Modeling of Pickering emulsion polymerization – PickEP

The aim of the present project is to develop a methodology for fundamental modeling of surfactant-free emulsion polymerization processes stabilized by inorganic particles, referred to as “Pickering emulsion polymerization (PickEP)”. The model should be able to describe the reaction kinetics in the different phases, mass transfer between phases (ex. radicals) and the evolution of the particle size distribution (PSD), which is an important end-use property of the latex. The sought model will be based on individual fundamental sub-models representing particle nucleation, growth, coagulation, partitioning of inorganic particles and the reaction kinetics. Sub-models are autonomous pieces that are individually identified and validated experimentally representing an elementary mechanism. In order to realize such an interdisciplinary objective, the present project involves specialists of physical chemistry, polymerization and process modeling/monitoring. The developed methodology should be applicable to different inorganic systems. The obtained model will improve process understanding and used in control strategies in order to improve the product quality, mainly to increase the solid contents of the latex for industrial interest.
Pickering stabilization has recently emerged as a new method to create colloidal nanocomposite particles by adsorption of solid particles at solid-liquid interfaces. The elaboration of composite latexes allows combining attributes of inorganic solids with the processing and handling advantages of organic polymers which allows for instance to improve mechanical and water-resistance properties of films. A collaboration between the Laboratoire de Chimie, Catalyse, Polymères et Procédés and Laboratoire d’Automatique et de Génie des Procédés allowed developing a process of producing silica-armored polystyrene latexes by soap-free emulsion polymerization. A poly(ethylene glycol) methyl ether methacrylate macromonomer was used to promote adsorption of silica particles on the surface of polystyrene latex particles (Sheibat-Othman and Bourgeat-Lami 2009). More recently, the method was applied successfully to clay particles (Bourgeat-Lami et al. 2010a and 2010b, Sheibat-Othman et al. 2011). These preliminary studies reveal that optimization of the process conditions is crucial to ensure the formation of stable colloidal suspensions of composite latexes. This system will be considered in this project for model development.
So far, there is no modeling studies devoted to Pickering emulsion polymerization systems, including PSD (while well established for conventional emulsifier-based emulsion polymerization, comprising PSD and molecular weight distribution). Though, there are fundamental differences between conventional and Pickering emulsion polymerization. First of all, the stabilization of Pickering emulsions takes place mainly by steric repulsions between adsorbed solid particles. The later form a rigid mechanical barrier that prevents the droplets from coalescence. Electrostatic phenomena do not significantly contribute to the colloidal stability. Adsorption of functional monomers to the solid particles surface controls the adsorption of these solid particles and the stabilization of the resulting polymer particles. Second, the nucleation mechanism is different since there are no micelles and it cannot be described by usual homogeneous nucleation models since nucleation does not depend on the concentration of inorganic particles (stabilizers in our system). Finally, the presence of inorganic particles on the surface of latex particles affects radical absorption and desorption which affects particle growth. Therefore, PickEP process deserves specific modeling.