Molecular dynamics study of water diffusivity at low concentrations in non-swollen and swollen polyurethanes

Molecular dynamics simulation was used to study the diffusivity of water at low concentrations in two types of amorphous polyurethanes PU1 and PU2. PU1 was made up of castor oil and 4,4′-diphenylmethano-diisocyanate (MDI), a polyurethane that is not swollen by water, and PU2 was made up of polyethylene glycol (PEG) with MDI, a polyurethane exhibiting a high degree of swelling by water. PU2 also has the ability to form a considerable amount of hydrogen bonds with water molecules. To mimic the non-swollen and swollen behavior, canonical (i.e., NVT) and isothermal-isobaric (i.e., NPT) ensembles were used for PU1 and PU2, respectively. Simulation results showed that the diffusivity of water in PU1 was insensitive to its concentration at low concentrations but decreased with increasing concentration for PU2. Further data analysis revealed that in the case of PU1, both the mean number of hops of water molecules from one free volume hole to another and the mean hopping distance did not vary with water concentration. In other words, at low concentrations, hopping of water molecules is not affected by the presence of other water molecules, suggesting that there is no blocking effect. In the case of PU2, the mean number of hops did decrease drastically in the concentration range of 0-2 wt% and leveled off at higher concentrations while the mean hopping distance exhibited a slight decrease. It was found that the observed behavior is mainly attributed to the hydrogen bonds formed between PU2 and water molecules. However, it is the swelling behavior (i.e., high ⅆK/ⅆc below 2 wt% but almost constant ⅆK/ⅆc above 2 wt%) leads to the concentration dependence profile of diffusion coefficient. Compared with the theory of Barrie and Platt, the computed diffusivity is much lower than that is predicted by the theory. This is because the theory does not include effects from the hydrogen bonds formed between PU2 and water molecules and the swelling behavior of PU2.