Synthesis, Characterization, and Gas Separation Properties of Polyimides Incorporating Fluorinated Non-coplanar Triphenylamine Units

A series of fluorinated copolyimides containing non-coplanar triphenylamine (TPA) units was synthesized from 4,4′-diamino-3′′,5′′-difluorotriphenylamine (DMTPA), 4,4′-oxydianiline (ODA), and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) through high-temperature polycondensation. The resulting copolyimides exhibited high thermal stability, good solubility in common organic solvents, optical transparency, and enhanced hydrophobicity. Wide-angle X-ray diffraction (WAXD) and molecular simulations confirmed that the bulky fluorinated TPA units increased interchain spacing and fractional free volume (FFV), thereby facilitating gas transport. Gas permeation measurements at 35 °C and 4 bar revealed that CO 2 permeability increased with TPA content, whereas CO 2 /N 2 selectivity remained nearly constant (~ 21). The best-performing membrane (DFPI-5) achieved a CO 2 permeability of 24.93 Barrer with a CO 2 /N 2 selectivity of 21.31, approaching the 2008 Robeson upper bound. These findings underscore the effectiveness of fluorinated, non-coplanar TPA moieties in tailoring polyimide membranes for advanced gas separation.