The Structure of Liquid Crystal Phases

Liquid crystal phases can be distinguished by examining their defect textures. What characteristic defects would indicate the presence of the nematic phase or the cholesteric phase  

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It is now well-established that for atomic fluids, far from the critical point, the atomic organisation is dictated by the repulsive forces while the longer range attractive forces serve to maintain the high density [34]. The investigation of systems of hard spheres can therefore be used as simple models for atomic systems they also serve as a basis for a thermodynamic perturbation analysis to introduce the attractive forces in a van der Waals-like approach [35]. In consequence it is to be expected that the anisotropic repulsive forces would be responsible for the structure of liquid crystal phases and numerous simulation studies of hard objects have been undertaken to explore this possibility [36]. 

P.S. Pershan, The Structure of Liquid Crystal Phases, World Scientific, Singapore, 1988. 

Also, further X-ray studies began to advance knowledge of the structure of liquid crystal phases, particularly smectics. The work of de Vries and Diele should be mentioned, and later on, notably that by Leve-lut and co-workers in France and Leadbet-ter in England (see, for example, [37, 38]), work which culminated in the 1980s in a clear structural elucidation and classification of smetic liquid crystals. This distin-... 

It follows from general considerations that the role of the shape of the filler particles during net-formation must be very significant. Thus, it is well-known that the transition from spherical particles to rod-like ones in homogeneous systems results in such radical structural effect as the formation of liquid-crystal phase. Something like that must be observed in disperse systems. 

Molecular mechanics force fields have largely been parameterised using the best available data from the gas phase and (in some cases) from liquid phase or solution data. The question therefore arises as to how applicable molecular mechanics force fields are to predicting structures of molecules in the liquid crystal phase. There is now good evidence from NMR measurements that the structure of liquid crystal molecules change depending on the nature of their... 

The crystallinity of liquid crystal phases refers to the large assortment of ways these micellar structures can be organized within a bulk phase. For example, spherical micelles of... 

Liquid Crystals Liquid-crystal phases may occur between the solid and the liquid phase. Cholesteryl myristate, for example, exists in a liquid-crystal phase between 71 and 85°C [6]. The appearance of liquid-crystal phases depends on the molecular structure. Compounds with elongated structures that are fairly rigid in the central part of the molecule are likely candidates for liquid crystals. The homologous series of p-alkoxybenzylidene-p-n-butylanilines is just one example for compounds with liquid-crystal phases. An excellent introduction to liquid crystals and their properties has been written by Collings [6]. 

The most comprehensive studies on shape-persistent Hekates have been performed on stilbenoid star-shaped molecules. Structures and mesomorphic properties are collected in Table 1. Core building blocks with only one repeating unit per arm and one flexible chain 26a,b, 28a-d, 30a did not show any liquid crystal properties [56-58]. In the series of two chain derivatives 28e, 30b,c the formation of meso-phases depend on the core [57-59]. The electron deficient triazine and the dicyanopyridine building block induced obviously columnar mesophases. The pyridine derivative 30b showed only a crystalline phase [58]. In the series of nine chain stars 26c-g [60-62] and 28f-n [57], the compounds formed columnar phases depending on the chain length of the peripheral chains. Propyloxy chains are too short but hexyloxy and dodecyloxy chains are sufficient for the formation of liquid crystal phases by nanosegregation [60-62]. This can be rationalised by a dense... 

The physical and chemical properties of photoisomerizable molecular films or photoisomerizable polymers are controlled by light. Photochemical control of the formation of liquid crystal phases, or sol-gel transitions,137 381 of polymers containing photoisomerizable components demonstrates signal regulation of the structure and properties of microscopic and macroscopic phases. Physicochemical properties of photoisomerizable membrane-mimetic assemblies such as liposomes,1241 mono-... 

The presence of liquid crystal structures at both the w-o and o-w interfaces in multiple emulsions has been investigated by Kavaliunas and Frank (31). Microscopic examination of w/o/w emulsions between crossed polarizers revealed the presence of liquid crystal phases at both inner (w-o) and outer (o-w) interfaces in a w/o/w system composed of water, p-xylene and nonylphenol diethylene glycol ether. Liquid crystalline phases were also detected in o/w/o emulsions at both interfaces. The presence of these liquid crystal structures was found to improve the stability of the emulsions markedly. Matsumoto (32, 33) have concluded that the oil layers in w/o/w systems are likely to be composed of or contain,at least in proximity to the aqueous phase,multilamellar layers of the lipophilic surfactant used in the formulation this is postulated in part to explain the rate of volume flux of water through the oily layer. 

