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CHAPTER – 2

LIQUID CRYSTALS

2.1 INTRODUCTION

Ordinary fluids are isotropic in nature. They have the same properties optically,

magnetically, electrically, etc. in all directions. Although the molecules which comprise

the fluid are generally anisometric in shape, this anisometry generally plays little role in

anisotropic macroscopic behavior (aside from viscosity). Nevertheless, there exists a

large class of highly anisometric molecules which gives rise to unusual, fascinating, and

potentially technologically relevant behavior. Liquid crystals are composed of moderate

sized organic molecules which tend to be elongated.

A liquid crystal molecule is often pictured as a rod. This view will often provide

important qualitative information about the macroscopic behavior of the system. Liquid

crystals exhibit a state of matter that is intermediate between the solid phase and the

isotropic liquid phase. The dual characteristics of liquid crystals exhibit flow properties

of ordinary liquids and anisotropic properties of solid crystals. Liquid crystals were first

observed by Reinitzer 19-21 in 1900. Liquid crystals are anisotropic materials and the

physical properties of the system vary with average alignment of the molecules.22-28The

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material is said to be isotropic when there is no alignment of molecules and is said to be

anisotropic when they are aligned. The melting heat energy required from solid to

liquid crystal is ten times the melting heat energy from liquid crystal to isotropic

liquid.22-26

Liquid crystalline phases are named according to their degree of molecular

ordering.29-30 Liquid Crystals are broadly classified as Thermotropic and Lyotropic.

Thermotropic liquid crystals are classified into Nematics, Cholesteric, Smectics,

Discotics 31 (or Canonic) and Bowlic 32. One type of liquid crystal molecule can

exhibit many different liquid crystal phases. The phase in which a pure liquid crystal

(with only one type of molecule) exists depends on the temperature. Pure liquid

crystals, or mixtures of them, in which the phase is controlled by temperature are called

thermo-tropic liquid crystals. The Brownian motion of the molecules increases with the

temperature, reducing the order in the material. At high temperature, orientation order is

lost and the material changes to the isotropic phase. When the temperature is decreased,

the material changes to the nematic phase. The temperature, at which the phase

transition occurs, is specific for each material and is called the nematic-isotropic

transition temperature or clearing point. By further lowering the temperature, the phase

can change to the smectic A and C phases and finally to the solid state. Each of the

phase transitions occurs at a specific temperature, but depending on the material,

additional phases can appear or some can be missing.

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Isotropic Phase

The molecules are randomly aligned and exhibit no long range order.

The isotropic phase has a low viscosity and will often appear to be crystal clear.

There is no long range positional or orientation order of the molecules, although this

sort of order may exist on very short length scales of order tens of Angstroms,

corresponding to a few molecular distances. For all practical purposes, the isotropic

phase macroscopically appears to be like any other isotropic liquid such as water.

Nematic Phase

The molecules in the nematic phase are oriented on an average along a particular

direction. In consequence, there is a macroscopic anisotropy in many material

properties, such as dielectric constants and refractive indices etc. This is the phase

which is used in many liquid crystal devices (e.g., the "twisted nematic" cell), because

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the average orientation may be manipulated with an electric or magnetic 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. Nematic liquid crystal media have

uniaxial symmetry, which means that in a homogeneous liquid crystal medium, a

rotation around the director does not make a difference. The bulk ordering has a

profound influence on the way light and electric fields behave in the material. Uniaxial

anisotropy results in different electrical and optical parameters, if considered along the

director or in a plane perpendicular to it. This gives rise to interesting technological

possibilities like reorientation of the molecules in an electric field and change of optical

birefringence of the molecules.

Smectic A Phase

The Smectic-A phase like the nematic, exhibits long range orientational order of

the molecules. In addition, it exhibits a layer like structure in one dimension, and thus is

often considered a two dimensional liquid (freedom of molecular motion within the

layer) and a quasi one-dimensional solid (hindered translation from one layer to the

next). The viscosity is rather high and this phase is generally not useful for electro optic

devices.

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Smectic C Phase

In this phase the molecules are tilted with respect to the layers, and the system is

now "biaxial" in character. The SmC phase has a two- fold symmetry axis perpendicular

to the tilt plane, and a mirror plane parallel to the tilt plane giving C2h symmetry, which

is non polar.

2.2 Ferroelectric LCS- Smectic C* (Chiral)

If the molecules are chiral, (lack inversion symmetry), Meyer, et. al1. 33

demonstrated on symmetry grounds that a polarization must exist parallel to the smectic

layers and perpendicular to the molecules. The magnitude of the polarization is

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determined by molecular considerations, although its existence depends solely on

symmetry. These materials can be used in rapidly switching electro optic shutters, with

response times in the microsecond range. The SmC* is a Polar Liquid Crystals.

Molecules with polar symmetry can exhibit a dipole moment.

2.3 Anti ferroelectric LC’s

Anti ferroelectric liquid crystals are similar to ferroelectric liquid crystals,

although the molecules tilt in an opposite sense in alternating layers. 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."

Ferroelectric and antiferroelectric 34properties in liquid crystals were usually

found in the chiral molecular systems. Until recently, chirality was considered to be

necessary to produce polar order in each layer, since the removed symmetry of mirror

planes is responsible for the genesis of ferroelectric and anti ferroelectric ordering.

