Accurately controlling the shell thickness in the core–shell microspheres with single silica core and poly (butyl acrylate) rubber shell via emulsion polymerization

The shell thickness of core–shell microspheres with a rigid core and soft rubber shell has been accurately controlled through the emulsion polymerization. When the amount of BA monomer and SiO 2 is fixed, the morphology of core–shell microspheres with SiO 2 core ( Φ  = 280 nm) and poly (butyl acrylate) (PBA) shell varies along with the emulsifier concentration ( C e ). If C e is lower than a low critical value C el , aggregated micelles with multiple SiO 2 cores are formed. And while C e is higher than an upper critical value C eu , isolated PBA micelles without SiO 2 core appear in the system. For a given amount of BA monomer and SiO 2 core, core–shell particles with single SiO 2 core can only be obtained at C el  <  C e  <  C eu . The value of C el or C eu increases with the BA monomer content when the SiO 2 amount is fixed. However, when the number of the emulsifier molecules absorbed onto the surface of the microspheres (Ʃ) is considered, the upper and lower limit Ʃ (Ʃ UC and Ʃ LC ) for the core–shell particles containing single SiO 2 core seems has nothing to do with the BA monomer content. In our system, the value of Ʃ UC is around ~ 3 × 10 6  μm −2 and Ʃ LC is about 1.3 × 10 6  μm −2 . When Ʃ is fixed at 2 × 10 6  μm −2 (Ʃ UC  > Ʃ > Ʃ LC ), the rubber shell thicknesses of core–shell particles with single SiO 2 core can vary from 8 to 57 nm along with the weight ratio of BA monomer to SiO 2 that increases from 1:10 to 1:1.