Effect of divalent ions on mixtures of like-charged polyelectrolytes

We study the rheology of mixtures of carboxymethyl cellulose and polyacrylate, two anionic polyelectrolytes in the presence of monovalent and divalent ions of the alkaline earth group. M²⁺ are shown to promote interactions between the two systems, the nature of which depends on the specific interactions between the carboxyate groups and the metal cations. Addition of NaPA to M²⁺CMC in salt-free solution leads to a non-monotonic dependence of the viscosity, entanglement modulus and longest relaxation time of the mixtures, presumably arising from an interplay of single chain collpase due to electrostatic screening and network strengthening resulting from associative interactions.