Heat-Induced Evolution of the Mesoscopic Structure of Dehydrated Poly(vinyl alcohol) Gel

Time-resolved small-angle X-ray scattering (SAXS) investigations were carried out in order to clarify the heat treatment effects on the structure of a dehydrated substance made from a crystalline-polymer (poly(vinyl alcohol)) hydrogel. The heat-treatments performed in the present study consisted of two procedures: an initial temperature increase to a target temperature (T _t) and a successive thermoregulation to maintain T_t. The evolution of the SAXS profile in the first procedure indicated that a mesoscopic structure, which probably originated from an inhomogeneity between crystalline and amorphous ingredients, began to develop around the glass transition temperature and became more distinct and large with increasing temperature. However, the SAXS profile showed almost no change for a long time during the second procedure. The latter feature was quite different from the crystallization processes of polymers without cross-linkage, which demonstrates how the cross-linkage hinders the thermal motion of the network polymers in the dehydrated gel.