A simple method for producing micron-sized, highly monodisperse polystyrene particles in aqueous media: Effects of impeller speed on particle size distribution

The single stage polymerization recently proposed for producing micrometer-sized polymer particles in aqueous media was carried out with styrene monomer, potassium persulfate initiator and dimethyldodecylbetaine amphoteric surfactant in ranges of impeller speed (250-500 rpm) and addition time of the surfactant to the system (5-30 min) at a monomer concentration of 1.1 mol/dm(3) H2O and initiator concentrations of 4 and 8 mmol/dm(3) H2O, Particle size distribution and monomer-to-polymer conversion were measured during the polymerizations. It was shown that the addition of small amount of the surfactant (0.5 mmol/dm(3) H2O) was effective to promote the coagulation of particles, which led to the production of monodisperse, micrometer-sized polymer particles. The particle size distribution significantly depended on impeller speed. At the lowest impeller speed of 250 rpm, secondary particles were generated during the reaction and the size distribution of final particles was bimodal. The generation of secondary particles is explained from an increase in electric surface potential of polymer particles during the reaction. At impeller speeds above 250 rpm, particle size distributions were unimodal throughout the reaction and the average diameter of final particles increased with a decrease in impeller speed. Particles produced at 300 rpm had an average diameter of 2 mum and the highest monodispersity with a coefficient of variation of size distribution of 3.4% that is much smaller than typical monodispersity criterion of 10%.