General Condition for Polymer Cononsolvency in Binary Mixed Solvents

Starting from a generic model based on the thermodynamics of mixing and abstracted from the chemistry and microscopic details of solution components, three consistent and complementary computational approaches are deployed to investigate the general condition for polymer cononsolvency in binary mixed solvents at the zeroth order. The study reveals χ_PS - χ_PC + χ_SC as the underlying universal parameter that regulates cononsolvency, where χ_αβ is the immiscibility parameter between the α- and β-component. Two disparate cononsolvency regimes are identified for χ_PS - χ_PC + χ_SC < 0 and χ_PS - χ_PC + χ_SC > 2, respectively, based on the behavior of the second osmotic virial coefficient at varying solvent mixture composition x _C. The predicted condition is verified using self-consistent field calculations by directly examining the polymer conformational transition as a function of x _C. It is further shown that in the regime χ_PS - χ_PC + χ_SC < 0, the reentrant polymer conformation transition is driven by maximizing the solvent-cosolvent contact, but in the regime χ_PS - χ_PC + χ_SC > 2, it is driven by promoting polymer and cosolvent contact. In-between the two regimes when neither effect is dominant, a monotonic response of polymer conformation to x _C is observed. Effects of the mean-field approximation on the predicted condition are evaluated by comparing the mean-field calculations with computer simulations. It shows that the fluctuation effects lead to a higher threshold value of χ_PS - χ_PC + χ_SC in the second regime, where local enrichment of cosolvent in polymer proximity plays a critical role.