading factors [18]. The introduction of fillers into a polymer brings about an essential decrease in the warm expansion coefficients joining those characteristics of polymers with practically identical properties of pottery and metals. The mechanical stability of polymers is shown in Fig. 19.3 [19].

19.1.2 Inspiration of polymers in daily life The polymers, a word that we find out about it a lot, is greatly essential and one can’t imagine the presence without it. Polymers are utilized to a great extent in things that we use in normal everyday life [20]. For many years, individuals utilized polymers for an astounding span of applications, however, these were used individually without additives nearly until World War II. There were sensibly couples of materials accessible to make of the articles required for an acculturated life. Glass, steel, wood, stone, and concrete were by far the most of advanced and jute, cotton, wood, and a few other agrarian things were used for dress or surface creation. The quick augmentation of the prominent for the made things displays the new materials. These new materials were polymers, and their effect on our present lifestyles is endless. Examples of the uses of polymers include articles of

19.1 Introduction

clothing created utilizing produced fibers, PE mugs, fiberglass, nylon course, plastic sacks, heart valves, polymer-based paints, PU foam cushions, epoxy sticks, silicone, and Teflon-secured utensils. The list is almost endless [21]. Polymers are gotten through substance response of monomers [22]. Monomers can respond with another atom from a similar kind or another composes in the appropriate condition to shape the polymer chain. This framework in nature is present in the structure of a number of common polymers, while constructed polymers are manmade. Polymers have been around us in the world since as far back as anyone can remember (e.g., starch, cellulose, etc.). Since the focal point of the 19th century, man-affected polymeric materials have been considered. Today, the polymer business has promptly become more prominent than copper, steel, aluminum, and other uncommon undertakings joined [23]. Both normal and manufactured polymers are surprisingly engaged with solace and assistance of human life and are in charge of life itself, for pharmaceutical, sustenance, correspondence, transportation, water system, holder, apparel, recording history, structures, interstates, and so forth. Honestly, it is difficult to imagine a human world without produced and customary polymers. In our consistently expanding innovative world, science assumes a vital job in giving answers for basic issues of nourishment, spotless and inexhaustible water, air, vitality, and wellbeing. The learning of polymers and related writings give both the data and experiences of their better comprehension in our life. The facts accumulated from essential science developments incite an improved understanding of polymers. This information consolidates the real, speculative, and rational thoughts shown in science. It is valuable to people who are experts and to those who would enjoy the opportunity to learn about medication, planning, material science, science, biomedical sciences, law, business, etc. [21,22]. Manufactured and basic polymers could be used as inorganic and normal polymers in bonds, coatings, elastomers, blends, strands, plastics, caulks, and stoneware generation composites. The essential rules that are connected to one polymer classification are connected to every single other class alongside a couple of basic principles. These basics are facilitated on the surface of polymer compositions [23]. It isn’t astounding that almost all material researchers and the greater part everything being equal and synthetic architects, countless, material technologists, mechanical designers, drug specialists and other logical gatherings are engaged with innovative work ventures identified with polymers. [24]. Also, the way that drug store, biomedicine, atomic science, organic chemistry, and biophysics are the fields that polymers and polymer science assume a critical job in the improvement of their new zones. It is evident why the investigation of goliath particles is a standout amongst the most went to and the quickest developing fields of science. Accordingly, it appears that polymer is certifiably not a particular interdisciplinary or part of science. Rather, it is a particular, expansive and one of a kind teach that could cover a few sections of science and a few other logical fields also. The fields of science have dependably turned out to be exceptionally dynamic when their interests to a related field inquire about gatherings prepared in one particular field turn. This has reliably been and later on, will be

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FIGURE 19.4 Polymers used in daily life.

especially substantial in polymer research works.The prerequisite in the polymer is the use of thoughts, science information, and systems to complex materials and macromolecules. This is a basic undertaking, and it requests the specific most ideal ways that science could give. [25]. Maybe polymer science, more than some other research field, traverses and cuts the conventional lines of all parts of science, physical science, material, building, drug store, and even medication. Furthermore, a newcomer to polymer science requires enough capacity to combine the huge learning from all previously mentioned fields. In like manner, this article has been made to exhibit the extraordinarily important and exceptional occupations of polymers in human life. Fig. 19.4 shows the polymers used in daily life.

19.1.3 Polymer uses in modern life Computer keyboards to advanced mobile phones, auto parts to espresso mug tops, restorative gadgets to satellites—plastics are a basic component to a great extent of the innovations that make current life conceivable. Seven kinds of plastics are the most widely recognized, flexible, and valuable on the planet. Unless you live

19.1 Introduction

in isolation, odds are that you have something made out of at least one of these sorts of plastics in your line of sight at this very moment.The American Chemistry Council focal points the seven most unavoidable plastics on earth, and you can visit their site for a delineation of each. Here, the segment of the confusing things-both reliably and marvelous-delivered utilizing these extraordinary seven plastics. 1. Polyethylene terephthalate (PET or PETE): The workhorse of the plastics world, PET is super valuable because of its toughness, temperature resilience, protection from debasement, and capacity to square dampness. Pretty much every kind of plastic container from refreshments to nutty spread and cleanser to clothing is made of PET. It is exceedingly recyclable, and ambitious producers presently make everything from floor coverings to attire out of reused PET containers. 2. High-density polyethylene (HDPE): As PET’s more grounded cousin, HDPE keeps grain crisp in boxes, makes home pipe frameworks less inclined to spill, is utilized in firecrackers, and is a most useful kind of fiber utilized in 3D printing. Like PET, HDPE is likewise recyclable. 3. Low-density polyethylene (LDPE): The primary sort of PE anytime delivered, LDPE makes the plastic rings that hold six-packs together. In film arrange it makes sure about meats, makes and bread shop in the grocery store. In film arrange it makes sure about meats, makes and bread shop in the grocery store. 4. PP: At PSI (Polymer System Inc), PP is important. It is utilized consistently in lab gear. It is a juggernaut in therapeutic applications, showing up in everything from pill jugs to demonstrative hardware, and inhalers to syringes. PP is additionally utilized in fixing containers and auto battery housings. 5. PS: With its good adaptability, PS can either be expelled as froth to make sheets (like the plate underneath cuts of meat in the market) or it tends to be hard and weak as when it is used to make plastic cutlery. PS gets your food scraps home securely as takeout compartments and helps keep your home safe when used to make smoke identifier lodgings. 6. Polyurethane (PUR): PUR gets around, appearing in everything from sleeping cushions to footwear. It is in like manner the most notable material used for energizing ride wheels, works outstandingly as assurance in homes and nuclear family machines, and is once in a while used in bandages and wound dressings. 7. Polymethyl methacrylate (PMMA): Broadly known as acrylic or acrylic glass, PMMA monitors, goldfish comfortable and aquarium home, is consistently used as impediments between the ice field and stands in hockey fields, and can help cascade patients recoup clear vision when a point of convergence made of PMMA is used to replace a developed, cloudy regular point of convergence inside the eye.

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19.2 Polymers in the environment 19.2.1 Polymers and their impacts in society: a general view Present day progression in advancement has redesigned great individual fulfillment in the general public. Polymer has expected an increasingly important activity right now. These are clear in the advanced modern therapeutic, transport, sensor, and agrarian hardware on the market, which are comprised of metals and altered polymers. Substitution of damage human parts with polymer, for example prostatic limbs, knee install and the new time of prescription movement. Polymers are currently altered by scientists and these adjustments modify their physical and compound properties for different societal needs, for example, sensors. These sensors are utilized in natural systems for detecting poisonous gases and contamination in streams and water. These sensors are additionally utilized in process plants for quality control. Progressing unrest in progress of batteries is made possible electrolyte. Low and powerful dry batteries are currently accessible for space utilization. Universal space stations presently utilize dry polymer electrolyte batteries for their activity. Polymers are utilized by the general public in therapeutics, sensors, transportation, farming, and so on.

19.2.2 Overview of environmental and societal applications of polymers Polymer usage in the environment is significant; the current situation of the world shows that polymers have an important role in human life. The utilization of polymers in environment is more; it is utilized in water purification, an adsorbent for toxic ions and heavy metals. The societal applications of polymers are numerous. Right now the everyday life is being with polymers. For example, polymers are used in medical applications, agricultural applications, hydrogen storage devices, space research materials, packaging devices, cooking utensils, solar cells, electrical instruments, and corrosion protective agents, etc. The lightweight of plastics makes for more fuel-efficient vehicles. It is estimated that every 10% reduction in vehicle weight results in a 5%
7% reduction in fuel usage. Current economic and environmental concerns make the creation of more fuel-efficient cars a top priority in the automotive industry. Other advantages of high-performance plastics used in transport vehicles include:

• • • • •

minimal corrosion, allowing for longer vehicle life; substantial design freedom, allowing for advanced creativity and innovation; flexibility in integrating components; safety, comfort, and economy; and recyclability. Here some of polymers and their uses in automobile industries are discussed.

19.2 Polymers in the environment

19.2.2.1 Polypropylene PP is a thermoplastic polymer used in a wide variety of applications. As a saturated addition polymer made from the monomer propylene, it is rugged and unusually resistant to many chemical solvents, bases, and acids. Its applications include automotive bumpers, chemical tanks, cable insulation, gas cans, carpet fibers, etc.

19.2.2.2 Polyurethane Solid PUR is an elastomeric material of exceptional physical properties including toughness, flexibility, and resistance to abrasion and temperature. PUR has a broad hardness range from eraser soft to bowling ball hard. Other PU characteristics include extremely high flex-life, high load-bearing capacity, and outstanding resistance to weather, ozone, radiation, oil, gasoline, and most solvents. It applications include flexible foam seating, foam insulation panels, elastomeric wheels and tires, automotive suspension bushings, cushions, electrical potting compounds, hard plastic parts, etc.

19.2.2.3 Polyvinyl chloride PVC has good flexibility, is flame retardant, and has good thermal stability, a high gloss, and low (to no) lead content. PVC molding compounds can be extruded, injection molded, compression molded, calendered, and blow molded to form a huge variety of products, either rigid or flexible depending on the amount and type of plasticizers used. Its applications include automobile instruments panels, sheathing of electrical cables, pipes, doors, etc.

19.2.2.4 Acrylonitrile butadiene Acrylonitrile butadiene styrene is a copolymer made by polymerizing styrene and acrylonitrile in the presence of polybutadiene. The styrene gives the plastic a shiny, impervious surface. The butadiene, a rubbery substance, provides resilience even at low temperatures. A variety of modifications can be made to improve its impact resistance, toughness, and heat resistance. Its application include automotive body parts, dashboards, wheel covers, etc.

19.2.2.5 Polyamide Nylon 6/6 is a general-purpose nylon that can be both molded and extruded. Nylon 6/6 has good mechanical properties and wear resistance. It is frequently used when a low cost, high mechanical strength, rigid, and stable material is required. Nylon is highly water absorbent and will swell in watery environments. Its applications include gears, bushes, cams, bearings, weather proof coatings, etc. In the medical field, polymers have a wide range of applications, among them tissue engineering is one of the most prominent usages of polymers. Poly

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(ε-caprolactone) (PCL) is additionally utilized in tissue engineering and other biomedical applications. PCL can be debased by microorganism, hydrolytic, enzymatic, or intracellular systems under physiological conditions. PCL is a semicrystalline polymer with a low glass-transition temperature of around 260 C. In this manner, it is dependably in the rubbery state and has high material porousness under physiological conditions. Be that as it may, PCL debases at a much slower rate than polylactic corrosive (PLA), Polyglycolic acid (PGA), and Poly Lactic-co-Glucolyic acid (PLGA), which makes PCL less charming for general tissue regeneration applications; be that as it may, it is logically engaging for long stretch installs and sedate movement systems. Polyanhydrides have been joined adequately from open, ease sources and have been controlled to meet alluring traits. Polyanhydrides are biocompatible and degradable in vivo into nontoxic diacid results that can be dispensed with from the body as metabolites. They were at first structured fundamentally for drug delivery applications, in light of the fact that these polymers are exceptionally hydrophobic and experience debasement through surface disintegration. Medications can be secured when installed in such polymers in light of the fact that no water enters before the polymer dissolves. They have been investigated for tissue designing frameworks also. Poly(glycerol sebacate) (PGS) is a generally new manufactured polymer; as a biodegradable and biocompatible polymer, it is progressively utilized in different biomedical applications. The Langer pack recently uncovered extraordinary biodegradable PGS incorporated for fragile tissue structuring in 2002. The beginning materials for the combination of PGS are glycerol and sebacic acid. Glycerol has been supported by the FDA for use as a humectant in sustenances; as the normal metabolic widely appealing in oxidation of medium-to long-chain unsaturated fats, sebacic acid has ended up being secured in vivo. PGS, which is moderately modest to deliver, shows thermoset elastomeric properties.

19.3 Polymer-based materials as a new direction for environmental remediations 19.3.1 Carbon-based polymeric composite materials for CO2 capture The gathering of natural permeable materials isn’t confined in the more than six social affairs, distinctive penetrable characteristic polymers/materials, including C3N4 [26], permeable natural enclosures [27
30], additionally, indicate the potential for gas catch and partition. The examination of natural permeable materials has been rapidly developing in the past decade, this is critical because of the decent variety of manufactured responses and rich strategies for surface functionalization for tuning the CO2 catch limits and CO2 selectivities for additionally understanding their relations. The essential research software engineers for enhancing porous natural polymers gas adsorption properties can be abridged into

19.3 Polymer-based materials

the accompanying subjects: (1) to expand materials CO2 catch limit, (2) to improve CO2/N2 selectivity, (3) the contemplations for the genuine application. The CO2 catch limits of the vast majority of polymers still fall behind that of carbon materials and metal natural structures (MOFs) altogether so far [31,32]. So upgrading the limit of CO2 at encompassing weight is by all accounts of incredible significance. The CO2 selectivity of polymers can be improved by doping CO2 affinitive useful gatherings, yet much of the time the presentation of utilitarian gatherings can debilitate materials CO2 catch capacity, maybe because of the volumes of practical gatherings which obstruct the space of pores, or the expansion of thickness to permeable polymers. Diverse strategies can be connected to tackle this issue, for example, picking low thickness practical gatherings, controlling the pore size to improve surface collaboration with CO2, and broadening polymers’ particular surface zone for giving more space to encourage functionalization. Up until this point, a portion of these strategies has been connected on a few polymers, which are demonstrated to have high CO2 selectivity while keeping great CO2 catch limit in the meantime, the mechanism of CO2 capture by porous hypercrosslinked ionic polymers was shown in Fig. 19.5 [33]. In the genuine application for CO2 catch at encompassing weight, particularly in the postburning techniques, a few themes must be taken genuine thought: (1) CO2 catch execution of materials at high temperature, (2) solidness of materials to dampness and high temperature, (3) cost of materials, (4) probability for scale up. The high-temperature execution of polymers merits additionally think about, considering the way that both the CO2 capture limits and CO2 selectivity’s of polymers will be debilitated at a higher temperature in generally circumstances [34]. On the opposite side, warm strength is the benefit of polymers when contrasted and MOFs, a large number of them can endure high temperature of in excess of 300 degrees in the environment of latent gases. The mugginess steadiness of polymers likewise pulls in considerations [35], and it very well may be an imperative research theme later on thinking about the earnest necessity for a

FIGURE 19.5 CO2 capture by the porous hypercrosslinked ionic polymers [33].

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genuine application. A few Performance optimized polymers (POP’s), for example, hyper-cross-linked polymers, demonstrate great potential for scale-up, as shabby reactants can be connected for their combination technique. This may illuminate a pertinent path to the improvement of future permeable materials. In a rundown, the permeable natural polymers indicate great potential for postignition CO2 catch. A few perspectives still need additionally think about keeping in mind the end goal to deliver more execution improved polymeric composites for genuine application later on, including expanding the CO2/N2 selectivity, augmenting CO2 catch capacity, lessening materials’ expense and enhancing union courses for scale up. Carbon materials reliably accept a basic part in our lives essentially through inventive applications and beginning of new materials [36
38]. The fundamental points of view in the advancement of our general populace are about related to advancement in the materials all through history [39]. For example, the Steel Age (mechanical distress), Bronze Age, Stone Age, Iron Age, silica and silicon age (correspondence uprising). All these mirrors that our bleeding edge human headway and step by step life in the 21st century depends upon a boundless arrangement of materials of changing degrees of progression [40]. Along these lines, materials science accepts a central part in our step by step life. Thusly, materials are central to new advancement that makes our lives flooding with accommodation, for example, exceptional electronic daily papers, viz., PC, cell phones, et cetera. The ordinary allotropic type of carbon will be carbon nanotubes (CNTs) and it has a tube formed structure [41]. Nanotubes are people from the fullerene helper family, and its name is gotten from their unfilled, broadened structure with the dividers encircled by one-molecule-thick sheets of carbon [42]. CNT have various bizarre properties, which are profitable for current science and development. In like manner, they have indicated potential application in the field of contraptions, optics, and assorted fields of materials science and improvement [43]. Specifically, inferable from their remarkable warm, electrical and mechanical properties and find critical uses in all fields of science thusly, the domain of the conductive polymer isn’t exclusion. A reasonable blend of CNTs with planning or nondriving polymers offers a particularly captivating course to strengthen macromolecular mixes and despite present new electronic properties in the polymer framework by changes or electronic exchanges between the two constituents (system and nanofillers). A couple of conductive polymer in perspective of CNTs has been made, and they have extraordinary driving properties [44]. In any case, the utilization of CNTs as a rough nanomaterial in various applications has been by and large obliged in light of their poor processability, insolubility, and infusibility. In any case, specific functionalization of CNTs can confine the above difficulties in the midst of polymer nanocomposites mix [45]. Materials science have been concentrating on two sorts of materials for the planning PCs (1) conductive composites in context of guaranteeing polymers, for example, PS or poly (methyl methacrylate), and so on, and (2) those made with driving polymers, for instance, polypyrrole (PPy), polyaniline (PANi), polythiophene, poly(3,4-ethylenedioxy thiophene), polyacetylene, 2,2-polybithiophene, etc. [35].

