Surface characterization of vinyl polymer-silica colloidal nanocomposites using X-ray photoelectron spectroscopy

We report the use of X-ray photoelectron spectroscopy (XPS) to characterize the surface compositions of colloidal nanocomposite particles. Each nanocomposite was synthesized by (co) polymerizing 4-vinylpyridine in the presence of an ultrafine silica sol. Thus, nitrogen and silicon were utilized as unique elemental markers for the (co)polymer and silica components, respectively, and the silicon/nitrogen atomic ratios determined by XPS were used to assess the surface compositions of the particles. For all the homopoly(4-vinylpyridine)-silica nanocomposites examined, the XPS surface compositions are comparable to the bulk compositions determined by thermogravimetric analyses and elemental microanalyses. This is consistent with the 'currant bun' particle morphologies observed by transmission electron microscopy (TEM) and indicates that the silica particles are uniformly distributed throughout the nanocomposite particles. In contrast, the particle surface of a poly(styrene-co-4-vinylpyridine)-silica nanocomposite is distinctly silica-rich, as judged by XPS; this suggests a core-shell morphology, with the silica component forming the shell and the hydrophobic copolymer forming the core. Both the 'currant bun' and core-shell particle morphologies are supported by TEM studies of nanocomposite particles sectioned using cryo-ultramicrotomy. A poly(methylmethacrylate-co-4-vinylpyridine)-silica nanocomposite shows an XPS surface composition which is intermediate between those found for the 'currant bun' particles and the core-shell particles. In view of its relatively high silica content, a 'raspberry' particle morphology, similar to that previously reported for conducting polymer-silica nanocomposites, is suggested. Finally, it is shown that, in the case of the poly(methyl methacrylate-co-4-vinylpyridine)-silica nanocomposite, it is possible to use the carbonyl carbon signal of the methyl methacrylate residues as an unambiguous marker for the copolymer component; the surface composition obtained from this alternative analysis is consistent with that calculated using the nitrogen XPS signal. This approach may be particularly useful for assessing the surface compositions of nanocomposites containing a relatively low (or zero) proportion of 4-vinylpyridine co-monomer.