1

CHAPTER I

INTRODUCTION TO LIQUID CRYSTALS

In this chapter, a brief introduction to liquid crystal (LC) materials and its sub

phases including ferroelectric liquid crystal (FLC), surface stabilized ferroelectric

liquid crystal (SSFLC), deformed helix ferroelectric liquid crystal (DHFLC) and

electroclinic liquid crystal (ELC) with their applications have been described in detail.

The emphasis in thesis is on the effect of nanomaterial on the dielectric and electro-

optical properties of FLCs and objectives of this work have been summarized at the end

of this chapter.

1.1 INTRODUCTION

Amazing, delicate, attractive, mysterious and dynamic are the words that describe the

rich class of liquid crystals (LCs). These materials are offering a new state (or phase)

of matter and breaking the general myth “Matter exists only in three states: solid,

liquid, and gas”. Now the immediate question comes into our mind is that “Why this

new phase of matter is designated as liquid crystal?”. The answer would not be

quickly understandable as the term ‘liquid crystal’ is intriguing and confusing in

itself. LC represents a special state of soft matter that is intermediate between highly

ordered crystalline solid and isotropic liquid phases. LCs are the anisotropic

molecules or their aggregates which demonstrate a long-range orientational order but

little or no positional order [1]. A substance in LC state is strongly anisotropic in

some of its properties and yet exhibits a certain degree of fluidity, hence, they show

the optical properties like solids due to anisotropy and flow properties like liquids.

These mesophases are mainly formed by some organic compounds with oriented

aromatic and aliphatic groups.

To explain the definition and classification of LCs, let us briefly look at the history of

their discovery. The first observation of this wonderful class of matter knocked the

door in 1850s by biologists and very soon physicists and chemists joined the fray.

German physicist Ottho Lehman and Austrian botanist Friedrich Reinitzer confirmed

the discovery in 1888 by observing the melting behaviour of an organic substance

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cholestryl benzoate, the derivate of cholesterol [2-5]. In 1900, Lehman with the

definition “It is a new state of matter possessing the optical properties like solids

and fluid properties like liquids” originated the term ‘liquid crystal’ for such phases

of matter.

These mesophases are more like liquid than a solid because the latent heat of LC to

liquid phase transition is very low, i.e., about 7 Cal/gm, while it is higher for solid to

LC, i.e., 65 Cal/gm. The less amount of latent heat in case of LC to liquid phase

transition suggests that it is more like liquid than solid. Therefore, they are named as

liquid first and crystal latter, i.e., ‘liquid crystal’ [2].

Overall, in the present scenario, the field of LCs has the huge impact on every aspects

of human life by their incredible scientific and technological developments. Recently,

LC materials having several new mesophases, dramatically influenced the other

emerging fields such as nanotechnology, nanoengineering, nanobiotechnology, etc.

and hence open several avenues to do further credential research in this field.

1.2 CLASSIFICATION OF LIQUID CRYSTALS

Since their discovery, a large number of LC composites have been synthesized and

characterized by means of physical and chemical methods. The shapes and molecular

arrangements of these LC composites are different to each other and by changing the

temperature or concentrations of constituent’s compounds or both, these LC

composites show different beautiful phases. Hence, there are two ways to categorize

the LC materials:

1) On the basis of geometrical structure of the molecules, and

2) On the basis of chemical composition.

The necessary condition for liquid crystalline properties to occur in the materials is

the geometrical (shape) anisotropy of its ingredient molecules [6]. On the basis of

geometrical structure of the molecules, they can be divided into rod like, lath like and

disc like which has shown in Fig. 1.1. The classical LCs are derived from rod like

molecules called ‘Calamitic’ (derived from the Greek word colomos = rod). At

present, there are about 20,000 rod like mesogenic compounds. This class of LC

compounds is the most important for practical applications. The LCs derived from

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disc like molecules were discovered independently by Chandrashekar et al. [7] and

Billard et al. in 1977 [8]. These were called ‘Discotic’ liquid crystals. Lath shaped

molecules are intermediate between rod and disc shaped molecules.

Figure 1.1: The schematic representation of geometrical structure of liquid

crystalline molecules: (a) Rod shape (b) Brick or Board shape (c) Bent-core

shape, and (d) Disc shape. Brick/Board and Bent-core shapes belong to the Lath

like structure.

Further, on the chemical basis, LCs can be classified mainly into two categories: (i)

thermotropic, and (ii) lyotropic. Thermotropic LCs are mesogenic compounds which

show liquid crystalline (i.e. mesomorphic) behavior in a particular temperature range.

On the other hand, lyotropic LCs show the mesomorphic behavior by changing the

concentration of the solute in a solvent e.g. water. Some compounds which are

capable to form lyotropic and thermotropic LCs are known as ‘amphotropic’

compounds [9]. Calamitic and discotic molecules are the most common forms in

which both thermotropic as well as lyotropic LCs exist [2].

1.2.1 THERMOTROPIC LIQUID CRYSTALS

The LCs, whose transitions between the phases occur due to the change in

temperature are called “Thermotropic LCs”. Table 1.1 depicts some of the important

thermotropic LC compounds discovered in the beginning.

(a) (b) (c) (d)

Disc Bent-core Brick/Board Rod

4

Table 1.1: Some important thermotropic LCs with their molecular structures

and phase transitions.

FIRST THERMOTROPIC LIQUID CRYSTAL

Line notation: Ph-COO-Cholest-5-ene

Common abbreviation: CB Phase Transitions: Cr. 150.5 N* 182.6 Iso.

FIRST ROOM TEMPERATURE NEMATIC LIQUID CRYSTAL

Line notation: CH3-O-Ph-CH=N-Ph-C4H9

Common abbreviation: MBBA Phase Transitions: Tg 72.0 Cr. 22.0 N 48.0 Iso.

FIRST FERROELECTRIC LIQUID CRYSTAL

Line notation: C10H21- O-Ph-CH=N-Ph-CH=CH-COO-CH2-CH-CH3-C2H5

Common abbreviation: DOBAMBC Phase Transitions: Cr. 76.0 C* 95 A 117.0 Iso.

43.0 H* 68.0

FIRST ANTI-FERROELECTRIC LIQUID CRYSTAL

Line notation: C8H17-O-Ph-Ph-COO-Ph-COO-CHMe-C6H13 Common abbreviation: MHPOBC

Phase Transitions: Cr.84.0 CA*18.4 Cγ 119.2 C*120.9 Cα 122.0 A148.0 Iso.

C 4H 9 N CH O CH 3

CH

N CHCH

C

O

O

CH 2

CH

CH 3

C2H5O

C10 H21

5

The term ‘thermotropic’ arises because transitions between different intermediate

submesophases of such LCs are most naturally affected by change in temperature.

