Phase Behavior and Viscoelasticity of Polyelectrolyte Coacervates Above the Binodal

Preparing coacervate samples at compositions above the binodal allows the polymer and salt concentrations in these materials to be varied independently. Here, we exploit this approach to investigate the dynamics of polystyrenesulfonate (PSS)/poly(diallyldimethylammonium chloride) (PDADMAC) coacervates with salt concentrations between 1.2 and 2 M and volume fractions of polymer between 0.1 and 0.25. Small-amplitude oscillatory shear rheology reveals that the relaxation times scale significantly more strongly with polymer volume fraction than has been previously assumed, highlighting the need to account for both salt and polymer-dependent contributions to coacervate dynamics. Additionally, we identify a second critical salt concentration above which the coacervates phase-separate again at high salt concentrations. Thermogravimetric analysis reveals significant differences in the tie line slope in this high-salt phase window compared to those seen in the low-salt regime. These results demonstrate that intentionally moving above the binodal is a powerful approach to understanding the composition dependence of complex coacervates and offers exciting new opportunities for understanding the physics of these systems.