The diffusion of simple penetrants in tangent site polymer melts

The diffuse behavior of penetrants in simple polymer melts was investigated by molecular dynamics simulation. For the case where the polymer melt cons isted of pearl-necklace chains, the diffusive behavior of the loose pearl p enetrants was seen to be qualitatively different than would be expected in realistic models of polymer melts. In particular, there was little or no "n on-Fickian" region; the variation of the diffusion coefficient with the pen etrant diameter was what one would expect for diffusion through small molec ular liquids; and, finally, the long time tail of the velocity autocorrelat ion displayed a "-3/2" power law form, also as in the small molecular liqui d case. When the chains' backbone motion was further constrained by the int roduction of a bond angle potential, the qualitative nature of the penetran t diffusion became more "polymer-like." A non-Fickian region developed; the diffusion coefficient varied more rapidly with penetrant diameter; and the velocity autocorrelation function developed a "-5/2" power law tail. (C) 1 999 American Institute of Physics. [S0021-9606(99)50345-0].