Degradation reactions during sulphonation of poly(styrene-co-acrylic acid) used as membranes

In this study, a random copolymer of poly(styrene-co-acrylic acid) (PS-AA) was synthesized in solution by radical polymerization and partially crosslinked with divinyl benzene to improve mechanical resistance. The copolymer (PS-AA) was sulphonated with different theoretical molar quantities (20-60%) of sulphuric acid (H2SO4) and acetyl sulphate (CH3COOSO3H). The sulphonated PS-AA materials were characterized with infrared spectroscopy (FTIR), insoluble material percentage by soxhlet extraction, molar mass by gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). FTIR indicated the presence of sulphonic groups bound into the copolymer matrix and some changes in specific bands. Gel percentage was considerably increased when H2SO4 was used as the sulphonation agent while a drop in molecular weight was detected by GPC with acetyl sulphate as the agent. The former effect is due to linkages through the sulphone groups and the latter as a consequence of a polymer matrix destructive side reaction. DSC thermograms show that sulphonation with H2SO4 decreases the Tg value, while CH3COOSO3H increases the transition, in comparison with the neat PS-AA. The latter could be due to the presence of ionic interactions in the copolymer. Side reactions during sulphonation of polymers are very important for the final physical properties. (C) 2014 Elsevier Ltd. All rights reserved.