1

GENERAL INTRODUCTION

______________________________________________ 1.1 Introduction

In the Early 1960’s the pioneering research efforts at RCA and Xerox brought

attention to possibility of using a relatively unknown class of organic materials, i.e.,

liquid crystals (LC’s), for modulating electro-optic properties of light for fabricating a

revolutionary new flat panel display. Today the flat panel liquid crystal display

devices, (LCD’s) have replaced the bulky and power-hungry cathode ray tubes

(CRTs) and thus changed the face of the modern office.

Liquid-crystalline material research has contributed significantly both to the

development of liquid-crystalline display (LCD) technology and also to the better

understanding of the phase behavior of soft condensed matter systems. Quantitative

knowledge of the physical properties is necessary, so that improved materials may be

devised for applications. Thus, attempts are continuously being made to study the

material properties of both pure compounds as well as their mixtures for a better

insight into the basic understanding of liquid crystalline behavior so that a newer and

more acceptable class of materials suitable for display devices emerges.

Any material, if the temperature is raised more and more, it generally changes

its state from solid to liquid and to gaseous. An Austrian botanist, Friedrich Reinitzer1

first observed liquid crystals in 1888. He discovered a strange material that exhibited

a mesophase between solid state and liquid state1. At a temperature of 145

OC, it

melted, becoming cloudy white and viscous. At a temperature of 179 OC, it became

isotropic and clear. The material he discovered was cholesteryl benzoate. On March

14, 1889, he wrote a letter to Otto Lehmann2, telling him about the two melting

points. Lehmann studied the material and discovered that the liquid at the mesophase

exhibited a double refraction effect, characteristic of a crystal. Because it shared

characteristics of both liquid and crystal, he named it “fliessende krystalle”and the

name “liquid crystal” was born2.

Liquid crystal is a state of matter that is neither a solid crystal nor an isotropic

liquid. The basic difference between crystals and liquids is that the molecules in a

2

crystal are ordered whereas in a liquid they are not. The existing order in a crystal is

usually both positional and orientational, i.e., the molecules are constrained both to

occupy specific sites in a lattice and to point their molecular axes in specific

directions. Contrary to this, the molecules in liquids diffuse randomly throughout the

sample container with the molecular axes tumbling wildly. The states of matter whose

symmetric and mechanical properties are intermediate between those of a crystalline

solid and an isotropic liquid are called `liquid crystals3-6

(LC). Liquid crystals exhibit

anisotropy in their mechanical, electrical, and optical properties, behaving in this

sense as solid crystal. Nevertheless, they have no ability to support shearing, and thus

they flow like ordinary liquids7.

This is why liquid crystals attain their importance in technological

applications, especially in electro optic switches and in liquid-crystal displays8.

Therefore, thermal stability of liquid-crystalline mesophase and its molecular ordering

are of great importance.

1.2 Chemical structure of liquid crystals

The role of molecular structure or geometry in liquid crystals is extensively

studied by Gray9. In order to exhibit the mesomorphism, the molecules should

possess highly geometrical anisotropy in shape, namely it should be either rod like or

disc like. The mosomorphic molecule mainly consists of three units such as core,

linking groups and terminal groups. The core should be highly rigid to form liquid

crystalline phases because it makes the interaction with other molecules anisotropic.

In addition to the core rigidity, there is a need to have certain flexibility for the

molecule in order to ensure reasonable low melting points and stability. This

flexibility can be obtained by attaching the terminal groups.

3

The general molecular structure is given as below

where R and R” are flexible terminal groups, while A and B are aromatic groups and

Z is the linking group. This part including aromatic rings and linking group together is

called as central rigid core. Z represents a linkage of the type –A=B- between two

benzene or hexane rings. The aromatic rings linked through a central group Z which

often involves a double bond or a triple bond to preserve the rigidity and linearity of

the molecules.

R and R” may be same or different and represent groups example, R = n-alkyl and R”

= n-alkoxy or viceversa, R and R” = n-alkyl

Generally the linking group used is as follows

Saturated groups, such as ethylene (C2H4), ester (-COO-)

Unsaturated groups containing double bonds, such as stilbene (-CH=N-),

azoxy (–N=N-) and schiff’s base (–CH=N-).

Unsaturated groups containing triple bond such as acetylene (-C≡C-), and

diacetylene (-C≡C-C≡C-) 10-13

.

1.3 Classification of liquid crystals

The Liquid crystal nature is exhibited by many number of different classes of

materials which includes organic compounds, amphiphilic organic compounds,

polymeric liquid crystals and colloidal dispersion of inorganic materials14, 15

.

