(670g) Co-Solvency Induced Micro- and Macro-Phase Behavior of Polymers in Mixed Solvents

Mixed solvents are commonly used in solution-phase processing of polymers. Highly non-trivial behaviors by polymers are often observed in solvent mixtures. An example of such is the co-solvency phenomenon, where polymers that collapse in two different poor solvents become soluble in their mixtures. Explanations based on chemistry-specific arguments are less than satisfactory in abstracting the essential physics that are involved in this intriguing phenomenon, and lacks the general applicability across systems of varying chemistries. In this study, by combining theoretical calculations and computer simulations, we are to establish a generic model for the description of co-solvency in polymer solutions. We show that by considering the two major contributions to the polymer-solvent interactions explicitly, i.e., the van der Waals-type dispersion interaction and the donor-acceptor-type associative interaction (e.g. hydrogen bonding), co-solvency can arise from the cross competitions among these effects when polymers are mixed with two solvents. Specifically, using the analytical model we first derive the osmotic second virial coefficient that describes the mean-field effective interactions between polymer segments in polymer solutions, and show that it exhibits a maximum as composition of the binary solvent mixture being varied. The collapse-swelling-collapse conformational transition of polymer in solution, as implied by the predicted osmotic second virial coefficient, is then tested and confirmed using single-chain computer simulations. The analytical model is then extended to determine the binodal boundaries of bulk polymer solutions with mixed solvents. Using the same set of parameters as for describing the collapse-swelling-collapse transition, we show that the calculated binodal boundaries agrees well with experiment results, exhibiting a soluble window sandwiched between two de-mixing regions. At last, we explore the implications of the predicted cosolvency on polymer self-assemblies in solutions by investigating its effects on the micellization of diblock copolymers in mixed solvents.