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LIQID CRYSTALS
What are liquid crystals?
There are three common states of matter, solid, liquid and gas- are different because the
molecules in each state have a different degree of order.
Crystalline solids
 Possess rigid arrangement of molecules
 Stay in a fixed position and orientation with small amount of variation from
molecular vibrations
 There are large forces holding the molecules in place
 Solids are difficult to deform
Liquid phase
 Molecules lack both position or orientation
 They are free to move in a random fashion
 Liquid phase has less order than the solid phase
 The intermolecular forces are only strong enough to keep the liquid molecules
fairly close together
 Liquids can be easily deformed
Liquid crystals
A liquid crystal is a fluid like a liquid, but is anisotropic in its optical and electromagnetic
characteristics like a solid. When the liquid crystal is formed from the isotropic state
some amount of positional or orientation order is gained. It is this order that accounts for
the anisotropies of the substance
Thus liquid crystal may be described as the distinct states of matter in which the degrees
of molecular ordering lie intermediate between the ordered crystalline state and the
completely disordered isotropic liquid.
In a liquid crystal the molecules possess orientation order, i.e., the molecules tend remain
oriented in a particular direction. The direction of preferred orientation in a liquid crystal
is called the director. In a liquid crystal phase they spend more time along the director
than in any other direction
Classification of liquid crystals
Liquid crystals are classified into two main categories`
1. Thermotropic liquid crystals
2. Lyotropic liquid crystals
Thermotropic liquid crystals are those, which exhibit liquid crystalline properties as the
temperature is varied. The liquid crystal to liquid transition and liquid-to-liquid crystal
transitions of a mesogenic material are essentially reversible. The mesophase which are
formed by both the heating and cooling cycles are thermodynamically stable are called
enatiotropic phases. But in some compounds, their thermotropic phases appears only
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during the cooling phase from the isotropic phase, but not on the heating process. These
mesophases, formed by the super cooling of the material below its melting point, are
meta-stable and are called monotropic phases.
Thermotropic liquid crystals can exist in three phases
1. Nematic phase
2. Cholesteric or chiral nematic phase
3. Smectic phase
Isotropic
Nematic
Smectic
The nematic phase
They exhibit long range molecular ordering but possess no positional ordering. The
preferred direction varies from point to point in the medium, but there is a uniform
alignment with respect to the long axis- director. The nematic phase is bifringent due to
the anisotropic nature. They also exhibit dielectric anisotropy
The cholesteric phase
In this structure the local molecular ordering is identical to that of the nematic phase, but
are exhibited my molecules possessing chiral centers. Chiral nematic liquid crystals are
also refereed as twisted nematic liquid crystals. Unlike in nematic phase where all the
molecules stay parallel to one another, in chiral nematic phase, the molecules arrange
themselves in such a way that a group of molecules align at different angles with respect
to their adjacent groups i.e., the director is not fixed in space, but rotates through out the
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sample forming a helical pattern as it changes its direction. The distance traveled by the
director as it completes one full turn is called the pitch of the liquid crystal. The twist
present in chiral nematic crystal makes them to exhibit spectacular optical properties,
which is made use of in displays. The most striking features of cholesteric mesophase is
strong optical activity and selective light reflection. The pitch is temperature dependent
and hence cholesteric phase is finding application in thermography
The nematic phase
Substances that form smectic phases are soap-like. In smectic mesophase, there is a small
amount of orientational order and also a small amount of positional order. The molecules
tend to point along the director and arrange themselves in layers. Based on the orientation
of the director there are many types of smectic phases. The interlayer force of attraction
in smectic phase is weak as compared with the lateral forces between molecules. As a
consequence the layers are able to slide over one another.
Discotic or columnar liquid phases
Liquid crystals formed by molecules, which have disc like or plate like structures are
classified as discotic liquid crystals. The simplest discotic phase is also called the nematic
discotic phase because there is oreintational order, but they lack positional order. There is
random motion of the molecules but on an average the axis perpendicular to the plane of
each molecule tend to orient along the director. In smectic discotic or columnar phase, in
addition to the oreintational order present, most of the molecules tend to position
themselves in columns. The columns are arranged in a hexagonal lattice resembling a set
of coins stalked into a pillar
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Chiral smectic
In a similar way to chiral nematics there are chiral forms of smectic phases. The tilted
director rotates from layer to layer forming a helical structure.