The transitions to the mesophases may be obtained in two different ways

either by the change of temperature or by the change of concentration of a solvent,

usually water

Z R” A B R

4

1.3.1. Thermotropic liquid crystals

Thermotropic liquid crystal phases are observed due to the change in

temperature. Thermotropic liquid crystals in which the transitions takes place

reversibly on both heating and cooling are the thermotropic liquid crystals which are

stable above the melting point of the compound are called enantiotropic. In certain

cases the phases are observed only on cooling, and the liquid crystalline state is stable

at temperature below the melting point. The phases are called monotropic.

Thermochronic liquid crystals are thermotropic in origin and have chiral (twisted)

structures16, 17

.

1.3.2. Lyotropic liquid crystals

Lyotropic phases are formed in the presence of a suitable solvent and also

concentration. The molecules that make up lyotropic liquid crystals are amphiphilic

or surfactants consisting of two distinct parts: a polar (often ionic) head and a non-

polar (often a hydrocarbon) tail. Following the rule of the like dissolve like, the head

is attached to water (or hydrophilic) and the tail is repelled by water (hydrophobic).

These amphiphilic molecules of material form liquid crystals on dissociation in

solvent18-20

. Deoxyribonucleic acid (DNA), certain viruses like tobacco mosaic virus

(TMV) form lyotropic mesophases when dissolved in an appropriate solvent (usually

water) in suitable concentration. Soaps and detergents when used with water form

liquid crystals.

1.3.3. Polymeric liquid crystals

Liquid crystalline polymers combine the self organization of the mesogenic

groups into the ordered structure of liquid crystalline phases with some typical

polymer properties such as freezing of the disordered polymer chain at glass transition

temperature. These can be prepared by incorporation of the anisotropic mesogenic

groups into polymeric systems. These are classified either as main chain liquid

crystalline polymers [MCLCP] or polymers with mesogenic side groups [SCLCP] or

as liquid crystalline elastomers21-24

.

5

Liquid crystalline materials in general may have various types of molecular

structure. Basing on the geometrical structure, the thermotropic liquid crystals can be

divided into rod like9, disc like

25, sanidic or brick-like or lath-like

26 and bowlic for

bowl-like27

. The liquid crystals derived from the rod-like molecules are called

calamitics. The liquid crystals formed from disc-shape molecules are known as

discotics. Intermediate between rod-like and disc-like molecules are the lath-like

species. The symmetry-based classification of liquid crystals was first made by

Friedel28

.

Liquid Crystals may be broadly classified as follows

Liquid Crystals

Thermotropic Lyotropic

Calamitic Discotic Sanidic bowlic

Nematic Cholestric Smectic

Cybotactic

Normal skewed SA SD SC SB (h) SI SF SL=B(cl) SJ SG SE S SM SO SQ SX

Nematics:

The first phase that condenses when isotropic liquid is cooled is called nematic

and also is known as high temperature liquid crystal phase. This phase possesses long

range orientational order. The phase is fluid and there is no long-range correlation of

the molecular centre of mass position. The long axis of the molecules on an average

are aligned about a specific direction, which is denoted by a unit vector, called the

director, . Nematic phase is apolar in nature i.e. and - are physically

equivalent. Thus the material is still anisotropic29-32

. In thermal equilibrium it has

6

∞/mm symmetry and is uniaxial. The important variable in nematic liquid crystal is

the order parameter, which measures how the molecules are aligned with the director.

The usual measure of this order is,

S = ½<3 cos2θ-1>

where S represents the thermal fluctuations of the individual molecular long axis and

the brackets <> denote a thermal averaging and θ is the angle between the molecule

(molecular long axis) and the director .

If the classical nematic has an additional short range smectic like ordering,

then it is called cybotactic nematic. The short range smectic like layers are either

orthogonal or tilted to the layer planes and they are denoted as normal cybotactic, Nnc

and skewed cybotactic, Nsc phases respectively. Biaxial nematics are also reported32,

33. The nematics exhibit schlieren, threaded marble, pseudo isotropic textures when

the sample is observed through crossed polarizers.

