The Preparation Of Core-Shell Structure Composite Particles By Semi-Continuous Soap-Free Emulsion Polymerization

Core-shell particles are prepared by direct polymerization of three monomers of styrene, methyl-methacrylate (MMA), and vinyl acetate (VAC) onto the hydrophilic surface of 30nm and 230nm silica particles without using any coupling agent in a semi-continuous process of emulsifier-free emulsion polymerization. After adding 30nm or 230nm silica particles, the polymerization is initiated by potassium persulfate (KPS) or 2,2'-Azobis(2-methyl- propionamidine) dihydrochloride(AIBA) in a stirred reactor with constant feed of monomer at 0.02,0.04,0.06 ml×min-1 . The semi-continuous processes are sampled each hour for four or five hours. The growth of the core-shell particles are measured by a laser particle size analyzer and the surfaces of the particles by FT-IR.When the seeded particles are 30nm silica nanoparticles and initiator KPS, results show that all of the three polymer shell can be synthesized directly onto the hydrophilic surface of the silica nanoparticles in aqueous solution in the monomer starved process. However, the particle growth differs in the size distribution depending on the hydrophobic characteristics of the three polymers. When the monomer is the most hydrophobic styrene, secondary particles are formed in the early stage of the process. Both PMMA and PVAC core-shell particles grow constantly on the hydrophilic surface of silica particles because of their better hydrophilic characteristics. The core-shell PMMA/silica nanoparticles are the most monodisperse. Similarly, crosslinked core-shell nanoparticles are also prepared by direct polymerization of monomers of styrene, and methyl-methacrylate (MMA) with a crosslinker of divinylbenzene onto the hydrophilic surface of 30nm silica nanoparticles.When the seeded particles are 230nm silica submicron particles and initiator KPS, results show that the growth of the core-shell particles differs depending on the hydrophobic characteristics of the three monomers and the silica surfaces. When the monomer is the most hydrophobic styrene, Polystyrene(PS) shells fall off from the hydrophilic surface of the silica particle in the reaction. However, after the silica particles are treated byγ-Methacryloxypropyltrimethoxysilane, PS shell is stable so that the PS/silica core-shell submicron-particles grow constantly. Both PMMA and PVAC shells grow constantly on the hydrophilic surface of silica submicron particles. The polydispersity(PDI) of the three core-shell composite submicron-particles increase with the reaction time although the increase is only slightly in the case of PMMA/silica particles.If the initiator changes to AIBA, PDI of the three core-shell composite nanoparticles increase with the reaction time and the emulsion become unstable. However, submicron particles are prepared by direct polymerization of three monomers onto the hydrophilic surface of 230nm silica submicron particles without any coupling agent. When the monomer is the most hydrophobic styrene, PS shells fall off from the hydrophilic surface of the silica particle in the reaction. Both PMMA and PVAC shells grow constantly on the hydrophilic surface of silica submicron particles. PDI of the three core-shell composite submicron-particles increase with the reaction time, which is probably caused by the uneven polymerization on the surface of the particles and the particle agglomeration during the transition of the charge reverse from negative silica seeded particles to positive polymerized particles.