SUBGLASS CHAIN DYNAMICS AND RELAXATION IN POLYETHYLENE - A MOLECULAR-DYNAMICS SIMULATION STUDY

Chain dynamics in amorphous polyethylene (PE) below the glass transiti on temperature is studied via molecular dynamics (MD) simulations. Ver y long, by current MD standards. trajectories (up to 450 ns) are gener ated. It is found that the time autocorrelation function (ACF) based o n pendant perpendicular dipoles placed at the methylene units agrees p romisingly well with the experimental gamma subglass dielectric proces s in dipole decorated PE. The central relaxation time of the dipolar A CF follows the gamma process on a map of experimental loss maxima vers us inverse temperature. When transformed to frequency domain, the isot hermal process represented by the ACF is of the broad proportions char acteristic of the experimental one. Analysis via several filtering pro cesses demonstrates clearly that all of the ACF decay is accomplished via conformational transitions. The transitions effective in ACF decay are found to be highly neighbor correlated, much more so than in the melt. Specifically, most of the correlations belong to the group of se veral +/- 2 next neighbor types that are common but not dominant in th e melt. Effects of nonergodicity in the glass on the torsional angle d ynamics were investigated. The torsional angle populations are fairly close to that expected from the explicit torsional potential invoked i n the simulation. However, the torsional angle trajectories show that individual bonds can have long residence times at angle values well re moved from the minima in the explicit potential. Conformational transi tions are found to and from these long-lived states that result from p artial freezing-in of the conformational state of the chain. (C) 1998 American Institute of Physics.