INTRODUCTION States of matter are distinct forms that different matters take on. Distinctions between states have been based on differences in molecular interrelationships. Solid is the state in which intermolecular attractions keep the molecules in fixed spatial relationships. Liquid is the state in which intermolecular attractions keep molecules in proximity, but do not keep the molecules in fixed relationships. Gas is that state in which the molecules are comparatively separated and intermolecular attractions have relatively little effect on their respective motions. Liquid crystal states have properties intermediate between mobile liquids and ordered solids. Liquid crystals are also not quite liquid and not quite solid. Physically, they are observed to flow like liquids, but they have some properties of crystalline solids. Liquid crystals can be considered to be crystals which have lost some or all of their positional order, while maintaining full orientational order.
Historical Perspective of Liquid Crystals Between 1850 and 1888, researchers in different fields such as chemistry, biology, medicine and physics found that several materials behaved strangely at temperatures near their melting points. It was observed that the optical properties of these materials changed discontinuously with increasing temperatures. W. Heintz, for example, reported in 1850 that stearin melted from a solid to a cloudly liquid at 52°C,changed at 58°C to an opaque and at 62.5°C to a clear liquid. Others reported observing blue colours when compounds synthesized from cholesterol were cooled. Biologists observed anisotropic optical behaviour in “liquid” biological materials, behaviour usually expected only in the crystal phase.
An important invention of the time was the heating state microscope by Otto Lehmann, a physicist from Karlsruhe, Germany. This microscope allowed control of the temperature of the sample. In a later version, with polarisers added, it became the standard equipment in every liquid crystal research laboratory. In 1888, an Austrian botanist named Friedrich Reinitzer, interested in the biological function of cholesterol in plants, was looking at the melting behaviour of an organic substance related to cholesterol. (The chemical structure of cholesterol was still unknown. Today we know that the observed substance was cholesteryl benzoate. He observed, as W. Heintz did with stearin 38 years before that the substance melted to a cloudy liquid at 145.5°C and became a clear liquid at 178.5°C. He repeated an earlier observation which showed that upon cooling the clear liquid, a brief appearance of blue colour could be seen at the transition temperature, and that a blue violet colour appeared just before crystallization. Discussion with Lehmann and others led to the identification of a new phase of matter called the liquid crystal phase. Research in the area of liquid crystals burgeoned in the years after 1888. Some of the more important results are listed below. The claim that a new phase of matter was found was challenged by important scientists like Walter Nernst and Gustav Tamman. They argued that the phenomenon could be explained as an effect of emulsion of two distinct phases or compound. Further experiments by Lehmann and others supported his theory. Later theoretical work by Emil Bose also gave rise to arguments against the hypothesis of an emulsion. Max Born suggested a theoretical description of liquid crystals as molecules with permanent electric dipoles. This theory unfortunately leads to faulty
results. In 1922 in Paris, France Georges Freidel suggested that the classification scheme which is used today with different phases of liquid crystals called nematic, smectic and cholesteric.
What are Liquid Crystals? Mesophases (liquid crystals and plastic crystals) are a state of order between crystals and liquids. They have imperfect long range orders of orientation and/or position. Thus, the can be fluid like a liquid and they can have anisotropic properties like crystals. Whereas the plastic crystals have a predominating positional order, the liquid crystals have a predominating orientational order. The main reasons for the formation of liquid crystalline phases are:
A simple geometrical form of the molecule: rods,discs or ball, which allow a closer packing in a mesophase ( monophilic liquid crystals).
An intramolecular contrast, which cases micro separation of different parts of the molecules (amphiphilic liquid crystals).
The simplest liquid crystalline phases are the nematic phases, which have no positional order. Sematic phases have layered structures. Unlike regular liquids, liquid crystals must have at least one degree of macroscopic order, even if it is only the angular alignment of a nematic. Liquid crystals lack the full three dimensional position and orientation order of ordinary crystals. In the isotropic phase, the particles are at random positions and orientations. In the nematic phase, the particles have an overall direction of orientation, but the positions of the individual particles are still random. The direction of alignment is called the director. The nematic order is
not perfect and there can be considerable fluctuation about the director. A cholesteric is similar to nematic, but in a certain direction the director twists about in a helix of a given pitch. In smectics, the particles form a set of parallel planes of regular spacing. Within a given plane of a smectic liquid crystal, the order is liquid-like. Several types of smectics have been found. In the columnar phase, the particles have liquid order within a regular two-dimensional lattice of columns; usually the lattice is hexagonal. Although many liquid crystals are composed of only one kind of molecule, they need not be. The main property needed is one dimension much different from another. The particles can be long rod-like objects or flat disks. Liquid crystals are composed of moderate size organic molecules which tend to be elongated and shaped like a cigar, although the literature is full of variety of other, highly exotic shapes as well. Because of their elongated shape, under appropriate conditions the molecules can exhibit orientational order, such that all the axes line up in a particular direction. In consequence, the bulk order has profound influences on the way light and electricity behave in the material. For example if the direction of the orientation varies in space, the orientation of the light (i.e. the polarization) can follow this variation. Under other conditions the molecules may form a stack of layers along one direction, but remain liquid like (in terms of the absence of translational order) within the layers.
Classification Of Liquid Crystals Liquid crystals can be classified into two main categories: thermotropic liquid crystals, and lyotropic liquid crystals. These two types of liquid crystals are distinguished by the mechanisms that drive their self-organization.
