1.1 Introduction
There are three common states of matter
that most people known about : solid, liquid and gas. The molecules in solid
exhibit both positional and orientational order which are completely destroyed
in liquid phase . Liquid crystals have properties that are intermediate between
the solid and liquid states . Normally when a crystalline solid is heated it
transforms to a liquid, but liquid crystals exhibit a number of intermediate
phases/phase or “mesophases” during transformation from the solid to liquid
state. They are therefore also known as mesogens.
This type of phase was first discovered
by Friedrich Reinitzer [1-2] in 1888.The term “mesomorphic” phases or
mesophases was first used by Georges Friedel [3-4] in 1922.An essential
requirement for mesomorphism to occur is that the molecule must be
geometrically anisotropic in shape(e.g. rod like or disc like to name only a
few).Owing to their remarkable properties , liquid crystals find widespread use
and application in modern day technology and their study is a field of
intensive research not only in pure but multidisciplinary sciences.
Several thousands of organic compounds
are known to form liquid crystals[5].Many organic , inorganic , organometallic and biological compounds exhibit
these states in which
the substance retains anisotropy in optical, dielectric, magnetic and other
properties(inherent in a crystal) in addition to possessing liquid like
properties as well. In Liquid Crystal Database 262869 properties of 85526
liquid crystal compounds have been enumerated[6] .
To understand the significance of liquid crystals
in terms of ordering of the molecules, we recall that in crystalline solids ,
the molecules have fixed orientation and the centre of mass of the molecules
are distributed on a three dimensional periodic lattice i.e. the crystal
possesses long range ordering in both position and orientation of the
molecules. In isotropic liquid, the molecules do not have positional or
orientational ordering, their positions and orientations are randomly distributed.
In liquid crystals the molecules exhibit a certain degree of orientational
ordering and may have in addition some degree of positional order giving rise
to different types of liquid crystalline phases. More precisely, in the liquid
crystalline state, the three dimensional translational ordering of the centre
of mass of molecules(which characterize the crystalline solid) is broken
entirely or partially and the type of order retained (e.g., one or two
dimensional positional ordering ,bond ordering or orientational ordering etc.)
characterizes or determines the type of liquid crystalline phase.
The vast majority of liquid crystals
are composed of non-spherical and rod or disc shaped molecules. The Figure below 1.1(a)
illustrates some molecular shapes.
Figure 1.1 (a)
Structure of liquid crystal of rod like and bent
core type molecules.
Presently many
other types of molecular shapes such as H-,Y-,T- etc (shown in Figures 1.1(b)
and (c) ) are also being synthesized and investigated. An interesting review
has been given by D.Demus[7] on this aspect. Several books, monographs and
review articles [8-10] have discussed at length the molecular structure and
physical properties of liquid crystals.
Figure 1.1(b) Structure of the columnar phase of disc
like molecules
Figure
1.1(c) Structure of the conical crystal
molecules
1.1.1Classification
of Liquid Crystals
The
transition to and from the intermediate phases may be brought about
by thermal process as in case of thermotropic liquid crystals or by the
influence of solvents by varying the concentration as in lyotropic liquid
crystals.
Liquid crystals
may thus be broadly classified under two categories: Lyotropic or Thermotropic.
1.2 Lyotropic
Liquid Crystal
Lyotropic liquid crystals are made up of two or more
components of which one is a solvent and the other an ampiphile (containing a
polar head group attached to one or more long hydrocarbon chains). Deoxy
ribonucleic acid (DNA),certain viruses and many synthetic polypeptides
dissolved in an appropriate solvent (usually water) are examples of a lyotropic
system.The inter molecular interaction between the solute and the polar head of
the solvent molecules is crucial in providing the stability of
ordered phases . A familiar example of a lyotropic liquid crystal is soap
solution ( sodium dodecyl sulphate in water).Living systems have an abundance
of lyotropic liquid crystals. Since this dissertation is not
concerned with lyotropicmesophases, no further discussion on them is presented
here.
