Surfactant-Free Emulsion Polymerization of Styrene in Ethanol–Water Mixtures: The Role of Mesostructures in the Formation of Polystyrene

Although the synthesis of monodisperse polystyrene nanoparticles (MPSPs) in surfactant-free systems has been widely investigated, the role of mesostructures in forming MPSPs is still unclear. Herein, the styrene/ethanol-water (St/EtOH-H₂O) ternary system with certain mesostructures was employed as a model to investigate the correlation between the mesostructure of systems and the properties of polystyrene products. The mesostructures of the ternary system, including styrene droplets, a sponge-like structure, and water droplets, were investigated by transmission electron microscope (TEM) with negative staining, dynamic light scattering (DLS), and nanoparticle tracking analysis (NTA). Classical molecular dynamics (MD) simulations showed the spontaneous aggregation of ethanol at the styrene-water interface, which may be one of the factors stabilizing the mesostructures. We demonstrated that the formation of MPSPs can only occur in the systems containing styrene droplets. Cooling the reactants of styrene droplet systems in the early stages of polymerization resulted in incomplete polystyrene nanospheres (PSNSs), indicating that the formation of MPSPs originated from the interface of styrene droplets. Subsequently, the MPSPs were gradually formed through self-templating polymerization. This study offers valuable insights into the preparation and understanding of the formation process of polymer nanoparticles in other similar surfactant-free systems.