Multiscale modeling of time-dependent CO_2 and N_2 permeation through a glassy polymer at steady and non-steady state

Abstract

Glassy polymers are often utilized used for the sepn. of mixts. of gases because of their high selectivity. The sorption behavior of gases in glassy polymers is commonly described math. with the dual-mode sorption model, however the phys. interpretation of the dual-mode sorption model remains disputed. Furthermore, the dual-mode model uses membrane properties measured at steady state, and so its utility for describing permeation under non-steady state conditions is unknown. This work reports a combined exptl.-theory-multiscale modeling investigation of the transport of two gases, CO_2 and N_2, through the glassy polymer poly(di-Me phenylene oxide) (PPO) to better understand the relationship of glassy polymer structure and dynamics with gas transport for both steadyand non-steady state conditions. The non-steady-state permeation behavior is investigated by measuring a transient pressure rise downstream as gas is transported through the polymer membrane. A multiscale model, consisting of phys.-based coupled reaction-diffusion kinetics, is developed and incorporates system properties at multiple length and time scales. We find that the multiscale model can reproduce time-dependent exptl. data at steady and non-steady state equally well using either a single reaction-diffusion transport mode, or an implementation of the dual-mode model. A key element of the model is a proper description of the gas concn. increase within the membrane as the external gas pressure rises, which is not instantaneous for this polymer. Because the multiscale model is phys. based, it is predictive and so is used to explore the plasticization behavior of PPO during permeation by CO_2.

Additional details