ABSTRACT
This diffusion of a polar penetrant in a polar polymeric material is a rather complex phenomenon, as evidenced by studying the transient penetration period preceding the sorption equilibrium. For instance, if a dry, homogeneous polyvinylalcohol film is exposed to water vapour (under a fixed pressure), its permeability to the vapour first declines with time. Plasticization of the accessible amorphous regions by sorbed water favours the growth of the adjacent crystalline domains, resulting in an increase in tortuosity. Suddenly, a steep increase in permeability is observed,
preceding a further progressive decline, and the phenomenon reiterates several times before the sorption equilibrium is finally attained. During the transient period, the permeability of the polyvinylalcohol film to water apparently increases and decreases in an irregular manner, with respect to time. It is probable that the repeated increases in permeability, thus observed at irregular intervals, originate from some mechanical relaxation occurring randomly when the penetrant (water) releases the hydrogen-bonds established between hydroxyl groups, located in regions where two different polymeric chains cross, in the amorphous regions of the polar material.
