Systematic\nInvestigation of Microstructures and Gas\nSeparation Performance of Thermally Rearranged Polybenzoxazole Membranes\nDerived from 9,9-Bis(3,4-dicarboxyphenyl)fluorene Dianhydride

Understanding the detailed thermal rearrangement (TR)\nprocess is\nvery important for achieving high-performance TR membranes for gas\nseparation. Here, four series of polyimides containing hydroxyl or\nacetyl groups (HPIs or AcPIs) and thermally rearranged polybenzoxazoles\n(TR-PBOs) were prepared using 9,9-bis(3,4-dicarboxyphenyl)fluorene\ndianhydride (BPAF), 3,3′-dihydroxybenzidine (HAB), and 2,2′-bis(3-amino-4-hydroxyphenyl)hexafluoropropane\n(APAF) monomers. The AcPIs had lower TR temperatures but lower final\nTR conversions compared to the corresponding HPIs. Specifically, the\nAcPIs exhibited two TR peaks with the first small one ranging from\n380 to 410 °C (∼30 °C lower than the HPIs) and the\nsecond major one ranging from ∼520 to 540 °C, which was\nabsent in HPIs. The AcPIs had a higher initial fractional free volume\n(FFV) but less FFV increments after TR, as compared to the corresponding\nHPIs. After TR, gas permeability increased by 3.8- to 118.6-folds\nfor the TR-PBOs, relative to the corresponding AcPIs or HPIs. Meanwhile,\nBPAF-APAF-Ac-450 displayed good CO<sub>2</sub>/CH<sub>4</sub> separation\nperformance, with its pure gas separation performance approaching\nthe 2008 Robeson’s upper bound, reliable resistance to physical\naging, and mixed-gas separation performance exceeding the 2018 upper\nbound. This work provides a good perspective for achieving high-performance\nTR membranes for gas separation applications.