1.3 Thermotropic
Liquid Crystals
The
termthermotropic arises because they exhibit transitions involving mesophases
which are usually affected by change in temperature . Thermotropic liquid
crystals are generally organic compounds with molecular structures
which may be approximated to rod-like or disc-like in shape.
Following the
nomenclature proposed originally by Friedel[3], the thermotropic liquid
crystals are broadly classified into three types of mesophases:
A. Nematic liquid crystals
B. Cholesteric liquid crystals
C. Smectic liquid crystal
They are
distinguished from each other by their different degree of translational and
/or orientational ordering.A more detailed account of the above mentioned
mesophases is given below.
1.3.1Nematic
liquid crystal
A liquid
crystalline material possessing only nematic phase is called nematic
liquid crystal. Liquid crystalline compounds in general
have translational, orientational and combination
of both translational and orientational ordering of individual
molecules. In nematic liquid crystals, there is only orientational ordering,
but no translational ordering between the molecules.
A schematic
representation of the arrangement of molecules in the nematic phase is shown in
Figure 1.2.
Figure1.2 The
arrangement of molecules in the nematic mesophase (a) made up of the rod-like
molecules (b) made up of disc-like molecules
The
characteristic feature of the nematic phase apparent from the figure is that
there is no long- range order of the centers of mass of the
molecules. Themolecules tend to orient along a common direction labeled by unit
vector “
”also known as director.The direction
of "
” is arbitrary in space and the states of director "” and -"” are indistinguishable i.e., there are just as many molecules in the upright
position as in the downright position . Only materials that do not distinguish
between left and right-handedness can form nematic phases.
The constituent must either be identical to its mirror
image (achiral) or the system must be a mixture of the right and
left-handed species in equal proportions i.e.,“ racemic” . The nematic
phase described above is uniaxial.
Bio
axialnematics also exist and were first observed in lyotropic systems[11].
Some nematics
possess a lamellar type of short-range order i.e., they consist of groups of
molecules called cybotatic groups in which the [12] molecular centers are
arranged in layers. This type of nematic is known as cybotaticnematic.
Nematic phase
is very sensitive to external fields, electric or magnetic and also to external
mechanical stress, which it translates into visible optical/electrical effects
and for which they find wide application in display devices.
1.3.2 Cholesteric
liquid crystal
In case
of an optically active material forming a nematic phase,
the preferred direction of the long molecular axis in a mono-domain (uniformly
oriented sample) is not constant over the whole sample. It would be
in a nematic phase, but displays a continuous twist as one travels through the
sample along the optic axis. Hence the name, “twisted nematic” or
“spontaneously twisted nematic” was given to this kind of phase. In this type
of mesophase, there is no long range order of the centers of mass of molecules
and the molecules prefer to lie next to each other in a slightly skewed
orientation i.e., the structure acquires a spontaneous twist about an axis
normal to the preferred molecular directions "” which has a helical symmetry. The
twist may be left or right-handed depending on the molecular conformation .
If the helical
axis be taken along z-axis,then
nx =cos(qoz
+ φ )
ny = sin(qoz
+ φ )
nz = 0
where both the
direction of the helix axis z and the magnitude of the phase φ are arbitrary
and qo is the pitch.
The structure is
periodic along z-axis and the spatial period L is equal to one half of its
pitch (since the states "”
and -“ ” are
equivalent).
Figure1.3Schematic
representation of chiral nematic phase
The pitch(p) of the helix is
temperature and/or concentration dependent and comparable to optical
wavelength .The spiral arrangement of the molecules in the cholesteric phase is
responsible for its unique optical properties. Cholesterics of low
pitch (less than ≈ 5000A0) exhibit what are known as blue phases.
These phases exist over a small temperature range(≈10C) between the
liquid crystal phase and isotropic liquid phase[13-14].
