THERMAL-PROPERTIES OF POLYSACCHARIDES AT LOW MOISTURE .1. AN ENDOTHERMIC MELTING PROCESS AND WATER-CARBOHYDRATE INTERACTIONS

The thermal properties of a broad range of polysaccharides containing 5-25% w/w water have been studied by differential scanning calorimetry and dynamic mechanical thermal analysis (DMTA). Following room temper ature conditioning, an endothermic event accompanied by material softe ning is observed at 45-80-degrees-C for all samples except those above their glass transition temperature. The temperature of the event is d etermined by thermal history and is apparently independent of polymer type or moisture content. The associated enthalpy increases with water content. Variable frequency DMTA analysis suggests a structural melti ng event rather than a relaxation process. The endothermic event is re covered over the days timescale after heating, and can be annealed to higher temperatures with increasing holding temperature. Results are i nterpreted in terms of a dynamic hydration model in which specific ene rgetic water-carbohydrate interactions occur but with a lifetime defin ed by their local effective microviscosity. The observation of the end otherm below glass transition temperatures suggests that in aqueous po lysaccharide glasses, enthalpic structures involving the solvent can b e made and broken.