Simultaneously enhanced gas separation and anti-aging performance of intrinsic microporous polyimide by dibromo substitution

Physical aging is a huge challenge hindering the practical gas separation application of high-performance intrinsic microporous polyimides (PIM-PIs). Herein, through precise molecular design, bromine was introduced into the PIM-PI chain to both improve its separation and anti-aging properties. To clarify the anti-aging effect of bromo groups in PIM-PIs, two fluorene-based diamine isomer monomers, 9,9-bis(3-bromo-4-aminophenyl)-fluorene (BBAPF), 2,7-dibromo-9,9-bis(4-aminophenyl)-fluorene (DBBAPF), and their corresponding PIM-PIs (BAPI, BBPI, DBPI) were synthesized. Compared with pristine BAPI membrane, the permeability of BBPI is improved about 50%, which can be attributed to the bromo groups near the amino group suppressing chain densification packing and providing more free volume element (FVE). In addition, the substituted bromo group increased the chain rigidity of the polymer and hindered its rotation, resulting in a lower release of fractional free volume (FFV) during aging process. Particularly, the permeability of the BBPI was maintained at approximately ∼70% with slightly increased selectivity after 250 days, while the pristine BAPI only retained about 50% permeability of N2 (CH4). Our findings proposed an effective strategy for designing microporous polymers with improved permeability and anti-aging properties for practical applications in gas separation membranes.