Figure1.4(a)&(b)Ordering
in a cholesteric liquid crystal. The molecules in successive layers are
oriented at a characteristic angle with respect to those in adjacent layers to
avoid repulsive interactions
1.3.3
Smectic liquid crystals
Thesmectic
phase exhibits layered structures with different molecular orientations and
configurations within each layer giving rise to different categories of smectic
phases. Figures 1.5(a) and 1.5(b) depict two such smectic phases;
smecticA and smecticC. The different layers have enough fluidity to slide over
each other, maintaining a well-defined inter-layer spacing that can be measured
by X-ray diffraction[15].The smectic phases are more ordered than thenematic
phase and generally exist at a temperature lower than nematic phase. Smectics
are characterized by both orientational order of the long molecular axis and
positional order along the direction. Depending on the type of
ordering within the layers, the smectics may be further classified
into different categories as mentioned earlier. The orientational order relative
to the layer planes and in-plane positional ordering of the
constituent molecules give rise to smectic phases and are more
ordered than the nematic phase.
Figure 1.5(a)
Molecular arrangements of smecticA and smecticC phases
Figure 1.5(b)
Schematic representation of smectic A
and smectic C phases
The different smectic phases may be further classified under
four sub groups. Two sub- groups can be defined where the molecules have their
long axis (on an average) normal to the layers. These two sub-groups are
distinguished by the degree of positional ordering [16] of the constituent
moleculese, smectic A and hexatic B having short range positional ordering belong to
one sub group, whereas crystal B and crystal E phases which are symmetric like
soft crystal modification belong [17-18] to the second sub group (with the
constituent molecules having long range positional ordering in three
dimension).The other two sub-groups have their constituent
molecules tilted relative to the layer planes. SmecticC, smectic I
and smectic F phases possessing this character and exhibiting short range
positional ordering belong to one sub group, while crystal G, crystal H,
crystal J and crystal K phases, possessing long range three dimensional
positional ordering belong to the other sub- group (Figure 1.6).Smectic A,
smecticC, smecticBhex, smecticI,smecticF are smectic liquid crystals, while
crystal B, E, G, H, J and K are crystal phases.
Figure 1.6 Sub
group of four smectic phases
If a compound exhibits both the nematic and
smectic phases, the nematic phase always appears on higher temperature side.
Some of the more common smectic phases are discussed below.
Smectic A
In the smectic A phase, the molecules are
upright in each layer with the molecular centres irregularly spaced in a
“liquid-like” fashion, i.e., the molecules are parallel to one another and are
arranged in [19-20] layers with molecular long axis normal to the layer planes
as shown in Figure 1.7.The inter layer attractions are weak as compared to the
lateral force between the molecules and in consequence the layers are able to
slide over one another relatively easily. Hence the smectic mesophase has fluid
properties, though it is more viscous than the nematic phase. The molecules are
free to rotate about their long axes. Since smectic A has infinite fold
rotational symmetry about the axes parallel to the layer normal, so this
mesophase is optically uniaxial. Thesmectic A layer spacing (d) is
close to full length (L) of the constituent molecules.If the molecules have
strong longitudinal dipole moment, anti-parallel nearest neighbor correlations
will result in the modification of the structure of smectic A [21-22] phase.
Figure1.7 Picture of the smecticA phase
The smectic A (SmA) phase can be further
divided into sub-phases viz.,Smectic A1 and SmecticA2.In case of SmecticA1,which is a conventional smectic A phase ,the molecules
have random head to tail orientations, where as smecticA2 is a bi-layer phase with anti-ferroelectric
ordering of constituent molecules .Smectic Ad [21-22] is a semi-bi-layer phase with partial molecular overlapping
due to associations and smectric A is defined as ribbon or
anti-phase as shown in Figure 1.8.
Figure 1.8 Sub-phases
of smectic A phase
The sm
phase has a modulated ferroelectric ordering
of the molecules within the layer giving a ribbon like structure .Polymorphism
of smectic A phase have been discussed in detail by several authors [23-25].
Biaxial smectic A (smectic Ab) phase has also been reported [26-27].
Details of the above topics have not been dealt with in this presentation .
