Effect of partitioning of monomer and emulsifier in aqueous media on particle formation in emulsion homopolymerization of hydrophobic and hydrophilic monomers with a nonionic emulsifier

Abstract

Styrene (hydrophobic monomer), methyl methacrylate (MMA, hydrophilic monomer, almost 100 times more soluble in water than styrene) and nonionic emulsifier are often used in general emulsion polymerizations in industrial production. To clarify the effect of stirring for 100 min at 70 °C before starting emulsion polymerization with a nonionic emulsifier (polyoxyethylene nonyl phenyl ether, E911) and potassium persulfate as the initiator for the particle formation, the emulsion homopolymerization of styrene and MMA was carried out at different stirring rates (0–500 r.p.m.) at 70 °C. Immediately before the start of the polymerization, in the case of styrene, partitioning of components in the system nearly reached an equilibrium state at 500 r.p.m., whereas the system was in a nonequilibrium state at 240 r.p.m. In the case of MMA, the system nearly reached an equilibrium state even at 240 r.p.m. Polystyrene particles were obtained as ∼100-nm-sized spherical particles at 240 r.p.m. and as ∼1-μm-sized ‘bur-like’ nonspherical particles at 500 r.p.m., whereas poly(MMA) particles were ∼100-nm-sized and almost spherical at both stirring rates. Monomer polarity strongly affected the partitioning of the monomer to the aqueous phase and the nonionic emulsifier to the monomer phase before starting the emulsion polymerization, resulting in the differences in particle formation and the incorporation of nonionic emulsifier inside the polymer particles. TEM photographs of PS (a, b) and PMMA (a′, b′) particles prepared by emulsion polymerization at 240 r.p.m. (a, a′) and 500 (b, b′) r.p.m.