Subglass chain dynamics and relaxation in polyethylene: A molecular dynamics simulation study

Chain dynamics in amorphous polyethylene (PE) below the glass transition temperature is studied via molecular dynamics (MD) simulations. Very long, by current MD standards. trajectories (up to 450 ns) are generated. It is found that the time autocorrelation function (ACF) based on pendant perpendicular dipoles placed at the methylene units agrees promisingly well with the experimental gamma subglass dielectric process in dipole decorated PE. The central relaxation time of the dipolar ACF follows the gamma process on a map of experimental loss maxima versus inverse temperature. When transformed to frequency domain, the isothermal process represented by the ACF is of the broad proportions characteristic of the experimental one. Analysis via several filtering processes 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 several +/- 2 next neighbor types that are common but not dominant in the melt. Effects of nonergodicity in the glass on the torsional angle dynamics were investigated. The torsional angle populations are fairly close to that expected from the explicit torsional potential invoked in the simulation. However, the torsional angle trajectories show that individual bonds can have long residence times at angle values well removed from the minima in the explicit potential. Conformational transitions are found to and from these long-lived states that result from partial freezing-in of the conformational state of the chain. (C) 1998 American Institute of Physics.