Thermotropic LCs exist in both rod and disc shaped molecules. Hence, the discussion

of different meosphases is in the following sequence (i) calamitic, (ii) discotic, and

(iii) bent-core. A vast majority of them are composed of calamitic or rod-like

molecules that exhibit a variety of phases as temperature is varied. In 1922, a french

scientist named George Friedel [10-12], proposed a classification scheme based on the

molecular arrangement. Friedel proposed that each mesophase can be further

classified into different phases such as (a) nematic, (b) cholesteric, and (c) smectic

phases, depending on the temperature variation of LC system [1, 3, 10, 13-14].

1.2.1.1 CALAMITIC MESOPHASES

The LC mesophases formed by elongated or rod like mesogens are known as calamitic

LC phases. The calamitic phases are the example of building blocks with more

complicated shape, which form exotic thermotropic LC phases. The calamitic LCs have

used in the most of practical applications. A typical calamitic mesogen consists of a

rigid core unit, ensuring the anisotropic character, together with flexible side chains as

shown in Fig.1.2, provides stabilising effects within the LC phases. The calamitic

phases have shown by various organic mesogenic compounds and can be further

classified into chiral and non-chiral phases. The molecular arrangements of LC phases

having chirality are found to be totally different than that of the LC phases without

chirality. So, it is necessary to discuss both the phases in detail.

Figure 1.2: The molecular structure of calamitic (i.e., 5CB) LC.

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1.2.1.1.1 NON-CHIRAL CALAMITIC PHASES

Thermotropic LCs made of non-chiral calamitic mesogens generally show two types of

temperature-dependent phases: (i) nematic, and (ii) smectic, in between crystalline and

isotropic phases.

(i) NEMATIC LIQUID CRYSTALS

Nematic (N) is the most common and simplest LC phase. The word “nematic” is

derived from the Greek word for “Thread” as thread like textures were observed in

such compounds, under the polarizing microscope [2]. These thread like topological

defects are also called as disclination lines. Figure 1.3 shows the optical micrograph of

characteristic thread like texture of nematic phase.

Figure 1.3: Optical micrograph of nematic phase having thread like texture under

the polarizing microscope.

In nematic mesophase, the molecules maintain a preferred orientational direction (i.e.

orientation order) as they diffuse throughout the sample, but there is no translational

(i.e. positional) order as shown in Fig. 1.4. The absence of translational order can be

confirmed by X-ray diffraction in which no diffraction peak is observed in case of

nematic LC. The absence of diffraction peak in X-ray diffraction means that the

molecules do not have long range positional order, so they can move throughout the

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⟩−⟨=⟩⟨= 1cos32

1)(cos 2

2 θθPS

material. The orientational order is defined by an average direction of molecules called

the ‘director’ and is denoted by the vector n. The states of director n and –n are

indistinguishable.

Figure 1.4: The schematic representation of arrangement of molecules in nematic

mesophase. Here, θθθθ is the angle between the long axis (L) of molecule and director

(n) of the sample.

However, the orientation of individual molecules differs significantly from the direction

of director (n) and hence an order parameter (S) is introduced to characterize the

nematic mesophase. The orientational order parameter S is based on the average of the

second order Legendre polynomial [15] and is defined as:

Where, < > denotes a thermal averaging and θ is the angle between the director (n) and

the long axis (L) of each molecule. There are three cases of the magnitude of S in a

sample: (i) for S = 0, all the molecules are randomly oriented about the director (n),

i.e. isotropic liquid, (ii) for S = 1, all the molecules are perfectly aligned with director,

θθθθ

n L

(1.1)

8

i.e. crystal, and (iii) S = 0.3 - 0.9 is the typical range for nematic phase of LC, which is

the result of kinetic molecular motion [16].

The typical behavior of temperature dependent order parameter (S) of a LC material in

nematic phase has shown in Fig.1.5. Here, Tc is the nematic to isotropic phase transition

temperature. We will discuss about transition temperature (Tc) in detail in the next

chapter (Chapter II) of the thesis.

Figure 1.5: The schematic representation of temperature dependent order

parameter (S) in the nematic mesophase.

Usually, nematic LCs are optically uniaxial but recently, it has been reported that

biaxial nematics are also possible. A biaxial nematic phase Nb may result due to further

breaking of rotational symmetry of the system around the director (n) [17-19].

Temperature (T/Tc)

Nematic Phase

Isotropic Phase

Transition Temperature (Tc)

S

9

(ii) SMECTIC LIQUID CRYSTALS

The word ‘smectic’ is basically derived from the Greek word for ‘soap’. The thick and

slippery material often found at the bottom of a soap dish is actually a type of smectic

LC and considered as the origin of the term ‘smectic’ [1, 2, 20]. Molecules in this phase

show additional degrees of translational order that was not present in the nematic phase.

In the smectic phase, the molecules maintain the general orientational order of

nematics, but also tend to align themselves in layers due to translation order; with a

well-defined interlayer spacing, which can be measured by X-ray diffraction [14]. The

interlayer attractions are weak compared to the lateral forces between the molecules, so

the layers are able to slide over each-other easily. This gives rise to fluidity to the

system with higher viscosity than nematics.

A large number of smectic mesophases (such as A, B, C, D, E, F, G, H, I, J, and K)

have been observed, depending on the way of layer formation as well as arrangement of

molecules inside layers [21]. The simplest phase among all smectic phases is smectic A

(Sm A) phase. In this phase, the average molecular axis is normal to the smectic layers.

Figure 1.6: The schematic representation of arrangement of molecules in smectic

mesophases: (a) smectic A (Sm A) and (b) smectic C (Sm C). Here, n and k are the

director and layer normal, respectively, and θθθθ is the tilt angle between n and k.

n k k

θθθθ

(a) (b)

Sm A Sm C

Smectic Layers

n

10

Table 1.2: Structural classification of smectic mesophases.

No. Mesophases Molecular arrangement

1. Smectic A Liquid like layers with molecules upright on the average and

negligible in-plane and interlayer correlations. An orientationally

ordered liquid on which a one-dimensional (1-D) density wave is

superimposed.

2. Smectic B Basically similar to Sm A phase but with higher in-plane ordering.

Two distinct types of smectic had been identified: (a) Crystal B, 3-D

crystal having hexagonal lattice and weak interlayer ordering, and

(b) Hexatic B, stack of interacting hexatic layers with short-range in

plane ordering and 3-D six-fold bond orientational ordering.

3. Smectic C Liquid like layers as in Sm A but with the molecules inclined with

respect to layer normal.

4. Smectic C* Chiral Sm C with twist axis normal to the layers.

5. Smectic D Cubic lattice with micelle type molecular arrangement and such

phase is usually observed between Sm A and Sm C or between Sm

C and isotropic phase.

