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Nematic mesophase molecular structure

The rapid rise in computer speed over recent years has led to atom-based simulations of liquid crystals becoming an important new area of research. Molecular mechanics and Monte Carlo studies of isolated liquid crystal molecules are now routine. However, care must be taken to model properly the influence of a nematic mean field if information about molecular structure in a mesophase is required. The current state-of-the-art consists of studies of (in the order of) 100 molecules in the bulk, in contact with a surface, or in a bilayer in contact with a solvent. Current simulation times can extend to around 10 ns and are sufficient to observe the growth of mesophases from an isotropic liquid. The results from a number of studies look very promising, and a wealth of structural and dynamic data now exists for bulk phases, monolayers and bilayers. Continued development of force fields for liquid crystals will be particularly important in the next few years, and particular emphasis must be placed on the development of all-atom force fields that are able to reproduce liquid phase densities for small molecules. Without these it will be difficult to obtain accurate phase transition temperatures. It will also be necessary to extend atomistic models to several thousand molecules to remove major system size effects which are present in all current work. This will be greatly facilitated by modern parallel simulation methods that allow molecular dynamics simulations to be carried out in parallel on multi-processor systems [115]. [Pg.61]

In order to avoid this section becoming too abstract, a selection of molecules which can form nematic or smectic liquid crystals is illustrated in Figure 7.2. For a discussion of how particular molecular structures lead to formation of particular mesophases, reference should be made to the work by Gray and Goodby [402] already cited or to Chapters 1 and 12 of Molecular Physics of Liquid Crystals edited by Luckhurst and Gray [28]. [Pg.138]

The article covers synthesis, structure and properties of thermotropic liquid-crystalline (LC) polymers with mesogenic side groups. Approaches towards the synthesis of such systems and the conditions for their realization in the LC state are presented, as well as the data revealing the relationship between the molecular structure of an LC polymer and the type of mesophase formed. Specific features of thermotropic LC polymers and copolymers of nematic, smectic and cholesteric types are considered. [Pg.173]

There is as yet no clear-cut relationship between the molecular structure of the mesogenic..unit and the type of mesophase it forms, but several generalizations can be made. Gray and Winsor have divided these factors into how the molecular structure (1) is conducive to liquid crystal formation, (2) affects the thermal stability of the mesophase, and (3) favors the occurrence of smectic versus nematic or cholesteric liquid crystals. [Pg.117]

A similar approach has been used to produce materials with a chiral (cholesteric) structure by performing the experiments described above in the presence of a low molecular weight chiral liquid crystalline material (Figure 9.6). The chiral material is not covalently attached to the network and can be removed subsequently to produce an imprinted chiral structure. As before, the polymer displays a nematic mesophase between the glass transition (Tg 33°C) and the transition to an isotropic fluid (rN, 128°C). [Pg.237]

These conjugated oligomers are likely to form organized molecular assemblies if coupled with suitable complementary structures. The dimers la and Ila-IId were made to react with various aromatic mono- and diamines in order to build mesogenic molecular structures capable of giving rise to thermotropic liquid ciystals ( ). When the dimer moiety was entirely furanic, no mesophase appeared, whereas the thiophenic and mixed moieties produced clear-cut nematic phases. [Pg.105]

The discussion of mixtures is worthy of further comment. It has been known for some time that mixtures of linear, nonmesomorphic molecules with nematic compounds are characterized by a sharp decrease in both the crystal-nematic and nematic-isotropic liquid transitions with increasing concentration of nonmesomorphic component. However, mixtures of two or more nematic compounds which possess subtle differences in molecular structure do not exhibit sharp decreases in the nematic-isotropic liquid transition temperatures with molar composition although eutectic points for the crystal-nematic transitions may be obtained. Thus, the nematic-isotropic liquid transition temperatures form a smooth curve over the entire range of molar composition. Similar smooth curve surfaces are observed in ternary mixtures. This remarkable stability of the nematic mesophase is undoubtedly due to the fact that all the molecules are oriented in the same direction resulting in the formation of a pseudo-lattice type structure. On the other hand, the molecules of a nonmesomorphic guest are randomly oriented in a nematic host and their presence results in the disruption of nematic order. [Pg.266]

In this section, a few of the mesophases that may be formed by materials having molecular structures that are rod-Uke in shape are briefly described. Similar aspects of these discussions can also be applied to disc-like systems, and so on. The phases formed by rod-like systems fall into two categories, nematic and lamellar. The nematic phase is generally not subdivided further, but as noted above the smectic state can be, and often is, in practice. [Pg.2789]


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