Study of Low-Frequency Motions in Polyethylene and the Paraffin Hydrocarbons by Neutron Inelastic Scattering

Abstract

The vibrational spectra between 800 and 30 cm—1, derived from measurements of the time-of-flight distributions of initially ``cold'' neutrons inelastically scattered from thin samples, are presented for highly crystalline polyethylene at sample temperatures below and above both the glass transition and the melting point, and for samples of branched, irradiated, and quenched polyethylene at room temperature. The acoustical torsion mode v9 with a high-frequency limit of 200 cm—1 and the acoustic deformation mode v5 with an estimated high-frequency limit of 550 cm—1 are observed in agreement with recent calculations of the normal modes of an infinite transplanar polyethylene chain. However, the measured spectra indicate that, although intermolecular forces appear to have a negligible influence, the distributions of frequencies associated with these modes are strongly influenced by the presence and by the phase of the disorder in the sample. Similar measurements for a series of n-paraffin hydrocarbons between C6H14 and C32H66 at room temperature indicate that while nearly free internal rotations take place in the liquid phase, these rotations are hindered in the solid phase. As the length of the chain increases, the frequency distribution of the vibrational motions in the solid n-paraffins approaches that observed in polyethylene at room temperature.