Simplified analysis of gas/polymer selective solubility behavior

Abstract

Abstract Interactions of penetrants with gas separation membrane materials can have both desirable and undesirable consequences. Improved permselectivity resulting from specific polymer/penetrant interactions must be balanced against potentially undesirable plasticization effects resulting from such interactions. Consideration is given to the utility of gas solubility measurements in low‐molecular‐weight solvents as an aid to understanding solubility data for polymer/gas systems. The solubility coefficients of gases in low‐molecular‐weight liquids and in polymeric media of similar chemical structure tend to be quite different in magnitude. “Equation‐of‐state” and combinatorial entropy differences for the free energy of mixing of gases with low‐molecular‐weight solvents and high‐molecular‐weight polymers presumably cause this discrepancy. The above complications are minimized and useful trends identified by considering the so‐called solubility selectivity comprised of ratios of absolute gas solubilities in both low‐and high‐molecular‐weight media of similar chemical natures. For example, both acetone and methyl acetate are highly sorptive of CO 2 and display a high degree of solubility selectivity for CO 2 relative to CH 4 . These two solvents have mass densities of carbonyl groups that are similar to that in cellulose acetate and poly(methyl methacrylate). Both of these polymers are also highly sorptive of CO 2 and have solubility selectivities for CO 2 relative to CH 4 that are similar to that of acetone and methyl acetate. Anomalous behavior is encountered in the case of Kapton poly(ether diimide), which contains a high density of carbonyl groups (four per repeat unit). On the basis of the behavior of the other carbonyl‐rich polymers, it was expected that Kapton would show high solubility selectivity for CO 2 relative to CH 4 , but this was not observed. A hypothetical explanation for the Kapton behavior is presented. In addition, possible advantages of the Kapton‐type behavior are discussed in the context of the reported good plasticization resistance of polyimides.