Properties and Applications of Superabsorbent Polymers: Smart Applications with Smart Polymers 9789819911011

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Sand Arpit Tuteja Jaya Editors

Properties and Applications of Superabsorbent Polymers Smart Applications with Smart Polymers

Properties and Applications of Superabsorbent Polymers

Sand Arpit · Tuteja Jaya Editors

Properties and Applications of Superabsorbent Polymers Smart Applications with Smart Polymers

Editors Sand Arpit School of Sciences Manav Rachna University Faridabad, Haryana, India

Tuteja Jaya School of Sciences Manav Rachna University Faridabad, Haryana, India

ISBN 978-981-99-1101-1 ISBN 978-981-99-1102-8 (eBook) https://doi.org/10.1007/978-981-99-1102-8 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

The book is written for all researchers in the field of polymer industry. The book features the basics and advanced technology involved in the synthesis and characterization of superabsorbent polymers and is a perfect summary or literature review of superabsorbent polymers’ application in versatile areas. Super absorbent polymers (SAPs) are the materials that are hydrophilic in nature and have the capacity to hold and retain the fluid with high efficiency. The common SAPs used are sugar-like white in appearance and employed in baby diapers/sanitary napkins/personal hygiene products. The SAPs are known for holding body fluid approx. 30 times of their original weight used in any diaper; along with holding and retaining the body fluid it keeps the skin dry and healthy. In previous times, cellulose fluff was used to absorb body fluid which has now been replaced by thinner SAPs in modern times. The book features the background, literature review, its types, synthesis methods, technologies involved in its synthesis, its experimental methods, physical and chemical properties, its applications in various sectors; recent research works, etc., the major of the literature found in SAP have involved the usage of SAP in disposable diapers/napkins, etc. Here, this book highlights its importance in heat resistance and treatment of industry effluents; SAPs’ potential application in agriculture field, drug delivery, nano-filtration, nano-medicines, and biomedical equipment/accessories. Owing to the variety of monomers present SAPs can be synthesized in various types. These SAPs are broadly classified into two types (i) synthetic SAPs (petrochemicalbased monomers) and (ii) natural SAPs (monomers based on renewable sources like polysachharide- and polypeptide-sbased). The SAPs available in market are based on acrylic acid or its salts based synthesized by inverse-suspension polymerization techniques or solution polymerization techniques. There are numerous internal and external factors that influence the synthesis of SAPs ultimately resulting in different physical and chemical properties. The quantification techniques to measure the swelling capacity, absorption capacity (absorption under body weight pressure and absorption without body weight pressure), and load capacity were discussed.

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In this book, our focus was not only to list the applications of SAPs but also was to connect the academic knowledge of SAPs with industrial application of SAPs. The book discusses the fundamental approach to the formation of cross-linked super absorbent polymers, its networking, and change in polymer network when it undergoes swelling, etc. These days various new SAPs are being synthesized and investigated for a particular applications or to respond to particular molecule. This is quite an emerging field and continuous researches are going on in this area to explore and invent new biodegradable SAPs to solve the concern of landfills. Faridabad, India

Sand Arpit Tuteja Jaya

Acknowledgements It gives us immense pleasure to acknowledge all of the contributors who have provided us their quality material to prepare this book. We pay our sincere gratitude to Honorable Vice-Chancellor Prof. I. K. Bhatt, Manav Rachna University, Faridabad, India. We are grateful to our beloved family members for their continuous support. Our sincere thanks to the publishing team and their efforts.

Contents

1

Introduction of Superabsorbent Polymers . . . . . . . . . . . . . . . . . . . . . . . Yahya Bachra, Fouad Damiri, Mohammed Berrada, Jaya Tuteja, and Arpit Sand

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Synthesis Methods of Superabsorbent Polymers and Factors Affecting Their Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Maria S. Lavlinskaya and Andrey V. Sorokin

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Experimental Methods of Superabsorbent Polymers: Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preeti Gupta and Roli Purwar

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Superabsorbent Polymers for Heat Resistance and Treatment of Industrial Effluents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amita Somya

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Superabsorbent Polymers Application in Agriculture Sector . . . . . . Jagdeep Singh, Ankit Kumar, and A. S. Dhaliwal

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Recent Advancements in Superabsorbent Polymers for Drug Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Cynthia Lizeth Barrera-Martínez, Lluvia Azhalea Guerrero-Hernández, Jorge Luis Sánchez-Orozco, Gladis Y. Cortez-Mazatan, H. Iván Meléndez-Ortiz, and René D. Peralta-Rodríguez

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Superabsorbent Polymers for the Development of Nanofiltration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Abel Inobeme, Alexander Ikechukwu Ajai, Jonathan Inobeme, Charles Oluwaseun Adetunji, Alfred Obar, John Tsado Mathew, John Olusanya Jacob, and Nkechi Nwakife

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Progressive Approach of SAPs in Disposable Hygiene Industry . . . . 171 Jaya Tuteja and Aparna Vyas

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Contents

Utility of Super-Absorbent Polymers in Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 Stephen Rathinaraj Benjamin and Eli José Miranda Ribeiro Júnior

10 Superabsorbent Polymer’s Role in Nanomedicines . . . . . . . . . . . . . . . 201 Patrícia Viera de Oliveira, Carlos Rafael Silva de Oliveira, Afonso Henrique da Silva Júnior, Alexandre José Sousa Ferreira, Nívea Taís Vila, Brenno Henrique Silva Felipe, and Joziel Aparecido da Cruz 11 Future Challenges and Opportunities in the Field of Superabsorbent Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 Fouad Damiri, Yahya Bachra, Mohammed Berrada, Jaya Tuteja, and Arpit Sand

Contributors

Charles Oluwaseun Adetunji Applied Microbiology, Biotechnology and Nanotechnology Laboratory, Department of Microbiology, Edo State University Uzairue, Edo, Nigeria Yahya Bachra Laboratory of Biomolecules and Organic Synthesis (BIOSYNTHO), Faculty of Sciences Ben M’Sick, Department of Chemistry, University Hassan II of Casablanca, Casablanca, Morocco; Laboratory of Analytical and Molecular Chemistry (LCAM), Faculty of Sciences Ben M’Sick, Department of Chemistry, University Hassan II of Casablanca, Casablanca, Morocco Cynthia Lizeth Barrera-Martínez Centro de Investigación en Química Aplicada, Saltillo, México Stephen Rathinaraj Benjamin Laboratory of Behavioral Neuroscience (LBN), Drug Research and Development Center (NPDM), Department of Physiology and Pharmacology, Federal University of Ceará (UFC), Porangabussu, Fortaleza, Ceará, Brazil Mohammed Berrada Laboratory of Biomolecules and Organic Synthesis (BIOSYNTHO), Faculty of Sciences Ben M’Sick, Department of Chemistry, University Hassan II of Casablanca, Casablanca, Morocco; Laboratory of Analytical and Molecular Chemistry (LCAM), Faculty of Sciences Ben M’Sick, Department of Chemistry, University Hassan II of Casablanca, Casablanca, Morocco Gladis Y. Cortez-Mazatan Centro de Investigación en Química Aplicada, Saltillo, México Joziel Aparecido da Cruz Department of Mining, Metallurgical and Materials Engineering, Federal University of Rio Grande do Sul–UFRGS, Porto Alegre, RS, Brazil

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Contributors

Afonso Henrique da Silva Júnior Department of Chemical Engineering and Food Engineering, Federal University of Santa Catarina-UFSC, Trindade Campus, Florianópolis—SC, Brazil Fouad Damiri Laboratory of Biomolecules and Organic Synthesis (BIOSYNTHO), Faculty of Sciences Ben M’Sick, Department of Chemistry, University Hassan II of Casablanca, Casablanca, Morocco; Laboratory of Analytical and Molecular Chemistry (LCAM), Faculty of Sciences Ben M’Sick, Department of Chemistry, University Hassan II of Casablanca, Casablanca, Morocco Carlos Rafael Silva de Oliveira Department of Textile Engineering, Federal University of Santa Catarina-UFSC, Blumenau Campus, Blumenau, SC, Brazil Patrícia Viera de Oliveira Department of Chemical Engineering and Food Engineering, Federal University of Santa Catarina-UFSC, Trindade Campus, Florianópolis—SC, Brazil A. S. Dhaliwal Department of Physics, Sant Longowal Institute of Engineering and Technology, Longowal, Punjab, India Brenno Henrique Silva Felipe Department of Textile Engineering, Federal University of Santa Catarina-UFSC, Blumenau Campus, Blumenau, SC, Brazil Alexandre José Sousa Ferreira Department of Textile Engineering, Federal University of Santa Catarina-UFSC, Blumenau Campus, Blumenau, SC, Brazil Lluvia Azhalea Guerrero-Hernández Centro de Investigación en Química Aplicada, Saltillo, México Preeti Gupta Discipline of Polymer Science and Chemical Technology, Department of Applied Chemistry, Delhi Technological University, Daulatpur, Delhi, India Alexander Ikechukwu Ajai Federal University of Technology, Minna, Nigeria Abel Inobeme Department of Chemistry, Edo State University Uzairue, Edo, Nigeria Jonathan Inobeme Department of Geography, Ahmadu Bello University, Zaria, Nigeria John Olusanya Jacob Federal University of Technology, Minna, Nigeria Eli José Miranda Ribeiro Júnior Department of Pharmacy, Faculty of CGESP (Centro Goiano de Ensino Superior), Goiânia, Goiás, Brazil Ankit Kumar Department of Physics, Sant Longowal Institute of Engineering and Technology, Longowal, Punjab, India

Contributors

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Maria S. Lavlinskaya Voronezh State University of Engineering Technologies, Voronezh, Russian Federation; Voronezh State University, Voronezh, Russian Federation John Tsado Mathew Department of Chemistry, Ibrahim Badamosi Babangida University, Lapai, Nigeria H. Iván Meléndez-Ortiz CONACyT—Centro de Investigación en Química Aplicada, Saltillo, México Nkechi Nwakife Federal University of Technology, Minna, Nigeria Alfred Obar Federal University of Technology, Minna, Nigeria René D. Peralta-Rodríguez Centro de Investigación en Química Aplicada, Saltillo, México Roli Purwar Discipline of Polymer Science and Chemical Technology, Department of Applied Chemistry, Delhi Technological University, Daulatpur, Delhi, India Arpit Sand School of Sciences, Manav Rachna University, Faridabad, India Jagdeep Singh Department of Physics, Sant Longowal Institute of Engineering and Technology, Longowal, Punjab, India Amita Somya Department of Chemistry, School of Engineering, Presidency Uniniversity, Bengaluru, India Andrey V. Sorokin Voronezh State University of Engineering Technologies, Voronezh, Russian Federation Jorge Luis Sánchez-Orozco Centro de Investigación en Química Aplicada, Saltillo, México Jaya Tuteja School of Sciences, Manav Rachna University, Faridabad, India Nívea Taís Vila Department of Textile Engineering, State University of MaringáUEM, Goioerê, PR, Brazil Aparna Vyas Department of Mathematics, Manav Rachna University, Faridabad, India

Abbreviations

AAc AAm AMPS APS CMC CRC DMW FPC HMI KPS LCST MBA NIPAm pAAcNa pAAm pAPTMACl pMAAc pNIPAAm SAP SDBS SPH

Acrylic acid Acryl amide 2-acrylamido 2-methylpropane sulfonic acid Ammonium persulfate Carboxyl methyl cellulose Centrifuge retention capacity Demineralized water Fire protection clothing Heavy metal ions Potassium persulfate Lower critical solution temperature N, N' -methylene bis acryl amide N-iso propyl acryl amide Sodium poly acrylic acid Poly acryl amide Poly(3-acryl amido propyl trimethyl ammonium chloride Poly methacrylic acid Poly N-Isopropyl acryl amide Superabsorbent polymer Sodium dodecyl benzene sulfonate Superabsorbent polymer hydrogel

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Chapter 1

Introduction of Superabsorbent Polymers Yahya Bachra, Fouad Damiri, Mohammed Berrada, Jaya Tuteja, and Arpit Sand

Abstract The materials known as superabsorbent polymers (SAPs) are all able to absorb and store substantial amounts of aqueous solutions. That fact has made these materials perfect for a diverse range of valuable applications in cosmetics formulation, agriculture, esthetics, food, medicine, and drug delivery. Due to their huge surface area and the increased accessibility of aqueous liquids to the hydrophilic groups of the polymer backbone, SAPs have a significant potential for absorbing water. Several parameters influence the swelling process, contributing to exceptional swelling capacity. As it is well-known, SAPs are the major constituent of sanitary care products. Furthermore, the consumption of highly absorbent healthcare products, such as adult incontinence products, female sanitary napkins, and baby diapers, is increasing every day worldwide due to the expansion of the world population. Accordingly, the performance testing of superabsorbent polymers has become in high demand. This is achieved by specific experimental methods of characterization of superabsorbent polymers. This chapter was devoted to a general introduction to superabsorbent polymers. The history of the development of superabsorbent polymers, the different methodologies and fundamentals of SAP design, and the classification of superabsorbent polymers have been discussed. Meanwhile, the interests, specificities, and implications of synthetic and polysaccharide-based SAPs were discussed. Finally, some highlights of the application areas of SAPs were also mentioned. Keywords Superabsorbent polymers (SAPs) · Evaluation · Swelling · Retention · Cross-linking

Y. Bachra (B) · F. Damiri (B) · M. Berrada Laboratory of Analytical and Molecular Chemistry (LCAM), Faculty of Sciences Ben M’Sick, Department of Chemistry, University Hassan II of Casablanca, Casablanca, Morocco e-mail: [email protected] F. Damiri e-mail: [email protected] J. Tuteja · A. Sand School of Sciences, Manav Rachna University, Faridabad 121002, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Arpit and T. Jaya (eds.), Properties and Applications of Superabsorbent Polymers, https://doi.org/10.1007/978-981-99-1102-8_1

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1.1 History of the Development of Superabsorbent Polymers Nowadays, the world’s elderly population has been growing continuously and will further increase in the foreseeable future because the average life span of human beings has been rising steadily over the past years, and the number of people living in the world is increasing. The prevalence of incontinence disorders in adults has also increased accordingly. Along with the rising human birth rate, this trend is expected to contribute to the global superabsorbent materials market throughout the forecast time frame [1]. The earliest model of SAP is over 80 years old. In the 1940s, the primary waterabsorbing polymer had obtained by synthesizing “divinylbenzene” as well as “acrylic acid” in solution, resulting in a networked material. The subsequent product exhibited some degree of swelling ability around 50wt.% [2, 3]. In the 1960s, a company named “Union Carbide” first commercialized SAPs for horticultural applications to accelerate the growth of sugar plants [4]. During the 1970s, they were developed to enable the cultivation of plants in deserts. Nevertheless, the advertising of these products was not very successful due to their high cost and unsatisfactory shelf life [5]. In 1978, in Japan, industrial production of SAP for feminine sanitary napkins began [6]. At the beginning of the 80s, in Japan, SAPs have been widely promoted for the baby diaper industry, which directed the development of new and more interesting high-swelling polymers; some substances with moderately long shelf life were exploited for agricultural purposes. The Europeans continued to develop superabsorbent materials for baby diapers during the 1980s [7, 8]. They innovatively mixed synthetic SAPs based on polyacrylate with a cellulose-based linting filler, which resulted in a much-reduced SAP in terms of quantity used, but which is characterized by a high water absorption capability. Then, in Japan, a lighter product with 4 to 5 g of SAP and a less linty charge has been commercialized [9]. Along with granular superabsorbent polymers, in the early 90 s, a company called “ARCO Chemical” started to industrialize superabsorbent fiber technology [10]. For the first time, a starch-grafted cross-linked polyacrylate polymer has been discovered and is being developed industrially by the Department of Agriculture in the United States (Agricultural Research Service) under the name “Super Slurper”. The water swelling performance of the Super Slurpers remained meaningly improved to exceed 400 times its own weight [11, 12]. Subsequently, as an alternative, Crosslinked polyacrylates have become the predominant SAP on the world market at present and have gradually substituted all formerly developed superabsorbents [13– 17]. Up until 2022, the majority of the SAP market remains occupied by sodium polyacrylate [18, 19]. The U.S. Department of Agriculture has given technical expertise to some U.S. corporations in order to further develop the core know-how. Following this directive, different functional groups grafted onto natural polymers have been investigated, such as acrylic acid, acrylamide, and polyvinyl alcohol (PVA). While, in Japan,

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they have initiated a new investigation on PVA, starch, carboxymethyl cellulose, and isobutylene maleic anhydride [4, 20, 21]. SAP technology has advanced considerably over the years, whereas the first-developed starch-grafted SAPs, with approximately 100 g of water/g of SAP, remained infrequently employed as they had low water uptake [22]. Currently, the SAPs used are generally cross-linked polymers based on acrylic acid which often are produced in the form of neutralized sodium salts. Notwithstanding, as mentioned above, the fact that acrylic SAPs have vastly improved water absorption properties, the material remains non-biodegradable. Because worldwide concerns about plastic pollution continue to grow, there is an increasing interest in developing SAP products based on natural polymers, including proteins and polysaccharides (e.g., starch, cellulose, pectin, chitin, alginate, chitosan, etc.) [23], which are considered to be biologically based, biodegradable, and therefore environmentally friendly materials. These materials typically gained their high water absorption capacity by grafting acrylic acid onto the primary chains of natural polymers [24, 25]. The investigation of SAPs has shown an increasing trend during the previous 40 years [26–29], as evidenced by the number of patents and research investigations published on “superabsorbent polymers” (Fig. 1.1). Furthermore, the advancement of more environmentally friendly SAPs, including biodegradable grades, that inhibit the production of long-lasting microplastics, is predictable to play an ever-growing role in the industrial and academic communities over the foreseeable horizon [16, 30, 31].

Fig. 1.1 Chemical structure and swelling performance of some commercial superabsorbent polymers

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1.2 Methodologies and the Fundamentals of SAPs Conception 1.2.1 Chemical and Structural Specifications A strongly hydrophilic substance with a cross-linked network structure would make up a “traditional” superabsorbent polymer, typically in the form of microspheres, that, while being subject to high pressure or stresses, can absorb and hold a significant volume of water or aqueous solutions within the microspheres [32, 33]. Also, these materials can absorb and retain up to about a thousand times their proper total weight in the water, although common hydrogels are limited to absorbing approximately the equivalent of 100% of their own weight in water [34–38]. However, compared to SAPs, hydrogels can lose most of the absorbed water when compressed. Due to their exceptional hydrophilicity, which is primarily caused by the presence of hydrophilic groups and a properly cross-linked structure, as well as their osmotic pressure-based absorption process, SAPs have unique properties [6]. As a result, the SAP network must have an adequate number of extremely hydrophilic organic groups in order to provide excellent water absorption capabilities. The functional groups in question are typically quite polar and, after being neutralized by metal cations, frequently ionic, (such as –OH, –CONH2 , –SO3 H/–SO3 –, etc.), and these groups may create hydrogen bonds with the water molecules directly (Fig. 1.2). Because of the powerful interactions between their hydrophilic groups and the water molecules, SAPs will dramatically expand when positioned in the liquid media. The polymer may eventually disintegrate in the media; hence the water absorption capacity cannot be limitless [5, 34, 39, 40]. Therefore, the formation of a 3D network through crosslinking is a requirement to prevent the hydrophilic polymers from being dissolved in the liquid solvent [26, 41, 42]. Fig. 1.2 Hydrogen bond scheme, case of anionic SAPs (carboxylate groups)

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1.2.2 Effect of the Cross-Linking Process Generally, the process of cross-linking was considered to be critical to obtaining stable SAPs, as a suitable equilibrium between water absorption properties, solubility, and structural resistance under pressure has to be maintained. Regarding the latter state, if SAP particles are not or not properly cross-linked, they deform easily or even agglomerate under load. Thus, the crosslinking density is considered the main parameter influencing the stability of SAPs [43, 44]. Nonetheless, it is appropriate to note that an exceeded cross-linking density should be carefully considered as it reduces the water uptake capacity. Mainly two types of crosslinking processes remain commonly applied in SAPs development, namely, bulk crosslinking and surface crosslinking. Bulk crosslinking, also known as base crosslinking, normally occurs during the polymerization phase in the superabsorbent polymer elaboration [45, 46]. In general, during free radical polymerization, cross-linking molecules can be incorporated into the backbone of the polymer chains. The above protocol normally gives SAPs significantly higher cross-linking densities and, consequently, higher gel strength and a more stable structure. However, because of the fairly extensive cross-linking network, which greatly reduces the free space to accommodate water molecules, their ability to absorb water may be negatively impacted. A “gel blocking” phenomenon developed once the liquid supply gaps were sealed off because the water’s ability to permeate the beads was stopped. Surface crosslinking appears to be a suitable strategy to maintain adequate gel strength in the shell layer and adequate water absorption capacity in the core in order to combat this issue [35, 47]. To clarify, surface crosslinking typically proceeds at the carboxyl group of a premolded base polymer. This phase of SAP manufacturing is frequently the final one. The polymer product is then dried, powdered, sized, and surface cross-linked after polymerization is finished. As a result of the surface crosslinking process, the crosslinking density on the surface of the polymer particles can be increased and they can be given a core–shell structure. On-surface crosslinking ensures the structural consistency of SAP, while less crosslinking in the core makes it more absorbent [6]. As it is now considered an important fact, it is of greatest importance to control the crosslinking process during the construction of SAP particles that are significantly influenced by the characteristics of a crosslinking agent. Crosslinkers (or crosslinking agents) are typically di-functional or multifunctional molecules. During SAP manufacture, UV irradiation or heating can start the crosslinking process between the functional groups of the premade polymer and the crosslinking agents. Diverse types of crosslinkers may be chosen depending on the reaction mechanisms. For carbon–carbon double bonds, for example, polymers were typically crosslinked with N,N ' -methylenebisacrylamide (MBA). Pendant functional groups (e.g., carboxylic acid, aldehyde, or hydroxyl groups) of the base polymer are typically crosslinked by crosslinking agents such as polyfunctional acids, diglycidyl ethers, polyhydric alcohols, or quaternary amines. MBA is currently the predominantly applied cross-linking agent for SAP production due to its excellent reactivity

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[48–50] with vinyl groups and their low toxicity after crosslinking [2, 20, 51]. The reactivity ratio of the crosslinking agents and the associated vinyl monomers is critical in defining the network topology of SAPs made of vinyl monomers, such as poly(acrylamide) and poly(acrylic acid) because it determines how quickly the SAPs will crosslink [50, 52]. In other words, does an acrylic acid molecule prefer to react with another molecule that is identical to its own or with something different, such as a cross-linking molecule? The reactivity ratios are a measuring scale of the affinity of each of the reaction components to react with one another. When the crosslinker has a high rate of reactivity, the majority of the crosslinker molecules are consumed during the initial stages of polymerization, and the later-formed polymer chains have a lower likelihood of being crosslinked and are hence more likely to become pull-out chains. Crosslinkers with low reactivity have the opposite propensity [17, 37]. Therefore, depending on the initial materials (natural macromolecules or vinyl monomers) and the specific structure of the network, different types of crosslinking agents may be chosen. Moreover, crosslinkers must be selected based on their cost when considering industrial SAPs [53].

1.2.3 Absorption and Retention of Liquids Principle As it is well-known, the performance of absorption and retention of liquids depends on and varies within the absorbing mechanism. The four primary types of mechanisms are listed in Table 1.1 [3]. Although the main characteristic of superabsorbent materials is the enormous absorption, specific methods are being developed for a significant assessment. To assess the water absorption capacity of SAPs, it is crucial to characterize four key Table 1.1 Main types of absorption mechanism of superabsorbent polymers Type Absorption mechanism

Example

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Reversible modification of the crystal structure

Anhydrous inorganic salts; silica gel [43, 54–56] …etc.

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Absorption via a macroporous Soft polyurethane sponge structure: Physical scavenging of water molecules by capillary forces

[51]

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Coordination of physical trapping Tissue paper of water molecules and hydration of functional groups

[2, 22]

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Association of physical scavenging Classic superabsorbent materials of water molecules, hydration of functional groups, combined with a natural dissolution of hydrophilic polymer portions

[6, 57, 58]

References

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parameters, namely free pressure absorption capacity (FSC), absorption capacity under centrifugal force (CRC), absorption capacity under pressure (AUL), and absorption rate (AR). There are numerous ways to measure it, including the sieve method, the filtration method, and the tea bag method, which is the most used. The following chapters will go into greater depth about these techniques.

1.3 Classification of Superabsorbent Polymers Superabsorbent polymers can be categorized according to different approaches. However, classification according to charge, type of monomer, and raw material origin is considered to be the most relevant classification type. The classification of SAPs according to the charge of the polymer chain makes a considerable difference in the absorption process. Since SAPs have a certain type of monomer unit as part of their chemical makeup, this classification makes the most material sense. They are also divided into groups based on the kind of monomer that was employed to create their chemical structure. As a result, the majority of SAPs can be classified into one of the three types listed below [1, 59]: (i) natural polymers grafted with polyacrylates (including cellulose, starch, chitosan, etc.); (ii) cross-linked polyacrylates and polyacrylamides; (iii) cross-linked copolymers of acrylates and acrylamides. Due to their non-ionic nature, SAPs based on polyacrylamide homopolymers and copolymers exhibit the best salt resistance performance, while SAPs based on cross-linked polyacrylates have the best absorption capacity. However, compared to acrylatebased SAPs, natural polymer-based SAPs have superior biodegradability and are more environmentally benign. Even in terms of water absorption, these environmentally friendly materials fall short of synthetic SAPs composed of polyacrylate. The main mechanisms underlying the non-ionic SAPs’ capacity to absorb water are, in general, the hydrophilic groups that are prevalent along the polymer chain and, subsequently, their interactions with water via dipolar Van der Waals interactions and/or hydrogen bonds. As a result, SAPs can be divided into two groups—ionic and non-ionic—based on the presence or absence of electrical charges located in the cross-linked chains [60, 61]. If the SAPs are present as metal salts, neutralization in the case of ionic SAPs will introduce metal ions into the polymer network and result in the creation of negatively charged hydrophilic groups, such as the carboxyl group. With the help of these groups, the negative charges neutralize one another electrostatically, causing the polymer network to expand and creating more space for the liquid to be absorbed [62]. Furthermore, Ionic SAPs typically have a higher capacity for absorption than their non-ionic counterparts. Poly(acrylic acid) is known as the most common anionic SAP in the industry, and polyacrylamide is known as the most common non-ionic species [63]. Generally, almost all SAPs could be divided into two main groups based on the origin of their raw materials: those with a fossil origin and those with a biological origin. Polymer degradation is the physical or chemical modification of a polymer as a result of physical, chemical, or biological processes that result in

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bond breaking and subsequent chemical changes [64]. Accordingly, the definition of biodegradation is the process of substances being broken down by microorganisms, which produces new biomass, CO2 , H2 O, and salts from organic compounds, as well as the recycling of carbon [65, 66]. The different processes of biodegradation will be discussed afterward.

