Perfluoropolymers have fundamentally distinct thermodynamic partitioning properties compared to those of their hydrocarbon counterparts. However, current upper bound theory assumes hydrocarbon solubility behavior for all polymers. Herein, the fundamental presupposition of invariance in solubility behavior to upper bound performance is critically assessed for perfluoropolymers and hydrocarbon polymers. By modifying solubility relationships, theoretical perfluoropolymer upper bounds are established, showing a positive shift of the upper bound front as a result of beneficial solubility selectivities for certain gas pairs, including N2/CH4, He/H2, He/N2, He/CH4, and He/CO2. Within the framework of the solution–diffusion model, an analysis is presented to compare two independent approaches often pursued in efforts to surpass the polymer upper bound: (a) achieving solubility selectivity via perfluoropolymers and (b) improving diffusion selectivity via rigid hydrocarbon polymers. This analysis demonstrates the significant benefit that can be achieved by considering both the chemical composition and morphology of solid‐state macromolecules when designing membrane materials.

Partially fluorinated polymers often exhibit exceptional membrane-based separation performance for a variety of gas pairs. While many gas transport studies focus on the incorporation of aliphatic fluorine groups (e.g., −CF3) on the polymer backbone, few studies have systematically investigated structure–property relationships for aromatic fluorine groups. Here, the effect of aliphatic and aromatic fluorine groups on solid-state morphology and gas transport is compared for structural analogues of 6FDA-based polyimides that contain either hydrogen or fluorine functional groups on the diamine monomer. Both fluorinated analogues displayed higher gas diffusivity compared to their hydrocarbon-based counterparts. However, the aromatic fluorinated analogue displayed a larger decrease in diffusivity selectivity due to weakened secondary interchain forces and a larger increase in interchain spacing, suggesting a greater extent of packing disruption resulting from increased steric hindrance associated with aromatic fluorine groups. This study establishes guiding principles for how carbon–fluorine bonds affect macromolecular packing structure and gas separation performance.

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.

Perfluoropolymers are a unique class of materials that display anomalous thermodynamic partitioning compared to hydrocarbon polymers and show exceptional separation performance for certain gas pairs. However, the molecular origin by which fluorine affects gas sorption is not well-understood, and the sorption behavior of partially fluorinated polymer analogues is rarely quantified. Here, we synthesized and characterized a series of structurally analogous poly­(ether imide)­s spanning from fully hydrocarbon to perfluorinated, which involved the synthesis of a perfluorinated dianhydride monomer. Sorption isotherms for multiple temperatures and gases were analyzed using the non-equilibrium lattice fluid model. The lattice fluid parameters were estimated from infinite dilution sorption data. The binary interaction parameter increased with polymer fluorine content for all gases, with CH4 showing the largest increase in unfavorable deviation from ideal mixing. Continuous trends for the enthalpic, entropic, and infinite dilution sorption selectivity with fluorine content were observed, wherein the increase in enthalpic selectivity was greater than the decrease in entropic selectivity, resulting in overall increased sorption selectivity for gas pairs where the less condensable gas is also the faster permeating gas (e.g., N2/CH4). Our findings connect the sorption behavior of hydrocarbon polymers and perfluoropolymers and provide mechanistic insight into the role of fluorine on gas sorption.