Introduction and historical overview What are Liquid Crystals? · Introduction and historical overview Research on liquid crystal has been involved in chemistry, physics, Biology,

Introduction Chapter-1

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Introduction and historical overview

Research on liquid crystal has been involved in chemistry, physics, Biology,

electric and electronic engineering and many other fields. Most of this research has been

reported by the universities and research institutions. The study of liquid crystals began in

1888 by Australian Botanist F. Reinitzer [1]. Liquid crystal materials are unique in their

properties and uses. As research into this field continues and as new applications are

developed, liquid crystals will play an important role in modern technology.

What are Liquid Crystals?

� The term ‘Liquid Crystals’ seems to be a self-contradiction as it suggest that a

substance is in two quite different state of matter at the same time.

� The two most common states of condensed matter are the isotropic liquid phase

and the crystalline solid phase.

� In a crystal, the molecules or atoms have both orientational and three-dimensional

positional order over a long range.

� In an isotropic liquid, however, the molecules have neither positional nor

orientational order, they are distributed randomly. There is no degree of order, so

three degrees of freedom are left. There is no preferred direction in a liquid, thus

the name isotropic.

� The transition from one state to another normally occurs at a very precise

temperature. � When pure crystalline solid is heated beyond its melting temperature, it undergoes

a single transition to isotropic liquid. e.g. ice-water is such a common phase

transition. � There are, however many organic compound that do not immediately transform to

liquid phase when heated beyond the melting temperature but exhibit more than a

single transition from solid to liquid showing the existence of one or more

intermediate phases, exhibiting the properties of both solids and liquids. � For examples p-azoxy anisole when heated does not transform into the liquid state

but adopts structure (turbid condition) that is both birefringence and fluid the

consistency varying with different compounds that of a paste to that of a freely

flowing liquid.


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� Transitions are definite and precisely reversible. � Materials undergoing such a phase transitions are called ‘Liquid Crystals’ [2].

History of liquid crystals

The discovery of liquid crystals is thought to have occurred nearly 150 years ago

although its significance was not fully realized until over a hundred years later. Around

the middle of the last century Virchow[3], Mettenheimer et al.[4] have found that the

nerve fiber they were studying formed a fluid substance when left in water which

exhibited a strange behaviour when viewed using polarized light. They did not realize

this was a different phase but they are attributed with the first observation of liquid

crystals. Later, in 1877, Further investigations of this phenomenon were carried out by

the German physicist O. Lehmann [5] who observed and confirmed, using the first

polarized optical microscope designed by himself, the existence of "crystals [which] can

exist with a softness that one could call them nearly liquid". He found that one substance

would change from a clear liquid to a cloudy liquid before crystallising but thought that

this was simply an imperfect phase transition from liquid to crystalline. The first reported


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documentation of the LC state was through an accidental observation by an Austrian

botanist, Friedrich Reinitzer [1] in 1888, working in the Institute of Plant Physiology at

the University of Prague. He observed “double melting" behaviour of cholesteryl

benzoate. The crystals of this material melted at 145.5 oC into a cloudy fluid, which upon

further heating to 178.5oC became clear. This discovery represented the first recorded

documentation of the LC phase. He was the first to suggest that this cloudy fluid was a

new phase of matter. He has consequently been given the credit for the discovery of the

liquid crystalline phase. Puzzled by his discovery, Reinitzer turned for help to the

German physicist Otto Lehmann, who was an expert in crystal optics. Lehmann became

convinced that the cloudy liquid had a unique kind of order. In contrast, the transparent

liquid at higher temperature had the characteristic disordered state of all common liquids.

Eventually he realized that the cloudy liquid was a new state of matter and coined the

name "liquid crystal," illustrating that it was something between a liquid and a solid,

sharing important properties of both. In a normal liquid the properties are isotropic, i.e.

the same in all directions. In a liquid crystal they are not; they strongly depend on

direction even if the substance itself is fluid. That new types of liquid crystalline states of

order were discovered. Up till 1890 all the liquid crystalline substances that had been

investigated naturally occurring and it was then that the first synthetic liquid crystal, p-

azoxyanisole, was produced by Gatterman and Ritschke. Subsequently more liquid

crystals were synthesized and it is now possible to produce liquid crystals with specific

predetermined material properties.

Maier and Saupe [6] formulated a microscopic theory of liquid crystals, Frank and

later Leslie and Ericksen developed continuum theories for static and dynamic systems

and in 1968 scientists from RCA first demonstrated a liquid crystal display [7]. The

interest in liquid crystals has grown ever since, partly due to the great variety of

phenomena exhibited by liquid crystals and partly because of the enormous commercial

interest and importance of liquid crystal displays.

Today, thanks to Reinitzer, Lehmann and their followers, we know that literally

thousands of substances have a diversity of other states. Some of them have been found

very usable in several technical innovations, among which liquid crystal screens and

liquid crystal thermometers may be the best known. In the 1960s, a French theoretical


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physicist, Pierre-Gilles de Gennes, who had been working with magnetism and

superconductivity, turned his interest to liquid crystals and soon found fascinating

analogies between liquid crystals and superconductors as well as magnetic materials. His

work was rewarded with the Nobel Prize in Physics 1991. The modern development of

liquid crystal science has since been deeply influenced by the work of Pierre-Gilles de

Gennes [8].

This new idea was challenged by the scientific community, and some scientists

claimed that the newly-discovered state probably was just a mixture of solid and liquid

components. But between 1910 and 1930 conclusive experiments and early theories

supported the liquid crystal concept at the same time. In 1922 the French scientist G.

Friedel produced the first classification scheme of LCs [9], dividing them into three

different types of mesogens (materials able to sustain mesophases), based upon the level

of order the molecules possessed in the bulk material:

1.nematic (from the Greek word nematos meaning "thread"),

2.Smectic (from the Greek word smectos meaning "soap"), and

3.Cholesteric (better defined as Chiral nematic)[10].

Following these first observations and discoveries, the scientific research turned

attention towards a growing number of compounds, which displayed liquid crystalline

properties. In order to establish a relationship between the molecular structure and the

exhibition of liquid crystalline properties, a series of systematic modifications of the

structures of mesogens was undertaken, leading, in 1973 [11], to the discovery of the

most technologically and commercially important class of LCs to date: the 4-alkyl-4'-

cyanobiphenyl (CB) of which an example, 4-pentyl-4'-cyanobiphenyl (5CB) 1 is

illustrated in Figure 1.

Figure 1

Figure 1. Molecular structure of 4-pentyl-4'-cyanobiphenyl (5CB) 1. (The transition

temperatures are expressed in oC).


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These are the materials, which still constitute the simple common displays found

in calculators or mobile phones. However, the numerous and increasingly sophisticated

applications, relying upon the use of liquid crystalline materials, require such a

complexity of superior properties to achieve improved devices performance, that the

quest for ever new LCs has grown enormously over the last three decades. Nowadays,

LCs play a dominant role in a large part of the display technology.

Liquid crystal is solid or liquid ?

It is sometimes difficult to determine whether a material is in a crystal or liquid

crystal state. The amount of energy required to cause the phase transition is called latent

heat of the transition and is useful to measure of how different the two phases are. In the

case of cholesteryl myristate, the latent heat of solid to liquid crystal is 65 calories/gram,

while the latent heat for liquid crystal to liquid transition is 7 calories/gram. These

numbers allow us to answer the question posed earlier. The smallness the latent heat of

liquid crystal to liquid phase transition is evidence that liquid crystal are more similar to

liquids than they are to solids. when a solid melts to a liquid crystal, it loses most of the

order it had and retains only a bit more order than a liquid possesses. This small amount

of order is then lost at the liquid crystal to liquid phase transition. The fact that liquid

crystals are similar to liquids with only a small amount of additional order, is the key to

understanding many physical properties that make them nature’s most delicate state of

matter [12].

Order Parameter

To quantify just how much order is present in a material, an order parameter (S) is defined. Traditionally, the order parameter is given as follows:

where theta is the angle between the director and the long axis of each molecule. The

brackets denote an average over all of the molecules in the sample. In an isotropic liquid,


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the average of the cosine terms is zero, and therefore the order parameter is equal to zero.

For a perfect crystal, the order parameter evaluates to one. Typical values for the order

parameter of a liquid crystal range between 0.3 and 0.9, with the exact value a function of

temperature, as a result of kinetic molecular motion. This is illustrated below for a

nematic liquid crystal material.

The tendency of the liquid crystal molecules to point along the director leads to a

condition known as anisotropy. This term means that the properties of a material depend

on the direction in which they are measured. For example, it is easier to cut a piece of

wood along the grain than against it. The anisotropic nature of liquid crystals is

responsible for the unique optical properties exploited by scientists and engineers in a

variety of applications.