Banana shaped liquid crystals
Apart from the cigar shaped molecules some more exotic shaped liquid crystals are also
reported. Below figure illustrates the stacking of banana shaped liquid crystals.
LC Phase
Nematic Phase
Example
OH
N
H3C
+
N
O
O
H3C
Cholesteric
Phase
CH3
CH3 (CH )
2 3
CH3
CH3
CH3
O
O
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CH3
(CH 2)7
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Smectic A
N
O
OEt
Smectic B
N
EtO
COOEt
COOH
Smectic C
O
C8H17-n
H37C18
Smectic D
O
COOH
O 2N
Smectic E
EtOOC
COOEt
N
Smectic F & G
O
H11C5
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C5H11
N
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H3C
Smectic H
H9C4
O
N
O
Discotic
O
R
R
O
O
O
O
R
R
O
O
O
R
O
R
O
O
Banana Shaped
molecules
O
O
O
Cl
O
N
N
Cl
HO
OH
Chemical constitution and liquid crystalline behaviour
Liquid crystalline behaviour depends on the rigidity at the center position of the
elongated molecule. If the molecule lacks rigidity, bending may occur along its lengths
destroying parallel arrangement of molecules resulting in no crystal formation
Long chain n-alkanes have elongated conformations, but the flexible alkyl chain can coil
and bend and hence no liquid crystal phase can be formed. On the other hand introduction
of a double bond makes the formation of mesophase molecules as the conjugates part
makes the molecule rigid
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E.g.: introduction of alternative double bonds
Aromatic compounds are polarizable, have a planar structure and are rigid. Introduction
of alkyl groups in the suitable positions make the molecule elongated and hence such
compounds can exhibit mesophase. The requirement for liquid crystal formation in
compounds with benzene rings is
1. The central part which links the benzene nuclei should itself be rigid
2. The entire molecule (i.e., the two rings and the central part) should constitute a
rod like structure
The central group may also involve a ring formed by dimerization through hydrogen
bonding, which makes it rigid.
HO
O
H3C
CH3
(CH 2)y
(CH 2)x
OH
n-alkanoic acid
O
mesophase not possible
H3C
O
CH
OH
C
C
OH
O
CH
CH3
Introduction of alternative double bonds make the molecule rigid
If the benzene rings are linked through para subtituents, mesophase formation is
favoures. But linkage through meta or ortho positions is not favorable. However,
introduction of an additional meta or ortho substitution to an existing para substituted
molecules favours mesophase formation.
The most common rigid cores, bridging groups and terminal groups found in rod-like
mesogens are given below
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R
X
A
B
Z
A & B HEXANE or AROMATIC RING
R
ALKYL SIDE GROUP
Z
LINKAGE GROUP
X
TERMINAL GROUP
Liquid crystalline behaviour in homogeneous series
A series of compounds of the same type wherein successive members differ in their
formula by a –CH2 group are called homologues series. The thermal stability of the
molecules of members of a homologues series changes with the number of carbon atoms
in the chain
PAA homologues series
The liquid crystal phase of p-azoxyanisole (PAA) is stable between 118 to 1350C. PAA
has more than 12 homologues, which are formed when –CH2 groups are added to its side
chain.
O
R
O
N
N
R
O
It can be seen that in general the transition temperature of the liquid crystal decreases
with the increase in the number of carbon atoms in the side chain. However, there is an
odd-even distribution of transition temperature and was found to decline with increase in
number of carbon atoms. When the alkyl group side chain contains one to six carbon
atoms liquid crystals show a nematic phase and when the number of carbon atoms is
greater than six, smectic phase is observed. Homologues containing seven and eight
carbon atoms, however, show a transition from solid to smectic to nematic before melting
to a liquid.
The high transition temperature of PAA series owing to the alkyl side group linkage
through oxygen atom makes them unsuitable for display applications. If alkyl groups are
attached directly to the benzene, p-alkylazoxybenzene series with lower transition
temperature is obtained.
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Another example of liquid crystal homologues having a low transition temperature is pmethoxybenzylidine, p-n-butylaniline (MBBA) series.
Altering the flexible side chain can alter the transition temperature of liquid crystal phase.
Biphenyl and terphenyl systems carrying highly polarizable groups such as nitro and
cyano groups are also show lower transition temperatures.
