Region superstructure

We have already mentioned that depending on composition, semicrystalline triblock copolymers can show some conflict between microphase separation and superstructure formation. In fact, one of the controversial aspects is the question whether block copolymers can or cannot exhibit spherulites. This is a relevant question because spherulitic structures greatly affect the ultimate mechanical properties, and the boundaries between adjacent spherulites are often weak points in mechanical performance. Kim et al. [125] studied the competition between crystallization within microphase-separated regions and reorganization into supermolecu-lar spherulites in semicrystalline PS-b-PB-b-PCL triblock copolymers. These authors found that the formation of spherulites is strongly affected by the thickness of the specimen in such a way that thin films crystallize into... 

In fig. 6a.b we can observe as examples how the user can define different areas of the ED pattern (lines, or regions around spots ) can select in order to speed up time of measurement and avoid unnecessary beam damage, or concentrate the study in selected group of reflections ( e.g symmetry-related reflections or superstructure reflections). 

This is, however, not the whole of the matter. The superstructure ordering of point defects the collection of interstitial ions along certain fines or sheets, as in Magneli s model for the precursor of his shear structures the temperature-dependent adjustment of composition of a nonstoichiometric phase at the boundary of the bivariant range the nucleation of a new phase of different stoichiometry—these depend on accumulating vacancies or interstitials in some regions of the crystal lattice at the expense of others. 

In the technique of spot-profile analysis LEED (SPA-LEED) [29, 30] intensity variations across LEED diffraction spots are measured. The technique provides information on both periodic and non-periodic arrangements of superstructure domains, terraces or facets, and strained regions. The interpretation of spot profiles is simplified due to the validity of the kinematic approximation, which is not the case for LEED I-V analysis. Deviations from simple structures resulting from defects produce characteristic modifications of the spot profile, so that a quantitative evaluation is possible. SPA-LEED has proven itself to be useful in the study of dynamic phenomena on surfaces such as phase transitions. Several examples of this capability are discussed in section 3 for Pb on Cu. 

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