Effect of Mixed Solvents on the Salt Resistance of Polyelectrolyte Complexes

Strongly interacting polyelectrolyte complexes (PECs) are a versatile class of materials whose physical states can be driven from glassy solids into low-viscosity liquid coacervates upon the addition of salt. However, many of these materials can display high stability towards salt doping using common monovalent salts, leading to general difficulties in accessing the entire spectrum of the complex-coacervate continuum to realize fully their potential in intended applications. In this work, the model PEC system we studied, composed of two strong styrenic polyelectrolytes, required exceptionally high salt concentrations, relative to more conventional polyelectrolyte pairings, to drive the complex from the solid to the fluid phase and ultimately into homogeneous solution. We discovered an unconventional approach of dramatically weakening the salt resistance of this system through switching the solvent from pure water into co-solvents of water and a fully miscible organic solvent constituent. Relying on thermalgravimetric analysis, we were able to systematically depict the binodal phase behaviors of this complex self-assembled under different co-solvent conditions. We believe the results we present in this study provide useful new insights into enriching and controlling the properties and functionalities of PECs through changing the solvent environment.