Sulfonation and Characterization of Poly(styrene-ethylene/butylene-styrene) Nanocomposite Membranes for Fuel Cell Applications

Poly(styrene-ethylene/butylene-styrene) (SEBS), with 30% weight fraction of polystyrene, was functionalized with sulfonic groups in order to increase its transport properties and create suitable proton exchange membranes for fuel cell (PEMFC) applications. Different sulfonation levels were obtained by varying the ratio of the sulfonating agent to polymer. The resulting sulfonated polymer was solvent casted and its properties were evaluated as a function of sulfonation level by different characterization techniques. In order to quantify and confirm the presence and coordination of the sulfonic groups, elemental analysis and fourier transform infrared spectroscopy measurements were performed. The thermal stability of the resulting membranes was determined by thermogravimetric analysis. The results show an increase in thermal stability upon sulfonation, suggesting that the additional ionic interconnection provides thermal stability to the polymer backbone.  In addition, water and methanol swelling of the membranes were analyzed as a function of time in order to determine their absorption limits.  Finally, transport properties of the membranes were studied by liquid phase methanol permeability experiments, suggesting that sulfonated SEBS has similar performance when compared to the state-of-the-art Nafion® and that further physical and chemical modifications with titanium dioxide (TiO₂) nanoparticles could lead to an even better performance.