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 m elts 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 b e almost entirely attributed to intramolecular dynamics throughout the Q ra nge 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 se lf terms and the cross terms localized to within a few repeat units along t he chain. It was found that the factor of 5 observed between the dynamics a t the first two peaks of S(Q) is part of a general trend largely independen t of whether S(Q) is at a minimum or a maximum. A comparison of S(Q,t) in t he region of the first peak in S(Q) and the P2C-H bond vector orientation a utocorrelation function F-C(t) suggests that the same molecular motions inf luence both the neutron spin echo and NMR T-1 relaxation experiments. [S106 3-651X(98)12712-5].