A study on the nucleation mechanisms under critical micelle concentration (CMC) in emulsion polymerization of methyl methacrylate (MMA)

Abstract

In this study, formulation of latex with methyl methacrylate (MMA) as a function of surfactant concentration ([S]) under critical micelle concentration (CMC) was investigated to explore the nucleation mechanism and the key to control the particle number (N _p) of latex in emulsion polymerization. Influences of surfactant concentration [S] on the conversion of monomer, particle size (PS), N _p, and surfactant surface coverage (θ) were studied, as well. Flocculation nucleation was considered as the main mechanism to generate particle nuclei when [S] was much less than CMC. The nucleation mechanism experienced an evolution of a flocculation nucleation-dominated region ([S] < 3.3 mmol L⁻¹) to a micellar nucleation-dominated region ([S] > 3.3 mmol L⁻¹). The contribution ratio of the flocculation nucleation was 86 % compared with the micellar nucleation. The particle number (N _p) was found to be controlled by the combination of micellar and flocculation nucleation-dominated regions under CMC, and the regulation range was about 1–6. The results can be used to guide the preparation of a bimodal high solid content (HSC) latex by creating a population of small particles in the presence of an initial population of larger ones in a controllable way. In this way, a controllable amount of large latex particles were prepared using a high emulsifier concentration, at first, and then the second population of small particles was formulated by secondary nucleation at a proper time through controlling surfactant concentration at a low level, followed by the growth of the two groups of the particles, and then the bimodal HSC latex can be prepared at low viscosity.