Types of LCs

Smectic

(Two dimensional order)

Nemetic

(One dimensional order)

Cholesteric

(Chosterol-derivatives)

(Helical structure)

Thermotropic Liquid crystals

(Non-amphiphilic)

Lyotrophic Liquid crystals

(Amphiphilic)

Liquid Crystals

Ordered fluid mesophase

(Solid-like liquids)

Plastic Crystals

Disordered Crystal mesophase

(Liquid-like solid)

Mesomorphic State


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Different types of molecules can form liquid crystalline phases. The common

structural feature is that these molecules are form anisotropic: one molecular axis is much

longer or wider than another one. The two major categories are:

1.Thermotropic LCs, whose mesophase formation is temperature (T) dependent, and

2. Lyotropic LCs, whose mesophase formation is concentration and solvent dependent.

Lyotropic LCs

Lyotropic LCs are two-component systems where an amphiphile is dissolved in a

solvent. In blends of different components phase transitions may also depend on

concentration and these liquid crystals are called lyotropic. Thus, lyotropic mesophases

are concentration and solvent dependent. The amphiphilic compounds are characterised

by two distinct moieties, a hydrophilic polar "head" and a hydrophobic "tail". Examples

of these kinds of molecules are soaps (Figure 2 a) and various phospholipids like those

present in cell membranes [13-15] (Figure 2 b).

Figure 2. Chemical structure and cartoon representation of (a) sodium dodecylsulfate

(soap) forming micelles, and (b) a phospholipids (lecitine), present in cell membranes, in

a bilayer lyotropic liquid crystal arrangement. Today, Lytropic liquid crystalline materials

have been widely used as display devices [16] and lytropics are also important for

biological systems, e.g. membranes [17-19].


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Thermotropic LCs

Fig. 3: liquid crystalline mesophases between the solid and isotropic liquid phase

Thermotropic transition 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.

Condensed matter which exhibit intermediate thermodynamic phases between the

crystalline solid and simple liquid state are now called liquid crystals or mesophases

(Fig.3). This fourth state of matter generally possess orientational or weak positional

order and thus reveals several physical properties of crystals but flow like liquids. If

transitions between the phases are given by temperature, they are called thermotropic

[20-21] While thermotropics are presently mostly used for technical applications [22].

The essential requirement for a molecule to be a thermotropic LC is a structure consisting

of a central rigid core (often aromatic) and a flexible peripheral moiety (generally

aliphatic groups). This structural requirement leads to two general classes of LCs:

1. Calamitic LCs, and

2. Discotic LCs

both of which have other molecular subclasses. Thermotropic liquid crystals can be

classified into two types:


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Enantiotropic liquid crystals: which can be changed into the liquid crystal state

from either lowering the temperature of a liquid or raising of the temperature of a solid or

mesomorphic transitions occur on heating the substance and these transition reveres in

the opposite direction on cooling. Such a mesophase is called the enantiotropic

mesophase.

Monotropic liquid crystals: which can only be changed into the liquid crystal state

from either an increase in the temperature of a solid or a decrease in the temperature of a

liquid, but not both or there are many compounds, which on heating do not exhibit

mesophase and directly pass into an isotropic liquid but on cooling, they exhibit a

mesophase is termed as monotropic mesophase.

This monotropic temperature is also reversible. In general, thermotropic mesophases

occur because of anisotropic dispersion forces between the molecules and because of

packing interactions. Although the term thermotropic and lyotropic are widely used, Gray

and Winsor [23] prefer the terms amphiphillic (for lyotropics) and non-amphiphillic (for

thermotropics ).

Polymorphism:

Many Liquid crystalline substances which have exclusively smectic mesophase

(structure) or exclusively nematic mesophase (structure). But some can exist as both

types of mesophase, smectic followed by nematic and they have definite transition

temperature defining the stability of the different phase, which are always reproducible.

There are substances possessing more than one smectic phase having sharp temperature

range of stability of different phases. This phenomenon is known as polymorphism.


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Fig.4: Schematical phase sequence of a liquid crystal. From left to right: smectic C

phase (tilt angle between layer normal and mean orientation of the molecules),

smectic A phase (layered structure, no tilt), nematic phase, isotropic phase.

Above the clearing temperature (Tc) the liquid crystal becomes an isotropic liquid. These

properties make liquid crystals an interesting object for the application of thermody-

namical methods.

Calamitic LCs

Calamitic or rod-like LCs are those mesomorphic compounds that possess an

elongated shape, responsible for the form anisotropy of the molecular structure, as the

result of the molecular length (l) being significantly greater than the molecular breadth

(b), as depicted in the cartoon representation in Figure 5.

Figure 5: Cartoon representation of calamitic LCs, where length(l) >> breadth(b).

Calamitic mesogens usually follow the general structural formula shown in Figure.6


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Figure.6 General structure of calamitic LCs.

R' and R" are often flexible terminal units such that at least one R group is an alkyl chain,

A, B, C, and D are used to generally describe ring systems (phenyl, cyclohexyl,

heteroaromatics, and heterocycles) and [L] represents the linking units, such as CH=N,

COO or N=N that can increase the length and flexibility of the molecule, whilst

preserving a compatible linear shape suitable for mesophase formation.

Calamitic LCs can exhibit two common types of mesophases:

1 Nematic, and

2. Smectic.

Nematic phase [24-33]

Figure 7. Cartoon representation of N phase

The word nematic is derived from the greek word “Nema” meaning thread like.

Under the polarsing microscope, the nematic phase is seen as thread schlieren texture.


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This is the most liquid like structure in which, contrary to isotropic liquids, one or two

molecular axes are oriented parallel to one another resulting in an orientational long-

range order and short positional order. Molecules can rotate by both the axes, the

molecules have several possibility of intermolecular mobility. Because of the high

mobility, the nematic phases have low viscosities. They are anisotropic with respect to

optical properties, viscosity, electrical and magnetic susceptibility, electrical and thermal

conductivity. The nematic substance separate as spherical drops form the melt or

solution, which coalesce to give the threaded structure.

The least ordered mesophase (the closest to the isotropic liquid state) is the

nematic (N) phase, where the molecules have only an orientational order. The molecular

long axis points on average in one favoured direction referred to as the director (Figure

7). The classical example of LC displaying a nematic mesophase is the 5CB 1 (Figure 1).

The molecules are oriented, on average, in the same direction referred to as the director,

with no positional ordering with respect to each other. The molecules in the nematic

phase are oriented on average along a particular direction. In consequence, there is a

macroscopic anisotropy in many material properties, such as dielectric constants and

refractive indices. This is the phase which is used in many liquid crystal devices (e.g., the

"twisted nematic" cell), because the average orientation may be manipulated with an

electric field and the polarization of light will follow the molecular orientation as it

changes through a cell. Typical response times are in the millisecond range.

Figure 8: (a) Schlieren texture of a nematic film with surface point defects (boojums). (b)

Thin nematic film on isotropic surface: 1-dimensional periodicity. Photos courtesy of

Oleg Lavrentovich http://www.lci.kent.edu/ALCOM/oleg.html. (c) Nematic thread-like

texture. After these textures the nematic phase was named, as “nematic” Photo courtesy

of Ingo Dierking.


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On optical examination of a nematic, one rarely sees the idealized equilibrium

configuration. Some very prominent structural perturbation appear as threads from which

nematics take their name. These threads are analogous to dislocations in solids and have

been termed disclinations by Frank.

Several typical textures of nematics are shown in Fig. (8). The first one is a

schlieren texture of a nematic film. This picture was taken under a polarization

microscope with polarizer and analyzer crossed. From every point defect emerge four

dark brushes. For these directions the director is parallel either to the polarizer or to the

analyzer. The colors are newton colors of thin films and depend on the thickness of the

sample. Point defects can only exist in pairs. One can see two types of boojums with

“opposite sign of topological charge”; one type with yellow and red brushes, the other

kind not that colorful. The difference in appearance is due to different core structures for

these defects of different “charge”.

The second texture is a thin film on isotropic surface. Here the periodic stripe

structure is a spectacular consequence of the confined nature of the film. It is a result of

the competition between elastic inner forces and surface anchoring forces. The surface

anchoring forces want to align the liquid crystals parallel to the bottom surface and

perpendicular to the top surface of the film. The elastic forces work against the resulting

“vertical” distortions of the director field. When the film is sufficiently thin, the lowest

energy state is surprisingly archived by “horizontal” director deformations in the plane of

the film. The current picture shows a 1-dimensional periodic pattern.

Many compounds are known to form nematic mesophase. A few typical examples

are sketched in Fig. (9). From a steric point of view, molecules are rigid rods with the

breadth to width ratio from 3:1 to 20:1.


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Figure 9: Typical compounds forming nematic mesophases: (PAA) p-azoxyanisole. From

a rough steric point of view, this is a rigid rod of length 20°A and width 5°A. The

nematic state is found at high temperatures (between 1160C and 1350C at atmospheric

pressure). (MMBA) N-(p-methoxybenzylidene)-p-butylaniline. The nematic state is

found at room temperatures (between 200C to 470C). Lacks chemical stability. (5CB) 4-

pentyl-4’-cyanobiphenyl. The nematic state is found at room temperatures (between 24°C

and 35°C).

Biaxial nematic

A biaxial nematic is a spatially homogeneous liquid crystal with three distinct

optical axes. This is to be contrasted to a simple nematic, which has a single preferred

axis, around which the system is rotationally symmetric. The symmetry group of a biaxial

nematic is D2h i.e. that of a rectangular right parallelepiped, having 3 orthogonal C2 axes

and three orthogonal mirror planes. In a frame co-aligned with optical axes the second

rank order parameter tensor of a biaxial nematic has the form

Where S is the standard nematic scalar order parameter T a measure of the biaxiality.

