Linear and nonlinear viscoelastic properties of aqueous solutions of cationic polyacrylamides

Linear and nonlinear viscoelasticity of aqueous solutions of cationic polya crylamides at different levels of salt addition have been examined over an extended polymer concentration regime. The polymers have different charge d ensity and the molecular weight of the polymers is high. The rheological fe atures of the systems depend on polymer concentration, charge density, and amount of added salt. The frequency dependences of the dynamic moduli canno t be described by a simple Maxwell model, but the rheological behavior is m ore complex with a distribution of relaxation modes. The power laws, descri bing the polymer concentration dependence of the longest relaxation time an d 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 polyme rs was observed. Nonlinear shear thinning behavior is found for all systems at higher shear rates. The magnitude of this effect depends on polymer con centration, charge density, and level of added salt. Thixotropy is only obs erved for solutions of the polymer of high charge density in the absence of added salt. Both, the linear and nonlinear rheological properties elucidat e the intricate interplay between polyelectrolyte effects and entanglements .