Session X36: Fantastic Polyelectrolytes and How They Behave in Coacervates

Structure and Dynamics of Polyelectrolyte Solutions and Coacervates

The shear rate dependence of viscosity is used to evaluate the concentration dependences of specific viscosity, relaxation time and terminal modulus for several polyelectrolytes in three solvents: deionized water, ethylene glycol and glycerol. Small-angle X-ray scattering determines the correlation length of these polyelectrolyte solutions from a peak in their scattering function, which changes as expected with solvent dielectric constant. In semidilute unentangled solutions, dynamics are described by the Rouse model and combined with the correlation length in different ways to determine the number density of chains, which allows calculation of the number-average molecular weight using a robust method that is insensitive to the presence of salt. The high viscosity glycerol solvent allows characterization of the viscoelastic response of high molecular weight polyelectrolytes over a wide range of frequency, showing Rouse character in unentangled solutions and a clear rubbery plateau in strongly entangled solutions.
A simple way to make a polyampholyte gel is to mix oppositely charged polyelectrolytes, which precipitate to form a sediment that is termed a coacervate. Hydrophilic coacervates (with roughly 70 % water in them) are reversible gels with labile ionic interactions and their dynamics are described by a sticky Rouse model. In both examples, while not everything is understood for these complicated forms of condensed matter, polymer physics can be used to gain important insights.
  

Phase Behavior and Viscoelasticity of Polyelectrolyte Coacervates at High Salt Concentrations

Separating the salt and polymer volume fraction-dependent dynamics of complex coacervates is challenging because changing the salt concentration of the sample typically also changes the polymer concentration in the polymer-rich phase. Here, we describe a way to address this challenge using a “salt-addition” method for preparation of complex coacervates that allows us to increase the salt concentration of the coacervate without significantly changing the polymer volume fraction. Using this method, we investigate the rheology of polystyrenesulfonate (PSS)/poly(diallyldimethylammonium chloride) (PDADMAC) coacervates at salt concentrations near and above the critical salt concentration. We find that the dependence of the relaxation times on salt concentration is similar to that observed in previous studies, but that the relaxation times scale significantly more strongly with polymer volume fraction than previously assumed. Additionally, we identify a second critical salt concentration above which the coacervates separate at high salt concentrations, and use thermogravimetric analysis to characterize the phase behavior of these materials in the high-salt regime. These results demonstrate that the salt addition method is a powerful approach for exploring and identifying new behaviors of coacervates in the high salt limit.   

Fantastic Entanglements between Polyelectrolytes in Solutions

We study the entanglement properties of flexible and semiflexible polyelectrolytes sodium polystyrene sulfonate and sodium carboxymehtyl cellulose. In dilute salt-free and excess-salt solutions these polymers adopt rod-like and expanded coil conformations respectively. The solvent’s ionic strength has a large impact on the conformation of polyelectrolytes, and the number of binary interchain contacts, quantified through the intrinsic viscosity and osmotic pressure respectively. While adding salt leads to a large decrease in polymer size, it does not affect their entanglement density and entanglement crossover. This contradicts packing models of polymer entanglement and suggests that the density of binary contacts in solution is not affected by the solvent quality. Based on this observation, we work out the reptation dynamics of polyelectrolytes in salt-free solution, which differ appreciably from earlier models.   

Structure and rheology of polyelectrolyte complex coacervates

Polyelectrolyte complexes are highly tunable materials that span from low-viscosity liquids (coacervates) to high-modulus solids with high water content, making them attractive as surface coating, membrane purification and bioadhesive materials. However, most of their properties and their effects with salt, pH, polymer ratio and temperature have only been qualitatively described. Here, we present an investigation of the structure and chain conformations, and rheological properties of polyelectrolyte complex (PEC) coacervates comprising biomimetic model polyelectrolytes. Systematic studies using small-angle X-ray scattering (SAXS) of the structure and chain behavior in liquid PEC coacervates revealed a physical description of these materials as strongly screened semidilute solutions of polyelectrolytes comprising oppositely charged chains. At the same time, solid PECs were found to be composed of hydrogen-bonding driven stiff ladder-like structures with large correlation lengths. While the liquid complexes behaved akin to semidilute polyelectrolyte solutions upon addition of salt, the solids were largely unaffected by it. Terminal relaxations of the chains in PEC coacervates were explored by rheology measurements. Excellent superposition of the dynamic moduli data was achieved by a time-salt superposition.
  

Fantastic Saloplastic

Oppositely-charged polymers spontaneously assemble into amorphous complexes or coacervates, driven mainly by the release of counterions. Macromolecules within these polyelectrolyte complex/coacervates, PECs, are well blended due to pairing of oppositely-charged repeat units Pol⁺ and Pol^-. These charge pairs can be quickly and reversibly broken by the addition of salt to the solution in which PECs are immersed, reducing their bulk modulus. This reversible doping by salt, termed saloplasticity, controls all of the physical and mechanical properties of PECs. This talk will illustrate the concept and origin of saloplasticity and show how a “stick association” theory of polymers can be extended to PECs to predict their dynamics as a function of salt doping.