Liquid-crystal

A liquid crystal is a special kind of material. It is a liquid because it is a fluid, but it is also a crystal because it is an ordered solid. It is a material in a state between liquid and solid. The simplest kind of liquid crystal consists of a single semirigid or mesogenic core attached to one or two terminal alkyl chains. It is the anisotropic interaction between the cores, normally of phenyl rings through short unsaturated linkages, that results in the hquid-crystal behavior. 

It is well known that matter exists in three different states depending upon the temperature sohd, hquid, and gas. Between the crystalline solid and the isotropic liquid (normal, isotropic hquid) there actually exists a series of transitions as temperature increases, giving rise to several new phases. These new phases have mechanical, optical, and stractural properties and are referred to as hquid-crystalline phases. Materials isolated from these phases are liquid crytals. A hquid crystal may, therefore, be as an intermediate phase or mesomorphic (meaning in between) phase which has hquidhke order in at least one direction and possesses a degree of anisotropy (also a kind of order). 

There are two major ways to describe the arrangement of molecules in a hquid crystal  

Orientational order— the symmetry axes of the ordering objects parallel to a well-deflned spatial direction, n, called the director 

Positional order—an integer number of lattice translational vectors which makes the system invariant under translation 

Liquid crystals (LC) are phase structures that are intermediate between liquid and crystal phases. They have also been mentioned as mesophases (Greek mesos = middle). Liquid crystals have an intermediate range of order between liquid and crystal phases (Soltis et al., 2004 Friberg, 1976). LC may be described as follows. If a pure substance, such as stearic acid, is heated, it melts at a very specific temperature. Heating a pure solid shows the following behavior  

Liquid crystals are substances that exhibit properties of both liquids, such as the ability to flow and to take on the shape of a container, and those of crystals, such as a regular arrangement of particles in a lattice. Some substances exhibit liquid crystal behavior when they are melted from a solid. As the temperature increases, the solid liquefies but retains some order in one or two dimensions. At a higher temperature, the ordered liquid becomes a more conventional liquid in which there is no consistent orientation of the molecules. 

Frederick Reinitzer discovered the first compound to exhibit liquid aystal behavior in 1888. Cholesteiyl benzoate was observed to form an ordered Uquid crystalline phase when melted, and this liquid crystalline phase became an ordinary liquid at higher temperatures  

The structure of cholesteryl benzoate is fairly rigid due to the presence of the fused rings and sp -hybridized carbon atoms. The molecule is relatively long compared to its width, too. Rigidity and this particular shape makes it possible for the cholesteryl benzoate molecules to arrange themselves in an orderly manner, in much the way that pencils, chopsticks, or tongue depressors can be arranged. 

Recal from Chapter 9 that there is free rotatkxi about cartxm-carbon single bonds (i.e., bonds between sp -hybridized carbon atoms), but rxJt about catfaotvcatbon double bonds (i.e., boixk between qj rybridizEd carbon atoms). 

A material that changes color as the temperature changes is called IhermodromK. 

Thermotropic liquid crystal polymers (LCI ) are of considerable current interest, because of their theoretical and technological aspects [1-3]. Evidently, a new class of polymers has been developed, combining anisotropic physical properties of the liquid crystalline state with diaracteristic polymer features. This unique combination promises new and interesting material properties with potential ai lications, for example in the field of high modulus fibers [4], storage technology, or non-linear optics [5]. 

Although considerable effort has centered around the synthetic design and macroscopic behaviour of these systems, relatively little attention has been focused on the dynamic and structural features of the mofecular units. Up to now, only limited information is available about molecular order and motion in the different polymer phases [. On the other hand, exactly these molecular a-operties are responsible for the macroscopic appearance of the systems. Aj arently, the knowledge of the molecular behavior as a function of the chemical structure or sample history offers a means to a directed design of LCPs with well-defined material properties. 

Cholesteric liquid crystals are compounds that go through a transition phase in which they flow like a liquid, yet retain much of the molecular order of a crystalline solid. Liquid crystals are able to reflect iridescent colors, depending on the temperature of their environment. Because of this property they may be applied to the surfaces of bonded assemblies and used to project a visual color picture of minute thermal gradients associated with bond discontinuities. Cholesteric crystals are potentially a simple, reliable, and economical method for evaluating bond defects in metallic composite structures.f Materials with poor heat-transfer properties are difficult to test by this method. The joint must also be accessible from both sides.  

