Liquid crystals nematic mesophase

Liquid crystals represent a transition between solid crystalline substances and isotropic liquids. On heating, mesophases are formed that have ordered structures which can be nematic, smectic or cholesteric. On further heating, the orientation is disturbed and the phases are converted into an isotropic liquid. The long structure of liquid crystals causes isomers with more drawn-out shapes to be readily dissolved in the ordered liquid crystal substrate ( mesophase ) thus yielding stronger sor-bat-sorbent interactions,... 

In some polymer liquid crystals, several mesophases can be identified. In main-chain liquid crystal polymers there is usually a transition from the crystal to a mesophase, whereas in more amorphous systems when a glass transition is present, the mesophase may appear after this transition has occurred. In multiple transition thermotropic systems, the increase in temperature leads to changes from the most-ordered to the least-ordered states, i.e., crystal (k) smectic (S) nematic (N) isotropic (i). 

On a microscopic scale just as for low molecular weight liquid crystals, nematic and cholesteric mesophases have been identified. Whether the smectic structure actually exists for polymeric systems is still open to question. However, it seems to be the macroscopic structure which accounts for the rheological properties of LCP. 

Calamitic liquid crystals—nematic and smectic mesophases... 

Calami tic Liquid Crystals—Nematic and Smectic Mesophases 45... 

Calami tic Liquid Crystals Nematic and Smectic Mesophases 67... 

The two most common molecular motifs that lead to liquid crystal phase behavior are the rod aud the disk. Clearly rodlike molecules have one unique axis that is longer than the other two, while diskUke molecules have one unique short axis and two longer axes (Figure 1). RodUke molecules organize into nematic or smectic phases, while disklike systems form nematic or colunmar phases. Figure 2 shows schematic diagrams of molecules arranged in a nematic, smectic A (SmA), and smectic C (SmC) liquid crystal phase (= mesophase). There are very many smectic phases of which SmA and SmC are only two. ... 

In liquid-phase carbonizations, the mechanisms are completely different from those in the solid phase. It is via liquid-phase carbonizations (but not all liquid phases) that graphitizable forms of carbon result. How does this come about The explanation takes us to a quite different subject area, that of anisotropic aromatic, discotic, nematic liquid crystals (called mesophase) formed as a result of growth and self-assembly of the constituent polycyclic aromatic molecules of the parent material. These usually are the highly aromatic coal-tar pitches, a liquid product from the making of metallurgical coke, from aromatic pitches synthesized by the petroleum industry as well as polycyclic aromatic model compounds. 

However, the stacking order of the graphitizable carbons from pitch is established via the formation and stacking of lamellar, nematic liquid crystals and mesophase from the isotropic fluid phase of the pitch carbonization (Section 2.7), such graphitizable carbons having relatively low surface areas (< 5 m g ). Hence, a gasifying gas has almost zero opportunity to penetrate the interior of a carbon and gasification will dominantly take place at external surfaces. 

Liquid crystals are classified by symmetry. As it is well known, isotropic liquids with spherically symmetric molecules are invariant under rotational, 0(3), and translational, T(3), transformations. Thus, the group of symmetries of an isotropic liquid is 0(3)xT(3). However, by decreasing the temperature of these liquids, the translational symmetry T(3) is usually broken corresponding to the isotropic liquid-solid transition. In contrast, for a liquid formed by anisotropic molecules, by diminishing the temperature the rotational symmetry is broken 0(3) instead, which leads to the ap>p)earance of a liquid crystal. The mesophase for which only the rotational invariance has been broken is called nematic. The centers of mass of the molecules of a nematic have arbitrary positions whereas the principal axes of their molecules are spontaneously oriented along a preferred direction n, as shown in Fig. 1. If the temperature decreases even more, the symmetry T(3) is also partially broken. The mesophases exhibiting the translational symmetry T(2) are called smectics (see Fig. 1), and those having the symmetry T(l) are called columnar phases (not shown). 

Rod-like liquid crystals [1] have been known for more than a hundred years, the first one, cholesteryl benzoate, being discovered in 1888 by Reinitzer. In materials of this type, nematic N, cholesteric N and different lamellar mesophases such as SmA, SmC, SmF, and SmI are obtained. Beside these classical liquid crystals, thermotropic mesophases - consisting of two-dimensional aromatic flat molecules - that exhibit various columnar phases (e.g., Col, Coif, Colob) have been known since 1977 [2, 3], In these two types of systems, the lamellar and columnar phases are observed separately. So, it was interesting to examine the mesomorphic properties of the hybrid molecules, i.e., molecules with a long rodlike rigid core ending in two half-disc moieties (Fig. 1). In fact, the phasmids [4,5] fill... 

Some liquid crystals exhibit mesophases between the isotropic phase and the chiral nematic (or cholesteric) phase. There may be up to three different spatial structures in a temperature interval as small as 1 K. These phases are optically not birefringent and they reflect preferentially short wavelengths that is why they are called blue phases. ... 

Nematic liquid crystals as thermotropic systans are also employed as orienting medium in the IR-LD analysis [65,66]. Thermotropy means that the liquid-crystal phase (mesophase) is implemented within a certain interval between the melting temperature and the point of clarification. Within these temperature limits, the liquid crystal molecules are located toward a certain direction, inducing orientation without any positional order (Figure 1.9). In their mesomorphic interval, the nematic liquid crystals are characterized by their lower fluidity and transparency as well as anisotropy in their optical properties snch as the double-beam refraction and dichroism. 

The rigid nature of the mesophase pitch molecules creates a strong relationship between flow and orientation. In this regard, mesophase pitch may be considered to be a discotic nematic liquid crystal. The flow behavior of liquid crystals of the nematic type has been described by a continuum theory proposed by Leslie [36] and Ericksen [37]. 

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]. 

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

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