Sorption of the Water Vapor by Vinyl Acetate-Vinyl Alcohol Copolymers

Abstract

Equilibrium water sorption isotherms for vinyl acetate-vinyl alcohol copolymers were obtained and the sorptive capacity of these copolymers was discussed with respect to the polar group content and its distribution.
For heterogeneously acetylated samples (B sample), a linear plot between the specific gravities and polymer composition was obtained. This suggests that B sample has almost the same micro structure in crystalline state as that of PVA. The sorptive capacity of both copolymers was similar at low humidity. At the higher relative humidity, however, the sorptive capacity of the homogeneously acetylated samples (R sample) was higher than that of B samples. It appears that the experimental results of B sample obeyed the BET equation between 10% to 45% RH, whereas the Flory-Huggins' solution mechanism may be applied to R sample over the whole range of relative humidity examined.
Further analysis of the sorption behavior was made in terms of the clustering function. At low humidity, for B sample, the values of the clustering function G₁₁/V₁ were less than -1 indicating the localized sorption of the water molecules on specific polymer sites. Moreover they reached values of more than -1 with the further increase of relative humidity. For R sample, on the contrary, the values of G₁₁/V₁1 were nearly -1, showing an ideal solution behavior over the whole range of relative humidity.