Synthesis of novel membranes of polyimide with side-chains and their pervaporation performance

Two novel aromatic diamines 3, 5-diaminobenzophenone (PDA) and 4-(trifluoromethyl) phenyl 3, 5-diaminobenzoate (FPDA) were mixed separately with 4, 4'-diaminodiphenyl ether (ODA) and 3, 5-diaminebenzoic acid (DABA) with different ratios, and then copolymerized separately with 4, 4'-oxydiphthalic anhydride (ODPA), 3, 3', 4, 4'-benzophenonetetracarboxylic dianhydride (BTDA) and 4, 4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) to give poly(amic acid)s, which were converted into cross-linked polyimides (PIs) with ethylene glycol (EG) by thermal imidization. The structures of these series of PIs were characterized by FT-IR, NMR and GPC. The effect of side-chain content on the thermal stability was investigated by DSC and TGA. The effects of the fluorine group on the properties and separation performance of side-chain polyimides were investigated. The membranes of polyimide obtained were employed in pervaporation separation of benzene/cyclohexane mixtures. The two side-chain polyimide membranes containing 6FDA have an optimal separation efficiency in terms of both flux and separation factor. The cross-linked 6FDA-FPDA/ODA/DABA (1:7:2) membranes show the most promising separation performance with a flux of 9.84 kg·μm·m^-2·h^-1 and separation factor of 6.1 for benzene/cyclohexane (50/50, mass ratio) at 50°C.