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

Liquid crystals can interact with light in several ways and such interactions have found wide application as shown in Table 5.1.  

The history of liquid crystals started with the pioneer works of Reinitzer and Lehmann (the latter constructed a heating stage for his microscope) at the end of the nineteenth century. Reinitzer was studying cholesteryl benzoate and found that this compound has two different melting points and undergoes some unexpected color changes when it passes from one phase to another [1]. In fact, he was observing a chiral nematic liquid crystal. 

As its name suggests, a liquid crystal is a fluid (liquid) with some long-range order (crystal) and therefore has properties of both states mobility as a liquid, self-assembly, anisotropism (refractive index, electric permittivity, magnetic susceptibility, mechanical properties, depend on the direction in which they are measured) as a solid crystal. Therefore, the liquid crystalline phase is an intermediate phase between solid and liquid. In other words, macroscopically the liquid crystalline phase behaves as a liquid, but, microscopically, it resembles the solid phase. Sometimes it may be helpful to see it as an ordered liquid or a disordered solid. The liquid crystal behavior depends on the intermolecular forces, that is, if the latter are too strong or too weak the mesophase is lost. Driving forces for the formation of a mesophase are dipole-dipole, van der Waals interactions, 71—71 stacking and so on. 

Liquid crystals are classified into two broad groups depending on the method used to get this state  

Thermotropic liquid crystals the temperature is the variable that determines which phase of matter exists. 

Lyotropic liquid crystals they display liquid crystalline behavior when mixed with another material in the right concentration (typically a solvent). They can also be a mixture of more than two components (e.g., cell membranes). 

When NMR spectra of compounds dissolved in liquid crystal solvents are studied, it is possible to obtain information that relates directly to the molecular structure (i.e. bond lengths and angles) of the compounds. 

The couplings normally observed in spectra obtained in fluid phases are the indirect couplings (/), transmitted via the electrons. But there are also direct dipolar couplings (D), which depend only on the geometrical relationships between pairs of nuclei, and are totally independent of the bonding relationships. However, these dipolar couplings are normally completely lost by the tumbling of the molecules, which is usually fast on the NMR timescale. 

In liquid crystals, which are compounds or mixtures that have phases between liquid and solid, the magnetic field of the NMR spectrometer leads to alignment of the solvent molecules and of their solute molecules, and this makes the direct dipolar couplings (D) observable. There are such couplings even between chemically equivalent nuclei, which therefore become magnetically non-equivalent. 

The spectra of even simple molecules can look very complicated in fact, a H NMR spectrum of benzene has more than 40 lines, as its spin system is [Ae] rather than [A]e- The analysis can be quite eomplex but, if successful, gives very precise relative distances between nuclei. For more detail you are referred to the section on liquid crystal NMR in the on-line supplementary material for Chapter 4. Other sources include a book [50], a review [51], and more recent book chapters on two-dimensional variants of the subject [52]. 

NMR spectra of KAsFg, with and without magic angle spinning. 

8 Collings, P. J., Liquid Crystals Nature s Delicate Phase of Matter, 2nd ed. Princeton University Press Princeton, 

FIGURE 3-3 Hexagonal (a), Lamellar (b), and Bicontinuous (c) liquid crystal structures. 

FIGURE 3-4 Phase diagram showing location of hexagonal (Hi), normal bicontinuous cubic (V)), and lamellar (La) liquid phases, aqueous nonmicellar solution (W), micellar solution (Li), liquid surfactant containing water (L2), and solid surfactant (S). 

The effect of temperature increase is typical for surfactants whose solubility increases with temperature increase, converting all liquid crystal phases to micellar solutions when the temperature is high enough. At high surfactant concentration and low temperature, solid surfactant may precipitate. 

Liquid crystal structures are important not only in the viscosity modification of surfactant solutions, but also in the stabilization of foams and emulsions, in detergency, in lubrication (Boschkova, 2002), and in other applications. 

We briefly discussed the origin and structure of liquid crystals in Section 4.13. The last decade has witnessed a surge of interest in liquid crystals because of their applications in display devices (devices that convert an electrical signal into visual information). The design of liquid crystal (LC) devices relies on the relation between the molecular structure and the phase behaviour (relative smectic-nematic tendency, NI etc.) as well as the physical properties of the liquid crystals (Chandrasekhar, 1994). 

