The Stabilization and Controlled Flocculation of Sterically-Stabilized Latices

Abstract

Virtually all the methods used for preparing polymer latices involve precipitation of polymer from solution during their formation. If distinct particles are to be formed, as opposed to large, ill-defined aggregates, then some form of stabilizing mechanism is required for the growing particles. In certain types of polymerization processes electrostatic stabilization alone may be sufficient. The most obvious example is aqueous emulsion polymerization used to produce fine dispersions of compact, lyophobic polymer particles (Figure 1a). Because of the long-range nature of the van der Waals (dispersion) attraction (V_A) in this case, a repulsion interaction of similar or greater range is required for stability to be achieved. Electrostatic interactions, arising from surface charge groups on the particles, achieve this requirement. If this electrostatic repulsion is removed or sufficiently reduced, such that the particles do come into primary contact (R=2a) then the force preventing further approach of the particle centers (partial coalescence) may be described as a “hard-sphere” repulsion (V_hs). However, because in most cases, and particularly with aqueous latices, there will be a large difference between the Hamaker constant of the particles and that of the medium, V_min will be of sufficient magnitude to allow irreversible aggregation (coagulation) to occur.