1.4 Interests, Specificities, and Implications of Synthetic and Polysaccharide-Based SAPs A relevant scientific classification to enable the investigation and development of SAPs. Therefore, the presence of ionic, non-ionic, ampholytic, or zwitterionic functional groups, covalent or physical cross-linking, or physical characteristics can all be used to categorize SAPs, as was already indicated (i.e., the morphology of the SAP). The three classes of natural, synthetic, and semi-synthetic (seminatural) SAPs represent the most significant division. Given that synthetic and semisynthetic SAPs are the most widely used, the monomers used in their formation for synthetic and semi-synthetic SAPs include 2-acrylamido-2-methylpropane sulfonic acid (AMPS), acrylamide (AM), methacrylic acid (MAA), dimethylaminoethyl methacrylate (DMAEMA), dimethylaminopropyl methacrylamide (DMAPMA), and others. Whereas they can be included in a network of cross-linked (co)polymers using a synthetic crosslinker like N,N ' -methylene bisacrylamide (MBA). By using graft polymerization to combine a synthetic element with a natural polymer backbone, it is conceivable to produce semi-synthetic or semi-natural SAPs [11, 67, 68]. In the latter example in the case, the synthetic monomers are naturally crosslinked by the natural backbone. Polysaccharides and proteins are examples of natural SAPs [69]. In order to create cell-interacting properties for biomedical applications, acylated proteins are frequently grafted onto other polymers, such as poly(acrylic acid) [70, 71]. Due to the limited applicability of proteins as such for SAP applications, polysaccharides have drawn interest. Biosynthesis, which takes place in both plants and animals, can be used to produce polysaccharides. In recent studies, it has also been revealed that bacteria produce polysaccharides like bacterial hyaluronan, gellan, or xanthan [25, 72]. Natural polymers currently used for SAPs include polysaccharides such as chitin [73], chitosan [38, 74, 75], cellulose [16, 75], starch [76], agarose [77], alginates [78], tragacanth [5, 33], tamarind [79] as well as proteins such as those based on soy, fish, and collagen [80–82]. Due to their biodegradable, easily accessible, biocompatible, nontoxic, renewable, and sustainable qualities, they have attracted growing interest. Natural polymers are also an affordable and sustainable alternative to crude oil, which is becoming more expensive and scarcer [83, 84]. They are less hazardous to the environment than synthetic SAPs because they are renewable. Functional groups like alcohols, carboxylic acids, and/or amines are present in water-soluble

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polysaccharides. These groups can also be used to crosslink or graft other polymers [85]. The biodegradability of natural SAPs and their main uses are covered in the sections of this chapter that follow.

1.5 Biodegradability of Superabsorbent Polymers Natural materials are mostly biodegradable, as is commonly understood. Thus, there are several steps involved in the biodegradation of polymeric materials, and each step has the potential to be the last [17, 84, 86]. Steps in a typical biodegradation process include: (i) Various physical and biological forces can cleave polymers into tiny fractions; (ii) microorganisms secrete catalytic agents that can break down polymers and lower their molecular weight (MW), a process named “depolymerization.” During this process, various slight molecular weight products are produced, including oligomers and monomers; (iii). Some of these small molecules bind to the microbial cells’ surface receptors, enabling the cells to absorb the chemicals via their plasma membrane. The other molecules are still present in the extracellular medium and are subject to different alterations; (iv). The process of integrating transported chemicals into microbial metabolism to produce energy, new biomass, storage vesicles, and a range of primary and secondary metabolites in the cytoplasm is referred to as “bioassimilation.” A subsequent process known as “mineralization” occurs after this and generates a variety of simple and complex compounds that can be discharged into the extracellular medium. Simple molecules from fully oxidized intracellular metabolites are released into the environment, including CO2 , CH4 , H2 O, and different salts [87]. The chemical and physical characteristics of polymers have a direct impact on how biodegradable they are. The surface properties of polymers, such as their surface area and their hydrophilic and hydrophobic characteristics, as well as their first-order structures, such as their chemical modification and molecular weight distribution, as well as their higher-order structures, such as their glass transition temperature, melting point, modulus of elasticity, crystallinity, and crystal structure, can all have an impact. Aliphatic polyesters and polycarbonates are two prevalent polymers with great potential for usage as biodegradable plastics because of the ester or carbonate bonds’ sensitivity to lipolytic enzymes and microbial degradation [88]. The chemical composition of SAPs as a functional polymer greatly influences how easily they degrade. If we use polyacrylate-based SAPs as an example, their polymeric backbone, which is made up entirely of carbon–carbon bonds, prevents them from degrading through biological processes. As the molecular weight of non-crosslinked polyacrylates rises, the rate of biodegradation declines. Other polymers, like aliphatic polyesters, are prone to this phenomenon. The stability of the network structure of SAPs increases with crosslink density, making them less susceptible to attack by microorganisms or enzymes. Crosslink density also affects the biodegradability of SAPs [89, 90]. The application of synthetic and natural SAPs, meanwhile, is covered in more detail in the section that follows.

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1.6 Highlights of SAP Application Areas To date, SAPs have been used in a wide range of applications, including the agricultural industry as water reservoirs, nutrient transporters, and soil conditioners to conserve water in dry areas [3, 91]. Moreover, SAPs are commonly used for hygienic purposes, mainly in diapers and sanitary napkins [92–99], for biomedical purposes such as drug delivery [13, 100, 101] and wound healing [102], conventional hydrogels, on the other hand, are more frequently used in situations where swelling is less crucial, like in disposable contact lenses [22, 103]. Water filtration and waterblocking strips are other applications. In these applications, a nonwoven fabric that has been top-coated with a binder and SAP is used [104]. In addition, waterproof tapes have also been used more recently in various stages of oil production [105]. The use of SAPs in mortar and concrete, particularly for self-sealing and self-healing of concrete cracks, is a potential remedy for the self-sealing issue [8, 106, 107]. Many studies have been conducted recently on the cost estimation and life cycle assessment of these superabsorbent polymers in comparison to other concrete self-healing mechanisms [108, 109]. Given this, the field of SAPs is already crowded with a veritable garden of applications, even though there is still space for many more. Grossmodo, as was already mentioned, natural polymers now dominate the global SAP market. In Table 1.2, the most popular natural polymers, such as cellulose chitosan, alginate, starch, lignin, guar gum, tragacanth gum, etc. are listed along with some recent examples of their various application in diverse areas.

1.7 Conclusions Superabsorbent polymers, which are literally hydrogels capable of absorbing up to several hundred times their own dry weight of liquids, are now among the most sought-after commercialized materials in the world. Although the history of their development dates back to the 1940s, several studies are still in progress, investigating their methodologies and the fundamental principles of their design in order to deepen the application areas of their strengths, including efforts to develop synthetic and polysaccharide-based SAPs. The subsequent chapters of the book will be devoted to exploring the main lines of research on superabsorbent polymers such as methods of synthesis of superabsorbent polymers and factors affecting their preparation, experimental methods of characterization of superabsorbent polymers, composites of superabsorbent polymers for heat resistance and absorbency, application of superabsorbent polymers in the agricultural sector, recent advances in superabsorbent polymers for drug delivery, superabsorbent polymers for nanofiltration development, progressive approach of superabsorbent polymers in the disposable hygiene industry, the usefulness of superabsorbent polymers in biomedical applications, the role of superabsorbent polymers in nano-drugs, and finally, future challenges and opportunities in superabsorbent polymers.

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Table 1.2 Benefits and drawbacks of natural source SAPs and their relevant applications Natural source of SAP

Applications

References

Benefits

Drawbacks

Starch

Slow release of fertilizer; drug delivery system

[76, 110–113]

Cellulose

Water reservoir in agriculture; personal care products

[114, 115]

Chitosan

Drug delivery system; bone tissue regeneration; water retaining agent

[116–119]

Renewable, biodegradable, biocompatible, less hazardous to the environment, easily available, long-lasting, and cell-adhesive

Required changes to create SAP; Methods of extraction required for the raw material; Delicate storage circumstances; Shelf-life

[120, 121]

Alginate

Wound healing

Lignin

Water purification [122]

Gelatin

Water purification [123]

Guar gum

Wound healing; vitamin delivery; moisture retainer; Hygiene products

Pectin

Oral drug delivery [127] systems

Tragacanth gum

Hygiene products, [5, 128] wound healing

[58, 124–126]

Karaya gum

In vitro release

[129]

Carrageenan

Drug delivery, tissue engineering, and wound healing

[121, 130]

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Chapter 2

Synthesis Methods of Superabsorbent Polymers and Factors Affecting Their Preparation Maria S. Lavlinskaya

and Andrey V. Sorokin

Abstract Superabsorbent polymers (SAPs) are hydrophilic cross-linked 3D materials capable to absorb and retain a huge amount of water. Typically, the amount of water absorbed by the SAPs is hundreds or even thousands of times greater than their mass. Due to these unique properties, they have attracted research attention and have found a wide application in the different fields of human activities. There are a lot of ways to form the three-dimensional structure of the SAPs including chemical and physical cross-linking. Also, SAPs can be classified into three main groups based on used raw materials. They are synthetic SAPs consisting of synthetic monomers; natural SAPs based on natural polymers, and composite or hybrid SAPs coupling synthetic and natural links. Differences in the obtaining ways and raw materials make it possible to obtain super absorbing materials of various properties that keep their key ability which is absorbing and retaining a great amount of water. It is well known that polymer properties greatly depend on their composition and structure, forming during the synthesis process. Hence the synthesis methods, raw materials such as monomers, cross-linkers, initiators, biodegradable nature components, and other reactants used for obtaining the superabsorbent polymers will be discussed in the present chapter.

2.1 Introduction According to the IUPAC definition, superabsorbent polymers (SAP) are polymers that can absorb and retain extremely large amounts of a liquid relative to their mass [1]. The liquid can be water or organics, however, most often, the term «superabsorbent polymer» refers to a polymer that absorbs water. Modern SAPs that swell

M. S. Lavlinskaya (B) · A. V. Sorokin Voronezh State University of Engineering Technologies, Voronezh, Russian Federation e-mail: [email protected] Voronezh State University, Voronezh, Russian Federation © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Arpit and T. Jaya (eds.), Properties and Applications of Superabsorbent Polymers, https://doi.org/10.1007/978-981-99-1102-8_2

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in an aqueous medium are hydrophilic hydrogels. Currently, the terms “hydrogel” and “superabsorbent” are almost synonymous; however, superabsorbents are characterized by a higher ability to swell in liquid media. The SAP equilibrium swelling ratio can achieve 1000 or more times. This characteristic is reached due to the presence of a larger number of hydrophilic groups such as carboxyl, amino, amide, hydroxyl, sulfonic, and other polar or ionogenic groups on the SAP polymeric network. So, the water absorbency of the SAPs is often responsible for the pH and ionic strength of the swelling media. Hence, they can possess sensitivity to external parameters like pH, ionic strength, solvent composition, light, and/or electrical fields. Moreover, swollen SAPs retain their dimensional cross-linking network which is a result of the physical or chemical process. The chemically cross-linked SAPs are obtained by the formation of the covalent bonds between the macromolecular chains, while physically cross-linked SAPs result from the holding of the polymeric chains by physical interactions, such as H-bonds, electrostatic or hydrophobic interactions, and Van der Waals forces. The SAP cross-linking type affects the equilibrium swelling ratio and swelling kinetics, mechanical and rheological properties, degradation rate, and other properties [2]. Due to their features, SAPs have been attached attention since the first research works were published in 1966–1967 in the USA. The first SAPs were graft copolymers of starch and partially hydrolyzed polyacrylonitrile. The copolymers obtained could swell up to 400 times in water, while the common absorbing materials used in these times, had a swelling ratio of 20 [3, 4]. After that, research in this field was started in Japan [5] and subsequently spread throughout the world. Based on the type of raw materials used to produce SAPs, they can be divided into three main groups: (i) synthetic SAPs, which are obtained using synthetic compounds; (ii) natural SAPs, obtained from natural polymers or precursors; (iii) semi-synthetic SAP combining both natural and synthetic fragments. The unique absorbing property of the SAPs is due to the three-dimensional polymeric network containing the functional groups reacting with the liquid molecules. So, the design of the SAP network is one of the key moments to create effective water-absorbing materials.

2.2 Methods of the SAP Cross-Linking 2.2.1 Chemical Cross-Linking The synthetic and semi-synthetic superabsorbent polymers are widely synthesized through the cross-linking process and it can be done either by the physical or chemical process. A great part of the industrially produced SAPs is chemically cross-linked, so it will be the first observed way of SAP obtaining.

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The most frequent technique used to obtain chemically cross-linked SAPs is free radical polymerization. It’s a cheap and fast process providing products with excellent properties. The conventional free radical copolymerization occurring during SAP formation includes the following steps: initiation, chain growth, and termination. In this process, free radicals are obtained and react with comonomers and cross-linker forming the 3D polymeric network. In the polymerization system, free radicals are formed by various mechanisms determined by the type of initiation. The most popular ways to initiate the SAP production by radical polymerization are substance-initiated and various radiation treatments, e.g., microwave, radiation, plasma, electron beam, etc. As a substance initiator, persulfates such as potassium (PPS), sodium (SPS), or ammonium (APS), are most often used. Moreover, to mitigate the oxidizing effect of persulfate, it is used in a mixture with reducing agents including sodium metabisulfite (SMB) or tetramethylethylenediamine (TMEDA) [6]. For the synthesis of the polysaccharidecontaining SAP cerium (IV) salts are often used [7]. In such polymerization, an additional stage appears which is graft polymerization of (co)monomers to polysaccharides backbone. Like any component of the reaction mixture, the substance initiator affects the properties of the resulting products. In [8] describes the effect of initiator concentration on porosity, mechanical and structural properties of acrylate superabsorbents. If the concentration of the initiator or initiator mixture is increased, the length of the resulting polymer chains and the elasticity of the gels are reduced with an increase in the gel turbidity. However, if the concentration of the initiator is high, it will result in the formation of short polymer chains, which will lead to difficulty in gelation. In the case of a decrease in the amount of initiator, it increases the elasticity and length of the polymer chain, but polymerization slows down, which allows the polymerization mixture to be enriched with oxygen. In total, this leads to the formation of a gel with high porosity and low mechanical strength. Experimentally it was found that the best results are achieved when using a mixture of persulfate-TMEDA at a concentration of 1–10 mM. Thus, by varying the concentration of the initiator or initiating mixture, it is possible to obtain a superabsorbent with the desired mechanical properties and porosity [8, 9]. Moreover, the type of initiator mixture and its amount also influence monomer conversion for SAP synthesis. Monomers which are commonly used for SAP synthesis are mostly toxic. This imposes additional restrictions on the content of unreacted ones in superabsorbent polymers used in agriculture and the production of personal hygiene products. Kabibri and co-authors [6] show that the residual monomer concentration and swelling capacity of acrylic SAP is strongly dependent on the type and concentration of the initiator. Initiation system dissociation rate was recognized as a key factor to obtain SAP with low residual monomer. It was found that in aqueous solution polymerization, the effect of a slowly dissociating system such as APS/TMEDA on decreasing the residual monomer was much higher than that of a rapidly dissociating system like APS/SMB. In the optimal conditions, residual monomer could be decreased up to 5327 ± 138 and 1715 ± 44 ppm for APS/SMBS and APS/TMEDA initiating systems, respectively. Moreover, it is worth

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emphasizing that in the industrial production of superabsorbent polymers, cheap and available ammonium or potassium persulfates are most often used. In laboratory practice, the Fenton reagent, which is a blend of iron (II) ions and hydrogen peroxide also a popular initiator. Polymerization under the radiation action does not require the introduction of additional components into the reaction system. Exposure to high-energy radiation promotes the formation of radicals on compounds containing C=C bonds. Peppas and Mikos showed that using high-energy radiation, superabsorbents based on acrylic monomers could be obtained. It has been shown that the irradiation dose is a decisive factor influencing the cross-link density, which affects the physicochemical characteristics of the resulting polymers, such as swelling and release kinetics [10]. Most often, 60 Co is used as a radiation source [2]. The published data presents many examples of obtaining water-swellable network polymers obtained using radiation. But most of the works are devoted to the synthesis of copolymer hydrogels based on acrylic acid copolymers [11]. Using radiation initiation, it is also possible to obtain superabsorbents with a porous structure [12, 13]. Electron beam irradiation is also widely used as a reagent less initiation. Due to the milder effect on the reactants used, it is applied to create superabsorbents with controlled structure, viscoelasticity, and thermal stability of polymers obtained from natural polymers, such as gelatin, agarose, carboxymethyl cellulose [14–16]. A combination of these two initiation methods has also been successfully used to produce a stimuli-responsive superabsorbent from vinyl methyl ether [17]. Photopolymerization is a process in which radicals are formed as a result of exposure to UV, visible, or IR radiation. This is also one of the varieties of radical polymerization, but photosensitive monomers or photoinitiators reacting in the conditions are required. These substances interact with radiation and induce radicals [18]. The radical formation can occur via photocleavage, cationic photopolymerization, and extraction of hydrogen atoms. Photopolymerization has the advantage which being a controlled gelation process. The main parameters affecting the efficiency of the process are exposure time and irradiation intensity. Superabsorbents based on carboxymethyl cellulose, sodium acrylate, and N,N-dimethylaminoethyl methacrylate were obtained by photopolymerization [18–20]. The main reason for SAP swelling is the hydrogen bond formation with water molecules. Therefore, the raw materials used for SAP synthesis must be able to form such bonds. The most common comonomer for synthetic and semi-synthetic SAPs are acrylates such as acrylic and methacrylic acids, sodium or potassium acrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, N-isopropylacrylamide, acrylonitrile, methyl methacrylate, 2-aminoethyl methacrylate, etc.; dicarboxylic acid derivatives such as maleic or itaconic acid [21]. However, ionogenic comonomers are preferred for hydrophilic SAP creation due to their higher swelling ability. Bouranis et al. have proposed the use of sulfonated polystyrene to provide a yield-enhancing superabsorbent in lettuce and tomatoes [22]. Other sulfonated derivatives of vinyl monomers also find use in the development of superabsorbent polymers [23, 24]. Worth noting is the widespread use of hydrophilic N-vinylpyrrolidone to create

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SAPs with excellent water-absorbing characteristics [25, 26]. However, for industrial SAP production, the most commonly applied comonomers are acrylate salts and acrylamide [27]. By including special links with stimulus-sensitive properties in the SAP composition, it becomes possible to obtain “smart” superabsorbents capable of changing their water-absorbance characteristics under the influence of the environment, such as pH, temperature, ionic strength, magnetic, and electric fields. Pourjavadi et al. developed intelligent pectin-based SAP with pH- and thermosensitive features. The SAP was prepared by free radical solution polymerization, pectin, 2-acrylamido-2methylpropane sulfonic, and acrylic acids were used as raw materials; APS was as an initiator. It was shown that with a change in temperature, pH, or ionic strength of the swelling medium, the equilibrium swelling ratio of the superabsorbent changes in the range of 22–484 g/g. The optimal conditions providing the maximal swelling are 37 ˚C in distilled water. With increasing the swelling medium temperature, the swelling capacity and rate rise. The authors assumed that the rise in swelling with the temperature growth is due to three main factors: increase in flexibility of polymer chains (i) providing better diffusion of water molecules into the superabsorbent network (ii), as well as pH of swelling medium, while the one is decreasing, the hydrogel hydrophilicity enhance (iii) [28]. The pH-sensitive SAPs based on carboxymethyl guar gum, acrylic acid, and acrylamide with potassium persulfate initiator were synthesized by free radical solution polymerization. It was found that maximum swelling achieves at pH 7. At pH = 3, the swelling ratio was low due to weak ionization of the functional groups and the polymer matrix was shrunk. At pH = 9, COOH groups of the SAP were dissociated completely providing the higher ions content in the gel rising electrostatic repulsion, and SAP networks collapse accompanied by the equilibrium swelling decreasing [29]. The SAP containing carboxymethyl agarose sodium salt and acrylamide which are cross-linked with N,N-methylene-bis-acrylamide is also shown pH-responsibility in swelling properties. The maximal equilibrium swelling ratio (ESR) achieved at pH 7.4 was 140 ± 5 g/g, while in an acidic medium ESR is minimal. The swelling properties of the synthesized SAPs were tested in a presence of different salts, namely, KCl, CaCl2 , MgCl2 , and FeCl3 . The saline solution concentrations were in the range of 0.5–1.5 wt%. All types of tested salts have a similar effect on the ESR; it is sharply decreased with a rise in salt concentrations. The ESRs in potassium chloride solutions were 21.3, 19.8, and 18 g/g for the concentrations 0.5, 1.0, and 1.5%, respectively. Minimum ESR was achieved in iron (III) chloride solutions: 4.08, 3.43, and 2.84 for 0.5, 1, and 1.5% solutions. The authors concluded that there is a trend in swelling with an ionic valency change and demonstrates a reverse and power-law relationship between the salt solution concentration and ESR of the superabsorbents [30]. Electro-responsive SAPs are similar to pH-responsive hydrogels as in both cases the sensitivity is related to the presence of ionic groups. An electrical or chemical potential can be created accordingly as ionic groups are attracted by oppositely charged electrodes. Depending on the charges of the ions and the electrodes, this can lead to either an increased or a reduced swelling degree, for example, using sodium alginate-g-polyacrylic acid [31]. Hosseinzadeh and Ramin produced a series

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of dual-stimuli-responsive starch-based superabsorbent nanocomposites (SANCs). The smart SAPs were obtained by the simultaneous formation of magnetic iron oxide nanoparticles (MIONs) and in situ radical solution polymerization of superabsorbents based on poly(acrylic acid-co-acryl amide) grafted onto starch backbones in the presence of graphene oxide (GO) nanosheets. It was found that the incorporation of GO nanoplates in the SANC polymeric network provided a valuable increase in swelling. The authors stated that the results obtained are due to hydrophilization of the SANCs by the GO structure, as well as its developed surface and plane structure had impacts. Moreover, the SNACs showed higher water uptake under a magnetic field provided by the magnetic iron oxide nanoparticles incorporated into the SNAC matrices. Also, the SNACs were characterized by a pH-responsibility in the range of pH values of swelling medium of 3–9. The results obtained demonstrated that the SNACs can act as smart core elements of magnetic-controlled drug delivery systems, sensors, and actuators [32]. Various natural or modified polysaccharides, such as chitin, chitosan, cellulose, carboxymethyl cellulose, carrageenans, gums, and alginates, are most often used as sources of biodegradable raw materials for the creation of semi-synthetic SAPs. The choice of polysaccharides is primarily due to their availability, prevalence, and renewability. In addition, polysaccharide macromolecules contain a large number of hydroxyl groups capable of forming hydrogen bonds [33]. The polysaccharides can be applied not only for chemically cross-linked SAPs but in physically crosslinked materials, which will be discussed in the next sub-chapter. The preparation of semi-synthetic chemically cross-linked SAPs containing polysaccharides requires the presence of a cross-linking agent. The initiating radicals can be generated both by material initiation and under the radiation action. The mechanism of dimensional network formation, in this case, is as follows: the radicals interact with the polysaccharide and monomers initiating graft polymerization. The formation of an SAP network occurs due to the interaction of growing macroradicals with a cross-linking agent. Radical free copolymerization conducting for synthetic and semi-synthetic SAP production often requires the use of cross-linkers, which are multifunctional compounds capable of forming a dimensional network. The bifunctional N,N ' methylene-bis-acrylamide is a common water-soluble cross-linker. Ethyleneglycole dimethacrylate, 1,1,1-trimethylolpropane triacrylate, and tetraalyloxy ethane are used as two-, three- and four-functional cross-linker agents, respectively [34]. Radical polymerization can be carried out by various methods and many of them are used to obtain superabsorbent polymers. The simplest approach to performing radical polymerization is bulk polymerization, which is a process that uses only a mixture of monomers and an initiator. Superabsorbents obtained in this technique are characterized by high mechanical strength, but the viscosity of the reaction mixture sharply increases and the heat resulting in the reaction is hard to spread. This leads to the formation of an uneven structure that differs in its volume in properties. Poly(acrylic acid) and poly(2-hydroxyethyl methacrylate)-based SAPs were obtained in this way. 2-hydroxyethyl methacrylate (HEMA) SAP had better mechanical properties compared to other polymers obtained [35]. In [36] it is reported

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an SAP synthesized by 2-hydroxyethyl methacrylate and sodium acrylate grafted bulk copolymerization. Researching on hydrogel properties, it was found that the pore size and pressure-sensitive adhesion force were dependent on 2-hydroxyethyl methacrylate and cross-linker content. The most common way to obtain hydrophilic superabsorbent polymers is solution polymerization. This is due to that all reagents used for SAP synthesis are soluble or swell well in water. The SAP formation is heterogeneous, so it is called precipitation polymerization. This technique provides easier control of the temperature and the molecular weight of the product obtained compared to bulk polymerization. The viscosity of the reaction mixture is lower than in the case of bulk polymerization. The solution polymerization is well-fitted for obtaining of the SAPs with excellent water-absorption properties. For example, semi-synthetic SAPs enriched in N-succinyl chitosan and N-maleoyl chitosan content were successfully obtained by free radical polymerization in an aqueous solutions. The equilibrium swelling ratio of the synthesized SAPs is up to 1144 g/g and it reduces with the carbohydrate content rise [37]. Also, solution polymerization was applied for the synthesis of the SAPs based on carboxymethyl cellulose and hexaethyl cellulose with the use of cross-linkers and initiators. Moreover, in many papers, acrylamide and acrylic acid were grafted to carboxymethyl cellulose in the condition of solution polymerization [38–41] or for synthetic SAP production [42]. Suspension polymerization is a handy tool when you want to get superabsorbent in the form of granules with a size less than 1 mm. In this process, the initiators and dispersants were dissolved in the aqueous media to form a stable (co)monomer droplet and the polymerization is triggered by radicals obtained from initiator thermal decomposition. The synthesis of the semi-synthetic superabsorbent polymer via inverse suspension polymerization used corn starch, acrylic acid, and acrylamide as reactants, cyclohexane as the continuous phase, N,N ' -methylene-bis-acrylamide as a cross-linking agent, and potassium persulfate as initiator is described in [43]. The average particle size of the SAPs obtained decreased with an increase of contain of Span65 or Span80 dispersant. Some SAP beads based on poly(hydroxyethyl methacrylate) have been prepared by this method [44, 45]. However, to modulate the properties of the resulting SAPs post-synthesis modification can be applied. For example, Ghsari et al. proposed that surface crosslinking is a post-treatment providing the absorbency under load (AUL) increase. Such a process is commonly performed through a conventional heating method. For the first time, the microwave method was applied for the surface treatment of the poly(sodium acrylate) SAPs. Diglycidyl compounds such as polyethylene diglycidyl ether (PEGDGE), ethylene glycol diglycidyl ether (EGDGE), and 1,4-butanediol diglycidyl ether (BDDGE) were applied as surface cross-linkers, and N,N-dimethyl aniline was utilized as a treatment catalyst. It was found that the application of the surface treatment solution catalyst led to AUL growth. The absorbance under a load of SAPs was enhanced from 14 g/g for non-modified polymer to 17.5, 19, and 20.7 g/g for SAPs treated with BDDGE, PEGDGE, and EGDGE, respectively [46]. The same approach was used by Kwon et al. to improve AUL for itaconic acid-based SAPs. Superabsorbent synthesized in the selected conditions had excellent water

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retention with centrifuge retention capacity values of 31.1 g/g and AUL values of 20.2 g/g [47]. For composite semi-synthetic and natural superabsorbents containing polysaccharide links, other approaches besides free radical polymerization are being opened for the creation of a dimensional network. It is well known that carbohydrate polymer macromolecules contain functional groups of various types. Among them are hydroxyl groups, which are characterized by all types of polysaccharides; carboxyl groups, contained in carboxymethyl cellulose, alginic and hyaluronic acids; amino groups presenting in chitosan, chitin, and hyaluronic acid, etc. Due to the presence of these and other types of functional groups, carbohydrates can interact with other compounds including other polysaccharides, and form three-dimensional networks. For example, chitosan reactive –OH and –NH2 groups involve in network formation by the reaction between the reactive groups and –COOH groups by condensation mechanism. The chitosan-based SAPs are the most studied polymers obtained by this method. Chitosan macromolecules cross-linked by dialdehydes, especially by glutaraldehyde, can absorb metal ions better than non-modified chitosan [48, 49]. The aldehyde groups react with the chitosan amine groups resulting in imine bonds that cross-link chitosan. Further was found that the reaction is two-stage: at the first stage glutaraldehyde molecules undergo condensation and dehydration to first form α,β-unsaturated aldehyde polymers, and the second stage is reacting with chitosan to form Schiff’s bases [50]. Another cross-linker commonly used for chitosan is epichlorohydrin [51] and genipin [52]. The chitosan dimensional network can be also obtained in a presence of enzymes such as horse radish peroxidase [53]. However, the chemical cross-linkers are usually toxic compounds, which restricted the use of the SAPs obtained. This prompted researchers to search for new ways to form a superabsorbent dimensional network. SAP was prepared through the cross-linking of chitosan with EDTA-urea adduct without initiators [54]. Chitosanbased SAPs were also synthesized by Diels–Alder click reaction of furan- or maleimide-derived chitosan [55] or by azide-alkyne click reactions [56]. So, it was shown, that the dimensional structure of the hydrophilic SAPs can be formed in different chemical ways. The most common of them is the radical polymerization of acrylates with or without polysaccharides, in a presence of chemical cross-linkers and initiators. Polysaccharide-based SAP can be prepared by different reactions of carbohydrate functional groups.

