Elucidation of the Miniemulsion Stabilization Mechanism and Polymerization Kinetics Christopher D. Anderson, E. David Sudol, Mohamed S. El-Aasser Emulsion Polymers Institute and Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015 Received 19 December 2002; accepted 29 January 2003 ABSTRACT: Styrene/hexadecane miniemulsions were polymerized at 50°C using a redox initiator. The miniemul- sions and their corresponding latexes were characterized in terms of size, polymerization rate, and surface properties. The resulting data were analyzed to elucidate the miniemul- sion stabilization and polymerization mechanisms. It was found that the free surfactant concentration exceeded the critical micelle concentration when large amounts of surfac- tant (60 mM sodium lauryl sulfate) were used, resulting in simultaneous micellar and droplet nucleation. Most surfac- tant was on the surface of the droplets (85%) or particles (95%). The fractional surface coverage was proportional to the surfactant concentration to the 0.55 power. Using a par- ticle diameter equation, the number of particles was calcu- lated to be proportional to the surfactant concentration to the 1.35 power. Through direct particle size measurements, a power of 1.38 was confirmed. The rate of polymerization was determined by reaction calorimetry to be proportional to the number of particles to the 0.59 power, in contrast to classical Smith–Ewart kinetics for conventional emulsions (1.0 power). The average number of radicals per particle was estimated from the rate and number data, and varied with the particle diameter to the 0.97 power. The observed kinetic dependencies were validated through an extension of Smith–Ewart theory. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3987–3993, 2003 Key words: miniemulsions; particle nucleation; polymeriza- tion kinetics; polystyrene; latexes INTRODUCTION Miniemulsions are submicron dispersions of mono- mer in water. Miniemulsion droplets are usually formed by the high-shear homogenization of a crude emulsion. Surfactant is used in sufficient quantity to provide the droplets with colloidal stability against coalescence. A hydrophobic costabilizer (often hexa- decane) retards the degradation by Ostwald ripening of the miniemulsion droplets. Oligoradicals enter di- rectly into the miniemulsion droplets to initiate poly- merization. Aqueous phase nucleation is avoided as a result of the low free surfactant concentration. Al- though predominant droplet nucleation was observed in submicron emulsions and documented in 1973, 1 the term “miniemulsion” did not appear in the literature until 1980. 2 Smith–Ewart 3 theory for conventional emulsion po- lymerizations states that the number of polymer par- ticles (N p ) and the rate of polymerization (R p ) vary with [SLS] 0.6 . It is not likely that miniemulsions follow the same trends as conventional emulsions, given that the nucleation mechanisms and surfactant distribu- tions differ substantially between the two cases. In a conventional emulsion, the majority of surfac- tant is in the water phase as micelles and free surfac- tant, and particles are nucleated primarily in the monomer-swollen micelles. The monomer droplets are large (1 m) and have relatively little interfacial area to adsorb surfactant. The droplets supply grow- ing polymer particles with monomer until they are exhausted. By contrast, in miniemulsions, the monomer drop- lets are small and the interfacial area is large, so that most of the surfactant resides on the droplet surface, and relatively little is present in the aqueous phase. This reduces the probability of homogeneous nucle- ation and eliminates micellar nucleation altogether because the free surfactant concentration is below the critical micelle concentration (cmc). Nucleation occurs primarily within the monomer droplets. The particle size is an important parameter in emul- sion polymerization, and control of the particle size is critical in industrial applications. It is possible to ma- nipulate the particle size through the amount of sur- factant and the level of shear used in preparing a miniemulsion. To understand the relationships among the particle size, the polymerization rate, and the sur- face properties, the surfactant concentration in a series Correspondence to: M. El-Aasser. Contract grant sponsor: National Science Foundation; contract grant numbers: CTS-9628783 and CTS-9980208. Contract grant sponsor: Emulsion Polymers Liaison Pro- gram. Journal of Applied Polymer Science, Vol. 90, 3987–3993 (2003) © 2003 Wiley Periodicals, Inc.