19.3 Polymer-based materials

The frequently utilized procedure for the relationship of CNT-based PCs is a fast blending of nanofillers in the dissolve or game-plan or by techniques for situ polymerization approach. A formed nanocomposite, for the most part, relies on the interfacial relationship among nanotube and polymer, which at last relies on the structured procedure. In this way, in situ system for nanocomposites blend drives the best interfacial contact diverged from the other point by point strategy for the main PCs [46]. The graphene-based driving PC materials have been pulling in much idea in view of the world class of graphene as nanofillers, giving these materials improved mechanical and electrical properties [47]. The delivered game plan of these nanomaterials is on an essential dimension the equal as the CNTs-based organizing polymer nanocomposites. Different graphene-based PPy, PANi, poly(3,4-ethylene dioxythiophene) (PEDOT) and poly(phenylenediamine), coordinating PCs have been spoken to the particular application [48]. These composites are showing high benefit in supercapacitors, solar cells, sensors, and differing contraptions. The bottleneck in the technique for these nanomaterials affiliation is gathering the probability of graphene. To evacuate this lacuna, various phenomenal endeavors and two or three methodologies have been made to plan graphene-dispersed composites in which graphene are homogeneously dispersed into polymer grids [49]. The carbon-based PCs are used as super capacitors [50], fuel cell electrodes [51], electrochemical actuators [52], memory devices [53], field emission devices [54] and lithium batteries [55].

19.3.2 Polymer-based membranes Polymeric layers are broadly utilized in film detachment forms for the upsides of their capacity to frame great film, adaptability, sturdiness, sparing, and astounding division properties; while, they have few entanglements, for example, constrained mechanical, compound, and warm obstruction, low mass transport (for thick polymeric layer) and also a poor enemy of fouling capacity incited by their surface properties. Arranging of an imaginative composite polymeric layer by fundamental alteration of the present cross section of polymer film materials to improve their vulnerability, perm-selectivity, and quality and other valuable properties would expect a basic part in film science and advancement. Polymeric nanocomposite layers are set up by mixing of nanosized fillers and polymer network to upgrade exhibitions, for example, high perm-selectivity, transitions, ideal surface morphology because of superb fouling obstruction contrasted and the unadulterated polymeric framework films [56,57]. In the past couple of years, polymer nanocomposite film has been amazing in its utilization for gas separation, desalination, water, and wastewater treatment, control gadget, etc. Authoritative properties of a polymeric nanocomposite layer are particularly dependent on the dissipating and course of the fortifying nanofiller in the relentless stage, and furthermore interfacial correspondence between the surface of the nanoscale filler and polymer lattice [58]. By joining nanoestimate fillers in the polymer arrangement before the layer creation, one can change arrangement rheology, modify

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FIGURE 19.6 Schematic illustration of interface-directed assembly of conjugated polymer membrane (A) interface-directed assembly of SERS-active uniform composite film architecture from PPI (
SH)-b-P3HT conjugated polymers and Au NPs and (B) hydrogen bonding induced micellization mediated fabrication of thiol groups functionalized PPI(
SH)- b-P3HT conjugated polymers [60].

stage reversal and control film morphology. Prearranged nanofillers for polymer nanocomposite film creation are CNTs, graphene oxide, Mg(OH)2, TiO2, SiO2, Al2O3, ZnO, earth and so on. Astounding upgrade of both layer penetrability and selectivity for subatomic species amid detachment could be accomplished by strengthening the nanofillers in polymer lattice [59]. The profoundly positive property upgrade as far as motion and selectivity for polymer nanocomposite layer rather than the ordinary filler strengthened polymer film is in all probability because of an additional locale at the nanofillers polymer interface as well as interruption of polymer chain pressing because of significant addition of free volume measure through which subnuclear transport would occur in the thick nanocomposite layer matrix, the working mechanism of the polymer membrane was shown in Fig. 19.6 [60]. In this manner, the two gas detachment and additionally broad scale water treatment of normal and business criticalness could be benefitted by using polymer nanocomposite film.

19.3.3 Magnetic polymer composites The polymer-based magnetic composite materials are the furthermost created and examined, attributable to their high flexibility and capacity to significantly adjust their inborn properties as an element of little natural changes [61]. Keen polymers are progressively assuming a vital part in an extensive variety of utilization,

19.3 Polymer-based materials

particularly in the biomedical applications (medicate transmission, biosensors, tissue engineering, dynamic conclusion), coatings (shrewd materials and filaments) and microelectronics (actuators, electromechanics). To be sure, the immense improvement of blend and triumphs in polymer science permit a legitimate plan of all around characterized macromolecules that can intertwine helps responsive building thwarts for all the recently referenced triggers. Notwithstanding, naturally attractive polymers are rare and for the most part, introduce poor viability. Fluid crystalline elastomers and polymers have been intended for uses as counterfeit muscles, in view of the fundamental thoughts suggested by De Gennes et al. [62]. Collective impacts of attractive introduction and temperature prompted stage progress (from nematic to the isotropic stage) are at the birthplace of volume changes and disfigurements. Notwithstanding their fascinating mechanical and warm properties, fluid precious stone polymers require extraordinary attractive fields for their arrangement (HB103 kA m-1) and current low exchanging rates. For sure, their reaction time is restricted by their high thickness, particularly in the high temperatures and mass are regularly required to achieve a suitable stage transition [63]. Opposing to frameworks constrained by dispersion forms (of either mass or heat), those specifically worked by the introduction to a field (be it electric or attractive) may have shorter reaction times, conceivably as quick as characteristic skeletal muscles [64]. To create exceptionally proficient attractive materials, the “doping” of polymer materials with attractive magnetic nanoparticles (MNPs), made of inorganic issue (frequently super paramagnetic press oxide Fe3O4 or g-Fe2O3, or “delicate” metallic iron, yet additionally “hard” attractive materials, e.g., Ni, Co, FeN, FePd, FePt. . .), gave off an impression of being the all the more engaging and effective arrangement. For sure, the attractive snapshot of these “little” magnets, substantially bigger than those of atomic magnets, enables them to react to feeble jolts (static or exchanging attractive field) with a huge impact (e.g., movement, warm age, attractive or optical flag). The subsequent composites that can be named “magnetic responsive polymer composites” (MRPCs) are the theme of this survey. The center is in this way not the same as the 2009 audit by Brazel [65] on “magnetothermal-responsive nanomaterials” devoted for the most part to thermo-touchy polymers related to a class of MRPCs, MNPs that will likewise be talked about here. Our approach is additionally not the same as Dai and Nelson’s 2010 instructional exercise audit on attractive PCs [66], and from the survey by Medeiros et al. on stimuli-responsive attractive particles for biomedical applications, in which the affectability to an attractive field was not constantly coupled to the next jolts (light, pH, electric field, temperature, ionic strength, etc.) [67]. In option to the specifying of the most recent chips away at the subject, we have settled on a very surprising methodology in fact, in view of uses as opposed to readiness techniques. Under our center, the enactment method of MRPCs dependably comprises in the utilization of an attractive field, either static or rotating. In our perspective of MRPCs, attraction must decide their center reactions as opposed to being extra usefulness.

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19.3.4 Ionic liquid based polymeric composites Polymeric ionic liquid (PIL) composites are an imperative and potential research field. CNT, graphene, graphite and diverse metal nanoparticles have been fused in poly(ionic liquid)s to frame nanocomposites. Graphene is a vital nanofillers, that is one molecule thick with two-dimensional honeycomb cross section. It has pulled in examine consideration because of remarkable mechanical, electronic, and warm properties. Zhou et al. [68] announced scattering of graphene sheets in the polymerized ionic fluid of 1-butyl-3-methylimidazolium hexafluorophosphate. The got PILs composites arrangement showed higher conductivity than the unadulterated ionic fluids. PIL with graphene sheet was professed to have potential application in vitality stockpiling gadgets, catalysis, solar cells, and so forth. Liu et al. [69] detailed ionic-liquid treated graphite sheets can be shed into functionalized graphene nanosheets. PS/graphene nanosheets composites were orchestrated by a fluid stage mix course. The composites showed permeation limit of 0.1 vol.% at room temperature. This composite indicated conductivity of 13.84 Sm21, which was 15 times higher than PS composites loaded up with single-walled CNT. Electroconductive delicate material has been set up from PILs and CNT [70]. CNTs have the capacity to associate with polymers through π 2 π connection [71]. Wang et al. [72] have announced PANi fiber containing CNT. An ionic fluid ethylmethyl imidazolium bis(trifluoromethanesulfonyl) amide (EMI TFSA) was utilized as an electrolyte. The electrochemical properties of PANi and PANi/CNT strands were researched by methods for cyclic voltammetry, a.c. impedance and galvanostatic charge
release methods. PANi fiber with 0.25 wt.% CNT content uncovered release limit of 12.1 mAh/g. An ongoing audit by Livi et al. has featured the part of PC and nanocomposite in the manufacture of PILs [73]. The poly(1-butyl-3-vinylimidazolium bromide) utilized a strong state electrolyte [74], poly(1-vinyl-3-butylimidazoliumbromide)graphene utilized as a biosensor [75], polyionic fluid utilized as a detachment layer for CO2 adsorption [76], basic polymer electrolyte utilizing poly(methylmethacrylate-co-vinylbenzylchloride) utilized as a fuel cell [77], functionalized polyionic fluid covered attractive nanoparticle utilized as a catalysis applications [78].

19.4 Polymer-based materials for societal applications 19.4.1 Polymers-based materials for agriculture and horticulture In the most recent period, one of the issues influencing the earth has been the expanded utilization of polymer materials and their ensuing transfer. Polymers have been utilized in incalculable uses with little thought for their definitive disposability [79,80]. Traditional polymers, for example, PP and PE continue for a long time after transfer. Worked for the whole deal, these polymers appear to be improper for applications in which plastics are utilized for brief eras and after

19.4 Polymer-based materials for societal applications

that arranged. Besides, plastics are frequently filthy by nourishment and other natural substances [81]. At the end of the day, the opposition of engineered polymers to the activity of living frameworks is winding up increasingly tricky in a few spaces where are utilized for a constrained timeframe before getting to be waste [82]. Among the different conceivable courses to wipe out polymeric squanders, biodegradation and bio reusing by means of bioadsorption are viewed as alluring answers for natural assurance when cremation isn’t plausible in light of the fact that it is a wellspring of unsuitable contamination. Biodegradable polymers (BPs) have been used reasonably, for instance, plastics substitutes for a couple of uses in the agribusiness field [83
85]. BPs, arranged in bioactive conditions, corrupt by the enzymatic activity of microorganisms, for example, microbes, organisms, and green growth and their polymer chains may likewise be separated by nonenzymatic procedures, for example, compound hydrolysis. Tragically, in the lion’s share of cases, the properties of regular polymers don’t fit the necessities of particular applications, and mixing with synthetic polymers is a course to a great extent used to pick up the coveted properties, the root soil structured mechanism of the polymers shown in Fig. 19.7 [86,87]. Advantageous possibilities for those applications are characteristic polymers, for example, starches, gelatins, alginates, agar, and cellulose subsidiaries, alongside manufactured BPs, for example, polycaprolactone (PCL), polylactide, and poly(vinyl alcohol) [88
91].

FIGURE 19.7 Polymers applications in agriculture [84].

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The proximity of water in the soil is basic to vegetation. Liquid water ensures the supporting of plants with nutritive segments, which makes it possible for the plants to procure a prevalent advancement rate. It is apparently captivating to manhandle the current water potential by reducing the incidents of water and moreover ensuring better living conditions for vegetation. Considering the water swallowing characteristics of super permeable polymer (SAP) materials, the potential results of its application in the agrarian field has dynamically been investigated to relieve certain provincial issues [92]. SAPs are exacerbated that ingest water and swell to commonly their unique size and weight. They are softly cross-connected systems of hydrophilic polymer chains. The system can swell in water and hold a lot of water while keeping up the physical measurement structure [93]. It was realized that monetarily utilized water-permeable polymeric materials utilized are fractional balance results of cross-connected polyacrylic acids, incomplete hydrolysis results of starch-acrylonitrile copolymers and starch
acrylic corrosive unite copolymers. At present, the material’s biodegradability is a critical focal point of the examination in this field due to the restored consideration towards natural insurance issues [94]. The half-life is by and large in the range 5
7 years, and they corrupt into ammonium, carbon dioxide, and water. The superabsorbent polymers and composites were used to improve the physical properties of soil in view of: 1. 2. 3. 4. 5. 6.

increasing their water-holding capacity and water use efficiency; water use efficiency; enhancing soil permeability and infiltration rates; reducing irrigation frequency and compaction tendency; stopping erosion and water run-off; Increasing plant performance (especially in structureless soils in areas subject to drought).

19.4.2 Polymer-based materials for packaging materials The basic properties of packaging materials are chosen by the physical and concoction attributes of the item, and additionally by the outside conditions in which the item is put away/transported. As polymers have a broad assortment of properties which can be exceptionally fitted as demonstrated by the thing need, they are the most appealing materials for packaging applications. PE, PVC, PS, PP and PET, are the most widely recognized packaging polymers, representing over 90% of the aggregate volume of polymers utilized in packaging [83]. Basic crude materials for generation of plastics are oil-based goods acquired from refining forms. Most of the standard, petroleum derivatives based polymers are nonbiodegradable and hereafter can cause natural contamination. Because of expanding ecological concerns, center around look into in bio-based practical plastic packaging materials has expanded essentially in the previous couple of years. The most promising bio-based polymers for future packaging applications are chitosan,

19.4 Polymer-based materials for societal applications

FIGURE 19.8 Polymers used for packaging applications.

cellulose, polyhydroxyalkanoates and PLA [95]. The fundamental elements of packaging can be extensively characterized into essential and optional capacities. Essential capacities, for example, those related with insurance, stockpiling, stacking, and transport of the item will require them to be packaging solid, watertight and ready to withstand any outside conditions that the capacity or transportation condition will force. Optional capacities, for example, those related with advancing offers of the item may require the packaging to be straightforward or to have great physical appearance (lustrous) to draw in client consideration. Data, for example, item fixings, nourishment content (of sustenance things) and maybe utilization directions ought to be put on the packaging, which will require the packaging material to be printable. Further, polymer recyclability can be a vital paradigm in deciding its appropriateness as packaging material particularly for volume applications [96,97]. Fig. 19.8 shows the types of polymers used in packaging purpose.

19.4.3 Polymeric materials for hydrogen storage purpose Most polymers have adequate conformational adaptability to fill space effectively and thus don’t have critical microporosity. Interestingly, our polymers of natural microporosity (PIMs) [98
100] are unbending and distorted macromolecules, entirely made out of melded ring segments, which pack space wastefully to leave interconnected atomic estimated voids, that is, microporosity. PIMs consolidate

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moderately high interior surface regions (BET1/4400
1050 m2 g-1) with the manufactured assorted variety of natural polymers. They can be readied utilizing a basic and productive dioxane shaping response either as insoluble system polymers, for example, those which consolidate cyclotricatechylene, hexaazatrinaphthylene, triptycene subunits and porphyrin or as dissolvable processable nonarrange polymers, for example, PIM-1 and PIM-7. The last give powerful self-standing movies when casting from the arrangement, reasonable for use as gas partition membranes [90] also, PIMs are promising for an extensive variety of utilization including heterogeneous catalysis [101
103], other layer separations [104,105] and the adsorption of natural compounds [106]. An elective way to deal with getting ready natural polymer-based microporous materials includes the escalated crosslinking of dissolvable swollen, chloromethylated PS dabs, by means of a Lewis corrosive intervened Friedel Craft alkylation [107]. These hypercrosslinked polymers can exhibit high surface zones and have been utilized as business adsorbents and for the production of particle trade Resins.

19.4.4 Polymer-based materials for corrosion control To build the corrosion protection efficiency of metals, copolymers were produced by various methods. Development of polymer mud nanocomposite materials that involved of poly(styrene-co-acrylonitrile) (PSAN) and layered (montmorillonite) MMT earth were effectively arranged by viably scattering the inorganic nanolayers of MMT dirt into the natural PSAN lattice by a regular in situ thermal polymerization [108]. PSAN and layered MMT mud were effectively arranged by successfully scattering the inorganic nanolayers of MMT mud into the natural PSAN framework by an ordinary in situ thermal polymerizations [109]. Polymeric movies containing some measure of liquor usefulness, of which poly (vinyl liquor) and poly(ethylene-co-vinyl liquor) are most normal, are known to be astounding obstructions to oxygen under low moistness conditions [110].The PPy and poly(pyrrole-co-anisidine) on 3102 aluminum compound and led various electrochemical tests demonstrated that the opposition of the copolymer covered Al fundamentally enhanced in 3.5% NaCl arrangement [111]. The erosion conduct of dissolvable self-doped copolymers of aniline and 4-amino-3-hydroxynaphthalene-1-sulfonic corrosive on press substrate was contemplated in the hydrochloric corrosive arrangement by electrochemical methods [112]. Table 19.1 shows the corrosion protection efficiency of some polymers.

