Thermotropic liquid-crystal-forming

Paleos, C. M., Tsiourvas, D. (1995), Thermotropic liquid-crystals formed by intermolecular hydrogen interactions, Angewandte Chemie - International Edition in English, 34, 1696-711. 

Fibers with very high strength and modulus can be fabricated from polymers that have a molecular structure in which the chains are packed in small cross-sectional areas with strong bonds and low elongation. However, some of these aromatic polymers have melting points that are higher than their decomposition temperatures due to the rigidity of their molecules. It is therefore impossible to process them in thermotropic liquid crystal form. 

Other interesting Langmuir monolayer systems include spread thermotropic liquid crystals where a foam structure forms on expansion from a collapsed state [23]. Spread monolayers of clay dispersions form a layer of overlapping clay platelets that can be subsequently deposited onto solid substrates [24]. 

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]. 

The liquid crystal state represents the fourth state of matter and exists between the solid and liquid states, which form its boundaries. The liquid crystal state is reached from the solid state either by the action of temperature (thermotropic liquid crystals) or of solvent (lyotropic liquid crystals) and it is the former that will be the subject of this chapter. 

The mixing of nematogenic compounds with chiral solutes has been shown to lead to cholesteric phases without any chemical interactions.147 Milhaud and Michels describe the interactions of multilamellar vesicles formed from dilauryl-phosphotidylcholine (DLPC) with chiral polyene antibiotics amphotericin B (amB) and nystatin (Ny).148 Even at low concentrations of antibiotic (molar ratio of DLPC to antibiotic >130) twisted ribbons are seen to form just as the CD signals start to strengthen. The results support the concept that chiral solutes can induce chiral order in these lyotropic liquid crystalline systems and are consistent with the observations for thermotropic liquid crystal systems. Clearly the lipid membrane can be chirally influenced by the addition of appropriate solutes. 

Lyotropic liquid crystals differ from thermotropic liquid crystals. They are formed by mesogens which are not the moleeules themselves but their hydrates or solvates as well as associates of hydrated or solvated molecules. In the presenee of water or a mixture of water and an organie solvent as the most important solvents for drug molecules, the degree of hydration or solvation, respectively, depends on the amphiphilic properties of a drug moleeule. Hydration of the mostly rod-shaped molecule— and the same holds for solvation—results in different geometries, eone or cylinder [5] (Fig. 3). 

The molecular structure of arsphenamin is a typical representative of a thermotropic mesogen. With its symmetrical arrangement of the atoms the same holds for disodium cromoglycate, DNCG [20], which forms both thermotropic liquid crystals and lyotropic mesophases in the presence of water. Micronized DNCG powder applied to the mucosa of the nose or the bronchi absorbs water from the high relative humidity of the respiration tract and is first transformed into a lyotropic mesophase and then into a solution depending on the amount of water available. 

Since this initial observation the field has expanded rapidly and there are numerous reports of cellulose derivatives that form lyotropic liquid crystals. Some of them form botii lyotropic and thermotropic liquid crystals. Gray (2) has tabulated various cellulose derivatives reported to form liquid crystals prior to early 1982. 

LCP phases are subdivided into thermotropic or lyotropic. Lyotropic liquid crystals are formed by macromolecules that show liquid crystalline behavior in solution. This behavior is strongly concentration dependent. Thermotropic liquid crystals are molecules that show liquid crystalline behavior above the melting point of their crystallites. 

When they are heated, mesogenic compounds do not melt directly from the highly ordered crystalline state to an isotropic liquid. They form instead, intermediate phases in which the molecules are orientated in a parallel direction and referred to as smectic (centers of the molecules organized in layers) or nematic (centers of the molecules distributed at random). Smectic and nematic mesophases are in turn divided into a variety of subgroups of thermotropic liquid crystals which will not be dealt with in detail in the present article. 

Acyclic carbohydrates with one alkyl chain are either derived from cyclitols or aldonic acids. Acyclic cyclitol derivatives 3 (X=0, S, NH, N(CH3), NHCO, N(CH3)C0 R=Q-Ci6 sug=Glc-ol, Man-ol) form thermotropic liquid crystals... 

