Charge Regulation Modulates Polyelectrolyte Viscoelasticity

Polyelectrolytes exhibit unique viscoelastic responses compared to uncharged polymers owing to electrostatic interactions of charges along their backbone. While standard interpretations of polyelectrolyte viscoelasticity assume passive, fixed polyelectrolyte charge, the actual polyelectrolyte charge can depend sensitively on the solution conditions and the polyelectrolyte extension state. Clearly, the electrochemical coupling of charge adsorption reactions can lead to salt-specific, pH and salt concentration dependent rheological properties of polyelectrolyte solutions, beyond non-specific screened electrostatic interactions. In fact, some previous studies found salt-specific viscosity and relaxation times in polyelectrolyte solutions, although no quantitative model was presented to explain the observed trends.



Here, we evaluate the roles of charge regulation via ionic adsorption and ionic screening on the rheological properties of aqueous polyelectrolyte solutions. Using steady shear and small amplitude oscillatory shear experiments, the viscoelasticity of polyelectrolyte solutions are probed with varying salt and polyelectrolyte concentrations, salt and polyelectrolyte identities, and pH. The behavior of polyelectrolytes with acid-base chemistry are contrasted with those that bind salt counterions through salt-specific reactions. We develop a theoretical model for dilute polyelectrolyte solutions, coupling polyelectrolyte extension to electrostatic interactions and ionic adsorption reactions, which is required in order to capture the trends across salts and polyelectrolytes. Ultimately, our analysis supports a framework in which the mechanics of viscoelastic fluid flow is coupled to microscopic electrostatic interactions that are modulated by the specific chemistry of the polyelectrolyte charge groups.