Columnar phases, thermotropic liquid crystals

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]. 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

Liquid crystalline phases can show not only long-range orientational order as nematic phases do but also long-range positional order. When this positional order is one-dimensional, the mesophase is called lamellar or smectic when it is two-dimensional, it is called columnar. The latter case is often found with thermotropic liquid-crystal disk-like molecules. Such molecules stack in columns that assemble on a 2-D lattice of hexagonal, rectangular, or oblique symmetry. The molecules in a given column only show 1-D liquid-Hke order and the uncorrelated columns are free to slide past each other, which ensures the mesophase fluidity [73]. 

The molecular organization in thermotropic liquid-crystal line phases is associated predominantly with a rigid anisometric architecture of the constituent single molecules. The triazines 13 were the first examples of electron donors that fotm columnar phases, which give rise to the induction of smectic liquid crystalline structures through donor-acceptor interactions (Figure 11). 

Thermotropic Liquid Crystals. - Organic molecules, having aromatic rings or unsaturations thus producing elongated shapes, and also polymeric molecules often show thermotropic phase behavior. Mesomorphism comprises typically nematic, smectic A and B, and cholesteric thermotropic phases, but in several cases columnar, discotic and rod-disk self-assembly shapes of thermosensitive mesogens have been observed. 

Figure 1 General structures of calamitic and discotic thermotropic liquid crystals. (A) Layered calamitic smectic liquid crystal. The structures of the various types depend on the local packing of the molecules, the extent of the packing, and the orientation of the long axes with respect to the layers. (B) Calamitic nematic liquid crystal. The molecules have no long-range order, and are only orientationally ordered. (C) Ordered columnar discotic liquid crystal. Disk-like molecules form ordered or disordered columns different column packings give rise to various mesophase structures. (D) Nematic-discotic liquid crystal phase. The disk-like molecules are only orientationally ordered.

Columnar phases are typically formed by either thermotropic liquid crystals made up of discotic mesogens stacking upon each other to form columns or by lyotropic liquid crystals composed of rod-like micelles. These columns or rods arrange into two-dimensionaUy correlated structures. Alongside the long axis of the columns... 

Thermotropic liquid crystal phases are formed by rod-like or disc-like molecules, either of which can have long-range order. In a liquid crystal phase, the anisotropic molecules tend to point along the same direction. This defines the director, n. With calamitic mesogens, the molecular long axes tend to lie parallel to the director, whereas the disc-like molecules in columnar phases are on average perpendicular to the director. 

The cubic phases of thermotropic liquid crystals display an independent class of liquid crystalline phases besides the smectic and columnar ones. The great variety of chemical structure leads to different cubic phases which can be expected to be constituted of molecular aggregates of different character. 

Primary silver thiolate compounds [AgSC H2 .,] (n=4, 6, 8, 10, 12, 16, 18), behave as thermotropic liquid crystals. On heating they display successively lamellar (SmA), cubic, and micellar mesophases. X-ray studies show that the micellar phase is an hexagonal columnar meso-phase [92]. 

In addition to the above lamellar, columnar and optically isotropic phases, there are also lyotropic nematic phases, which usually involve mixtures of a charged amphiphilic, such as simple soap, with an alkanol (a weaker amphiphilic where the head group is an alcohol), together with water and a simple salt. They are termed nematic because, like thermotropic nematics, their optical axes are easily oriented by external magnetic fields. In contrast to thermotropic nematics, the basic units of lyotropic nematics are molecular aggregates with dimensions of about 2-10 nm. In lyotropics, the nematic phase is much less usual than in thermotropic liquid crystals. Lyotropic nematics... 

Seo, S. H. Park, J. H. Tew, G. N. Chang, J. Y. (2007). Thermotropic liquid crystals of IH-imidazole amphiphiles showing hexagonal columnar and micellar cubic phases. Tetrahedron Lett., 48,6839-6844. 

Stiff rod-like helical polymers are expected to spontaneously form a thermotropic cholesteric liquid crystalline (TChLC) phase under specific conditions as well as a lyotropic liquid crystal phase. A certain rod-like poly(f-glutamate) with long alkyl side chains was recently reported to form a TChLC phase in addition to hexagonal columnar and/or smectic phases [97,98]. These properties have already been observed in other organic polymers such as cellulose and aromatic polymers. 

Understanding the organization of disc-shaped mesogens in columnar phases is of practical interest because of their useful optoelectronic applications [168-170]. It will be instructive to explore the role of fluctuations in influencing transport properties of thermotropic discotic liquid crystals—in particular, in the columnar phase. 

Mesophases can be locked into a polymer network by making use of polymerizable LCs [59]. These molecules contain moieties such as acryloyl, diacety-lenic, and diene. Self-organization and in situ photopolymerization under UV irradiation will provide ordered nanostmctured polymers maintaining the stable LC order over a wide temperature range. A number of thermotropic liquid crystalline phases, including the nematic and smectic mesophases, have been successfully applied to synthesize polymer networks. Polymerization of reactive lyotropic liquid crystals also have been employed for preparation of nanoporous polymeric materials [58, 60]. For the constmction of nanoporous membranes, lyotropics hexagonal or columnar, lamellar or smectic, and bicontinuous cubic phases have been used, polymerized, and utilized demonstrated in a variety of applications (Fig. 2.11). 

According to Fig. 3, classical thermotropic smectic phases of amphotropic liquid crystals are (SmA ), colunmar hexagonal (Col ), bicontinuous cubic (Cub, i), or discontinuous cubic (Cubjis) [169]. All these meso-phases include a disclination surface between the hydrophilic and the lipophilic parts of the unordered molecules. This surface can be uncurved (SmA), curved in one direction (columnar), curved in two directions with the same sign (discontinuous cubic), or curved in two directions with opposite sign (bicontinuous cubic). 

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