Review pubs.acs.org/Biomac
Chitin Whiskers: An Overview Jian-Bing Zeng,* Yi-Song He, Shao-Long Li, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China. ABSTRACT: Chitin is the second most abundant semicrystalline polysaccharide. Like cellulose, the amorphous domains of chitin can also be removed under certain conditions such as acidolysis to give rise to crystallites in nanoscale, which are the so-called chitin nanocrystals or chitin whiskers (CHWs). CHW together with other organic nanoparticles such as cellulose whisker (CW) and starch nanocrystal show many advantages over traditional inorganic nanoparticles such as easy availability, nontoxicity, biodegradability, low density, and easy modification. They have been widely used as substitutes for inorganic nanoparticles in reinforcing polymer nanocomposites. The research and development of CHW related areas are much slower than those of CW. However, CHWs are still of strategic importance in the resource scarcity periods because of their abundant availability and special properties. During the past decade, increasing studies have been done on preparation of CHWs and their application in reinforcing polymer nanocomposites. Some other applications such as being used as feedstock to prepare chitosan nanoscaffolds have also been investigated. This Article is to review the recent development on CHW related studies.
1. INTRODUCTION Chitin is a naturally occurring polysaccharide and is well-known to consist of 2-acetamido-2-deoxy-D-glucose via a β (1−4) linkage (Figure 1A). As the second most abundant biopolymer after cellulose, chitin is mainly synthesized via a biosynthetic way by an enormous number of living organisms such as shrimp, crab, tortoise, and insects1 and can also be synthesized by a nonbiosynthetic pathway through Chitinase-catalyzed polymerization of a chitobiose oxazoline derivative. 2−4 Chitosan, as the most important derivative of chitin, can be prepared by deacetylation of chitin. Chitin and chitosan have many excellent properties including biocompatibility, biodegradability, nontoxicity, absorption properties, and so on, and thus they can be widely used in a variety of areas such as biomedical applications, agriculture, water treatment, and cosmetics. In addition, there is a reactive amino group on the structure units of chitosan (Figure 1B). The amino group makes chitosan much easier to be modified by chemical reaction than cellulose. A lot of derivatives of chitosan with different functional properties have been synthesized by chemical modification. There are many reviews focusing on the properties, modifications, and applications of chitin and chitosan.5−10 This Article focuses on the other aspects of chitin: chitin nanoparticles and their applications especially in reinforcing polymer nanocomposites. Polymer nanocomposites refer to multiphase materials consisting of a polymer matrix and nanofillers. They show distinctive properties even at small of loading nanofillers, due to the nanometric size effect, in comparison with virgin polymer and conventional polymer composites.11 Following the successful development of nylon−clay nanocomposites in 1985,12 the polymer nanocomposites have attracted a great © 2011 American Chemical Society
deal of interest from both academic and industrial fields due to their appealing unique properties.13−18 Conventionally used nanofillers are inorganic compounds, especially layered nanoclays, such as montmorillonite (MMT) and layered double hydroxides (LDH), with homogeneous dispersion in polymer matrix, which are able not only to reinforce mechanical properties but also to improve barrier properties and other properties such as flame retardancy of polymer matrix.19−24 The strong aggregation tendency usually prevents the homogeneous dispersion, which is the prerequisite for improvement of physical properties, of the nanoclays within the polymer matrix.25 Renewable semicrystalline polysaccharides like cellulose, chitin, and starch consisting of crystalline domains and amorphous domains are possible candidates for organic nanofillers because the amorphous domains can be removed under certain conditions such as acidolysis and the crystalline domains with high modulus can be isolated in nanoscale. Then, the nanosized crystalline particles, usually known as polysaccharides nanocrystals, can be used as reinforcing nanofillers in polymer nanocomposites. Because Favier et al reported the use of cellulose nanocrystals as reinforcing nanofillers in poly(styrene-co-butyl acrylate) (poly(S-co-BuA))-based nanocomposites in 1995,26 the polysaccharides nanocrystals have drawn a large number of interest in polymer nanocomposites due to their excellent intrinsic properties such as nanosized dimensions, high surface area, biodegradability, nontoxicity, Received: November 6, 2011 Revised: December 5, 2011 Published: December 12, 2011 1
dx.doi.org/10.1021/bm201564a | Biomacromolecules 2012, 13, 1−11
Biomacromolecules
Review
Figure 1. Chemical structures of chitin (A) and chitosan (B).
Figure 2. Structures of α-chitin (A) and β-chitin (B). Reprinted with permission from ref 6. Copyright 2006 Elsevier.
