Indirect sulfonation of telechelic poly(styrene-ethylene-butylene-styrene) via chloromethylation for preparation of sulfonated membranes as proton exchange membranes

Abstract

. The indirect sulfonation, via chloromethylation, of poly(styrene-(ethylene-butylene)-styrene) (polySEBS), under mild conditions, is here reported as an alternative route for the conventional use of chlorosulfonic acid. This indirect sulfonation reaction is an effective route to insert sulfonic groups in the aromatic rings of SEBS to impart a proton exchange capability. The chloromethylated polySEBS was chemically modified by the isothiouronium grouPp, afterward hydrolyzed and oxidized to generate sulfonic acid groups selectively into the aromatic portion (polystyrene) of the polySEBS, to a great extent. The chloromethylated and sulfonated polymeric membranes were characterized by NMR, FT-IR, water uptake, TGA, ion exchange capacity (IEC), and ion conductivity. The obtained results show that as the oxidation time increased in performic acid, the water uptake achieved up to 79.6% due to the conversion of isothiouronium to the sulfonic acid groups into the polymer structure. Furthermore, the sample after 7 hours of oxidation reaction (sSEBS-7H) showed 59% of sulfonation, determined by RMN, and had an IEC value of 1.46 meq/g and also an ion conductivity value of 18.7 mS/cm at RT , which are 46 and 75% higher than those of Nafion 115, a commercial polymer conventionally used for proton exchange membranes (PEM). Thus, the as-prepared sSEBS-7H membrane, via chloromethylation, can be used for PEM since it exhibits good ionic conductivity and structural stability.