Rheology changes of Laponite aqueous dispersions due to the addition of sodium polyacrylates of different molecular weights

The rheology, at rest and under flow, and the zeta potential of aqueous Laponite dispersions are studied in the presence of sodium polyacrylates of different molecular weights. Oscillatory measurements, performed in the linear viscoelastic region, show that the storage modulus is reduced significantly in presence of low molecular weight polyacrylate, while it shows similar values to the dispersion with no sodium polyacrylate in presence of higher molecular weights. The rheological behavior under flow was determined by applying sudden shear rate-changes, from which the steady state viscosities, the start-up viscosities and the thixotropic behavior were determined: (1) the steady state viscosities were found to fit the Oswald-de Waele power law model. (2) After the sudden shear rate-changes, it was possible to model the start-up viscosities, by introducing the concept of structural viscosity and aggregate alignment. The aggregate alignment was found to fit a power law with the applied shear rate. High molecular weights of sodium polyacrylate promoted aggregation or more structured aggregates because polyacrylate chains are long enough to form interparticular bridges connecting the Laponite particles. (3) The thixotropic times the net aggregate breakdown and buildup process follow a first order kinetic equation.