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.

Effect of isopropanol cosolvent on the rheology and spinnability of aqueous polyacrylic acid solutions

“…In a previous study by Jimenez et al, 45,46 the addition of glycerol, a less polar solvent than water, was found to indeed result in a deviation of the scaling of aqueous PAA solutions from a salt‐free polyelectrolyte solutions. A similar variation was also observed for NaCMC solutions on the addition of glycerol 45,46 . Our observations here on IPA deviates from these previous reports.…”

“…This observation, however, deviates from previous findings on water–glycerol mixtures for both PAA and NaCMC solutions. In contrast to IPA, the scaling of both was found to be unaffected by the addition of glycerol 45,46 . They have argued such insensitivity of λ E scaling in extension unlike in shear is due to a similar stretched/elongated conformation in extensional flow where the intra‐chain interactions—electrostatic, hydrodynamic, and excluded volume interactions—are screened 45 .…”

“…Furthermore, as discussed before, extensional rheological properties play an important role in the electrospinning process. Previous studies on the extensional rheology of polyelectrolyte solutions in water–alcohol mixtures, though fewer compared to in aqueous media, reported the effect of alcohol on the extensional flow properties to be distinct from shear 44–47 . Despite their importance, previous studies examining the effect of water–alcohol composition on fiber morphology of PAA solutions have only considered shear rheology in correlating to the fiber morphology, and are rarely reported even in studies on using aqueous or other media.…”

“…Despite their importance, previous studies examining the effect of water–alcohol composition on fiber morphology of PAA solutions have only considered shear rheology in correlating to the fiber morphology, and are rarely reported even in studies on using aqueous or other media. Studies focused on extensional rheology are also fewer on polyelectrolyte solutions in water–alcohol mixtures 44–47 compared to in water/polar media 48–51 …”

Salt Effect on the Viscosity of Semidilute Polyelectrolyte Solutions: Sodium Polystyrenesulfonate

“…Examples of polyelectrolytes include biological molecules such as DNA, RNA, or hyaluronic acid as well as synthetic ones such as polystyrenesulfonate, carboxymethyl cellulose, or poly­(acrylic acid). Solution properties of polyelectrolytes are controlled by the chain degree of polymerization, fraction and distribution of the ionic groups, solvent quality for the polymer backbone, polymer and added salt concentrations, and solution pH. − …”