Molecular dynamics simulation study of the mechanisms of water diffusion in a hydrated, amorphous polyamide

Atomistic, molecular dynamics simulations of water diffusion in a hydrated, amorphous polyamide have been carried out to study the effects of polymer dynamics, cross-linking, and hydrogen bonding interactions on the molecular mechanisms of diffusion. This polymer was selected as a model for the discriminating layer of FT-30 reverse osmosis membranes, used commercially for water desalination. Analysis of the configurations generated during the simulations shows that, at the relatively high water content studied, a continuous water phase is formed which permeates the polymer and consists of more than 90% of all waters of hydration. Water diffusion takes place by distinct 'jump-like' movements between weakly localized sites in this continuous phase. The jump length is on average ~3Å, independent of the system studied, and is most likely defined by the local, cooperative rearrangement of water molecules. However, the jump frequency, or equivalently, the rate of water diffusion varies from system to system, and depends on polymer dynamics and cross-linking density, and more significantly, on water-water hydrogen bonding interactions.