Molecular understanding of CO2-induced plasticization of a polyimide membrane by a combination of experiment and simulation study

Abstract

Plasticization phenomenon is well-recognized for polymeric membranes in gas separation processes. Extensive studies have shown that plasticization essentially originates from the interactions between gas and membrane which significantly affect the performance of membrane. Plasticization is usually encountered in the separation of natural gas, which involves highly condensable gases such as CO2, H2S, H2O, and hydrocarbons. Beyond a critical feed pressure, the permeabilities of both CO2 and CH4 increase, and consequently a loss of permselectivity. The fundamental understanding of CO2-induced plasticization is critical to the enhancement of separation efficacy and the development of high-performance polymeric membranes. The question remains unsolved whether there is a physical property of significance indicating the critical threshold of plasticization. In this work, experimental measurements and fully atomistic simulation are carried out to examine the CO2-induced plasticization of a polyimide membrane synthesized from 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 4,4′-oxydianiline (ODA). With increasing feed pressure, the permeability of CO2 in the 6FDA-ODA membrane initially decreases, crosses a minimum, and then increases. The radial distribution functions between CO2 and polyimide atoms reveal that the imide groups are the preferential sorption sites, followed by the ether and CF3 groups. The experimental and simulated sorption isotherms of CO2 are in fairly good agreement. At low loadings, CO2 molecules are largely trapped with small mobility. With increasing loading, the polyimide membrane exhibits a depressed glassy transition temperature, a dilated volume, an increased fractional free volume. In addition, larger and more interconnected voids appear and the mean radius of voids increases with increasing CO2 loading. Consequently, the mobility of both CO2 molecules and polymer chains is enhanced. Based on molecular displacement, the percentages of three types of motions (jumping, trapped, and continuous) are estimated for CO2. The continuous motion contributes dominantly to CO2 diffusion. At a high loading, the ether groups in polyimide chains exhibit a significant effect on plasticization. It is therefore suggested that the plasticization could be suppressed by substituting the ether groups. The microscopic information of this current study is particularly useful for the quantitative understanding of plasticization.