Preparation of micronsized polystyrene beads by seeded emulsifier-free, emulsion polymerization without swelling process

Micron sized monodisperse crosslinked polystyrene (PS) beads were prepared by a multi-stage emulsion polymerization using styrene monomer, divinylbenzene (DVB) crosslinking agent and potassium persulfate initiator in the absence oi emulsifier. The beads crosslinked with 10 mol% DVB at 80 degrees C were uniformly formed in the early stage of reaction and gradually grown with the reaction time. The lower reaction temperature, the larger the beads sizes were obtained. Particle nucleation of the pre-existing polymer seeds using a multi-stage emulsion polymerization at 60 degrees C was required to bring monodisperse crosslinked PS beads of sizes greater than 2 mu m. As the particle size grows, the number of free radicals around the growing particles increases and the conversion rate of the next stage was accelerated. It is possible that the continuous polymerization at inner part of the bead surface leads to spherical particles. On the other hand, phase separation during the growth of the seed particles or the capture of the free radical at the beads surface may result in the formation of protrusion on the beads surface.