Enhanced droplet nucleation in styrene miniemulsion polymerization. 3. Effect of shear in miniemulsions that use cetyl alcohol as the cosurfactant

The effect on the polymerization kinetics of the level of shear imparted during homogenization of styrene miniemulsions prepared with cetyl alcohol was studied. It was noted that, at relatively low levels of shear, predissolving polystyrene into the monomer prior to homogenization did not result in an enhancement in the kinetics. At high levels of shear, there was a large enhancement in the rate of polymerization that results from predissolving polymer. These conflicting results were correlated to droplet stability measurements. Droplet sizes formed in the size region produced by the low levels of shear have been shown to be stable with time while the droplet sizes formed by the high level of shear have been shown to be unstable with time. The combination of the kinetic results and the droplet stability measurements was taken as evidence that the dominant mechanism causing "enhanced droplet nucleation" is the preservation of the droplet number prior to the polymerization. Numerous results are also presented that show that the rate of miniemulsion polymerization is directly proportional to the droplet number available at the time of polymerization. With a better understanding of the nucleation mechanisms in miniemulsion polymerization, future formulations can be designed to ensure all nucleation in these systems occurs in the monomer droplets.