SPECIAL REPORT
LiQUiCl
crystals η colorful state of matter Glenn H. Brown, Kent State University, and Peter P. Crooker, University of Hawaii
Liquid crystals have been known for a century or more. But only in the past 20 years or so have their unusual properties—especially the color changes that they can undergo—found widespread application. Once mere chemical curiosities, they now are common as the es sential materials in many electrically controlled display devices. The numerical displays on wristwatches, pocket calculators, thermometers, games, clocks, and many other electrical and electronic devices depend on them. Only displays based on cathode-ray tubes have a bigger share of the display market. Now, moreover, they may be on the verge of finding even more uses—in forming television pictures, for example, or as displays for telephones and on automo bile dashboards. They appear to be ideal, too, as displays for portable instruments incorporating microproces sors. In addition, the development of Du Pont's Kevlar high-performance fiber may open the door to further progress in preparing macromolecular liquid crystals having exceptionally great strength. More and more, too, liquid crystals are being found in living structures, where they seem to control biological processes. For chemists and physicists studying the theory of matter, liquid crystals are of great interest. They occupy a sort of grey area between true solids and true liquids. Their behavior, therefore, has an important role in fur thering knowledge of such processes as ordering and phase transition. 24
January 31, 1983 C&EN
Molecules of disk-shaped liquid crystals are packed as cylinders set in a hexagon
Although the uniqueness of the liquid crystalline (or mesomorphic) state was not recognized until the late 1800s, papers written in the early 1800s describe systems that fit the properties we now ascribe to it. Liquid crystal science really begins, however, in 1888 with the researches of Friedrich Reinitzer, an Austrian botanist who prepared cholesteryl benzoate and found it has two interesting properties. When heated, its crystal lattice collapses at 145 °C to form a turbid liquid (liquid crystal). At 179 °C, the turbid liquid disappears and an isotropic liquid is formed. On cooling, the process is reversed. Reinitzer's second and even more interesting observation was that the color of the turbid liquid changes from red to blue as its temperature increases, and from blue back to red as the system is cooled. Reinitzer's choice of compound was fortuitous; not all liquid crystals change color with a change in temperature. Nevertheless, many organic compounds adopt structures with properties intermediate between those of a true crystal and those of a true liquid as they pass from the solid to the liquid state. That is why they are called liquid crystals and the intermediate phase (or phases) is described as liquid crystalline. Although liquid crystals exhibit certain aspects of both the solid and liquid states, they also possess properties that are not found in either liquids or solids. Their ordering properties, for example, can be controlled by ordinary magnetic and/or electric fields. Some have optical activity of a magnitude without parallel in any solid, liquid, or gas. And some change color as a result of the sensitivity of their structure to temperature. These properties are the basis for liquid crystals' practical applications. In addition to forming naturally at certain temperatures, liquid crystalline structures also can be formed when certain compounds are treated with a controlled amount of water or other polar solvent. These biréfringent systems contain certain rather large molecules, such as potassium myristate, and also are found in some biological systems. A hypothetical organic molecule that exhibits liquid crystallinity when heated or cooled appears as follows:
Several general statements can be made about the molecular geometry of such organic thermotropic liquid crystalline compounds. For one thing, the majority of liquid crystalline compounds have aromatic nuclei that are polarizable, planar, and rigid. In addition, the central group (X) connecting the two benzene rings in the molecule usually contains either a multiple bond along the long axis of the molecule or a system of conjugated
double bonds, or it involves a dimerization of carboxyl groups. The central group constitutes a lathlike or rodlike core of the molecule. The length of the molecule is greater than its diameter (assuming a cylindrical geometry). A strong polar group near the center of the molecule and along the molecular axis generally enhances liquid crystallinity. Finally, weak polar groups at the extremities of the molecule (Λ and B) enhance liquid crystallinity. Some liquid crystals, such as the cholesteryl esters, possess a macroscopic twist and are formed by chiral molecules. In addition to liquid crystals formed from such rodor lathlike molecules, discotic liquid crystalline struc tures composed of large platelike molecules also occur—for example, at high temperatures during the carbonization of graphitic materials such as coal and petroleum. Disklike molecules are not new to the or-
Molecules in thermotropic liquid crystals are arranged in an orderly structure
Ordinary nematic structure
Isotropic structure (unordered, nonliquid crystalline)
Smectic A structure
Cholesteric structure
SmecticC structure
January31, 1983 C&EN
25
Special Report Structures of lyotropic liquid crystals are formed by amphiphilic molecules and water
MmÊÊÊWêt
Lamellar packing
Regular and inverted cylindrical packing
Cubic packing of spheres
Note: Ionic head is represented by a circle, and organic tail by a wiggly line; water is an integral part of the structure.
