A dual mode interpretation of the kinetics of penetrant-induced swelling and deswelling in a glassy polymer

In this study, a unified dual mode description of penetrant sorption and desorption kinetics in a glassy polymer was extended to the study of the kinetics of penetrant-induced swelling and deswelling below the glass transition concentration. The swelling of cellulose acetate (CA) induced by the sorption of vapor phase acetonitrile, and the subssuent deswelling during desorption, was measured using time-resolved FTIR-ATR (Fourier Transform Infrared-Attenuated Total Reflectance) spectroscopy. Equilibrium swelling as a function of penetrant activity from the spectroscopic measurements was confirmed with conventional length elongation measurements (dilatometry). From equilibrium measurements that traversed the glass transition concentration, the partial molar volume both below and above the transition was determined. Using the partial molar volume obtained above the glass transition and assuming that only the dissolved population contributes to dilation, the swelling kinetics can be predicted from the dual mode framework presented here. However, the deswelling kinetics cannot be predicted, suggesting that a fraction of the holes formed upon sorption in the unified model must collapse during contraction of the polymer upon desorption.