Gel-sol transition of poly(vinyl alcohol) hydrogels formed by freezing and thawing

Poly(vinyl alcohol) (PVA) hydrogels were prepared by freezing and thawing. Freezing and thawing cycle (n) was varied from 1 to 5. Gel-sol transition of the above hydrogels was measured using a highly sensitive differential scanning calorimeter (DSC). Phase transition of hydrogels was also investigated by simultaneous X-ray diffractometry (XRD) and DSC. Gel-sol transition temperature was observed in a temperature range from 55 to ca. 90 °C. Enthalpy of gel-sol transition was varied in a range from ca. 0.4 to 2 J g−1. Peak temperature and enthalpy of gel-sol transition increased with increasing n. By simultaneous measurement of X-ray diffractometry (XRD) and DSC, it is confirmed that no structural change is observed for either ice or water before and after gel formation. It is concluded that PVA molecules are aggregated during ice formation and hydrogen bonding is established between molecular chains. By increase of freezing and thawing cycle, residual free molecular chains are successively included in junction zones.