Relaxations in Chitin: Evidence for a Glass Transition

Relaxations in chitin have been investigated in the temperature range 298-523 K using impedance spectroscopy in the frequency range 10(-1)-10(8) Hz. The objective was to detect a glass-transition temperature for this naturally occurring, semicrystalline polysaccharide. The impedance study was complemented with X-ray diffraction, thermogravimetric, and differential scanning calorimetry measurements. Preliminary impedance data treatment includes the subtraction of the dc conductivity contribution, the exclusion of contact and interfacial polarization effects, and obtaining a condition of minimum moisture content for further analysis. when all these aspects are taken into account, two relaxations are clearly revealed in the impedance data. For the first time, evidence is presented for a relaxation process, which exhibits a non-Arrhenius temperature dependence, in dry alpha-chitin (similar to 0.1% moisture content), and likely represents the primary alpha-relaxation. This evidence suggests a glass transition temperature for chitin of 335 +/- 10 K estimated on the basis of the temperature dependence of the conductivity and of the relaxation time. A second relaxation in dry alpha-chitin, not previously reported in the literature, is observed from 353 K to the onset of thermal degradation (similar to 483 K) and is identified as the sigma-relaxation often associated with proton mobility. It exhibits a normal Arrhenius-type temperature dependence with activation energy of 113 +/- 3 kJ/mol. The latter has not been previously reported in the literature. A high frequency secondary beta-relaxation is also observed with Arrhenius activation energy of 45 +/- 1 kJ/mol. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 932-943, 2009