When does silica exchange occur between vinyl polymer-silica nanocomposite particles and sterically stabilized latexes?

The redistribution of silica nanoparticles between "core-shell" polymer-silica nanocomposites and sterically stabilized latexes is investigated using a combination of electron microscopy, disk centrifuge photosedimentometry (DCP), and X-ray photoelectron spectroscopy (XPS). Facile exchange of silica nanoparticles occurs on addition of sterically-stabilized polystyrene (or poly(2-vinylpyridine)) latex to polystyrene-silica (or poly(2-vinylpyridine)-silica) nanocomposite particles previously prepared by heteroflocculation. In contrast, no silica exchange occurs after such a latex "challenge" if similar polymer/silica nanocomposite particles are prepared via in situ polymerization. Silica redistribution can be confirmed by post mortem electron microscopy studies, which are facilitated if the original nanocomposite and latex particles differ sufficiently in their mean diameters. Ideally, XPS requires a unique elemental marker for the nanocomposite particle cores, which become progressively more exposed if silica exchange occurs. DCP is a particularly convenient in situ technique for assessing whether or not silica exchange has occurred. If no silica exchange occurs, there is little or no change in the nanocomposite and latex size distributions. On the other hand, silica redistribution always results in a larger mean particle diameter for the (partially) silica-coated latex particles relative to the original bare latex. In addition, incipient flocculation is typically observed after silica exchange. Like electron microscopy, DCP studies are aided if there is a significant difference in particle diameter between the original polymer-silica nanocomposite particles and the added latex. Moreover, silica redistribution can be prevented for heteroflocculated polymer-silica nanocomposite particles under certain conditions. For example, although silica exchange is observed at pH 10 when adding sterically-stabilized polystyrene (or poly(2-vinylpyridine)) latex to heteroflocculated poly(2-vinylpyridine)-silica particles, it does not occur at pH 5. Presumably, this is due to greater electrostatic attraction between the cationic P2VP cores and the anionic silica nanoparticles at this lower pH.