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

Cononsolvency is an intriguing phenomenon where a homopolymer solution undergoes phase separation in a binary solvent mixture, even though both solvents are individually good for the polymer and mutually miscible. While typically observed in systems with a lower critical solution temperature (LCST), cononsolvency also occurs in some upper critical solution temperature (UCST) systems. 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 UCST systems. However, the role of solvent entropy, particularly the size ratio between solvent and cosolvent molecules, has been largely overlooked. Extending recent work by one of us [Macromolecules 57, 4298 (2024); 58, 2472 (2025)], we incorporate these entropic effects into the ternary FH theory and find that they significantly broaden the parameter space where cononsolvency occurs in UCST systems.