Dielectric relaxations of chitosan: The effect of water on the α-relaxation and the glass transition temperature



In this work thermal relaxations of chitosan are reported by using a novel methodology that includes subtraction of the dc conductivity contribution, the exclusion of contact and interfacial polarization effects, and obtaining a condition of minimum moisture content. When all these aspects are taken into account, two relaxations are clearly revealed in the low frequency side of the impedance data. We focus on the molecular motions in neutralized and non-neutralized chitosan analyzed by dielectric spectroscopy in the temperature range from 25 to 250 °C. Low and high frequency relaxations were fitted with the Havriliak and Negami model in the 10−1 to 108 Hz frequency range. For the first time, the low frequency α-relaxation associated with the glass-rubber transition has been detected by this technique in both chitosan forms for moisture contents in the range 0.05 to 3 wt % (ca. 18–62 °C). A strong plasticizing effect of water on this primary α-relaxation is observed by dielectric spectroscopy and is supported by dynamic mechanical analysis measurements. In the absence of water (<0.05 wt %) the α-relaxation is obscured in the 20–70 °C temperature range by a superposition of two low frequency relaxation processes. The activation energy for the σ-relaxation is about 80.0–89.0 kJ/mol and for β-relaxation is about 46.0–48.5 kJ/mol and those values are in agreement with that previously reported by other authors. The non-neutralized chitosan possess higher ion mobility than the neutralized one as determined by the frequency location of the σ-relaxation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2259–2271, 2009