Investigation of emulsion gel polymerization of polystyrene using Saponin-based biosurfactants

Emulsion gel polymerization offers an efficient approach to enhance polymerization processes by allowing lower processing temperatures, resulting in both environmental and industrial advantages. This research employed biosurfactants, particularly crude soapnut extract (SE), which contains saponin-based surfactants as a replacement for traditional synthetic surfactants such as cetyltrimethylammonium bromide (CTAB), used in the emulsion gel polymerization of styrene. Polymerization involved styrene as the monomer, azobisisobutyronitrile (AIBN) as the initiator, fumed silica (FS) as the gelling agent, and SE as the surfactant. Saponin-based biosurfactants were obtained through maceration extraction of soapnut fruit ( Sapindus sp. ), and polymerizations were performed at room temperature for 24 h. The polymerizations were carried out in two stages: first, a crude SE solution was used to evaluate the feasibility of polymerization using saponins in emulsion gels; subsequently, freeze-dried SE (FD-SE) powder was utilized to optimize the surfactant concentration. In both stages, monomer-to-polymer conversions exceeded 80%, with optimal surfactant amounts identified as 1.5 mL crude SE and 0.4 g FD-SE in 0.5 mL of water for 2.0 mL of styrene. Scanning electron microscopy (SEM) and ImageJ analysis showed that PS prepared with SE formed polydisperse, agglomerated particles (6–26 µm), likely due to the complex composition of crude SE affecting droplet stabilization. Differential scanning calorimetry (DSC) analysis showed a glass transition temperature (T _g ) of 96°C, which was lower than that of polystyrene synthesized using CTAB-based surfactants. Gel permeation chromatographic (GPC) analysis confirmed that all samples possessed sufficiently high molar masses for T _g to be independent of molecular mass, suggesting that the reduction likely arises from residual impurities or structural variations affecting chain packing. Despite this, the study successfully demonstrated the potential for synthesizing polystyrene at ambient temperature using saponin-based surfactants extracted from soapnut, highlighting the promise of exploring other natural surfactants for emulsion gel polymerization.