Degree of Sulfonation (DS) - Determination by Different Analytical Techniques in Ionomers Derived from Polystyrene

Polymers with ion charges anchored to their structure, known as ionomers, are slightly modified materials with notable applications for electrochemical processes. However, the characterization and quantification of those sulfonic groups grafted to the polymer is uncommon, which can lead to inaccurate data regarding their performance. A styrene-co-butyl acrylate copolymer (80:20) was synthesized and further sulfonated with concentrated sulfuric acid. Analytical techniques such as proton nuclear magnetic resonance (¹H NMR) and thermogravimetric analysis (TGA) were employed to characterize ionomers and used to calculate the degree of sulfonation (DS). Membranes were prepared by casting from the sulfonated copolymer, using two different solvent systems; then, the ion exchange capacity (IEC) was evaluated to be able to calculate DS values. For the ionomer, a DS of 35.2% was obtained by ¹H NMR and 38.4% by TGA, while from membranes IEC results the DS ranged from 4.9% to 28%, depending on the solvent system used for membrane preparation. The DS evaluation in the ionomer is similar for both analytical techniques used, since measuring the presence of sulfonic groups; however, the most popular IEC technique to measure DS value must be taken with care, since the method is more related to the structural condition, where the ionic groups are available to exchange, highly affected by the membrane preparation method.