Novel side-chain-type sulfonated diphenyl-based poly(arylene ether sulfone)s with a hydrogen-bonded network as proton exchange membranes

Abstract

A new bisphenol monomer, 3,3′,5,5′-tetramethoxy-4,4′-dihydroxybiphenyl, was synthesized and copolymerized to prepare diphenyl-based poly(arylene ether sulfone) copolymers containing tetra-methoxy groups (MOPAES). After converting the methoxy group to the reactive hydroxyl group, the resulting side-chain-type sulfonated copolymers (SOPAES) with a hydrogen bonded network were obtained by a sulfobutylation reaction. The copolymers were characterized and confirmed by ¹H NMR, FT-IR, thermogravimetric analysis (TGA) and small-angle X-ray scattering. The water uptake, proton and methanol transport properties of the resulting membranes were also determined for fuel cell applications. These SOPAES series membranes showed high proton conductivity in the range of 0.032–0.054 and 0.084–0.142 S cm⁻¹ at 25 and 80 °C under hydrated conditions, respectively. SOPAES-40 (IEC = 1.38 mequiv. g⁻¹) showed comparable proton conductivity with Nafion 117 in the hydrated state. The methanol permeability of these membranes was in the range of 1.58–4.29 × 10⁻⁷ cm² s⁻¹, which is much lower than Nafion (1.55 × 10⁻⁶ cm² s⁻¹). It should be noted that the intra/inter hydrogen bonds formed between sulfonic acid and hydroxyl groups or between hydroxyl and hydroxyl groups improved the mechanical properties and reduced the methanol permeability of the membranes effectively. A combination of suitable proton conductivity, low water uptake, and low methanol crossover for selected SOPAES indicates that they are good candidates as proton exchange membrane materials for fuel cells.