Molecular Dynamics Simulation on Diffusion of 13 Kinds of Small Molecules in Polyethylene Terephthalate

Understanding the diffusion of migrants in polyethylene terephthalate (PET) and calculating the diffusion coefficients are very important for migration research. In this study, the diffusion coefficients of 13 kinds of small molecules with molecular weights ranging from 32 to 339 g/mol in amorphous PET are calculated based on molecular dynamics (MD) simulation. By comparison of diffusion coefficients simulated by MD simulation techniques, predicted by the Piringer model and by experiments, the accuracy of the Piringer model and MD simulation techniques for the estimation of diffusion coefficients of migrants in PET is evaluated. The MD simulation shows that D-simu is very close to D-exp, within one order of magnitude of the experimental diffusion coefficients except for a few samples. The possible reasons for the differences among D-simu, D-pred and D-exp are analysed from the molecular weight and temperature. The results show that the Piringer-model-predicted values at high temperatures overestimate significantly higher than that at lower temperatures. The activation energy is calculated by the Arrhenius equation, which shows the relationship between diffusion coefficient and temperature. It is shown that the MD simulation yields acceptable activation energy. The study suggests that MD simulation may be a useful approach to calculate the diffusion coefficients of small molecules in PET. Copyright (C) 2010 John Wiley & Sons, Ltd.