Effects of Solvent Entropy on Homopolymer Cononsolvency in Binary Solvent Mixtures Predicted by Flory–Huggins Theory

Cononsolvency generally refers to the phase separation of a homopolymer solution in mixtures of two miscible good solvents. While commonly observed in systems with a lower critical solution temperature (LCST), it also occurs in systems lacking specific interactions or LCST behavior. Recent coarse-grained polymer models and the classical Flory-Huggins (FH) theory used to study cononsolvency consider only chain connectivity, excluded-volume repulsion, and isotropic van der Waals attractions, with constant nonbonded interaction parameters─making them especially suitable for systems exhibiting an upper critical solution temperature (UCST). However, the role of solvent entropy, particularly the size ratio between the solvent and cosolvent molecules, has been largely overlooked. Extending recent work by one of us (Zhang, P. Macromolecules 2024, 57, 4298; ibid. 2025, 58, 2472), we incorporate this entropic effect into the ternary FH theory and find that it significantly broadens the parameter space where cononsolvency occurs in UCST systems.