Chapter 2: Liquid CrystalsStates between crystalline and

isotropic liquid

Liquid Crystals, 1805-1922. Before discovery of LC, Lehmann designed a microscope that could be used to monitor phase transition process.

1888 by Prof. Reinitzer, a botanist, University of Prague, Germany

C11H23O CO2H

C S N I

84.5o 128o 139.5o

Phase Transition first defined by Georges Freidel in 1922

The ordering parameterS=1/2<3cos2-1>

S=0, isotropicS=1, OrderedNematic, S=0.5-0.6

Classification of Smectic Liquid Crystals

A type: molecular alignment perpendicular to the surface of the layer, but lack of order within the layer.

B type: molecular alignment perpendicular to the surface of the layer, having order within the layer.

C type: having a tilted angle between molecular alignment and the surface of the layer.

Smectic B Liquid Crystals

Smectic C Liquid Crystals

Smectic A Liquid Crystals

More Detailed Classification of Smectic Phases

Nematic Liquid Crystals

Cholesteric Phase Liquid Crystals

Polymeric Liquid Crystal

Advantages of Nematic Phase and Cholesteric Phase LC

For Display Propose

Low ViscosityFast Response Time

Discotic Liquid Crystals

Response to Electric and Magnetic Fields

External Electric Field and Dielectric Properties of LC molecules

Dielectric Constant

L = C = q/V

Flow of ions in the presence of electric field

Internal Field Strength E = E0 – E’

S = 0 1 > S > 0

Alignment of LC molecules in Electric Field

Dielectric Anisotropy and Permanent Dipole Moment

Dielectric Anisotropy and Induced Dipole Moment

easily polarized

Molecular axis

induced is large is large

induced is small

is small

+ -r//

+

-

r

dielectric constant along the direction perpendicular to the molecular axis

dielectric constant along the direction parallel to the molecular axis

Light is a high frequency electromagnetic wave and will only

polarize electron cloud.

In general, = > 0 or

Positive> 0 (10 to 20) Negative < 0 (-1 to -2)

For high electrical resistance materials, n is proportional

to 1/2

n = n n > 0 in general

n is a very important parameter for a LC device. Larger the n value, thinner the LC device and faster the response time

O

S C N

C5H11

= +33

C - N - I76 98

O

O C7H15

C

N

C5H11 = - 4.0

C - N - I45 101

Examples

Magnetic Susceptibility and Anisotropy

Most of the organic molecules have closed-shell structure which is diamagnetic. In particular, the aromatic component will lead to a ring current that against the external magnetic field. Therefore the magnetic susceptibility is negative

//

large

small

Light as Electromagnetic Wave

Plane Polarized light can be resolved into Ex and Ey

Birefringence

Ordinary light travels in the crystal with the same speed v in all direction. The refractive index n0=c/v in all direction are identical.

Extraordinary light travels in the crystal with a speed v that varies with direction.The refractive index n0=c/v also varies with different direction

Generation of polarized light by crystal birefringence

Interaction of Electromagnetic Wave with LC Molecules

E field

Induced dipole by electromagnetic wave

Propagation of the light is hindered by the molecule

Speed of the light is slowed down

= C ///

//

E field

Induced dipole by electromagnetic wave

Propagation of the light parallel to the molecular axis

Change of the speed is relatively small

// = C// /

//

Circular Birefringence

Reflection of Circular Polarized Light

Devices for Liquid Crystal Display

Designs of LC cell

Electronic DriveAM: active matrix; TFT: thin film transistor; MIM: metal-insulator-metal

Alignment of LC molecules in a Display Device

Dynamic Scattering Mode LCD Device

Twisted Nematic (TN) Device 1971 by Schadt

Optical Response of a Twisted Nematic (TN) Device

Applied voltages and optical response

Super Twisted Nematic (STN) LC Device 1984 by Scheffer

By addition of appropriate amounts of chiral reagent

Twisted by 180-270 o

N:Number of row for scanningVs: turn on voltageVns:turn off voltage

Sharp change in the voltage-transmittance curve

Electrically Controlled Birefringence (ECB) Device (DAP type)

Black and White RF-STN Device

Optical response of Nematic LC in a Phase-ChangeGuest-Host Type Device (by G. Heilmeier)

Phase Change (PC) in a Guest Host (GH) LC Device

In-Plane Switching (IPS) type LC Device

Polymer Dispersed Liquid Crystal (PDLC) Device

Polymeric Nematic LC Materials

Active Matrix LCD

Structure of a typical LC Display

Hybrid Aligned Nematic (HAN) type

Fast response time,Upto ms scale.

