Miniemulsion polymerization of styrene stabilized by nonionic surfactant and reactive cosurfactant

A nonionic surfactant, nonylphenol polyethoxylate with an average of 40 ethylene oxides per molecule (NP-40), in combination with a reactive cosurfactant such as dodecyl methacrylate (DMA) or stearyl methacrylate (SMA) was used to prepare stable styrene miniemulsions in this study. Like conventional cosurfactants such as cetyl alcohol (CA) and hexadecane (HD), the reagent DMA or SMA may act as a cosurfactant in stabilizing the homogenized miniemulsions. Furthermore, the methacrylate group can be chemically incorporated into latex particles in subsequent miniemulsion polymerization, as shown by the IR spectra. The rate of Ostwald ripening for these miniemulsions in the decreasing order is: CA > DMA > KD similar to SMA. This trend correlates well with the water solubility of these cosurfactants. The lower the levels of NP-40 and DMA, the greater is the degree of Ostwald ripening. This is because the DMA concentration is not high enough to counteract diffusional degradation of monomer droplets. Polystyrene latex particles were produced via both monomer droplet nucleation and homogeneous (or micellar nucleation for the system exhibiting strong Ostwald ripening and/or droplet coalescence during the very early stage of polymerization. On the other hand, monomer droplet nucleation becomes more important for the system showing weak Ostwald ripening. Temperature has an important influence on the shelf-life of miniemulsions and subsequent polymerization. This is attributed to the fact that hydrogen bonds between the polyethylene oxide part of NP-40 and water are quite sensitive to changes in temperature.