Investigation of the crystallinity of freeze/thaw poly(vinyl alcohol) hydrogels by different techniques

The crystallinity of freeze/thaw poly(vinyl alcohol) (PVA) hydrogels, either fresh or aged or obtained by dipping dried freeze/thaw gel samples in water immediately after their preparation, was investigated by using different techniques. Free induction decays obtained from 1H NMR experiments provide the most accurate measurement of the degree of crystallinity of these systems. Values thus obtained are in a good agreement with data obtained by X-ray diffraction for all the samples under study. The degrees of crystallinity, determined by using differential scanning calorimetry (DSC), instead, are lower than those obtained by the other two methods, for all the gel samples, but the aged gels. This result is due to the occurrence of the gel-sol transition during the heating scan which is characterized by the endothermic melting of the crystallites and the exothermic solubilization and solvation of PVA chains in water. In as-prepared and rehydrated gels, the endothermic and exothermic effects overlap, which leads to an underestimated value of the degree of crystallinity. For aged samples, the crystallites are larger and more perfect; the corresponding melting endotherms are narrower and shifted toward higher temperatures, which permits the separation of the endothermic and exothermic effects and leads to a more accurate measurement of the degree of crystallinity. Thus, the comparative analysis of the degree of crystallinity in PVA hydrogels measured by different techniques provides indirect information concerning their complex structure.