The interaction of liquid crystals with neighbor phases (gas, liquid, solid) is a very interesting problem relevant to their electrooptical behavior. The structure of liquid crystalline phases in close proximity to an interface is different from that in the bulk, and this surface structure changes boundary conditions and influences the behavior of a liquid crystal in bulky samples. The nematic phase is especially sensitive to external agents, in particular, to surface forces, and the majority of papers devoted to the surface properties of mesophases have been carried out on nematics. In addition, the nematic phase is of great importance from the point of view of applications in electrooptical devices. Thus, in this chapter, we will concentrate on surface properties of nematics, though the properties of the other phases will not be skipped either. 

The description of liquid-crystal phases by S mmetry point groups applies to the time average of the structure. 

Recently, polyoxyethylene oleyl ethers were made of high-purity oleyl alcohol (99.7%) synthesized from high-purity oleic acid [3]. In this paper, the effect of hydrophilic chain length on the phase behavior and the structures of liquid crystals in polyoxyethylene oleyl ether/water system were investigated by means of phase study and small-angle X-ray scattering (SAXS). 

In concentrated solution, DNA fragments can form lyotropic liquid crystal phases. Short fragments behave like rods, and so the formation of liquid crystal phases is possible. On increasing concentration (above 160 mg/ml for 50 nm DNA in physiological salt solutions), cholesteric and hexagonal columnar phases may be observed (see Chapter 5 for a discussion of these structures) Just below the cholesteric phase, a blue phase is sometimes observed. This phase is named for the colour arising from the double twist cylinders that result from the packing of helices onto a cubic lattice. 

The structure of liquid crystals can broadly be classified as nematic, cholesteric and smectic, see Fig. 1. None of them have full three-dimensional (3-D) positional order, but some degree of orientational order. Most often the constituent molecules are elongated, as indicated in Fig. 1, but distinctly flat molecules make up the socalled discotic liquid crystals. The nematic phase has only orientational ordering of the molecules. The collection of molecules have one symmetry axis called the director n. The cholesteric phase has only orientational order, formed by the constituent chiral molecules. The director twists with a pitch comparable to the wavelength of light. 

The response of liquid crystal molecular orientation to an electric field is another major characteristic utilised for many years in industrial applications [44] and more recently in studies of electrically-induced phase transitions [45]. The ability of the director to align along an external field again results from the electronic structure of the individual molecules. 

It is clear from the forgoing discussions that the important material properties of liquid crystals are closely related to the details of the structure and bonding of the individual molecules. However, emphasis in computer simulations has focused on refining and implementing intermolecular interactions for condensed phase simulations. It is clear that further work aimed at better understanding of molecular electronic structure of liquid crystal molecules will be a major step forward in the design and application of new materials. In the following section we outline a number of techniques for predictive calculation of molecular properties. 

In the previous sections, we have seen how computer simulations have contributed to our understanding of the microscopic structure of liquid crystals. By applying periodic boundary conditions preferably at constant pressure, a bulk fluid can be simulated free from any surface interactions. However, the surface properties of liquid crystals are significant in technological applications such as electro-optic displays. Liquid crystals also show a number of interesting features at surfaces which are not seen in the bulk phase and are of fundamental interest. In this final section, we describe recent simulations designed to study the interfacial properties of liquid crystals at various types of interface. First, however, it is appropriate to introduce some necessary terminology. 

Hopefully with this brief introduction the reader will be able to appreciate fully the chapters which follow and which have been written by experts in the field. The complexity and beauty of the liquid crystalline phase has attracted many able scientists and the applications of liquid crystals in the electronics industry have provided a secure funding base for the subject. This is therefore still a field which is expanding rapidly and many research avenues remain to be explored by newcomers. Perhaps after reading these volumes of Structure and Bonding you will be tempted to join this exciting endeavour. 

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