Elelctrooptical effects in liquid crystals
Nematic liquid crystals are the simplest forms with rod like molecular structure and align
themselves spontaneously along the director
Dielectric anisotropy () is defined as the difference between the dielectric constant
parallel and perpendicular to the director. The optical anisotropy (n) is defined as the
difference between the refractive indices parallel and perpendicular to the director. These
two properties are important for the electro-optic effects in liquid crystals
Effects of electric fields
The director in a liquid crystal is free to point in any direction. But when a film of liquid
crystals is placed between two plates of certain materials, the director is forced to point
along a perpendicular direction. When a thin film of liquid crystal is sandwiched between
two glass plates, the molecules close to the glass surface are forced to orient themselves
parallel to the surface of the glass sheets.
Before sandwiching
After sandwiching
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In the absence of an electric field (below a threshold value) the directors at other layers
are also aligned parallel to the surfaces giving a homogeneous texture. But when an
electric field is applied perpendicular to the molecules near the center of the crystal orient
themselves along the applied field. The deformity begins at a threshold vale of the
strength of the applied field and increases with increase in the strength of the field. This
deformity brings about a significant change in the optical characteristics of the liquid
crystals and are made use in liquid crystal displays
Original orientation
Situation in electrical
field
Result electrical field
Result strong electrical
field
Effects of light
When light is incident on two crossed polarizes no light emerges as the light coming out
of the first polarizer is completely absorbed by the second polarizer and hence appears
dark. When a pair of crossed polarizers is filled with a twisted nematic liquid crystal
having a positive dielectric anisotropy, the twisted structure acts like a wave-guide and
gradually rotates the plane of polarization of light by 90o . Hence a linearly polarized light
incident on the cell emerges linearly polarized but in an orthogonal direction resulting in
a bright appearance. The 90o twist in the cell is lost when a sufficiently strong electric
field is applied to the cell. Hence the cell appears to dark between the two crossed
polarizers. However the following conditions should be met to see the elelctrooptic
effects
(i)
The plane of polarization of the incident light should be parallel or
perpendicular at the surface of the cell
(ii)
The product of the optical anisotropy, n and the pitch P should be greater
than the wavelength of the incident light
n x P  
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Applications of liquid crystals
(i) DISPLAYS
All information displays utilizes the ability to control light, in order to function. By
controlling what parts of a display are bright and what parts are dark, information is
passed to the user.
Regardless of the complexity of the display, the basic working
principle remains the control of light from small area of the display. This can be done in
two ways- active displays and passive displays
Active display
 Each area is equipped with the ability to generate light
 Different areas are made to produce light by hitting only these areas with the
electron beam
E.g.: cathode ray tube (CRT), light emitting diode (LED)
Passive display
 Does not generate light by itself
 Controls the amount of light that passes through
 Utilizes ambient light
 Do not consume electrical power in order to generate light
 Ideal for battery operated equipment
E.g.: liquid crystal display (LCD)
In LCDs in order to enhance the difference in the brightness between dark (turned off)
and bright (turned on) areas. Dichoric dyes are used to give desired coloures to the
display with a good contrast
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(ii) THERMOGRAPHY
Liquid crystals can be used to measure temperature utilizing the selective reflection
property of chiral nematic liquid crystals when a light is incident on a cholesteric
mesophase parallel to the axis of rotation of the helix, radiation of very small wavelength
range corresponding to
n x P = R
gets divided into two beams one of the beam is reflected and the other is transmitted. If
the wavelength range R is in the visible region, the reflection give rise to light colours.
The R and hence the colour of the reflected light depends on the temperature of the
mesophase. Temperature dependence of the colour reflected by liquid crystals has been
utilized for application in thermography.
(iii) DETECTION OF AIR POLLUTANTS.
It has been observed that the colour of liquid crystals changes in the presence of
impurities. When impurities diffuse in to the cholesteric liquid crystal film, the pitch is
altered and hence the colour changes. This property is used in the detection of
contaminants in the atmosphere.
(iv) Solitary wave propagation
A high intensity laser beam injected in a liquid crystal can produce a local reorientation
of the director molecules. In this way the light produces its own wave-guide and the laser
beam will not diffract but stays confined in a narrow beam. The soliton application can
lead to an addressable liquid crystal wave-guide to switch light between several optical
fibers.
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