Types of Liquid Crystals

Substances that form liquid crystals are often composed of rod-shaped molecules that are somewhat rigid. In the liquid phase, these molecules are oriented randomly. In the liquid crystalline phase, by contrast, the molecules are arranged in specific patterns as 

Long axes of molecules aligned, but ends are not aligned 

Molecules aligned in layers, long axes of molecules perpendicular to layer planes 

Molecules pack into layers, long axes of molecules in one layer rotated relative to die long axes in the layer above it 

This chapter is organized as follows. The various types of liquid crystals are introduced in Section 5.2. Some important characteristics of liquid crystalline materials that result from the anisometry of liquid crystal molecules are discussed in Section 5.3. Then, in Section 5.4, the identification of liquid crystal phases is considered. Orientational order is a defining characteristic of thermotropic liquid crystals, and Section 5.5 is devoted to it. Section 

6 is concerned with the elastic properties of liquid crystals and Section 

7 with phase transitions. The chapter concludes in Section 5.8 with a brief review of applications of thermotropic liquid crystals, particularly in displays. 

Thermotropic liquid crystal phases are formed by anisotropic molecules with long-range orientational order, and in many types of structure some degree of translational order. The main types of mesogens are those that are rod-like or calamitic and those that are disc-like or discotic. 

An understanding of the correlation between molecular structure and physical properties of thermotropic mesogens is important to optimize parameters such as the operating temperature range of liquid crystal displays. 

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We are all familiar with tire tliree states of matter gases, liquids and solids. In tire 19tli century the liquid crystal state was discovered [1 and 2] tliis can be considered as tire fourtli state of matter [3].The essential features and properties of liquid crystal phases and tlieir relation to molecular stmcture are discussed here. Liquid crystals are encountered in liquid crystal displays (LCDs) in digital watches and otlier electronic equipment. Such applications are also considered later in tliis section. Surfactants and lipids fonn various types of liquid crystal phase but this is discussed in section C2.3. This section focuses on low-molecular-weight liquid crystals, polymer liquid crystals being discussed in tire previous section. 

U 13 Identify the different types of liquid crystals (Section 5.16). 

Also there seems to be a certain time delay between photoreaction and complete recovery of the nematic phase. This problem is relevant to molecular mobility in liquid crystals as a function of temperature, rubbing condition, external electric field and most importantly, the type of liquid crystal. Research is now being undertaken on direct determination of molecular mobility by fluorescence technique. 

There are two possibilities for the detection of the interferences the film detection and the registration of x-ray counts with scintillation counters or position-sensitive detectors. However, the SAXD method does not detect interferences from which the interlayer spacings can be calculated. It rather makes it possible from the sequence of the interferences to decide the type of liquid crystal [13,14]. 

Different types of liquid crystals exhibit different rheological properties [16,17]. With an increase in organization of the microstructure of the liquid crystal its consistency increases and the flow behavior becomes more viscous. The coefficient of dynamic viscosity r, although a criterion for the viscosity of ideal viscous flow behavior (Newtonian systems), is high for cubic and hexagonal liquid crystals but fairly low for lamellar ones. However, the flow characteristics are not Newtonian but plastic or pseudoplastic, respectively. 

Molecules which form other types of liquid crystal phases are also finding... 

Liquid crystals, as the name implies, are condensed phases in which molecules are neither isotropically oriented with respect to one another nor packed with as high a degree of order as crystals they can be made to flow like liquids but retain some of the intermolecular and intramolecular order of crystals (i.e., they are mesomorphic). Two basic types of liquid crystals are known lyotropic, which are usually formed by surfactants in the presence of a second component, frequently water, and thermotropic, which are formed by organic molecules. The thermotropic liquid-crystalline phases are emphasized here they exist within well-defined ranges of temperature, pressure, and composition. Outside these bounds, the phase may be isotropic (at higher temperatures), crystalline (at lower temperatures), or another type of liquid crystal. Liquid-crystalline phases may be thermodynamically stable (enantiotropic) or unstable (monotropic). Because of their thermodynamic instability, the period during which monotropic phases retain their mesomorphic properties cannot be predicted accurately. For this reason it is advantageous to perform photochemical reactions in enantiotropic liquid crystals. 

Shibaev, V. P., Moiseenko, V. M., Plate, N. A. Thermotropic liquid crystalline polymers, 3, Comb-like polymers with side chains simulating the smectic type of liquid crystals. Makromol. Chem. 181, 1381 (1980)... 

Nature also uses liquid crystals. The protein solution, or silk, spun by spiders to make webs is actually a type of liquid crystal. The well-known strength of the web depends on the crystal structure of the molecules in the liquid silk. 

Two types of liquid crystal polymers (LCP s) can be distinguished lyotropic and thermotropic. The first type is formed in a solution this is done when the polymer has such a high melting point (low AS, see 4.2), that it cannot be handled in the molten condition without being degraded. If this limitation is not present, then the orientation can be brought about in the melt in such a case we have a thermotropic LCP. 

Since the end of the seventies, interest in cyclotriveratrylene has moved towards the use of its cone shaped structure for applications in various fields, including investigations of the electronic transitions of the benzene chromophore via UV and CD spectroscopy, studies in the area of host-guest chemistry, synthesis of new types of liquid crystals, and searches for new three-dimensional organic charge-transfer materials. These works have been made possible because efficient synthetic... 

It is well-known that the tendency to form a liquid-crystalline phase is most pronounced for those substances which molecules have an elongated shape. Stiff-chain macromolecules are obviously good examples of this kind. Their asymmetry can be so large that they can form liquid-crystalline phase not only in the bulk but also in the solution. In the latter case, liquid crystals are called polymeric lyotropic liquid crystals. It is the theory of this type of liquid crystals that will be considered in the present paper. 

The key role played by conformations in these reactions prompted investigation of results in liquid crystals n0). However, results obtained with octane-4,5-dione and l,6-diphenylhexane-3,4-dione in several types of liquid crystals did not differ appreciably from those obtained in hexane solution. 

We do not give a detailed classification of the types of liquid crystals, since the systems under consideration mainly form nematic or in some cases cholesteric phase. The latter phase belongs, in principle, to the same range of liquid crystals as the nematic phase, since there is no phase transition between them (unlike smectic-nematic phase transition). 

We do not give a detailed classification of the types of liquid crystals, since the systems under consideration mainly form nematic or in some cases cholesteric phase. 

Figure 2.18 (a) Some typical mesogen groups, (b) Schematic representation of types of liquid crystal polymers according to the location of the mesogen groups in the main chain (right) or as side substituents (left). 

The passage of light through a twisted nematic (TN) cell type of liquid crystal display (LCD) when the current to the electrodes is off (top) and when it is on (bottom). Liquid crystai dispiay iiiustration by Hans Cassidy. Courtesy of Gale Research. 

Drug Type of liquid crystal Formula References... 

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