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.