A DYNAMIC ROTATIONAL ISOMERIC STATE APPROACH FOR EXTENSION OF THE TIME-SCALE OF THE LOCAL DYNAMICS OBSERVED IN FULLY ATOMISTIC MOLECULAR-DYNAMICS SIMULATIONS - APPLICATION TO POLYBUTADIENE

The dynamic rotational isomeric state (DRIS) formalism has been utiliz ed to predict the local dynamics of amorphous cis- and trans-polybutad iene at bulk density from short-time molecular dynamics (MD) simulatio ns at 425 K. The rates for transitions between rotational isomeric sta tes have been calculated from the initial slopes of time-delayed trans ition (conditional) probability curves extracted from the MD simulatio n. First- (independent), second- (pairwise dependent), and third- (tri plewise dependent) order conformational transitions have been incorpor ated into the DRIS formalism. Conformational and orientational correla tion functions have been evaluated. The comparison of DRIS results wit h MD simulations indicates that this approach may be advantageously us ed to predict the time evolution of bond isomeric states and the contr ibution of transitions between these states to conformational correlat ion functions. The first-order conformational kinetics is the major fa ctor controlling the relaxation in the latter. Cross-correlation funct ions are not reproduced as well, because they are dominated by torsion al librations within rotational isomeric states, and such librations a re not incorporated in the DRIS analysis. The prediction of anisotropi c character of the segmental motions is also satisfactory to an import ant extent, but it still awaits some more consideration in the choice of the size of the kinetic segment, along with the accurate input of c ooperative motions arising from both intra- and intermolecular interac tions in a MD simulation, Information which would otherwise be extract ed from the statistical analysis of very long trajectories of MD simul ations may become readily obtainable from DRIS. (C) 1996 American Inst itute of Physics.