Effect of benzene side groups on polyimide membranes containing spiro-bis-benzimidazole scaffold structures for gas separation

Two novel spiro-bis-benzimidazole-based diamines were synthesized and polymerized with commercial 4,4′-(hexafluoroisopropylidene)diphthalic anhydride dianhydride (6FDA) to produce two types of spiro-bis-benzimidazole-based polyimides (PIM-PBZIs). The resulting polymer membranes, PIM-PBZI-1 and PIM-PBZI-2, exhibited excellent thermal and mechanical properties, including glass transition temperature (T g ) of 417 and 413 °C, 5 % thermal decomposition temperature (T d5% ) above 500 °C, tensile strength of 72 and 74 MPa, and an elongation at break of 6.6 and 5.4 %. Additionally, these PIM-PBZIs were confirmed as novel microporous polymers with BET surface areas of 473–546 m 2 /g and pore sizes ranging from 4 to 9 Å. Furthermore, PIM-PBZI-1, modified with N -phenyl, demonstrated excellent organic solubility and gas separation performance, with CO 2 /CH 4 gas pair permeability selectivity exceeding the 1991 Robeson upper bound, providing an effective pathway for the development of high-performance gas separation membranes.