Emulsion Polymerization of Styrene to Polystyrene Nanoparticles with Self-Emulsifying Nanodroplets as Nucleus

The mechanism of the emulsion polymerization of styrene to polystyrene nanoparticles (PSNPs) remains a subject of debate. Herein, a series of reaction parameters with different surfactant concentrations, monomer contents, temperatures, and equilibration times were investigated to understand the formation mechanism of PSNPs, which demonstrate a correlation between the properties of PSNPs and the mesostructure of the premix. Cooling the model systems with self-emulsifying nanodroplets (SENDs) in the early reaction stages resulted in the hollow polystyrene spheres (H-PSSs), ruptured PSNPs, and dandelion-like PSNPs, further indicating that the oil nanodroplets are the key sites for the formation of PSNPs. Therefore, the oligomer radicals generated in the medium at the early polymerization stages are preferentially captured by the oil–H₂O interface, and once in the oil droplets, the oligomeric radical continues to grow until colliding with another radical or monomers run out. The self-emulsifying oil nanodroplets with a particle size of 100–300 nm are formed under high temperature and low surfactant concentration conditions and serve as nucleation sites. This study not only contributes to a profound understanding of the correlation between the mesostructure and nucleation pathways but also paves the way for the flexible regulation of the monodispersity and morphology of PSNPs.