Role of intermolecular interactions for upper and lower critical solution temperature behaviors in polymer solutions: Molecular simulations and thermodynamic modeling

We investigated the role of intermolecular interactions as a thermodynamic driver of phase transition behavior in polymer solutions. Molecular simulations based on a Monte-Carlo sampling technique were used to directly evaluate the intermolecular energy between specific molecules. The thermal energy behavior of several polymer solutions was determined from the simulation results. The degree of miscibility and types of liquid–liquid equilibrium (LLE), including upper critical solution temperature (UCST) and lower critical solution temperature (LCST) phase transition behavior, were studied qualitatively. A quantitative description based on a thermodynamic model was also applied to generate temperature–composition phase diagrams, and the simulated energy values were directly incorporated into the model. A modified double lattice (MDL) model with chain length dependence was adopted as a reference thermodynamic framework, and the calculated coexistence curves were fairly close to the phase transition boundary of reported experiments. The proposed modeling technique can serve as a screening tool that enables the development of stable formulations.