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High molecular weight polyelectrolytes are commonly used as drag reducers, viscosity enhancers, and drilling fluids. An understanding of polyelectrolyte solution rheology under various solvent conditions is critical to their efficient use in industry. Currently, the dynamics of entangled polyelectrolyte solutions are poorly understood. Polyelectrolyte concentrations ranging from dilute to concentrated in both water and salt solution were studied using oscillatory and shear rheology. The viscosity as a function of concentration for xanthan gum in both salt free solution and in 50 mM NaCl was measured and compared to a scaling theory for polyelectrolytes. In general, the zero shear rate viscosity and the degree of shear thinning increase with polymer concentration. In addition, shear thinning was observed in the dilute regime in both solvents. In salt free solution, four concentration regimes of viscosity scaling and three associated critical concentrations were observed (c* = 70 ppm, ce = 400 ppm, and cD = 2000 ppm). In salt solution, only three concentration regimes and two critical concentrations were observed (c* = 200 ppm and ce Å 800 ppm). In the presence of salt, the polymer chain structure collapses and occupies much less space resulting in higher values of the overlap and entanglement concentrations. The observed viscosity-concentration scaling is in very good agreement with theory in the semidilute unentangled and semidilute entangled regimes in both salt free and 50 mM NaCl solution.

Next, the viscosity of xanthan and several other polyelectrolytes was measured in both salt free solutions and solutions in the high salt limit. At low polymer concentrations (i.e., semidilute unentangled and dilute regimes), the zero shear rate viscosity decreases as much as 100-fold upon addition of salt. However, as polymer concentration increases, the viscosity difference between polymer in salt free and in salt solution diminishes. Next, the zero shear rate viscosity becomes independent of added salt at the critical polyelectrolyte concentration cD. Above cD, the addition of salt increases the zero shear rate viscosity for xanthan (up to 300%), carrageenan (140%), welan (35%), and hydrolyzed polyacrylamide (10%). Further, the magnitudes of the dynamic moduli increase with addition of salt above cD. The increase in viscosity in salt solution and magnitude of cD appear to be heavily influenced by the molecular characteristics of the polymers such as monomer weight and molecular structure.

Next, rheological properties of entangled xanthan solutions above the critical concentration cD were examined in a number of inorganic salt solutions of varying ionic size and charge. The effect of salt counterion (i.e., anion) size and valency on the magnitude of the viscosity increase is elucidated. A hypothesis that larger salt counterions produce higher viscosities is confirmed for both monovalent and divalent salts. For counterions of similar ionic radius and differing valency, the counterion carrying the higher charge produces a higher viscosity. Lastly, an alternative hypothesis incorporating ion bridging between polymer chains is proposed to explain the effect of counterion valency in the observed viscosity differences.

Finally, the drag reduction properties of the polyelectrolyte xanthan are explored in differing solvent environments (salt free versus salt solution) and delivery configurations (homogeneous versus stock solution dilution). The ability to reduce the frictional drag in turbulent flow in pipes and channels by addition of a small amount of a high molecular weight polymer has application in myriad industries and processes. In a turbulent pipe flow, significant drag reduction is measured for xanthan concentrations as low as 20 ppm. Drag reduction effectiveness is greatly increased when an entangled xanthan solution is diluted to a concentration in the dilute regime (less than 70 ppm) compared to solutions prepared in the dilute regime. The increased drag reduction effectiveness is hypothesized to be attributed to residual entanglements and network structure present in the diluted solution. Dynamic rheological measurements of the elastic modulus (GÕ) agree with the hypothesis of residual entanglements as the diluted solutions exhibit greater elasticity than solutions prepared at the dilute concentration. Drag reduction effectiveness is unchanged by the presence of salt when the stock solution concentration is sufficiently above the critical concentration cD. Lastly, the drag reduction effectiveness decreases with time when diluted from an entangled stock solution but remains greater than the homogeneous case after five hours.

In total, one PhD student and three undergraduates have participated in the project. A PhD has been awarded to the graduate student and is currently working at the Sandia National Laboratories. One of the undergraduates (MiKayla Henry) is currently a graduate student in Australia. Professor Liberatore leveraged the PRF grant to start his research group and is now working on applying for promotion and tenure.