Full color reflective display

References(1) Liquid Crystals, P. J. Collings, Princeton(2) Introduction to liquid crystals, P. J. Collings and M. Hird, Taylor and Francis(3) Flat Panel Displays, J. A. Connor, RSC.

Structure of rigid rod like liquid crystal molecules

Core group: usually aromatic or alicyclic; to make the structure linear and rigidLinker: maintaining the linearity and polarizability anisotropic.Terminal Chain: usually aliphatic chain, linear but soft so that the melting point could be reduced. Without significant destroy the LC phase. Note that sometimes one terminal unit is replaced by a polar group to provide a more stable nematic phase.Side group: to control the lateral interaction and thereore enhance the chance for nematic. Note that large side groups will weaken the lateral interaction

Linker A, B -(CH=N)-; -(N=N)--(N=NO)-; -(O-C=O)-

Terminal Group X, YNon-polar flexible groups

-R, -OR, -O2CRPolar rigid group

-CN, -CO2H, -NO2, -F, -NCS

Core Group

Common components for LC molecules

Side Branch-F, -Cl, -CN, -CH3

Character of LC molecules

(1) Rod like or Discotic(2) Empirical Length/Diameter parameter for LC phase

4 (Flory theory predicted critical L/D ratio = 6.4; Onsager theory predicted critical L/D ratio = 3.5)

(3) Having polar or highly polarizable moiety(4) Large enough rigidity to maintain the rod or discotic l

ike structure upon heating(5) Chemically stable.(6) Phase transition temperature is determined by H an

d S. At TCN or TNI, Go = Ho –TSo= 0. Therefore TCN= Ho

CN/SoCN and TNI= Ho

NI/SoNI

n

L/D > 4 Ti > Tm (nematic)

L

D

No. of Phenyl ring L/D Ti Tm2 2 773 3 2134 3.9 3205 4.8 445 3886 5.5 565 438

When the length of the molecules increases, van der Waal’s interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.

In the above examples, the critical L/D is around 4. When L/D = 1, 2, or 3, no LC phase was observed.

O

O

O

O

n DL

n L/D Ti Tm

1 3.8 1322 5.1 254 1763 6.4 464 220

Nematic phase could not be observed until L/D >4

Flexible linker

6-10 o67 o

6-10 o67 o

This type of linker group is more flexible. Entropy gain is more effective in isotropic liquid state. Therefore SN-I is relatively large, leading to a low Ti. In the presence case, even for the LC molecules having the L/D upto 5.1, the Ti is only 254 oC

Other Options for the core group.

Thermal Stability:

Low TC-N; high TN-I

larger T = TN-I - TC-N , higher the stability of the LC state

In general, shorter the LC molecule, lower the phase transition temperature it has.For LC molecule contains more polarizable aromatic cores, or longer the body, Vander Waals interactions between LC molecules will increase. This will lead to higher thermal stability.

Crystal Nematic LC Isotropic LiquidTC-N

TN-I

T

(1) Nematogenic: structures that lead to nematic phase as the only LC phase

(2) Smectogenic: smectic phase is the only mesophase exhibited

(3) Calamitic: Both nematic and smectic phases would exhibited.

Smectic Phase

Smectic LC phase: Lamellar close packing structure are favored by a symmetrical molecular structure; Wholly aromatic core-alicyclic core each with two terminals alkyl/alkoxyl chains compatible with the core ten to pack well into a layer-like structures and generates smectic phase.

Long alkyl/alkoxyl chain would lead to strong lateral interactions that favors lamellar packing smectic phase formation.

RO

OHR

O

HO

R = C5H11 TCN = 88; TNI=126.5

R = C8H17O TCS = 101; TSN = 108; TNI=147

R = C10H21O TCS = 97; TSN = 122; TNI=142

Terminal groups for smectic phase

(1) Salts from RCO2H/RNH2

(2) Terminal groups contain -CO2R, -CH=CHCOR, -CONH2, -OCF3, -Ph, -NHCOCH3, -OCOCH3

N CH

C8H17O X

N CH

MeO XShort chain

Terminal group for nematic

For Smectic Phase

NHCOCH3 > Br > Cl > F > NMe2 > RO > H > NO2 > OMe

For nematic Phase

NHCOCH3 > OMe > NO2 > RO > Br~ Cl > NMe2 > Me >F > H

-CN,-NO2 -MeO are nematogen: poor smectic/good nematic-NHCOCH3, halogen, -NR2, good smectic/nematic

Nematic Phase.