swelling behaviors of the two kinds of chitins. A lot of polar molecules such as water and alcohol are able to penetrate βchitin easily,6 whereas they cannot penetrate that of α-chitin, where only stronger swelling agents like aliphatic diamines can intercalate.57,58 It is reported that the intracrystalline swelling of β-chitin in water or alcohol is reversible but is irreversible in strong acid solutions, in which the intrasheet hydrogen bonds are destroyed and the crystalline phase seems completely disappeared, but the crystallinity is restored and the α-chitin crystals are formed by recrystallization after removal of the acid.6,58,59 The β-to-α form conversion suggests that α-chitin is more thermodynamically stable than β-chitin. α-Chitin is usually used to prepare chitin nanocrystals because of its abundant availability; β-chitin has also been utilized to prepare chitin nanocrystals, and individual chitin nanocrystals with high aspect ratio (length to width ratio) are easier to be prepared from β-chitin than from α-chitin mainly due to their different intersheet hydrogen bonds.60
renewability, low density, and easy modification conferred by the large amount of surface hydroxyl groups.27−37 Some recent reviews have been reported on preparation, properties, modification, and application of cellulose and starch nanocrystals.25,27,38−41 However, no review, as far as we know, has been published exclusively on chitin whiskers except for being simply mentioned in some nanocomposites reviews.42−45 In this sense, a detailed overview on preparation, modification and application of chitin whiskers should be presented and will be anticipated to be helpful for future related studies.
2. CRYSTAL STRUCTURES AND SWELLING OF CHITIN Native chitin is a crystalline polymer, which occurs as three allomorphs, that is, the α, β, and γ forms, depending on the resources.46−48 The γ form was occasionally observed, which is thought as a variant of the α form.49 α-Chitin is by far the most stable and abundant form because it widely exists in a lot of living organisms such as fungal and yeast cell walls, krill, lobster, and crab tendons and shells, shrimp shells, and insect cuticle.6 In contrast, β-chitin is less abundant and is present in squid pens and tubeworms.50−52 The proposed crystal structures of α-chitin and β-chitin are shown in Figure 2.6 In both structures, the chitin chains are organized in sheets, and the sheets are tightly held by a number of intrasheet hydrogen bonds.53−56 This tight network, dominated by the rather strong C−O···NH hydrogen bonds, maintains the chains at a distance of ∼0.47 nm along the a parameter of the unit cell.6,7,54 Some intersheet hydrogen bonds occur along the b direction of the unit cell in α-chitin, but no intersheet hydrogen bond occurs for β-chitin. The different characteristics account for the different intersheet
3. CHITIN WHISKER The structure of chitin is very analogous to cellulose. They both are supporting materials for living bodies and are found to arrange in living plants or animals with sizes increasing from simple molecules and highly crystalline fibrils on the nanometer level to composites on the micrometer level upward.60 Therefore, they intrinsically have the potential to be converted to crystalline nanoparticles and nanofibers and to find application in nanocomposite fields. Chitin has been known to form microfibrillar arrangements in living organisms.61 These fibrils with diameters from 2.5 to 25 nm depending on 2
dx.doi.org/10.1021/bm201564a | Biomacromolecules 2012, 13, 1−11
Biomacromolecules
Review
isolation of chitin nanocrystals is also based on acid hydrolysis.63 Disordered and low lateral ordered regions of chitin are preferentially hydrolyzed and dissolved in the acid solution, whereas water-insoluble, highly crystalline residues that have a higher resistance to acid attack remain intact. Therefore, following an acid hydrolysis that removes disordered and low lateral ordered crystalline defects, chitin rodlike whiskers are produced.26,27 Unlike tunicin whiskers, which can only be prepared by hydrolysis in strong sulfuric acid (H2SO4) solutions,26,69 CHWs can be prepared by hydrolysis in HCl solutions. Revol et al.62 and Li et al.70 reported an approach toward preparation of suspension of chitin crystallites through acid hydrolysis in detail. In the approach, purified chitin was hydrolyzed by 3 N HCl at the boil for 90 min; after acid hydrolysis, the suspensions were diluted with deionized water, followed by centrifugation and decanting of the supernatant; this process was repeated several times until the suspension spontaneously transformed into a colloidal state. The obtained crystallites were rodlike particles with average size of 200 ± 20 nm in length and 8 ± 1 nm in width. Because of the nanoscale size, the crystals are reasonably named by nanocrystals or whiskers. On the basis of these procedures, whiskers have recently been prepared from many chitins of different origins such as squid pen chitin,71 riftia tubes,72 crab shells,73−78 and shrimp shells.79−87 The detailed information for CHWs prepared from different origins of chitins is summarized in Table 1. The
their biological origins are usually embedded in a protein matrix.62 CHW can be prepared from chitins isolated from chitin containing living organisms by the similar method toward preparation of cellulose whisker through hydrolysis in strong acid aqueous medium. On the basis of preparation of cellulose crystallites suspension, Marchessault et al.63 for the first time reported a route for preparing suspension of chitin crystallite particles in 1959. In the method, purified chitin was first treated within 2.5 N hydrochloric acid (HCl) solutions under reflux for 1 h, the excess acid was decanted, and then distilled water was added to obtain the suspension. They found that acidhydrolyzed chitin spontaneously dispersed into rodlike particles that could be concentrated to a liquid crystalline phase and selfassemble to a cholesteric liquid crystalline phase above a certain concentration.62,64 3.1. Isolation of Chitin from Shell Wastes. Before going to prepare CHW, chitin has to be isolated from the wastes of living animals, in which chitin does not occur alone, whereas chitin always coexists with some other compounds. At present, the major sources of chitin in industry are the shell wastes of crabs and shrimps. The shell wastes are mainly made up of chitin (20−30%), proteins (30−40%), calcium carbonate (30− 50%), and lipids and astaxanthin (