The first report of a biaxial nematic appeared in 2004 [34, 35] based on a boomerang

shaped oxadiazole bent-core mesogen. The biaxial nematic phase for this particular

compound only occurs at temperatures around 200°C and is preceded by as yet

unidentified smectic phases.

It is also found that this material can segregate into chiral domains of opposite

handedness [36] for this to happen the boomerang shaped molecules adopt a helical

superstructure.


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In one azo bent-core mesogen as shown below in which a thermal transition is

found from a uniaxial Nu to a biaxial nematic Nb mesophase [37]. This transition is

observed on heating from the Nu phase with Polarizing optical microscopy as a change in

Schlieren texture and increased light transmittance and from x-ray diffraction as the

splitting of the nematic reflection. The transition is a second order with low energy

content and therefore not observed in differential scanning calorimetry. The positional

order parameter for the uniaxial nematic phase is 0.75 to 1.5 times the mesogen length

and for the biaxial nematic phase 2 to 3.3 times the mesogen length.

Another strategy towards biaxial nematic is the use of mixtures of classical rod

like mesogens and disk like discotic mesogens. The biaxial nematic phase is expected to

be located below the minimum in the rod-disk phase diagram. In one study [38] a

miscible system of rods and disks is actually found although the biaxial nematic phase

remains elusive.

Smectic phases[39-43]

The word "Smectic" is derived from the Greek word for soap. This seemingly

ambiguous origin is explained by the fact that the thick, slippery substance often found at

the bottom of a soap dish is actually a type of smectic liquid crystal. Molecules in this

phase show a degree of translational order not present in the nematic. Smectic phase

(Liquid Crystal) retain a two dimensional order. In the smectic phase the layer of the

molecules are quite flexible.

Smectic phase gives focal conic texture. It extends all over the specimen and

when examined under polarised light it gives a fan-like appearance. It is unaffected by

magnetic and electric fields. A number of different type of smectic liquid crystals are

known which differ from each other in the way of layer formation. The increased order

means that the smectic state is more "solid-like" than the nematic. Smectic – A, B, C, D,

E, F, G, H, I. A number of different classes of smectics have been recognized.


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Figure 10. Cartoon representation of (a) the SmA phase, and (b) the SmC phase.

In Smectic A: It has a layer structure inside the layers, the molecules are parallel their

long axes perpendicular to the plane. These are optically uniaxial and hence homeotropic

texture extinguishes light between crossed polarizes. It gives focal conic texture (or

batonnets).

Smectic-C: (Titled)

Smectic –C phase is closely related to Smectic-A phase. Smectic-C is a tilted (as

shown above) from Smectic-A. The major difference between the two is the tilt (inclined)

of the molecular long axes with respect to the layers. This phase is optically biaxial.

(Monoclinic symmetry) therefore, it is impossible to have homeotropic texture. It exhibit

schlieren texture. It can also form focal conic texture. Broken fan shaped texture. In this

phase the molecules are tilted with respect to the layers, and the system is now "biaxial"

in character

An example of a molecular structure displaying a smectic mesophase is given by

the quaterphenyl derivative [44] illustrated in Figure.11, where the presence of such an

extended aromatic core, characterised by a large phenyl (ph) system, is responsible for

the establishment of lateral stacking interactions between adjacent molecules, resulting in

a layered organisation (SmA and SmC).


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Cr1 Cr2 Cr3 Cr4 SmC SmA I123 166 180 293 324 327

Figure.11 4,4"'-Bis-nonyloxy-[1,1';4',1";4",1''']quaterphenyl 2 exhibiting SmA and SmC

phases. (The transition temperatures are expressed in oC).

In general a smectic, when placed between glass slides, does not assume the

simple form. The layers, preserving their thickness, become distorted and can slide over

one another in order to adjust to the surface conditions. The optical properties (focal

conic texture) of the smectic state arise from these distortions of the layers. Typical

textures formed by smectics are shown in Fig. (12) [45].

(a) (b) (c)

Figure 12: (a,b) Focal-conic fan texture of a smectic A liquid crystal (courtesy of Chandrasekhar S., Krishna Prasad and Gita Nair) (c) Focal-conic fan texture of a chiral smectic C liquid crystal. Smectic C* (Chiral) - Ferroelectric

The nematic and Smectic-A (SmA) liquid crystal phases are too symmetric to

allow any vector order, such as ferroelectricity. The tilted smectics, however, do allow

ferroelectricity if they are composed of chiral molecules. The pictures below show the

original ferroelectric LC[46-53], DOBAMBC.

In the simplest case, the Smectic-C (SmC), the average long molecular axis is

tilted from the layer normal z by a fixed angle but the molecules are free to rotate on the

so-defined tilt cone. The phase has a C2 symmetry axis perpendicular to both the


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molecular director and the layer normal. The molecules exhibit a net spontaneous

polarization along this axis. The magnitude of the polarization depends on temperature,

generally decreasing as the tilt angle goes to zero at the SmC-SmA phase transition. The

following figure shows the geometry of the chiral SmC phase.

Figure 13: Chiral SmC phase:

Ferroelectric liquid crystals (FLCs) also exhibit a sponteneous helixing of the

polarization, so that over macroscopic distances (a few microns, say) the polarization

averages to zero.

Since the coupling of the polarization to applied fields is linear in the field, this

means that FLCs can be made to switch quickly (typically within a few microseconds)

and in a bipolar manner. This makes FLCs ideally suited to electrooptic applications.

FLCs are now included in several display technologies [52, 54-57], the most popular of

which use the surface stabilized (SSFLC) geometry.

Surface-Stabilized Ferroelectric Liquid Crystals

Although the molecular director in bulk ferroelectric

liquid crystals (FLCs) adopts a helical structure, Noel

Clark and Sven Lagerwall found in 1980 that by

confining the LC material between closely-spaced

glass plates (spaced closer than the ferroelectric helix

pitch), the natural helix could be suppressed. This

principle is illustrated in the polarized micrograph above, where helix lines are largely

absent in the thinner (upper right) part of the cell. Clark and Lagerwall found that the

smectic layers were oriented approximately perpendicular to the glass. Furthermore, they

discovered that such cells could be switched rapidly between two optically distinct, stable

states simply by alternating the sign of an applied electric field. The electro-optic


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properties of an SSFLC depend strongly on the layer geometry as well as on the nature of

the orienting properties of the bounding glass plates [53,54]. SSFLCs are being studied in

many research laboratories throughout the world. They form the basis for the

development of optical shutters, phase plates, and of high-resolution color displays.

Antiferroelectric LCs

Antiferroelectric liquid crystals are similar to ferroelectric

liquid crystals, although the molecules tilt in an opposite

sense in alternating layers as show in figure. In consequence,

the layer-by-layer polarization points in opposite directions.

These materials are just beginning to find their way into

devices, as they are fast, and devices can be made

"bistable"[58-64].

Cholesteric Phases (Chiral nematic)

The cholesteric (or chiral nematic) liquid crystal phase is typically composed of

nematic mesogenic molecules containing a chiral center which produces intermolecular

forces that favour alignment between molecules at a slight angle to one another[65-72].

This leads to the formation of a structure which can be visualized as a stack of very thin

2-D nematic-like layers with the director in each layer twisted with respect to those above

and below. In this structure, the directors actually form in a continuous helical pattern

about the layer normal as illustrated by the black arrow in the following figure and

animation. The black arrow in the animation represents director orientation in the

succession of layers along the stack.


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Nematic Chiral Nematic

Fig.14 The molecules shown are merely representations of the many chiral nematic mesogens lying in the slabs of infinitesimal thickness with a distribution of orientation around the director. The phase was first observed in cholesterol derivatives, hence it is known as cholesteric phase.

Various colour changes can be observed by winding or unwinding the helix. This

can be done by means of changing temperature, mechanical disturbance like pressure or

shear. Liquid Crystals of this type is mostly optically active. The cholesteric liquid

crystals are optically uniaxial with negative character, it can scatter the light to give

bright colour and it shows strong rotalory power. Three type of texture are generally

observed in cholesteric phases. 1. Focal conic texture 2. Planar texture and

3. Blue phase (N*-Phase).

Pitch:

An important characteristic of the cholesteric mesophase is the pitch. The pitch, p,

is defined as the distance it takes for the director to rotate one full turn in the helix as

illustrated in the above animation. A byproduct of the helical structure of the chiral

nematic phase, is its ability to selectively reflect light of wavelengths equal to the pitch

length, so that a color will be reflected when the pitch is equal to the corresponding

wavelength of light in the visible spectrum. The effect is based on the temperature

dependence of the gradual change in director orientation between successive layers

(illustrated above), which modifies the pitch length resulting in an alteration of the

wavelength of reflected light according to the temperature. The angle at which the


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director changes can be made larger and thus tighten the pitch, by increasing the

temperature of the molecules, hence giving them more thermal energy. Similarly,

decreasing the temperature of the molecules increases the pitch length of the chiral

nematic liquid crystal.

