Mechanistic understanding of CO2-induced plasticization of a polyimide membrane: A combination of experiment and simulation study

Abstract

Experimental measurements and fully atomistic simulations 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 plasticization pressure is estimated to be at approximately 8 atm. 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 glass transition temperature, a dilated volume and an increased fractional free volume. In addition, larger and more interconnected voids appear and the mean radius of voids increases from 2.5 to 3.3 Å 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 in the membrane. The continuous motion contributes predominantly to CO2 diffusion. At a high loading, the ether groups in the 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 study is particularly useful for the quantitative understanding of plasticization. © 2010 Elsevier Ltd.