Impact of divalent ions on the rheology and aggregation of semidilute polyelectrolyte solutions

We report rheology and light scattering data for the Na+, Mg2+, Ca2+, Mn2+, Co2+, Ba2+ salts of carboxymethyl cellulose in aqueous solutions. The viscosity as a function of molar polymer concentration falls into a single curve for all divalent salts. Compared NaCMC, divalent salts display a lower viscosities at low concentrations (in the non-entangled regime), suggesting less expanded chains. Above the entanglement crossover, solutions with divalent counterions display viscosities an order of magnitude larger than NaCMC because interchain crosslinks form by electrostatic bridging.

DLS measurements on semidilute reveal a bimodal decay function, where the relative amplitudes of the two modes vary with counterion valence, size as well as with the filter size employed and the time after filtration. These variables (except for counterion valency) do not affect the solution viscosity, indicating that polyelectrolyte clusters contain a small fraction of the total number of chains in solution.