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SURFACE MODIFICATION OF BIOPOLYMERS: AN OVERVIEW

Manju Kumari Thakur1, Ashvinder Kumar Rana2, Yang Liping3, Amar Singh Singha4, and Vijay Kumar Thakur5

1 Division of Chemistry, Govt. Degree College Sarkaghat, Himachal Pradesh University, Shimla, Himachal Pradesh, India

2 Department of Chemistry, Sri Sai University, Palampur, Himachal Pradesh, India

3 Polymer Engineering and Catalysis, Institute of Chemical and Engineering Sciences, Singapore, Singapore

4 Department of Chemistry, National Institute of Technology, Hamirpur, Himachal Pradesh, India

5 School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, USA

1.1 INTRODUCTION

Among various materials used in our everyday life, polymers play the most imperative role along with their use in a number of industries for versatile applications [1-3]. Polymers have been known to play a key role in the economy of the most of the countries of modern world since last century [4, 5]. Polymers have been frequently classified into natural and synthetic polymers [6, 7]. Although natural polymers were frequently used by the people of earlier civilization for a number of applications directly/indirectly, usage of synthetic polymers has dominated the modern world especially the last few decades [8-11]. Synthetic polymers have even replaced some of the commonly used metallic materials due to their enormous advantages such as light weight, chemical/water resistant, versatility, decent mechanical/thermal properties, and easy tailor ability [12-15]. Both natural and synthetic polymers can be easily distinguished depending upon their inherent properties and their structural property relationship [16]. However, during the last few years, sustainable development through the use of environmentally friendly materials has become the hottest topic of conversation as well as research all around the globe [17, 18]. In this direction, the usage of materials that can be procured from our nature is rising rapidly for a number of applications [19, 20]. In fact the materials obtained from the nature are becoming a potentially viable alternative to a number of traditional synthetic materials that are generally synthesized from petroleum-based resources [21-23]. The effective utilization of the materials obtained from nature offers a number of challenges for their successful usage as well as offers new opportunities from the economic and environmental point of view [24, 25]. The past few decades has seen a pronounced advancement in the development of new eco-friendly materials that are procured from bio-based biopolymers for vast applications [26-28]. Different kinds of bio-based biopolymers represent a renewable feedstock of materials for different usage [29, 30]. The renewable feedstock of biopolymers extensively depends upon the availability of bio-based resources in different regions of the world, the new developments in the use of these materials, and the agricultural production as most of the biopolymers are directly/indirectly related to the field of agriculture [31, 32]. Applications of any biopolymer material in a particular application stresses on the specific physical, chemical, thermal, mechanical, economic, and degradation properties so as to offer significant advantages over their synthetic counterpart [33, 34]. In addition to these requirements, the easy availability of these biopolymeric materials is one of the most significant parameters in their commercialization as it is directly related to the final cost of the material in the market [35, 36]. Different kinds of biopolymer-based materials found in the nature can play one of the key roles in the modern industries to make the final product green [37, 38]. The use of biopolymer-based materials ranges from house hold applications to advanced applications in the defense [39, 40]. Different kinds of biopolymers depending upon their compositions can be used in a number of applications as follows: biomedical (e.g., stent, drug-delivery vehicles), food packaging, polymer composites for structural applications, as electrolyte for energy storage in super capacitor/battery, adhesives, cosmetic industries, and most frequently in textile industries [41, 42].

1.2 STRUCTURES OF SOME COMMERCIALLY IMPORTANT BIOPOLYMERS

Among the various biopolymer materials, a few materials such as natural cellulosic fibers, starch, agar, chitosan, and poly(3-hydroxyalkanoates) (PHAs), are being used in a number of applications [24, 43-49] . In the following section, we briefly describe some of the commercially important biopolymers, as their detailed introduction along with their modification/applications has been given in the upcoming chapters.

