| Abstract
| - The surfactant-free synthesis of colloidal dispersions of vinyl polymer−silica nanocomposite particlesin aqueous media using a batch emulsion polymerization protocol has been previously described [Percy,M. J.; et al. Langmuir2000, 16, 6913]. In the present work 2-hydroxypropyl methacrylate [HPMA] wascopolymerized with 4-vinylpyridine [4VP] using ammonium persulfate in the presence of an ultrafine silicasol. 4VP is used as an auxiliary in these syntheses; the strong interaction of this basic monomer with theacidic surface of the silica particles is essential for successful nanocomposite particle formation. HPMAmonomer was selected since it has appreciable water solubility (up to 13% at 20 °C), but HPMA homopolymeris water-insoluble. This unusual solubility behavior ensured that these nanocomposite syntheses wereconducted under true dispersion polymerization conditions. In view of the success of these syntheses, weconclude that emulsion monomer droplets and micelles are not a prerequisite for the formation ofnanocomposite particles. Both thermogravimetric analysis and elemental microanalyses were used todetermine the silica contents of the nanocomposite particles, which ranged from 5% to 42% by mass anddepended on the proportion of 4VP in the comonomer feed. Under the conditions investigated the minimumamount of 4VP auxiliary required was around 15%. Depending on the synthesis conditions, the meanparticle diameter of the HPMA−4VP/SiO2 particles varied from 205 to 330 nm, as judged by disk centrifugephotosedimentometry. Scanning electron microscopy studies of selected nanocomposites indicated thatdiscrete particles were obtained on freeze-drying, partial particle coalescence occurred on drying at ambienttemperature, and complete particle coalescence occurred on annealing at 50 °C. The transmittance of theannealed films was greater than 80% over the entire visible wavelength range, indicating a high degreeof dispersion for the ultrafine silica sol within the film. Aqueous electrophoresis measurements combinedwith X-ray photoelectron spectroscopy studies indicated that the surface compositions of these HPMA−4VP/SiO2 particles are silica-rich.
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