Lower Critical Solution Temperature Behavior in Polyelectrolyte Complex Coacervates

In light of recent experimental observations of lower critical solution temperature (LCST) in polyelectrolyte complex coacervates (Ali, S. et al. ACS Macro Lett. 2019, 8, 289-293), we explore its possible mechanisms on the basis of a slight modification of our theory (Adhikari, S. et al. J. Chem. Phys. 2018, 149, 163308). We explore the consequences of the temperature dependence of the solvent dielectric constant (ε) and the solvent-polymer interaction parameter (χ) on the complex coacervates' phase behavior. The results show that the temperature dependence of the solvent dielectric constant and solvent-polymer interaction parameter can result in a complex phase behavior involving two disjoint unstable regions on the temperature (T)-polyelectrolyte concentration (ϕ _p) plane. Comparison of phase diagrams constructed for different possible temperature dependencies of ε and χ shows that the experimentally observed LCST behavior is obtained only if the solvent dielectric constant decreases and the solvent-polymer interaction parameter increases with increasing temperature. Preferential partitioning of salt into the polyelectrolyte poor phase is predicted for all possible combinations of temperature dependencies of χ and ε considered in this work.