Synthesis and characterization of bis(phenyl)fluorene-based cardo polyimide membranes for H₂/CH₄ separation

Three bis(phenyl)fluorene-based cardo diamine monomers with different side groups (–CF₃, –H, and –CH₃) were synthesized and used to synthesize three 6FDA-based polyimides (6FDA-FBPF, 6FDA-BPF, and 6FDA-MBPF). The influence of bis(phenyl)fluorene cardo moiety and different side groups on the glass transition temperature (T_g), thermal stabilities, and chain packing of the synthesized polyimides were systematically studied. The gas permeabilities and selectivities of three bis(phenyl)fluorene-based polyimide membranes were studied and correlated with their fractional free volumes and chain packing conditions. As a result, the obtained polyimides showed promising performance for hydrogen separation. Among the three polyimides, 6FDA-FBPF had the highest gas permeabilities which exhibited H₂ permeability of 151.12 Barrer and H₂/CH₄ selectivity of 62.19. The high gas separation performance of 6FDA-FBPF mainly attributed to the ineffective chain packing via the incorporation of bi(phenyl)fluorene cardo moiety and introduction of bulky CF₃ side groups into the polyimide backbones that is favorable to formation hourglass-shaped pores.