ganic chemist: Porphyrins form liquid crystals, and chlorophyll in selected concentrations of water also has been described as liquid crystalline. Probably the first relatively small disklike molecules prepared in a laboratory were benzenehexa-n-alkanoates and hexa-alkoxy triphenylans. In discotic compounds, the disks are stacked aperiodically, one on top of the other, to form liquidlike columns, the different columns forming a hexagonal array. The structure therefore has translational order in two dimensions but not in the third.
Classification of liquid crystals Liquid crystals are classified as either lyotropic or thermotropic, depending on the principal way by which the order of the parent solid state is destroyed. Lyotropic liquid crystals are commonly prepared by the action of a solvent on a solid. They are further divided into several categories, according to their structure. They often are two-component systems composed of water and amphiphilic compounds. However, multicomponent systems also are common in lyotropic liquid crystals. An example similar to liquid crystals found in living systems is a mixture of lecithin, cholesterol, bile salts, and water. Amphiphilic compounds have a polar head (ionic) that tends to dissolve in water (hydrophilic) and an organic tail that is water insoluble (hydrophobic). Two common types of molecular geometries are found. Sodium stéarate is typical of one type of molecule, in which the molecule's polar head is attached to one long hydrophobic tail. A second type of molecule has its polar head attached to two hydrophobic tails. The hydrophobic groups either lie side-by-side in a "clothespin" structure or at an acute angle to each other to form a wedge-shaped molecule. An example of the latter type 26
January 31, 1983 C&EN
of molecule is dioctyl sodium sulfosuccinate (Aerosol OT): Ç2H5
?
CH3—(CH2)3—CH— •CH2—OC—CH2 CH3"
(012)3
GT* C2H5
CH2
- O C — C H ( S 0 3 - ) Na +
II ο
Starting with water and the crystalline form of an amphiphile, a series of structures can be generated, ranging from the crystal to a true solution. With certain combinations, the polymorphic mesophases formed show lamellar molecular packing (packing in layers), cubic molecular packing, or hexagonal molecular packing. Removing water can reverse the order of mesophase formation, as follows: Lamellar + Η 2 θ Cubic + Η 2 θ Hexagonal > > 114UIU liquid lliquid ' liquid 114UIU (< iquid - H ^ O „ r o ^ 1 - HH92O ^ . x r o ^ l - HH92O ™,oJ.„1 -H20 crystal " ° crystal ~ ° crystal +Η2θ
< Solid G -* V 2 -* M 2 No known systems exhibit all of the mesophases, however. And other, less common packings exist. For example, an initial viscous (Sic) phase has been identi fied. And molecules with lamellar packing can be tilted to the planes of the layer. Thermotropic liquid crystals, which form naturally over a specific temperature range, are further classified as either nematic or smectic. "Nematic" comes from the Greek word for thread; nematic crystals have a thread like pattern when viewed through a microscope with crossed polarizers. "Smectic," from the Greek for soap, describes these crystals' greasy or soapy properties. Nematic liquid crystals are the type now most widely used in display devices. In typical nematic liquid crys tals, the only structural restriction is that the long axes of the molecules maintain a parallel or nearly parallel arrangement over macroscopic distances. The direction of the alignment axis (indicated by a unit vector known as the director) is arbitrary in space. The nematic phase always changes on heating to an isotropic liquid. This transition is weakly first order; the enthalpy of the transition generally lies between 0.1 and 1.0 kcal per mole. Although nematic liquids have an infinite-fold symmetry axis and are, therefore, uniaxial, the orientation of the individual molecules along this axis is not perfect. A measure of the degree of orientation can be expressed by the order parameter, S, where S = ^[3cos20-1] θ denotes the angle between a molecule's long axis and the nematic symmetry axis and the brackets indicate a January 31, 1983 C&EN 27
