Vibrational dynamics of solid poly(ethylene oxide)

Molecular-dynamics simulations of crystalline poly(ethylene oxide) have been carried out in order to study its vibrational properties. The vibrational density of states has been calculated using a normal mode analysis and also through the velocity autocorrelation function of the atoms. Results agree well with experimental spectroscopic data. System size effects in the crystalline state, studied through a comparison between results for 16 unit cells and that for one unit cell, have shown important differences in the features below 100cm^-1. Effects of interchain interactions are examined by a comparison of the spectra in the condensed state to that obtained for an isolated oligomer of ethylene oxide. Calculations of the local character of the modes indicate the presence of collective excitations for frequencies lower than 100cm^-1, in which around 8 to 12 successive atoms of the polymer backbone participate. The backbone twisting of helical chains about their long axes is dominant in these low-frequency modes.