Infrared dichroism/dichroic ratio

In 1958, Beer et al. re-examined the infrared dichroism situation and remeasured the dichroic ratio for several of the peptide bands in collagen. On the basis of studies on model compounds, they calculated the directions of the transition moments for the principal bands in the peptide link and, using the atomic coordinates for various proposed collagen models, the inclination of these transition moment vectors to the fiber axis. Since Beer (1956) had shown that the dichroism of a partially oriented polymer may be considered equivalent to that of a sample containing fully aligned and completely random portions, a disorientation parameter (/) characteristic of the degree of order of the sample could be calculated for each band. The spread of the values obtained for this parameter, which is characteristic of the sample only and not of the bands used to determine it,... 

Another use of infrared spectroscopy employs linearly polarized infrared radiation to determine information about oriented samples, an experiment that has been termed infrared dichroism. The absorbance will be a maximum when the electric field vector and the dipole moment vector are in the same direction, and the absorbance will be zero when the two are perpendicular. Only uniaxial orientation will be considered because this situation is normally found in the literature. R, the dichroic ratio, is defined as follows ... 

Carbonate tons and Enamel. Elliott (1965) has used polarized infrared absorption spectra to study longitudinal sections of human tooth enamel in order to determine whether carbonate ions can substitute for hydroxyl ions in the enamel. His conclusion was that carbonate ions substitute to a very limited extent for hydroxyl ions. The evidence consisted of certain bands in the infrared spectra of enamel (Fig. 19.12) which coincide with those of the synthetic apatite in which this substitution is known to have taken place (Fig. 19.13). In a hydroxyapatite that had been reacted with carbon dioxide at 1000°C, carbonate ion had absorption bands at 878, 1463, and 1528cm , and the hydroxyapatite 3570 cm band (OH ) had disappeared (Fig. 19.13). Elliott examined enamel which had been heated at 1000°C in carbon dioxide and measured the dichroism of the out-of-plane deformations at the 879 cm" mode (Fig 19.14). From the dichroic ratio he was able to calculate that the plane of the carbonate ion is nearly parallel to the c-axis of the apatite. Elliott et al. (1948) have given the dichroic ratio applicable to this case as... 

The dependence of absorbency of polarized radiation on the direction of polarization. For polymers the magnitude of dichroism, expressed as dichroic ratio, depends on the orientation of the radiation absorbing groups, thus the macromolecules. In consequence, the infrared dichroism is a powerful method to measure the molecular orientation. 

Ultimate dichroic ratio Dynamic infrared linear dichroism Observation period Time... 

Observations of infrared dichroism on pol3rmeric systems were first made about 1950 using pols peptides (53,54). Fraser (55,56) developed its use to measure orientation in polymers quantitatively and establish a relationship of dichroic ratio D to the Hermans orientation factor equivalent to equation 2. The first infrared dichroism studies on polyethylene were performed in 1954 by Stein and co-workers (57,58). The article (58) discusses the determination of uniaxial orientation in polyethylene films using wide-angle x-ray (wax) diffraction, birefringence, and infrared dichroism, and explicitly states the interrelation of the former two measurements through... 

The infrared absorption bands of syndiotactic PVC are classified into three groups from their dichroic properties. When the film is fully stretched, most of the bands show dichroism as shown in Fig. 1 (draw ratio 7). When the film is only partly stretched, some of the bands show the dichroism different from that observed for the fully stretched film. Fig. 1 (draw ratio 2.5) gives the result. This dichroism inversion is observed for four bands [J]. 

---