Particle Nucleation during Microemulsion Polymerization of Methyl Methacrylate

The evolution of the reaction conversion and particle size distribution during the microemulsion polymerization of methyl methacrylate (MMA) is used to determine the particle nucleation mechanisms. A pseudo-3-component oil-in-water microemulsion is formed with water, MMA, and a mixture of dodecyltrimethylammonium bromide (DTAB) and diodecyldimethylammonium bromide (DDAB) in a 3:1 weight ratio as surfactant. Polymerization is initiated with either an oil-soluble or a water-soluble initiator and conversion followed either by measurement of the unpolymerized monomer concentration in samples taken during the reaction or by direct on-line densimetry. A two-stage process is observed. The first stage, described by a very slow increase in conversion, is attributed mainly to homogeneous nucleation, and the second stage, characterized by a much higher rate of conversion, involves continuous nucleation and is governed mainly by a micellar-entry mechanism.