The Basis of Orientation Measurements by Raman Spectroscopy

The information provided by the Raman spectrum of an oriented polymer differs from its infrared counterpart because of the fundamentally different processes involved in the generation of the spectra. In the infrared absorption process, as already noted, the absorption intensity is dependent on the angle between the electric vector and the direction of the dipole moment change. The Raman spectrum results from inelastic photon scattering details of which are determined by changes in the polarizability of the chemical bonds involved. Polarizability is a tensor quantity, which results in complications but, in principle, provides additional information. As we have seen, infi ared spectroscopy involves only one beam of polarized radiation, and the fraction of the nufotion absorbed by a molecule depends only on the orientation of the molecule with respect to the polarisation vector of the radiation. However, Raman scattering involves two beams of radiation, those of illumination and collection, and the scattered intensity depends on the orientation of the molecule with respect to the polarisation vectors of both beams, whidi may, of course, be different. This necessitates more detailed measurements in order to obtain the relevant information. 

It may be shown that the Raman measurements are capable of yielding information on both cos 0 and cos d . The availability of cos 0 data can be valuable is distinguishing between the differing types of stress deformation mechanisms that have been proposed. However, an interpretation of the band intensities in terms of cos 0 and cos 0 is possible only when the principal components of the derived polarisability tensor are known. This information is often not available and assumptions must then be made these then render the method non-absolute. Examples of this approach will be considered briefly below. 

The interpretation of detailed orientation measurements by Raman spectroscopy has led to the use of various abbreviated procedures. However, unlike the use of infrared dichroic ratios for comparative purposes, simplified Raman procedures present pitfalls for the unwary and must be used with due care and attention. 

It is useful at this juncture to note the capabilities of the other major methods for assessing orientation in polymers. Birefringence measurements yield values for cos 0 and broad line NMR provides cos 0 and cos 0 , together with cos 0) in certain favourable circumstances. X-ray diffraction measurements define orientation uniquely, and so give values for cos 0 , where n takes on all even values. Infrared spectroscopy and birefringence measurements yield the 

In the infrared, plane polarized radiation is required, and is provided by passing the radiation through a polarizer transparent in the wavelength domain of interest. 

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