Microstructure of poly(vinyl alcohol) hydrogels investigated with differential scanning calorimetry

The physical state of water in poly(vinyl alcohol) (PVA) hydrogels was studied with differential scanning calorimetry to elucidate their microstructure. PVA hydrogels were prepared after three different methods, viz. chemical crosslinking with glutaraldehyde, annealing of dried film and crystallization at low temperature. The water in PVA hydrogels could be classified into three types: free water, intermediate water and bound water. The concentration of bound water is practically not dependent on the total water content of the hydrogels when prepared by low‐temperature crystallization or annealing of the PVA film. The gel prepared by low‐temperature crystallization contained less bound water than the gel obtained by annealing of a cast film. Furthermore, the concentration of bound water increases as the degree of polymerization (DP) of PVA increases, whereas the reverse tendency was observed for the intermediate water. From these results it was concluded that the low‐temperature crystallization method results in a larger free space between the PVA microcrystallites and a larger size of microcrystallites than the annealing method. It is also likely that PVA of higher DP provides a larger amount of free PVA chains which are not involved in microcrystallites.