Nanoparticles of polystyrene latexes by semicontinuous microemulsion polymerization using mixed surfactants

Nanosized polystyrene (PS) latexes stabilized by the mixture of cationic/cationic, anionic/anionic, or anionic/cationic surfactants of various types with high weight ratios of PS to surfactant (ca. 10:1) have been successfully synthesized by a semicontinuous microemulsion polymerization process. For cationic or anionic systems, spherical latex particles with a weight-averaged diameter (D _w) ranging from about 22 to 53 nm were nearly linearly dependent on the weight ratio of the mixed surfactants with similar charges. Their particle size distributions were rather uniform (D _w/D _n < 1.20). For a system with oppositely charged surfactants at nonequimolar ratios, it could produce stable PS particles up to 94 nm in diameter. High molar masses (M _w) of PS ranging from 1.1 to 1.9 × 10 ⁶ g/mol could easily be obtained for all three systems investigated. For both cationic/cationic and anionic/anionic surfactant systems, the number of PS particles per milliliter of latex (N _p) generated in the very early stage of O/W microemulsion remained rather constant throughout the polymerization. This was controlled by using only 1 wt% of mixed surfactants and the continuous addition of a small amount of styrene. The present polymerization method allows one to synthesize nanoparticles of PS or other polymers of high polymer/surfactant weight ratios at some particle sizes that are unable to achieve them with a single type of surfactant.