Raman tensors for the 1614 and 633 cm−1 modes of bis(2‐hydroxyethyl) terephthalate

Abstract

AbstractRaman spectroscopy has been used extensively over the last 12 years in the study of molecular orientation in polymers and liquid crystals. It has become increasingly clear that fundamental information about the anisotropy of the Raman scattering from molecules is needed, including its dependence on internal field effects. This paper reports the results of a study of the Raman scattering due to two benzene ring modes in a single crystal of a model compound for poly(ethylene terephthalate) (PET). The ratios of the observed scattered intensities for various scattering geometries and polarizations have been corrected in several alternative ways for internal field effects, before being used to evaluate the ratios of the Raman tensor components. It was not possible, however, to decide which of the internal field corrections considered was the most appropriate and the results were in some cases significantly different for different corrections. Several alternative assumptions were also made about the orientations of the principal axes of the Raman tensor with respect to the symmetry axes of the para‐disubstituted benzene rings. Even when complete freedom of the axes was allowed, the greatest departure of any principal axis from the corresponding symmetry axis of the ring was found to be only 9±4° for the 1614 cm−1 mode whereas the greatest departure for the 633 cm−1 mode was 13±5°, supporting other evidence that the latter mode is less localized to the benzene ring than the former. The implications of the results for the use of Raman spectroscopy in studying molecular orientation in PET are discussed.