19.4.5 Polymer-based materials for medical and biomedical applications As far back as four decades since the primary display of conducting polymers (CPs) during the 1970s [113,114], the centrality and interests in this extraordinary class of typical materials have developed exponentially. With an electrical

Coating materials

Corrosion monitoring technique

Polyaniline Polypyrrole

Tafel extrapolation, EIS Potentiodynamic polarization Chronoamperometry Potentiodynamic polarization Galvanostatic polarization CV or galvanostatic

Polyaniline Polyaniline Polyaniline Polyaniline

Protection efficiency

References

Not measured 90%

118 119

Not measured Not measured

120 121

Oxalic acid

97.6%

122

H2SO4

Not measured

123

Type of metal used

Medium used

Mild steel Pt,Au,Cu,Ti SS/V2A (SS type of V2A), Fe Steel Mild steel

HCl 20 nonaqueous and aqueous electrolytes Oxalic acid H3PO4

AA3004 (Al alloy type of 3004) Mild steel

19.4 Polymer-based materials for societal applications

Table 19.1 The corrosion protection efficiency of some polymers and its properties.

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conductivity drawing closer those of metals and inorganic semiconductors and furthermore simple status and extraordinary process limit of ordinary polymers [115], CPs have immense potential for a broad assortment of uses [116
118]. These highlights, joined with the glorious in vitro and in vivo biocompatibility of CPs [119
121], have incited they are being constantly examined for different biomedical applications, especially for tissue engineering, cure movement, bioimaging, and bio detecting [122
127]. Especially for tissue engineering, with their electrical incitement, CPs can manage distinctive cell works on, including cell connection, game plan, extension, detachment, and empower the recuperation of hurt tissues, for instance, bone, nerve, skin, and myocardium tissues [128
132]. While CPs have been viewed as promising biomaterials, their pragmatic bio applications and clinical interpretations are up ‘til now astounded by various restraints (prominently, the poor dissolvability and nonbiodegradability of ordinary CPs) [133,134]. For the outline, when brought into a physiological territory as introduce materials for tissue building applications, the nonbiodegradability of existing CPs may show fundamental issues. Specifically, the disappointment of CPs to corrupt may defer their stay in vivo, which hence, may trigger a heartbreaking provocative response. Massive undertakings have along these lines been proposed for the association of biodegradable electro dynamic polymers with astounding biodegradability [135]. Truly, up to now, biodegradability remains a champion among other ideal focuses of the upgrade of CPs for biomedical applications. Energetically, CPs with biodegradable trademark has been persistently recognized through different arrangement and producing techniques. A standout amongst the most dependable undertakings to recognize mostly biodegradable CP composites was through the mixing of CPs with normal BPs, for instance, PLA, PCL, and PU [136
138]. Despite the incredible bounce forward accomplished by strategies for this mixing strategy, the biodegradability of the resultant composites for down to business in vivo biomedical applications is up ‘til now a long way from worthy. To address this, continuous years have seen the improvement of different sharp procedures in the arrangement of biodegradable CPs. These join essentially the main oligomer-based course of action of straight, star-shaped, hyper fanned, and other complex biodegradable electro dynamic polymeric structures and furthermore the mix of biodegradable CPs through the circuit of balanced monomers or the coordination of degradable monomer units and conjugated linkers [139]. The upgrade of these frameworks has, truly, massively expanded the contraption hold of biodegradable CPs and opened up improvement open passages for the accommodating utilization of their biomedical applications. This diagram would like to give a wide layout of the constant advances in the improvement of biodegradable CPs and their biomedical applications. The nuts and bolts of coordinating and BPs are first shown and after that taken after by exchanges on the certifiable techniques beginning at now being utilized to make biodegradable CPs. The potential biomedical uses of biodegradable CPs, especially for tissue building, regenerative medication, and biomedical imaging and likewise biomedical additions, bioelectronics contraptions, and customer

19.5 Polymers

FIGURE 19.9 Polymers used for medical applications.

equipment, are then included. This review article over the long haul completes up with an abstract and perspectives on the present troubles and future open entryways going up against the headway and practical employment of biodegradable CPs. Fig. 19.9 shows the polymers used in medical and biomedical applications.

19.5 Polymers: recent trends, strategic changes, economic and market demands In a world facing international challenges like aggressive population, food security, and temperature change, our societies have to be compelled to select and believe the foremost economical solutions so as to ensure a property development. Europe has started a progress from a straight towards around and asset affordable society and furthermore the unmistakable attributes of plastics empower them to make an astounding commitment towards this social gathering change, due to

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their skillfulness and their high resource potency, plastics became key materials in strategic sectors like building & construction, packaging, renewable energy, transportation, medical devices or perhaps sports, to say however many. In addition, polymers have empowered development in a few elective segments allowing the occasion of stock and arrangements that probably won’t exist nowadays while not these materials. Polymeric materials and plastic merchandise area unit extraordinarily resource economically on their service life, serving to North American nation to avoid waste matter, to save lots of energy and to decrease carbon dioxide emissions. At the tip of their utilization life they’ll be fixed or reutilized, anyway, at last, they can end up waste, and this waste is extremely a substitution asset that must be put back inside the existing cycle of plastics, shutting so the circle of Circular Economy. Be that as it may, to gain from the absolute capability of plastics at the tip of their first life, we have to push for the most feasible alternative of waste administration, encouraging use, exploitation vitality recuperation as an integral choice and prohibiting the swamp store of any recoverable plastic waste. Europe’s forcefulness and resource strength should be what we zone unit endeavor for. Polymers materials and also the industry will definitively create a major contribution during this overarching goal.

At an amazing tip, polymers zone unit still awfully important assets which will be improved into new feedstock or into vitality.

19.5.1 Economic development of polymeric products The expense of polymers should be investigated dispassionately and tended to, on the grounds that as a polymer is growing, more thoughts regarding polymers and the need to grasp their utilization is essential and vital, along these lines, economic concerns must be tended to, in light of the fact that the eventual fate of each polymer item is exclusively subject to its cost intensity [140], and society’s capacity to pay for it on the grounds that the greater part of the polymers are exorbitant and since oil-based polymers are less expensive, business set out on their use without considering the ecological factors rather the benefit [141]. In created and creating nations, governments and NGO’s are acquainting activities planned with advance, illuminate individuals and advance instruction by giving examination stipends, give space to the application and sufficient improvement of polymers [142]. Most of the countries everywhere throughout the world and their strategy producers bolster work in the zone of polymers look into, with governments all around intrigued. This writing survey gives data giving mindfulness and advancement made accessible in the generation, application and improvement of BP materials and some critical data should have been investigated about biodegradable materials.

19.7 Consequences of the nonbiodegradable polymers

19.6 Polymers: future impacts on energy and solar cells Solar cells utilizing organic material as the dynamic layer changing over a photon stream into an electron stream have been known and revealed for a long while [143
145] while the term polymer, solar cells is generally later with a history that basically length the primary decade of the new centuries [146]. Among this era, the field has seen a close exponential development in the quantity of distributed logical articles and references with a yearly production and reference rate in the year 2009 of, separately, 900 and 22,000 (while looking through the subject “polymer sun oriented cells” in ISI web of science). A few other pursuit terms can be utilized and they all mirror a similar example of steeply expanding yearly rates. This can be credited to two main considerations: the bigger accessibility of research assets because of expanding universal spotlight on vitality, condition, and environmental change and the way that polymer solar cells are moving quickly in the direction of commercialization. It likewise demonstrates that the field in no way, shape or form has achieved development or a steady business stage as the last is regularly gone before by stagnation in the quantity of distributed logical articles. The field has been surveyed more than one hundred times when seen extensively with a delegate set of audits [147
150], a few extraordinary issues devoted to polymer sun oriented cells [151
154], various book sections and a few reading material and monographs have been distributed [155,156]. Their perspectives range from expansive diagrams of the field with later particular surveys managing subsets of the distributed in general information, for example, pair cells [157,158], dependability [159], preparing [160], low band hole materials [161] half breed sunlight based cells [162] and novel ideas [163]. This is demonstrative of huge potential and the enormous collection of data accessible and look into movement warrant promote examination of the polymer sunlight based cell as an innovation with regards to business, market and licensed innovation. The improvement in polymer sun-powered cells is quick. Polymer-fullerene solar cells have a huge elite among others. The accompanying polymer sun oriented cells have the best exhibitions of polymer solar cells and its properties like PCE—control transformation proficiency, Voc—open circuit voltage, FF—fill factor and Jsc—short out current, are given in Table 19.2.

19.7 Consequences of the nonbiodegradable polymers derived from renewable resources Some polymer materials are conveyed totally or not entirely from manageable (the term unlimited here is incredibly confined, as was prior to elucidated) unrefined materials. A few examples are recorded below. Braskem—green PE: is the ordinary PE, however, got from ethylene carried with ethanol from sugar cane.

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Table 19.2 Currently used polymers for the improvement of solar cell. Active materials

PCE

Voc (V)

FF (%)

Jsc (mA/cm2)

Reference

P3HT:PCBMs PSiF-DBT:PCBM PTB1:P71CBM PCPDTBT:PCBM Hyberbranched CdSe nanoparticle:P3HT OC1C10-PPV:CdSe tetrapods PCPDTBT:PCBM/TiOx/ PEDOT:PSS/P3HT:PCBM PCDTBT:PC70BM/TiOx

5 5.4 5.3 5.2 2.18

0.6 0.9 0.56 0.62 0.6

60 50.7 63.3 55 50

11 9.5 15 16.2 7

175 176 177 178 179

2.8 6.5

0.76 1.24

40 67

9.1 7.8

180 181

6

0.88

66

10.6

182

P3HT:PCBMs—[poly(3-hexyl)thiophene:[6,6]-phenylC61-butyricacidmethylester]; PSiF-DBT:PCBM— [poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole]: [6,6]phenylC61-butyricacidmethylester]]; PTB1:P71CBM—[poly(4,8-bis(octyloxy)benzo(1,2-b:4,5-b0 ) dithiophene-2,6-diyl](2-(dodecylcarbonyl)thieno(3,4-b)thiophenediyl: [6,6]-phenylC71butyricacidmethylester]; PCPDTBT:PCBM—[poly(2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4b0 ]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)): [6,6]-phenylC61-butyricacidmethylester]; hyberbranched CdSe nanoparticle:P3HT [hyperbranched CdSe nanoparticle: poly(3hexyl)thiophene]; OC1C10-PPV:CdSe tetrapod’s:[poly(2-methoxy-5-(3’7’-dimethyloctyloxy)p-phenylenevinylene): methanofullerene:CdSe tetrapod’s]; PCPDTBT:PCBM/TiOx/PEDOT:PSS/P3HT:PCBM—[poly(2,6-(4,4bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0 ]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)):[6,6]henylC61-butyricacidmethylester]/TiOx/poly(2,3-dihydrothieno-1,4-dioxin):poly(styrenesulfonate)/poly(3hexyl)thiophene: [6,6]-phenylC61-butyricacidmethylester; PCDTBT:PC70BM/TiOx—[poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0 ]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)):[6,6]-phenylC70butyricacidmethylester/TiOx].

DuPont—PTT-poly(trimethylene terephthalate): one of its monomers, 1,3-propanediol, is acquired from corn or sugar beets. Coca-Cola Co: Plant Bottle: PET container produced using ethylene glycol got from liquor got from sugar cane and molasses. Moreover, the PP top is somewhat smaller. The extent of crude materials acquired from nonrenewable energy sources (oil, coal, and gas) and got from plants can be found through investigation of the extent of carbon-12 to carbon-14 present in the polymer since petroleum products contain basically no carbon-14 whose half-life is around 5730 years. We may create wealth from waste depending upon the chemical characterization and simplicity of isolation and extraction. It is a procedure by which plastic materials that would somehow end up strong waste are gathered, isolated, or handled and reused as crude materials or completed products. Industrialists and scientists are playing a functioning job in limiting the measure of plastic materials that end up in the landfill with activities over the globe went for stimulating advancement in a wide range of reusing and recovery alternatives. As per World Bank estimates, 1.4 billion tons of waste is produced internationally every year, and around 10% of it is plastic. The dumping of plastic waste collecting has been restricted as of now in a few sectors of the

19.8 Recyclability, biodegradability, and reusability

FIGURE 19.10 Systematic representation of polymer degradation.

globe, but still, incidences seem without strict regulation. In this way, the incredible substance of plastic, metallic, and electronic waste produced every year escapes into nature as opposed to being landfilled, burned, or reused. The plastic business additionally trusts that an ideal waste gathering plan including intermixed plastic accumulation may prompt better outcomes and through and through increment reusing rates if an appropriate reusing system is set up. Fig. 19.10 shows the systematic representation of polymer degradation.

19.8 Recyclability, biodegradability, and reusability of polymeric products Primary mechanical reusing is the direct reuse of uncontaminated disposed of the polymer into another item without loss of properties [164]. Much of the time,

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essential mechanical reusing is led by the producer itself for postindustrial waste. Accordingly, this procedure is regularly named shut circle reusing [165]. On a basic level, postconsumer (PC) waste can be likewise exposed to essential reusing; be that as it may, for this situation, some of the extra difficulties may emerge, for example, the need of specific gathering and unpleasant (manual) arranging. Such issues may altogether expand the expenses of recyclates. Along these lines, when all is said in done, this strategy is disliked among recyclers. Exact substance and grade of purity of end of life and PC steam are every now and again not known; subsequently, they are handled through auxiliary mechanical reusing, which includes partition/refinement as opposed to essential reusing [166]. Just as on account of essential reusing ordinarily just thermoplastic polymers can be reprocessed. The polymer is not changed amid the optional reusing, yet its atomic weight falls attributable to chain scissions, which happen within the sight of water and follow measures of acids. This may result in a decrease in mechanical properties. This marvel can, in any event, be mostly neutralized by serious drying, utilization of vacuum degassing, and utilization of difference balancing out added substances, the Fig. 19.11 shows the most common polymer recycling method [167]. The term BPs typically allude to an assault by microorganisms on nonwaterdissolvable polymer-based materials. This infers the biodegradation of polymers is normally a heterogeneous procedure [168]. On account of an absence of waterdissolvability and the measure of the polymer atoms, microorganisms can’t transport the polymeric material specifically into the cells, where most biochemical procedures happen; rather, they should initially discharge extracellular catalysts which depolymerize the polymers outside the cells [169]. As a result, if the molar

FIGURE 19.11 Schematic illustration of most common polymer recycling method.

19.9 Polymeric products disposal ways and its impacts

FIGURE 19.12 General methods of polymer degradation.

mass of the polymers can be adequately decreased to produce water-solvent intermediates, these can be transported into the microorganisms and nourished into the proper metabolic pathways [170]. Therefore, the finished results of these metabolic procedures incorporate water, carbon dioxide, and methane, together with another biomass. The extracellular chemicals are too huge to even think about penetrating profoundly into the polymer material, thus act just on the polymer surface; subsequently, the biodegradation of polymers is normally a surface disintegration process. Fig. 19.12 shows the general polymeric degradation procedure [171].

19.9 Polymeric products disposal ways and its impacts The polymeric waste disposed of by landfilling, incineration recycling, and biodegradable methods. On the occasion of polymeric waste disposal, the main factor is entering of microplastics in the environment is the biggest issue. Water for human utilization originates from different freshwater sources which are liable to presentation to microplastics entering the earth through different courses. Microplastics are generally detailed in surface waters (waterways, lakes, and rivers) and are ordinarily revealed in water bodies close urbane and additionally

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populated regions just as remote territories. Microplastics may enter the drinking water supply form any of these water sources; ponder estimating of microplastics in crude water drawn by drinking water treatment plants from repositories and a stream. The microplastics are proposed to enter oceanic situations by spills from mechanical movement, natural corruption of disposed of plastic things, clothes washer effluents conveying manufactured filaments effluents conveying microplastics found in makeup and from the physical wear of plastic things being used. The nearness of microplastics in barometrical examples has driven scientists to propose climatic transport and statement by wind or precipitation, giving a course to seagoing situations including surface waters for drinking water extraction, and with consequences for water collecting. Wastewater treatment plants can be effective in expelling huge rates of microplastics from the fluid portion, yet because of substantial heaps of microplastics entering wastewater treatment plants, the outpouring of microplastics in treated effluent can even now be critical. The muck portion has been found to contain microplastics and is normally utilized for farming purposes, as is dealt with wastewater, giving another course smaller scale plastics into surface waters. The section of microplastics form earthly situations into groundwater may require further investigation, given the difference between microplastics centralizations of an Environmental Protection Agency production (up to 6500 particles/m3 in untreated private well water tests), and of an investigation of water sources (fixation up to 7 particles/m3).

