Nematic molecular structure

Currently, theories are not yet able to predict the transition temperatures based on molecular structure of the constituent molecules. However, for several compounds there is considerable empirical data relating the transition temperature between isotropic and nematic phases (Tni) to molecular structure. Higher implies greater nematic stability. For example, it is... 

This article reviews progress in the field of atomistic simulation of liquid crystal systems. The first part of the article provides an introduction to molecular force fields and the main simulation methods commonly used for liquid crystal systems molecular mechanics, Monte Carlo and molecular dynamics. The usefulness of these three techniques is highlighted and some of the problems associated with the use of these methods for modelling liquid crystals are discussed. The main section of the article reviews some of the recent science that has arisen out of the use of these modelling techniques. The importance of the nematic mean field and its influence on molecular structure is discussed. The preferred ordering of liquid crystal molecules at surfaces is examined, along with the results from simulation studies of bilayers and bulk liquid crystal phases. The article also discusses some of the limitations of current work and points to likely developments over the next few years. 

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]. 

Zugenmaier and Heiske [60] presented the crystal and molecular structures of the homologous series of 4 -(hydroxy-l-n-alkoxy)-4-cyanobi-phenyls (CBO(CH2)nOH) n = 4, 5, 7-11). The chemical structure of the compounds is shown in Fig. 1. All compounds of the series exhibit a nematic phase. The crystal and molecular data of the investigated compounds CBO(CH2)nOH and some derivatives are presented in Table 3. 

We briefly discussed the origin and structure of liquid crystals in Section 4.13. The last decade has witnessed a surge of interest in liquid crystals because of their applications in display devices (devices that convert an electrical signal into visual information). The design of liquid crystal (LC) devices relies on the relation between the molecular structure and the phase behaviour (relative smectic-nematic tendency, NI etc.) as well as the physical properties of the liquid crystals (Chandrasekhar, 1994). 

Next, we examine the term i2. In a gas-like single segment approximation, this term can be replaced by 1212. The molecular conformation statistics are independent of each other. This might be due to the fact that in the absence of a three-dimensional lattice-potential, nematic shifts of neighboring segments are very likely to occur. In this approximation the configuration does not depend on which individual pair of molecules k, 1 is picked out The molecular structure factor is independent of the indexes k and L Hence 1 inter, d can be written as... 

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]. 

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. 

Fig. 6 Molecular structure of compound 6 and a change of its observed photoluminescent color on the nematic-smectic liquid-crystalline phase transition...

A. di Matteo, S. M. Todd, G. GottareUi, G. Solladie, V. E. Williams and R. P. Lemieux, Correlation between molecular structure and helicity of induced chiral nematics in terms of short-range and electrostatic-induction interactions. The case of chiral biphenyls, J. Am. Chem. Soc., 123 (2001) 7842-7851. 

L. C. Snyder and S. Meiboom, Molecular structure of cyclopropane from its proton NMR in a nematic solvent.. Chem. Phys., 1967, 47(4), 1480-1487. 

Figure 2.6 The molecular structure of a typical calamitic, thermotropic nematic liquid crystal. The diameter of the molecule measured in the x-y plane is symmetrical about the z-axis due to very rapid rotation about the molecular long axis.

The correlation between molecular structure, liquid crystal transition temperatures and physical properties of the nematic phase of these materials of relevance to individual types LCDs is dealt with extensively in Chapter 3. 

Table 3.3 Molecular structures, transition temperatures (°C) and dielectric anisotropy (Ae measured at 20°C) for nematic liquid crystals (15-27)...

The intention of this brief survey has been to demonstrate that besides the "classical" aspects of isotropic polymer solutions and the amorphous or partially crystalline state of polymers, a broad variety of anisotropic structures exist, which can be induced by definable primary structures of the macromolecules. Rigid rod-like macromolecules give rise to nematic or smectic organization, while amphiphilic monomer units or amphiphilic and incompatible chain segments cause ordered micellar-like aggregation in solution or bulk. The outstanding features of these systems are determined by their super-molecular structure rather than by the chemistry of the macromolecules. The anisotropic phase structures or ordered incompatible microphases offer new properties and aspects for application. 

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