Chain-Increment Approach to the Mutual Miscibility of Polymers with All-Atom Molecular Simulation

Abstract

The mutual miscibility of polymers was addressed with all-atom molecular dynamics (MD) simulation and the energy-representation theory of solvation. The chain-increment method was extended to polymer mixtures, and the mixing free energy was related to the incremental free energy for the constituent polymer species that is the free-energy change of turning on the interaction of a monomer in the tagged polymer chain with the surrounding molecules. The free energy was then computed for the mixing of polyethylene (PE) and poly(vinylidene difluoride) (PVDF) and of poly(vinyl alcohol) (PVA) and polyvinylpyrrolidone (PVP). It was seen that the incremental free energy is insensitive to the location of the monomer in the inner part of the polymer chain and depends on the mixing ratio of the polymer system in parallel with the average interaction energy of the incremented monomer with the surrounding polymers. The intermolecular interactions of each polymer species were further analyzed at atomic resolution. The PVDF–PVDF interaction was found to become less favorable (more positive) upon addition of PE in the mixture of PE and PVDF, making them immiscible with each other. PVP favors PVA more than PVP, on the other hand, leading to the miscibility of PVA and PVP. The connection of our formulation of the mixing free energy with the Flory–Huggins expression is also described.