Molecular relaxation in Chitosan films in GHz frequency range
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Molecular relaxation in Chitosan films in GHz frequency range Siva Kumar-Krishnan, Evgen Prokhorov and Gabriel Luna-Barcenas CINVESTAV-Queretaro, Libramiento Norponiente 2000, Queretaro, QRO 76230, MEXICO
ABSTRACT The molecular relaxations behavior of chitosan (CS) films in the wide frequency range of 0.1-3x109 Hz (by using three different impedance analyzers) have been investigated in the temperature range of -100C to 1200C using Dielectric Spectroscopy (DS). Additionally to the low frequency molecular relaxations such as D and β relaxations, for the first time, high frequency (13 GHz) relaxation process has been observed in the chitosan films. This relaxation exhibits Arrhenius-type dependence in the temperature range of -100 C to 540C with negative activation energy -2.7 kJ/mol. At temperatures above 540C, the activation energy changes from -2.7 kJ/mol to +4.4 kJ/mol. Upon cooling, the activation energy becomes negative again with a value of -1.2 kJ/mol. The bound water between chitosan molecules strongly modifies molecular motion and the relaxation spectrum, giving rise to a new relaxation at the frequency at ca. 1 GHz. In situ FTIR analysis has shown that this relaxation related to the changes in vibration of the –OH, NH and –CO functional groups. INTRODUCTION The molecular motion and dielectric relaxation dynamics over high frequency range and the effect of moisture content in polysaccharide based biopolymer networks is an interesting and fundamentally important and not-well understood problem [1-3]. Dielectric spectroscopy (DS) is a powerful technique for the investigation of structure and dielectric relaxation phenomena and rotational dynamics of the molecules in polymers. It is noteworthy that, the physical and chemical properties of biopolymers can be significantly changed by the presence of small amounts of water [2]. The increasing numbers of studies of dielectric measurements in polysaccharides are controversially discussed about the shape of the dielectric spectra, number of relaxation process and their interpretation [4,5]. Among the main causes for the discussion are the very complex supramolecular structures of polysaccharides such as intra and intermolecular hydrogen bonds and the strong influence of water content [6]. The presence of water can significantly distort the results, giving rise to additional relaxation process in the material. Dielectric relaxation studies on different polysaccharides have been reported in literature, mainly for cellulose [7] and startch [8]. Einfeldt et al [9,10] have reported the dielectric relaxations found in variety of polysaccharides that exhibit similar relaxation processes. Although a great deal of dielectric data exists for frequencies in the megahertz region and below, there exist a small number of articles about relaxation properties of polymers in the GHz range. Difficulties arise when measuring high resistivity polymer material with relatively low dielectric
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constant in the GHz range. Monitoring the temperature is also difficult due to convention su
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