Elucidating the Role of Fluorine Content on Gas Sorption Properties of Fluorinated Polyimides

Abstract

Hydrocarbon and perfluorinated polymers display distinct thermodynamic partitioning characteristics. These differences enable perfluoropolymers to outperform hydrocarbon polymers for many membrane-based gas separations, but the mechanism in which fluorine affects gas sorption and sorption selectivity in polymers is still not well understood. To bridge the existing gap in our fundamental understanding of sorption in hydrocarbon and perfluorinated polymers, this study investigates gas sorption across a range of temperatures, pressures, and gas species for four polyimides containing varying fluorine content. Observed improvements in sorption selectivity for the highly fluorinated polymers were analyzed through the dual-mode model and were found to result primarily from increased Henry sorption selectivity. Additionally, analysis of the energetics of sorption revealed a greater enthalpic penalty for Henry sorption in highly fluorinated polymers. Finally, consistent with the anomalous solubility behavior observed for hydrocarbon–perfluorocarbon liquid mixtures, our results indicate that fluorination appears to affect bulk penetrant–polymer mixing through unfavorable mixing interactions.