Revisit to the self-assembled hybrid acrylate/silica core-shell structured particles in the presence of unmodified silica particles

Abstract

Polyacrylate/silica nanocomposite particles are synthesized by soap-free emulsion polymerization using negatively charged silica particles, where the anionic initiator, potassium peroxydisulfate (KPS), is used. The influence of the content of hydrophilic silica particles on the composite particle growth and conversion is investigated. Nucleation process of unmodified silica-stabilized emulsion polymerization is studied by characterizing the structures of composite particles forming at different polymerization stages with transmittance electron microscope (TEM) and laser light scattering (DLS). The results show that the presence of silica particles on latex particles reinforces the barrier to radical absorption and retards the polymerization rate and therefore the growth of the latex particles. However, the composite particle size does not decrease much when the content of silica sol is increased to more than 5wt%. A mechanism is proposed to explain the formation of the core-shell structured particles when an electrostatic repulsive force between the silica and polyacrylate particles is present. The polyacrylate precursors forming in the early stage of the soap-free emulsion polymerization are unstable and tend to aggregate and absorb silica particles to lower their interfacial free energy, thus forming the composite particles. In contrast to the monomer droplets dispersed in the sole acrylic soap free emulsion polymerization, the monomer droplets dispersed in the aqueous system containing silica sol can work as polymerization loci due to their relatively small size and the large quantity (or the large specific surface areas), hence enhancing the ability to capture radicals.