Coagulation size of freezable water in poly(vinyl alcohol) hydrogels formed by different freeze/thaw cycle periods

Abstract

The coagulation size of freezable water in a poly(vinyl alcohol) (PVA) hydrogel was investigated as a function of the freeze/thaw cycle period using thermal analysis. The melting temperature of ice in the gel shifted to a lower temperature than that of normal ice. This temperature depression can be interpreted in terms of the coagulation size of freezable water in the gel. For longer freeze/thaw cycle periods, the coagulation size became larger. In addition, the weight content of freezable water estimated from the melting enthalpy of ice increased with an increasing length of the freeze/thaw period. These results suggested an expansion of the coagulation size of freezable water. As for the molecular structure of PVA, the nature of the hydrogen bonds was investigated for a PVA/deuterated water (D2O) gel by infrared spectroscopy. A component analysis of the OD-stretching mode of D2O showed a decrease in multiple hydrogen bonds and an increase in the free OD-stretching mode over longer freeze/thaw periods. This led to the transformation from non-freezable water to freezable water. Therefore, it was concluded that the coagulation size of freezable water expands and the aggregation of PVA increases as a result of lengthening the freeze/thaw cycle. © The Society of Polymer Science, Japan (SPSJ) All rights reserved.