Liquid-crystal thermotropic

1 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. 

Thermotropic phases are those that occur in a certain temperature range. If the temperature is raised too high, thermal motion will destroy the delicate cooperative ordering of the LC phase, pushing the material into a conventional isotropic liquid phase. At too low a temperature, most LC materials will form a conventional (though anisotropic) crystal. Many thermotropic LCs exhibit a variety of phases as the temperature is changed. For instance, a particular mesogen may exhibit various smectic and nematic (and finally isotropic) phases as temperature is increased. 

The chiral pitch refers to the distance (along the director) over which the mesogens undergo a full 360° twist (the structure repeats itself every half pitch since the positive and negative directions along the director are equivalent). The pitch may be varied by adjusting temperature or adding other molecules to the LC fluid. For many types 

There are some materials that exhibit more than one transition in changing from the solid to the liquid state. The molecular ordering in these mesophases lies between that of a solid and that of an isotropic liquid. This kind of material is known as a liquid crystal. Liquid crystals show physical properties in both the solid and liquid states. There are two types of liquid crystal one is lyotropic, whose phase transition is caused by changing the concentration or pH value of the solution, and the other is thermotropic, whose phase transition occurs on changing the temperature. 

There is a large variety of thermotropic liquid crystals, and generally each of them may exhibit several mesophases as the temperature changes. The transition behavior of these mesophases is complex. DSC and DTA methods are particularly suitable for the detection and study of the mesophase. 

X-ray diffractometry, polarization microscopy and mixture testingt are important in the identification of mesophases of liquid crystals while DSC and DTA are the most valuable aids in revealing and confirming phase transition. Simultaneous polarization microscopy-DSC instruments are now commercially available. 

DSC peak area (i.e. transition enthalpy) can be used, to some extent, to identify the mesophase near the transition temperature. Many studies have showed that the transition enthalpy (kjmol ) for N— 1, for example, is 0.08-9.6 S - I, 3-13 Sc I, 10-43 S - N, 0.2-4.6 Sc- N, 0.7-9.6 S - S, 0.4-4.6 Sb S, 1.8-10 S(-- Sa, 0.04-2.8 Sp- S, 0.2-0.5 and so on. The melting enthalpy for crystals is usually higher than that for liquid crystals by at least 10-100-fold. A highly sensitive and reliable instrument is therefore necessary in order to observe calorimetrically the phase transition for liquid crystals. 

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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]. 

Ruths M, Steinberg S and Israelachvili J N 1996 Effects of confinement and shear on the properties of thin films of thermotropic liquid crystal Langmuir M 6637-50... 

Liquid crystal phases can be divided into two classes. Thermotropic liquid crystal phases are fonned by pure... 

Hamley I W, Garnett S, Luckhurst G R, Roskilly S J, Pedersen J S, Richardson R M and Seddon J M 1996 Orientational ordering in the nematic phase of a thermotropic liquid crystal A small angle neutron scattering study J. Chem. Phys. 104 10 046-54... 

Vertogen G and de Jeu W H 1988 Thermotropic Liquid Crystals, Fundamentals (Heidelberg Springer). 

D. Aciemo and M. R. Nobile, Thermotropic Liquid Crystal Polymer Blends, (F. P. La Mantia, ed.), Technomic Publishing, Lancaster (1993). 

The raw materials from which di-D-fructose dianhydrides can be obtained in appreciable yield are readily available from comparatively inexpensive agricultural feedstocks. Thus, these compounds are attractive as chiral-starting materials for chemical synthesis. Their stability to acid and heat, and their relative rigidity, because of the conformational constraints covered here, are also features that might be exploited during syntheses.119 A series of variously substituted di-D-fructose dianhydrides has been prepared,119 starting from 6,6 -dideoxy-6,6 -di-halosucroses. The properties of these and other derivatives of di-D-fructose dianhydrides are summarized in Tables XIV-XX. Two of these derivatives, 48 and 56, exhibit thermotropic liquid-crystal properties.119... 

Molecular Structure in Thermotropic Liquid Crystal Phases. 58... 

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]. 

Leadbetter AJ (1987) In Gray GW (ed) Thermotropic liquid crystals, chap 1. Wiley, New York... 

A compound which displays liquid crystal properties is referred to as a mesogen and said to exhibit mesomorphism. Liquid crystals may be considered either as disordered solids or ordered liquids, and their properties are very dependent on temperature and the presence or absence of solvent. In thermotropic liquid crystals the phases of the liquid crystals may be observed to change as the temperature is increased. In lyotropic liquid crystals the ordered crystalline state is disrupted by the addition of a solvent, which is very commonly water. For these systems temperature changes may also be... 

The mesophases of thermotropic liquid crystals are described as calamitic if the constituent molecules are rod-like and columnar, if the constituent molecules, which often have a disc like shape(discotic), stack into columns. 

Hird, M. and Toyne, K.J. (1998) Fluoro Substitution in Thermotropic Liquid Crystals. Molecular Crystals and Liquid Crystals, 323, 1-67. 

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. 

Thermotropic liquid crystals may be classified as either low molar mass (i.e. non-polymeric), or high molar mass (i.e. polymeric) and within each of these broad classifications, there are several sub-classifications. 

The family of primary silver thiolate compounds AgSC H2n+1 ( = 4, 6, 8, 10, 12, 16, or 18), which in the solid state consists of 2x,[AgSR] layers, behaves as thermotropic liquid crystals. On heating, they display successively lamellar (smectic A), cubic, and micellar mesophases.969... 

Kevlar has the highest tensile strength and is often used as a reinforcing fibre in composites with, e.g., epoxy, PEEK. The thermotropic liquid crystal polymer Vectran is made by melt polymerisation of p-acetoxybenzoic acid and 6-acetoxy-2-naphthoic acid, (the corresponding hydroxy acids decompose on melting). Because of its liquid crystal properties the polymer can be spun into fibres from the melt. Kevlar is spun from a solution in concentrated sulfuric acid, and can be melt drawn to give a high modulus (stiff) polymer. Vectran ... 

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. 

McDonnel, D. G. In Thermotropic Liquid Crystals. Gray, G. W. (Ed.). Wiley, Chichester, 1987, pp. 120-125, gives a clear discussion of this phenomenon. 

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