In this work, we report the synthesis, comprehensive characterization, and gas separation evaluation of a new series of fluorinated copolyimides (FCPs) based on 9,9-bis(trifluoromethyl)-2,3,6,7-xanthenetetracarboxylic dianhydride (6FCDA) and 4,4′-oxydiphthalic anhydride (ODPA) with different diamine components (ODA and TFDB). The copolyimides were synthesized via a two-step chemical imidization and solution-casting process, yielding high molecular weight polymers with narrow polydispersity. Structural and thermal analyses-including ¹H NMR, ATR-FTIR, DSC, TGA, and X-ray diffraction-confirmed complete imidization and allowed for fine tuning of polymer chain packing and fractional free volume (FFV). Gas permeabilities and permselectivities for various gas pairs (C₂H₄/C₂H₆ and C₃H₆/C₃H₈) were studied at three different temperatures (25°C, 35°C, and 45°C) and feed pressures of 20, 40, 60, and 80 psia. The results indicate that FCP membranes with a higher concentration of fluorine exhibited lower chain packing density and higher permeability, whereas those with segregated perfluoroalkyl side chains demonstrated both higher permeability and relatively improved permselectivity. Additionally, carbon molecular sieve (CMS) membranes using controlled pyrolysis were fabricated from these copolyimides and tested at room temperature. Gas permeability measurements were performed for all four gases, which provided a comparative performance evaluation of CMS versus the present FCP membranes. These findings provide valuable insights into the structure–property relationships of fluorinated copolyimides and offer guidance for designing high-performance membranes for challenging gas separations. Future work will focus on membrane fabrication and process optimization for scalable industrial applications.