Special Report Thermotropic liquid crystals have either nematic or smectic structures Class
Optical properties
Textures
Structure
Examples
NEMATICS Ordinary nematic
Uniaxially positive
Schlieren; threaded; marbled; pseudoisotropic; homogeneous
Parallelism of long molecular axes
p-Azoxyanisole; p-methoxybenzylidene; p-(n-butyl)aniline
Cholestericnematic
Uniaxially negative or isotropic; optically active
Focal conic with Grandjean steps; homogeneous; isotropic
Nematic packing in planes; superimposed twist in direction perpendicular to long axes of molecules
Cholesteryl nonanoate
Blue phase
Isotropic; optically active
Platelet with Grandjean steps
Cubic
Cholesteryl nonanoate
STRUCTURED SM ECTICS Smectic Β
Uniaxially positive
Mosaic; stepped drops; pseudo-isotropic; homogeneous
Layer structure; molecular axes orthogonal to the layers; hexagonal arrangement within the layers
Ethy lethoxybenzy 1 ideneaminocinnamate; terephthal-bis-butylaniline
Smectic Ε
Biaxially positive
Mosaic; pseudo-isotropic
Layer structure; molecular axes orthogonal to the layers; ordered arrangement within the layers
Di-n-propylterphenyldicarboxylate
Smectic G
Biaxially positive
Mosaic
Layer structure with ordered arrangement within the layers
2-(4-n-Pentylphenyl)-5-(4-npentyloxyphenyl)pyrimidine
Smectic H
Biaxially positive
Fan
Tilted analogue of smectic F
Smectic 1
Biaxially positive
Mosaic; fan-shaped texture with stripes across the fans
Hexagonal correlation inthe-plane and the tilt direction is uniform toward neighboring molecules
4-r>-Pentylbenzenethio-4'-noctyloxybenzoate
1
UNSTRUCTURED SMECTICS Smectic A
Uniaxially positive
Focal conic (fan-shaped or polygon); stepped drops; homogeneous; pseudo-isotropic
Layer structure; molecular axes orthogonal to the layers; random arrange ment within the layers
Diethylazoxybenzoate
Smectic C
Biaxially positive
Broken focal conic; schlieren; homogeneous
Layer structure; molec ular axes tilted to the layers; random arrange ment within the layers
Dodecyloxyazoxybenzene
Smectic D
Isotropic
Isotropic; mosaic
Cubic structure
4'-Octadecyloxy-3'-nitrodiphenyl-4-carboxylic acid
Smectic F
Biaxially positive
Schlieren; broken focal conic
Layer structure
2-(4-rt-Pentylphenyl)-5-(4rt-pentyloxy phenyl )pyrimidine
Source: "Liquid Crystals and Biological Systems" by J. J. Wolken and G. H. Brown
. - ....... .. thermal average. Experimental values of S range from about 0.4 at the nematic-isotropic point to about 0.6 in the nematic liquid at lower temperatures. Chiral liquid crystals, first observed with cholesteryl esters, are known as cholesteric or chiral-nematic crys tals. Nonsteroidal molecules that exhibit optical activity also may show the cholesteric structure. Cholesteric liquid crystals are formed by some opti cally active organic compounds, or mixtures of such 28
January 31, 1983 C&EN
compounds, or by mixing optically active compounds with ordinary nematic liquid crystals. They are miscible with ordinary nematic liquid crystals and have a local nematic packing of the molecules. The director, how ever, is not fixed; instead, it rotates spatially about an axis perpendicular to itself. Thus, cholesteric molecules have a helical structure, and the distance for a 360° turn of the director, commonly referred to as the pitch, may be of the order of a wavelength of light. The periodicity of
such a structure leads to spectacular color effects. And the sensitivity of cholesterics to temperature and ra diation is opening up many potential uses for them as sensors or displays. Cholesteric compounds with pitches less than about 700 nm exhibit a recently recognized phase, called the blue phase, between the isotropic and cholesteric re gions. The temperature range of the blue phase is typi cally about 1 °C; in that range a blue phase sample re flects the colors of light that would be associated with a cubic structure having a lattice parameter about the same as the pitch of the cholesteric phase.
