Effects of heat treatment on the permeability of polyvinyl alcohol films to a hydrophilic solute.

Polyvinyl alcohol (PVA) films may be useful as release-controlling membrane systems. Untreated, they are readily permeable to water and hydrophilic drugs. Because heating has been used to increase crystallinity and thus reduce the solubility and swelling in water of PVA films, we have studied the effects of heat on the permeability of PVA films to a water-soluble drug marker. Heat treatment was varied in the temperature range 100-200 degrees C for 1 h. The effect of time of heating was studied at 100 degrees C for 0.5-160 h. After pre-equilibration with water (heat-treated membranes remained intact, untreated ones dissolved), membrane permeabilities to methylene blue in aqueous solution (37 degrees C) were determined in a rotating diffusion cell. Permeabilities decreased with increased heating times (0.98-0.039 cm X min-1 for 0.5-160 h at 100 degrees C, respectively). Heating in air or N2 produced similar results. Further dramatic decreases in permeability occurred with increasing pretreatment temperatures; membrane permeability fell by a factor of approximately 500 with increasing temperature in the range 100-200 degrees C. There was no evidence of decomposition at temperatures less than or equal to 190 degrees C for 1 h. Results were consistent with literature reports of heat-induced increases in crystallinity. Membranes were simple to prepare and permeability could be controlled without recourse to chemical manipulation.