This makes it possible to build a liquid crystal thermometer that displays the

temperature of its environment by the reflected color. Mixtures of various types of these

liquid crystals are often used to create sensors with a wide variety of responses to

temperature change. Such sensors are used for thermometers often in the form of heat

sensitive films to detect flaws in circuit board connections, fluid flow patterns, condition

of batteries, the presence of radiation or in novelties such as "mood" rings.

Figure 15: (a) Cholesteric fingerprint texture. The line pattern is due to the helical

structure of the cholesteric phase, with the helical axis in the plane of the substrate. Photo

courtesy of Ingo Dierking. (b) A short-pitch cholesteric liquid crystal in Grandjean or

standing helix texture, viewed between crossed polarizers. The bright colors are due to

the difference in rotatory power arising from domains with different cholesteric pitch

occuring on rapid cooling close to the smectic A* phase where the pitch strongly diverges

with decreasing temperature. Photo courtesy of Per Rudqvist. (c) Long-range orientation


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of cholesteric liquid crystalline DNA mesophases occurs at magnetic field strengths

exceeding 2 Tesla. The image presented above illustrates this long-range order in DNA

solutions approaching 300 milligrams per milliliter. Parallel lines denoting the periodicity

of the cholesteric mesophase appear at approximately 45-degrees from the axis of the

image boundaries.

Discotic LCs

In 1977, a second type of mesogenic structure, based on discotic (disc-shaped)

molecular structures was discovered. The first series of discotic compounds to exhibit

mesophase belonged to the hexa-substituted benzene derivatives 1 (Figure 16)

synthesised by S. Chandrasekhar et al. [73-76]

Figure 16. Molecular structure of the first series of discotic LCs discovered: the benzene-

hexa-n-alkanoate derivatives.

Similarly to the calamitic LCs, discotic LCs possess a general structure

comprising a planar (usually aromatic) central rigid core surrounded by a flexible

periphery, represented mostly by pendant chains (usually four, six, or eight), as illustrated

in the cartoon representation in Figure 17. As can be seen, the molecular diameter (d) is

much greater than the disc thickness (t), imparting the form anisotropy to the molecular

structure[77-85].

Figure 17. Cartoon representation of the general shape of discotic LCs, where d>>t.


23

Discotic LCs, as well as calamitic LCs, can show several types of mesophases, with

varying degree of organisation. The two principle mesophases are:

1. Nematic discotic and

2. Columnar.

Nematic discotic phase

Nematic discotic (ND) is the least ordered mesophase [77], where the molecules

have only orientational order being aligned on average with the director as illustrated in

figure 18. There is no positional order.

Figure 18. Cartoon representation of the ND phase, where the molecules are aligned in

the same orientation, with no additional positional ordering.

Columnar phases

Disk-shaped mesogens can orient themselves in a layer-like fashion known as the

discotic nematic phase. If the disks pack into stacks, the phase is called a discotic

columnar. The columns themselves may be organized into rectangular or hexagonal

arrays [86], see Fig. (21).


24

Chiral discotic phases, similar to the chiral nematic phase, are also known . The columnar

phase is a class of liquid-crystalline phases in which molecules assemble into cylindrical

structures to act as mesogens.

Figure 19: (a) hexagonal columnar phase Colh (with typical spherulitic texture); (b) Rectangular phase of a discotic liquid crystal (c) hexagonal columnar liquid-crystalline phase.

Figure 20: Typical discotics: derivative of a hexabenzocoronene and 2,3,6,7,10,11- hexakishexyloxytriphenylene. K(70K) Colh(100K) I.

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. Since recent findings provide a number of columnar liquid crystals

consisting of non-discoid mesogens, it is more common now to classify this state of

matter and compounds with these properties as columnar liquid crystals.

Figure 21: (1) Columnar phase formed by the disc-shaped molecules and the most common arrangements of columns in two-dimensional lattices: (a) hexagonal, (b)


25

rectangular, and (c) herringbone. (2,3) MD simulation results: snapshot of the hexabenzocoronene system with the C12 side chains. Aromatic cores are highlighted. Both top and side views are shown. T = 400 K, P = 0.1MPa. [78]

Columnar liquid crystals are grouped by their structural order and the ways of

packing of the columns. Nematic columnar liquid crystals have no long-range order and

are less organized than other columnar liquid crystals. Other columnar phases with long-

range order are classified by their two-dimensional lattices: hexagonal, tetragonal,

rectangular, and oblique phases. The discotic nematic phase includes nematic liquid

crystals composed of flat-shaped discotic molecules without long-range order. In this

phase, molecules do not form specific columnar assemblies but only float with their short

axes in parallel to the director (a unit vector which defines the liquid-crystalline

alignment and order).

In the years following the discovery of the first discotic mesogens, further

investigations lead to the synthesis of a vast number of new discotic LCs [87-98]

Figure 22. Molecular structure of some discotic mesogens: 2,3,6,7,10,11-

hexakishexyloxytriphenylene 3 [87-93], 3,10-dipentylperylene discogen derivative 4

[94], 2,3,7,8,12,13-hexakispentyloxy-10,15-dihydro- 5H-tribenzo [a,d,g] cyclononene


26

(bowl-shaped discotic) 5 [95-97], porphyrin metallomesogen 6 [98]. (The transition

temperatures are expressed in oC, and the mesophase in brackets represents a monotropic

transition).

Polycatenar LC's

Polycatenar mesogens [99-102] represent a hybrid class of thermotropic LCs,

which can be described with intermediate molecular features between classical rod-like

and disc-like mesogens. Schematically the central core of polycatenar LCs comprises a

calamitic region, with half-discs on the extremities (Figure 23). This hybrid molecular

structure allows both calamitic and columnar phases to be generated, depending on the

specific molecular structure of the components.

Figure 23. Cartoon representation of the architectural molecular structure of polycatenar

LCs.

Polycatenar molecules possess a number of flexible alkyl chain substituents,

which varies from two to six (bi- to hexa-catenar compounds). Bi- catenar LCs are in

most of the cases classical rod- like molecules, like compound 7 (Figure 24). Examples

of bi-, tri-, tetra- and hexa-catenar LCs are shown in Figures 24-27 [103-106].

Figure 24. Molecular structure of two bi-catenar mesogens 4-pentyl-4'-pentyl biphenyl 7

and 4'-[(3'', 4’’-bis-hexyloxy-benzylidene)-amino]-4-carbonitrile 8.


27

Figure 25. Molecular structure of a tri-catenar mesogen.

Figure 26. Molecular structure of tetra-catenar mesogens 2, 2’-bipyridine derivative 10

and liquid crystalline 3, 4-dioctyloxystilbazole silver complex 11.

Figure 27. Molecular structure of a hexa-catenar mesogen 12.

Compound 8 and 9 show close similarity to the class of LCs named swallow-tailed LCs

and compounds 10 and 11 show similarity to the bi-swallow-tailed LCs

Twist-Grain boundary phase (TGB)

The TGB phase was Proposed by Renn and Lubensky [107] and discovered by

Goodby et al. [108,109]. These are smectic phase where arrays of defects from part of the

ordered structure. The nature of the SmC and SmC* TGB structures and their relationship

to larger scale superstructures are still open issues[110,111].


28

Banana-shaped LCs

Banana-shaped LCs are similar to calamitic LCs [112-119], but contain a

molecular kink.. They have an elongated shape, with the molecular length being

significantly larger than the molecular breadth. These LCs as well as generating the

mesophases associated with calamitic LCs, generate a set of their own mesophases. These

mesophases have been given the nomenclature of B1-B7 depending on the order of

discovery. They are closely related to the smectic phases, for instance in the B2

mesophase the molecules are tilted as in the SmC mesophase but also resemble to the

SmA phase.

In smectic liquid crystals Banana-shaped molecules on smectic layers (smectic C ) have a

spontaneous polarization, P.

Liquid crystal 'blue phases' –recent advances

Liquid crystal 'blue phases' are highly fluid self-

assembled three-dimensional cubic defect

structures that exist over narrow temperature

ranges in highly chiral liquid crystals[120-126].

The characteristic period of these defects is of the

order of the wavelength of visible light, and they


29

give rise to vivid specular reflections that are vivid specular reflections that are

controllable with external fields. Blue phases may be considered as examples of tuneable

photonic crystals with many potential applications. The disadvantage of these materials,

as predicted theoretically and proved experimentally, is that they have limited thermal

stability: they exist over a small temperature range (0.5–2 °C) between isotropic and

chiral nematic (N*) thermotropic phases, which limits their practical applicability.

Effect of Chemical Constitution on Mesomorphism:

Most of the rod-like liquid crystalline compounds consist of two or more rings,

which are directly bonded to one another or connected by linking groups. The chemical

structure of many mesogens can be represented by the general formula-I

R1 O O L1 L2 O R2

Z1 Z2 Z3

Here L is linking group. The molecule may have terminal substituent (R) and

lateral substituent (Z). O-Aromatic/Alicyclic/Heterocyclic rings/ cores.

The core is usually a relatively stiff unit, compared to the terminal lateral

substituents in most cases are small units such as halogens, methyl, methoxy, hydroxy,

cyano groups etc. however, now liquid crystals with long lateral substituents are known.

Effect of Core:

The major anisotropy of molecules, which is necessary for their mesogenity,

results from the cores, which are also responsible for relatively high melting

temperatures.