THERMOTROPIC LIQUID CRYSTALS Thermotropic transactions occur in most liquid crystals, and they are defined by the fact that the transitions to the liquid crystalline state are induced thermally. That is, one can arrive at the liquid crystalline state by raising the temperature of a solid and/or lowering the temperature of a liquid. Thermotropic liquid crystals can be classified into two types: enantiotropic liquid crystals, which can be changed into liquid crystal state from either lowering the temperature of a liquid or raising of the temperature of a solid, and monotropic liquid crystals, which can only be changed into the liquid or raising of the temperature of a solid or a decrease in the temperature of a liquid, but not both. In general, thermotropic mesophases occur because of anisotropic dispersion forces between the molecules and because of packing interactions. t1
t2
Crystalline
Liquid Crystalline or
Isotropic Liquid
Solid State
Mesomorphic State
State
t4
t5 Enantiotropic Phase t1
Crystalline Solid State
Isotropic Liquid State t4
Liquid Crystal or Mesomorphic State Monotropic State
t3
LYOTROPIC LIQUID CRYSTALS A liquid crystalline material is called lyotropic if phases having long ranged orientational order are induced by the addition of a solvent. Historically the term was used to describe materials composed of amphiphilic molecules. Such molecules comprise a water- loving „hydrophilic‟ head group attached to a water-hating „hydrophobic‟ group. Typical hydrophobic groups are saturated or unsaturated hydrocarbon chains. Examples of amphiphilic compounds are the salts of fatty acids, phospholipids. Many simple amphiphiles are used as detergents. They can be classified as: I. II.
Micellar cubic Hexagonal phase
III.
Lamellar phase
IV.
Bicontinuous phase
Types of Liquid Crystalline Phases SMECTIC PHASES Smectic liquid crystals are turbid and viscous substances. Their molecules are arranged into equidistant two-dimensional sheets. The long axis of the molecules is perpendicular to or tilted by a certain angle to the face of the layers. The lateral spacing between the molecules within a layer may be regular or random. Hydrogen bonding between the polar groups links the molecules laterally and cooperatively to form head-to-head and tai-to-tail bilayers. This class is further separated into several subclasses, due to subtle differences.
In the smectic phase, the molecules maintain the general orientational order and also tend to align themselves in layers or planes. Motion is restricted to within these planes and separate planes are observed to flow past each other. Many compounds are observed to show more than one type of smectic phases, as many as 12 of these variations have been identified. According to the chronological order, phases have been designated with code letters A,B,C.....,M Smectic A phase: In the smectic phase A mesophase, the director is perpendicular to the
smectic plane, and there is no particular positional order in the layer. Smectic C phase: The smectic C phases are closely related to smectic A phases. The most
important difference is the tilt of the molecular long axes with respect to the normal. Here, the molecular ability of the molecules is similar to that of the smectic A phase. However, the rotation around the long axes is slightly more hindered than in smectic A phase.
NEMATIC PHASES The simplest liquid crystal phase is called the nematic phase (N). It is characterized by a high degree of long range orientational order but no translational order. Molecules in a nematic phase spontaneously order with their long axes roughly parallel. A schematic diagram of a nematic phase is shown in the figure. A uniformly aligned nematic has a preferred direction, often described in terms of a unit vector called the director. More generally a bulk nematic will contain domains. The orientation of the director is constant in each domain but is different in different domains. Viewed under a polarizing microscope the defect regions linking these domains appear as dark threads.
CHOLESTERIC PHASE
A cholesteric liquid crystal is a type of a liquid crystal with a helical structure and which is therefore chiral. Cholesteric liquid crystals are also known as chiral nematic liquid crystals. They organise in layers with no positional ordering within layers, but a director axis which varies with layers. The variation of the director axis tends to be periodic in nature. The period of this variation is known as the pitch, p. The pitch is varying with temperature and it can also be affected by the boundary conditions when the chiral nematic liquid crystals are sandwiched between two substrate planes. Examples of compounds known to form cholesteric phases are cholesteryl benzoate.
DISCOTIC PHASE The columnar phase is a class of liquid-crystalline planes in which molecules assemble into cylindrical structures to act as mesogens. Originally, these kinds of liquid crystals were called discotic liquid crystals because the columnar structures are composed of flat-shaped discotic molecules stacked one-dimensionally. The first discotic liquid crystal was found in 1977 by the Indian researcher Sivaramakrishna Chandrasekhar. This molecule has one central benzene ring surrounded by six alkyl chains. Since then, a large number of discoid mesogenic compounds have been discovered in which triphenylene,porphyrin, phthalocyanine, and other aromatic molecules are involved. The typical columnar liquid-crystalline molecules have a pi-electronrich aromatic core attached by flexible alkyl chains. This structure is attracting particular attention for potential molecular electronics in which aromatic parts transport electrons or holes and alkyl chains act as insulating parts.
Structural Features Of The Liquid Crystals
Molecular Geometry is very important factor which influences to a great extent the liquid crystalline properties. The thermotropic mesogens vary in chemical constituitions, however, there are certain common features based on molecular geometry of the anisotropy. Generally molecules of mesogens are elongated and rod-like in shape. They are loosely packed in a regular manner giving rise to a parallel arrangement in crystalline state along their major axis. In such long molecules presence of polar terminal, middle or lateral groups and polarisable units in p.p.‟positions along with benzene rings play important role in endowing the mesogens with liquid crystalline properties. The figure below shows the molecular structure of a typical rod-like liquid crystal molecule. It consists of two or more ring systems connected by a central linkage group.
Terminal group---
---linkage group---
Typical shape of a liquid crystal molecule
--Side chain