(1) Due to its fast response time, the nematic LC phase is technologically the most important of the many different types of LC phase

(2) The smectic phases are lamellar in structure and more ordered than the nematic phase.

(3) The smectic phases are favored by an symmetrical molecular structure.

(4) Any breaking of the symmetry or where the core is long relative to the overall molecular length tends to destabilized the smectic formation and facilitate the nematic phase formation.

(1) At least two rings are required to enable the generation of LC phase.

(2) The nematic phase tends to be the phase exhibited when the conditions for the lamellar packing (smectic) cannot be met.

(3) Molecular features for nematic phase: (a) breaking of the symmetry or (b) short terminal chain.

C N TiTm Ti

24 35

130 239

84 127

68 130

71 (52)

204

95

3.5

34

NC R

H

H

NC H

H

R

No LC phase

Stereochemistry of alicyclic systems

CNC5H11

CNC5H11

CNC5H11

CNC5H11

C N ITm Ti

48

24

31

62

61

35

55

100

Change in the core structure of one phenyl ring for a range of non-aromatic rings only leads to increasing Tm and Ti, indicating that packing effect is more important than the polarizability effect for nematic phase. The ring functions in a space-filling manner, preventing the molecule form tumbling and maintaining the orientational ordering.

Heteroatomeffects

The heteroatoms enhances the polarity and higher melting point are seen. Nematic phase transition temperature is low than the melting point. The large sulfur atom further disrupts the nematic packing. The flexible sulfur containing ring gains more entropy from N to I and therefore lead to lower TNI.

UnsymmetricalTNI = 19 oC

Flat moleculeTNI = 55 oC

Symmetrical but rings are perpendicularTNI = 28 oC

MM2 space-filling models

The TCN and TNI orders: dicyclooctane > cyclohexane > phenyl

MM2 calculation

Linearstructure

Bent structure

Extending the number of the rings

Linking group:

Linking groups are used to extend the length and polarizability anisotropy of the molecular core in order to enhance the LC phase stability by more than any increase in melting point, producing wider LC phase ranges.

(A) Linking group should maintain the linearity of the molecule.

(CH2)nR R

R = N CH

OCH3

where

n Tm Ti

0 266 >390

1 - -

2 171 3123 - -

4 156 2705 - -

Odd number of CH2: Bent

Even number of CH2: Linear

(b) Linker groups that connect aromatic core units with the conjugation extended over the longer molecules. This could enhance the polarizability anisotropy.

Other common linker groups

O

O

O O

O

e.g.

C5H11

O CN

O

C5H11

CN

Tm Ti

48

30

79

51

Amide linker cannot be used due to the strong hydrogen bond interactions that lead to high melting temperature

Terminal Flexible Long Chain:

The function of the terminal flexible long chain is to suppress the melting point without serious destroying the LC phase.

Lateral Substitution

Lateral substitution is important in both nematic/smectic systems. However, because of the particular disruption to the lamellar packing, necessary for smectic phases, lateral substitution nearly always reduces smectic phase stability more than nematic phase stability except when the lateral substitutions lead to a strong dipole-dipole interaction.

CO2HC8H17O

X Not quite linear for some substituents

Electronic effects arising from the lateral groups

CO2HRO

R = Me or Et

Doesn't show LC properties

RO

O

OH

OR or R'

O

HO

LC

Mixing of two Components may generate a LC phase

Mixture of two Components

N C4H9

RO

MBBA R = MeEBBA R = Et

A mixture of MBBA (60%) and EBBA (40%) would lead to LC at room temperature

Cl

H

CH3(CH2)12CO2

H

Left

Right

Temperature Dependent Rotation of the Cholesteric Phase

Main Chain Liquid Crystal Polymer

mesogenic unit flexible linker

Side Chain Liquid Crystal Polymer

Polymer Backbone

Polymer Backbone

Terminally attached

Laterally attached

Combined Liquid Crystal Polymer

Lyotropic Liquid Crystal Polymers

Fairly rigid rod like polymers; but soluble in certain solvents to form a LC phase

NHHN

O O

Kelver

HN

O

PBA

Dissolve and LC formation

Fiber formation to give high tensile strength fibers

Common Components for Lyotropic Liquid Crystals

N

ON

O N

SN

S

Examples

N

SN

S nPoly(p-phenylenebenzobisthiazole) PBT

Soluble in PPA or H2SO2 and could be fabricated as high tensile strength polymeric wires

N

ON

O n