Water-, salt-, and temperature-induced phase transitions in polyelectrolyte complexes

Oppositely charged polyelectrolytes may experience a solid-liquid (precipitate) or liquid-liquid (coacervate) phase separation upon aqueous assembly. This phase separation is influenced by polymer properties such as molecular weight and charge density, solvent properties such as pH and ionic strength, and environmental properties such as temperature and humidity. Understanding polyelectrolyte complexes is valuable for advancing drug delivery systems, underwater adhesives, and multi-compartment cells. Many theories define the coacervate-solution phase boundary exist, but a gap in the knowledge of the coacervate-precipitate boundary remains. In this study, using turbidity, conductivity measurements, thermogravimetric analysis (TGA), and optical microscopy (OM), we identify the phase boundaries of poly(diallyldimethyl ammonium)/poly(acrylic acid) complexes. We observe reversible temperature-induced phase transitions using OM and quantify the water content in both macro- and micro-phase separated coacervates as a function of temperature and salt. These results provide more understanding of salt, water, and polymer contributions to phase transitions.