6. Smectic E 3-D orthorhombic crystal with interlayer herringbone structure.

7. Smectic F Titled hexatic structure with C centered monoclinic (a>b) with in-

plane. short-range positional correlation and weak interlayer positional

correlation.

8. Smectic G 3-D crystal with C-centered monoclinic (a>b).

9. Smectic H 3-D crystal with C-centered monoclinic (a>b) and having herringbone

structure.

10. Smectic I Titled hexatic C-centered monoclinic (b>a) with slightly greater in

plane correlation than Sm F.

11. Smectic J 3-D crystal with C centered mono clinic (b>a) structure.

12. Smectic K 3-D monoclinic (b>a) structure with herringbone structure.

On decreasing temperature, Sm A phase may transform into a phase possessing lower

symmetry. The breaking or lowering of symmetry may lead to the tilting of molecules

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Solid (Cryst.) H K E G J F B (Cryst.) I B (Hex.) C A

N Blue Phases (BPs) Isotropic

Smectic Phases

Nematic Phases

relative to the smectic layers and such formed phase is known as smectic C (Sm C)

phase. Figures 1.6(a) and 1.6(b) show the arrangement of molecules in Sm A and Sm C

phases, respectively. There are various other types of smectic phases which are listed in

Table 1.2.

If the above phases exist, they appear according to the sequence rule, which in

generalized form may be written as:

The above-mentioned generalized sequence rule has been derived from the behaviour of

many different compounds. However, there is no single compound, in which the

complete sequence can be realized. Therefore, this generalized sequence could be

considered as hypothetical.

1.2.1.1.2 CHIRAL CALAMITIC PHASES

Before starting the discussion about chiral calamitic mesophases, let us first preview

chiral molecule. A chiral molecule (originated from the Greek word for ‘hand’) is

basically a molecule that can not be superimposed on its mirror image because of

symmetry breaking. Chiral molecules are optically active in the sense that they can

rotate the plane of polarized light [1, 3, 22]. Importantly, chiral compounds are able to

form mesophases with structures related to those of non-chiral substances, however,

with different properties. Chiral calamitic phases can be of such types: (i) non-

ferroelectric or chiral nematic (ii) ferroelectric, and (iii) antiferroelectric and

ferrielectric phases.

(i) NON-FERROELECTRIC OR CHIRAL NEMATIC PHASES

When a chiral molecule is incorporated into the nematic mesophase, the complete

structure can be visualized as a stack of very thin 2-D nematic layers with the director

in each layer twisted with respect to consecutive layers [23]. This further induces a

12

helical director configuration in which the director rotates throughout the sample and

hence termed as twisted nematic (Nt) or chiral nematic (N*). Since this mesophase was

invented in derivatives of cholesterol, therefore are also known as ‘Cholesteric Liquid

Crystals’ [1]. An important characteristic of the cholesteric mesophase is its pitch (p),

which can be defined as the distance travelled by the director to complete one full

rotation (i.e., 360º) in

Figure 1.7: The schematic representation of molecular arrangement in chiral

nematic mesophase for half pitch (p/2). Here, n is the director and p is the pitch of

the mesophase.

the helix along the layer normal to reach the same molecular orientation. Figure 1.7

shows the schematic representation of molecular arrangement of cholesteric LCs.

Cholesterics with a small pitch value (less than about 5000 Ǻ) exhibit blue phases. In a

very small temperature region (typically less than 0.5oC) between cholesteric and

isotropic phase, few organic compounds with sufficiently high twist can display up to

three different types of phases termed as “blue phases” [20, 24-26]. These phases

exhibit cubic symmetry and the basic structure of blue phases must be strongly related

n

n

n

n

n

p/2

13

to cholesteric one [9]. These materials exhibit temperature dependent birefringence

because of which they are being used in highly sensitive thermometers and the high

rotatory power of cholesteric LCs makes them potentially useful for different

applications.

(ii) FERROELECTRIC SMECTIC PHASES

All chiral smectic phases with tilted structure exhibit ferroelectric properties due to their

low symmetry. Due to the presence of chirality, they are able to exhibit spontaneous

polarization (Ps) and are known as ferroelectric liquid crystals (FLCs) [27-29].

Spontaneous polarization (Ps) is the signature and defining characteristic of FLCs. The

permanent electric polarization, P, is perpendicular to the director (n) and parallel to the

smectic layers. When the molecule is chiral, successive tilted smectic layers show a

gradual change in the direction of tilt, such that director precesses about the layer

normal from layer to layer and always lying on the surface of a hypothetical cone of

angle 2θ. The angle around the circle of precession is known as azimuthal angle (Φ).

This angle creates a helical structure in tilted chiral smectic C (Sm C*) mesophases

with the pitch (p) being distance along the layer normal needed to reach the same

molecular orientation [Fig. 1.8(a)]. Figure 1.8(b) shows that Ps is perpendicular to the

director (n) and parallel to the smectic layers. It is worth to mention here that Sm C*

mesophases possess PS microscopically but not macroscopically, due to the helical

arrangement of the director. Hence, chirality produces the helical structure which is the

reason behind the existence of a spontaneous molecular polarization.

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(a)

(b)

Figure 1.8: The schematic representation of: (a) spiralling of the director in chiral

smectic C (Sm C*) phase and (b) basic geometry of Sm C* phase showing layer

normal (k), tilt angle (θ), molecular director (n), director (c), spontaneous

polarization (Ps), along (k. x, n) plane and azimuthal angle Φ of Ps about k.

Directions of both n and Ps vary spatially in a helical manner.

Further discussion related to the achievement of non-zero macroscopic polarization in

these mesophases are given in subsequent sections.

θ

φ

k

z n

c

x

y

Smectic layers

15

(iii) ANTI-FERROELECTRIC AND FERRIELECTRIC PHASES

In antiferroelectric LCs, the director (n) always lies in the layer plane and the

spontaneous polarization vector (Ps) is perpendicular to it but in the subsequent layer

the director is pointed in opposite direction. Thus because of an equal number of

polarization vectors pointing up and down, the spontaneous polarization (Ps) averages

out to zero even for the unwound (i.e., nonhelical) state. 4-(1-methylheptyloxycalbonyl)

phenyl 4'-octyloxybiphenyl-4-carboxylate was the first antiferroelectric LC found by a

Japanese group in 1986 [30-32].

Ferroelectric Anti-ferroelectric Ferrielectric

Figure 1.9: The schematic representation of the arrangement of molecules in

ferroelectric, anti-ferroelectric, and ferrielectric Sm C* phases.

