Block structure defects

An unsatisfactory aspect of this synthesis is the quite low degree of polymerization a maximum of 20 aromatic rings. Moreover, in addition to the predominant 2,7-coupling of the building blocks, other types of coupling can occur leading to structural defects. 

Structural Imperfections. In many respects HREM has had a greater impact upon our knowledge of the nature of the atomic reorganization at crystalline imperfections than any other single technique. One of the very first contributions of HREM as a new analytical and structural tool was described in the paper by Iijimia (42) in 1971 on 2 10 29 v -ewe< down to its b - axis. Structural faults, arising from subtle fluctuations in composition, could be clearly seen in the block-structure (based on NbO octahedra) which is a feature of this ternary oxide system. More than a decade later similar materials are yielding to active scrutiny by HREM, and Horiuchi (43), for example, has shown how point defects may be directly viewed... 

It must be pointed out that defects in block structures giving rise to nonstoichiometry are possible. Both Wadsley defects and point defects are known in block structure phases. Nonstoichiometry is introduced by the insertion of rows or columns... 

Shear structure with two sets of planar defects—block structure... 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

In order to strengthen a first line of defence made up of the acceptability of structure damage during past and future operations and structure defect tolerance, an in-situ inspection programme was carried out on the major reactor block structures upper hangers, conical shell and the core cover plug. 

The ability to generate structural defects in reaction intermediates (e.g., leading to lacunary structures) for example by removing building blocks from (large) intermediates due to the presence of appropriate reactants. 

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