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Résumé |
Contrary to common belief, the hydrodynamic interactions (HI) in polymer melts
are not screened beyond the monomer length and are important in transient
regimes. We show that the viscoelastic HI effects (VHI) lead to anomalous
dynamics of a tagged chain in an unentangled melt at $t < t_N$ ($t_N$, the Rouse
time). The chain centre-of-mass (CM) mean-square displacement is enhanced (as
compared to the Rouse diffusion) by a large factor increasing with chain length.
We develop an analytical theory of VHI-controlled chain dynamics yielding
negative CM velocity autocorrelation function which quantitatively agrees with
our MD simulations without any fitting parameter. It is also shown that the
Langevin friction force, when added in the model, strongly affects the short-$t$
CM dynamics which, however, can remain strongly enhanced. The transient VHI
effects thus provide the dominant contribution to the subdiffusive CM motion
universally observed in simulations and experiments on polymer melts. |
