Oppositely charged polyelectrolytes in solution spontaneously associate into hydrated complexes or coacervates, PECs. The morphology, stability, and properties of PECs depend strongly on their ion content, which moderates the “sticky” reversible interactions between Pol+ and Pol– oppositely charged repeat units. Here, it is shown that the distribution of ions between a PEC and the aqueous solution in which it is immersed is accurately predicted by the Donnan equilibrium. For ideal, stoichiometric mixing of polyelectrolytes, corresponding to an enthalpy of complexation ΔH PEC → 0, the salt, MA, concentration inside the PEC, [MA]PEC, is equal to the solution salt concentration, [MA]s. Isothermal calorimetry measurements along a Hofmeister series show that if mixing is exothermic, [MA]PEC < [MA]s, while for endothermic association of Pol+ and Pol–, [MA]PEC > [MA]s. A set of simple self-consistent expressions illustrate PEC salt response without consideration of net Coulombic or electrostatic forces between charged species. ΔH PEC exactly predicts deviations from ideal Donnan equilibria, which are connected to the equilibria between associated or intrinsic pairs of Pol+Pol– and extrinsic Pol+A– and Pol–M+ pairs, where counterions compensate polyelectrolyte charges. The equilibrium constant K pair for Pol+Pol– pair formation is shown to be proportional to the volume charge density of the hydrated, ion-free complex. K pair may also be used to estimate the critical salt concentration at which polyelectrolytes completely dissociate.

The linear viscoelastic responses for a series of polyelectrolyte complexes, PECs, made from pairs of poly [3-(methacryloylamino)propyltrimethylammonium chloride], a polycation, and poly(sodium methacrylate), a polyanion, having various molecular weights were measured. Time−temperature superposition (TTS) for broad and narrow molecular weight distributions revealed entangled behavior at low salt concentration for the longer polyelectrolytes studied. All characteristic lifetimes were slowed by "sticky" dynamics of positive, Pol + and negative, Pol − , pairing. Time−temperature−salt doping superposition (TTSS) was achieved by considering the dual effects of increasing salt concentration on PECs: the partner lifetimes of Pol + and Pol − were inversely proportional to [NaCl], as was the population of Pol + Pol − pairs. Relaxation times for polymer partnering, entanglement, and reptation were measured directly on some systems. Whereas the intrinsic (in the absence of salt ions) lifetime for Pol + Pol − pairs was determined to be on the order of 1 × 10 −4 s, salt doping provided a faster, extrinsic, channel for relaxation at the monomer scale. The time−salt shift factor was decomposed into contributions from Pol + Pol − partner lifetimes, the number density of Pol + Pol − pairs, and the volume fraction of polymer.

The spontaneous association of oppositely charged polyelectrolytes is an example of liquid−liquid phase separation. The resulting hydrated polyelectrolyte complexes or coacervates, both termed "PECs", display a wide range of viscosities. In addition to the usual dependence of viscosity on molecular weight and volume fraction expected for condensed neutral polymers, PECs also contain dense charge pairing between positive, Pol + , and negative, Pol − , repeat units. These "stickers" slow polymer chain dynamics on multiple length scales. Pol + Pol − charge pairs may be broken by the addition of salt to solutions contacting PECs, reducing viscosity ("saloplasticity"). Here, the dynamics of matched pairs of a polycation, poly(methacryloylaminopropyltrimethylammonium chloride), and polyanion, sodium poly(methacrylate), with molecular weights considerably above the entanglement concentration, were measured as a function of temperature and salt concentration. The dynamics of NaCl ions in PECs were also determined and correlated to the segmental relaxation times, which control viscosity. A suite of relaxation times corresponding to ion, monomer, Pol + Pol − pair exchange, entanglement, and reptation was determined or estimated. The zero-shear viscosity, η 0 , was found to be an unusually strong function of molecular weight, with the scaling η 0 ∼ M 5 . A polymer coil size, measured by small-angle neutron scattering, was used in concert with new quantitative expressions to provide a good fit of theory to experiment for this unusual scaling.

The properties of polyelectrolyte complexes, PECs, made from blended polycations, Pol+, and polyanions, Pol–, are routinely studied under conditions where they are at least partially swollen with water. Water plasticizes PECs, transforming them from an intractable, glassy, and brittle state when dry to tough and viscoelastic when wet. In the present work the supreme efficiency of water, compared to other solvents on a polarity scale, in swelling a PEC is illustrated. Using a PEC of poly­(diallyl­dimethyl­ammonium) and poly­(styrene­sulfonate) with precisely determined density, we show that swelling tracks a Dimroth–Reichardt polarity scale until the molecular volume exceeds ∼50 Å3, whereupon the degree of swelling drops sharply. Long-term (>1 year) swelling of this PEC in pure water reveals an instability, wherein the material substantially inflates, generating large pores even though T < T g. The mechanism for this instability is attributed to a small population of counterions, resulting from slight nonstoichiometry of polyelectrolytes, as well as the polymers themselves, a contribution estimated using Des Cloizeaux’s theory of osmotic pressure for overlapping chains. Low concentrations of salt in the bathing solution are enough to overcome the osmotic pressure within the PEC, and it remains dimensionally stable over the long time periods studied. The universal practice of rinsing PECs, whether they are in macroscopic or thin-film morphology, in pure water should be re-evaluated.

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