Pendant-group cross-linked highly sulfonated co-polyimides for proton exchange membranes

• Novel pendant-group cross-linkable highly sulfonated co-polyimides were synthesized.
• The crosslinking reaction was performed at 260 °C.
• During the curing process, there was no elimination of sulfonic acid groups.
• After crosslinking, the performances of the membranes were significantly enhanced.

A series of pendant-group cross-linkable sulfonated co-polyimides (SPIs) with high degree of sulfonation were synthesized via a one-pot poly-condensation reaction from 1,3-bis(2-trifluoromethyl-4-aminophenoxy)-5-(2,3,5,6-tetrafluoro-4-vinylphenoxy)benzene (6FTFPB), 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6FAPB), 4,4′-diamindodiphenyl ether-2,2′-disulfonic acid (ODADS) and 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA). Tough and flexible membranes were obtained from the SPIs by a solvent casting method. The cross-linking reaction of the pendant tetrafluorostyrol groups in the SPIs was performed at 260 °C. During the thermal treatment, no loss of the sulfonic acid groups was observed. The performance of the membranes was significantly enhanced after curing reaction. For instance, the proton conductivity, water uptake, swelling ratio and methanol permeability of the Cured-SPI-20 at room temperature were 85 mS cm−1, 33.3%, 8.9% and 4.93×10−7 cm2 s−1, respectively. Moreover, the cross-linked membranes also exhibited improved chemical resistance, oxidative stability and mechanical property. This investigation demonstrates that the pendant cross-linked polyimide membranes with high degree of sulfonation may be promising for proton exchange membrane fuel cell technology.

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