Infrared spectroscopy chain orientation

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... 

A series of SAMs formed on Au from mono- and dithiol conjugated aromatic molecules was characterized by cyclic voltammetry, grazing incidence Fourier transform infrared spectroscopy, contact angle measurement, and ellipsometry.43 The analyses indicated that the molecular orientation of conjugated phenylene- and thophene-based dithiols became less tilted with respect to the surface normal as the chain length of the organic molecules increased. 

In addition to describing the conformation of the hydrocarbon chains for amphiphilic molecules at the A/W interface, external reflectance infrared spectroscopy is also capable of describing the orientation of the acyl chains in these monolayers as a function of the monolayer surface pressure. The analysis of the orientation distribution for an infrared dipole moment at the A/W interface proceeds based on classical electromagnetic theory of stratified layers (2). In particular, when parallel polarized radiation interacts with the A/W interface, the resultant standing electric field has contributions from both the z component of the p-polarized radiation normal to the interface, as well as the x component of the p-polarized radiation in the plane of the interface. The E field distribution for these two... 

Modern methods of vibrational analysis have shown themselves to be unexpectedly powerful tools to study two-dimensional monomolecular films at gas/liquid interfaces. In particular, current work with external reflection-absorbance infrared spectroscopy has been able to derive detailed conformational and orientational information concerning the nature of the monolayer film. The LE-LC first order phase transition as seen by IR involves a conformational gauche-trans isomerization of the hydrocarbon chains a second transition in the acyl chains is seen at low molecular areas that may be related to a solid-solid type hydrocarbon phase change. Orientations and tilt angles of the hydrocarbon chains are able to be calculated from the polarized external reflectance spectra. These calculations find that the lipid acyl chains are relatively unoriented (or possibly randomly oriented) at low-to-intermediate surface pressures, while the orientation at high surface pressures is similar to that of the solid (gel phase) bulk lipid. 

The study of the raesophases by X-ray diffraction, electron microscopy, infrared spectroscopy and circular dichroism20-2S has shown that the structure is always lamellar and can be described as follows the lamellar structure consists of plane, parallel, and equidistant sheets of thickness d each sheet results from the superposition of two layers one of thickness dA formed by the polyvinyl chains in a more or less random coil conformation, the other with a thickness dB formed by the polypeptide chains in an a helix conformation, oriented perpendicular to the plane of the layers, arranged in a bidimensional hexagonal array, and generally folded. 

The study by low-angle X-ray diffraction of these mesophases provides X-ray patterns exhibiting a set of sharp lines with Bragg spadngs characteristic of a layered structure. This lamellar structure results from the superposition of plane, parallel, and equidistant sheets each sheet contains two layers one of thickness dA formed by the carbohydrate blocks, the other of thickness dB formed by the polypeptide blocks, there is a partition of the solvent between the two blocks 70% DMSO is localized in the carbohydrate layer284. Furthermore, the polypeptide chains are in a a-helix conformation as is demonstrated by infrared spectroscopy and X-ray diffraction and are oriented perpendicular to the plane of the sheets and assembled in a bidimensional hexagonal lattice as is revealed by X-rays284. ... 

Practical problems associated with infrared dichroism measurements include the requirement of a band absorbance lower than 0.7 in the general case, in order to use the Beer-Lambert law in addition infrared bands should be sufficently well assigned and free of overlap with other bands. The specificity of infrared absorption bands to particular chemical functional groups makes infrared dichroism especially attractive for a detailed study of submolecular orientations of materials such as polymers. For instance, information on the orientation of both crystalline and amorphous phases in semicrystalline polymers may be obtained if absorption bands specific of each phase can be found. Polarized infrared spectroscopy can also yield detailed information on the orientational behavior of each component of a pol3mier blend or of the different chemical sequences of a copoljnner. Infrar dichroism studies do not require any chain labelling but owing to the mass dependence of the vibrational frequency, pronounced shifts result upon isotopic substitution. It is therefore possible to study binary mixtures of deuterated and normal polymers as well as isotopically-labelled block copolymers and thus obtain information simultaneously on the two t3q>es of units. 

Infrared dichroism has been successfully applied to characterize the orientational relaxation of linear and branched polyst3rrene chains as well as binary blends of long and short chains. By deuterating some chains or parts of chains, infrared spectroscopy provides a method of analyzing the orientational behaviour of the different species and consequently probe the molecular relaxation mechanisms. 

Polymers may exhibit a biaxial orientation. The segmental orientation function is in this case a function of two angular variables, i.e. /(, v), as shown in Figure 10.7. The in-plane orientation is different in the zx and zy planes (Figure 10.7). There are several methods commonly used to determine chain orientation in-plane birefringence, wide angle X-ray diffraction, small angle X-ray diffraction, infrared spectroscopy and sonic modulus measurements. In the case of uniaxial orientation there is only... 

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