Precipitated, colloidal and fumed silica are composed of ramified mass fractal aggregates of amorphous nanoparticles. These aggregates are used to reinforce many polymers such as polydimethylsiloxane (PDMS) and polybutadiene elastomers. Silica/polymer nanocomposites are incompatible systems with dispersion and distribution driven by mixing. Under the right conditions silica aggregates can display varying degrees of correlation up to the formation of colloidal crystalline domains that can be used as elastically tunable optical diffraction devices [1]. Silica can also be randomly distributed with no specific correlation between aggregates [2]. Correlations are driven by Coulombic interactions associated with surface hydroxyl groups. The conditions under which silica aggregates, or particles, correlate depend on the surface charge density, Debye screening length, mixing intensity, and the local nano-scale concentration of aggregates or particles [2,3]. Silica correlations in polymer nanocomposites can be studied using small-angle X-ray scattering. For PDMS, correlated nanocomposites can also be produced by in situ formation of aggregates through an excess of sol-gel crosslinking agents such as tetra ethyl ortho silicate which can hydrolyze to amorphous SiO<sub>2</sub> under basic or acidic conditions (away from the isoelectric point) [4].

Correlations in silica nanocomposites

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