Molecular dynamics simulation of diffusion of O2 and CO2 in amorphous poly(ethylene terephthalate) and related aromatic polyesters

The diffusion of small molecules through polymers is important in many areas of polymer science, such as gas barrier and separation membrane materials, polymeric foams, and in the processing and properties of polymers. Molecular simulation techniques have been applied to study the diffusion of oxygen and dioxide of carbon as small molecule penetrants in models of bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. A bulk amorphous configuration with periodic boundary conditions is generated into a unit cell whose dimensions are determined for each of the simulated aromatic polyesters in the cell to have the experimental density. The aim for this research is to explore and investigate the diffusion of gases through bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. The diffusion coefficients for O₂ and CO₂ were determined via NVE molecular dynamics simulations using the Dreiding 2.21 molecular mechanics force field over a range of temperatures (300, 500 and 600 K) using up to 30 ns simulation time. We have focussed on the influence of the temperature, polymer dynamics, number of aromatic rings, ortho-, meta-, para-isomers, density and free volume distribution on the diffusion properties. Correlation of diffusion coefficients with free volume, temperature, number of aromatic rings, ortho-, meta- and para-isomers was found.