Synthesis and characterization of vinyl polymer-silica colloidal nanocomposites

Colloidal dispersions of polymer-silica nanocomposite particles were synthesized in high yield by homopolymerizing 4-vinylpyridine (4VP) in the presence of an ultrafine silica sol using a free-radical initiator in aqueous media at 60 degrees C. Copolymerization of 4VP with methyl methacrylate and styrene also produced colloidally stable nanocomposite particles, in some cases for comonomer feeds containing as little as 6 mol % 4VP. However, homopolymerization of styrene or methyl methacrylate in the presence of the silica sol did not produce nanocomposite particles in control experiments. Thus a strong acid-base interaction between the silica sol and the (co)polymer appears to be essential for nanocomposite formation. Transmission electron microscopy studies confirmed the presence of the ultrafine silica sols within the nanocomposite particles, which typically exhibited "currant-bun" particle morphologies. This is in contrast to the "raspberry" particle morphologies previously reported for conducting polymer-silica nanocomposite particles. The average silica contents and mean particle diameters of the vinyl (co)polymer-silica nanocomposites were surprisingly insensitive to the synthesis conditions, as judged by thermogravimetric analysis and disk centrifuge photosedimentometry studies, respectively. The latter technique also indicated that some of the copolymer-silica dispersions were appreciably flocculated, although the degree of dispersion could be improved by redispersion in alkaline media. H-1 NMR spectroscopy studies on the extracted nanocomposites confirmed incorporation of the 4VP comonomer, with reasonable agreement between copolymer compositions and comonomer feeds being obtained. Aqueous electrophoresis measurements confirmed that the surface of the 4VP-silica particles is polymer-rich, which is consistent with their currant-bun morphology. Time-resolved photon correlation spectroscopy studies during nanocomposite formation showed that particle growth occurred rapidly, with particles reaching their final size after approximately 1 h. Doubling the 4VP monomer concentration at a fixed 4VP/silica ratio led to an increase in particle size from 150 to 220 nm.