Nanosecond-time-scale reversibility of dilation induced by carbon dioxide sorption in glassy polymer membranes

Abstract

Many models have been proposed to describe the sorption isotherms of small gases in glassy polymers at equilibrium. However, the sorption kinetics to reach this equilibrium are dependent on the nature of the penetrant and the swelling of the matrices. In the present work, large-scale molecular dynamics (MD) simulations are used to describe CO2 sorption-induced dilation kinetics at very short time scales in 6FDA-6FpDA polyimide membranes. The latter are subjected to two cycles of CO2 sorption and post-degassing relaxation, which mimick the sorption-degassing experiments with large integral pressure steps. The glassy membranes dilate during sorption, mainly because of an increase in the longitudinal strain. Upon degassing, they systematically relax to a metastable state, with some residual extra-volume and improved solubility for the penetrant, which depends on the swelling behavior history. These subtle changes agree with the qualitative model based on variations in the relative fractions of Fickian and relaxational dilation sites as a function of the swelling history. Both sorptions also exhibit differences at very short times, with a enhanced void-space and concentration of CO2 sorbed along with a lower polymer mobility for the conditioned one, even if competitive sorption counterbalances the expected increase in gas mobility.