Smectic C
The structure of the smectic
C phase
is closely related to that of
the smectic A phase .The
molecules are arranged in layers but tilted [28-29] with respect to layer
normal i.e., with the long axis to the layer normal as shown in Figure1.9. The angle of tilt may remain constant or may vary with temperature. The centers of
gravity of the molecules are positioned randomly and the molecules are free to rotate around their long axes.
Figure1.9 Picture of the smectic C phase
The layer thickness is
significantly less than the molecular length and depends on the angle of tilt.
The layer thickness is d = Lcosβ, where L is the molecular length and β the angle of tilt. Due to the tilt of the director, the smecticC phase is optically biaxial.
There is no long range
positional ordering of the molecules though orientational ordering appears to
be long range. Depending on the nature of the angle of tilt and its variations
with temperature , de Vries [30] has subdivided the smecticC phase into various
categories. The different categories except for the smectic* phase not been
discussed here.
Smectic C*
There are chiral forms
of smectic phases which are similar to chiral nematics. The molecular arrangements in a chiral smecticC
phase, denoted by smectic C* is depicted in Figure 1.10. In this case the tilted director rotates from
layer to layer forming a helical structure. This helix may be suppressed by
placing the liquid crystal in a cell where the material is sandwiched between
two glass plates. Such systems are defined as surface stabilized system.
In this phase, the tilt direction of the
mesogensrotates along the layers. In the figure, the different layers
are coloured differently for convenience. The diagram to the far right shows
the twisting of the mesogens and emphasizes the chiral twisting of the
director.
Figure 1.10 Molecular
arrangements of smectic C* mesophase
Smectic B
The molecules of this type
of liquid crystal are free to rotate about their long axis, however the
rotational freedom of their short axes are restricted due to the close packing
of the molecules[31].The structure of
this mesophase may be optically uniaxial or biaxial.There is no long
range translational order in the direction of the molecular axis.
There are two kinds of
smectic B: hexatic smectic B and crystal smectic B. In case of the hexatic
smectic B phase, the in-plane positional order is short-range, but the bond
order is long range whereas in case of the crystal smectic B phase long range
positional order exists within the layers, but the layers are weakly attached
to each other and very weak forces are able to impose plastic deformations
(Figures 1.11 (a) and (b)).
The
smectic D phase has a cubic structure [32-33]and
would appear to be an exception to the
rule that the smectics have layered structure, in fact the smectic D is no
longer considered to be liquid crystalline.
Figure1.11(a) Schematic structure of smecticB phase
Figure1.11(b) Schematic structure of crystalB phase
There are other smectic phases such as
smecticI, smecticF,smecticG and smecticH etc. Some of them are discussed
below. The hexagonal nature of the
smecticB phase generates two tilted analogues called smecticI and smectic F
phases where molecules are tilted.
Smectic I
In this phase the positional ordering of molecules is short
rangein nature [34-37].Out of plane correlations between the molecular
positions are weak and smectic I phase has long range bond orientational
order.The tilt in molecular long axis in smecticI phase is directed towards an apex of the
hexagonal packing net. The structure of smectic I phase is shown in Figure 1.12.
Figure 1.12 Structure
of smecticI phase
Smectic F
The smectic F phase is
observed to be very similar to smecticI phase[35].In the smectic F phase the
tilt direction is directed towards an edge of the net as shown in Figure1.13.The
smecticI andsmectic F differ in the extent of in-plane ordering, the smectic
F phase has slightly longer correlation length than smectic I phase [38-39].
Crystal B
The molecules in crystal B phase are arranged in layers with the long
axis normal to the layer planes [40-41].The molecules have long range translational order and long range bond
orientational order. X-ray studies indicate that the molecules are not
laterally separated to allow free re-orientational motion about their long axis
to occur. The inter-layer stacking of molecules in crystal B shows some variations
in the inter-layer stacking. Transitions in different packing structures can
occur with variation of temperature.
