Emergent Viscoelasticity in Polyelectrolyte Complex Coacervates: Relaxation, from Monomer to Entangled Polymer Chains*

Polyelectrolyte complex coacervates, PECs, are associated charged polymers with charge pairing between positive, Pol⁺, and negative, Pol^-, repeat units. The Pol⁺Pol^- dynamic physical crosslinks are responsible for increasing the viscosity of PECs on the micro- and macro- scale. Even in liquid-like PECs, often termed coacervates, relaxation times spanning 10 orders of magnitude are found at various length scales. This presentation will discuss the measurements of Pol⁺Pol^- pair relaxation time, about 10^-9 s, to entangled polymer viscosity with an entanglement time of ~50 s. In PECs without added salt the mechanism of segment relaxation is much slower: pair exchange, with a lifetime of about 0.3 s. This lifetime controls the dynamics of the rest of the chain. “Doping” by added salt breaks these Pol⁺Pol^- pairs and accelerates chain motion, while having little influence on the pair lifetime. Therefore, “saloplasticity,” the plasticization of PECs with salt doping, stems from a (completely reversible) reduction in crosslink density and not from a decrease in the Pol⁺Pol^- lifetime by electrostatic screening of charges. A full picture of polymer coils within PECs, obtained by small angle neutron scattering of deuterated polyelectrolyte, enables quantitative analysis according to “sticky reptation” models.