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

Abstract

Raman 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(ethy1ene 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 9f4' for the 1614 em-' mode whereas the greatest departure for the 633 cm-' mode was 13f So, 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. INTRODUCIIONIt is well known that molecular orientation can profoundly affect the mechanical and other properties of solid polymers. For this reason, a great deal of research has been directed towards obtaining an understanding of how particular deformation processes lead to particular types of distributions of molecular orientation and to how a particular type of distribution affects the properties. Many methods have been developed for characterizing the orientation distributions, including the measurement of refractive indices,' x-ray diffraction intensities,' visible and infrared dichroi~m,~ polarized fluorescence," broad-line NMR absorption' and polarized Raman scattering intensities.6 The advantages of Raman scattering over the other methods are: (i) it gives information directly about the polymer chains or about specific parts of the chains; (ii) it is not restricted to either the crystalline or non-crystalline regions; and (iii) it can give more information about the degree and type of orientation than is obtainable by techniques such as infrared dichroism which share advantages (i) and (ii).The price to be paid for the extra information is the extra complexity of the experimental technique and of the analysis of the results compared with, for instance, those required in using the infrared method. Nevertheless, a number of polymers have been studied satisfactorily, including polyethylene: poly(methy1 methacrylate):...

High resolution electron microscopy of Bis‐(‐2‐Hydroxyethyl) terephthalate crystalline polymers

“…The so-called ␣ form is the most stable and common one and was found in the present case according to the X-ray and FTIR results. The usual crystallographic habit (Lewis and Bower, 1987) of the ␣ form is schematically depicted in Figure 4, where it can be observed that the largest (100) faces are parallel to the b and c axes, which coincides with the SAD patterns found in the present study and which consistently appeared in the mentioned zones axis (Fig. 3b).…”

“…3b). A projection of the structure parallel to the b axis, according to some detailes X-ray diffraction (Miyake, 1957) and Raman (Lewis and Bower, 1987) studies, is shown schematically in Figure 5, where two independent molecules, A and B, form the basis of the structure. Positions C and D are obtained from A and B by symmetry operations of the P2 1 /a group.…”

Molecular Orientation in Polyester Films Using Polarized Laser Raman and Fourier Transform Infrared Spectroscopies and X-Ray Diffraction

“…(b) The 1616 cm -1 line of the benzene ring vibration (C 1 −C 4 ) was used in our calculations for the following reasons: (i) it is the strongest line in the Raman spectrum, (ii) it occurs as an isolated and well-localized line in the Raman spectrum of para-disubstituted benzene ring, with very small changes of frequency observed in a variety of related “model” compounds, , as also confirmed by vibrational calculations, and (iii) it is not directly affected by conformational changes and is expected to give a reliable indication of the overall chain orientation . Thus, it is very likely that one of the principal components of the Raman tensor coincides with the chain axis for this vibration (the deviation, estimated to be about 6° 21 on the average, is considered very small, since it does not change the resulting P 2 values significantly; this is also supported from literature data on PET with the same Raman band, ,, where for a deviation of about 19°, the results were not much affected).…”

Quantitative polarized Raman spectroscopy in highly turbid bone tissue

“…The Raman intensity ratios ͑R x = I xy / I xx and R y = I yx / I yy ͒ for the mineral and amide I bands were corrected for the influence of sample birefringence ͑reflectivity, internal field, divergence͒ by assuming the typical collagen and mineral birefringence values of 3 ϫ 10 −3 and 7 ϫ 10 −3 , respectively. 18,26 Statistical tests were performed on band intensities and intensity ratios using two-tailed unpaired t-tests to compare the effect of NA and to compare wild-type with oim/oim specimens. A value of p Ͻ 0.05 was considered significant.…”