The question of predicting the type of smectic polymorph that a given mesogen 

More-definitive structural correlations have been established in the nematic-cholesteric systems. Following Gray (1983), we summarize them below  

Substitution at lateral positions in the core structure generally decreases NI-For instance, in the series of carboxylic acids  

Correlations between molecular structure and the LC state have been fruitfully employed to design 4-alkyl and 4-alkoxy-4 -cyanobiphenyls, which form the basis for electrooptic display devices. The reasoning behind the design of these nematogens is as follows. In a family of nematogens of the general formula 

Hyperbranched polymers have been used to make unimolecular micelles and novel liquid crystals (see below for a discussion of liquid crystals). A major advantage of hyperbranched systems is that their synthesis is a single step It seems likely that for some applications, hyperbranched systems will perform well, but for others, the precision of a dendritic system will be required. 

Liquid crystals represent a state of matter that is intermediate between the crystalline and the liquid state. Because of this intermediate nature, the liquid crystal phase is sometimes referred to as a mesophase, and molecules that form liquid crystals are called meso-gens. Mesogens can be small molecules or polymers. The crystalline state is associated with complete molecular order and extremely high barriers to reorienting any molecule in the crystal, as long as we are well below the melting temperature. The liquid state involves molecules that are randomly oriented and rapidly reorienting. In liquid crystals there is molecu- 

Liquid crystals. A. Generic description of select phases. A rod-shaped mesogen can form a smectic or a nematic phase. A disk-shaped m can form a columnar/discotic phase. B. Illu.stration of how the dirertor of a cholesteric phase rotates as one moves through the material, giving rise to a helical pitch. Note that the oval in this part of the figure is not a sinci mesogenic molecule, but rather represents the director of a thin section of the cholesteric phase. 

Temperature plays a critical role here. The mesophases occupy an intermediate temperature regime. At low temperatures we have crystals at high temperatures we have liquids. In the intermediate temperature range, liquid crystalline behavior is possible. Tuning this intermediate temperature range, so that it is appropriately broad or narrow and does or does not encompass room temperature, is a key component of liquid crystal research. 

Schematic description of a simple LCD display. The colored arrows represent the direction of propagation of the light, while the black arrows represent the direction of polarization. A. With the twisted nematic between the two perpendicular polarizers, the polarization induced by the liquid crystal allows the light to pass. B. When an external electric field aligns the liquid crystal parallel to the first polarizer (indicated by the dipole arrow) the light cannot pass through the second polarizer and so there is no reflected light. 

Before considering the type of crystal with which everyone is familiar, namely the solid crystalline body, it is worth while mentioning a state of matter which possesses the flow properties of a liquid yet exhibits some of the properties of the crystalline state. 

Although liquids are usually isotropic, some 200 cases are known of substances that exhibit anisotropy in the liquid state at temperatures just above their melting point. These liquids bear the unfortunate, but popular, name liquid crystals the term is inapt because the word crystal implies the existence of a precise space lattice. Lattice formation is not possible in the liquid state, but some form of molecular orientation can occur with certain types of molecules under certain conditions. Accordingly, the name anisotropic liquid is preferred to liquid crystal . The name mesomorphic state is used to indicate that anisotropic liquids are intermediate between the true liquid and crystalline solid states. 

The mesomorphic state is conveniently divided into two main classes. The smectic (soap-like) state is characterized by an oily nature, and the flow of such liquids occurs by a gliding movement of thin layers over one another. Liquids in the nematic (thread-like) state flow like normal viscous liquids, but mobile threads can often be observed within the liquid layer. A third class, in which 

Strong optical activity is exhibited, is known as the cholesteric state some workers regard this state as a special case of the nematic. The name arises from the fact that cholesteryl compounds form the majority of known examples. 

For further information on this subject, reference should be made to the relevant references listed in the Bibliography at the end of this chapter. 