Cholesteric or spontaneously twisted nematic

Cholesteric LC’s are regarded as special kind of nematics. The nematic phase

exhibited by an optically active compound is called spontaneously twisted nematic or

cholesteric. The name cholesteric is given to these compounds as the first materials

exhibiting this phase are from cholesterol derivatives. In this phase, the molecules

prefer to lie next to each other in a slightly skewed orientation. This induces helical

director configuration in which the director rotates through the material. These are

composed of helical aggregation of the molecules. Such a structure is envisaged as

being composed of sheets of molecules and within each sheet the molecules behave as

in nematic phase and has an average direction defined by a director . To quantify

7

the degree of twist, the pitch length, P of the helix is defined by convention as the

longitudinal distance in which the director has made one complete 360o revolution.

The pitch length varies with time. Each molecule is skewed at some average angle

with respect to its neighbors in a sheet above and below. Two such helical

arrangements are possible one right-handed, other left-handed, but one will have a

lower energy than the other34-37

. These are uniaxial (both positive and negative).

These exhibit focal conic with Grandjean, homogeneous and pseudo isotropic

textures. The molecular arrangement is shown below.

Smectics

Smectic is the name coined by G.Friedel for certain mesophases with

mechanical properties reminiscent of soaps. From a structural point of view, all the

smectics are layered structures, with well defined inter-layer spacing which can be

measured by X-ray diffraction technique28

. Smectic phases are characterized by both

long range orientational order as well as short range positional order. With simple

changes in mutual arrangement of the layers and with changes (in the state of order)

inside the layers, a variety of smectic phases can be obtained38-41

. The smectics

exhibit subtle polymorphism than nematics. No single compound is found to exhibit

all the smectic phases (SA to SX) so far.

There are various modifications of the smectic phases, each with different

combination of in-plane order and tilt angle. These have been named as the smectic A,

B, C, D, E, F, G, H, I, J and K phases designated as, Smectic-A42-47

, Smectic-B48-49

,

Smectic-C50-53

, Smectic-D54-55

, Smectic-E55-59

, Smectic-F60-64

, Smectic-G, Smectic-

8

H65-66

, Smectic-I44

, Smectic-J44

and Smectic-K44

or SA, SB, SC, SD, SE, SF, SG, SH, SI,

SJ, and SK, with the letters denoting the chronological order of their discovery. The

thermal sequence of their occurrence is SA, SD, SC, SB, SE, SI, SF, SG, SG (SJ), SH and

SH (SK) with decreasing temperature. SB, SG, SJ, SE, SH and SK are found to be more

crystalline than remaining liquid crystalline phases

The structural characteristics of smectic phases of classic liquid crystalline

compounds is given in Table 1.1.

Table 1.1 The structural characteristics of smectic phases of classic liquid crystalline

compounds

Phase Phase

type

Molecular

Packing

Molecular

Orientation

In-plane

Correlation

Layer

correlation

Bond-

orientation

order

Dimensionality

A fluid random orthogonal Short-range None Short-range -----------------

B

(Hexatic) hexatic hexagonal orthogonal Short-range None Long- range 3D hexatic

Crystal

B

crystal Hexagonal orthogonal long-range long-range long-range

3D

C fluid random tilted Short-range None Long-range -----------------

D Plastic rods ----------- long-range ------------- ------------- isotropic

E crystal orthorhom

bic orthogonal long-range long-range long-range 3 D

F hexatic Pseudo

hexagonal Tilt to side Short-range None long-range 3D hexatic

G crystal Pseudo

hexagonal Tilt to side long-range long-range long-range 3 D

H crystal

Herringbon

e Tilted* long-range long-range long-range 3 D

Monoclinic

I Pseudo

hexagonal Tilt to apex

Possibly long

range None long-range

Stacked 2D

crystal & 3D

hexatic

J(G’) crystal

Pseudo

hexagonal Tilt to apex long-range long-range long-range 3 D

K(H’) crystal

Herringbon

e Tilted* long-range long-range long-range 3 D

Monoclinic

9

Discotics

Mesogens with disc-like molecules are called discotics. The existence was

theoretically predicted in 1970 and mesomorphism in discotic materials (hexa

alkonoyl benzenes) was first reported in 1977 by Chandrasekhar et al25

. The

molecules of these classes are flat and disc-like in shape and able to be packed in

different structures. The classification of columnar phase is given below.

The tilted variants are also possible. Similar to rod shaped molecules the disc-

shaped molecules can also form a nematic phase characterized by a long range

orientational order, but no positional order with the director parallel to the disc

normal. In chiral compounds, the discotic nematic exhibits a twisted structure67-70

.

Sanidic

These are characterized by boards. Depending up on the relative size of the

main axes, these molecules can be derived from either rod-like or disc-like molecules.

