Systematic Investigation of Microstructures and Gas Separation Performance of Thermally Rearranged Polybenzoxazole Membranes Derived from 9,9-Bis(3,4-dicarboxyphenyl)fluorene Dianhydride

Understanding the detailed thermal rearrangement (TR) process is very important for achieving high-performance TR membranes for gas separation. Here, four series of polyimides containing hydroxyl or acetyl groups (HPIs or AcPIs) and thermally rearranged polybenzoxazoles (TR-PBOs) were prepared using 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF), 3,3′-dihydroxybenzidine (HAB), and 2,2′-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (APAF) monomers. The AcPIs had lower TR temperatures but lower final TR conversions compared to the corresponding HPIs. Specifically, the AcPIs exhibited two TR peaks with the first small one ranging from 380 to 410 °C (∼30 °C lower than the HPIs) and the second major one ranging from ∼520 to 540 °C, which was absent in HPIs. The AcPIs had a higher initial fractional free volume (FFV) but less FFV increments after TR, as compared to the corresponding HPIs. After TR, gas permeability increased by 3.8- to 118.6-folds for the TR-PBOs, relative to the corresponding AcPIs or HPIs. Meanwhile, BPAF-APAF-Ac-450 displayed good CO₂/CH₄ separation performance, with its pure gas separation performance approaching the 2008 Robeson’s upper bound, reliable resistance to physical aging, and mixed-gas separation performance exceeding the 2018 upper bound. This work provides a good perspective for achieving high-performance TR membranes for gas separation applications.