The core consists of rings that are connected to one another either directly or by

linking groups. Any ring that allows a stretched configuration of the molecules can be

used. More complex ring systems cholesterol is also used.

The oldest known liquid crystal have benzene rings as core. The increase in

number of benzene rings generally results in the increase of melting temperatures. Also,

the mesogenity of the compound increases with the number of linearly connected rings.

Due to the large conjugated aromatic- systems, the intermolecular attractions of the

molecules are very large giving rise to high melting temperatures.


30

N N

> >N

>

N

N>

N

N>

N

NN>

N N

NN

Decrease in mesogenity

Influence of nitrogen substituents on mesogenity.

The cyclohexane ring is non-aromatic and flexible compared to benzene ring. The

flexibility of the central ring has some negative influence on mesogenity. Bicyclooctane

derivatives have much stronger nematogenity as compared to the cyclohexane core.

Effect of Linking Groups:

Small chemical groups between the rings of liquid crystal molecule can increase

the length of the molecule while preserving the linear shape. However, when the linking

groups produce a bent molecular shape, the mesogenic potential of the molecule is

diminished.

Besides the geometry of the molecules, additional effects such as conjugative

interaction of the linking groups with aromatic groups, effects due to polarity of the

linking groups etc. also play an important role in liquid crystalline of a molecule. The

effects of linking groups can be quite different in aromatic and non-aromatic compounds,

as in the case of non- aromatic compounds, there are no conjugative effects, however, the

effect of terminal substituents may sometimes overcome this effect.

Effect of Terminal Substituents:

Terminally substituted compounds exhibit more stable mesophases compared to

unsubstituted mesogenic compounds. The most common terminal substituents are the

alkyl and alkoxy groups. The behavior within the homologous series shows that in

general there is an alteration of TN-I temperatures.

This can be explained by the alteration of the length to breadth ratio. Fig.28 shows

a typical six-member ring, with an attached alkyl chain.


31

1

2 3

1090

Fig.28: Alteration effect in a terminal alkyl chain

The attachment of an odd numbered carbon atom substituent increases the length

to breadth ratio more than does the attachment of an even numbered carbon atom

substituent. This principle behavior seen in alkyl chain can also be found in other flexible

chains.

X-rays and other methods have been used to show that compounds containing

strongly polar groups like -CN and -NO2 from double molecules that exist in equilibrium

with single molecules [127-129]. Due to such dimerization, the breadth increases by the

factor of 2 and length only by a factor of 1.1~1.4. Hence, the effective L/B ratio should

be reduced. But, highly polar compounds have a much higher density than low polar

compounds [130,131]. This accounts for the increase in clearing temperature. The

halogens and isothio-cyanato groups introduce relatively large positive dielectric

anisotropy into the molecules however, there is no association [132].

Branched terminal substituent also affects mesomorphism. The effect of a branch

depends substantially on its position in a chain. When the branch is nearer the centre of

the molecules the clearing temperature is lowered. When -CH2 group in the terminal

chains are replaced by an oxygen atom, clearing temperature decreases. Oxygen atom

seems to reduce the stiffness of the chain.

The terminal group efficiency order which has been compiled for Smectic phase

in rod-like aromatic system is:

-Ph > -Br > -Cl > -F > -NMe2 > -Me > -H >-NO2 > -OMe > -CN and the nematic group

efficiency order is, -Ph > -NHCOCH3 >-CN > -OCH3 >-NO2 > - Cl > - Br > -N (CH3)2 >

-CH3 > - F .

Intermolecular Hydrogen Bonding

Intermolecular hydrogen bonding interactions have shown great potential in the

preparation of new liquid crystalline systems especially thermotropic LCs [133, 134].

They have been used as links, connecting two independent molecular components. These


32

form anisotropic molecules, which complies with the main characteristic of liquid crystal

molecules. Most of these systems are based on pyridine and acid derivatives [135].

The hydrogen bond in the liquid crystal field enables molecular components that

do not themselves exhibit the property, form supramolecular species, which show the

liquid crystal behaviour. Also these liquid crystal moieties have greatly enhanced

mesomorphic range [136].

Physical properties of liquid crystals

Fig. 29: physical properties of liquid crystals

As a result of orientational order, most physical properties of liquid crystals are

anisotropic[137-139] and must be described by second rank tensors. Examples are the

heat diffusion, the magnetic susceptibility, the dielectric permittivity or optical

birefringence[140]. Additionally, there are new physical qualities, which do not appear in

simple liquids as e.g. elastic or frictional torques (rotational viscosity) acting on static or

dynamic director deformations, respectively.

The most remarkable features of liquid crystals with respect to applications are

due to their anisotropic optical properties. Nematics, and SmAs are uniaxial, SmCs

weakly biaxial. Cholesterics give rise to Bragg reflections if the helix pitch is in the


33

magnitude of the light wavelength. As mentioned above these properties are carried by a

fluid, soft material, and therefore are extremely sensitive against external perturbations.

Orientational order and hence birefringence can be manipulated easily e.g. with the help

of rather weak magnetic, electric or optical fields, leading to huge magneto-optical,

electro-optical and opto-optical effects [141,142]. The most successful application are

liquid crystal displays well-known from wrist watches, pocket calculators or flat screens

of laptop computer which take advantage of electro-optical effects. More recently, it

turned out that orientational order can be also affected by optical fields leading to rather

sensitive opto-optical effects and nonlinear optical properties, which are important e.g.

for all-optical switching and other photonic devices in future optical information

technologies [143, 144].

Birefringence in Liquid Crystals

Liquid crystals are found to be birefringent, due to their anisotropic nature. That

is, they demonstrate double refraction (having two indices of refraction). Light polarized

parallel to the director has a different index of refraction (that is to say it travels at a

different velocity) than light polarized perpendicular to the director. In the following

diagram, the blue lines represent the director field and the arrows show the polarization

vector.

Thus, when light enters a birefringent material, such as a nematic liquid crystal

sample, the process is modeled in terms of the light being broken up into the fast (called

the ordinary ray) and slow (called the extraordinary ray) components. Because the two

components travel at different velocities, the waves get out of phase. When the rays are

recombined as they exit the birefringent material, the polarization state has changed

because of this phase difference.


34

Light traveling through a birefringent medium will

take one of two paths depending on its polarization.

Liquid Crystal Textures

The term texture refers to the orientation of liquid crystal molecules in the vicinity

of a surface. Each liquid crystal mesophase can form its own characteristic textures,

which are useful in identification. We consider the nematic textures here. If mesogenic

materials are confined between closely spaced plates with rubbed surfaces (as described

above) and oriented with rubbing directions parallel, the entire liquid crystal sample can

be oriented in a planar texture, as shown in the following diagram. Mesogens can also be

oriented normal to a surface with the use of appropriate polymer films or in the presence

of an electric field applied normal to the surface, giving rise to the homeotropic texture,

as illustrated below.

Experimental Identification of Liquid Crystals

Liquid crystal phases can be identified by a variety of techniques [145] like

optical polarizing microscope, differential scanning calorimetry, X-ray analysis,

miscibility studies, neutron scattering studies, cryo-transmission electron microscopy

[146], nuclear magnetic resonance[147] and fabry-perot Scattering studies [148]. A few

of these are described here.


35

Differential Scanning Calorimetry (DSC)

Heat is needed to melt a crystalline solid to a liquid crystalline phase. The heat is

measured using a DSC instrument. Although DSC cannot identify the type of phase, it

provides valuable information like the exact transition temperatures and the enthalpies of

the different phases [149].

Polarizing Microscope

In a polarising microscope, the light is polarized by passing it through a

polarizing filter. It then passes through the sample, and then through a second polarizing

filter called the analyzer. When a liquid crystal material is placed on a microscope slide

with a cover slip and the slide is heated and viewed using a polarizing microscope,

textures characteristic of each type of liquid crystal can be seen. Cooling the liquid can

also yield these textures when liquid crystal phases are present [44].

X-ray Crystallography

This can be used to study the extent of translational or positional order, and thus

infer the type of liquid crystal phase [150].

Extended X-ray absorption fine structure spectroscopy(EXAFS)

EXAFS was used to investigate the local structure of the polar spines of metal ion

soaps in the columnar liquid crystalline state [151].

Applications of liquid crystals

Display application of liquid crystals

The most common application of liquid crystal

technology is liquid crystal displays (LCDs.)[152-157]

This field has grown into a multi-billion dollar

industry, and many significant scientific and

engineering discoveries have been made. Liquid

crystal display devices consisting of digital readouts

are used in watches , calculators , and several other instruments like mobile and many

household electric appliances [158]. Some liquid crystal substances could be useful in

computer industry, for making new computer elements with high memory capacity.


36

Liquid crystals displays (LCDS) [159] had a humble beginning with wrist

watches in the seventies.Continued research and development in this multidisciplinary

field have resulted in display with increased size and complexity.After three decades of

growth in performance, LCDs now offer a formidable challenge to cathod ray tubes

(CRT). Liquid crystal display (LCDs) have many adventages over other display types.