Smectic structures Of the nine known smectic structures, eight have molecular packings arranged in strata or layers, with each structure possessing a characteristic packing be tween and within layers. The ninth smectic structure, known as smectic D, is optically isotropic and has cubic, not layer, packing. Structured and unstructured smectics can be differ entiated by the order within their layers. The structured smectics have a crystallike arrangement of molecules in each layer; each layer forms a regular two-dimensional lattice. In unstructured smectics, the molecules are po sitioned randomly in each layer. In some smectics, the molecules are perpendicular to the layers (orthogonal smectics); in others, the molecules are tilted (tilted smectics). Orthogonal smectics include the smectic A, B, and Ε phases. Tilted smectics include the smectics C, F, G, H, and I. (The letter designations merely indicate the chronological order in which the various smectic phases were first observed.) Smectic A liquid crystals are the least ordered of the orthogonal smectic phases. The molecules are arranged in layers, but the molecules in the layer have no struc ture. Recent x-ray experiments show that the layer re flections are almost completely due to one spatial fre quency; hence the "layers" are really a sinusoidal vari ation in the density. Furthermore, high-resolution studies of the x-ray line shapes show that the positional correlation of the layers dies away algebraically with distance rather than remaining constant, as in a true crystal. Thus, a smectic A crystal does not possess true long-range order, a result confirming the 1937 predic tion by Lev D. Landau, a Russian physicist, that a three-dimensional crystal cannot consist of a one-di mensional density wave. Smectic C is like smectic A in that the molecules are randomly positioned in planes. The molecules are all tilted away from the layer perpendicular in the same direction, however, so that smectic C is optically biaxial. The smectic Β structure consists of layers of molecules and, in addition, hexagonal packing of the molecules in the layers. One form of smectic Β has the molecular axis perpendicular to the layers; it is optically uniaxial. A similar form, now called smectic G, has its molecules tilted in the layers; it is biaxial. The same questions of long-range order that exist in smectic A also apply to smectic Β and are currently under intense investiga tion.
Schlieren texture of a nematic liquid crystal viewed be tween crossed polarizers shows singular points, character ized by dark brushes, that are threads viewed on end where the molecules are aligned with either polarizer The molecules of the smectic D phase are thought to be packed hexagonally into spherical arrangements. The spheres are then packed in a cubic order. Other more complex models also have been proposed. This phase is optically isotropic. The smectic Ε in-plane packing is rectangular, with long-range positional correlations. Smectic Ε is often iden tified as a soft crystal. Its structure might be regarded as bordering between solids and liquid crystals. The smectic F phase has short-range hexagonal cor relations in the plane, and the molecules are tilted uni formly toward neighboring molecules. This molecular packing is like the smectic C phase, but more ordered. The two smectic structures G and H are tilted analogs of smectic Β and smectic E, respectively. The smectic I phase exhibits hexagonal correlations in the plane, with the direction of tilt of its molecules uniform toward neighboring molecules. A variation of ordinary smectics may occur when a tilted smectic liquid crystal is formed from optically ac tive compounds or is doped with a small amount of a chiral compound. A macroscopically chiral texture is formed wherein each layer of molecules is twisted through a small angle relative to the adjacent layers. The twist is cumulative and may translate throughout the January 31, 1983 C&EN
29
Special Report entire system, resulting in a strongly optically active liquid crystal. Smectic phases can be identified by x-ray analysis and by using a polarizing microscope. Many thermotropic liquid crystals pass through more than one mesomorphic phase on heating from the solid to the isotropic phase. Such liquid crystalline com pounds are said to be polymorphous. Raising the temperature of a thermotropic liquid crystal results in the progressive destruction of its mo lecular order. Because a smectic Β phase is more ordered than a smectic C, and a smectic C phase is more ordered than a nematic structure, for a compound exhibiting smectics A, B, C, and nematic phases, the order of sta bility is solid -* SB -+ Se -* SA ~~* nematic -* isotropic An interesting variation of this scheme is the re-en trant nematic phenomenon discovered by Patricia E. Cladis of Bell Laboratories. Mixtures formed with two liquid crystals exhibit with decreasing temperature a sequence of nematic, smectic, and then nematic phases. The nematic phase that occurs at the lowest temperature is called the re-entrant nematic. The same phenomenon has been observed with single compounds under pres sure.
