Particle nucleation loci in styrene miniemulsion polymerization using alkyl methacrylates as the reactive cosurfactant

A water-insoluble blue dye was used to study the particle nucleation mechanisms involved in the styrene miniemulsion polymerizations stabilized by sodium dodecyl sulfate (SDS) along with a reactive cosurfactant (e.g., dodecyl methacrylate (DMA) or stearyl methacrylate (SMA)). A mass balance was established to determine the number of latex particles originating from the monomer droplets (N-d) and that of primary particles generated in the aqueous phase (N-w). The accuracy of this method relies on producing a stable miniemulsion during the reaction. About 55% of the monomer droplets initially present in the reaction mixture (N-m,N-i) can be successfully converted into latex particles for the system stabilized by SDS/DMA. The value of N-d is much smaller than N-w. Homogeneous nucleation plays an important role in the reaction kinetics, but it becomes less significant when the DMA concentration is increased. On the other hand, the value of N-m,N-i is much larger when the more hydrophobic SMA is chosen as the cosurfactant. As a consequence, nucleation of primary particles in the aqueous phase is greatly reduced. Nevertheless, only 49% of the original monomer droplets can be converted into latex particles during polymerization. The value of N-w is comparable to that of N-d. Thus, the relatively large population of primary particles generated via homogeneous nucleation cannot be neglected for the SMA stabilized system. In addition, increasing the initiator concentration may enhance the degree of homogeneous nucleation during the early stage of polymerization.