Studies on bound water in poly(vinyl alcohol). Hydrogel by DSC and FT-NMR

The phase transition of water bound in poly(vinyl alcohol)(PVA) hydrogel was measured by differential scanning calorimetry. The cross-link density calculated from the swelling ratio of PVA hydrogel ranged from 3 × 10−5 to 1 × 10−3 according to the total dose of 60Co γ-ray irradiation. From the enthalpy of melting, it was found that water in PVA gel can be classified into at least three groups; ca. 1–1.5 mol non-freezing water, 5–6 mol freezing bound water, and the rest free water, corresponding to each hydroxyl group of PVA. The characteristic properties of polymer hydrogel were attributed to the large number of molecules of freezing bound water in the network. The amount of free water depended markedly upon cross-link density; however, it was not apparent in PVA gel with cross-link density >2.0 × 10−4. The decreases in the nuclear magnetic resonance relaxation times of water and PVA were related to the cross-link density and were found to level off at a cross-link density 2.0 × 10−4.