Enhanced Nucleation of Monomer Droplets in Miniemulsion Polymerization

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Abstract - El-Aasser - 9628783 The potential advantages of applying miniemulsion technology to the preparation of latexes through free radical polymerization of vinyl monomers have not been realized, one reason being that only a fraction (circa 20%) of the initial droplets (50 - 500 nm) are nucleated to become polymer particles. This limits the control of the particle size and copolymer composition by requiring diffusion of monomers from the droplets which are not nucleated to the growing polymer particles. This is a particular disadvantage for monomers with low water solubility which do not readily diffuse through the aqueous phase (e.g., macromonomers). Recent developments in the miniemulsion polymerization of styrene have shown that the addition of a small amount of polymer to the miniemulsion droplets enhances their nucleation which can reach 100% under certain conditions, thereby increasing the polymerization rate and resulting particle number. This phenomenon has been attributed to an increased rate of radical entry into these droplets which improves their ability to compete with the existing polymer particles for aqueous phase free radicals. The mechanistic explanation for this phenomenon and the extension of the method to other monomer systems are the focus of this research. A combination of techniques will be applied in this investigation including continuous measurements of the reaction rate via reaction calorimetry and monitoring the evolution of the number of particles through the particle size distribution. The nature of the added polymer will be varied in terms of its molecular weight and the functionality of its end-groups in order to determine whether it is an interfacial phenomenon or a ObulkO phenomenon. Likewise, the composition of the droplet/water interface will be varied and characterized. In addition to the normal water-soluble initiator (i.e., persulfate), oil-soluble initiators will be used to determine the effect of the origin of the free radicals (aqueous vs. droplet phase) on particle nucleation. Other monomer/comonomer systems will also be investigated. And finally, mathematical modeling will be used in conjunction with the above findings to aid in mechanistic understanding of the miniemulsion polymerization process as a function of the variables chosen in these studies.