Free Radical Entry and Secondary Nucleation in Styrene Emulsion Polymerization: A DLVO Approach

ABSTRACT A comprehensive model of free radical entry into particles during emulsion polymerization, recently reported in this journal, was modified to include the calculation of radical‐particle interactions based on the DLVO theory as well as the simplified calculation of secondary nucleation. Electrostatic repulsion was found to play a crucial role, causing changes in the size of the predominant entering oligomer radical, whether due to variations in ionic strength or surfactant coverage ( θ ). As θ increases to ∼0.1, the energy barrier for the entry of water‐soluble radicals raises sharply, causing their entry frequency ( ρ ) to decrease drastically and be controlled by diffusion. For θ >∼0.1, ρ becomes constant, and the predominant entering species are primary particles, whose formation is the rate‐determining step. It was also found that the onset of secondary nucleation occurs at a critical θ value rather than a critical particle concentration. Based on the results obtained and their agreement with experimental data, key mechanistic aspects of radical entry into the system under study are proposed, which contradict several postulates of one of the most accepted theories of free radical entry. These findings also suggest that nucleation mechanisms above and below the surfactant's CMC should be revisited.