Inelastic Neutron Scattering of Biaxially-Oriented Polyethylene Terephthalate (BoPET) Films

Polyethylene terephthalate (PET) has received considerable attention in recent years due to its ability to be generated from renewable materials that are biologically-based. Using the VISION spectrometer at the Spallation Neutron Source at Oak Ridge National Laboratory the inelastic neutron scattering (INS) spectrum has been measured for a thin sheet of biaxially oriented PET (BoPET) at low temperatures with the highest resolution attainable to date. Comparisons with earlier INS measurements of polyethylene films at the Rutherford Appleton Laboratory (J. Chem. Soc., Faraday Trans., 1996,92, 1941-1946) will be made. CASTEP (DFT) was used to optimize the structure of a model PET crystal and the corresponding vibrational spectrum was generated from coordinates and force constants. The INS spectrum in the current study of BoPET was generated using the classical Wilson GF matrix method. The phonon density of states was used to correct for dispersion in the vibrational spectrum. The calculated INS spectrum using this approach shows good agreement with the experimentally obtained results.