In 1888 Frederick Reinitzer, an Austrian botanist, discovered that the organic compound cholesteryl benzoate has an interesting and unusual property, shown in FIGURE 11.31. Solid cholesteryl benzoate melts at 145 forming a viscous milky liquid then at 179 °C the milky liquid becomes clear and remains that way at temperatures above 179 °C. When cooled, the clear liquid turns viscous and milky at 179 °C, and the milky liquid solidifies at 145 °C. Reinitzer s work represents the first systematic report of what we call a liquid crystal, the term we use today for the viscous, milky state. 

Instead of passing directly from the solid phase to the liquid phase when heated, some substances, such as cholesteryl benzoate, pass through an intermediate liquid crystalline phase that has some of the structure of solids and some of the freedom of motion of liquids. Because of the partial ordering, liquid crystals may be viscous and possess properties intermediate between those of solids and those of liquids. The region in which they exhibit these properties is marked by sharp transition temperatures, as in Reinitzer s sample. 

Today liquid crystals are used as pressure and temperature sensors and as the display element in such devices as digital watches and laptop computers. They can be used for these applications because the weak intermoiecuiar forces that hold the molecules together in the liquid crystalline phase are easily affected by changes in temperature, pressure, and electric fields. 

Long axes of molecules aligned, but ends are not aligned 

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

Lithium behaves differently from sodium or cesium also in the monooctanoin Li/H2 0 system. Quadrupole splittings for all the alkali metal cations are reported in decylammonium mesophases. A recent report finds Na quadrupole splittings in a nonaqueous liquid crystal in which the sodium is complexed by the cryptand C222, and this is believed to indicate that the splittings arise because of ordering effects rather than electrostatic interactions.  

Although the complete atomistic simulation of ensembles of mesogenic molecules is within reach of present computational facilities, the traditional treatment of liquid crystals in molecular dynamics or Monte Carlo simulations makes use of the Gay-Berne potential, an ingenious computational machine whose aspects deserve to be described here for their epistemological implications. An ordinary Lennard-Jones (LJ) potential, equation 4.38 or 4.40, can be written as a function of the distance between two particles, /Jy, the well depth e and the equilibrium separation a. An ellipsoidal object is identified by the position of its centroid and by an orientation unit vector u, and the Gay-Berne (GB) potential is a modified U that takes into account the anisotropy of the ellipsoid, both in energy and equilibrium separation  

In standard modeling, the parameters of the force field are optimized to reproduce experimental properties of real substances, e.g. heats of sublimation or of melting, and the parametric force field is then used to predict unknown values of the same and other physical quantities. In the study of liquid crystals by the above formalism the logical path is sometimes different one asks for certain properties, and then tries to find the parameters that would be needed to produce such a result. Consider, for example, the clearing point between a smectic phase and the liquid the transition is easily characterized by the evolution in temperature of the second moment of the distribution of molecular orientation vector u with respect to a principal direction e [30]  

Surfactant solutions at concentrations close to the cmc are clear and isotropic that is, the 

Modified from J. S. Clunie, J. F. Goodman and P. C. Symons, Trans. Farad. 

A second category of liquid crystals is the type produced when certain substances, notably the esters of cholesterol, are heated. These systems are referred to as thermotropic liquid crystals and, although not formed by surfactants, their properties will be described here for purposes of comparison. The formation of a cloudy liquid when cholesteryl benzoate is heated to temperatures between 145 and 179°C was first noted in 1888 by the Austrian botanist Reinitzer. The name liquid crystal was applied to this cloudy intermediate phase because of the presence of areas with crystal-like molecular stmcture within this solution. 

Although the compounds that form thermotropic liquid crystalline phases are of a variety of chemical types such as azo compounds, azoxy compounds or esters, the molecular geometries of the molecule have some characteristic features in that they are 

The arrangement of the elongated molecules in thermotropic liquid crystals is generally recognisable as one of three principal types  

Check The pressure and temperature at the critical point are higher than those at the triple point, which is expected. Methane is the principal component of natural gas. So it seems reasonable that it exists as a gas at 1 atm and 0 °C. 

Use the phase diagram of methane to answer the following questions. (a) What is the normal boiling point of methane (b) Over what pressru-e range does solid methane sublime (c) Above what temperature does liquid methane not exist  

Reinitzer s work represents the first systematic report of what we call a liquid crystal, the term we use today for the viscous, milky state that some substances exhibit 

Weinheim New York Chichester Brisbane Singapore Toronto 

John W. Goodby School of Chemistry Germany 

Universitat Hamburg Martin-Luther-King-Platz 6 

Merck Ltd. Liquid Crystals Merck House Poole BH15 ITD UK. Germany 

This book was carefully produced. Nevertheless, authors, editors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. 

