Modified Sulfonated Poly(arylene ether sulfone) Membranes Prepared via a Radiation Grafting Method for Fuel Cell Application

Abstract

Polymer modifications involving crosslinking and grafting by radiation have been widely researched for use in biopolymers, hydrogels, heat-resisting electric wires, vulcanization, polymer recycling, gas separation, and pervaporation membranes because of their advantages over traditional chemical crosslinking and grafting methods, including a catalyst-free reaction, post-modification at room temperature for solid polymers, and short modification times and steps. Various recent studies have also utilized radiation modification techniques to prepare proton exchange membranes (PEMs). PEMs are membranes which have the ability to selectively transfer protons generated by electrochemical reactions from the anode to the cathode in fuel cells. For this purpose, polymers having strong acidic functional group as proton carriers (e.g. sulfonic acid groups) have generally been used. There are two main strategies to introduce sulfonic acid groups into polymers. One is to introduce the groups directly to a polymer (post-sulfonation or polymersulfonation) and the other is to perform polymerization with a sulfonated monomer (monomer-sulfonation). To date, preirradiation and post-sulfonation methods, in which sulfonic acid groups are introduced after crosslinking or grafting non-sulfonated polymers by irradiation, have been used in radiation-induced processes of PEMs in order to prevent unfavorable membrane damage such as decomposition of sulfonic acid groups. However, this method has structural limitations for PEMs because the types of target polymer for irradiation are restricted to perfluorinated or partially fluorinated polymers and aliphatic polymers. In addition, crosslinking and grafting agents should have sites available for post-sulfonation. Whereas, few studies have been conducted on radiation-induced sulfonated polymers (post-irradiation method) despite the advantage that various types of polymers, prepared with various sulfonated and nonsulfonated monomers, can be used as a matrix for radiation modification. In this study, we investigated a post-irradiation method for PEMs and tried to improve PEM performance by preparing a radiation-grafted sulfonated polymer. Sulfonated polyarylene ether sulfone (SPAES) was used as a polymer matrix for irradiation. Here, the sulfonated polymer with a perfluorinated backbone (e.g. Nafion) was excluded due to its serious decomposition resulting from the chain scission effect caused by irradiation. Due to its additional acid group, acrylic acid was used as a grafting agent to increase the proton conduction properties of the resulting polymer.