Sorption/desorption properties of water vapour in poly(2-hydroxyethyl methacrylate): 2. Two-stage sorption models

Abstract

The two-stage integral sorption data for water vapour in a glassy poly(2-hydroxyethyl methacrylate) (PHEMA) membrane are analysed by the variable-surface concentration model, the diffusion-relaxation model, and the diffusion-reaction model. Satisfactory fitting of the data and similar kinetic and equilibrium parameters are obtained using these models. The diffusion coefficient for water is about 5.0 × 10 −7 cm 2s −1 in the glassy polymer at 37°C and is near 1.5–2 times higher as the polymer goes over the glass-rubber transition region due to water sorption. The relaxation rate constant is between 1.0 and 5.1 × 10 5s −1 and is increased as the sorbed concentration or the vapour activity of water increases. The equilibrium ratio constant is between 0.15 and 0.21 for the cases in which the polymer is characterized to be in the glassy state, and between 0.06 and 0.1 for the case in which the polymer passes the glass-rubber transition. When the two-stage sorption prevails, the diffusion-relaxation model approximates the limiting cases of the other two models. The diffusion-reaction model, based on Fickian diffusion and a reversible first order reaction kinetics for the penetrant transfer between the immobilized and mobile modes, is superior to the other two in terms of completeness of description of physical phenomena.