Observed crystal orientation factor

Figure 11. The observed crystal orientation factor (filled circles) of the 80/20 PI/PEI molecular composite in comparison with the prediction of the floating rod model (the solid line).

It pays to process the data as it is collected, to observe how far the crystal diffracts in different crystal orientations and how well it merges in terms of R factor in resolution shells and overall cumulative R factor. 

The single hydrogen of the trianion was found to occupy only one of the two octahedral sites rather than being randomly disordered in the crystalline state in both octahedral sites, with an occupancy factor of 0.5 for each position. This observed crystal ordering of the polar C v trianions, which are identically oriented... 

For crystalline samples under plane-wave illumination, a diffraction pattern is observed as a spot pattern. The individual spots depend on the crystal orientation, its structure factor, and obey Bragg s law. Bragg s law states that the difference between the scattered k and the incident ko wave vector is equal to a vector g of the reciprocal lattice ... 

Fig. 6.9. The origin of liquid crystal alignment on rubbed polymer surfaces (A) Liquid crystal molecules have highly anisotropic charge distributions with their tt system preferentially oriented perpendicular to their long axis, which gives an ensemble of oriented (nematic) liquid crystals an asymmetric charge distribution (B). This can be characterized by molecular orientation factors (fa,fb,fc) describing the preferential orientation of the tt system. For the experimentally observed alignment directions the anisotropic charge distribution of the liquid crystal is oriented parallel to the one of the rubbed polyimide (C) and polystyrene (D) surface, which optimizes their interaction energy [3].

Dislocation-dissociation in quartz was first observed by McLaren ef al. [32] in a natural dry quartz deformed at 500 °C, and has been studied most recently by Cordier and Doukhan [33] in a synthetic crystal containing 100 at. ppm [H]/[Si]. By using a crystal oriented to have high Schmid factors for (0001)1/3 (1120)basal slip, 1120 [0001] prismplaneslip, and 1010 1/3 (1213) pyramidal slip, and deformed under a hydrostatic pressure of 1.0-1.1 GPa, Cordier and Doukhan found a flow stress at 500 °C of almost 3 GPa, which decreased to 2 GPa at 900 °C. These high flow stresses correspond to a significant fraction of the shear modulus and reflect the fact that the quartz is relatively dry. Slip at 500 °C was heterogeneous (in slip bands). 

It is, thus, observed that the initial orientation factor and the temperatures of crystallization or annealing are the two important parameters. It was also observed that the rates of strain-induced crystallization are higher by an order of magnitude than the crystallization rate from isotropic polymer melt. 

The manner in which a metal deforms after its yield strength has been exceeded by the applied stress depends on many factors. Parameters controlling the deformation process include the alloy s composition, its class (i.e., whether a, a -i- p, or P), its condition (i.e., whether quenched— e.g., P r, a + P-annealed, low-temperat ire aged, etc.), and the rate and temperature at which the deformation is carried out. Some observables or results of the deformation process include the anomalous stress-strain behavior alluded to above and discussed in Chapter 12,phase transformation under stress (i.e., transformation-assisted deformation), and texturization (i.e., the development of preferential crystal orientation or the formation of deformation cells or subbands in response to heavy cold work). 

In summary, then, it is necessary to measure the fiaction of crystals, the crystalline orientation factor the amorphous orientation factor and possibly the size and size distribution of crystals in order to relate polymer structure to polymer properties. Although the extent of crystallinity is generally measured using density or heat-of-fusion methods, orientation is determined with the help of optical birefringence, dichroism, sonic modulus, or x-ray diffraction [60]. The size of crystals is observed with an optical or electron microscope. 

Numerous observations of the effect in ionic crystals were carried out by Mineev and Ivanov in the Soviet Union [76M01]. This is a class of crystals in which a number of materials factors can be confidently varied. By choice of crystallographic orientation, various slip directions can be invoked. By choice of various crystals other physical factors such as dielectric constant, ionic radius, and an electronic factor thought to be representative of dielec-... 

The intensity of the EPR resonance absorption is a measure of the number of paramagnetic centres present [346], while the type of line observed and the measured g factor are indications of the interactions of the paramagnetic particles and of their distribution within the matrix. Such spectra are much more sensitive to changes in crystal field and atomic orientations than X-ray diffraction and are not dependent upon crystallinity [347]. The nature of the paramagnetic particles may be discerned from the superfine structure of the spectrum. 

Fig. 5.17 Comparison of the experimental PVDOS determined from NIS measurements on Fe (TPP)(NO) (upper panel) with the PVDOS predicted on the basis of DFT calculations using the B3LYP (center panel) and BP86 (lower panel) functionals. Blue traces represent the PPVDOS Dp (v)for oriented crystals (see Appendix 2, Part III, 3 of CD-ROM), scaled by a factor of 3 for comparison with the total PVDOS Dpe(v)of unoriented polycrystalline powder (red traces). Since the X-ray beam direction k lies 6° from the porphyrin plane, modes involving Fe motion in the plane of the porphyrin are enhanced, and modes with Fe motion primarily normal to the plane are suppressed, in the scaled oriented crystal PVDOS relative to the powder PVDOS. In-plane Fe modes dominate the 200-500 cm range of the data, while Fe motion in modes observed at 74, 128, and 539 cm is predominantly out-of-plane. Crosshatching in the upper panel indicates the area attributable to acoustic modes. In the lower two panels, the Fe-NO bend/stretch modes predicted at 386 and 623 cm , have been artificially shifted to the observed 539 cm frequency to facilitate comparison with the experimental results. Predicted PVDOS are convolved with a 10 cm Gaussian (taken from [101])...

NFS spectra recorded at 300 K for -cut and c-cut crystals are shown in Fig. 9.17 [48]. The/factors for the two orientations were derived from the speed-up of the nuclear decay (i.e., from the slope of the time-dependent intensity in Fig. 9.17a and from the slope of the envelope in Fig. 9.17b). The factors obtained f ( P = 0.122 (10) and f = 0.206(10) exhibit significant anisotropic vibrational behavior of iron in GNP. This anisotropy in f is the reason for the observed asymmetry in the line intensity of the quadrupole doublet (in a conventional Mossbauer spectrum in the energy domain) of a powder sample of GNP caused by the Goldanskii-Karyagin effect [49]. 

Reinitzer discovered liquid crystallinity in 1888 the so-called fourth state of matter.4 Liquid crystalline molecules combine the properties of mobility of liquids and orientational order of crystals. This phenomenon results from the anisotropy in the molecules from which the liquid crystals are built. Different factors may govern this anisotropy, for example, the presence of polar and apolar parts in the molecule, the fact that it contains flexible and rigid parts, or often a combination of both. Liquid crystals may be thermotropic, being a state of matter in between the solid and the liquid phase, or they may be lyotropic, that is, ordering induced by the solvent. In the latter case the solvent usually solvates a certain part of the molecule while the other part of the molecule helps induce aggregation, leading to mesoscopic assemblies. The first thermotropic mesophase discovered was a chiral nematic or cholesteric phase (N )4 named after the fact that it was observed in a cholesterol derivative. In hindsight, one can conclude that this was not the simplest mesophase possible. In fact, this mesophase is chiral, since the molecules are ordered in... 

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