Some aspects of two stage diffusion in polymer films and membranes

Abstract

The two-stage sorption data for dichloromethane vapour in a cellulose triacetate films and membranes are analysed by the variable-surface concentration model and the diffusion relaxation model. Satisfactory fitting of the data and similar kinetic parameters are obtained using these models. The diffusion coefficient for dichloromethane is about 2.5 x 10-11 m2 s-1 in the glassy polymer at 40 C and is near 1.5 2.5 times higher as the polymer goes over the glass-rubber transition region due to solvent sorption. The relaxation rate constant is between 3.0 and 18 x 10-4 s -1 and is increased as the sorbed concentration or the vapour activity of solvent increases. When the two-stage non-Fick sorption prevails, the diffusion-relaxation model approximates the limiting cases of the both models.