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Dynamical studies of fully oriented deuteropolyethylene by inelastic neturon scattering

Publication Date
DOI: 10.1016/0032-3861(82)90097-0
  • Spectrometry
  • Neutron Scattering
  • Inelastic
  • Phonon Dispersion Curves
  • Deuteropolyethylene
  • Musicology
  • Physics


Abstract Experimental parameters for interchain force fields of polyethylene are measured by neutron triple axis spectrometry and provide some of the first anisotropic information about the forces between carbon chains in simple polymers. Preparation of a bulk specimen of deuteropolyethylene with single crystal texture is reported for the first time. Near Brillouin-zone centre frequencies for both interchain optic and acoustic phonons are derived from neutron scattering studies. These measurements are combined with four frequencies obtained by Raman spectroscopy to give a total of seven experimental parameters for the interchain force field of polyethylene. These are then used to test the available force models. Overall fits of 9 and 19% are provided to these data by two Urey-Bradley force fields. Calculations of the interchain phonon dispersion curves of deuteropolyethylene based on an atom-atom potential parameter model are described. Intermolecular force constants which are derived from an analytical ‘sixexp’ form for the potential between nonbonded atoms are used. These reproduce a wide range of structural and thermodynamic data for many hydrocarbons. This rigid chain model using already fixed parameters fits the observed dynamical data within an average error of 5%. This is significantly better than the fits obtained using the Urey-Bradley force fields with adjustable parameters. The close reproduction of the experimental elastic constants gives support to the calculation of the related elastic moduli perpendicular to the chain axis. Tensile elastic moduli calculated along the two crystallographic axes perpendicular to the chain axis in the orthorhombic cell are very similar, in contrast with earlier predictions. The calculated values of 8 × 10 10 and 9 × 10 10 dyne cm −2 for b and a moduli agree closely with an average interchain modulus de4rived for the crystal from bulk measurements using a two phase ‘sandwich’ model of composite polyethylene.

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