Decoupling the Effects of Charge Density and Hydrophobicity on the Phase Behavior and Viscoelasticity of Complex Coacervates

Complex coacervation is an entropically driven, associative liquid-liquid phase separation that results in a polymer-rich coacervate and a polymer-poor supernatant. A number of studies have looked into the effect of salt concentration on the phase behavior and mechanical response of the resulting coacervate. However, the effect of copolymer chemistry on the phase behavior and viscoelasticity of complex coacervates is not well understood. To understand the influence of copolymer chemistry, we developed a library of oppositely-charged methacrylate copolymers of varying charge density and hydrophobicity, with which we studied the coacervate phase behavior and viscoelasticity as a function of salt concentration. Our results show that polymer charge density and hydrophobicity drastically affect the phase behavior, with charge density dictating the salt stability and hydrophobicity controlling the polymer concentration of the complexes. We used small amplitude oscillatory shear to study the viscoelastic response of complex coacervates, and time-salt superposition to examine the dependance of salt concentration. We take advantage of different copolymer chemistries to construct time-salt-copolymer master curves that have not been shown before in the literature. Our phase diagrams and rheological data show evidence of charge-dominated and hydrophobicity-dominated regimes. Finally, we highlight how copolymer chemistry can be used to tune the mechanical properties of complex coacervates.