Raman scattering, polymer orientation

J Purvis, DI Bower, IM Ward. Molecular orientation in PET studied by polarized Raman scattering. Polymer 14 398-400, 1973. 

The ease of sample handling makes Raman spectroscopy increasingly preferred. Like infrared spectroscopy, Raman scattering can be used to identify functional groups commonly found in polymers, including aromaticity, double bonds, and C bond H stretches. More commonly, the Raman spectmm is used to characterize the degree of crystallinity or the orientation of the polymer chains in such stmctures as tubes, fibers (qv), sheets, powders, and films... 

Polarized analysis There is useful spectral information arising from the analysis of polarization of Raman scattered light. This, typically called as polarized analysis, provides an insight into molecular orientation, molecular shape, and vibrational symmetry. One can also calculate the depolarization ratio. Overall, this technique enables correlation between group theory, symmetry, Raman activity, and peaks in the corresponding Raman spectra. It has been applied successful for solving problems in synthetic chemistry understanding macromolecular orientation in crystal lattices, liquid crystals or polymer samples and in polymorph analysis. 

Polarized fluorescence arises from a different optical interaction with matter than does Raman scattering, but as in Raman scattering, the wavelength of the emitted light differs from that of the incident light. Also like Raman scattering, fluorescence polarization also allows one to measure both second and fourth moments of the orientation of specific bonds in a molecule. Monnerie (1987) has pioneered the application of fluorescence polarization methods to polymer dynamics. 

In the spectra of nonrandomly oriented polymers, the intensity and polarization of the Raman scattered light are dependent on the polarization of the exciting light relative to the orientation of the molecules (35). If the orientation of the molecules is known, comparison of spectra recorded with different polarizations of the exciting light yields useful information about the directionality of the vibrational motions. Conversely, information about molecular orientation can be gained if the directionality of the vibrations is... 

The degree of microcrystallite orientation in crystalline films can be determined from careful analysis of the polarized Raman scattering line intensities. Since the induced polarization (P) and incident electric field (E) of the probe laser are three-dimensional vectors related by the polarizability tensor (o), individual elements of the tensor ( ij) be determined from the line intensities establishing molecular orientation in materials. Molecular chain orientation in polymer glasses has been demonstrated by Raman and infrared dichroism studies (12). 

The theory of the method is rather complicated, because the amplitude of Raman scattering is described by a second-rank tensor, so it will not be discussed here. Just like for fluorescence, P2 cosff)) and (P4(cosd)), or cos 6) and (cos" 6), can, at least in principle, be obtained for the simplest type of uniaxial orientation distribution and these values now refer directly to the molecules of the polymer itself. In practice it is often necessary to make various simplifying assumptions. For biaxially oriented samples several other averages can be obtained. 

The first treatment of the theory of the intensities and polarisation effects to be expected in the Raman scattering from an oriented polymer sample appears to be that given by Cornell and Koenig." This treatment must be regarded at best as a very rough approximation, since the tensor nature of the effect is not taken into account properly. Snyder has given a correct account of the theory for rather special distributions of orientations of the Raman scatterers but his work concentrates on the information that can be obtained about the Raman tensors if the orientation distribution is known. The only treatment that has considered how much information can in principle be obtained about the distribution of orientations and what measurements are necessary to obtain it is that of Bower and Bower and Purvis. " In this treatment the similarities and differences between the theories of the fluorescence and Raman methods are apparent and an account of it follows. 

Raman spectroscopy has been used to probe interactions occurring in PAni nanotube [23-24] composites, the orientation of nanotube bundles within a matrix [25, 26], and the efficiency of load transfer from the host matrix to SWCNTs [27,28]. Unlike X-ray diffraction (XRD) methods [12], Raman spectroscopy can detect very low concentrations of SWCNTs in a polymer matrix [29,30]. The degree of orientation of aligned nanotubes can be estimated by polarized Raman spectroscopy due to the presence of a strong resonance Raman scattering effect [31,32]. Polarized Raman spectroscopy in combination with a mathematical model [33] has been employed to characterize the orientational order of nanotubes in polymers [34]. Using this model, the polarized Raman intensity of nanotubes is correlated with the orientation order parameters of SWCNTs in a utuaxially oriented system. An orientation distribution function can then be obtained. 

In oriented polymer samples, polarized Infrared and Raman spectroscopy are also very useful in determining bond orientations in the crystals and oriented amorphous regions (28-34). Furthermore, surface enhanced Raman scattering techniques can also be used to obtain information regarding the molecular orientation at surfaces and interfaces. 

DI Bower. Investigation of molecular orientation distributions by polarized Raman scattering and polarized fluorescence. J Polym Sci Polym Phys Ed 10 2135-2153, 1972. 

Any molecular property which is anisotropic, i.e., that shows a direcional dependence, is per se capable of providing information on orientation both in solids and in appropriate melts or liquids. Such properties include optical birefringence, infrared polarization, anisotropic Raman scattering, broad line NMR and X-ray diffraction, each of which has been exploited in the study of polymers. These various approaches yield differing amounts of information, for fundamental reasons in order to appreciate why this is so it is necessary to consider their theoretical basis, and also to have a convenient mathematical framework by which to quantify degrees of orientation. These two topics will now... 

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. 

The polarization of Raman scattered light also contains useful structural information. This property can be measured by using plane polarized light and a polarization analyzer. Spectra recorded with the analyzer set both parallel and perpendicular to the excitation plane can be used to calculate the depolarization ratio of each vibrational mode. This provides insight into molecular orientation and the symmetry of the vibrational modes, as well as information about molecular shape. It is often used to determine macromolecular orientation in crystal lattices, liquid crystals, or polymer samples. 

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