Preparation and characterization of α-chitin whisker-reinforced poly(vinyl alcohol) nanocomposite films with or without heat treatment
Introduction
Nanocomposites are a relatively new class of composites with at least one phase having a dimension in the vicinity of 1–1000 nm. As most of the present-day nanofillers used to prepare nanocomposites with synthetic polymeric materials are inorganic [1], their processability, biocompatibility, and biodegradability are much more limited than those of naturally organic ones. In nature, a large number of animals and plants synthesize extracellular high-performance skeletal biocomposites that consist of a matrix reinforced by fibrous biopolymers [2]. Cellulose is a classical example where the reinforcing elements exist as whisker-like microfibrils that are biosynthesized and deposited in a continuous manner [3]. Favier et al. [4] was the first to prepare cellulose whiskers from tunic of tunicate Microcosmus fulcatus by acid hydrolysis (i.e. tunicin whiskers) and use them as reinforcing nanofillers in a copolymer of styrene and butyl acrylate (i.e. poly(S-co-BuA) latex). Since then, tunicin whiskers have been used as reinforcing nanofillers in poly(β-hydroxyoctanoate) (PHO) [5], poly(hydroxyalkanoate) (PHA) [6], plasticized maize starch [7], and poly(oxyethylene) [8]. Cellulose whiskers from wheat straw can also be prepared and have been used as reinforcing nanofillers in poly(S-co-BuA) latex [9].
Whiskers from other polysaccharides, such as starch [10] and chitin [11], can also be prepared. Unlike tunicin whiskers which can only be prepared by hydrolysis in strong sulfuric acid (H2SO4) solutions [4], [5], [6], [7], [8], [9], [11], both starch and chitin whiskers can be prepared by hydrolysis in hydrochloric acid (HCl) solutions [10], [11]. Chitin is a high molecular weight biopolymer found predominantly in exoskeleton shells of arthropods as well as the internal flexible backbone of cephalopods. Chemically, chitin molecules consist of N-acetyl-d-glucosamine units. Chitin is known to be non-toxic, odorless, biocompatible with living tissues, and biodegradable [12]. Chitin whiskers have been successfully prepared from crab shells [11], [13], [14], squid pens [15], and tubes of Riftia pachyptila tubeworms [16]. Regardless of the chitin sources, the commonly-used hydrolytic condition for obtaining chitin whiskers is 3 N HCl at the boil for 90 min under vigorous stirring [11], [13], [14], [15], [16].
Since there have not been prior reports on the preparation of chitin whiskers from shrimp shells, α-chitin whiskers were then prepared by acid hydrolysis of α-chitin from shells of Penaeus merguiensis shrimps in the present contribution. Nanocomposite films using the obtained α-chitin whiskers as nanofillers were prepared by solution-casting from mixtures of poly(vinyl alcohol) (PVA) solution and α-chitin whisker suspensions. PVA was chosen as the matrix material, due to its excellent film-forming ability, chemical resistance, and good transparency. Due to such properties, PVA has found wide applications in membrane technologies [17]; however, its actual utilization is limited by its affinity to water, which leads to high degree of swelling and even complete solubilization of the resulting membranes. Consequently, the presence of α-chitin whiskers with or without heat treatment was expected to improve the resistance to water of the resulting nanocomposite films. Furthermore, thermal and mechanical properties of the nanocomposite films were also investigated.
Section snippets
Materials
Shells of Penaeus merguiensis shrimps were donated from Surapon Foods Public Co., Ltd. (Thailand). Poly(vinyl alcohol) (PVA), having the weight-average molecular weight of about 72,000 Da and the degree of hydrolysis of about 97.5–99.5%, was purchased from Sigma-Aldrich (USA). An aqueous solution of sodium hydroxide (NaOH) (50% w/w) was donated from KPT Corporation Co., Ltd. (Thailand). Anhydrous NaOH pellets, sodium borohydride (NaBH4), glacial acetic acid (99.8% w/w), and hydrochloric acid
Morphological appearance and sizes of α-chitin whiskers
Fig. 1 shows a TEM image of a dilute suspension of α-chitin whiskers from acid hydrolysis of Penaeus merguiensis shrimp shells. Such a suspension exhibited a colloidal behavior. This is due to the presence of the positive charges (NH3+) on the whisker surface, which resulted from the protonation of the amino groups of chitin in acidic conditions [11]. The suspension contained chitin fragments consisting of both individual microcrystals and aggregated microcrystals. The chitin fragments
Conclusions
Chitin whisker-reinforced nanocomposites films were prepared by blending suspensions of α-chitin whiskers from Penaeus merguiensis shrimp shells with solution of poly (vinyl alcohol) (PVA). The as-prepared α-chitin whiskers consisted of slender parallel rods with a broad distribution in both length and width. The average length and width of these whiskers were about 417 and 33 nm, respectively, with the average aspect ratio between the length and the width being about 17. Incorporation of
Acknowledgements
Partial support from the Petroleum and Petrochemical Technology Consortium (through a Thai governmental loan from the Asian Development Bank) and the Petroleum and Petrochemical College, Chulalongkorn University is acknowledged. The authors would like to thank M. Peesan for her assistance on Fourier-transformed infrared spectroscopic measurements.
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