The structure of antiferroelectric LCs is evidenced by the fact that when a strong

electric field is applied to this phase, the layer ordering is perturbed and phase returns to

a normal ferroelectric phase. Moreover, in the switching of antiferroelectric phases

three states are produced: one antiferroelectric and two ferroelectric [33]. This tristable

switching occurs at a defined electric field and thus the presence of a sharp switching

threshold may be useful in display applications, which require multiplexing with grey

scales. The structure of ferroelectric Sm C* phase is repeated at every 360° rotation of

helix, whereas the helical structure of antiferroelectric phase repeats itself after every

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180° rotation. Therefore, the phase appears to have a relatively short pitch that can be

change quite significantly by the change in temperature.

In the ferrielectric smectic phase, the layers are stacked in such a way that there is a net

overall spontaneous polarization (Ps). This is due to the fact that numbers of layers

having polarization (Ps) in one direction are not exactly equal to the number of layers

having equal polarization (Ps) but in opposite direction. It has also been suggested that

the stacking of layers has two interpenetrating sub-lattices. Moreover, there will be

alternating layer structures, i.e., two layers tilted to the right and one to the left, and this

arrangement repeating itself throughout the bulk of the mesophase. Thus, the

ferrielectric phase has a considerable polarization. The difference among ferro,

antiferro, and ferrielectric Sm C* mesophases can be easily understood from the Fig.

1.9.

1.2.1.2 DISCOTIC MESOPHASE

The first identification of a discotic mesophase in benzene-hexa-n-alkanoates

compounds was reported by Chandrasekhar et al. in the year 1977 [7]. The schematic

representation of molecular structure of a discotic LC (i.e., benzene-hexa-n-alkanoate

derivatives) has shown in Fig. 1.10.

Figure 1.10: Molecular structure of benzene-hexa-n-alkanoate derivatives (discotic

LC).

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However, there were evidences that mesomorphic phases formed by disc shaped

molecules were realized as far back as the year 1960 by Brooks and Taylor [34]. Since

then, a large number of discotic compounds have been synthesized and a variety of

discotic mesophases discovered [35-37]. From the structural point of view, the discotic

LCs can be divided into two distinct categories: nematic and the columnar [Fig. 1.11].

Moreover, there are few other modified forms of discotic mesophase, as shown in Fig.

1.11 [34-36].

Figure 1.11: The schematic representation of arrangement of disc-like molecules:

(a) upright columnar structure (b) its hexagonal and (c, d) rectangular

modifications (e) tilted columnar structure (f) its face-centered lattice, and (g)

shows the nematic phase of disc-like molecules.

Figures 1.11(a-g) show the both phases (i.e., nematic and columnar) of the disc-like

molecules. Nematic phase is the simplest discotic phase because there is orientational

order of the discs without any long-range translational order. The molecules move

around quite randomly, but on average the axis perpendicular to plane of each molecule

(a)

(g)

(f) (e)

(d) (b) (c)

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tends to orient along the director. The nematic mesophase is optically negative (∆n < 0),

unlike rod shaped nematic mesophase (∆n > 0). Thus, these mesophases have recently

recognized for their commercial applications such as (i) optical compensating films

used to enlarge the viewing angle and enhance the contrast ratio of multibillion dollar

LCD industry, and (ii) high strength and high performance carbon fibres used in

industrial applications [38]. In chiral nematic discotic LCs, the director rotates in a

helical fashion throughout the sample just as in the case of rod-like chiral nematic LCs.

In columnar mesophase, the discotic molecules are stacked up to form columns. The

columnar mesophase has been found useful for semiconducting devices such as field

effect transistors, organic photovoltaic, organic light emitting diodes, etc. [9, 39, 40]. A

smectic-like lamellar phase has also been reported but the disposition of molecules in

the layers of this phase has not yet been resolved.

1.2.1.3 BENT CORE MESOPHASES

As given in the previous sections that thermotropic LCs made by rod (i.e.,calamitic) and

disc shaped mesogens exhibit both nematic and chiral nematic mesophases. In addition

to nematic mesophase, rod shaped mesogens also show smectic (i.e. lamellar)

mesophases while disc shaped mesogens show columnar mesophases. The bent-core or

banana shaped mesogens are intermediate between rod and disc shaped mesogens and

exhibit both nematic and smectic mesophases.

Figure 1.12: Molecular structure of a typical bent-core or banana-shaped LC

mesophase.

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The nature and characteristic features of bent-core mesophases [41-44] are entirely

different from calamitic/discotic mesophases. Such striking features placed the bent-

core LCs into a separate and novel area of research in the field of LC based technology.

Figure 1.12 shows the molecular structure of a typical banana shaped or bent-core LC

mesophase.

1.2.2 LYOTROPIC LIQUID CRYSTALS

The LCs, in which the phase transitions occur due to the change in the concentration

of either solute (i.e., mixture of amphiphilic compounds) or solvent (e.g. water) or

both, are known as lyotropic LCs [45-50]. An amphiphilic molecule consists of a

hydrophilic head group (ionic/non-ionic) attached with a hydrophobic group. A

familiar example of amphiphilic molecule is soap (sodium dodecyl sulphate), which is

shown in Fig. 1.13. When an amphiphilic compound is mixed in a highly polar

solvent, most common water, amphiphilic molecules start to form aggregates through

self assembly process driven by hydrophobic effect [48, 49].

Figure 1.13: Molecular structure of sodium dodecyl-sulphate (soap), an example

of lyotropic LC.

In most of the cases, such aggregation of amphiphilic molecules occurs, only when

the concentration of amphiphiles exceeds then a critical concentration.

The lyotropic LCs are as interesting and important as thermotropic LCs. In lyotropic,

although the concentration of one component with respect to the other is most

important but the temperature is still an important feature in phase determination.

Lyotropic LCs are fluid phases possessing considerable molecular order as

thermotropic LCs, but are different in the sense that these are systems of two or more

20

components being composed of large organic molecules dissolved in water. They are

also different because the formation of the basic structures of lyotropic is due to the

interaction of the organic molecules with the aqueous solvent that is most crucial in

providing the stability of these ordered phases. Also, the lyotropic assembly can be

observed in every cell membrane in our body due to the systematic arrangement of

phospholipids/water mixture. Lyotropic LCs are generally used in cosmetics,

medicine, templates for direct synthesis of organic/inorganic nanostructures,

development of network and cellular morphologies in soft materials. These type of

LCs are also important from technological point of view [51-54].

1.3 FERROELECTRIC LIQUID CRYSTAL

Ferroelectricity in a material is the manifestation of lowering in the symmetry: as

symmetric phases can not possess a net spontaneous polarization (Ps). The

spontaneous polarization (Ps) occurs in ferroelectric LCs, must be invariant under

symmetry operations of the medium which requires the polar axis to be along a

unique rotation axis (only one may exist) to which no perpendicular plane of

reflection symmetry belongs. Most crystal classes (22 out of 32) contain too many

symmetry elements to allow for ferroelectricity.

