Plasticization resistant gas separation membranes derived from polyimides exhibiting polyethylene-oxide moieties

Abstract

Despite exhibiting excellent thermal/chemical stability, mechanical robustness, and easy processability, which makes them attractive candidates for gas separation applications, aromatic polyimides are susceptible to plasticization. This study proposes an effective approach to suppress CO2-induced plasticization in a high fractional free volume (FFV) polyimide (PI*) derived from 4,4’-(hexafluoroisopropylidene)diphthalic anhydride and 2,4,6-trimethylphenyldiamine. PI* was first modified by incorporating 3,5-diaminobenzoic acid (DABA) and polyethylene oxide segments (PEO) into the polymer backbone, with the goal of partially removing PEO through high temperature (450 °C) pyrolysis, thereby increasing the membrane FFV and, simultaneously, providing enhanced plasticization resistance via DABA cross-linking. To mitigate the effects of themomechanical history, the membranes were aged prior to the thermal treatment process. However, the great enhancement in plasticization resistance was not accompanied by a parallel improvement in the separation performance relative to PI*. To address this shortcoming, a microporous organic polymer (POP) was blended with the polyimides to fabricate mechanically-robust mixed matrix membranes (MMMs). Remarkably, MMM plasticization was suppressed even without the aid of thermal crosslinking for PEO-containing PI*, which was rationalized by molecular simulation invoking the formation of hydrogen bonds between the lactam groups of the POP and PEO moieties.