Anchoring the Dianhydride Structure to Simultaneously Enhance the Permeability and Selectivity of the Polybenzoxazole Membranes

The fine structure of microporous polymer materials significantly affects their membrane separation performance. However, the regulation of bulky monomers complicates the precise elucidation of the structure–property relationship in these materials. In this study, we proposed an anchored twisted monomer strategy to achieve atomic-level optimization of polybenzoxazole (PBO) membranes, significantly enhancing the gas separation performance. Based on the pristine dianhydride 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), the ether-oxygen bonds were precisely introduced to anchor the distorted dianhydride structure. Two hydroxyl-containing polyimides (PIs), 6FCBI (with ether-oxygen bonds) and 6FBI (without ether-oxygen bonds), were synthesized and subsequently converted into PBO thin films (6FCBO and 6FBO) via thermal treatment. The incorporation of ether-oxygen bonds effectively modulates polymer chain rigidity and influences the stacking of PBO chains, resulting in increased free volume and microporosity. Compared to conventional thermal rearrangement membranes, the 6FCBO membranes demonstrate an anti-trade-off effect in gas separation performance, enhancing both gas permeability and selectivity. Notably, 6FCBO achieves a CO₂ permeability of 2540 Barrer and a CO₂/CH₄ selectivity of 28.8, exceeding the 2008 Robeson upper bound. This work provides a precise structural optimization strategy for microporous polymeric materials, offering valuable insights and guidance for the design of advanced gas separation membranes.