In general, very high symmetry of the common phases, nematic (D∞h, uniaxially

birefringent) and smectics (Sm A: D∞; Sm C: C2h, biaxially birefringent) is not

compatible with a spontaneous macroscopic polarization (Ps). However, a Sm C phase

composed of chiral molecules (C*) has the symmetry allowing it to be ferroelectric

because now the inversion symmetry and reflection or mirror symmetry have

vanished [55].

The possibility of a ferroelectric phase in LCs was first envisaged by Alfred Saupe in

1969 [29]. Later, R. B. Meyer gave the first theoretical prediction of ferroelectricity

(based on the rigorous symmetry arguments) in the Sm C* mesophase (one of the

tilted chiral smectic mesophases) in 1974 [56]. The symmetry argument of Meyer

goes as: the Sm C phase has monoclinic symmetry, the point group for which contains

not only a two-fold rotation axis parallel to the layers and normal to long molecular

axis but also reflection plane normal to the two-fold axis and a centre of inversion.

The reflection plane and centre of inversion are eliminated due to the presence of

21

Cr76 °C

Sm C* Iso.117 °C

63 °C

93 °CSm A

Sm H*40 °C

C NC10H21O C

H

C

H H

CO2 CHC2H5

CH3

CH2

*

chiral molecules (not super imposable on their mirror image). The remaining single

two-fold axis allows the existence of a permanent dipole moment parallel to this axis.

If all the molecules are identical then a net polarization of at most a few Debye per

molecule will be produced. With these considerations in mind, the first chiral material

DOBAMBC (decycloxybenzylidene p'-amino 2 choloropropyl cinnamate) was

synthesized in 1975 [27]. The chemical structure and phase sequence of the first FLC

(DOBAMBC) have been shown in Fig. 1.14.

Figure 1.14: The chemical structure and phase sequence of first FLC

(DOBAMBC) material. Here, asterisk (*) on C shows the presence of chirality in

carbon.

This was just the beginning and was followed by studies of the nature of helical

structure of the spontaneous polarization (Ps) field, pyroelectric effect, electroclinic

effect, coupling of polarization to flow, orientational fluctuation and field responses in

freely suspended films, polarization reversal and electro-optic effects, etc. All these

studies were summarized in the reviews by Meyer, Durand, and Martinot-Lagarde

[57, 58].

1.3.1 INCLUSION OF FERROELECTRICITY IN LIQUID CRYSTALS

The director (n) plays a crucial role in local symmetry consideration, e.g., in defining

the local symmetry axis. In solids any descriptive vector fits well into the definition of

a polarization of the medium. However, in LCs, it is likewise fundamental that in the

formation of all properties, the medium is invariant against sign reversal in the

director.

n - n, symmetry operation (1.2)

22

If this invariance was not required, the ferroelectricity would have been found a long

ago in the common NLCs. Thus, director (n) is not a variable that leads itself as a

direction for introducing polarization symmetry, except in the opposite sense: if the

medium has a macroscopic polarization (P), any component of it along director (n)

clearly violates the condition (1.2). Therefore, in a FLC, as long as the invariance

condition (1.2) is valid, the polarization vector is always locally perpendicular to the

director,

P ⊥⊥⊥⊥ n (1.3)

(a) Ordinary liquid with optically active molecules (rotation around any axis is a

symmetry operation)

(b) Rotation 180 º around the y-axis

P = (Px, Py, Pz) → (-Px, Py, -Pz)

P = (0, Py, 0)

(c) Rotation 90 ºaround the z-axis

P = (0, Py, 0) → (0, -Py, 0)

P = (0, 0, 0)

Figure 1.15: The isotropic liquid showing that it cannot possess an intrinsic

macroscopic polarisation.

X

y

Z

-Z

P

y

-X

P

y

Z

y

-X X

23

The polarization vector (P), which is an intrinsic property of FLCs, must remain

invariant by all symmetry operations that leave the medium invariant. Consider the

case of an ordinary liquid with optically active molecules as shown in Fig. 1.15(a).

The medium has the full rotational symmetry with no reflections allowed. Under 180o

rotation around the y-axis, any assumed polarization vector (P) is forced to change

sign in its x and z components, which therefore must be zero for polarization vector

(P) to remain invariant as shown in Fig. 1.15(b). Figure 1.15(c) shows that the

remaining polarization vector (Py) is transformed into polarization vector (Px) at a

subsequent 90o rotation around the z-axis, and so likewise has to vanish. Thus, an

isotropic liquid can be optically active but not ferroelectric. The same argument can

be applied to nematic and Sm A phases [55]. The only fundamental difference

between Sm C and Sm C* is the lack of mirror symmetry of latter phase. The Sm C*

phase can be made in many ways for instance, by synthesizing a chiral variant of a

molecule which is known for its stable Sm C phase, or by doping a Sm C phase with

any chiral molecule soluble in it, symbolically it can be written as follows:

Sm C + * → Sm C*.

The dopant can be a molecule with one or several asymmetric carbons, which is

usually the case, but it can also be a molecule without asymmetric centers, having

chirality built into its integral structure. Neither doping nor chiralizing of molecules

normally changes the phase transition properties of medium.

However, the lack of mirror symmetry of medium now gives rise to summing up of

dipolar contributions in a certain direction for every molecule with appropriate

dipoles. Figure 1.16 shows a small portion of Sm C and Sm C* phases, respectively,

introducing once again the coordinates needed for the present purpose [55].

In case of Sm C phase, the basic symmetry elements are a two fold rotation axis

parallel to the plane of smectic layers and perpendicular to the long molecular axis, a

mirror plane normal to the rotation axis and a center of symmetry which corresponds

to a point group C2h. But if the molecules are chiral in nature, the mirror plane and

center of symmetry disappear reducing the symmetry to a point group C2. In this case,

there is a possibility of obtaining a spontaneous polarization along the rotation axis if

the molecule has a permanent dipole moment perpendicular to the long axis and free

rotation around this axis is hindered by the surrounding molecules.

24

Figure 1.16: The schematic representation to show the non-ferroelectric &

ferroelectric nature of Sm C and Sm C* mesophases, by using symmetry

arguments. ‘Z’ and ‘n’ are considered as layer normal and director of the

mesophase, respectively. The symmetry arguments are applied in single layer of

Sm C and Sm C* mesophases.

