Dilute polyelectrolyte solutions: recent progress and open questions

Abstract

Polyelectrolytes are a type of polymers possessing ionic groups on their repeating units. Since counterions can dissociate from the polymer backbone, polyelectrolyte chains are strongly influenced by the electrostatic interactions....

Rheological Properties of Concentrated Sodium Polystyrenesulfonate in Aqueous Salt Solutions

“…In this article, we report rheological measurements for aqueous solutions of sodium polystyrenesulfonate (NaPSS) with added sodium chloride (NaCl), the canonical model polyelectrolyte system. While dilute and modestly concentrated NaPSS solutions in excess NaCl conform to the widely accepted view of neutral polymer-like behavior, ,− we observe several unexpected phenomena in the high added salt, high polymer concentration region: 1) the specific viscosity of solutions increases with increasing added salt concentration. 2) The specific viscosity scales with molar mass following the reptation exponent of η sp ∝ M w 3 in excess salt, and η sp ∝ M (Rouse) in DI water, 3) Solutions display strong and sudden shear thickening behavior reminiscent of shear-induced gelation and discontinuous shear thickening of colloids , that is typically attributed to jamming, 4) Step strain-rate experiments show that beyond a critical shear rate, the apparent viscosity of solutions becomes unstable after an induction time, the value of which depends on the applied shear rate.…”

“…The R g values at c s = 3 M correspond to R g 2 /N ≃ 15.5 Å 263 and R g 2 /N ≃ 11.5 Å 2 (our work), which are ∼20−40% higher than the dimensions of NaPSS under θ-solvent conditions. 2 Increases in the specific viscosity with increasing added salt have been reported by Matsumoto et al for poly(ionic liquids) in ionic-liquid/organic solvent solutions. 65−67 These data, which have been interpreted in terms of the underscreening theory of Perkin and co-workers 68,69 show qualitatively different behavior from ours in that the onset of the η sp increase occurs at fixed ionic strength, so that at low polymer concentrations, c s,u is essentially independent of the polymer concentration, in contrast to the behavior observed in Figure 1c.…”

“…When a salt is added to the solvent media, the mobile ions preferentially locate themselves around oppositely charged ones, leading to Debye–Hückel type screening, with a characteristic spatial range approaching the Debye screening length κ –1 . For sufficiently high ionic strengths, electrostatics are predicted to become short-ranged and their effects are similar to excluded volume interactions in nonionic polymers. ,, The analogy between polyelectrolytes in excess salt and neutral polymers in good solvent is supported by extensive data sets for the conformational and hydrodynamic properties of dilute polyelectrolytes. ,− Data for nondilute solutions, which generally cover only modestly high polymer and added salt concentrations, broadly support this view. ,− …”

“…In salt-free polar solvents, counterions dissociate from the polymer, leading to strong repulsion between like-charged groups along the backbone via long-ranged electrostatic forces, and chains adopt highly extended configurations. 1,2 Their thermodynamic properties are controlled by dissociated counterions, leading to osmotic pressures several orders of magnitude larger than neutral polymers 3−5 and phase boundaries which are largely independent of the chain molar mass, a unique feature among polymeric systems. 6 The rheology of salt-free polyelectrolytes displays several unusual features, including the existence of a broad concentration range over which chains interpenetrate but do not entangle.…”

“…They share properties both with simple electrolytes and with nonionic polymers. In salt-free polar solvents, counterions dissociate from the polymer, leading to strong repulsion between like-charged groups along the backbone via long-ranged electrostatic forces, and chains adopt highly extended configurations. , Their thermodynamic properties are controlled by dissociated counterions, leading to osmotic pressures several orders of magnitude larger than neutral polymers − and phase boundaries which are largely independent of the chain molar mass, a unique feature among polymeric systems . The rheology of salt-free polyelectrolytes displays several unusual features, including the existence of a broad concentration range over which chains interpenetrate but do not entangle. ,,− By contrast, in nonpolar media with low permittivity, the so-called ionomer regime, dipolar attraction between groups with condensed counterions dominates. − Temporary associations between chains increase the relaxation times in solutions and in the bulk state, yielding markedly different properties from those of polyelectrolytes .…”