Thermal and rheological properties of poly(vinyl alcohol) hydrogels prepared by repeated cycles of freezing and thawing

Differential scanning calorimetry and scanning electron microscopy were carried out in order to study the relation between the structure and properties of poly(vinyl alcohol) hydrogels prepared by repeated cycles of freezing-thawing. The effect of the number N of repeated cycles of freezing-thawing, of the degree of saponification (DS), and of the degree of polymerization (DP) on structure and properties was examined. The elastic modulus and the endothermic enthalpy, estimated from the area surrounded by the endothermic peak, increase with increasing N, DS, and DP. The melting temperature T_m shifts to higher temperatures with increasing DS and DP, but it is almost independent of N for gels with high DS or DP. In PVA gels with lower DP or DS, T_m shifts to higher temperatures with increasing N. The temperature dependence of the elastic modulus is discussed on the basis of a theory on thermoreversible gels.