Sm C Sm C*

θ

n

θ

n Z Z

25

In both Sm C and Sm C* phase, an 180˚ rotation around the y-axis is an allowed

symmetry operation [59]. Under this rotation the polarization vector (P) transforms

according to

(Px, Py, Pz) → (-Px, Py, -Pz)

Thus we can only allow the possibility of a polarization vector

P = (0, Py, 0) to exist.

In the Sm C phase, the xz plane is a mirror plane as shown in Fig. 1.16. The

corresponding symmetry operation transforms the polarization vector (P) according to

(0, Py, 0) → (0, -Py, 0)

Showing, that Py must be equal to zero and it excluded the possibility of

ferroelectricity in Sm C phase. However, in the case of Sm C* phase the molecules

are chiral, i.e., they are no longer mirror images of themselves. Thus, the xz plane is

not any longer a mirror plane and therefore, symmetry permits the system to exhibit a

permanent polarization in the y direction, i.e., direction perpendicular to the direction

of the tilt.

Therefore, the reduced symmetry and interactions due to chirality across the layer

planes defines that the molecular tilt direction will twist through successive layers,

which leads to the rotation of spontaneous polarization that is always parallel to

smectic layers as well as perpendicular to the long molecular axis and to the direction

of tilt. These are the results in a helicoidal structure with a pitch (p), which is the

distance along z-axis needed to reach the same molecular orientation.

Thus, the macroscopic polarization of Sm C* phase averages to zero however, the

system will exhibit a local spontaneous polarization (Ps) according to the following

equations:

Px = - PsinΦ

Py = PcosΦ (1.4)

Pz = 0

Where, Px, Py, and Pz are the x, y, and z component of spontaneous polarization (Ps)

and Φ is the azimuthal angle.

Hence, the net value of macroscopic polarization (P) is zero in the bulk sample. The

ferroelectric properties are in a way lying hidden in the helicoidal structure. Such

types of ferroelectrics are called improper ferroelectrics or pseudo ferroelectrics. But

26

this helix can be unwound either by applying electric field or by surface interactions,

which make them proper ferroelectrics with a net spontaneous polarization (Ps).

Moreover, these symmetry arguments indicate that ‘All chiral tilted smectics can be

expected to be ferroelectric’. That means ferroelectricity is not only expected in the

Sm C* mesophase but also in other chiral versions of tilted smectic mesophases

having lower symmetry (such as Sm F*, G*, H*, I*, J*, K*, and O* mesophases).

1.3.2 COMPARISON WITH SOLID FERROELECTRICS

The phenomenon of ferroelectricity was discovered by J. Valasek in Rochelle salt for

the first time in 1921. Thereafter, several ferroelectric materials have been studied and

some laws (such as Curie-Weiss law) have been defined for phase transition, etc.

There are interesting similarities and dissimilarities between FLCs and well studied

solid ferroelectrics [57, 60]. Making a comparative study, solid ferroelectrics are

inorganic compounds, whereas FLCs are organic compounds. Molecules in solid

ferroelectrics are arranged in domains and further lead to zero net dipole moment due

to their random orientation. In FLCs, molecules are arranged in a helicoidal manner

due to which the net dipole moment is zero. Typical values of spontaneous

polarization (Ps) for solid ferroelectrics lie around 10-7-10-5 C/cm2 while in case of

FLCs the corresponding values are around 10-9-10-7 C/cm2. Coercive field is of the

order of kV/cm in case of solid ferroelectrics, but is comparatively less in case of

FLCs. The Curie Weiss law is obeyed over a wide range of temperature in solid

ferroelectrics, whereas in FLCs, it is obeyed in a very narrow range of temperature. In

solid ferroelectrics, polarization (P) is most often the primary order parameter of

ferroelectric phase. On the other hand, in FLCs, polarization (P) is the secondary

order parameter and it is a structural by product of the symmetry of tilted chiral

smectics. Solid ferroelectrics generally exhibit fast (~10 ns) switching with low

refractive index changes (∆n ~ 10-3) while FLC switching is slower (~1 µs) but with a

much larger change in refractive index (∆n ~ 0.1). In BaTiO3 and related materials,

the bireferingence ∆n is zero in cubic paraelectric phase and ∆n ≠ 0 in ferroelectric

phase which is directly related to the lattice distortion introduced at the transition. In

FLCs, there is hardly any practical difference between the birefringence values of Sm

27

A* and Sm C* phases. The difference is that in the Sm C* phase the anisotropic axis

(n) can be switched around two new directions in space because of its rigid relation

with local polarization P.

FLCs can be further classified into three types: (a) surface stabilized (b) deformed

helix, and (c) electroclinic.

1.3.3 SURFACE STABILIZED FERROELECTRIC LIQUID CRYSTALS

In 1980, Clark and Lagerwall proposed a way to suppress the helix and developed the

surface stabilized ferroelectric liquid crystal (SSFLC) concept [61]. SSFLCs are

basically the chiral tilted smectic mesophases (such as Sm C*) with unwound helical

structure by using surface interactions and having electrically switchable macroscopic

polarization (P) in two stable states [55, 61]. The surface stabilization can be achieved

by keeping the sample cell thickness (d) less than the pitch (p) (i.e., d < p) of Sm C*

mesophase of considered FLC compound. The switching between two surface

stabilized states under externally applied electric field (± E) can be understood by

considering the switching of director (n) on the hypothetical cone of angle 2θ, as

demonstrated in Fig. 1.17. The switching response of SSFLC is faster (~ µs) than

nematic LC (~ms) which is due to the direct linear coupling between polarization (P)

and external electric field (E). Further, due to the in-plane switching, the viewing

angle of FLC based electro-optical devices is more than the nematic case. Overall,

SSFLCs have shown their profound impact on the display devices (such as micro

displays, etc.) as well as non-display applications (high speed phase/amplitude

modulation, optical signal processing, and optical computing, etc.) due to faster

switching response, lower threshold voltage, better optical contrast, wider viewing

angle, high nonlinear optical coefficients, and static memory effects [55, 62-65]. The

SSFLCs have further been discussed in Chapter III of the present thesis.

28

Figure 1.17: The schematic representation to show the switching of FLC director

(n) between two stable states on a hypothetical cone of angle 2θθθθ, under externally

applied electric field (±±±± E). The polarization (P) is shown in two opposite

directions [into (××××) and out of the plane (••••) of paper]. The layer normal (k) is

parallel to the z axis (i.e. z |||||||| n).

1.3.4 DEFORMED HELIX FERROELECTRIC LIQUID CRYSTALS

As discussed in previous sections, the ferroelectrics based on Sm C* are called

improper ferroelectrics or pseudo ferroelectrics due to complete helix. However, this

helix can be unwound either by applying electric field or by surface interactions,

which make them proper ferroelectrics with a net spontaneous polarization (Ps).

