Effect of counterion and solvent type on the scattering and rheology of polyelectrolyte solutions

Polyelectrolytes are an important class of materials that play a role in many biological processes and find use in various industrial processes. Recently, the role of counter-ion solvation on the properties of polyelectrolyte solutions has received renewed attention. Here we use Small-Angle neutron Scattering (SANS) and rheology to study solutions of carboxymethyl cellulose (CMC) with different counterions of varying size and hydrophobicity in aqueous and organic media. The counterions used belong to two classes: alkaline and tetra-alkyl ammonium.

Scattering profiles display an upturn at low-Q and a correlation peak at mid-Q. With increasing polymer concentration and increasing alkyl chain length, the peak of the TAA salts showed a decreasing prominence. While alkaline salts of CMC are only soluble in water, TBACMC can be dissolved in a wide variety of organic media. We use this feature to study the dependence of the correlation peak and solution viscosity on solvent permittivity. It was observed that with a decreasing solvent dielectric constant, the peak shifted towards lower-Q values and eventually disappeared, giving a scattering profile similar to that of a neutral polymer. Viscosity data are also interpreted as a polyelectrolyte-to-neutral polymer transition.