Synthesis of Polymer/Silica Hybrid Latexes by Surfactant-Free RAFT-Mediated Emulsion Polymerization

The reversible addition fragmentation chain transfer (RAFT) polymerization technique was used to synthesize random copolymers of poly(ethylene glycol) methyl ether acrylate) (PEGA) and n-butyl acrylate (BA) and terpolymers of acrylic acid (AA), PEGA and BA with a trithiocarbonate reactive end-group. These macromolecular RAFT agents (macro-RAFTS) were subsequently adsorbed at the surface of size-monodisperse colloidal silica particles with diameters varying between 40 and 450 nm. Adsorption isotherms for both macro-RAFTs could be well fitted to the Langmuir adsorption model, the AA-based macro-RAFT agent showing however a lower maximum adsorption. The adsorbed macro-RAFT agents were subsequently chain extended with a mixture of methyl methacrylate (MMA) and BA by starved feed emulsion polymerization. Cryo-TEM analysis of the resulting hybrid latexes synthesized in the presence of the P(AA-co-PEGA-co-BA) terpolymers resulted in multipod-like particles while the P(PEGA-co-BA) copolymers showed the formation of individually and multiencapsulated silica particles depending on the silica particle size. Decreasing the total silica surface area available by decreasing the silica concentration or by increasing the silica particle size resulted in limited coagulation of the latex particles due to a less efficient use of the free nonadsorbing macro-RAFT agent. The feeding process also had a strong impact on particle morphology, and snowman-like particles could be successfully achieved under batch conditions. The use of commercial silica particles instead of homemade silica led to armored latexes illustrating the determinant role of the surface properties of the macro-RAFT-coated inorganic particles in controlling hybrid particle morphology. At last, core shell particles with a rigid silica core and a soft copolymer shell were obtained for the first time by polymerizing a film-forming monomer mixture showing the high potential of the P(PEGA-co-BA) macro-RAFT agent for the elaboration of polymer-encapsulated silica particles for coating applications.