2.2.2 Physical Cross-Linking Methods Physical methods of dimensional network formation include a molecular assembly occurring by cross-linking with hydrogen or ionic bonds, or hydrophobic interactions between macromolecules. In these conditions, superabsorbent polymers can be obtained at low temperatures, in contrast to methods used at ambient temperatures, and without the use of toxic organic solvents. However, due to the structure formed

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by the weak physical interactions, the mechanical properties of such SAP are lower compared to chemically cross-linked ones. Ionotropic gelation is one of the common and simple ways to form 3D polymeric structures. It is electrostatic cross-linking of the ionized macromolecules by opposite multi-charged ions. The SAPs based on alginic acid are usually obtained by crosslinking sodium alginate aqueous solutions with Ca2+ ions [57]. This process mostly occurs via ionic interactions of the metal cations and carboxyl groups of alginate G-blocks [58]. From the mechanical point of view, ionotropically obtained alginate hydrogels are relatively strong, however, fragile. Depending on the synthetic strategy, the water total amount contained in the alginate hydrogels achieves 99% of the material’s own mass. Moreover, such gels couple the properties of solids and solutions. They have clearly defined interfaces in the aqueous media, and in the swelled state has properties like those of a regular solution [59]. Two techniques are providing crosslinking ions presence in alginate chains: diffusion gelation and internal gelation [60]. In the case of diffusion gelation, multiply charged cations diffuse into the alginate solution. When alginate is introduced into the multi-charged cation medium, gelation occurs on the interfaces, and then the cross-linker ions diffuse inside the alginate solution. The driving force of the process is the diffusion of metal ions inside the resulting 3D network. The forming gels have an inhomogeneous composition, and it is difficult to control the completeness of the reaction. When using the method of integral gelation, a controlled release of the cross-linker ion occurs, which makes it possible to obtain a homogeneous three-dimensional structure. This method commonly uses water-insoluble salts, such as calcium carbonate or calcium sulfate, and the release of the cross-linker cation is achieved by changing the pH of the medium. However, the gelation rate, in this case, is quite low [61]. Moreover, ionized macromolecules can interact with opposite-charged ones resulting in inter polyelectrolyte complexes (IPEC) forming through ionic bonds. The formation of IPECs results from Coulombic interactions of oppositely charged polyelectrolytes, leading to interpolymer ionic condensation. The driving force results mainly from the release of molecular counterions into a polyelectrolyte-depleted solution, which increases the entropy of the system [62]. The other type of physical macromolecular interaction is polyelectrolyte complex (PEC) formation. The difference between PEC and IPEC is the nature of the driving force of the complex formation. PECs formation consists of two or three steps. The first one is an instantaneous and random primary complex with significant distortions of polymer chain configuration obtained. Then, the secondary complex is formed by the rearrangement of existing linkages within intra-complexes. At this stage, the formation of hydrogen and electrostatic bonds, hydrophobic interactions, etc. occur. Moreover, in some cases, complexes obtained can form new non-covalent bonds, mainly hydrophobic interactions, leading to various stable structures: entangled aggregates, fibrils, ordered networks, etc. [63]. Due to the ability of some IPECs to swell in aqueous media, they also can be classified as superabsorbents. For example, the IPEC of chitosan and sodium alginate swells up to 700% [64]. IPECs of chitosan and carboxymethyl cellulose sodium salt may swell up to 1000% and their water-absorbance ability depends on carboxymethyl

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cellulose sodium salt. With an increase in the amount of the latter, the swelling ability also rises [65]. Cationic chitosan can form dimensional structure in a presence of tripolyphosphate (TPP) [66]. Further, it was shown that chitosan-based gel formation strongly depends on the pH of TPP solution. While pH < 6 gel formation occurs due to the electrostatic inotropic process, and at pH > 7 coacervation is dominated [67]. Cross-linking can also proceed by hydrogen bond formation. Such interactions are present between OH-groups in cellulose macromolecules and can form H-bond with water molecules to create a dimensional structure [68]. Such SAPs are very sensitive to changes in the concentration of polymers, temperature, solvent nature, and the degree of association of functional groups. The higher concentration of polymers promotes more hydrogen bonding and a more rigid and stable gel prepared [69]. Olad et al. reported SAP production by mixing the polyethylene glycol and carboxymethyl cellulose solution, forming hydrogen bonds between the oxygen of glycol and hydrophilic OH-group of cellulose derivative [70]. The strong hydrogen bonds may be also formed during the freeze/thaw cycles: either during initial reagent freezing or storage of the prepared SAPs in the frozen state or during the thawing of the frozen ones. It is the cheapest and simplest process applied to obtaining natural SAPs. This process is need to be repeated two or more times the achieve stable physically cross-linked SAPs. The freeze/thaw cycle method is used mostly for preparing SAPs based on cellulose or its derivatives. Guan et al. [71] described SAPs were prepared from hemicelluloses, polyvinyl alcohol, and nano-chitin through 0, 1, 3, 5, 7, and 9 times of freeze/thaw cycles. It was found the increase of freezing/thawing cycles makes the SAP structure stiffer, the thermal property becomes more stable, and a crystalline degree becomes higher. However, at the third freeze/thaw cycle, the swelling ability of the SAP decreased due to the formation of the lamellar structure. Hydrophobic interactions can lead to dimensional structure formation. Supper et al. reported the mechanism of chitosan gelation through interactions with βglycerophosphate, glucose-1-phosphate, and glucose-6-phosphate. The presence of the polyol phosphates raises the pH of chitosan solution, and gelation or phase separation has not occurred. It was concluded that this is due to the proton transfer from chitosan polycation to the polyol phosphates and the formation of a protective hydration layer of polyol phosphates around the chitosan chains. With an increase in temperature to 37 °C, the protective hydration layer is destroyed, and it is leading to gelation due to hydrophobic interactions between the chitosan macromolecules [72]. Gelation of chitosan in a hydroalcoholic medium consisting of alcohol, preferably an alkane diol, water, and hydrochloric acid has also been studied. Apart from hydrophobic interactions, hydrogen bonding was also theorized to be responsible for the formation of the three-dimensional network [73]. Alkylated cellulose polymers demonstrate hydrophobic interactions between the OH-group and the alkylated chain. Cellulose gets hydrated at low temperature while the OH-group makes interaction with each other and turns to hydrogel at high temperature [74]. Physically cross-linked SAPs also may possess a stimuli-responsibility such as thermo- or pH-sensitivity. The thermo-responsible hydrogel of chitosan and agarose

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was prepared by the dissolution of agarose in chitosan-acetic acid solutions with different concentrations at 50 °C. The hydrogels obtained in this study gel form when the temperature reduced from 50 °C to 37 °C becoming transparent to opaque. The thermo-responsive behavior of the hydrogels obtained is due to agarose undergoing a reversible gelling process retaining its rheological properties. With temperature reductions, agarose polymeric chains change conformation from random coils to double helices, which interact with each other forming a physical gel [75]. Various PECs may be characterized by pH-sensitivity in aqueous media, especially if they consist of weak polyelectrolytes. The pH values of the medium have a significant influence on the ionization of the ones, which, in turn, affects the interactions of weak polyelectrolytes and their phase separation [76]. Wu & Delair [77] have found that the PEC of hyaluronic acid and chitosan undergoes a decrease in particle size and electrophoretic mobilities in the pH range of 4–5.5. The authors associated these facts with the deprotonation of the chitosan primary amino groups initiating the shrinking of the hydrogel particle chitosan shell. Meng et al. successfully fabricated novel chitosan–sodium polyacrylate (PAAS) polyelectrolyte complex hydrogels (CPG) by interacting chitosan and sodium polyacrylate with epichlorohydrin cross-linking agent by inhibiting the protonation effect of chitosan in alkali/urea aqueous solution. The swelling features of the hydrogel obtained with different compositions were investigated systematically in different media. The ESR of chitosan hydrogel in water sharply rises from 46.3 to 404.8 g/g with an increase in the polyacrylate content. The semi-synthetic complex hydrogels are characterized by different water uptake depending on the pH values and salt concentration of the swelling medium indicating smart-polymer properties. Also, synthesized CPGs have adequate mechanical strength, good biocompatibility, and the possibility of in vitro biodegradability [78]. Hu et al. obtained self-assembled different composition salecan/chitosan lactate (CL) IPEC hydrogels without any toxic cross-linker agent use. The preparation process was rapid and reliable, the duration of complex preparation was about 2 h. It was shown that a driving force of the self-assembly process was electrostatic interactions of carbohydrates. The obtained PEC hydrogels had a well-defined porous structure with excellent water-absorbance properties. The water uptake was raised as the salecan/chitosan lactate proportion grew. At pH 6.8, hydrogel had the lowest water absorbance of 57.0 g/g, and the highest one was 76.6 g/g. The polymer also demonstrated pH-responsive swelling: with the pH values increase, the hydrogel ESR rises. Also, it was found that the water uptake of the hydrogels obtained significantly depends on polymer micromorphology [79]. Devi & Kakati fabricated the porous microparticles of physically cross-linked hydrogel with different particle sizes were fabricated by polyelectrolyte interactions of two biopolymers, namely, sodium alginate and gelatin. The hydrogel microparticles are characterized by pH-sensitivity reflected in particle size changes. The particle size on swelling at pH = 7.4 was twice that at pH = 1.2 demonstrating the pH-sensitivity [80]. Physical cross-linking is a common way to obtain SAPs formed by hydrogen bonds, electrostatic or hydrophobic interactions, or their cooperative action.

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2.3 Strategies to Improve the Swelling Ratio of the Semi-Synthetic SAPs Equilibrium swelling ratio which is the maximum amount of liquid that a superabsorbent can absorb and hold is a key characteristic. Therefore, in most research devoted to superabsorbents, the influence of various factors on this parameter is studied in detail. Among the most frequently studied [81–85] are the composition of the comonomer mixture, the amount of the cross-linking agent and initiator, and the amount and the nature of the biodegradable component. A published data analysis shows that for synthetic superabsorbents based on acrylic monomers only, the dominant factor determining the value of the equilibrium swelling ratio is the content of the cross-linking agent in the initial polymerization mixture. For semi-synthetic superabsorbents containing both acrylate monomers and biodegradable components, such factors are the content of the cross-linker, the nature and the amount of the polysaccharide. A detailed explanation of the impact of these parameters is done by Suo et al. [86]. In his work, the semi-synthetic SAPs based on carboxymethyl cellulose, acrylic acid (AA), and acrylamide (AAm) with different compositions were prepared by solution radical polymerization. N,N ' -methylene-bis-acrylamide (MBAAm), and potassium persulfate–sodium metabisulfite mixture were used as cross-linker and initiator, respectively. The research on initiator concentration influence on water absorbance shows that the equilibrium swelling ratio of the SAPs increases to achieve a maximum of 910 at 1 wt% initiator concentration, and then decreases with an initiator amount growth (Fig. 2.1A). The authors attribute this to the fact that a smaller initiator amount is not enough for effective dimensional network formation, which leads to low swelling. In the case of using large amounts of initiator, low molecular weight polymers are formed, and side processes such as homopolymerization also occur, which also reduces the swelling ratio. The optimal concentration of the MBAAm cross-linker is 0.75 wt% providing maximal water absorption (Fig. 2.1B). At the MBAAm content interval of 0.1–0.75 wt%, the absorption capacity is growing due to the formation of a cross-linked graft copolymer with a 3D network and another cross-linked insoluble homo- or copolymers. At MBAAm content higher than 0.75% wt a sharp decrease of swelling ratio is observed ascribed to forming the densely cross-linked structure, which cannot be expanded and hold a large quantity of water. The influence of the comonomers’ ratio also was discussed (Fig. 2.1C). With an increase of AA content to AAm, the equilibrium swelling ratio rises. This is due to the non-ionogenic nature of AAm, which can form internetwork hydrogen bonds acting as additional cross-linkers and increasing the rigidity of the resulting network [86]. The effect of polysaccharide nature and its content on the equilibrium swelling ratio of the semi-synthetic SAP is researched in [37]. The authors obtained SAPs based on AA, AAm, and chitosan or water-soluble chitosan (Cht) compounds, namely N-succinoyl chitosan (NSCht) and N-maleoyl chitosan (NMCht). It was shown that chitosan-derivative-based SAPs are characterized by a higher equilibrium swelling

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Fig. 2.1 The influence of initiator (A) or cross-linker (B) content and comonomer ratio (C) on water absorbance of the SAP based on carboxymethyl cellulose. The figures are from Suo et al. paper [86]

ratio compared to Cht-based ones. Due to their structure, the affinity to the water of NSCht and NMCht is higher compared to Cht. However, it was found that despite the type of carbohydrate used, the equilibrium swelling rate of the SAPs obtained is decreased with an increase in polysaccharide content. These data correlate with results obtained by other researchers [87], and the reason for this trend is inter- and intramolecular hydrogen bonds the formation of which is characteristic of polysaccharide macromolecules. This increases the overall rigidity of the SAP network and restricts its ability to stretch, accumulate, and retain water [87]. So, it can be concluded that high network rigidity caused by high cross-linker content or the formation of additional internetwork hydrogen bonds is the main reason for decreasing the water-absorbance ability of the SAPs. On the other hand, a common approach to controlling polymer chain/network rigidity properties is the use of plasticizers. By reducing intermolecular and intersegmental interactions, they change the structure and some important polymer characteristics, e.g. the glass transition temperature, crystallinity, etc. [88–90]. Lin et al. [90] investigated the effect of industrial plasticizers, namely dibutyl phthalate, diethyl phthalate, glycerol triacetate, and tributyl citrate, on the equilibrium swelling ratio of Eudragit resins based on (meth)acrylates. It was shown that in the presence of the hydrophilic plasticizer glycerol triacetate, the swelling ratio increases by more than two times. The review [89] provides information that the use of various structure plasticizers has a positive effect on both the moisture absorbing properties and the mechanical properties of polysaccharide films. The influence of dibutyl succinate (DiBuSuc) plasticizer on the equilibrium swelling ratio of SAPs based on a sodium salt of carboxymethyl cellulose (NaCMC) was researched in [91]. The semi-synthetic SAPs containing 10 or 20 wt% of Na-CMC with a molecular weight of 10 000 were produced by free radical solution polymerization with the use of PPS initiator, MBAAm cross-linker; AA, and AAm comonomers. DiBu-Suc in the amount of 0, 2, 5, and 10 wt% was milled with SAPs obtained. It was shown that the presence of 5 wt% of DiBu-Suc is optimal and increases the equilibrium swelling ratio to 21 and 51% for SAPs containing 10 and 20 wt% of Na-CMC, respectively. The authors explained this by the selective interaction

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Fig. 2.2 The mechanism of equilibrium swelling ratio growth in a presence of dibutyl succinate. The figure is from paper [91]

of dibutyl succinate with moieties of the SAP network forming intramolecular Hbond increasing the flexibility of the polymeric network and the availability of water sorption centers (Fig. 2.2) The hypothesis was confirmed by FTIR spectroscopy, glass transition temperature shifts measured by differential scanning calorimetry, and XRD [91]. Another approach positively affecting the superabsorbent equilibrium swelling ratio is to increase the availability of polymer network centers interacting with water molecules by increasing the polymer surface area. Kabiri et al. [92] used porogens to increase the SAP surface. The synthetic superabsorbent hydrogels based on partially neutralized acrylic acid and acrylamide with adding acetone, sodium bicarbonate, or their mixture in the reaction feed were prepared. In comparison with an SAP obtained without porogen, acetone, and sodium hydrocarbonate increased the ESR up to 43–55% and 111–131% of the blank sample, respectively. The application of the porogens cooperatively, a significant synergistic effect in the swelling rate occurred: reaction blends were foamy and synthesized products are characterized by the clearly defined porous structure. However, when one porogen was utilized, the foam formation stage was demonstrated only in a part of the synthesis. The apparent volume of the as-synthesized porous products obtained with two porogens was more than that of the ones produced with one porogen. The scanning electron microscopy research of the SAP surface morphology confirmed that the use of acetone and sodium hydrocarbonate cooperatively formed highly porous and developed structures (Fig. 2.3). Kuang et al. also synthesized SAPs with sodium bicarbonate porogen and poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymer (PF127) stabilizer use [93]. Modified starch or the one in combination with acrylic acid was used as raw materials for preparing natural SAPs or semi-synthetic SAPs denoted as SPH/ASS or SPH/ASS-AA, respectively. APS-TEMED mixture acting as an initiator. The authors showed that porogen use allows for achieving a uniform porous structure of the SAPs (Fig. 2.4). Due to the porous structure, synthesized superabsorbent hydrogels reach an equilibrium swelling during 60– 440 s. However, water-absorbance ability decreases with an increase in modified starch content despite the hydrophilic starch modification and porogen presence.

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Fig. 2.3 The SEM images of the surface of the SAPs were prepared without porogens (A), in a presence of acetone (B), sodium bicarbonate (C), and their mixture (D). The figures are from [92]

It is well known, that the methods used to isolate and dry a polymeric material can largely impact its final physical and chemical characteristics. An analysis of the published research data shows that the most common method for drying the SAPs obtained is convection treatment with a stream of warm air with preliminary exposure of the polymer in C1-C2 alcohols or acetone to remove moisture excess. However, drying under such conditions can take quite a long time, up to several days, which is unprofitable in industrial scale and slows down laboratory research. A promising approach to removing excess water is freeze drying, which maintains the position of the molecules as in a solution or swollen state, thus leaving the active sites more accessible for interaction with water. Etminani-Isfahani et al. used a lyophilization of the synthesized semi-synthetic SAP to create a porous structure. The synthesized SAP is characterized by excellent water-absorbency properties reaching an equilibrium swelling of 800.37 g/g in deionized water and 78.02 g/g in 1 wt% NaCl solution [84]. Thus, it was shown that the nature and quantity of used reactants have a significant impact on the water-absorbance properties of the semi-synthetic superabsorbent polymers. To improve it, the use of plasticizers, porogens, lyophilization, or a combination of them will make it possible to influence the structure of semi-synthetic superabsorbents and increase their equilibrium swelling ratio. Thus, this chapter shows that by varying the composition or production techniques of superabsorbent polymers, it becomes possible to influence their various practically significant properties and, first of all, the equilibrium swelling ratio.

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Fig. 2.4 Photographs (a) and SEM micrographs (b) of dried starch-based SAPs. The figure is from [93]

This work is supported by the Council on Grants of the President of the Russian Federation for State Support to Young Russian scientists—Candidates of Sciences, Grant number is MK-2517.2022.1.3

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Chapter 3

Experimental Methods of Superabsorbent Polymers: Characterization Preeti Gupta and Roli Purwar

Abstract Superabsorbent polymers (SAPs) have proven to be very promising materials in all areas where there is a need to retain and/or release water or aqueous medium as required, such as agriculture (water reservoir), drug delivery, baby diapers, pollution control, sanitary napkins etc. The basic chemistry of SAPs is provided by FTIR analysis while the connectivity and the foreign species present are confirmed by other techniques of instrumental analysis like NMR, EDX, OES, AAS etc. Their particle size distribution is assessed by laser diffraction. An essential feature for SAPs is their ability to hold huge amounts of water or aqueous medium in comparison to their own weight. Their morphology in dry, as well as swollen state, is observed using SEM and ESEM, but these techniques provide only qualitative information. The quantitative information about their swelling and de-swelling is obtained with the help of swelling kinetics. Therefore, SAP samples are pre-tested for their absorptivity as well as the kinetics of absorption and desorption. In this chapter, we emphasized not only the basic analytical techniques and sorptivity tests, but also focused on thermodynamics as well as rheological tests. Tests showing their porosity, soluble fraction, salt resistance, residual monomer, ionic sensitivity and degradability have also been discussed. Keywords Polymer characterization · Sol–gel fraction · Gravimetric estimation

3.1 Introduction The enormous de and re-absorption capacity and shape retention of SAP’s (superabsorbent polymers), the hydrophilic admixtures, discriminate them from other absorbent materials and hence they found a number of applications in various areas such as agriculture (water reservoir), drug delivery, baby diapers, pollution control, sanitary napkins, cement-based construction material, as scaffolds in tissue P. Gupta · R. Purwar (B) Discipline of Polymer Science and Chemical Technology, Department of Applied Chemistry, Delhi Technological University, Bawana Road Shahabad, Daulatpur Delhi 110042, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Arpit and T. Jaya (eds.), Properties and Applications of Superabsorbent Polymers, https://doi.org/10.1007/978-981-99-1102-8_3

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Fig. 3.1 Characterization of Super Absorbent Polymers

engineering, in biosensors etc. The SAPs designed for such applications should be pre-tested for their quantitative as well as qualitative aspects as shown in Fig. 3.1. Some of these tests are common to all types of SAPs while others are application based. Therefore, we can classify these tests in the following manner: (i) (ii)

General characterization (Regular tests) Application specific characterization.

In this chapter, we will discuss about these tests one by one. We will also explore the limitations, if any, of these tests.

3.2 General Characterization (Regular Tests) Some tests such as structural characterization, morphological studies, swelling and de-swelling studies, porosity, degradability, ionic sensitivity etc. are common for all types of SAPs and hence categorised as regular tests. In the following section, we will discuss how and by using which techniques these tests are performed. 1. 2. 3. 4. 5. 6. 7.

Solubility of the SAP sample Structural characterization & morphological studies Swelling and de-swelling studies Network parameters Swelling kinetics Mechanical characterization (Quantitative analysis of free and bound water) Other tests.

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3.2.1 Solubility of the SAP Sample or Sol–Gel Fraction The total fraction of an SAP sample that dissolves in water such as non-cross-linked oligomers, unreacted monomers etc. is referred as its sol fraction. It can simply be determined by extracting the SAP samples in distilled water in the following two ways: (i)

In one method a weighed quantity of the SAP sample is immersed into distilled water (to form a dispersion of about 1% concentration) for about 16 to 48 h at room temperature and then the sample is taken out and dried to obtain its dry weight. The loss in the weight of SAP sample gives the soluble fraction of the sample [1]. The gel fraction is then calculated using the following relation [2] Gel fraction =

(ii)

Wd × 100 Wi

(3.1)

where W d is the weight of SAP sample after extracting with water and drying and W i is the initial weight of the dry SAP sample. Centrifugation method—In this method, the sample is dispersed into the distilled water to prepare a 12% solution by weight. It is then centrifuged for about 25 min at the rate of 2500 rpm. The solution is filtered using a glass fiber filter of known weight (pore size 1.2 microns). The weight of the filter is determined after drying it to 105 ºC for 1h and then cooling it in a silica gel based desiccator to remove the moisture (if any) completely. The filter containing SAP sample is again dried under similar conditions (i.e., at 105 ºC for 1h followed by cooling in a silica gel based desiccator) and weighed when a constant weight is reached [2]. The gel fraction is then calculated using the following relation Gel fraction =

W1 − W2 × 100 C

(3.2)

where W 1 is the weight of filter, W 2 is the weight of filter containing SAP sample and C is concentration of the solution. Depending upon the particle size of SAP sample, filter with different mesh size can be used.

3.2.2 Structural Characterization and Morphological Studies Following techniques can be used for the structural characterization and morphological studies of the SAP sample: (i)

Fourier transform infrared (FTIR) technique—The chemical structure of SAP samples and any change in it such as introduction of cross-links, presence

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(iii)

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of foreign materials like metals, dyes, drugs etc. is identified and confirmed by this technique. This method is based upon the excitation of the bonds present in the structure of sample by absorbing IR radiations of corresponding frequencies. The FTIR spectra provides a fingerprint of the sample. The sample is prepared by KBr pellet method [3] but used directly when FTIR is used in ATR (attenuated total reflection) mode [4, 5]. It is usually the case with films or the sample which cannot be crushed to turn into pellet form. In KBr pellet method, a small amount of finely pulverized sample is mixed with KBr (or any other alkali metal halide like CsI, which is generally used in low wavelength regions) and kept into a pellet shaped die. On applying a force of about 8 tons under vacuum, pellet is formed which is used for scanning purposes. Formation of a broken pellet indicates that vacuum is not sufficient. While measurements are taken, first start an empty run to make the background correction. The scanning is usually performed in the frequency range of 500–4000 cm−1 and at a resolution of 4 cm−1 . By comparing the bands of two samples e.g., cross-linked and non-cross-linked samples or an SAP sample before or after the adsorption of pollutant etc., the introduction of cross-links or foreign materials can be confirmed [6]. Scanning electron microscopy (SEM)—This technique is useful for observing surface topography, porosity, composition and other properties like conductivity etc. of the SAP sample [7]. In this method, first, the dry SAP sample (which is generally non-conducting) is made conducting by gold sputter coating and then fixed using an adhesive tape. (In case of conducting samples this step is not required). The sample is then mounted into the SEM and SEM micrographs are obtained. The SEM images are generally analysed using J-image software for the particle size, pore size, fiber diameter etc. The swollen sample is first freeze dried and then subjected to the above procedure. Comparison of the SEM images of dried and swollen samples clearly indicate the change in the morphology of the SAP sample in wet condition [6]. Environmental scanning electron microscopy (ESEM)—This technology is more useful for studying the morphology of swollen SAP sample. In addition to morphology, it also provides percentage elemental composition and its distribution on the surface of SAP samples. The swollen SAP samples are mounted on the carbon disks which are supported by the aluminium stub. The disks are then kept on the ESEM in cooling mode. The temperature of the equipment is adjusted to 3–5 ºC and pressure is maintained above the saturated water pressure (0.76 to 0.84 kPa). The micrographs are observed in the vapor pressure range of 0.33 to 0.56 kPa. The micrographs are collected using a secondary electron detector in gas form [8]. Transmission electron microscopy (TEM)—This technique is mainly used to study the morphology of nano-sized particles and/or very small quantities of the sample. TAM has better resolution (50 pm) and much greater magnification (10,000,000×) than SEM. In this method, the sample is dissolved in a suitable solvent like ethanol and then taken into a carbon coated copper grid for testing.

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(v)

(vi)

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The grids are mounted into the TEM instrument and set up is started. The TEM micrographs are observed, analysis of which gives a clear topology of the sample [9]. Atomic force microscopy (AFM)—It gives detailed information about the nanometer range events. The scanning is done using a multimode scanning probe microscope having a nanoscope IIIa controller and EV scanner of 125 μm (Veccon instrument. Inc). The scanner is calibrated along x–y plane with the help of 1 μm etched glass and along z-axis using 200 nm height as a standard for calibration. This method is good enough in “tapping mode” in the case of soft samples because the lateral forces are totally reduced in this method. Silicon nitride (NP-20, Veeco) and silica tips (TESP, Vecco) are used to apply light tapping for contact mode. In this mode, the force applied to maintain the contact of the probe and the scanning surface should be minimum [10]. The test sample is prepared by dissolving 10 mg of the SAP sample in one liter of millipure or ultrapure water and 5μL of it is taken in a freshly cleaved mica with the help of a pipette. The excess of water is removed by either enclosing the mica in a petri dish for 24 h at 50% relative humidity or flowing nitrogen. A root mean square voltage of about 2.5–3 V is applied and set point is adjusted to obtain a clear image. The set point is usually about 1 V less than the root mean square voltage. The scan rate is fixed at 2–3 Hz to obtain height and amplitude. The images can be processed with the help of a nanoscope version 4.43 r8 software. These images are analysed to obtain the particle diameter of the sample. Confocal laser scanning microscopy (CLSM)—This technique gives a complete idea about the SAP morphology in aqueous or solvent media. It is a powerful tool for analysing pore size of the SAP sample [11]. This technique is used in two modes- reflectance mode and fluorescence mode.

In this technique, the swollen sample is examined at a certain sample penetration depth (say 100 μm) with the help of 20× dipping lens. During the whole experiments, the sample is kept immersed into the clear solution. Scanning gives a native 3-D image of the SAP sample when cross-sectional image is taken along z-axis. In the fluorescence mode, the samples are excited by the light (of certain wavelength) using a filter of 500–550 nm. The pore size is analysed using J-image software. Topography of the sample is obtained by constructing the 3-D structure from the pieces of the image.

