chapter two liquid crystals

7/29/2019 Chapter Two Liquid Crystals

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Chapter 2: Liquid Crystals

States between crystalline and

isotropic liquid


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Liquid Crystals, 1805-1922.

Before discovery of LC, Lehmann designed a microscope that

could be used to monitor phase transition process.


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


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C11H23O CO2H

C S N I

84.5o

128o

139.5o

Phase Transition first defined by

Georges Freidel in 1922


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The ordering parameter

S=1/2

S=0, isotropic

S=1, Ordered

Nematic, S=0.5-0.6


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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.


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Smectic B Liquid Crystals


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Smectic C Liquid Crystals


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Smectic A Liquid Crystals


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More Detailed Classification of Smectic Phases


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Nematic Liquid Crystals


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Cholesteric Phase Liquid Crystals


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Polymeric Liquid Crystal


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Advantages of Nematic Phase and Cholesteric Phase LC

For Display Propose

Low ViscosityFast Response Time


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Discotic Liquid Crystals


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Response to Electric and Magnetic Fields


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External Electric Field and Dielectric Properties of LC molecules


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Dielectric Constant

k0L = C = q/V


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Flow of ions in the presence of electric field

Internal Field StrengthE = E0E


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S = 0 1 > S > 0

Alignment of LC molecules in Electric Field


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m m

Dielectric Anisotropy and Permanent Dipole Moment


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Dielectric Anisotropy and Induced Dipole Moment

easily polarized

Molecular axis

minduced is large is large

minduced 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


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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 generaln is a very important parameter for a LC device.

Larger the n value, thinner the LC device and faster the

response time


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O

S C NC5H11

= +33C - N - I

76 98

O

O C7H15

C

N

C5H11 = - 4.0

C - N - I

45 101

Examples


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Magnetic Susceptibility and Anisotropy


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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 magneticfield. Therefore the magnetic susceptibility is negative

//

large

small


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Light as Electromagnetic Wave

Plane Polarized light can be resolved into Ex and Ey


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Birefringence


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Ordinary light travels in thecrystal with the same speed v in

all direction.

The refractive index n0=c/v in

all direction are identical.

Extraordinary lighttravels in the crystalwith a speed v that varies with direction.

The refractive index n0=c/v also varies

with different direction


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Generation of polarized light by crystal birefringence


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Interaction of Electromagnetic Wave with LC Molecules


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E field

Induced dipole

by electromagnetic wave

Propagation of the light is hindered by the molecule

Speed of the light is slowed down

= C / //

//


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E field

Induced dipole

by electromagnetic wave

Propagation of the light parallel to the molecular axis

Change of the speed is relatively small

// = C// /

//


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Circular Birefringence


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R fl ti f Ci l P l i d Li ht


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Reflection of Circular Polarized Light


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Devices for Liquid Crystal Display


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Designs of LC cell

Electronic Drive

AM: active matrix; TFT: thin film transistor;

MIM: metal-insulator-metal


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Alignment of LC molecules in a Display Device


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Dynamic Scattering Mode LCD Device


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Twisted Nematic (TN) Device 1971 by Schadt


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Optical Response of a Twisted Nematic (TN) Device

Applied voltages and optical response

S T i t d N ti (STN) LC D i 1984 b S h ff


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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 scanning

Vs: turn on voltage

Vns:

turn off voltage


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Sharp change in the voltage-transmittance curve

Electrically Controlled Birefringence (ECB) Device (DAP type)


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Black and White

RF-STN Device


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

In Plane Switching (IPS) type LC Device


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In-Plane Switching (IPS) type LC Device

Polymer Dispersed Liquid Crystal (PDLC) Device


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Polymer Dispersed Liquid Crystal (PDLC) Device


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Polymeric Nematic LC Materials

Active Matrix LCD


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Active Matrix LCD


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Structure of a typical

LC Display


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Hybrid Aligned Nematic (HAN) type


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Hybrid Aligned Nematic (HAN) type

Fast response time,Upto ms scale.


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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


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Structure of rigid rod like liquid crystal molecules

Core group: usually aromatic or alicyclic; to make the structure linear and rigid

Linker: maintaining the linearity and polarizability anisotropic.

Terminal Chain: usually aliphatic chain, linear but soft so that the melting point could bereduced. 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

Common components for LC molecules


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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


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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

like structure upon heating

(5) Chemically stable.

