Solution rheology of polyelectrolytes with monovalent and divalent counterions

We report solution rheology data for the sodium, magnesium and calcium salts of polystyrene sulfonate (PSS) and carboxymethyl cellulose (CMC) in salt-free aqueous solutions as a function of polymer concentration and molar mass. Below the overlap concentration, both polymers are found to exhibit rod-like conformations for high degrees of polymerization. The chain dimensions for the divalent salts are significantly smaller than for the monovalent ones, suggesting larger electrostatic blobs. This is consistent with the stronger binding of divalent ions to the chains inferred from conductivity data. In the semidilute regime, the correlation length for NaCMC and the various salts PSS follows the scaling prediction of ξ ∼ c^−1/2.  The divalent salts of CMC do not exhibit a peak in their scattering function and therefore extracting the correlation length is challenging.

The variation of the specific viscosity with the degree of  polymerization in the unentangled regime  (η_sp ∼ N) is consistent with Rouse-like dynamics for both monovalent and divalent salts of PSS. At low polymer concentrations Fuoss-law behaviour (η_sp ∼ c^1/2) is observed. At intermediate and high polymer concentrations, the specific viscosity displays a much stronger concentration dependence as the results of entanglements and/or local friction effects. The lower effective charge density observed for the divalent salts of CMC enhances interchain associations, which in turn facilitates gel formation at high polymer concentrations.