A comprehensive rheological study of thermally activated suspensions of polymer-grafted silica nanoparticles*

This experimental study reveals the broad range of interesting rheological characteristics of suspensions of covalently grafted poly(ethylene glycol) methyl ether (mPEG) polymers on silica nanoparticles in the presence of a secondary solvent. The rheology of this system is governed by Brownian motion of the core particles and polymer-brush induced interactions. At small core volume fractions, these suspensions exhibit Newtonian rheology with a viscosity that increases with volume fraction similar to other well-studied low-Peclet number Brownian suspensions. With increasing the shear rate and particle concentration, the suspensions begin to shear thin as the hydrodynamic stresses are sufficient to drive overlap between the polymer brushes of neighboring particles. At large core volume fractions, these systems exhibit soft glassy rheology. In this case, the system has an apparent yield stress which we attribute to the free energy barrier arising as the mixture of solvent and tethered polymers tries to fill the interstices between a deformed array of cores. We measure the apparent yield stress in the polymer grafted suspension and observe the history dependent rheology occurring as thermal fluctuations aid their yielding. We also investigate the effect of changing the synthetic parameters (e.g.: grafting density, core volume fraction, solvent molecular weight, etc.) on the apparent yield stress of this class of materials.