Knowledge Bank

When a low energy incident neutron is scattered by a given sample, there is energy exchange between this neutron and thermally excited phonons in the sample. The energy distribution of a monoenergetic incident neutron beam after a single scattering event provides, therefore, information concerning the molecular motions in the sample. Since there are no selection rules governing this energy exchange process, the scattered neutron energy spectrum is proportional to the density of states of the sample phonons. Thus, when the slope of the frequency-wave vector relations (dispersion curves) goes to zero, a discontinuity and/or a peak is expected in the neutron spectrum corresponding respectively to an acoustical or an optical mode of vibration. In polyethylene, Marlex 6050 (85% crystalline), a discontinuity was observed at $200cm-1$ in the neutron spectrum corresponding to a skeletal torsion $\nu p$ predicted by the calculation. Several peaks were also observed between 220 and $550cm-1$ which can be explained if interchain forces are taken into account in the calculation of the dispersion relation for the skeletal deformation $\nu 6$ (bending and stretching). The peaks observed between 40 and $200cm-1$ in the neutron spectrum were attributed to lattice modes. In polyethylene glycol, three sharp discontinuities are observed in the neutron spectrum together with several peaks. A comparison with infrared and Raman data is made in an attempt to assign these various modes of vibration. The neutron spectra of polypropylene samples with various degree of tacticity and of polyoxymethylene will also be presented.