Introduction and historical overview What are Liquid Crystals? · Introduction and historical...
Transcript of Introduction and historical overview What are Liquid Crystals? · Introduction and historical...
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
Introduction Chapter-1
22
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.
Introduction Chapter-1
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).
Introduction Chapter-1
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)
Introduction Chapter-1
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
Introduction Chapter-1
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.
Introduction Chapter-1
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].
Introduction Chapter-1
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
Introduction Chapter-1
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.
Introduction Chapter-1
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.
Introduction Chapter-1
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
Introduction Chapter-1
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
Introduction Chapter-1
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.
Introduction Chapter-1
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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.
Introduction Chapter-1
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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.
Introduction Chapter-1
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.
Introduction Chapter-1
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].
Introduction Chapter-1
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].
Introduction Chapter-1
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
Introduction Chapter-1
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
Introduction Chapter-1
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
Introduction Chapter-1
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
Introduction Chapter-1
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
Introduction Chapter-1
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 anti- ferroelectric
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
Introduction Chapter-1
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-
Introduction Chapter-1
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-
Introduction Chapter-1
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
Introduction Chapter-1
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.
Introduction Chapter-1
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).
Chapter-4 Synthesis, characterization and mesophase behavior of new liquid crystalline
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).
Introduction Chapter-1
50
Chapter-5 Synthesis, characterization and mesophase behavior of new liquid crystalline
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).
Introduction Chapter-1
51
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