TY - JOUR
TI - Peculiar behavior of the ester carbonyl vibrational modes in anisotropic aliphatic and semi-aromatic polyesters
AU - Andrikopoulos, K.S.
AU - Chrissanthopoulos, A.
AU - Soto Beobide, A.
AU - Iconomopoulou, S.M.
AU - Moschopoulou, H.
AU - Voyiatzis, G.A.
JO - Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
PY - 2022
VL - 269
TODO - null
SP - null
PB - Elsevier B.V.
SN - 1386-1425
TODO - 10.1016/j.saa.2021.120710
TODO - Anisotropy;  Aromatic compounds;  Aromatization;  Circular dichroism spectroscopy;  Density functional theory;  Dichroism;  Dipole moment;  Esters;  Polyesters;  Tensors, Aromatic polyesters;  DFT calculation;  Dichroism spectroscopy;  Ester carbonyl;  Ester carbonyl vibrational mode;  Infrared dichroism spectroscopy;  Infrared dichroisms;  Polarized Raman spectra;  Segmental orientation;  Vibrational modes, Fourier transform infrared spectroscopy, ester;  polyester, anisotropy;  Raman spectrometry;  vibration, Anisotropy;  Esters;  Polyesters;  Spectrum Analysis, Raman;  Vibration
TODO - The current work reports on a systematic study related to the vibrational modes of the ester carbonyl group in drawn polyesters. We have observed and try to explain how the presence of aromatic units in the molecular structure substantially affects the respective elements of the Raman tensor in contrast to the dipole moment derivative vector which is only marginally influenced. The work is based on the collection of polarized Raman spectra and FTIR dichroism measurements on the one hand and on DFT calculations on the other. The experimental data were obtained from uniaxially stretched aliphatic and semi-aromatic polyesters. The calculations were applied on relevant oriented oligomers and allowed the extraction: (i) of reliable Raman/FTIR vibrational spectra and (ii) the components of the dipole moment derivative and Raman tensor of the vibrational modes and in particular the ones involving the ester carbonyl group. Experimental data indicate that the intensity of the ester carbonyl band is considerably enhanced in the Raman spectra of semi-aromatic polyesters, which results from a considerable enhancement of the related coupling coefficient. Furthermore, the angles of the principle Raman tensor axis are rotated so that the element of the tensor with the greatest value is oriented towards the direction designated by the segment. The latter explains the peculiar experimentally indicated anisotropy, through the ester carbonyl stretching, for the case of semi-aromatic polyesters, which is totally different with that observed in the aliphatic ones. © 2021 Elsevier B.V.
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