Texture and colors Liquid crystals have a wide variety of macroscopic textures. Each of their many phases—nematic, cholesteric, blue, and smectic—has its own structure which, in turn, can support its own family of structural defects. The defects of a particular structure influence the texture one sees in viewing a liquid crystal through a Liquid crystals having structured layers, such as this polarizing microscope. Other factors affecting a partic smectic H, produce a mosaic texture; each color domain ular liquid crystal's texture are the technique by which essentially is a single undeformed crystal Magnification it is prepared and the method by which the texture is is 60 times observed. Molecules aligned perpendicular to the sample surface will appear to be isotropic. If, on the other hand, the molecular alignment is parallel to the surface, the sample will be optically biréfringent. Electric and magnetic I fields, as well as velocity fields, can (8 affect the alignment. Thermal history also is important; a desired texture often can be formed by growing it out of a previously formed, more easily alignable phase. Texture appearance also is greatly affected by the way the crystal is illuminated and observed. Most textures are biréfringent and therefore are observed easily in transmitted light with the sample between crossed polarizers. Without the polarizers, the texture cannot be discerned. Cholesteric and blue-phase textures are best seen in reflection using circularly polarized light hav- Fan texture is possible when a smectic A liquid crystal, in this case 4'-n-octyl' ing the same rotational sense as the oxybenzylidene-4-chloroaniline, is observed through cross polarizers. Magnifisample. Observation of a right- cation is 96 times 30
January 31, 1983 C&EN
I handed structure with left circularly
polarized light produces no selective reflection at all. Colors observed in a liquid crystal texture may be caused by three mechanisms. A liquid crystal—usually one with chiral molecules—that contains a spatial periodicity in its structure that is compatible with a ! wavelength of visible light will form visible Bragg reflections, for example, in the same way x-rays are diffracted by crystals. Colors also can be produced by the interference of transmitted polarized light when a biréfringent texture is placed between two polarizers. These colors may be changed by manipulating the polarizers or by holding the polarizers fixed and realigning Elliptical focal conic texture is seen when a smectic A liquid crystal, formed by the liquid crystal, either mechani4'-n-butyloxybenzylidene-4-aminopropiophenone, is viewed through crossed cally or with an external field. polarizers. Magnification is 320 times In addition, dichroic dye molecules (such as methyl red or azo dyes of the aminothiazole type), when dissolved in liquid crystals, tend to orient in the direction of the liquid crystal molecules. The dye molecules absorb light polarized in one direction. They may be rotated by an electric field into or out of alignment with the polarization direction of a light beam to produce colors. The simplest texture is one in which the whole liquid crystal appears optically uniform. Nematics and some smectics can be aligned in parallel by treating the substrate with polyvinyl alcohol or an organosilane and rubbing it in one direction. The molecules tend to align with their long axes in the direction they are rubbed. Similar effects can be achieved when silicon monoxide is evaporated on a glass or quartz substrate at a large angle from the normal. (This is known as skew evaporation.) On the other hand, treatment of the substrate with a surfactant, such as lecithin, will cause perpendicular alignment of the molecules. Color effects can then be achieved by interference of polarized light or by the use of dichroic dye molecules. Cholesteric molecules can be uniformly aligned only with the pitch axis perpendicular to the surface. The color of this structure is caused by a Bragglike reflection associated with the periodicity of the cholesteric spiral and can be observed without polarizers. Nematic liquid crystals not uniformly aligned appear turbid. In films greater than 0.1 mm thick, they show threadlike singularities (called disclinations) when viewed between crossed polarizers. In thinner films, a schlieren texture with pointlike singularities is seen. These singularities are threads viewed on end and may be characterized by the number of dark lines (brushes) that appear when they are observed between crossed polarizers. Points with two or four brushes are common. Platelet texture is formed by blue-phase liquid crystals, Simultaneous rotation of the polarizer and analyzer alwith red, yellow, and violet colors corresponding, respeclows one to distinguish between positive and negative tively, to reflection from each of three cubic planes of the points, depending on whether the brushes rotate in the crystal. This back-reflection photomicrograph was made using white incident light and crossed polarizers same (positive) or opposite (negative) sense.
