Influence of Molecular Weight and Shape on Diffusion Coefficients of Small Molecules in Polyethylene Terephthalate

In this study, the diffusion coefficients of 13 kinds of small molecules with molecular weights ranging from 32 to 339 g.mol(-1) in amorphous PET are calculated based on molecular dynamics (MD) simulation. The results suggest that diffusion coefficient of migrant depends not only the molecular weight but also the shape of migrant molecules. Further, the free volume of polymer matrix is calculated using Connolly surface method. The results show that some small free volume cavities conjoin together and form the larger cavities which facilitate the diffusion of migrant molecules in polymer matrix. The diffusion trajectories suggest that the molecules in first class move actively, but the molecules in class third class move limitedly and the movement mobility of molecules in second class is between that of first class and third class. The diffusion trajectories of small molecules strongly depend on the shape and molecular weight of migrant molecules, which is consistent with the diffusion coefficients.