Polymer Electrolyte Membranes Derived from New Sulfone Monomers with Pendent Sulfonic Acid Groups

Abstract

New monomers containing two or four pendent phenyl groups were synthesized by bromination of bis(4-fluorophenyl) sulfone, followed by Suzuki coupling with benzeneboronic acid. The resulting monomers were converted to the corresponding sulfonated monomers having two or four pendent sulfonic acid groups, predominately at the p-phenyl position. Aromatic nucleophilic substitution (SNAr) polycondensation using the di- and tetrasulfonated monomers provided sulfonated poly(arylene ether sulfone) copolymers S2-PAES-xx and S4-PAES-xx, respectively, where xx refers to the molar ratio of the sulfonated to non-sulfonated pendent phenyl monomer. Copoly(arylene ether sulfone)s based on the corresponding non-sulfonated monomers were also synthesized for a parallel study on postpolymerization sulfonation of these copolymers. Postsulfonation occurred predominately at the para-pendent phenyl site, and the reactions were complete within a short time (about 30 min), without evidence of chain degradation. Flexible and tough membranes having high mechanical strength were obtained by solution casting of all four series of copolymers. The copolymers with two or four pendent sulfonic acid groups had high proton conductivities in the range of 44-142 mS/cm for S2-PAES-xx and 51-158 mS/cm for S4-PAES-xx at room temperature, respectively. The methanol permeabilities of these copolymers were in the range of 0.8 x 10(-8)-15.0 x 10(-7) cm(2)/s, which is lower than Nation (16.7 x 10(-7) cm(2)/s). The S4-PAES-xx membranes displayed better properties (lower water uptake and higher proton conductivities) than the S2-PAES-xx membranes, which can be attributed to the more blocky architecture of the sulfonic acid groups in the S4 membranes. A combination of high proton conductivities, low water uptake, and low methanol permeabilities for some of the obtained copolymers indicated that they are good candidate materials for proton exchange membrane in fuel cell applications.