Synthesis and characterization of sulfonated poly(aryl ether sulfone) containing pendent quaternary ammonium groups for proton exchange membranes

A new biphenol monomer, 2-(2-(dimethylamino)ethyl)-3,3′-bis(4-hydroxyphenyl)isoindolin-1-one (CPPH), was easily prepared by the reaction of phenolphthalein with N,N-dimethylethane-1,2-diamine. Novel polyampholyte, sulfonated poly(aryl ether sulfone)s with pendent alkyl ammonium groups PESQ-x-y, were prepared by copolymerization of CPPH, 4,4′-biphenol, 3,3′-disulfonated-4,4′-dichlorodiphenyl sulfone, and 4,4′-dichlorodiphenyl sulfone, and this was followed by reaction with iodomethane. PESQ-x-y was able to form uniform and tough membranes by simple solution casting. Moreover, the material exhibited a low water uptake as well as a decent dimensional stability in water, even at high temperature (e.g. 100 °C), as a result of the strong electrostatic interactions between the alkyl ammonium and sulfonic acid groups. The PESQ-10-50 copolymer membrane with an ion-exchange capacity (IEC) of 1.65 mequiv./g, displayed a lower water uptake (58.9%), but a higher proton conductivity (0.082 S/cm) than a PES-40 membrane without alkyl ammonium groups (IEC = 1.67 mequiv./g, WU = 85.4%, σ = 0.069 S/cm) at 20 °C.