Abstract
Fluorinated 6FCDA-based copolyimides (FCPs) were synthesized using varied monomer compositions, specifically HQDEA-TADPS-DAM to ODA ratios of 1:1, 1:3, and 3:1 to systematically engineer the porosity, rigidity, and thermal stability of their corresponding carbon molecular sieve (CMS) membranes for light olefin/paraffin separations. Owing to their excellent solubility in polar aprotic solvents, all FCPs enabled the fabrication of defect-free dense films, which were subsequently pyrolyzed at 550 °C under argon to yield structurally robust CMS membranes. Dual-mode sorption analysis combined with Clausius-Clapeyron modeling revealed a pronounced affinity of olefins to the polymer matrices, driven primarily by Langmuir-type adsorption mechanisms. The as-cast FCP films exhibited ultrahigh gas permeabilities (e.g., ∼1.3 × 106 to 8.3 × 106 Barrer for C2H4 and C3H6), albeit with low selectivities, consistent with solution-diffusion transport mechanisms. In contrast, the CMS membranes displayed strong molecular sieving behavior, with reduced permeabilities (e.g., C2H4 ∼377 Barrer; C3H6 ∼311 Barrer) but significantly enhanced olefin selectivities (C2H4/C2H6 = 9–10; C3H6/C3H8 = 16–18). Monomer composition was shown to play a critical role in dictating chain packing, fractional free volume, segmental mobility, and resistance to plasticization and physical aging in both the polyimide and the resulting CMS membranes. This study demonstrates a rational monomer design strategy for tailoring microstructure and performance in CMS membranes, offering a scalable pathway toward high-performance and stable gas separation technologies.
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•Monomer-tuned fluorinated copolyimides were synthesized using varied rigid/flexible diamine ratios, enabling tailored microstructure and chain packing.•First demonstration of CMS membranes derived from 6FCDA-based fluorinated copolyimides.•Pyrolyzed CMS membranes exhibited high C2H4/C2H6 and C3H6/C3H8 selectivity.•Dual-mode sorption and isosteric heat (Qst) analysis confirmed strong olefin affinity via Langmuir-type adsorption.