Entanglement density and crossovers of polyelectrolyte solutions

We present steady shear and oscillatory rheology data for flexible sodium polystyrene sulfonate (NaPSS) and semiflexible sodium carboxymethyl cellulose as a function of polymer concentration, degree of polymerisation and added salt. Semidilute non-entangled dynamics are well described by the Rouse model, in agreement with Dobrynin's model for flexible polyelectrolytes. Addition of salt leads to a decrease in chain dimensions and therefore in solution viscosity, the effect being much more pronounced for NaPSS than NaCMC, which can be understood as resulting from the intrinsic rigidity of NaCMC preventing chain collapse at high salt.

The entanglement concentration of NaCMC is found to be only weakly dependent on added salt concentration. This contrasts with the overlap crossover which increases by several orders of magnitude from salt-free conditions to 0.1 M NaCl. Further, we find that the entanglement density remains constant upon addition of salt despite significant changes in polymer conformation. Our results for entangled polyelectrolyte in salt-free and salt solutions strongly contradict several predictions of the current scaling models for polymer entanglement.