1
1 î
î
January 31, 1983 C&EN
31
Special Report
(a)
Cholesteric liquid crystals display vivid colors The brilliant colors produced by cho lesteric liquid crystals (as well as by some other chiral crystal systems) result from the unusual optical properties of such materials. In a cholesteric, the molecules are arranged as in diagram (a). Although the symmetry at any single layer of the crystal is uniaxial, the di rector, ή, which indicates the alignment of the molecules, describes a helix in space. The diagram shows one pitch, the distance for a complete rotation of the director. A liquid crystal system of this type has a spatially rotating dielectric ellipsoid, as shown in (b). The ellipsoids are identical, each having a major axis parallel to the molecular orientation and two equal minor axes perpendicular to the mo lecular orientation. As the molecules rotate in space, so do the dielectrical properties. The difference (Δη) between the refractive indexes corresponding to the major axis (r\\\) and the minor axes ( n ± ) of the ellipsoids is much less than the average of these indices (n); typi cally, Δη/η is about 0.03. Light incident along the pitch axis of the crystal generally propagates as two independent waves, one right circularly polarized and the other left circularly polarized. In a cholesteric with righthanded chirality, the left circularly
polarized wave spirals in an opposite direction to the cholesteric helix, "senses" n\\ and n ± often, and as a re sult behaves like a wave propagating through a medium with a refractive index equal to n. The right-hand circularly polarized light spirals in the same di rection as the helix; consequently, it shows anomalous behavior similar to that of other waves in periodic media. As plane-polarized light passes through the cholesteric, its polarization changes as its wavelength increases. Light of wavelengths shorter than the pitch of the liquid crystal remains linearly polarized, but the spatial rotation of the dielectric ellipsoid causes a rotation of the plane of polarization in the same direction as the helix. This rotation de creases as the wavelength increases until the wavelength reaches a value nearly equal to the pitch; the right cir cularly polarized component of the wave then begins to "sense" more strongly the twist of the dielectric spiral also, so that the rotatory power increases rap idly—as shown in graph (c) for a cho lesteric liquid crystal with a pitch of 500 nm. The reflectivity of this component likewise increases rapidly—as shown in graph (d)—becoming 100% in the region where the wavelength is about
Smectics and cholesterics that do not have uniform textures generally exhibit some form of focal conic texture. Each focal conic arrangement contains an el liptical disclination line and, in a perpendicular plane, a hyperbolic disclination that passes through the focus of the ellipse. When the ellipses are parallel to the sur face, elliptical figures appear. When the hyperbolae are parallel to the surface, a "fan" texture results. Optically, focal conic textures scatter light in all directions and have a strong overall depolarizing effect on incident light. An interesting mosaic texture is produced by smectic Β and other liquid crystals that have structured layers that do not deform into focal conies. The domains are essentially single, undeformed smectic crystals; corre spondingly, the colors, which are the result of the in terference of polarized light, are constant over any do main but vary from domain to domain. An unusual platelet texture is formed when individual blue-phase crystals having different crystal planes are aligned with the sample surface. Distinct, characteristic colors are reflected, each corresponding to a Bragglike reflection from one of the cubic planes of the blue phase structure. 32
January 31, 1983 C&EN
iS^^
equal to the pitch. At this wavelength, the remaining transmitted light is all left circularly polarized. For wavelengths longer than the pitch, reflectivity de creases again and the rotatory power, which now is strong in the opposite sense as the helix, becomes strongly left
