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

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.