Synthesis and characterization of vinyl polymer-silica colloidal nanocomposites

Colloidal dispersions of polymer-silica nanocomposite particles were synthe sized in high yield by homopolymerizing 4-vinylpyridine (4VP) in the presen ce of an ultrafine silica sol using a free-radical initiator in aqueous med ia at 60 degrees C. Copolymerization of 4VP with methyl methacrylate and st yrene also produced colloidally stable nanocomposite particles, in some cas es for comonomer feeds containing as little as 6 mol % 4VP. However, homopo lymerization of styrene or methyl methacrylate in the presence of the silic a sol did not produce nanocomposite particles in control experiments. Thus a strong acid-base interaction between the silica sol and the (co)polymer a ppears to be essential for nanocomposite formation. Transmission electron m icroscopy studies confirmed the presence of the ultrafine silica sols withi n the nanocomposite particles, which typically exhibited "currant-bun" part icle morphologies. This is in contrast to the "raspberry" particle morpholo gies previously reported for conducting polymer-silica nanocomposite partic les. The average silica contents and mean particle diameters of the vinyl ( co)polymer-silica nanocomposites were surprisingly insensitive to the synth esis conditions, as judged by thermogravimetric analysis and disk centrifug e photosedimentometry studies, respectively. The latter technique also indi cated that some of the copolymer-silica dispersions were appreciably floccu lated, although the degree of dispersion could be improved by redispersion in alkaline media. H-1 NMR spectroscopy studies on the extracted nanocompos ites confirmed incorporation of the 4VP comonomer, with reasonable agreemen t between copolymer compositions and comonomer feeds being obtained. Aqueou s electrophoresis measurements confirmed that the surface of the 4VP-silica particles is polymer-rich, which is consistent with their currant-bun morp hology. Time-resolved photon correlation spectroscopy studies during nanoco mposite formation showed that particle growth occurred rapidly, with partic les reaching their final size after approximately 1 h. Doubling the 4VP mon omer concentration at a fixed 4VP/silica ratio led to an increase in partic le size from 150 to 220 nm.