The Bulk Phase Behavior of Ternary Mixtures of Polymers and Two Good Solvents

Mixed solvents are extensively utilized in diverse processes for characterizing, separating, and assembling polymers. However, understanding the thermodynamics of these polymer solutions remains rudimentary. One example is the so called cononsolvency effect, under which polymers in a good solvent undergo reentrant coil-to-globule-to-coil transition upon addition of a good cosolvent. Only recently, computer simulations have clarified the generic nature of the effect. Yet, the phase behavior of the solutions associated with such effect remains uncharacterized. In this study, we combine the self-consistent field (SCF) calculations and the Gibbs-ensemble simulations that employ identical models to determine the binodal curves of ternary mixtures of polymers and two good solvents. It is found that adding a small amount of cosolvent to solution can induce a 1^st-order phase separation. The coexisting densities predicted by the SCF calculations matches those from the simulations, pointing to the leading role of the mean-field density correlations in such polymer cononsolvency behavior. A transition back to the homogeneous phase is observed as the cosolvent composition further increases that is predicted to be of 1^st-order in the SCF but appears as a 2^nd-order transition in simulations.