Figure
1.13 Picture of phase
Structure of smecticF
Figure1.14 Molecular arrangements in crystal E phase
Crystal E
The
molecules are arranged with their long axis normal to the layer planes. They
are arranged locally in orthorhombic array
[42].Bi-layer structure of
crystalE is also available [43].The rotation of the molecules
about their long axis is not free and therefore the molecules
are arranged in herringbone array. The molecules have long range periodic order both inside the layers and out
of plane.
Crystal J and Crystal G
The crystal J and crystal G
phases are the crystalline analogues of smectic I and smecticF phases. The
molecules in crystal J phase are
arranged in layers with their molecular long axes tilted relative to their
layer planes and posses their long range positional ordering within the layers
and also between the layers. The molecules are packed in a pseudo-hexagonal
structure when viewed down the tilt direction. CrystalG phase has similar structure as the crystalJ phase except that the tilt of the moleculesis
directed towards the edge of the hexagonal packing array where in J phase the tilt is towards
the apex of the hexagonal net [44-45].
Crystal H and Crystal K
In the crystal H and crystal
K phases the molecules are arranged in layers and there is a long
range positional ordering inside and between
the layers. In crystal H phase the molecular packing is monoclinic with the tilt towards the short
edge of the packing net whereas in the crystal K phase
the tilt is towards the longer
edge of the packing array.
The crystal H and K phases are analogous to the crystalE except that the molecules are
tilted with respect to the layer planes. Variants of H
and K phases are possible with the tilt making an angle with edges of
monoclinic unit cell[46].
The crystalJ,
G, E, K and H phases are essentially crystalline. However ,the molecular
dynamics are quite different from
those observed in crystals, e.g,the molecules undergo rapid re-orientational motion about their long axes. The rapid re-orientational
motionof the molecules about their long
axes has led to describe these phases as soft crystals.
Table
1.1 Progression of order from nematic to
smectic phases
|
Isotropic
|
|
|
Nematic
|
Molecular Orientational order
|
|
smecticA
|
smecticC
|
Positional order normal to
layer
|
|
hexaticB
|
crystalF
|
smecticI
|
Bond orientational order
|
|
crystalF
|
crystalG
|
crystalJ
|
Positional order within
layer
|
|
crystalE
|
crystalH
|
crystalK
|
Asymmetrical axial site
symmetry
|
1.4
Ferroelectric Liquid Crystals
Different types of smectic crystal mesophases
are found e.g.,SmC*, SmI*, SmF* and crystal smectic phases J*,G*,K* and
H*. The constituent molecules of these phases
are chiral and their long axes are tilted with respect to the layer planes. Chiral
compounds with tilted structures exhibit ferroelectric properties .Intensive investigations have been carried out on ferroelectric liquid
crystals over last two decades for applications in fast switching flat panel
displays or optical light modulators. Excellent review articles, books and
monographs [47-52] are available on ferroelectric liquid crystals.
The mostcommonly exhibited tilted chiral smectic
phase is SmC*.Due to its reduced symmetry the phase can be spontaneously
polarized.
There are other chiral smectic phases viz.,
anti-ferroelectric chiral smectic C (SmC*anti) phase and the
ferroelectric chiral smectic C (SmC*ferri) phase.The structures of
SmC*,SmC*antiand SmC*ferriare depicted in Figure1.15.In
SmC*antiphase,the constituent molecules have tilted lamellar
structure of the ferroelectric SmC* phase, but the tilt direction of the
molecular long axes alternates from layer to layer. The spontaneous
polarization of SmC*anti is zero.
SmC*ferri phase has alternating tilted
structure, however the alternation is not symmetrical and more layers are
tilted in one direction than the other and generates spontaneous polarization, the magnitude of
which depends on the degree of alternation of tilt direction.
