Steric hindrance effects of diverse substituents in spirobisindane-based polyimides for gas separation

Development of polymer membranes that combine high processability with superior gas permselectivity remains a critical challenge. In this work, a series of spirobisindane-based polyimides bearing diverse substituents (SPIRO-PIs) were synthesized by polymerization with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA). Substituents including H, -CF 3 , and -CH 3 were introduced to disrupt chain packing, reduce packing efficiency and systematically tuning the fractional free volume (FFV) and gas separation properties of the resulting polyimides. The obtained SPIRO-PIs exhibited excellent solubility in common organic solvents and good processability. Gas separation measurements showed that SPIRO-CH 3 -6FDA exhibits a CO 2 permeability of 578 Barrer and a CO 2 /CH 4 selectivity of 17.4, while SPIRO-CF 3 -6FDA exhibits a CO 2 permeability of 428 Barrer and a CO 2 /CH 4 selectivity of 22.9. Furthermore, SPIRO-CH 3 -6FDA displays a high BET surface area of 629 m 2 g −1 , mainly attributed to the enhanced steric hindrance induced by methyl substituents. These SPIRO-PIs also demonstrated excellent mechanical strength and outstanding thermal stability, highlighting their potential as promising candidates for high-performance gas separation membranes.