Fabrication of high-performance gas separation membranes via fluorenyl-induced and thermal rearrangement-enhanced microporosity

To overcome the intrinsic permeability–selectivity trade-off of polymeric gas separation membranes, a synergistic strategy was developed that integrates the fluorenyl-induced intrinsic microporosity (PIMs) effect with thermal rearrangement (TR)-enhanced structural evolution. A fluorene-containing diamine, 9,9-bis(4-aminophenyl) fluorene (FDA), and three o-hydroxyl-containing diamines with distinct bridging architectures — 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (6FAP), 2,2-bis(3-amino-4-hydroxyphenyl) propane (APA), and 3,3′-dihydroxybenzidine (HAB) — were incorporated into a 6FDA (4,4′-(hexafluoroisopropylidene) diphthalic anhydride)-based polyimide backbone, followed by thermal rearrangement. Combining microstructural characterization and molecular dynamics simulations, the study revealed that contorted fluorenyl units effectively suppressed dense chain packing and generated a pre-existing microporous framework, while the incorporation of 6FAP promoted micropore interconnection and stabilization during thermal rearrangement. Consequently, the optimized TR membrane with a 6FAP-to-FDA ratio of 4:6 exhibited a CO 2 permeability of 432.2 Barrer and a CO 2 /CH 4 selectivity of 77.2, representing an approximately 6.4-fold increase in permeability and a 1.6-fold increase in selectivity over the PI membrane without o-hydroxyl-containing diamines. Notably, the membrane also exhibited desirable mechanical properties, including a high tensile strength (>35 MPa) and good flexibility. Furthermore, it demonstrated reliable mixed-gas separation capability and long-term operational stability.