1.2.1 Natural Fibers

Among the various fibers available naturally/synthetically, natural cellulosic fibers are of much importance due to their intrinsic properties [48-50]. These fibers have been reported to be used by human beings for thousands of years ago starting from early civilization in the formation of bridges for on-foot passage as well as in naval ships to biomedical in the present time [48-50]. Depending on their extraction as well as on the part of the plant from which they are taken, their properties vary considerably [50]. Figure 1.1 shows the schematic representation of natural fibers [51].

FIGURE 1.1 Structure of lignocell ulosic natural fiber.

Reproduced with permission from Ref. [51]. © 2013 Elsevier.

Natural cellulosic fibers primarily contain cellulose, hemicellulose, and lignin as their primary constituent and have been well researched as well as documented in the existing literature [48]. Figure 1.2 shows the structure of cellulose found in natural fibers. Cellulose (a nonbranched polysaccharide) is the prime constituent of all lignocellulosic natural fibers and has been found to exist in two crystalline forms, namely, cellulose I and II [48-51]. Cellulose is a linear condensation polysaccharide that comprises a d-anhydro glucopyranose units joined by ß-1,4-glycosidic bonds. On the other hand, hemicelluloses are composed of a combination of 5- and 6-ring carbon ring sugars and have been found to remain associated with cellulose even after the removal of lignin [49, 50]. As opposed to the structure of cellulose, hemicelluloses exhibit a branched structure and consist of mixtures of polysaccharides with much lower molecular weight compared to cellulose [48-50].

FIGURE 1.2 Structure of cellulose.

Reproduced with permission from Refs. [48-50]. © Elsevier.

Among different constituents of natural cellulosic fibers, lignin is one of the highly branched components. It is a complex chemical compound present in huge quantities in the cell walls of plants. It is the main binding agent for components of the plants and serves as a matrix to the embedded cellulose fibers along with hemicellulose. The structure of lignin is highly branched that consists of phenyl propane units. These units are organized in a complex three-dimensional structure linked together through numerous types of carbon-carbon and ether bonds.

1.2.2 Chitosan

Chitosan is another most significant biopolymer that is derived from chitin (produced by many living organisms) [1, 3, 52] . Chitin is the second most abundant natural polymer available on earth after cellulose and is found in a number of organisms from crustaceans such as lobsters, crabs, shrimp, and prawns along with insects to some types of fungi [31, 34, 52]. Chitin is a nitrogen-rich polysaccharide and a high-molecular-weight linear polymer composed of N-acetyl-d-glucosamine (N-acetyl-2-amino-2-deoxy-d-glucopyranose) units linked by ß-d-(l??4) bonds. Figure 1.3 shows the comparative chemical structure of chitin, chitosan, and cellulose [49, 52].

FIGURE 1.3 Structure of chitosan, chitin, and cellulose.

Reproduced with permission from Ref. [52]. © 2013 Elsevier.

Chitosan is most frequently produced from chitin by deacetylation process [31, 34, 49, 52]. Figure 1.4 shows the scheme for the extraction of chitosan from chitin.

FIGURE 1.4 Deacetylation of chitin to chitosan.

Reproduced with permission from Ref. [52]. © 2013 Elsevier.

Chitosan has been found to exhibit a basic character and is one of the highly basic polysaccharides compared to other natural polysaccharides that are acidic such as cellulose, agar, pectin, dextrin, and agarose [24, 29-53]. The degree of deacetylation and the charge neutralization of ─NH2 groups along with ionic strength have been found to control the intrinsic pKa value of chitosan. Figure 1.5 shows the schematic illustration of the versatility of chitosan.

FIGURE 1.5 Schematic illustration of chitosan's versatility. At high pH (above 6.5), chitosan's amine groups are deprotonated and reactive. At low pH (<6.7), chitosan's amines are protonated, confirming the polycationic behavior of chitosan.

Reproduced with permission from Ref. [52]. © 2013 Elsevier.

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