Abstract
The low‐frequency skeletal and intermolecular modes (below 900 cm^−1^) have been studied for a sample of “as‐polymerized” Ziegler‐Natta polyethylene for which extended‐chain conformations have been proposed. These results have been compared with data from a sample of stirred‐solution‐crystallized polyethylene and with previous measurements for highly crystalline samples of folded‐chain polyethylene. Measurements were also made of the temperature dependence of these modes for the as‐polymerized material and of the polarization dependence of these modes for an oriented sample of solution‐stirred polyethylene. In the neutron spectra and the corresponding derived frequency distributions, the observed major “singular” frequencies for both the skeletal and intermolecular modes are in reasonable accord with those previously observed for folded‐chain samples of high crystallinity and in Raman measurements. In addition, these frequencies agree with those predicted by theory for the singular frequencies corresponding to the phase‐frequency relationships for the skeletal and intermolecular modes of an infinitely extended chain in a crystal. However, in the spectra of the extended‐chain materials, these singular frequencies, in general, appear sharpened and enhanced in intensity relative to those for previously reported results on folded‐chain polyethylenes; consequently, assignment of their characteristic frequencies is easier. In addition, evidence is presented to show that, for the extended‐chain materials, these frequencies broaden less rapidly with increasing temperature, and the corresponding average vibrational amplitudes are smaller than those observed for highly crystalline samples of folded‐chain material. It is suggested that in these materials the reduction of the number of chain folds and the increased intramolecular ordering give rise to increased thermal stability of the configurations with regard to segmental rotation and to decreased vibrational amplitudes relative to folded‐chain materials. When the aspolymerized materials is preheated above the normal melting point for polyethylene, the neutron spectra revert to those more characteristic of the folded‐chain materials, indicating that a relaxation of chain extensions has occurred. However, on the basis of these results, on quantitative estimates of the characteristic relaxation time can be presented. Additional frequencies, besides those predicted by theory for the skeletal and intermolecular modes and those associated with either multiphonon contributions or the presence of chain folds, are observed. The origin of these frequencies remains unclear but may warrant further theoretical consideration.