Abstract
A new series of fluorinated copolyimides (FCPs) based on 9,9-bis(trifluoromethyl)-2,3,6,7-xanthenetetracarboxylic dianhydride (6FCDA) was synthesized using structurally diverse diamines, namely, N,N′-[1,1′-Biphenyl]-4,4′-diylbis[4-(4-aminophenoxy)benzamide] (HQDEA), 4,4′-oxydianiline (ODA), 2,4-diaminomesitylene (DAM), and 3,3,5,5-tetramethyl-4,4′-diaminodicyclohexylmethane (TADPS), in order to systematically investigate the relationships between polymer structure, physical properties, and gas separation performance. The resulting copolyimides (e.g., 6FCDA–HQDEA–ODA, 6FCDA–HQDEA–DAM, and 6FCDA–TADPS–ODA) were evaluated as membrane materials for light olefin/paraffin separations. By varying the rigidity, polarity, and steric hindrance of the diamine components, a structure–mobility–affinity (SMA) framework was established, providing a rational basis for correlating polymer microstructure with gas transport behavior. All FCPs exhibited excellent thermal stability, with glass transition temperatures (Tg) exceeding 290 °C and 5 % weight loss decomposition temperatures (Td5 %) above 480 °C. The Dual-Mode Sorption Model was employed to analyze their gas sorption and permeation behavior. The FCPs were processed, and membrane thin films were prepared by using NMP and THF as casting solvents. Among them, the 6FCDA–HQDEA–ODA membrane cast from THF displayed the highest thermal stability and tunable chain packing, resulting in an ethylene permeability of up to 2.21 × 106 Barrer and a C2H4/C2H6 selectivity of 3.45 at 1.5 bar and 35 °C. In contrast, incorporation of the rigid and sterically hindered DAM diamine in the 6FCDA–HQDEA–DAM copolyimide, cast from THF, yielded the highest C3H6/C3H8 selectivity of 8 under the same conditions. These results are attributed to enthalpy-favored sorption at imide carbonyl and ether linkage sites, as well as enhanced polar confinement arising from the increased free volume introduced by the bulky DAM structure. Furthermore, solvent casting conditions significantly influenced membrane morphology and separation performance, highlighting the critical role of processing parameters in FCP membrane fabrication.
FCPs membranes, engineered via molecularly tailored diamines, achieve high selectivity for olefin/paraffin pairs offering an eco-friendly alternative to fluorinated systems. [Display omitted]
•A new series of 6FCDA-based fluorinated copolyimides (FCPs) was synthesized using HQDEA, ODA, DAM, and TADPS monomers.•The Structure–Mobility–Affinity (SMA) framework was introduced to rationalize the enthalpic and entropic gas–polymer interactions.•6FCDA–HQDEA–DAM achieved exceptional C3H6/C3H8 selectivity of >8 via polar domain confinement.•6FCDA–HQDEA–ODA exhibited record ethylene permeability of 2.6 × 10⁶ Barrer (at 1.5 bar, NMP), while a C₂H₄/C₂H₆ selectivity of 3.45 was achieved at 1.5 bar, THF.