3.2.3 Swelling and De-Swelling Studies In this section, we will discuss the various methods that can be used for testing the swelling and de-swelling properties of a SAP sample. These methods are of

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great significance from the practical point of view for the academic and industrial researchers. (i)

Gravimetric method—It is a highly convenient, fast and conventional method that can be used when the sample is available in limited quantity (0.1 to 0.3 g). In this method, a bag made of non-woven fabric, generally a tea bag (that are commonly available), is used. The bag is pre-wetted with the test liquid and then placed in another dry cloth to remove excess or weakly bound water or liquid. The bag is then weighed and a calculated quantity of SAP sample was kept into it. The bag is inserted into the test liquid (usually water or buffer of definite pH or saline solution) for a fixed period of time. Then the bag is hanged to remove excess of water until no liquid drop is dropped off and weighed. The swelling capacity is then calculated by using the following equation S=

(ii)

(3.3)

where W 0 is the initial weight of tea bag containing sample, and W 1 is the weight of the tea bag after removing excess water [1]. Centrifuge method—This is the more accurate and more widely reported method as most of the inter-particle liquid is also being removed during centrifugation but the swelling values obtained from it are lower. Unlike the tea bag method here a non-woven fabric bag is used in place of tea bag and the excess of water is removed using a centrifugal separator. First, the procedure is done with empty bag and then repeated by taking sample in the bag. The swelling capacity is calculated by putting the weights obtained in the following equation: s=

(iii)

W1 − W0 W0

W2 − W0 − W1 W1

(3.4)

where W 0 = Weight of the empty bag, W 1 = Weight of the SAP sample, W 2 = Weight of swollen bag with sample [1]. Sieve method—This method can be used only when sample is available in excess. In this method, SAP sample is poured in excess of test liquid and stirred with a mild magnet till the equilibrium swelling is achieved. The sample is then filtered through a weighed 100 mesh wire gauze (sieve). The excess water is then removed using a soft open cell polyurethane foam (For this the foam is rubbed under the gauze and squeezed) till the gel loses its sleepiness when sieve is held vertically. The standard deviation of this method was found to be ±2.1%. The swelling capacity in this method at any time t is calculated by using the following equation

3 Experimental Methods of Superabsorbent Polymers: Characterization

S=

(iv)

(W2 + W0 ) − (W0 + W1 ) W1

47

(3.5)

where W 1 = Weight of dry SAP sample, W 2 = Weight of swollen SAP sample, W 0 = Weight of the sieve [1] Tissue paper wiping method—This method is mostly used while testing the swelling behaviour of hydrogel films and to study their swelling kinetics. In this method, the weighed SAP sample of fixed dimensions (3 × 4mm2 ) is plunged into the test liquid and after a regular time interval the sample is taken out of the test liquid, wiped between the layers of tissue paper and weighed. The process is repeated till the equilibrium swelling (constant value of swelling) is not obtained [12]. The percentage swelling is calculated using the following formula Percent swelling, S% =

(W2 − W1 ) × 100% W1

(3.6)

where, W 2 = Weight of swollen SAP sample and W 1 = Initial weight of dry SAP sample. / Swelling ratio(α) = S% Seq

(v)

(3.7)

where Seq = Equilibrium swelling of the SAP sample at a certain temperature and pH. Optical microscopy—This is a good method for providing in-depth knowledge of the variation of the internal structure of the sample as a function of time during the course of swelling. The pH sensitive behaviour of the SAP samples is determined using the Nikon AZ100 microscope (Surrey UK) with a numerical aperture of 0.5 × 0.05. Its lens is fitted with a green excitation fluorescence filter and a dichromatic mirror [8]. It works on the principle of change in surface area on swelling. For testing, the SAP sample is placed in the buffer solution (of test pH) at a fixed temperature and its surface area is measured at different time intervals. Knowing the surface area, the optical swelling ratio is calculated using the following equation: φ0 =

At − A0 At

(3.8)

where A0 is the initial surface area of the sample and At is the surface area at time t. Thus, we can determine the optical swelling ratio at different time periods. Change in fluorescence density is predicted with the help of LCS software version 2.6.1.

48

P. Gupta and R. Purwar

3.2.4 Network Parameters In order to justify the swelling behaviour of SAP samples, their network parameters like number average molecular mass between cross-links (Mc ), cross-link density (q) and mesh size (ξ) are calculated in the following manner. (i)

Number average molecular mass between cross-links (Mc )

For calculating M c first measure the volume of the SAP sample in dry and swollen states. The ratio of these two volumes gives the polymer volume fraction of the SAP sample. (If the SAP sample is in the form of a film, measure its volume from its dimensions) [13]. Polymer volume fraction, vf =

vd vs

(3.9)

And calculate the specific volume of the polymer using the following formula Specific volume of the polymer v =

volume of the polymer mass

(3.10)

Knowing the values of vf and apply the following equation to determine the number average molecular mass between cross-links (Mc ) [13–15]. 1 =− Mc

ν vm

[ ( ] ) ln 1 − v f + v2,s + χ v2f ( 1 ) v 3f − 21 v f

(3.11)

where, V m = molar volume of water≅ molar mass of water = 18.1 g mol−1 [16] χ = Flory Huggins polymer –water interaction parameter ≅ 0.5 [16, 17], υ = Specific volume of the SAP sample, υ f = SAP volume fraction in the swollen condition. (ii)

Cross-link density, q

To determine the cross-linking ratio, q first calculate the molar mass of the repeating unit from the moles and molar masses of monomers and cross-linkers using the following relation [18]: Σ nM Mr = Σ n Or Mr =

n 1 × M1 + n 2 × M2 + n 3 × M3 n1 + n2 + n3

(3.12) (3.13)

where n 1 , n 2 , n 3 are the number of moles of monomer 1, monomer 2, and cross-linker respectively and M1 , M2 , M3 are the molar masses of monomer 1, monomer 2, and cross-linker respectively.

3 Experimental Methods of Superabsorbent Polymers: Characterization

49

Knowing the values of M r and M c the cross-link density, q is calculated in terms of mole fraction of cross-linked units using the following relation [19]: q= (iii)

Mr Mc

(3.14)

Mesh size (ξ)

Following relation is used to determine the mesh size (ξ) of the SAP sample √ ξ=

2Cn Mc l Mr

−1/3 υ2,s

(3.15)

where l = C–C bond length along the polymer backbone = 1.54 Å [16]. C n = weight average of C n values of chains of monomer 1 and monomer 2 in accordance to their mole ratio used in the sample. Note that the C n values from the literature for poly (acrylamide) chain and poly (acrylic acid) chain are found to be 2.74 and 6.7 respectively [14].

3.2.5 Swelling Kinetics The swelling data obtained from the swelling studies are fitted into different models such as Peppas-model, Higuchi-model, zero-order, first-order and second-order kinetic equations [4, 20]. The equations related to these models along with their interpretation are shown in Table 3.1. Table 3.1 Swelling kinetics study of SAP sample Model/equation applied Model/equation Peppas-model

Mt M∞

=

Higuchi-model

Mt M∞

= kt 2

Interpretation

kt n

(i) Fickian diffusion (n < 0.5), solvent diffusion rate < chain relaxation rate (ii) Non- Fickian diffusion (0.5 < n < 1), diffusion rate > relaxation rate (iii) Case II transport (n ≥ 1), solvent diffusion rate is approximately equal to the chain relaxation rate

1

Water intake mainly by diffusion

Zero-order

Mt = M∞ + kt

Penetration rate is slow

First-order

α = (1 − Ae−kt )

Swelling α water content present in the sample

Second-order kinetics

t Mt

=

(

1 2 k M∞

)

+

(

1 M∞

)

t Swelling α (water content present in the sample)2

50

P. Gupta and R. Purwar

Where Mt /M∞ = Normalized swelling ratio, n = Swelling exponent indicating mode of movement of penetrate, k = Constant for SAP sample, t = Time taken in Swelling. The models / equation for which the value of correlation coefficient (R2 ) is found to be the highest, is supposed to be the best fit model for exhibiting the swelling kinetics of the SAP sample.

3.2.6 Mechanical Characterization (Quantitative Analysis of Free and Bound Water) SAP samples are hard to hold between the clamps owing to their slippery nature and have very small elastic modulus (in the range of kilopascal). However ordinary mechanical testing machines measure elastic modulus in the range of MPa (megapascal) to GPa (gigapascal). Because of these difficulties, the mechanical strength of SAPs (in wet condition) cannot be measured by ordinary mechanical testing machines [21]. For this some specific equipment are required, which are as follows: (i)

Thermo gravimetric analysis (TGA)—This technique is used to check the thermal behaviour of the SAP samples. It is based upon the principle of rate of weight change as a function of temperature or time. This technique can be used to obtain the following information about a material under different conditions as depicted in Fig. 3.2:

For TGA analysis, 10–12 mg of sample is required (however if it is a composite or blend, 15–20 mg sample is needed). The heating (or cooling) rate may be kept from

Fig. 3.2 Thermogravimetric analysis techniques

3 Experimental Methods of Superabsorbent Polymers: Characterization

51

10 to 20 ºC/min. The heating is generally done in the inert atmosphere of nitrogen in order to avoid oxidation and other unwanted reactions in the reaction chamber, but air is used for checking thermal degradation which is accomplished by oxidation (oxidative stability) or adsorption. All the results should be taken under the same atmosphere because change in atmosphere changes the results. The inert gas flow rate is maintained at 40 mL/min. For deep measurement, the initial temperature is taken as 25–30 ºC in order to detect the moisture entrapped into the SAP sample and the final temperature may vary from 600 to 1000 ºC depending upon the nature of the material. In TGA a descending curve indicates weight loss which may occur due to the processes like decomposition, evaporation, reduction, desorption etc., whereas ascending curve indicates an increase in weight which may occur due to oxidation or adsorption. The TGA curve gives the degradation temperature of a substance which is determined by drawing tangents. In the case of copolymers, a multistep degradation indicates the degradation of some segments at low temperature while others at high temperature. The major weight loss at higher temperature indicates a higher stability of the sample. The various types of plots obtained during TGA analysis along with their explanations are shown in Table 3.2. We can understand this by taking an example of poly (acrylamide-co-acrylic acid) SAP. Its TGA curve shows a multistep degradation process with stable intermediates as shown in Fig. 3.3 [22]. It is a three step degradation process. The initial weight loss in the first step may be due to loss of moisture content or volatile materials or soft segments of the polymeric chain. The weight loss in second step may be the result of Table 3.2 Various TGA plots and their interpretations TGA plot

Observation

Interpretation

No mass change over the entire range

Degradation temperature is greater than the temperature range of the instrument

Rapid initial weight loss

Drying or desorption

Single step decomposition

Shows degradation temperature and sample can be safely used below this temperature

Multistep degradation with stable intermediates

Stepwise degradation, different segments of the polymer chain have different stability

Multistep degradation with unstable intermediates

May be due to high heating rate or unstable intermediate

Gain in mass

Oxidation or adsorption

Initial gain in mass and then loss

Oxidation followed by decomposition

52

P. Gupta and R. Purwar

Fig. 3.3 TGA thermogram of poly (acrylamide-co-acrylic acid) SAP

partial thermal degradation or de-cross-linking of the SAP sample and the weight loss in the third step may occur due to the complete degradation of the polymer chain [23]. If a residue remains after burning the char in air, it indicates the presence of some inorganic material like fillers in the polymer. To study decomposition kinetics using TGA, the sample is heated with different rates ranging from 2.5 to 20 °C in the atmosphere of nitrogen. With increase in heating rate, the onset of degradation temperature moved towards higher temperature. This gives a relation between the time and temperature of the degradation of a sample. (ii)

Differential scanning calorimetry (DSC)—This technique is also used to check the thermal properties but in terms of heat change as a function of temperature i.e., the difference in the heats given, to maintain the temperature difference between the reference and the sample 0ºC, is obtained in the form of a plot. It is an important and popular technique because it gives accurate results and requires very low quantity of the sample (Fig. 3.4).

DSC results are greatly affected by the experimental conditions, therefore selection of correct experimental conditions is essential. In case of polymers, a minute Fig. 3.4 Schematic diagram showing flux DSC

3 Experimental Methods of Superabsorbent Polymers: Characterization

53

quantity i.e., 6 to 8 mg of the sample is required for DSC analysis whereas a higher quantity i.e., 8–10 mg is required in case of composite or blend. The scanning is carried out under the atmosphere of nitrogen by varying the temperature within certain limits. The initial and final temperatures are selected in such a way that the initial temperature is about 30–40 °C below the glass transition temperature of the sample and final temperature is any point below its degradation temperature in order to avoid the degradation of the sample inside the pan. Heating/cooling rate is also selected carefully because a very high heating/cooling rate results in poor resolution. For best results, the optimum heating rate is 5–20 °C/min and the optimum cooling rate is 5–10 °C/min. In DSC, the heat-cool-reheat method is of great significance. The plots obtained may have two types of peaks: An upside peak indicates the exothermic nature/reaction of the sample and downside peak indicates its endothermic nature/reaction. Analysing the peaks provides sharp information about the substance (Fig. 3.5). In general, polymers or SAPs are semi-crystalline i.e., they have crystalline as well as amorphous nature, so they give two endothermic and one exothermic transition peak as shown in Fig. 3.6. The first endothermic peak is the glass transition temperature Tg of the sample and the second one represents the melting point of the sample. Indeed, the glass transition temperature is not a fixed temperature but it is a range of temperatures in which the sample changes from the hard glassy state to soft glassy state. It is obtained by drawing a tangent. A cross-linked sample has a higher glass transition temperature (Tg) than a non-cross-linked sample [24]. The exothermic transition represents the crystallization of the sample. Appearance of more than one sharp endothermic peak may be due to the presence of crystals of

Fig. 3.5 Exothermic and endothermic peaks in DSC thermogram

54

P. Gupta and R. Purwar

Fig. 3.6 DSC analysis of a semi-crystalline sample

different sizes in the sample. However, only a sharp endothermic peak indicates the homogeneity in the size of the crystals. Degree of crystallinity can also be calculated from the endothermic melting peaks using the following formula: Degree of crystallinity, X c =

ΔHm × 100% ΔH 0

(3.16)

where ΔH m is the heat of fusion of the sample and ΔH o is the heat of fusion of 100% crystalline sample. Higher the crystallinity, the better the mechanical properties. (iii)

(iv)

Nuclear magnetic Resonance(NMR)—This is a popular and non-invasive technique to investigate the molecular structure of the SAP sample. It provides an information about the interchange of water molecules between the free and bound water. Solid state NMR is used for analysing SAP samples. Rheological Studies—The rheological studies of the SAP sample provide information about the network structure, gelation kinetics, mechanical properties like gel strength etc. These studies also provide an idea about the presence of cross-links, association, or entanglement in the structure of SAP sample. There are several types of rheometers such as coquette type (which has a rotating bob inside the fixed cylindrical cup), Poiseuille type (in which fluid flows in between two stationary restricted surfaces), cone-plate type (in which a solid cone rotates on the top of a smooth horizontal surface), parallel plate type (in which one plate is stationary and other is reciprocating) [2].

Out of these, parallel plate rheometers are commonly used. For rheological studies, the SAP sample is dissolved in a solvent like distilled water or PBS buffer solution with vigorous stirring. The swollen gel particles are then placed onto the lower plate of the rheometer and the upper plate is lowered slowly till a certain gap and then, measurements are started. The plot gives the storage modulus (G’) and loss modulus (G”) as a function of time at the oscillatory frequency of 1 Hz and fixed strain. Water

3 Experimental Methods of Superabsorbent Polymers: Characterization Table 3.3 Rheological interpretations

Feature

Interpretation

G''

Liquid is flowing freely

>

G'

Increasing G’

Further cross-linking

G* does not vary with oscillatory frequency

Covalently cross-linked networks

G' > >> G''

Gel strength is measured

55

evaporation is checked using a solution trap. The point of intersection of G’ and G” is called the gel point. After a certain period of time (120 min) complex shear modulus (G*) is obtained. The sample is then removed, and the procedure is repeated with the swollen SAP sample. The value of storage modulus and loss modulus can provide an idea about the cross-link and gel strength of the superabsorbent (Table 3.3). Keeping the strain constant (0.05), the frequency sweep experiments are performed as a function of frequency (0.1–10 Hz). Finally, the stress is varied from 10–1 to 104 Pa to perform amplitude sweep experiments. The latter experiments are performed to confirm that all the above measurements are performed in the linear viscoelastic region of the sample [1].

3.2.7 Other Tests (i)

(ii)

Environmental sensitivity—Some SAPs behave differently in different environmental conditions like pH, temperature, ionic strength etc., and hence referred as smart SAPs. To check whether an SAP is smart or not, the swelling tests, as mentioned above, are carried out by varying the environmental conditions. To check pH responsiveness, buffer solutions of different pHs (like acidic pH 4, neutral 7 and alkaline 8 etc.) are used and to check temperature responsiveness, the swelling experiments are performed under different temperature conditions. However, if swelling is same in all the cases, the SAP is not environment sensitive. Porosity—The porosity of the SAP sample is checked using liquid displacement method. In this method, the known volume of the solvent usually nhexane (because it shows unhindered motion through interlinked pores and does not affect the morphology of the SAP sample) is taken in a measuring cylinder, and SAP sample of specific size is immersed into it for a period of about 10 min. The rise in volume is noted [6]. The swollen sample is taken out and final volume of the remaining solvent is noted. From these data, the porosity is calculated by using the following formula % Porosity =

(v0 − v2 ) × 100 v1 − v2

(3.17)

56

(iii)

P. Gupta and R. Purwar

where V 0 = Initial volume of solvent i.e., n-hexane, V 1 = Volume of solvent after dipping the SAP sample, V 2 = Final volume of the solvent when SAP sample is removed. Ionic sensitivity—To check the ionic sensitivity of an SAP sample, it is placed in the given ionic liquid for a particular period of time and its solution absorbency (As ) is calculated. Similar experiment is performed with distilled water to calculate the water absorbency (Aw ) of the sample. The swelling factor, f is calculated by using the following relation: f =1−

As Aw

(3.18)

A higher f value indicates a low absorption capacity in the given salt or ionic solution. Therefore the SAPs having lower value of f are preferred for the absorption of the salt solution. A negative f value means increased absorption capacity in the given salt solution [1].

3.3 Application Specific Characterization This section includes the characterization which are performed only when SAP is used for that particular purpose. Some such characterization techniques are as follows:

3.3.1 For Removing Pollutant/Contaminant Like Heavy Metal Ion or Dye from Waste Water When SAPs are used for wastewater treatment i.e., for the removal of heavy metals or dyes from water, they are characterized by the following characterization methods: (i)

Pollutant/contaminant adsorption capacity—The adsorption capacity of a particular pollutant/contaminant can be determined quantitatively by placing the SAP sample in contact with that particular pollutant/contaminant (dye or metal ions like Cu2+ ) for a particular period of time. For this, a solution of the known concentration of the pollutant/contaminant is prepared and the SAP sample is placed into it for a particular period of time. Then the sample is taken out and the concentration of the remaining pollutant/contaminant solution is checked by means of spectrophotometry. The process is repeated till the equilibrium adsorption is achieved. Knowing the initial and final concentrations of the pollutant/contaminants solution, the removal efficiency of the SAP sample, its equilibrium adsorption capacity, retention capacity and percent pollutant/contaminant uptake can be calculated using the following relations [25]:

3 Experimental Methods of Superabsorbent Polymers: Characterization

Removal efficiency, R =

C0 − Ce × 100 C0

Equilibrium adsorption capacity, qe =

(3.19) (3.20)

Wp Ws

(3.21)

Wp × 100 W0

(3.22)

Retention capacity, Q r = %Pollutant uptake, Pu =

C0 − Ce ×V Ws

57

where C 0 and C e are the initial and equilibrium concentrations of the pollutant/contaminant solutions, V is the volume of pollutant/contaminant solution, W s is the weight of dry SAP sample, W 0 and W p are the quantity of pollutant/contaminant in the feed and SAP sample respectively. The adsorption capacity of an SAP sample may also vary with the factors like temperature, pH and concentration of pollutant/contaminant solution, period of contact and ionic strength of the contaminants. To check the effect of these factors on adsorption capacity, the above experiment is performed by varying these parameters and the results are compared. E.g., to check whether the SAP is temperature responsive, repeat the above adsorption experiment by performing it under different temperature conditions (like at 10ºC, 20ºC, 30ºC and so on) and calculate the removal efficiency of the SAP sample and its equilibrium adsorption capacity in each case [25]. Compare the results to check the effect of increase in temperature on the adsorption capacity of pollutant/contaminant. In the same way, to check whether the SAP is pH responsive, repeat the above adsorption experiment by performing it using pollutant/contaminant solution of different pHs and calculate the removal efficiency of the SAP sample and its equilibrium adsorption capacity in each case. Compare the results to check the effect of change in pH on the adsorption capacity of pollutant/contaminant [25]. Similarly, vary the concentrations of the pollutant/contaminant solutions or ionic strengths and compare the results [25]. To check the effect of ionic strength, generally, salt solutions of different concentrations are used. As the ionic strength increases, adsorption capacity decreases if the force of attraction between the pollutant and SAP are electrostatic interaction [26, 27]. (ii)

Adsorption kinetics—In addition to high adsorption capacity the SAP sample must have high rate of adsorption which is determined by the study of their adsorption kinetics. To study adsorption kinetics the adsorption data at different times are fitted into various kinetic models and the model with highest value of regression coefficient (R2 ) is considered as the best fit model. These models along with their linear equation and expected interpretation are shown in Table 3.4.

The acrylic based natural SAPs have a greater rate of adsorption than their synthetic counterparts.

58

P. Gupta and R. Purwar

Table 3.4 Various models to study adsorption kinetics Model

Linear equation

Pseudo-first order

ln (qe − qr ) = ln qe − k1 t ln(qe − qr )V st k is linear function of initial concentration of solute, Slope gives adsorption rate constant and intercept gives desorption rate constant [28]

Pseudo second order

t qr

=

1 qe t

+

1 k2 qe2

√ Inter-particle diffusion qt = kdi t + C

Elovich

(iii)

(iv)

qt =

1 1 β (lnαβ) + β lnt

Plot

t qr

V st

√ qt V s t

qt V slnt

Interpretation

k is complex function of initial concentration of solute, Chemisorption by ion exchange or covalent bonding [29] Initially external mass transfer followed by inter-particle pore diffusion [25, 30] Chemical adsorption, heterogeneous adsorbing surface [31]

Distribution of pollutant/contaminant in the SAP samples—The distribution of pollutant/contaminant within the SAP sample at equilibrium adsorption is determined by fitting the adsorption data i.e., equilibrium concentration and equilibrium adsorption at a particular temperature in different adsorption isotherms. These isotherms and the corresponding assumptions are given in Table 3.5. The isotherm with the highest value of regression coefficient (R2 ) is considered as the best fit isotherm and shows the distribution of pollutant/contaminant in the SAP sample. Adsorption Mechanism- The pollutant/contaminant gets adsorbed on the SAP sample by any of the following types of interactions like hydrogen bonding, complexation reaction, electrostatic interactions, hydrophobic interactions between the non-polar pollutant/contaminant and the SAP, ion exchange method, π–π interaction or a combination of these [26]. Various techniques like FTIR, XPS etc., can be used to determine the type of interaction(s) between the pollutant (adsorbate) and the SAP sample (adsorbent) e.g., (a)

FTIR—The use and scanning conditions of this technique have been discussed earlier in this chapter. When the plane (which has no pollutant/contaminants i.e., dye or metal ion) and pollutant/contaminants loaded samples are analysed using FTIR, the bands in the spectra clearly reveal the appearance or disappearance of particular type of

3 Experimental Methods of Superabsorbent Polymers: Characterization

59

Table 3.5 Different adsorption isotherms and corresponding assumptions Isotherm

Linear equation

Plot

Assumptions

Langmuir

Ce qe

Ce qe

Monolayer adsorption, Homogeneous distribution of pollutant Uniform heat of adsorption [32]

Freundlich

ln(qe ) =

=

Ce qm

ln(k f ) +

+

1 qm K c

RT bT

Redlich-Peterson

ln(qe ) V s ln(C e )

Multilayer adsorption, non-uniform distribution of pollutant and non-uniform heat of adsorption [33]

qe = C e

Uniform distribution of binding energies [33]

1 n ln(C e )

qe =

Tempkin

Vs Ce

lnk T +

RT bT

lnCe

( ) ln K R P Cqee − 1 =

( ) lnCe Vs ln K R P Cqee − 1

β R P (lnCe ) + ln(a R P )

(b)

(v)

A combination of Langmuir and Freundlich [33]

bond(s) or functional group(s), on the basis of which the mechanism can be predicted. For example, Orozco-Guareno et al. analysed the FTIR spectra of plane and Cu2+ loaded poly (Aac-co-Aam) hydrogel and confirms a complexation reaction during the adsorption of copper ion by the involvement of the -NH2 and -OH groups of the adsorbent (hydrogel) [34]. XPS (X-ray photoelectron spectroscopy)—Wang et al. used this technique to analyse the adsorption of Sr2+ ion on various hydrogel samples and reported that weakening or disappearance of Na+ peak after the adsorption of heavy metal like Sr2+ confirms that the adsorption follows ion exchange mechanism [35].

Feasibility of adsorption- The feasibility of adsorption process can be checked with the help of thermodynamic parameters like standard enthalpy change (ΔHo ), standard entropy change (ΔSo ) and standard Gibbs free energy change (ΔGo ) [27]. The standard enthalpy change (ΔHo ) and standard entropy change (ΔSo ) are related with the equilibrium constant (K c ) by the following relation [32]: ln K c =

ΔH 0 ΔS 0 + RT R

(3.23)

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P. Gupta and R. Purwar

where R is the universal gas constant. The values of standard enthalpy change (ΔHo ), standard entropy change (ΔSo ) can be obtained as the slope and intercept of the ln K c Vs 1/T graph [30]. The high value of ΔSo reveals that randomness increases on the interface of pollutant/contaminants and SAP sample during adsorption. A negative value of ΔHo shows an exothermic adsorption process and its positive value indicates that the adsorption is endothermic. Knowing the values of standard enthalpy change (ΔHo ) and standard entropy change (ΔSo ), the value of standard Gibbs free energy change (ΔGo ) can be calculated by the following relation: ΔG o = ΔH o − T ΔS o

(vi)

(3.24)

A negative value of standard Gibbs free energy change (ΔGo ) indicates the adsorption process is feasible while its positive value shows that no adsorption takes place. SAP regeneration- It is an essential feature of economically affordable SAPs. The SAPs are generally regenerated by treating them with certain acids (HCl, HNO3 ) or alkalis (NaOH). The SAP regeneration is greatly affected by the concentration of the acid/ alkali used, its volume and treatment time. Therefore the optimum conditions for the regeneration of a particular SAP can be determined by varying these parameters one by one and plotting the results obtained. Desorption efficiency is calculated with the help of quantity of pollutant/contaminants desorbed Md and that of sorbed, Ms using the following relation [32]: De = Or De =

Md × 100 Ms

Cr vr × 100 (C0 − Ce )v

(3.25) (3.26)

where C r is the concentration of pollutant/contaminants solution after regeneration C o and C e are the initial and equilibrium concentrations of the pollutant/contaminants solution, V and V r are the volume of feed solution and regeneration volume respectively [36]. Desorption ratio is calculated from desorption and adsorption capacities using the relation: Dr =

qd qe

where qd and qe are desorption and adsorption capacities respectively.

(3.27)

3 Experimental Methods of Superabsorbent Polymers: Characterization

61

3.3.2 For Personal Care Products (i)

(ii)

Residual Monomers—This test is used while using SAP for personal care purposes like sanitary napkins, baby diapers, drug delivery devices etc. In general, acrylic acid or its salts based SAPs are used for making these healthcare devices and the allowed safe level of acrylic acid or acrylate in these devices is now less than 300 ppm. There are several methods to determine the residual monomer fraction in the SAP sample but HPLC is one of the preferred methods used for this purpose. In this method, the total residual monomer is removed as the acid or its salt while extracted with the extractant which is usually a solution of orthophosphoric acid (OPA) [37]. Swelling—This has been discussed earlier in this chapter.

