Influence of Aliphatic and Aromatic Fluorine Groups on Gas Permeability and Morphology of Fluorinated Polyimide Films

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.