MOLECULAR-DYNAMICS SIMULATION OF MOISTURE DIFFUSION IN POLYETHYLENE BEYOND 10-NS DURATION

Molecular-dynamics (MD) simulation of the diffusion of water in amorph ous polyethylene has been performed. In order to obtain information ab out the fundamental diffusion behavior of water, the constant particle number, constant pressure, and constant temperature (NPT) MD was empl oyed for three model systems differing in the number of water molecule s and methylene group in the polyethylene chains. Time duration beyond 10 ns showed that the water molecules came together forming water clu sters of various sizes. Although the self-diffusion coefficient D-self estimated from the mean-square displacement curve scattered considera bly in the order from 10(-8)-10(-5) cm(2)/s, D-self for the water clus ter is generally one order lower in magnitude than that of a single wa ter. However, the linearity of the mean-square curve for the larger sy stem which contained 4000 carbon atoms was very good, and D-self obtai ned for ten water molecules up to 11 ns duration showed 7.1x10(-7) cm( 2)/s which was in good agreement with the experimental value. (C) 1997 American Institute of Physics.