In this work, a facile method was presented for the preparation of magnetic SiO₂/PSt hollow composite microspheres via double in situ miniemulsion polymerization. Our approach was based on the in situ miniemulsion polymerization of organic monomers and in situ formation of silica from precursor (tetraethoxysilane, TEOS) within miniemulsion monomers droplets simultaneously. Firstly, monodisperse hydrophobic magnetic nanoparticles were synthesized by coprecipitation method as the literature reported. Then the as-prepared magnetic nanoparticles (MNP) were added in the mixtures of styrene (St), divinyl benzene (DVB), TEOS, and γ-methacryloxypropyl trimethoxy silane (MPS) to be used as the oil phase. Magnetic nanoparticles were restricted in miniemulsion microreactor droplets via the miniemulsification process by sodium dodecyl sulfate (SDS) aqueous solution as a water phase. After the monomers polymerized, TEOS phase is compressed and restricted as a liquid core due to phase separation between TEOS and the growing polystyrene. When the ammonia was added, silica was in situ formed by the hydrolysis-condensation of TEOS under basic conditions. Because the volume of TEOS decreased dramatically, the original TEOS phase shrank into silica and led to the formation of hollow structure. Hollow composite microspheres were characterized by Fourier-transformed infrared spectrum (FT-IR), transmission electron microscopy (TEM), thermogravimetry and differential analyses (TGA/DSC) and vibrating sample magnetometer (VSM), respectively. The results showed that in the absence of magnetic nanoparticles SiO₂/PSt hollow composite microspheres with particle sizes in range of 300～600 nm were prepared successfully. With the addition of hydrophobic magnetic nanoparticles, the magnetic SiO₂/PSt composite microspheres were fabricated conveniently with magnetic nanoparticles embedded in the polymeric shell, which remained the same hollow structure as the SiO₂/PSt hollow composite microspheres. The inner void size of composite microspheres could be easily controlled by using varied TEOS contents in the miniemulsion formulation. The entrapment efficiency of magnetic nanoparticles in SiO₂/PSt composite microspheres reached up to 86%. The magnetic SiO₂/PSt hollow composite microspheres were superparamagnetic and their saturation magnetizations were 14.7 emu/g. The magnetic hollow composite microspheres may provide a very promising vehicle for drug delivery, catalysis, and photoelectric materials.

Key words: magnetic, hollow, miniemulsion, composite microspheres, in situ