Viscosity of bimodal charge-stabilized polymer dispersions

Monodisperse electrostatically stabilized polymer latices with particles di ameters of 310 and 120 nm were synthesized and used to prepare binary mixtu res with bimodal size distribution. The zero shear viscosity of these bimod al, charge stabilized polymer latices was studied at different salt concent rations. In contrast to hard sphere colloidal suspensions, no minimum in vi scosity was found as a function of the mixing ratio of small and large part icles. Instead the viscosity increased when the fraction of small particles increased, which is due to the direct Coulomb interaction. In order to com pare the results with data from hard sphere systems, we used an effective v olume fraction phi (eff). The experimentally determined volume fraction at the divergence of the zero shear viscosity is compared with the hard sphere value in order to define phi (eff). The effective volume of the particles was then used to calculate the effective volume fraction of the binary mixt ures containing small and large particles. When using phi (eff), a minimum of the viscosity was found at a composition of similar to 30% of small part icles similar to the behavior of bimodal hard-sphere suspensions. The volum e fraction at maximum packing could be calculated by theoretical models onl y at high salt concentration. The models underestimate the maximum volume f raction at low salt concentration. (C) 2000 The Society of Rheology. [S0148 -6055(00)00106-1].