Microscopic investigations crystal shape

Optical microscopic examinations of the reactant at room temperature can provide information on the shapes and sizes of the crystallites and structural information from features of observed symmetry. The degree of perfection of the crystallites can be assessed and damage, major defects and inclusions may be identified. Surface defects, such as the points of emergence of dislocations, may be revealed by etching the sample surface with a suitable solvent. Cleaving of a crystal gives two closely related faces and one section may be etched to reveal the location of defects while the other section is partially decomposed and then re-examined [30,31 ] to enable the distributions of the different surface features (usually dislocations and nuclei) to be compared. Such techniques have been used to investigate the role of defects in the initiation of decomposition [32,33]. 

We have described lattice spin models for the simulation of polymer-dispersed liquid crystals. The biggest advantage of Monte Carlo simulations is the possibility of investigating the system at a microscopic level, and to calculate thermodynamic properties and their specific order parameters suitable for different types of PDLC. Molecular organizations can be investigated by calculating the order parameters point by point across the droplet. Moreover, it is possible to calculate experimental observables like optical textures and, as discussed here, NMR line shapes. We have given an overview of the method and some applications to models of PDLC with radial and bipolar boundary conditions, and considered the effect of orientational and translational diffusion on the spectra. We have examined in particular under what conditions the NMR spectra of the deuterated nematic can provide reliable information on the actual boundaries present in these submicron size droplets. 

These same immunoglobulin crystals have also been the subject of a study in the electron microscope. The molecule in projection appears to have a shape varying between a Y and a T, in agreement with previous electron microscope studies and the X-ray diffraction investigation. 

So far we have discussed the macroscopic symmetry elements that are manifested by the external shape of the three-dimensional patterns, that is, crystals. They can be studied by investigating the symmetry present in the faces of the crystals. In addition to these symmetry elements there are two more symmetry elements that are related to the detailed arrangements of motifs (atoms or molecules in actual crystals). These symmetry elements are known as microscopic symmetry elements, as they can only be identified by the study of internal arrangement of the motifs. As X-ray or electron diffraction can reveal the internal structures, these symmetry arrangements can only be identified by X-ray, Electro or Neutron diffraction. Obviously, they are not revealed in the external shape of the pattern. These symmetry elements are classified as microscopic symmetry elements. There are two such types of synunetry elements (i) glide plane of symmetry and (ii) screw axis of synunetry. 

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