Poly(arylene ether sulfone) proton exchange membranes with flexible acid side chains

New monomers containing two or four pendent methoxyphenyl groups were synthesized by bromination of bis(4-fluorophenyl)sulfone, followed by Suzuki coupling with 4-methoxybenzeneboronic acid. Copoly(arylene ether sulfone)s containing methoxyphenyl groups (2-MPAES-xx and 4-MPAES-xx) based on the corresponding monomers were prepared by aromatic nucleophilic substitution (SNAr) polycondensation. After demethylation of the methoxy group to the reactive hydroxyl group, the respective side chain-type sulfonated copolymers (2-SPAES-xx and 4-SPAES-xx) were obtained by sulfobutylation. Flexible and tough membranes having good mechanical strength were obtained by solution casting of all copolymers. The sulfonated copolymers with two or four pendent sulfobutoxyphenyl groups had high proton conductivities in the range of 0.108–0.258 S/cm for 2-SPAES-xx and 0.135–0.194 S/cm for 4-SPAES-xx at 80 °C, respectively. The methanol permeabilities of these membranes were in the range of 1.59 × 10−7–4.69 × 10−7 cm2/s, which is much lower than Nafion® (15.5 × 10−7 cm2/s). 2-SPAES-xx and 4-SPAES-xx display comparatively better balance between proton conductivity and water swelling than the analogous poly(arylene ether sulfone)s containing pendent sulfophenyl groups. A combination of high proton conductivities, low water uptake, and low methanol permeabilities for some of the obtained copolymers indicated that they have some of the requisite properties for proton exchange membranes in fuel cell applications.

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► Poly(arylene ether sulfone) PEMs with two or four flexible sulfobutyl groups.
► Excellent balance between proton conductivity and dimensional swelling.
► Nanophase separated morphology with interconnected hydrophilic clusters of 1–3 nm.
► Better conductivity-swelling behavior compared with analogous poly(arylene ether sulfone)s with inflexible side-chain sulfonic acid.