Molecular organization nematic liquid crystals

Nuclear magnetic resonance (NMR), in particular, deuterium NMR, has proven to be a valuable technique for determining the nature of molecular organization in liquid crystals. The utility of the NMR technique derives from the fact that the relevant NMR interactions are entirely intramolecular, i.e. the dominant interaction is that between the nuclear quadrupole moment of the deuteron and the local electric-field gradient (EFG) at the deuterium nucleus. The EFG tensor is a traceless, axially symmetric, second-rank tensor with its principal component along the C—D bond. In a nematic fluid rapid anisotropic reorientation incompletely averages the quadrupolar interaction tensor q, resulting in a nonzero projection similar to the result in Eq. (5.6) ... 

The development of organic nematic liquid crystals of other than only calamitic molecular structures has brought forward valuable mat als of several disc-/star- or hoop-like shapes. The more than one hundred examples discussed here and compiled in six tables provide evidence for the successful research activities in this branch of scientific interest during the past twenty-five years. 

Liquid crystals are classified into lyotropic and thermotropic crystals depending on the way in which the mesomorphic phase is generated. Lyotropic liquid-crystalline solvents are formed by addition of controlled amounts of polar solvents to certain amphiphilic compounds. Thermotropic liquid-crystalline solvents, simply obtained by temperature variations, can be further classified into nematic, smectic, and cholesteric solvents depending on the type of molecular order present. Liquid crystals are usually excellent solvents for other organic compounds. Nonmesomorphic solute molecules may be incorporated into liquid-crystalline solvents without destruction of the order prevailing in the liquid-crystalline matrix (Michl and Thulstrup, 1986). Ordered solvent phases such as liquid crystals have also been used as reaction media, particularly for photochemical reactions (Nakano and Hirata, 1982). 

In contrast to conventional nematic liquid crystals, the molecular units are disk shaped and range widely in size, even when the mesophase is produced by the pyrolysis of pure organic compounds (17). Many of the molecules are volatile or reactive in the temperature range over which the mesophase is fluid, and the evolution of gaseous species usually causes the mesophase to be extensively deformed by bubble percolation before it congeals to a solid semicoke. 

Abstract Monte Carlo simulations of lattice spin models are a powerful method for the investigation of confined nematic liquid crystals and allow for a study of the molecular organization and thermod3mamics of these systems. Investigations of models of polymer-dispersed liquid cr3rstals are reviewed devoting particular attention to the calculation of deuterium NMR spectra from the simulation data. 

Figure 5.2. Monolayers of the amphiphile 1-monopalmitoyl-( )-glycerol at the air-water interface assemble in domains in which the molecular tilt azimuth is organized in star-shaped patterns. It is possible to preserve this order during the transfer on to a solid support. LB monolayers of this material have been utilized for the anchoring of nematic liquid crystals. The order within the monolayer determines the order within the bulk phase of the nematic liquid crystal (LC). The image here shows the LC cell between crossed polarizers. (From J. Fang, U. Gehlert, R. Shashidar and C. Knobler, Langmuir (1999), 15, 297)...

Liquid crystals (LCs) are organic liquids with long-range ordered structures. They have anisotropic optical and physical behaviors and are similar to crystal in electric field. They can be characterized by the long-range order of their molecular orientation. According to the shape and molecular direction, LCs can be sorted as four types nematic LC, smectic LC, cholesteric LC, and discotic LC, and their ideal models are shown in Fig. 23 [52,55]. 

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