(382h) Rheology and Mesoscale Network Structure of Entangled Polystyrene-Organoclay Solutions

Many of the desirable properties of polymer-clay nanocomposite materials are believed to derive from the microstructure induced by processing. Here, we explore the rich rheological behavior of these material systems by working with entangled polystyrene-organoclay solutions, drawing analogies to both entangled polymer and dispersion rheology.

Solutions are comprised of high molecular weight, narrow molecular weight distribution polystyrene (Mw=8.42 x 106 g/mol), organically modified montmorillonite clay from which excess free surfactant has been removed, and p-xylene. The polymer concentration has been fixed relative to the amount of solvent such that the average number of entanglements per chain is approximately 10 in a pure polystyrene solution. Organoclay has been added to similarly entangled solutions at loadings of 0.89 and 2.68 wt% relative to solvent. Pure organoclay dispersions of similar loadings are studied as reference materials.

Various deformation histories, including oscillatory shear, steady shear, and step-strain, are imposed in a mechanical rheometer to examine the influence of organoclay filler on polymer relaxation dynamics. The sample with 0.89 wt% organoclay exhibits a divergent viscosity at low shear rates similar to that observed for more highly filled polymer nanocomposite melts; however, the linear viscoelastic response remains dominated by the polymer, and the Cox-Merz rule is satisfied. At 2.68 wt% organoclay loading, the rheology is dominated by organoclay, as evidenced by pronounced yield behavior, incomplete relaxation following step-strain, and failure of Cox-Merz. Yield behavior suggests the existence of a mesoscale organoclay network that is stabilized by the elasticity of the polymer. A novel wet SEM technique provides direct imaging of the inferred mesoscale organoclay network in the quiescent state. Rheological data are discussed in the context of that network and classic entangled polymer rheology.