Crystal defects considerations

MgCl2-Supported Catalysts. Examination of polymerizations with TiCl catalysts has estabUshed that only a small percentage of titanium located on lateral faces, edges, and along crystal defects is active (52) (see Titanium and titanium alloys). This led to the recognition that much of the catalyst mass acted only as a support, promoting considerable activity aimed at finding a support for active titanium that would not be detrimental to polymer properties. 

Effect of Crystal Defects on Corrosion—General Considerations... 

Active Dissolution and Crystal Defects—Energy Considerations... 

The usefulness of quadrupolar effects on the nuclear magnetic resonance c I 7 yi nuclei in the defect solid state arises from the fact that point defects, dislocations, etc., give rise to electric field gradients, which in cubic ciystals produce a large effect on the nuclear resonance line. In noncubic crystals defects of course produce an effect, but it may be masked by the already present quadrupole interaction. Considerable experimental data have been obtained by Reif (96,97) on the NMR of nuclei in doped, cubic, polycrystalline solids. The effect of defect-producing impurities is quite... 

General Considerations. In the well-known photodimerization of anthracenes in liquid solution 9, lO /lO, 9 -dimers (head-to-tail [4 + 4]) are formed in most cases. However, there have also been instances where head-to-head-photodimers (9,9 /10,10 ) are produced [19], and these were overseen previously. The solid phase photoreactions of anthracenes charged the topochemical postulate [7] for decades with hitherto unsolvable difficulties. All examples that contradict this assumption were eliminated without hesitation from the scope of topochemistry and termed to be crystal defect reactions, because the topochemically allowed processes were taken as support for topochemistry without further proof. The later provision, that the dimerizations occur within reaction cavities in the bulk of the crystal [20], did not help in this respect. A summary of the various arguments is given in Ref. 8. From examples 7 to 8 only b and perhaps c formally meet... 

Combinations of rigid and flexible segments complicate crystal structures considerably. In particular is was shown in Sect. 5.1 that seemingly rigid p-phenyl sequences may still show partial and complete ring-flips about the ring axes. The motion and introduction of defects based on ring-flips is much less cooperative than found for many flexible molecules. 

In general, the smaller the particle size, the more rapidly dehydration occurs. This can be justified on the basis of surface area and/or, in some cases, the existence of crystal defects. The latter is particularly true for substances that have been subjected to high-energy particle size reduction processes. The impact of morphology may be more subtle and is certainly not independent of size considerations. The balance between the two mechanisms has been a topic of discussion, but very little quantitative work has been done to elucidate the relative contribution to the dehydration process. 

The maximum possible resolution of the liquid crystal technique obtained for pores in an Si02 layer, about 100 nm thick, was 0.01 fim at 7 60 V [84, 86]. Such a high sensitivity of the method is due to the fact that the area of deformation in a liquid crystal layer considerably exceeds the defect in size. In other words, the layer acts as a magnifying lens. 

Traditional introductory materials science texts, concerned mostly with metals and ceramics, devote considerable space to crystal structures and crystal defects. These topics have been studied for polymers, but they are hardly mentioned here. Why are they much less important in polymers ... 

The number and kind of defects in a given specimen, as well as the crystal habit and with it the proportion of different crystal faces exposed, will in general depend in considerable degree on the details of preparation. The production of a standard sample of a given chemical substance, having reproducible adsorptive behaviour, remains therefore as much an art as a science. 

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