Figure 1.15
Pictures of different phases of ferro, ferriand anti-ferro-electric
phases
1.5 Columnar Phases
The columnar liquid
crystal phases or generally referred to as discotic mesophase was first observed by Chandrasekhar et al[53].Columnar liquid crystal
phases are different from the other
previous types of liquid crystals
because the molecules are shaped like discs instead of long rods. These
mesophases are characterized by stacked
columns of molecules. The columns are packed together to form a two dimensional
lattice. The arrangement of the molecules within the columns themselves leads
to new mesophase. The columnar liquid crystal phase is shown in Figure1.17.
Figure 1.17
Pictures of columnar phases of
liquid crystal
On the other hand, the nematic phase has an
orientationally ordered arrangement of the discs without any long –range
translational order. Unlike the classical nematic of rod-like molecules,this
mesophase is optically negative. A cholesteric or twisted nematic phase has
also been identified. The polymorphism in columnar phases is analogous in many
ways to the polymorphism in smectic mesophases
exhibited by calamatic materials. The picture of discotic nematic and discotic [54-59] columnar phase
is shown in Figure 1.18.
Figure 1.18 Pictures of columnar phases of (a) discotic nematic and (b) discotic columnar phase
1.6 Polymer Liquid Crystals
The molecules are
built up of a large number of chemically bonded repeating structural units in
polymer liquid crystals. These mesogenic units should be attached with
appropriate functional end groups in order to obtain a polymer. Structurally
there are two types of polymer liquid crystals: main chain polymer and side
chain polymer. The mesogenic group repeats via the linking unit in the main
chain polymer liquid crystal. The nature of the mesophase depends rather sensitively on the backbone ,the
mesogenic unit and the linking unit. When the mesogenic repeating units are rod-like,
mesophases similar to the nematic, cholesteric and smectic types of calamatic
liquid crystals are observed[60-63]. With discotic mesogenic
unit, other new kinds of mesophases are generated [64-67] viz., sandic nematic or columnar nematic phases.
1.7 Induced
Smectic and Re-entrant Nematic Phase
Binary or multi component liquid crystal mixtures
have an interesting feature of formation
of relatively more ordered phases, the
so called induced smectic phase from
components which show only nematic phase
in their pure liquid crystalline
state [68-77].The induced phases occur in binary mixtures with one
component having a strong terminal polar group and another with weak or
non-polar terminal groups. Mixtures of cyano compounds have been found to
induce smectic Cphase[78] .
Cladis [79]
observed a counter phenomenon where the less ordered nematic phase
reappeared at a temperature lower than the more ordered smectic phase
in binary mixtures of certain mesogenic compounds containing a
terminal cyano group. This newtype
of nematicphase is known as re-entrant nematic phase[80].Re-entrant nematic
phase has also been observed in pure compounds containing a terminal cyano group, at normal atmospheric pressure [81-84].
1.8 Liquid
Crystals, their Importance and their
Applications
The special properties of liquid
crystals - having their molecules arranged in order, but still being able to
flow like a liquid have been used to advantage in the development of many
technological devices.
In recent years the interest
in the field of research in liquid crystal has enhanced remarkably due to its
immense possibilities for practical applications as well as for its role in
biological systems. These materials have unusual structure, physiochemical and
thermo-physical properties and the dependence of their properties on
temperature for the rmotropic mesogens forms an interesting study to all
research workers in the field of liquid crystals.
Presently research on liquid
crystal is being conducted amongst other reasons, also for the development of
visual display devices based on various electro-optical effects. The main
advantage of liquid crystal display over other types of display devices is that
they operate at low voltage and therefore input power consumption is relatively
low. Fast switching time, wide-angle viewing, large area display by small
volume, design flexibility, high resolution, wide temperature range, high
contrast ratio, lower weight and variety in working material are special
advantages of liquid crystal displays[85-86].
An LCD consists primarily of
two glass plates with some liquid crystal material between them. There is no
bulky picture tube. This makes LCDs practical for applications where size (as
well as weight) are important. From the thin film wrist watch and pocket
calculator to an advanced computer screen, this type of display has evolved
into an important and versatile “interface”.