19.10 Waste to wealth future perspectives of ecofriendly polymer materials development and usage In a couple of nations, isolation units are being developed that may occupy nonrecyclable waste from landfill for the generation of a sustainable power source. Later on, it is normal that the plant will in like manner create an inexhaustible wellspring of hydrogen for business reason. An open-get to office is starting new headings for concerting items and materials, for instance, biomass into highreview energizes and vitality. The new office may build the measure of superb container review plastics. The ecological effects delivered by regular polymers on the planet are currently plainly watched. As a result, these materials, especially plastic sacks, have persevered through various attacks in a couple of countries, and elective solutions for their use have been stimulated. In any case, to date, no legitimate and comprehensive response for the substitution of normal polymer materials has risen. Likewise, all things considered, the best approach to be taken is accurately this: to propel the tolerable assortment of materials available, as shown by the adjacently arranged assortments of the planet. Dependent upon characteristic conditions, available unrefined materials, adjacent social orders, current parks, etc., exceptional polymeric or not polymeric materials may be picked as the most reasonable for a given masses at a given time ever. The effects

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19.11 Conclusion Despite developing alert, polymers are basic to present day life. Polymers made conceivable the improvement of PCs, phones, and the majority of the lifesaving advances of the present-day solution. Lightweight and useful for protection, polymers help spare nonrenewable energy sources utilized in warming and in transportation. Maybe most essential, reasonable polymers raised the way of life and made material wealth all the more promptly accessible. Without polymers numerous belonging that we remove for allowed may be from going after everything except the most extravagant Americans. Supplanting normal materials with polymers has made a significant number of our belonging less expensive, lighter, more secure, and more grounded. Since obviously, polymers have a significant place in our lives, a few researchers are endeavoring to make polymers more secure and more manageable. A few pioneers are creating biopolymers, which are produced using plant trims rather than nonrenewable energy sources, to make substances that are more ecologically amicable than ordinary polymers. Others are attempting to make polymers that are genuinely biodegradable. A few trendsetters are scanning for approaches to make reusing more productive, and they even want to consummate a procedure that believers polymers once more into the nonrenewable energy sources from which they were determined. These trailblazers perceive that polymers are not impeccable but rather that they are an essential and important piece of our future.

Acknowledgment Dr. M. Rajan is grateful to the Department of Science and Technology, Science and Engineering Research Board (Ref: YSS/2015/001532; New Delhi, India) and Indian Council of Medical Research (ICMR), New Delhi (File No. 5/3/8/350/2018-ITR) for providing financial support.

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Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A AA. See Amino acids (AA) Abbe number, 427 Abrasion resistance, 135
136 ABS. See Acrylonitrile butadiene styrene (ABS) Absorption, 153
154, 295
296 Acacia gum, 287 Acetaldehyde, 530 Acetate, 262 fiber, 346
347 chemistry, 346 properties, 347 uses, 347 Acetyl tributyl citrate, 525
526 Acetylation, 306
307 Acetylene-derived polymer application for personal care, 559 Acid hydrolysis, 306
308 Acid
base equilibrium in dilute solutions, 231
233, 241 Acrylic, 601 fiber, 347
348 chemistry, 348 properties, 348 structure of polyacrylonitrile, 348f uses, 348 Acrylic glass. See Acrylic Acrylonitrile butadiene, 603 Acrylonitrile styrene acrylate (ASA), 145
147 Acrylonitrile butadiene styrene (ABS), 145
147, 603 Active packaging, 537
539 Acyclic polyols, 574 Adaptive optics (AO), 425 Adaptive surrogate modeling-based platform, 50 Additive printing, 477 Adenomatous polyps, 296 Advanced polymer-based materials, 474
476 Advaxt, 574 AFM. See Atomic force microscopy (AFM) AFM-IR. See Atomic force microscopy-infrared spectroscopy (AFM-IR) AG. See Arabinogalacturonan (AG) Agar, 287, 578 Agar
agar, 576
577 Agricultural waste materials, 572 Agriculture, polymer-based materials for, 610
612, 611f

AIBN. See Azobisisobutyronitrile (AIBN) Aldosterone hormone, 295
296 Alginate, 287, 289t, 577
578 Alginic acid, 550 Alkyl-modified silicones, 557
558 Alkyl-substituted siloxanes, 557
558 Alkyldiammonium hydroxypropyl (or ethyl) hydrolyzed protein, 554 Alkyne-modified silicon, 554
556 Aloe barbadensis, 576
577 α-(1,4) glycoside bonds, 259 Aluminum trioxide (Al2O3), 607
608 Alzamic acid, 550 AMBER. See Assisted Model Building with Energy Refinement (AMBER) Amebiasis, 309 American Chemistry Council, 600
602 Amide group, polymers containing IR analysis of, 84
85 Amine, carboxyl and hydroxyl groups chemical analysis of, 72
73 Amines, 530 Amino acids (AA), 195
196, 579 protonation, 230
231 Amodimethicone, 557 Amorphous polymers, 23 AMP-isostearoyl hydrolyzed protein, 554 Amphoteric polysaccharides, 551
552, 552f AMPK/GSK3β/GS signaling pathway, 264 Amylase enzyme, 575
576 Amylopectin, 304
305, 577
578 chemical structures, 305f Amyloplasts, 575 Amylose, 289t, 304
305, 577
578 chemical structures, 305f molecular weight, 305 Amylose
lipid complexation, 306
308 Angle light scattering (SALS), 27, 30
32, 31f, 32f Animal cellulose, 575 Animal-based polymers, 568
569 Anionic polysaccharides, 550 Annealing, 308
309 Antibacterial, 263 Anticancer drugs, 299
300 polysaccharides as carriers, 287
288, 289t Antidiabetic, 264 Antimicrobial packaging, 537
538 Antioxidants, 263

635

636

Index

Antitumor, 264
266 AO. See Adaptive optics (AO) Apparent pKa, 235 of His residues, 239 Arabic, 550 Arabinogalacturonan (AG), 265
266 Aramids, 350
351, 350f chemistry, 350 properties, 350
351 uses, 351 Aromatic group polymers, IR analysis of, 80 Articulated robot, 397 Artificial polymers, 2 ASA. See Acrylonitrile styrene acrylate (ASA) Ascending colon, 288
296 Asilone, 577
578 ASIMO robot, 395 Assam Bora rice starch, 319
320 Assisted Model Building with Energy Refinement (AMBER), 208
210 Associative mechanism, 549 Athletics, 511
512 Atom transfer radical polymerization (ATRP), 93
94 Atomic force microscopy (AFM), 21
22, 45
47, 47f Atomic force microscopy-infrared spectroscopy (AFM-IR), 159 Atomic oxygen (ATOX), 459, 462
464 energies for chemical bonds, 464t and organic materials, 463f Atomistic MD method, 208, 210 ATOX. See Atomic oxygen (ATOX) ATRP. See Atom transfer radical polymerization (ATRP) Attenuated total reflectance IR spectrometer (ATR-IR spectrometer), 76 Avastin. See Bevacizumab Azo-polymer technique, 274
275 Azobisisobutyronitrile (AIBN), 94
95, 99

B Bacteroides, 275
276 Bagasse, 532
533 Bakelite, 593
594 Barium titanate (BaTiO3), 379 BBL. See Polybenzimidazobenzophenanthroline (BBL) Beads, 267
269, 319 Beer’s law, 76 Beeswax, 583 Bending modulus. See Flexural modulus of polymers Bending test, 138
139

Benzene, 530 Benzoxadiazole, 383 β-lactoglobulin
pectin nanoparticles, 272 Bevacizumab, 299
300, 301t Biaxial orientation (BOPP), 379 Bifidobacterium, 262 Bifidobacterium bifidum, 262 Bifidobacterium longum, 262 B. longum BB-46, 262 Bifunctional monomer, 548
549 Bilayer films, 578 Binary protein mixtures, 250 Binocular stereo microscopes, 38
39 Bioactive dietary fiber, 259
266, 260f, 261f Bioactive proteins, 258
259 Bioavailability, 273
274 Biobased polyester, 537 Biodegradability of polymeric products, 621
623 Biodegradable plastic, 11
12 Biodegradable polymers (BPs), 117, 528, 531
533, 534t, 610
611, 622
623 advantages and disadvantages, 535t Biomedical applications, 614
617 Biomethane, 554
556 Bioplastics, 532 Biopolymer(s) and active packaging, 537
539 alginates, 577
578 hybrids, 533
535 materials, 525, 532 methods for biopolymers production, 537 Biosensors, 386, 387f 3,3’,4,4’-Biphenyldianhydride (BPDA), 157 Birefringence, 427 Bis(2-ethylhexyl)adipate, 525
526 Bisodol, 577
578 BLS. See Brillouin scattering (BLS) BN. See Boron nitride (BN) “Boil and bite” type mouth guards, 508 BOPP. See Biaxial orientation (BOPP) Boron nitride (BN), 159
160 Bovine serum albumin, 269 BPDA. See 3,3’,4,4’-Biphenyldianhydride (BPDA) BPs. See Biodegradable polymers (BPs) Brachytherapy, 299 Bragg equation, 172 Braskem, 619
621 Brillouin scattering (BLS), 429 Bromide (Br), 105
106 Bronze Age, 606
607 1,3-Butadiene, 530 Butylated hydroxytoluene, 525
526 Butylene 1, 4-cyclohexane dicarboxylate, 537 Butyrate, 262, 306

Index

C 13

C correlation spectroscopy (COSY), 168
169 13 C NMR spectroscopy, 97 CA. See Cellulose acetate (CA)Citric acid (CA) Caco-2 cells, 265 Calcium carbonate, 577
578 Calcium ions, 267
269, 271
272 Calcium pectinate beads, 267
269 Camptosar. See Irinotecan Cancer, 265
266, 273
274 chemotherapeutics, 287
288 colon, 296
300 colorectal, 273
274, 296, 309 nanotherapeutics, 320
321 Candelilla wax, 583 Canoes, 515 Capecitabine, 299
300 Capecitabine and oxaliplatin (CAPOX), 299
300 Carbohydrate polymers, 304 Carbohydrates, 1, 258
259, 572
573, 574t Carbon dioxide (CO2), 372
373, 528, 530 Carbon fiber (CF), 143, 353
355, 515. See also Manmade fibers on bobbin, 355f chemistry, 353 composite materials, 511 partial product data sheet, 354t properties, 353 uses, 353
355 Carbon fiber
reinforced polymer (CFRP), 353, 499
500 Carbon materials, 606
607 Carbon nanotubes (CNTs), 402, 409
411, 606
608 CNT-reinforced polymers, 26
27 Carbon-based polymeric composite materials for CO2 capture, 604
607 Carbon-reinforced polymers, 513 Carbonate functional group, 494 ions, 271
272 Carboxybetaine ester, 586
587 Carboxyl group, chemical analysis of, 72
73 Carboxylic acid group, 82
84 Carboxymethyl chitin, 550 Carboxymethyl glucan, 550 Carboxymethylcellulose, 550, 575
576, 578 Carnauba wax, 583 Carrageenan, 287, 550, 577
578 Cartesian robot, 397 Casein, 258
259 Casting method, 7 Cationic guar, 550
551

Cationic hydroxyethyl cellulose, 550
551 Cationic hydroxyl propyl guar, 550
551 Cationic polymer, 269 Cationic polysaccharides, 550
551, 550f Cecum, 288, 295
296 Cellophane, 575
576 Cellulose, 287, 571
573, 575, 593
594, 598
600, 612
613 chains, 571 chemical structure, 571f gum, 550 linear acyl esters, 188 nitrate, 575
576 Cellulose acetate (CA), 145
147, 332, 575
576 Cetearyl methicone, 557
558 Cetyl dimethicone copolyol, 557
558 CF. See Carbon fiber (CF) CFRP. See Carbon fiber
reinforced polymer (CFRP) CG DPD. See Coarse-grained DPD (CG DPD) Chain entanglement, 548 Charge transfer polymers, 370
371 CHARMM. See Chemistry at Harvard Macromolecular Mechanics (CHARMM) Charpy impact test, 143
144 Chemical analysis of polymers IR spectroscopy, 74
85 mass spectrometry, 101
108 molecular weight determination, 70
74 nuclear magnetic resonance spectroscopy, 85
101 Chemical equilibrium, 231
232, 235
237 Chemical free energy, 241 Chemical shift, 89
90, 90t Chemistry at Harvard Macromolecular Mechanics (CHARMM), 208
210 Chewing gums, 585
586 China National Space Administration (CNSA), 458 Chitin, 287, 532, 559
560, 575, 582 Chitosan, 269, 287, 289t, 532, 535t, 550
551, 578
579, 612
613 chitosan-calcium pectinate beads, 269 cosmetic use, 559
560 nanofilms, 536
537 Chlorine, 530, 593 Chlorotrifluoethylene (CTFE), 145
147 Chondroitin sulfate, 550 Cis-1,4-polyisoprene, 258
259 Cis-polyacetylene, 368 Cisplatin, 301t Citric acid (CA), 195
196 Citrus pectin, 264, 269

637

638

Index

CLCP. See Cross-linked liquid crystalline polymer (CLCP) Cleansing products, 554 Clostridia, 275
276 Clostridium difficile, 269 Clostridium perfringens, 262 CNSA. See China National Space Administration (CNSA) CNTs. See Carbon nanotubes (CNTs) CO2/N2 selectivity, 606
607 Coarse-grained DPD (CG DPD), 207
208 Coating methods, 378, 584 Codonopsis pilosula, 265
266 Coefficient of friction, 132
133 Cohesive wear process, 136 Colectomy, 298
299 Collagens, 580 nutritional facts about great-lakes, 581f Colon, 275
276 anatomy and physiology, 288
296, 295f Colon cancer, 296
300 colonic pH profiles, 296t experimental chemotherapy, 301t grading systems, 298f screening tests, 298f statistics, 297
298 treatment modes, disadvantages, and limitations, 298
300 Colon-specific drug delivery system, 300
304, 309
318 Colonic drug delivery system, 309 Colonic polyps, 296 Color, 427 Colorectal cancer, 273
274, 296, 309 Coloring lotions, 554 Compatibilizers, 24
25 Composites, 26, 490
491, 569
570 Concrete, 470
472, 470f, 598
600 Conditioners, 550
551, 554 Conducting polymers (CPs), 367
369, 402, 412, 614
617. See also Semiconducting polymers advantages, 369 features, 367
368 structure, 368
369 traditional sequences, 367 conductive polymer and metal conductor, 368f Confocal scanning laser microscopy, 39
40 Conjugated conducting polymers, 371
372 charge transport polymer, 371
372 Consumable foodstuffs, 572 Consumable polysaccharides, 572
573 Contact lenses, 230

Control transformation proficiency (PCE), 619 Coordination polymers (CPs), 445 Copolymers, 584 analysis by 1H NMR spectroscopy, 99
101 hydrogel films, 250 Core
shell constituents, 587 Cornstarch, 577
578 Corrosion control, 614 Cortex philodendron extract (CPE), 538 Cosmetics, 545
546 polymer/surfactant interactions, 547 products, 545 use of polymers in, 546f, 547
560 examples and case studies, 558
560 polysaccharide-based polymers, 549
552 proteins, 552
554 silicones, 554
558 synthetic polymers, 547
549 COSY. See 13C correlation spectroscopy (COSY) Cotton, 571 Counterion confinement, 241 Coupling constant, 90
91 Covalent cross-linking, 548
549 CPE. See Cortex philodendron extract (CPE) CPs. See Conducting polymers (CPs)Coordination polymers (CPs) Crack propagation, 133
134, 134f Crazing, 126
130 behavior in deformed HIPS, 129f behavior of PP, 128f Creep, 122
123 Crohn’s diseases, 309 Cross-linked liquid crystalline polymer (CLCP), 406 Crosslinked weak polyacid hydrogels, 229 Crosslinking, 1
2, 306
308 Cryoscopy, 71 Crystalline polymers, 14, 22
23 Crystallinity, 142, 595 Crystallization point, 34
35 CTFE. See Chlorotrifluoethylene (CTFE) Curcumin-loaded micellar nanoparticles, 301t Cutin, 258
259 Cyanoacrylate, 492 Cyclic polyols, 574 Cycling, 516
517 Cyclodextrin, 289t Cyclomethicones, 556 Cyclomethycaine, 554
556 Cyclopentadiene (Cp), 105
106 Cylindrical robot, 397 Cytochrome c, 241 adsorption, 243

Index

D DABA. See 3,5-d-Iaminobenzoic acid (DABA) DAM. See 2,4,6-trimethyl-1,3-diaminobenzene (DAM) Damping, 139
140 Data acquisition and control robotic system, 397 DCJTB. See 4-(Dicyanomethylene)-2-tert-butyl-6 (1,1,7,7-tetramethyljulolidyl-9-enyl)-4Hpyran (DCJTB) dd. See Doublet of doublet (dd) DE. See Degree of esterification (DE) DEAs. See Dielectric elastomer actuators (DEAs) Decyl dimethicone, 557
558 Deficient robot, 395 Definitive natural polymers, 593
594 Deformation energy, 132 Degradable synthetic polymers, 257 Degree of crystallinity, 142
143 Degree of esterification (DE), 259
262 Delta inulin, 574 Denier, 337 Density functional theory (DFT), 153 Density functional theory, 207 Deoxyribonucleic acid, 258
259 Desamido collagen, 554 Descending colon, 288
295 Dextran, 287, 289t DFT. See Density functional theory (DFT) Diabetes, 264 1,5-Diaminonaftalene functionalized with poly (Niso-propyl) acrylamide (Napht-NPNIPAM), 386 Diarylethene hydroxypropyl MC, 552
553 4-(Dicyanomethylene)-2-tert-butyl-6(1,1,7,7tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), 435
436 Dielectric elastomer actuators (DEAs), 404
405, 405f Dielectrics, 379
382 dielectric constant of cellulose/reduced graphene oxide, 382f Differential scanning calorimetry (DSC), 27, 33
35, 50
51, 181
196 applications, 188
196 PEG networks, 195f solution DSC exotherms of poly(ethylene-co1-hexene), 192f thermograms for PEN/PC 50/50, 194f thermograms of cellulose esters, 191f thermograms of chitosan and acylated chitosans, 193f Differential thermal analysis (DTA), 181
183 applications, 188
196 4,5-Dihydroxy-2-cyclopenten-1-one, 265
266 Dimethicone, 546
547, 554
557

