Orientation transmission method

Figure 4 Evolution of the orientation during and after the deformation of PS films at different temperatures above Tg. Time-resolved birefringence results with up to a 2 ms time resolution were obtained using the transmission method. Reproduced with permission from Messe et al. [15]. Copyright Elsevier 2001.

Some diffractometers can accommodate samples held in capillary tubes usually made of glass or quartz. Only a few milligrams of sample are required. The capillary is rapidly rotated to help insure the distribution of the particles in all possible crystallite orientations. The X-ray beam is perpendicular to the sample and passes through it (transmission method). Some discretion must be used, as the presence of heavy elements in the sample will increase the absorption. 

The Schulz transmission method [9.10] involves the use of a divergent, rather than a parallel, incident beam and a counter slit narrow enough to intercept only the central portion of the diffracted beam. For these conditions Schulz derived an equation, analogous to Eq. (9-9), relating the intensity diffracted by a random specimen (for brevity random specimen will be written for specimen with randomly oriented grains ) to the angular setting a. He also showed that the intensity diffracted by a random specimen was constant within a few percent up to an a value of about 30° for a sufficiently thin specimen nt = 0.4 to 0.7) and small values of 6 (less than about 20°) under these conditions the correction equation is not needed. 

Fig. 9-17 Pole-figure goniometer. The specimen shown is positioned for measurements by the transmission method, and a simple change in the orientation of the specimen holder allows measurements by the Schulz reflection method. The x-ray tube is seen here end-on. This instrument is designed for automatic operation. (Courtesy of Siemens Corporation.)...

Diffracted intensities, proportional to pole densities, may be put on a times-random basis by comparing them with intensities diffracted by a random specimen [9.20]. The random specimen should be of the same material as the textured specimen and, for a transmission method, it should have the same value of fit if not, a correction has to be made that will depend on the transmission method involved. The random specimen itself is usually made by compressing and sintering a powder [9.11, 9.12]. The randomness of grain orientation in this specimen must be checked by determining its diffraction pattern with a diffractometer in the usual way the measured integrated intensities of all lines should agree with those calculated by Eq. (4-21). 

Consider the diffraction geometry for a = 0 in the transmission method for determining preferred orientation and for a = 90° in the reflection method. Let rjnf be the infinite thickness required in the reflection method, and assume ti is that thickness which would diffract 99 percent of the intensity diffracted by a specimen of truly infinite thickness. Let opt be the optimum thickness for the transmission method. 

In solids there are two general methods used for the determination of An. One is the transmission method while the other is a compensator technique. Each has advantages and disadvantages and thus the method chosen depends highly on the experimental circumstances as well as on the material itself. It is worth pointing out that the above discussion applies to both uniaxially and biaxially oriented systems. Two other methods not to be discussed here are interference microscopy and refractometry. ... 

A comparison of the spectra measured by the reflection-absorption method and that measured by the transmission method was made by using machine oil as a test sample the results are shown in FigurelO.6. The sample oil was thinly spread onto a chromium-coated glass substrate without any particular measure to control the orientation of component molecules of the sample. The spectrum obtained by the reflection-absorption measurement shown in (a) of Figure 10.6 agrees satisfactorily with the absorbance spectmm shown... 

RA and transmission techniques [3], and applied it to the studies of LB films of cadmium stearate [3], azobenzene-containing long-chain fatty acids and their barium salts [4], dipalmitoylphosphatidylcholine (DPPC) [5], and polyion complexes [6]. Furthermore, we explored the relationship between the molecular orientation evaluated by this method and pyroelectricity in alternating (noncentrosymmetric) Y-type LB films consisting of a phenylpyrazine-containing long-chain fatty acid and deuterated stearic acid and of their barium salts [7]. 

In the normal-incident transmission measurements of LB films deposited on transparent substrates, the electric vector of the infrared beam is parallel to the film surface (Figure 5A). Therefore, only absorption bands which have the transition moments parallel to the film surface can be detected by this method. On the other hand, in the above-mentioned RA measurements, in which the p-polarized infrared beam is incident upon the LB film prepared on Ag-evaporated substrates at a large angle of incidence, we have a strong electric field perpendicular to the film surface as shown in Figure 5B. Therefore, in this case, only absorption bands which have the transition moments perpendicular to the film surface can be detected with a large intensity enhancement. Thus, if the molecules are highly oriented in the LB films, the peak intensities of particular bands should be different between the transmission and RA spectra. 

On the basis of these experimental results and discussion, we tried to establish a convenient and useful method for quantitative evaluation of the molecular orientation in thin LB films using FT-IR transmission and RA spectra. Here, we assume a uniaxial orientation of the transition moment with an angle < > around the normal axis, Z, to the LB film surface i. e. the XY plane (Figure 8). In this case, the ratio of the absorbance of a particular band in the transmission spectrum, AT, to that of the same band in the RA spectrum, AR, is given by... 

Solid state spectra can be measured by the Attenuated Total Reflectance or Multiple Internal Reflection methods as well as by simple transmission techniques (222). Brooker (223) has reported the discovery of the translational vibrations of NaN02, NaNOs, and CaCOs (calcite). The orientation of the bonds in (U02)(N0s)2. 6 H2O has been ascertained from the spectra obtained in this manner (223). 

The structural anisotropy in crystalline or structurally ordered BPDA-PFMB films was studied in this laboratory with wide-angle X-ray diffraction (WAXD) methods. In brief, WAXD experiments were designed to examine both the reflection and transmission modes of thin-fihn samples. In addition, uniaxially oriented polyimide fiber WAXD patterns were obtained to aid in the identification of the film structure. The film WAXD pattern obtained from the reflection mode corresponded well to the fiber pattern scanned along the equatorial direction (Figure 16.3), " which indicates that the reflection mode pattern represents the (hkQ) diffractions. On the other hand, as shown in Figure 16.4, the (001) diffractions were predominant in the film WAXD pattern obtained via the transmission mode. This pattern corresponded to the fiber pattern scanned along the meridian direction. These experimental observations clearly indicate that the c-axes of the crystals are preferentially oriented parallel to the film surface however, within the film, they are randomly oriented. 4.2 5 j( should be pointed out that the WAXD experiments are only sensitive to crystalline or ordered structures in polyimide films. They do not provide any information on the amorphous regions. 

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