They are flate and compact, posses extremely low power consumption (Microwats per

square centimeter in the case of the twisted Nematic display), their colour and contrast

does not fade with an increase in the illumination intensity. They work both in

transmitive and reflective modes in a wide operating temperature range and with a long

life time. Because that, LCDs are the most economically produced display systems.

LCDs have a brilliant future in high defination TV system, personal computer, measuring

devices etc. The most widely used electro optics effects in display are the twist, super

twist and guest host modes.

There are many types of liquid crystal displays, each with unique properties. The

most common LCD that is used for everyday items like watches and calculators is called

the twisted nematic (TN) display. This device consists of a nematic liquid crystal

sandwiched between two plates of glass. A special surface treatment is given to the glass

so that the director at the top of the sample is perpendicular to the director at the bottom.

This configuration sets up a 90 degree twist into the bulk of the liquid crystal, hence the

name of the display. The underlying principle in a TN display (shown below) is the

manipulation of polarised light. The left image shows that when light enters the TN cell,

the polarisation state twists with the director of the liquid crystal material. For example,

consider light polarised parallel to the director at the top of the sample. As it travels

through the cell, its polarisation rotates with the molecules. When the light emerges, its

polarisation has rotated 90 degrees from when it entered.


37

Principle of twisted nematic LCDs

The right image shows that the application of an electric current to these liquid

crystals will "untwist" them to varying degrees, depending on the voltage. These liquid

crystals are most popular for LCDs because they react predictably to electric current in

such a way as to control light passage. Depending on the field strength, twisted nematic

displays can switch between light and dark states, or somewhere in between (greyscale).

How the molecules respond to a voltage is the important characteristic of this type of

display. This link shows the optical response curve of a TN LCD. It also shows the

response curve of a super-twisted nematic (STN) LCD, which rotates the director of the

liquid crystal by 270 degrees instead of 90 degrees, and has some technical advantages

over ordinary TN displays. Active matrix LCDs and STN (super twisted Nematic) LCDs

are leading display technologies for portable application such as notebook computers.

New LCD device configuration and new LCD operation modes require improved liquid

crystal materials. Advance liquid crystalline material had to be developed in order to

fulfill the requirements of higher resolution and large size LCDs. For simple calculator

and watch displays TN (Twisted Nematic) mixture based on cyanobiphenyls are used.

These materials were first invented by G.W. Gray 25 years ago [160]. Broad range TN

mixtures with improved viewing angle using phenylcyclohexanes [161,162] were then

used for automative application. The introduction of STN displays requires materials with

large dielectric anisotropy, eg. cyanoesters with lateral fluoro substitution [163]. Thin

film technology (TFT) displays requires liquid crystalline materials with high stability

like fluorinated liquid crystals [164, 165].


38

The use of polymer liquid crystal (PLC’s) in the display industries is an exciting

area of research. A twisted nematic polymer liquid crystal cell can be used to make

energy efficient displays. A layer is use to selectively melt portions of the display into the

liquid crystal phase. The orientation of the cell can be used to make energy efficient

displays. The orientation of the cell is then chosen by applying a field across it, just as in

an ordinary twisted nematic liquid crystal cell. When the polymer cools down and

hardens into a glass, the mesogens will be locked in that configuration and the field can

be turned off. Side chain polymer liquid crystal exhibit good properties for application in

optically nonlinear devices including optical wave guides and electrooptic modulators in

poled polymeric slab-waveguides. More device are expected to be fabricated from PLCs

in the future like optically-addressed spatial light modulators, tunable notch filters,

optical amplifiers and laser beam deflectors. The properties of ferroelectric chiral SmC

phase make this material useful for films with application in non linear optics. A review

on the use of liquid crystals in laser optics [166, 167] has also appeared in the literature.

Owsik et al. [168] have reported the use of chiral liquid crystals in layer engineering.

Thermal mapping and non-destructive testing

Chiral nematic (cholesteric) liquid crystals reflect light with a wavelength equal to

the pitch. Because the pitch is dependent upon temperature, the color reflected also is

dependent upon temperature. Liquid crystals make it possible to accurately gauge

temperature just by looking at the color of the thermometer. By mixing different

compounds, a device for practically any temperature range can be built.

More important and practical applications have been developed in such diverse

areas as medicine and electronics. Special liquid crystal devices can be attached to the

skin to show a "map" of temperatures. This is useful because often physical problems,

such as tumors, have a different temperature than the surrounding tissue. Liquid crystal

temperature sensors can also be used to find bad connections on a circuit board by

detecting the characteristic higher temperature. The sensitivity of cholesteric liquid

crystals to react to pressure as well as temperature by colour change is used to make some

very interesting publicity materials and toys. Cholesteric liquid crystals can be used as an

analytical tool to detect the presence of very small amounts of gases or vapours by colour

changes to the extent of about 1 ppm. [169].


39

A film of cholesteryl liquid crystal [170] may be applied to large uneven area.

This makes it an ideal tool for thermal mapping and non-destructive testing. The great

deal of flexibility in the color play range allows for a great diversity in potential

applications ranging from food processing to electronics and space applications e.g.

thermo chromic paints have been used on primed circuit boards to examine overheating

of components. The area in which liquid crystal thermograph is of use in non destructive

testing continued to grow due to the development on new chiral smectic materials which

offer improved performance over the cholesteryl esters used in early applications.

Thermo chromic liquid crystals are extensively used in medical applications,

forehead thermometers also known as ‘fever strips’ are based on different thermo

chromic liquid crystal materials, thermal mapping of various areas of the body has been

used as a diagnostic technique for a wide ranging group of medical conditions in which a

temperature differential near the skin surface may be related to the disorder subcutaneous

and intracutaneous malignant tumors are typically 0.9–3.30C warmer than the

surrounding tissues. Therefore, thermograph is an interesting candidate for cancer

screening.

Medicinal Uses

Cholesteric liquid crystal mixtures have also been suggested for measuring body

skin temperature, to outlines tumours etc. Any inflammation or construction of the

vessels will naturally affect the temperature of the skin: this will help in the location of

inflammation, since the warmer areas will outline by the colour pattern.

In gynecology, where there is a possibility that a cessarian section may be

necessary, liquid crystals can be used to locate the plecenta, thus avoiding the need for x-

ray. Hence it is useful in controlled drug delivery [171]. Recently their biomedical

applications such as protein binding [172], phospholipids labeling [173] and inmicrobe

detection [174] have been demonstrated. In psychology, cholesteric liquid crystals could

be used in lie detectors.

Optical Imaging and Liquid Crystal Interactions with Nanostructure

An application of liquid crystals that is only now being explored is optical

imaging and recording. This technology is still being developed and is one of the most

promising areas of liquid crystal research. The use of the mesomorphic state for the


40

organisation of nanoparticles opens up the utilisation of techniques employed for

fabrication of large panel displays, or alternatively if higher ordered LC phases are used,

for the controlled bottom-up self organisation in two- or three-dimensional lattices,

depending on the type of mesophase. This might be particularly valuable in applications

associated with the optical, magnetic or conducting properties of nanoparticles [175-179].

Liquid Crystal in Chromatography [180]

The molecular structure of liquid crystals allow for their application in gas and

liquid chromatography as highly elective stationary phase. A number of models have

been developed to describe more quantitatively the enhancement in selectivity that is

obtained form the anisotropic orientational ordering of liquid crystals [181-191]. Earlier

use of liquid crystals as stationary phases in gas chromatography are available [192-197]

which can be prepared from either monomer or side chain polymeric liquid crystals.

Liquid Crystal as Solvents in Spectroscopy [198]

Liquid crystalline media, particularly pneumatics, provide the bulk molecular

orientation necessary for observation of spectroscopic details analogous to those obtained

in solid state experiments, these media have been widely used as solvents in NMR, EPR

and Optical spectroscopic studies an oriented molecules. A few reviews in this area of

application have appeared [199, 200].

Liquid Crystal as Solvents in chemical reactions

Thermotropic liquid crystals have been used as solvents to after course or bi-

molecular thermal and photochemical reactions. The unique anisotropic properties of

liquid crystals are utilized to control the efficiency and specificity in micro synthesis

elucidation of reaction mechanism etc. factor that are importance in defining the ability of

liquid crystals to control solute reactivity have been reviewed to be able to choose the

liquid crystals of proper morphology as a solvent [201-204].

Guest-Host type Display

A pleochoric dye is added to Nematic host and the dye molecule gets oriented

parallel to the host molecule since the colours of the dye depends on the orientation of the

dye molecule an electric field causing reorientation in the Nematic host will causes a

colour change. Guest-host displays and their use in military air crafts. Liquid crystal

windows were also prepared to control the solar energy by fabricating it with phase


41

change guest-host mixture [205]. Liquid crystals glass cover is used to control the

transmittance on light incident side at the solar cell. Solar cell having this cover maintains

constant out put voltage under varying load situation [206].

Other Liquid Crystal Applications

Liquid crystals have a multitude of other uses. They are used for nondestructive

mechanical testing of materials under stress. This technique is also used for the

visualization of RF (radio frequency) waves in waveguides. They are used in medical

applications where, for example, transient pressure transmitted by a walking foot on the

ground is measured. Low molar mass (LMM) liquid crystals have applications including

erasable optical disks, full color "electronic slides" for computer-aided drawing (CAD),

and light modulators for color electronic imaging.

