Dissolution and Diffusion of Small Molecules in the Amorphous and Crystalline States of Polymers Studied with All-Atom Molecular Dynamics Simulation

The dissolution and diffusion were addressed in connection to the permeation of water, carbon dioxide, and nitrogen through polyethylene, polystyrene, poly(ethylene terephthalate), and cellulose in their amorphous and crystalline states. All-atom molecular dynamics simulation was conducted to analyze the dissolution free energy and diffusion coefficient, and the former was decomposed into the contributions from intermolecular-interaction components of electrostatic, van der Waals, and excluded volume (cavity formation). It was found that the excluded-volume component can describe the difference in the dissolution free energy between the amorphous and crystalline states and that diffusion is promoted when paths of molecular sizes are involved in the polymer medium. The permeability was shown to be larger in the amorphous state than in the crystalline state for each combination of the permeant and polymer, and molecular-sized cavities in a crystalline polymer lead to suppressed permeation through enhanced dissolution and inhibited diffusion.