Increasing viscosity in entangled polyelectrolyte solutions by the addition of salt

Abstract

The viscosity of several polyelectrolytes is measured in both salt free solutions and solutions in the high salt limit. At low polymer concentrations, the zero shear rate viscosity decreases as much as 100-fold upon addition of a monovalent salt, namely NaCl. However, as polymer concentration increases, the viscosity difference between polymer in salt free and in monovalent salt solution diminishes. Further, the zero shear rate viscosity becomes independent of added monovalent salt at the critical polyelectrolyte concentration c_D. Above c_D, the addition of monovalent salt increases the zero shear rate viscosity of the entangled polyelectrolyte solutions. The viscosity increase agrees with viscosity scaling theory for polyelectrolytes in the entangled regime. Polyelectrolytes exhibiting an increase in viscosity above c_D in the presence of monovalent salt include three natural anionic polyelectrolytes (xanthan, carrageenan, welan), one synthetic anionic polyelectrolyte (hydrolyzed polyacrylamide), and one natural cationic polyelectrolyte (chitosan). Generally, these polyelectrolytes are relatively high molecular weight (>1 M Dalton), which makes c_D experimentally accessible (e.g., c_D = 0.2 wt% for xanthan). The magnitude of the viscosity increase is as high as 300% for xanthan and nearly independent of monovalent salt concentration in the high salt limit. The increase in viscosity in monovalent salt solution and magnitude of c_D appear to be heavily influenced by the molecular characteristics of the polymers such as monomer weight, molecular structure, and chain conformation.

Introduction

Polyelectrolytes are macromolecules whose repeat units bear an ionizable group. In polar solvents, the ionization of these groups along the backbone results in a macromolecule carrying an electrostatic charge. Charged polymer systems are encountered daily in nature in the form of biological polymers such as DNA, polypeptides, and proteins. Polyelectrolytes are also used extensively in many important industrial applications as food additives, flocculants, drilling fluids and drag reducers. Charged polymer solution rheology is complex due, in part, to the polymer’s sensitivity to the presence of ions in solution [1], [2], [3], [4], [5], [6], [7]. Several theories have been proposed to explain the observed differences in properties between polyelectrolytes and neutral polymers [8], [9], [10], [11], [12], [13], [14], [15]. However, the majority of the theories presented are focused on the polymer in the dilute and semidilute concentration regimes. Little theory is available for polyelectrolytes in the entangled and concentrated regimes. Since polyelectrolytes are so vital to myriad different industries, a fundamental understanding of the solution rheology in various solvent conditions and across all concentration regimes is critical.

The dynamics, and therefore rheology, of polyelectrolyte solutions are determined in large part by the configuration of the polyelectrolyte chains in solution. The polymer chain configuration is determined, in turn, by the solution conditions (i.e., solvent quality, ionic strength, temperature, etc.) [3], [9]. Further rheological complexity is added by factors such as chain orientation, chain stiffness, chain overlap and entanglement, and electrostatic interactions between polymer chains (many of which are not well understood for polyelectrolytes). In salt free solution, strong Coulombic repulsions between like charges along the polyelectrolyte backbone stretch and elongate individual chains. The polymers are surrounded by a cloud of counterions that exactly balances the charge on the chains so the solution maintains charge neutrality. The extended configuration of the polymer chains, together with the electrostatic interactions, accounts for some of the interesting rheological properties of polyelectrolyte solutions. Because of the charge balance in salt free solution, the polyelectrolyte is very sensitive to the presence of added salt ions.

The addition of counterions, normally in the form of ionizable salt molecules, screens the electrostatic repulsions between charges along the backbone of the polyelectrolyte chain. The screening of the electrostatic interactions, in turn, allows the chain to fold up and assume a smaller, more compact conformation. The degree to which a polymer molecule’s conformation shrinks may be dependent on the stiffness of the polymer chain in solution [1], [16]. Since the molecular configuration changes, the solution rheology must also change. The addition of salt to polyelectrolyte solutions has been shown to have dramatic effects on the solution rheology [3], [4], [5], [6], [7], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. In the dilute and semidilute concentration regimes, the collapsed polymer chains lead to a large reduction in both viscosity and dynamic rheological properties [1], [7], [22]. If enough salt is added (i.e., high salt limit is attained), electrostatic effects are predicted to be negligible and neutral polymer behavior is predicted [9], [10]. The high salt limit is defined to be the limit where the number of added salt ions is greater than the number of free counterions in solution. Despite these predictions, little experimental work has been done to examine polyelectrolyte rheology and the effect of added monovalent salt on polyelectrolyte rheology in the entangled concentration regime.

Recent publications preface the work presented here and summarize the dilute and semidilute behavior of the polyelectrolyte xanthan in salt free and salt solutions [1], [27], [28]. Specifically, three critical concentrations for xanthan were determined in salt free solution (Fig. 1), namely the overlap concentration c∗, the entanglement concentration c_e, and c_D [1]. Interestingly, the changes in viscosity near the overlap concentration for xanthan also corresponded to a similar increase in the drag reducing properties of the polyelectrolyte in salt free solution [27]. The scaling of the zero shear rate viscosity (η₀) in the semidilute unentangled and semidilute entangled concentration regimes is well described by theory for polyelectrolytes in a good solvent [1]. Above c_D, the viscosity scaling is well described by theory for neutral polymers in a good solvent.

In 50 mM NaCl (i.e., in the high salt limit), xanthan’s zero shear rate viscosity decreased in the dilute and semidilute concentration regimes by as much as 89% (Fig. 1). Further, two critical concentrations, c∗ and c_e, were determined in NaCl solution, and the viscosity scaling agreed well with theory for neutral polymer in a theta solvent [1]. Also, a study of different salts, varying cation size and valency, showed larger increases in viscosity in the entangled regime for divalent cations compared to monovalent ions of the same ionic radius [28]. A recent review of the rheology of dilute and semidilute polymer solutions addresses the current state of the science in this area [29], and clearly states “Entanglement in polyelectrolyte solutions is not yet well understood”. In the present manuscript, details of a new and interesting rheological phenomena in entangled polyelectrolyte solutions is presented.

The predicted viscosity-concentration scaling in the entangled regime (i.e., above c_D in salt free solution and above c_e in monovalent salt solution) is stronger in monovalent salt solution than in salt free solution (14/3 vs. 15/4, Fig. 1). Despite the significantly smaller viscosity of xanthan in NaCl near c_e (∼800 ppm), the scaling theory predicts that the viscosity in the monovalent salt solution should exceed the salt free solution at some higher concentration. The present work confirms the prediction that viscosity of polyelectrolyte solutions can be greater in monovalent salt solution than salt free solution at the same polymer concentration.

In this manuscript, we focus on the changes in rheological behavior of polyelectrolytes in the entangled regime observed when a monovalent salt is introduced into the system. We expand upon the work summarized for xanthan discussed above, then expand the study to include several other polyelectrolytes including natural (both anionic and cationic) and synthetic polyelectrolytes. For each polymer studied, we show that the solution viscosity is measurably increased by adding a monovalent salt for polymer concentrations above c_D. A detailed presentation of steady and oscillatory shear rheology for xanthan is followed by a summary of the viscosity increase for a number of other polyelectrolytes.