Crystal random

Powder, single crystal, randomly oriented, or aligned film... 

In most studies of surface chemistry, it is common practice to devote a small fraction of the total effort to the preparation of the surface and then much effort to an elaborate measurement of rates and other surface properties. Obviously, the measurement is no better than the preparation of the surface, and if the surface is contaminated, strained, or has an unknown crystal orientation, the measurements can have very little meaning. Furthermore, since metals as ordinarily used consist of minute crystals randomly arranged, measurements made on such specimens are composite quantities and tell little about the reactivity of a particular type of structure. It is therefore highly important that the preparation of the catalyst specimen be carried out with great care, and this is generally a tedious undertaking. 

In a final interesting example of solid state [2+2] photocycloaddition chemistry involving the sec-butyl group, it was found that the optically pure compound 6e can crystallize randomly in two dimorphic modifications, A and B, as shown in Scheme 5... 

Orientation of crystals—random or preferred, local or general. 

Keywords Mesophase Crystallization Random coil Folded-chain fringed-micellar grains Flow... 

Copolymers are often sufficiently irregular not to be able to crystallize. Random copolymers of short repeating units, such as vinyl polymers, and mass fractions of 0.3-0.7 are normally amorphous since sequences of identical repeating units must commonly be two to five nanometers long to crystallize, and cocrystallization of different repeating units is not very frequent (see Sect. 5.1.10). 

Fig. 8. Temperature dependence of the degree of preferred orientation by IR spectroscopy in a liquid crystal random copolyester (V). The orientation is explained as the order parameter, S = (/ 2(cos or)). O, Oriented samples in the nematic state , oriented samples quenched to the solid from the nematic state A, X-ray measurement for comparison made at 260°C.

Crystal Structures.—Crystallization, a pre-requisite for diffraction studies, is a notoriously and unpredictably difficult exercise with new proteins. Carter and Carter have introduced a method which searches a large number of experimental variables (e.g., buffer components and pH) that can influence rates of crystallization. Random combinations of the variables are used to attempt crystallization and the resulting precipitated protein scored on an arbitrary crystallinity scale. After applying the appropriate statistic, a complete factorial experiment is set up using the conditions which promoted crystallization in the first experiment. 

Sea salt consists of quite large single crystals randomly grown together. Inclusions of mother liquor inside the crystals or between the grown-together crystals, as well as gypsum as cocrystals, are reasons for the limited purity of around 98 wt%. 

Figure 15.23 A polycrystaUine solid. This microscopic-level photograph shows small, orderly crystals, randomly arranged in solid sphalerite (zinc sulfide). 

There is little reorganization in a crystal of potassium Kurrol s salt even at temperatures near melt temperatures, because there are no phase transitions in potassium Kurrol s salt crystals. Random cross-linkages may form and disappear as equilibrium is reached. In sodium systems, crystals of Kurrol s salt do suffer phase transitions. At a phase transition, chaos of melts is recaptured over an interval of transition. 

There has been considerable argument over the years concerning the way in which folding occurs and the nature of the fold plane. The different models that have been suggested to account for folding in polymer crystals are illustrated schematically in Fig. 4.11. The models range from random re-entry ones, where a molecule leaves and re-enters a crystal randomly, to adjacent re-entry models, whereby molecules leave and re-enter the crystals in adjacent positions. Two particular adjacent re-entry models... 

A side effect of a compatibilizer is heterogeneization of the polyethylene matrix. This is related to selective migration of unbound compatibilizer into those regions of the matrix where the semicrystalline nanostructure is predominantly made from imperfect crystals randomly distributed in the amorphous phase. 

Hachiboshi et al were able to crystallize random copolymers of ethylene terephthalate/ ethylene isophthalate over the complete composition range. The wide-angle X-ray patterns of these copolymers change systematically with co-unit content and it is concluded that the two units can cocrystallize and can form a new unit cell. A complete phase diagram, involving both the solidus and liquidus composition, was determined and is illustrated in Figure 23. The solidus was determined by... 

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