Liquid crystals containing chiral molecules form macroscopically chiral structures—specifically, cho lesteric, blue-phase, and chiral smectic structures—that can exhibit striking color effects. A planar cholesteric, for example, has a structure that is uniaxial in any given plane but spatially describes a helix. Light incident along the axis of the helix will propagate as two independent waves, one essentially right circularly polarized and the other left circularly polarized. One wave spirals in the same sense as the helix; the other spirals in an opposite sense. The behavior of incident light that is plane polarized will change as its wavelength increases. For wavelengths much shorter than the pitch of the structure (the distance of one complete rotation of the helix), the combined wave remains linearly polarized, but the direction of polarization is rotated by the spatial rotation of the helix. The rotatory power is extremely high—360,000° per mm for a sample with a pitch of 1 μιη. As the wavelength is increased to a value nearly equal to the pitch, the right circularly polarized wave increasingly is reflected, with reflectivity becoming 100% where the wavelength is about equal to the pitch. In this region, the transmitted
(c)
;*>)
2000 Ε Ε ν_
ω ο. % ι—
-1000
Ο S Ο
-2000 (d)
1
£ c . 2 Q>
υ υ 0.5 ω ο
DC Ο
0 450
Source: Dwight W. Berreman, Bell Laboratories
and diminishes as the wavelength be comes greater than the pitch. The nar rowness of the region of total reflection causes the reflected color to appear very pure. This behavior is quite different than that of other materials. The rotatory
475
500
525
550
Wavelength, nm
power of solid crystals is relatively small—22° per mm in quartz, for ex ample—with a small monotonie wave length variation. The rotatory power of a cholesteric is orders of magnitude larger, and it diverges and changes sign with changing wavelengths.
wave is then solely left circularly polarized. Because the width of this region of 100% reflectivity is quite narrow, the reflected color appears very pure. For wavelengths longer than the pitch, the reflectivity decreases. The above description accounts for the main features of the optical properties of cholesterics. Additional structure is added to the reflectivity when the sample is very thin or when the cholesteric spiral is distorted by external fields. For waves propagated at angles to the pitch axis, the selectively reflected wavelength generally decreases with increasing angle. A simple theory ex plains this effect by assuming that Bragg reflection oc curs from the cholesteric planes. The blue phase of cholesterics reflects only certain discrete colors that have wavelength ratios compatible with Bragg reflections from a cubic lattice. Two addi tional facts—that the lattice parameter is near the cho lesteric pitch and that the Bragg reflections are circularly polarized—indicate that a unit cell is composed of cholestericlike spirals fitted together in cubic fashion. Several models have been proposed, most of which contain a regular array of disclinations, but the bluephase structure is still an unsolved problem.
The narrowness of the region of total reflection also is unusual and is the ori gin of the vivid colors reflected by cholesterics. Similar color effects seen in nature—on some beetle wings, for in stance—are produced by the same mechanism.
The widest use of liquid crystals currently is in display devices. They consume very little power and are reliable, versatile, and easy to read under ambient illumination. They are used in watches, pocket calculators, and other semistatic panel displays. More involved displays, par ticularly for flat panel television screens, are being de veloped. Typically, a small amount of nematic liquid crystal is placed in a thin, flat optical cell. The cell walls, which are coated with a transparent conducting film, are treated carefully to control the direction in which the long liquid crystal molecules are oriented. Application of an electric field to a certain region of the cell then disturbs the orientation of that region in one of a number of different ways, thereby changing its optical appear ance. Many mechanisms are possible. For example, the ap plied electric field may cause conduction, thereby scrambling the molecular order and rendering the ma terial reflective. Or the field may rotate the molecules, changing the sample's birefringence. It is also possible to dissolve a dichroic dye into the liquid crystal so that application of the field rotates the liquid crystal and January 31, 1983 C&EN
33
Special Report
Thermotropic liquid crystalline compounds typically have aromatic nuclei Formula
Name
Liquid crystalline range, °C
CLASSICAL NEMATIC LIQUID CRYSTALS H
I
C=N
ChUO
λ
//~C 4 H 9 -r?
λ
/r— C4H9-n
4-Methoxybenzylidine-4'-n-butylaniline (MBBA)
21-47°
Ο
t
ChLO
N=N
CH30
N=N^v
α
ο
^-C6H13—