3.3.3 As Drug Delivery Devices (i)

(ii)

(iii)

Drug loading and release profile—Drug can be loaded into the SAP sample either at the time of their preparation or by placing the SAP sample into the drug solution of certain concentration (usually 1mgmL−1 ) for certain period of time. The latter one is a better method to avoid any chemical treatment to drug. In this method, the amount of drug loaded into the SAP sample is determined using UV–Vis spectrophotometer. For this, a calibration curve for the drug is plotted and using this curve the concentration of the remaining solution is determined. The concentration (amount) of drug loaded is calculated by subtracting the concentration of the remaining solution from the initial concentration of the drug solution. Drug entrapment efficiency—The drug entrapment into an SAP sample is checked using X-ray power diffraction (XRD) technique. For this, the drug and drug loaded SAP are analysed with the help of a diffratometer. The analysis is done in the angle range of 2θ = 5º–90º with the scanning rate 0.001º/s and their diffraction patterns are compared [9]. The drug individually has a sharp peak but when loaded within the SAP sample, diffused peaks should be there for confirming the amorphous morphology of drug loaded SAP or entrapment of drug into the SAP sample. Drug release kinetics—To study drug release kinetics, the drug loaded and unloaded samples are placed separately in 20 mL distilled water or buffer solution of certain pH. After a regular interval of time (i.e., 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 45 min, 1 h, 1.15 h 1.30 h, 2.0 h and so on), 2 mL aliquot is taken out and at the same time, 2 mL distilled water or fresh buffer solution is added to maintain the volume of the solution constant. Continue this process for about 10 h and then take an aliquot after 24 and 48 h. These aliquots are then tested using UV–Vis spectrophotometer. Here the aliquots of unloaded sample taken at different time intervals are used as reference and that of the loaded sample as tests. On running the set up the concentration is

62

P. Gupta and R. Purwar

obtained [6]. These concentrations are changed into percentage cumulative release using following relation Percentage cumulative release =

(iv)

(v)

(vi)

Ve C1 + V0 Ct × 100 m

(3.28)

where m is the mass of drug loaded SAP sample, V e and V 0 are the sampling and initial volumes and C 1 and C t are the initial concentration and concentration at time t respectively. These cumulative release along with the corresponding time are fitted into the following kinetics models and the model having highest value of R2 (regression coefficient) is supposed to be the best fit model. The interpretations related to different kinetic models along with the related equations are tabulated in Table 3.6. pH responsiveness—To check the pH responsiveness, the drug release tests are performed using the buffer solutions of different pHs (like acidic pH 4, neutral 7 and alkaline 8 etc.). In case if one constituent of the SAP is an acid, choose one pH below the pKa of the acid and the other above it. Variation in the release profile at different pHs clearly confirms the pH responsiveness of the SAP sample. However, if the release profile is same in all the buffer solutions, the sample is not pH responsive. Figure 3.7 shows Fickian and non-Fickian drug release behaviour. Drug interaction with SAP sample—It is done by using FTIR technique as described above. A comparison of the spectra of unloaded and drug/nutrient loaded sample clearly indicates whether the drug interact with the SAP sample or not. Antimicrobial activity—Zone inhibition method can be used to check the antimicrobial behaviour of the drug loaded SAP samples. In this method, the

Table 3.6 Various kinetic models and Interpretation from them Model/equation applied Model/equation

Interpretation

Peppas-model

Mt M∞

Higuchi-model

Mt = M∞ + k H t 1/2

Drug release is mainly by diffusion [38, 38]

Zero-order

Mt = M∞ + kt

Slow drug release profile (due to no disaggregation of SAP) [40]

First-order

α = (1 − Ae−kt )

Drug release α water content present in the sample [40]

Second-order

t Mt

=

=

kt n

(

1 2 k M∞

(i) Fickian diffusion (n < 0.5) drug diffusion rate < relaxation rate of the hydrogel matrix (ii) Non-Fickian diffusion (0.5 < n < 1) drug diffusion rate > relaxation rate of the hydrogel matrix (iii) Case II transport (n ≥ 1)

)

+

(

1 M∞

) t Drug release α (water content present in the sample)2 [40]

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Fig. 3.7 Schematic diagram showing Fickian and non-Fickian Drug Release

agar plates are cultured with gram positive and gram negative bacteria and disks of drug loaded SAP samples that have been sterilized in laminar air flow under ultra-violet lamp for a period of about 24 h, are mounted at the center of the agar plate. The plates are then sealed with parafilm tape to avoid contamination. The sealed plates are incubated for 24 h at a temperature of 37ºC. Formation of a clear zone of inhibition (the zone which has no bacterial activity) below and around the SAP sample gives a qualitative idea about the release of drug and also confirms the antimicrobial nature of the loaded SAP sample. The drug release by different samples can be compared qualitatively by measuring the diameter of the zone of inhibition [9].

References 1. Zohuriaan-mehr, M.J., Kabiri, K.: Superabsorbent Polymer Materials: A Review. Iran. Polym. J. 17, 451–477 (2008). http://journal.ippi.ac.ir 2. Azeera, M., Vaidevi, S., Ruckmani, K.: Characterization Techniques of Hydrogel and Its Applications. In: Cellulose-Based Superabsorbent Hydrogels. pp. 1–24 (2018). https://doi.org/10. 1007/978-3-319-76573-0_25-1 3. Starsinic, M., Taylor, R.L., Jr, W., Painter, P.C.: FTIR STUDIES OF SARAN CHARS. Carbon N. Y. 21, 69–74 (1982). https://doi.org/10.1016/0008-6223(83)90158-6 4. Ilgin P, Ozay H, Ozay O (2019) A new dual stimuli responsive hydrogel : Modeling approaches for the prediction of drug loading and release profile. Eur Polym J 113:244–253. https://doi. org/10.1016/j.eurpolymj.2019.02.003 5. Madejova, J.: FTIR techniques in clay mineral studies. Vib. Spectrosc. 31, 1–10 (2003). https:// doi.org/10.1016/S0924-2031(02)00065-6

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25. Bhattacharyya, R., Ray, S.K.: Adsorption of industrial dyes by semi-IPN hydrogels of Acrylic copolymers and sodium alginate. J. Ind. Eng. Chem. 22, 92–102 (2015). https://doi.org/10. 1016/j.jiec.2014.06.029 26. Akter, M., Bhattacharjee, M., Dhar, A.K., Rahman, F.B.A., Haque, S., Rashid, T.U., Kabir, S.M.F.: Cellulose-Based Hydrogels for Wastewater Treatment: A Concise Review Gels. 7, 1–28 (2021). https://doi.org/10.3390/gels7010030 27. Wang Y, Zeng L, Ren X, Song H, Wang A (2010) Removal of Methyl Violet from aqueous solutions using poly (acrylic acid-co-acrylamide)/attapulgite composite. J Environ Sci 22:7–14. https://doi.org/10.1016/S1001-0742(09)60068-1 28. Azizian, S.: Kinetic models of sorption : a theoretical analysis. J. Colloid Interface Sci. 276, 47–52 (2004). https://doi.org/10.1016/j.jcis.2004.03.048 29. Ho Y (2006) Review of second-order models for adsorption systems. J Hazard Mater 136:681– 689. https://doi.org/10.1016/j.jhazmat.2005.12.043 30. Nandi BK, Goswami A, Purkait MK (2009) Applied Clay Science Removal of cationic dyes from aqueous solutions by kaolin : Kinetic and equilibrium studies. Appl Clay Sci 42:583–590. https://doi.org/10.1016/j.clay.2008.03.015 31. Wu, F., Tseng, R., Juang, R.: Characteristics of Elovich equation used for the analysis of adsorption kinetics in dye-chitosan systems. Chem. Eng. J. J. 150, 366–373 (2009). https://doi. org/10.1016/j.cej.2009.01.014 32. Mittal H, Ray SS (2016) A study on the adsorption of methylene blue onto gum ghatti / TiO2 nanoparticles-based hydrogel nanocomposite. Int J Biol Macromol 88:66–80. https://doi.org/ 10.1016/j.ijbiomac.2016.03.032 33. Zheng Y, Wang A (2009) Evaluation of ammonium removal using a chitosan-g-poly(acrylic acid)/rectorite hydrogel composite. J. Hazard. Mater. J. 171:671–677. https://doi.org/10.1016/ j.jhazmat.2009.06.053 34. Orozco-Guareño E, Santiago-Gutiérrez F, Morán-Quiroz JL, Hernandez-Olmos SL, Soto V, de la Cruz W, Manríquez R, Gomez-Salazar S (2010) Removal of Cu(II) ions from aqueous streams using poly(acrylic acid-co-acrylamide) hydrogels. J Colloid Interface Sci 349:583–593. https:// doi.org/10.1016/j.jcis.2010.05.048 35. Wang M, Xu L, Peng J, Zhai M, Li J, Wei G (2009) Adsorption and desorption of Sr ( II ) ions in the gels based on polysaccharide derivates. J. Hazard. Mater. J. 171:820–826. https://doi. org/10.1016/j.jhazmat.2009.06.071 36. Khan M, Lo IMC (2017) Removal of ionizable aromatic pollutants from contaminated water using nano γ-Fe2O3 based magnetic cationic hydrogel: Sorptive performance, magnetic separation and reusability. J Hazard Mater 322:195–204. https://doi.org/10.1016/j.jhazmat.2016. 01.051 37. Jamshidi A, Beigi FAK, Kabiri K (2005) Optimized HPLC determination of residual monomer in hygienic SAP hydrogels. Polym Test 24:825–828. https://doi.org/10.1016/j.polymertesting. 2005.07.007 38. Dorota, W., Krzak, J., Macikowski, B., Berkowski, R.: Evaluation of the Release Kinetics of a Pharmacologically Active Substance from Model Intra-Articular Implants Replacing the Cruciate Ligaments of the Knee. Materials (Basel). 12, 1–13 (2019). https://doi.org/10.3390/ ma12081202 39. Paul DR (2011) Elaborations on the Higuchi model for drug delivery. Int J Pharm 418:13–17. https://doi.org/10.1016/j.ijpharm.2010.10.037 40. Samaha, D., Shehayeb, R., Kyriacos, S.: Modeling and comparison of dissolution profiles of diltiazem modified-release formulations. Dissolution Technol. 16, 41–46 (2009). https://doi. org/10.14227/DT160209P41

Chapter 4

Superabsorbent Polymers for Heat Resistance and Treatment of Industrial Effluents Amita Somya

Abstract The superabsorbent polymers or hydrogels (SAHs) are hydrophilic threeD polymer matrix that have the potential to expand, absorb, and hold a tremendous amount of aqua molecules from aqueous solutions also, and exhibit improved sorption ability and these characteristics have made them enable to adsorb heavy metal ions (HMI), dyes, and furthermore organic foulants away from wastewaters. Consequently, SAHs have recently, attracted a lot of interest from researchers who want to explore their utilization to treat wastewater. The present chapter explains the elimination of toxic heavy metal pollutant ions from industrial effluents in detail in order to fathom the swelling characteristics, industrial effluents treatment as a whole, heavy metal pollutant ions and synthetic dyes elimination from these effluents, specifically. The application of these hydrogels in fabrication of clothing for thermal comfort has also been explained which is the most recent one employed in textile industries. Keywords Hydrogels · Acrylic acid · 3-D network · Amphoteric

4.1 Introduction Superabsorbent polymers—commonly known as SAPs [1] or hydrogels are flexible polymer chains that are loosely conjoined to create three-dimensional networks that transport dissociated, ionic substances. Depending on zero or positive electrical charge present on cross-linked networks, like ion exchangers [2], SAPs are classified [3] into nonionic, ionic, amphoteric and zwitterionic. Essentially, these are the materials which are capable to absorb fluids up to 15 times of their dry weight, with or without a load, such as brines, synthetic urine, electrolyte solution, and water bodily fluids like blood, sweat, and urine. They are polymers exhibiting hydrophilicity as one of their characteristics. They are composed of water-insoluble groups like hydroxyl, carboxamide, carboxylic acid, amine, imide groups etc. and cross-linked polyelectrolytes. Owing to ionic characteristics and interlinked network structure, they are A. Somya (B) Department of Chemistry, School of Engineering, Presidency Uniniversity, Bengaluru 560064, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Arpit and T. Jaya (eds.), Properties and Applications of Superabsorbent Polymers, https://doi.org/10.1007/978-981-99-1102-8_4

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efficient to absorb huge amounts of aqua and various aqueous solutions without dissolving by hydration of aqua particles through H- links. They have been used in various fields like, in sanitary [4] by Ma et al., in drug delivery systems [5] by S Mohammad and S Fatemeh, in immobilization of protein and cells [6] by Li et al., in tissue engineering [7], in agriculture and horticulture and for wastewater treatment processes [8] because of their extraordinary water retention ability which is well defined in terms of outstanding hydrophilic characteristics, great swelling ratio, biocompatibility and plentiful in occurrence [9]. However, they find extensive application in hygienic products such as baby diapers and napkins [10] which were commercialized in USA and Japan at the start of the decade from 1980. The worldwide demand for superabsorbent hydrogels is expanding day by day but, owing to lack of biodegradability and biocompatibility, it has been a great challenge for the researchers to develop environment friendly and multi-application oriented SAPs which could be overcome by an introduction of natural polymers into the metrics of synthetic materials in order to develop biocompatible and bio-degradable SPH, however, these hydrogels have been found to have inferior mechanical stability. Hence, to overcome this obstacle, Spagnol et al. [11] have used petroleum-based monomers with higher thermal, swelling and mechanical capacities relative to normal polymers, such as acrylic acid (AAc) and acrylamide (AAm) for the production of hydrogels. In comparison to other conventional adsorbents, these SPHs could be an exemplary choice for the extraction of heavy or transition metal pollutant ions away from defiled aqua due to their high swelling capacity, ease of manoeuvring, synthesis with variable combinations by tuning the characteristics, and application [12].

4.2 Application of SAPs in Heat Resistance There are wealthy literature dedicated to the various properties of SAPs like swelling behavior, absorbency under load (AUL), water retention capacity, permeability, cross-linking effect on properties, centrifuge retention capacity (CRC), antibacterial property etc. leading to their applications in health and hygiene, agriculture, wastewater treatment, sanitary and many others. However, only few reports are available exploring their thermal behavior. Thermal properties were studied by Hafida et al. [13] on chitosan-g- poly(acrylamide)/montmorillonite superabsorbent polymer composites and Kim et al. [14] on Itaconic acid based superabsorbent polymer composites applying Cellulose by thermal gravimetric analysis. The output of these studies has not been much explored in terms of applications, except in textile industry i.e., fabrication of clothings for thermal comfort. Additionally, some recent research investigations explored the potential application of SAP materials in performance textiles, such as the adjacent to skin layer fabric of fire protection clothing (FPC), to enhance sweat absorption for enhanced thermal comfort [15, 16]. By utilizing SAP’s hydrophilic nature, it is envisaged that the coating’s integration will improve moisture vapour transmission for thermal comfort. A method for designing chemical barrier clothing with simultaneous protection performance and thermo-physiological

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wear comfort is the production of protective coating by integrating SAP. Bhuiyan et al. [17] developed cotton fabrics by coating polyurethane integrated with superabsorbent polymer with thermal comfort. By utilizing the hydrophilic characteristics of the SAP, this embedded SAP in coating has increased evaporative transmittance up to a greater extent which has been boon for the users in terms of thermal support. Additionally, by padding the PU-SAP coating with fluoropolymers, an omniphobic facet was created. This surface exhibited great permeability to perspiration vapours and was found to be repellent to water, oils, and many other aqueous solutions those are having a very low surface tension. Hence, these coatings have not only attracted the researchers for exploring their applications in fabrication to offer thermal comfort, but also, for chemical protection. Thermal resistance for these fabrics against radiative and contact heat transfer was also well explored. Shaid et al. [18] developed an easy operating, low cost and convenient instrument which can investigate the performance of clothing in terms of radioactive and contact heat safeguarding in various configurations.

4.3 Application of SAPs in Treatment of Industrial Effluents One of the largest risks to civilization today, is environment pollution which is incredibly complicated and huge due to unchecked technological advancements brought about by humans. Owing to enhancement in urbanization and industrialization, water is getting polluted. Discharge of industrial effluents in the surface water has been noticed as one of the reasons among. Consequently, a lot of toxic metals, organic pollutants and dyes are released into the water systems which are the cause for decay of water animals and bad impact on other living organisms depending on water. Hence, before wastewater is discharged into freshwater bodies, the most essential objective is to eliminate harmful heavy metal pollutant ions since these ions can directly or indirectly cause a multitude of health issues in humans and aquatic animals [19]. Mercury, Cadmium, Lead and Nickel [20–22] are the most toxic metals among other heavy metals, found owing to their implications on living organisms. Mercury being the most noxious metal [20] which plunges into surroundings via many industries, such as paints and pharmaceuticals, paper and pulp industry, molding processes, chlor-alkali plants etc. Its contamination is much more dangerous since it can affect the lungs, digestive system, or blood streams. Even Hg(II) fumes are very harmful, and few mercury combinations are extremely volatile or insoluble, posing possible risks to human beings. The other poisonous heavy metal is cadmium which builds up and is retained by the body, leading to kidney damage, lung cancer, and demineralization of the bones [21]. Many products, including electroplating, storage batteries, vapour lamps, and some solders, employ cadmium. Two to four hours after exposure, symptoms may not start to appear. Fatigue, headaches, nausea, vomiting, cramps in the stomach, diarrhoea, and fever can all result from excessive exposure of cadmium.

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Both man-made and natural sources introduce lead (Pb) into the environment. The soil contains lead that is naturally occurring. The enormous growth in lead use over the last couple of decades has raised environmental challenges due to its toxicity. It occupies a significant position among the harmful elements found whose traces have been detected in human cells. The primary origin of its exposure are lead paints used to paint steel, water storage tanks, washbasins, kitchen sinks, lead pipes utilized to link plumbing fixtures, and some plastic pipes containing stabilizers made of lead compounds. Nickel (Ni) is an immunotoxic and carcinogenic substance that can have a range of negative health consequences on the body, including contact dermatitis, cardiovascular illness, respiratory tract cancer, asthma, and lung fibrosis, based on the dosage and time span of exposure. Comparatively, excessive Ni exposure has more detrimental consequences on humans, including birth abnormalities, heart disease, respiratory failure, pulmonary embolism [22] etc. Therefore, in order to eliminate these heavy metal pollutant ions coming from drain water, an appropriate, simple and advanced method must be used. Flotation, electrochemical deposition, adsorption and ion exchange have been just a few of the established methods used to eliminate heavy metal pollutants from industrial effluents. The pervasively used method for the elimination of heavy metal pollutants from industrial wastewaters has been the chemical precipitation method. However, it fails in case disruptive ions are present, like complex formers in the wastewaters. Among all methods, adsorption has been observed as a convenient and simple technique in order to remove toxic metal pollutant species from the effluents. In recent years, several materials and procedures, such as ion exchange resins [20, 23–28], usage of surfactant media [29–31] etc. was developed for the elimination of the hazardous heavy metal pollutant ions coming from wastewater which is based on adsorption processes. However, the shortcomings of conventional adsorbents necessitated the development of innovative materials that were inexpensive, biocompatible, biodegradable, simple to synthesize, easy to regenerate, and recyclable with high efficacy. The aforementioned limitations can thus, be overcome by researches on the development of SAHs. But the sky-high costs and stagnant nature of the materials restrains their employment in large scale water treatment processes. Hence, to overcome the shortcomings of the above said materials, applications of a number of superabsorbent systems have been explored for different approaches for eg, diapers applications [32] by Cordella et al., elimination of organic synthetic dyes [33] by Lai et al. in 2017, water purification [34] by Ahmad et al. in 2016 etc. The superabsorbent hydrogels (SAHs) consist of three-D polymer networks with hydrophilic properties that possess the capacity to expand, absorb, and hold a tremendous amount of aqua in aqueous medium being a thousand folds multiplied as compared to its original dry state while depicting enhanced sorption capacity. These hydrogels can adsorb heavy metal ions (HMI), organic foulants and synthetic dyes away from wastewater because of their excellent sorption capacity. The collapse and dominance of hydrophobic interconnections between these macromolecule chains, which are heavily dependent on the kind and combination of the constituents, is what causes SAHs to be in a dry state. But, when these dry hydrogels are in contact with aqua or any other aqueous

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solution, it inflates rapidly to a size that is noticeably huge holding on to water inside the three-D network. The best goals for SAHs synthesis are to initially absorb aqua molecules and then, preserve that aqua interior of the gels for a very long period of time [35]. These hydrogel’s capacity to swell is caused by the appearance of electrostatic and physical interconnections between aqua molecules and various macromolecule chains. It is the appearance of only hydrophilic moieties in these hydrogels on polymer chains which molds this network as hydrophilic and perceptive with regard to environmental temperature [36]. The presence of ionic groups [37] such as sulfonic acid (-SO3H), carboxylic acid (-COOH), hydroxyl (-OH) and amines (-NH2), inside the network is too accountable for the swelling property of these macromolecules networks by going through ionization. It enlarges the size of hydrogels by generating electrostatic repulsion forces and by enhancing osmotic pressure inside polymer networks. Therefore, swelling property of SAHs has made them enable for exploring their implementations for elimination of venomous heavy metal pollutant ions away from the drain water. Schematic presentation of swelling and adsorption of toxic metal pollutant species in SAH networks is depicted while keeping in aqueous solution, in Fig. 4.1. The mechanism of elimination of heavy or transition metal pollutant ions from wastewater by superabsorbent hydrogels involves two main processes—the diffusion and electrostatic force of attraction. In contrast to the ionizable groups, which establish a Coulomb’s force of attraction along heavy metal pollutant ions and exclude these ions from the aqueous solutions, the diffusion mechanism results in the perforation of ions within SAH networks throughout a concentration slope. The hydrogels

Fig. 4.1 Swelling and adsorption of metal ions on SAH kept in aqueous solution containing metal ions

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respond to miscellaneous environmental impetus, for instance, alterations in pressure, pH, temperature, light intensity etc. [38–40]. A volume phase change with reversible properties is then brought on by these stimuli in an aqueous medium [41]. As a result, SAHs have recently attracted a lot of interest from researchers who want to utilize them to treat wastewater. For the synthesis of SAHs, a variety of polymerization techniques are used, including suspension, grafting and free radical etc. Free radical polymerization process has been one of them and is frequently employed. For the onestep synthesis procedure, accelerators, cross-linkers and free radical initiators are required. Monomers can be cationic, anionic, or neutral. In order for initiators to produce free radicals, accelerators are needed and to accomplish this either temperature has to be raised or suitable stimulating substances as an example N,N,N' ,N' tetramethylethylenediamine (TEMED) is used. Luqman et al. [42] has recently utilized a variety of combinations of AAc and AAm to develop superabsorbent hydrogels that have good thermal and mechanical properties [28]. They have synthesized superabsorbent hydrogel (SAH) by using varying compositions of AAc and AAm in which N, N-methylene bis-acrylamide (MBA) has been cross-linked using usual free radical polymerization procedure, also they explored these synthesized SAHs in a variety of environmental conditions to remove a few heavy metal pollutant ions from drain water, including cadmium (II), nickel (II), copper (II), and cobalt (II). The AAc and AAm components present in the structure reveal pH and temperature-dependent co-ducts in SPHs. These materials are found to exhibit high stability towards heat and phase transition of second order. In-depth research has been done on the influence of monomer arrangement on the elimination capacity, elimination efficiency, elimination selectivity and swelling ratio of SPH towards various heavy metal pollutant species. Liu et al. [43] used N,N' -methylene bis acryl amide (MBA) as the cross-linking reagent and ammonium persulfate (APS) as an initiator to create a new SAH i.e., chitosan-g-poly(acrylic acid)/sodium humate. Similarly, in order to create a superabsorbent hydrogel, Thakur and Arotiba [44] created poly acrylic acid (pAAc) grafted over sodium alginate (SA-cl-PAA). These hydrogels were created by grafting process of AAc monomer over the sodium alginate polymer in which potassium persulfate (KPS) and MBA were employed as initiator and cross-linking reagents, respectively. Superabsorbent hydrogels have special characteristics which are dependent on the fundamental modifications of their meshwork and, this unique property facilitates the hydrogel protean for different implementations. The most significant characteristic of these hydrogels among others is the reversible swelling and deswelling transition which takes place at the hands of the hydrophilic nature of macromolecule chains present in the structure and empowers them to interconnect with water molecules. Therefore, SAHs expand or swell as a result of the ingress or uptake of aqua molecules inside the polymer network. The swelling ratio (Eq. 4.1) can be used to estimate the SAHs’ ability to absorb water, and this ability is temperature-dependent. The hydrogels consisting of thermo-sensitive monomers such as vinylcaprolactam, Nisopropyl acryl amide (NIPAAm), acrylamide etc. exhibit the reversible swelling

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characteristics by the difference in temperature of the reaction mixture. These superabsorbent hydrogels exhibit various behaviours on higher and lower ranges of the lower critical solution temperature (LCST). The elimination of metal and extractant complex utilizing temperature-reliant hydrogel set-up was described where temperatures exceeding LCST, it was followed by desorption process from the network of poly N- Isopropyl acryl amide (pNIPAAm). Copper (II), a transition metal pollutant ion, sodium dodecyl benzene sulfonate (SDBS), an anionic surfactant and (pNIPA) hydrogel were used as the model systems for this investigation, and the latest temperature-swing solid-phase extraction (TS-SPE) method was successfully established [45]. % Swelling = Ws− Wd/ Wd × 100

(4.1)

where, Ws and Wd being the weight of hydrogel samples after and before swelling, respectively. The active sites of a SAH are supposed to be because of different variety of functional groups present in its structure, carrying positive and/or negative charges which in turn, helds responsible for swelling and deswelling processes through ionization of the active sites on changing the pH of used aqueous medium. For instance, protonation and dehydronation of the hydroxyl and carboxylic acid groups in the hydrogel network occur at lower and higher pH levels of the reaction medium, respectively. Dehydronation results in the surface becoming negatively charged and electrostatic repulsions happening at higher pH, which in turn leads the SAH network to expand or swell. Similar to this, the process is reversed when the medium’s low pH results in protonation of the active sites, which causes surface charge to vanish and the SAH network to collapse. On contrary, in the case of basic groups such as amines, similar results were observed but in the opposite direction. The significant applications of pH-dependent hydrogels have been explored in drug delivery, elimination of dyes, heavy metal pollutant ions and organic pollutant species away from industrial wastewater. The present chapter explains the significance of SAHs in the ministration of industrial effluents particularly, the elimination of heavy metal pollutant ions away from drain water. Javed R et al. [46] has explored how charge on the active sites affects the efficacy of hydrogels to eliminate heavy metal pollutant ions. The influence of charge over active sites in hydrogels in the extraction of heavy metal pollutant ions was investigated throughout the manufacture of anionic; poly methacrylic acid (pMAAc), neutral; poly acryl amide (pAAm) and cationic; poly(3-acryl amido propyl trimethyl ammonium chloride (pAPTMACl) hydrogels. The produced hydrogels have been found to follow the order P(MAAc) > P(AAm) > P. (APTMACl) in terms of elimination of heavy metal pollutant ions. Güçlü et al. [47] synthesized starch graft acrylic acid/montmorillonite (S-gAA/MMT) superabsorbent composite hydrogels and applied the same materials exclusively, as superabsorbent hydrogels for elimination of copper (II) and lead (II) ions. An elimination tendency of copper (II) ions was reported to be higher as compared to lead (II) ions. A physically cross-linked CMC-Chitosan hydrogel

