Solvent Polarity Effect on Physicochemical and Morphological Properties for Casted Sulfonated (Styrene‐co‐butyl Acrylate) Ionomer Membranes

Abstract The most critical component in electrochemical cells is the ion exchange membrane (IEM), it must combine high ionic conductivity, mechanical stability, easy fabrication, and low cost. Alternative polymer membranes, such as styrene copolymers, meet the expected performance and are often prepared by casting methods for research. Surprisingly the solvent effect is rarely evaluated, though it can influence final properties. A styrene‐co‐butyl acrylate copolymer was synthesized and sulfonated with sulfuric acid. FTIR, 1 H, and 13 C NMR tracked copolymer formation and successful sulfonation. The membrane performance was characterized through contact angle, water uptake, ion exchange capacity (IEC), and ionic conductivity. Morphology was examined using scanning electron microscopy (SEM). The degree of sulfonation (DS) was calculated using results from 1 H NMR, thermogravimetric analysis (TGA), and IEC evaluations. FTIR and NMR indicated effective copolymer formation and sulfonation, with a minor degradation during the sulfonation procedure. Membranes from polar solvents exhibited superior performance in contact angle, water uptake, ion exchange capacity (IEC), and ionic conductivity. The DS values were 30.3% by 1 H NMR and 37.9% by TGA, but ranged from 4.9% to 28.45% when calculated from IEC, depending on ionomer/solvent affinity. Solvents or their mixtures also induced pore formation in the membranes, as observed by SEM.