Solution Properties of Polyelectrolytes with Divalent Counterions

Divalent ions are known to strongly influence the phase behavior, conformation, and flow properties of polyelectrolytes in solution. Here, we report small-angle neutron scattering and viscosity data for the magnesium salt of polystyrene sulfonate (MgPSS) in salt-free water and compare these results with analogous measurements for NaPSS. In a dilute solution, both salts are found to exhibit a rod-like conformation for high degrees of polymerization, but the chain dimensions of MgPSS are significantly smaller than for NaPSS, which is consistent with the lower charge density of MgPSS found from conductivity data. In the semidilute regime, the correlation length scales as ξ ≃ 3.7c–1/2 nm for NaPSS and ξ ≃ 5.4c–1/2 nm for MgPSS. At high concentrations, deviations to weaker power laws are observed for both salts. The variation of the viscosity with the degree of polymerization (ηsp ∼ N) is consistent with unentangled, Rouse-like dynamics. Fuoss-law-type behavior (ηsp ∼ c1/2) is observed at low polymer concentrations, which turns into a stretched exponential dependence at high c, perhaps due to an increase in the local friction experienced by polymer segments.