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was also created by Zhao and Mitomo [48] and utilised to absorb copper (II) ions from wastewater. The electrostatic force of attraction between (+)ve copper ions, carboxylic acid and amine groups, being the active sites of manufactured hydrogels, causes copper (II) ions to be absorbed on hydrogels. Chitosan-CMC was able to remove up to 169.5 mg/g of copper (II) ions. As a result, copper (II) ions can be removed using these hydrogels. Sodium poly acrylic acid (pAAcNa) hydrogel was produced by Yung et al. [49] using simple, inexpensive methods and employed it in the elimination of Cu (II) ions away from aqueous medium. The maximal absorption capacity of copper (II) ions which they stated was the maximum uptake competence mentioned in literature, was discovered to be 243.91 mg/g. A hydrogel with an interpenetrating meshwork was created by Tang et al. [50] to trap heavy metal pollutant ions. The author reported that superabsorbents in particular, were found very effective in the elimination of nickel (II), chromium (II) and cadmium (II) ions away from aqueous solutions. Chromium (II) and cadmium (II) ions were eliminated after the nickel (II) metal ions, according to the study, which found 102.34 mg/g of nickel (II) metal ions were eliminated. By preparing a solution of AAc monomer in DMW, neutralising it with alkali solution, and then scattering the clay (bentonite) powder, Bulut et al. [51] created the SAH of acrylic acid based monomer. N,N' - methylene bis acryl amide (MBA) was added to the reaction vessel as a cross-linking reagent while APS as an initiator to speed up the reaction, which was then, allowed to run for two hours at 70 °C. The author explored the sample for elimination of heavy metal pollutant ions such as lead (II), cadmium (II), nickel (II) and copper (II) ions away from aqueous solutions. The sample’s maximum reported adsorption capacities for Pb (II), Ni (II), Cd (II) and Cu (II) ions at 273 K were 1666.67, 270.27, 416.67, and 222.22 mg g-1, respectively. Hosseinzadeh and Tabatabai Asl [52] reported a new poly(pyrrole) graft based magnetic hydrogel nanocomposite (MHN) and the material was found to be an efficacious adsorbent for elimination of deadly chromium (VI) ions away from solutions in water. Polycarboxylated starch based hydrogels were created by Chauhan et al. [53] and used for the sorption of copper ions. For the sorption of copper (II) ions, this hydrogel revealed a stimuli responsive character, dependent on the three-D network of these hydrogels where polycarboxylated hydrogel acted as a chelating agent and thus caused chelation with copper (II) ions. In situ copolymerization was used to create the novel SAH nanocomposites that Ka¸sgöz et al. [54] reported using clay and sodium salt of acrylamide (AAm)-2acrylamido2-methylpropane sulfonic acid (AMPS). The heavy metal ions copper (II), lead (II) and cadmium (II) ions could be successfully eliminated from the aqueous medium using the produced materials. Table 4.1 lists how well various hydrogel samples removed copper divalent metal ions in comparison. In recent past years, it has been observed that adsorption, compared to other water filtration techniques, is quick, inexpensive, simple to use, and a very effective method/technique without creating undesired byproducts [55, 56]. Hydrogels made from natural polymers [57] are shown to be harmless, eco-friendly substances that can be treated as biodegradable or renewable polymers with lower toxicity. By using

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Table 4.1 Adsorption of various hydrogel samples for Cu (II) ions Name

Adsorption of Copper (II) ions (milligram/gram)

References

Starch-graft-acrylicacid/montmorillonite(S-gAAc/MMT)

130.1

[47]

Chitosan-CMC

169.6

[48]

Sodium poly acrylic acid (pAAcNa)

243.9

[49]

Acrylic acid/clay

222.2

[51]

Polycarboxylated starch (pCS)

128

[53]

Acrylamide(AAm)-2-acrylamido-2-methylpropane sulfonic acid (AMPS) sodium salt/clay

81.66

[54]

gamma radiation to create hydrogels out of a mixture of potato starch and AAc, Bhuyan et al. [58] evaluated the properties of the hydrogels with and without the addition of alkali solution and exclusively used to eliminate dye from wastewater samples. A few other kinds of hydrogels, including micelle-laden [59], scavenging nanocomposite [60], chitosan [61], and porous nanocomposite [62], have recently been employed for water treatment. Among these, chitosan based hydrogels find extensive applications in the elimination of synthetic dyes like methylene blue and congo red away from wastewaters [63–66]. Further SAH has been reported where they are explored to find applications in the removal of chromium. (VI) ions [67, 68], dye removal [69, 70], desalination [71], removal of trace metal ions [72] etc. Gary W. Roach [73] utilized superabsorbent polymers (SAPs) for the exclusion of contaminants and wastewaters by mixing with sufficient amount of wastewater in order to absorb substantially and to stave off it from leaving a defined containment zone. The water was subsequently removed from the SAP, leaving behind a mostly dry polymer composition of the wastewater pollutants and the SAP. The polymer content is either thrown away or, if appropriate, used as plant fertilizer. Surprisingly, the SAP worked by binding to the ammonia and reducing the amount of ammonia gas in the air around the effluent to lessen the stench associated with wastewaters that include ammonia. A heat exchanger may be utilized during the evaporation process to create cool, fresh air that is used to cool buildings, especially those used for production and animal rearing.

4.3.1 Factors Influencing the Adsorption of Heavy Metal Pollutant Ions on SAHs The elimination of toxic metal pollutants from industrial effluents via adsorption or absorption using SAHs is not so simple. Several factors affect on how well SAHs

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can eliminate heavy metal ions from these industrial effluents. Few important factors among those have been discussed here that depict a direct effect on the elimination of heavy metal pollutant ions.

4.3.1.1

Effect of pH

The most important factor among all has been the pH of the aqueous solutions from which elimination of toxic metal pollutants has to take place. Guilherme et al. [41, 74] used the free radical polymerization approach to create polysaccharide superabsorbent (MPSA) hydrogel with few modifications by firstly, treating potassium hydroxide with AAc, adding a specified quantity of modified gum arabic (MAG), AAm and 84 mmol of sodium persulfate (SPS) used as a precursor. Two metal ions, i.e., copper (II) and lead (II) were removed using these synthesized SAHs as an ionic absorbent and based on the findings, it has also been reported that swelling of SAHs depends on the pH of the medium used throughout. When copper (II) and lead (II) were removed from a hydrated medium, it was noticed that the elimination of same metals increased as pH range rose out from three to five which is caused by the dehydronation of carboxylic acid groups in hydrogel network that are above the pKa value (4.0). As a result, the alkaline media produced an anionic environment that has in turn, created an ionic link with the + ve metal ions present in that medium and favored elimination procedures. Peng et al. reported the preparation of other pHsensitized hydrogels by grafting acrylic acid (AA) onto hemicellulose containing xylene. These samples were used to filter out heavy metal ions from the media, including lead (II), cadmium (II) and zinc (II) ions. According to the authors, the sample’s greatest adsorption capabilities for the heavy metal ions were as: 859 mg/g for lead (II), 495 mg/g for cadmium (II) and 274 mg/g for zinc (II). The scientists also stated that these increased capacities of hydrogels to eliminate heavy metals from solutions were the result of medium’s increased pH value, which is brought on by the appearance of the carboxylic acid functional group. These scientists also stated that the appearance of a carboxylic acid functional group in that medium increased pH of that medium that in turn, increased an ability of hydrogels to absorb heavy metals out from the solutions. At high pH levels, the carboxylic acid group deprotonated to COO− ; as a result, the hydrogel network has more active sites available to eliminate the +ve heavy metal ions and the reverse was observed at lower pH levels.

4.3.1.2

Effect of Inceptive Concentration of Heavy Metals

Another factor which plays a key role in the elimination of toxic metal pollutant species from industrial wastewaters is the initial amount of the same metals present in hydrated solutions. Agnihotri and Singhal [75] described a unique method of synthesizing SAH using sodium humate (SH), sodium alginate (SA) and acrylic

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acid (AA), (SH/SA/AAc) to create superabsorbent hydrogels named, poly (AAc-coNaAlg-co-SH) by a copolymerization method in the appearance of MBA by heating at the temperature of 70 °C for six hours. These SAHs were employed as sorbents to absorb dyes—MB and CV and heavy metal ions like lead (II), iron (II) and copper (II) out from aqueous solutions. These researchers noticed that the adsorption capacity in mg/g rose as per inceptive volume of heavy metal ions increased. This is because the heavy metal ions created a force that caused the mass to be transported from the hydrated media to adsorbent surface. They plotted the figures [75] based on an investigation on variation in adsorption capability as the function of inceptive concentration of metals. For instance, the trends are plotted for initial concentration of copper (II) ions by using AAc/NaAlg/SH SAHs taking various amounts of SAH against adsorption capability. Similarly, they plotted for initial concentration of iron (II) ions by taking AAc/NaAlg/SH superabsorbent hydrogels along with the varied volumes of SAH and for initial concentration of lead (II) ions by taking AAc/NaAlg/SH superabsorbent hydrogels along with the varying amounts of SAH against adsorption capability.

4.3.1.3

Effect of Heat on Elimination of Heavy Metals

It is important to know the optimum temperature in order to do optimum elimination of toxic metal pollutants from wastewaters as temperature is an important parameter in the aforesaid processes. The preparation of poly acrylamide copolymerized with acrylic acid (poly AAm-co- AA/Na zeolite) superabsorbent hydrogels for the sequential elimination of heavy metal ions such as cadmium (II) and lead (II) was investigated by Zendehdel et al. [76]. They also investigated the impact of temperature (between 285 and 323 K) on the elimination of same heavy metal ions from hydrated solutions. The adsorption of heavy metal ions was found to be increased by enhancing the temperature from a range of 285 K to 303 K, which suggests that the process is endothermic. The reverse process i.e., desorption of same metals from the hydrogel network, however, caused the adsorption of heavy metal ions to decline as temperature was raised further, up to 323 K.

4.3.1.4

Effect of an Adsorbent Dose on the Elimination of Heavy Metals

Another important factor which leads over the others is the appropriate adsorbent dose which can eliminate the maximum amount of toxic metal pollutants from the wastewater samples. Sharma and Tiwari [77] prepared copolymer p(IA-co-AAm) consisting of acryl amide and itaconic acid (IA) and the synthesized samples were very well evaluated under various environmental circumstances for an elimination of manganese (II) ions from polluted aqua. They investigated and provided a brief report on how the amount of adsorbent affected on the elimination of manganese (II) ions. It was discovered that boosting the doses of absorbent ranging from 0.04 g to

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0.10 g resulted in a greater percentage of manganese (II) ions being removed from contaminated wastewater.

4.4 Conclusion The increasing industrialization is only enhancing the manufacturing operations but promoting the market for economic expansion also. On the contrary, these industries grievously contaminate the natural water bodies with a number of toxic metals and other pollutant species. Hence, before the effluent is discharged into streams and rivers, harmful heavy metals must be treated and removed. Superabsorbent hydrogels are more widely employed in the adsorption of heavy metal pollutant ions owing to their easy handling, inexpensive, furthermore effectiveness, biodegradable, reusable, and recyclable characteristics. These superabsorbent hydrogels’ more recent ability to eliminate heavy metal pollutant ions has caught the attention of researchers in this field. Additionally, they find it suitable for fabricating clothing to get thermal comfort. Acknowledgements The author is grateful and thanks to the Chancellor, Presidency University, Bengaluru for the seed grant RIC/funded projects/IR 1, dated 11.07.2018. Thanks are also due to Mr. Amit Prakash Varshney, Mr. Anirudh and Miss. Anushi Varshney for supporting to compile this work.

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Chapter 5

Superabsorbent Polymers Application in Agriculture Sector Jagdeep Singh, Ankit Kumar, and A. S. Dhaliwal

Abstract Super Absorbent Polymers (SAPs) are the new type of three-dimensional macro cross-linked hydrophilic polymers, which swell without dissolving on contacting with water or other biological fluids and by osmosis, will uptake about 100,000% of its own weight in a short span of time. In the soil, they form granules to enhance its properties and for slow release of agrochemicals. The various characteristics of these super absorbent polymers overcome the problems related to the degradation process, environmental and health issues. This chapter would prove to be a comprehensive review on various SAPs (natural and synthetic), their properties, preparation techniques, modification with specific functions and their applications with remarkable advantages in agriculture sector. Here the emphasis is given to the two key applications of SAPs, i.e., water absorbance and retention and as control release devices for agrochemicals. The future developments of SAPs have also been discussed in this chapter for providing a background for their amazing properties/performance in the field of agriculture. Keywords Synthetic polymers · Hydrophilic · Agriculture · Fertilizers

5.1 Introduction Green revolution has resulted into rise in agricultural productivity by the use of high yielding diversities of seeds, pesticides and fertilizers [1, 2]. This productivity has been achieved by the exploiting our natural resources such as water, soil and using fertilizers and pesticides tremendously [3–5]. The chemical fertilizers have been extensively used to attain maximum production of crop and its usages rises up every year by 2.5 million metric tons [6–8]. The constant and excessive use of chemical fertilizers has led to several adverse effects such as environmental pollution, extinction of a useful organism species, pest resurgence [9–11]. The uptake capacity/utilization efficacy of plant of these fertilizers to undergo biological action J. Singh · A. Kumar · A. S. Dhaliwal (B) Department of Physics, Sant Longowal Institute of Engineering and Technology, Longowal 148106, Punjab, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Arpit and T. Jaya (eds.), Properties and Applications of Superabsorbent Polymers, https://doi.org/10.1007/978-981-99-1102-8_5

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is usually not more than 50% [12–14]. Above 50% of this fertilizer leaks to the environment posing severe environmental hazards and its minimum concentration for the sustenance of biological activity has also been decreased [15, 16]. For this, chemical fertilizers have been applied again and again in the fields which causes several environmental and health problems [17, 18]. Hence, to overcome this problem and achieve food security for the increasing population of world, a new sustainable innovative strategy has been needed for agriculture and in these techniques, fertilizer has been entrapped inside the polymer and its controlled diffusion in an optimum manner to attain maximum output is gained at the minimal cost [19–21]. The increasing global demand for water resulting in the shortage of water in many regions of the world [22–24]. Due to this, water management is very important issues faced by all the countries in arid and semi-arid areas [25–27]. Water plays crucial role in agricultural practices for effective crop production and optimum amount is required for the growth of plants [28]. Presently, 70% of fresh water has been used in the agricultural sector in most regions of the world and resulting in to water scarcity [29, 30]. As a result of droughts and water shortages there has been soil desertification and salinization, challenging the sustainable development of food security and agriculture [31–33]. Hence, it is a significant application of these hydrogels and polymeric materials for water retention in soil. They can minimize the consumption of water for the irrigation purposes in dry regions, improve retention of fertilizer in soil, increasing the plant growth by reducing the death rate [34–37]. Super Absorbent Polymers (SAPs) are three-dimensional, water swellable, hydrophilic, cross-linked network, which can absorb and hold an enormous volume of water i.e., hundred times the dry weight of the polymer without dissolving and shape changing [38–43]. The swelling or ability to absorb large quantity of aqueous solutions is the remarkable property of SAPs and alteration in the chains by hydration and dehydration has been displayed in Fig. 5.1. The comparison of water absorbency of some common absorbent material with commercial SAP developed by Rahab Resin Co. (Iranian manufacturer) has been described below (Fig. 5.2):

Fig. 5.1 The volume phase transition of SAPs by hydration and dehydration

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Fig. 5.2 Comparison of water absorbency of various absorbents

SAPs can be categorised as natural (polysaccharide and polypeptide based) and synthetic (chemical based) [44, 45]. Chitosan, alginate, collagen, dextran, cellulose, chitin, etc. and some synthetic polymers such as acrylic acid, methacrylic acid, polyethylene glycol, vinyl acetate, polyvinyl alcohol, and various acrylate are a few natural polymers for the preparation of SAPs (detailed discussion of these material has been present in proceeding sections) [46, 47]. The various inherent properties of SAPs described below in Fig. 5.3, which makes them an appropriate aspirant for many applications in agriculture. Duo to above said excellent features of these materials it has capability to increase the availability of water to plant, act as soil conditioners, reduce bulk density and mitigate drought stress [48–51]. In addition to these, by controlled transport of water and nutrients the plant growth has been indorsed [52, 53]. Also, SAPs can act as a reservoir holding on to nutrients from fertilizer preventing them from leaching from the root zone [44, 54]. The important application of SAPs has been their incorporation into soil to increase water retention and consequently improve plant growth, increase permeability and releasing of agrochemical at optimum rates [55–58]. The rate of the releasing has been alternated by changing the polymer type, environmental conditions and types of soils, pH etc. [59–61]. This chapter deals with the recent developments in SAPs (natural and synthetic), classifications, preparation techniques and their properties and its applications with remarkable advantages in agriculture sector. The synthesis procedure employed for the polymer structure and its properties will be discussed in detail and remarkable findings in this direction are also highlighted. Moreover, the detailed discussion of applications of SAPs in agricultural sector has been done. Here the emphasis is given to the two key applications of SAPs, i.e., water retention and control release of fertilizer and pesticides. The future developments of

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Fig. 5.3 Characteristics of SAPs for application in agriculture sector

SAPs have also been discussed in this chapter for providing a background for their amazing properties/performance in the field of sustainable agriculture.

5.2 Classifications To retain moisture in the soil and to decrease water utilization during irrigation, superabsorbent polymers (SAPs) can be used effectively due to their super high-water absorption and retention capacity [62, 63]. Based on their mode of derivation, SAPs can be categorized into natural and synthetic polymers. There are various advantages and disadvantages of these SAPs as natural polymers e.g., cellulose and starch have the advantage of degradability but due to the low rate of water absorption, they have to use in a larger quantity. Whereas SAPs with synthetic polymers e.g., polyacrylic acid (PAA) and polyacrylamide (PMA) are favoured due to their cost-effectiveness, durability and high rate of absorption but because of their non-degradable nature, they can damage the plant growth [64]. The detailed description of natural origin and synthetic origin as described in the next sections.

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5.2.1 Natural SAPs Natural polysaccharides are biological macromolecules of natural origin (algal, plant, animal) formed via glycosidic linkages of monosaccharides. Natural Polysaccharides has been used tremendously in the various applications because of their excellent properties such as high molecular weight, high solubility, pH stability, nontoxicity, and gelling characteristics [65]. Polysaccharides have a variety of reactive functional groups in their chemical composition, including hydroxyl, amino, and carboxylic acid groups. Due to these functional group, the chemical modification has been taking place easily [66]. The polymer based on natural polysaccharides preferred over semi synthetic and synthetic polymer due to its characteristics, such as, inertness, swellability, easily availability, biocompatibility and biodegradability. Among polysaccharides, guar gum, cellulose, starch, chitosan, and xanthan gum are the significant polysaccharide described in the proceeding section and its structures are described in Fig. 5.4.

5.2.1.1

Guar Gum

The Guar gum (GG) is obtained from endosperm of the guar plant, Cyamopsis tetragonalobus of Leguminaceae family and is edible in nature, [67]. It is a non-ionic, branched-chain polymer, having straight-chain mannose units joined together by βD-(1–4) linkages having α-D-galactopyranose units attached to this linear chain by (1–6) linkages. GG is included in the food and drug administration standards. GG is often used as a thickener, binder of free water and used in ice cream mixer as a stabilizer. It has an effective application as a viscosifier in soups, sauces and gravies

Fig. 5.4 Chemical structure of SAPs based on natural origin

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etc. alone or in combination with xanthan gum [68]. It is used in diverse in fields due to its properties, such as, hydrophilic, biodegradable, excellent biocompatibility, eco-friendly nature, low cost, abundant availability and suitability [69]. Recently, the hydrogel based on GG has been used as release device. In order to induce the stimuli effect in this hydrogel, chemical modification and incorporation nanofiller inside the polymer matrix has been carried out.

5.2.1.2

Cellulose

Cellulose is the most prevalent organic compound that comes from biomass [70]. There are many natural sources of the origin of cellulose such as wood, plant fibres, marine animals etc. [71]. Cellulose consists of a number of hydrophilic groups on the chain, which makes it a “water retention agent” for agriculture applications, in building materials [72]. Because of the highly dense crystalline structure of pure cellulose, it shows low absorbency (below 30 g/g) in filtered water. The water absorbency can be increased by modifying the crystalline structure of cellulose through etherification, esterification and sulfonation of hydroxyl groups [73–76]. In 2019 Jiang Song et al. [77] studied the etherification reaction with 2-chloroethanol and cellulose. The maximum water absorbance capacity of the hydroxyethyl cellulose attained the value of 240 g/g as compared to 200 g/g of wheat straw implanted on the poly(acrylic-co-acrylamide) suggesting the effect of etherification. The esterification of cellulose has been achieved by condensation of the carboxylic acid or acyl chlorides with a cellulosic group [76]. Further, the maximum water absorbency of cellulose after esterification crosslinking with 1,2,3,4-butane tetracarboxylic dianhydride was found to be 987 g/g [78]. Further, the water absorbance capacity of modified cellulose can be increased by combining it with synthetic SAPs to improve salt resistance and mechanical properties [79, 80].

5.2.1.3

Xanthan Gum

Xanthan gum (XG) was discovered in 1950s at the National Centre for Agricultural Utilization (previously known as Northern Regional Research Centre) of the United Sates Department of Agriculture (USDA). It is non-toxic and non-sensitizing and it does not inhibit growth or cause eye or skin irritation. Due to this, United States Food and Drug Administration (FDA) has approved xanthan for use in food additive. It is an anionic, natural edible polysaccharide derived from Xanthomonas campestris. Repeating pentasaccharide units consisting of two D-mannopyranosyl units, two Dglucopyranosyl units and one D-glucopyranosyluronic acid are the constituents of the primary structure of xanthan [81]. 1–4 linked β-D-glucopyranosyl units are the backbone of xanthan. It has been widely used in the pharmaceutical industries due to their excellent properties such as high molecular weight, high solubility, pH stability, nontoxicity, and gelling characteristics [82, 83].

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Starch

In nature, starch is the second most prevalent biomass after cellulose. The primary sources of starch are maize, wheat, and potatoes [84]. It is a biomacromolecule made up by polymerization of glucose mainly used as a water-retaining agent due to its strength in hydrophilicity, degradability, and environmental friendliness. The starch is weak in mechanical stability, salt tolerance, and water absorbency (below 10 g/g in distilled water). The water absorbance capacity of starch is improved by grafting with acrylic acid (AA). The water absorbency of the AA polymerized sulfamic acidmodified starch in deionized water was 1026 g/g due to the adsorption and transfer of water molecules by the sulfonic acid groups [85]. The ratio of amylose/amylopectin affects the properties of the starch. If the amount of amylopectin is more than viscosity increases which decrease the mobility of chains. Whereas an increase in the content of amylopectin increases grafting efficiency and swelling capacity [86]. The storage modulus of cassava starch is found to improve when 2-acrylamido-2-methyl1-propane sulfonic acid (AMPS) was added forming the perfect network structure. The swelling ratio in distilled water reached 1200 g/g [87]. By using hybrid starch and inorganic fillers, water absorbency is found to be improved [88]. Further, by the addition of chemically modified natural char nanoparticles (NCNPs) into starch SAPs the water absorbency was found to increase (390 g/g in distilled water) [89].

5.2.1.5

Chitosan

Chitosan is mainly found in the outer regions of crabs and shrimps and on the cell walls of fungi [90]. It is made of glucosamine building blocks with a linear polysaccharide chain. It is preferred as a water retention agent due to its fundamental properties such as excellent biocompatibility, biodegradability, and repeatability. The low water absorbency (below 10 g/g) in chitosan can be enhanced by chemical modifications [91]. Fang et al. polymerize the (2-pyridyl) acetyl chitosan chloride PACS) with acrylic acid and acrylamide in an aqueous solution and found that water absorbency increased to 615 g/g (distilled water) and 44 g/g (0.9 wt% NaCl solution) [92]. Further water absorbency and salt tolerance of AA-grafted chitosan were found to be improved after the increase in the number of amino groups on the chitosan backbone [93].

5.2.2 Synthetic SAPs Synthetic polymers such as Polyacrylic acid, Polyacrylamide, and Polyvinyl alcohol used to make SAPs have better water absorption and retention properties as compared to natural polymers [94, 95]. Synthetic SAPs are preferred over natural SAPs because the extraction process of synthetic SAPs is easy. As synthetic polymers don’t degrade in soil and thus remain in the soil for many years causing environmental issues. In

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Fig. 5.5 Chemical structure of SAPs based on synthetic origin

order to balance the antagonistic association between water absorption and retention and biodegradability, synthetic SAPs are combined with natural polymers [96, 97]. Synthetic SAPs based on Polyacrylic acid (PAA), Polyacrylamide (PAM) and Polyvinyl alcohol (PVA) are the important and described in the proceeding section and its structures are described in Fig. 5.5.

5.2.2.1

Polyacrylic Acid (PAA)

Polyacrylic acid (PAA) is one of the most used synthetic polymers as a water retention agent due to its high-water absorbency (300–500 g/g in water). However, the salt resistance and degradability are less as compared to natural polymers. Recent studies have shown that on increasing the mechanical strength and salt tolerance capacity of SAPs by adding clay such as kaolin and bentonite is reported [98, 99]. Yang Xiang et al. examined the water absorbency of a semi-interpenetrating polymer network (semi-IPN) slow-release fertilizer and reported a value of 68 g/g in tap water [100]. There are further studies on the addition of natural polymers to the cross-linked copolymer systems of AA and 2-acrylamido-2-methyl propyl sulfonic acid (AMPS) to attain better absorbency and degradability [101–103]. Sodium humate, which is a natural organic material with a large number of carboxylates and phenolic hydroxyls, when grafted on P(AA-co-AMPS) the water absorbency increased to 1380 g/g in distilled water [104].

5.2.2.2

Polyacrylamide (PAM)

Polyacrylamide (PAM) is the most preferred synthetic SAP after PAA because it shows a similar water absorbency capacity as PAA. But its utilization is greatly hampered due to poor mechanical qualities and limited salt tolerance. Therefore, in the last few years, new methods have been put forward to improve the properties of PAM [105]. Sufen Zhang et al. studied the water absorbency in starchgraft-polyacrylamide (St-g-PAM) superabsorbent synthesised using an electron beam of 10 meV irradiation and reported the absorption of 1452 g/g and 83 g/g for distilled water and saline solution, respectively [106]. Further, the addition of hydrophobic monomer increases the water absorbency of synthetic polymers.

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The product obtained after grafting sodium 2-acrylamido-tetradecanesulfonate, hydrophobic monomer, soluble in starch and AM shows the water absorbency of 2537 g/g in distilled water and 93 g/g in saline solution [107]. Further, the mechanical properties of this combination were found to improve.

5.2.2.3

Polyvinyl Alcohol (PVA)

The synthetic hydrophilic polymer, Polyvinyl alcohol (PVA) has a medium water absorbency as compared to PMA. Whereas the harmful effect of PVA is less as it is degraded by bacterial enzymes. A decrease in water evaporation in the soil is overserved when 30% PVA aqueous solution is sprayed [108]. On the formation of semi-interpenetrating polymer network (semi-IPN) by the use of 3 wt% PVA and hemicelluloses-g-AA/bentonite matrix (HAB), the absorbency was found to be increased. The maximum absorbency was found to be 1200 g/g in distilled water which is 25% higher than the absorbency of HAB [109]. Weishuai Wang et al. studied the slow-release fertilizer (CSC-g-AA/APP, CSC-g-AA/PVA-APP), based on corn straw cellulose (CSC) polymer and linear polyvinyl alcohol (PVA) and reported the slow-release of N and P to soil with good water absorbency of 303 g/g in distilled water [110]. Further, cross-linking of PVA and borax and then adding Moringa oleifera causes the swelling ratio of 1163 g/g in deionized water and maintains better water retention capacity under high temperatures [111].

5.3 Preparation Techniques of SAPs The properties of SAP are dependent upon the preparation technique used for it and also on the original source used for preparation. A number of commonly used techniques for the preparation of SAP including solution polymerization, bulk polymerization, inverse suspension and radiation polymerization.

5.3.1 Bulk Polymerization It is the simplest technique which comprises the polymerization of monomers by initiators, radiation, or by heat in the absence of solvents and dispersants [112]. The choice of appropriate initiator is determined by the solvent and monomer type used for polymerization. Due to high monomer concentration the rate and degree of polymerization are high. An increase in the viscosity is observed as the reaction proceeds which causes the heat production in the course of polymerization. These issues can be avoided by controlling reactions at lower temperatures and low concentration of initiators [113]. High molecular weight polymers are produced by

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bulk polymerization with high purity, which is the salient feature of this technique [114].

