Cross polarization defined

Table II highlights the 3C NMR relaxation times that reflect molecular motions and help define the physical properties of the cutin polymer. For those solid-like carbons that cross polarized, considerable motional freedom was evidenced for (CH2)n and CH2OCOR groups, on both MHz and kHz timescales, by the short values of Tj(C) and Tip(C), respectively. By contrast, the CHOCOR moiety was more restricted dynamically as judged from its long value of TX(C) low-frequency motions in particular were implicated by the strong dependence of Tip(C) on Bi. These latter groups...

With the programs in the appendixes one can compute elements of the scattering matrix for spheres and, consequently, all the quantities defined in the preceding paragraph. The cross polarization vanishes for spherical particles, and the following relations hold ... 

Figure 9.3 Conventional 13C cross-polarization magic angle spinning (CP/MAS) solid-state NMR spectra of mangrove leaves at different stages of decomposition in estuarine waters. Differences in carbon functionality are defined by the following key characteristic resonances carbohydrates (50-90 ppm) carboxyl (175 ppm) aromatic (90-170 ppm). (Modified from Benner et al., 1990.)...

These different contrast mechanisms can all be used to reveal the scale of liquid crystalline polymer microstructures. In specimens that exhibit a mosaic texture, and in those that contain predominantly planar defects, domain size is easily defined in terms of areas that uniformly show extinction between crossed polars. However, if the defects are predominantly linear, as in specimens that exhibit schlieren textures, such simple characterization of microstructural scale is no longer possible. Here it is more convenient to look at the length of disclination line per unit volume, which is equivalent to the number of lines intersecting unit area, and analogous to the dislocation density as defined for crystalline solids. Good contrast is essential in order to obtain an accurate count. Technologically, microstructural scale is of growing interest because of its relevance to processability, mechanical properties and optical transparency. 

The spectra were obtained at a frequency of 100 MHz and with a MAS frequency of 12.487 kHz corresponding to the n = 1 rotational resonance that is the spinning speed was equal to the difference in isotropic shifts between the labeled sites. The time shown denotes the interval between inversion of the labeled methylene resonance, which occurs immediately after cross polarization, and application of the observation pulse. Thus, 0 ms corresponds to the initial nonequilibrium state established by the selective inversion. The rate of this decrease may be modeled to determine the distance between the labeled sites (Fig. 23.24 (bottom)). Comparison of magnetization exchange data (filled circle) for a37-38 along with the calculated curves for four distances 4.74 A (dotted line), 4.0 A (dashed line), 3.9 A (solid line) and 3.8 A (dot-dashed line). The 4.74 A distance would be expected for an idealized antiparallel j8-strand. The best fit to the data is 3.9 A. This result is for the undiluted a37-38 sample. To eliminate possible effects due to intermolecular interactions, measurements were performed on isotopically pure samples and on samples in which the doubly labeled peptide was diluted 1 5 and 1 10 in unlabeled peptide. This produced a corrected distance for a37-38 of 4.0 A and in all cases, the distance could be defined to within... 

The effect of additives on the asphaltene from the Catalytic Incorporated (Cat. Inc.) coal liquid product was studied. Asphaltene is defined as the pentane insoluble but benzene soluble part of the coal liquid. The fractionation procedure has been described in detail elsewhere(l) and is shown schematically in Figure 1. Some work was also done with A240 petroleum pitch. Elemental analysis for the Wyoming sub-bituminous coal. Cat. Inc. coal liquid product, and Cat. Inc. asphaltene and A240 petroleum pitch are shown in Table I. Measured amounts of the additive compounds to be studied were added to the Cat. Inc. asphaltene and petroleum pitch. The samples were pyrolyzed and the pyrolysis residues examined by cross polarized light microscopy. Elemental analyses of the residues were done. 

Knowledge of the electrooptic behavior of the FLCPs is of the utmost importance for display device applications. One relevant parameter in this respect is the response time. As for the spontaneous polarization, the determination of the response time requires a uniformly aligned sample. The test cell is placed between crossed polarizers so that one tilt direction is parallel to the direction of one polarizer. The electrooptic effect is achieved by applying an external electric field across the cell, which switches the side chains from one tilt direction to the other as the field is reversed. A photodiode measures the attenuation of a laser beam when the cell is switched between the two states. Generally, the electrooptical response time is defined as the time corresponding to a change in the light intensity from 10 to 90% when the polarity of the applied field is reversed ( 10-9o)-... 

The tradeoff between angular viewing dependence and N is shown explicitly in Fig. 2 where measured contrast ratio is plotted as a function of the angle of incidence 0 in the principal viewing plane for a typical TN-LCD with E-7 biphenyl liquid crystal mixture between crossed polarizers. The principal viewing plane is defined by the sample normal and the liquid crystal director midway between the cell electrodes. Contrast ratio is defined as the ratio of cell transmission I with Vjjg applied to transmission with V3 applied. 

In the limiting case of high voltage applied to a "perfect" cell between crossed polars, the transmittance should be dictated by the dichroism of the polars. Defining dichroism of a polar as the difference between absorbances of the polar in crossed and parallel orientations to a perfectly polarized beam, it should be apparent that polar dichroism would have to be somewhat greater than 5 to achieve a contrast ratio of 10 under the above conditions. Literature on commercially available polars does... 

Optical properties High birefringence, perfect cleavage, bird s-eye maple appearance and lack of colour identify pyrophyllite. Biaxial (-), P= 1.588°,a= 1.553°, Y= 1-600, 5 = 0.047, 2H=52°-62° Talc is similar to muscovite, chlorite and pyrophyllite, but has a smaller 2V and often appears smeared or poorly defined when viewed under crossed polars. Biaxial (-), P = 1.58°, a =1.54°, 7=1.58, 5 = 0.05,2H=6°-30°... 

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