Configurational defect

The first results of computer-based assessment system application show that the benefits are obvious for repaired (without heat treatment) welds and complex defect configurations defect with height local increasing, group of defects, case analysis of defects interference and possible joining. 

To illustrate this point it is particularly instructive to examine the configurational defects existing in isotactic polypropylene. The steric control during polymerization can in principle be attributed to two different factors the influence of the chirality of the last entered unit (this case falls within the discussion of symmetric chains reported previously) or that of the catalytic site. The chirality of the latter may be preexisting or may arise at the moment of polymer-... 

The H3 center is connected with a pair of nitrogen atoms, which captures one vacancy, while the H4 center is principally the same but with a slightly different configuration. Defect S2 is beheved to be due to a combination of two vacancies and one nitrogen atom, while S3 is connected with a combination of several nitrogen atoms and a vacancy and SI with a vacancy-nitrogen pair. 

If all the bonds follow in the same sequence, i.e., ( + )( — )( + )( — )( + ) (-) or ( + ) (—) ( —) ( + ) (+) (—) (-) ( + ), then one speaks of holotactic polymers. Holotactic, monotactic polymers thus show only isotactic or syndiotactic diads, whereas real polymers always possess a number of configurational defects, i.e., they contain iso- as well as syndiotactic diads (see also Section 3.2). The bonds of real polymers, therefore, do not always follow in the same sequence, i.e., they can be ( + ) ( —) (-h) (—) ( —) (-h) ( + ) ( —), etc. The configurational defects make it necessary to bear in mind the randomness of the configuration when dealing with real structures. 

Fig. 5. Fourier transform Raman spectra of various syndiotactic polypropylenes with known configurational defects (a) experimental (b) theoretical.

Effects of Molecular Configurational Defects on Crystallization, Melting, and Annealing... 

No ZN-catalyst is completely stereospecific. The best heterogeneous ZN-catalyst produces polypropylene which contain configurational defects (2% to 5% racemic diads ). The stereoselectivity is sufficiently high so that 99+% of the PP are insoluble in refluxing n-heptane and have T j, (melting temperature)2.165 C. Such products are commercially referred to as isotactic PP (i-PP). 

Fig. 11. Resolved normal mode relaxation of PI-3 sample. The lines represent the behaviour predicted by the Rouse model including for the actual sample polydispiersity (soUd line) and without it (dotted line). Triangles correspond to the difference between the experimental losses and the Rouse model predictions. The inset shows schematically how the presence of configuration defects allows fluctuations of the whole chain dipole moment without variation of the end-to-end vector. Reprinted with permsision from Riedel et al, Macromolecules 42, 8492-8499 Copyright 2009 American Chemical Society...

In catalyst systems developed for polypropylene like the single site metallocene catalysts, highly isotactic polypropylene structures with configurational defects are obtained by a chain-end mechanism of stereocontrol. For catalytic polymerizations where the tacticity is high, configurational defects can be recognized. 

Volumetric defects clusters of atoms of different chemical composition, ordered regions, clusters of point defects, divacancies, trivacancies, impurity-atom vacancy or interstitial complexes, small regions of disorder, extended long-range configurational defects, initial stages of spinodal decomposition. 

A fifth type of imperfection in MX compounds is for an M atom to exchange positions with an X atom, that is, the configurational defect mentioned in the introduction. 

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