(6) Phase transition temperature is determined by Hand S. At TCN or TNI, Go = HoTSo= 0.Therefore TCN= HoCN/SoCN and TNI= HoNI/SoNI

L


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n

L/D > 4 Ti > Tm (nematic)

D

No. of Phenyl ring L/D Ti Tm

2 2 773 3 213

4 3.9 320

5 4.8 445 388

6 5.5 565 438


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When the length of the molecules increases, van der

Waals 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 6-10 o67 o


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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


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6-10 o67 o

This type of linker group is more flexible. Entropy gain ismore 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 254o

C

Other Options for the core group.


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Other Options for the core group.

Thermal Stability:


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Thermal Stability:

Low TC-N; high TN-IlargerT = 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 LCmolecules will increase. This will lead to higher thermal

stability.

Crystal Nematic LC Isotropic Liquid

TC-N TN-IT


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(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.

S ti Ph


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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 packwell 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

OH

RO

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


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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

T i l f ti


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N CH

MeO XShort chain

Terminal group for nematic

F S Ph


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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


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Nematic Phase.

(1) Due to its fast response time, the nematic LC phase is

technologically the most importantof the many differenttypes 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.


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(1) At least two rings are requiredto enable the generation of

LC phase.

(2) The nematic phase tends to be the phase exhibited whenthe conditions for the lamellar packing (smectic) cannotbe met.

(3) Molecular features for nematic phase: (a) breaking of the

symmetry or (b) short terminal chain.


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C N TiTm Ti

24 35

130 239

84 127

68 130

71 (52)

204

95

3.5

34

Stereochemistry of alicyclic systems


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NC R

H

H

NC H

H

R

No LC phase

Stereochemistry of alicyclic systems

C N ITm Ti


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CNC5H11

CNC5H11

CNC5H11

CNC5H11

C N I

48

24

31

62

61

35

55

100

Change in the core structure of one phenyl ring for a range ofnon-aromatic rings only leads to increasing Tm and Ti, indicating

that packing effect is more important than the polarizability effectfor nematic phase. The ring functions in a space-filling manner,preventing the molecule form tumbling and maintaining theorientational ordering.


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H


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Heteroatom

effects

The heteroatoms enhances the polarity and higher melting point are seen. Nematicphase 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.

MM2 space-filling models


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UnsymmetricalTNI = 19

oCFlat moleculeTNI = 55

oCSymmetrical butrings areperpendicularTNI = 28

oC


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The TCN and TNI orders: dicyclooctane > cyclohexane > phenyl

MM2 calculation


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Linear

structure

Bent

structure


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Extending the number of the rings

Linking group:


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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 - -

2 171 312

3 - -

4 156 270

5 - -


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Odd number of

CH2: Bent

Even number

of CH2: Linear

(b) Linker groups that connect aromatic core units with


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the conjugation extended over the longer molecules.

This could enhance the polarizability anisotropy.

Other common linker groups


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O

O

O O

O

e.g.

C5H11

O CN

O

C5H11

CN

Tm Ti

48

30

79

51Amide linker cannot be used due to the strong hydrogen bond

interactions that lead to high melting temperature

Terminal Flexible Long Chain:


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The function of the terminal flexible long chain is to suppress

the melting point without serious destroying the LC phase.


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Lateral Substitution


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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.


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X Not quite linear for

some substituents


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CO2HC8H17Osome substituents


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Electronic effects arising from the lateral groups


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g g p

Mixing of two Components may generate a LC phase


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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


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Mixture of two Components

N C4H9

RO

MBBA R = Me

EBBA R = Et

A mixture of MBBA (60%) and

EBBA (40%) would lead to LC at

room temperature

Temperature Dependent Rotation of the Cholesteric Phase


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Cl

H

CH3(CH2)12CO2

H

Left

Right

Temperature Dependent Rotation of the Cholesteric Phase

i Ch i i id C l l


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Main Chain Liquid Crystal Polymer

mesogenic unit flexible linker

Side Chain Liquid Crystal Polymer

Polymer

BackbonePolymer

Backbone

Terminally attached

Laterally attached

C bi d Li id C t l P l


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Combined Liquid Crystal Polymer


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Lyotropic Liquid Crystal Polymers

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

NHHN

O O

Kelver

HN

O

PBA

Dissolve and


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Dissolve andLC formation

Fiber formation to give high tensile strength fibers

Common Components for Lyotropic Liquid Crystals

N

ON

O N

SN

S

Examples


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N

SN

Sn

Poly(p-phenylenebenzobisthiazole) PBT

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

N

ON

On

chapter two liquid crystals

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