Poly(styrene-co-acrylonitrile) based proton conductive membranes

Sulfonated poly(styrene-co-acrylonitrile) (PSAN–SO₃H) membranes were obtained by sulfonation of the original styrene–acrylonitrile copolymer, in different molar ratios, and characterized by vibrational spectroscopy (FTIR), thermal analyses (TGA and DSC) and electrochemical impedance spectroscopy (EIS). The thermal stability of the sulfonated polymers exhibited a dependence on the sulfonation degree and reached 261 °C for samples up to 1:4 (sulfonating agent to styrene unit). FTIR spectra showed the covalent incorporation of sulfonic groups at the styrene units, confirming the PSAN–SO₃H formation. Vibrational spectra also indicated the presence of hydronium ions and dissociated sulfonic groups, indicating the existence of mobile protons for ion conduction. DSC analyses evidenced two glass transition temperatures (T_g), one associated with an ion-water domain and other with the chain backbone glass transition. The maximum conductivity of PSAN–SO₃H membranes at ambient temperature was about 10⁻³ Ω⁻¹ cm⁻¹, achieving 10⁻² Ω⁻¹ cm⁻¹ at 80 °C. The conductivity dependency on the temperature was found to be linear, similarly to other sulfonic acid polymers described on the literature, and the water uptake reaches 45.7% of the polymer mass, against 18.9% of the original copolymer.