Abstract
Freezing behavior of cross-linked polymer gels was studied mainly by differential scanning calorimetry. Heat evolution during warming, due to ice crystallization, was sometimes observed below 0°C following precedent cooling to -50°C. It was found, further, that water compartmentalized by the cross-linked network structure remains unfrozen during cooling when the size of the compartments is appropriately small, and that it is the unfrozen compartmentalized water which crystallizes during subsequent warming. The mechanism of anomalous ice crystallization observed during warming was, then, investigated relating to change of the polymer network structure. In this process, it became clear that not only the size of the compartments but also continuity of water between adjacent compartments and flexibility of polymer chains affect freezing behavior of gels. These three factors seem, however, to be interrelated with each other via the density of cross-links of polymers. Therefore, it seems to be necessary to analyse characteristic of compartmentalized water more closely for the clarification of the freezing mechanism of gels. Moreover, the importance of the concept of compartmentalized water in gels, which may form a phase intermediate between continuous and discrete phases, was pointed out in this paper.
