Synergistic Thermal Rearrangement and In Situ Decarboxylation Crosslinking: Structure Tailoring of Copolyimide Membranes for Enhanced Gas Separation

Abstract To enhance the gas separation performance of polyimide membranes, the synergy of thermal rearrangement (TR) and in situ decarboxylation crosslinking represents a distinctive fabrication strategy. Herein, a diamine monomer functionalized with both carboxyl and hydroxyl groups, namely 2-(3,6-bis(4-amino-3-hydroxyphenoxy)-9H-xanthen-9-yl)benzoic acid (BAHXBA), was synthesized. This diamine was then copolymerized with 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAP) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) at varying molar ratios. A set of polyimide (PI) membranes, designated as PI(BAHXBA-6FAP), were fabricated via thermal imidization at 300 °C, followed by annealing at 450 °C to construct thermally rearranged membranes, TR(BAHXBA-6FAP). The high-temperature thermal treatment induces in situ benzoxazole cyclization and moderate crosslinking derived from side-chain carboxyl decarboxylation. The copolymer composition effectively tunes chain-stacking distance and thermal properties: elevated 6FAP content increases d-spacing, and higher thermal treatment temperature also leads to a noticeable increase in d-spacing. For the TR(BAHXBA-6FAP) membrane with a BAHXBA-to-6FAP molar ratio of 5:5, gas permeability coefficients reach 1177 (H2), 341 (O2), 1116 (CO2), 51 (CH4) and 43 (N2) Barrer. Its ideal H2/N2 selectivity exceeds the 2008 Robeson upper bound, and the O2/N2 selectivity surpasses the corresponding 2015 upper bound. This work provides a facile structural tailoring strategy via coupled thermal rearrangement and decarboxylation crosslinking for high-performance polyimide separation membranes.