Formulation and stability mechanisms of polymerizable miniemulsions

Polystyrene latexes in a size range of 30-180 nm can be synthesized by polymerization in miniemulsions where the particle size responds to the amount of the surfactant sodium dodecyl sulfate (SDS) which was varied between 0.5 and 50 wt % with respect to monomer. Turbidity and surface tension measurements were used to characterize the miniemulsification by ultrasonication and the approach to a steady state of droplet size. Since size equilibrium seems to be established by a rate equation of collision-induced droplet fusion and ultrasound fission, miniemulsions realize the minimal droplet size for the distinct amount of surfactant; i.e., they are "critically stabilized" with respect to collisions (tau(2) process). Complete stability against Ostwald ripening (tau(1) process) is obtained by the addition of a hydrophobe, which was varied over a broad structural range. The efficiency of this hydrophobic agent is given mainly by a very low water solubility. The growth of the critically stabilized miniemulsion droplets is usually slower than the polymerization time; therefore in ideal cases, a 1:1 copy of droplets to particles is obtained, and the critically stabilized state is frozen. The critical surface coverage of these particles with SDS molecules was determined and depends strongly on the particle size: the smaller the particle size, the higher the required surface coverage with surfactant. The good accessibility of these systems allows to establish a general relation between the macroscopic surface tension of the latex and the microscopic coverage of the particles which is expected to hold true for all latexes. Finally, the characteristics of polymerization in miniemulsions were compared with those of the corresponding processes in macro- and microemulsions.