Tailored 6FCDA Copolyimides for Microporosity-Controlled CMS Membranes in C ₃ H ₆ /C ₃ H ₈ Separation

A series of 6FCDA-based fluorinated copolyimides (FCPs) were synthesized via controlled polycondensation by combining rigid aromatic diamines and fluorinated, ether-bridged dianhydrides to tailor the polymer backbone for enhanced gas separation performance. The structure–property design strategy involved the copolymerization of 2,2′-dimethylbenzidine, 1,4-bis(3,4-dicarboxyphenoxy)benzene dianhydride and a mixture of 2,4,6-trimethyl-1,3-phenylenediamine and 4,4′-oxidianiline. This combination was employed to disrupt polymer chain packing, increase the fractional free volume (FFV), and improve solubility in polar aprotic solvents, thereby facilitating polymer processing and membrane fabrication. Additionally, the resulting copolyimides exhibited excellent thermal stability, making them suitable candidates for further pyrolysis to produce carbon molecular sieve membranes (CMSs). Single-gas permeation measurements for propylene (C3H6) and propane (C3H8) at 35 °C revealed dual-mode sorption behavior and pressure-dependent transport characteristics. Among the synthesized polymers, the polymer with a DAM/ODA molar ratio of 3:1 (FCP-31) exhibited a C3H6 permeability of 271 Barrer and an ideal C3H6/C3H8 selectivity of 3 at 35 °C and 1 bar under pure-gas conditions. After 60 days of physical aging at 25 °C under dry N2, the C3H6 permeability decreased to 199 Barrer, whilel selectivity increased to about 4 at 1 bar. Carbonization of the FCPs produced CMS membranes with significantly enhanced selectivity. Fresh CMS membranes demonstrated a C3H6 permeability of 8 Barrer and an ideal C3H6/C3H8 selectivity of 16 at 1 bar. However, both permeability and selectivity decreased with increasing feed pressure up to 6 bar. After 60 days of ambient storage, the CMS membranes showed a reduced C3H6 permeability of 5 Barrer and an increased selectivity of 19, indicative of physical aging effects. These findings highlight the critical influence of the monomer structure on the design of 6FCDA-based fluorinated copolyimides and their derived CMS membranes, demonstrating their potential for efficient C3H6/C3H8 separation.