Fine Structures and States of Water in Poly(vinyl alcohol) Hydrogels Prepared by Repeated Freezing and Thawing.

Investigations have been made into the relationship between structure and states of water in unfrozen poly (vinyl alcohol) (PVA) hydrogels prepared by repeated freezing and thawing of aqueous solution with various PVA concentrations. The states of water were investigated by using a pulsed nuclear magnetic resonance spectroscopy. Every unfrozen PVA hydrogel gave only one spin-lattice relaxation time (T-1) value. T-1 of all unfrozen hydrogels were smaller than that of free water and decreased with increasing PVA concentration. These mean that the molecular motion of water in the unfrozen PVA hydrogels are more restrained than those of the free water molecules, and the mobility decreases with increasing PVA concentration. The activation energy, which was estimated from Ahrrenius type plot, for the motion of water molecules of all unfrozen PVA hydrogels was smaller than that of free water and decreased with increasing PVA concentration. These mean that hydrogen bonding force between water molecules of all unfrozen PVA hydrogels are smaller than that of free water and decreases with increasing PVA concentration. It was clarified by comparing with the results of fine structure of the unfrozen PVA hydrogels which have been studied in a previous paper that with increasing number of PVA chain in the uncrystallized region of unfrozen PVA hydrogels, the restrain of mobility of water molecules increased and hydrogen bonding force weakened.