Effects of bulky 2,2′-substituents in dianhydrides on the microstructures and gas transport properties of thermally rearranged polybenzoxazoles

The chain interactions of hydroxyl polyimide (HPI) precursors, such as chain packing and π-π stacking interactions, significantly affect the microstructures and gas separation performance of the resulting thermally rearranged polybenzoxazoles (TR-PBOs). In this study, HPIs with various 2,2′-substituents in their anhydrides (BPDA-AP, PBPDA-AP, and 12FPBPDA-AP) were prepared through polycondensation of three dianhydrides (4,4′-biphenyltetracarboxylic dianhydride (BPDA), 2,2′-diphenyl-4,4′,5,5′-biphenyltetracarboxylic dianhydride (PBPDA), and 2,2′-bis(3″,5″-ditrifluoromethylphenyl)-4,4′,5,5′-biphenyltetracarboxylic dianhydride (12FPBPDA)) with 2,2′-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (AP). The dihedral angles and rotational barriers for PBPDA and 12FPBPDA were considerably larger than those for BPDA. Thus, the inter- and intra-molecular chain interactions were more pronounced for BPDA-AP than for PBPDA-AP and 12FPBPDA-AP. Consequently, the glass transition and TR temperatures of the HPIs were in the order of BPDA-AP > PBPDA-AP > 12FPBPDA-AP, whereas fractional free volume (FFV) and interchain distances followed the opposite trend. After TR, the FFV and interchain distances of the TR-PBO from BPDA-AP increased more than those from PBPDA-AP and 12FPBPDA-AP, whereas the π-π stacking interactions in BPDA-AP were well maintained. Consequently, BPDA-AP-450 exhibited the highest increase in gas permeability relative to its HPI precursor but the poorest plasticization resistance among fully converted TR-PBOs. This work demonstrated that chain packing played a crucial role in the TR behavior, microstructures, and gas transport properties of TR-PBOs. [Display omitted] •4,4′-Biphenyltetracarboxylic dianhydride (BPDA) and its 2,2′-disubstituted analogues were studied.•Thermally rearranged polybenzoxazoles (TR-PBOs) derived from BPDA and its bulky analogues were prepared.•TR-PBOs made with BPDA showed better gas permeability than those made from the bulky analogues.•TR-PBOs made with BPDA showed good potential for H2- and CO2-related separations.•Changes in FFV, chain packing, and π-π stacking interactions during TR transformation were noted.