Linear and nonlinear viscoelastic properties of aqueous solutions of cationic polyacrylamides

Linear and nonlinear viscoelasticity of aqueous solutions of cationic polyacrylamides at different levels of salt addition have been examined over an extended polymer concentration regime. The polymers have different charge density and the molecular weight of the polymers is high. The rheological features of the systems depend on polymer concentration, charge density, and amount of added salt. The frequency dependences of the dynamic moduli cannot be described by a simple Maxwell model, but the rheological behavior is more complex with a distribution of relaxation modes. The power laws, describing the polymer concentration dependence of the longest relaxation time and the zero-shear viscosity, revealed a stronger concentration dependence of these parameters as the salt concentration increased. At sufficiently high salt concentration, a behavior resembling that of entangled neutral polymers was observed. Nonlinear shear thinning behavior is found for all systems at higher shear rates. The magnitude of this effect depends on polymer concentration, charge density, and level of added salt. Thixotropy is only observed for solutions of the polymer of high charge density in the absence of added salt. Both, the linear and nonlinear rheological properties elucidate the intricate interplay between polyelectrolyte effects and entanglements.