Rheology of highly swollen chitosan/polyacrylate hydrogels

Recently we reported on chitosan hydrogel systems having excellent laser da mage resistance, The measured laser damage threshold LDT was better than BK 7 glass and quartz, commonly used inorganic optical materials, and 20 to 35 times higher than commercial PMMA, a popular polymer optical material. In this study, we continue our investigation of the phase transition behavior of water within the hydrogels by means of oscillatory shear rheology. The c rystallization and melting behavior of ice in these hydrogels is greatly af fected by the internal structure of the network. Changes in this structure are probed by comparing differences in the rheological measurements at temp eratures above and below freezing. Trends among the measured shear storage modulus (G'), shear loss modulus (G") and shear loss tangent (tan delta) ar e shown to be related to the mobility of water within the gels. The activat ion energy associated with the melting of ice in a particular hydrogel was found to be significantly higher than the melting enthalpy of pure ice, sug gesting imperfection in the ice crystals and/or a low degree of crystallini ty in the hydrogel. (C) 1999 Elsevier Science Ltd. All rights reserved.