Universal Features of the Melt Elasticity of Interacting Polymer Nanocomposites

We study the structure and linear viscoelasticity of interacting polymer nanocomposites based on mixtures of poly(ethylene oxide) and fumed silica particles. The filler is dispersed within the polymer using two different techniques which lead to different dispersion states. The analysis of the dynamic response of our systems highlights the formation of a stress-bearing network above a critical volume fraction, Phi(c). Extending a two-phase model used to describe weakly interacting systems, we show that above Phi(c) the melt-state elasticity of the composites arises from the independent contributions of a polymer-particle network and a viscous matrix. We also find that, although Phi(c) depends on the initial state of dispersion, the network elasticity scales with volume fraction following a universal power-law, with an exponent v approximate to 1.8. Such a scaling law has been recently predicted for the stress-bearing mechanism governed by polymer-mediated interactions.