Dimethiconol, 556
557 Dimethyl silicone fluids, 556
557 Dimethylene, 554
556 Dioxin, 530 Dip coating, 584 Dipolar/acid base interaction, 547 Direct observation methods, 37
45 AFM, 45
47 OM, 38
40 SEM, 40
42 STM, 44
45 TEM, 42
44 Dispersive forces, 547 Displacement of chemical equilibrium, 235
237 sensor, 138 Disposable nonwovens, 333 Dissipative particle dynamics (DPD), 207 simulations, 213
217 2,2’-Dithiopyridine (DTP), 95
97 Diverting colostomy, 298
299 DMA. See Dynamic mechanical analysis (DMA) DMTA. See Dynamic mechanical thermal analysis (DMTA) Double emulsification, 319
320 Doublet of doublet (dd), 92
93 Doxorubicin, 299
300, 301t model, 272
273 DPD. See Dissipative particle dynamics (DPD) Drug releasing agent, 586 Dry spinning, 335
336 Dry wicking, 519 DSC. See Differential scanning calorimetry (DSC) DTA. See Differential thermal analysis (DTA) DTP. See 2,2’-Dithiopyridine (DTP) Dual crosslinking strategy, 269 DuPont, 619
621 Durable nonwovens, 333 Dynamic mechanical analysis (DMA), 27, 35
36, 36f Dynamic mechanical thermal analysis (DMTA), 136
140 instrument, 138f modes of experiments, 139f

E E-GMAx. See Ethylene-glycidyl methacrylate copolymers (E-GMAx) E/P/EP copolymer. See Polyethylene/ polypropylene/ethylene
propylene copolymer (E/P/EP copolymer) EACs. See Electroactive ceramics (EACs) EAP. See Electroactive polymers (EAP) Eb. See Elongation to break (Eb) Ebullioscopy, 71

639

640

Index

EC. See Electrochromic (EC) Economic development of polymeric products, 618 Edible caseinate films, 581
582 Edible coatings, 567
568 Edible films, 578 Edible polymers, 569 Edible polypeptide films, 579
580 Edible starch-based films, 577
578 EELS. See Electron energy-loss spectroscopy (EELS) Effective stress, 129 Egg-box model, 259
262 “Egg-box” conformation, 267 Elastic scattering, 30 Elastic solids, 118 Elastomer-coated polyester, 515 Elastomers, 1
2, 10, 515, 595
596 Electric robots, 396 Electro-strictive polymers, 403
404, 404f Electroactive ceramics (EACs), 400
402, 476
477, 477t mechanism of electroactive polymers, 401
402 Electroactive polymers (EAP), 400, 402f, 476
477, 477t Electrochromic (EC), 437 Electromagnetic radiation (EM radiation), 153
154 Electron energy-loss spectroscopy (EELS), 179 Electron microscopy, 126
127 Electronic electroactive polymers, 402
408 DEAs, 404
405 electro-strictive polymers, 403
404, 404f ferroelectric polymers, 407
408 flexible robotic arm ofCLCP, 407f liquid crystal elastomers, 405
406, 406f piezoelectric polymers, 403 Electronic(s) conducting polymers, 368 controls, 399 polymers in conducting polymers, 367
369 semiconducting polymers, 369
372 Electrorheological fluids, 412 Electrospinning, 8, 537 Electrospray ionization (ESI), 69
70 Electrospray ionization-mass spectrometry (ESIMS), 102 applications, 103
108 formation of stable cycloadduct, 109f peroxypivalates utilized for free radical polymerization of MMA, 104f reaction between cyclopentadiene-functionalized PEG and benzylmethyldithioformate, 110f Electrostatic interaction, 547

Ellagitannins, 572 Elongation to break (Eb), 340 Eloxatin. See Oxaliplatin EM radiation. See Electromagnetic radiation (EM radiation) EMI TFSA. See Ethylmethyl imidazolium bis (trifluoromethanesulfonyl) amide (EMI TFSA) Encapsulation, 584 End group analysis, 71
74 Endocavitary therapy, 299 Energy, future impacts of polymers, 619 Engineering plastics, 488 Engineering thermoplastics. See Modified thermoplastics Enterobacteria, 275
276 Enterococci, 275
276 Enterococcus faecalis, 537 Environmental Protection Agency, 623
624 Enzymatic debranching, 306
308 Enzymes, 258
259 amylase, 575
576 enzyme-modified pectin, 265
266 Epichlorhydrin, 269 Epichlorohydrin, 308 Epithelial cells, 295 Epoxy, 414
415 gums, 593 resin, 495 EPR. See Ethylene-copropylene rubber (EPR) EPR-g-GMAx. See Glycidyl methacrylate-grafted EPR (EPR-g-GMAx) EPR-g-MA. See Maleic anhydride-grafted EPR (EPR-g-MA) EPS. See Expanded polystyrene (EPS) Equipment design, 487 Equisetum arvense, 263 Equisetum sylvaticum, 263 ESA. See European Space Agency (ESA) Escherichia coli, 262
263 ESI. See Electrospray ionization (ESI) ESI-MS. See Electrospray ionization-mass spectrometry (ESI-MS) Ester group polymers, IR analysis of, 81
82 Ether-modified cellulose, 551 Ethylene vinyl acetate (EVA), 413
414 Ethylene-copropylene rubber (EPR), 135 Ethylene-glycidyl methacrylate copolymers (EGMAx), 135 Ethylmethyl imidazolium bis (trifluoromethanesulfonyl) amide (EMI TFSA), 610 Eudragit, 269 Eudragit S100, 271
272

Index

European Space Agency (ESA), 458 EVA. See Ethylene vinyl acetate (EVA) EXAFS. See Extended X-ray absorption fine structure (EXAFS) Expanded polystyrene (EPS), 495 Extended X-ray absorption fine structure (EXAFS), 172 Extensible robot control language (XRCL), 399 External beam radiation therapy, 299 Extrusion, 8 blowing, 537 extrusion
spheronization, 319
320

F Fabric materials, 487 Fabricated polymers, 593 Fatigue, 133
134 crack propagation, 133
134, 134f Fats, 582 FDA. See Food and Drug Administration (FDA) 6FDA. See (4,40-Hexafluoroisopropylidene) diphthalic anhydride (6FDA) Female lac bug (Laccifer lacca), 583 Ferroelectric polymers, 407
408 Ferroelectricity, 373
374 FF. See Fill factor (FF) Fiber density, 342 identification, 340
342 burn test, 341
342 chemical test, 341 density test, 342 microscopy test, 341 stain test, 342 Fiber-reinforced composites, 490
491 Fiber-reinforced polymer (FRP), 471
472 composites, 26
27 Fiberglass composite material, 490 Fibroin protein, 553 Ficus pumila, 264 Field theory, 207 Field-based approaches, 207 Filaments, 593 Fill factor (FF), 619 Filled polymers, 369
370 Fire-resistant fabric, 517 Flavoring agent, 581
582 Flexible electronics, 377
378 Flexo printing, 378 Flexural modulus of polymers, 125
126, 125f Fluorescence sensors, 438
439 Fluorescent polymeric nanoparticles (FPNs), 446
447 Fluorine, 593

5-Fluorouracil (FU), 270, 299
300, 586
587 FU-loaded pectin nanospheres and vesicles, 271
272 5-Fluorouracil and leucovorin calcium (FU-LV), 299
300 Foam polymer, 520 Folate, 274
275 receptor, 274
275 FOLFIRI. See Leucovorin calcium, 5-fluorouracil, and irinotecan hydrochloride (FOLFIRI) Food food-grade biopolymers, 537
538 grade extracts, 569 hydrocolloids, 537
538 packaging, 525
526 spoilage avoidance, 536 Food and Drug Administration (FDA), 586 6FDA-based PIs, 183
186 6FDA-DAM, 172
173 Food polymers, 570
587 development of, 587 lipids, 582
583 polypeptides, 579
582 polysaccharides, 570
579 food storage polysaccharides, 575 mucosubstances, 576
579 structural polysaccharides, 575
576 synthetic and composite, 583
587 Foodstuffs, 569, 573t Force field potential, 208
209 Formaldehyde, 529
530 Fourier-transform infrared spectroscopy (FT-IR spectroscopy), 69
70, 154 FPNs. See Fluorescent polymeric nanoparticles (FPNs) Fracture and fracture mechanics, 130
132, 131f Free-radical emulsion polymerization method, 502 Freeze drying process, 273 Friedel
Crafts reaction, 24
25 FRP. See Fiber-reinforced polymer (FRP) FT-IR spectroscopy. See Fourier-transform infrared spectroscopy (FT-IR spectroscopy) FU. See 5-Fluorouracil (FU) FU-LV. See 5-Fluorouracil and leucovorin calcium (FU-LV) Fuel cells, 372
373 Fungal melanin concentrations, 537 Fungus cellulose, 575 Furans, 530 Fusilev. See Levoleucovorin

G Galactomannas, 287 Galactose residues, 263

641

642

Index

Galectin-3, 264
266 Gamma inulin, 574 Gas permeation behavior, 202 Gas sensors, 385, 385f Gaviscon, 577
578 Gel permeation chromatography (GPC), 69
70 Gel pH, 241 Gel spinning, 336 Gelatin, 554, 580 gelatin-coated pills, 584
585 Gelidiella, 576
577 Gelidium, 576
577 Gellan, 287 Generally recognized as safe (GRAS), 262 GEO. See Geostationary Earth orbit (GEO) Geopolymer concrete, 472
473, 475t high-performance polymer-based materials, 475t in loose and compacted conditions, 475f SEM microscopy of geopolymer binders, 474f SEM observations, 473f Geostationary Earth orbit (GEO), 464
465 Germinal coupling, 91
92 GFRP. See Glass-reinforced polymer (GFRP) Ginseng pectin, 264 GL. See d-Glucono-δ-lactone (GL) Gladstone
Dale relation, 428 Glass, 487, 527, 598
600 containers, 527 fiber
reinforced composite materials, 491 glass-reinforced materials, 514 Glass-reinforced polymer (GFRP), 513
514 Gloss, 428 Glucoamylase, 308 Glucokinase, 264 d-Glucono-δ-lactone (GL), 195
196 Glucose, 572
573 Glucose-6-phosphatase, 264 Gluten protein, 580
581 Glycerol, 603
604 Glycidyl methacrylate (GMA), 99
100 Glycidyl methacrylate-grafted EPR (EPR-gGMAx), 135 Glycogen, 575 Glycosaminoglycan, 287, 569
570 GMA. See Glycidyl methacrylate (GMA) GO. See Graphene oxide (GO) Golf attire, 519 polymers, 512
513 GPC. See Gel permeation chromatography (GPC) Graphene, 610 graphene-based PPy, 607
608 Graphene oxide (GO), 174, 607
608 Graphite fiber
reinforced polymers, 487

GRAS. See Generally recognized as safe (GRAS) Gravure printing, 378 Green robots, 396 Groningen Machine for Chemical Simulations (GROMACS), 210, 215 GS. See Substituted galacturonans (GS) Guar gum, 289t, 578
579, 586 guar gum-grafted lysine-β-cyclodextrin, 586
587 Guar-based materials, 551 Gum arabic, 578
579 Gutta-percha, 493 Gymnastics, 511
512

H 1

H NMR spectroscopy copolymer analysis, 99
101 end groups analysis, 93
97, 94f molecular weight determination, 97
99 Hair care, 554, 558
559 Hair keratins, 554 Hair straightening products, 554 Hard polyethene, 500
501 Hawthorn pectin penta-oligogalacturonides, 263 HDC. See Huisgen 1,3-dipolar cycloaddition (HDC) HDPE. See High density polyethylene (HDPE) Heat resistant materials, 595
596 Heat
moisture treatment, 308
309 Helicobacter pylori, 263 Hemicellulose, 576
577 Hemp, 571 HepG2 cancer cells, 265
266, 272
273 Heteronuclear multiple bond correlation spectroscopy (HMBC spectroscopy), 168
169 Heteronuclear single quantum correlation spectroscopy (HSQC spectroscopy), 168
169 Heteropolysaccharide chitin, 575 (4,40-Hexafluoroisopropylidene) diphthalic anhydride (6FDA), 172
173 HG. See Homogalacturonan (HG) High density polyethylene (HDPE), 14, 29
30, 145
147, 500
501, 525, 601 Highly Optimized Object-oriented Many-particle Dynamics—Blue Edition (HOOMD-blue), 210, 215 High methoxyl pectin (HMP), 259
262 High refraction diamond turning (HRDT), 430 High-amylopectin starch. See Waxy starch High-amylose corn starch/pectin blend, 319
320 High-impact polypropylene (HIPP), 159 High-impact polystyrene (HIPS), 128

Index

High-performance fibers, 350
355 aramids, 350
351, 350f carbon fiber, 353
355 polyolefins, 355
358, 357f UHMWPE, 352
353 High-resolution IR spectra, 77 High-resolution NMR spectroscopy, 99 HIPP. See High-impact polypropylene (HIPP) Hippophae rhamnoides, 265 HIPS. See High-impact polystyrene (HIPS) Histidine-tag adsorption to pH-responsive hydrogels, 237
241, 245 modifying pH inside hydrogel, 239
241 nonmonotonic function of pH, 237
239 HMBC spectroscopy. See Heteronuclear multiple bond correlation spectroscopy (HMBC spectroscopy) HMP. See High methoxyl pectin (HMP) Homogalacturonan (HG), 259 HG-rich pectin, 265 Homogeneous blend, 25 Horticulture, polymer-based materials, 610
612, 611f HPESU. See Hydrophilic polyethersulfone (HPESU) HPMC. See Hydroxypropyl methyl cellulose (HPMC) HRDT. See High refraction diamond turning (HRDT) HSQC spectroscopy. See Heteronuclear single quantum correlation spectroscopy (HSQC spectroscopy) HT-29 cancer cells, 265 HTPB. See Hydroxyl terminated polybutadiene (HTPB) Huisgen 1,3-dipolar cycloaddition (HDC), 164
165 Hyaluronic acid, 550, 576
577, 586 Hybrid manipulator, 396 Hydraulic robots, 396 Hydrochloric acid, 530 Hydrocolloids, 537
538, 569
570 Hydrogels, 229, 319 Hydrogen (H2), 85
86, 372 Hydrolyzed protein, 552
553 Hydrolyzed wheat protein, 554 Hydrophilic effect, 547 Hydrophilic polyethersulfone (HPESU), 176
177 Hydrophilic polymers. See Hydrocolloids Hydrophobic effect, 549 Hydrothermal treatment, 306
309 Hydroxyl group chemical analysis, 72
73 IR analysis of hydroxyl group polymers, 80
81

Hydroxyl propyl MCs, 558 Hydroxyl propyl methyl cellulose, 586
587 Hydroxyl terminated polybutadiene (HTPB), 167 Hydroxypropyl cellulose, 578 Hydroxypropyl methyl cellulose (HPMC), 552
553, 578

I 3,5-d-Iaminobenzoic acid (DABA), 172
173 Ice hockey, polymers in, 506 IEAPs. See Ionic electroactive polymers (IEAPs) IFR. See Intumescent flame retardant (IFR) Immunoglobin of egg yolk (IgY), 269 In situ intracapsular pellet coating, 270 In vivo rat study, 269 Indica rice, 307 Indirect observation methods, 27
36 DMA, 35
36 DSC, 33
35 SALS, 30
32 SAXS, 32
33 XRD, 28
30 Indomethacin in calcium pectinate gel beads, 267
269 Industrial sector, 416 Inelastic scattering, 30 Inflatable bases, 468
470, 469f Infrared spectroscopy (IR spectroscopy), 73
85, 153
165. See also Nuclear magnetic resonance spectroscopy (NMR spectroscopy) applications, 156
165 infrared spectra of chitosan and acylated chitosans, 166f IR analysis of aromatic group polymers, 80 IR analysis of ester group polymers, 81
82 IR analysis of hydroxyl group polymers, 80
81 IR analysis of polymers containing amide group, 84
85 IR analysis of polymers containing carboxylic acid group, 82
84 IR analysis of saturated polymers, 76
79 IR analysis of unsaturation polymers, 79
80 vibrational frequencies for different functional groups, 87t Inorganic polymers, 257 Insoluble fibrous collagen, 553 Insoluble proteins, 553 Insusceptible vests, 595
596 Interfacial wear process, 136 Internet of things (IoT), 14
15 smart cities in application domain, 16f Intra-network electrostatic repulsions, 235
237 Intraocular lenses (IOLs), 434

