Pinching Dynamics and Extensional Rheology of Polyelectrolyte Solutions

Biological macromolecules like proteins, DNA and polysaccharides, as well as many industrial polymers, are classified as polyelectrolytes, for in solution, the repeat units in their backbone have charge-bearing groups, surrounded by a cloud of counter-ions. Many polyelectrolytes are used as rheology modifiers in paints, pharmaceuticals, and cosmetics. Understanding of, and control over, the influence of added polyelectrolytes on complex free surface flows encountered in printing as well as spraying, dispensing, and microfluidic drops formation are both scientifically and technologically significant problems. Quantitative studies of capillary-driven thinning and pinch-off dynamics of semidilute polyelectrolyte solutions, and their response to extensional flows are relatively rare, and are the focus of this contribution. Here the pinch-off dynamics and extensional rheology of three model polyelectrolytes: poly(sodium 4-styrenesulfonate) (NaPSS), poly(acrylic acid) (PAA), and sodium carboxymethyl cellulose (NaCMC) in different solvents and ionic systems are characterized by using Dripping-onto-Substrate rheometry. Unlike shear relaxation time that decreases with increasing concentration for unentangled, semidilute solutions, the extensional relaxation time of PAA increases with an exponent of 1/2 and exhibits 3/2 for entangled systems. The specific viscosity of all three polymers show higher sensitivity to salt and glycerol in semidilute unentangled solutions compared to entangled systems, whereas the influence on extensional relaxation time and viscosity is less prominent. Using NaPSS in glycerol/water, we show that the specific viscosity and extensional relaxation time values obtained on addition of salt normalized by their salt-free value converge to universal master-curves if plotted as a function of ratio of salt concentration to molar concentration of monomers. Lastly, we find that the addition of glycerol changes the concentration-dependent increase in specific viscosity, and overall change in extensional viscosity due to corresponding changes in dielectric constant and solvent viscosity. The experiments and analysis show how the interplay of stretching due to electrostatics and hydrodynamics influence the shear and extensional rheology response as well as pinching dynamics and processability of polyelectrolyte dispersions. This contribution details how the influence of polymer (type, concentration, molecular weight), salt and solvent affect the rheology of polyelectrolyte dispersions and is expected to help in advancements of macromolecular engineering of multicomponent formulations.