Miniemulsion polymerization is used to describe heterogeneous polymerization in those systems in which monomer droplets are substantially smaller than found in a typical conventional emulsion polymerization. These polymerizations exhibit some characteristics of suspension and emulsion polymerization systems. In each type of polymerization, monomer droplets are dispersed in a continuous aqueous phase in which the monomer is insoluble or only slightly soluble. Surfactants and other colloidal stabilizers are used to stabilize the droplets and prevent particle agglomeration. While both suspension and emulsion polymerizations are free radical reactions, each proceeds by a unique mechanism. Initiators for free radical suspension reactions are oil-soluble. The monomer droplets are approximately 1 mm in diameter and each behaves as a small batch reactor, with the monomer reacting to produce polymer. Since the number of free radicals per particle is quite high, the kinetics are those of a bulk free radical polymerization. In conventional emulsion polymerization moderate shear in the presence of surfactant leads to the formation of a stable emulsion containing 10 micron monomer droplets and surfactant micelles about 0.01 micron in diameter. These micelles are formed from excess surfactant in the continuous phase and contains a small amount of monomer. Because of their relative size and number, the total surface area of micelles is much greater than the total surface area of the droplets. Thus free radicals in the aqueous phase enter micelles much more frequently than droplets, resulting in particle nucleation primarily from micelles. The primary locus of polymerization is the polymer particles thus formed. The particles eventually grow to a size of about 0.10 micron forming a final product, referred to as a latex which is a stable dispersion of polymer particles in the continuous aqueous phase. In miniemulsion polymerization, high shear is used in combination with surfactants and cosurfactants to produce a fine emulsion of monomer droplets. The monomer droplets are substantially smaller than those found in a typical emulsion polymerization. Reduction of the monomer droplet diameter causes a corresponding increase in the total droplet surface area. This has the effect of making the monomer droplets competitive with the micelles for radical capture since radical absorption is competitive on the basis of relative surface area. The objective of this research is to investigate applications of miniemulsions to: (i) produce current latex products with a higher level of efficiency and uniformity, and (ii) develop new latex products based on miniemulsion technology. Specific areas which will be investigated are: (1) the use of polymer as a cosurfactant; (2) the use of droplet nucleation to effect more robust particle nucleation in systems in which micellar nucleation tends to be unstable with respect to impurities; (3) the use of the miniemulsion process to produce polymers of narrow copolymer composition distribution from monomers which are highly water-insoluble; (4) the use of simultaneous micellar and droplet nucleation to produce lattices with unique particle size distributions; and (5) the impact of "accidental" cosurfactants such as dodecyl mercaptan on conventional emulsion polymerization processes.
