The static structure factor $S\left(Q\right)\mathrm{}$ and the coherent dynamic structure factor $S(Q,t)\mathrm{}$ are calculated from molecular dynamics simulations of polyisoprene melts and compared with neutron scattering results [R. Zorn, D. Richter, B. Farago, B. Frick, F. Kremer, U. Kirst, and L. J. Fetters, Physica B 180&181, 534 (1992)]. Both the shape and the absolute time scale of the calculated $S(Q,t)\mathrm{}$ are consistent with experimental results. The decay of $S(Q,t)\mathrm{}$ can be almost entirely attributed to intramolecular dynamics throughout the Q range studied $(1.2\mathrm{}<~Q<~3.0{\AA }^{-1}),$ i.e., the full $S(Q,t)\mathrm{}$ can be approximated by considering only the self terms and the cross terms localized to within a few repeat units along the chain. It was found that the factor of 5 observed between the dynamics at the first two peaks of $S\left(Q\right)\mathrm{}$ is part of a general trend largely independent of whether $S\left(Q\right)\mathrm{}$ is at a minimum or a maximum. A comparison of $S(Q,t)\mathrm{}$ in the region of the first peak in $S\left(Q\right)\mathrm{}$ and the $P_2\mathrm{C}\mathrm{—}\mathrm{H}$ bond vector orientation autocorrelation function $F_C\mathrm{}\left(t\right)\mathrm{}$ suggests that the same molecular motions influence both the neutron spin echo and NMR $T_1$ relaxation experiments.

• Received 7 August 1998

DOI:https://doi.org/10.1103/PhysRevE.59.623