Constructing Gas Molecule Transport Channels in Thermally Rearranged Multiblock Poly(benzoxazole-co-imide) Membranes for Effective CO₂/CH₄ Separation

A series of multiblock poly(benzoxazole-co-imide) (PBOI) membranes were prepared via thermal rearrangement of their corresponding multiblock copolyimide (CPI) precursors based on 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 2,2'-bis(3-amino-4-hydroxy-phenyl) hexafluoropropane (APAF), and 5-amino-2-(4-aminobenzene) benzimidazole (BIA). For ortho-hydroxy-functionalized CPI precursors, the difference in stacking behavior between the 6FDA-APAF and 6FDA-BIA blocks results in a weak microphase separation in CPI membranes, whereas the random precursor exhibits a homogeneous morphology, which has been identified by dynamic mechanical analysis, small-angle X-ray scattering, and atomic force microscopy analysis. After the subsequent thermal rearrangement at 420 degrees C, the microphase separation was also observed in the multiblock poly(benzoxazole-coimide) (TR-PBOI) membranes with an enlarged domain size. The impacts of the sequence structure on the membrane phase-separation behavior and mechanical and gas separation properties were investigated. Comparatively, an obvious increase in the CO2/CH4 gas separation property from the random to TR-PBOI membranes was observed. Specially, the resultant multiblock A(40)B(40)-TR-420 membrane possesses a CO2 permeability of 92 Barrer and a CO2/CH4 selectivity of 54.7, which is substantially higher than the values of the random-PBOI-420 membrane (CO2 permeability of 40.2 Barrer and CO2/CH4 of 58) and those of recently reported TR-PBOI membranes. The appealing gas separation performance of the multiblock PBOI membranes can be attributed to their microphase-separated morphology, in which continuous percolating microcavities formed in the thermal rearrangement reaction provide transport channels for gas molecules. The present study demonstrates that the modification of the micromorphological structure of membranes can effectively tune their final gas separation properties.