Enhanced gas separation performance of polyimide membranes through nucleophilic ring-opening crosslinking with diepoxides

Crosslinking is an effective strategy to enhance both gas separation performance and stability of membranes. Herein, we introduced hydroxyl (-OH) and amine (-NH2) groups into the polyimide backbone by copolymerizing 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF) and Tris (4-aminophenyl)amine (TAPA) with DAM monomers. Two diepoxide crosslinkers were then used to crosslink the polyimide membranes through nucleophilic ring-opening reactions, and the resulting gas separation performance and membrane stability were assessed. Crosslinking with the bulky 1,4-Bis(glycidyloxy)benzene (BGOB) crosslinker reduced O2 permeabilities to 56 Barrer for OH-containing polyimide membranes and 32.7 Barrer for NH2-containing counterpart, while significantly increasing O2/N2 selectivities to 5.4 and 5.5, respectively. These membranes successfully exceeded the 1991 upper bound and approached the 2008 upper bound for O2/N2 separation. The diethylene glycol diglycidyl ether (PGGE)-crosslinked polyimide membranes exhibited superior CO2/N2 separation properties compared to their BGOB-crosslinked counterparts, attributed to the CO2-philic nature of the PGGE crosslinker. Additionally, the crosslinked membranes demonstrated great anti-aging performance over 120 days and strong resistance to CO2-induced plasticization under elevated pressures ranging from 2 to 12 bar. Overall, this innovative diepoxide crosslinking method improved the gas separation performance, and effectively addressed the challenge of membrane stability under harsh conditions.