Picture of L C D
In general, LCDs use
much less power than their cathode-ray tube (CRT) counterparts. Many LCDs are
reflective, meaning that they use only ambient light to illuminate the display.
Even displays that do require an external light source (i.e. computer displays)
consume much less power than CRT devices.
Liquid crystal
displays however have some drawbacks, and these are the subject of
intense research currently. Problems with viewing angle, contrast ratio, and
response time still need to be improved. However with the rate of technological
innovation, this day may not be too far into the future, when above mentioned
drawbacks will be removed.
Picture of Liquid
Crystal Thermometer
Another type of liquid
crystalline property that has been put to use is in the working of liquid crystal thermometers. Chiral
nematic (cholesteric) liquid crystals reflect light with a wavelength equal to
the pitch. Because the pitch is dependent upon temperature, the reflected colour
also is dependent upon temperature. Liquid crystals make it possible to
accurately gauge temperature using
colour as an indicator. By mixing
different liquid crystal compounds, a
device for measuring practically any
temperature range can be developed.
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.
Optical imaging and
recording are other applications of
liquid crystals. In this technology, a liquid crystal cell is placed between
two layers of photoconductor. Light is applied to the photoconductor, which
increases the material's conductivity. This causes an electric field to develop
in the liquid crystal corresponding to the intensity of the light. The electric
pattern can be transmitted by an electrode, which enables the image to be
recorded. This technology is still being developed and is one of the most
promising areas of liquid crystal research.
Picture of optical
image by Liquid Crystal
Liquid crystals 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 may be measured.
Liquid crystalline
solutions with non-mesogenic guest molecules are easily oriented in electric or
magnetic field as a result of which bulk samples of highly oriented solute
molecules can be prepared. Liquid crystals are therefore used as solvent for
organic molecules in nuclear magnetic resonance (NMR) measurements and infrared
spectroscopy. Liquid crystals are also used as solvent in gas-liquid
chromatography (GLC) for the separation of two geometric isomers such as m- and
p-xyline.
Liquid Crystal can be
used as a smart window . When liquid crystalline molecules are not arranged in
regular pattern then light can not pass through it and it behaves like an
opaque object. The same object with proper arrangement when powered by
electricity then molecules can arrange in regular pattern and light can pass
through the liquid crystalline material and sample becomes transparent .
Liquid crystals are
also fundamentally important to life. DNA and cell membranes have liquid
crystal phases.
Properties of liquid
crystals may be modified or tailored as per requirement by synthesizing
mixtures of liquid crystals. This is the new window of investigation where we can manufacture a material which
exactly fill our demand.
References
[1] F.Reintizer, MonatschChem., 9,421 (
1888).
[2] F.Reintizer, “History of Liquid Crystals”, Ann
Physik, 27, 213(1908).
[3] G.Friedel,AnnPhysique.,18,273 (1922).
[4] G.Friedel,“Colloid Chemistry”,Ed.J.Alexandee,The
ChemicalCatalogue Company,Inc.,New.York.,Vol 1,p.102ff.(1926).
[5] D.Demus,
H.Demus and H.Zaschke, “Flussige Kristalle
in Tabellen”, VEB DeutscherVerlag
fur Grundst off industrie, Leipzig(1974). D.Demus and H.Zaschke, “Flussige Kristalle in Tabellen”, Vol.II, VEBDeutscher Verlag fur Grundst off
industrie, Leipzig(1984).
[6] V.Vill,“Liq. Cryst.Database”,Ver.4.4, LCI
Publisher, GmbH, Humberg (2003).
[7] D.Demus,
Mol. Cryst. Liq.Cryst., 364, 25(2001).
[8] P.G.de
Gennes, “The Physics of Liquid crystals”, Clarendon Press, Oxford (1974).
[9] S.
Chandrasekhar , “ Liquid Cryststals”, 2nd Edition, Cambridge
University Press, Cambridge (1992).
[10] G.W.
Gray and
J.W. Goodby, Eds., “Smectic Liquid
Crystals”,Leonard-Hill,London (1984).
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