Neutron-Scattering Study of the Intramolecular and Crystalline Modes of Polyethylene

Abstract

The neutron spectra and the derived frequency distributions for polyethylene are compared with recent calculations. The skeletal-deformation and -torsion modes appear at 550±50 and 200±10 cm−1, respectively, in agreement with both theory and previous measurements. Five additional weak maxima, observed below 190 cm−1, correlate with predicted intermolecular frequencies and are observed to persist to 350°K (well above the glassy-transition interval) in both a high-density and a strongly branched polyethylene. In contrast, the skeletal torsion and deformation modes broaden and decrease in intensity with increasing temperature and with increasing branching disorder. With annealed samples, the skeletal torsion sharpens with increasing density (i.e., increased order), but the intermolecular modes do not. A spectrum taken normal to the direction of chain alignment in a specimen that had been cold drawn to the yield point showed a sharp, well-resolved, skeletal-torsion mode but no skeletal-deformation mode. This agrees with a previous assignment of the deformation as a longitudinal and the torsion as a transverse mode. The interchain modes were split and shifted in frequency in the drawn sample, presumably because of interchain disordering. Eight additional weak maxima were observed between 800 and 200 cm−1 in high-density polyethylene. The nature of the molecular motions associated with these modes is uncertain, but it is suggested that they involve coupling with the skeletal modes.