Propylene/propane plasticization in polyimide membranes

Polyimides have been considered as attractive polymers to be used in membrane based separation processes to separate propylene/propane mixtures. However, propane and propylene tend to plasticize polyimide membranes. In this study, we used molecular simulation techniques to gain insight on the relationship between molecular structure and plasticization of polyimides due to the sorption of propane and propylene molecules. Three fluorinated polyimides with different backbone rigidities were considered for this purpose: 6FDA-ODA, 6FDA-DPX and 6FDA-DAM (6FDA: 4,4-hexafluoroisopropylidene-diphthalic anhydride; DAM: 2,4,6-trimethyl-m-phenylene diamine ODA: and 4,4-oxydianiline DPX: 2,5-dimethyl-p-phenylenediamine). A combination of molecular dynamics and Monte Carlo methods was used to estimate sorption induced changes in the polymer matrices. Structural analyses such as radial distribution function, free volume, and local dynamics analyses were carried out to better understand the plasticization mechanism. Results showed that propane and propylene sorption capacities also depend on the polymer-penetrant interactions in addition to the fractional free volume of the polymer which has been shown as the dominant factor for CO₂ sorption on the same polyimides in our previous work. The polymer-penetrant interactions are more dominant for propylene than propane due to high polarizability of propylene resulting from π-electron complexation. The analysis of preferential sorption sites in both single and binary gas adsorption reveals that site preferences are altered in binary gas adsorption due to the competition between propane and propylene molecules. The dianhydride structure contain the most preferential sorption site and hence is plasticized in the single gas adsorption case, whereas, the most preferential sorption site moves to the diamine structure in case of binary sorption, especially in more flexible polyimides such as 6FDA-ODA.