A Generic Explanation of the Mechanism of Co-solvency*

Mixed solvents are extensively utilized in solution processing of polymers. In solvent mixtures, polymers are often observed to exhibit highly non-trivial behaviors. One example is co-solvency --- polymers that collapse in two different poor solvents become soluble in their mixtures. Explanations based on chemistry-specific arguments are less than satisfactory in carving out a clear physical picture of this intriguing phenomena. In this study, we conduct theorical calculations and computer simulations based on a generic polymer solution model in order to offer a clear account of the driving mechanism of co-solvency. We show that co-solvency results from the composite nature of polymer-solvent interactions, made up of the van der Waals type interactions and associations such as hydrogen bonding. Competition of the two effects gives rise to collapsed conformation when polymers are mixed with each solvent individually. In binary solvent mixtures, cross competitions among the four factors can lead to a swollen polymer conformation at suitable solvent compositions. Implications of the predicted collapse-swelling-collapse transition on the bulk solution phase behavior is further explored using the generic model. The obtained phase diagram compares well with existing experiment data.