We isolated α-chitin, β-chitin, and γ-chitin from natural resources by a chemical method to investigate the crystalline structure of chitin. Its characteristics were identified with Fourier transform infrared (FTIR) and solid-state cross-polarization/magic-angle-spinning (CP–MAS) 13C NMR spectrophotometers. The average molecular weights of α-chitin, β-chitin, and γ-chitin, calculated with the relative viscosity, were about 701, 612, and 524 kDa, respectively. In the FTIR spectra, α-chitin, β-chitin, and γ-chitin showed a doublet, a singlet, and a semidoublet at the amide I band, respectively. The solid-state CP–MAS 13C NMR spectra revealed that α-chitin was sharply resolved around 73 and 75 ppm and that β-chitin had a singlet around 74 ppm. For γ-chitin, two signals appeared around 73 and 75 ppm. From the X-ray diffraction results, α-chitin was observed to have four crystalline reflections at 9.6, 19.6, 21.1, and 23.7 by the crystalline structure. Also, β-chitin was observed to have two crystalline reflections at 9.1 and 20.3 by the crystalline structure. γ-Chitin, having an antiparallel and parallel structure, was similar in its X-ray diffraction patterns to α-chitin. The exothermic peaks of α-chitin, β-chitin, and γ-chitin appeared at 330, 230, and 310, respectively. The thermal decomposition activation energies of α-chitin, β-chitin, and γ-chitin, calculated by thermogravimetric analysis, were 60.56, 58.16, and 59.26 kJ mol−1, respectively. With the Arrhenius law, ln β was plotted against the reciprocal of the maximum decomposition temperature as a straight line; there was a large slope for large activation energies and a small slope for small activation energies. α-Chitin with high activation energies was very temperature-sensitive; β-Chitin with low activation energies was relatively temperature-insensitive. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3423–3432, 2004
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