Miniaturization of anisotropic composite particles incorporating a silica particle smaller than 100 nm

Two-step aqueous polymerizations with a water-soluble initiator of potassium persulfate were conducted to prepare anisotropic composite particles incorporating a silica core smaller than 100 nm. The two-step polymerization consisted of the first polymerization to coat the silica cores with cross-linked polymethylmethacrylate (PMMA) shell and the second polymerization to protrude a polystyrene (PSt) bulge from the core–shell particles. The concentration of ionic comonomer of sodium p-styrenesulfonate (NaSS) in the first polymerization was an important factor to stabilize the core–shell particles during the second polymerization as well as the first one, and an appropriate concentration of NaSS could prepare the anisotropic composite particles incorporating a single core. Another important factor for small, anisotropic composite particles was duration time for swelling the core–shell particles with the second monomer of styrene. Extension of the duration time from 2 to 4 h facilitated protrusion of the PSt bulge from the particles incorporating a 44-nm silica core. The composite particles were also employed to fabricate anisotropic hollow particles. Chemical etching of silica component in the composite particles with hydrofluoric acid successfully created anisotropic hollow polymer particles with a cavity size corresponding to the silica cores.