Environmentally adaptable pathway to emulsion polymerization for monodisperse polymer nanoparticle synthesis

An environmentally adaptable method is proposed for emulsion polymerization to produce monodisperse polymer nanoparticles at surfactant concentrations much lower than conventional emulsion polymerization. The present method is based on an idea of employing a surfactant with a low critical micelle concentration (CMC) to generate a large number of polymer particles in the presence of micelles at low surfactant concentrations and to cease the particle generation by the micelle disappearance in early reaction stage for attainment of high monodispersity of particles. A hydrophobic monomer of styrene (St) and a less hydrophobic monomer of methyl methacrylate (MMA) were used in the present emulsion polymerization at a total monomer concentration of 0.3 M with an initiator of ammonium persulfate (APS) and a long alkyl chain surfactant of sodium octadecyl sulfate (SOS). The polymerization was conducted at a high APS concentration of 20 mM that gave the CMC as low as 0.085 mM for SOS. An increase in the monomer mole fraction of MMA increased the number of polymer particles, while the high monomer mole fraction of MMA failed to stabilize the dispersion of particles. The polymerization for a monomer ratio of St/MMA = 1/1 could produce monodisperse polymer particles of 33 nm at a surfactant concentration of 1 mM that was higher than its CMC (0.085 mM) but much lower than several ten mM of surfactant concentrations in conventional emulsion polymerization.