Side-chain sulfonated random and multiblock poly(ether sulfone)s for PEM applications

Abstract

Copoly(arylene ether sulfone)s from 4,4′-difluorodiphenyl sulfone, 4,4′-dihydroxydiphenyl sulfone bis-trimethylsilylether, and 2,5-diphenylhydroquinone bis-trimethylsilylether were obtained by nucleophilic displacement polycondensation with high molecular weights. While the random copolymers were obtained with narrow molecular weight distributions, the molecular weight distributions of the multiblock copolymers were much broader, probably indicating branching reactions. Random as well as multiblock copolymers showed a single glass transition temperature ( T g) at around 230 °C. Upon sulfonation with concentrated sulfuric acid, the T gs (from samples in the protonated form) were shifted to higher temperatures. NMR spectra and the determined ion-exchange capacities, which were close to the theoretical values, indicated that the pendant phenyl rings of the 2,5-diphenylhydroquinone moieties in the polymer backbone were sulfonated selectively. Membranes prepared from N-methyl-2-pyrrolidone solutions were transparent and soft. The water uptake at room temperature increased from 30% to 80% with increasing IEC. Samples with an IEC ⩾1.8 mmol/g swell to a high extend (sMBC) or even dissolve in water (sRC) at elevated temperatures. While the proton conductivities of the low IEC samples were lower than or close to that of Nafion®, the conductivities of the high IEC samples were superior to that of Nafion®. In general membranes from block copolymers showed similar water uptake and similar dimensional changes but higher proton conductivities as compared to samples from random copolymers with similar monomer composition and ion-exchange capacities.