(551c) A Unified Understanding of Cononsolveny of Polymers in Binary Solvent Mixtures

Standard Random Phase Approximation (RPA) model are applied to investigate cononsolvency of polymers in mixtures of two good solvents. The model reveals two mechanisms that can give rise to the cononsolvency effect. It is shown that the overall solvent quality for polymers will be reduced by local compositional fluctuations given a mismatch in theaffinities between polymers and the two solvents, or by preferential mixing between the solvents and cosolvents. The RPA model further predicts the critical magnitudes of the affinity-mismatch and the preferential interaction strength between the two solvents, beyond which cononsolvency will be manifested through either polymer conformational changes or phase instability. The two types of cononsolvency are distinguished by the solvent composition at which maximum immiscibility are predicted to occur. The maximum immiscibility occurs with the cosolvents being of a minor fraction if cononsolvency is driven by affinity-mismatch, but of an equal fraction if solvent-cosolvent preferential mixing is the responsible mechanism. Interplay of the two driving forces gives rise to a reentrant behavior in which cononsolvency of the two types switching from one to the other through a “conventional” region where overall solvent quality varies merely monotonically with the solvent composition. The RPA model provides a general and unified framework for rationalizing and understanding findings from previous studies on cononsolvency of polymers in solvent mixtures. Findings of such highlight the complex role played by solvents in determining conformation and miscibility of polymers in multi-solvent mixtures, a problem of fundamental and practical interest in diverse applications of materials science.