Liquid crystals calamitic mesogen phases

Of all liquid crystalline phases, the nematic phase is the phase with the highest symmetry, i.e. Dooh, and the least order. As shown in Fig. 3.3a, b, the mesogens solely possess orientational order. Positional order of the mass centers does not occur in this phase. Nematic phases are usually built up by either rod-like or disc-like mesogens. For thermotropic liquid crystals these mesogens are therefore calamitic or discotic molecules, respectively. In both cases the phase is simply denoted with the abbreviation N. For lyotropics, the notation typically distinguishes between nematic phases Nc, which are formed by rod-like micelles, and nematic phases Np, which are composed of disc-like micelles. 

Thennotropic liquid crystal phases are fonned by anisotropic molecules witli long-range orientational order and in many types of stmcture witli some degree of translational order. The main types of mesogen are Arose tlrat are rodlike or calamitic and Arose Arat are disclike or discotic. 

Many technological applications of liquid crystals, as in electro-optic display devices, are based on multicomponent mixtures. Such systems offer a route to the desired material properties which cannot be achieved simultaneously for single component systems. Mixtures also tend to exhibit a richer phase behaviour than pure systems with features such as re-entrant nematic phases [3] and nematic-nematic transitions possible. In this section, we describe simulations which have been used to study mixtures of thermotropic calamitic mesogens. 

The liquid crystal phases of calamitic mesogens fall into two types - nematic (N) and smectic (Sm). The nematic phase is the most disordered of the liquid crystal phases and possesses only orientational order, so that the long axes of the molecules are correlated in one direction (known as the director, n) while being positioned randomly (Fig. 2A). There are several smectic phases and these differ from the nematic phase in possessing partial posi-... 

Thermotropic liquid crystals come in two types calamitic and discotic. Calamitic phases (from the Greek for tube ) are all those that are caused by rod-like mesogens. The more recently characterised discotic phases are caused by disc-like species. Calamitic phases may be either nematic (from the Greek for thread ), smectic (from the Greek for soap ) or cholesteric (named after the cholesterol derivatives such as 13.4, which exhibit this behaviour). 

In contrast to calamitic mesogens, discotic liquid crystals are built from disk-like molecules that can arrange into different structures, such as the discotic nematic mesophase, the discotic columnar mesophase, or the discotic hexagonal meso-phase. 

Summary Cyclic siloxanes substituted with mesogenic groups, which are connected to the backbone by aliphatic spacers, exhibit liquid crystalline (LC) phases as the classic calamitic liquid crystals. 

The true liquid-crystal mesophases of calamitic mesogens are divided into two classes—the nematic and smectic phases and these will be described in turn. There is, in addition, a series of crystal smectic phases that are not really liquid-crystal phases but which for many years were classified as such these will be mentioned briefly below. 

Whereas monophilic liquid crystals can show a high diversity of smectic phases (SmA-SmQ), the amphotropic liquid crystals normally exhibit only the SmA phase. Tilted smectic phases are only observed in a few cases. The first indication of possibly tilted phases was given in 1933 for thallium stearate [ 170]. A disordered SmC phase was also clearly deseribed for mesogens containing a classical calamitic core aside to their amphiphilic structure [171]. Monophilic liquid crystals can show various ordered tilted smectic phases, for example, smectic I, F, G, J, H, and K. In the case of lipids only one mesophase, the j8 phase,... 

The above shown structure has amesogenic core, (hard central segment) correlated with dynamic packing of anisometric shapes. The flexible tales, often hydrocarbon chains, extend from the mesogenic core and facihtate the transformatirHi from the sohd state to the liquid crystaUine phase. 2. A calamitic liquid crystal... 

Rod-shaped molecules forming liquid crystalline phases are called calamitic . A prominent example of such a calamitic molecule is terephthal-bis-(p-butylaniline) (TBBA) [2]. Its chemical structure is shown in the upper left comer of Fig. 3.1. The molecule possesses a rigid aromatic core as well as flexible alkyl chains. The aromatic core favors a parallel packing of the molecules, while the flexible chains keep them from crystallizing. These intermolecular interactions, as well as entropic effects and steric interactions between the mesogens, promote the formation of mesophases, as discussed by Onsager [3]. The mesophases formed by calamitic mesogens frequently possess a layered structure, but different phase types are also possible. 

Fig. 3.1 Building blocks of thermotropic and lyotropic liquid crystalline phases. The upper part of the figure shows two examples of typical thermotropic mesogens. Calamitic mesogens, such as terephthal-bis-(p-butylaniline) (TBBA) [2], can be represented by prolate ellipsoids or rigid rods, while discotic mesogens, such as benzene-hexa-n-octanoate (BH8) [4], are usually described by oblate ellipsoids or discs. The lower part of the figure shows the typical surfactant molecule sodium dodecyl sulfate (SDS), which forms lyotropic phases with water [5], Such a surfactant molecule is basically composed of a polar head group and a flexible hydrophobic tail. These amphiphilic molecules aggregate into different types of micelles, which are the actual mesogens of lyotropic liquid crystals. The shape of the micelles depends mainly on the solvent concentration...

There exist also cubic arrangements of mesogens (called So phases in calamitic liquid crystals). Such isotropic supramolecular structures typically are found... 

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