Tailoring the microporosity and gas separation property of soluble polybenzoxazole membranes derived from different regioisomer monomers

Abstract

New polymers and the relationship between their molecular structure and gas separation performance are crucial in membrane based gas separation. In the study, a range of bisbenzoxazole containing diamines with spirobisindane as building block were synthesized, and consequently, their corresponding four soluble spirobisindane-bisbenzoxazole-polyimides (SPBO-PIs) (5a-6FDA to 5bb-6FDA) were obtained. These SPBO-PIs exhibit superior solubility, brilliant mechanical performance which shows high tensile strengths (about 80 MPa) and attractive elongation at break (7.6–12.8 %), and good thermal stability (Tg up to 491 °C). The SPBO-PIs from para-amino and symmetric linkages in the polymeric main chains has superior thermal and mechanical properties, slightly lower density (1.24, 1.25 vs 1.27, 1.29 cm3 g−1), higher surface area (561, 437 vs 364, 318 m2 g−1), bigger micropore (5.74, 5.60 vs 5.53, 5.49 Å), and larger free volume (0.302, 0.284 vs 0.282, 0.277) than the corresponding mata-amino and asymmetric linkages. The SPBO-PI from amine isomers at para-linked (5a-6FDA) exhibits better CO2/CH4 and CO2/N2 separation performance than the SPBO-PI from amine isomers at meta-linked (5aa-6FDA), meanwhile, the SPBO-PI from symmetric oxazole isomers (5aa-6FDA) shows higher permeability than the asymmetric oxazole isomers (5bb-6FDA). The 5b-6FDA exhibits reasonable anti-plasticization properties, and 97.3 Barrer for CO2 permeability and 24.1 of CO2/CH4 mixed-gas selectivity at 20 bar. The study about SPBO-PI membranes shed light on molecular design of high performance polyimide and polybenzoxazole-based membranes.