Towards the understanding of the unusual rheological response of polymer nanocomposites

It is well known, that the properties of polymers can be improved by the incorporation of nano-additives in the polymeric matrix, forming nanocomposite materials that often possess innovative physicochemical properties compared to the initial materials and / or the respective conventionally filled systems. In this work, we investigate the rheological response of different series of poly(ethylene oxide) / silica, PEO/SiO2, nanocomposites by shear rheology. More specifically, spherical SiO2 nanoparticles of two different radii are dispersed in PEOs of different molecular weights covering the regime from unentangled to highly-entangled systems. Nanocomposites over a wide range of compositions were developed in order to investigate both the effect of the size and concentration of the nanoparticles and the effect of the molecular weight of the polymer matrix, on the rheological properties of the nanocomposite materials. For all systems three different concentration regimes are observed with distinctly different rheological response; a ‘polymer-like’ regime, where the behavior resembles the one of the respective polymers, a ‘weak gel-like’ regime at higher nanoparticle content, where enhanced polymer / particle interactions modify the behavior, and a ‘strong gel-like’ one characterized by percolated networks of nanoparticles formed within the polymer matrices. The ‘polymer-like’ regime is especially interesting since a significant reduction of the melt viscosity is observed, which is attributed to a dilution of the entanglement network of the polymer chains due to the presence of the nanoparticles.