Surface interactions generated the SSFLC effect while the application of electric field

(unwinding of helix by applying electric field) gave a new kind of FLCs which were

called deformed helix ferroelectric liquid crystals (DHFLCs) [66-68]. DHFLCs are

very short-pitched FLCs in which grey level (brightness level of pixel representing its

light from black to white) can be easily realised as the pitch (p) is linearly dependent

k

n

z

x

y

+θθθθ

P ×××× P

- θθθθ

2 θ θ θ θ

n

+ E - E

•

Smectic Layers

29

on the applied electric field. The electro-optic effect observed in DHFLC is due to the

distortion of the helix by an applied electric field as shown in Fig.1.18 [69].

Figure 1.18: Schematic diagram showing the distortion of helix of DHFLC

materials by application of electric field perpendicular to the helical axis.

When an electric field is applied perpendicular to helical axis, the helix is deformed

and this deformation of helix is due to the coupling of molecular polarization with

applied electric field. The helix becomes completely unwound above a certain

threshold voltage (Eth). The spontaneous polarization (PS) and the threshold voltage

(Eth) are related to each other by the following equation:

(1.5)

Where, K is the torsional elastic constant and L is the unperturbed pitch of the helix.

Thus, from the above relation, when the value of spontaneous polarization (PS) is

extremely large then the value of Eth will be very low.

The fast response time and low driving voltage of DHFLCs make them popular

among the other FLCs. DHFLCs have been found to possess long lasting thickness

independent memory effect (under certain conditions) [70], whereas SSFLCs show

the memory effect that depends strongly on the thickness of the samples [61].

E > 0 E = 0 E < 0

Ps = 0

s

thPL

KE

2∝

30

1.3.5 ELECTROCLINIC LIQUID CRYSTALS

After the discovery of ferroelectricity in tilted chiral smectic phases in 1975, Garoff

and Meyer discovered the electroclinic effect in Sm A phase [71, 72]. A symmetry

argument similar to one which predicted ferroelectricity in a Sm C* phase [27],

described the origin of the electroclinic effect in a Sm A phase composed of chiral

molecules and such LCs are known as “electroclinic liquid crystals” (ELCs). The

application of an electric field parallel to the layers of Sm A* phase biases the free

rotation of molecules and therefore produces a non-zero average of transverse

component of molecular polarization (P). When such a dipole moment is present, a

tilt of long molecular axis (the director) is induced in a plane perpendicular to the

dipole moment. In an aligned Sm A* sample, a tilt of the director is directly related to

a tilt of optic axis; therefore electroclinic effect results in a linear electro-optic

response [71].

The origin of induced spontaneous polarization in Sm A* mesophase is explained

through Landau theory [73], in which the free energy density (g) expression under

externally applied electric field (E) can be written as:

ϕθθθ cos

4

1

2

1 42cEbag −+=

where, g = free energy density, a = )( cTT −α = first coefficient of Landau free

energy expression, b = second coefficient of free energy expression, θ = electric field

induced tilt angle and treated as main order parameter, c = structure coefficient or

electroclinic coupling constant between θ & E , and ϕ = angle between E and the

direction of the induced ferroelectric polarization (P).

As the field induced θ is generally small in Sm A* mesophase, 4θ and ϕ terms can

be neglected, therefore, the Eq. 1.7 can be approximated as:

θθ cEag −= 2

2

1

For the minima of above expression,

0=∂

∂

θ

g, cEa =θ

(1.6)

(1.7)

31

So, )( cTT

cE

a

cE

−==

αθ

Eec=θ

Here, ce is defined as “electroclinic coefficient” and given by:

where, α = susceptibility coefficient controlling the molecular tilt and independent of

chirality, and cT is Sm A* to Sm C* phase transition temperature.

Thus, the induced polarization (P) in the direction of applied field can be obtained as:

)(

2

cTT

Ecc

E

gP

−==

∂

∂−=

αθ (1.11)

The linear coupling between induced tilt and applied external electric field [See Eq.

1.9] in ELCs offer several electro-optical applications with faster (~sub-

microseconds) switching response and grey scales [73-75].

Researchers have not paid much attention for the commercialization of devices based

on the electroclinic effect. First, it might be due to shrinking of the smectic layers

which occurs at the transition from orthogonal Sm A* phase to tilted Sm C* phase.

Secondly, the formation of chevron structure or so-called “zig-zag” defects and a

reduction in effective optical tilt that seriously degrades the quality of electro-optic

devices [73-75, 76]. However, recently, a number of ELC materials are identified

which shows non-layer shrinkage (NLS) and free from above-mentioned defects [77-

80]. These materials are termed as “de Vries ELCs/FLCs” [77] and the special feature

of de Vries FLCs is their substantial/NLS near Tc of Sm A* to Sm C* phase

transition. NLS in de Vries FLCs is explained through a different type of Sm A*

phase structure, where LC molecules are tilted very similar to Sm C* phase, however,

tilt directions in different smectic layers are randomly oriented (i.e., randomization of

tilt).

Generally, the phase transitions in de Vries FLCs could be of first and second order

[81, 82]. In first order phase transition, there is a considerably large change in optical

(1.9)

(1.10) )( c

cTT

ce

−=

α

(1.8)

32

and dielectric behavior in the vicinity of Tc. On the contrary, second order phase

transition is a continuous play of disorderliness and orderliness gradually transferring

to the other near Tc. Thus, de Vries FLC materials are characterized by the

electroclinic effect in the Sm A phase. It is basically the amount of order created in

molecular tilt on increase in the electric field applied, which gives the behavior of

electroclinic effect in LC material. Several theoretical and experimental studies [74,

76, 78, 83-91] have been carried out on these intriguing de Vries FLCs. In these

reports, the evidences for randomization have been presented on the basis of X-ray

diffraction, textural, dielectric, and optical birefringence studies.

Recently, it has been reported that de Vries FLCs show thickness-independent

memory effect, random mode, and non-validity of Curie-Weiss law, mainly due to

randomization [92-95]. Further, the effect of doping of different nanomaterials [such

as graphene oxide (GO), zinc oxide (ZnO), etc.] and isotropic solvent (such as water,

ethanol, etc.) on the physical properties of de Vries FLCs has also been observed [96-

100]. We have studied and presented the results based on such materials (de Vries) in

Chapters IV and VI of this thesis.

1.4 OTHER TYPES OF LIQUID CRYSTALS

1.4.1 POLYMER LIQUID CRYSTALS

Polymer liquid crystals (PLCs) are a class of materials that combine the properties of

polymers with those of LCs. These ‘hybrid materials’ show the same mesophases

characteristic of ordinary LCs, yet retain many of the useful and versatile properties of

polymers. For normally flexible polymers to display LC characteristics, rod-like or

disk-like elements must be incorporated into their chains [101-103]. There are two

types of PLCs (a) main chain polymer liquid crystals (MCPLC) and (b) side chain

polymer liquid crystals (SCPLCs). The schematic presentation of both MCPLC and

SCPLCs has given in Fig 1.19.

