## 3 Results and Discussion  
The introduction of sulfonic groups (-SO\({}_{3}\)H) into the polymer chain can significantly enhance important properties, such as hydrophilicity, permeability, morphology, and antifouling capacity [28; 29]. In this study, the primary goal in modifying R-HIPS membranes was to make them less susceptible to fouling by changing them from a hydrophobic to hydrophilic nature through varying concentrations of -SO\({}_{3}\)H. The experimental sulfonation degrees (SDs) of R-HIPS membranes ranged from 0% to 5.9%, closely aligning with the theoretical SD, as can be seen in Table 1. Initial experimental tests for membrane fabrication were conducted using a higher SD (10, 15, and 20%) of recycled HIPS. However, at these sulfonation levels, HIPS becomes partially soluble in water and alcohols, which complicates both the membrane formation process and their application in water treatment.  
The reaction mechanism for HIPS sulfonation with chlorosulfonic acid is a typical electrophilic aromatic substitution, which introduces a sulfonic acid group on the benzene rings of the polystyrene fragment, as depicted in Scheme 1 [25].  
FTIR spectroscopy was employed to verify the chemical structures of R-HIPS and sulfonated R-HIPS-3 and R-HIPS-5. Figure 1 shows that the aromatic C-H stretching vibrations appeared at 3082, 3058, and 3024 cm\({}^{-1}\). Additionally, the peaks centered at 2921 cm\({}^{-1}\) and 2850 cm\({}^{-1}\) corresponded to the symmetric and asymmetric stretching of C-H bonds of vinyl groups, respectively [14]. The spectra also displayed vibrations at 1601 cm\({}^{-1}\) and 1492 cm\({}^{-1}\), which were attributed to aromatic C=C stretching. Furthermore, all spectra exhibited characteristic polybutadiene signals between 994 and 720 cm\({}^{-1}\), attributed to the deformation bands of vinyl-1,2, _trans_-1,4, and cis-1,4 configurations [30]. The peaks around 1184 and 1042 cm\({}^{-1}\), assigned to the antisymmetric and symmetric vibrations of the SO\({}_{3}\)\({}^{-}\) group [31], may overlap with absorption peaks from additives in R-HIPS, complicating the identification of those signals. Moreover, a broad peak between 3100 and 3650 cm\({}^{-1}\) was observed, corresponding to O-H stretching vibrations from hydrogen bonds. This peak was intensified with an increasing sulfonation degree, indicating that higher sulfonation enhances polymer hydrophilicity.  
Changes in the hydrophobic character of R-HIPS due to sulfonation could modify the thermodynamic equilibrium for the R-HIPS solution behavior between the polymer/solvent/non-solvent system during the membrane formation process [25]. As reported in the literature, R-HIPS membranes with macrovoids in their structure and larger cavities in the sublayer are obtained using deionized water as non-solvent in asymmetric membrane preparation, due to an instantaneous demixing [14; 15; 19; 20]. As a result, the formation of macrovoids and larger cavities can indeed compromise the mechanical stability of R-HIPS membranes. Thus, it is important to consider alternative non-solvents or look into adjusting the casting  
Figure 1: ATR-FTIR spectra of R-HIPS and sulfonated R-HIPS.  
conditions to obtain delayed demixing processes to avoid the formation of macrovoids and large cavities. In this case, after several tests, commercial vinegar/isopropanol (15/85% \(v/v\)) containing 0.12% NaHCO\({}_{3}\) was selected as the coagulation bath to reduce macroporous formation in the membrane's cross-section and promote the formation of the asymmetric membrane with a sponge-like porous structure. This formulation closely follows those reported by [PERSON] et al. for asymmetric membranes preparation from sulfonated polymers [24]. Additionally, this coagulation bath was chosen because it was not favorable to use water as a non-solvent for sulfonated membranes formation.