An integral equation theory for polymer solutions: Explicit inclusion of the solvent molecules

Abstract

Self-consistent Polymer Reference Interaction Site Model (PRISM) calculations and molecular dynamics (MD) simulations were performed on athermal solutions of linear polymers. Unlike most previous treatments of polymer solutions, we explicitly included the solvent molecules. The polymers were modeled as tangent site chains and the solvent molecules were taken to be spherical sites having the same intermolecular potential as the polymer sites. The PRISM theory was solved self-consistently for both the single chain structure and intermolecular correlations as a function of chain length and concentration. The rms end-to-end distance from PRISM theory was found to be in agreement with corresponding MD simulations, and exhibited molecular weight dependence in accordance with scaling predictions in the dilute and concentrated solution limits. The presence of explicit solvent molecules had a significant effect on the packing of the polymer by inducing additional structure in the intermolecular radial distribution function between polymer sites. Using the direct correlation functions from the athermal solution and the random phase approximation, we were able to estimate the spinodal curves for solutions when polymer and solvent attractions were turned on. We found significant deviations from Flory–Huggins theory that are likely due to compressibility and nonrandom mixing effects.