The determinants of latex monodispersity in emulsion polymerizations

Abstract

The kinetic factors that determine the monodispersity of polystyrene latices produced by emulsion polymerization were explored, both experimentally and theoretically, in order to understand why certain surfactants (e.g., Aerosol MA) generate latices with a higher degree of monodispersity than others (e.g., sodium dodecyl sulfate). Three surfactants, Aerosol MA, Aerosol OT, and sodium dodecyl sulfate, were found experimentally to generate latices whose particle size distributions at the end of the nucleation period, although different, were all positively skewed. This skewness is in conformity with the coagulative nucleation theory. Experiments also showed that the free radical entry rate coefficient in the subsequent growth of the latex particles was inversely related to the number of latex particles generated, the latter being smaller for aerosol MA than for sodium dodecyl sulfate. Theoretical calculations based on coagulative nucleation theory showed that neither the duration of the nucleation period nor the form of the particle production rate curves was an important determinant of monodispersity at long times (although it may exert large effects at earlier times). In contrast, the monodispersity of the final latex produced was found theoretically to be critically dependent upon the entry rate parameter, the coefficient of variation of the latex being reduced as the entry rate coefficient increased. It can be inferred that Aerosol MA favors the production of a monodisperse latex primarily because it generates relatively few latex particles during nucleation: as a consequence, the free radical entry rate coefficient is relatively high (varying approximately inversely with particle number), which is conducive to the production of a monodisperse latex. Thus those properties of a surfactant that are able directly or indirectly to influence growth kinetics in emulsion polymerizations are important determinants of the size and size distribution of the latex particles ultimately generated. © 1987 Academic Press, Inc. All rights reserved.