Thermotropic liquid crystals are divided into two principal types, by considering the shape of the units forming the mesophase (Figure 7.1) ... 

Lyotropic Liquid Crystals, Some molecules in a sulvcni form phases with orientational antl/or positional order. In these systems, the transition from one phase to another can occur due to a change of concentration, so they arc given the name lyotropic liquid cry stals Of course temperature can also cause phase transitions in these systems, so this aspect of thermotropic liquid crystals is shared hy lyotropics. The real distinctiveness of lyotropic-liquid crystals is the fact that at least two very different species of molecules nttisl be present for these structures to form... 

Mesophases Induced by Association of Complementary Molecular Components. A common type of molecular species that form thermotropic liquid crystals possesses an axial rigid core fitted with flexible chains at each end. One may then imagine splitting the central core into two complementary halves e and 3, whose association would generate the mesogenic supermolecule, as schematically represented in Figure 38. 

Liquid crystalsare an intermediate state in which the molecules in a crystal can undergo a secondary phase transition to a mesophase, which gives them mobility in 1-2 directions. They are birefringent, but possess low properties like a liquid phase. Lyotropic liquid crystals form on uptake of water into a system that increases its mobility, and thermotropic liquid crystals can be disrupted by heating above a transition temperature. Cromolyn sodium (Cox et al., 1971), the HMG-CoA reductase inhibitor SQ33600 (Brittain et al., 1995), and the leukotriefienffagonist L-660,711 (Vadas et al., 1991) are examples of pharmaceuticals that can form liquid crystals. 

One of the most classic examples of chiral expression in thermotropic liquid crystals is that of the stereospecific formation of helical fibres by di-astereomers of tartaric acid derivatised either with uracil or 2,6-diacylamino pyridine (Fig. 9) [88]. Upon mixing the complementary components, which are not liquid crystals in their pure state, mesophases form which exist over very broad temperature ranges, whose magnitude depend on whether the tartaric acid core is either d, l or meso [89]. Electron microscopy studies of samples deposited from chloroform solutions showed that aggregates formed by combination of the meso compounds gave no discernable texture, while those formed by combinations of the d or l components produced fibres of a determined handedness [90]. The observation of these fibres and their dimensions makes it possible that the structural hypothesis drawn schematically in Fig. 9 is valid. This example shows elegantly the transfer of chirality from the molecular to the supramolecular level in the nanometer to micrometer regime. 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

PDES) is a thermotropic liquid-crystal material. It exists in the mesomorphic state for a certain temperature range after melting of the crystalline phase (i5). This property is due primarily to the side chains. This material has two crystalline forms, a and P, and these crystalline forms go through isomorphic transitions from al to a2 and from pi to P2 when the temperature increases. These transitions occur because of ethyl group reorientation (i6), which has an activation energy of 9.3 kcal/mol. The previous experimental results (16) did not identify the initial and final orientations. In this study, we identified these orientations by using conformational analysis. 

The range of polymers which were found to be able to form liquid crystalhne systems has been considerably extended. Poly(Y-benzyl-L-glutamate) and its analogs, as well as para-aromatic polyamides, exhibit this property in solutions, which served the basis for relating them to lyotropic liquid crystals (see Sect. 2.1). Subsequently, the classes of polymers were found which exhibited such a transition during a change of temperature thermotropic liquid crystals). 

In this connection let us consider a fragment of a schematic phase diagram in the region of high concentrations of a polymer capable of forming the liquid crystalUne phase (Fig. 2). In a crystalUne polymer containing no solvent (100% polymer, vf), the transition from the crystalline state (c) to the Uquid crystalline state (Ic) must take place at the temperature T -.ic and further transition into isotropic state (i), at the temperature Such transitions are called thermotropic, and the system formed at I, is called the thermotropic liquid crystal. The transition to the Uquid crystalline state can also occur by adding to a polymer a solvent at a temperature below T -. c-... 

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