643

644

Index

Intrinsic viscosity (IV), 343 Intumescent flame retardant (IFR), 48 Inulin, 289t, 572
574 IOLs. See Intraocular lenses (IOLs) Ion-selective sensors, 385
386 Ionic conducting polymers, 367 Ionic conductivity, 371 Ionic electroactive polymers (IEAPs), 402, 408
412 carbon nanotubes, 409
411 conductive polymers, 412 electrorheological fluids, 412 ionic polymer gels, 411
412 ionic polymer
metal composites, 408
409 Ionic liquid based polymeric composites, 610 Ionic polymer gels (IPGs), 402, 411
412 nanotubes suitable for robotic applications, 410t Ionic polymer
metal composites (IPMC), 402, 408
409, 409f Ionic polymers, 370 Ionic surfactants, 547 Ionizing radiation, 464 Ionomers polymers. See Ionic polymers Ionotropic gelation, 271 IoT. See Internet of things (IoT) IPGs. See Ionic polymer gels (IPGs) IPMC. See Ionic polymer
metal composites (IPMC) IR spectroscopy. See Infrared spectroscopy (IR spectroscopy) Irinotecan, 299
300 hydrochloride, 299
300 Iron Age, 606
607 IRS-1/PI3K/Akt/GSK3β/GS insulin signaling pathway, 264 Isoelectric point (pI), 230, 242 IV. See Intrinsic viscosity (IV)

J Japan Aerospace Exploration Agency’s (JAXA), 458 Jsc. See Short out current (Jsc)

K Karaya, 550 Keratan sulfate, 576
577 Ketoprofen, 267
271 Kevlar, 595
596 string, 511 Klebsiella pneumoniae, 263 Kraft paper, 527

L Laccifer lacca. See Female lac bug (Laccifer lacca)

Lactide, 50 Lactobacillus, 262, 275
276 L. acidophilus, 262 L. bravis, 262 L. paracasei, 262 L. rhamnosus, 262 LAMMPPS, 215 Land vegetal polysaccharides, 287 Landfills, 11 Langley Research Center (LaRC), 474
475 Laparoscopic resection, 299 Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS), 210 Larmor frequency, 89 Lasers, 434
437 Latex, 488 Lather enhancer cellulose in personal care, 558 Lauryl dimethicone, 557
558 LCEs. See Liquid crystal elastomers (LCEs) LCM. See Liquid composite modeling (LCM) LCST. See Low critical solution temperature (LCST) LDPE. See Low-density polyethylene (LDPE) LEDs. See Light-emitting diodes (LEDs) LEFM. See Linear elastic fracture mechanics (LEFM) Left colon, 295
296 LEO. See Low Earth orbit (LEO) Lepton
based polymers, 559 Lesions, 296 Leucovorin calcium, 299
300 Leucovorin calcium, 5-fluorouracil, and irinotecan hydrochloride (FOLFIRI), 299
300 Leucovorin calcium, 5-fluorouracil, irinotecan hydrochloride, and bevacizumab (FOLFIRI-BEVACIZUMAB), 299
300 Leucovorin calcium, 5-fluorouracil, irinotecan hydrochloride, and cetuximab (FOLFIRICETUXIMAB), 299
300 Levoleucovorin, 299
300 Lewis corrosive intervened Friedel Craft alkylation, 613
614 Lifestyle induced diseases, 485 Light microscopy, 40 Light-emitting diodes (LEDs), 367, 423 Lignins, 572, 574 Linear elastic fracture mechanics (LEFM), 131
132 Linear low-density polyethylene (LLDPE), 188
190 Linear polyamide, 72 Linear variable differential transformer (LVDT). See Displacement—sensor Lipids, 258
259, 569
570, 582
583 films, 583

Index

structure, 583f Lipopolysaccharide, 576
577 Liquid composite modeling (LCM), 597 Liquid crystal elastomers (LCEs), 402, 405
406, 406f Liquid-state NMR, 166
167 Listeria monocytogenes, 263 Litmus lichen (LLE), 538
539 Lizardfish, 569
570 LLDPE. See Linear low-density polyethylene (LLDPE) LLE. See Litmus lichen (LLE) LMP. See Low methoxyl pectin (LMP) Local excision, 298
299 Local pH, 234
235 Lorentz
Lorenz equation, 428 Low critical solution temperature (LCST), 229, 383 Low-density polyethylene (LDPE), 14, 23
24, 500
501, 525, 601 Low Earth orbit (LEO), 459 Low methoxyl pectin (LMP), 259
262 Lower critical solubility temperature. See Low critical solution temperature (LCST) Lynch syndrome, 296
297 Lysozyme, 230, 241 adsorption, 243

M MAA. See Methacrylic acid (MAA) Macrocellulose, 578 Macromolecules, 23
25 Magnesium ions, 271 Magnetic nanoparticles (MNPs), 608
609 Magnetic polymer composites, 608
609 Magnetic responsive polymer composites (MRPCs), 608
609 MALDI. See Matrix assisted laser desorption/ ionization mass spectrometry (MALDI) Maleic anhydride grafted poly(lactic acid) (PLA-gMA), 82 Maleic anhydride-grafted EPR (EPR-g-MA), 135 Mammalian gelatins, 584
585 Manipulation robotic system, 397 Manmade fibers, 332, 342
358. See also Carbon fiber (CF) acetate fiber, 346
347 acrylic fiber, 347
348 high-performance fibers, 350
355 modacrylic fiber, 348
349 nylon, 345
346, 346f polyester, 343
345 spandex fiber, 349
350 Manufactured polymers, 593

MAP. See Modified atmosphere packaging (MAP) Marine vegetal polysaccharides, 287 Mass spectrometry (MS), 101
108 ESI, 102 applications, 103
108 MALDI, 102
103 applications, 103
108 Material technology, 485
487 Matrix assisted laser desorption/ionization mass spectrometry (MALDI), 69
70, 102
103 applications, 103
108 formation of stable cycloadduct, 109f peroxypivalates utilized for free radical polymerization of MMA, 104f Matrix tablet, 267 MB. See Melt blown (MB) MC. See Methyl cellulose (MC) MC method. See Monte Carlo method (MC method) MCC. See Microcrystalline cellulose (MCC) MCF-7 cells, 265 MCPDB. See S-methoxycarbonylphenylmethyl dithiobenzoate (MCPDB) MD. See Molecular dynamics (MD) Mechanical analysis of polymers, 146t abrasion resistance, 135
136 brittleness values for polymers, 145t coefficient of friction, 132
133 crazing and shear yielding, 126
130 degree of crystallinity, 142
143 DMTA, 136
140 elongation at break, 126 fatigue and fatigue crack propagation, 133
134 flexural modulus, 125
126 fracture and fracture mechanics, 130
132 molecular weight, 141 processing methods, 144
147 storage modulus of polymers, 144t stress
strain behavior, 119
121 temperature, 143
144 tensile strength, 124
125 time
temperature dependence, 123
124 toughness, 135 viscoelasticity, 121
123 Medical applications, polymer-based materials, 614
617 Medical sector robots, 416 MEH-PPV. See Poly (2-methoxy-5-(20 ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) Melt blown (MB), 336 Melt mix, 537 Melt spinning, 334
335

645

646

Index

Melt spinning (Continued) filaments exiting through spinneret holes of spinneret pack, 334f polypropylene fiber tenacity, 335f Membrane osmometry, 71 Metal natural structures (MOFs), 604
606 Metals, 489
490 Methacrylic acid (MAA), 231
232, 239 Methandienone, 559 Methane (CH4), 528 Methocel cellulose ethers, 558 Methyl cellulose (MC), 552
553 Methyl methacrylate (MMA), 94
95 Methylcellulose, 578 Mg(OH)2, 607
608 Michael conjugate addition, 265
266 Microbial-degraded system, 275
276 Microbial/fungi polysaccharides, 287 Microcontroller systems, 399 Microcrystalline, 578 Microcrystalline cellulose (MCC), 196, 270
271 Microgels, 229 Micronized powders, 553 Microparticles as drug carrier, 319
320 Microplastics, 5
6, 623
624 Microscopic techniques, 69
70, 126
127 Microsphere-tipped micropillars of polydimethylsiloxane (MSMPs-PDMS), 439 Military robots, 417
418 Milk protein, 552
553, 581
582 Mineral-based particles, 26
27 Mining robots in, 418 MIP. See Molecular imprinted polymer (MIP) MMA. See Methyl methacrylate (MMA) MMT. See Montmorillonite (MMT) MNPs. See Magnetic nanoparticles (MNPs) Mobile phones, 512 Mobile robotic system, 397 Modacrylic fiber, 348
349 chemistry, 348 properties, 348
349 uses, 349 Modern tennis balls, 511 Modified atmosphere packaging (MAP), 529
530 Modified carbohydrates, 572
573, 574t Modified thermoplastics, 9 Modified velocity-Verlet algorithm, 215 Modulus of elasticity, 125
126 MOFs. See Metal natural structures (MOFs) Moisture wicking fabrics, 519 Molding method, 7
8 Molecular dynamics (MD), 207 simulations, 208
213

Molecular imprinted polymer (MIP), 437
438 protocol for glucose sensor fabrication using, 438f Molecular mechanics, 208
209, 209f Molecular theory (MT), 207, 217
221, 231 Molecular weight (MW), 117
118, 141 determination of polymers, 70
74 by end group analysis, 71
73 by 1H NMR spectroscopy, 97
99 number average molecular weight determination, 73
74 Monolithic metals, 490 Monomers, 2, 545
546, 598
600 Monte Carlo method (MC method), 207 Montmorillonite (MMT), 614 Morphology analysis applications, 47
58 AFM images of poly(n-butyl acrylate), 56f AFM images of polyvinylidene fluoride/FeO surface, 57f optical microscopy images of dewetting stages of PS film, 51f SEM images of co-PA fiber mats fabrication, 54f SEM images of foams and distribution of pore size, 52f SEM images of linear-LDPE foam and linearLDPE foam, 53f TEM images of aluminium
polyimide laminate interfaces, 56f 3D profilometry images of electrospun co-PA fibers, 55f XRD graph of LDPE and LDPE, 48f XRD of PA/melamine polyphosphate composite, 49f characterization methods, 27
47 direct observation methods, 37
45 indirect observation methods, 27
36 polymer morphology, 21
27 amorphous polymers, 23 crystalline polymers, 22
23 polymer blends, 24
26 polymer composites, 26
27 semicrystalline polymers, 23
24 Motor sports, polymers, 515
516 Motors, 399 Movies, 593 MRPCs. See Magnetic responsive polymer composites (MRPCs) MS. See Mass spectrometry (MS) MSMPs-PDMS. See Microsphere-tipped micropillars of polydimethylsiloxane (MSMPs-PDMS) MT. See Molecular theory (MT)

Index

Mucopolysaccharides, 576
577 Mucoproteins, 576
577 Mucosa in large intestine, 295 Mucus, 576
577 Mugs, 593 Multi-unit formulations, 309
318 Multiparticulate dosage forms, 318
319 Multisensors, 386 Multiwalled carbon nanotube-PA nanocomposite (MWCNT-PA), 212
213 Multiwalled CNTs (MWCNTs), 409
410 Mung grams, 582 MW. See Molecular weight (MW) MWCNT-PA. See Multiwalled carbon nanotubePA nanocomposite (MWCNT-PA) MWCNTs. See Multiwalled CNTs (MWCNTs) Myoglobin, 241 adsorption, 243 protein net charge behavior, 243f

N N,N-Dimethyl amino ethyl methacrylate, 584 N,N-Methylenebisacrylamide-cross-linked starch microparticles, 319
320 N-alkanes, 525
526 N-carboxymethyl chitosan, 586 N-isopropylacrylamide (NIPAM), 383 N-phenyl methacrylamide (PMA), 99
100 Nafion, 190
192 Nanocomposites, 536, 587
588 Nanomaterials, 535
537 Nanomedicine, 271 Nanoparticles, 271
273, 320
321 Nanoprotein, 586 Nanoscale crystallites, 153
154 Nanoscale Molecular Dynamics (NAMD), 210 Nanospray drying technique, 273 Nanotechnology, 320
321, 536 National Aeronautics and Space Administration (NASA), 458 Native proteins, 553 Natural fibers, 331 Natural gut string, 511 Natural materials, 487 Natural polymeric foods, 567
568 Natural polymers, 2
3, 117, 258
259, 258f, 569 Natural proteins, 579
580 Natural rubber, 1
2 Natural tracks, 507 Natural-origin tannins, 572 Near infrared region (NIR), 75 Neoplasms, 296 Neoplastic polyps, 296 Neoprene, 488, 502, 509, 515

Neta sol products, 552
553 Newton’s equations of motion, 208 Nickelocene (NiCp2), 105
106 NIPAM. See N-isopropylacrylamide (NIPAM) NIR. See Near infrared region (NIR) Nitrogen, 257, 593 Nivolumab, 299
300 NLFM. See Nonlinear fracture mechanics (NLFM) NLO. See Nonlinear optical (NLO) Nomex, 517 Nonbioactive proteins, 258
259 Nonbiodegradable polymers consequences, 619
621 Noncompostability, 528 Nonconsumable polysaccharides, 572
573 Nondigestible polysaccharides, 572
573 Nonionic polysaccharides, 551, 551f Nonionic surfactants, 547 Nonlinear fracture mechanics (NLFM), 131
132 Nonlinear optical (NLO), 423
424 Nonlinear optics, 441
442 Nonwovens, 333 processing, 336
337 melt blown, 339f spunbond nonwoven process, 338f Novel polymers, 478
480 TEM images for SWNT/PANI composites, 479f NS398, 301t Nuclear magnetic resonance spectroscopy (NMR spectroscopy), 69
70, 85
101, 153, 166
167. See also Infrared spectroscopy (IR spectroscopy) applications, 167
171 13 C-NMR spectra of chitosan in F3CCOOD/ D2O and H-chitosan, 171f 1 H and 13C assignment for PA-TR, and 2D correlations, 168f 1 H-NMR spectra of chitosan in F3CCOOD/ D2O and H-chitosan, 170f liquid-state 19F NMR spectra of HTPB, 167f chemical shift, 89
90, 90t copolymer analysis by 1H NMR spectroscopy, 99
101 end groups analysis by 1H NMR spectroscopy, 93
97 molecular weight determination by 1H NMR spectroscopy, 97
99 NMR splitting pattern in methyl acrylate, 92f nuclear Zeeman splitting, 87
89, 88f spin
spin coupling, 90
93, 91f Nuclear powered robots, 396 Nuclear Zeeman splitting, 87
89, 88f Nylon, 345
346, 346f, 503, 593
594 automobile airbag, 347f

647

648

Index

Nylon (Continued) chemistry, 345
346 fabric, 515 fiber, 332, 346 Nylon 6/6, 603 properties, 346 string, 510 taffeta
coated amino acid polymer, 485 uses, 346

O ODA. See 4,4’-Oxidianiline (ODA) OFETs. See Organic field-effect transistors (OFETs) Offset printing, 378 Oils, 582 Okra, 576
577 OLED. See Organic light-emitting diode (OLED) OM. See Optical microscopy (OM) Opdivo. See Nivolumab Open circuit voltage (Voc), 619 Open-loop manipulator, 396 Optical lenses, 432
434, 435f fabrication process for dual-wavelength random laser, 436f Optical microscopy (OM), 27, 38
40, 341 comparison of bright field and dark field modes, 39f Optical profilometry, 40 Optical sensors, 437
439 Optics, polymers in, 429
430 applications, 424f, 430
447 lasers, 434
437 nonlinear optics, 441
442 optical lenses, 432
434, 435f optical sensors, 437
439 photocatalysis, 444
445 POF, 430
432 solar cells, 442
444, 442f waveguide fabrication, 439
440 characterization, 428
429 Abbe refractometer, 428 Brillouin spectroscopy, 429 photoluminescence spectroscopy, 428 Raman spectroscopy, 429 UV
visible absorption spectroscopy, 428 future perspective and challenges, 447 properties, 426
428, 426f abbe number, 427 birefringence, 427 color, 427 gloss, 428 refractive index, 427 transparency, 427

Optoelectronics, 374
377 inorganic and organic materials, 374t OPVs. See Organic photovoltaic cells (OPVs) Oral colon-specific drug delivery systems, 270, 273
276 Oral drug administration, 309 Organic field-effect transistors (OFETs), 437 Organic light-emitting diode (OLED), 365
366 Organic materials, 465
467 Organic photovoltaic cells (OPVs), 374 Organic polymers, 117 Organic π-conjugated materials, 215 Organofunctional silicone, 554
556 Ovalbumin, 258
259 Oxaliplatin, 299
300 4,4’-Oxidianiline (ODA), 157 Oxygen, 257, 593

P P-glycoprotein, 320
321 p-phenylenediamine (PDA), 157 P4HB. See Poly(4-hydroxybutyrate) (P4HB) PA. See Polyacetylene (PA)Polyamide (PA) PAA. See Poly(acrylic acid) (PAA) PAAs. See Poly(amic acid) (PAAs) Packaging, 525 food, 526 materials, 527
537 biodegradable polymers, 531
533 nanomaterials, 535
537 polymers, 529
531 synthetic polymers and biopolymers hybrids, 533
535 polymer-based materials for packaging materials, 612
613 PAI. See Polyamide-imide (PAI) PAM. See Polyacrylamide (PAM) PAMPS. See Poly (2-acrylamido-2-methylpropane sulfonic acid) (PAMPS) PAN. See Polyacrylonitrile (PAN) Pan. See Polyaniline (PANI) PANI. See Polyaniline (PANI) Panitumumab, 299
300 Paper, 527 Paraffin wax, 583 Parallel manipulator, 396 Paris law, 134 Particle-based approaches, 207 Particle-reinforced polymer composites, 26
27 PBO. See Poly benzoxazole (PBO) PBT. See Polybutadiene (PBT) PBXPG composite films. See Poly(ether-blockamide)/polyethylene glycol composite films (PBXPG composite films)

