Isatin-based microporous polymer membranes with tunable building blocks: Understanding structural effects on gas transport properties and plasticization behavior

In this study, we synthesized microporous polymer membranes using electrophilic isatin and nucleophilic aromatic building blocks via super-acid-catalyzed Friedel‒Crafts polycondensation to investigate the influence of polymer chain packing structures on gas transport properties and plasticization characteristics. By tuning the aromatic building blocks without modifying isatin, we controlled key structural parameters, including repeating unit chain length, planar conformation, rotational freedom, and inter-segmental interactions. The thermal and mechanical properties, fractional free volume, and chain packing structures of the synthesized polymers were thoroughly characterized to elucidate the impact of structural variations on gas separation performance. Further, variations in gas separation properties owing to different building blocks were elucidated through structural characterization. Notably, among the synthesized polymer membranes, the one with planar building blocks demonstrated the highest selectivity between H₂ and CO₂, attributed to its reduced symmetry and relatively polar characteristics. Meanwhile, the microporous polymer membrane with restricted rotational freedom exhibited the highest dimensional stability and plasticization resistance, enhancing stability during gas separation. Overall, this study provides fundamental insights into the structure–property relationships of isatin-based microporous polymers, offering guidance for the rational design of polymer materials for efficient and stable gas separation.