Crystal irregularity

Fig. 18 Partially interactive powder mixture between sodium chloride (A) and micronized etilefrine HCl. The picture (C) is an extension of the picture (B). Etilefrine HCl settle at crystal irregularities of the cubic sodium chloride.

An appropriate internal standard for MALDI must compensate not only for any crystallization irregularities but also for subsequent desorption and gas-phase effects. In choosing an internal standard, the relative polarities of the analytes and internal standard as well as their solvent solubilities should be considered (Sleno and Volmer, 2006). Structural similarities should reflect the gas-phase behavior of the involved molecules and extend to solubility. Naturally, an isotope-labeled standard is the ideal choice since its chemical behavior is nearly identical to its unlabeled counterpart (Gusev et al., 1996). Such a standard guarantees identical crystallization and gas-phase behavior of the analyte and internal standard (Kang et al., 2001). 

Crystallization can usually, but not always, occur if the polymer structure is regular. Except in those cases where isomorphous replacement is possible irregular structures cannot crystallize. If it is desired to avoid crystallization, irregularities may be introduced in the following ways ... 

As various irregularities of crystals lattice are discussed above, it is now necessary to analyze their effect and importance in heterogeneous catalysis. There are at least two facts confirming the relation of crystal irregularity with the catalytic active center on catalyst surface. One of them is on those sites where the dislocations and surface point defects occur, and the atomic arrangements would differ from the others sites in catalyst surface, while surface atomic space and the properties of stereochemistry would remain the important factors to decide the catalytic activity. 

FIGURE 9.1 Resin-bond grinding (RVG ) diamond crystals—irregularly shaped, friable. 

The melting and boiling points of the aluminium halides, in contrast to the boron compounds, are irregular. It might reasonably be expected that aluminium, being a more metallic element than boron, would form an ionic fluoride and indeed the fact that it remains solid until 1564 K. when it sublimes, would tend to confirm this, although it should not be concluded that the fluoride is, therefore, wholly ionic. The crystal structure is such that each aluminium has a coordination number of six, being surrounded by six fluoride ions. 

Deviations from the Avrami equation are frequently encountered in the long time limit of the data. This is generally attributed to secondary nucleation occurring at irregularities on the surface of crystals formed earlier. 

Polymer crystals most commonly take the form of folded-chain lamellae. Figure 3 sketches single polymer crystals grown from dilute solution and illustrates two possible modes of chain re-entry. Similar stmctures exist in bulk-crystallized polymers, although the lamellae are usually thicker. Individual lamellae are held together by tie molecules that pass irregularly between lamellae. This explains why it is difficult to obtain a completely crystalline polymer. Tie molecules and material in the folds at the lamellae surfaces cannot readily fit into a lattice. 

Halogenated Butyl Rubber. Halogenation at the isoprene site ia butyl mbber proceeds by a halonium ion mechanism leading to a double-bond shift and formation of an exomethylene alkyl haUde. Both chlorinated and brominated mbber show the predominate stmcture (1) (>80%), by nmr, as described eadier (33,34). Halogenation of the unsaturation has no apparent effect on the isobutylene backbone chains. Cross-linked samples do not crystallize on extension due to the chain irregularities introduced by the halogenated isoprene units. 

When the polymerization is mn at less than about 20°C, the resulting polymer is hard and tough and has valuable properties as an adhesive. Polymer made at higher temperature, with more chain irregularities, tends to be much slower crystallizing, and is more suitable for mechanical goods appHcations. 

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