Plasticization phenomena can significantly reduce the performance of polymeric membranes in highpressure applications. Polyetherimides (PEIs) are a promising group of membrane materials that combine relatively high CO 2 /CH 4 selectivities with high chemical and thermal stability. In this work sorption, swelling, and mixed gas separation performance of 3,3 0 ,4,4 0 -oxydiphthalic dianhydride (ODPA)based PEI polymers, with 1, 2 or 3 para-aryloxy substitutions in the diamine moeiety, is investigated under conditions where commercial membranes suffer from plasticization. Particular focus is on the influence of the amount of para-aryloxy substitutions and the film thickness. Results are compared with those of commercially available polymeric membrane materials (sulphonated PEEK, a segmented blockco-polymer PEBAX and the polyimide Matrimid).
The glassy polymers display increasing CO 2 sorption with increasing T g . The larger extent of sorption results from a larger non-equilibrium excess free volume. Swelling of the polymers is induced by sorption of CO 2 molecules in the non-equilibrium free volume as well as from molecules dissolved in the matrix. Dilation of the polymer is similar for each molecule sorbed. Correspondingly, the partial molar volume of CO 2 is similar for molecules present in both regions.
Mixed gas separation experiments with a 50/50% CO 2 /CH 4 feed gas mixture showed high CO 2 /CH 4 selectivities for the ODPA PEI films at elevated pressure. This shows that these materials could potentially be interesting for high-pressure gas separation applications, although additional gas permeation experiments using different feed gas compositions and thin films are required.