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 utilized to predict the local dynamics of amorphous cis- and trans-polybutadiene at bulk density from short-time molecular dynamics (MD) simulations at 425 K. The rates for transitions between rotational isomeric states have been calculated from the initial slopes of time-delayed transition (conditional) probability curves extracted from the MD simulation. First- (independent), second- (painvise dependent), and third- (triplewise dependent) order conformational transitions have been incorporated into the DRIS formalism. Conformational and orientational correlation functions have been evaluated. The comparison of DRIS results with MD simulations indicates that this approach may be advantageously used to predict the time evolution of bond isomeric states and the contribution of transitions between these states to conformational correlation functions. The first-order conformational kinetics is the major factor controlling the relaxation in the latter. Cross-correlation functions are not reproduced as well, because they are dominated by torsional librations within rotational isomeric states, and such librations are not incorporated in the DRIS analysis. The prediction of anisotropic character of the segmental motions is also satisfactory to an important extent, but it still awaits some more consideration in the choice of the size of the kinetic segment, along with the accurate input of cooperative motions arising from both intra- and intermolecular interactions in a MD simulation. Information which would otherwise be extracted from the statistical analysis of very long trajectories of MD simulations may become readily obtainable from DRIS.