As new properties and types of liquid crystals are investigated and researched,

these materials are sure to gain increasing importance in industrial and scientific

applications.

Liquid crystal polymers

Liquid crystal polymers (LCPs) are a unique class of wholly aromatic polyester

polymers that provide previously unavailable high performance properties [207-210]. A

number of LCP resins were produced in the 1970s which displayed order in the melt

phase analogous to that exhibited by non-polymeric liquid crystals. The structure of LCPs

consists of densely packed fibrous polymer "chains" that provide self-reinforcement

almost to the melting point. However, the commercial introduction of liquid crystal

polymer resins did not occur until 1984, at which time LCPs could not be injection

molded. Today, LCPs can be melt processed on conventional equipment at fast speeds

with excellent replication of mold details. A relatively unique class of partially crystalline

aromatic polyesters based on p-hydroxybenzoic acid and related monomers. Liquid

crystal polymers are capable of forming regions of highly ordered structure while in the

liquid phase. However, the degree of order is somewhat less than that of a regular solid

crystal. Typically LCPs have outstanding mechanical properties at high temperatures,

excellent chemical resistance, inherent flame retardancy and good weather ability. Liquid

crystal polymers come in a variety of forms from sinterable high temperature to injection

moldable compounds. LCPs are exceptionally inert. They resist stress cracking in the


42

presence of most chemicals at elevated temperatures, including aromatic or halogenated

hydrocarbons, strong acids, bases, ketones, and other aggressive industrial substances.

Hydrolytic stability in boiling water is excellent. Environments that deteriorate the

polymers are high-temperature steam, concentrated sulfuric acid, and boiling caustic

materials.

Liquid Crystal of high - Strength fibers

An application of polymer liquid crystal that has been successfully developed for

industry is the area of high strength fibers. eg. Kevlar fibers, which are used to make such

things as helmets and bulletproof vests, is just one example of the use of polymer liquid

crystal in application calling for strong, lightweight materials.

Ordinary polymers have never able to demonstrate the stiffness necessary to

complete against traditional materials like steel. It has been observed that polymers with

long strength chains are significantly stronger than their tangled counter parts. Main

chain liquid crystal polymers are well suited to ordering processes. For example, the

polymer can be oriented in the desired liquid crystal phase and then quenched to create a

highly ordered, strong solid. As these technologies continue to develop an increasing

variety of new materials with strong and light-weight properties will become available.

The recent discovery of ferroelectricity and antiferroelectricity on compounds

composed of achiral banana-shaped molecules may extend the application of liquid

crystal in the field of display technology. Work is in progress in many laboratories

throughout the world in order to understand clear the structure and properties of these

mesophases and to develop application of such materials.

Typical LCP applications[211-216]

� Electrical/Electronic applications

� Automotive applications

� Parts, Engineering

� Containers, Food appliances

� Industrial applications

� Connectors

� Optical applications

� Parts, Thin-walled


43

Advantages of LCP

� High heat resistance

� Flame retardant

� Chemical resistance

� Dimensional stability

� Mold ability

� Heat aging resistance

� Adhesion

� Low viscosity

� Wieldable

� Low cost

Disadvantages of LCP

� Form weak weld lines

� Highly anisotropic properties

� Drying required before processing

� High Z-axis thermal expansion coefficient

New Liquid Crystal Composite Materials for Photorefractive Applications[217-223]

The photorefractive effect is an energy efficient method through which image

storage and retrieval can occur with outstanding image quality and high density. The

photorefractive effect results from optically induced directional charge transport within

the material. When the charges trap, an electric field is produced which modulates the

material’s index of refraction. When this effect is properly harnessed through a laser

induced grating, image storage and retrieval can occur. Commercially available

photorefractive materials currently available consist of inorganic ferroelectric crystals

which are expensive and time consuming to grow. Liquid crystals represent a simpler and

more economical alternative to presently available materials. Furthermore, liquid crystals

allow for greater versatility due to the ease with which different chromophores can be

utilized to “sensitize” the material to different laser wavelengths. These materials also

possess significant advantages for dynamic holography due to their low photon flux

requirements and ease of hologram erasure.

Liquid crystals through the application of magnetic fields, providing superior

grating resolution compared to liquid crystals aligned solely through surfactant


44

techniques. Liquid crystals that operate in the near infrared spectral region, which is

required for studies related to imaging biological tissue. This was accomplished using

substituted phthalocyanines as electron donors and pyromellitimide as an electron

acceptor. Polymer /liquid crystal composites whose photoconductive mechanism occurs

in large part by intrachain electron hole trans-port, rather than solely through ion

diffusion. This permits faster formation of the photo-refractive grating at smaller fringe

spacings.

Biphenyl:

In general, the molecules of a liquid crystalline compounds are elongated, rod or

lath shaped thin and often flat, possessing middle and terminal polar groups. Molecules

which form liquid crystal have dipole in their structure often a strong dipole towards the

centre and weak dipole towards the end of the molecules. When more than two benzene

rings are linked through more than one central group, the liquid crystalline properties

enhance the most. However, linearity and rigidity are increased by linking up the benzene

rings directly and thus biphenyl provides a rich source of liquid crystals which are

thermally more stable than those benzene substituted analogues. It plays an important

role in the formation of liquid crystal having ferroelectric and antiferroelectric

properties. [224-231] which are widely used in display device and/or electro-optical

devices nowadays.

The 4-alkyl/alkoxy-4’-cyanobiphenyls occupy a unique place in the development

of liquid crystals as the first stable compounds to exhibit the nematic phase at room

temperature as single components or in mixtures which were suitable for use in twisted

nematic display devices [162, 232, 233]. The commercial nematic mixtures of biphenyls,

which often include a small amount of a 4-alkyl-4’-cyanoterphenyl are still used widely

in display devices more than 25 years after their discovery; in such a rapidly developing

and changing area it is a remarkable conformation of the revolution in display technology

which these compounds stimulated.

To achieve high ∆n, molecules that contain highly polar groups and high electron

density, such as biphenyl rings the preferred candidates. High birefringence (∆n) liquid

crystals (LCs) are useful in super twisted nematic LC displays [234] polarizer-free

reflective displays such as polymer-dispersed LCs [235], cholesterics [236], holographic


45

switching devices [237], polarizers and directional reflectors [238, 239]. Apart from these

display applications, these materials are also useful for laser beam steering using optical

phased arrays and for spatial light modulators [240–242]. The ∆n values of the LC

materials are determined mainly by their electron conjugation, differential oscillator

strength and order parameter [243]. Several molecular structures with high ∆n values, e.g.

diphenyldiacetylene [244, 245].

Gray et al. [246] have prepared 4`-n alkoxy –diphenyl -4 carboxylic acids & their

mesomorphic behavior compared with that of the 4-n-alkoxy benzoic acids. They exhibit

mesophases of much greater relative thermal stability than simple benzoic acids. Joseph

et al. [247] have synthesized homologues series, namely, p-n-alkoxy benzylidene –p`-

aminoaceto phenones having biphenyl moiety. Bailey & Bates [248] have reported new

thermotropic liquid crystals having isocyano and haloalkynyl biphenyls. Chiellini et al.

[249] have reported new thermotropic polyurethanes having biphenyl moiety. Hogan et

al. [250] have reported polar compounds having biphenyl moiety with nematic properties.

Demus et al. [251-254] have synthesized the terminal branches in the 3-position at the

opposite end with respect to the polar cyano group, also synthesized one homologous

series of bi-swallow-tailed compounds and lateral central linkage substituted liquid

crystalline compounds having biphenyl moiety. Malthete et al. [255] have synthesized

hemiphasmidic compound containing 3,4,5-trisubstituted aromatic ring showing biaxial

nematic phase (Nb), in addition to a uniaxial one (Nu), has been detected for the first time

having biphenyl moiety. Cox et al. [256] have synthesized the homologous series which

exhibits liquid crystalline properties. Adam et al. [257] have reported metal complexes

with a linear molecular shape that, in principle, are tail-to-tail twins containing biphenyl

moiety. Chandrasekhar et al. [258] have reported Cu-metal complexes containing

biphenyl moiety. Centore et al. [259] have reported 4,4'-biphenylene-bis(oxycarbony1

butyric acid). Vora and Prajapati [260] have reported liquid crystalline homologous series

with biphenyl nucleus. Hall et al. [261] have synthesized some branched

alkyloxycarbonylphenyl esters of 3-(4'-n-alkyl- and -alkoxy-biphenyl-4-yl)propanoic

acids and their laterally fluorinated analogues. Parghi et al. [262] have synthesized series

of fluorinated antiferroelectric liquid crystals having biphenyl moiety. V. Surendranath

[263] has reported new liquid crystalline dimesogens of biphenyl ring with cholesterol.

