COUPLING BETWEEN DIFFERENT MODES IN LOCAL CHAIN DYNAMICS - A MODAL CORRELATION-ANALYSIS

A correlation analysis is proposed for studying the coupling between l ow- and high-frequency rotational motions in polymers. Molecular dynam ics simulations are used to generate trajectories of a polyethylene-li ke polymer chain. Contributions to the observed trajectories from char acteristic modes of various frequencies are identified by extensive us e of filtering techniques. Correlations between modes are represented in terms of a modal correlation function. The latter is expressed as t he sum of two contributions, reflecting the degrees of coupling and in coherence, respectively, between various modes. The term modal is used throughout to prevent confusion with the widely studied time-correlat ion functions in the literature. The analysis of the kinetic-energy tr ajectories of backbone atoms indicates that after an initial relaxatio n taking place within picoseconds, the kinetic-energy fluctuations are adequately described by a few lowest-frequency modes of the total spe ctrum. Strong mode-mode coupling, both coherent and incoherent, is obs erved among the modes in the range nu < 20 cm(-1), whereas the correla tions among modes with frequencies 20 < nu/cm(-1) < 30 are mainly expr essed by incoherent motions increasing with the separation between ato ms along the chain contour. The flow of kinetic energy between backbon e atoms is found to take place mostly through modes of comparable freq uencies. However, communication between modes of distinct frequencies does also exist, which is particularly pronounced during bond rotation al jumps from one isomeric state to another and gives rise to localiza tion of the motion in space.