5.3.2 Solution Polymerization The free radical initiated polymerization or solution polymerization is commonly used to synthesise SAPs which involve the polymerization of acrylic acid and its salts. The initiation in this polymerization occurs by a redox initiator system, thermal, or UV-irradiation [115–118]. When the resultant polymers can dissolve in water, the process is known as homogeneous solution polymerization and if cannot dissolve in water, then the process is known as heterogeneous polymerization. This technique is preferred over bulk polymerization due to monitoring over reaction temperature by controlling viscosity using solvent [119, 120]. The gel-like elastic product is produced after a fast exothermic reaction which is washed with distilled water/ethanol to remove impurities, unreacted monomers, and initiator. After that formed gel is pulverized and sieved to attain the required particle size. This method is less preferred due to the formation of irregular particle size, lack of reaction control, and handling of rubbery/solid products [121]. Despite being few demerits, the solution polymerization method is preferred because it is a faster and more economical method to produce SAPs [122].

5.3.3 Inverse Suspension Polymerization Inverse suspension polymerization also known as dispersion polymerization involves a water-in-oil process which is opposite to the commonly used method of oil-inwater, which is why this technique is called “inverse-suspension polymerization”. A spherical-shaped SAP is produced by using this method with particle size between 1 micron to 1 mm and thus lower the grinding issues which is an advantage over other techniques. This method involves the homogenous mixture of the monomers, and the initiator is disseminated in the hydrocarbon phase. Further, the particle size of resins and form are primarily controlled by the viscosity of the monomer solution, rotor design, and dispersant type [123–126]. To create SAPs with a high capacity for swelling and quick absorption kinetics, the inverse-suspension method is a highly adaptable and versatile one [127]. A water-soluble initiator exhibits more efficiency as compared to an oil-soluble initiator. After the dissolution of initiator in the dispersed phase, all the particles act as isolated micro-batch polymerization reactor [121]. By filtration or centrifugation, the resultant micro spherical particles from the continuous organic phase are simply removed. These granules or beads are instantly produced into powder that flows freely after drying. These particles or beads will instantly produce a free-flowing powder after drying. When compared to the solution approach, the inverse-suspension procedure exhibits additional benefits in

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addition to the special flowing characteristics of these beads. There are more options for altering particle structure or morphological alteration, as well as improved control over the reaction heat removal and ab initio regulation of particle-size distribution [45].

5.3.4 Radiation Polymerization Radiation polymerization involves the polymerization using the high energy radiations such as gamma rays and electron beams to produce SAPs [128–130]. Due to the irradiation of aqueous solution radicals are created on the polymer chains [131]. Hydroxyl radicals are created because of the radiolysis of water molecules, and they attack polymer chains to create macro radicals. A cross-linked structure is created when these macro radicals unite once more to form covalent bonds. This method is beneficial over the others as the final product is moderately pure because of the absence of use of a chemical initiator. Further, the grafting initiated by radiations is advantageous as it takes less time than grafting initiated by chemicals, preventing time wastage [132]. The advantages of the above discussed synthesis procedures have been described in Table 5.1.

5.4 Evaluation of Performance of SAPs SAPs has many characteristics properties as discussed earlier and the performance of these materials has been evaluated through water absorbency (%), water retention rate and loading and release kinetics. Table 5.1 Comparison of different preparation techniques Name of the preparation techniques

Characterization

Bulk polymerization

This technique produces high molecular weight polymers with high purity [114]. But the viscosity increases as the reaction proceeds which causes the heat during polymerization

Solution polymerization

This method is preferred because it is a faster and more economical method to produce SAPs. But avoided due to the formation of irregular particle size, lack of reaction control, and handling of rubbery/solid products [121]

Inverse suspension This method is used to produce spherical-shaped SAP with particle size polymerization between 1 micron to 1 mm and thus lower the grinding issues Radiation polymerization

The final product produced through this method is moderately pure because no chemical initiator is used

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5.4.1 Water Absorbency Performance The swelling performance of the SAPs has been executed at different reaction parameters such as diverse temperatures, various time intervals and different pH in distilled water or saline solution. For this, immersion of dried SAPs samples in excessive water at a given physicochemical condition until the achievement of swelling equilibrium. The swollen sample is then taken out from water, afterwards it is gently wiped for removing excess liquid and then it is weighed. The percentage swelling (Ps) has been calculated using the Eq. 5.1 [19]. PS (% Swelling) =

WS − Wd × 100 Wd

(5.1)

where WS and Wd are the weights of the swollen and dry SAPs samples, respectively.

5.4.2 Water Retaining Potential The water retaining potential of SAPs has been examined in different soil samples. Known amount of SAPs has been crushed well and then mixed with some known amount of different soil samples and placed in plastic container with slowly addition of water. Then, by using analytical weighing machine resulting mixture is weighed. For several days these samples are kept in shade at room temperature and the containers are weighed everyday until there is no any noticeable weight loss is observed. The following equation is used to calculate the water evaporation ratio (W %) of different soil samples [19]: ( W% =

Wi − Wf V

) × 100

(5.2)

where Wi and Wf are initial and final weight of container and V is volume of water taken in mL.

5.4.3 Loading and Release Kinetics of Agrochemical The SAPs has been placed in the saturated aqueous solution of agrochemical for 24-h to attain the swelling equilibrium and a hot air over is used to dry it until no noticeable change in weight is observed. The following equation has been used to calculate the loading percentage [133]: loading percentage =

m1 − m0 × 100 m0

(5.3)

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where m 1 and m 0 are the weights of loaded and unloaded sample, respectively. The release profile of agrochemical in aqueous solution as releasing medium has been studied by placing the loaded SAPs in beaker and releasing concentration has been quantified. The mathematical modelling of releasing which quantify the mechanism of diffusion through SAPs has been described as below:

5.4.3.1

Mathematical Analysis of Releasing of Agrochemical

The releasing has been predicted using Korsmeyer–Peppas mathematical modelling which can predict the Fickian and Non-Fickian diffusion. The mathematical form of this model which is based on Fick’s law is expressed as [134, 135]: Mt = kt n M∞

(5.4)

Here Mt and M∞ represent the amounts of agrochemical released at time t and equilibrium time, respectively. Here ‘k’ is gel characteristic constant of SAPs system and ‘n’ is the diffusion exponent, which categorized the different release mechanism. For n ≤ 0.5, release is Fickian; release is non-Fickian or anomalous when the value of n lies between 0.5–1.0. The ( n)= 1, is said to be Case II diffusion [136]. The slope and intercept of the plot ln MM∞t vs. ln(t) gives the value of n and k, respectively. The diffusion process through the SAPs has been describe by Fick’s first and second laws. The values of initial, average and later diffusion coefficient are calculated from the following equations [137, 138]. Mt =4× M∞

(

Dt πl 2

)0.5 (5.5)

where D is the diffusion coefficient, Mt /M∞ is fractional release by SAPs, Mt and M∞ are the Agrochemical releasing at time t and at equilibrium, l is the thickness of the sample. The initial diffusion coefficient (Di ) is calculated from the slope of the plot between Mt /M∞ versus t 0.5 using equation [135]. ( Di =

(slope)l 2 π 2

) (5.6)

The average diffusion coefficient can be calculated from the following equation [139]: DA =

0.049 × l 2 t 0.5

(5.7)

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where t 0.5 is the time required for 50% releasing of Agrochemical. The later diffusion coefficient is determining from the following equation [140]: Mt =1− M∞

(

8 π2

)

(

−π 2 Dt exp l2

) (5.8)

) ( The slope of the plot between ln 1 − MM∞t and t has been used for determination of later diffusion coefficient using equation [141]. ( DL =

slope×l 2 8

) (5.9)

5.5 Application of SAPs in Agriculture Sector The SAPs have many features discussed earlier, which make them as an appropriate candidate as a controlled/slow-release device, water utilization by superabsorbent polymers and others application such as improving the soil properties and enhances plant growth. The detailed discussion on these applications is as below.

5.5.1 Controlled/ Slow Release (C/SRs) Devices The SAPs can be used as device for controlled and slow-release fertilizers (C/SRFs) that can gradually release soluble nutrients over a long period of time [142]. The SRFs formulation has many advantages as compared to normal water-soluble fertilizer which quickly dissolve and leach upon applying to media [143]. Moreover, granular fertilizers (dry, pellets) also quickly dissolve in medium when exposed to moisture, can harm plants, and have high leaching rates [144]. These SRFs has been used in nursery production in order to lessen nutrient losses brought on by intense irrigation or rainfall as well as wide temperature variations [145]. Controlled-release fertilisers (CRFs) consist of encapsulated solutions in a semi-permeable membrane, surrounded by a hydrophobic polymeric coating [146, 147]. The fundamental idea behind slow-release chemical fertilizers is that they gradually release the nutrients they contain, allowing for optimal uptake and usage while avoiding losses from leaching, volatilization, or excessive growth. The SRFs have many advantages as compared to tradition fertilizers system and are described in Fig. 5.6 [148–150]. The hydrophobic polymeric coating on SRFs assists in limiting exposure of the core of the SRFs device to moisture, allowing for slow release of nutrients by diffusion mechanism as described in Fig. 5.7 [151, 152].

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Fig. 5.6 Advantages of SAPs system

Fig. 5.7 Mechanism of slow release of nutrients (SRFs system)

The sulphur coating based SRFs are relatively inexpensive and has low solubility and allow the diffusion of nutrients through small pore opening on coating material [153]. The advanced SRFs system contains polymer coatings which can control the releasing of nutrients. The releasing rate has been adjusted by changing the coating thickness as well as by changing the composition of polymer. A product with a thicker polymeric coating will have a longer lifespan [154]. Devices for releasing fertilizer based on hydrogel have grown in popularity recently. They are composed of fertilizer enclosed in a polymer network as a microcapsule or granule. They can be used successfully for fertilizer encapsulation. In addition to delivering a delayed release of fertiliser, hydrogel-based fertilizer delivery systems bond the soil and improve its ability to retain water [155–157]. Various controlled release formulation based on the natural polymers such as alginate, lignin, ethyl cellulose and starch are reported by many researchers and these formulations have provided the slow release of agrochemicals. A lot of work has been reported on the encapsulation of pesticides into starch for the controlled release of insecticide and endosulfan [158, 159].

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In 2002, Bajpai and Giri prepared a polymeric network of carboxymethyl cellulose (CMC), polyethylene glycol (PEG) and cross linked polyacrylamide (PAM) and they used as a carrier of agrochemicals (such as KNO3 ) [160]. They showed that the loaded hydrogel continuously releases KNO3 up to 10 days. The release rate is increases with increasing the concentration of CMC, PEG and AM. Later, in 2003, Bajpai and Giri also synthesized the carboxyl methylcellulose-g-polyacrylamide hydrogels via a free radical polymerization method for release of potassium nitrate as agrochemical. It has been discovered that the swelling ratio increases in the concentration range (0.2–0.4 g) of CMC, followed by a decrease [161]. Slow-release nitrogen fertilizer (SSRNF) made by inverse suspension polymerization was reported by Liu et al. in 2006. The results show that the product has good slow-release properties as well as excellent soil moisture preservation capacity, which could significantly increase the simultaneous use of fertilizer and water resources [162]. In 2007, B. Singh, Sharma, and Gupta develops hydrogels based on starch poly(acrylamide) and starch poly(acrylic acid) for the regulated and sustained release of agrichemicals. They concluded that the release of thiram from the hydrogels happens in a very regulated manner and that it remained constant from 60 to 108 h [163]. The release of thiram reduces as the cross-linker concentration in the polymer matrix increases. In 2008, they have created an agrochemical delivery system based on starch and poly(methacrylic acid) for its regulated and sustained release. According to the findings, the release of thiram from hydrogels increases over time and, after a given period, occurs in a very controlled and sustained manner [158]. In 2009 Liang et al. developed a novel wheat straw-g-poly(acrylic acid) (WS/PAA) superabsorbent composite by using the graft polymerization process. The synthesized superabsorbent has good swelling behaviour and as a controlled device for releasing of urea in water and soil [164]. In 2010, Aouada et al. investigated the polyacrylamide and methylcellose hydrogel as a controlled release of paraquat. In general, the initial rate of paraquat release was rapid, but it gradually decreased over several days, demonstrating that paraquat on the surface of hydrogels dispersed rapidly after the gel swelled. Later, with the paraquat content remaining constant between 15 and 46 days, the cumulative release took place in a very regulated and sustained manner. The matrix swelling and network chain density both had an impact on the paraquat release capacity. The continuous release of the herbicide paraquat can be delayed for up to 45 days using this hydrogel-based carrier [165]. Ni et al. (2011) reported slow-release formulations of nitrogen fertilizer based on natural attapulgite (APT) clay, ethylcellulose (EC) film, and sodium carboxymethylcellulose/hydroxyethylcellulose (CMC/HEC) hydrogel. These results revealed that the synthesized hydrogels can effectively reduce nutrient loss, improve use efficiency of water, and prolong irrigation cycles in drought-prone environments [166]. In Rashidzadeh, Olad, and Reyhanitabar’s (2015) work, commercial NPK fertilizer was coated with a hydrogel/clinoptilolite (Hyd/CL) nanocomposite to create a new core–shell slow-release fertilizer formulation. They demonstrated that, in comparison to uncoated fertilizer, the release of fertilizer from Hyd/CL nanocomposite-coated fertilizer granules into soil occurs in a more regulated manner [167].

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Giroto et al. (2017) fabricated a nanocomposite based on starch/urea (TPSUr) blends as a matrix in which hydroxyapatite particles (Hap) were dispersed at ratios 50 and 20% Hap. The controlled release experiment is conducted in order to inspect the nanocomposite properties. The results revealed that there is interaction in between the fertilizer components and the morphological changes in the nanocomposites and synthesized nanocomposites provided a controlled release of urea [168]. Olad et al. (2018) investigated the gradual release of NPK fertilizer formulation through the in situ free radical graft polymerization of salep with an acrylic acid (AA) monomer in the presence of montmorillonite (MMT) [169]. Its swelling and water retention behaviour was investigated. The outcomes showed that the synthetic hydrogels had good water-retention abilities as well as pH and salt-sensitive swelling behaviours. The presence of MMT caused the formulation system to release the nutrient in a more regulated manner, according to fertiliser release trials. In 2018, Ghobashy et al. reported synthetic and natural hydrogel-based acrylamide and gelatin by gamma radiation polymerization technique as fertilizers carrier and soil conditioner [170]. The outcomes showed that the chemical conversion of (PAAM/G) into (PAAMCOOK/G) could significantly increase water absorbency and has a good impact when used as a carrier for fertilizer and a soil conditioner. Table 5.2 represent the various SAPs developed for controlled release of fertilizer or other agrochemicals.

5.5.2 Soil–Water Dynamics The use of SAPs for soil water retention is an important application. The use of SAPs can decrease the amount of water needed for irrigation in arid areas, improve fertilizers retention in the soil, lower the death rate of plants, and increase plant development. Additionally, by promoting soil permeability and infiltration rates, reducing erosion and water runoff, raising plant productivity, and boosting soil aeration, it can also improve the physical qualities of soil. Various researchers have been studied the influence of various SAPs on soil –water properties and characteristics in lab of field scale experimentation. In 2018, Abrisham et al. [182] reported that the application of SAP (g dm−3 ) increased the available water content up to 68.5% and decreased soil bulk density by 25.5% and soil infiltration rate by 21.5%. In addition, the application of SAPs can significantly increase the soil cation exchange capacity up to 31% as compared to the control. In a sandy loam soil study by Rostampour et al. (2013), 225 kg ha−1 (0.015% BM (by mass)) raised leaves’ relative water content from 61 to 78% while 75 kg ha−1 (0.003% BM), the lowest application of SAP, only increased 6%. Yang et al. (2015) reported the 110-day drip irrigation on a variety of soil types, including built soil (sand, loam, and silt) amended with 0.6% BM (by mass) in the depth of 20 to 30 cm. The findings show that the SAP altered layer had a 12.5% volumetric water content, compared to only 3.5 and 4.5% for the layers above and below, demonstrating that SAP had an impact on both downward water transport and retention [184]. Yu et al. (2012) [185] reported the application of 0.5% BM of SAP on loamy sand, sandy loam, sandy clay loam, and clay loam soils. After seven hours

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Table 5.2 Examples of various SAPs used for Controlled releasing Name of SAPs

Name of agrochemical

Release property used

References

XG/Poly(AA)/AgNPs

Potassium chloride

Diffusion control

[19]

SAPSRPF

Phosphate

Diffusion control

[171]

P-CMS-g-PAM

Phosphorous

Diffusion control

[172]

Poly(PEG-co-PLGA)

Metolachlor

Diffusion control

[173]

Starch-modified poly(acrylic acid)

Urea

Diffusion control

[174]

IPN Based on Pre-vulcanized Natural Rubber and Cassava Starch

Urea

Diffusion control

[175]

P(AM-co-NHMA)

Urea

Diffusion control

[176]

Xanthan gum

Biofertilizer

Diffusion control

[135]

Wheat straw-g-poly(acrylic acid)

Urea

Diffusion control

[164]

MSB/PAA/PHR

Potassium

Diffusion control

[177]

Poly(acrylamide-co-acrylic Acid)/AlZnFe2 O4

Potassium

Diffusion control

[178]

Gelatin-Tapoica/polyacrylamide

KNO3

Diffusion control

[179]

Zeolite reinforced Cellulose-Na + -g-cl-poly(AAm)

Phosphate

pH responsive

[180]

NaAlg-g-poly(AA-co-AAm)/RHA

Nitrogen (N), phosphorous (P), and potassium (K) (NPK)

pH responsive

[181]

Zeolite reinforced Cellulose-Na + -g-cl-poly(AAm)

Phosphate

pH responsive

[180]

Xanthan gum

[133]

of drying at 60 °C, the soils samples retained 0.12–0.35 g/g of water as compared to soils with no SAP. Banedjschafie and Durner (2015) reported the significantly increment of plant available water from the control (0.005 g g−1 ) by 0.56, 0.20, 0.27 g g−1 , respectively under the SAP application to dune sand at rates of 0.3, 0.6 and 1% SAP + soil [186]. Li et al. [187] stated that plant available water significantly increased for the first two stages of wheat growth from 12 to 14.4–15.1% and 10 to 12.6–14.8%. Moreover, in another studies Li et al. reported the application of dry SAP having 200 Kgha−1 to a loam soil surface which significantly increased water content for various stages of wheat growth from 13 to 14.8–15.2%, 10 to 12–13.9% and 6 to 9.8–10.1%. Similarly, Dehkordi et al., 2018 [188] reported the application of 180 kgha−1 (0.008% BM) SAP (wet and dry) which can significant increase the water holding capacity from 11.6 to 15.8–16.2%, at the stem elongation growth stage, similar increases were seen in heading stage. Wilske et al. [189] (2014) reported the application of 0.05% BM which significantly reduced soil water holding capacity of a loamy-sandy texture range of soils and also degraded by 0.45 to 0.76% after six

5 Superabsorbent Polymers Application in Agriculture Sector Table 5.3 Examples of various SAPs used as soil conditioner and water retention in soil

Name of SAPs

101 References

Poly(acrylamide-co-acrylic Acid)/AlZnFe2 O4

[178]

Single and double-coated urea with PAN-based PAAc

[191]

Borassus aethiopum starch and Maesopsis eminii [192] NPK fertilizer modified with PVA and CS

[193]

Acrylamide and Acrylic Acid Hydrogels

[194]

PVP/CMC

[195]

Crosslinked cellulose-acrylamide

[196]

months. In 2010, Bai et al. [190] reported the wetting and drying cycles SAP application of 0.05, 0.1, 0.2 and 0.3% increased water content by 6.2 to 32.8% relative to the control whose water content range was 14.3 to 85.6%. In summary, the utilization of SAPs can influence soil–water dynamic and determining the right rate of water to crop and influenced by soil texture, method of application, and water availability. Table 5.3 represents the large number of SAPs used as soil conditioner and water retention in soil and for soil–water dynamics.

5.5.3 Effects on Soil Properties The utilization of SAPs in a soil has been alteration technique that can improve the properties of soil [197]. SAPs can readily absorb water, improve water retention, and preserve soil water by inhibiting evaporation and also water absorbed by SAPs can be use by crops in dry condition [190, 198]. The use of SAP also reduces soil erosion and runoff, and also improve soil structure and enhance aggregate properties, soil infiltration and fertilizer conservation [199–201]. The detailed discussion on enhancement in the soil properties by virtue of SAPs has been described as below:

5.5.3.1

Bulk Density

Bulk density is the significant parameter whose value depend upon soil bulk density include soil organic matter, soil texture, and the density of soil minerals. After four wetting–drying cycles on a sandy clay loam soil, Bai et al. (2010) reported a significant drop in bulk density from 1.27 to 1.17 g cm−3 [190]. According to Hou et al. [202], the bulk density of a sandy loam soil decreased from 1.72 and 1.71 g cm−3 to 1.63 and 1.56 g cm−3 , respectively, after 90 kg ha−1 of a 0.1% SAP + Soil mixture was applied. In 2021, Silva et al. stated the 10% reduction in the bulk density by adding 0.5% of SAP [203]. Prabha et al. (2014) reported the bulk density reduction from 1.05 to 1.00 g cm−3 with 0.5% BM application under two watering intervals

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with porosity increasing from 67.7 to 77.9% [204]. Hence from the above studies it has been confirmed that SAPs application on soils can reduce the bulk density.

5.5.3.2

Chemical Properties

The application of SAPs in field may influence the soil chemical properties, such as pH, CEC (cation exchange capacity), EC (electrical conductivity), and nutrient retention. Various researcher examines the influence of SAPs on the changes in the soil chemical properties with varying dosage of SAPs, time of application, buffering capacity of the soil etc. These parameters are significant because the growth of plants have been affected. Prabha et al., 2014 [204] reported the change in the pH of soil by the application of SAP. The 0.5% powdered SAP has been applying to sandy clay loam soil for two watering intervals over the two-week study period. Even though the pH of the soil did not change between watering intervals, it did decrease from 6.04 to 5.7 to 5.8, and this fall was attributable to the carboxylate ions present in the SAP. The impact on soil pH may have been amplified by SAP’s larger surface area. Bai et al. [190] described the utilisation of sodium polyacrylate based SAP at a rate of 0.3% BM which does not change the pH of soil. But, the usage of potassium polyacrylate applied at 0.05% BM initially increased soil pH but after 12 days pH decreased relative to the control soil sample. The change in the soil pH has been attributed to the different chemical composition of SAPs applied, as well as wetting and drying process. The use of SAPs can also affect the EC (electrical conductivity) of soil sample. In 2014, Prabha et al. [204] reported the increment in the EC of soil sample with the application rate of 0.5% SAP on sandy clay loam soil. The controlled soil sample had an EC of 2.95 dSm−1 , whereas after 13 weeks a treated soil increased EC to 5.3 dSm−1 . The increase in the EC of soil has been attributed to the presence of acids and ions present in SAP. Bai et al., [189] reported the increase in EC with changing water states in the initial stage but after 12 and 21 days after application, the EC significantly decreased 51.7–65.5%, and 42.5–51.3%, respectively, relative to the control soil sample having EC is 0.3 dSm−1 for two polymer types. Hence it may conclude that SAPs effects the EC of soil samples. Similar to EC the application of SAPs can also alter the cation exchange capacity of soil samples which may influence soil fertility and plant growth. Abrisham et al. [182] reported the application of SAP having amount of 3 g dm−3 (0.2% BM) to a sandy loam soil which increased cation exchange capacity from 6.2 to 8.2. But, no change in CEC has been observed at the rate of 1 g dm−3 (0.07% BM) of SAP. Hence from the above discussion, it may conclude that chemical properties of soil have been altered by the application of various SAPs.

5.5.3.3

Biological Properties

The primary utilization of SAP to conserve water and promote plant growth while several researchers also revealed that it enhances the microbial activity of soil and also

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assist in the adaptation of the ecosystem and are critical to the long-term viability of plants which improve the crop productivity. In 2014, Li et al. [187] reported the dry and wet application of 200 kg ha−1 of SAP with loam soil + water can improve the bacterial abundance significantly. Similar to this, 0.5% BM added to a built soil boosted the numbers of bacteria and fungi by 16% and 18%, respectively [204]. In well-oxygenated soil, this process happens quickly as soil-associated bacteria, primarily glucose and sucrose units, often break down polysaccharides into smaller components. When the soil aeration level was less than 12%, sugar beet yield was reported to have decreased by Wallace and Garn in 1986 [205]. Therefore, a high quantity of polysaccharides that create a favourable environment for anaerobic microorganisms is primarily responsible for optimal soil aggregation. Due to inadequate oxygenation caused by the higher cohesive forces in soil, anaerobic bacteria as well as more sophisticated organisms like fungi can thrive there. The microbial activity has been characterized by microbial biomass carbon (MBC) and soil microbial respiration (SMR). In 2018, Dehkordi et al., reported that the application of SAP at 180 kg ha−1 with 0.3 m3 of water has no significant change in MBC and decline in SMR is recorded. The increment in MBC and SMR in other growth stages has been attributed to the utilization of SAP as carbon source by microbes. Apart from the encouraging microbial growth, In order to increase the capacity of plants to absorb nitrogen and other mineral nutrients, SAP can also increase the competition between microbes and plant roots [206]. Although soil aeration increases productivity, the soil cannot hold onto water. Incorporating the SAP into soil can improve its water retention and also resulting in more favourable growing conditions for the plant which also allows nutrients and ions to be transported rapidly through the soil [207, 208]. Plasma membrane, which includes steroid, protein, phospholipid, and other molecules, makes up the surface of a plant’s root [209]. By enhancing hydrogen bonds or dipole–dipole interactions, the SAPs have also improved the interaction between soil particles and roots [210, 211]. The active flow of water and nutrients to plants through negatively charged clay surfaces with anionic functional groups (Ca2+ , Fe3+ , and Al3+ ) may have contributed to this interaction [212]. Hence it may conclude that SAPs can improve microbial abundance and activity of soil surface through aggregate formation.

5.5.3.4

Yield and Plant Growth Characteristics

In addition to properties of soil, the application of SAP can also improve the plant yields or their production. In 2015, Montesano et al. [213], described the use of SAP having 2 gL−1 based on cellulose in sandy soil which enhances the cucumber total fresh biomass and fruit fresh biomass from 840 and 494 g per plant, respectively. Furthermore, Salavati et al., 2018 [214] also reported the improved potato cultivars tuber yield at 10, 15, 20 and 25 cm depth with SAP applied in furrow at 80 kg ha−1 to a clay loam soil and the highest yield has been found at the 25 cm sowing depth. Hou et al., 2017 [202] also reported the similar results with the application of SAP having amount 60 and 90 kg ha−1 in a sandy loam soil which yielded potatoes, 38.2

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and 50.5% higher than the control situation. According to Rostampour et al., 2013 [59], the application of SAP at rates of 75, 150, or 225 kg ha−1 only caused the dry matter of the sorghum to rise when there was a water deficit in sandy loam soil. In 2018, Eneji et al. [73] applied the SAP at a rate of 30 kg ha−1 to a sandy loam soil, which enhanced corn biomass by 99% when irrigation was insufficient, 39% when irrigation was moderate, and only 11% when irrigation was fully applied. In 2019, Grabinski et al., [215] reported the three-year study of SAP having amount 30 kg ha−1 to wheat crop can improved the wheat grain yield relative to the control even in the years of water deficit. In 2017, Dehkordi et al. [216] reported the usage of SAP under stress conditions with salt (NaCl), drought, and combined (NaCl + drought). The results revealed that encapsulated dry weight increased from 52.56 g to 45.56, 44.14, and 47.68 g, respectively, and this increment in the dry weight may be attributed to the SAPs mitigation of salt and drought. In 2017, Satriani et al. [217] reported biodegradable cellulose-based SAP in the management of water irrigation systems in bean crop cultivations in areas affected by water scarcity. In literature only few reports are available which indicate the negative impact of SAP on crops. The use of acrylate-based SAP for seed germination, soil pot culture, hydroponic experiments, and maintaining the moisture level of agricultural management was described by Chen et al. in 2016 [218]. However, the results showed that SAPs impeded maize growth and changed the shape of the roots, which may harm plants. The high concentration of both AA and Na+ in the SAPs, which hinders plant growth, may be to blame for the harm to the plants. Hence, it may be concluded that the use of SAPs has been beneficial to the growth of plant, if proper rate of application, type of soils has been stated properly.