Index

PC. See Polycarbonate (PC) PC waste. See Postconsumer waste (PC waste) Pc-rubber. See Neoprene PC4-phenol. See Phenol-terminated polycarbonate (PC4-phenol) PCE. See Control transformation proficiency (PCE) PCL. See Polycaprolactone (PCL) PCs. See Polymer composites (PCs) PDA. See p-phenylenediamine (PDA) PDF. See Probability distribution function (PDF) PDMS. See Polydimethylsiloxane (PDMS) PDPA. See Polydiphenylamine (PDPA) PDS. See Pyridyl disulfide (PDS) PE. See Polyethylene (PE) PEBAX. See Poly(ether-block-amide) (PEBAX) Pectin, 287, 289t, 300
304, 550, 578 as bioactive dietary fiber, 259
266, 260f, 261f antibacterial, 263 antidiabetic, 264 antioxidants, 263 antitumor, 264
266 prebiotic, 262 hydrogels, 586 methylesterases, 269 natural polymers, 258
259, 258f oral colon-specific drug delivery mechanism, 273
276 pectin-based oral drug delivery system, 266
273 beads, 267
269 nanoparticles, 271
273 pellets, 270
271 tablets, 267 pectin
methotrexate nanoparticles, 271 pectin
silica beads, 269 synthetic polymers, 257 Pectinase, 270 PEDOT. See Poly(3,4-ethylene dioxythiophene) (PEDOT) PEDOT:PSS. See Poly (3,4-ethylenedioxythiophene) doped with poly (styrenesulfonate) (PEDOT:PSS) PEEK. See Polyether ether ketone (PEEK) PEF. See Polyethylene furanoate (PEF) PEFCs. See Polymer electrolyte fuel cells (PEFCs) PEG. See Polyethylene glycol (PEG) PEI. See Polyetherimide (PEI) Pellets, 270
271, 321 PEN. See Poly(ethylene 2,6-naphthalate) (PEN) PEO. See Polyethylene oxide (PEO) Perfluoroalkoxy alkanes (PFA), 145
147 Personal care industries, 545
546 PES. See Polyethersulfone (PES)

PESU. See Polyethersulfone (PES) PET. See Polyethylene terephthalate (PET) PETE. See Polyethylene terephthalate (PET) Petrochemical polymers, 527
528 Petroleum plastics, 525 Petroleum-based plastic, 11
12 PEVA. See Poly(ethylene-vinyl acetate) (PEVA) PFA. See Perfluoroalkoxy alkanes (PFA) PGS. See Poly(glycerol sebacate) (PGS) pH in magnitude of adsorption, 242
245 sensors, 384
385 pH-dependent system, 275
276 pH-responsive hydrogels, 229
230 acid
base equilibrium in dilute solutions, 231
233 histidine-tag adsorption to, 237
241 protein adsorption, 241
250 protonation of weak polyacid hydrogel films, 233
237 pH-sensing film, 538
539 PHAs. See Polyhydroxyalkanoates (PHAs) PHB. See Polyhydroxybutyrate (PHB) Phenol-terminated polycarbonate (PC4-phenol), 164
165 Phosphoenolpyruvate carboxykinase, 264 Phosphorous, 593 Phosphoryl chloride, 308 Photocatalysis, 444
445 Photodynamic therapy, 446 Photon radiation, 21
22 Phthalates, 525
526 Physical activities, 485
486 Physical and thermal analysis of polymer gas permeation behavior, 202 IR and Raman spectroscopy, 154
165 NMR spectroscopy, 166
171 quantum chemical calculations, 197
201 SEM and TEM, 177
181 TGA and DSC, 181
196 X-ray analysis, 172
177 PI. See Polyimide (PI) Piezoelectric materials, 373
374 Piezoelectric polymers, 15, 403 PIL. See Polymeric ionic liquid (PIL) PIMs. See Polymers of natural microporosity (PIMs) PLA. See Polylactic acid (PLA) PLA/silver nanocomposite films, 536 Planar manipulator, 397 Plant polysaccharides, 574 Plant-based polymers, 568
569 Plantago ovata, 576
577 Plastic(s), 487, 515, 529
530

649

650

Index

Plastic(s) (Continued) bags, 530 deformations, 126
127 jugs, 593 packaging, 530, 531f production, 5 PLEDs. See Polymer light-emitting diodes (PLEDs) PMA. See N-phenyl methacrylamide (PMA) PMAA. See Poly(methacrylic acid) (PMAA) PMDA. See Pyromelliticdianhydride (PMDA) PMMA. See Polymethyl methacrylate (PMMA) PNC. See Polymer nanocomposites (PNC) Pneumatic robots, 396 Pneumatics, 399 PNIPAM. See Poly(N-isopropyl acrylamide) (PNIPAM) POF. See Polymer optical fiber (POF) Polarized light microscopy, 39 Polarized optical microscopy, 126
127 Pole vaulting, 513
514 Poly (2-acrylamido-2-methylpropane sulfonic acid) (PAMPS), 411 Poly (2-methoxy-5-(20 -ethylhexyloxy)-1,4phenylene vinylene) (MEH-PPV), 435
436 Poly (3,4-ethylene-dioxythiophene) doped with poly (styrene-sulfonate) (PEDOT:PSS), 437 Poly (p-phenylene vinylene) (PPV), 367 Poly (vinyl carbazole) (PVK), 370
371 Poly (vinylidene fluoride 2) (PVF2), 407
408 Poly benzoxazole (PBO), 168
169 Poly trimethyl benzene ethersulfone (TPESU), 176
177 Poly-(2-hydroxyethyl methacrylate), 585
586 Poly(1-butyl-3-vinylimidazolium bromide), 610 Poly(1-vinyl-3-butylimidazoliumbromide) graphene, 610 Poly(3,4-ethylene dioxythiophene) (PEDOT), 606
608 Poly(4-hydroxybutyrate) (P4HB), 141 Poly(acrylic acid) (PAA), 229, 257 Poly(amic acid) (PAAs), 157 Poly(dimethylsiloxane), 257 Poly(ether-block-amide) (PEBAX), 537 Poly(ether-block-amide)/polyethylene glycol composite films (PBXPG composite films), 537 Poly(ethylene 2,6-naphthalate) (PEN), 192
193 Poly(ethylene glycol), 257 Poly(ethylene-co-vinyl liquor), 614 Poly(ethylene-vinyl acetate) (PEVA), 498
499, 508 Poly(glycerol sebacate) (PGS), 603
604 Poly(maleic anhydride), 257

Poly(methacrylic acid) (PMAA), 229 Poly(methylmethacrylate-co-vinylbenzylchloride), 610 Poly(N-isopropyl acrylamide) (PNIPAM), 95, 96f, 229 Poly(oxalate), 257 Poly(phenylenediamine), 607
608 Poly(propylene glycol), 257 Poly(styrene-co-acrylonitrile) (PSAN), 614 Poly(trimethylene terephthalate) (PTT), 619
621 Poly(vinyl acetate), 257 Poly(vinyl liquor), 614 Polyacetylene (PA), 367, 606
607 Polyacrylamide (PAM), 433 Polyacrylonitrile (PAN), 348, 411, 499
500 Polyamide (PA), 49, 503, 529
530, 603
604 Polyamide-imide (PAI), 145
147 Polyanhydrides, 603
604 Polyaniline (PANI), 367, 478
480, 606
608 Polybenzimidazobenzophenanthroline (BBL), 174 Polybenzoxazole, 515 2,2-Polybithiophene, 606
607 Polybutadiene (PBT), 145
147 Polybutylene, 594 PPTF, 533 Polycaprolactone (PCL), 157
158, 603
604, 610
611 Polycarbonate (PC), 23, 164
165, 371
372, 431
432, 494
495, 529
530, 594 Polychloroprene. See Neoprene Polydimethylsiloxane (PDMS), 156
157, 434, 502
503, 546
547 polymers, 556
557 Polydiphenylamine (PDPA), 162
164, 188 Polyelectrolyte complexation, 271 Polyesters, 72
73, 82, 258
259, 343
345, 586
587, 594 chemistry, 343 Dacron polyester sail cloth, 344f olyester fiber, 343 properties, 343
344 string, 510 swimming pool filter cartridge, 345f uses, 344
345 Polyether ether ketone (PEEK), 143, 381
382 Polyetherimide (PEI), 145
147 Polyethersulfone (PES), 145
147, 176
177 Polyethylene (PE), 3
5, 22
23, 119
121, 159, 379, 406, 496, 527
528, 594
596, 610
613 Polyethylene furanoate (PEF), 531 Polyethylene glycol (PEG), 73, 445, 537, 547
548, 584 Polyethylene oxide (PEO), 547
548, 584

Index

Polyethylene terephthalate (PET), 4, 73, 135, 343, 343f, 380
381, 525, 527
528, 601, 612
613 container, 619
621 Polyethylene/polypropylene/ethylene
propylene copolymer (E/P/EP copolymer), 160
162 Polyhydroxyalkanoates (PHAs), 258
259, 531, 612
613 Polyhydroxybutyrate (PHB), 157
158 Polyimide (PI), 145
147, 157 Polyisoprenes, 258
259 Polylactic acid (PLA), 53, 531, 603
604, 612
613 Polylactide, 610
611 Polymer composites (PCs), 26
27, 584
585, 597 Polymer electrolyte fuel cells (PEFCs), 372 Polymer light-emitting diodes (PLEDs), 367 Polymer nanocomposites (PNC), 8, 207
208 Polymer optical fiber (POF), 430
432, 431f, 432f Polymer solar cells (PSCs), 367, 442 Polymer-based materials for agriculture and horticulture, 610
612, 611f for corrosion control, 614 for environmental remediations carbon-based polymeric composite materials for CO2 capture, 604
607 CO2 capture by porous hypercrosslinked ionic polymers, 605f interface-directed assembly of conjugated polymer membrane, 608f ionic liquid based polymeric composites, 610 magnetic polymer composites, 608
609 polymer-based membranes, 607
608 for medical and biomedical applications, 614
617 for packaging materials, 612
613 Polymeric materials for hydrogen storage purpose, 613
614 Polymer-based membranes, 607
608 Polymer-fullerene solar cells, 619 “Polymer-to-fabric”, 336 Polymer/surfactant interactions in cosmetics, 547 Polymeric ionic liquid (PIL), 610 Polymeric/polymer(s), 1, 3, 69, 117, 525, 529
531, 545, 593 biodegradability, 621
623 blends, 24
26 characteristics, 595
597 chemical analysis IR spectroscopy, 74
85 mass spectrometry, 101
108 molecular weight determination, 70
74 nuclear magnetic resonance spectroscopy, 85
101

classification and production, 3
10, 9t elastomers, 10 microplastic particles isolated from seawater samples, 7f thermoplastics, 9 thermosets, 9
10 corrosion protection efficiency, 615t degradation methods, 623f fabrication methods, 7
8 recyclability, 612
613, 621
623, 622f reusability, 621
623 requirements using physical, mechanical, and chemical methods, 13
14 2D network of polymeric structure, 594f Polymerization, 1
2, 70, 319
320, 502 Polymers of natural microporosity (PIMs), 613
614 PIM-1, 613
614 PIM-7, 613
614 Polymethyl methacrylate (PMMA), 23, 76, 179, 411
412, 427, 529
530, 601 Polymethylpentene, 594 Polynucleotides, 258
259 Polyolefins, 4, 355
358, 357f, 504 chemistry, 355
356 properties, 357 uses, 357
358 Polyols, 574 Polyoxalate-cross-linked chitosan nanocarriers, 586
587 Polypectomy, 298
299 Polypeptides, 569
570, 579
582 structure, 580f Polyphenol, 263 Polyphenylene oxide (PPO), 145
147 Polyphenylene sulfate (PPS), 196, 198f, 380 Polyphenylene sulfide (PPS), 145
147 Polyphenylsulfone (PPSU), 176
177 Polyplexes, 584 Polypropylene (PP), 22
23, 73, 127
128, 355, 488, 525, 594, 601, 603, 610
613 Polypropylene 2,5-thiophenedicarboxylate (PPTF), 533 Polypropylene carbonates (PPC), 529
530 Polypropylene oxide, 584 Polyps, 296 Polypyrrole (PPy), 159
160, 367, 606
607 Polysaccharide-based polymers, 545
546, 549
552 amphoteric polysaccharides, 551
552, 552f anionic polysaccharides, 550 cationic polysaccharides, 550
551, 550f nonionic polysaccharides, 551, 551f

651

652

Index

Polysaccharides, 258
259, 287, 304, 570
579, 573t, 586 as anticancer drug carriers, 287
288, 289t colon anatomy and physiology, 288
296 colon cancer, 296
300 colon-specific drug delivery, 300
304 food storage polysaccharides, 575 mucosubstances, 576
579 sources, 288f starch as drug carrier, 304
321 structural polysaccharides, 575
576 Polysiloxane, 257 copolymer, 554 Polystyrene (PS), 23, 128, 257, 430, 525, 594
596, 601, 612
613 Polysulfone (PSU), 145
147 Polytetrafluoroethylene (PTFE), 50
51, 53 Polythiophene (PTh), 367, 606
607 PTh-based cationic conjugated polyelectrolytes, 186
187 Polyurethane (PU), 53, 145
147, 257, 384
385, 488, 496
497, 595
596, 601, 603 Polyvinyl acetate, 584
585 Polyvinyl alcohol (PVA), 177, 411, 529
530, 547
548, 610
611 Polyvinyl chloride (PVC), 498, 525, 527, 594, 597, 603, 612
613 Polyvinyl formal (PVF), 529
530 Polyvinylidene difluoride (PVDF), 145
147, 373, 403, 424
425 Polyvinylidene fluoride. See Polyvinylidene difluoride (PVDF) Polyvinylpyrrolidone (PVP), 435
436, 547
548, 559 Pomegranates, 572 Postconsumer waste (PC waste), 621
622 Potassium bromide (KBr), 76 Potato starch, 577
578 Power supply, 399 PP. See Polypropylene (PP) PPC. See Polypropylene carbonates (PPC) PPF, 533 PPO. See Polyphenylene oxide (PPO) PPS. See Polyphenylene sulfate (PPS) Polyphenylene sulfide (PPS) PPSU. See Polyphenylsulfone (PPSU) PPTF. See Polypropylene 2,5thiophenedicarboxylate (PPTF) PPV. See Poly (p-phenylene vinylene) (PPV) PPy. See Polypyrrole (PPy) Prebiotic, 262 Pressed pellet technique, 76 Primary mechanical reusing, 621
622 Printable electronics, 378
379

Probability distribution function (PDF), 217 Prodrugs, 274
275 Programming languages, 399 Propensity of polymeric material. See Abrasion resistance Propionate, 262 Proprioceptive sensors, 399 Protein adsorption to pH-sensitive hydrogels, 241
250 adsorption from binary protein mixtures, 250 pH and salt concentration in magnitude of adsorption, 242
245 protein charge regulation, 245
246 protein model and solution titration curves, 241
242 protonation of amino acids after adsorption, 246
249 Protein hydrolysates, 553 of wheat, 554 Proteins, 1, 258
259, 545
546, 552
554, 571
572 additives, 554 in cleansing products, 554 in hair care, 554 protein-based polymeric foods, 569
570 in skin care, 553 Proton dissociation constant, 231
232 Protonation of amino acids after adsorption, 246
249 of weak polyacid hydrogel films, 233
237 displacement of chemical equilibrium, 235
237 local pH, 234
235 Proximity sensors, 399 PS. See Polystyrene (PS) PSAN. See Poly(styrene-co-acrylonitrile) (PSAN) PSCs. See Polymer solar cells (PSCs) Pseudomonas aeruginosa, 263, 537 Pseudomonas putida, 537 PSU. See Polysulfone (PSU) PTFE. See Polytetrafluoroethylene (PTFE) PTh. See Polythiophene (PTh) PTT. See Poly(trimethylene terephthalate) (PTT) PU. See Polyurethane (PU) Pullulan, 287 PUR. See Polyurethane (PU) Purified chitin, 582 PVA. See Polyvinyl alcohol (PVA) PVC. See Polyvinyl chloride (PVC) PVDF. See Polyvinylidene difluoride (PVDF) PVF. See Polyvinyl formal (PVF) PVF2. See Poly (vinylidene fluoride 2) (PVF2) PVK. See Poly (vinyl carbazole) (PVK) PVP. See Polyvinylpyrrolidone (PVP)

Index

Pyridyl disulfide (PDS), 95
97 2-Pyrilidone, 559 Pyromelliticdianhydride (PMDA), 157

Q QCCs. See Quantum chemical calculations (QCCs) QDs. See Quantum dots (QDs) Quantitative analysis, 207 Quantum chemical calculations (QCCs), 153 applications, 197
201 temperature-dependent THz spectra of PCL, 200f basic principle, 197 Quantum dots (QDs), 436
437 Quaternary protein, 552
553

R R2 Robotnaut (humanoid robot), 395 Radiation therapy, 299 Radical polymerization technique, 94
95 RAFT. See Reversible addition-fragmentation chain transfer (RAFT) Raman spectroscopy, 153
165 applications, 156
165 Rapidly digestible starch, 305
306 Rassum’s Universal Robots (RUR), 393 Ravel on, 554
556 Rayon, 575
576 Rectum, 288 Recyclability of polymeric products, 621
623 Recycling, 11 Reduced graphene oxide (rGO), 174 Redundant robot, 395 Refractive index, 427 Regorafenib, 299
300 Reinforced carbon fiber, 513
514 Reinforcements, 2 Remotely operated vehicles (ROVs), 417 Renewable resources, 532
533, 619
621 Reppe process, 559 Resistant starch, 305
306 acetylation, 307 acid hydrolysis, 307
308 amylose
lipid complexation, 308 crosslinking, 308 enzymatic debranching, 308 hydrothermal treatment, 308
309 Retrogradation process, 308 Reusability of polymeric products, 621
623 Reversible addition-fragmentation chain transfer (RAFT), 94
95 agents, 106
107 polymerization, 95 RG I. See Rhamnogalacturonan I (RG I)