However, Niori .et al.35 discovered that achiral bent-shaped molecules (so called banana

molecules) can also form polar smectic layers and exhibit ferroelectric and anti

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ferroelectric behavior with electro-optic switching although the molecules themselves

are not chiral. Since this discovery, a number of achiral bent shaped molecules have

been reported 36-40 and bent-shaped molecules with chiral terminal groups have also

been studied extensively in order to investigate the relationship between the terminal

chain and phase structure.41-44 One of the most widely investigated bent-shaped

mesophase is the B2 phase because of its polar switching.44 In the B2 phase, the

molecules are tilted from the layer normal, resulting in unique layer chirality.45

Depending on tilt and polar correlation between adjacent layers four phase structures

differing in chirality and polarity are formed.43 These are distinguished using the

nomenclature SmC S, AP F,A. Here, the first two subscripts, S and A, specify

synclinicity and anti clinicity, and second two subscripts, F and A, specify ferroelectric

and anti ferroelectric respectively. Moreover, in accordance with the switching current

measurement of these molecules most of them exhibit the anti ferroelectric meso phase

at the ground state.

2.4 Banana-shaped Liquid Crystals

Bent-core liquid crystals were synthesized by Vorlander 46 in 1930’s. But, they

were not recognized as interesting materials for a long time, since they are bad

molecules for liquid crystals. Matsunaga et al. 47 synthesized the bent-core mesogens in

1993, but reported only the mesomorphic properties. Moreover, at the same time, Cladis

48 and Brand displayed the model structure of SmCP, which has C2v symmetry and is

similar to the B2 phase realized in Banana shaped mesogens. 1996 Takezoe and

Watanabe et al. report these "banana-shaped" molecules produce ferroelectric phases,

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starting a wave of banana mania in the FLC community 48-49. In 1997 the Boulder

Group proposed the chiral SmC P layer structure for the B2.

Banana-shaped liquid crystals are the first ferroelectric and anti-ferroelectric

liquid crystals, which contain no chiral carbon. Banana-shaped liquid crystals introduce

chirality to the systems, although they have no chiral carbon, particularly in the B4-like

phase. Banana-shaped mesogens show not only the highest second-order nonlinear optic

susceptibility in liquid crystals but also large chiral nonlinear optic effect such as chiral

Pockels (electrogyration) effect and SHG-CD (PRL).

The main feature of molecules such as their symmetry C2v in this case and

therefore its polarisation is in the direction of the C2 axis. Subsequently interchangeable

ferroelectric states may be induced. Banana-shaped LCs have a much faster switching

time, as these types of LCs can re-orientate in an electric field, not by a 900 turn, but, by

processing around an axis to realign themselves (figure- 10c). This process does not

require as much energy and can occur much faster.

Banana shaped liquid crystals

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N

R O

N

OO

OO Banana shaped liquid crystal molecule

C2VR O Bend unit

Terminal chain

Linear rod like unit

Bent-core liquid crystals

Discotic phase

Discotic system can be made chiral by incorporating a chiral unit into one or

more of the peripheral units that surround the discotic core. This compound exhibits

solely a chiral nematic discotic phase (ND*) phase because the steric effect of the

branched chains at the chiral centre disrupt the ability of the molecules to pack in

columns. The liquid crystal tendency depends critically on the type of chiral peripheral

chain.

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Discotic shaped liquid crystal molecule

Ferro electricity, resulting from a spontaneous macroscopic electric polarization,

is a property which was first reported by Meyer 33 to occur in a fluid, liquid crystalline

phase. Until recently, Ferro electricity in liquid crystals was based on a tilted

arrangement of homo chiral molecules in layers (e.g. smectic C phase) which generates

C2v symmetry and allows the occurrence of a spontaneous electric polarization. In

recent years such ferroelectric liquid crystals have attracted considerable interest

because of their unique switching properties and their technical applications, for

example, in fast-switching electro-opticaldevices.50 As predicted by theory, Ferro

electricity is not restricted to chiral tilted phases.51–53 In 1996 Niori et al.54 reported on

Ferro electricity in a smectic phase formed by bow-shaped (‘banana-shaped’) non-chiral

molecules. Later on, anti ferroelectric switching behavior was found for this compounds

55-57.Not only is the special electro optical behavior of these non conventional Liquid

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crystals of interest. But these molecules also represent a new subfield of thermo tropic

liquid crystals, different from the classical types such as calamitic and disc-like

mesogens. Till now, all banana-shaped liquid crystals which exhibit (anti)ferroelectric

switching behavior have had a rather uniform structure, and they usually comprise 1, 3-

phenylene bis’ benzoates incorporating at least one Schiff-base unit 54-57. Therefore, a

major drawback of these compounds is their limited thermal, hydrolytic and

photochemical stability. Further more, these special mesophases occur at rather high

temperatures.

Splay-cell Anti-parallel rubbed twisted nematic

2.5 Calculation of the director pattern in a liquid crystal medium

A liquid crystal medium prefers a uniform director distribution. A variation of

the director in space induces an increase of the free energy. According to the elastic

theory for liquid crystals, the elastic energy related to the variation of the director 'n' in

space can be written as

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With the three elastic constants k11, k22 and k33. This equation is known as the

Oseen-Frank distortion energy. The three terms in the equation are related to distortion

due to splay, twist and bend respectively as illustrated in the figure below. General

deformations are a mixture of these three types.

Calculations of the equilibrium director distribution involve minimizing the total

free energy of the volume. The total energy of a liquid crystal is made up of three

components: the elastic energy density (as described above), the interface energy

related to the alignment of the director at the surfaces of the considered volume and the

electric energy density

related to the interaction of the applied electric field and the director of the liquid crystal

molecule.