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 less than or equal to Q less than or equal to 3.0 Angstrom(-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 F-C(t) suggests that the same molecular motions influence both the neutron spin echo and NMR T-1 relaxation experiments. [S1063-651X(98)12712-5].