RG II. See Rhamnogalacturonan II (RG II) rGO. See Reduced graphene oxide (rGO) Rhamnogalacturonan I (RG I), 259 Rhamnogalacturonan II (RG II), 259 Rheology modifiers, 551 Ribonucleic acid (RNA), 32
33, 258
259, 297
298 Right colon, 295
296 RNA. See Ribonucleic acid (RNA) Robo ML. See Robotic markup language (Robo ML) Robot efficiency, 399 general purpose, 395 Robotic markup language (Robo ML), 399 Robotics, 393 classification, 395
397, 396f applications, 397 degrees of freedom, 395 drive technology, 396 kinematic structure, 396 motion characteristics, 397 workspace geometry, 397 components, 397
399, 398f driving high-current loads from logic controllers, 399 electronic controls, 399 languages, 399 mechanical platform, 398 microcontroller systems, 399 motors, 399 pneumatics, 399 power supply, 399 sensors, 398
399 in composites, 415 industrial sector, 416 medical sector, 416 military applications, 417
418 mining, 418 miscellaneous applications, 416 polymeric sensors, 415 space applications, 416
417 terrestrial applications, 416 types, 400
415 electroactive materials, 400
402 electronic electroactive polymers, 403
408 underwater applications, 417 Roomba (robotic vacuum cleaner), 395 Rotary screen printing, 378 ROVs. See Remotely operated vehicles (ROVs) Rubber, 488, 493, 515
516, 548
549 tree latex, 593
594 RUR. See Rassum’s Universal Robots (RUR) Rutin, 272

653

654

Index

S S-methoxycarbonylphenylmethyl dithiobenzoate (MCPDB), 94
95 Sack paper, 527 Salmonella typhimurium, 262
263 SALS. See Angle light scattering (SALS) Salt concentration, 235
237, 241 in magnitude of adsorption, 242
245 SAN. See Styrene acrylonitrile (SAN) SAP. See Super permeable polymer (SAP) Saturated polymers, 76
79 SAXS. See Small angle X-ray scattering (SAXS) SB. See Spunbond (SB) Scanning electron microscopy (SEM), 24
25, 40
42, 41f, 153
154, 177
178 applications, 178
181 Fe-SEM images of PPy, BN/Ag, and PPy/ Ag@BN, 180f SEM images of PI fibers, 182f Scanning probe microscopy, 126
127 Scanning tunneling microscopy (STM), 27, 44
45, 45f SCARA robot. See Selective compliance assembly robot arm robot (SCARA robot) Sceroglycan, 287 SCF theory. See Self-consistent field theory (SCF theory) SEC. See Size exclusion chromatography (SEC) Secondary xylem, 489 Selective compliance assembly robot arm robot (SCARA robot), 397 Self-assembly complexation, 271 method, 272
273 Self-consistent field theory (SCF theory), 207 Self-healing polymers, 12, 13f SEM. See Scanning electron microscopy (SEM) Semiconducting polymers, 369
372. See also Conducting polymers (CPs) applications, 372
386 dielectrics, 379
382 flexible electronics, 377
378 fuel cells, 372
373 optoelectronics, 374
377 piezoelectric materials, 373
374 printable electronics, 378
379 sensors, 382
386 charge transfer polymers, 370
371 conjugated conducting polymers, 371
372 filled polymers, 369
370 ionic polymers or ionomers, 370 representation for classification, 369f Semicrystalline, 142

membrane, 569 polymers, 23
24 Semicrystalline/semicrystalline mixes, 597 Sensors, 382
386, 398
399 biosensors, 386, 387f gas, 385, 385f ion-selective, 385
386 multisensors, 386 pH, 384
385 relative resistances, 384f stress, 386 temperature, 383 Serial robot, 396 Sericin hydrolysate in shampoo, 554 Serum albumin, 554 Shakey (mobile robot), 394
395 Shampoos, 554 Shape memory alloys (SMAs), 476, 477t Shape memory polymers (SMPs), 476, 512 Shapeless/semicrystalline, 597 Shear yielding, 126
130 Shellac coatings, 585 Shellac resins, 583 Shielding effect of mobile ions, 235
237 Short out current (Jsc), 619 Sigmoid colon, 288
295 Silica, 554
556, 606
608 Silicare skin lotion, 546
547 Silicon, 557
558, 593 age, 606
607 Silicon nitride (Si3N4), 46
47 Silicones, 502
503, 545
547, 554
558 alkyl-modified, 557
558 amodimethicone, 557 cyclomethicones, 556 dimethicone, 556
557 Silver (Ag), 536 Simulation approaches to polymer research DPD simulations, 213
217 molecular dynamics simulations, 208
213 molecular theory, 217
221 passivation properties of PEG-coated substrate surfaces, 220f streptavidin
biotin binding, 220f Single emulsification technique, 319
320 Single-point diamond turning (SPDT), 430 Single-unit formulations, 309
318 Single-walled CNTs (SWCNTs), 409
410 siRNA. See Small interfering RNA (siRNA) Size exclusion chromatography (SEC), 73
74 Skiing, 505
506 Slowly digestible starch, 305
306 SMA. See Styrene maleic anhydride (SMA)

Index

Small angle X-ray scattering (SAXS), 27, 32
33, 33f Small interfering RNA (siRNA), 301t Smart biomaterials, 15 Smart materials, 14
15 SMAs. See Shape memory alloys (SMAs) SMPs. See Shape memory polymers (SMPs) Snowboarding, 505 Sodium alginate, 577
578 Sodium bicarbonate, 577
578 Sodium carboxymethylcellulose, 550 Sodium caseinate, 554 Sodium chloride (NaCl), 76 Sodium trimetaphosphate, 308 Sodium tripolyphosphate, 308 Soft polyethene, 500
501 Solar cells, 442
444, 619 Solar ultraviolet radiation, 464
465 Solid-state NMR, 166
167 Solubility, 341 Soluble collagen, 554 Soluble keratin, 554 Soluble proteins, 553 Solution casting, 537 Solution proteins, 246 Solvent evaporation technique, 319
320 Sony Aibo (animal robot), 395 Soy protein, 579
582 Soy protein isolate (SPI), 535t Space exploration demonstration of InSight collecting data on Mars, 459f environment of Earth, Moon, and Mars, 460t inorganic polymers as building materials, 465
467, 466f atomic formulation of polydimethylsiloxane, 466f mechanical properties of adhesive materials, 468t landing sites, 458f novel polymers, 478
480 research needs and future directions, 476
478 artificial muscle and sensor array, 477f flexible and energy harvesting polymers, 478 processing and printing, 477 representation of preparation of composites, 479f utilization of robotics, 476
477 space environment effect on polymers, 460
465, 461t atomic oxygen, 462
464 ionizing radiation, 464 solar ultraviolet radiation, 464
465 thermal cycling, 462

vacuum, 461
462 space environments, actions, and conditions, 459
460 space resources, 467
468 possible asteroidal materials, 469t Spandex, 519 Spandex fiber, 349
350 chemistry, 349 polymerization reaction of polyurethane, 349f properties, 349 uses, 350 Spatial manipulator, 397 SPDT. See Single-point diamond turning (SPDT) Spectroscopic techniques, 153 Spherical manipulator, 397 Spherical robot, 397 SPI. See Soy protein isolate (SPI) SPI-based films with PLA, 533 Spin
spin coupling, 90
93, 91f Sports, 485 materials evolution in sports from traditional to composites composite materials, 490
491 metals, 489
490 wood, 489 motor sports, polymers in, 515
516 polymeric sports surfaces, 506
507 soft and hard polyethene, 500
501 Surlyn copolymer, 493
494 polymers in athletics and gymnastics, 511
512 polymers in sports footwear, 520 sports footwear, 520 sports protection equipment, 507
510 protection for head, 508
509 protection for mouth, 508 sportswear, 517
519 tennis, 510
511 trans-1,4-Polyisoprene, 493 vectran, 492
493 water sports, 514
515 winter sports, 504
506 Sportswear, 517
519 golf attire, 519 thermal properties of sportswear, 519 Sprague Dawley rats, 271
272 Spray congealing, 319
320 Spray drying, 273, 319
320 Spraying, 584 Spun yarn, 333 Spunbond (SB), 336 Stanford Cart, 394
395 Staphylococcus aureus, 263, 538 Staple fiber, 333, 335

655

656

Index

Starch, 287, 289t, 535t, 571
573, 575, 577
578, 593
594, 598
600 chemical structure, 571f as drug carrier, 304
321 pharmaceutical applications, 309, 310t physicochemical properties, 304
305 preparations of resistant starch, 306
309, 307f resistant starch, 305
306 as oral colon-specific drug carrier, 309
321 beads, 319 hydrogels, 319 microparticles, 319
320 nanoparticles, 320
321 pellets, 321 starch-based substances, 568 Stearodiacetin, 583 Stearoxy dimethicone, 557
558 Stearyl dimethicone, 557
558 Steel, 489
490, 598
600 Steel Age, 606
607 Stimuli-responsive polymers, 14, 15f Stivarga. See Regorafenib STM. See Scanning tunneling microscopy (STM) Stone, 598
600 Stone Age, 606
607 Storage proteins, 258
259 Straight polymers, 593 Stress sensors, 386 Stress
strain behavior of polymeric materials, 119
121 Strontium titanate (SrTiO3), 478 Structural polymers, 4 Structural polysaccharides, 570, 572
573, 575
576 Structural techniques, 27
28 Structureless polymers, 595 Styrene acrylonitrile (SAN), 145
147, 433 Styrene maleic anhydride (SMA), 145
147 Styrofoam, 595
596 Suberin, 258
259 Substituted galacturonans (GS), 259 Sudden Date (silicone-based lotion spray), 546
547 Sugar beet pectin, 269 Sugar polyols, 574 Sulfur, 257, 593 Super permeable polymer (SAP), 612 Superparamagnetic iron oxide, 271
272 Surface morphology, 21 Surfactants, 547 Surfboards, 515 Surgery, 298
299 Surlyn copolymer, 493
494

Sustainability, 525 SWCNTs. See Single-walled CNTs (SWCNTs) Synthesized polymers, 3 Synthetic cellulose derivatives, 578 nanocrystals, 583
584 Synthetic elastomers, 10 Synthetic food polymers, 583
587 “Synthetic gut”. See Nylon string Synthetic leather, 488 Synthetic polymers, 2, 257, 533
535, 545, 547
549, 568
569 in cosmetics, 548f thickening by associative mechanism, 549 by chain entanglement, 548 by covalent cross-linking, 548
549 Synthetic tracks, 507

T Tablets, 267 Tamarind seed polysaccharide (TSP), 538
539 Targeting attributes, 287
288 TDI. See 2,4- and 2,6-tolylene toluene diisocyanate (TDI) TeCEA. See Tetrachloroethyl acrylate (TeCEA) Telechelic, 164
165 TEM. See Transmission electron microscopy (TEM) Temperature of polymers, 143
144 Tenacity, 340 Tennis, 510
511 Kevlar string, 511 natural gut string, 511 nylon string, 510 polyester string, 510 Tensile strength of polymers, 124
125, 124f Terrestrial robots, 416 Tetrachloroethyl acrylate (TeCEA), 155 Textile fibers, 342 characterization and testing, 337
342 density, 337
339 fiber identification, 340
342 fiber structure and morphology, 340 mechanical properties, 340 Textiles, 342
358 characterization and identification techniques, 342 fiber manufacturing, 333
337 dry spinning, 335
336 gel spinning, 336 melt spinning, 334
335 nonwovens processing, 336
337 wet spinning, 336

Index

manmade fibers, 332 Textured filament yarn, 333 TFAA. See Trifluoroacetic anhydride (TFAA) TGA. See Thermogravimetry (TGA) Thermal cycling, 462 Thermally rearranged polymer (TR polymer), 168
169 Thermo pressing and casting, 537 Thermo-optic (TO) devices, 424
425 Thermogravimetry (TGA), 181
196 applications, 183
188 temperature dependence of weight loss of polydiphenylamine, 190f thermal gravimetric analysis of PPy and PPy/ Ag/BN, 189f Thermoplastic hydroxypropyl starch (TPS), 196 Thermoplastics, 9, 10f, 366, 412
414 Thermosensitive polymer, 229 Thermosets, 9
10 polymers, 257, 593 Thickeners, 551 Thickening, 551 by associative mechanism, 549 by chain entanglement, 548 by covalent cross-linking, 548
549 Thin hydrogel films, 233 Thiolated pectin, 270
271 Thiostrepton, 301t Three-dimension (3D), 465 printing, 528 of polymers, 14
15 SrTiO3, 478 “Tie molecules”, 23 Time
temperature dependence of polymers, 123
124 Tissue biodistribution study, 272
273 Titania (TiO2), 536, 607
608 Titanium dioxide (TiO2), 478 2,4- and 2,6-tolylene toluene diisocyanate (TDI), 158
159 TOPs. See Transparent optical polymers (TOPs) Total mesorectal excision, 299 Toughness of polymer, 135 TPESU. See Poly trimethyl benzene ethersulfone (TPESU) TPS. See Thermoplastic hydroxypropyl starch (TPS) TR polymer. See Thermally rearranged polymer (TR polymer) Tragacanthin gum, 550 TRAIL. See Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) Trans-1,4 polymer, 493 Trans-1,4-polyisoprene, 258
259

Trans-1,4-Polyisoprene, 493 Trans-polyacetylene, 368 Transmission electron microscopy (TEM), 42
44, 43f, 153
154, 177
178 applications, 178
181 images of HIPP-1 and HIPP-2, 183f Transparency, 427 Transparent optical polymers (TOPs), 424
425 Transverse colon, 288
296 TrFE. See Trifluoroethylene (TrFE) Triamines, 495 Tributyl aconitate, 525
526 Trifluoroacetic anhydride (TFAA), 167 Trifluoroethylene (TrFE), 403
404 2,4,6-trimethyl-1,3-diaminobenzene (DAM), 172
173 TSP. See Tamarind seed polysaccharide (TSP) Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), 301t Type I prodrugs, 274
275 Type II prodrugs, 274
275

U Ulcerative colitis, 309 Ultrahigh molecular weight polyethylene (UHMWPE), 336, 351, 505 chemistry, 352 properties, 352
353 uses, 353 Ultraviolet (UV), 459 Ultraviolet-ozone (UVO), 156
157 Underwater robots, 417 Unimate (programmable robot), 394
395 Unsaturated polymers, 79
80 UV. See Ultraviolet (UV) UVO. See Ultraviolet-ozone (UVO) UV
vis spectroscopy, 153

V Vaccines, 586 Vacuum, 461
462 outgassing levels, 462t VAL. See Variable assembly language (VAL) Van der Waals forces, 23, 547 Vapor pressure osmometry, 73
74 Variable assembly language (VAL), 399 Vectibix. See Panitumumab Vectra, 492 Vectran, 492
493 Velocity-Verlet algorithm, 210 Vesicles, 584 Vicinal coupling, 91
92 Viscoelasticity of polymers, 121
123 Viscometry, 97

657

658

Index

Vision sensors, 399 Voc. See Open circuit voltage (Voc) Vulcanization of rubber, 548
549

W Warm strategies, 597
598 Waste to wealth future perspectives, 624
625 Water (H2O), 531 for human utilization, 623
624 polymers in water sports, 514
515 water-soluble synthetic polymers, 547
548 Waveguide fabrication, polymers in, 439
440 prototype of flexible strain sensor, 440f Wavenumber scale, 155 WAXD. See Wide-angle X-ray diffraction (WAXD) Waxy starch, 304 Wet spinning, 336 Wheat gluten, 581
582 Wheat protein, 553
554, 581
582 Whey protein, 579 Whey protein isolate (WPI), 535t Wide-angle X-ray diffraction (WAXD), 172
173 scans of polyimide membranes, 174f Wood, 489, 598
600 resins, 583 Wound-type golf balls, 512 WPI. See Whey protein isolate (WPI)

X X-ray analysis, 172

applications, 172
177 spectra of PES membranes, 177f XRD patterns of PPy, BN, BN/Ag, and PPy/ Ag/BN, 176f X-ray diffraction (XRD), 27
30 X-ray photoelectron spectroscopy (XPS), 174 X-ray wavelength-dispersive spectroscopy, 172 Xanthan gum, 287, 550, 578
579 Xanthate, 575
576 XELIRI. See Xeloda and irinotecan hydrochloride (XELIRI) Xeloda. See Capecitabine Xeloda and irinotecan hydrochloride (XELIRI), 299
300 Xeloda and oxaliplatin (XELOX), 299
300 XPS. See X-ray photoelectron spectroscopy (XPS) XRCL. See Extensible robot control language (XRCL) XRD. See X-ray diffraction (XRD) Xylan, 289t

Y Young’s modulus, 439

Z Zein protein, 535t, 580
581 Ziegler
Natta LLDPE, 188
190 Zimm plot, 31
32 Zinc ions, 267
269 Zinc oxide (ZnO), 607
608