Properties of Polystyrene/Acrylic Acid Membranes After Sulphonation Reactions

Abstract

Sulphonation reaction in styrenic membranes is a common method used to graft ionic groups needed to enhance proton conductivity in polymeric electrolytes (PEM). A synthesized copolymer of styrene and acrylic acid (94/6 % mol) was partially crosslinked using a trifunctional monomer, trimethylolpropane trimethacrylate (TMPTMA) (0.01% mol), like a crosslinking agent. The obtained material was subjected to different sulphonation reactions in order to prepare several PEM membranes. The sulphonation reactions used sulphuric acid (H2SO4, 98%) at 50, 75 and 100 %molar ratio considering amount of styrene rings, during 1 and 2 hours of reaction time. Fouriertransformed infrared spectroscopy (FTIR) spectra were obtained to identify specific chemical groups in these materials. Thermogravimetric analysis (TGA) and Dynamic Mechanical Analysis (DMA) were used to evaluate the thermal stability and mechanical effects after sulphonation reactions. Water absorption and its effect on proton conductivity by means of Electrochemical Impedance Spectroscopy (EIS) were also evaluated. FTIR spectra show characteristic bands corresponding to sulphone groups within macromolecular structure. Sulphonating agent concentration and/or sulphonation time induce higher glass transition temperatures and no changes in thermal stability when comparing with the no sulphonated material. Latter condition also allows higher water absorption and an enhancement in proton conductivity.