Polarized IR Radiation

Polarization mechanisms, 10 21 Polarized ir radiation, 24 234 p-Polarized light, 24 115 s-Polarized light, 24 115... 

IR spectroscopy is not only useful for determining the chemical constitution of polymers. It additionally provides profound information on chain orientation and on the orientation of attached lateral substituents of polymers. In this case, polarized IR radiation is applied which is only absorbed by an IR-active bond if the plane in which the electrical field vector E of the IR beam oscillates is parallel to the transition dipole moment p of the vibration to be excited. If, on the other hand, the transition dipole moment p is perpendicular to the electrical field vector E of the IR beam no absorption is observed. Using this effect, the degree of orientation of a polymer sample (film, fiber) can be estimated by comparing the intensity at maximum /(11) and at minimum I ) absorption, i.e., the dichroic ratio. 

Another way to obtain information about molecular structure is by means of polarized IR radiation (see Section 3.3.10). This method has the potential advantage of providing the direction of the vibrating dipole. There are also disadvantages some kind of oriented sample is... 

Fig. 54. Time-resolved FTIR spectra in polarized IR radiation of p-xylene sorption into sili-calite-1 at 353 K. From bottom to top, the spectra correspond to the uptake of 0,1,2,3 and 4 molecules of p-xylene per unit cell. The insert above the spectra shows schematically a silicaUte-1 single crystal with the two types of straight and zigzag (or sinusoidal) channels (reproduced from [949]).

For chiral molecules a small difference in the magnitude of absorption of left- and right-circularly polarized IR radiation is observed. This is known as vibrational circular dichroism and, since the effect can be observed from each normal mode, absolute stereochemical information can be obtained from the entire molecule. This is different from the UV analog of this effect, where there may often be only one chromophore present. 

The biggest changes that have occurred in recent years have been in instrumentation, and the chapter on IR experimental techniques has been revised extensively. We have avoided detailed descriptions of instruments rather we have described in broad terms the basic features of instrumentation. These include discussions of interferometers that are used in Fourier transform IR spectrometers, and discussions of grating monochromators that are used in dispersive IR spectrometers. Infrared spectra can be run on gases, liquids, or solids most of the techniques used in running these samples are discussed. Solid sampling techniques are the most diversified. Sections are included on quantitative analysis, internal reflection spectroscopy, and the use of polarized IR radiation. 

Figure 1 Energy-level diagram illustrating the definition of VCD as the difference in the absorbance of a molecule for left versus right circularly polarized IR radiation in a vibrational transition between states gO and g in the ground electronic state.

Other uses of IR spectroscopy include monitoring reactions through the appearance or loss of an absorption and measurements of orientation by use of polarized IR radiation. 

In order to determine the polymer chain orientation, infrared measurements of transmittance or absorbance must be performed with polarized IR radiation, parallel or perpendicular to the mechanical draw direction, for a particular absorption band of the spectrum. Fig. 7.11 shows the FTIR spectra obtained for two orthogonal polarization directions, parallel and perpendicular to the draw direction of a poly(vinylidene fluoride) (PVDF) polymer sample. It is possible to observe that the overall spectra are similar with neither modes being totally suppressed nor new modes seeming to appear. Nevertheless, the amount of IR radiation absorbed by some particular modes are clearly different for IR spectra obtained with radiation polarized in the perpendicular or parallel to the polymer draw direction (Fig. 7.11). 

By using linearly polarized IR radiation, the orientation of the functional groups in a polymer system can be measured. 

Figure 33 F-IR polarization spectra of the prealigned FLOP (47 °C) at different polarities (IR radiation polarized at 45° to the rubbing direction (see Figure 32)).

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