33

Figure 1.19: Schematic representation to show the molecular structures of main

chain polymer LC (MCPLC) and side chain polymer LCs (SCPLCs).

MCPLCs are formed when the mesogens are themselves part of the main chain of a

polymer, e.g. poly (hydroxynapthoic acid). Conversely, SCPLCs are formed when the

mesogens are connected as side-chains to the polymer by a flexible “BRIDGE” called

spacer [104]. The molecular structure of a typical type of MCPLC and SCPLC has

been shown in Fig.1.20.

(a)

(b)

Figure 1.20: The molecular structure of typical (a) MCPLC and (b) SCPLC.

34

1.4.2 POLYMER DISPERSED LIQUID CRYSTALS

Polymer dispersed liquid crystals (PDLCs) are relatively a new class of materials that

hold promise for many applications ranging from switchable windows to projection

displays. PDLCs consist of LC droplets that are dispersed in a solid polymer matrix.

The resulting material is a sort of “SWISS CHEESE” polymer with LC droplets

filling in the holes and these tiny droplets are responsible for the unique behavior of

such material. These materials are used in a variety of electro-optical applications

ranging from displays to light shutters [9, 105].

The following pictures are of the thermally-induced phase separation (TIPS) process

taken at three different times such as t1, t2, and t3 and t1 < t2 < t3. [Fig.1.21]. From the

figures one can see that the droplets grow in size as time passes.

Time → (t1) (t2) (t3)

Figure 1.21: Typical PDLC diagrams for TIPs process at three different times.

1.4.3 POLYMER STABILIZED LIQUID CRYSTALS

The previous section on PDLCS, dealt with a higher polymer concentration range

(above about 20 wt %). In display applications, these materials create problems with

hazy images for obliquely incident light. Rapid development is occurring in the low

polymer concentration range (10 weight % or less) because of the ability to form

polymer networks which stabilize LC textures throughout the bulk of a device and

improve its electro-optical performance and such LCs are known as polymer

stabilized liquid crystals (PSLCs) [106].

35

1.5 APPLICATIONS OF LIQUID CRYSTALS

LCs have been specially recognized as mysterious and beautiful materials in the field

of soft matter. The LCs provide to scientists and engineers, one of the most active and

fascinating fields of scientific and industrial research. In other words, LCs are of

demand in various disciplines such as chemistry, biology, physics, and engineering, to

harness their versatile features simultaneously [1, 2, 107, 108, 104, and 109].

LCs based display devices have been utilized in a wide range of instruments such as

watches, calculators, portable colour televisions (TVs), and laptop computer screens,

etc. [Fig. 1.22]. The liquid crystal displays (LCDs) are famous due to their flatness,

low power consumption/low threshold voltage, and portable characteristics as

compared to other displays such as cathode ray tubes (CRTs). Apart from these

features, several setbacks such as viewing angle, switching response, resolution, and

size of the display, etc., have been encountered in the early days. But in the recent

years, significant improvements have been made and as a result, now LCDs have

more than $60 billion per year business in the display industry which is just a example

of commercial success [110, 111].

There are numerous non-display applications, such as: spatial light modulators

(SLMs) [112-114], tunable filters [115-120], photonic crystal based devices and fibers

[121-125], mirrorless lasing/LC laser [126-128], optical computing & signal

processing [129-134], biomedical [135-137], and nanotechnology/nanoengineering

[138-140], of LCs also [Fig. 1.22]. Spatial light modulators (SLMs) have been

extensively used for wave front correction/compensation, beam steering, generation

of holograms through computer, high speed projectors, spatial filters, and so on.

Tunable filters based on LCs provide the electrically tunable transmission/reflection

of particular band of electromagnetic radiation.

36

Wrist Watch

Spatial Light ModulatorTunable Filters

Photonic Crystal FiberMirrorless Lasing

Desktop Monitor

Displays

Non

-

Displays

No

n-D

isp

lays

Figure 1.22: The snapshots of some display and non-display applications of LCs

[from http://www.google.co.in/images].

The LC droplets are currently being used at NASA, USA to combine beams of light

from multiple telescopes, which is a good way to search for distant planets around

other stars. Thus, LCs have number of potential uses besides being used in display

applications.

37

1.6 IMPORTANCE OF THE PRESENT STUDIES IN THE

CURRENT SCENARIO

We know that an idea is nothing but a spark of intelligence; we can call it a brainstorm

or power infinite. The present chapter gives us an introduction to the field of LCs. The

journey begins from the accidental discovery of a cloudy liquid before the isotropic

liquid phase and the curiosity led to the development of a whole new subject of LCs.

Since then there have been numerous studies by scientists to understand and

characterize the rich variety of LC mesophases. This chapter explains the various

classifications of LCs on the basis of shape, molecular arrangement or structural

behaviour.

FLCs have faster response and show a number of other features such as grey scale and

bistability that nematics do not exhibit. A special type of FLC known as ELC has

been studied with interest lately due to their extra-ordinary characteristics. ELCs do

not exhibit chevron defects near the transition temperature so can be used where

better optical contrast is required.

The present thesis deals with broad and systematic investigations based on structural,

dielectric, and electro-optical studies of pure and nano-material doped FLCs (i.e.,

conventional and de Vries) to look at their fundamental and applied aspect.

Chapter II of the thesis, mainly deals with the experimental techniques used for the

preparation and characterization of FLC samples. It is further extended to provide new

ways to obtain perfect homeotropic (HMT) alignments in ELCs either by surface

treatment or without any surface treatment of substrates (Chapter IV) and observation

of good memory effect in FLCs (Chapter III). Dielectric, textural and electro-optical

measurements have been the various tools to establish the behaviour of these materials.

The main objective of the present thesis is to understand the collective dielectric

relaxation processes in the FLCs and to investigate the basic mechanism behind such

behaviour in FLCs, either in their pure form or doped with some nano-material,

respectively (Chapter VI and IV).

In this thesis, we report the investigations carried out on the nano-materials (ZnO NPs

and GO, etc.) doped FLCs to enhance the transition temperature from Sm A* to Sm

38

C* and to enhance the performance of electro-optical properties of FLC based

devices. These studies were also applied to have fast response LC devices based on

CuO decorated multi-walled carbon nanotubes (MWCNTs) doped FLC material

(Chapter V). All these fundamental and applied studies have been discussed in detail

in the successive chapters.

39

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