Effect of Ionic Strength on the Rheological Properties of Multisticker Associative Polyelectrolytes

The rheology of hydrophobically modified polyelectrolytes containing small hydrophobic blocks randomly distributed along the hydrophilic backbone has been investigated in aqueous salt solutions by means of steady-flow, creep, and oscillatory experiments. The polymers contain acrylamide (≈86 mol %), sodium 2-acrylamido-2-methyl-1-propanesulfonate (≈12 mol %), and N,N-dihexylacrylamide units (≈2 mol %). The rheological behavior in the presence of electrolyte for two polymers with two different hydrophobic block lengths (N H = 3 or 7 monomer units per block) was compared to that obtained for salt-free systems. At a fixed salt concentration, the critical concentration at the onset of viscosity enhancement does not depend on the length of the hydrophobic segments in the polymers and is located in the vicinity of the critical overlap concentration of the corresponding hydrophobe-free polymer. This is in strong contrast to the behavior observed for the same polymers in pure water, for which the onset of viscosity enhancement shifts toward lower concentrations as the hydrophobic block length is increased. Below the critical entanglement concentration, the presence of salt influences the dynamics of the polymers, resulting in significant reductions of the zero-shear viscosity and the plateau modulus, the terminal relaxation time being less affected by the addition of electrolyte. In contrast, in the entangled regime, the rheological behavior in salt solutions is not very different from that in the salt-free systems. The properties of these associative polyelectrolytes have been analyzed in the framework of the available theories, that is, either the sticky Rouse model or the sticky reptation model, depending on the concentration range.