Phospholipids and glycolipids exhibit both thermotrophic and lyotropic mesomorphism. The temperature at which such phase transitions are observed depends on the head group, the hydrocarbon chain length, and the degree and type of unsaturation present. Unsaturation tends to lower the transition temperature, as do shorter chains. This subject has been reviewed by Saupe (1973). 

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A Shearographic Inspection System Using a Liquid Crystal Phase Modulator. 

A nematic liquid crystal cell, based on Merck Licrilite E202, was used in these experiments. The rod like liquid crystal molecules preferentially aligned themselves with each other and to an alignment surface in the liquid crystal device. Any birefringence. An, was given as the difference between the two orthogonal refractive indices. As a consequence, any resulting... 

Figure 2. Schematic of the experimental arrangement used for inspection of aluminium plate. Lenses are shown as LI and L2, mirrors as Ml, M2 and M3, and liquid crystal cell as LC...

There has been much activity in the study of monolayer phases via the new optical, microscopic, and diffraction techniques described in the previous section. These experimental methods have elucidated the unit cell structure, bond orientational order and tilt in monolayer phases. Many of the condensed phases have been classified as mesophases having long-range correlational order and short-range translational order. A useful analogy between monolayer mesophases and die smectic mesophases in bulk liquid crystals aids in their characterization (see [182]). 

S. Chandrasekhar, Liquid Crystals, 2nd ed., Cambridge University Press, 1992. 

SAMs are generating attention for numerous potential uses ranging from chromatography [SO] to substrates for liquid crystal alignment [SI]. Most attention has been focused on future application as nonlinear optical devices [49] however, their use to control electron transfer at electrochemical surfaces has already been realized [S2], In addition, they provide ideal model surfaces for studies of protein adsorption [S3]. 

D. O. Shah and W. C. Hsieh, Microemulsions, Liquid Crystals and Enhanced Oil Recovery, in Theory, Practice, and Process Principles for Physical Separations, Engineering Foundation, New York, 1977. 

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 method has been extended to mixtures of hard spheres, to hard convex molecules and to hard spherocylinders that model a nematic liquid crystal. For mixtures m. subscript) of hard convex molecules of the same shape but different sizes. Gibbons [38] has shown that the pressure is given by... 

Another important application of perturbation theory is to molecules with anisotropic interactions. Examples are dipolar hard spheres, in which the anisotropy is due to the polarity of tlie molecule, and liquid crystals in which the anisotropy is due also to the shape of the molecules. The use of an anisotropic reference system is more natural in accounting for molecular shape, but presents difficulties. Hence, we will consider only... 

The liquid-crystal transition between smectic-A and nematic for some systems is an AT transition. Depending on the value of the MacMillan ratio, the ratio of the temperature of the smectic-A-nematic transition to that of the nematic-isotropic transition (which is Ising), the behaviour of such systems varies continuously from a k-type transition to a tricritical one (see section A2.5.91. Garland and Nounesis [34] reviewed these systems in 1994. 

Syimnetrical tricritical points are also found in the phase diagrams of some systems fomiing liquid crystals. 

One class of large molecules that was investigated relatively early was liquid crystals [37, 38], and in particular the group 4-n-alkyl-4 -cyanbiphenyl (mCB). These molecules fonu a highly crystalline surface adlayer, and STM images clearly show the characteristic shape of the molecule (figure B 1.19.8). 

Forster J S and Frommer J E 1988 Imaging of liquid crystals using a tunnelling microscope Nature 333 542... 

Smith D P E, Hdrber H, Gerber Ch and Binnig G 1989 Smectic liquid crystal monolayers on graphite observed by scanning tunnelling microscopy Science 245 43... 

Idziak S H J ef a/1994 The x-ray surface forces apparatus structure of a thin smectic liquid crystal film under confinement Science 264 1915-8... 

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... 

Wefers M M and Nelson K A 1995 Analysis of programmable ultrashort waveform generation using liquid-crystal spatial light modulators J. Opt. Soc. Am. B 12 1343-62... 

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