Linear rheology of polymer nanocomposites with polymer-grafted nanoparticles

Abstract

Polymer nanocomposites with polymer grafted nanoparticles (NPs) are of great interest since they have improved properties relative to their unfilled analogs. While it is well-appreciated that nanocomposite flow behavior is critically affected by filler dispersion, the roles of the matrix and the polymeric grafts remain unexplored. We perform linear oscillatory rheology on composites of polystyrene-grafted silica NPs in two different polystyrene matrices which have similar dispersion states and glass transition temperatures, Tg. When the matrix viscosity is considerably lower than that of the neat NP melt, the composite viscosities follow the scaling ideas applicable to star/linear polymer mixtures, well above the Einstein prediction. The higher matrix molecular weight shows lower viscosity, essentially following the Einstein relationship, presumably because faster modes of relaxation become available. We conclude that there is a deep, currently unexploited, analogy between the viscosity of polymer-grafted NP and star polymers.