Mechanical properties and morphology of polystyrene-co-acrylic acid synthesized as membranes for fuel cells

Two copolymers of poly(styrene-co-acrylic acid) (PS-AA) were synthesized in solution by radical polymerization and partially crosslinked by adding, either trimethylol propane trimethacrylate (TMPTMA) or divinylbenzene (DVB) to improve mechanical resistance. Copolymers were sulfonated with theoretical molar quantities of sulfuric acid (H2SO4 = 170%) and two different amounts of silver sulfate (Ag2SO4 = 0.11 or 0.055%). Materials were dissolved in three different solvent compositions: copolymer + THF, copolymer + THF+55% DMSO and copolymer + THF+110% DMSO and used to prepare membranes prepared by casting. Membranes were characterized by Thermomechanical Analysis (TMA), Scanning Electron Microscopy (SEM), Ionic Exchange Capacity (IEC) and Water Uptake (WU) capacity. Addition of DMSO to THF during casting procedure has an important increment on IEC and WU results; however, mechanical resistance is considerably reduced. SEM images show almost no pores in the membranes casted from THF alone, while increasing the amount of DMSO enhances porosity. Such phenomenon is responsible for reduced mechanical property (brittleness), as seen by TMA.