The Fabrication of Very Small Miniemulsion Latexes from N-Stearoylglutamate and Lauryl Methacrylate: Evidence for Droplet Budding

Miniemulsion polymerization is a new way to synthesize polymer nanoparticles in the size range of 40–500 nm with high morphological uniformity and chemical flexibility. Focusing on the very small particles, miniemulsion polymerization already steps into the range of microemulsion polymerization, however, with a minimum expense of surfactant which is up to a factor of 10 lower than in microemulsion formulations with similar particle size. The name ‘‘miniemulsion’’ refers to systems where small monomer droplets in a continuous phase are created by using high shear and are stabilized by a combination of surfactant and an osmotic pressure agent. In a second step, these droplets are polymerized under conservation of the droplet number. The most effective systems with respect to particle size are based on the ionic surfactant sodium dodecyl sulfate (SDS). Usually, both miniemulsion droplets and polymer particles are just incompletely covered with surfactant (which is the basis of the high efficiency), which can be detected by the rather high surface tensions of the resulting polymer dispersions. It was, however, observed that the coverage of the droplets by surfactant depends on the droplet size: the smaller the droplets, the higher the required surface coverage is in order to obtain stable droplets. This is why miniemulsion polymerization is somewhat selflimiting towards smaller particle sizes. Exchange of the SDS to other ionic or nonionic surfactants has only led to systems with lower performance. First exploitations of cationic surfactants such as octadecyl pyridinium bromide (however, with incomplete dispersion) showed only broad particle size distributions. Recent work on steady state miniemulsions revealed that miniemulsions on the basis of cetyltrimethylammonium chloride give narrowly distributed stable cationic latex particles being slightly larger than those with the SDS system. Employment of charged biosurfactants, such as cholic acids, lipids, and short polysaccharides, has indeed broadened the range of accessible surface structures, but did not lower the accessible particle size. It is the purpose of the present contribution to show that another bio-based surfactant with a functional and Full Paper: A new, biodegradable surfactant with high efficiency,N-stearoyl-di(sodium)glutamate (SDG), was used for miniemulsion polymerization of styrene and lauryl methacrylate (LMA). In the case of styrene, SDG produces latexes at least as small as those produced by sodium dodecyl sulfate (SDS), over a range of concentrations. In the case of LMA, a combination of SDG with an appropriate costabilizer allows one to obtain very small particles. With potassium persulfate (KPS) as initiator, the SDG/LMA system deviates significantly from the ideal miniemulsion, forming extremely small particles, which was speculatively attributed to a budding phenomenon. The new, biodegradable surfactant, N-stearoyl-di(sodium)glutamate. 1966 Macromol. Chem. Phys. 2003, 204, 1966–1970