Solvent Properties Influence the Rheology and Pinching Dynamics of Polyelectrolyte Solutions: Thickening the Pot with Glycerol and Cellulose Gum

Abstract

Charge-bearing groups that ionize in solution influence the macromolecular conformations, interactions, and dynamics of polyelectrolytes. The rich and complex shear rheological response of polyelectrolyte solutions and the influence of salt and pH are the focus of many exquisite theoretical and experimental studies. Surprisingly, the influence of solvent properties on the rheological response of polyelectrolyte solutions is not well studied, which motivates this study. Here, we tune the solvent properties by varying the glycerol fraction in glycerol/water mixtures used for dissolving cellulose gum or sodium carboxymethyl cellulose (NaCMC). The rate-dependent steady shear viscosity measured using torsional rheometry shows that cellulose gum and glycerol thicken the pot of aqueous food or pharmaceutical formulations, but the effect is not additive. The thickening behavior is often assessed or tuned in applications and processing by contrasting the delay in pinching of necks formed by dripping or stretching of liquid bridges. As streamwise velocity gradients associated with extensional flows arise in such pinching necks, extensional rheology measurements are needed but are rare due to the longstanding characterization challenges. Here, we utilize dripping-onto-substrate (DoS) rheometry protocols we developed to visualize and analyze the capillarity-driven pinching dynamics and characterize the extensional rheological response. We characterize the influence of NaCMC concentration and glycerol fraction on the shear and extensional rheological response and pinching dynamics of semi-dilute polyelectrolyte solutions. We find that solvent choice and properties offer pragmatic options for macromolecular engineering of formulations in the food, pharmaceutical, and personal care industries that rely on polyelectrolytes as additives and thickeners.

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