Calculation of the coherent dynamic structure factor of polyisoprene from molecular dynamics simulations

The static structure factor $S(Q)$ and the coherent dynamic structure factor $S(Q,t)$ 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 $S(Q,t)$ are consistent with experimental results. The decay of $S(Q,t)$ can be almost entirely attributed to intramolecular dynamics throughout the Q range studied (1.2<~Q<~3.0^-1), i.e., the full $S(Q,t)$ 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 $S(Q)$ is part of a general trend largely independent of whether $S(Q)$ is at a minimum or a maximum. A comparison of $S(Q,t)$ in the region of the first peak in $S(Q)$ and the P₂C---H bond vector orientation autocorrelation function FC(t) suggests that the same molecular motions influence both the neutron spin echo and NMR T₁ relaxation experiments.