Super- and sub-Einstein intrinsic viscosities of spherical nanoparticles in concentrated low molecular weight polymer solutions

Abstract

We explore the intrinsic viscosity of 40 nm silica particles in concentrated solutions of low molecular weight hydroxyl terminated polyethylene glycol (PEG), polyethylene glycol dimethyl ether (PEGDME) and polytetrahydrofuran (PTHF). Our goal is to develop a correlation between the intrinsic viscosity and the strength of the polymer segment-surface attractions, ε_pc. We show when the particles are large compared to the polymer radius of gyration and the polymer molecular weight is below the entanglement molecular weight, the intrinsic viscosity can be larger or smaller than the Einstein value of 2.5 expected for spheres with hard surfaces experiencing no-slip boundary conditions at the fluid/particle interface. Extensive small angle X-ray scattering studies are undertaken to extract ε_pc and establish the thermodynamic state of particle dispersion. We demonstrate a monotonic dependence of intrinsic viscosity on ε_pc with weaker polymer segment-particle surface interactions leading to sub-Einstein intrinsic viscosities. When interpreted in terms of a continuum model, our results suggest that the variations in intrinsic viscosity can be understood in terms of adsorbed layers which can have different viscosities from that of bulk fluid and which can slip at the particle surface. Increased layer viscosities are encouraged by large ε_pc while slip and or low viscosities are encouraged by low ε_pc.