Synthesis of vinyl polymer-silica colloidal nanocomposites via aqueous dispersion polymerization

The surfactant-free synthesis of colloidal dispersions of vinyl polymer-silica nanocomposite particles in aqueous media using a batch emulsion polymerization protocol has been previously described [Percy, M. J.; et al. Langmuir 2000, 16, 6913]. In the present work 2-hydroxypropyl methacrylate [HPMA] was copolymerized with 4-vinylpyridine [4VP] using ammonium persulfate in the presence of an ultrafine silica sol. 4VP is used as an auxiliary in these syntheses; the strong interaction of this basic monomer with the acidic surface of the silica particles is essential for successful nanocomposite particle formation. HPMA monomer was selected since it has appreciable water solubility (up to 13% at 20 degreesC), but HPMA homopolymer is water-insoluble. This unusual solubility behavior ensured that these nanocomposite syntheses were conducted under true dispersion polymerization conditions. In view of the success of these syntheses, we conclude that emulsion monomer droplets and micelles are not a prerequisite for the formation of nanocomposite particles. Both thermogravimetric analysis and elemental microanalyses were used to determine the silica contents of the nanocomposite particles, which ranged from 5% to 42% by mass and depended on the proportion of 4VP in the comonomer feed. Under the conditions investigated the minimum amount of 4VP auxiliary required was around 15%. Depending on the synthesis conditions, the mean particle diameter of the HPMA-4VP/SiO2 particles varied from 205 to 330 nm, as judged by disk centrifuge photosedimentometry. Scanning electron microscopy studies of selected nanocomposites indicated that discrete particles were obtained on freeze-drying, partial particle coalescence occurred on drying at ambient temperature, and complete particle coalescence occurred on annealing at 50 degreesC. The transmittance of the annealed films was greater than 80% over the entire visible wavelength range, indicating a high degree of dispersion for the ultrafine silica sol within the film. Aqueous electrophoresis measurements combined with X-ray photoelectron spectroscopy studies indicated that the surface compositions of these HPMA-4VP/SiO2 particles are silica-rich.