Synthesis and properties of cis/trans mixtures of bis(4-hydroxyphenyl)-1,2-diphenylethylene containing sulfonated poly(ethersulfone)s proton exchange membranes

Conjugated cis/trans isomer-based polymer membranes were prepared with 15–25 mol% of bis(4-hydroxyphenyl)-1,2-diphenylethylene (BHDPE). BHDPE has a conjugated nonplanar conformation structure containing the peripheral four aromatic rings which facilitate the formation of π-π interactions. A series of sulfonated poly(ethersulfone)s (S-PBHDPE) was synthesized with concentrated sulfuric acid. The sulfonation took place selectively on the BHDPE unit, and simultaneously on the side-chain and main-chain phenyl groups. The S-PBHDPEs consisted of a 4:6 ratio of cis and trans form mixtures. The S-PBHDPE membrane structures and characteristics were evaluated by 1H NMR spectroscopy, and their thermal stabilities evaluated. The ion exchange capacity (IEC), water uptake (WU) and proton conductivity of the membranes were evaluated as a function of the molar percentage of the sulfonic acid group. The WU of the synthesized S-PBHDPE membranes ranged from 21 to 52%, compared with 28% for Nafion 211®. The S-PBHDPE membranes exhibited proton conductivities (80 °C, RH100) of 42.7–125.6 mS/cm, compared with 137.4 mS/cm for Nafion 211®. The surface morphologies of S-PES 40 and S-PBHDPE 20 membranes were studied by AFM images.