Duffy et al. [264] reported synthesis and evaluation of nematic 4-alkenyloxy- and 4-


46

alkenoyloxy-4'-cyanobiphenyls. Marius Kölbel and Carsten Tschierske [265] have

reported a novel class of amphotropic mesogens displaying SA-polymorphism, nematic

and lyotropic columnar phases having biphenyl ring. Guittard et al. [266] have reported

synthesis and thermotropic liquid crystal partially fluorinated materials derived from

biphenyl incorporating an ester connector. Liao et al. [267] have reported synthesis and

mesomorphic properties of fluoro and isothiocyanato biphenyl tolane liquid crystals. Wen

et al. [268] reported synthesis and phase-transition of 4-alkoxycarbonylphenyl 4'-n-

alkoxy-2,3,5,6-tetrafluorobiphenyl-4-carboxylates. Wild et al. [269] have reported

synthesis and mesomorphic behaviour of wedge-shaped nematic liquid crystals with

flexoelectric properties containing biphenyl moiety. WU and Lin [270] have reported

antiferroelectric liquid crystals having a semi-fluorinated alkane positioned at the chiral

tail having biphenyl moiety. Makarov et al. [271] have reported thermotropic liquid

crystals based on ferrocenylbiphenyl and ferrocenylterphenyl. Jeon et al. [224] have

reported chiral smectic C phases exhibited by biphenyl resorcylate and vanillate

derivatives. Swansburg et al. [272] have reported synthesis and characterization of liquid

crystals containing a non-activated 1',3',3'-trimethylspiro\[2H-1-benzopyran-2,2-indoline]

group containing biphenyl moiety. Manickam et al. [273] have reported introduction of

bis-discotic and bis-calamitic mesogenic addends to C60 having biphenyl moiety.

Barmatov et al. [228] have reported induction of the cholesteric mesophase in hydrogen-

bonded blends of polymers with a low molecular mass chiral dopant having biphenyl

moiety. Eagle et al. [274] have reported a study of the stability and phase behaviour of

some smectic liquid crystalline biphenyl derivatives. Guillermain and Gallot [225] have

reported synthesis and liquid crystalline structures of poly(L-lysine) containing

undecanamidobiphenyl units in the side chains. Zhang et al. [275] have reported the

synthesis and thermotropic liquid crystalline behaviour of novel main chain poly(aryl

ether ketone) containing a lateral phenyl group having biphenyl moiety. Czuprynski et al.

[276] have reported carborane-containing liquid crystals: a comparison of 4-octyloxy-4’-

(12-pentyl-1,12-dicarbadodecaboran-1-yl) biphenyl with its hydrocarbon analogues.

Cooray et al. [230] have reported novel antiferroelectric liquid crystals with a

phenylpiperazine moiety in the mesogenic core structure having biphenyl ring. Campo et

al. [277] have reported thermal properties of non-symmetric bibenzoate liquid crystalline

dimers. Geng et al. [278] have reported structure and liquid crystalline properties of 5-


47

[(4'-heptoxy-4-biphenylyl)carbonyloxy]-1-pentyne. Cui et al. [279] have reported

synthesis and thermal behaviour of liquid crystalline pyridinium bromides containing a

biphenyl core. Xu et al. [280] have reported Synthesis and characterization of novel

ferroelectric liquid crystals and copolymers containing biphenyl azobenzene and / or

phenyl biphenyl carboxylate mesogenic groups. Schulte et al. [281] have reported

development of non-reactive fluorine-rich biphenyl molecules and their incorporation

into a PDLC system. Hattori and Uryu [282] have reported synthesis and characterization

of polymerizable photochromic liquid crystals containing a spiro-oxazine group

containing biphenyl core. Chen and Wu [283] have reported synthesis and

characterization of new ferroelectric liquid crystals containing oligomethylene spacers.

Pal et al. [284] have reported phase transitions in novel disulphide-bridged

alkoxycyanobiphenyl dimers. Chambers et al. [285] have reported laterally fluorinated

phenyl biphenylcarboxylates; versatile components for ferroelectric smectic C mixtures.

Campbell et al. [286] have reported polar 2-alkoxyethoxy-substituted nematic liquid

crystals containing biphenyl core. Lin and Hsu [287] have reported synthesis of high

temperature cholesteric copolysiloxanes having biphenyl moiety and their use as

stationary phases for high resolution gas chromatography. Imrie et al. [288] have reported

six new oligomeric nematic liquid crystals are reported consisting of a triphenylene-based

core attached to which are six 4-cyanobiphenyl units via flexible alkyl spacers. Hartley

and Lemieux [289] have reported ferroelectric liquid crystals induced by atropisomeric

biphenyl dopants: the effect of chiral perturbations on achiral dopants. Rauch et al. [290]

have reported glass forming banana-shaped compounds having biphenyl moiety: Vitrified

liquid crystal states. Catanescu and Chien [291] have reported high birefringence

difluoroisothiocyanate biphenyl tolane liquid crystals. Jaishi and Mandal [292] have

reported optical microscopy, DSC and X-ray diffraction studies in binary mixtures of 4-

pentyloxy-4'-cyanobiphenyl with three 4,4'- di(alkoxy) azoxybenzenes. Svensson et al.

[293] have reported effects of nitro substituents on the properties of a ferroelectric liquid

crystalline side chain polysiloxane having biphenyl core. Gray et al. [294] have reported

the synthesis and transition temperatures of some 4'-alkyl- and 4'-alkoxy-4-cyano-3-

fluorobiphenyls. Xie and Zhang [295] have reported synthesis and characterization of

side-chain liquid-crystalline poly(ethyleneimines) with cyanobiphenyl groups. Petrenko

and Goodby [296] have reported V-Shaped switching and interlayer interactions in


48

ferroelectric liquid crystals having biphenyl core. Cseh and Mehl [297] have reported

structure–property relationships in nematic gold nanoparticles having biphenyl core.

Keith et al. [298] have reported the influence of shape and size of silyl units on the

properties of bent-core liquid crystals—from dimers via oligomers and dendrimers to

polymers having biphenyl moiety. Zeng et al. [299] have reported testing the triple

network structure of the cubic Im3m (I) phase by isomorphous replacement and model

refinement using biphenyl ring. Drzewinski [300] has reported nitroarenes-the simple

way to liquid crystalline fluoroalkyl-aryl ethers having biphenyl moiety. Yang et al. [301]

have reported Synthesis and physical properties of a main-chain chiral smectic thiol-ene

oligomer having biphenyl moiety. Li et al. [302] have reported effect of a biphenyl side

chain of polyimide on the pretilt angle of liquid crystal molecules: molecular simulation

and experimental studies. Song et al. [303] have reported high birefringence lateral

difluoro phenyl tolane liquid crystals having biphenyl core. Florjanczyk et al. [304] have

reported the influence of structural changes of the n-substituent on liquid crystalline

behaviour of ester imides having biphenyl core. Diez et al. [305] have reported dielectric

studies of a laterally-linked siloxane ester dimmer having biphenyl moiety. Yang et al.

[306] have reported synthesis and mesomorphic properties of several series of fluorinated

ester liquid crystals containing biphenyl moiety.


49

Research Objective and outline of thesis

The aim of the research is to synthesized and characterized new liquid crystalline

compounds and explores their importance. From the above survey we inspired to

synthesize liquid crystal compound containig biphenyl moiety due to their importance

and applications in various field. In fact, we see structure and geometry of compound

through the window of spectroscopy. The down side of this is that we see only as much

as the concepts allow us to see, but we overcome all limitation by investigates with

specific characterization and attest all the compounds. The thermal properties and

mesophase study of all the compounds achieved by differential scannig calorimetry and

optical polarizing microscope. The data obtained by above study were described with

theoretical and practical screening.

Chapter-2 Synthesis, characterization and mesomorphic properties of new liquid

crystalline compounds of biphenyl ring involoving α-methyl schiff base as a central

linkage and 2-methyl 5-amino thiazole, 1-amino 4-methyl piperazine and 4-cyano aniline

as a terminal group. All these compounds were characterized by elemental analyses and

spectroscopic techniques (UV-Visible, FT-IR, 1H NMR and Mass spectra). Their

mesomorphic properties were measured by optical polarized microscopy and differential

scanning calorimetry (DSC).

Chapter-3 Synthesis, characterization and mesophase behavior of new liquid crystalline

compounds having chalcone as a central linkage derived from 1-(4'-butoxybiphenyl-4-yl)

ethanone and n- alkoxy benzaldehyde. All these compounds were characterized by

elemental analyses and spectroscopic techniques (UV-Visible, FT-IR, 1H NMR, 13C

NMR and Mass spectra). Their mesomorphic properties were measured by optical

polarized microscopy and differential scanning calorimetry (DSC).


compounds having azo-ester as central linkages and naphthalene, 4-cyano aniline and 4-

Flouro aniline as a terminal group. All these compounds were characterized by elemental

analyses and spectroscopic techniques (UV-Visible, FT-IR, 1H NMR, 13C NMR and

Mass spectra). Their mesomorphic properties were measured by optical polarized

microscopy and differential scanning calorimetry (DSC).


50


compounds having azo-cinnamate as central linkages and naphthalene, 4-cyano aniline

and 4-Flouro aniline as a terminal group. All these compounds were characterized by

elemental analyses and spectroscopic techniques (UV-Visible, FT-IR, 1H NMR, 13C

NMR and Mass spectra). Their mesomorphic properties were measured by optical

polarized microscopy and differential scanning calorimetry (DSC).


51

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Introduction and historical overview What are Liquid Crystals? · Introduction and historical overview Research on liquid crystal has been involved in chemistry, physics, Biology,

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