Derived from Greek word for board, these phases are called sanidic. The rod-like and

disc-like liquid materials are characterized by the rotational symmetry of the

molecular models, which means the anisotropy of the models can be described by two

main axes. The more general case would be the consideration of molecular models

with three different main axes. The structure of sanidic phases can be characterized

by three translational periods (in accordance with length, breadth and thickness) of

scattering maxima, which cause three molecular units by investigation with X-rays71

.

10

Bowlics

Mesogens with bowl-like molecules forms the bowlic liquid crystal. These

contain molecules, which are three dimensional in a physical sense and can exhibit

mesophases as the one-dimensional rod-like and two-dimensional disc-like bowlic

compounds. Lin first proposed72

these bowl like molecules. Pyramidal73

shape and

cone-shaped74

are later reported. X-ray diffraction studies75

performed to investigate

the structure of these phases showed either classical columnar or a hexagonal or

oblique lattice in a two dimensional array with a disordered array in three dimensions.

11

1.4. Statement of the problem

The research on materials is mainly directed towards the design, fabrication and synthesis

of novel systems and the study of the physical properties of these for the usage in fundamental

and applied research for their applications which are useful for the society. These include the

fascinating and novel liquid crystal materials also. These systems function as both ordered

crystalline solids and isotropic liquids. That is they have the flow as well as anisotropic nature.

The research interest in these materials is two fold. On one hand new properties are expected and

on the other hand the study of interaction of different systems (namely the effect of end chains,

rigidity of the core, position of certain polar atoms like N, O, Cl etc, in the molecule) is

fascinating and interesting. Now the research is mainly directed towards the design and

fabrication of such materials for their use in chronological and scientific interest, which involve

the characterization and study of their properties with different experimental techniques

including density and optical measurements.

In recent years, liquid crystals have become lucrative and alternative candidates for the

electro optic displays in the place of conventional inorganic materials, as they are easy to design

and also for synthesis. However, these materials underline much importance to an experimental

solid-state chemist and condensed matter physicist, as they are representative of low-dimensional

systems, workable at ambient temperatures. On the other side, if the information of these phase

transitions is systematically studied in the domain of fundamental molecular forces and the

higher archive of fundamental scaling laws, it gives totality of understanding in exploitation of

their application viabilities. Moreover, these materials exhibit polymorphism involving phase

transitions of rare occurrence and far awaited theoretical expectations at the inter phase one, two,

and three –dimensional melting.

The Schiff’s base N-(p-n-alkoxybenzylidene)-p-n-alkoxy anlines popularly known as

nO.Om’s where the chain number on both sides can be varied from 1 to 18 to realize a variety of

phase sequences (variants). The quenching of oxygen atom in the molecular moiety on either

side or on both sides will provide a variety of changes in phases or the clearing temperatures or

both. That is, the electro negative oxygen atom plays an important role in deciding the clearing

temperature as well as the phase variants. It is well established that the oxygen atom in the

molecular moiety enhances the clearing temperatures as well as the nematic phase abundance in a

homologous series.

12

The earlier systematic studies are carried out at CLCRE, Acharya Nagarjuna University

and the literature survey on a number of nO.Om, nO.m, n.Om and n.m compounds regarding the

phase transitions across different LC phases and the molecular polarizability studies which

includes the estimation of molecular polarizability anisotropy of a liquid crystal material

employing the modified Lippincott - δ function and molecular vibrational models, motivated the

author to synthesize N-(p-n-heptyloxybenzylidene)-p-n-alkoxy anlines with alkoxy chain

number, m = 3, 5, 6, 7, and 9 with the molecular formula.

These liquid crystal materials show different phase variants namely N, N-C, N-C-I giving

rise to different phase transformations, such as isotropic to nematic, nematic to smectic-C, and

smecti-C to smectic-I phases.

The order of phase transitions can be studied by number of experimental techniques. The

author used the density studies to arrive at the order of the phase tansformation.

These experimental and semi empirical theoretical calculations enabled for the study of

temperature variation of orientational nematic order parameter, S in the nematic phase.

In the estimation of order parameter, S the author used different methods and field

models. The author used the birefringence data along with density results to understand the

nature of phase transformations and for the determination of molecular polarizabilities of the

molecule employing different local field models which the molecule experiences in the nematic

phase. Further, the author studied the variation of order parameter, S with temperature in all the

compounds and able to discuss the results with the body of the data available. The comparative

study of S from different methods on these compounds and the compounds available in literature

is studied.

Further, different thermodynamic parameters are estimated from the density data for these

compounds and compared the results with the other compounds available in literature.

C7H15O OCmH2m+1 CH = N

13

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14

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