Chemical Stability of HIPS During Sulfonation Reactions: Impact of Sulfonation Agent on Side Reactions

Abstract

Sulfonated aromatic polymers (SAPs) are functionalized materials with ion-transport capabilities, suitable for various electrolytic applications. A common route to their synthesis is direct sulfonation reactions. However, the selection of the sulfonating agent influences the occurrence of side reactions and polymer stability. In this study, commercial HIPS was chemically modified via direct sulfonation using two different sulfonating agents: concentrated sulfuric acid (S-HIPS) and acetyl sulfate (A-HIPS). The resulting materials were characterized by Fourier-Transform infrared spectroscopy (FTIR), nuclear magnetic resonance (¹H and ¹³C NMR), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA). FTIR spectra confirmed sulfonation through the appearance of characteristic bands of the SO₃H group, but rubber double bond signals disappeared in S-HIPS and A-HIPS. The degree of sulfonation (DS) value varies when calculated from TGA or proton NMR data, suggesting the presence of side reactions during both sulfonation processes. Sulfone-type and rubber crosslinking processes were detected as side reactions, confirmed by RMN, TGA, and DMA. Acetyl sulfate was a more effective sulfonating agent for HIPS, yielding higher DS, fewer side reactions, and better stability.