Synthesis and gas permeation properties of hyperbranched polyimides membranes from a novel (A₂+B₂B'+B₂)-type method

A series of hyperbranched polyimides were successfully synthesized by A₂+B₂B'+B₂ copolymerization from 2,2-bis(3,4-dicarboxylphenyl) hexafluoropropane dianhydride (6FDA), 2,4,6-triaminopyrimidine (TAP) and 4,4'-oxydianiline (ODA). Self-standing polyimides membranes with sufficient mechanical properties were achieved by incorporation of difunctional B₂ comonomer into the hyperbranched polymer structure. The degree of branching was estimated using ¹H NMR spectroscopy. The morphology structure of the prepared polyimides was evaluated by wide-angle X-ray diffraction patterns, and the d-spacing values first increased gradually to the highest value 5.73Å with increasing the content of TAP, and then decreased. The densities of these films were tested and the difference in density is probably due to the different inter-macromolecular packing efficiency. Both the 5% weight loss at temperatures and the glass transition temperatures (Tg) of the obtained polyimides decreased with decreasing the content of ODA comonomer. Similar tendency was found for tensile strength and elongation at break. Gas transport measurements showed the prepared TAP-ODA-6FDA hyperbranched polyimides exhibited lower permeabilities compared with ODA-6FDA linear polyimides, but higher O₂/N₂ and CO₂/N₂ selectivities. So they are expected to apply in the field of gas separation.