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.