Microstructure and rheology of polymer-colloid mixtures

A wide variety of industries control the interparticle interactions by polymer addition to tune the stability and rheological behavior of colloidal dispersions. However, the effect of added polymer on the macroscopic properties of colloids is not well understood. Here, we present a systematic experimental study of the effect of polymer induced interactions on the rheology, microstructure and phase behavior of a polymer stabilized colloidal dispersions and present a theoretical framework for predicting these properties from the apriori knowledge of interparticle forces.; Rheological, small angle neutron scattering (SANS) and phase behavior measurements are presented for a dispersion of silica nanoparticles in aqueous gelatin solution. The adsorbed polymer layer and the resulting large excluded volume dominates the rheology. A micromechanical model for the interaction potential and the zero-shear viscosity of polyampholyte-stabilized colloids is proposed and tested against model system measurements. The model is demonstrated to provide a master curve for literature data for the zero-shear viscosity of polyampholyte-stabilized colloidal dispersions. Analysis of the potential of interaction through SANS measurements and rheology, coupled with the observed phase behavior demonstrates the presence of weak attractions attributed to depletion forces arising from free polyampholyte in the suspending medium. The attractions are modeled with the Asakura-Oosawa potential. Independent validation of the potential is provided by quantitative prediction of SANS and phase behavior. A new semi-empirical, predictive model for the low shear viscosity of colloids is proposed and validated against measurements on model dispersions. This rheological constitutive relation is shown to provide an improved description of the low shear viscosity of mixtures of adsorbing polyampholyte and colloids. Excellent quantitative predictions of zero shear viscosity and microstructure validates the approach. The model is extended to other systems in literature to demonstrate the model's efficacy and limitations.; Finally, the effect of polymer on the high shear rheology of colloidal dispersions is examined. Gelatin stabilized systems are found to obey the Cross model for describing their shear thinning behavior. A predictive model for shear thickening in polymer-stabilized dispersions is derived and the model is found to successfully describe the shear thickening transition in polymer stabilized colloids from literature.