Production of nanoparticles: polymerization termination and molecular weights during radical-induced emulsifier-free emulsion polymerization (heterogeneous polymerization)

Polymethylmethacrylate (PMMA) nanoparticles were produced by radical-induced emulsifier-free emulsion poly-merization (heterogeneous polymerization) in aqueous solution using γ-rays, ammonium persulphate (APS) and potassium persulphate as initiators. The conditions of polymerization were varied with regard to the concentration of monomer and initiator, the temperature of reaction, and the properties of the reaction mixture. The molecular weights of the polymers were determined by gel permeation chromatography (GPC) and by viscosimetry. Increasing amounts of monomer increased the molecular weight up to a concentration of 3% MMA. Monomer concentrations higher than 3% led to a decrease in the molecular weight most likely due to the high viscosity and diffusion distances in the resulting polymer particles. The concentration of initiator (APS) up to a concentration of 0.3% decreased the molecular weight without significant change in the molecular weight distribution of the resulting polymers. Magnesium sulphate, potassium chloride as well as higher viscosities of the reaction media led to an increase in the molecular weight. In contrast, an increase in reaction temperature and addition of potassium nitrate decreased the molecular weights. The molecular weights determined by GPC were confirmed by a comparison with molecular weights resulting from viscosimetry.