Defect groups formation

Establishment of the detailed microstructure of PVC has attracted considerable interest. This has been spurred by the desire to rationalize the poor thermal stability of the polymer (Chapter 1). Many reviews have appeared on the chemical microstruclure of PVC and the mechanisms of defect group formation.56 60... 

HM-MOR was dealuminated with phosgene between 673 and 973 K. Chemical analysis of samples combined with A1 NMR and Si NMR data allowed us to quantify the Si/Al ratio, the concentration of extra-framework aluminium and framework vacancies. The Si NMR spectra reveal progressive dealumination with increasing temperature, accompanied by formation of SiCl and Si(OH)2 defect groups. The former yields quantitatively SiOH groups. The result showed unambiguously that framework reconstruction occurred during dealumination. 

It is observed that a much larger relative amount of defect groups is formed when hexamethonium ions are incorporated in the ZSM-48 zeolitic structure. The results can be rationalized, if it is supposed that each terminal trimethylammonium groups leads to one missing tetrahedral site in the zeolitic structure, therefore inducing the formation of 4 hydroxyl nests. 

It is clear that a large number of parameters influence the formation of the sheets in a tubule and that their relative importance is still unknown, as is also the cause of the occurrence of the defects responsible for the eventual polygonization of the sheets. Although the model presented here highlights the necessity of including, as one of the parameters, the uncertainty 8r or dd on the separation of successive cylindrical sheets, it is impossible to predict with absolute certainty the final characteristics of any of these sheets, symmetric or not, on the basis of the characteristics of the previous one. Nevertheless, a number of features of their structure, such as the presence or absence of helicity, and the presence of groups of sheets with nearly the same angle of pitch, can be explained and quantified. 

Significant stimulated emission is only found for the pristine side of the sample. From these results it was concluded that the photoinduccd absorption that suppresses die stimulated emission is directly or indirectly caused by the presence of oxygen-related defects. It was shown earlier that the effect of photooxidation on the emission properties of PPV can be explained by the formation of carbonyl-groups that act as sLrong electron acceptors leading to an efficient dissociation of the plioh excited slate 29). It can be concluded that the dissociated pair near the defcci leads to the strong photoinduccd absorption. The observation that... 

Generally, during hydroboration polymerization of dicyano compounds, the formation of the borazine structures that have a six-membered boron-nitrogen ring (scheme 19a) and dihydroborated end groups (scheme 19b) as a structural defect is unavoidable. The borazine cross-linked structures often cause the gelation, and dehydroboration causes a decrease in molecular weight. 

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