MESH SIZE AND DIFFUSIVE CHARACTERISTICS OF SEMICRYSTALLINE POLY(VINYL ALCOHOL) MEMBRANES PREPARED BY FREEZING-THAWING TECHNIQUES

Semicrystalline poly(vinyl alcohol) membranes were prepared by freezing/thawing of aqueous PVA solutions at - 20 degrees C/+25 degrees C for up to ten cycles. The ensuing membranes were analyzed by swelling, mechanical and thermal studies. During the freezing/thawing process, a crystalline phase was formed by the slow crystallization process. The crystalline PVA fraction was a function of the number of cycles and duration of each cycle. The volume-based crystalline fraction of PVA on a wet basis varied from 0.052 to 0.116. The equilibrium volume swelling ratio varied from 4.48 to 9.58 and was a function of the degree of crystallinity. The approximate mesh size of the amorphous network structure was also determined. Transport of theophylline and FITC-dextran was studied and the solute diffusion coefficients were determined. It was shown that solute transport was a function of the crystalline PVA fraction of the membranes and the mesh size of the networks formed. Comparison of theophylline transport in these films and membranes prepared by conventional annealing showed a significant densification of the membranes processed by freezing/thawing techniques.