5.6 Conclusion Superabsorbent polymers based on natural and synthetic source materials have been successfully demonstrated (laboratory to field studies) to have many possibilities for agricultural related applications due to its unique properties. The application of SAPs has improved and prolong soil water holding capacity and increase time between irrigation events. Due to this, it can reduce the consumption of water for the irrigation in dry regions, reduction in the death rate of plant and also increase the growth of plant. The SAPs can also act as a controlled release device for various agrochemicals. The utilization of CRFs can reduce the leaching, runoff and volatilization. The implementations of SAPs on large field scale are limited due to its production cost of the material. It may be concluded that in future new environment friendly, economical or hybrid polymeric materials should be developed in order to minimize the loss of agrochemicals in the environment. However, systematic research work on smart/hybrid SAPs and large field test has been executed for commercialization of these materials on an industrial scale with recyclability and reusability.

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Chapter 6

Recent Advancements in Superabsorbent Polymers for Drug Delivery Cynthia Lizeth Barrera-Martínez, Lluvia Azhalea Guerrero-Hernández, Jorge Luis Sánchez-Orozco, Gladis Y. Cortez-Mazatan, H. Iván Meléndez-Ortiz, and René D. Peralta-Rodríguez Abstract Superabsorbent polymers (SAPs) are advanced materials that have a crosslinked three-dimensional network structure, which endows attractive properties for a wide range of applications including daily hygiene, agriculture, construction, food, and health care, among others. In recent years, there has been a huge demand for new materials that could be used as drug carriers in alternative treatment to existing and emerging diseases for drug delivery. SAPs whether of synthetic (petrochemical based), natural (polysaccharide and polypeptide based) or semi-synthetic (a mixture of synthetic and natural polymers) origin are especially interesting to be used as drug delivery systems since their unique structure gives them the ability to absorb and retain aqueous solutions up to 1000 times their own weight. After SAPs swell (smart stimuli response), the drug molecules can be slowly released from the polymeric matrix and distributed throughout the body in a controlled released manner of the encapsulated drugs for an extended period. Additionally, SAPs are able to control drug loading and release, degradation, and flexibility; some SAPs may possess stimuli sensitivity and mechanical strength which in many cases depend on their crosslinking degree. The above mentioned characteristics have led research groups to investigate these polymers and how their properties could fulfill the requirements to be used as efficient drug carriers. This chapter is aimed to provide an introduction, C. L. Barrera-Martínez · L. A. Guerrero-Hernández · J. L. Sánchez-Orozco · G. Y. Cortez-Mazatan · R. D. Peralta-Rodríguez (B) Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna No. 140, Col. San José de los Cerritos, Saltillo 25294, México e-mail: [email protected] L. A. Guerrero-Hernández e-mail: [email protected] J. L. Sánchez-Orozco e-mail: [email protected] G. Y. Cortez-Mazatan e-mail: [email protected] H. I. Meléndez-Ortiz (B) CONACyT—Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna No. 140, Col. San José de los Cerritos, Saltillo 25294, México e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Arpit and T. Jaya (eds.), Properties and Applications of Superabsorbent Polymers, https://doi.org/10.1007/978-981-99-1102-8_6

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generalities, and review of the latest and most outstanding literature reports about SAPs applied as drug delivery systems. Keywords Superabsorbent polymers · Drug delivery · Controlled release · Encapsulation · Smart SAPs

6.1 Introduction Scientific research and development has focused on pursuing new materials with water-retention and water-absorption capacities that provide solutions not only to everyday problems but also in specialized applications. In this context, polymers play an important role in the development of these materials, especially those classified as superabsorbent polymers (SAPs). SAPs (Fig. 6.1) are materials that perform a swelling process that gives them the ability to absorb and retain a large volume of water and aqueous fluids [1]. The swelling process of SAPs is ascribed to different interactions between the functional groups of the polymer chain and the surrounding medium (water) including electrostatic forces, hydrogen bonds, and covalent bonds [2]. Historically speaking, in 1938, a water-absorbent polymer based on acrylic acid and divinylbenzene was synthesized for the first time via polymerization in aqueous medium. Later in 1950, Wichterle and Lim synthesized an absorbent polymer used for contact lenses production based on poly(2-hydroxyethyl methacrylate). However, it was not until 1978, in Japan, when SAPs were first commercialized to be used as napkins [3]. Since then, SAPs have been used predominantly in personal care products as baby diapers, feminine sanitary napkins, adult underwear, and disposable hygiene items. According to global SAPs market statistics, these products raised 9.7 USD billions in 2021 and is projected to grow to 13.1 USD billions in 2026 [4]. In the specialized literature, SAPs are often confused with polymer-based hydrogels; however, despite that both materials absorb and retain large volume of water, SAPs have the ability to absorb aqueous solutions up to 1000 times its own weight compared to hydrogels that only absorb 20–100 times [5]. Additionally, the waterabsorption capacity is directly regulated by the type and degree of cross linking: SAPs that are generally prepared with a low cross-linking density, allowing a high absorption capacity due to the fact that water molecules diffuse more easily through low-density cross-linking networks compared with highly cross-linked hydrogels that show a rigid structure [3]. However, although there are notable differences between both materials, specialized literature often uses indiscriminately these terms as synonyms. SAPs can be classified into three main groups: synthetic, natural, and semisynthetic materials that are generally made with cross-linked hydrophilic polymers which have polar or ionic groups (such as carbonyl, hydroxyl, amide, and amine groups) in their backbone structure making them hygroscopic materials [1]. In the last decade, SAPs have attracted the attention of research groups worldwide due to their unique characteristics that can be applied in different specialized areas such

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Fig. 6.1 General characteristics of SAPs

as agriculture, food technology, tissue engineering, and drug delivery systems [6], among other applications. SAPs applied in the biomedical field have been used as surgical caps and wound dressings. However, their swelling ability makes them excellent candidates for drug administration since the polymeric matrix could absorb or incorporate the drugs within it and release them in the physiological environment improving the bioavailability of the drug. Furthermore, depending on their chemical composition, SAPs are stimuli-responsive materials and, thus, they could be sensitive to changes in pH, temperature, pressure, light, and other stimulus. [3]. This chapter will focus on presenting the most recent advances about synthetic, natural, and semi-synthetic SAPs applied to drug delivery systems.

6.2 Synthetic Superabsorbent Polymers for Drug Delivery Synthetically obtained SAPs are materials with superior mechanical resistance and durability. Synthetic SAPs are classified according to their electrical charge into zwitterionic, amphoteric, non-ionic, and ionic electrolytes, the latter being the most common [7]. There are different types of polymerization processes to obtain these polymers, one of the most notable techniques is mass polymerization which allows to obtain high purity and high molecular weight polymers in a fast and simple manner. In mass polymerization, high monomer concentrations and the use of photosensitive or thermal initiators are required (free-radical polymerization) avoiding the use of additional solvents. Suspension polymerization is similar to the prior technique with the difference that it begins through an initiator thermal decomposition process to obtain spherical polymer products. Another procedure is polymerizations in aqueous

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media which can be homogeneous (solution) or heterogeneous (emulsion, dispersion, etc.) and its main advantage is that it is possible to achieve better control in viscosities and molecular weight of the synthesized polymers. Physical methods are other SAP synthesis techniques including freezing and thawing cycles and free-radical polymerization initiated by high-energy electromagnetic rays and electrons [8]. Synthetic SAPs have been used for drug administration in smart hydrogels allowing active ingredients to be trapped and released at the site of action by stimuli response such as pH, temperature, electrolytic, and/or magnetic fields which could achieve better bioavailability of active ingredient, biodegradability and reduction of toxicity. Some of these hydrogels have been used for cancer therapy [9]. In terms of drug encapsulation techniques in synthetic SAPs, it has been used a post-loading technique which is carried out by the drug absorption into the hydrogel and its subsequent drying. On the other hand, the in situ technique involves that the polymerization is done with a mixture of the active ingredients, monomer, and initiator, which allows drug encapsulation into the hydrogel network [7]. Another application of synthetic SAPs related with drugs is its use as coating material in oral tablets due to its superabsorbent capacities which allows them to swell, and hence tablet disintegration occurs, accelerating drug release. One of the limitations of synthetic SAPs in this applications is that these polymers increase the viscosities of the formulations being the pharmaceutical form for administration one of the main variables to consider, e.g., an inappropriate route would be drug administration via injection [10]. The following section presents the main advances on selected synthetic SAPs used as drug delivery systems.

6.2.1 Poly(Acrylic Acid) (PAA) and Poly(Methacrylic Acid) (PMA) Superabsorbent polymeric materials have evolved over the years, from their invention in 1960 starting with external uses as diapers to the present, where they play a very important role in terms of health when used for drug oral administration and release. Synthetic SAPs are used for different applications according to their properties, e.g., diapers, absorbent pads, and drug delivery devices. Regarding acrylic acid, this monomer contains an ionizable group that gives it strong absorption properties due to ion–dipole interactions with water molecules. The poly(acrylic acid) is highly soluble in water, and for this reason, the use of a cross-linking agent is necessary since it will help to retain the main polymer structure preventing its dissolution in water, making them candidates to be used as superabsorbent materials [11]. PAA (Fig. 6.2) can be obtained through free-radical polymerization of acrylic acid by a cross-linking and neutralizing process. Cross-linking process between covalent bonds could limit water absorption in superabsorbent polymers and this is due to the formation of a rigid network. In order to improve water absorption, it could be recommended employing nanoparticles as physical cross-linking agents that promote the formation of less

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Fig. 6.2 Chemical structure of PAA

rigid networks, and therefore obtaining flexible polymers. One example of this is the use of calcium hydroxide, Ca(OH)2 , nanoparticles as physical cross linker, and potassium hydroxide (KOH) as neutralizer agent. Conversely, the use of a chemical cross-linking agent such as N,N’-methylene-bis-acrylamide (MBA) gives rigidity and hardness to polymer chains which reduces the amount of water absorbed due to the formation of covalent bonds between the monomer and cross linker, the amount of water absorbed by SAPs depends on the flexibility of the chains [12]. SAPs are novel and intelligent materials and are being used as dressings in skin wounds, especially in those acute or chronic wounds caused by burns or ulcers, they do not only have protecting functions in wound, but also improve the healing process by absorbing blood and/or fluids. Additionally, SAPs can act as drug suppliers that allow wound to heal even faster accelerating the healing process. SAPs must meet certain characteristics and properties such as being intelligent, multifunctional materials capable to absorb fluids, maintain a humid environment, and be able to swell and retain liquids; further, SAPs must be biocompatible, biodegradable, and hemocompatible to be in contact with wounds. Fang et al. [13] have investigated the use of a combination of acrylic acid with chitosan, which is a linear polysaccharide with excellent biocompatibility, biodegradability, and antimicrobial activity properties, and managed to synthesize an absorbent polymer as a result of the copolymerization of acrylic acid on chitosan. Authors achieved a high water-absorption capacity (615 g/g) increasing antimicrobial activity against E. coli and S. aureus showing a maximum inhibition rate for both bacteria between 66 and 71%, respectively [13]. Yang et al. made tablets with hydrogel microparticles of the superabsorbent polyacrylic acid polymer and the active ingredient ketoprofen. The main objective of this research was to obtain a rapidly disintegrating tablet. The reported results showed that the absorption capacity of the SAP contributed to promote the super disintegration of the tablet and increase the release of ketoprofen [14]. Synthetic SAPs are also used in the treatment of diseases such as diabetes for the release of insulin. Usually, insulin is administered to the patient by a subcutaneous injection by means of small syringes, due to its inherent instability in the gastrointestinal (GI) tract and its low permeability across the membranes. SAPs can break down these barriers as long as they are pH sensitive by encapsulating insulin, protecting it in the stomach and releasing it into the colon. To develop these materials, it was necessary to consider polymers that were biocompatible such as PAA and PMA that have the ability to open narrow epithelial bonds. Additionally, these polymers

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possess excellent mechanical resistance which helps to reduce erosion. Synthetic SAPs, as mentioned above, have low stability, and thus it is necessary to combine them with another polymer to form an interpenetrating or semi-interpenetrating network (semi-IPN), an example of this is sodium carboxymethylcellulose (CMC) which has advantages such as enzymatic immobilization and tissue engineering due to its low toxicity, biocompatibility as well as low cost. Li et al. (2020) prepared pH-sensitive smart hydrogels by free-radical polymerization of methacrylic acid in the presence of carboxymethylcellulose and N, N’-methylenebis (acrylamide) (NNMBA) as a cross-linking agent. These CMC/PMA hydrogels were loaded with insulin, placed in contact with a solution of pH 1.2 (HCl buffer/NaCl) and pH of 6.8 (KH2 PO4 /NaOH buffer), centrifuged them at a temperature of 37 °C at 100 rpm filling with their respective buffer solutions to take care of the volume of dissolution. They found that the hydrogels showed high biocompatibility with a favored release of insulin at pH 1.2 decreasing the blood glucose levels when they are applied in vivo in diabetic rats [15].

6.2.2 Poly(Ethylene Glycol) (PEG) PEG (Fig. 6.3) is a synthetic polyether obtained from the condensation polymerization of ethylene oxide and water, it is considered one of the main synthetic polymers to prepare SAPs, highly soluble in water and organic solvents, biocompatible, and nontoxic and because of these properties, it has been approved by the Food Drug Administration (FDA) for its clinical use. PEG has promising applications in drug administration [16–18]. Ijaz et al. (2018) obtained a hydrogel through radical polymerization between PEG and acrylic acid using benzoyl peroxide as initiator. The formulations were made by varying the relations of polymer, monomer, and cross-linking agent (methylene bis-acrylamide) at different pHs. Subsequently, these formulations were loaded with the antidepressant drug venlafaxine. The results showed that increasing the amount of acrylic acid at neutral pH, the increase in the swelling rate was favored improving the controlled release of the drug (zero-order kinetics) [16]. In 2019, another group of researchers made hydrogels with poly(aspartic acid) and PEG functionalized with hydrazine and loaded with the anticancer drug, doxorubicin. Results showed that the synthesized polymer has self-repair capacity, biodegradability, and biocompatibility. Release tests showed that during the first 12 h, the release of doxorubicin increased and was controlled for 36 h [19]. Researchers at the Laboratory of Polymer Synthesis of the University of Zanjan, Iran, in 2020 synthesized PEG and PEG-poly(caprolactone) (PCL) hydrogels; these hydrogels were loaded with diclofenac sodium to determine drug release profiles. Formulations were made using PEG of different molecular weights and varying the amounts of PCL. Results showed a direct relationship between molecular weight of PEG and swelling capacity due to the increase in cross-linking density; therefore, release rate decreased. Formulations with higher amount of PCL showed a decrease in release rate due to the increase

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Fig. 6.3 Chemical structure of PEG

Fig. 6.4 Chemical structure of PAM

in its hydrophobicity. PEG SAPs have greater swelling capacity when compared to PEG-PCL materials [20].

6.2.3 Poly(Acrylamide) (PAM) PAM (Fig. 6.4) is one of the most important synthetically obtained superabsorbent polymers, which has been shown to present water-absorption capacities around 300– 500 g/g. SAPs based on PAM are generally obtained by photopolymerization of acrylamide forming networks which improve their elasticity and affinity in water. These polymers have the potential to release drugs and can be used for biomedical applications in implants and tissue regeneration [21, 22]. Ravindra et al. (2012) obtained SAPs based on PAM polymer with ammonium persulfate as initiator, N, N´methylene-bis-acrylamide, and potassium poly(acrylamide-co-acrylate) to encapsulate antihistamine triprolidene hydrochloride [23]. Serafim et al. (2014) encapsulated antibiotic, nafcillin sodium in a porous superabsorbent PAM scaffold using a photoinitiated polymerization method and N, N’-methylene-bis-acrylamide as crosslinking agent. SAPs showed greater affinity for water at different pHs and good degradability properties. SAPs with high absorption and swelling capacities were the ones that released the greatest amount of antibiotic (91% in 10 h), demonstrating its potential for drug release applications [21].

6.2.4 Sodium Polyacrylate (SPA) SPA (Fig. 6.5) is a superabsorbent hydrophilic polymer obtained from the polymerization of the acrylic sodium salt, it has a high water-absorption capacity, due to its carbonyl and sodium groups [24]. Yang and Park (2018) created implant-mediated

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Fig. 6.5 Chemical structure of SPA

drug delivery systems (IMDDS) using SPA to release dexamethasone. IMDDS is a novel technology that allows local drug delivery. This implant was tested in vivo in a population of 18 rabbits and results showed a drug release in an accelerated manner in the first hour of the study followed by a sustained release for 2 weeks, showing good drug bioavailability [25].

6.2.5 Polyvinyl Alcohol (PVA) PVA (Fig. 6.6) is a flexible water-soluble SAP, highly biocompatible and biodegradable. This polymer is composed of hydroxyl groups which make it highly reactive and allow it to interact with other materials. One of the most notable applications of PVA in biomedical applications is in anticancer drug delivery systems [17, 26]. In 2019, researchers at the University of Tabriz synthesized PVA chemically cross-linked using epichlorohydrin for ibuprofen release evaluation. Additionally, copper oxide nanoparticles were added to PVA as an antimicrobial agent and their effect on drug encapsulation and release was evaluated. The findings of this evaluation showed that the presence of copper oxide nanoparticles contracts the polymer network decreasing drug loading and release rates [27]. Synthetic SAPs are also known as superabsorbent resins that compared to natural polymers have better absorbent capacities; however, one of their main disadvantage is the negative environmental impact. For this reason, the use of natural SAPs or at least a mixture of synthetic and natural SAPs can help to reduce the environmental pollution and increase its biodegradability capacity improving other physicochemical properties [17, 22]. Fig. 6.6 Chemical structure of PVA

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6.3 Natural Superabsorbent Polymers for Drug Delivery The use of natural polymers for the preparation of SAPs has recently emerged due to the poor environmentally friendly properties of synthetic polymers [28]. This drawback has increased the development of new SAPs derived from natural materials and additives. Natural polymers have some advantages including biodegradability, biocompatibility, non-toxicity, and, in many cases, low cost which is advantageous for some specific applications such as controlled drug delivery, cell cultivation, agriculture, water purification, tissue engineering, wound dressings, and personal hygiene. [5]. Recently, natural polymers have gained great interest in the biomedical field since they are renewable, sustainable, and easy to metabolize by the human body [29]. Among the most studied natural polymers used in the synthesis of SAPs are found some polysaccharides such as chitosan, alginate, cellulose, carrageenan, starch, gum, and gelatin [30, 31]. Additionally, some proteins as well as some poly(amino acids) have been reported in the preparation of these materials [1, 32, 33]. However, SAPs based on natural polymers may exhibit some limitations in their extraction, processability, and reactivity. These inconveniencies can be avoided after their modification using different techniques and methods including cross linking, blending with synthetic polymers, functionalization to endow with reactive groups, etc. [5].

6.3.1 Chitosan Chitosan (CS) is a versatile cationic natural polymer obtained from the partial deacetylation of chitin (the second most abundant natural polysaccharide). This natural polymer has in its chemical structure units of D-glucosamine and N-acetylD-glucosamine (Fig. 6.7). Both the hydroxyl and amine groups facilitate its reactivity, and as a consequence, a variety of functional organic groups can be attached generating derivatives, which are useful for different applications [30, 34, 35].

Fig. 6.7 Chemical structure of chitosan

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This polymer has shown good antibacterial, antiviral, and antifungal properties as well as biocompatibility and biodegradability, which depends on the degree of acetylation, molecular weight, concentration, etc. In addition, its cationic nature allows the interaction with enzymes, cells, nucleic acids, etc. All these features make it an interesting biomaterial to obtain SAPs with potential biomedical applications [30, 35]. In this sense, some SAPs based on chitosan have been reported. Zamani et al. reported the synthesis of a series of renewable porous SAPs based on carboxymethylchitosan by cross linking and gelation with glutaraldehyde in aqueous solution. The best result reported by these authors was 107 (g/g) after exposing the hydrogel during 1 h in pure water [36]. Sorokin et al. synthesized SAPs based on the derivatives of N-maleoylchitosan and N-succinylchitosan by free-radical polymerization in an aqueous solution [37]. Also, they found that the N-succinylchitosan derivative showed higher swelling than the N-maleoylchitosan derivative SAPs reaching swelling values of up to 1144 g/g. Additionally, CS-based SAPs have been recently copolymerized with other natural polymers such as starch [38, 39], cellulose [40], and carrageenan [41]. Moreover, composites based on CS and pectin with montmorillonite clay have been reported as SAPs [42]. CS-based SAPs have been synthesized for their use in biomedical applications, particularly as drug delivery systems [43, 44]. Ismail et al. reported the synthesis of hydrogels based on CS and acrylic acid to obtain carrier materials for the drug chlortetracycline hydrochloride, which is a tetracycline with broad-spectrum antibacterial and antiprotozoal activity [43]. These authors found that the release of this drug was strongly influenced by the pH of the solution. For their part, Pana et al. synthesized semi-interpenetrating networks with hydrogel features based on CS and oligomers of D-glucose and D-mannose and 2-hydroxyethylmethacrylate [45] and found that these materials were able to load a significant amount of the antibiotic levofloxacin. However, a “burst effect” during the first 5 h was observed in the release profile. More recently, Silva et al. developed edible and non-cytotoxic films produced through the combination of CS and virgin coconut oil [46]. Authors claim that these films can act as drug delivery vehicles in which the release of the active compound could be controlled, or as a topical patch for wound care due to its superabsorbent properties.

6.3.2 Alginate Alginate is an anionic linear polysaccharide (extracted from brown seaweed) composed of units of mannuronic and guluronic acids arranged either as consecutive blocks or random distribution linked by a 1, 4-glucosidic linkage (Fig. 6.8). This polymer is commercially available as a sodium salt (sodium alginate, SA) and can be physically cross linked and form hydrogels by the addition of multivalent cations (e.g., calcium, barium) via metal bridges between the guluronic acid residues and adjacent chains becoming insoluble in water [47, 48].

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Fig. 6.8 Chemical structure of alginate

Due to their properties such as non-toxicity, biocompatibility, biodegradability, high water absorption, and relatively low cost, this natural polymer has been extensively used in construction, food, agricultural, and biomedical fields [49– 52]. In addition, SA exhibits excellent antibacterial effects upon ionic bonding [53]. However, SA-based hydrogels, like most natural-based hydrogels, have poor mechanical strength, which restricts their use. In this sense, some chemical and physical methods have been reported to avoid this drawback including grafting and semi-interpenetrating polymerizations, polymer blending, and functionalization with versatile organic groups. SAPs based on alginate and their copolymers (with other natural polymers) as well as composite materials have been reported [54–56]. Ye et al. prepared biodegradable SAPs based on SA and carrageenan using calcium chloride as cross-linking agent [57]. The obtained material showed fast swelling (swelling ratio of 13.1 g/g in 0.9% saline water in just 5 min) and high mechanical strength (maximal tensile strength of 12.8 MPa). For their part, Olad et al. synthesized SAPs with good reswelling capability composed of montmorillonite and SA grafted with synthetic polymers in the form of semi-interpenetrating networks by free-radical graft polymerization [58]. These nanocomposites exhibited high swelling, 618 g/g, which depended on pH value, salt solution type, and concentration. More recently, Choi et al. synthesized biocompatible superabsorbent sponges using alginate, carboxymethyl cellulose, and dextran to produce biomedical materials that can absorb blood and body fluids [59]. These authors found that the incorporation of dextran (a hydrophilic natural polymer) caused an increase in the swelling ratio of sponges and concluded that these materials can be used in wound treatment and intelligent drug delivery systems. In addition, the use of SAPs based on this natural polymer has been reported as drug delivery systems [60, 61]. Chang prepared pH-sensitive SAPs based on SA, starch, and poly(acrylic acid) for the controlled delivery of diclofenac [62]. Chang found that the incorporation of starch and the synthetic polymer greatly influenced the swelling behavior and cumulative release of diclofenac. Furthermore, the presence of these polymers delayed or avoided the disintegration of pristine SA beads.

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Chang concluded that these SAPs are very promising drug delivery carriers for oral administration in the intestinal tract. More recently, Arafa et al. (2022) synthesized antimicrobial superabsorbent hydrogels by cross linking SA with a derivative of CS for using as urea-controlled delivery system [63]. These SAPs were able to retain water, control the release of urea, as well as prevent the growth of pathogens.

6.3.3 Starch Starch is one of the cheapest and the third most abundant carbohydrate biopolymers in nature and the main sources are maize, wheat, and potatoes, and it is the most common constituent of the human diet. This biopolymer is non-toxic, biocompatible, biodegradable, and composed of a mixture of amylose (15–25%) and amylopectin (75–85%). Amylopectin is a highly branched polymeric chain consisting of relatively short branches of α-D-(1–4)-glucopyranose that are interlinked by α-D-(1–6)glycosidic linkages which contains 20–25 glucose units on average whereas amylose is a linear polymer of glucose with α-1,4 linkages and forms the amorphous region in starch (Fig. 6.9) [64]. However, starch has strong hydrophilicity, low mechanical properties, poor processability, and high viscosity which is hindering its performance as SAPs. These drawbacks can be avoided by modifying starch through different techniques such as graft copolymerization, physical or chemical modifications such as blending, and derivation since starch has available hydroxyl groups, which exhibit high reactivity to some chemical reactions [31]. The first commercially available SAP was developed by Sanyo Chemical Co., Ltd. in 1978 [5]. This SAP was prepared by grafting acrylic acid onto starch followed by a neutralization step. Since then, some reports about the preparation of SAPs based on starch have been reported [65]. Teli et al. reported the preparation of SAPs

Fig. 6.9 Chemical structure of starch

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based on starch grafted with acrylic acid and acrylamide, which exhibited water absorption in the range of 45–85 g/g [66]. After saponification of this product, a high amount of water absorption (up to 400 g/g) was obtained. In addition, the water absorption was dependent on pH reaching maximum water absorption at pH values of 7–8. On the other hand, Zhang et al. synthesized degradable rice starchbased SAPs grafted with poly(acrylic acid) with high water-absorption capacity (up to 439 g/g) [67]. They found that the content of amylopectin improved the waterabsorption capacity. In addition, they evaluated the effect of some salt ions on the water-absorption capacity. This capacity decreased more in the presence of aluminum than potassium salt. For their part, Olad et al. obtained a superabsorbent nanocomposite of starch-graft-poly(acrylic acid-co-acrylamide)/polyvinylalcohol) filled with clinoptilolite [68]. The equilibrium swelling capacity of the semi-IPN superabsorbent nanocomposite was 364 g/g, which was slightly lower than that obtained by Teli et al. However, the incorporation of clinoptilolite increased the equilibrium swelling, water retention, and mechanical strength by comparing with the pristine polymer. Few works about the use of SAPs based on starch used as drug delivery systems have been reported [69–71]. Avval et al. grafted acrylamide and hydroxyethylacrylate onto starch by free-radical polymerization and atom transfer radical polymerization (ATRP) for the controlled release of the antibiotic cephalexin [70]. These authors found that the copolymers obtained by ATRP showed a more sustained drug release compared with those obtained by free-radical polymerization.

6.3.4 Natural Gums The term “gum” is adapted to define a group of naturally occurring polysaccharides owing to their aptitude to form either “gel” or “viscous solution”. As the previously described polysaccharides, gums are biocompatible and biodegradable and have been used in the biomedical field in applications such as tissue engineering, wound management, and drug delivery [72]. The development of SAPs especially from the natural gums is receiving attention since they have the ability to undergo easy chemical modifications and possess many advantages as raw materials or blending with other ceramics and synthetic polymers [31]. Among the most common and widely used gums are guar, Arabic, tragacanth, and xanthan gums. Guar gum is a natural non-ionic, biodegradable complex polymer having β-Dmannopyranosyl units linked 1–4 with single-membered α-D-galactopyranosyl units as side branches while the gum Arabic is a complex mixture of macromolecules of different size and composition characterized by a high proportion of D-galactose and L-arabinose (~97%) and a low proportion of proteins (