POLY(VINYL ALCOHOL) HYDROGELS PREPARED BY FREEZING-THAWING CYCLIC PROCESSING

A novel method of preparation of strong poly(vinyl alcohol) (PVA) hydrogels was developed without utilization of chemical crosslinking or other reinforcing agents. Aqueous solutions containing 10-15 wt% PVA were frozen at - 20-degrees-C for 1 24 h. They were subsequently thawed at 23-degrees-C for up to 24 h. This process was repeated four more times and the weight change and degree of swelling were measured after each freezing-thawing cycle. Strong thermoreversible hydrogels exhibiting mechanical integrity were obtained with 15 wt % solutions frozen at - 20-degrees-C for 24 h and thawed at 23-degrees-C for any time period. The process of reinforcement of these thermoreversible hydrogels was a densification of the macromolecular structure. The densification process was more prominent for gels that had been exposed to up to five freezing-thawing cycles. Compressive creep studies indicated that the gel compliance was relatively constant over a wide range of stresses and decreased as the number of freezing-thawing cycles increased.