Polymer Dynamics, Rheology & Processing: Flexibility, Extensibility, and Charge

Biological macromolecules like proteins, DNA, polysaccharides, and many industrial polymers, are classified together as polyelectrolytes. In solution, the repeat units in their backbone are decorated with dissociated, charge-bearing ionic groups, surrounded by a cloud of counter-ions. Even though a large number of polyelectrolytes are processed or used as rheology modifiers in paints, pharmaceuticals, fertilizers, pesticides, and cosmetics, the shear and extensional rheology response of the charged macromolecular solutions is not as well understood as for their uncharged counterparts, and motivate this study. We characterize the pinching dynamics as well as shear and extensional rheology of solutions of four model polyelectrolytes poly(sodium 4-styrenesulfonate) (NaPSS), poly(acrylic acid) (PAA), xanthan gum (XG), and sodium carboxymethylcellulose (NaCMC) as a function of solvent and salt concentration. We identify universalities in the shear and extensional rheology response of salt-added unentangled semi-dilute solutions and examine the influence of solvent properties on polyelectrolyte dynamics. Unlike shear relaxation time that decreases with an increase in polymer concentration in the unentangled, semidilute salt-free solutions, the measured extensional relaxation time always shows an increase with polymer concentration for the unentangled systems. We elucidate the influence of both electrostatic and hydrodynamic interactions and stretching of macromolecules on stickiness, printability, jettability, stringiness, and overall processability.