Formulation and stability mechanisms of polymerizable miniemulsions

Polystyrene latexes in a size range of 30-180 nm can be synthesized by poly merization 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 measure ments were used to characterize the miniemulsification by ultrasonication a nd the approach to a steady state of droplet size. Since size equilibrium s eems to be established by a rate equation of collision-induced droplet fusi on and ultrasound fission, miniemulsions realize the minimal droplet size f or the distinct amount of surfactant; i.e., they are "critically stabilized " with respect to collisions (tau(2) process). Complete stability against O stwald ripening (tau(1) process) is obtained by the addition of a hydrophob e, which was varied over a broad structural range. The efficiency of this h ydrophobic 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 t o particles is obtained, and the critically stabilized state is frozen. The critical surface coverage of these particles with SDS molecules was determ ined and depends strongly on the particle size: the smaller the particle si ze, the higher the required surface coverage with surfactant. The good acce ssibility of these systems allows to establish a general relation between t he 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 thos e of the corresponding processes in macro- and microemulsions.