Charge Density- and Hydrophobicity-Dependent Dynamics of Polyelectrolyte Complex Coacervates

We use a library of polyacrylamide-based polymers to investigate the roles of charge density and hydrophobicity in determining the phase behavior and viscoelasticity of poleyelectrolyte complex coacervates. The polymers, which are made by post-polymerization functionalization of poly(N-acryloxy succinimide), have charged monomer fractions between 60 and 100%, with the balance comprised of either hydrophilic (acrylamide) or hydrophobic (butyl acrylamide) comonomers. We identify the critical salt concentration of the coacervates by optical turbidity, and characterize their degree of swelling and viscoelasticity via thermogravimetric analysis and small amplitude oscillatory shear rheology, respectively. We find that as the charge density of the polymers is decreased, the critical salt concentration and volume fraction of polymer in the coacervates also decrease, and the relaxation dynamics speed up. Interestingly, these properties depend only on the charge density, and not on the hydrophobicity of the comonomer. This suggests that hydrophobic interactions are much weaker than the entropic forces driving coacervate formation, and may provide a new window into understanding the extent to which polymer concentration and electrostatic interactions determine coacervate dynamics.